On the Origin of Species
by
Charles Darwin
Part 3 out of 9
common. Hence, the struggle for the production of new and modified
descendants, will mainly lie between the larger groups, which are all
trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and
seizing on many new places in the polity of Nature, will constantly
tend to supplant and destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finally tend to disappear.
Looking to the future, we can predict that the groups of organic
beings which are now large and triumphant, and which are least broken
up, that is, which as yet have suffered least extinction, will for a
long period continue to increase. But which groups will ultimately
prevail, no man can predict; for we well know that many groups,
formerly most extensively developed, have now become extinct. Looking
still more remotely to the future, we may predict that, owing to the
continued and steady increase of the larger groups, a multitude of
smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on
Classification, but I may add that on this view of extremely few of
the more ancient species having transmitted descendants, and on the
view of all the descendants of the same species making a class, we can
understand how it is that there exist but very few classes in each
main division of the animal and vegetable kingdoms. Although extremely
few of the most ancient species may now have living and modified
descendants, yet at the most remote geological period, the earth may
have been as well peopled with many species of many genera, families,
orders, and classes, as at the present day.
SUMMARY OF CHAPTER.
If during the long course of ages and under varying conditions of
life, organic beings vary at all in the several parts of their
organisation, and I think this cannot be disputed; if there be, owing
to the high geometrical powers of increase of each species, at some
age, season, or year, a severe struggle for life, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions
of existence, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, I think it would
be a most extraordinary fact if no variation ever had occurred useful
to each being's own welfare, in the same way as so many variations
have occurred useful to man. But if variations useful to any organic
being do occur, assuredly individuals thus characterised will have the
best chance of being preserved in the struggle for life; and from the
strong principle of inheritance they will tend to produce offspring
similarly characterised. This principle of preservation, I have
called, for the sake of brevity, Natural Selection. Natural selection,
on the principle of qualities being inherited at corresponding ages,
can modify the egg, seed, or young, as easily as the adult. Amongst
many animals, sexual selection will give its aid to ordinary
selection, by assuring to the most vigorous and best adapted males the
greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other
males.
Whether natural selection has really thus acted in nature, in
modifying and adapting the various forms of life to their several
conditions and stations, must be judged of by the general tenour and
balance of evidence given in the following chapters. But we already
see how it entails extinction; and how largely extinction has acted in
the world's history, geology plainly declares. Natural selection,
also, leads to divergence of character; for more living beings can be
supported on the same area the more they diverge in structure, habits,
and constitution, of which we see proof by looking at the inhabitants
of any small spot or at naturalised productions. Therefore during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their chance
of succeeding in the battle of life. Thus the small differences
distinguishing varieties of the same species, will steadily tend to
increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and
space should be related to each other in group subordinate to group,
in the manner which we everywhere behold--namely, varieties of the
same species most closely related together, species of the same genus
less closely and unequally related together, forming sections and
sub-genera, species of distinct genera much less closely related, and
genera related in different degrees, forming sub-families, families,
orders, sub-classes, and classes. The several subordinate groups in
any class cannot be ranked in a single file, but seem rather to be
clustered round points, and these round other points, and so on in
almost endless cycles. On the view that each species has been
independently created, I can see no explanation of this great fact in
the classification of all organic beings; but, to the best of my
judgment, it is explained through inheritance and the complex action
of natural selection, entailing extinction and divergence of
character, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long
succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner as
species and groups of species have tried to overmaster other species
in the great battle for life. The limbs divided into great branches,
and these into lesser and lesser branches, were themselves once, when
the tree was small, budding twigs; and this connexion of the former
and present buds by ramifying branches may well represent the
classification of all extinct and living species in groups subordinate
to groups. Of the many twigs which flourished when the tree was a mere
bush, only two or three, now grown into great branches, yet survive
and bear all the other branches; so with the species which lived
during long-past geological periods, very few now have living and
modified descendants. From the first growth of the tree, many a limb
and branch has decayed and dropped off; and these lost branches of
various sizes may represent those whole orders, families, and genera
which have now no living representatives, and which are known to us
only from having been found in a fossil state. As we here and there
see a thin straggling branch springing from a fork low down in a tree,
and which by some chance has been favoured and is still alive on its
summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and
overtop on all sides many a feebler branch, so by generation I believe
it has been with the great Tree of Life, which fills with its dead and
broken branches the crust of the earth, and covers the surface with
its ever branching and beautiful ramifications.
CHAPTER 5. LAWS OF VARIATION.
Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long lost characters.
Summary.
I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser
degree in those in a state of nature--had been due to chance. This, of
course, is a wholly incorrect expression, but it serves to acknowledge
plainly our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive
system to produce individual differences, or very slight deviations of
structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have
remarked in the first chapter--but a long catalogue of facts which
cannot be here given would be necessary to show the truth of the
remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being
functionally disturbed in the parents, I chiefly attribute the varying
or plastic condition of the offspring. The male and female sexual
elements seem to be affected before that union takes place which is to
form a new being. In the case of "sporting" plants, the bud, which in
its earliest condition does not apparently differ essentially from an
ovule, is alone affected. But why, because the reproductive system is
disturbed, this or that part should vary more or less, we are
profoundly ignorant. Nevertheless, we can here and there dimly catch a
faint ray of light, and we may feel sure that there must be some cause
for each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in
that of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on
islands or near the coast. So with insects, Wollaston is convinced
that residence near the sea affects their colours. Moquin-Tandon gives
a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several
other such cases could be given.
The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties.
Thus the species of shells which are confined to tropical and shallow
seas are generally brighter-coloured than those confined to cold and
deeper seas. The birds which are confined to continents are, according
to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.
When a variation is of the slightest use to a being, we cannot tell
how much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during
many generations, and how much to the direct action of the severe
climate? for it would appear that climate has some direct action on
the hair of our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same
species under the same conditions. Such facts show how indirectly the
conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.
EFFECTS OF USE AND DISUSE.
