LAWS OF VARIATION.
Effects of external conditions. Use and disuse, combined with natural
selection; organs of flight and of vision. Acclimatisation. Correlation
of growth. Compensation and economy of growth. False correlations.
Multiple, rudimentary, and lowly organised structures variable. Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable.
Species of the same genus vary in an analogous manner. Reversions to
long lost characters. Summary.
I have hitherto sometimes spoken as if the variations—so common and
multiform in organic beings under domestication, and in a lesser degree
in those in a state of nature—had been due to chance. This, of course,
is a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or very slight deviations of structure,
as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have remarked
in the first chapter—but a long catalogue of facts which cannot be here
given would be necessary to show the truth of the remark—that the
reproductive system is eminently susceptible to changes in the
conditions of life; and to
this system being functionally disturbed in the parents, I chiefly
attribute the varying or plastic condition of the offspring. The male
and female sexual elements seem to be affected before that union takes
place which is to form a new being. In the case of “sporting” plants,
the bud, which in its earliest condition does not apparently differ
essentially from an ovule, is alone affected. But why, because the
reproductive system is disturbed, this or that part should vary more or
less, we are profoundly ignorant. Nevertheless, we can here and there
dimly catch a faint ray of light, and we may feel sure that there must
be some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that
of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on islands
or near the coast. So with insects, Wollaston is convinced that
residence near the sea affects their colours. Moquin-Tandon gives a
list of plants which when growing near the sea-shore have their leaves
in some degree fleshy, though not elsewhere fleshy. Several other such
cases could be given.
The fact of varieties of one species, when they range
into the zone of habitation of other species, often acquiring in a very
slight degree some of the characters of such species, accords with our
view that species of all kinds are only well-marked and permanent
varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined to
cold and deeper seas. The birds which are confined to continents are,
according to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during many
generations, and how much to the direct action of the severe climate?
for it would appear that climate has some direct action on the hair of
our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly the
conditions of life must act. Again, innumerable instances are known to
every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.
_Effects of Use and Disuse_.—From the facts alluded to in the first
chapter, I think there can be little doubt that use in our domestic
animals strengthens and enlarges certain parts, and disuse diminishes
them; and that such modifications are inherited. Under free nature, we
can have no standard of comparison, by which to judge of the effects of
long-continued use or disuse, for we know not the parent-forms; but
many animals have structures which can be explained by the effects of
disuse. As Professor Owen has remarked, there is no greater anomaly in
nature than a bird that cannot fly; yet there are several in this
state. The logger-headed duck of South America can only flap along the
surface of the water, and has its wings in nearly the same condition as
the domestic Aylesbury duck. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly wingless
condition of several birds, which now inhabit or have lately inhabited
several oceanic islands, tenanted by no beast of prey, has been caused
by disuse. The ostrich indeed inhabits continents and is exposed to
danger from which it cannot escape by flight, but by kicking it can
defend itself from enemies, as well as any of the smaller
quadrupeds. We may imagine that the early progenitor of the ostrich had
habits like those of a bustard, and that as natural selection increased
in successive generations the size and weight of its body, its legs
were used more, and its wings less, until they became incapable of
flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described as
not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient evidence
to induce us to believe that mutilations are ever inherited; and I
should prefer explaining the entire absence of the anterior tarsi in
Ateuchus, and their rudimentary condition in some other genera, by the
long-continued effects of disuse in their progenitors; for as the tarsi
are almost always lost in many dung-feeding beetles, they must be lost
early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are very
frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr. Wollaston, lie much concealed,
until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira
itself; and especially the extraordinary fact, so strongly insisted on
by Mr. Wollaston, of the almost entire absence of certain large groups
of beetles, elsewhere excessively numerous, and which groups have
habits of life almost necessitating frequent flight;—these several
considerations have made me believe that the wingless condition of so
many Madeira beetles is mainly due to the action of natural selection,
but combined probably with disuse. For during thousands of successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight will oftenest have been blown to sea and
thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects, their
wings not at all reduced, but even enlarged. This is quite compatible
with the action of natural selection. For when a new insect first
arrived on the island, the tendency of natural selection to enlarge or
to reduce the wings, would depend on whether a greater number of
individuals were saved by successfully battling with the winds, or by
giving up the attempt and rarely or never flying. As with mariners
shipwrecked near a coast, it would have been better for the good
swimmers if they had been able to swim still further, whereas it would
have been better for the bad swimmers if they had not been able to swim
at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse, but
aided perhaps by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its
habits than the mole; and I was assured by a Spaniard, who had often
caught them, that they were frequently blind; one which I kept alive
was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not indispensable to animals with subterranean habits, a
reduction in their size with the adhesion of the eyelids and growth of
