The Origin of Species
Chapter 5: Laws of Variation
by Charles Darwin
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 character 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, &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 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 ship-wrecked 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 antennae or palpi, as a compensation for
blindness. Notwithstanding such modifications, we might expect still to see in the
cave-animals of America, affinities to the other inhabitants of that continent, and in
those of Europe, to the inhabitants of the European continent. And this is the case with
some of the American cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding country. It would be most
difficult to give any rational explanation of the affinities of the blind cave-animals to
the other inhabitants of the two continents on the ordinary view of their independent
creation. That several of the inhabitants of the caves of the Old and New Worlds should be
closely related, we might expect from the well-known relationship of most of their other
productions. Far from feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is
the case with the blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved, owing to the less
severe competition to which the inhabitants of these dark abodes will probably have been
exposed.
Acclimatisation
Habit is hereditary with plants, as in the period of flowering, in the amount of rain
requisite for seeds to germinate, in the time of sleep, &c., and this leads me to say
a few words on acclimatisation. As it is extremely common for species of the same genus to
inhabit very hot and very cold countries, and as I believe that all the species of the
same genus have descended from a single parent, if this view be correct, acclimatisation
must be readily effected during long-continued descent. It is notorious that each species
is adapted to the climate of its own home: species from an arctic or even from a temperate
region cannot endure a tropical climate, or conversely. So again, many succulent plants
cannot endure a damp climate. But the degree of adaptation of species to the climates
under which they live is often overrated. We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and from the number of
plants and animals brought from warmer countries which here enjoy good health. We have
reason to believe that species in a state of nature are limited in their ranges by the
competition of other organic beings quite as much as, or more than, by adaptation to
particular climates. But whether or not the adaptation be generally very close, we have
evidence, in the case of some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures, or becoming acclimatised: thus the pines
and rhododendrons, raised from seed collected by Dr Hooker from trees growing at different
heights on the Himalaya were found in this country to possess different constitutional
powers of resisting cold. Mr Thwaites informs me that he has observed similar facts in
Ceylon, and analogous observations have been made by Mr H. C. Watson on European species
of plants brought from the Azores to England. In regard to animals, several authentic
cases could be given of species within historical times having largely extended their
range from warmer to cooler latitudes, and conversely; but we do not positively know that
these animals were strictly adapted to their native climate, but in all ordinary cases we
assume such to be the case; nor do we know that they have subsequently become acclimatised
to their new homes.
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 means combined, is a very obscure question. That habit or
custom has some influence I must believe, both from analogy, and from the incessant advice
given in agricultural works, even in the ancient Encyclopaedias of China, to be very
cautious in transposing animals from one district to another; for it is not likely that
man should have succeeded in selecting so many breeds and sub-breeds with constitutions
specially fitted for their own districts: the result must, I think, be due to habit. On
the other hand, I can see no reason to doubt that natural selection will continually tend
to preserve those individuals which are born with constitutions best adapted to their
native countries. In treatises on many kinds of cultivated plants, certain varieties are
said to withstand certain climates better than others: this is very strikingly shown in
works on fruit trees published in the United States, in which certain varieties are
habitually recommended for the northern, and others for the southern States; and as most
of these varieties are of recent origin, they cannot owe their constitutional differences
to habit. The case of the Jerusalem artichoke, which is never propagated by seed, and of
which consequently new varieties have not been produced, has even been advanced for it is
now as tender as ever it was -- as proving that acclimatisation cannot be effected! The
case, also, of the kidney-bean has been often cited for a similar purpose, and with much
greater weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by frost, and then
collect seed from the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the experiment cannot be
said to have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has been published how
much more hardy some seedlings appeared to be than others.
On the whole, I think we may conclude that habit, use, and disuse, have, in some cases,
played a considerable part in the modification of the constitution, and of the structure
of various organs; but that the effects of use and disuse have often been largely combined
with, and sometimes overmastered by, the natural selection of innate differences.
