The Variation of Animals and Plants under Domestication
by
Charles Darwin
Part 7 out of 12
the best endowed in any respect would continue multiplying; and so onwards,
always progressing, sometimes in one direction, and sometimes in another,
towards the excellently co-ordinated structure of the male elk. To make this
clear, let us reflect on the probable steps, as shown in the twentieth
chapter, by which our race and dray horses have arrived at their present state
of excellence; if we could view the whole series of intermediate forms between
one of these animals and an early unimproved progenitor, we should behold a
vast number of animals, not equally improved in each generation throughout
their entire structure, but sometimes a little more in one point, and
sometimes in another, yet on the whole gradually approaching in character to
our present race or dray horses, which are so admirably fitted in the one case
for fleetness and in the other for draught.
Although natural selection would thus (25/37. Mr. Herbert Spencer 'Principles
of Biology' 1864 volume 1 pages 452, 468 takes a different view; and in one
place remarks: "We have seen reason to think that, as fast as essential
faculties multiply, and as fast as the number of organs that co-operate in any
given function increases, indirect equilibration through natural selection
becomes less and less capable of producing specific adaptations; and remains
fully capable only of maintaining the general fitness of constitution to
conditions." This view that natural selection can do little in modifying the
higher animals surprises me, seeing that man's selection has undoubtedly
effected much with our domesticated quadrupeds and birds.) tend to give to the
male elk its present structure, yet it is probable that the inherited effects
of use, and of the mutual action of part on part, have been equally or more
important. As the horns gradually increased in weight the muscles of the neck,
with the bones to which they are attached, would increase in size and
strength; and these parts would react on the body and legs. Nor must we
overlook the fact that certain parts of the skull and the extremities would,
judging by analogy, tend from the first to vary in a correlated manner. The
increased weight of the horns would also act directly on the skull, in the
same manner as when one bone is removed in the leg of a dog, the other bone,
which has to carry the whole weight of the body, increases in thickness. But
from the fact given with respect to horned and hornless cattle, it is probable
that the horns and skull would immediately act on each other through the
principle of correlation. Lastly, the growth and subsequent wear and tear of
the augmented muscles and bones would require an increased supply of blood,
and consequently increased supply of food; and this again would require
increased powers of mastication, digestion, respiration, and excretion.
COLOUR AS CORRELATED WITH CONSTITUTIONAL PECULIARITIES.
It is an old belief that with man there is a connection between complexions
and constitution; and I find that some of the best authorities believe in this
to the present day. (25/38. Dr. Prosper Lucas apparently disbelieves in any
such connection; 'L'Hered. Nat.' tome 2 pages 88-94.) Thus Dr. Beddoe by his
tables shows (25/39. 'British Medical Journal' 1862 page 433.) that a relation
exists between liability to consumption and the colour of the hair, eyes, and
skin. It has been affirmed (25/40. Boudin 'Geograph. Medicale' tome 1 page
406.) that, in the French army which invaded Russia, soldiers having a dark
complexion from the southern parts of Europe, withstood the intense cold
better than those with lighter complexions from the north; but no doubt such
statements are liable to error.
In the second chapter on Selection I have given several cases proving that
with animals and plants differences in colour are correlated with
constitutional differences, as shown by greater or less immunity from certain
diseases, from the attacks of parasitic plants and animals, from scorching by
the sun, and from the action of certain poisons. When all the individuals of
any one variety possess an immunity of this nature, we do not know that it
stands in any sort of correlation with their colour; but when several
similarly coloured varieties of the same species are thus characterised,
whilst other coloured varieties are not thus favoured, we must believe in the
existence of a correlation of this kind. Thus, in the United States purple-
fruited plums of many kinds are far more affected by a certain disease than
green or yellow-fruited varieties. On the other hand, yellow-fleshed peaches
of various kinds suffer from another disease much more than the white-fleshed
varieties. In the Mauritius red sugar-canes are much less affected by a
particular disease than the white canes. White onions and verbenas are the
most liable to mildew; and in Spain the green-fruited grapes suffered from the
vine-disease more than other coloured varieties. Dark-coloured pelargoniums
and verbenas are more scorched by the sun than varieties of other colours. Red
wheats are believed to be hardier than white; and red-flowered hyacinths were
more injured during one particular winter in Holland than other coloured
varieties. With animals, white terriers suffer most from the distemper, white
chickens from a parasitic worm in their tracheae, white pigs from scorching by
the sun, and white cattle from flies; but the caterpillars of the silk-moth
which yield white cocoons suffered in France less from the deadly parasitic
fungus than those producing yellow silk.
The cases of immunity from the action of certain vegetable poisons, in
connexion with colour, are more interesting, and are at present wholly
inexplicable. I have already given a remarkable instance, on the authority of
Professor Wyman, of all the hogs, excepting those of a black colour, suffering
severely in Virginia from eating the root of the Lachnanthes tinctoria.
According to Spinola and others (25/41. This fact and the following cases,
when not stated to the contrary, are taken from a very curious paper by Prof.
Heusinger in 'Wochenschrift fur Heilkunde' May 1846 s. 277. Settegast 'Die
Thierzucht' 1868 page 39 says that white or white-spotted sheep suffer like
pigs, or even die from eating buckwheat; whilst black or dark-woolled
individuals are not in the least affected.), buckwheat (Po1ygonum fagopyrum),
when in flower, is highly injurious to white or white-spotted pigs, if they
are exposed to the heat of the sun, but is quite innocuous to black pigs.
According to two accounts, the Hypericum crispum in Sicily is poisonous to
white sheep alone; their heads swell, their wool falls off, and they often
die; but this plant, according to Lecce, is poisonous only when it grows in
swamps; nor is this improbable, as we know how readily the poisonous principle
in plants is influenced by the conditions under which they grow.
Three accounts have been published in Eastern Prussia, of white and white-
spotted horses being greatly injured by eating mildewed and honeydewed
vetches; every spot of skin bearing white hairs becoming inflamed and
gangrenous. The Rev. J. Rodwell informs me that his father turned out about
fifteen cart-horses into a field of tares which in parts swarmed with black
aphides, and which no doubt were honeydewed, and probably mildewed; the
horses, with two exceptions, were chestnuts and bays with white marks on their
faces and pasterns, and the white parts alone swelled and became angry scabs.
The two bay horses with no white marks entirely escaped all injury. In
Guernsey, when horses eat fool's parsley (Aethusa cynapium) they are sometimes
violently purged; and this plant "has a peculiar effect on the nose and lips,
causing deep cracks and ulcers, particularly on horses with white muzzles."
(25/42. Mr. Mogford in the 'Veterinarian' quoted in 'The Field' January 22,
1861 page 545.) With cattle, independently of the action of any poison, cases
have been published by Youatt and Erdt of cutaneous diseases with much
constitutional disturbance (in one instance after exposure to a hot sun)
affecting every single point which bore a white hair, but completely passing
over other parts of the body. Similar cases have been observed with horses.
(25/43. 'Edinburgh Veterinary Journal' October 1860 page 347.)
We thus see that not only do those parts of the skin which bear white hair
differ in a remarkable manner from those bearing hair of any other colour, but
that some great constitutional difference must be correlated with the colour
of the hair; for in the above-mentioned cases, vegetable poisons caused fever,
swelling of the head, as well as other symptoms, and even death, to all the
white, or white-spotted animals.
CHAPTER 2.XXVI.
LAWS OF VARIATION, continued.--SUMMARY.
THE FUSION OF HOMOLOGOUS PARTS.
THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS.
COMPENSATION OF GROWTH.
MECHANICAL PRESSURE.
RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE
OVARY, AS INDUCING VARIATION.
ANALOGOUS OR PARALLEL VARIETIES.
SUMMARY OF THE THREE LAST CHAPTERS.
THE FUSION OF HOMOLOGOUS PARTS.
Geoffroy Saint-Hilaire formerly propounded what he called la loi de l'affinite
de soi pour soi, which has been discussed and illustrated by his son, Isidore,
with respect to monsters in the animal kingdom (26/1. 'Hist. des Anomalies'
1832 tome 1 pages 22, 537-556; tome 3 page 462.), and by Moquin-Tandon, with
respect to monstrous plants.
This law seems to imply that homologous parts actually attract one another and
then unite. No doubt there are many wonderful cases, in which such parts
become intimately fused together. This is perhaps best seen in monsters with
two heads, which are united, summit to summit, or face to face, or Janus-like,
back to back, or obliquely side to side. In one instance of two heads united
almost face to face, but a little obliquely, four ears were developed, and on
one side a perfect face, which was manifestly formed by the fusion of two
half-faces. Whenever two bodies or two heads are united, each bone, muscle,
vessel, and nerve on the line of junction appears as if it had sought out its
fellow, and had become completely fused with it. Lereboullet (26/2. 'Comptes
Rendus' 1855 pages 855, 1039.), who carefully studied the development of
double monsters in fishes, observed in fifteen instances the steps by which
two heads gradually became united into one. In all such cases it is now
thought by the greater number of capable judges that the homologous parts do
not attract each other, but that in the words of Mr. Lowne (26/3. 'Catalogue
of the Teratological Series in the Museum of the R. Coll. of Surgeons' 1872
page 16.): "As union takes place before the differentiation of distinct organs
occurs, these are formed in continuity with each other." He adds that organs
already differentiated probably in no case become united to homologous ones.
M. Dareste does not speak (26/4. 'Archives de Zoolog. Exper.' January 1874
page 78.) quite decisively against the law of soi pour soi, but concludes by
saying, "On se rend parfaitement compte de la formation des monstres, si l'on
admet que les embryons qui se soudent appartiennent a un meme oeuf; qu'ils
s'unissent en meme temps qu'ils se forment, et que la soudure ne se produit
que pendant la premiere periode de la vie embryonnaire, celle ou les organes
ne sont encore constitues que par des blastemes homogenes."
By whatever means the abnormal fusion of homologous parts is effected, such
cases throw light on the frequent presence of organs which are double during
an embryonic period (and throughout life in other and lower members of the
same class) but which afterwards unite by a normal process into a single
medial organ. In the vegetable kingdom Moquin-Tandon (26/5. 'Teratologie Veg.'
1841 livre 3.) gives a long list of cases, showing how frequently homologous
parts, such as leaves, petals, stamens, and pistils, flowers, and aggregates
of homologous parts, such as buds, as well as fruit, become blended, both
normally and abnormally, with perfect symmetry into one another.
THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS.
Isidore Geoffroy (26/6. 'Hist. des Anomalies' tome 3 pages 4, 5, 6.) insists
that, when any part or organ is repeated many times in the same animal, it is
particularly liable to vary both in number and structure. With respect to
number, the proposition may, I think, be considered as fully established; but
the evidence is chiefly derived from organic beings living under their natural
conditions, with which we are not here concerned. Whenever such parts as the
vertebrae or teeth, the rays in the fins of fishes, or the feathers in the
tails of birds, or petals, stamens, pistils, or seeds, are very numerous, the
number is generally variable. With respect to the structure of multiple parts,
the evidence of variability is not so decisive; but the fact, as far as it may
be trusted, probably depends on multiple parts being of less physiological
importance than single parts; consequently their structure has been less
rigorously guarded by natural selection.
COMPENSATION OF GROWTH, OR BALANCEMENT.
This law, as applied to natural species, was propounded by Goethe and Geoffroy
Saint-Hilaire at nearly the same time. It implies that, when much organised
matter is used in building up some one part, other parts are starved and
become reduced. Several authors, especially botanists, believe in this law;
others reject it. As far as I can judge, it occasionally holds good; but its
importance has probably been exaggerated. It is scarcely possible to
distinguish between the supposed effects of such compensation, and the effects
of long-continued selection which may lead to the augmentation of one part,
and simultaneously to the diminution of another. Anyhow, there can be no doubt
that an organ may be greatly increased without any corresponding diminution of
an adjoining part. To recur to our former illustration of the Irish elk, it
may be asked what part has suffered in consequence of the immense development
of the horns?
It has already been observed that the struggle for existence does not bear
hard on our domesticated productions, and consequently the principle of
economy of growth will seldom come into play, so that we ought not to expect
to find with them frequent evidence of compensation. We have, however, some
such cases. Moquin-Tandon describes a monstrous bean (26/7. 'Teratologie Veg.'
page 156. See also my book on 'The Movements and Habits of Climbing Plants'
2nd edition 1875 page 202.), in which the stipules were enormously developed,
and the leaflets apparently in consequence completely aborted; this case is
interesting, as it represents the natural condition of Lathyrus aphaca, with
its stipules of great size, and its leaves reduced to mere threads, which act
as tendrils. De Candolle (26/8. 'Memoires du Museum' etc. tome 8 page 178.)
has remarked that the varieties of Raphanus sativus which have small roots
yield numerous seed containing much oil, whilst those with large roots are not
productive in oil; and so it is with Brassica asperifolia. The varieties of
Cucurbita pepo which bear large fruit yield a small crop, according to Naudin;
whilst those producing small fruit yield a vast number. Lastly, I have
endeavoured to show in the eighteenth chapter that with many cultivated plants
unnatural treatment checks the full and proper action of the reproductive
organs, and they are thus rendered more or less sterile; consequently, in the
way of compensation, the fruit becomes greatly enlarged, and, in double
flowers, the petals are greatly increased in number.
With animals, it has been found difficult to produce cows which yield much
milk, and are afterwards capable of fattening well. With fowls which have
large top-knots and beards the comb and wattles are generally much reduced in
size; though there are exceptions to this rule. Perhaps the entire absence of
the oil-gland in fantail pigeons may be connected with the great development
of their tails.
MECHANICAL PRESSURE AS A CAUSE OF MODIFICATIONS.
