The Variation of Animals and Plants under Domestication
by
Charles Darwin
Part 6 out of 12
for instance, it has been clearly proved, more especially by Mr. J.A. Allen,
that, with birds, many species differ in tint, size of body and of beak, and
in length of tail, in proceeding from the North to the South; and it appears
that these differences must be attributed to the direct action of temperature.
(23/46. Professor Weismann comes to the same conclusion with respect to
certain European butterflies in his valuable essay 'Ueber den Saison-
Dimorphismus' 1875. I might also refer to the recent works of several other
authors on the present subject; for instance to Kerner's 'Gute und schlechte
Arten' 1866.) With respect to plants I will give a somewhat analogous case:
Mr. Meehan (23/47. 'Proc. Acad. Nat. Soc. of Philadelphia' January 28, 1862.),
has compared twenty-nine kinds of American trees with their nearest European
allies, all grown in close proximity and under as nearly as possible the same
conditions. In the American species he finds, with the rarest exceptions, that
the leaves fall earlier in the season, and assume before their fall a brighter
tint; that they are less deeply toothed or serrated; that the buds are
smaller; that the trees are more diffuse in growth and have fewer branchlets;
and, lastly, that the seeds are smaller--all in comparison with the
corresponding European species. Now considering that these corresponding trees
belong to several distinct orders, and that they are adapted to widely
different stations, it can hardly be supposed that their differences are of
any special service to them in the New and Old worlds; and if so such
differences cannot have been gained through natural selection, and must be
attributed to the long continued action of a different climate.
GALLS.
Another class of facts, not relating to cultivated plants, deserves attention.
I allude to the production of galls. Every one knows the curious, bright-red,
hairy productions on the wild rose-tree, and the various different galls
produced by the oak. Some of the latter resemble fruit, with one face as rosy
as the rosiest apple. These bright colours can be of no service either to the
gall-forming insect or to the tree, and probably are the direct result of the
action of the light, in the same manner as the apples of Nova Scotia or Canada
are brighter coloured than English apples. According to Osten Sacken's latest
revision, no less than fifty-eight kinds of galls are produced on the several
species of oak, by Cynips with its sub-genera; and Mr. B.D. Walsh (23/48. See
Mr. B.D. Walsh's excellent papers in 'Proc. Entomolog. Soc. Philadelphia'
December 1866 page 284. With respect to the willow see ibid 1864 page 546.)
states that he can add many others to the list. One American species of
willow, the Salix humilis, bears ten distinct kinds of galls. The leaves which
spring from the galls of various English willows differ completely in shape
from the natural leaves. The young shoots of junipers and firs, when punctured
by certain insects, yield monstrous growths resembling flowers and fir-cones;
and the flowers of some plants become from the same cause wholly changed in
appearance. Galls are produced in every quarter of the world; of several sent
to me by Mr. Thwaites from Ceylon, some were as symmetrical as a composite
flower when in bud, others smooth and spherical like a berry; some protected
by long spines, others clothed with yellow wool formed of long cellular hairs,
others with regularly tufted hairs. In some galls the internal structure is
simple, but in others it is highly complex; thus M. Lacaze-Duthiers (23/49.
See his admirable 'Histoire des Galles' in 'Annal. des Sc. Nat. Bot.' 3rd
series tome 19 1853 page 273.) has figured in the common ink-gall no less than
seven concentric layers, composed of distinct tissue, namely, the epidermic,
sub-epidermic, spongy, intermediate, and the hard protective layer formed of
curiously thickened woody cells, and, lastly, the central mass, abounding with
starch-granules on which the larvae feed.
Galls are produced by insects of various orders, but the greater number by
species of Cynips. It is impossible to read M. Lacaze-Duthiers' discussion and
doubt that the poisonous secretion of the insect causes the growth of the
gall; and every one knows how virulent is the poison secreted by wasps and
bees, which belong to the same group with Cynips. Galls grow with
extraordinary rapidity, and it is said that they attain their full size in a
few days (23/50. Kirby and Spence 'Entomology' 1818 volume 1 page 450; Lacaze-
Duthiers ibid page 284.); it is certain that they are almost completely
developed before the larvae are hatched. Considering that many gall-insects
are extremely small, the drop of secreted poison must be excessively minute;
it probably acts on one or two cells alone, which, being abnormally
stimulated, rapidly increase by a process of self-division. Galls, as Mr.
Walsh (23/51. 'Proc. Entomolog. Soc. Philadelphia' 1864 page 558.) remarks,
afford good, constant, and definite characters, each kind keeping as true to
form as does any independent organic being. This fact becomes still more
remarkable when we hear that, for instance, seven out of the ten different
kinds of galls produced on Salix humilis are formed by gall-gnats
(Cecidomyidae) which "though essentially distinct species, yet resemble one
another so closely that in almost all cases it is difficult, and in most cases
impossible, to distinguish the full-grown insects one from the other." (23/52.
Mr. B.D. Walsh ibid page 633 and December 1866 page 275.) For in accordance
with a wide-spread analogy we may safely infer that the poison secreted by
insects so closely allied would not differ much in nature; yet this slight
difference is sufficient to induce widely different results. In some few cases
the same species of gall-gnat produces on distinct species of willows galls
which cannot be distinguished; the Cynips fecundatrix, also, has been known to
produce on the Turkish oak, to which it is not properly attached, exactly the
same kind of gall as on the European oak. (23/53. Mr. B.D. Walsh ibid 1864
pages 545, 411, 495; and December 1866 page 278. See also Lacaze-Duthiers.)
These latter facts apparently prove that the nature of the poison is a more
powerful agent in determining the form of the gall than the specific character
of the tree which is acted on.
As the poisonous secretion of insects belonging to various orders has the
special power of affecting the growth of various plants; as a slight
difference in the nature of the poison suffices to produce widely different
results; and lastly, as we know that the chemical compounds secreted by plants
are eminently liable to be modified by changed conditions of life, we may
believe it possible that various parts of a plant might be modified through
the agency of its own altered secretions. Compare, for instance, the mossy and
viscid calyx of a moss-rose, which suddenly appears through bud-variation on a
Provence-rose, with the gall of red moss growing from the inoculated leaf of a
wild rose, with each filament symmetrically branched like a microscopical
spruce-fir, bearing a glandular tip and secreting odoriferous gummy matter.
(23/54. Lacaze-Duthiers ibid pages 325, 328.) Or compare, on the one hand, the
fruit of the peach, with its hairy skin, fleshy covering, hard shell and
kernel, and on the other hand one of the more complex galls with its
epidermic, spongy, and woody layers, surrounding tissue loaded with starch
granules. These normal and abnormal structures manifestly present a certain
degree of resemblance. Or, again, reflect on the cases above given of parrots
which have had their plumage brightly decorated through some change in their
blood, caused by having been fed on certain fishes, or locally inoculated with
the poison of a toad. I am far from wishing to maintain that the moss-rose or
the hard shell of the peach-stone or the bright colours of birds are actually
due to any chemical change in the sap or blood; but these cases of galls and
of parrots are excellently adapted to show us how powerfully and singularly
external agencies may affect structure. With such facts before us, we need
feel no surprise at the appearance of any modification in any organic being.
[I may, also, here allude to the remarkable effects which parasitic fungi
sometimes produce on plants. Reissek (23/55. 'Linnaea' volume 17 1843; quoted
by Dr. M.T. Masters, Royal Institution, March 16, 1860.) has described a
Thesium, affected by an Oecidium, which was greatly modified, and assumed some
of the characteristic features of certain allied species, or even genera.
Suppose, says Reissek, "the condition originally caused by the fungus to
become constant in the course of time, the plant would, if found growing wild,
be considered as a distinct species or even as belonging to a new genus." I
quote this remark to show how profoundly, yet in how natural a manner, this
plant must have been modified by the parasitic fungus. Mr. Meehan (23/56.
'Proc. Acad. Nat. Sc., Philadelphia' June 16, 1874 and July 23, 1875.) also
states that three species of Euphorbia and Portulaca olereacea, which
naturally grow prostrate, become erect when they are attacked by the Oecidium.
Euphorbia maculata in this case also becomes nodose, with the branchlets
comparatively smooth and the leaves modified in shape, approaching in these
respects to a distinct species, namely, the E. hypericifolia.]
FACTS AND CONSIDERATIONS OPPOSED TO THE BELIEF THAT THE CONDITIONS OF LIFE ACT
IN A POTENT MANNER IN CAUSING DEFINITE MODIFICATIONS OF STRUCTURE.
I have alluded to the slight differences in species naturally living in
distinct countries under different conditions; and such differences we feel at
first inclined to attribute, probably often with justice, to the definite
action of the surrounding conditions. But it must be borne in mind that there
exist many animals and plants which range widely and have been exposed to
great diversities of climate, yet remain uniform in character. Some authors,
as previously remarked, account for the varieties of our culinary and
agricultural plants by the definite action of the conditions to which they
have been exposed in the different parts of Great Britain; but there are about
200 plants (23/57. Hewett C. Watson 'Cybele Britannica' volume 1 1847 page
11.) which are found in every single English county; and these plants must
have been exposed for an immense period to considerable differences of climate
and soil, yet do not differ. So, again,, some animals and plants range over a
large portion of the world, yet retain the same character.
[Notwithstanding the facts previously given on the occurrence of highly
peculiar local diseases and on the strange modifications of structure in
plants caused by the inoculated poison of insects, and other analogous cases;
still there are a multitude of variations--such as the modified skull of the
niata ox and bulldog, the long horns of Caffre cattle, the conjoined toes of
the solid-hoofed swine, the immense crest and protuberant skull of Polish
fowls, the crop of the pouter-pigeon, and a host of other such cases--which we
can hardly attribute to the definite action, in the sense before specified, of
the external conditions of life. No doubt in every case there must have been
some exciting cause; but as we see innumerable individuals exposed to nearly
the same conditions, and one alone is affected, we may conclude that the
constitution of the individual is of far higher importance than the conditions
to which it has been exposed. It seems, indeed, to be a general rule that
conspicuous variations occur rarely, and in one individual alone out of
millions, though all may have been exposed, as far as we can judge, to nearly
the same conditions. As the most strongly marked variations graduate
insensibly into the most trifling, we are led by the same train of thought to
attribute each slight variation much more to innate differences of
constitution, however caused, than to the definite action of the surrounding
conditions.
We are led to the same conclusion by considering the cases, formerly alluded
to, of fowls and pigeons, which have varied and will no doubt go on varying in
directly opposite ways, though kept during many generations under nearly the
same conditions. Some, for instance, are born with their beaks, wings, tails,
legs, etc., a little longer, and others with these same parts a little
shorter. By the long-continued selection of such slight individual differences
which occur in birds kept in the same aviary, widely different races could
certainly be formed; and long-continued selection, important as is the result,
does nothing but preserve the variations which arise, as it appears to us,
spontaneously.
In these cases we see that domesticated animals vary in an indefinite number
of particulars, though treated as uniformly as is possible. On the other hand,
there are instances of animals and plants, which, though they have been
exposed to very different conditions, both under nature and domestication,
have varied in nearly the same manner. Mr. Layard informs me that he has
observed amongst the Caffres of South Africa a dog singularly like an arctic
Esquimaux dog. Pigeons in India present nearly the same wide diversities of
colour as in Europe; and I have seen chequered and simply barred pigeons, and
pigeons with blue and white loins, from Sierra Leone, Madeira, England, and
India. New varieties of flowers are continually raised in different parts of
Great Britain, but many of these are found by the judges at our exhibitions to
be almost identical with old varieties. A vast number of new fruit-trees and
culinary vegetables have been produced in North America: these differ from
European varieties in the same general manner as the several varieties raised
in Europe differ from one another; and no one has ever pretended that the
climate of America has given to the many American varieties any general
character by which they can be recognised. Nevertheless, from the facts
previously advanced on the authority of Mr. Meehan with respect to American
and European forest-trees it would be rash to affirm that varieties raised in
the two countries would not in the course of ages assume a distinctive
character. Dr. M. Masters has recorded a striking fact (23/58. 'Gardener's
Chronicle' 1857 page 629.) bearing on this subject: he raised numerous plants
of Hybiscus syriacus from seed collected in South Carolina and the Holy Land,
where the parent-plants must have been exposed to considerably different
conditions; yet the seedlings from both localities broke into two similar
strains, one with obtuse leaves and purple or crimson flowers, and the other
with elongated leaves and more or less pink flowers.
We may, also, infer the prepotent influence of the constitution of the
organism over the definite action of the conditions of life, from the several
cases given in the earlier chapters of parallel series of varieties,--an
important subject, hereafter to be more fully discussed. Sub-varieties of the
several kinds of wheat, gourds, peaches, and other plants, and to a limited
extent sub-varieties of the fowl, pigeon, and dog, have been shown either to
resemble or to differ from one another in a closely corresponding or parallel
manner. In other cases, a variety of one species resembles a distinct species;
or the varieties of two distinct species resemble one another. Although these
parallel resemblances no doubt often result from reversion to the former
characters of a common progenitor; yet in other cases, when new characters
first appear, the resemblance must be attributed to the inheritance of a
similar constitution, and consequently to a tendency to vary in the same
manner. We see something of a similar kind in the same monstrosity appearing
and reappearing many times in the same species of animal, and, as Dr. Maxwell
Masters has remarked to me, in the same species of plant.]
We may at least conclude, that the amount of modification which animals and
plants have undergone under domestication does not correspond with the degree
to which they have been subjected to changed circumstances. As we know the
parentage of domesticated birds far better than of most quadrupeds, we will
glance through the list. The pigeon has varied in Europe more than almost any
other bird; yet it is a native species, and has not been exposed to any
extraordinary change of conditions. The fowl has varied equally, or almost
equally, with the pigeon, and is a native of the hot jungles of India. Neither
the peacock, a native of the same country, nor the guinea-fowl, an inhabitant
of the dry deserts of Africa, has varied at all, or only in colour. The
turkey, from Mexico, has varied but little. The duck, on the other hand, a
native of Europe, has yielded some well-marked races; and as this is an
aquatic bird, it must have been subjected to a far more serious change in its
habits than the pigeon or even the fowl, which nevertheless have varied in a
much higher degree. The goose, a native of Europe and aquatic like the duck,
has varied less than any other domesticated bird, except the peacock.
Bud-variation is, also, important under our present point of view, in some few
cases, as when all the eyes on the same tuber of the potato, or all the fruit
on the same plum-tree, or all the flowers on the same plant, have suddenly
varied in the same manner, it might be argued that the variation had been
definitely caused by some change in the conditions to which the plants had
been exposed; yet, in other cases, such an admission is extremely difficult.