fur over them, might in such case be an advantage; and if so, natural
selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the eye
is gone; the stand for the telescope is there, though the telescope
with its glasses has been lost. As it is difficult to imagine that
eyes, though useless, could be in any way injurious to animals living
in darkness, I attribute their loss wholly to disuse. In one of the
blind animals, namely, the cave-rat, the eyes are of immense size; and
Professor Silliman thought that it regained, after living some days in
the light, some slight power of vision. In the same manner as in
Madeira the wings of some of the insects have been enlarged, and the
wings of others have been reduced by natural selection aided by use and
disuse, so in the case of the cave-rat natural selection seems to have
struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common
view of the blind animals having been separately created for the
American and European caverns, close similarity in their organisation
and affinities might have been expected; but, as 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
Professor Dana; and some of the European cave-insects are very closely
allied to those of the surrounding country. It would be most difficult
to give any rational explanation of the affinities of the blind
cave-animals to the other inhabitants of the two continents on the
ordinary view of their independent creation. That several of the
inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of
their other productions. Far from feeling any surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the inhabitants of these
dark abodes will probably have been exposed.
_Acclimatisation_.—Habit is hereditary with plants, as in the period of
flowering, in the amount of rain requisite for seeds to germinate, in
the time of sleep, etc., and this leads me to say a few words on
acclimatisation. As it is extremely common for species of the same
genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single
parent, if this view be correct, acclimatisation must be readily
effected during long-continued descent. It is notorious that each
species is adapted to the climate of its own home: species from an
arctic or even from a temperate region cannot endure a tropical
climate, or conversely. So again, many succulent plants cannot endure a
damp climate. But the degree of adaptation of species to the climates
under which they live is often overrated.
We may infer this from our frequent inability to predict whether or not
an imported plant will endure our climate, and from the number of
plants and animals brought from warmer countries which here enjoy good
health. We have reason to believe that species in a state of nature are
limited in their ranges by the competition of other organic beings
quite as much as, or more than, by adaptation to particular climates.
But whether or not the adaptation be generally very close, we have
evidence, in the case of some few plants, of their becoming, to a
certain extent, naturally habituated to different temperatures, or
becoming acclimatised: thus the pines and rhododendrons, raised from
seed collected by Dr. Hooker from trees growing at different heights on
the Himalaya, were found in this country to possess different
constitutional powers of resisting cold. Mr. Thwaites informs me that
he has observed similar facts in Ceylon, and analogous observations
have been made by Mr. H. C. Watson on European species of plants
brought from the Azores to England. In regard to animals, several
authentic cases could be given of species within historical times
having largely extended their range from warmer to cooler latitudes,
and conversely; but we do not positively know that these animals were
strictly adapted to their native climate, but in all ordinary cases we
assume such to be the case; nor do we know that they have subsequently
become acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly
fertile (a far severer test) under them, may be used as an argument
that a large proportion of other animals, now in a state of nature,
could easily be brought to bear widely different climates. We must not,
however, push the foregoing argument too far, on account of the
probable origin of some of our domestic animals from several wild
stocks: the blood, for instance, of a tropical and arctic wolf or wild
dog may perhaps be mingled in our domestic breeds. The rat and mouse
cannot be considered as domestic animals, but they have been
transported by man to many parts of the world, and now have a far wider
range than any other rodent, living free under the cold climate of
Faroe in the north and of the Falklands in the south, and on many
islands in the torrid zones. Hence I am inclined to look at adaptation
to any special climate as a quality readily grafted on an innate wide
flexibility of constitution, which is common to most animals. On this
view, the capacity of enduring the most different climates by man
himself and by his domestic animals, and such facts as that former
species of the elephant and rhinoceros were capable of enduring a
glacial climate, whereas the living species are now all tropical or
sub-tropical in their habits, ought not to be looked at as anomalies,
but merely as examples of a very common flexibility of constitution,
brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopædias
of China, to be very cautious
in transposing animals from one district to another; for it is not
likely that man should have succeeded in selecting so many breeds and
sub-breeds with constitutions specially fitted for their own districts:
the result must, I think, be due to habit. On the other hand, I can see
no reason to doubt that natural selection will continually tend to
preserve those individuals which are born with constitutions best
adapted to their native countries. In treatises on many kinds of
cultivated plants, certain varieties are said to withstand certain
climates better than others: this is very strikingly shown in works on
fruit trees published in the United States, in which certain varieties
are habitually recommended for the northern, and others for the
southern States; and as most of these varieties are of recent origin,
they cannot owe their constitutional differences to habit. The case of
the Jerusalem artichoke, which is never propagated by seed, and of
which consequently new varieties have not been produced, has even been
advanced—for it is now as tender as ever it was—as proving that
acclimatisation cannot be effected! The case, also, of the kidney-bean
has been often cited for a similar purpose, and with much greater
weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by
frost, and then collect seed from the few survivors, with care to
prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.
