On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER V.
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 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.
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.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 131-170. ]
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