Correlation of Growth
I mean by this expression that the whole organisation is so tied together during its
growth and development, that when slight variations in any one part occur, and are
accumulated through natural selection, other parts become modified. This is a very
important subject, most imperfectly understood. The most obvious case is, that
modifications accumulated solely for the good of the young or larva, will, it may safely
be concluded, affect the structure of the adult; in the same manner as any malconformation
affecting the early embryo, seriously affects the whole organisation of the adult. The
several parts of the body which are homologous, and which, at an early embryonic period,
are alike, seem liable to vary in an allied manner: we see this in the right and left
sides of the body varying in the same manner; in the front and hind legs, and even in the
jaws and limbs, varying together, for the lower jaw is believed to be homologous with the
limbs. These tendencies, I do not doubt, may be mastered more or less completely by
natural selection: thus a family of stags once existed with an antler only on one side;
and if this had been of any great use to the breed it might probably have been rendered
permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere; this is often
seen in monstrous plants; and nothing is more common than the union of homologous parts in
normal structures, as the union of the petals of the corolla into a tube. Hard parts seem
to affect the form of adjoining soft parts; it is believed by some authors that the
diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape
of their kidneys. Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes, according to
Schlegel, the shape of the body and the manner of swallowing determine the position of
several of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure. M. Is. Geoffroy
St Hilaire has forcibly remarked, that certain malconformations very frequently, and that
others rarely coexist, without our being able to assign any reason. What can be more
singular than the relation between blue eyes and deafness in cats, and the tortoise-shell
colour with the female sex; the feathered feet and skin between the outer toes in pigeons,
and the presence of more or less down on the young birds when first hatched, with the
future colour of their plumage; or, again, the relation between the hair and teeth in the
naked Turkish dog, though here probably homology comes into play? With respect to this
latter case of correlation, I think it can hardly be accidental, that if we pick out the
two orders of mammalia which are most abnormal in their dermal coverings, viz. Cetacea
(whales) and Edentata (armadilloes, scaly ant-eaters, &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 difference in the ray and
central florets of, for instance, the daisy, and this difference is often accompanied with
the abortion of parts of the flower. But, in some Compositous plants, the seeds also
differ in shape and sculpture; and even the ovary itself, with its accessory parts,
differs, as has been described by Cassini. These differences have been attributed by some
authors to pressure, and the shape of the seeds in the ray-florets in some Compositae
countenances this idea; but, in the case of the corolla of the Umbelliferae, it is by no
means, as Dr Hooker informs me, in species with the densest heads that the inner and outer
flowers most frequently differ. It might have been thought that the development of the
ray-petals by drawing nourishment from certain other parts of the flower had caused their
abortion; but in some Compositae there is a difference in the seeds of the outer and inner
florets without any difference in the corolla. Possibly, these several differences may be
connected with some difference in the flow of nutriment towards the central and external
flowers: we know, at least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an instance of this, and of
a striking case of correlation, that I have recently observed in some garden pelargoniums,
that the central flower of the truss often loses the patches of darker colour in the two
upper petals; and that when this occurs, the adherent nectary is quite aborted; when the
colour is absent from only one of the two upper petals, the nectary is only much
shortened.
With respect to the difference in the corolla of the central and exterior flowers of a
head or umbel, I do not feel at all sure that C. C. Sprengel's idea that the ray-florets
serve to attract insects, whose agency is highly advantageous in the fertilisation of
plants of these two orders, is so far-fetched, as it may at first appear: and if it be
advantageous, natural selection may have come into play. But in regard to the differences
both in the internal and external structure of the seeds, which are not always correlated
with any differences in the flowers, it seems impossible that they can be in any way
advantageous to the plant: yet in the Umbelliferae these differences are of such apparent
importance the seeds being in some cases, according to Tausch, orthospermous in the
exterior flowers and coelospermous in the central flowers, that the elder De Candolle
founded his main divisions of the order on analogous differences. Hence we see that
modifications of structure, viewed by systematists as of high value, may be wholly due to
unknown laws of correlated growth, and without being, as far as we can see, of the
slightest service to the species.