On the whole, I think we may conclude that habit,
use, and disuse, have, in some cases, played a considerable part in the
modification of the constitution, and of the structure of various
organs; but that the effects of use and disuse have often been largely
combined with, and sometimes overmastered by, the natural selection of
innate differences.
_Correlation of Growth_.—I mean by this expression that the whole
organisation is so tied together during its growth and development,
that when slight variations in any one part occur, and are accumulated
through natural selection, other parts become modified. This is a very
important subject, most imperfectly understood. The most obvious case
is, that modifications accumulated solely for the good of the young or
larva, will, it may safely be concluded, affect the structure of the
adult; in the same manner as any malconformation affecting the early
embryo, seriously affects the whole organisation of the adult. The
several parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the same
manner; in the front and hind legs, and even in the jaws and limbs,
varying together, for the lower jaw is believed to be homologous with
the limbs. These tendencies, I do not doubt, may be mastered more or
less completely by natural selection: thus a family of stags once
existed with an antler only on one side; and if this had been of any
great use to the breed it might probably have been rendered permanent
by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of
the petals of the corolla into a tube. Hard parts seem to affect the
form of adjoining soft parts; it is believed by some authors that the
diversity in the shape of the pelvis in birds causes the remarkable
diversity in the shape of their kidneys. Others believe that the shape
of the pelvis in the human mother influences by pressure the shape of
the head of the child. In snakes, according to Schlegel, the shape of
the body and the manner of swallowing determine the position of several
of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure.
M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist,
without our being able to assign any reason. What can be more singular
than the relation between blue eyes and deafness in cats, and the
tortoise-shell colour with the female sex; the feathered feet and skin
between the outer toes in pigeons, and the presence of more or less
down on the young birds when first hatched, with the future colour of
their plumage; or, again, the relation between the hair and teeth in
the naked Turkish dog, though here probably homology comes into play?
With respect to this latter case of correlation, I think it can hardly
be accidental, that if we pick out the two orders of mammalia which are
most abnormal in their dermal coverings, viz. Cetacea (whales) and
Edentata (armadilloes, scaly ant-eaters, etc.), that these are likewise
the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility
and, therefore, of natural selection, than that of the difference
between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the
difference in the ray and central florets of, for instance, the daisy,
and this difference is often accompanied with the abortion of parts of
the flower. But, in some Compositous plants, the seeds also differ in
shape and sculpture; and even the ovary itself, with its accessory
parts, differs, as has been described by Cassini. These differences
have been attributed by some authors to pressure, and the shape of the
seeds in the ray-florets in some 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
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
seeds which were a little better fitted to be wafted further, might get
an advantage over those producing seed less fitted for dispersal; and
this process could not possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, “in order to spend on one side, nature is forced to
economise on the other side.” I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to fatten
readily. The same varieties of the cabbage do not yield abundant and
nutritious foliage and a copious supply of oil-bearing seeds. When the
seeds in our fruits become atrophied, the fruit itself gains largely in
size and quality. In our poultry, a large tuft of feathers on the head
is generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying
to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure. I can
thus only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another and is thus
protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary
manner with the Proteolepas: for the carapace in all other cirripedes
consists of the three highly-important anterior segments of the head
enormously developed, and furnished with great nerves and muscles; but
in the parasitic and protected Proteolepas, the whole anterior part of
the head is reduced to the merest rudiment attached to the bases of the
prehensile 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.