As new characters sometimes appear by bud-variation, which do not occur in the
parent-species or in any allied species, we may reject, at least in these
cases, the idea that they are due to reversion. Now it is well worth while to
reflect maturely on some striking case of bud-variation, for instance that of
the peach. This tree has been cultivated by the million in various parts of
the world, has been treated differently, grown on its own roots and grafted on
various stocks, planted as a standard, trained against a wall, or under glass;
yet each bud of each sub-variety keeps true to its kind. But occasionally, at
long intervals of time, a tree in England, or under the widely different
climate of Virginia, produces a single bud, and this yields a branch which
ever afterwards bears nectarines. Nectarines differ, as every one knows, from
peaches in their smoothness, size, and flavour; and the difference is so great
that some botanists have maintained that they are specifically distinct. So
permanent are the characters thus suddenly acquired, that a nectarine produced
by bud-variation has propagated itself by seed. To guard against the
supposition that there is some fundamental distinction between bud and seminal
variation, it is well to bear in mind that nectarines have likewise been
produced from the stone of the peach; and, reversely, peaches from the stone
of the nectarine. Now is it possible to conceive external conditions more
closely alike than those to which the buds on the same tree are exposed? Yet
one bud alone, out of the many thousands borne by the same tree, has suddenly,
without any apparent cause, produced a nectarine. But the case is even
stronger than this, for the same flower-bud has yielded a fruit, one-half or
one-quarter a nectarine, and the other half or three-quarters a peach. Again,
seven or eight varieties of the peach have yielded by bud-variation
nectarines: the nectarines thus produced, no doubt, differ a little from one
another; but still they are nectarines. Of course there must be some cause,
internal or external, to excite the peach-bud to change its nature; but I
cannot imagine a class of facts better adapted to force on our minds the
conviction that what we call the external conditions of life are in many cases
quite insignificant in relation to any particular variation, in comparison
with the organisation or constitution of the being which varies.
It is known from the labours of Geoffroy Saint-Hilaire, and recently from
those of Dareste and others, that eggs of the fowl, if shaken, placed upright,
perforated, covered in part with varnish, etc., produce monstrous chickens.
Now these monstrosities may be said to be directly caused by such unnatural
conditions, but the modifications thus induced are not of a definite nature.
An excellent observer, M. Camille Dareste (23/59. 'Memoire sur la Production
Artificielle des Monstruosites' 1862 pages 8-12; 'Recherches sur les
Conditions, etc., chez les Monstres' 1863 page 6. An abstract is given of
Geoffroy's Experiments by his son, in his 'Vie, Travaux' etc. 1847 page 290.),
remarks "that the various species of monstrosities are not determined by
specific causes; the external agencies which modify the development of the
embryo act solely in causing a perturbation--a perversion in the normal course
of development." He compares the result to what we see in illness: a sudden
chill, for instance, affects one individual alone out of many, causing either
a cold, or sore-throat, rheumatism, or inflammation of the lungs or pleura.
Contagious matter acts in an analogous manner. (23/60. Paget 'Lectures on
Surgical Pathology' 1853 volume 1 page 483.) We may take a still more specific
instance: seven pigeons were struck by rattle-snakes (23/61. 'Researches upon
the Venom of the Rattle-snake' January 1861 by Dr. Mitchell page 67.): some
suffered from convulsions; some had their blood coagulated, in others it was
perfectly fluid; some showed ecchymosed spots on the heart, others on the
intestines, etc.; others again showed no visible lesion in any organ. It is
well known that excess in drinking causes different diseases in different men;
but in the tropics the effects of intemperance differ from those caused in a
cold climate (23/62. Mr. Sedgwick 'British and Foreign Medico-Chirurg. Review'
July 1863 page 175.); and in this case we see the definite influence of
opposite conditions. The foregoing facts apparently give us as good an idea as
we are likely for a long time to obtain, how in many cases external conditions
act directly, though not definitely, in causing modifications of structure.
SUMMARY.
There can be no doubt, from the facts given in this chapter, that extremely
slight changes in the conditions of life sometimes, probably often, act in a
definite manner on our domesticated productions; and, as the action of changed
conditions in causing indefinite variability is accumulative, so it may be
with their definite action. Hence considerable and definite modifications of
structure probably follow from altered conditions acting during a long series
of generations. In some few instances a marked effect has been produced
quickly on all, or nearly all, the individuals which have been exposed to a
marked change of climate, food, or other circumstance. This has occurred with
European men in the United States, with European dogs in India, with horses in
the Falkland Islands, apparently with various animals at Angora, with foreign
oysters in the Mediterranean, and with maize transported from one climate to
another. We have seen that the chemical compounds of some plants and the state
of their tissues are readily affected by changed conditions. A relation
apparently exists between certain characters and certain conditions, so that
if the latter be changed the character is lost--as with the colours of
flowers, the state of some culinary plants, the fruit of the melon, the tail
of fat-tailed sheep, and the peculiar fleeces of other sheep.
The production of galls, and the change of plumage in parrots when fed on
peculiar food or when inoculated by the poison of a toad, prove to us what
great and mysterious changes in structure and colour, may be the definite
result of chemical changes in the nutrient fluids or tissues.
We now almost certainly know that organic beings in a state of nature may be
modified in various definite ways by the conditions to which they have been
long exposed, as in the case of the birds and other animals in the northern
and southern United States, and of American trees in comparison with their
representatives in Europe. But in many cases it is most difficult to
distinguish between the definite result of changed conditions, and the
accumulation through natural selection of indefinite variations which have
proved serviceable. If it profited a plant to inhabit a humid instead of an
arid station, a fitting change in its constitution might possibly result from
the direct action of the environment, though we have no grounds for believing
that variations of the right kind would occur more frequently with plants
inhabiting a station a little more humid than usual, than with other plants.
Whether the station was unusually dry or humid, variations adapting the plant
in a slight degree for directly opposite habits of life would occasionally
arise, as we have good reason to believe from what we actually see in other
cases.
The organisation or constitution of the being which is acted on, is generally
a much more important element than the nature of the changed conditions, in
determining the nature of the variation. We have evidence of this in the
appearance of nearly similar modifications under different conditions, and of
different modifications under apparently nearly the same conditions. We have
still better evidence of this in closely parallel varieties being frequently
produced from distinct races, or even distinct species; and in the frequent
recurrence of the same monstrosity in the same species. We have also seen that
the degree to which domesticated birds have varied, does not stand in any
close relation with the amount of change to which they have been subjected.
To recur once again to bud-variations. When we reflect on the millions of buds
which many trees have produced, before some one bud has varied, we are lost in
wonder as to what the precise cause of each variation can be. Let us recall
the case given by Andrew Knight of the forty-year-old tree of the yellow
magnum bonum plum, an old variety which has been propagated by grafts on
various stocks for a very long period throughout Europe and North America, and
on which a single bud suddenly produced the red magnum bonum. We should also
bear in mind that distinct varieties, and even distinct species,--as in the
case of peaches, nectarines, and apricots,--of certain roses and camellias,--
although separated by a vast number of generations from any progenitor in
common, and although cultivated under diversified conditions, have yielded by
bud-variation closely analogous varieties. When we reflect on these facts we
become deeply impressed with the conviction that in such cases the nature of
the variation depends but little on the conditions to which the plant has been
exposed, and not in any especial manner on its individual character, but much
more on the inherited nature or constitution of the whole group of allied
beings to which the plant in question belongs. We are thus driven to conclude
that in most cases the conditions of life play a subordinate part in causing
any particular modification; like that which a spark plays, when a mass of
combustibles bursts into flame--the nature of the flame depending on the
combustible matter, and not on the spark. (23/63. Professor Weismann argues
strongly in favour of this view in his 'Saison-Dimorphismus der
Schmetterlinge' 1875 pages 40-43.)
No doubt each slight variation must have its efficient cause; but it is as
hopeless an attempt to discover the cause of each, as to say why a chill or a
poison affects one man differently from another. Even with modifications
resulting from the definite action of the conditions of life, when all or
nearly all the individuals, which have been similarly exposed, are similarly
affected, we can rarely see the precise relation between cause and effect. In
the next chapter it will be shown that the increased use or disuse of various
organs produces an inherited effect. It will further be seen that certain
variations are bound together by correlation as well as by other laws. Beyond
this we cannot at present explain either the causes or nature of the
variability of organic beings.
CHAPTER 2.XXIV.
LAWS OF VARIATION--USE AND DISUSE, ETC.
NISUS FORMATIVUS, OR THE CO-ORDINATING POWER OF THE ORGANISATION.
ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS.
CHANGED HABITS OF LIFE.
ACCLIMATISATION WITH ANIMALS AND PLANTS.
VARIOUS METHODS BY WHICH THIS CAN BE EFFECTED.
ARRESTS OF DEVELOPMENT.
RUDIMENTARY ORGANS.
In this and the two following chapters I shall discuss, as well as the
difficulty of the subject permits, the several laws which govern Variability.
These may be grouped under the effects of use and disuse, including changed
habits and acclimatisation--arrest of development--correlated variation--the
cohesion of homologous parts-the variability of multiple parts--compensation
of growth--the position of buds with respect to the axis of the plant--and
lastly, analogous variation. These several subjects so graduate into one
another that their distinction is often arbitrary.
It may be convenient first briefly to discuss that coordinating and reparative
power which is common, in a higher or lower degree, to all organic beings, and
which was formerly designated by physiologists as nisus formativus.
[Blumenbach and others (24/1. 'An Essay on Generation' English translation
page 18; Paget 'Lectures on Surgical Pathology' 1853 volume 1 page 209.) have
insisted that the principle which permits a Hydra, when cut into fragments, to
develop itself into two or more perfect animals, is the same with that which
causes a wound in the higher animals to heal by a cicatrice. Such cases as
that of the Hydra are evidently analogous to the spontaneous division or
fissiparous generation of the lowest animals, and likewise to the budding of
plants. Between these extreme cases and that of a mere cicatrice we have every
gradation. Spallanzani (24/2. 'An Essay on Animal Reproduction' English
translation 1769 page 79.) by cutting off the legs and tail of a Salamander,
got in the course of three months six crops of these members; so that 687
perfect bones were reproduced by one animal during one season. At whatever
point the limb was cut off, the deficient part, and no more, was exactly
reproduced. When a diseased bone has been removed, a new one sometimes
"gradually assumes the regular form, and all the attachments of muscles,
ligaments, etc., become as complete as before." (24/3. Carpenter 'Principles
of Comp. Physiology' 1854 page 479.)
This power of regrowth does not, however, always act perfectly; the reproduced
tail of a lizard differs in the form of the scales from the normal tail: with
certain Orthopterous insects the large hind legs are reproduced of smaller
size (24/4. Charlesworth 'Mag. of Nat. Hist.' volume 1 1837 page 145.): the
white cicatrice which in the higher animals unites the edges of a deep wound
is not formed of perfect skin, for elastic tissue is not produced till long
afterwards. (24/5. Paget 'Lectures on Surgical Pathology' volume 1 page 239.)
"The activity of the nisus formativus," says Blumenbach, "is in an inverse
ratio to the age of the organised body." Its power is also greater with
animals, the lower they stand in the scale of organisation; and animals low in
the scale correspond with the embryos of higher animals belonging to the same
class. Newport's observations (24/6. Quoted by Carpenter 'Comp. Phys.' page
479.) afford a good illustration of this fact, for he found that "myriapods,
whose highest development scarcely carries them beyond the larva of perfect
insects, can regenerate limbs and antennae up to the time of their last
moult;" and so can the larvae of true insects, but, except in one order, not
in the mature insect. Salamanders correspond in development with the tadpoles
or larvae of the tailless Batrachians, and both possess to a large extent the
power of regrowth; but not so the mature tailless Batrachians.
Absorption often plays an important part in the repair of injuries. When a
bone is broken and does not unite, the ends are absorbed and rounded, so that
a false joint is formed; or if the ends unite, but overlap, the projecting
parts are removed. (24/7. Prof. Marey's discussion on the power of co-
adaptation in all parts of the organisation is excellent. 'La Machine Animale'
1873 chapter 9. See also Paget 'Lectures' etc. page 257.) A dislocated bone
will form for itself a new socket. Displaced tendons and varicose veins
excavate new channels in the bones against which they press. But absorption
comes into action, as Virchow remarks, during the normal growth of bones;
parts which are solid during youth become hollowed out for the medullary
tissue as the bone increases in size. In trying to understand the many well-
adapted cases of regrowth when aided by absorption, we should remember that
almost all parts of the organisation, even whilst retaining the same form,
undergo constant renewal; so that a part which is not renewed would be liable
to absorption.
Some cases, usually classed under the so-called nisus formativus, at first
appear to come under a distinct head; for not only are old structures
reproduced, but new structures are formed. Thus, after inflammation "false
membranes," furnished with blood-vessels, lymphatics, and nerves, are
developed; or a foetus escapes from the Fallopian tubes, and falls into the
abdomen, "nature pours out a quantity of plastic lymph, which forms itself
into organised membrane, richly supplied with blood-vessels," and the foetus
is nourished for a time. In certain cases of hydrocephalus the open and
dangerous spaces in the skull are filled up with new bones, which interlock by
perfect serrated sutures. (24/8. These cases are given by Blumenbach in his
'Essay on Generation' pages 52, 54.) But most physiologists, especially on the
Continent, have now given up the belief in plastic lymph or blastema, and
Virchow (24/9. 'Cellular Pathology' translation by Dr. Chance 1860 pages 27,
441.) maintains that every structure, new or old, is formed by the
proliferation of pre-existing cells. On this view false membranes, like
cancerous or other tumours, are merely abnormal developments of normal
growths; and we can thus understand how it is that they resemble adjoining
structures; for instance, that a "false membrane in the serous cavities
acquires a covering of epithelium exactly like that which covers the original
serous membrane; adhesions of the iris may become black apparently from the
production of pigment-cells like those of the uvea." (24/10. Paget 'Lectures
on Pathology' volume 1 1853 page 357.)
No doubt the power of reparation, though not always perfect, is an admirable
provision, ready for various emergencies, even for such as occur only at long
intervals of time. (24/11. Paget ibid page 150.) Yet this power is not more
wonderful than the growth and development of every single creature, more
especially of those which are propagated by fissiparous generation. This
subject has been here noticed, because we may infer that, when any part or
organ is either greatly increased in size or wholly suppressed through
variation and continued selection, the co-ordinating power of the organisation
will continually tend to bring again all the parts into harmony with one
another.]