We may often falsely attribute to correlation of growth, structures which are common to
whole groups of species, and which in truth are simply due to inheritance; for an ancient
progenitor may have acquired through natural selection some one modification in structure,
and, after thousands of generations, some other and independent modification; and these
two modifications, having been transmitted to a whole group of descendants with diverse
habits, would naturally be thought to be correlated in some necessary manner. So, again, I
do not doubt that some apparent correlations, occurring throughout whole orders, are
entirely due to the manner alone in which natural selection can act. For instance, Alph.
De Candolle has remarked that winged seeds are never found in fruits which do not open: I
should explain the rule by the fact that seeds could not gradually become winged through
natural selection, except in fruits which opened; so that the individual plants producing
seeds which were a little better fitted to be wafted further, might get an advantage over
those producing seed less fitted for dispersal; and this process could not possibly go on
in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period, their law of
compensation or balancement of growth; or, as Goethe expressed it, 'in order to spend on
one side, nature is forced to economise on the other side.' I think this holds true to a
certain extent with our domestic productions: if nourishment flows to one part or organ in
excess, it rarely flows, at least in excess, to another part; thus it is difficult to get
a cow to give much milk and to fatten readily. The same varieties of the cabbage do not
yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. When the
seeds in our fruits become atrophied, the fruit itself gains largely in size and quality.
In our poultry, a large tuft of feathers on the head is generally accompanied by a
diminished comb, and a large beard by diminished wattles. With species in a state of
nature it can hardly be maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not, however, here give
any instances, for I see hardly any way of distinguishing between the effects, on the one
hand, of a part being largely developed through natural selection and another and
adjoining part being reduced by this same process or by disuse, and, on the other hand,
the actual withdrawal of nutriment from one part owing to the excess of growth in another
and adjoining part.
I suspect, also, that some of the cases of compensation which have been advanced, and
likewise some other facts, may be merged under a more general principle, namely, that
natural selection is continually trying to economise in every part of the organisation. If
under changed conditions of life a structure before useful becomes less useful, any
diminution, however slight, in its development, will be seized on by natural selection,
for it will profit the individual not to have its nutriment wasted in building up an
useless structure. I can thus only understand a fact with which I was much struck when
examining cirripedes, and of which many other instances could be given: namely, that when
a cirripede is parasitic within another and is thus protected, it loses more or less
completely its own shell or carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all other cirripedes
consists of the three highly-important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic and protected
Proteolepas, the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antennae. Now the saving of a large and complex
structure, when rendered superfluous by the parasitic habits of the Proteolepas, though
effected by slow steps, would be a decided advantage to each successive individual of the
species; for in the struggle for life to which every animal is exposed, each individual
Proteolepas would have a better chance of supporting itself, by less nutriment being
wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long run in reducing
and saving every part of the organisation, as soon as it is rendered superfluous, without
by any means causing some other part to be largely developed in a corresponding degree.
And, conversely, that natural selection may perfectly well succeed in largely developing
any organ, without requiring as a necessary compensation the reduction of some adjoining
part.
