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.
2 How much direct effect difference of climate, food,
&c., 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, &c.:
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.
3 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.
4 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.
5 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.
6 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.
7 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.
8 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.
9 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.
10 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.
11 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.
12 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.
13 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, &c., 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 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, &c.), that
these are likewise the most abnormal in their teeth.
20 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.
21 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.
22 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.
23 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.
24 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.
25 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.
26 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 Prof. 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.
27 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.
28 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.
29 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.
30 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, &c., 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.
31 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.
32 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.
33 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.
34 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.
35 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.
36 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.
37 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.
38 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.
39 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.
40 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.
41 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.
42 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
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.
43 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.
44 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.
45 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, &c., inhabiting the most distant
parts of the world.
46 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.
47 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.
48 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.
49 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, &c.,
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.
50 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.