ON THE EFFECTS OF THE INCREASED USE AND DISUSE OF ORGANS.
It is notorious, and we shall immediately adduce proofs, that increased use or
action strengthens muscles, glands, sense-organs, etc.; and that disuse, on
the other hand, weakens them. It has been experimentally proved by Ranke
(24/12. 'Die Blutvertheilung, etc. der Organe' 1871 as quoted by Jaeger 'In
Sachen Darwin's' 1874 page 48. See also H. Spencer 'The Principles of Biology'
volume 2 1866 chapters 3-5.) that the flow of blood is greatly increased
towards any part which is performing work, and sinks again when the part is at
rest. Consequently, if the work is frequent, the vessels increase in size and
the part is better nourished. Paget (24/13. 'Lectures on Pathology' 1853
volume 1 page 71.) also accounts for the long, thick, dark-coloured hairs
which occasionally grow, even in young children, near old-standing inflamed
surfaces or fractured bones by an increased flow of blood to the part. When
Hunter inserted the spur of a cock into the comb, which is well supplied with
blood-vessels, it grew in one case spirally to a length of six inches, and in
another case forward, like a horn, so that the bird could not touch the ground
with its beak. According to the interesting observations of M. Sedillot
(24/14. 'Comptes Rendus' September 26, 1864 page 539.), when a portion of one
of the bones of the leg of an animal is removed, the associated bone enlarges
till it attains a bulk equal to that of the two bones, of which it has to
perform the functions. This is best exhibited in dogs in which the tibia has
been removed; the companion bone, which is naturally almost filiform and not
one-fifth the size of the other, soon acquires a size equal to or greater than
that of the tibia. Now, it is at first difficult to believe that increased
weight acting on a straight bone could, by alternately increasing and
diminishing the pressure, cause the blood to flow more freely in the vessels
which permeate the periosteum and thus supply more nutriment to the bone.
Nevertheless the observations adduced by Mr. Spencer (24/15. H. Spencer 'The
Principles of Biology' volume 2 page 243.), on the strengthening of the bowed
bones of rickety children, along their concave sides, leads to the belief that
this is possible.
The rocking of the stem of a tree increases in a marked manner the growth of
the woody tissue in the parts which are strained. Prof. Sachs believes, from
reasons which he assigns, that this is due to the pressure of the bark being
relaxed in such parts, and not as Knight and H. Spencer maintain, to an
increased flow of sap caused by the movement of the trunk. (24/16. Ibid volume
2 page 269. Sachs 'Text-book of Botany' 1875 page 734.) But hard woody tissue
may be developed without the aid of any movement, as we see with ivy closely
attached to an old wall. In all such cases, it is very difficult to
distinguish between the effects of long-continued selection and those which
follow from the increased action of the part, or directly from some other
cause. Mr. H. Spencer (24/17. Ibid volume 2 page 273.) acknowledges this
difficulty, and gives as an instance the thorns on trees and the shells of
nuts. Here we have extremely hard woody tissue without the possibility of any
movement, and without, as far as we can see, any other directly exciting
cause; and as the hardness of these parts is of manifest service to the plant,
we may look at the result as probably due to the selection of so-called
spontaneous variations. Every one knows that hard work thickens the epidermis
on the hands; and when we hear that with infants, long before birth, the
epidermis is thicker on the palms and soles of the feet than on any other part
of the body, as was observed with admiration by Albinus (24/18. Paget
'Lectures on Pathology' volume 2 page 209.), we are naturally inclined to
attribute this to the inherited effects of long-continued use or pressure. We
are tempted to extend the same view even to the hoofs of quadrupeds; but who
will pretend to determine how far natural selection may have aided in the
formation of structures of such obvious importance to the animal?
[That use strengthens the muscles may be seen in the limbs of artisans who
follow different trades; and when a muscle is strengthened, the tendons, and
the crests of bone to which they are attached, become enlarged; and this must
likewise be the case with the blood-vessels and nerves. On the other hand,
when a limb is not used, as by Eastern fanatics, or when the nerve supplying
it with nervous power is effectually destroyed, the muscles wither. So again,
when the eye is destroyed the optic nerve becomes atrophied, sometimes even in
the course of a few months. (24/19. Muller 'Phys.' English translation pages
54, 791. Prof. Reed has given ('Physiological and Anat. Researches' page 10) a
curious account of the atrophy of the limbs of rabbits after the destruction
of the nerve.) The Proteus is furnished with branchiae as well as with lungs:
and Schreibers (24/20. Quoted by Lecoq in 'Geograph. Bot.' tome 1 1854 page
182.) found that when the animal was compelled to live in deep water, the
branchiae were developed to thrice their ordinary size, and the lungs were
partially atrophied. When, on the other hand, the animal was compelled to live
in shallow water, the lungs became larger and more vascular, whilst the
branchiae disappeared in a more or less complete degree. Such modifications as
these are, however, of comparatively little value for us, as we do not
actually know that they tend to be inherited.
In many cases there is reason to believe that the lessened use of various
organs has affected the corresponding parts in the offspring. But there is no
good evidence that this ever follows in the course of a single generation. It
appears, as in the case of general or indefinite variability, that several
generations must be subjected to changed habits for any appreciable result.
Our domestic fowls, ducks, and geese have almost lost, not only in the
individual but in the race, their power of flight; for we do not see a young
fowl, when frightened, take flight like a young pheasant. Hence I was led
carefully to compare the limb-bones of fowls, ducks, pigeons, and rabbits,
with the same bones in the wild parent-species. As the measurements and
weights were fully given in the earlier chapters I need here only recapitulate
the results. With domestic pigeons, the length of the sternum, the prominence
of its crest, the length of the scapulae and furculum, the length of the wings
as measured from tip to tip of the radii, are all reduced relatively to the
same parts in the wild pigeon. The wing and tail feathers, however, are
increased in length, but this may have as little connection with the use of
the wings or tail, as the lengthened hair on a dog with the amount of exercise
which it has habitually taken. The feet of pigeons, except in the long-beaked
races, are reduced in size. With fowls the crest of the sternum is less
prominent, and is often distorted or monstrous; the wing-bones have become
lighter relatively to the leg-bones, and are apparently a little shorter in
comparison with those of the parent-form, the Gallus bankiva. With ducks, the
crest of the sternum is affected in the same manner as in the foregoing cases:
the furculum, coracoids, and scapulae are all reduced in weight relatively to
the whole skeleton: the bones of the wings are shorter and lighter, and the
bones of the legs longer and heavier, relatively to each other, and relatively
to the whole skeleton, in comparison with the same bones in the wild-duck. The
decreased weight and size of the bones, in the foregoing cases, is probably
the indirect result of the reaction of the weakened muscles on the bones. I
failed to compare the feathers of the wings of the tame and wild duck; but
Gloger (24/21. 'Das Abandern der Vogel' 1833 s. 74.) asserts that in the wild
duck the tips of the wing-feathers reach almost to the end of the tail, whilst
in the domestic duck they often hardly reach to its base. He remarks also on
the greater thickness of the legs, and says that the swimming membrane between
the toes is reduced; but I was not able to detect this latter difference.
With the domesticated rabbit the body, together with the whole skeleton, is
generally larger and heavier than in the wild animal, and the leg-bones are
heavier in due proportion; but whatever standard of comparison be taken,
neither the leg-bones nor the scapulae have increased in length proportionally
with the increased dimensions of the rest of the skeleton. The skull has
become in a marked manner narrower, and, from the measurements of its capacity
formerly given, we may conclude, that this narrowness results from the
decreased size of the brain, consequent on the mentally inactive life led by
these closely-confined animals.
We have seen in the eighth chapter that silk-moths, which have been kept
during many centuries closely confined, emerge from their cocoons with their
wings distorted, incapable of flight, often greatly reduced in size, or even,
according to Quatrefages, quite rudimentary. This condition of the wings may
be largely owing to the same kind of monstrosity which often affects wild
Lepidoptera when artificially reared from the cocoon; or it may be in part due
to an inherent tendency, which is common to the females of many Bombycidae, to
have their wings in a more or less rudimentary state; but part of the effect
may be attributed to long-continued disuse.]
From the foregoing facts there can be no doubt that with our anciently
domesticated animals, certain bones have increased or decreased in size and
weight owing to increased or decreased use; but they have not been modified,
as shown in the earlier chapters, in shape or structure. With animals living a
free life and occasionally exposed to severe competition the reduction would
tend to be greater, as it would be an advantage to them to have the
development of every superfluous part saved. With highly-fed domesticated
animals, on the other hand, there seems to be no economy of growth, nor any
tendency to the elimination of superfluous details. But to this subject I
shall recur.
Turning now to more general observations, Nathusius has shown that with the
improved races of the pig, the shortened legs and snout, the form of the
articular condyles of the occiput, and the position of the jaws with the upper
canine teeth projecting in a most anomalous manner in front of the lower
canines, may be attributed to these parts not having been fully exercised. For
the highly-cultivated races do not travel in search of food, nor root up the
ground with their ringed muzzles. (24/22. Nathusius 'Die Racen des Schweines'
1860 s. 53, 57; 'Vorstudien...Schweineschadel' 1864 s. 103, 130, 133. Prof.
Lucae supports and extends the conclusions of Von Nathusius: 'Der Schadel des
Maskenschweines' 1870.) These modifications of structure, which are all
strictly inherited, characterise several improved breeds, so that they cannot
have been derived from any single domestic stock. With respect to cattle,
Professor Tanner has remarked that the lungs and liver in the improved breeds
"are found to be considerably reduced in size when compared with those
possessed by animals having perfect liberty" (24/23. 'Journal of Agriculture
of Highland Soc.' July 1860 page 321.); and the reduction of these organs
affects the general shape of the body. The cause of the reduced lungs in
highly-bred animals which take little exercise is obvious; and perhaps the
liver may be affected by the nutritious and artificial food on which they
largely subsist. Again, Dr. Wilckens asserts (24/24. 'Landwirth. Wochenblatt'
No. 10.) that various parts of the body certainly differ in Alpine and lowland
breeds of several domesticated animals, owing to their different habits of
life; for instance, the neck and fore-legs in length, and the hoofs in shape.
[It is well known that, when an artery is tied, the anastomosing branches,
from being forced to transmit more blood, increase in diameter; and this
increase cannot be accounted for by mere extension, as their coats gain in
strength. With respect to glands, Sir J. Paget observes that "when one kidney
is destroyed the other often becomes much larger, and does double work."
(24/25. 'Lectures on Surgical Pathology' 1853 volume 1 page 27.) If we compare
the size of the udders and their power of secretion in cows which have been
long domesticated, and in certain breeds of the goat in which the udders
nearly touch the ground, with these organs in wild or half-domesticated
animals, the difference is great. A good cow with us daily yields more than
five gallons, or forty pints of milk, whilst a first-rate animal, kept, for
instance, by the Damaras of South Africa (24/26. Andersson 'Travels in South
Africa' page 318. For analogous cases in South America see Aug. St.-Hilaire
'Voyage dans la Province de Goyaz' tome 1 page 71.), "rarely gives more than
two or three pints of milk daily, and, should her calf be taken from her, she
absolutely refuses to give any." We may attribute the excellence of our cows
and of certain goats, partly to the continued selection of the best milking
animals, and partly to the inherited effects of the increased action, through
man's art, of the secreting glands.
It is notorious that short-sight is inherited; and we have seen in the twelfth
chapter from the statistical researches of M. Giraud-Teulon, that the habit of
viewing near objects gives a tendency to short-sight. Veterinarians are
unanimous that horses are affected with spavins, splints, ringbones, etc.,
from being shod and from travelling on hard roads, and they are almost equally
unanimous that a tendency to these malformations is transmitted. Formerly
horses were not shod in North Carolina, and it has been asserted that they did
not then suffer from these diseases of the legs and feet. (24/27. Brickell
'Nat. Hist. of North Carolina' 1739 page 53.)]
Our domesticated quadrupeds are all descended, as far as is known, from
species having erect ears; yet few kinds can be named, of which at least one
race has not drooping ears. Cats in China, horses in parts of Russia, sheep in
Italy and elsewhere, the guinea-pig formerly in Germany, goats and cattle in
India, rabbits, pigs, and dogs in all long-civilised countries have dependent
ears. With wild animals, which constantly use their ears like funnels to catch
every passing sound, and especially to ascertain the direction whence it
comes, there is not, as Mr. Blyth has remarked, any species with drooping ears
except the elephant. Hence the incapacity to erect the ears is certainly in
some manner the result of domestication; and this incapacity has been
attributed by various authors (24/28. Livingstone quoted by Youatt on 'Sheep'
page 142. Hodgson in 'Journal of Asiatic Soc. of Bengal' volume 16 1847 page
1006 etc. etc. On the other hand Dr. Wilckens argues strongly against the
belief that the drooping of the ears is the result of disuse: 'Jahrbuch der
deutschen Viehzucht' 1866.) to disuse, for animals protected by man are not
compelled habitually to use their ears. Col. Hamilton Smith (24/29.
'Naturalist's Library' Dogs volume 2 1840 page 104.) states that in ancient
effigies of the dog, "with the exception of one Egyptian instance, no
sculpture of the earlier Grecian era produces representations of hounds with
completely drooping ears; those with them half pendulous are missing in the
most ancient; and this character increases, by degrees, in the works of the
Roman period." Godron also has remarked "that the pigs of the ancient
Egyptians had not their ears enlarged and pendent." (24/30. 'De l'Espece' tome
1 1859 page 367.) But it is remarkable that the drooping of the ear is not
accompanied by any decrease in size; on the contrary, animals so different as
fancy rabbits, certain Indian breeds of the goat, our petted spaniels, blood-
hounds, and other dogs, have enormously elongated ears, so that it would
appear as if their weight had caused them to droop, aided perhaps by disuse.
With rabbits, the drooping of the much elongated ears has affected even the
structure of the skull.
The tail of no wild animal, as remarked to me by Mr. Blyth, is curled; whereas
pigs and some races of dogs have their tails much curled. This deformity,
therefore, appears to be the result of domestication, but whether in any way
connected with the lessened use of the tail is doubtful.
The epidermis on our hands is easily thickened, as every one knows, by hard
work. In a district of Ceylon the sheep have "horny callosities that defend
their knees, and which arise from their habit of kneeling down to crop the
short herbage, and this distinguishes the Jaffna flocks from those of other
portions of the island;" but it is not stated whether this peculiarity is
inherited. (24/31. 'Ceylon' by Sir J.E. Tennent 1859 volume 2 page 531.)