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 Professor
Owen’s expression, seems to be a sign of low organisation; the
foregoing remark seems connected with the very general opinion of
naturalists, that beings low in the scale of nature are more variable
than those which are higher. I presume that lowness in this case means
that the several parts of the organisation have been but little
specialised for particular functions; and as long as the same part has
to perform diversified work, we can perhaps see why it should remain
variable, that is, why natural selection should have preserved or
rejected each little deviation of form less carefully than when the
part has to serve for one special purpose alone. In the same way that a
knife which has to cut all sorts of things may be of almost any shape;
whilst a tool for some particular object had better be of some
particular shape. Natural selection, it should never be forgotten, can
act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to
the general subject of rudimentary and aborted organs; and I will here
only add that their variability seems to be owing to their uselessness,
and therefore to natural selection having no power to check deviations
in their structure. Thus
rudimentary parts are left to the free play of the various laws of
growth, to the effects of long-continued disuse, and to the tendency to
reversion.
_A part developed in any species in an extraordinary degree or manner,
in comparison with the same part in allied species, tends to be highly
variable_.—Several years ago I was much struck with a remark, nearly to
the above effect, published by Mr. Waterhouse. I infer also from an
observation made by Professor Owen, with respect to the length of the
arms of the ourang-outang, that he has come to a nearly similar
conclusion. It is hopeless to attempt to convince any one of the truth
of this proposition without giving the long array of facts which I have
collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowance for
them. It should be understood that the rule by no means applies to any
part, however unusually developed, unless it be unusually developed in
comparison with the same part in closely allied species. Thus, the
bat’s wing is a most abnormal structure in the class mammalia; but the
rule would not here apply, because there is a whole group of bats
having wings; it would apply only if some one species of bat had its
wings developed in some remarkable manner in comparison with the other
species of the same genus. The rule applies very strongly in the case
of secondary sexual characters, when displayed in any unusual manner.
The term, secondary sexual characters, used by Hunter, applies to
characters which are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and
females; but as females more rarely offer remarkable secondary sexual
characters, it applies
more rarely to them. The rule being so plainly applicable in the case
of secondary sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual manner—of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse’s remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of
several of the species is so great, that it is no exaggeration to state
that the varieties differ more from each other in the characters of
these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly
to hold good in this class. I cannot make out that it applies to
plants, and this would seriously have shaken my belief in its truth,
had not the great variability in plants made it particularly difficult
to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or
manner in any species, the fair
presumption is that it is of high importance to that species;
nevertheless the part in this case is eminently liable to variation.
Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see
no explanation. But on the view that groups of species have descended
from other species, and have been modified through natural selection, I
think we can obtain some light. In our domestic animals, if any part,
or the whole animal, be neglected and no selection be applied, that
part (for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a nearly uniform character. The breed will then be
said to have degenerated. In rudimentary organs, and in those which
have been but little specialised for any particular purpose, and
perhaps in polymorphic groups, we see a nearly parallel natural case;
for in such cases natural selection either has not or cannot come into
full play, and thus the organisation is left in a fluctuating
condition. But what here more especially concerns us is, that in our
domestic animals those points, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the breeds of the pigeon; see what a prodigious
amount of difference there is in the beak of the different tumblers, in
the beak and wattle of the different carriers, in the carriage and tail
of our fantails, etc., these being the points now mainly attended to by
English fanciers. Even in the sub-breeds, as in the short-faced
tumbler, it is notoriously difficult to breed them nearly to
perfection, and frequently individuals are born which depart widely
from the standard. There may be truly said to be a constant struggle
going on between, on the one hand, the tendency to reversion to a less
modified state, as well as an innate tendency to further
variability of all kinds, and, on the other hand, the power of steady
selection to keep the breed true. In the long run selection gains the
day, and we do not expect to fail so far as to breed a bird as coarse
as a common tumbler from a good short-faced strain. But as long as
selection is rapidly going on, there may always be expected to be much
variability in the structure undergoing modification. It further
deserves notice that these variable characters, produced by man’s
selection, sometimes become attached, from causes quite unknown to us,
more to one sex than to the other, generally to the male sex, as with
the wattle of carriers and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has undergone
an extraordinary amount of modification, since the period when the
species branched off from the common progenitor of the genus. This
period will seldom be remote in any extreme degree, as species very
rarely endure for more than one geological period. An extraordinary
amount of modification implies an unusually large and long-continued
amount of variability, which has continually been accumulated by
natural selection for the benefit of the species. But as the
variability of the extraordinarily-developed part or organ has been so
