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 {132} 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, &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.
The fact of varieties of one species, when they range {133} 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 {134} the conditions of
life 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 {135} 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 me 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 frequently blown to sea and perish;
that the beetles in Madeira, as observed by Mr. Wollaston, lie much
concealed, {136} until the wind lulls and the sun shines; that the
proportion of wingless beetles is larger on the exposed Desertas 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 would 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. {137}
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 {138} 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 Schiödte 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 Schiödte 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 antennæ 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 {139} 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, &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. {140} 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 {141} 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 Encyclopædias of China, to be very
{142} 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, {143} 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 variations.
_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 {144} 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 covering,
viz. Cetacea (whales) and Edentata (armadilloes, scaly anteaters, &c.),
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 {145} 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 Compositæ countenances this idea;
but, in the case of the corolla of the Umbelliferæ, 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 Compositæ 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 {146} 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
Umbelliferæ 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 {147} 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 {148} 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 antennæ. 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. {149}
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 vertebræ 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.
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 {150} 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 {151} 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 {152} 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 {153} 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 {154} 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 {155} 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 {156} 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 {157} 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 Engidæ, 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 {158} 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 slight 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 having been thus adapted for secondary
sexual, and for ordinary specific purposes. {159}
_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 {160} 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 {161} 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 {162} 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 {163}
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 the 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 {164} 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 racehorse 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 {165} 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.
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: according to Mr. Gosse, in
certain parts of the United States about nine out of ten mules have striped
legs. I once saw a mule with its legs so much striped that any one would at
first 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 Morton'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 his 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 {166} 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 {167}
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 {168} 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; {169} 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 {170} 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.
* * * * *
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