The mucous membrane which lines the stomach is continuous with the external
skin of the body; therefore it is not surprising that its texture should be
affected by the nature of the food consumed, but other and more interesting
changes likewise follow. Hunter long ago observed that the muscular coat of
the stomach of a gull (Larus tridactylus) which had been fed for a year
chiefly on grain was thickened; and, according to Dr. Edmondston, a similar
change periodically occurs in the Shetland Islands in the stomach of the Larus
argentatus, which in the spring frequents the cornfields and feeds on the
seed. The same careful observer has noticed a great change in the stomach of a
raven which had been long fed on vegetable food. In the case of an owl (Strix
grallaria), similarly treated, Menetries states that the form of the stomach
was changed, the inner coat became leathery, and the liver increased in size.
Whether these modifications in the digestive organs would in the course of
generations become inherited is not known. (24/32. For the foregoing
statements see Hunter 'Essays and Observations' 1861 volume 2 page 329; Dr.
Edmondston, as quoted in Macgillivray 'British Birds' volume 5 page 550:
Menetries as quoted in Bronn 'Geschichte der Natur' b. 2 s. 110.)
The increased or diminished length of the intestines, which apparently results
from changed diet, is a more remarkable case, because it is characteristic of
certain animals in their domesticated condition, and therefore must be
inherited. The complex absorbent system, the blood-vessels, nerves, and
muscles, are necessarily all modified together with the intestines. According
to Daubenton, the intestines of the domestic cat are one-third longer than
those of the wild cat of Europe; and although this species is not the parent-
stock of the domestic animal, yet, as Isidore Geoffroy has remarked, the
several species of cats are so closely allied that the comparison is probably
a fair one. The increased length appears to be due to the domestic cat being
less strictly carnivorous in its diet than any wild feline species; for
instance, I have seen a French kitten eating vegetables as readily as meat.
According to Cuvier, the intestines of the domesticated pig exceed greatly in
proportionate length those of the wild boar. In the tame and wild rabbit the
change is of an opposite nature, and probably results from the nutritious food
given to the tame rabbit. (24/33. These statements on the intestines are taken
from Isidore Geoffroy Saint-Hilaire 'Hist. Nat. Gen.' tome 3 pages 427, 441.)
CHANGED AND INHERITED HABITS OF LIFE.
This subject, as far as the mental powers of animals are concerned, so blends
into instinct, that I will here only remind the reader of such cases as the
tameness of our domesticated animals--the pointing or retrieving of dogs--
their not attacking the smaller animals kept by man--and so forth. How much of
these changes ought to be attributed to mere habit, and how much to the
selection of individuals which have varied in the desired manner,
irrespectively of the special circumstances under which they have been kept,
can seldom be told.
We have already seen that animals may be habituated to a changed diet; but
some additional instances may be given. In the Polynesian Islands and in China
the dog is fed exclusively on vegetable matter, and the taste for this kind of
food is to a certain extent inherited. (24/34. Gilbert White 'Nat. Hist.
Selborne' 1825 volume 2 page 121.) Our sporting dogs will not touch the bones
of game birds, whilst most other dogs devour them with greediness. In some
parts of the world sheep have been largely fed on fish. The domestic hog is
fond of barley, the wild boar is said to disdain it; and the disdain is
partially inherited, for some young wild pigs bred in captivity showed an
aversion for this grain, whilst others of the same brood relished it. (24/35.
Burdach 'Traite de Phys.' tome 2 page 267 as quoted by Dr. P. Lucas 'L'Hered.
Nat.' tome 1 page 388.) One of my relations bred some young pigs from a
Chinese sow by a wild Alpine boar; they lived free in the park, and were so
tame that they came to the house to be fed; but they would not touch swill,
which was devoured by the other pigs. An animal when once accustomed to an
unnatural diet, which can generally be effected only during youth, dislikes
its proper food, as Spallanzani found to be the case with a pigeon which had
been long fed on meat. Individuals of the same species take to new food with
different degrees of readiness; one horse, it is stated, soon learned to eat
meat, whilst another would have perished from hunger rather than have partaken
of it. (24/36. This and several other cases are given by Colin 'Physiologie
Comp. des Animaux Dom.' 1854 tome 1 page 426.) The caterpillars of the Bombyx
hesperus feed in a state of nature on the leaves of the Cafe diable, but,
after having been reared on the Ailanthus, they would not touch the Cafe
diable, and actually died of hunger. (24/37. M. Michely de Cayenne in 'Bull.
Soc. d'Acclimat.' tome 8 1861 page 563.)
It has been found possible to accustom marine fish to live in fresh water; but
as such changes in fish and other marine animals have been chiefly observed in
a state of nature, they do not properly belong to our present subject. The
period of gestation and of maturity, as shown in the earlier chapters,--the
season and the frequency of the act of breeding,--have all been greatly
modified under domestication. With the Egyptian goose the rate of change with
respect to the season has been recorded. (24/38. Quatrefages 'Unite de
l'Espece Humaine' 1861 page 79.) The wild drake pairs with one female, the
domestic drake is polygamous. Certain breeds of fowls have lost the habit of
incubation. The paces of the horse, and the manner of flight of certain breeds
of the pigeon, have been modified and are inherited. Cattle, horses, and pigs
have learnt to browse under water in the St. John's River, East Florida, where
the Vallisneria has been largely naturalised. The cows were observed by Prof.
Wyman to keep their heads immersed for "a period varying from fifteen to
thirty-five seconds." (24/39. 'The American Naturalist' April 1874 page 237.)
The voice differs much in certain kinds of fowls and pigeons. Some varieties
are clamorous and others silent, as the Call and common duck, or the Spitz and
pointer dog. Every one knows how the breeds of the dog differ from one another
in their manner of hunting, and in their ardour after different kinds of game
or vermin.
With plants the period of vegetation is easily changed and is inherited, as in
the case of summer and winter wheat, barley, and vetches; but to this subject
we shall immediately return under acclimatisation. Annual plants sometimes
become perennial under a new climate, as I hear from Dr. Hooker is the case
with the stock and mignonette in Tasmania. On the other hand, perennials
sometimes become annuals, as with the Ricinus in England, and as, according to
Captain Mangles, with many varieties of the heartsease. Von Berg (24/40.
'Flora' 1835 b. 2 page 504.) raised from seed of Verbascum phoeniceum, which
is usually a biennial, both annual and perennial varieties. Some deciduous
bushes become evergreen in hot countries. (24/41. Alph. de Candolle 'Geograph.
Bot.' tome 2 page 1078.) Rice requires much water, but there is one variety in
India which can be grown without irrigation. (24/42. Royle 'Illustrations of
the Botany of the Himalaya' page 19.) Certain varieties of the oat and of our
other cereals are best fitted for certain soils. (24/43. 'Gardener's
Chronicle' 1850 pages 204, 219.) Endless similar facts could be given in the
animal and vegetable kingdoms. They are noticed here because they illustrate
analogous differences in closely allied natural species, and because such
changed habits of life, whether due to habit, or to the direct action of
external conditions, or to so-called spontaneous variability, would be apt to
lead to modifications of structure.
ACCLIMATISATION.
From the previous remarks we are naturally led to the much disputed subject of
acclimatisation. There are two distinct questions: Do varieties descended from
the same species differ in their power of living under different climates? And
secondly, if they so differ, how have they become thus adapted? We have seen
that European dogs do not succeed well in India, and it is asserted (24/44.
Rev. R. Everest 'Journal As. Soc. of Bengal' volume 3 page 19.), that no one
has there succeeded in keeping the Newfoundland long alive; but then it may be
argued, and probably with truth, that these northern breeds are specifically
distinct from the native dogs which flourish in India. The same remark may be
made with respect to different breeds of sheep, of which, according to Youatt
(24/45. Youatt on 'Sheep' 1838 page 491.), not one brought "from a torrid
climate lasts out the second year," in the Zoological Gardens. But sheep are
capable of some degree of acclimatisation, for Merino sheep bred at the Cape
of Good Hope have been found far better adapted for India than those imported
from England. (24/46. Royle 'Prod. Resources of India' page 153.) It is almost
certain that all the breeds of the fowl are descended from one species; but
the Spanish breed, which there is good reason to believe originated near the
Mediterranean (24/47. Tegetmeier 'Poultry Book' 1866 page 102.), though so
fine and vigorous in England, suffers more from frost than any other breed.
The Arrindy silk moth introduced from Bengal, and the Ailanthus moth from the
temperate province of Shan Tung, in China, belong to the same species, as we
may infer from their identity in the caterpillar, cocoon, and mature states
(24/48. Dr. R. Paterson in a paper communicated to Bot. Soc. of Canada quoted
in the 'Reader' 1863 November 13.); yet they differ much in constitution: the
Indian form "will flourish only in warm latitudes," the other is quite hardy
and withstands cold and rain.
[Plants are more strictly adapted to climate than are animals. The latter when
domesticated withstand such great diversities of climate, that we find nearly
the same species in tropical and temperate countries; whilst the cultivated
plants are widely dissimilar. Hence a larger field is open for inquiry in
regard to the acclimatisation of plants than of animals. It is no exaggeration
to say that with almost every plant which has long been cultivated, varieties
exist which are endowed with constitutions fitted for very different climates;
I will select only a few of the more striking cases, as it would be tedious to
give all. In North America numerous fruit-trees have been raised, and in
horticultural publications,--for instance, in that by Downing,--lists are
given of the varieties which are best able to withstand the severe climate of
the northern States and Canada. Many American varieties of the pear, plum, and
peach are excellent in their own country, but until recently, hardly one was
known that succeeded in England; and with apples (24/49. See remarks by Editor
in 'Gardener's Chronicle' 1848 page 5.), not one succeeds. Though the American
varieties can withstand a severer winter than ours, the summer here is not hot
enough. Fruit-trees have also originated in Europe with different
constitutions, but they are not much noticed, because nurserymen here do not
supply wide areas. The Forelle pear flowers early, and when the flowers have
just set, and this is the critical period, they have been observed, both in
France and England, to withstand with complete impunity a frost of 18 deg and
even 14 deg Fahr., which killed the flowers, whether fully expanded or in bud,
of all other kinds of pears. (24/50. 'Gardener's Chronicle' 1860 page 938.
Remarks by Editor and quotation from Decaisne.) This power in the flower of
resisting cold and afterwards producing fruit does not invariably depend, as
we know on good authority (24/51. J. de Jonghe of Brussels in 'Gardener's
Chronicle' 1857 page 612.), on general constitutional vigour. In proceeding
northward, the number of varieties which are found capable of resisting the
climate rapidly decreases, as may be seen in the list of the varieties of the
cherry, apple, and pear, which can be cultivated in the neighbourhood of
Stockholm. (24/52. Ch. Martius 'Voyage Bot. Cotes Sept. de la Norvege' page
26.) Near Moscow, Prince Troubetzkoy planted for experiment in the open ground
several varieties of the pear, but one alone, the Poire sans Pepins, withstood
the cold of winter. (24/53. 'Journal de l'Acad. Hort. de Gand' quoted in
'Gardener's Chronicle' 1859 page 7.) We thus see that our fruit-trees, like
distinct species of the same genus, certainly differ from each other in their
constitutional adaptation to different climates.
With the varieties of many plants, the adaptation to climate is often very
close. Thus it has been proved by repeated trials "that few if any of the
English varieties of wheat are adapted for cultivation in Scotland" (24/54.
'Gardener's Chronicle' 1851 page 396.); but the failure in this case is at
first only in the quantity, though ultimately in the quality, of the grain
produced. The Rev. M.J. Berkeley sowed wheat-seed from India, and got "the
most meagre ears," on land which would certainly have yielded a good crop from
English wheat. (24/55. Ibid 1862 page 235.) In these cases varieties have been
carried from a warmer to a cooler climate; in the reverse case, as "when wheat
was imported directly from France into the West Indian Islands, it produced
either wholly barren spikes or furnished with only two or three miserable
seeds, while West Indian seed by its side yielded an enormous harvest."
(24/56. On the authority of Labat quoted in 'Gardener's Chronicle' 1862 page
235.) Here is another case of close adaptation to a slightly cooler climate; a
kind of wheat which in England may be used indifferently either as a winter or
summer variety, when sown under the warmer climate of Grignan, in France,
behaved exactly as if it had been a true winter wheat. (24/57. MM. Edwards and
Colin 'Annal. des Sc. Nat.' 2nd series Bot. tome 5 page 22.)
Botanists believe that all the varieties of maize belong to the same species;
and we have seen that in North America, in proceeding northward, the varieties
cultivated in each zone produce their flowers and ripen their seed within
shorter and shorter periods. So that the tall, slowly maturing southern
varieties do not succeed in New England, and the New English varieties do not
succeed in Canada. I have not met with any statement that the southern
varieties are actually injured or killed by a degree of cold which the
northern varieties can withstand with impunity, though this is probable; but
the production of early flowering and early seeding varieties deserves to be
considered as one form of acclimatisation. Hence it has been found possible,
according to Kalm, to cultivate maize further and further northwards in
America. In Europe, also, as we learn from the evidence given by Alph. De
Candolle, the culture of maize has extended since the end of the last century
thirty leagues north of its former boundary. (24/58. 'Geograph. Bot.' page
337.) On the authority of Linnaeus (24/59. 'Swedish Acts' English translation
1739-40 volume 1. Kalm in his 'Travels' volume 2 page 166 gives an analogous
case with cotton-plants raised in New Jersey from Carolina seed.), I may quote
an analogous case, namely, that in Sweden tobacco raised from home-grown seed
ripens its seed a month sooner and is less liable to miscarry than plants
raised from foreign seed.
With the Vine, differently from the maize, the line of practical culture has
retreated a little southward since the middle ages (24/60. De Candolle
'Geograph. Bot.' page 339.); but this seems due to commerce being now easier,
so that it is better to import wine from the south than to make it in northern
districts. Nevertheless the fact of the vine not having spread northward shows
that acclimatisation has made no progress during several centuries. There is,
however, a marked difference in the constitution of the several varieties,--
some being hardy, whilst others, like the muscat of Alexandria, require a very
high temperature to come to perfection. According to Labat (24/61. 'Gardener's
Chronicle' 1862 page 235.), vines taken from France to the West Indies succeed
with extreme difficulty, whilst those imported from Madeira or the Canary
Islands thrive admirably.
Gallesio gives a curious account of the naturalisation of the Orange in Italy.