great and long-continued within a period not excessively remote, we
might, as a general rule, expect still to find more variability in such
parts than in other parts of the organisation, which have remained for
a much longer period nearly constant. And this, I am convinced, is the
case. That the struggle between natural selection on the one hand, and
the tendency to reversion and variability on the other hand, will in
the
course of time cease; and that the most abnormally developed organs may
be made constant, I can see no reason to doubt. Hence when an organ,
however abnormal it may be, has been transmitted in approximately the
same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to my theory, for an
immense period in nearly the same state; and thus it comes to be no
more variable than any other structure. It is only in those cases in
which the modification has been comparatively recent and
extraordinarily great that we ought to find the _generative
variability_, as it may be called, still present in a high degree. For
in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner
and degree, and by the continued rejection of those tending to revert
to a former and less modified condition.
The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic. To
explain by a simple example what is meant. If some species in a large
genus of plants had blue flowers and some had red, the colour would be
only a specific character, and no one would be surprised at one of the
blue species varying into red, or conversely; but if all the species
had blue flowers, the colour would become a generic character, and its
variation would be a more unusual circumstance. I have chosen this
example because an explanation is not in this case applicable, which
most naturalists would advance, namely, that specific characters are
more variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera.
I believe this explanation is partly, yet only indirectly, true; I
shall, however, have to return
to this subject in our chapter on Classification. It would be almost
superfluous to adduce evidence in support of the above statement, that
specific characters are more variable than generic; but I have
repeatedly noticed in works on natural history, that when an author has
remarked with surprise that some _important_ organ or part, which is
generally very constant throughout large groups of species, has
_differed_ considerably in closely-allied species, that it has, also,
been _variable_ in the individuals of some of the species. And this
fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes
variable, though its physiological importance may remain the same.
Something of the same kind applies to monstrosities: at least Is.
Geoffroy St. Hilaire seems to entertain no doubt, that the more an
organ normally differs in the different species of the same group, the
more subject it is to individual anomalies.
On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same part
in other independently-created species of the same genus, be more
variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the case
in another manner:—the points in which all the species of a genus
resemble each other, and in which they differ from the species of some
other genus, are called generic characters; and these characters in
common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection will
have modified several species, fitted to more or less widely-different
habits, in exactly the same manner: and as these so-called generic
characters have been inherited from a remote period, since that period
when the species first branched off from their common progenitor, and
subsequently have not varied or come to differ in any degree, or only
in a slight degree, it is not probable that they should vary at the
present day. On the other hand, the points in which species differ from
other species of the same genus, are called specific characters; and as
these specific characters have varied and come to differ within the
period of the branching off of the species from a common progenitor, it
is probable that they should still often be in some degree variable,—at
least more variable than those parts of the organisation which have for
a very long period remained constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which
is less rigid in its action than ordinary selection, as it does not
entail death, but only gives fewer offspring to the less favoured
males. Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had
a wide scope for action, and may thus readily have succeeded in giving
to the species of the same group a greater amount of difference in
their sexual characters, than in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the
same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature, the
relation can hardly be accidental. The same number of joints in the
tarsi is a character generally common to very large groups of beetles,
but in the 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 the several species to their several places in the economy
of nature, and likewise to fit the two sexes of the same species to
each other, or to fit the males and females to different habits of
life, or the males to struggle with other males for the possession of
the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;—that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in
its congeners; and the not great degree of variability in a part,
however extraordinarily it may be developed, if it be common to a whole
group of species;—that the great variability of secondary sexual
characters, and the great amount of difference in these same characters
between closely allied species;—that secondary sexual and ordinary
specific differences are generally displayed in the same parts of the
organisation,—are all principles closely connected together. All being
mainly due to the species of the same group having descended from a
common progenitor, from whom they have inherited much in common,—to
parts which have recently and largely varied being more likely still to
go on varying than parts which have long been inherited and have not
varied,—to natural selection having more or less completely, according
to the lapse of time, overmastered the tendency to reversion and to
further variability,—to sexual selection being less rigid than ordinary
selection,—and to variations in the same parts having been accumulated
by natural and sexual selection, and thus adapted for secondary sexual,
and for ordinary specific purposes.