In some few cases there is reason to believe that mere mechanical pressure has
affected certain structures. Vrolik and Weber (26/9. Prichard 'Phys. Hist. of
Mankind' 1851 volume 1 page 324.) maintain that the shape of the human head is
influenced by the shape of the mother's pelvis. The kidneys in different birds
differ much in form, and St. Ange (26/10. 'Annales des Sc. Nat.' 1st series
tome 19 page 327.) believes that this is determined by the form of the pelvis,
which again, no doubt, stands in close relation with their power of
locomotion. In snakes, the viscera are curiously displaced, in comparison with
their position in other vertebrates; and this has been attributed by some
authors to the elongation of their bodies; but here, as in so many previous
cases, it is impossible to disentangle a direct result of this kind from that
consequent on natural selection. Godron has argued (26/11. 'Comptes Rendus'
December 1864 page 1039.) that the abortion of the spur on the inner side of
the flowers in Corydalis, is caused by the buds at a very early period of
growth whilst underground being closely pressed against one another and
against the stem. Some botanists believe that the singular difference in the
shape both of the seed and corolla, in the interior and exterior florets in
certain Compositous and Umbelliferous plants, is due to the pressure to which
the inner florets are subjected; but this conclusion is doubtful.
The facts just given do not relate to domesticated productions, and therefore
do not strictly concern us. But here is a more appropriate case: H. Muller
(26/12. "Ueber fotale Rachites" 'Wurzburger Medicin. Zeitschrift' 1860 b. 1 s.
265.) has shown that in shortfaced races of the dog some of the molar teeth
are placed in a slightly different position to that which they occupy in other
dogs, especially in those having elongated muzzles; and as he remarks, any
inherited change in the arrangement of the teeth deserves notice, considering
their classificatory importance. This difference in position is due to the
shortening of certain facial bones and the consequent want of space; and the
shortening results from a peculiar and abnormal state of the embryonal
cartilages of the bones.
[RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE
OVARY, AS INDUCING VARIATION.
In the thirteenth chapter various peloric flowers were described, and their
production was shown to be due either to arrested development, or to reversion
to a primordial condition. Moquin-Tandon has remarked that the flowers which
stand on the summit of the main stem or of a lateral branch are more liable to
become peloric than those on the sides (26/13. 'Teratologie Veg.' page 192.);
and he adduces, amongst other instances, that of Teucrium campanulatum. In
another Labiate plant grown by me, viz., the Galeobdolon luteum, the peloric
flowers were always produced on the summit of the stem, where flowers are not
usually borne. In Pelargonium, a SINGLE flower in the truss is frequently
peloric, and when this occurs I have during several years invariably observed
it to be the central flower. This is of such frequent occurrence that one
observer (26/14. 'Journal of Horticulture' July 2, 1861 page 253.) gives the
names of ten varieties flowering at the same time, in every one of which the
central flower was peloric. Occasionally more than one flower in the truss is
peloric, and then of course the additional ones must be lateral. These flowers
are interesting as showing how the whole structure is correlated. In the
common Pelargonium the upper sepal is produced into a nectary which coheres
with the flower-peduncle; the two upper petals differ a little in shape from
the three lower ones, and are marked with dark shades of colour; the stamens
are graduated in length and upturned. In the peloric flowers, the nectary
aborts; all the petals become alike both in shape and colour; the stamens are
generally reduced in number and become straight, so that the whole flower
resembles that of the allied genus Erodium. The correlation between these
changes is well shown when one of the two upper petals alone loses its dark
mark, for in this case the nectary does not entirely abort, but is usually
much reduced in length. (26/15. It would be worth trial to fertilise with the
same pollen the central and lateral flowers of the pelargonium, or of other
highly cultivated plants, protecting them of course from insects: then to sow
the seed separately, and observe whether the one or the other lot of seedlings
varied the most.)
Morren has described (26/16. Quoted in 'Journal of Horticulture' February 24,
1863 page 152.) a marvellous flask-shaped flower of the Calceolaria, nearly
four inches in length, which was almost completely peloric; it grew on the
summit of the plant, with a normal flower on each side; Prof. Westwood also
has described (26/17. 'Gardener's Chronicle' 1866 page 612. For the
Phalaenopsis see ibid 1867 page 211.) three similar peloric flowers, which all
occupied a central position on the flower-branches. In the Orchideous genus,
Phalaenopsis, the terminal flower has been seen to become peloric.
In a Laburnum-tree I observed that about a fourth part of the racemes produced
terminal flowers which had lost their papilionaceous structure. These were
produced after almost all the other flowers on the same racemes had withered.
The most perfectly pelorised examples had six petals, each marked with black
striae like those on the standard-petal. The keel seemed to resist the change
more than the other petals. Dutrochet has described (26/18. 'Memoires...des
Vegetaux' 1837 tome 2 page 170.) an exactly similar case in France, and I
believe these are the only two instances of pelorism in the laburnum which
have been recorded. Dutrochet remarks that the racemes on this tree do not
properly produce a terminal flower, so that (as in the case of the
Galeobdolon) their position as well as structure are both anomalies, which no
doubt are in some manner related. Dr. Masters has briefly described another
leguminous plant (26/19. 'Journal of Horticulture' July 23, 1861 page 311.),
namely, a species of clover, in which the uppermost and central flowers were
regular or had lost their papilionaceous structure. In some of these plants
the flower-heads were also proliferous.
Lastly, Linaria produces two kinds of peloric flowers, one having simple
petals, and the other having them all spurred. The two forms, as Naudin
remarks (26/20. 'Nouvelles Archives du Museum' tome 1 page 137.), not rarely
occur on the same plant, but in this case the spurred form almost invariably
stands on the summit of the spike.
The tendency in the terminal or central flower to become peloric more
frequently than the other flowers, probably results from "the bud which stands
on the end of a shoot receiving the most sap; it grows out into a stronger
shoot than those situated lower down." (26/21. Hugo von Mohl 'The Vegetable
Cell' English translation 1852 page 76.) I have discussed the connection
between pelorism and a central position, partly because some few plants are
known normally to produce a terminal flower different in structure from the
lateral ones; but chiefly on account of the following case, in which we see a
tendency to variability or to reversion connected with the same position. A
great judge of Auriculas (26/22. The Rev. H.H. Dombrain in 'Journal of
Horticulture' 1861 June 4 page 174; and June 25 page 234; 1862 April 29 page
83.) states that when one throws up a side bloom it is pretty sure to keep its
character; but that if it grows from the centre or heart of the plant,
whatever the colour of the edging ought to be, "it is just as likely to come
in any other class as in the one to which it properly belongs." This is so
notorious a fact, that some florists regularly pinch off the central trusses
of flowers. Whether in the highly improved varieties the departure of the
central trusses from their proper type is due to reversion, I do not know. Mr.
Dombrain insists that, whatever may be the commonest kind of imperfection in
each variety, this is generally exaggerated in the central truss. Thus one
variety "sometimes has the fault of producing a little green floret in the
centre of the flower," and in central blooms these become excessive in size.
In some central blooms, sent to me by Mr. Dombrain, all the organs of the
flower were rudimentary in structure, of minute size, and of a green colour,
so that by a little further change all would have been converted into small
leaves. In this case we clearly see a tendency to prolification--a term which
I may explain, for those who have never attended to botany, to mean the
production of a branch or flower, or head of flowers, out of another flower.
Now Dr. Masters (26/23. 'Transact. Linn. Soc.' volume 23 1861 page 360.)
states that the central or uppermost flower on a plant is generally the most
liable to prolification. Thus, in the varieties of the Auricula, the loss of
their proper character and a tendency to prolification, also a tendency to
prolification with pelorism, are all connected together, and are due either to
arrested development, or to reversion to a former condition.
The following is a more interesting case; Metzger (26/24. 'Die Getreidearten'
1845 s. 208, 209.) cultivated in Germany several kinds of maize brought from
the hotter parts of America, and he found, as previously described, that in
two or three generations the grains became greatly changed in form, size, and
colour; and with respect to two races he expressly states that in the first
generation, whilst the lower grains on each head retained their proper
character, the uppermost grains already began to assume that character which
in the third generation all the grains acquired. As we do not know the
aboriginal parent of the maize, we cannot tell whether these changes are in
any way connected with reversion.
In the two following cases, reversion comes into play and is determined by the
position of the seed in the capsule. The Blue Imperial pea is the offspring of
the Blue Prussian, and has larger seed and broader pods than its parent. Now
Mr. Masters, of Canterbury, a careful observer and a raiser of new varieties
of the pea, states (26/25. 'Gardener's Chronicle' 1850 page 198.) that the
Blue Imperial always has a strong tendency to revert to its parent-stock, and
the reversion "occurs in this manner: the last (or uppermost) pea in the pod
is frequently much smaller than the rest; and if these small peas are
carefully collected and sown separately, very many more, in proportion, will
revert to their origin, than those taken from the other parts of the pod."
Again, M. Chate (26/26. Quoted in 'Gardener's Chronicle' 1866 page 74.) says
that in raising seedling stocks he succeeds in getting eighty per cent to bear
double flowers, by leaving only a few of the secondary branches to seed; but
in addition to this, "at the time of extracting the seeds, the upper portion
of the pod is separated and placed aside, because it has been ascertained that
the plants coming from the seeds situated in this portion of the pod, give
eighty per cent of single flowers." Now the production of single-flowering
plants from the seed of double-flowering plants is clearly a case of
reversion. These latter facts, as well as the connection between a central
position and pelorism and prolification, show in an interesting manner how
small a difference--namely, a little greater or less freedom in the flow of
sap towards one part of the plant--determines important changes of structure.]
ANALOGOUS OR PARALLEL VARIATION.
By this term I mean that similar characters occasionally make their appearance
in the several varieties or races descended from the same species, and more
rarely in the offspring of widely distinct species. We are here concerned, not
as hitherto with the causes of variation, but with the results; but this
discussion could not have been more conveniently introduced elsewhere. The
cases of analogous variation, as far as their origin is concerned, may be
grouped, disregarding minor subdivisions, under two main heads; firstly, those
due to unknown causes acting on similarly constituted organisms, and which
consequently have varied in a similar manner; and secondly, those due to the
reappearance of characters which were possessed by a more or less remote
progenitor. But these two main divisions can often be separated only
conjecturally, and graduate, as we shall presently see, into each other.
[Under the first head of analogous variations, not due to reversion, we have
the many cases of trees belonging to quite different orders which have
produced pendulous and fastigiate varieties. The beech, hazel, and barberry
have given rise to purple-leaved varieties; and, as Bernhardi remarks (26/27.
'Ueber den Begriff der Pflanzenart' 1834 s. 14.), a multitude of plants, as
distinct as possible, have yielded varieties with deeply-cut or laciniated
leaves. Varieties descended from three distinct species of Brassica have their
stems, or so-called roots, enlarged into globular masses. The nectarine is the
offspring of the peach; and the varieties of peaches and nectarines offer a
remarkable parallelism in the fruit being white, red, or yellow fleshed--in
being clingstones or freestones--in the flowers being large or small--in the
leaves being serrated or crenated, furnished with globose or reniform glands,
or quite destitute of glands. It should be remarked that each variety of the
nectarine has not derived its character from a corresponding variety of the
peach. The several varieties also of a closely allied genus, namely the
apricot, differ from one another in nearly the same parallel manner. There is
no reason to believe that any of these varieties have merely reacquired long-
lost characters; and in most of them this certainly is not the case.
Three species of Cucurbita have yielded a multitude of races which correspond
so closely in character that, as Naudin insists, they may be arranged in
almost strictly parallel series. Several varieties of the melon are
interesting from resembling, in important characters, other species, either of
the same genus or of allied genera; thus, one variety has fruit so like, both
externally and internally, the fruit of a perfectly distinct species, namely,
the cucumber, as hardly to be distinguished from it; another has long
cylindrical fruit twisting about like a serpent; in another the seeds adhere
to portions of the pulp; in another the fruit, when ripe, suddenly cracks and
falls into pieces; and all these highly remarkable peculiarities are
characteristic of species belonging to allied genera. We can hardly account
for the appearance of so many unusual characters by reversion to a single
ancient form; but we must believe that all the members of the family have
inherited a nearly similar constitution from an early progenitor. Our cereal
and many other plants offer similar cases.
With animals we have fewer cases of analogous variation, independently of
direct reversion. We see something of the kind in the resemblance between the
short-muzzled races of the dog, such as the pug and bull-dog; in feather-
footed races of the fowl, pigeon, and canary-bird; in horses of the most
different races presenting the same range of colour; in all black-and-tan dogs
having tan-coloured eye-spots and feet, but in this latter case reversion may
possibly have played a part. Low has remarked (26/28. 'Domesticated Animals'
1845 page 351.) that several breeds of cattle are "sheeted,"--that is, have a
broad band of white passing round their bodies like a sheet; this character is
strongly inherited, and sometimes originates from a cross; it may be the first
step in reversion to an early type, for, as was shown in the third chapter,
white cattle with dark ears, dark feet and tip of tail, formerly existed, and
now exist in feral or semi-feral condition in several quarters of the world.
Under our second main division, namely, of analogous variations due to
reversion, the best cases are afforded by pigeons. In all the most distinct
breeds, sub-varieties occasionally appear coloured exactly like the parent
rock-pigeon, with black wing-bars, white loins, banded tail, etc.; and no one
can doubt that these characters are due to reversion. So with minor details;
turbits properly have white tails, but occasionally a bird is born with a
dark-coloured and banded tail; pouters properly have their primary wing-
feathers white, but not rarely a "sword-flighted" bird appears, that is, one
with the few first primaries dark-coloured; and in these cases we have
characters proper to the rock-pigeon, but new to the breed, evidently
appearing from reversion. In some domestic varieties the wing-bars, instead of
being simply black, as in the rock-pigeon, are beautifully edged with
different zones of colour, and they then present a striking analogy with the
wing-bars in certain natural species of the same family, such as Phaps
chalcoptera; and this may probably be accounted for by all the species of the
family being descended from the same remote progenitor and having a tendency
to vary in the same manner. Thus, also, we can perhaps understand the fact of
some Laugher-pigeons cooing almost like turtle-doves, and for several races
having peculiarities in their flight, since certain natural species (viz., C.
torquatrix and palumbus), display singular vagaries in this respect. In other
cases a race, instead of imitating a distinct species, resembles some other
race; thus, certain runts tremble and slightly elevate their tails, like
fantails; and turbits inflate the upper part of their oesophagus, like pouter-
pigeons.
