On the Origin of Species by Means of Natural Selection
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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 specialised
for any particular purpose, and perhaps in polymorphic groups, we see a
nearly parallel natural case; for in such cases natural selection either
has not or cannot come into full play, and thus the organisation is left in
a fluctuating condition. But what here more especially concerns us is,
that in our domestic animals those points, which at the present time are
undergoing rapid change by continued selection, are also eminently liable
to variation. Look at the breeds of the pigeon; see what a prodigious
amount of difference there is in the beak of the different tumblers, in the
beak and wattle of the different carriers, in the carriage and tail of our
fantails, &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.
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