 Chapter
5 - 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
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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