It seems to be a rule, as remarked by Is. Geoffroy St Hilaire, both in varieties and in
species, that when any part or organ is repeated many times in the structure of the same
individual (as the vertebrae in snakes, and the stamens in polyandrous flowers) the number
is variable; whereas the number of the same part or organ, when it occurs in lesser
numbers, is constant. The same author and some botanists have further remarked that
multiple parts are also very liable to variation in structure. Inasmuch as this
'vegetative repetition,' to use 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 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 specialized for any particular purpose, and perhaps in polymorphic groups, we see a
nearly parallel natural case; for in such cases natural selection either has not or cannot
come into full play, and thus the organisation is left in a fluctuating condition. But
what here more especially concerns us is, that in our domestic animals those points, which
at the present time are undergoing rapid change by continued selection, are also eminently
liable to variation. Look at the breeds of the pigeon; see what a prodigious amount of
difference there is in the beak of the different tumblers, in the beak and wattle of the
different carriers, in the carriage and tail of our fantails, &c., these being the
points now mainly attended to by English fanciers. Even in the sub-breeds, as in the
short-faced tumbler, it is notoriously difficult to breed them nearly to perfection, and
frequently individuals are born which depart widely from the standard. There may be truly
said to be a constant struggle going on between, on the one hand, the tendency to
reversion to a less modified state, as well as an innate tendency to further variability
of all kinds, and, on the other hand, the power of steady selection to keep the breed
true. In the long run selection gains the day, and we do not expect to fail so far as to
breed a bird as coarse as a common tumbler from a good short-faced strain. But as long as
selection is rapidly going on, there may always be expected to be much variability in the
structure undergoing modification. It further deserves notice that these variable
characters, produced by man's selection, sometimes become attached, from causes quite
unknown to us, more to one sex than to the other, generally to the male sex, as with the
wattle of carriers and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an extraordinary manner in
any one species, compared with the other species of the same genus, we may conclude that
this part has undergone an extraordinary amount of modification, since the period when the
species branched off from the common progenitor of the genus. This period will seldom be
remote in any extreme degree, as species very rarely endure for more than one geological
period. An extraordinary amount of modification implies an unusually large and
long-continued amount of variability, which has continually been accumulated by natural
selection for the benefit of the species. But as the variability of the
extraordinarily-developed part or organ has been so great and long-continued within a
period not excessively remote, we might, as a general rule, expect still to find more
variability in such parts than in other parts of the organisation, which have remained for
a much longer period nearly constant. And this, I am convinced, is the case. That the
struggle between natural selection on the one hand, and the tendency to reversion and
variability on the other hand, will in the course of time cease; and that the most
abnormally developed organs may be made constant, I can see no reason to doubt. Hence when
an organ, however abnormal it may be, has been transmitted in approximately the same
condition to many modified descendants, as in the case of the wing of the bat, it must
have existed, according to my theory, for an immense period in nearly the same state; and
thus it comes to be no more variable than any other structure. It is only in those cases
in which the modification has been comparatively recent and extraordinarily great that we
ought to find the generative variability, as it may be called, still present in a
high degree. For in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner and degree, and by
the continued rejection of those tending to revert to a former and less modified
condition.
The principle included in these remarks may be extended. It is notorious that specific
characters are more variable than generic. To explain by a simple example what is meant.
If some species in a large genus of plants had blue flowers and some had red, the colour
would be only a specific character, and no one would be surprised at one of the blue
species varying into red, or conversely; but if all the species had blue flowers, the
colour would become a generic character, and its variation would be a more unusual
circumstance. I have chosen this example because an explanation is not in this case
applicable, which most naturalists would advance, namely, that specific characters are
more variable than generic, because they are taken from parts of less physiological
importance than those commonly used for classing genera. I believe this explanation is
partly, yet only indirectly, true; I shall, however, have to return to this subject in our
chapter on Classification. It would be almost superfluous to adduce evidence in support of
the above statement, that specific characters are more variable than generic; but I have
repeatedly noticed in works on natural history, that when an author has remarked with
surprise that some important organ or part, which is generally very constant
throughout large groups of species, has differed considerably in closely-allied
species, that it has, also, been variable in the individuals of some of the
species. And this fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind applies to
monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain no doubt, that the
more an organ normally differs in the different species of the same group, the more
subject it is to individual anomalies.
On the ordinary view of each species having been independently created, why should that
part of the structure, which differs from the same part in other independently-created
species of the same genus, be more variable than those parts which are closely alike in
the several species? I do not see that any explanation can be given. But on the view of
species being only strongly marked and fixed varieties, we might surely expect to find
them still often continuing to vary in those parts of their structure which have varied
within a moderately recent period, and which have thus come to differ. Or to state the
case in another manner: the points in which all the species of a genus resemble each
other, and in which they differ from the species of some other genus, are called generic
characters; and these characters in common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection will have modified
several species, fitted to more or less widely-different habits, in exactly the same
manner: and as these so-called generic characters have been inherited from a remote
period, since that period when the species first branched off from their common
progenitor, and subsequently have not varied or come to differ in any degree, or only in a
slight degree, it is not probable that they should vary at the present day. On the other
hand, the points in which species differ from other species of the same genus, are called
specific characters; and as these specific characters have varied and come to differ
within the period of the branching off of the species from a common progenitor, it is
probable that they should still often be in some degree variable, at least more variable
than those parts of the organisation which have for a very long period remained constant.