From the facts alluded to in the first chapter, I think there can be
little doubt that use in our domestic animals strengthens and enlarges
certain parts, and disuse diminishes them; and that such modifications
are inherited. Under free nature, we can have no standard of
comparison, by which to judge of the effects of long-continued use or
disuse, for we know not the parent-forms; but many animals have
structures which can be explained by the effects of disuse. As
Professor Owen has remarked, there is no greater anomaly in nature
than a bird that cannot fly; yet there are several in this state. The
logger-headed duck of South America can only flap along the surface of
the water, and has its wings in nearly the same condition as the
domestic Aylesbury duck. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly
wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast of prey, has
been caused by disuse. The ostrich indeed inhabits continents and is
exposed to danger from which it cannot escape by flight, but by
kicking it can defend itself from enemies, as well as any of the
smaller quadrupeds. We may imagine that the early progenitor of the
ostrich had habits like those of a bustard, and that as natural
selection increased in successive generations the size and weight of
its body, its legs were used more, and its wings less, until they
became incapable of flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described
as not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient
evidence to induce us to believe that mutilations are ever inherited;
and I should prefer explaining the entire absence of the anterior
tarsi in Ateuchus, and their rudimentary condition in some other
genera, by the long-continued effects of disuse in their progenitors;
for as the tarsi are almost always lost in many dung-feeding beetles,
they must be lost early in life, and therefore cannot be much used by
these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are
very frequently blown to sea and perish; that the beetles in Madeira,
as observed by Mr. Wollaston, lie much concealed, until the wind lulls
and the sun shines; that the proportion of wingless beetles is larger
on the exposed Dezertas than in Madeira itself; and especially the
extraordinary fact, so strongly insisted on by Mr. Wollaston, of the
almost entire absence of certain large groups of beetles, elsewhere
excessively numerous, and which groups have habits of life almost
necessitating frequent flight;--these several considerations have made
me believe that the wingless condition of so many Madeira beetles is
mainly due to the action of natural selection, but combined probably
with disuse. For during thousands of successive generations each
individual beetle which flew least, either from its wings having been
ever so little less perfectly developed or from indolent habit, will
have had the best chance of surviving from not being blown out to sea;
and, on the other hand, those beetles which most readily took to
flight will oftenest have been blown to sea and thus have been
destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects,
their wings not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For when a new insect
first arrived on the island, the tendency of natural selection to
enlarge or to reduce the wings, would depend on whether a greater
number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with
mariners shipwrecked near a coast, it would have been better for the
good swimmers if they had been able to swim still further, whereas it
would have been better for the bad swimmers if they had not been able
to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse,
but aided perhaps by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its
habits than the mole; and I was assured by a Spaniard, who had often
caught them, that they were frequently blind; one which I kept alive
was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not indispensable to animals with subterranean habits, a
reduction in their size with the adhesion of the eyelids and growth of
fur over them, might in such case be an advantage; and if so, natural
selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the
eye is gone; the stand for the telescope is there, though the
telescope with its glasses has been lost. As it is difficult to
imagine that eyes, though useless, could be in any way injurious to
animals living in darkness, I attribute their loss wholly to disuse.
In one of the blind animals, namely, the cave-rat, the eyes are of
immense size; and Professor Silliman thought that it regained, after
living some days in the light, some slight power of vision. In the
same manner as in Madeira the wings of some of the insects have been
enlarged, and the wings of others have been reduced by natural
selection aided by use and disuse, so in the case of the cave-rat
natural selection seems to have struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its
work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the
common view of the blind animals having been separately created for
the American and European caverns, close similarity in their
organisation and affinities might have been expected; but, as Schiodte
and others have remarked, this is not the case, and the cave-insects
of the two continents are not more closely allied than might have been
anticipated from the general resemblance of the other inhabitants of
North America and Europe. On my view we must suppose that American
animals, having ordinary powers of vision, slowly migrated by
successive generations from the outer world into the deeper and deeper
recesses of the Kentucky caves, as did European animals into the caves
of Europe. We have some evidence of this gradation of habit; for, as
Schiodte remarks, "animals not far remote from ordinary forms, prepare
the transition from light to darkness. Next follow those that are
constructed for twilight; and, last of all, those destined for total
darkness." By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more
or less perfectly obliterated its eyes, and natural selection will
often have effected other changes, such as an increase in the length
of the antennae or palpi, as a compensation for blindness.
Notwithstanding such modifications, we might expect still to see in
the cave-animals of America, affinities to the other inhabitants of
that continent, and in those of Europe, to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding
country. It would be most difficult to give any rational explanation
of the affinities of the blind cave-animals to the other inhabitants
of the two continents on the ordinary view of their independent
creation. That several of the inhabitants of the caves of the Old and
New Worlds should be closely related, we might expect from the
well-known relationship of most of their other productions. Far from
feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference
to the reptiles of Europe, I am only surprised that more wrecks of
ancient life have not been preserved, owing to the less severe
competition to which the inhabitants of these dark abodes will
probably have been exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in the
amount of rain requisite for seeds to germinate, in the time of sleep,
etc., and this leads me to say a few words on acclimatisation. As it
is extremely common for species of the same genus to inhabit very hot
and very cold countries, and as I believe that all the species of the
same genus have descended from a single parent, if this view be
correct, acclimatisation must be readily effected during
long-continued descent. It is notorious that each species is adapted
to the climate of its own home: species from an arctic or even from a
temperate region cannot endure a tropical climate, or conversely. So
again, many succulent plants cannot endure a damp climate. But the
degree of adaptation of species to the climates under which they live
is often overrated. We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and
from the number of plants and animals brought from warmer countries
which here enjoy good health. We have reason to believe that species
in a state of nature are limited in their ranges by the competition of
other organic beings quite as much as, or more than, by adaptation to
particular climates. But whether or not the adaptation be generally
very close, we have evidence, in the case of some few plants, of their
becoming, to a certain extent, naturally habituated to different
temperatures, or becoming acclimatised: thus the pines and
rhododendrons, raised from seed collected by Dr. Hooker from trees
growing at different heights on the Himalaya, were found in this
country to possess different constitutional powers of resisting cold.
Mr. Thwaites informs me that he has observed similar facts in Ceylon,
and analogous observations have been made by Mr. H. C. Watson on
European species of plants brought from the Azores to England. In
regard to animals, several authentic cases could be given of species
within historical times having largely extended their range from
warmer to cooler latitudes, and conversely; but we do not positively
know that these animals were strictly adapted to their native climate,
but in all ordinary cases we assume such to be the case; nor do we
know that they have subsequently become acclimatised to their new
homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly fertile (a far severer test)
under them, may be used as an argument that a large proportion of
other animals, now in a state of nature, could easily be brought to
bear widely different climates. We must not, however, push the
foregoing argument too far, on account of the probable origin of some
of our domestic animals from several wild stocks: the blood, for
instance, of a tropical and arctic wolf or wild dog may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered
as domestic animals, but they have been transported by man to many
parts of the world, and now have a far wider range than any other
rodent, living free under the cold climate of Faroe in the north and
of the Falklands in the south, and on many islands in the torrid
zones. Hence I am inclined to look at adaptation to any special
climate as a quality readily grafted on an innate wide flexibility of
constitution, which is common to most animals. On this view, the
capacity of enduring the most different climates by man himself and by
his domestic animals, and such facts as that former species of the
elephant and rhinoceros were capable of enduring a glacial climate,
whereas the living species are now all tropical or sub-tropical in
their habits, ought not to be looked at as anomalies, but merely as
examples of a very common flexibility of constitution, brought, under
peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopaedias
of China, to be very cautious in transposing animals from one district
to another; for it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts: the result must, I think, be due to
habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which
are born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are
said to withstand certain climates better than others: this is very
strikingly shown in works on fruit trees published in the United
States, in which certain varieties are habitually recommended for the
northern, and others for the southern States; and as most of these
varieties are of recent origin, they cannot owe their constitutional
differences to habit. The case of the Jerusalem artichoke, which is
never propagated by seed, and of which consequently new varieties have
not been produced, has even been advanced--for it is now as tender as
ever it was--as proving that acclimatisation cannot be effected! The
case, also, of the kidney-bean has been often cited for a similar
purpose, and with much greater weight; but until some one will sow,
during a score of generations, his kidney-beans so early that a very
large proportion are destroyed by frost, and then collect seed from
the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the
experiment cannot be said to have been even tried. Nor let it be
supposed that no differences in the constitution of seedling
kidney-beans ever appear, for an account has been published how much
more hardy some seedlings appeared to be than others.