_Distinct species present analogous variations; and a variety of one
species often assumes some of the characters of an allied species, or
reverts to some of the characters of an early progenitor_.—These
propositions will be most readily understood by looking to our domestic
races. The most distinct breeds of pigeons, in countries most widely
apart, present sub-varieties with reversed feathers on the head and
feathers on the feet,—characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter, may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on
by similar unknown influences. In the vegetable kingdom we have a case
of analogous variation, in the enlarged stems, or roots as commonly
called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white
rump, a bar at the end of the tail, with the outer feathers externally
edged near their bases with white. As all these marks are
characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I think confidently
come to this conclusion, because, as we have seen, these coloured marks
are eminently liable to appear in the crossed offspring of two distinct
and differently coloured breeds; and in this case there is nothing in
the external conditions of life to cause the reappearance of the
slaty-blue, with the several marks, beyond the influence of the mere
act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations—some say, for a dozen or even a
score of generations. After twelve generations, the proportion of
blood, to use a common expression, of any one ancestor, is only 1 in
2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this very small proportion of foreign blood.
In a breed which has not been crossed, but in which _both_ parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might be, as
was formerly remarked, for all that we can see to the contrary,
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations
distant, but that in each successive generation there has been a
tendency to reproduce the character in question, which at last, under
unknown favourable conditions, gains an ascendancy. For instance, it is
probable that in each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character
being inherited for an endless number of generations, than in quite
useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce
a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an unimportant
nature, for the presence of all important characters will be governed
by natural selection, in accordance with the diverse habits of the
species, and will not be left to the mutual action of the conditions of
life and of a similar inherited constitution. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to lost ancestral characters. As, however, we never know the
exact character of the common ancestor of a group, we could not
distinguish these two
cases: if, for instance, we did not know that the rock-pigeon was not
feather-footed or turn-crowned, we could not have told, whether these
characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blueness was a case of
reversion, from the number of the markings, which are correlated with
the blue tint, and which it does not appear probable would all appear
together from simple variation. More especially we might have inferred
this, from the blue colour and marks so often appearing when distinct
breeds of diverse colours are crossed. Hence, though under nature it
must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous
variations, yet we ought, on my theory, sometimes to find the varying
offspring of a species assuming characters (either from reversion or
from analogous variation) which already occur in some other members of
the same group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable species
in our systematic works, is due to its varieties mocking, as it were,
some of the other species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two other forms,
which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is afforded by parts or organs of an important
and uniform nature occasionally varying so as to acquire, in some
degree, the character of the same part or organ in an allied species. I
have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and
seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those on the legs of a
zebra: it has been asserted that these are plainest in the foal, and
from inquiries which I have made, I believe this to be true. It has
also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and
outline. A white ass, but _not_ an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
Blyth and others, occasionally appear: and I have been informed by
Colonel Poole that the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but Dr.