In connexion with the present subject, I will make only two other remarks. I think it
will be admitted, without my entering on details, that secondary sexual characters are
very variable; I think it also will be admitted that species of the same group differ from
each other more widely in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly displayed, with the
amount of difference between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual characters is not
manifest; but we can see why these characters should not have been rendered as constant
and uniform as other parts of the organisation; for secondary sexual characters have been
accumulated by sexual selection, which is less rigid in its action than ordinary
selection, as it does not entail death, but only gives fewer offspring to the less
favoured males. Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had a wide scope for
action, and may thus readily have succeeded in giving to the species of the same group a
greater amount of difference in their sexual characters, than in other parts of their
structure.
It is a remarkable fact, that the secondary sexual differences between the two sexes of
the same species are generally displayed in the very same parts of the organisation in
which the different species of the same genus differ from each other. Of this fact I will
give in illustration two instances, the first which happen to stand on my list; and as the
differences in these cases are of a very unusual nature, the relation can hardly be
accidental. The same number of joints in the tarsi is a character generally common to very
large groups of beetles, but in the Engidae, as Westwood has remarked, the number varies
greatly; and the number likewise differs in the two sexes of the same species: again in
fossorial hymenoptera, the manner of neuration of the wings is a character of the highest
importance, because common to large groups; but in certain genera the neuration differs in
the different species, and likewise in the two sexes of the same species. This relation
has a clear meaning on my view of the subject: I look at all the species of the same genus
as having as certainly descended from the same progenitor, as have the two sexes of any
one of the species. Consequently, whatever part of the structure of the common progenitor,
or of its early descendants, became variable; variations of this part would it is highly
probable, be taken advantage of by natural and sexual selection, in order to fit the
several species to their several places in the economy of nature, and likewise to fit the
two sexes of the same species to each other, or to fit the males and females to different
habits of life, or the males to struggle with other males for the possession of the
females.
Finally, then, I conclude that the greater variability of specific characters, or those
which distinguish species from species, than of generic characters, or those which the
species possess in common; that the frequent extreme variability of any part which is
developed in a species in an extraordinary manner in comparison with the same part in its
congeners; and the not great degree of variability in a part, however extraordinarily it
may be developed, if it be common to a whole group of species; that the great variability
of secondary sexual characters, and the great amount of difference in these same
characters between closely allied species; that secondary sexual and ordinary specific
differences are generally displayed in the same parts of the organisation, are all
principles closely connected together. All being mainly due to the species of the same
group having descended from a common progenitor, from whom they have inherited much in
common, to parts which have recently and largely varied being more likely still to go on
varying than parts which have long been inherited and have not varied, to natural
selection having more or less completely, according to the lapse of time, overmastered the
tendency to reversion and to further variability, to sexual selection being less rigid
than ordinary selection, and to variations in the same parts having been accumulated by
natural and sexual selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.
Distinct species present analogous variations; and a variety of one species often
assumes some of the characters of an allied species, or reverts to some of the characters
of an early progenitor.
These propositions will be most readily understood by looking to our domestic races.
The most distinct breeds of pigeons, in countries most widely apart, present sub-varieties
with reversed feathers on the head and feathers on the feet, characters not possessed by
the aboriginal rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in the pouter, may
be considered as a variation representing the normal structure of another race, the
fantail. I presume that no one will doubt that all such analogous variations are due to
the several races of the pigeon having inherited from a common parent the same
constitution and tendency to variation, when acted on by similar unknown influences. In
the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or
roots as commonly called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common parent: if this be not
so, the case will then be one of analogous variation in two so-called distinct species;
and to these a third may be added, namely, the common turnip. According to the ordinary
view of each species having been independently created, we should have to attribute this
similarity in the enlarged stems of these three plants, not to the vera causa of
community of descent, and a consequent tendency to vary in a like manner, but to three
separate yet closely related acts of creation.