It is a common circumstance to find certain coloured marks persistently
characterising all the species of a genus, but differing much in tint; and the
same thing occurs with the varieties of the pigeon: thus, instead of the
general plumage being blue, with the wing-bars black, there are snow-white
varieties with red bars, and black varieties with white bars; in other
varieties the wing-bars, as we have seen, are elegantly zoned with different
tints. The Spot pigeon is characterised by the whole plumage being white,
excepting a spot on the forehead and the tail; but these parts may be red,
yellow, or black. In the rock-pigeon and in many varieties the tail is blue,
with the outer edges of the outer feathers white; but in the sub-variety of
the monk-pigeon we have a reversed style of coloration, for the tail is white,
except the outer edges of the outer feathers, which are black. (26/29.
Bechstein 'Naturgeschichte Deutschlands' b. 4 1795 s. 31.)
With some species of birds, for instance with gulls, certain coloured parts
appear as if almost washed out, and I have observed exactly the same
appearance in the terminal dark tail-bar in certain pigeons, and in the whole
plumage of certain varieties of the duck. Analogous facts in the vegetable
kingdom could be given.
Many sub-varieties of the pigeon have reversed and somewhat lengthened
feathers on the back part of their heads, and this is certainly not due to
reversion to the parent-species, which shows no trace of such structure: but
when we remember that sub-varieties of the fowl, turkey, canary-bird, duck,
and goose, all have either topknots or reversed feathers on their heads; and
when we remember that scarcely a single large natural group of birds can be
named, in which some members have not a tuft of feathers on their heads, we
may suspect that reversion to some extremely remote form has come into action.
Several breeds of the fowl have either spangled or pencilled feathers; and
these cannot be derived from the parent-species, the Gallus bankiva; though of
course it is possible that one early progenitor of this species may have been
spangled, and another pencilled. But, as many gallinaceous birds are either
spangled or pencilled, it is a more probable view that the several domestic
breeds of the fowl have acquired this kind of plumage from all the members of
the family inheriting a tendency to vary in a like manner. The same principle
may account for the ewes in certain breeds of sheep being hornless, like the
females of some other hollow-horned ruminants; it may account for certain
domestic cats having slightly-tufted ears, like those of the lynx; and for the
skulls of domestic rabbits often differing from one another in the same
characters by which the skulls of the various species of the genus Lepus
differ.
I will only allude to one other case, already discussed. Now that we know that
the wild parent of the ass commonly has striped legs, we may feel confident
that the occasional appearance of stripes on the legs of the domestic ass is
due to reversion; but this will not account for the lower end of the shoulder-
stripe being sometimes angularly bent or slightly forked. So, again, when we
see dun and other coloured horses with stripes on the spine, shoulders, and
legs, we are led, from reasons formerly given, to believe that they reappear
through reversion to the wild parent-horse. But when horses have two or three
shoulder-stripes, with one of them occasionally forked at the lower end, or
when they have stripes on their faces, or are faintly striped as foals over
nearly their whole bodies, with the stripes angularly bent one under the other
on the forehead, or irregularly branched in other parts, it would be rash to
attribute such diversified characters to the reappearance of those proper to
the aboriginal wild horse. As three African species of the genus are much
striped, and as we have seen that the crossing of the unstriped species often
leads to the hybrid offspring being conspicuously striped--bearing also in
mind that the act of crossing certainly causes the reappearance of long-lost
characters--it is a more probable view that the above-specified stripes are
due to reversion, not to the immediate wild parent-horse, but to the striped
progenitor of the whole genus.]
I have discussed this subject of analogous variation at considerable length,
because it is well known that the varieties of one species frequently resemble
distinct species--a fact in perfect harmony with the foregoing cases, and
explicable on the theory of descent. Secondly, because these facts are
important from showing, as remarked in a former chapter, that each trifling
variation is governed by law, and is determined in a much higher degree by the
nature of the organisation, than by the nature of the conditions to which the
varying being has been exposed. Thirdly, because these facts are to a certain
extent related to a more general law, namely, that which Mr. B.D. Walsh
(26/30. 'Proc. Entomolog. Soc. of Philadelphia' October 1863 page 213.) has
called the "Law of EQUABLE VARIABILITY," or, as he explains it, "if any given
character is very variable in one species of a group, it will tend to be
variable in allied species; and if any given character is perfectly constant
in one species of a group, it will tend to be constant in allied species."
This leads me to recall a discussion in the chapter on Selection, in which it
was shown that with domestic races, which are now undergoing rapid
improvement, those parts or characters vary the most, which are the most
valued. This naturally follows from recently selected characters continually
tending to revert to their former less improved standard, and from their being
still acted on by the same agencies, whatever these may be, which first caused
the characters in question to vary. The same principle is applicable to
natural species, for, as stated in my 'Origin of Species' generic characters
are less variable than specific characters; and the latter are those which
have been modified by variation and natural selection, since the period when
all the species belonging to the genus branched off from a common progenitor,
whilst generic characters are those which have remained unaltered from a much
more remote epoch, and accordingly are now less variable. This statement makes
a near approach to Mr. Walsh's law of Equable Variability. Secondary sexual
characters, it may be added, rarely serve to characterise distinct genera, for
they usually differ much in the species of the same genus, and they are highly
variable in the individuals of the same species; we have also seen in the
earlier chapters of this work how variable secondary sexual characters become
under domestication.
SUMMARY OF THE THREE PREVIOUS CHAPTERS ON THE LAWS OF VARIATION.
In the twenty-third chapter we saw that changed conditions occasionally, or
even often, act in a definite manner on the organisation, so that all, or
nearly all, the individuals thus exposed become modified in the same manner.
But a far more frequent result of changed conditions, whether acting directly
on the organisation or indirectly through the reproductive system, is
indefinite and fluctuating variability. In the three last chapters, some of
the laws by which such variability is regulated have been discussed.
Increased use adds to the size of muscles, together with the blood-vessels,
nerves, ligaments, the crests of bone and the whole bones, to which they are
attached. Increased functional activity increases the size of various glands,
and strengthens the sense-organs. Increased and intermittent pressure thickens
the epidermis. A change in the nature of the food sometimes modifies the coats
of the stomach, and augments or decreases the length of the intestines.
Continued disuse, on the other hand, weakens and diminishes all parts of the
organisation. Animals which during many generations have taken but little
exercise, have their lungs reduced in size, and as a consequence the bony
fabric of the chest and the whole form of the body become modified. With our
anciently domesticated birds, the wings have been little used, and they are
slightly reduced; with their decrease, the crest of the sternum, the scapulae,
coracoids, and furculum, have all been reduced.
With domesticated animals, the reduction of a part from disuse is never
carried so far that a mere rudiment is left; whereas we have reason to believe
that this has often occurred under nature; the effects of disuse in this
latter case being aided by economy of growth, together with the intercrossing
of many varying individuals. The cause of this difference between organisms in
a state of nature, and under domestication, probably is that in the latter
case there has not been time sufficient for any very great change, and that
the principle of economy of growth does not come into action. On the contrary,
structures which are rudimentary in the parent-species, sometimes become
partially redeveloped in our domesticated productions. Such rudiments as
occasionally make their appearance under domestication, seem always to be the
result of a sudden arrest of development; nevertheless they are of interest,
as showing that rudiments are the relics of organs once perfectly developed.
Corporeal, periodical, and mental habits, though the latter have been almost
passed over in this work, become changed under domestication, and the changes
are often inherited. Such changed habits in an organic being, especially when
living a free life, would often lead to the augmented or diminished use of
various organs, and consequently to their modification. From long-continued
habit, and more especially from the occasional birth of individuals with a
slightly different constitution, domestic animals and cultivated plants become
to a certain extent acclimatised or adapted to a climate different from that
proper to the parent-species.
Through the principle of correlated variability, taken in its widest sense,
when one part varies other parts vary, either simultaneously, or one after the
other. Thus, an organ modified during an early embryonic period affects other
parts subsequently developed. When an organ, such as the beak, increases or
decreases in length, adjoining or correlated parts, as the tongue and the
orifice of the nostrils, tend to vary in the same manner. When the whole body
increases or decreases in size, various parts become modified; thus, with
pigeons the ribs increase or decrease in number and breadth. Homologous parts
which are identical during their early development and are exposed to similar
conditions, tend to vary in the same or in some connected manner,--as in the
case of the right and left sides of the body, and of the front and hind limbs.
So it is with the organs of sight and hearing; for instance, white cats with
blue eyes are almost always deaf. There is a manifest relation throughout the
body between the skin and various dermal appendages, such as hair, feathers,
hoofs, horns, and teeth. In Paraguay, horses with curly hair have hoofs like
those of a mule; the wool and the horns of sheep often vary together; hairless
dogs are deficient in their teeth; men with redundant hair have abnormal
teeth, either by deficiency or excess. Birds with long wing-feathers usually
have long tail-feathers. When long feathers grow from the outside of the legs
and toes of pigeons, the two outer toes are connected by membrane; for the
whole leg tends to assume the structure of the wing. There is a manifest
relation between a crest of feathers on the head and a marvellous amount of
change in the skull of various fowls; and in a lesser degree, between the
greatly elongated, lopping ears of rabbits and the structure of their skulls.
With plants, the leaves, various parts of the flower, and the fruit, often
vary together to a correlated manner.
In some cases we find correlation without being able even to conjecture what
is the nature of the connection, as with various monstrosities and diseases.
This is likewise the case with the colour of the adult pigeon, in connection
with the presence of down on the young bird. Numerous curious instances have
been given of peculiarities of constitution, in correlation with colour, as
shown by the immunity of individuals of one colour from certain diseases, from
the attacks of parasites and from the action of certain vegetable poisons.
Correlation is an important subject; for with species, and in a lesser degree
with domestic races, we continually find that certain parts have been greatly
modified to serve some useful purpose; but we almost invariably find that
other parts have likewise been more or less modified, without our being able
to discover any advantage in the change. No doubt great caution is necessary
with respect to this latter point, for it is difficult to overrate our
ignorance on the use of various parts of the organisation; but from what we
have seen, we may believe that many modifications are of no direct service,
having arisen in correlation with other and useful changes.
Homologous parts during their early development often become fused together.
Multiple and homologous organs are especially liable to vary in number and
probably in form. As the supply of organised matter is not unlimited, the
principle of compensation sometimes comes into action; so that, when one part
is greatly developed, adjoining parts are apt to be reduced; but this
principle is probably of much less importance than the more general one of the
economy of growth. Through mere mechanical pressure hard parts occasionally
affect adjoining parts. With plants the position of the flowers on the axis,
and of the seeds in the ovary, sometimes leads, through a more or less free
flow of sap, to changes of structure; but such changes are often due to
reversion. Modifications, in whatever manner caused, will be to a certain
extent regulated by that co-ordinating power, or so-called nisus formativus,
which is in fact a remnant of that simple form of reproduction, displayed by
many lowly organised beings in their power of fissiparous generation and
budding. Finally, the effects of the laws which directly or indirectly govern
variability, may be largely regulated by man's selection, and will so far be
determined by natural selection that changes advantageous to any race will be
favoured, and disadvantageous changes will be checked.
Domestic races descended from the same species, or from two or more allied
species, are liable to revert to characters derived from their common
progenitor; and, as they inherit a somewhat similar constitution, they are
liable to vary in the same manner. From these two causes analogous varieties
often arise. When we reflect on the several foregoing laws, imperfectly as we
understand them, and when we bear in mind how much remains to be discovered,
we need not be surprised at the intricate and to us unintelligible manner in
which our domestic productions have varied, and still go on varying.
CHAPTER 2.XXVII.
PROVISIONAL HYPOTHESIS OF PANGENESIS.
PRELIMINARY REMARKS.
FIRST PART:
THE FACTS TO BE CONNECTED UNDER A SINGLE POINT OF VIEW, NAMELY, THE VARIOUS
KINDS OF REPRODUCTION.
REGROWTH OF AMPUTATED PARTS.
GRAFT-HYBRIDS.
THE DIRECT ACTION OF THE MALE ELEMENT ON THE FEMALE.
DEVELOPMENT.
THE FUNCTIONAL INDEPENDENCE OF THE UNITS OF THE BODY.
VARIABILITY.
INHERITANCE.
REVERSION.
SECOND PART:
STATEMENT OF THE HYPOTHESIS.
HOW FAR THE NECESSARY ASSUMPTIONS ARE IMPROBABLE.
EXPLANATION BY AID OF THE HYPOTHESIS OF THE SEVERAL CLASSES OF FACTS SPECIFIED
IN THE FIRST PART.
CONCLUSION.
In the previous chapters large classes of facts, such as those bearing on bud-
variation, the various forms of inheritance, the causes and laws of variation,
have been discussed; and it is obvious that these subjects, as well as the
several modes of reproduction, stand in some sort of relation to one another.
I have been led, or rather forced, to form a view which to a certain extent
connects these facts by a tangible method. Every one would wish to explain to
himself, even in an imperfect manner, how it is possible for a character
possessed by some remote ancestor suddenly to reappear in the offspring; how
the effects of increased or decreased use of a limb can be transmitted to the
child; how the male sexual element can act not solely on the ovules, but
occasionally on the mother-form; how a hybrid can be produced by the union of
the cellular tissue of two plants independently of the organs of generation;
how a limb can be reproduced on the exact line of amputation, with neither too
much nor too little added; how the same organism may be produced by such
widely different processes, as budding and true seminal generation; and,
lastly, how of two allied forms, one passes in the course of its development
through the most complex metamorphoses, and the other does not do so, though
when mature both are alike in every detail of structure. I am aware that my
view is merely a provisional hypothesis or speculation; but until a better one
be advanced, it will serve to bring together a multitude of facts which are at
present left disconnected by any efficient cause. As Whewell, the historian of
the inductive sciences, remarks:--"Hypotheses may often be of service to
science, when they involve a certain portion of incompleteness, and even of
error." Under this point of view I venture to advance the hypothesis of
Pangenesis, which implies that every separate part of the whole organisation
reproduces itself. So that ovules, spermatozoa, and pollen-grains,--the
fertilised egg or seed, as well as buds,--include and consist of a multitude
of germs thrown off from each separate part or unit. (27/1. This hypothesis
has been severely criticised by many writers, and it will be fair to give
references to the more important articles. The best essay which I have seen is
by Prof. Delpino, entitled 'Sulla Darwiniana Teoria della Pangenesi, 1869' of
which a translation appeared in 'Scientific Opinion' September 29, 1869 and
the succeeding numbers. He rejects the hypothesis, but criticises it fairly,
and I have found his criticisms very useful. Mr. Mivart ('Genesis of Species'
1871 chapter 10.) follows Delpino, but adds no new objections of any weight.