On the whole, I think we may conclude that habit, use, and disuse,
have, in some cases, played a considerable part in the modification of
the constitution, and of the structure of various organs; but that the
effects of use and disuse have often been largely combined with, and
sometimes overmastered by, the natural selection of innate
differences.
CORRELATION OF GROWTH.
I mean by this expression that the whole organisation is so tied
together during its growth and development, that when slight
variations in any one part occur, and are accumulated through natural
selection, other parts become modified. This is a very important
subject, most imperfectly understood. The most obvious case is, that
modifications accumulated solely for the good of the young or larva,
will, it may safely be concluded, affect the structure of the adult;
in the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several
parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the
same manner; in the front and hind legs, and even in the jaws and
limbs, varying together, for the lower jaw is believed to be
homologous with the limbs. These tendencies, I do not doubt, may be
mastered more or less completely by natural selection: thus a family
of stags once existed with an antler only on one side; and if this had
been of any great use to the breed it might probably have been
rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants; and nothing is more
common than the union of homologous parts in normal structures, as the
union of the petals of the corolla into a tube. Hard parts seem to
affect the form of adjoining soft parts; it is believed by some
authors that the diversity in the shape of the pelvis in birds causes
the remarkable diversity in the shape of their kidneys. Others believe
that the shape of the pelvis in the human mother influences by
pressure the shape of the head of the child. In snakes, according to
Schlegel, the shape of the body and the manner of swallowing determine
the position of several of the most important viscera.
The nature of the bond of correlation is very frequently quite
obscure. M. Is. Geoffroy St. Hilaire has forcibly remarked, that
certain malconformations very frequently, and that others rarely
coexist, without our being able to assign any reason. What can be more
singular than the relation between blue eyes and deafness in cats, and
the tortoise-shell colour with the female sex; the feathered feet and
skin between the outer toes in pigeons, and the presence of more or
less down on the young birds when first hatched, with the future
colour of their plumage; or, again, the relation between the hair and
teeth in the naked Turkish dog, though here probably homology comes
into play? With respect to this latter case of correlation, I think it
can hardly be accidental, that if we pick out the two orders of
mammalia which are most abnormal in their dermal coverings, viz.
Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.),
that these are likewise the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of
utility and, therefore, of natural selection, than that of the
difference between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the difference in the ray and
central florets of, for instance, the daisy, and this difference is
often accompanied with the abortion of parts of the flower. But, in
some Compositous plants, the seeds also differ in shape and sculpture;
and even the ovary itself, with its accessory parts, differs, as has
been described by Cassini. These differences have been attributed by
some authors to pressure, and the shape of the seeds in the
ray-florets in some Compositae countenances this idea; but, in the
case of the corolla of the Umbelliferae, it is by no means, as Dr.
Hooker informs me, in species with the densest heads that the inner
and outer flowers most frequently differ. It might have been thought
that the development of the ray-petals by drawing nourishment from
certain other parts of the flower had caused their abortion; but in
some Compositae there is a difference in the seeds of the outer and
inner florets without any difference in the corolla. Possibly, these
several differences may be connected with some difference in the flow
of nutriment towards the central and external flowers: we know, at
least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an
instance of this, and of a striking case of correlation, that I have
recently observed in some garden pelargoniums, that the central flower
of the truss often loses the patches of darker colour in the two upper
petals; and that when this occurs, the adherent nectary is quite
aborted; when the colour is absent from only one of the two upper
petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel's idea that the ray-florets serve to attract insects,
whose agency is highly advantageous in the fertilisation of plants of
these two orders, is so far-fetched, as it may at first appear: and if
it be advantageous, natural selection may have come into play. But in
regard to the differences both in the internal and external structure
of the seeds, which are not always correlated with any differences in
the flowers, it seems impossible that they can be in any way
advantageous to the plant: yet in the Umbelliferae these differences
are of such apparent importance--the seeds being in some cases,
according to Tausch, orthospermous in the exterior flowers and
coelospermous in the central flowers,--that the elder De Candolle
founded his main divisions of the order on analogous differences.
Hence we see that modifications of structure, viewed by systematists
as of high value, may be wholly due to unknown laws of correlated
growth, and without being, as far as we can see, of the slightest
service to the species.
We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and,
after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted to
a whole group of descendants with diverse habits, would naturally be
thought to be correlated in some necessary manner. So, again, I do not
doubt that some apparent correlations, occurring throughout whole
orders, are entirely due to the manner alone in which natural
selection can act. For instance, Alph. De Candolle has remarked that
winged seeds are never found in fruits which do not open: I should
explain the rule by the fact that seeds could not gradually become
winged through natural selection, except in fruits which opened; so
that the individual plants producing seeds which were a little better
fitted to be wafted further, might get an advantage over those
producing seed less fitted for dispersal; and this process could not
possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, "in order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to
fatten readily. The same varieties of the cabbage do not yield
abundant and nutritious foliage and a copious supply of oil-bearing
seeds. When the seeds in our fruits become atrophied, the fruit itself
gains largely in size and quality. In our poultry, a large tuft of
feathers on the head is generally accompanied by a diminished comb,
and a large beard by diminished wattles. With species in a state of
nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists,
believe in its truth. I will not, however, here give any instances,
for I see hardly any way of distinguishing between the effects, on the
one hand, of a part being largely developed through natural selection
and another and adjoining part being reduced by this same process or
by disuse, and, on the other hand, the actual withdrawal of nutriment
from one part owing to the excess of growth in another and adjoining
part.
I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under a
more general principle, namely, that natural selection is continually
trying to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure. I
can thus only understand a fact with which I was much struck when
examining cirripedes, and of which many other instances could be
given: namely, that when a cirripede is parasitic within another and
is thus protected, it loses more or less completely its own shell or
carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all
other cirripedes consists of the three highly-important anterior
segments of the head enormously developed, and furnished with great
nerves and muscles; but in the parasitic and protected Proteolepas,
the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antennae. Now the saving of a
large and complex structure, when rendered superfluous by the
parasitic habits of the Proteolepas, though effected by slow steps,
would be a decided advantage to each successive individual of the
species; for in the struggle for life to which every animal is
exposed, each individual Proteolepas would have a better chance of
supporting itself, by less nutriment being wasted in developing a
structure now become useless.
Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as
it is rendered superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary
compensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both
in varieties and in species, that when any part or organ is repeated
many times in the structure of the same individual (as the vertebrae
in snakes, and the stamens in polyandrous flowers) the number is
variable; whereas the number of the same part or organ, when it occurs
in lesser numbers, is constant. The same author and some botanists
have further remarked that multiple parts are also very liable to
variation in structure. Inasmuch as this "vegetative repetition," to
use Professor Owen's expression, seems to be a sign of low
organisation; the foregoing remark seems connected with the very
general opinion of naturalists, that beings low in the scale of nature
are more variable than those which are higher. I presume that lowness
in this case means that the several parts of the organisation have
been but little specialised for particular functions; and as long as
the same part has to perform diversified work, we can perhaps see why
it should remain variable, that is, why natural selection should have
preserved or rejected each little deviation of form less carefully
than when the part has to serve for one special purpose alone. In the
same way that a knife which has to cut all sorts of things may be of
almost any shape; whilst a tool for some particular object had better
be of some particular shape. Natural selection, it should never be
forgotten, can act on each part of each being, solely through and for
its advantage.
Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to
the general subject of rudimentary and aborted organs; and I will here
only add that their variability seems to be owing to their
uselessness, and therefore to natural selection having no power to
check deviations in their structure. Thus rudimentary parts are left
to the free play of the various laws of growth, to the effects of
long-continued disuse, and to the tendency to reversion.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,
IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.
Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr. Waterhouse. I infer also from an observation
made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of this
proposition without giving the long array of facts which I have
collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware
of several causes of error, but I hope that I have made due allowance
for them. It should be understood that the rule by no means applies to
any part, however unusually developed, unless it be unusually
developed in comparison with the same part in closely allied species.
Thus, the bat's wing is a most abnormal structure in the class
mammalia; but the rule would not here apply, because there is a whole
group of bats having wings; it would apply only if some one species of
bat had its wings developed in some remarkable manner in comparison
with the other species of the same genus. The rule applies very
strongly in the case of secondary sexual characters, when displayed in
any unusual manner. The term, secondary sexual characters, used by
Hunter, applies to characters which are attached to one sex, but are
not directly connected with the act of reproduction. The rule applies
to males and females; but as females more rarely offer remarkable
secondary sexual characters, it applies more rarely to them. The rule
being so plainly applicable in the case of secondary sexual
characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner--of which fact I think
there can be little doubt. But that our rule is not confined to
secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse's remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of
several of the species is so great, that it is no exaggeration to
state that the varieties differ more from each other in the characters
of these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly
to hold good in this class. I cannot make out that it applies to
plants, and this would seriously have shaken my belief in its truth,
had not the great variability in plants made it particularly difficult
to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or
manner in any species, the fair presumption is that it is of high
importance to that species; nevertheless the part in this case is
eminently liable to variation. Why should this be so? On the view that
each species has been independently created, with all its parts as we
now see them, I can see no explanation. But on the view that groups of
species have descended from other species, and have been modified
through natural selection, I think we can obtain some light. In our
domestic animals, if any part, or the whole animal, be neglected and
no selection be applied, that part (for instance, the comb in the
Dorking fowl) or the whole breed will cease to have a nearly uniform
character. The breed will then be said to have degenerated. In
rudimentary organs, and in those which have been but little
specialised for any particular purpose, and perhaps in polymorphic
groups, we see a nearly parallel natural case; for in such cases
natural selection either has not or cannot come into full play, and
thus the organisation is left in a fluctuating condition. But what
here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the breeds of the pigeon; see what a prodigious amount of difference
there is in the beak of the different tumblers, in the beak and wattle
of the different carriers, in the carriage and tail of our fantails,
etc., these being the points now mainly attended to by English
fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is
notoriously difficult to breed them nearly to perfection, and
frequently individuals are born which depart widely from the standard.
There may be truly said to be a constant struggle going on between, on
the one hand, the tendency to reversion to a less modified state, as
well as an innate tendency to further variability of all kinds, and,
on the other hand, the power of steady selection to keep the breed
true. In the long run selection gains the day, and we do not expect to
fail so far as to breed a bird as coarse as a common tumbler from a
good short-faced strain. But as long as selection is rapidly going on,
there may always be expected to be much variability in the structure
undergoing modification. It further deserves notice that these
variable characters, produced by man's selection, sometimes become
attached, from causes quite unknown to us, more to one sex than to the
other, generally to the male sex, as with the wattle of carriers and
the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has
undergone an extraordinary amount of modification, since the period
when the species branched off from the common progenitor of the genus.
This period will seldom be remote in any extreme degree, as species
very rarely endure for more than one geological period. An
extraordinary amount of modification implies an unusually large and
long-continued amount of variability, which has continually been
accumulated by natural selection for the benefit of the species. But
as the variability of the extraordinarily-developed part or organ has
been so great and long-continued within a period not excessively
remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation,
which have remained for a much longer period nearly constant. And
this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and
variability on the other hand, will in the course of time cease; and
that the most abnormally developed organs may be made constant, I can
see no reason to doubt. Hence when an organ, however abnormal it may
be, has been transmitted in approximately the same condition to many
modified descendants, as in the case of the wing of the bat, it must
have existed, according to my theory, for an immense period in nearly
the same state; and thus it comes to be no more variable than any
other structure. It is only in those cases in which the modification
has been comparatively recent and extraordinarily great that we ought
to find the GENERATIVE VARIABILITY, as it may be called, still present
in a high degree. For in this case the variability will seldom as yet
have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of
those tending to revert to a former and less modified condition.
The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic. To
explain by a simple example what is meant. If some species in a large
genus of plants had blue flowers and some had red, the colour would be
only a specific character, and no one would be surprised at one of the
blue species varying into red, or conversely; but if all the species
had blue flowers, the colour would become a generic character, and its
variation would be a more unusual circumstance. I have chosen this
example because an explanation is not in this case applicable, which
most naturalists would advance, namely, that specific characters are
more variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera.
I believe this explanation is partly, yet only indirectly, true; I
shall, however, have to return to this subject in our chapter on
Classification. It would be almost superfluous to adduce evidence in
support of the above statement, that specific characters are more
variable than generic; but I have repeatedly noticed in works on
natural history, that when an author has remarked with surprise that
some IMPORTANT organ or part, which is generally very constant
throughout large groups of species, has DIFFERED considerably in
closely-allied species, that it has, also, been VARIABLE in the
individuals of some of the species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy St. Hilaire seems
to entertain no doubt, that the more an organ normally differs in the
different species of the same group, the more subject it is to
individual anomalies.