During many centuries the sweet orange was propagated exclusively by grafts,
and so often suffered from frosts, that it required protection. After the
severe frost of 1709, and more especially after that of 1763, so many trees
were destroyed, that seedlings from the sweet orange were raised, and, to the
surprise of the inhabitants, their fruit was found to be sweet. The trees thus
raised were larger, more productive, and hardier than the old kinds; and
seedlings are now continually raised. Hence Gallesio concludes that much more
was effected for the naturalisation of the orange in Italy by the accidental
production of new kinds during a period of about sixty years, than had been
effected by grafting old varieties during many ages. (24/62. Gallesio 'Teoria
della Riproduzione Veg.' 1816 page 125; and 'Traite du Citrus' 1811 page 359.)
I may add that Risso (24/63. 'Essai sur l'Hist. des Orangers' 1813 page 20
etc.) describes some Portuguese varieties of the orange as extremely sensitive
to cold, and as much tenderer than certain other varieties.
The peach was known to Theophrastus, 322 B.C. (24/64. Alph. de Candolle
'Geograph. Bot.' page 882.) According to the authorities quoted by Dr. F.
Rolle (24/65. 'Ch. Darwin's Lehre von der Entstehung' etc. 1862 s. 87.), it
was tender when first introduced into Greece, and even in the island of Rhodes
only occasionally bore fruit. If this be correct, the peach, in spreading
during the last two thousand years over the middle parts of Europe, must have
become much hardier. At the present day different varieties differ much in
hardiness: some French varieties will not succeed in England; and near Paris,
the Pavie de Bonneuil does not ripen its fruit till very late in the season,
even when grown on a wall; "it is, therefore, only fit for a very hot southern
climate." (24/66. Decaisne quoted in 'Gardener's Chronicle' 1865 page 271.)
I will briefly give a few other cases. A variety of Magnolia grandiflora,
raised by M. Roy, withstands a temperature several degrees lower than that
which any other variety can resist. With camellias there is much difference in
hardiness. One particular variety of the Noisette rose withstood the severe
frost of 1860 "untouched and hale amidst a universal destruction of other
Noisettes." In New York the "Irish yew is quite hardy, but the common yew is
liable to be cut down." I may add that there are varieties of the sweet potato
(Convolvulus batatas) which are suited for warmer, as well as for colder,
climates. (24/67. For the magnolia see Loudon's 'Gardener's Mag.' volume 13
1837 page 21. For camellias and roses see 'Gardener's Chronicle' 1860 page
384. For the yew 'Journal of Hort.' March 3, 1863 p 174. For sweet potatoes
see Col. von Siebold in 'Gardener's Chronicle' 1855 page 822.)]
The plants as yet mentioned have been found capable of resisting an unusual
degree of cold or heat, when fully grown. The following cases refer to plants
whilst young. In a large bed of young Araucarias of the same age, growing
close together and equally exposed, it was observed (24/68. The Editor
'Gardener's Chronicle' 1861 page 239.), after the unusually severe winter of
1860-61, that, "in the midst of the dying, numerous individuals remained on
which the frost had absolutely made no kind of impression." Dr. Lindley, after
alluding to this and other similar cases, remarks, "Among the lessons which
the late formidable winter has taught us, is that, even in their power of
resisting cold, individuals of the same species of plants are remarkably
different." Near Salisbury, there was a sharp frost on the night of May 24,
1836, and all the French beans (Phaseolus vulgaris) in a bed were killed
except about one in thirty, which completely escaped. (24/69. Loudon's
'Gardener's Mag.' volume 12 1836 page 378.) On the same day of the month, but
in the year 1864, there was a severe frost in Kent, and two rows of scarlet-
runners (P. multiflorus) in my garden, containing 390 plants of the same age
and equally exposed, were all blackened and killed except about a dozen
plants. In an adjoining row of "Fulmer's dwarf bean" (P. vulgaris), one single
plant escaped. A still more severe frost occurred four days afterwards, and of
the dozen plants which had previously escaped only three survived; these were
not taller or more vigorous than the other young plants, but they escaped
completely, with not even the tips of their leaves browned. It was impossible
to behold these three plants, with their blackened, withered, and dead
brethren all around, and not see at a glance that they differed widely in
constitutional power of resisting frost.
This work is not the proper place to show that wild plants of the same
species, naturally growing at different altitudes or under different
latitudes, become to a certain extent acclimatised, as is proved by the
different behaviour of their seedlings when raised in another country. In my
'Origin of Species' I have alluded to some cases, and I could add many others.
One instance must suffice: Mr. Grigor, of Forres (24/70. 'Gardener's
Chronicle' 1865 page 699. Mr. G. Maw gives ('Gardener's Chronicle' 1870 page
895) a number of striking cases; he brought home from southern Spain and
northern Africa several plants, which he cultivated in England alongside
specimens from northern districts; and he found a great difference not only in
their hardiness during the winter, but in the behaviour of some of them during
the summer.), states that seedlings of the Scotch fir (Pinus sylvestris),
raised from seed from the Continent and from the forests of Scotland, differ
much. "The difference is perceptible in one-year-old, and more so in two-year-
old seedlings; but the effects of the winter on the second year's growth
almost uniformly make those from the Continent quite brown, and so damaged,
that by the month of March they are quite unsaleable, while the plants from
the native Scotch pine, under the same treatment, and standing alongside,
although considerably shorter, are rather stouter and quite green, so that the
beds of the one can be known from the other when seen from the distance of a
mile." Closely similar facts have been observed with seedling larches.
[Hardy varieties would alone be valued or noticed in Europe; whilst tender
varieties, requiring more warmth, would generally be neglected; but such
occasionally arise. Thus Loudon (24/71. 'Arboretum et Fruticetum' volume 3
page 1376.) describes a Cornish variety of the elm which is almost an
evergreen, and of which the shoots are often killed by the autumnal frosts, so
that its timber is of little value. Horticulturists know that some varieties
are much more tender than others: thus all the varieties of the broccoli are
more tender than cabbages; but there is much difference in this respect in the
sub-varieties of the broccoli; the pink and purple kinds are a little hardier
than the white Cape broccoli, "but they are not to be depended on after the
thermometer falls below 24 deg Fahr.;" the Walcheren broccoli is less tender
than the Cape, and there are several varieties which will stand much severer
cold than the Walcheren. (24/72. Mr. Robson in 'Journal of Horticulture' 1861
page 23.) Cauliflowers seed more freely in India than cabbages. (24/73. Dr.
Bonavia 'Report of the Agri.-Hort. Soc. of Oudh' 1866.) To give one instance
with flowers: eleven plants raised from a hollyhock, called the Queen of the
Whites (24/74. 'Cottage Gardener' 1860 April 24 page 57.) were found to be
much more tender than various other seedlings. It may be presumed that all
tender varieties would succeed better under a climate warmer than ours. With
fruit-trees, it is well known that certain varieties, for instance of the
peach, stand forcing in a hot-house better than others; and this shows either
pliability of organisation or some constitutional difference. The same
individual cherry-tree, when forced, has been observed during successive years
gradually to change its period of vegetation. (24/75. 'Gardener's Chronicle'
1841 page 291.) Few pelargoniums can resist the heat of a stove, but Alba
Multiflora will, as a most skilful gardener asserts, "stand pine-apple top and
bottom heat the whole winter; without looking any more drawn than if it had
stood in a common greenhouse; and Blanche Fleur seems as if it had been made
on purpose for growing in winter, like many bulbs, and to rest all summer."
(24/76. Mr. Beaton in 'Cottage Gardener' March 20, 1860 page 377. Queen Mab
will also stand stove heat. See 'Gardener's Chronicle' 1845 page 226.) There
can hardly be a doubt that the Alba Multiflora pelargonium must have a widely
different constitution from that of most other varieties of this plant; it
would probably withstand even an equatorial climate.
We have seen that according to Labat the vine and wheat require
acclimatisation in order to succeed in the West Indies. Similar facts have
been observed at Madras: "two parcels of mignonette-seed, one direct from
Europe, the other saved at Bangalore (of which the mean temperature is much
below that of Madras), were sown at the same time: they both vegetated equally
favourably, but the former all died off a few days after they appeared above
ground; the latter still survive, and are vigorous, healthy plants." "So
again, turnip and carrot seed saved at Hyderabad are found to answer better at
Madras than seed from Europe or from the Cape of Good Hope." (24/77.
'Gardener's Chronicle' 1841 page 439.) Mr. J. Scott of the Calcutta Botanic
Gardens, informs me that seeds of the sweet-pea (Lathyrus odoratus) imported
from England produce plants, with thick, rigid stems and small leaves, which
rarely blossom and never yield seed; plants raised from French seed blossom
sparingly, but all the flowers are sterile; on the other hand, plants raised
from sweet-peas grown near Darjeeling in Upper India, but originally derived
from England, can be successfully cultivated on the plains of India; for they
flower and seed profusely, and their stems are lax and scandent. In some of
the foregoing cases, as Dr. Hooker has remarked to me, the greater success may
perhaps be attributed to the seeds having been more fully ripened under a more
favourable climate; but this view can hardly be extended to so many cases,
including plants, which, from being cultivated under a climate hotter than
their native one, become fitted for a still hotter climate. We may therefore
safely conclude that plants can to a certain extent become accustomed to a
climate either hotter or colder than their own; although the latter cases have
been more frequently observed.]
We will now consider the means by which acclimatisation may be effected,
namely, through the appearance of varieties having a different constitution,
and through the effects of habit. In regard to new varieties, there is no
evidence that a change in the constitution of the offspring necessarily stands
in any direct relation with the nature of the climate inhabited by the
parents. On the contrary, it is certain that hardy and tender varieties of the
same species appear in the same country. New varieties thus spontaneously
arising become fitted to slightly different climates in two different ways;
firstly, they may have the power, either as seedlings or when full-grown, of
resisting intense cold, as with the Moscow pear, or of resisting intense heat,
as with some kinds of Pelargonium, or the flowers may withstand severe frost,
as with the Forelle pear. Secondly, plants may become adapted to climates
widely different from their own, from flowering and fruiting either earlier or
later in the season. In both these cases the power of acclimatisation by man
consists simply in the selection and preservation of new varieties. But
without any direct intention on his part of securing a hardier variety,
acclimatisation may be unconsciously effected by merely raising tender plants
from seed, and by occasionally attempting their cultivation further and
further northwards, as in the case of maize, the orange and the peach.
How much influence ought to be attributed to inherited habit or custom in the
acclimatisation of animals and plants is a much more difficult question. In
many cases natural selection can hardly have failed to have come into play and
complicated the result. It is notorious that mountain sheep resist severe
weather and storms of snow which would destroy lowland breeds; but then
mountain sheep have been thus exposed from time immemorial, and all delicate
individuals will have been destroyed, and the hardiest preserved. So with the
Arrindy silk-moths of China and India; who can tell how far natural selection
may have taken a share in the formation of the two races, which are now fitted
for such widely different climates? It seems at first probable that the many
fruit-trees which are so well fitted for the hot summers and cold winters of
North America, in contrast with their poor success under our climate, have
become adapted through habit; but when we reflect on the multitude of
seedlings annually raised in that country, and that none would succeed unless
born with a fitting constitution, it is possible that mere habit may have done
nothing towards their acclimatisation. On the other hand, when we hear that
Merino sheep, bred during no great number of generations at the Cape of Good
Hope--that some European plants raised during only a few generations in the
cooler parts of India, withstand the hotter parts of that country much better
than the sheep or seeds imported directly from England, we must attribute some
influence to habit. We are led to the same conclusion when we hear from Naudin
(24/78. Quoted by Asa Gray in 'Am. Journ. of Sc.' 2nd series January 1865 page
106.) that the races of melons, squashes, and gourds, which have long been
cultivated in Northern Europe, are comparatively more precocious, and need
much less heat for maturing their fruit, than the varieties of the same
species recently brought from tropical regions. In the reciprocal conversion
of summer and winter wheat, barley, and vetches into each other, habit
produces a marked effect in the course of a very few generations. The same
thing apparently occurs with the varieties of maize, which, when carried from
the Southern States of America, or into Germany, soon became accustomed to
their new homes. With vine-plants taken to the West Indies from Madeira, which
are said to succeed better than plants brought directly from France, we have
some degree of acclimatisation in the individual, independently of the
production of new varieties by seed.
The common experience of agriculturists is of some value, and they often
advise persons to be cautious in trying the productions of one country in
another. The ancient agricultural writers of China recommend the preservation
and cultivation of the varieties peculiar to each country. During the
classical period, Columella wrote, "Vernaculum pecus peregrino longe
praestantius est." (24/79. For China see 'Memoire sur les Chinois' tome 11
1786 page 60. Columella is quoted by Carlier in 'Journal de Physique' tome 24
1784.)
I am aware that the attempt to acclimatise either animals or plants has been
called a vain chimera. No doubt the attempt in most cases deserves to be thus
called, if made independently of the production of new varieties endowed with
a different constitution. With plants propagated by buds, habit rarely
produces any effect; it apparently acts only through successive seminal
generations. The laurel, bay, laurestinus, etc., and the Jerusalem artichoke,
which are propagated by cuttings or tubers, are probably now as tender in
England as when first introduced; and this appears to be the case with the
potato, which until recently was seldom multiplied by seed. With plants
propagated by seed, and with animals, there will be little or no
acclimatisation unless the hardier individuals are either intentionally or
unconsciously preserved. The kidney-bean has often been advanced as an
instance of a plant which has not become hardier since its first introduction
into Britain. We hear, however, on excellent authority (24/80. Messrs. Hardy
and Son in 'Gardener's Chronicle' 1856 page 589.) that some very fine seed,
imported from abroad, produced plants "which blossomed most profusely, but
were nearly all but abortive, whilst plants grown alongside from English seed
podded abundantly;" and this apparently shows some degree of acclimatisation
in our English plants. We have also seen that seedlings of the kidney-bean
occasionally appear with a marked power of resisting frost; but no one, as far
as I can hear, has ever separated such hardy seedlings, so as to prevent
accidental crossing, and then gathered their seed, and repeated the process
year after year. It may, however, be objected with truth that natural
selection ought to have had a decided effect on the hardiness of our kidney-
beans; for the tenderest individuals must have been killed during every severe
spring, and the hardier preserved. But it should be borne in mind that the
result of increased hardiness would simply be that gardeners, who are always
anxious for as early a crop as possible, would sow their seed a few days
earlier than formerly. Now, as the period of sowing depends much on the soil
and elevation of each district, and varies with the season; and as new
varieties have often been imported from abroad, can we feel sure that our
kidney-beans are not somewhat hardier? I have not been able, by searching old
horticultural works, to answer this question satisfactorily.