Dr. Bastian ('The Beginnings of Life' 1872 volume 2 page 98) says that the
hypothesis "looks like a relic of the old rather than a fitting appanage of
the new evolution philosophy." He shows that I ought not to have used the term
"pangenesis," as it had been previously used by Dr. Gros in another sense. Dr.
Lionel Beale ('Nature' May 11, 1871 page 26) sneers at the whole doctrine with
much acerbity and some justice. Prof. Wigand ('Schriften der Gesell. der
gesammt. Naturwissen. zu Marburg' b. 9 1870) considers the hypothesis as
unscientific and worthless. Mr. G.H. Lewes ('Fortnightly Review' November 1,
1868 page 503) seems to consider that it may be useful: he makes many good
criticisms in a perfectly fair spirit. Mr. F. Galton, after describing his
valuable experiments ('Proc. Royal Soc.' volume 19 page 393) on the
intertransfusion of the blood of distinct varieties of the rabbit, concludes
by saying that in his opinion the results negative beyond all doubt the
doctrine of Pangenesis. He informs me that subsequently to the publication of
his paper he continued his experiments on a still larger scale for two more
generations, without any sign of mongrelism showing itself in the very
numerous offspring. I certainly should have expected that gemmules would have
been present in the blood, but this is no necessary part of the hypothesis,
which manifestly applies to plants and the lowest animals. Mr. Galton, in a
letter to 'Nature' (April 27, 1871 page 502), also criticises various
incorrect expressions used by me. On the other hand, several writers have
spoken favourably of the hypothesis, but there would be no use in giving
references to their articles. I may, however, refer to Dr. Ross' work, 'The
Graft Theory of Disease; being an application of Mr. Darwin's hypothesis of
Pangenesis' 1872 as he gives several original and ingenious discussions.)
In the First Part I will enumerate as briefly as I can the groups of facts
which seem to demand connection; but certain subjects, not hitherto discussed,
must be treated at disproportionate length. In the Second Part the hypothesis
will be given; and after considering how far the necessary assumptions are in
themselves improbable, we shall see whether it serves to bring under a single
point of view the various facts.
PART I.
Reproduction may be divided into two main classes, namely, sexual and asexual.
The latter is effected in many ways--by the formation of buds of various
kinds, and by fissiparous generation, that is by spontaneous or artificial
division. It is notorious that some of the lower animals, when cut into many
pieces, reproduce so many perfect individuals: Lyonnet cut a Nais or
freshwater worm into nearly forty pieces, and these all reproduced perfect
animals. (27/2. Quoted by Paget 'Lectures on Pathology' 1853 page 159.) It is
probable that segmentation could be carried much further in some of the
protozoa; and with some of the lowest plants each cell will reproduce the
parent-form. Johannes Muller thought that there was an important distinction
between gemmation and fission; for in the latter case the divided portion,
however small, is more fully developed than a bud, which also is a younger
formation; but most physiologists are now convinced that the two processes are
essentially alike. (27/3. Dr. Lachmann also observes ('Annals and Mag. of Nat.
History' 2nd series volume 19 1857 page 231) with respect to infusoria, that
"fissation and gemmation pass into each other almost imperceptibly." Again,
Mr. W.C. Minor ('Annals and Mag. of Nat. Hist.' 3rd series volume 11 page 328)
shows that with Annelids the distinction that has been made between fission
and budding is not a fundamental one. See also Professor Clark's work 'Mind in
Nature' New York 1865 pages 62, 94.) Prof. Huxley remarks, "fission is little
more than a peculiar mode of budding," and Prof. H.J. Clark shows in detail
that there is sometimes "a compromise between self-division and budding." When
a limb is amputated, or when the whole body is bisected, the cut extremities
are said to bud forth (27/4. See Bonnet 'Oeuvres d'Hist. Nat.' tome 5 1781
page 339 for remarks on the budding-out of the amputated limbs of
Salamanders.); and as the papilla, which is first formed, consists of
undeveloped cellular tissue like that forming an ordinary bud, the expression
is apparently correct. We see the connection of the two processes in another
way; for Trembley observed with the hydra, that the reproduction of the head
after amputation was checked as soon as the animal put forth reproductive
gemmae. (27/5. Paget 'Lectures on Pathology' 1853 page 158.)
Between the production, by fissiparous generation, of two or more complete
individuals, and the repair of even a very slight injury, there is so perfect
a gradation, that it is impossible to doubt that the two processes are
connected. As at each stage of growth an amputated part is replaced by one in
the same state of development, we must also follow Sir J. Paget in admitting,
"that the powers of development from the embryo, are identical with those
exercised for the restoration from injuries: in other words, that the powers
are the same by which perfection is first achieved, and by which, when lost,
it is recovered." (27/6. Ibid pages 152, 164.) Finally, we may conclude that
the several forms of budding, fissiparous generation, the repair of injuries,
and development, are all essentially the results of one and the same power.
SEXUAL GENERATION.
The union of the two sexual elements seems at first sight to make a broad
distinction between sexual and asexual generation. But the conjugation of
algae, by which process the contents of two cells unite into a single mass
capable of development, apparently gives us the first step towards sexual
union: and Pringsheim, in his memoir on the pairing of Zoospores (27/7.
Translated in 'Annals and Mag. of Nat. Hist.' April 1870 page 272.), shows
that conjugation graduates into true sexual reproduction. Moreover, the now
well-ascertained cases of Parthenogenesis prove that the distinction between
sexual and asexual generation is not nearly so great as was formerly thought;
for ova occasionally, and even in some cases frequently, become developed into
perfect beings, without the concourse of the male. With most of the lower
animals and even with mammals, the ova show a trace of parthenogenetic power,
for without being fertilised they pass through the first stages of
segmentation. (27/8. Bischoff as quoted by von Siebold "Ueber Parthenogenesis"
'Sitzung der math. phys. Classe.' Munich November 4, 1871 page 240. See also
Quatrefages 'Annales des Sc. Nat. Zoolog.' 3rd series 1850 page 138.) Nor can
pseudova which do not need fertilisation, be distinguished from true ova, as
was first shown by Sir J. Lubbock, and is now admitted by Siebold. So, again,
the germ-balls in the larvae of Cecidomyia are said by Leuckart (27/9. 'On the
Asexual Reproduction of Cecidomyide Larvae' translated in 'Annals and Mag. of
Nat. Hist.' March 1866 pages 167, 171.) to be formed within the ovarium, but
they do not require to be fertilised. It should also be observed that in
sexual generation, the ovules and the male element have equal power of
transmitting every single character possessed by either parent to their
offspring. We see this clearly when hybrids are paired inter se, for the
characters of both grandparents often appear in the progeny, either perfectly
or by segments. It is an error to suppose that the male transmits certain
characters and the female other characters; although no doubt, from unknown
causes, one sex sometimes has a much stronger power of transmission than the
other.
It has, however, been maintained by some authors that a bud differs
essentially from a fertilised germ, in always reproducing the perfect
character of the parent-stock; whilst fertilised germs give birth to variable
beings. But there is no such broad distinction as this. In the eleventh
chapter numerous cases were advanced showing that buds occasionally grow into
plants having quite new characters; and the varieties thus produced can be
propagated for a length of time by buds, and occasionally by seed.
Nevertheless, it must be admitted that beings produced sexually are much more
liable to vary than those produced asexually; and of this fact a partial
explanation will hereafter be attempted. The variability in both cases is
determined by the same general causes, and is governed by the same laws. Hence
new varieties arising from buds cannot be distinguished from those arising
from seed. Although bud-varieties usually retain their character during
successive bud-generations, yet they occasionally revert, even after a long
series of bud-generations, to their former character. This tendency to
reversion in buds, is one of the most remarkable of the several points of
agreement between the offspring from bud and seminal reproduction.
But there is one difference between organisms produced sexually and asexually,
which is very general. The former pass in the course of their development from
a very low stage to their highest stage, as we see in the metamorphoses of
insects and of many other animals, and in the concealed metamorphoses of the
vertebrata. Animals propagated asexually by buds or fission, on the other
hand, commence their development at that stage at which the budding or self-
dividing animal may happen to be, and therefore do not pass through some of
the lower developmental stages. (27/10. Prof. Allman speaks ('Transact. R.
Soc. of Edinburgh' volume 26 1870 page 102) decisively on this head with
respect to the Hydroida: he says, "It is a universal law in the succession of
zooids, that no retrogression ever takes place in the series.") Afterwards,
they often advance in organisation, as we see in the many cases of "alternate
generation." In thus speaking of alternate generation, I follow those
naturalists who look at this process as essentially one of internal budding or
of fissiparous generation. Some of the lower plants, however, such as mosses
and certain algae, according to Dr. L. Radlkofer (27/11. 'Annals and Mag. of
Nat. Hist.' 2nd series volume 20 1857 pages 153-455), when propagated
asexually, do undergo a retrogressive metamorphosis. As far as the final cause
is concerned, we can to a certain extent understand why beings propagated by
buds should not pass through all the early stages of development; for with
each organism the structure acquired at each stage must be adapted to its
peculiar habits; and if there are places for the support of many individuals
at some one stage, the simplest plan will be that they should be multiplied at
this stage, and not that they should first retrograde in their development to
an earlier or simpler structure, which might not be fitted for the then
surrounding conditions.
From the several foregoing considerations we may conclude that the difference
between sexual and asexual generation is not nearly so great as at first
appears; the chief difference being that an ovule cannot continue to live and
to be fully developed unless it unites with the male element; but even this
difference is far from invariable, as shown by the many cases of
parthenogenesis. We are therefore naturally led to inquire what the final
cause can be of the necessity in ordinary generation for the concourse of the
two sexual elements.
Seeds and ova are often highly serviceable as the means of disseminating
plants and animals, and of preserving them during one or more seasons in a
dormant state; but unimpregnated seeds or ova, and detached buds, would be
equally serviceable for both purposes. We can, however, indicate two important
advantages gained by the concourse of the two sexes, or rather of two
individuals belonging to opposite sexes; for, as I have shown in a former
chapter, the structure of every organism appears to be especially adapted for
the concurrence, at least occasionally, of two individuals. When species are
rendered highly variable by changed conditions of life, the free intercrossing
of the varying individuals tends to keep each form fitted for its proper place
in nature; and crossing can be effected only by sexual generation; but whether
the end thus gained is of sufficient importance to account for the first
origin of sexual intercourse is extremely doubtful. Secondly, I have shown
from a large body of facts, that, as a slight change in the conditions of life
is beneficial to each creature, so, in an analogous manner, is the change
effected in the germ by sexual union with a distinct individual; and I have
been led, from observing the many widely-extended provisions throughout nature
for this purpose, and from the greater vigour of crossed organisms of all
kinds, as proved by direct experiments, as well as from the evil effects of
close interbreeding when long continued, to believe that the advantage thus
gained is very great.
Why the germ, which before impregnation undergoes a certain amount of
development, ceases to progress and perishes, unless it be acted on by the
male element; and why conversely the male element, which in the case of some
insects is enabled to keep alive for four or five years, and in the case of
some plants for several years, likewise perishes, unless it acts on or unites
with the germ, are questions which cannot be answered with certainty. It is,
however, probable that both sexual elements perish, unless brought into union,
simply from including too little formative matter for independent development.
Quatrefages has shown in the case of the Teredo (27/12. 'Annales des Sc. Nat.'
3rd series 1850 tome 13.), as did formerly Prevost and Dumas with other
animals, that more than one spermatozoon is requisite to fertilise an ovum.
This has likewise been shown by Newport (27/13. 'Transact. Phil. Soc.' 1851
pages 196, 208, 210; 1853 pages 245, 247.), who proved by numerous
experiments, that, when a very small number of spermatozoa are applied to the
ova of Batrachians, they are only partially impregnated, and an embryo is
never fully developed. The rate also of the segmentation of the ovum is
determined by the number of the spermatozoa. With respect to plants, nearly
the same results were obtained by Kolreuter and Gartner. This last careful
observer, after making successive trials on a Malva with more and more pollen-
grains, found (27/14. 'Beitrage zur Kenntniss' etc. 1844 s. 345.), that even
thirty grains did not fertilise a single seed; but when forty grains were
applied to the stigma, a few seeds of small size were formed. In the case of
Mirabilis the pollen grains are extraordinarily large, and the ovarium
contains only a single ovule; and these circumstances led Naudin (27/15.
'Nouvelles Archives du Museum' tome 1 page 27.) to make the following
experiments: a flower was fertilised by three grains and succeeded perfectly;
twelve flowers were fertilised by two grains, and seventeen flowers by a
single grain, and of these one flower alone in each lot perfected its seed:
and it deserves especial notice that the plants produced by these two seeds
never attained their proper dimensions, and bore flowers of remarkably small
size. From these facts we clearly see that the quantity of the peculiar
formative matter which is contained within the spermatozoa and pollen-grains
is an all-important element in the act of fertilisation, not only for the full
development of the seed, but for the vigour of the plant produced from such
seed. We see something of the same kind in certain cases of parthenogenesis,
that is, when the male element is wholly excluded; for M. Jourdan (27/16. As
quoted by Sir J. Lubbock in 'Nat. Hist. Review' 1862 page 345. Weijenbergh
also raised ('Nature' December 21, 1871 page 149) two successive generations
from unimpregnated females of another lepidopterous insect, Liparis dispar.