On the ordinary view of each species having been independently
created, why should that part of the structure, which differs from the
same part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner:--the points in which all the species of a
genus resemble each other, and in which they differ from the species
of some other genus, are called generic characters; and these
characters in common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection
will have modified several species, fitted to more or less
widely-different habits, in exactly the same manner: and as these
so-called generic characters have been inherited from a remote period,
since that period when the species first branched off from their
common progenitor, and subsequently have not varied or come to differ
in any degree, or only in a slight degree, it is not probable that
they should vary at the present day. On the other hand, the points in
which species differ from other species of the same genus, are called
specific characters; and as these specific characters have varied and
come to differ within the period of the branching off of the species
from a common progenitor, it is probable that they should still often
be in some degree variable,--at least more variable than those parts
of the organisation which have for a very long period remained
constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which is less rigid in its action than ordinary
selection, as it does not entail death, but only gives fewer offspring
to the less favoured males. Whatever the cause may be of the
variability of secondary sexual characters, as they are highly
variable, sexual selection will have had a wide scope for action, and
may thus readily have succeeded in giving to the species of the same
group a greater amount of difference in their sexual characters, than
in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the
same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my
list; and as the differences in these cases are of a very unusual
nature, the relation can hardly be accidental. The same number of
joints in the tarsi is a character generally common to very large
groups of beetles, but in the Engidae, as Westwood has remarked, the
number varies greatly; and the number likewise differs in the two
sexes of the same species: again in fossorial hymenoptera, the manner
of neuration of the wings is a character of the highest importance,
because common to large groups; but in certain genera the neuration
differs in the different species, and likewise in the two sexes of the
same species. This relation has a clear meaning on my view of the
subject: I look at all the species of the same genus as having as
certainly descended from the same progenitor, as have the two sexes of
any one of the species. Consequently, whatever part of the structure
of the common progenitor, or of its early descendants, became
variable; variations of this part would it is highly probable, be
taken advantage of by natural and sexual selection, in order to fit
the several species to their several places in the economy of nature,
and likewise to fit the two sexes of the same species to each other,
or to fit the males and females to different habits of life, or the
males to struggle with other males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in
common;--that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with
the same part in its congeners; and the not great degree of
variability in a part, however extraordinarily it may be developed, if
it be common to a whole group of species;--that the great variability
of secondary sexual characters, and the great amount of difference in
these same characters between closely allied species;--that secondary
sexual and ordinary specific differences are generally displayed in
the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same
group having descended from a common progenitor, from whom they have
inherited much in common,--to parts which have recently and largely
varied being more likely still to go on varying than parts which have
long been inherited and have not varied,--to natural selection having
more or less completely, according to the lapse of time, overmastered
the tendency to reversion and to further variability,--to sexual
selection being less rigid than ordinary selection,--and to variations
in the same parts having been accumulated by natural and sexual
selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE
SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head
and feathers on the feet,--characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter, may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on
by similar unknown influences. In the vegetable kingdom we have a case
of analogous variation, in the enlarged stems, or roots as commonly
called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white rump, a bar at the end of the tail, with the
outer feathers externally edged near their bases with white. As all
these marks are characteristic of the parent rock-pigeon, I presume
that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We may I
think confidently come to this conclusion, because, as we have seen,
these coloured marks are eminently liable to appear in the crossed
offspring of two distinct and differently coloured breeds; and in this
case there is nothing in the external conditions of life to cause the
reappearance of the slaty-blue, with the several marks, beyond the
influence of the mere act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations--some say, for a dozen or even a
score of generations. After twelve generations, the proportion of
blood, to use a common expression, of any one ancestor, is only 1 in
2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this very small proportion of foreign blood.
In a breed which has not been crossed, but in which BOTH parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might be, as
was formerly remarked, for all that we can see to the contrary,
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but
that in each successive generation there has been a tendency to
reproduce the character in question, which at last, under unknown
favourable conditions, gains an ascendancy. For instance, it is
probable that in each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character
being inherited for an endless number of generations, than in quite
useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce
a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an
unimportant nature, for the presence of all important characters will
be governed by natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual action of
the conditions of life and of a similar inherited constitution. It
might further be expected that the species of the same genus would
occasionally exhibit reversions to lost ancestral characters. As,
however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two cases: if, for instance, we
did not know that the rock-pigeon was not feather-footed or
turn-crowned, we could not have told, whether these characters in our
domestic breeds were reversions or only analogous variations; but we
might have inferred that the blueness was a case of reversion, from
the number of the markings, which are correlated with the blue tint,
and which it does not appear probable would all appear together from
simple variation. More especially we might have inferred this, from
the blue colour and marks so often appearing when distinct breeds of
diverse colours are crossed. Hence, though under nature it must
generally be left doubtful, what cases are reversions to an anciently
existing character, and what are new but analogous variations, yet we
ought, on my theory, sometimes to find the varying offspring of a
species assuming characters (either from reversion or from analogous
variation) which already occur in some other members of the same
group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable
species in our systematic works, is due to its varieties mocking, as
it were, some of the other species of the same genus. A considerable
catalogue, also, could be given of forms intermediate between two
other forms, which themselves must be doubtfully ranked as either
varieties or species; and this shows, unless all these forms be
considered as independently created species, that the one in varying
has assumed some of the characters of the other, so as to produce the
intermediate form. But the best evidence is afforded by parts or
organs of an important and uniform nature occasionally varying so as
to acquire, in some degree, the character of the same part or organ in
an allied species. I have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and
seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those on the legs of a
zebra: it has been asserted that these are plainest in the foal, and
from inquiries which I have made, I believe this to be true. It has
also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and
outline. A white ass, but NOT an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
Blyth and others, occasionally appear: and I have been informed by
Colonel Poole that the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but
Dr. Gray has figured one specimen with very distinct zebra-like bars
on the hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of ALL
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut: a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a bay horse. My
son made a careful examination and sketch for me of a dun Belgian
cart-horse with a double stripe on each shoulder and with leg-stripes;
and a man, whom I can implicitly trust, has examined for me a small
dun Welch pony with THREE short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
the breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and
bay Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago
in the south. In all parts of the world these stripes occur far
oftenest in duns and mouse-duns; by the term dun a large range of
colour is included, from one between brown and black to a close
approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species--one of which, the dun, was striped;
and that the above-described appearances are all due to ancient
crosses with the dun stock. But I am not at all satisfied with this
theory, and should be loth to apply it to breeds so distinct as the
heavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race,
etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and
horse is particularly apt to have bars on its legs. I once saw a mule
with its legs so much striped that any one at first would have thought
that it must have been the product of a zebra; and Mr. W. C. Martin,
in his excellent treatise on the horse, has given a figure of a
similar mule. In four coloured drawings, which I have seen, of hybrids
between the ass and zebra, the legs were much more plainly barred than
the rest of the body; and in one of them there was a double
shoulder-stripe. In Lord Moreton's famous hybrid from a chestnut mare
and male quagga, the hybrid, and even the pure offspring subsequently
produced from the mare by a black Arabian sire, were much more plainly
barred across the legs than is even the pure quagga. Lastly, and this
is another most remarkable case, a hybrid has been figured by Dr. Gray
(and he informs me that he knows of a second case) from the ass and
the hemionus; and this hybrid, though the ass seldom has stripes on
its legs and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Welch pony, and even had some
zebra-like stripes on the sides of its face. With respect to this last
fact, I was so convinced that not even a stripe of colour appears from
what would commonly be called an accident, that I was led solely from
the occurrence of the face-stripes on this hybrid from the ass and
hemionus, to ask Colonel Poole whether such face-stripes ever occur in
the eminently striped Kattywar breed of horses, and was, as we have
seen, answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears--a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We
see this tendency to become striped most strongly displayed in hybrids
from between several of the most distinct species. Now observe the
case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races) of a
bluish colour, with certain bars and other marks; and when any breed
assumes by simple variation a bluish tint, these bars and other marks
invariably reappear; but without any other change of form or
character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks
to reappear in the mongrels. I have stated that the most probable
hypothesis to account for the reappearance of very ancient characters,
is--that there is a TENDENCY in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen that in
several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the
horse-genus! For myself, I venture confidently to look back thousands
on thousands of generations, and I see an animal striped like a zebra,
but perhaps otherwise very differently constructed, the common parent
of our domestic horse, whether or not it be descended from one or more
wild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents,
but other species of the genus. To admit this view is, as it seems to
me, to reject a real for an unreal, or at least for an unknown, cause.