On the whole the facts now given show that, though habit does something
towards acclimatisation, yet that the appearance of constitutionally different
individuals is a far more effective agent. As no single instance has been
recorded either with animals or plants of hardier individuals having been long
and steadily selected, though such selection is admitted to be indispensable
for the improvement of any other character, it is not surprising that man has
done little in the acclimatisation of domesticated animals and cultivated
plants. We need not, however, doubt that under nature new races and new
species would become adapted to widely different climates, by variation, aided
by habit, and regulated by natural selection.
[ARRESTS OF DEVELOPMENT: RUDIMENTARY AND ABORTED ORGANS.
Modifications of structure from arrested development, so great or so serious
as to deserve to be called monstrosities, are not infrequent with domesticated
animals, but, as they differ much from any normal structure, they require only
a passing notice. Thus the whole head may be represented by a soft nipple-like
projection, and the limbs by mere papillae. These rudiments of limbs are
sometimes inherited, as has been observed in a dog. (24/81. Isid. Geoffroy
Saint-Hilaire 'Hist. Nat. des Anomalies' 1836 tome 2 pages 210, 223, 224, 395;
'Philosoph. Transact.' 1775 page 313.)
Many lesser anomalies appear to be due to arrested development. What the cause
of the arrest may be, we seldom know, except in the case of direct injury to
the embryo. That the cause does not generally act at an extremely early
embryonic period we may infer from the affected organ seldom being wholly
aborted,--a rudiment being generally preserved. The external ears are
represented by mere vestiges in a Chinese breed of sheep; and in another
breed, the tail is reduced "to a little button, suffocated in a manner, by
fat." (24/82. Pallas quoted by Youatt on 'Sheep' page 25.) In tailless dogs
and cats a stump is left. In certain breeds of fowls the comb and wattles are
reduced to rudiments; in the Cochin-China breed scarcely more than rudiments
of spurs exist. With polled Suffolk cattle, "rudiments of horns can often be
felt at an early age" (24/83. Youatt on 'Cattle' 1834 page 174.); and with
species in a state of nature, the relatively great development of rudimentary
organs at an early period of life is highly characteristic of such organs.
With hornless breeds of cattle and sheep, another and singular kind of
rudiment has been observed, namely, minute dangling horns attached to the skin
alone, and which are often shed and grow again. With hornless goats, according
to Desmarest (24/84. 'Encyclop. Method.' 1820 page 483: see page 500, on the
Indian zebu casting its horns. Similar cases in European cattle were given in
the third chapter.), the bony protuberance which properly supports the horn
exists as a mere rudiment.
With cultivated plants it is far from rare to find the petals, stamens, and
pistils represented by rudiments, like those observed in natural species. So
it is with the whole seed in many fruits; thus, near Astrakhan there is a
grape with mere traces of seeds, "so small and lying so near the stalk that
they are not perceived in eating the grape." (24/85. Pallas 'Travels' English
Translat. volume 1 page 243.) In certain varieties of the gourd, the tendrils,
according to Naudin, are represented by rudiments or by various monstrous
growths. In the broccoli and cauliflower the greater number of the flowers are
incapable of expansion, and include rudimentary organs. In the Feather
hyacinth (Muscari comosum) in its natural state the upper and central flowers
are brightly coloured but rudimentary; under cultivation the tendency to
abortion travels downwards and outwards, and all the flowers become
rudimentary; but the abortive stamens and pistils are not so small in the
lower as in the upper flowers. In the Viburnum opulus, on the other hand, the
outer flowers naturally have their organs of fructification in a rudimentary
state, and the corolla is of large size; under cultivation, the change spreads
to the centre, and all the flowers become affected. In the compositae, the so-
called doubling of the flowers consists in the greater development of the
corolla of the central florets, generally accompanied with some degree of
sterility; and it has been observed (24/86. Mr. Beaton in 'Journal of
Horticulture' May 21, 1861 page 133.) that the progressive doubling invariably
spreads from the circumference to the centre,--that is, from the ray florets,
which so often include rudimentary organs, to those of the disc. I may add, as
bearing on this subject, that with Asters, seeds taken from the florets of the
circumference have been found to yield the greatest number of double flowers.
(24/87. Lecoq 'De la Fecondation' 1862 page 233.) In the above cases we have a
natural tendency in certain parts to be rudimentary, and this under culture
spreads either to, or from, the axis of the plant. It deserves notice, as
showing how the same laws govern the changes which natural species and
artificial varieties undergo, that in the species of Carthamus, one of the
Compositae, a tendency to the abortion of the pappus may be traced extending
from the circumference to the centre of the disc as in the so-called doubling
of the flowers in the members of the same family. Thus, according to A. de
Jussieu (24/88. 'Annales du Museum' tome 6 page 319.), the abortion is only
partial in Carthamus creticus, but more extended in C. lanatus; for in this
species only two or three of the central seeds are furnished with a pappus,
the surrounding seeds being either quite naked or furnished with a few hairs;
and lastly in C. tinctorius, even the central seeds are destitute of pappus,
and the abortion is complete.
With animals and plants under domestication, when an organ disappears, leaving
only a rudiment, the loss has generally been sudden, as with hornless and
tailless breeds; and such cases may be ranked as inherited monstrosities. But
in some few cases the loss has been gradual, and has been effected partly by
selection, as with the rudimentary combs and wattles of certain fowls. We have
also seen that the wings of some domesticated birds have been slightly reduced
by disuse, and the great reduction of the wings in certain silk-moths, with
mere rudiments left, has probably been aided by disuse.]
With species in a state of nature, rudimentary organs are extremely common.
Such organs are generally variable, as several naturalists have observed; for,
being useless, they are not regulated by natural selection, and they are more
or less liable to reversion. The same rule certainly holds good with parts
which have become rudimentary under domestication. We do not know through what
steps under nature rudimentary organs have passed in being reduced to their
present condition; but we so incessantly see in species of the same group the
finest gradations between an organ in a rudimentary and perfect state, that we
are led to believe that the passage must have been extremely gradual. It may
be doubted whether a change of structure so abrupt as the sudden loss of an
organ would ever be of service to a species in a state of nature; for the
conditions to which all organisms are closely adapted usually change very
slowly. Even if an organ did suddenly disappear in some one individual by an
arrest of development, intercrossing with the other individuals of the same
species would tend to cause its partial reappearance; so that its final
reduction could only be effected by some other means. The most probable view
is, that a part which is now rudimentary, was formerly, owing to changed
habits of life, used less and less, being at the same time reduced in size by
disuse, until at last it became quite useless and superfluous. But as most
parts or organs are not brought into action during an early period of life,
disuse or decreased action will not lead to their reduction until the organism
arrives at a somewhat advanced age; and from the principle of inheritance at
corresponding ages the reduction will be transmitted to the offspring at the
same advanced stage of growth. The part or organ will thus retain its full
size in the embryo, as we know to be the case with most rudiments. As soon as
a part becomes useless, another principle, that of economy of growth, will
come into play, as it would be an advantage to an organism exposed to severe
competition to save the development of any useless part; and individuals
having the part less developed will have a slight advantage over others. But,
as Mr. Mivart has justly remarked, as soon as a part is much reduced, the
saving from its further reduction will be utterly insignificant; so that this
cannot be effected by natural selection. This manifestly holds good if the
part be formed of mere cellular tissue, entailing little expenditure of
nutriment. How then can the further reduction of an already somewhat reduced
part be effected? That this has occurred repeatedly under Nature is shown by
the many gradations which exist between organs in a perfect state and the
merest vestiges of them. Mr. Romanes (24/89. I suggested in 'Nature' (volume 8
pages 432, 505) that with organisms subjected to unfavourable conditions all
the parts would tend towards reduction, and that under such circumstances any
part which was not kept up to its standard size by natural selection would,
owing to intercrossing, slowly but steadily decrease. In three subsequent
communications to 'Nature' (March 12, April 9, and July 2, 1874), Mr. Romanes
gives his improved view.) has, I think, thrown much light on this difficult
problem. His view, as far as it can be given in a few words, is as follows:
all parts are somewhat variable and fluctuate in size round an average point.
Now, when a part has already begun from any cause to decrease, it is very
improbable that the variations should be as great in the direction of increase
as of diminution; for the previous reduction shows that circumstances have not
been favourable for its development; whilst there is nothing to check
variations in the opposite direction. If this be so, the long continued
crossing of many individuals furnished with an organ which fluctuates in a
greater degree towards decrease than towards increase, will slowly but
steadily lead to its diminution. With respect to the complete and absolute
abortion of a part, a distinct principle, which will be discussed in the
chapter on pangenesis, probably comes into action.
With animals and plants reared by man there is no severe or recurrent struggle
for existence, and the principle of economy will not come into action, so that
the reduction of an organ will not thus be aided. So far, indeed, is this from
being the case, that in some few instances organs, which are naturally
rudimentary in the parent-species, become partially redeveloped in the
domesticated descendants. Thus cows, like most other ruminants, properly have
four active and two rudimentary mamma; but in our domesticated animals, the
latter occasionally become considerably developed and yield milk. The
atrophied mammae, which, in male domesticated animals, including man, have in
some rare cases grown to full size and secreted milk, perhaps offer an
analogous case. The hind feet of dogs naturally include rudiments of a fifth
toe, and in certain large breeds these toes, though still rudimentary, become
considerably developed and are furnished with claws. In the common Hen, the
spurs and comb are rudimentary, but in certain breeds these become,
independently of age or disease of the ovaria, well developed. The stallion
has canine teeth, but the mare has only traces of the alveoli, which, as I am
informed by the eminent veterinarian Mr. G.T. Brown, frequently contain minute
irregular nodules of bone. These nodules, however, sometimes become developed
into imperfect teeth, protruding through the gums and coated with enamel; and
occasionally they grow to a fourth or even a third of the length of the
canines in the stallion. With plants I do not know whether the redevelopment
of rudimentary organs occurs more frequently under culture than under nature.
Perhaps the pear-tree may be a case in point, for when wild it bears thorns,
which consist of branches in a rudimentary condition and serve as a
protection, but, when the tree is cultivated, they are reconverted into
branches.
CHAPTER 2.XXV.
LAWS OF VARIATION, continued.--CORRELATED VARIABILITY.
EXPLANATION OF TERM CORRELATION.
CONNECTED WITH DEVELOPMENT.
MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED SIZE
OF PARTS.
CORRELATED VARIATION OF HOMOLOGOUS PARTS.
FEATHERED FEET IN BIRDS ASSUMING THE STRUCTURE OF THE WINGS.
CORRELATION BETWEEN THE HEAD AND THE EXTREMITIES.
BETWEEN THE SKIN AND DERMAL APPENDAGES.
BETWEEN THE ORGANS OF SIGHT AND HEARING.
CORRELATED MODIFICATIONS IN THE ORGANS OF PLANTS.
CORRELATED MONSTROSITIES.
CORRELATION BETWEEN THE SKULL AND EARS.
SKULL AND CREST OF FEATHERS.
SKULL AND HORNS.
CORRELATION OF GROWTH COMPLICATED BY THE ACCUMULATED EFFECTS OF NATURAL
SELECTION.
COLOUR AS CORRELATED WITH CONSTITUTIONAL PECULIARITIES.
All parts of the organisation are to a certain extent connected together; but
the connection may be so slight that it hardly exists, as with compound
animals or the buds on the same tree. Even in the higher animals various parts
are not at all closely related; for one part may be wholly suppressed or
rendered monstrous without any other part of the body being affected. But in
some cases, when one part varies, certain other parts always, or nearly
always, simultaneously vary; they are then subject to the law of correlated
variation. The whole body is admirably co-ordinated for the peculiar habits of
life of each organic being, and may be said, as the Duke of Argyll insists in
his 'Reign of Law' to be correlated for this purpose. Again, in large groups
of animals certain structures always co-exist: for instance, a peculiar form
of stomach with teeth of peculiar form, and such structures may in one sense
be said to be correlated. But these cases have no necessary connection with
the law to be discussed in the present chapter; for we do not know that the
initial or primary variations of the several parts were in any way related:
slight modifications or individual differences may have been preserved, first
in one and then in another part, until the final and perfectly co-adapted
structure was acquired; but to this subject I shall presently recur. Again, in
many groups of animals the males alone are furnished with weapons, or are
ornamented with gay colours; and these characters manifestly stand in some
sort of correlation with the male reproductive organs, for when the latter are
destroyed these characters disappear. But it was shown in the twelfth chapter
that the very same peculiarity may become attached at any age to either sex,
and afterwards be exclusively transmitted to the same sex at a corresponding
age. In these cases we have inheritance limited by both sex and age; but we
have no reason for supposing that the original cause of the variation was
necessarily connected with the reproductive organs, or with the age of the
affected being.
In cases of true correlated variation, we are sometimes able to see the nature
of the connection; but in most cases it is hidden from us, and certainly
differs in different cases. We can seldom say which of two correlated parts
first varies, and induces a change in the other; or whether the two are the
effects of some common cause. Correlated variation is an important subject for
us; for when one part is modified through continued selection, either by man
or under nature, other parts of the organisation will be unavoidably modified.
From this correlation it apparently follows that with our domesticated animals
and plants, varieties rarely or never differ from one another by a single
character alone.
One of the simplest cases of correlation is that a modification which arises
during an early stage of growth tends to influence the subsequent development
of the same part, as well as of other and intimately connected parts. Isidore
Geoffroy Saint-Hilaire states (25/1. 'Hist. des Anomalies' tome 3 page 392.
Prof. Huxley applies the same principle in accounting for the remarkable,
though normal, differences in the arrangement of the nervous system in the
Mollusca, in his paper on the Morphology of the Cephalous Mollusca in 'Phil.
Transact.' 1853 page 56.) that this may constantly be observed with
monstrosities in the animal kingdom; and Moquin-Tandon (25/2. 'Elements de
Teratologie Veg.' 1841 page 13.) remarks, that, as with plants the axis cannot
become monstrous without in some way affecting the organs subsequently
produced from it, so axial anomalies are almost always accompanied by
deviations of structure in the appended parts. We shall presently see that
with short-muzzled races of the dog certain histological changes in the basal
elements of the bones arrest their development and shorten them, and this
affects the position of the subsequently developed molar teeth. It is probable
that certain modifications in the larvae of insects would affect the structure
of the mature insects. But we must be careful not to extend this view too far,
for during the normal course of development, certain species pass through an
extraordinary course of change, whilst other and closely allied species arrive
at maturity with little change of structure.