These females did not produce at most one-twentieth of their full complement
of eggs, and many of the eggs were worthless. Moreover the caterpillars raised
from these unfertilised eggs "possessed far less vitality" than those from
fertilised eggs. In the third parthenogenetic generation not a single egg
yielded a caterpillar.) found that, out of about 58,000 eggs laid by
unimpregnated silk-moths, many passed through their early embryonic stages,
showing that they were capable of self-development, but only twenty-nine out
of the whole number produced caterpillars. The same principle of quantity
seems to hold good even in artificial fissiparous reproduction, for Hackel
(27/17. 'Entwickelungsgeschichte der Siphonophora' 1869 page 73.) found that
by cutting the segmented and fertilised ova or larva of Siphonophorae (jelly-
fishes) into pieces, the smaller the pieces were, the slower was the rate of
development, and the larvae thus produced were by so much the more imperfect
and inclined to monstrosity. It seems, therefore, probable that with the
separate sexual elements deficient quantity of formative matter is the main
cause of their not having the capacity for prolonged existence and
development, unless they combine and thus increase each other's bulk. The
belief that it is the function of the spermatozoa to communicate life to the
ovule seems a strange one, seeing that the unimpregnated ovule is already
alive and generally undergoes a certain amount of independent development.
Sexual and asexual reproduction are thus seen not to differ essentially; and
we have already shown that asexual reproduction, the power of regrowth and
development are all parts of one and the same great law.
REGROWTH OF AMPUTATED PARTS.
This subject deserves a little further discussion. A multitude of the lower
animals and some vertebrates possess this wonderful power. For instance,
Spallanzani cut off the legs and tail of the same salamander six times
successively, and Bonnet (27/18. Spallanzani 'An Essay on Animal Reproduction'
translated by Dr. Maty 1769 page 79. Bonnet 'Oeuvres d'Hist. Nat.' tome 5 part
1 4to. edition 1781 pages 343, 350.) did so eight times; and on each occasion
the limbs were reproduced on the exact line of amputation, with no part
deficient or in excess. An allied animal, the axolotl, had a limb bitten off,
which was reproduced in an abnormal condition, but when this was amputated it
was replaced by a perfect limb. (27/19. Vulpian as quoted by Prof. Faivre 'La
Variabilite des Especes' 1868 page 112.) The new limbs in these cases bud
forth, and are developed in the same manner as during the regular development
of a young animal. For instance, with the Amblystoma lurida, three toes are
first developed, then the fourth, and on the hind-feet the fifth, and so it is
with a reproduced limb. (27/20. Dr. P. Hoy 'The American Naturalist' September
1871 page 579.)
The power of regrowth is generally much greater during the youth of an animal
or during the earlier stages of its development than during maturity. The
larvae or tadpoles of the Batrachians are capable of reproducing lost members,
but not so the adults. (27/21. Dr. Gunther in Owen 'Anatomy of Vertebrates'
volume 1 1866 page 567. Spallanzani has made similar observations.) Mature
insects have no power of regrowth, excepting in one order, whilst the larvae
of many kinds have this power. Animals low in the scale are able, as a general
rule, to reproduce lost parts far more easily than those which are more highly
organised. The myriapods offer a good illustration of this rule; but there are
some strange exceptions to it--thus Nemerteans, though lowly organised, are
said to exhibit little power of regrowth. With the higher vertebrata, such as
birds and mammals, the power is extremely limited. (27/22. A thrush was
exhibited before the British Association at Hull in 1853 which had lost its
tarsus, and this member, it was asserted, had been thrice reproduced; having
been lost, I presume, each time by disease. Sir J. Paget informs me that he
feels some doubt about the facts recorded by Sir J. Simpson ('Monthly Journal
of Medical Science' Edinburgh 1848 new series volume 2 page 890) of the
regrowth of limbs in the womb in the case of man.)
In the case of those animals which may be bisected or chopped into pieces, and
of which every fragment will reproduce the whole, the power of regrowth must
be diffused throughout the whole body. Nevertheless there seems to be much
truth in the view maintained by Prof. Lessona (27/23. 'Atti della Soc. Ital.
di Sc. Nat.' volume 11 1869 page 493.), that this capacity is generally a
localised and special one, serving to replace parts which are eminently liable
to be lost in each particular animal. The most striking case in favour of this
view, is that the terrestrial salamander, according to Lessona, cannot
reproduce lost parts, whilst another species of the same genus, the aquatic
salamander, has extraordinary powers of regrowth, as we have just seen; and
this animal is eminently liable to have its limbs, tail, eyes and jaws bitten
off by other tritons. (27/24. Lessona states that this is so in the paper just
referred to. See also 'The American Naturalist' September 1871 page 579.) Even
with the aquatic salamander the capacity is to a certain extent localised, for
when M. Philipeaux (27/25. 'Comptes Rendus' October 1, 1866 and June 1867.)
extirpated the entire fore limb together with the scapula, the power of
regrowth was completely lost. It is also a remarkable fact, standing in
opposition to a very general rule, that the young of the aquatic salamander do
not possess the power of repairing their limbs in an equal degree with the
adults (27/26. Bonnet 'Oeuvres Hist. Nat.' volume 5 page 294, as quoted by
Prof. Rolleston in his remarkable address to the 36th annual meeting of the
British Medical Association.) but I do not know that they are more active, or
can otherwise better escape the loss of their limbs, than the adults. The
walking-stick insect, Diapheromera femorata, like other insects of the same
order, can reproduce its legs in the mature state, and these from their great
length must be liable to be lost: but the capacity is localised (as in the
case of the salamander), for Dr. Scudder found (27/27. 'Proc. Boston Soc. of
Nat. Hist.' volume 12 1868-69 page 1.), that if the limb was removed within
the trochanto-femoral articulation, it was never renewed. When a crab is
seized by one of its legs, this is thrown off at the basal joint, being
afterwards replaced by a new leg; and it is generally admitted that this is a
special provision for the safety of the animal. Lastly, with gasteropod
molluscs, which are well known to have the power of reproducing their heads,
Lessona shows that they are very liable to have their heads bitten off by
fishes; the rest of the body being protected by the shell. Even with plants we
see something of the same kind, for non-deciduous leaves and young stems have
no power of regrowth, these parts being easily replaced by growth from new
buds; whilst the bark and subjacent tissues of the trunks of trees have great
power of regrowth, probably on account of their increase in diameter, and of
their liability to injury from being gnawed by animals.
GRAFT-HYBRIDS.
It is well known from innumerable trials made in all parts of the world, that
buds may be inserted into a stock, and that the plants thus raised are not
affected in a greater degree than can be accounted for by changed nutrition.
Nor do the seedlings raised from such inserted buds partake of the character
of the stock, though they are more liable to vary than are seedlings from the
same variety growing on its own roots. A bud, also, may sport into a new and
strongly-marked variety without any other bud on the same plant being in the
least degree affected. We may therefore infer, in accordance with the common
view, that each bud is a distinct individual, and that its formative elements
do not spread beyond the parts subsequently developed from it. Nevertheless,
we have seen in the abstract on graft-hybridisation in the eleventh chapter
that buds certainly include formative matter, which can occasionally combine
with that included in the tissues of a distinct variety or species; a plant
intermediate between the two parent-forms being thus produced. In the case of
the potato we have seen that the tubers produced from a bud of one kind
inserted into another are intermediate in colour, size, shape and state of
surface; that the stems, foliage, and even certain constitutional
peculiarities, such as precocity, are likewise intermediate. With these well-
established cases, the evidence that graft-hybrids have also been produced
with the laburnum, orange, vine, rose, etc., seems sufficient. But we do not
know under what conditions this rare form of reproduction is possible. From
these several cases we learn the important fact that formative elements
capable of blending with those of a distinct individual (and this is the chief
characteristic of sexual generation), are not confined to the reproductive
organs, but are present in the buds and cellular tissue of plants; and this is
a fact of the highest physiological importance.
DIRECT ACTION OF THE MALE ELEMENT ON THE FEMALE.
In the eleventh chapter, abundant proofs were given that foreign pollen
occasionally affects in a direct manner the mother-plant. Thus, when Gallesio
fertilised an orange-flower with pollen from the lemon, the fruit bore stripes
of perfectly characterised lemon-peel. With peas, several observers have seen
the colour of the seed-coats and even of the pod directly affected by the
pollen of a distinct variety. So it has been with the fruit of the apple,
which consists of the modified calyx and upper part of the flower-stalk. In
ordinary cases these parts are wholly formed by the mother-plant. We here see
that the formative elements included within the male element or pollen of one
variety can affect and hybridise, not the part which they are properly adapted
to affect, namely, the ovules, but the partially-developed tissues of a
distinct variety or species. We are thus brought half-way towards a graft-
hybrid, in which the formative elements included within the tissues of one
individual combine with those included in the tissues of a distinct variety or
species, thus giving rise to a new and intermediate form, independently of the
male or female sexual organs.
With animals which do not breed until nearly mature, and of which all the
parts are then fully developed, it is hardly possible that the male element
should directly affect the female. But we have the analogous and perfectly
well-ascertained case of the male element affecting (as with the quagga and
Lord Morton's mare) the female or her ova, in such a manner that when she is
impregnated by another male her offspring are affected and hybridised by the
first male. The explanation would be simple if the spermatozoa could keep
alive within the body of the female during the long interval which has
sometimes elapsed between the two acts of impregnation; but no one will
suppose that this is possible with the higher animals.
DEVELOPMENT.
The fertilised germ reaches maturity by a vast number of changes: these are
either slight and slowly effected, as when the child grows into the man, or
are great and sudden, as with the metamorphoses of most insects. Between these
extremes we have every gradation, even within the same class; thus, as Sir J.
Lubbock has shown (27/28. 'Transact. Linn. Soc.' volume 24 1863 page 62.)
there is an Ephemerous insect which moults above twenty times, undergoing each
time a slight but decided change of structure; and these changes, as he
further remarks, probably reveal to us the normal stages of development, which
are concealed and hurried through or suppressed in most other insects. In
ordinary metamorphoses, the parts and organs appear to become changed into the
corresponding parts in the next stage of development; but there is another
form of development, which has been called by Professor Owen metagenesis. In
this case "the new parts are not moulded upon the inner surface of the old
ones. The plastic force has changed its course of operation. The outer case,
and all that gave form and character to the precedent individual, perish and
are cast off; they are not changed into the corresponding parts of the new
individual. These are due to a new and distinct developmental process," etc.
(27/29. 'Parthenogenesis' 1849 pages 25, 26. Prof. Huxley has some excellent
remarks ('Medical Times' 1856 page 637) on this subject in reference to the
development of star-fishes, and shows how curiously metamorphosis graduates
into gemmation or zoid-formation, which is in fact the same as metagenesis.)
Metamorphosis, however, graduates so insensibly, into metagenesis, that the
two processes cannot be distinctly separated. For instance, in the last change
which Cirripedes undergo, the alimentary canal and some other organs are
moulded on pre-existing parts; but the eyes of the old and the young animal
are developed in entirely different parts of the body; the tips of the mature
limbs are formed within the larval limbs, and may be said to be metamorphosed
from them; but their basal portions and the whole thorax are developed in a
plane at right angles to the larval limbs and thorax; and this may be called
metagenesis. The metagenetic process is carried to an extreme point in the
development of some Echinoderms, for the animal in the second stage of
development is formed almost like a bud within the animal of the first stage,
the latter being then cast off like an old vestment, yet sometimes maintaining
for a short period an independent vitality. (27/30. Prof. J. Reay Greene in
Gunther's 'Record of Zoolog. Lit.' 1865 page 625.) If, instead of a single
individual, several were to be thus developed metagenetically within a pre-
existing form, the process would be called one of alternate generation. The
young thus developed may either closely resemble the encasing parent-form, as
with the larvae of Cecidomyia, or may differ to an astonishing degree, as with
many parasitic worms and jelly-fishes; but this does not make any essential
difference in the process, any more than the greatness or abruptness of the
change in the metamorphoses of insects.
The whole question of development is of great importance for our present
subject. When an organ, the eye, for instance, is metagenetically formed in a
part of the body where during the previous stage of development no eye
existed, we must look at it as a new and independent growth. The absolute
independence of new and old structures, although corresponding in structure
and function, is still more obvious when several individuals are formed within
a previous form, as in the cases of alternate generation. The same important
principle probably comes largely into play even in the case of apparently
continuous growth, as we shall see when we consider the inheritance of
modifications at corresponding ages.
We are led to the same conclusion, namely, the independence of parts
successively developed, by another and quite distinct group of facts. It is
well known that many animals belonging to the same order, and therefore not
differing widely from each other, pass through an extremely different course
of development. Thus certain beetles, not in any way remarkably different from
others of the same order, undergo what has been called a hyper-metamorphosis--
that is, they pass through an early stage wholly different from the ordinary
grub-like larva. In the same sub-order of crabs, namely, the Macroura, as
Fritz Muller remarks, the river cray-fish is hatched under the same form which
it ever afterwards retains; the young lobster has divided legs, like a Mysis;
the Palaemon appears under the form of a Zoea, and Peneus under the Nauplius-
form; and how wonderfully these larval forms differ from one another, is known
to every naturalist. (27/31. Fritz Muller 'Fur Darwin' 1864 s. 65, 71. The
highest authority on crustaceans, Prof. Milne-Edwards, insists ('Annal. des
Sci. Nat.' 2nd series Zoolog. tome 3 page 322) on the difference in the
metamorphosis of closely-allied genera.) Some other crustaceans, as the same
author observes, start from the same point and arrive at nearly the same end,
but in the middle of their development are widely different from one another.
Still more striking cases could be given with respect to the Echinodermata.
With the Medusae or jelly-fishes Professor Allman observes, "The
classification of the Hydroida would be a comparatively simple task if, as has
been erroneously asserted, generically-identical medusoids always arose from
generically-identical polypoids; and, on the other hand, that generically-
identical polypoids always gave origin to generically-identical medusoids." So
again, Dr. Strethill Wright remarks, "In the life-history of the Hydroidae any
phase, planuloid, polypoid, or medusoid, may be absent." (27/32. Prof. Allman
'Annals and Mag. of Nat. Hist.' 3rd series volume 13 1864 page 348; Dr. S.
Wright ibid volume 8 1861 page 127. See also page 358 for analogous statements
by Sars.)