It makes the works of God a mere mockery and deception; I would almost
as soon believe with the old and ignorant cosmogonists, that fossil
shells had never lived, but had been created in stone so as to mock
the shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case
out of a hundred can we pretend to assign any reason why this or that
part differs, more or less, from the same part in the parents. But
whenever we have the means of instituting a comparison, the same laws
appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between
species of the same genus. The external conditions of life, as climate
and food, etc., seem to have induced some slight modifications. Habit
in producing constitutional differences, and use in strengthening, and
disuse in weakening and diminishing organs, seem to have been more
potent in their effects. Homologous parts tend to vary in the same
way, and homologous parts tend to cohere. Modifications in hard parts
and in external parts sometimes affect softer and internal parts. When
one part is largely developed, perhaps it tends to draw nourishment
from the adjoining parts; and every part of the structure which can be
saved without detriment to the individual, will be saved. Changes of
structure at an early age will generally affect parts subsequently
developed; and there are very many other correlations of growth, the
nature of which we are utterly unable to understand. Multiple parts
are variable in number and in structure, perhaps arising from such
parts not having been closely specialised to any particular function,
so that their modifications have not been closely checked by natural
selection. It is probably from this same cause that organic beings low
in the scale of nature are more variable than those which have their
whole organisation more specialised, and are higher in the scale.
Rudimentary organs, from being useless, will be disregarded by natural
selection, and hence probably are variable. Specific characters--that
is, the characters which have come to differ since the several species
of the same genus branched off from a common parent--are more variable
than generic characters, or those which have long been inherited, and
have not differed within this same period. In these remarks we have
referred to special parts or organs being still variable, because they
have recently varied and thus come to differ; but we have also seen in
the second Chapter that the same principle applies to the whole
individual; for in a district where many species of any genus are
found--that is, where there has been much former variation and
differentiation, or where the manufactory of new specific forms has
been actively at work--there, on an average, we now find most
varieties or incipient species. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
sexes of the same species, and specific differences to the several
species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through
an extraordinary amount of modification since the genus arose; and
thus we can understand why it should often still be variable in a much
higher degree than other parts; for variation is a long-continued and
slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants--which on my view must be a very slow process, requiring a
long lapse of time--in this case, natural selection may readily have
succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters
of their ancient progenitors. Although new and important modifications
may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of
nature.
Whatever the cause may be of each slight difference in the offspring
from their parents--and a cause for each must exist--it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more
important modifications of structure, by which the innumerable beings
on the face of this earth are enabled to struggle with each other, and
the best adapted to survive.
CHAPTER 6. DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so
grave that to this day I can never reflect on them without being
staggered; but, to the best of my judgment, the greater number are
only apparent, and those that are real are not, I think, fatal to my
theory.
These difficulties and objections may be classed under the following
heads:--
Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we
believe that natural selection could produce, on the one hand, organs
of trifling importance, such as the tail of a giraffe, which serves as
a fly-flapper, and, on the other hand, organs of such wonderful
structure, as the eye, of which we hardly as yet fully understand the
inimitable perfection?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct as that
which leads the bee to make cells, which have practically anticipated
the discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism in
separate chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent or other less-favoured forms with which it comes into
competition. Thus extinction and natural selection will, as we have
seen, go hand in hand. Hence, if we look at each species as descended
from some other unknown form, both the parent and all the transitional
varieties will generally have been exterminated by the very process of
formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record;
and I will here only state that I believe the answer mainly lies in
the record being incomparably less perfect than is generally supposed;
the imperfection of the record being chiefly due to organic beings not
inhabiting profound depths of the sea, and to their remains being
embedded and preserved to a future age only in masses of sediment
sufficiently thick and extensive to withstand an enormous amount of
future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur only
rarely, and after enormously long intervals. Whilst the bed of the sea
is stationary or is rising, or when very little sediment is being
deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural collections have been
made only at intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive
intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species have
descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent
and all the transitional varieties between its past and present
states. Hence we ought not to expect at the present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But in
the intermediate region, having intermediate conditions of life, why
do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can be
in large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that almost every
continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have
been formed on strictly continuous areas; though I do not doubt that
the formerly broken condition of areas now continuous has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and
finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains,
and sometimes it is quite remarkable how abruptly, as Alph. De
Candolle has observed, a common alpine species disappears. The same
fact has been noticed by Forbes in sounding the depths of the sea with
the dredge. To those who look at climate and the physical conditions
of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away
insensibly. But when we bear in mind that almost every species, even
in its metropolis, would increase immensely in numbers, were it not
for other competing species; that nearly all either prey on or serve
as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing
physical conditions, but in large part on the presence of other
species, on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are already
defined objects (however they may have become so), not blending one
into another by insensible gradations, the range of any one species,
depending as it does on the range of others, will tend to be sharply
defined. Moreover, each species on the confines of its range, where it
exists in lessened numbers, will, during fluctuations in the number of
its enemies or of its prey, or in the seasons, be extremely liable to
utter extermination; and thus its geographical range will come to be
still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each
has a wide range, with a comparatively narrow neutral territory
between them, in which they become rather suddenly rarer and rarer;
then, as varieties do not essentially differ from species, the same
rule will probably apply to both; and if we in imagination adapt a
varying species to a very large area, we shall have to adapt two
varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist
in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with
varieties in a state of nature. I have met with striking instances of
the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information
given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur,
they are much rarer numerically than the forms which they connect.
Now, if we may trust these facts and inferences, and therefore
conclude that varieties linking two other varieties together have
generally existed in lesser numbers than the forms which they connect,
then, I think, we can understand why intermediate varieties should not
endure for very long periods;--why as a general rule they should be
exterminated and disappear, sooner than the forms which they
originally linked together.
For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on
both sides of it. But a far more important consideration, as I
believe, is that, during the process of further modification, by which
two varieties are supposed on my theory to be converted and perfected
into two distinct species, the two which exist in larger numbers from
inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will always
have a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved. It
is the same principle which, as I believe, accounts for the common
species in each country, as shown in the second chapter, presenting on
an average a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three varieties of
sheep to be kept, one adapted to an extensive mountainous region; a
second to a comparatively narrow, hilly tract; and a third to wide
plains at the base; and that the inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than
the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the
place of the less improved hill breed; and thus the two breeds, which
originally existed in greater numbers, will come into close contact
with each other, without the interposition of the supplanted,
intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are
very slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity of the country can be better
filled by some modification of some one or more of its inhabitants.