Another simple case of correlation is that with the increased or decreased
dimensions of the whole body, or of any particular part, certain organs are
increased or diminished in number, or are otherwise modified. Thus pigeon
fanciers have gone on selecting pouters for length of body, and we have seen
that their vertebrae are generally increased not only in size but in number,
and their ribs in breadth. Tumblers have been selected for their small bodies,
and their ribs and primary wing-feathers are generally lessened in number.
Fantails have been selected for their large widely-expanded tails, with
numerous tail-feathers, and the caudal vertebrae are increased in size and
number. Carriers have been selected for length of beak, and their tongues have
become longer, but not in strict accordance with the length of beak. In this
latter breed and in others having large feet, the number of the scutellae on
the toes is greater than in the breeds with small feet. Many similar cases
could be given. In Germany it has been observed that the period of gestation
is longer in large than in small breeds of cattle. With our highly-improved
breeds of all kinds, the periods of maturity and of reproduction have advanced
with respect to the age of the animal; and, in correspondence with this, the
teeth are now developed earlier than formerly, so that, to the surprise of
agriculturists, the ancient rules for judging of the age of an animal by the
state of its teeth are no longer trustworthy. (25/3. Prof. J.B. Simonds on the
Age of the Ox, Sheep, etc. quoted in 'Gardener's Chronicle' 1854 page 588.)
CORRELATED VARIATION OF HOMOLOGOUS PARTS.
Parts which are homologous tend to vary in the same manner; and this is what
might have been expected, for such parts are identical in form and structure
during an early period of embryonic development, and are exposed in the egg or
womb to similar conditions. The symmetry, in most kinds of animals, of the
corresponding or homologous organs on the right and left sides of the body, is
the simplest case in point; but this symmetry sometimes fails, as with rabbits
having only one ear, or stags with one horn, or with many-horned sheep which
sometimes carry an additional horn on one side of their heads. With flowers
which have regular corollas, all the petals generally vary in the same manner,
as we see in the complicated and symmetrical pattern, on the flowers, for
instance, of the Chinese pink; but with irregular flowers, though the petals
are of course homologous, this symmetry often fails, as with the varieties of
the Antirrhinum or snapdragon, or that variety of the kidney-bean (Phaseolus)
which has a white standard-petal.
In the Vertebrata the front and hind limbs are homologous, and they tend to
vary in the same manner, as we see in long and short legged, or in thick and
thin legged races of the horse and dog. Isidore Geoffroy (25/4. 'Hist. des
Anomalies' tome 1 page 674.) has remarked on the tendency of supernumerary
digits in man to appear, not only on the right and left sides, but on the
upper and lower extremities. Meckel has insisted (25/5. Quoted by Isid.
Geoffroy ibid tome 1 page 635.) that, when the muscles of the arm depart in
number or arrangement from their proper type, they almost always imitate those
of the leg; and so conversely the varying muscles of the leg imitate the
normal muscles of the arm.
In several distinct breeds of the pigeon and fowl, the legs and the two outer
toes are heavily feathered, so that in the trumpeter pigeon they appear like
little wings. In the feather-legged bantam the "boots" or feathers, which grow
from the outside of the leg and generally from the two outer toes, have,
according to the excellent authority of Mr. Hewitt (25/6. 'The Poultry Book'
by W.B. Tegetmeier 1866 page 250.), been seen to exceed the wing-feathers in
length, and in one case were actually nine and a half inches long! As Mr.
Blyth has remarked to me, these leg-feathers resemble the primary wing-
feathers, and are totally unlike the fine down which naturally grows on the
legs of some birds, such as grouse and owls. Hence it may be suspected that
excess of food has first given redundancy to the plumage, and then that the
law of homologous variation has led to the development of feathers on the
legs, in a position corresponding with those on the wing, namely, on the
outside of the tarsi and toes. I am strengthened in this belief by the
following curious case of correlation, which for a long time seemed to me
utterly inexplicable, namely, that in pigeons of any breed, if the legs are
feathered, the two outer toes are partially connected by skin. These two outer
toes correspond with our third and fourth toes. (25/7. Naturalists differ with
respect to the homologies of the digits of birds; but several uphold the view
above advanced. See on this subject Dr. E.S. Morse in 'Annals of the Lyceum of
Nat. Hist. of New York' volume 10 1872 page 16.) Now, in the wing of the
pigeon or of any other bird, the first and fifth digits are aborted; the
second is rudimentary and carries the so-called "bastard-wing;" whilst the
third and fourth digits are completely united and enclosed by skin, together
forming the extremity of the wing. So that in feather-footed pigeons, not only
does the exterior surface support a row of long feathers, like wing-feathers,
but the very same digits which in the wing are completely united by skin
become partially united by skin in the feet; and thus by the law of the
correlated variation of homologous parts we can understand the curious
connection of feathered legs and membrane between the two outer toes.
Andrew Knight (24/8. A. Walker on 'Intermarriage' 1838 page 160.) has remarked
that the face or head and the limbs usually vary together in general
proportions. Compare, for instance, the limbs of a dray and race horse, or of
a greyhound and mastiff. What a monster a greyhound would appear with the head
of a mastiff! The modern bulldog, however, has fine limbs, but this is a
recently-selected character. From the measurements given in the sixth chapter,
we see that in several breeds of the pigeon the length of the beak and the
size of the feet are correlated. The view which, as before explained, seems
the most probable is, that disuse in all cases tends to diminish the feet, the
beak becoming at the same time shorter through correlation; but that in some
few breeds in which length of beak has been a selected point, the feet,
notwithstanding disuse, have increased in size through correlation. In the
following case some kind of correlation is seen to exist between the feet and
beak: several specimens have been sent to Mr. Bartlett at different times, as
hybrids between ducks and fowls, and I have seen one; these were, as might be
expected, ordinary ducks in a semi-monstrous condition, and in all of them the
swimming-web between the toes was quite deficient or much reduced, and in all
the beak was narrow and ill-shaped.
With the increased length of the beak in pigeons, not only the tongue
increases in length, but likewise the orifice of the nostrils. But the
increased length of the orifice of the nostrils perhaps stands in closer
correlation with the development of the corrugated skin or wattle at the base
of the beak, for when there is much wattle round the eyes, the eyelids are
greatly increased or even doubled in length.
There is apparently some correlation even in colour between the head and the
extremities. Thus with horses a large white star or blaze on the forehead is
generally accompanied by white feet. (25/9. 'The Farrier and Naturalist'
volume 1 1828 page 456. A gentleman who has attended to this point, tells me
that about three-fourths of white-faced horses have white legs.) With white
rabbits and cattle, dark marks often co-exist on the tips of the ears and on
the feet. In black and tan dogs of different breeds, tan-coloured spots over
the eyes and tan-coloured feet almost invariably go together. These latter
cases of connected colouring may be due either to reversion or to analogous
variation,--subjects to which I shall hereafter return,--but this does not
necessarily determine the question of their original correlation. Mr. H.W.
Jackson informs me that he has observed many hundred white-footed cats, and he
finds that all are more or less conspicuously marked with white on the front
of the neck or chest.
The lopping forwards and downwards of the immense ears of fancy rabbits seems
partly due to the disuse of the muscles, and partly to the weight and length
of the ears, which have been increased by selection during many generations.
Now, with the increased size and changed direction of the ears not only has
the bony auditory meatus become changed in outline, direction, and greatly in
size, but the whole skull has been slightly modified. This could be clearly
seen in "half-lops"--that is, in rabbits with only one ear lopping forward--
for the opposite sides of their skulls were not strictly symmetrical. This
seems to me a curious instance of correlation, between hard bones and organs
so soft and flexible, as well as so unimportant under a physiological point of
view, as the external ears. The result no doubt is largely due to mere
mechanical action, that is, to the weight of the ears, on the same principle
that the skull of a human infant is easily modified by pressure.
The skin and the appendages of hair, feathers, hoofs, horns, and teeth, are
homologous over the whole body. Every one knows that the colour of the skin
and that of the hair usually vary together; so that Virgil advises the
shepherd to look whether the mouth and tongue of the ram are black, lest the
lambs should not be purely white. The colour of the skin and hair, and the
odour emitted by the glands of the skin, are said (25/10. Godron 'Sur
l'Espece' tome 2 page 217.) to be connected, even in the same race of men.
Generally the hair varies in the same way all over the body in length,
fineness, and curliness. The same rule holds good with feathers, as we see
with the laced and frizzled breeds both of fowls and pigeons. In the common
cock the feathers on the neck and loins are always of a particular shape,
called hackles: now in the Polish breed, both sexes are characterised by a
tuft of feathers on the head, and through correlation these feathers in the
male always assume the form of hackles. The wing and tail-feathers, though
arising from parts not homologous, vary in length together; so that long or
short winged pigeons generally have long or short tails. The case of the
Jacobin-pigeon is more curious, for the wing and tail feathers are remarkably
long; and this apparently has arisen in correlation with the elongated and
reversed feathers on the back of the neck, which form the hood.
The hoofs and hair are homologous appendages; and a careful observer, namely
Azara (25/11. 'Quadrupedes du Paraguay' tome 2 page 333.), states that in
Paraguay horses of various colours are often born with their hair curled and
twisted like that on the head of a negro. This peculiarity is strongly
inherited. But what is remarkable is that the hoofs of these horses "are
absolutely like those of a mule." The hair also of their manes and tails is
invariably much shorter than usual, being only from four to twelve inches in
length; so that curliness and shortness of the hair are here, as with the
negro, apparently correlated.
With respect to the horns of sheep, Youatt (25/12. 'On Sheep' page 142.)
remarks that "multiplicity of horns is not found in any breed of much value;
it is generally accompanied by great length and coarseness of the fleece."
Several tropical breeds of sheep which are clothed with hair instead of wool,
have horns almost like those of a goat. Sturm (25/13. 'Ueber Racen,
Kreuzungen' etc. 1825 s. 24.) expressly declares that in different races the
more the wool is curled the more the horns are spirally twisted. We have seen
in the third chapter, where other analogous facts have been given, that the
parent of the Mauchamp breed, so famous for its fleece, had peculiarly shaped
horns. The inhabitants of Angora assert (25/14. Quoted from Conolly in 'The
Indian Field' February 1859 volume 2 page 266.) that "only the white goats
which have horns wear the fleece in the long curly locks that are so much
admired; those which are not horned having a comparatively close coat." From
these cases we may infer that the hair or wool and the horns tend to vary in a
correlated manner. (25/15. In the third chapter I have said that "the hair and
horns are so closely related to each other, that they are apt to vary
together." Dr. Wilckens ("Darwin's Theorie" 'Jahrbuch der Deutschen Viehzucht'
1866 1. Heft) translates my words into "lang- und grobhaarige Thiere sollen
geneigter sein, lange und viele Horner zu bekommen" and he then justly
disputes this proposition; but what I have really said, in accordance with the
authorities just quoted, may, I think, be trusted.) Those who have tried
hydropathy are aware that the frequent application of cold water stimulates
the skin; and whatever stimulates the skin tends to increase the growth of the
hair, as is well shown in the abnormal growth of hair near old inflamed
surfaces. Now, Professor Low (25/16. 'Domesticated Animals of the British
Islands' pages 307, 368. Dr. Wilckens argues ('Landwirth. Wochenblatt' Nr. 10
1869) to the same effect with respect to domestic animals in Germany.) is
convinced that with the different races of British cattle thick skin and long
hair depend on the humidity of the climate which they inhabit. We can thus see
how a humid climate might act on the horns--in the first place directly on the
skin and hair, and secondly by correlation on the horns. The presence or
absence of horns, moreover, both in the case of sheep and cattle, acts, as
will presently be shown, by some sort of correlation on the skull.
With respect to hair and teeth, Mr. Yarrell (25/17. 'Proceedings Zoolog. Soc.'
1833 page 113.) found many of the teeth deficient in three hairless "Egyptian
dogs," and in a hairless terrier. The incisors, canines, and the premolars
suffered most, but in one case all the teeth, except the large tubercular
molar on each side, were deficient. With man several striking cases have been
recorded (25/18. Sedgwick 'Brit. and Foreign Medico-Chirurg. Review' April
1863 page 453.) of inherited baldness with inherited deficiency, either
complete or partial, of the teeth. I may give an analogous case, communicated
to me by Mr. W. Wedderburn, of a Hindoo family in Scinde, in which ten men, in
the course of four generations, were furnished, in both jaws taken together,
with only four small and weak incisor teeth and with eight posterior molars.
The men thus affected have very little hair on the body, and become bald early
in life. They also suffer much during hot weather from excessive dryness of
the skin. It is remarkable that no instance has occurred of a daughter being
thus affected; and this fact reminds us how much more liable men are in
England to become bald than women. Though the daughters in the above family
are never affected, they transmit the tendency to their sons; and no case has
occurred of a son transmitting it to his sons. The affection thus appears only
in alternate generations, or after longer intervals. There is a similar
connection between hair and teeth, according to Mr. Sedgwick, in those rare
cases in which the hair has been renewed in old age, for this has "usually
been accompanied by a renewal of the teeth." I have remarked in a former part
of this volume that the great reduction in the size of the tusks in domestic
boars probably stands in close relation with their diminished bristles, due to
a certain amount of protection; and that the reappearance of the tusks in
boars, which have become feral and are fully exposed to the weather, probably
depends on the reappearance of the bristles. I may add, though not strictly
connected with our present point, that an agriculturist (25/19. 'Gardener's
Chronicle' 1849 page 205.) asserts that "pigs with little hair on their bodies
are most liable to lose their tails, showing a weakness of the tegumental
structure. It may be prevented by crossing with a more hairy breed."
In the previous cases deficient hair, and teeth deficient in number or size,
are apparently connected. In the following cases abnormally redundant hair,
and teeth either deficient or redundant, are likewise connected. Mr. Crawfurd
(25/20. 'Embassy to the Court of Ava' volume 1 page 320.) saw at the Burmese
Court a man, thirty years old, with his whole body, except the hands and feet,
covered with straight silky hair, which on the shoulders and spine was five
inches in length. At birth the ears alone were covered. He did not arrive at
puberty, or shed his milk teeth, until twenty years old; and at this period he
acquired five teeth in the upper jaw, namely, four incisors and one canine,
and four incisor teeth in the lower jaw; all the teeth were small. This man
had a daughter who was born with hair within her ears; and the hair soon
extended over her body. When Captain Yule (25/21. 'Narrative of a Mission to
the Court of Ava in 1855' page 94.) visited the Court, he found this girl
grown up; and she presented a strange appearance with even her nose densely
covered with soft hair. Like her father, she was furnished with incisor teeth
alone. The King had with difficulty bribed a man to marry her, and of her two
children, one, a boy fourteen months old, had hair growing out of his ears,
with a beard and moustache. This strange peculiarity has, therefore, been
inherited for three generations, with the molar teeth deficient in the
grandfather and mother; whether these teeth would likewise fail in the infant
could not then be told.