According to the belief now generally accepted by our best naturalists, all
the members of the same order or class, for instance, the Medusae or the
Macrourous crustaceans, are descended from a common progenitor. During their
descent they have diverged much in structure, but have retained much in
common; and this has occurred, though they have passed through and still pass
through marvellously different metamorphoses. This fact well illustrates how
independent each structure is from that which precedes and that which follows
it in the course of development.
THE FUNCTIONAL INDEPENDENCE OF THE ELEMENTS OR UNITS OF THE BODY.
Physiologists agree that the whole organism consists of a multitude of
elemental parts, which are to a great extent independent of one another. Each
organ, says Claude Bernard (27/33. 'Tissus Vivants' 1866 page 22.), has its
proper life, its autonomy; it can develop and reproduce itself independently
of the adjoining tissues. A great German authority, Virchow (27/34. 'Cellular
Pathology' translated by Dr. Chance 1860 pages 14, 18, 83, 460.), asserts
still more emphatically that each system consists of an "enormous mass of
minute centres of action...Every element has its own special action, and even
though it derive its stimulus to activity from other parts, yet alone effects
the actual performance of duties...Every single epithelial and muscular fibre-
cell leads a sort of parasitical existence in relation to the rest of the
body...Every single bone-corpuscle really possesses conditions of nutrition
peculiar to itself." Each element, as Sir J. Paget remarks, lives its
appointed time and then dies, and is replaced after being cast off or
absorbed. (27/35. Paget 'Surgical Pathology' volume 1 1853 pages 12-14.) I
presume that no physiologist doubts that, for instance, each bone-corpuscle of
the finger differs from the corresponding corpuscle in the corresponding joint
of the toe; and there can hardly be a doubt that even those on the
corresponding sides of the body differ, though almost identical in nature.
This near approach to identity is curiously shown in many diseases in which
the same exact points on the right and left sides of the body are similarly
affected; thus Sir J. Paget (27/36. Ibid page 19.) gives a drawing of a
diseased pelvis, in which the bone has grown into a most complicated pattern,
but "there is not one spot or line on one side which is not represented, as
exactly as it would be in a mirror, on the other."
Many facts support this view of the independent life of each minute element of
the body. Virchow insists that a single bone-corpuscle or a single cell in the
skin may become diseased. The spur of a cock, after being inserted into the
ear of an ox, lived for eight years, and acquired a weight of 396 grammes
(nearly fourteen ounces), and the astonishing length of twenty-four
centimetres, or about nine inches; so that the head of the ox appeared to bear
three horns. (27/37. See Prof. Mantegazza's interesting work 'Degli innesti
Animali' etc. Milano 1865 page 51 tab. 3.) The tail of a pig has been grafted
into the middle of its back, and reacquired sensibility. Dr. Ollier (27/38.
'De la Production Artificielle des Os' page 8.) inserted a piece of periosteum
from the bone of a young dog under the skin of a rabbit, and true bone was
developed. A multitude of similar facts could be given. The frequent presence
of hairs and of perfectly developed teeth, even teeth of the second dentition,
in ovarian tumours (27/39. Isidore Geoffroy Saint-Hilaire 'Hist. des
Anomalies' tome 2 pages 549, 560, 562; Virchow ibid page 484. Lawson Tait 'The
Pathology of Diseases of the Ovaries' 1874 pages 61, 62.), are facts leading
to the same conclusion. Mr. Lawson Tait refers to a tumour in which "over 300
teeth were found, resembling in many respects milk-teeth;" and to another
tumour, "full of hair which had grown and been shed from one little spot of
skin not bigger than the tip of my little finger. The amount of hair in the
sac, had it grown from a similarly sized area of the scalp, would have taken
almost a lifetime to grow and be shed."
Whether each of the innumerable autonomous elements of the body is a cell or
the modified product of a cell, is a more doubtful question, even if so wide a
definition be given to the term, as to include cell-like bodies without walls
and without nuclei. (27/40. For the most recent classification of cells, see
Ernst Hackel 'Generelle Morpholog.' b. 2 1866 s. 275.) The doctrine of omnis
cellula e cellula is admitted for plants, and widely prevails with respect to
animals. (27/41. Dr. W. Turner 'The Present Aspect of Cellular Pathology'
'Edinburgh Medical Journal' April 1863.) Thus Virchow, the great supporter of
the cellular theory, whilst allowing that difficulties exist, maintains that
every atom of tissue is derived from cells, and these from pre-existing cells,
and these primarily from the egg, which he regards as a great cell. That
cells, still retaining the same nature, increase by self-division or
proliferation, is admitted by every one. But when an organism undergoes great
changes of structure during development, the cells, which at each stage are
supposed to be directly derived from previously existing cells, must likewise
be greatly changed in nature; this change is attributed by the supporters of
the cellular doctrine to some inherent power which the cells possess, and not
to any external agency. Others maintain that cells and tissues of all kinds
may be formed, independently of pre-existing cells, from plastic lymph or
blastema. Whichever view may be correct, every one admits that the body
consists of a multitude of organic units, all of which possess their own
proper attributes, and are to a certain extent independent of all others.
Hence it will be convenient to use indifferently the terms cells or organic
units, or simply units.
VARIABILITY AND INHERITANCE.
We have seen in the twenty-second chapter that variability is not a principle
co-ordinate with life or reproduction, but results from special causes,
generally from changed conditions acting during successive generations. The
fluctuating variability thus induced is apparently due in part to the sexual
system being easily affected, so that it is often rendered impotent; and when
not so seriously affected, it often fails in its proper function of
transmitting truly the characters of the parents to the offspring. But
variability is not necessarily connected with the sexual system, as we see in
the cases of bud-variation. Although we are seldom able to trace the nature of
the connection, many deviations of structure no doubt result from changed
conditions acting directly on the organisation, independently of the
reproductive system. In some instances we may feel sure of this, when all, or
nearly all the individuals which have been similarly exposed are similarly and
definitely affected, of which several instances have been given. But it is by
no means clear why the offspring should be affected by the exposure of the
parents to new conditions, and why it is necessary in most cases that several
generations should have been thus exposed.
How, again, can we explain the inherited effects of the use or disuse of
particular organs? The domesticated duck flies less and walks more than the
wild duck, and its limb-bones have become diminished and increased in a
corresponding manner in comparison with those of the wild duck. A horse is
trained to certain paces, and the colt inherits similar consensual movements.
The domesticated rabbit becomes tame from close confinement; the dog,
intelligent from associating with man; the retriever is taught to fetch and
carry; and these mental endowments and bodily powers are all inherited.
Nothing in the whole circuit of physiology is more wonderful. How can the use
or disuse of a particular limb or of the brain affect a small aggregate of
reproductive cells, seated in a distant part of the body, in such a manner
that the being developed from these cells inherits the characters of either
one or both parents? Even an imperfect answer to this question would be
satisfactory.
In the chapters devoted to inheritance it was shown that a multitude of newly
acquired characters, whether injurious or beneficial, whether of the lowest or
highest vital importance, are often faithfully transmitted--frequently even
when one parent alone possesses some new peculiarity; and we may on the whole
conclude that inheritance is the rule, and non-inheritance the anomaly. In
some instances a character is not inherited, from the conditions of life being
directly opposed to its development; in many instances, from the conditions
incessantly inducing fresh variability, as with grafted fruit-trees and
highly-cultivated flowers. In the remaining cases the failure may be
attributed to reversion, by which the child resembles its grandparents or more
remote progenitors, instead of its parents.
Inheritance is governed by various laws. Characters which first appear at any
particular age tend to reappear at a corresponding age. They often become
associated with certain seasons of the year, and reappear in the offspring at
a corresponding season. If they appear rather late in life in one sex, they
tend to reappear exclusively in the same sex at the same period of life.
The principle of reversion, recently alluded to, is one of the most wonderful
of the attributes of Inheritance. It proves to us that the transmission of a
character and its development, which ordinarily go together and thus escape
discrimination, are distinct powers; and these powers in some cases are even
antagonistic, for each acts alternately in successive generations. Reversion
is not a rare event, depending on some unusual or favourable combination of
circumstances, but occurs so regularly with crossed animals and plants, and so
frequently with uncrossed breeds, that it is evidently an essential part of
the principle of inheritance. We know that changed conditions have the power
of evoking long-lost characters, as in the case of animals becoming feral. The
act of crossing in itself possesses this power in a high degree. What can be
more wonderful than that characters, which have disappeared during scores, or
hundreds, or even thousands of generations, should suddenly reappear perfectly
developed, as in the case of pigeons and fowls, both when purely bred and
especially when crossed; or as with the zebrine stripes on dun-coloured
horses, and other such cases? Many monstrosities come under this same head, as
when rudimentary organs are redeveloped, or when an organ which we must
believe was possessed by an early progenitor of the species, but of which not
even a rudiment is left, suddenly reappears, as with the fifth stamen in some
Scrophulariaceae. We have already seen that reversion acts in bud-
reproduction; and we know that it occasionally acts during the growth of the
same individual animal, especially, but not exclusively, if of crossed
parentage,--as in the rare cases described of fowls, pigeons, cattle, and
rabbits, which have reverted to the colours of one of their parents or
ancestors as they advanced in years.
We are led to believe, as formerly explained, that every character which
occasionally reappears is present in a latent form in each generation, in
nearly the same manner as in male and female animals the secondary characters
of the opposite sex lie latent and ready to be evolved when the reproductive
organs are injured. This comparison of the secondary sexual characters which
lie latent in both sexes, with other latent characters, is the more
appropriate from the case recorded of a Hen, which assumed some of the
masculine characters, not of her own race, but of an early progenitor; she
thus exhibited at the same time the redevelopment of latent characters of both
kinds. In every living creature we may feel assured that a host of long-lost
characters lie ready to be evolved under proper conditions. How can we make
intelligible and connect with other facts, this wonderful and common capacity
of reversion,--this power of calling back to life long-lost characters?
PART II.
I have now enumerated the chief facts which every one would desire to see
connected by some intelligible bond. This can be done, if we make the
following assumptions, and much may be advanced in favour of the chief one.
The secondary assumptions can likewise be supported by various physiological
considerations. It is universally admitted that the cells or units of the body
increase by self-division or proliferation, retaining the same nature, and
that they ultimately become converted into the various tissues and substances
of the body. But besides this means of increase I assume that the units throw
off minute granules which are dispersed throughout the whole system; that
these, when supplied with proper nutriment, multiply by self-division, and are
ultimately developed into units like those from which they were originally
derived. These granules may be called gemmules. They are collected from all
parts of the system to constitute the sexual elements, and their development
in the next generation forms a new being; but they are likewise capable of
transmission in a dormant state to future generations and may then be
developed. Their development depends on their union with other partially
developed or nascent cells which precede them in the regular course of growth.
Why I use the term union, will be seen when we discuss the direct action of
pollen on the tissues of the mother-plant. Gemmules are supposed to be thrown
off by every unit, not only during the adult state, but during each stage of
development of every organism; but not necessarily during the continued
existence of the same unit. Lastly, I assume that the gemmules in their
dormant state have a mutual affinity for each other, leading to their
aggregation into buds or into the sexual elements. Hence, it is not the
reproductive organs or buds which generate new organisms, but the units of
which each individual is composed. These assumptions constitute the
provisional hypothesis which I have called Pangenesis. Views in many respects
similar have been propounded by various authors. (27/42. Mr. G.H. Lewes
('Fortnightly Review' November 1, 1868 page 506) remarks on the number of
writers who have advanced nearly similar views. More than two thousand years
ago Aristotle combated a view of this kind, which, as I hear from Dr. W. Ogle,
was held by Hippocrates and others. Ray, in his 'Wisdom of God' (2nd edition
1692 page 68), says that "every part of the body seems to club and contribute
to the seed." The "organic molecules" of Buffon ('Hist. Nat. Gen.' edition of
1749 tome 2 pages 54, 62, 329, 333, 420, 425) appear at first sight to be the
same as the gemmules of my hypothesis, but they are essentially different.
Bonnet ('Oeuvres d'Hist. Nat.' tome 5 part 1 1781 4to edition page 334) speaks
of the limbs having germs adapted for the reparation of all possible losses;
but whether these germs are supposed to be the same with those within buds and
the sexual organs is not clear. Prof. Owen says ('Anatomy of Vertebrates'
volume 3 1868 page 813) that he fails to see any fundamental difference
between the views which he propounded in his 'Parthenogenesis' (1849 pages 5-
8), and which he now considers as erroneous, and my hypothesis of pangenesis:
but a reviewer ('Journal of Anat. and Phys.' May 1869 page 441) shows how
different they really are. I formerly thought that the "physiological units"
of Herbert Spencer ('Principles of Biology' volume 1 chapters 4 and 8 1863-64)
were the same as my gemmules, but I now know that this is not the case.
Lastly, it appears from a review of the present work by Prof. Mantegazza
('Nuova Antologia, Maggio' 1868), that he (in his 'Elementi di Igiene' Ediz. 3
page 540) clearly foresaw the doctrine of pangenesis.)
Before proceeding to show, firstly, how far these assumptions are in
themselves probable, and secondly, how far they connect and explain the
various groups of facts with which we are concerned, it may be useful to give
an illustration, as simple as possible, of the hypothesis. If one of the
Protozoa be formed, as it appears under the microscope, of a small mass of
homogeneous gelatinous matter, a minute particle or gemmule thrown off from
any part and nourished under favourable circumstances would reproduce the
whole; but if the upper and lower surfaces were to differ in texture from each
other and from the central portion, then all three parts would have to throw
off gemmules, which when aggregated by mutual affinity would form either buds
or the sexual elements, and would ultimately be developed into a similar
organism. Precisely the same view may be extended to one of the higher
animals; although in this case many thousand gemmules must be thrown off from
the various parts of the body at each stage of development; these gemmules
being developed in union with pre-existing nascent cells in due order of
succession.