Gray has figured one specimen with very distinct zebra-like bars on the
hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of _all_
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut: a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a
bay horse. My son made a careful examination and sketch for me of a dun
Belgian cart-horse with a double stripe on each shoulder and with
leg-stripes; and a man, whom I can implicitly trust, has examined for
me a small dun Welch pony with _three_ short parallel stripes on each
shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined the
breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and bay
Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago in
the south. In all parts of the world these stripes occur far oftenest
in duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to
cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species—one of which, the dun, was striped; and
that the above-described appearances are all due to ancient
crosses with the dun stock. But I am not at all satisfied with this
theory, and should be loth to apply it to breeds so distinct as the
heavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race,
etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and
horse is particularly apt to have bars on its legs. I once saw a mule
with its legs so much striped that any one at first would have thought
that it must have been the product of a zebra; and Mr. W. C. Martin, in
his excellent treatise on the horse, has given a figure of a similar
mule. In four coloured drawings, which I have seen, of hybrids between
the ass and zebra, the legs were much more plainly barred than the rest
of the body; and in one of them there was a double shoulder-stripe. In
Lord Moreton’s famous hybrid from a chestnut mare and male quagga, the
hybrid, and even the pure offspring subsequently produced from the mare
by a black Arabian sire, were much more plainly barred across the legs
than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass seldom has stripes on its legs and the
hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those
on the dun Welch pony, and even had some zebra-like stripes on the
sides of its face. With respect to this last fact, I was so convinced
that not even a stripe of colour appears from what would commonly be
called an accident, that I was led solely from the occurrence of the
face-stripes on this hybrid from the ass and hemionus,
to ask Colonel Poole whether such face-stripes ever occur in the
eminently striped Kattywar breed of horses, and was, as we have seen,
answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears—a tint which approaches to that of the general colouring of the
other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids from
between several of the most distinct species. Now observe the case of
the several breeds of pigeons: they are descended from a pigeon
(including two or three sub-species or geographical races) of a bluish
colour, with certain bars and other marks; and when any breed assumes
by simple variation a bluish tint, these bars and other marks
invariably reappear; but without any other change of form or character.
When the oldest and truest breeds of various colours are crossed, we
see a strong tendency for the blue tint and bars and marks to reappear
in the mongrels. I have stated that the most probable hypothesis to
account for the reappearance of very ancient characters, is—that there
is a _tendency_ in the young of each successive generation to produce
the long-lost character, and that this tendency, from unknown causes,
sometimes prevails. And we have just seen that in several species of
the horse-genus the stripes are either plainer or appear more commonly
in the young than in the old. Call the breeds of pigeons, some of which
have bred true for centuries, species; and how exactly parallel is the
case with that of the species of the horse-genus!
For myself, I venture confidently to look back thousands on thousands
of generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our
domestic horse, whether or not it be descended from one or more wild
stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me,
to reject a real for an unreal, or at least for an unknown, cause. It
makes the works of God a mere mockery and deception; I would almost as
soon believe with the old and ignorant cosmogonists, that fossil shells
had never lived, but had been created in stone so as to mock the shells
now living on the sea-shore.
_Summary_.—Our ignorance of the laws of variation is profound. Not in
one case out of a hundred can we pretend to assign any reason why this
or that part differs, more or less, from the same part in the parents.
But whenever we have the means of instituting a comparison, the same
laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between
species of the same genus. The external conditions of life, as climate
and food, etc., seem to have induced some slight modifications. Habit
in producing constitutional differences,
and use in strengthening, and disuse in weakening and diminishing
organs, seem to have been more potent in their effects. Homologous
parts tend to vary in the same way, and homologous parts tend to
cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed,
perhaps it tends to draw nourishment from the adjoining parts; and
every part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable
to understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to
any particular function, so that their modifications have not been
closely checked by natural selection. It is probably from this same
cause that organic beings low in the scale of nature are more variable
than those which have their whole organisation more specialised, and
are higher in the scale. Rudimentary organs, from being useless, will
be disregarded by natural selection, and hence probably are variable.
Specific characters—that is, the characters which have come to differ
since the several species of the same genus branched off from a common
parent—are more variable than generic characters, or those which have
long been inherited, and have not differed within this same period. In
these remarks we have referred to special parts or organs being still
variable, because they have recently varied and thus come to differ;
but we have also seen in the second Chapter that the same principle
applies to the whole individual; for in a district where many species
of any genus are found—that is, where there has been much former
variation and differentiation, or where the manufactory of new specific
forms has been actively at work—there, on an average, we now find most
varieties or incipient species. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
sexes of the same species, and specific differences to the several
species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we
can understand why it should often still be variable in a much higher
degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had
time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants—which on my view must be a very slow process, requiring a
long lapse of time—in this case, natural selection may readily have
succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifications may
not arise from reversion and analogous variation, such modifications
will add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the offspring
from their parents—and a cause for each must exist—it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face
of this earth are enabled to struggle with each other, and the best
adapted to survive.