A parallel case of a man fifty-five years old, and of his son, with their
faces covered with hair, has recently occurred in Russia. Dr. Alex. Brandt has
sent me an account of this case, together with specimens of the extremely fine
hair from the cheeks. The man is deficient in teeth, possessing only four
incisors in the lower and two in the upper jaw. His son, about three years
old, has no teeth except four lower incisors. The case, as Dr. Brandt remarks
in his letter, no doubt is due to an arrest of development in the hair and
teeth. We here see how independent of the ordinary conditions of existence
such arrests must be, for the lives of a Russian peasant and of a native of
Burmah are as different as possible. (25/22. I owe to the kindness of M.
Chauman, of St. Petersburg, excellent photographs of this man and his son,
both of whom have since been exhibited in Paris and London.)
Here is another and somewhat different case communicated to me by Mr. Wallace
on the authority of Dr. Purland, a dentist: Julia Pastrana, a Spanish dancer,
was a remarkably fine woman, but she had a thick masculine beard and a hairy
forehead; she was photographed, and her stuffed skin was exhibited as a show;
but what concerns us is, that she had in both the upper and lower jaw an
irregular double set of teeth, one row being placed within the other, of which
Dr. Purland took a cast. From the redundancy of teeth her mouth projected, and
her face had a gorilla-like appearance. These cases and those of the hairless
dogs forcibly call to mind the fact, that the two orders of mammals--namely,
the Edentata and Cetacea--which are the most abnormal in their dermal
covering, are likewise the most abnormal either by deficiency or redundancy of
teeth.
The organs of sight and hearing are generally admitted to be homologous with
one another and with various dermal appendages; hence these parts are liable
to be abnormally affected in conjunction. Mr. White Cowper says "that in all
cases of double microphthalmia brought under his notice he has at the same
time met with defective development of the dental system." Certain forms of
blindness seem to be associated with the colour of the hair; a man with black
hair and a woman with light-coloured hair, both of sound constitution, married
and had nine children, all of whom were born blind; of these children, five
"with dark hair and brown iris were afflicted with amaurosis; the four others,
with light-coloured hair and blue iris, had amaurosis and cataract conjoined."
Several cases could be given, showing that some relation exists between
various affections of the eyes and ears; thus Liebreich states that out of 241
deaf-mutes in Berlin, no less than fourteen suffered from the rare disease
called pigmentary retinitis. Mr. White Cowper and Dr. Earle have remarked that
inability to distinguish different colours, or colour-blindness, "is often
associated with a corresponding inability to distinguish musical sounds."
(25/23. These statements are taken from Mr. Sedgwick in the 'Medico-Chirurg.
Review' July 1861 page 198; April 1863 pages 455 and 458. Liebreich is quoted
by Professor Devay in his 'Mariages Consanguins' 1862 page 116.)
Here is a more curious case: white cats, if they have blue eyes, are almost
always deaf. I formerly thought that the rule was invariable, but I have heard
of a few authentic exceptions. The first two notices were published in 1829
and relate to English and Persian cats: of the latter, the Rev. W.T. Bree
possessed a female, and he states, "that of the offspring produced at one and
the same birth, such as, like the mother, were entirely white (with blue eyes)
were, like her, invariably deaf; while those that had the least speck of
colour on their fur, as invariably possessed the usual faculty of hearing."
(25/24. Loudon's 'Mag. of Nat. Hist.' volume 1 1829 pages 66, 178. See also
Dr. P. Lucas 'L'Hered. Nat.' tome 1 page 428 on the inheritance of deafness in
cats. Mr. Lawson Tait states ('Nature' 1873 page 323) that only male cats are
thus affected; but this must be a hasty generalisation. The first case
recorded in England by Mr. Bree related to a female, and Mr. Fox informs me
that he has bred kittens from a white female with blue eyes, which was
completely deaf; he has also observed other females in the same condition.)
The Rev. W. Darwin Fox informs me that he has seen more than a dozen instances
of this correlation in English, Persian, and Danish cats; but he adds "that,
if one eye, as I have several times observed, be not blue, the cat hears. On
the other hand, I have never seen a white cat with eyes of the common colour
that was deaf." In France Dr. Sichel (25/25. 'Annales des Sc. Nat.' Zoolog.
3rd series 1847 tome 8 page 239.) has observed during twenty years similar
facts; he adds the remarkable case of the iris beginning, at the end of four
months, to grow dark-coloured, and then the cat first began to hear.
This case of correlation in cats has struck many persons as marvellous. There
is nothing unusual in the relation between blue eyes and white fur; and we
have already seen that the organs of sight and hearing are often
simultaneously affected. In the present instance the cause probably lies in a
slight arrest of development in the nervous system in connection with the
sense-organs. Kittens during the first nine days, whilst their eyes are
closed, appear to be completely deaf; I have made a great clanging noise with
a poker and shovel close to their heads, both when they were asleep and awake,
without producing any effect. The trial must not be made by shouting close to
their ears, for they are, even when asleep, extremely sensitive to a breath of
air. Now, as long as the eyes continue closed, the iris is no doubt blue, for
in all the kittens which I have seen this colour remains for some time after
the eyelids open. Hence, if we suppose the development of the organs of sight
and hearing to be arrested at the stage of the closed eyelids, the eyes would
remain permanently blue and the ears would be incapable of perceiving sound;
and we should thus understand this curious case. As, however, the colour of
the fur is determined long before birth, and as the blueness of the eyes and
the whiteness of the fur are obviously connected, we must believe that some
primary cause acts at a much earlier period.
The instances of correlated variability hitherto given have been chiefly drawn
from the animal kingdom, and we will now turn to plants. Leaves, sepals,
petals, stamens, and pistils are all homologous. In double flowers we see that
the stamens and pistils vary in the same manner, and assume the form and
colour of the petals. In the double columbine (Aquilegia vulgaris), the
successive whorls of stamens are converted into cornucopias, which are
enclosed within one another and resemble the true petals. In hose-in-hose
flowers the sepals mock the petals. In some cases the flowers and leaves vary
together in tint: in all the varieties of the common pea, which have purple
flowers, a purple mark may be seen on the stipules.
M. Faivre states that with the varieties of Primula sinensis the colour of the
flower is evidently correlated with the colour of the under side of the
leaves; and he adds that the varieties with fimbriated flowers almost always
have voluminous, balloon-like calyces. (25/26. 'Revue des Cours Scientifiques'
June 5, 1869 page 430.) With other plants the leaves and fruit or seeds vary
together in colour, as in a curious pale-leaved variety of the sycamore, which
has recently been described in France (25/27. 'Gardener's Chronicle' 1864 page
1202.), and as in the purple-leaved hazel, in which the leaves, the husk of
the nut, and the pellicle round the kernel are all coloured purple. (25/28.
Verlot gives several other instances 'Des Varietes' 1865 page 72.) Pomologists
can predict to a certain extent, from the size and appearance of the leaves of
their seedlings, the probable nature of the fruit; for, as Van Mons remarks
(25/29. 'Arbres Fruitiers' 1836 tome 2 pages 204, 226.) variations in the
leaves are generally accompanied by some modification in the flower, and
consequently in the fruit. In the Serpent melon, which has a narrow tortuous
fruit above a yard in length, the stem of the plant, the peduncle of the
female flower, and the middle lobe of the leaf, are all elongated in a
remarkable manner. On the other hand, several varieties of Cucurbita, which
have dwarfed stems, all produce, as Naudin remarks, leaves of the same
peculiar shape. Mr. G. Maw informs me that all the varieties of the scarlet
Pelargoniums which have contracted or imperfect leaves have contracted
flowers: the difference between "Brilliant" and its parent "Tom Thumb" is a
good instance of this. It may be suspected that the curious case described by
Risso (25/30. 'Annales du Museum' tome 20 page 188.), of a variety of the
Orange which produces on the young shoots rounded leaves with winged petioles,
and afterwards elongated leaves on long but wingless petioles, is connected
with the remarkable change in form and nature which the fruit undergoes during
its development.
In the following instance we have the colour and the form of the petals
apparently correlated, and both dependent on the nature of the season. An
observer, skilled in the subject, writes (25/31. 'Gardener's Chronicle' 1843
page 877.), "I noticed, during the year 1842, that every Dahlia of which the
colour had any tendency to scarlet, was deeply notched--indeed, to so great an
extent as to give the petals the appearance of a saw; the indentures were, in
some instances, more than a quarter of an inch deep." Again, Dahlias which
have their petals tipped with a different colour from the rest of the flower
are very inconstant, and during certain years some, or even all the flowers,
become uniformly coloured; and it has been observed with several varieties
(25/32. Ibid 1845 page 102.) that when this happens the petals grow much
elongated and lose their proper shape. This, however, may be due to reversion,
both in colour and form, to the aboriginal species.
In this discussion on correlation, we have hitherto treated of cases in which
we can partly understand the bond of connection; but I will now give cases in
which we cannot even conjecture, or can only very obscurely see, the nature of
the bond. Isidore Geoffroy Saint-Hilaire, in his work on Monstrosities,
insists (25/33. 'Hist. des Anomalies' tome 3 page 402. See also M. Camille
Dareste 'Recherches sur les Conditions' etc. 1863 pages 16, 48.), "que
certaines anomalies coexistent rarement entr'elles, d'autres frequemment,
d'autres enfin presque constamment, malgre la difference tres-grande de leur
nature, et quoiqu'elles puissent paraitre COMPLETEMENT INDEPENDANTES les unes
des autres." We see something analogous in certain diseases: thus in a rare
affection of the renal capsules (of which the functions are unknown), the skin
becomes bronzed; and in hereditary syphilis, as I hear from Sir J. Paget, both
the milk and the second teeth assume a peculiar and characteristic form.
Professor Rolleston, also, informs me that the incisor teeth are sometimes
furnished with a vascular rim in correlation with intra-pulmonary deposition
of tubercles. In other cases of phthisis and of cyanosis the nails and finger-
ends become clubbed like acorns. I believe that no explanation has been
offered of these and of many other cases of correlated disease.
What can be more curious and less intelligible than the fact previously given,
on the authority of Mr. Tegetmeier, that young pigeons of all breeds, which
when mature have white, yellow, silver-blue, or dun-coloured plumage, come out
of the egg almost naked; whereas pigeons of other colours when first born are
clothed with plenty of down? White Pea-fowls, as has been observed both in
England and France (25/34. Rev. E.S. Dixon 'Ornamental Poultry' 1848 page 111;
Isidore Geoffroy 'Hist. Anomalies' tome 1 page 211.), and as I have myself
seen, are inferior in size to the common coloured kind; and this cannot be
accounted for by the belief that albinism is always accompanied by
constitutional weakness; for white or albino moles are generally larger than
the common kind.
To turn to more important characters: the niata cattle of the Pampas are
remarkable from their short foreheads, upturned muzzles, and curved lower
jaws. In the skull the nasal and premaxillary bones are much shortened, the
maxillaries are excluded from any junction with the nasals, and all the bones
are slightly modified, even to the plane of the occiput. From the analogous
case of the dog, hereafter to be given, it is probable that the shortening of
the nasal and adjoining bones is the proximate cause of the other
modifications in the skull, including the upward curvature of the lower jaw,
though we cannot follow out the steps by which these changes have been
effected.
Polish fowls have a large tuft of feathers on their heads; and their skulls
are perforated by numerous holes, so that a pin can be driven into the brain
without touching any bone. That this deficiency of bone is in some way
connected with the tuft of feathers is clear from tufted ducks and geese
likewise having perforated skulls. The case would probably be considered by
some authors as one of balancement or compensation. In the chapter on Fowls, I
have shown that with Polish fowls the tuft of feathers was probably at first
small; by continued selection it became larger, and then rested on a fibrous
mass; and finally, as it became still larger, the skull itself became more and
more protuberant until it acquired its present extraordinary structure.
Through correlation with the protuberance of the skull, the shape and even the
relative connection of the premaxillary and nasal bones, the shape of the
orifice of the nostrils, the breadth of the frontal bone, the shape of the
post-lateral processes of the frontal and squamosal bones, and the direction
of the bony cavity of the ear, have all been modified. The internal
configuration of the skull and the whole shape of the brain have likewise been
altered in a truly marvellous manner.
After this case of the Polish fowl it would be superfluous to do more than
refer to the details previously given on the manner in which the changed form
of the comb has affected the skull, in various breeds of the fowl, causing by
correlation crests, protuberances, and depressions on its surface.
With our cattle and sheep the horns stand in close connection with the size of
the skull, and with the shape of the frontal bones; thus Cline (25/35. 'On the
Breeding of Domestic Animals' 1829 page 6.) found that the skull of a horned
ram weighed five times as much as that of a hornless ram of the same age. When
cattle become hornless, the frontal bones are "materially diminished in
breadth towards the poll;" and the cavities between the bony plates "are not
so deep, nor do they extend beyond the frontals." (25/36. Youatt on 'Cattle'
1834 page 283.)
It may be well here to pause and observe how the effects of correlated
variability, of the increased use of parts, and of the accumulation of so-
called spontaneous variations through natural selection, are in many cases
inextricably commingled. We may borrow an illustration from Mr. Herbert
Spencer, who remarks that, when the Irish elk acquired its gigantic horns,
weighing above one hundred pounds, numerous co-ordinated changes of structure
would have been indispensable,--namely, a thickened skull to carry the horns;
strengthened cervical vertebrae, with strengthened ligaments; enlarged dorsal
vertebrae to support the neck, with powerful fore-legs and feet; all these
parts being supplied with proper muscles, blood-vessels, and nerves. How then
could these admirably co-ordinated modifications of structure have been
acquired? According to the doctrine which I maintain, the horns of the male
elk were slowly gained through sexual selection,--that is, by the best-armed
males conquering the worse-armed, and leaving a greater number of descendants.
But it is not at all necessary that the several parts of the body should have
simultaneously varied. Each stag presents individual characteristics, and in
the same district those which had slightly heavier horns, or stronger necks,
or stronger bodies, or were the most courageous, would secure the greater
number of does, and consequently have a greater number of offspring. The
offspring would inherit, in a greater or less degree, these same qualities,
would occasionally intercross with one another, or with other individuals
varying in some favourable manner; and of their offspring, those which were
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