Physiologists maintain, as we have seen, that each unit of the body, though to
a large extent dependent on others, is likewise to a certain extent
independent or autonomous, and has the power of increasing by self-division. I
go one step further, and assume that each unit casts off free gemmules which
are dispersed throughout the system, and are capable under proper conditions
of being developed into similar units. Nor can this assumption be considered
as gratuitous and improbable. It is manifest that the sexual elements and buds
include formative matter of some kind, capable of development; and we now know
from the production of graft-hybrids that similar matter is dispersed
throughout the tissues of plants, and can combine with that of another and
distinct plant, giving rise to a new being, intermediate in character. We know
also that the male element can act directly on the partially developed tissues
of the mother-plant, and on the future progeny of female animals. The
formative matter which is thus dispersed throughout the tissues of plants, and
which is capable of being developed into each unit or part, must be generated
there by some means; and my chief assumption is that this matter consists of
minute particles or gemmules cast off from each unit or cell. (27/43. Mr.
Lowne has observed ('Journal of Queckett Microscopical Club' September 23,
1870) certain remarkable changes in the tissues of the larva of a fly, which
makes him believe "it possible that organs and organisms are sometimes
developed by the aggregation of excessively minute gemmules, such as those
which Mr. Darwin's hypothesis demands.")
But I have further to assume that the gemmules in their undeveloped state are
capable of largely multiplying themselves by self-division, like independent
organisms. Delpino insists that to "admit of multiplication by fissiparity in
corpuscles, analogous to seeds or buds...is repugnant to all analogy." But
this seems a strange objection, as Thuret (27/44. 'Annales des Sc. Nat.' 3rd
series Bot. tome 14 1850 page 244.) has seen the zoospore of an alga divide
itself, and each half germinated. Haeckel divided the segmented ovum of a
siphonophora into many pieces, and these were developed. Nor does the extreme
minuteness of the gemmules, which can hardly differ much in nature from the
lowest and simplest organisms, render it improbable that they should grow and
multiply. A great authority, Dr. Beale (27/45. 'Disease Germs' page 20.), says
"that minute yeast cells are capable of throwing off buds or gemmules, much
less than the 1/100000 of an inch in diameter;" and these he thinks are
"capable of subdivision practically ad infinitum."
A particle of small-pox matter, so minute as to be borne by the wind, must
multiply itself many thousandfold in a person thus inoculated; and so with the
contagious matter of scarlet fever. (27/46. See some very interesting papers
on this subject by Dr. Beale in 'Medical Times and Gazette' September 9, 1865
pages 273, 330.) It has recently been ascertained (27/47. Third Report of the
R. Comm. on the Cattle Plague as quoted in 'Gardener's Chronicle' 1866 page
446.) that a minute portion of the mucous discharge from an animal affected
with rinderpest, if placed in the blood of a healthy ox, increases so fast
that in a short space of time "the whole mass of blood, weighing many pounds,
is infected, and every small particle of that blood contains enough poison to
give, within less than forty-eight hours, the disease to another animal."
The retention of free and undeveloped gemmules in the same body from early
youth to old age will appear improbable, but we should remember how long seeds
lie dormant in the earth and buds in the bark of a tree. Their transmission
from generation to generation will appear still more improbable; but here
again we should remember that many rudimentary and useless organs have been
transmitted during an indefinite number of generations. We shall presently see
how well the long-continued transmission of undeveloped gemmules explains many
facts.
As each unit, or group of similar units, throughout the body, casts off its
gemmules, and as all are contained within the smallest ovule, and within each
spermatozoon or pollen-grain, and as some animals and plants produce an
astonishing number of pollen-grains and ovules (27/48. Mr. F. Buckland found
6,867,840 eggs in a cod-fish ('Land and Water' 1868 page 62). An Ascaris
produces about 64,000,000 eggs (Carpenter's 'Comp. Phys.' 1854 page 590). Mr.
J. Scott, of the Royal Botanic Garden of Edinburgh, calculated, in the same
manner as I have done for some British Orchids ('Fertilisation of Orchids'
page 344), the number of seeds in a capsule of an Acropera and found the
number to be 371,250. Now this plant produces several flowers on a raceme, and
many racemes during a season. In an allied genus, Gongora, Mr. Scott has seen
twenty capsules produced on a single raceme; ten such racemes on the Acropera
would yield above seventy-four millions of seed.), the number and minuteness
of the gemmules must be something inconceivable. But considering how minute
the molecules are, and how many go to the formation of the smallest granule of
any ordinary substance, this difficulty with respect to the gemmules is not
insuperable. From the data arrived at by Sir W. Thomson, my son George finds
that a cube of 1/10000 of an inch of glass or water must consist of between 16
million millions, and 131 thousand million million molecules. No doubt the
molecules of which an organism is formed are larger, from being more complex,
than those of an inorganic substance, and probably many molecules go to the
formation of a gemmule; but when we bear in mind that a cube of 1/10000 of an
inch is much smaller than any pollen-grain, ovule or bud, we can see what a
vast number of gemmules one of these bodies might contain.
The gemmules derived from each part or organ must be thoroughly dispersed
throughout the whole system. We know, for instance, that even a minute
fragment of a leaf of a Begonia will reproduce the whole plant; and that if a
fresh-water worm is chopped into small pieces, each will reproduce the whole
animal. Considering also the minuteness of the gemmules and the permeability
of all organic tissues, the thorough dispersion of the gemmules is not
surprising. That matter may be readily transferred without the aid of vessels
from part to part of the body, we have a good instance in a case recorded by
Sir J. Paget of a lady, whose hair lost its colour at each successive attack
of neuralgia and recovered it again in the course of a few days. With plants,
however, and probably with compound animals, such as corals, the gemmules do
not ordinarily spread from bud to bud, but are confined to the parts developed
from each separate bud; and of this fact no explanation can be given.
The assumed elective affinity of each gemmule for that particular cell which
precedes it in due order of development is supported by many analogies. In all
ordinary cases of sexual reproduction, the male and female elements certainly
have a mutual affinity for each other: thus, it is believed that about ten
thousand species of Compositae exist, and there can be no doubt that if the
pollen of all these species could be simultaneously or successively placed on
the stigma of any one species, this one would elect with unerring certainty
its own pollen. This elective capacity is all the more wonderful, as it must
have been acquired since the many species of this great group of plants
branched off from a common progenitor. On any view of the nature of sexual
reproduction, the formative matter of each part contained within the ovules
and the male element act on each other by some law of special affinity, so
that corresponding parts affect one another; thus, a calf produced from a
short-horned cow by a long-horned bull has its horns affected by the union of
the two forms, and the offspring from two birds with differently coloured
tails have their tails affected.
The various tissues of the body plainly show, as many physiologists have
insisted (27/49. Paget 'Lectures on Pathology' page 27; Virchow 'Cellular
Pathology' translated by Dr. Chance pages 123, 126, 294. Claude Bernard 'Des
Tissus Vivants' pages 177, 210, 337; Muller 'Physiology' English translation
page 290.), an affinity for special organic substances, whether natural or
foreign to the body. We see this in the cells of the kidneys attracting urea
from the blood; in curare affecting certain nerves; Lytta vesicatoria the
kidneys; and the poisonous matter of various diseases, as small-pox, scarlet-
fever, hooping-cough, glanders, and hydrophobia, affecting certain definite
parts of the body. It has also been assumed that the development of each
gemmule depends on its union with another cell or unit which has just
commenced its development, and which precedes it in due order of growth. That
the formative matter within the pollen of plants, which by our hypothesis
consists of gemmules, can unite with and modify the partially developed cells
of the mother-plant, we have clearly seen in the section devoted to this
subject. As the tissues of plants are formed, as far as is known, only by the
proliferation of pre-existing cells, we must conclude that the gemmules
derived from the foreign pollen do not become developed into new and separate
cells, but penetrate and modify the nascent cells of the mother-plant. This
process may be compared with what takes place in the act of ordinary
fertilisation, during which the contents of the pollen-tubes penetrate the
closed embryonic sac within the ovule, and determine the development of the
embryo. According to this view, the cells of the mother-plant may almost
literally be said to be fertilised by the gemmules derived from the foreign
pollen. In this case and in all others the proper gemmules must combine in due
order with pre-existing nascent cells, owing to their elective affinities. A
slight difference in nature between the gemmules and the nascent cells would
be far from interfering with their mutual union and development, for we well
know in the case of ordinary reproduction that such slight differentiation in
the sexual elements favours in a marked manner their union and subsequent
development, as well as the vigour of the offspring thus produced.
Thus far we have been able by the aid of our hypothesis to throw some obscure
light on the problems which have come before us; but it must be confessed that
many points remain altogether doubtful. Thus it is useless to speculate at
what period of development each unit of the body casts off its gemmules, as
the whole subject of the development of the various tissues is as yet far from
clear. We do not know whether the gemmules are merely collected by some
unknown means at certain seasons within the reproductive organs, or whether
after being thus collected they rapidly multiply there, as the flow of blood
to these organs at each breeding season seems to render probable. Nor do we
know why the gemmules collect to form buds in certain definite places, leading
to the symmetrical growth of trees and corals. We have no means of deciding
whether the ordinary wear and tear of the tissues is made good by means of
gemmules, or merely by the proliferation of pre-existing cells. If the
gemmules are thus consumed, as seems probable from the intimate connection
between the repair of waste, regrowth, and development, and more especially
from the periodical changes which many male animals undergo in colour and
structure, then some light would be thrown on the phenomena of old age, with
its lessened power of reproduction and of the repair of injuries, and on the
obscure subject of longevity. The fact of castrated animals, which do not cast
off innumerable gemmules in the act of reproduction, not being longer-lived
than perfect males, seems opposed to the belief that gemmules are consumed in
the ordinary repair of wasted tissues; unless indeed the gemmules after being
collected in small numbers within the reproductive organs are there largely
multiplied. (27/50. Prof. Ray Lankester has discussed several of the points
here referred to as bearing on pangenesis, in his interesting essay, 'On
Comparative Longevity in Man and the Lower Animals' 1870 pages 33, 77, etc.)
That the same cells or units may live for a long period and continue
multiplying without being modified by their union with free gemmules of any
kind, is probable from such cases as that of the spur of a cock which grew to
an enormous size when grafted into the ear of an ox. How far units are
modified during their normal growth by absorbing peculiar nutriment from the
surrounding tissues, independently of their union with gemmules of a distinct
nature, is another doubtful point. (27/51. Dr. Ross refers to this subject in
his 'Graft Theory of Disease' 1872 page 53.) We shall appreciate this
difficulty by calling to mind what complex yet symmetrical growths the cells
of plants yield when inoculated by the poison of a gall-insect. With animals
various polypoid excrescences and tumours are generally admitted (27/52.
Virchow 'Cellular Pathology' translated by Dr. Chance 1860 pages 60, 162, 245,
441, 454.) to be the direct product, through proliferation, of normal cells
which have become abnormal. In the regular growth and repair of bones, the
tissues undergo, as Virchow remarks (27/53. Ibid pages 412-426.), a whole
series of permutations and substitutions. "The cartilage cells may be
converted by a direct transformation into marrow-cells, and continue as such;
or they may first be converted into osseous and then into medullary tissue; or
lastly, they may first be converted into marrow and then into bone. So
variable are the permutations of these tissues, in themselves so nearly
allied, and yet in their external appearance so completely distinct." But as
these tissues thus change their nature at any age, without any obvious change
in their nutrition, we must suppose in accordance with our hypothesis that
gemmules derived from one kind of tissue combine with the cells of another
kind, and cause the successive modifications.
We have good reason to believe that several gemmules are requisite for the
development of one and the same unit or cell; for we cannot otherwise
understand the insufficiency of a single or even of two or three pollen-grains
or spermatozoa. But we are far from knowing whether the gemmules of all the
units are free and separate from one another, or whether some are from the
first united into small aggregates. A feather, for instance, is a complex
structure, and, as each separate part is liable to inherited variations, I
conclude that each feather generates a large number of gemmules; but it is
possible that these may be aggregated into a compound gemmule. The same remark
applies to the petals of flowers, which are sometimes highly complex
structures, with each ridge and hollow contrived for a special purpose, so
that each part must have been separately modified, and the modifications
transmitted; consequently, separate gemmules, according to our hypothesis,
must have been thrown off from each cell or unit. But, as we sometimes see
half an anther or a small portion of a filament becoming petali-form, or parts
or mere stripes of the calyx assuming the colour and texture of the corolla,
it is probable that with petals the gemmules of each cell are not aggregated
together into a compound gemmule, but are free and separate. Even in so simple
a case as that of a perfect cell, with its protoplasmic contents, nucleus,
nucleolus, and walls, we do not know whether or not its development depends on
a compound gemmule derived from each part. (27/54. See some good criticisms on
this head by Delpino and by Mr. G.H. Lewes in the 'Fortnightly Review'
November 1, 1868 page 509.)
Having now endeavoured to show that the several foregoing assumptions are to a
certain extent supported by analogous facts, and having alluded to some of the
most doubtful points, we will consider how far the hypothesis brings under a
single point of view the various cases enumerated in the First Part. All the
forms of reproduction graduate into one another and agree in their product;
for it is impossible to distinguish between organisms produced from buds, from
self-division, or from fertilised germs; such organisms are liable to
variations of the same nature and to reversions of the same kind; and as,
according to our hypothesis, all the forms of reproduction depend on the
aggregation of gemmules derived from the whole body, we can understand this
remarkable agreement. Parthenogenesis is no longer wonderful, and if we did
not know that great good followed from the union of the sexual elements
derived from two distinct individuals, the wonder would be that
parthenogenesis did not occur much oftener than it does. On any ordinary
theory of reproduction the formation of graft-hybrids, and the action of the
male element on the tissues of the mother-plant, as well as on the future
progeny of female animals, are great anomalies; but they are intelligible on
our hypothesis. The reproductive organs do not actually create the sexual
elements; they merely determine the aggregation and perhaps the multiplication
of the gemmules in a special manner. These organs, however, together with
their accessory parts, have high functions to perform. They adapt one or both
elements for independent temporary existence, and for mutual union. The
stigmatic secretion acts on the pollen of a plant of the same species in a
wholly different manner to what it does on the pollen of one belonging to a
distinct genus or family. The spermatophores of the Cephalopoda are
wonderfully complex structures, which were formerly mistaken for parasitic
worms; and the spermatozoa of some animals possess attributes which, if
observed in an independent animal, would be put down to instinct guided by
sense-organs,--as when the spermatozoa of an insect find their way into the
minute micropyle of the egg.