And such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants becoming slowly
modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one
time, we ought only to see a few species presenting slight
modifications of structure in some degree permanent; and this
assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties
between the several representative species and their common parent,
must formerly have existed in each broken portion of the land, but
these links will have been supplanted and exterminated during the
process of natural selection, so that they will no longer exist in a
living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these
intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process of
further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they connect;
for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but
the very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found
only amongst fossil remains, which are preserved, as we shall in a
future chapter attempt to show, in an extremely imperfect and
intermittent record.
ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS
AND STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for
instance, a land carnivorous animal could have been converted into one
with aquatic habits; for how could the animal in its transitional
state have subsisted? It would be easy to show that within the same
group carnivorous animals exist having every intermediate grade
between truly aquatic and strictly terrestrial habits; and as each
exists by a struggle for life, it is clear that each is well adapted
in its habits to its place in nature. Look at the Mustela vison of
North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very
little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in closely
allied species of the same genus; and of diversified habits, either
constant or occasional, in the same species. And it seems to me that
nothing less than a long list of such cases is sufficient to lessen
the difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from
others, as Sir J. Richardson has remarked, with the posterior part of
their bodies rather wide and with the skin on their flanks rather
full, to the so-called flying squirrels; and flying squirrels have
their limbs and even the base of the tail united by a broad expanse of
skin, which serves as a parachute and allows them to glide through the
air to an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own country,
by enabling it to escape birds or beasts of prey, or to collect food
more quickly, or, as there is reason to believe, by lessening the
danger from occasional falls. But it does not follow from this fact
that the structure of each squirrel is the best that it is possible to
conceive under all natural conditions. Let the climate and vegetation
change, let other competing rodents or new beasts of prey immigrate,
or old ones become modified, and all analogy would lead us to believe
that some at least of the squirrels would decrease in numbers or
become exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in the
continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of
natural selection, a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also,
furnished with an extensor muscle. Although no graduated links of
structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Lemuridae, yet I can see no difficulty in
supposing that such links formerly existed, and that each had been
formed by the same steps as in the case of the less perfectly gliding
squirrels; and that each grade of structure had been useful to its
possessor. Nor can I see any insuperable difficulty in further
believing it possible that the membrane-connected fingers and fore-arm
of the Galeopithecus might be greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned, would convert
it into a bat. In bats which have the wing-membrane extended from the
top of the shoulder to the tail, including the hind-legs, we perhaps
see traces of an apparatus originally constructed for gliding through
the air rather than for flight.
If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have surmised that birds might have existed
which used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the
land, like the penguin; as sails, like the ostrich; and functionally
for no purpose, like the Apteryx. Yet the structure of each of these
birds is good for it, under the conditions of life to which it is
exposed, for each has to live by a struggle; but it is not necessarily
the best possible under all possible conditions. It must not be
inferred from these remarks that any of the grades of wing-structure
here alluded to, which perhaps may all have resulted from disuse,
indicate the natural steps by which birds have acquired their perfect
power of flight; but they serve, at least, to show what diversified
means of transition are possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing
that we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they had
been inhabitants of the open ocean, and had used their incipient
organs of flight exclusively, as far as we know, to escape being
devoured by other fish?
When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural
selection. Furthermore, we may conclude that transitional grades
between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many
subordinate forms, for taking prey of many kinds in many ways, on the
land and in the water, until their organs of flight had come to a high
stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition
will always be less, from their having existed in lesser numbers, than
in the case of species with fully developed structures.
I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case
occurs, it would be easy for natural selection to fit the animal, by
some modification of its structure, for its changed habits, or
exclusively for one of its several different habits. But it is
difficult to tell, and immaterial for us, whether habits generally
change first and structure afterwards; or whether slight modifications
of structure lead to changed habits; both probably often change almost
simultaneously. Of cases of changed habits it will suffice merely to
allude to that of the many British insects which now feed on exotic
plants, or exclusively on artificial substances. Of diversified habits
innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over
one spot and then proceeding to another, like a kestrel, and at other
times standing stationary on the margin of water, and then dashing
like a kingfisher at a fish. In our own country the larger titmouse
(Parus major) may be seen climbing branches, almost like a creeper; it
often, like a shrike, kills small birds by blows on the head; and I
have many times seen and heard it hammering the seeds of the yew on a
branch, and thus breaking them like a nuthatch. In North America the
black bear was seen by Hearne swimming for hours with widely open
mouth, thus catching, like a whale, insects in the water. Even in so
extreme a case as this, if the supply of insects were constant, and if
better adapted competitors did not already exist in the country, I can
see no difficulty in a race of bears being rendered, by natural
selection, more and more aquatic in their structure and habits, with
larger and larger mouths, till a creature was produced as monstrous as
a whale.
As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other
species of the same genus, we might expect, on my theory, that such
individuals would occasionally have given rise to new species, having
anomalous habits, and with their structure either slightly or
considerably modified from that of their proper type. And such
instances do occur in nature. Can a more striking instance of
adaptation be given than that of a woodpecker for climbing trees and
for seizing insects in the chinks of the bark? Yet in North America
there are woodpeckers which feed largely on fruit, and others with
elongated wings which chase insects on the wing; and on the plains of
La Plata, where not a tree grows, there is a woodpecker, which in
every essential part of its organisation, even in its colouring, in
the harsh tone of its voice, and undulatory flight, told me plainly of
its close blood-relationship to our common species; yet it is a
woodpecker which never climbs a tree!
Petrels are the most aerial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, its manner of swimming,
and of flying when unwillingly it takes flight, would be mistaken by
any one for an auk or grebe; nevertheless, it is essentially a petrel,
but with many parts of its organisation profoundly modified. On the
other hand, the acutest observer by examining the dead body of the
water-ouzel would never have suspected its sub-aquatic habits; yet
this anomalous member of the strictly terrestrial thrush family wholly
subsists by diving,--grasping the stones with its feet and using its
wings under water.
He who believes that each being has been created as we now see it,
must occasionally have felt surprise when he has met with an animal
having habits and structure not at all in agreement. What can be
plainer than that the webbed feet of ducks and geese are formed for
swimming? yet there are upland geese with webbed feet which rarely or
never go near the water; and no one except Audubon has seen the
frigate-bird, which has all its four toes webbed, alight on the
surface of the sea. On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by membrane. What seems
plainer than that the long toes of grallatores are formed for walking
over swamps and floating plants, yet the water-hen is nearly as
aquatic as the coot; and the landrail nearly as terrestrial as the
quail or partridge. In such cases, and many others could be given,
habits have changed without a corresponding change of structure. The
webbed feet of the upland goose may be said to have become rudimentary
in function, though not in structure. In the frigate-bird, the
deeply-scooped membrane between the toes shows that structure has
begun to change.
