The Origin of Species
by Charles Darwin
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Chapter 5 - Laws of Variation
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.
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