With pigeons, however, we have another case, namely, the occasional appearance in all
the breeds, of slaty-blue birds with two black bars on the wings, a white rump, a bar at
the end of the tail, with the outer feathers externally edged near their bases with white.
As all these marks are characteristic of the parent rock-pigeon, I presume that no one
will doubt that this is a case of reversion, and not of a new yet analogous variation
appearing in the several breeds. We may I think confidently come to this conclusion,
because, as we have seen, these coloured marks are eminently liable to appear in the
crossed offspring of two distinct and differently coloured breeds; and in this case there
is nothing in the external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on the laws of
inheritance.
No doubt it is a very surprising fact that characters should reappear after having been
lost for many, perhaps for hundreds of generations. But when a breed has been crossed only
once by some other breed, the offspring occasionally show a tendency to revert in
character to the foreign breed for many generations some say, for a dozen or even a score
of generations. After twelve generations, the proportion of blood, to use a common
expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it is generally
believed that a tendency to reversion is retained by this very small proportion of foreign
blood. In a breed which has not been crossed, but in which both parents have lost
some character which their progenitor possessed, the tendency, whether strong or weak, to
reproduce the lost character might be, as was formerly remarked, for all that we can see
to the contrary, transmitted for almost any number of generations. When a character which
has been lost in a breed, reappears after a great number of generations, the most probable
hypothesis is, not that the offspring suddenly takes after an ancestor some hundred
generations distant, but that in each successive generation there has been a tendency to
reproduce the character in question, which at last, under unknown favourable conditions,
gains an ascendancy. For instance, it is probable that in each generation of the
barb-pigeon, which produces most rarely a blue and black-barred bird, there has been a
tendency in each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no more abstract
improbability in a tendency to produce any character being inherited for an endless number
of generations, than in quite useless or rudimentary organs being, as we all know them to
be, thus inherited. Indeed, we may sometimes observe a mere tendency to produce a rudiment
inherited: for instance, in the common snapdragon (Antirrhinum) a rudiment of a fifth
stamen so often appears, that this plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to have descended from
a common parent, it might be expected that they would occasionally vary in an analogous
manner; so that a variety of one species would resemble in some of its characters another
species; this other species being on my view only a well-marked and permanent variety. But
characters thus gained would probably be of an unimportant nature, for the presence of all
important characters will be governed by natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual action of the conditions of life
and of a similar inherited constitution. It might further be expected that the species of
the same genus would occasionally exhibit reversions to lost ancestral characters. As,
however, we never know the exact character of the common ancestor of a group, we could not
distinguish these two cases: if, for instance, we did not know that the rock-pigeon was
not feather-footed or turn-crowned, we could not have told, whether these characters in
our domestic breeds were reversions or only analogous variations; but we might have
inferred that the blueness was a case of reversion, from the number of the markings, which
are correlated with the blue tint, and which it does not appear probable would all appear
together from simple variation. More especially we might have inferred this, from the blue
colour and marks so often appearing when distinct breeds of diverse colours are crossed.
Hence, though under nature it must generally be left doubtful, what cases are reversions
to an anciently existing character, and what are new but analogous variations, yet we
ought, on my theory, sometimes to find the varying offspring of a species assuming
characters (either from reversion or from analogous variation) which already occur in some
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, come 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, 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 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; but traces of it, as stated by Mr Blyth and others, occasionally appear:
and I have been informed by Colonel Poole that foals of this species are generally striped
on the legs, and faintly on the shoulder. The quagga, though so plainly barred like a
zebra over the body, is without bars on the legs; but Dr Gray has figured one specimen
with very distinct zebra-like bars on the hocks.
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, &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. 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, &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 specialized 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 specialized, 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.