The antagonism which has long been observed (27/55. Mr. Herbert Spencer
('Principles of Biology' volume 2 page 430) has fully discussed this
antagonism.), with certain exceptions, between growth and the power of sexual
reproduction (27/56. The male salmon is known to breed at a very early age.
The Triton and Siredon, whilst retaining their larval branchiae, according to
Filippi and Dumeril ('Annals and Mag. of Nat. Hist.' 3rd series 1866 page 157)
are capable of reproduction. Ernst Haeckel has recently ('Monatsbericht Akad.
Wiss. Berlin' February 2, 1865) observed the surprising case of a medusa, with
its reproductive organs active, which produces by budding a widely different
form of medusa; and this latter also has the power of sexual reproduction.
Krohn has shown ('Annals and Mag. of Nat. Hist.' 3rd series volume 19 1862
page 6) that certain other medusae, whilst sexually mature, propagate by
gemmae. See also Kolliker 'Morphologie und Entwickelungsgeschichte des
Pennatulidenstammes' 1872 page 12.)--between the repair of injuries and
gemmation--and with plants, between rapid increase by buds, rhizomes, etc.,
and the production of seed, is partly explained by the gemmules not existing
in sufficient numbers for these processes to be carried on simultaneously.
Hardly any fact in physiology is more wonderful than the power of regrowth;
for instance, that a snail should be able to reproduce its head, or a
salamander its eyes, tail, and legs, exactly at the points where they have
been cut off. Such cases are explained by the presence of gemmules derived
from each part, and disseminated throughout the body. I have heard the process
compared with that of the repair of the broken angles of a crystal by re-
crystallisation; and the two processes have this much in common, that in the
one case the polarity of the molecules is the efficient cause, and in the
other the affinity of the gemmules for particular nascent cells. But we have
here to encounter two objections which apply not only to the regrowth of a
part, or of a bisected individual, but to fissiparous generation and budding.
The first objection is that the part which is reproduced is in the same stage
of development as that of the being which has been operated on or bisected;
and in the case of buds, that the new beings thus produced are in the same
stage as that of the budding parent. Thus a mature salamander, of which the
tail has been cut off, does not reproduce a larval tail; and a crab does not
reproduce a larval leg. In the case of budding it was shown in the first part
of this chapter that the new being thus produced does not retrograde in
development,--that is, does not pass through those earlier stages, which the
fertilised germ has to pass through. Nevertheless, the organisms operated on
or multiplying themselves by buds must, by our hypothesis, include innumerable
gemmules derived from every part or unit of the earlier stages of development;
and why do not such gemmules reproduce the amputated part or the whole body at
a corresponding early stage of development?
The second objection, which has been insisted on by Delpino, is that the
tissues, for instance, of a mature salamander or crab, of which a limb has
been removed, are already differentiated and have passed through their whole
course of development; and how can such tissues in accordance with our
hypothesis attract and combine with the gemmules of the part which is to be
reproduced? In answer to these two objections we must bear in mind the
evidence which has been advanced, showing that at least in a large number of
cases the power of regrowth is a localised faculty, acquired for the sake of
repairing special injuries to which each particular creature is liable; and in
the case of buds or fissiparous generation, for the sake of quickly
multiplying the organism at a period of life when it can be supported in large
numbers. These considerations lead us to believe that in all such cases a
stock of nascent cells or of partially developed gemmules are retained for
this special purpose either locally or throughout the body, ready to combine
with the gemmules derived from the cells which come next in due succession. If
this be admitted we have a sufficient answer to the above two objections.
Anyhow, pangenesis seems to throw a considerable amount of light on the
wonderful power of regrowth.
It follows, also, from the view just given, that the sexual elements differ
from buds in not including nascent cells or gemmules in a somewhat advanced
stage of development, so that only the gemmules belonging to the earliest
stages are first developed. As young animals and those which stand low in the
scale generally have a much greater capacity for regrowth than older and
higher animals, it would also appear that they retain cells in a nascent
state, or partially developed gemmules, more readily than do animals which
have already passed through a long series of developmental changes. I may here
add that although ovules can be detected in most or all female animals at an
extremely early age, there is no reason to doubt that gemmules derived from
parts modified during maturity can pass into the ovules.
With respect to hybridism, pangenesis agrees well with most of the ascertained
facts. We must believe, as previously shown, that several gemmules are
requisite for the development of each cell or unit. But from the occurrence of
parthenogenesis, more especially from those cases in which an embryo is only
partially formed, we may infer that the female element generally includes
gemmules in nearly sufficient number for independent development, so that when
united with the male element the gemmules are superabundant. Now, when two
species or races are crossed reciprocally, the offspring do not commonly
differ, and this shows that the sexual elements agree in power, in accordance
with the view that both include the same gemmules. Hybrids and mongrels are
also generally intermediate in character between the two parent-forms, yet
occasionally they closely resemble one parent in one part and the other parent
in another part, or even in their whole structure: nor is this difficult to
understand on the admission that the gemmules in the fertilised germ are
superabundant in number, and that those derived from one parent may have some
advantage in number, affinity, or vigour over those derived from the other
parent. Crossed forms sometimes exhibit the colour or other characters of
either parent in stripes or blotches; and this occurs in the first generation,
or through reversion in succeeding bud and seminal generations, of which fact
several instances were given in the eleventh chapter. In these cases we must
follow Naudin (27/57. See his excellent discussion on this subject in
'Nouvelles Archives du Museum' tome 1 page 151.) and admit that the "essence"
or "element" of the two species,--terms which I should translate into the
gemmules,--have an affinity for their own kind, and thus separate themselves
into distinct stripes or blotches; and reasons were given, when discussing in
the fifteenth chapter the incompatibility of certain characters to unite, for
believing in such mutual affinity. When two forms are crossed, one is not
rarely found to be prepotent in the transmission of its characters over the
other; and this we can explain by again assuming that the one form has some
advantage over the other in the number, vigour, or affinity of its gemmules.
In some cases, however, certain characters are present in the one form and
latent in the other; for instance, there is a latent tendency in all pigeons
to become blue, and, when a blue pigeon is crossed with one of any other
colour, the blue tint is generally prepotent. The explanation of this form of
prepotency will be obvious when we come to the consideration of Reversion.
When two distinct species are crossed, it is notorious that they do not yield
the full or proper number of offspring; and we can only say on this head that,
as the development of each organism depends on such nicely-balanced affinities
between a host of gemmules and nascent cells, we need not feel at all
surprised that the commixture of gemmules derived from two distinct species
should lead to partial or complete failure of development. With respect to the
sterility of hybrids produced from the union of two distinct species, it was
shown in the nineteenth chapter that this depends exclusively on the
reproductive organs being specially affected; but why these organs should be
thus affected we do not know, any more than why unnatural conditions of life,
though compatible with health, should cause sterility; or why continued close
interbreeding, or the illegitimate unions of heterostyled plants, induce the
same result. The conclusion that the reproductive organs alone are affected,
and not the whole organisation, agrees perfectly with the unimpaired or even
increased capacity in hybrid plants for propagation by buds; for this implies,
according to our hypothesis, that the cells of the hybrids throw off
hybridised gemmules, which become aggregated into buds, but fail to become
aggregated within the reproductive organs, so as to form the sexual elements.
In a similar manner many plants, when placed under unnatural conditions, fail
to produce seed, but can readily be propagated by buds. We shall presently see
that pangenesis agrees well with the strong tendency to reversion exhibited by
all crossed animals and plants.
Each organism reaches maturity through a longer or shorter course of growth
and development: the former term being confined to mere increase of size, and
development to changed structure. The changes may be small and insensibly
slow, as when a child grows into a man, or many, abrupt, and slight, as in the
metamorphoses of certain ephemerous insects, or, again, few and strongly-
marked, as with most other insects. Each newly formed part may be moulded
within a previously existing and corresponding part, and in this case it will
appear, falsely as I believe, to be developed from the old part; or it may be
formed within a distinct part of the body, as in the extreme cases of
metagenesis. An eye, for instance, may be developed at a spot where no eye
previously existed. We have also seen that allied organic beings in the course
of their metamorphoses sometimes attain nearly the same structure after
passing through widely different forms; or conversely, after passing through
nearly the same early forms, arrive at widely different mature forms. In these
cases it is very difficult to accept the common view that the first-formed
cells or units possess the inherent power, independently of any external
agency, of producing new structures wholly different in form, position, and
function. But all these cases become plain on the hypothesis of pangenesis.
The units, during each stage of development, throw off gemmules, which,
multiplying, are transmitted to the offspring. In the offspring, as soon as
any particular cell or unit becomes partially developed, it unites with (or,
to speak metaphorically, is fertilised by) the gemmule of the next succeeding
cell, and so onwards. But organisms have often been subjected to changed
conditions of life at a certain stage of their development, and in consequence
have been slightly modified; and the gemmules cast off from such modified
parts will tend to reproduce parts modified in the same manner. This process
may be repeated until the structure of the part becomes greatly changed at one
particular stage of development, but this will not necessarily affect other
parts, whether previously or subsequently formed. In this manner we can
understand the remarkable independence of structure in the successive
metamorphoses, and especially in the successive metageneses of many animals.
In the case, however, of diseases which supervene during old age, subsequently
to the ordinary period of procreation, and which, nevertheless, are sometimes
inherited, as occurs with brain and heart complaints, we must suppose that the
organs were affected at an early age and threw off at this period affected
gemmules; but that the affection became visible or injurious only after the
prolonged growth, in the strict sense of the word, of the part. In all the
changes of structure which regularly supervene during old age, we probably see
the effects of deteriorated growth, and not of true development.
The principle of the independent formation of each part, owing to the union of
the proper gemmules with certain nascent cells, together with the
superabundance of the gemmules derived from both parents, and the subsequent
self-multiplication of the gemmules, throws light on a widely different group
of facts, which on any ordinary view of development appears very strange. I
allude to organs which are abnormally transposed or multiplied. For instance,
a curious case has been recorded by Dr. Elliott Coues (27/58. 'Proc. Boston
Soc. of Nat. Hist.' republished in 'Scientific Opinion' November 10, 1869 page
488.) of a monstrous chicken with a perfect additional RIGHT leg articulated
to the LEFT side of the pelvis. Gold-fish often have supernumerary fins placed
on various parts of their bodies. When the tail of a lizard is broken off, a
double tail is sometimes reproduced; and when the foot of the salamander was
divided longitudinally by Bonnet, additional digits were occasionally formed.
Valentin injured the caudal extremity of an embryo, and three days afterwards
it produced rudiments of a double pelvis and of double hind-limbs. (27/59.
Todd 'Cyclop. of Anat. and Phys.' volume 4 1849-52 page 975.) When frogs,
toads, etc., are born with their limbs doubled, as sometimes happens, the
doubling, as Gervais remarks (27/60. 'Compte Rendus' November 14, 1865 page
800.), cannot be due to the complete fusion of two embryos, with the exception
of the limbs, for the larvae are limbless. The same argument is applicable
(27/61. As previously remarked by Quatrefages in his 'Metamorphoses de
l'Homme' etc. 1862 page 129.) to certain insects produced with multiple legs
or antennae, for these are metamorphosed from apodal or antennae-less larvae.
Alphonse Milne-Edwards (27/62. Gunther 'Zoological Record' 1864 page 279.) has
described the curious case of a crustacean in which one eye-peduncle
supported, instead of a complete eye, only an imperfect cornea, and out of the
centre of this a portion of an antenna was developed. A case has been recorded
(27/63. Sedgwick 'Medico-Chirurg. Review' April 1863 page 454.) of a man who
had during both dentitions a double tooth in place of the left second incisor,
and he inherited this peculiarity from his paternal grandfather. Several cases
are known (27/64. Isid. Geoffroy Saint-Hilaire 'Hist. des Anomalies' tome 1
1832 pages 435, 657; and tome 2 page 560.) of additional teeth having been
developed in the orbit of the eye, and, more especially with horses, in the
palate. Hairs occasionally appear in strange situations, as "within the
substance of the brain." (27/65. Virchow 'Cellular Pathology' 1860 page 66.)
Certain breeds of sheep bear a whole crowd of horns on their foreheads. As
many as five spurs have been seen on both legs of certain Game-fowls. In the
Polish fowl the male is ornamented with a topknot of hackles like those on his
neck, whilst the female has a top-knot formed of common feathers. In feather-
footed pigeons and fowls, feathers like those on the wing arise from the outer
side of the legs and toes. Even the elemental parts of the same feather may be
transposed; for in the Sebastopol goose, barbules are developed on the divided
filaments of the shaft. Imperfect nails sometimes appear on the stumps of the
amputated fingers of man (27/66. Muller 'Phys.' English Translation volume 1
1833 page 407. A case of this kind has lately been communicated to me.) and it
is an interesting fact that with the snake-like Saurians, which present a
series with more and more imperfect limbs, the terminations of the phalanges
first disappear, "the nails becoming transferred to their proximal remnants,
or even to parts which are not phalanges." (27/67. Dr. Furbringer 'Die Knochen
etc. bei den schlangenahnlichen Sauriern' as reviewed in 'Journal of Anat. and
Phys.' May 1870 page 286.)
Analogous cases are of such frequent occurrence with plants that they do not
strike us with sufficient surprise. Supernumerary petals, stamens, and
pistils, are often produced. I have seen a leaflet low down in the compound
leaf of Vicia sativa replaced by a tendril; and a tendril possesses many
peculiar properties, such as spontaneous movement and irritability. The calyx
sometimes assumes, either wholly or by stripes, the colour and texture of the
corolla. Stamens are so frequently converted into petals, more or less
completely, that such cases are passed over as not deserving notice; but as
petals have special functions to perform, namely, to protect the included
organs, to attract insects, and in not a few cases to guide their entrance by
well-adapted contrivances, we can hardly account for the conversion of stamens
into petals merely by unnatural or superfluous nourishment. Again, the edge of
a petal may occasionally be found including one of the highest products of the
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