He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me
only restating the fact in dignified language. He who believes in the
struggle for existence and in the principle of natural selection, will
acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being vary ever so little,
either in habits or structure, and thus gain an advantage over some
other inhabitant of the country, it will seize on the place of that
inhabitant, however different it may be from its own place. Hence it
will cause him no surprise that there should be geese and
frigate-birds with webbed feet, either living on the dry land or most
rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should
be woodpeckers where not a tree grows; that there should be diving
thrushes, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest possible degree. Yet reason
tells me, that if numerous gradations from a perfect and complex eye
to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist; if further, the eye does vary ever
so slightly, and the variations be inherited, which is certainly the
case; and if any variation or modification in the organ be ever useful
to an animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, can hardly be
considered real. How a nerve comes to be sensitive to light, hardly
concerns us more than how life itself first originated; but I may
remark that several facts make me suspect that any sensitive nerve may
be rendered sensitive to light, and likewise to those coarser
vibrations of the air which produce sound.
In looking for the gradations by which an organ in any species has
been perfected, we ought to look exclusively to its lineal ancestors;
but this is scarcely ever possible, and we are forced in each case to
look to species of the same group, that is to the collateral
descendants from the same original parent-form, in order to see what
gradations are possible, and for the chance of some gradations having
been transmitted from the earlier stages of descent, in an unaltered
or little altered condition. Amongst existing Vertebrata, we find but
a small amount of gradation in the structure of the eye, and from
fossil species we can learn nothing on this head. In this great class
we should probably have to descend far beneath the lowest known
fossiliferous stratum to discover the earlier stages, by which the eye
has been perfected.
In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this
low stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets,
within each of which there is a lens-shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and
which properly act only by excluding lateral pencils of light, are
convex at their upper ends and must act by convergence; and at their
lower ends there seems to be an imperfect vitreous substance. With
these facts, here far too briefly and imperfectly given, which show
that there is much graduated diversity in the eyes of living
crustaceans, and bearing in mind how small the number of living
animals is in proportion to those which have become extinct, I can see
no very great difficulty (not more than in the case of many other
structures) in believing that natural selection has converted the
simple apparatus of an optic nerve merely coated with pigment and
invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit
that a structure even as perfect as the eye of an eagle might be
formed by natural selection, although in this case he does not know
any of the transitional grades. His reason ought to conquer his
imagination; though I have felt the difficulty far too keenly to be
surprised at any degree of hesitation in extending the principle of
natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We
know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with a nerve sensitive to
light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers
of different densities and thicknesses, placed at different distances
from each other, and with the surfaces of each layer slowly changing
in form. Further we must suppose that there is a power always intently
watching each slight accidental alteration in the transparent layers;
and carefully selecting each alteration which, under varied
circumstances, may in any way, or in any degree, tend to produce a
distincter image. We must suppose each new state of the instrument to
be multiplied by the million; and each to be preserved till a better
be produced, and then the old ones to be destroyed. In living bodies,
variation will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out with
unerring skill each improvement. Let this process go on for millions
on millions of years; and during each year on millions of individuals
of many kinds; and may we not believe that a living optical instrument
might thus be formed as superior to one of glass, as the works of the
Creator are to those of man?
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to my theory, there has been much
extinction. Or again, if we look to an organ common to all the members
of a large class, for in this latter case the organ must have been
first formed at an extremely remote period, since which all the many
members of the class have been developed; and in order to discover the
early transitional grades through which the organ has passed, we
should have to look to very ancient ancestral forms, long since become
extinct.
We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus the
alimentary canal respires, digests, and excretes in the larva of the
dragon-fly and in the fish Cobites. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might easily
specialise, if any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone, and thus wholly
change its nature by insensible steps. Two distinct organs sometimes
perform simultaneously the same function in the same individual; to
give one instance, there are fish with gills or branchiae that breathe
the air dissolved in the water, at the same time that they breathe
free air in their swimbladders, this latter organ having a ductus
pneumaticus for its supply, and being divided by highly vascular
partitions. In these cases, one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then
this other organ might be modified for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a wholly different purpose, namely respiration. The
swimbladder has, also, been worked in as an accessory to the auditory
organs of certain fish, or, for I do not know which view is now
generally held, a part of the auditory apparatus has been worked in as
a complement to the swimbladder. All physiologists admit that the
swimbladder is homologous, or "ideally similar," in position and
structure with the lungs of the higher vertebrate animals: hence there
seems to me to be no great difficulty in believing that natural
selection has actually converted a swimbladder into a lung, or organ
used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiae have wholly
disappeared--the slits on the sides of the neck and the loop-like
course of the arteries still marking in the embryo their former
position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some
quite distinct purpose: in the same manner as, on the view entertained
by some naturalists that the branchiae and dorsal scales of Annelids
are homologous with the wings and wing-covers of insects, it is
probable that organs which at a very ancient period served for
respiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that
I will give one more instance. Pedunculated cirripedes have two minute
folds of skin, called by me the ovigerous frena, which serve, through
the means of a sticky secretion, to retain the eggs until they are
hatched within the sack. These cirripedes have no branchiae, the whole
surface of the body and sack, including the small frena, serving for
respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the
sack, in the well-enclosed shell; but they have large folded
branchiae. Now I think no one will dispute that the ovigerous frena in
the one family are strictly homologous with the branchiae of the other
family; indeed, they graduate into each other. Therefore I do not
doubt that little folds of skin, which originally served as ovigerous
frena, but which, likewise, very slightly aided the act of
respiration, have been gradually converted by natural selection into
branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes
had become extinct, and they have already suffered far more extinction
than have sessile cirripedes, who would ever have imagined that the
branchiae in this latter family had originally existed as organs for
preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible
to conceive by what steps these wondrous organs have been produced;
but, as Owen and others have remarked, their intimate structure
closely resembles that of common muscle; and as it has lately been
shown that Rays have an organ closely analogous to the electric
apparatus, and yet do not, as Matteuchi asserts, discharge any
electricity, we must own that we are far too ignorant to argue that no
transition of any kind is possible.
The electric organs offer another and even more serious difficulty;
for they occur in only about a dozen fishes, of which several are
widely remote in their affinities. Generally when the same organ
appears in several members of the same class, especially if in members
having very different habits of life, we may attribute its presence to
inheritance from a common ancestor; and its absence in some of the
members to its loss through disuse or natural selection. But if the
electric organs had been inherited from one ancient progenitor thus
provided, we might have expected that all electric fishes would have
been specially related to each other. Nor does geology at all lead to
the belief that formerly most fishes had electric organs, which most
of their modified descendants have lost. The presence of luminous
organs in a few insects, belonging to different families and orders,
offers a parallel case of difficulty. Other cases could be given; for
instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with a sticky gland at the end,
is the same in Orchis and Asclepias,--genera almost as remote as
possible amongst flowering plants. In all these cases of two very
distinct species furnished with apparently the same anomalous organ,
it should be observed that, although the general appearance and
Back to Full Books
|