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frequently differ in their inner and outer flowers. It might have been
thought that the development of the ray-petals by drawing nourishment
from the reproductive organs causes their abortion; but this can hardly be
the sole cause, for in some compositae the seeds of the outer and inner
florets differ, without any difference in the corolla. Possibly these several
differences may be connected with the different flow of nutriment toward
the central and external flowers. We know, at least, that with irregular
flowers those nearest to the axis are most subject to peloria, that is, to
become abnormally symmetrical. I may add, as an instance of this fact, and
as a striking case of correlation, that in many pelargoniums the two upper
petals in the central flower of the truss often lose their patches of darker
color; and when this occurs, the adherent nectary is quite aborted, the
central flower thus becoming peloric or regular. When the color is absent
from only one of the two upper petals, the nectary is not quite aborted but is
much shortened.

With respect to the development of the corolla, Sprengel's idea that the
ray-florets serve to attract insects, whose agency is highly advantageous, or
necessary for the fertilization of these plants, is highly probable; and if so,
natural selection may have come into play. But with respect to the seeds,
it seems impossible that their differences in shape, which are not always
correlated with any difference in the corolla, can be in any way beneficial;
yet in the umbelliferae these differences are of such apparent importance —
the seeds being sometimes orthospermous in the exterior flowers and
ccelospermous in the central flowers — that the elder De Candolle founded
his main divisions in the order on such characters. Hence modifications of
structure, viewed by systematists as of high value, may be wholly due to
the laws of variation and correlation, without being, as far as we can
judge, of the slightest service to the species.


We may often falsely attribute to correlated variation 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 in some necessary
manner correlated. Some other correlations are apparently due to the
manner in which natural selection can alone act. For instance, Alph. de
Candolle has remarked that winged seeds are never found in fruits which
do not open; I should explain this rule by the impossibility of seeds gradually
becoming winged through natural selection, unless the capsules were open:
for in this case alone could the seeds, which were a little better adapted to
be wafted by the wind, gain an advantage over others less well fitted for
wide dispersal.


The elder Geoffroy and Goethe propounded, at about the same time,
their law of compensation or balancement of growth; or, as Goethe ex-
pressed it, "in order to spend on one side, nature is forced to economize 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, how-
ever, here give any instances, for I see hardly any way of distinguishing
between the eff'ects, on the one hand, of a part being largely developed
through natural selection and another and adjoining part being reduced
by the same process or by disuse, and, on the other hand, the actual with-
drawal 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
economize every part of the organization. If, under changed conditions of
life, a structure, before useful, becomes less useful, its diminution will be
favored, for it wiU profit the individual not to have its nutriment wasted in
building up a useless structure. I can thus only understand a fact with


which I was much struck when examining cirripedes, and of which many
analogous instances could be given: namely, that when a cirripede is
parasitic within another cirripede, 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 seg-
ments 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, 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 would have a better chance of supporting
itself, by less nutriment being wasted.

Thus, as I beHeve, natural selection will tend in the long-run to reduce
any part of the organization, as soon as it becomes, through changed habits,
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 an organ without requiring
as a necessary compensation the reduction of some adjoining part.


It seems to be a rule, as remarked by Is. Geoffroy Saint-Hilaire, both
with varieties and species, that when any part or organ is repeated many
times in the same individual (as the vertebrae in snakes, and the stamens
in polyandrous flowers) the number is variable; whereas the same part or
organ, when it occurs in lesser numbers, is constant. The same author, as
well as some botanists, have further remarked that multiple parts are
extremely liable to vary in structure. As "vegetable repetition," to use Pro-
fessor Owen's expression, is a sign of low organization, the foregoing state-
ments accord with the common opinion of naturalists, that beings which
stand low in the scale of nature are more variable than those which are
higher. I presume that lowness here means that the several parts of the
organization have been but little specialized 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 not
have preserved or rejected each little deviation of form so carefully as
when the part has to serve for some one special purpose. In the same way
that a knife which has to cut all sorts of things may be of almost any shape;
while a tool for some particular purpose must be of some particular shape.
Natural selection, it should never be forgotten, can act solely through and
for the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly variable.


We shall have to recur to this subject; and I will here only add that their
variability seems to result from their uselessness, and consequently from
natural selection having had no power to check deviations in their structure.


Several years ago I was much struck by a remark to the above effect made
by Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the truth
of the above 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 allowances for them. It
should be understood that the rule by no means applies to any part, how-
ever unusually developed, unless it be unusually developed in one species
or in a few species in comparison with the same part in many closely allied
species. Thus, the wing of the bat is a most abnormal structure in the class
of mammals, but the rule would not apply here, because the whole group
of bats possesses wings; it would apply only if some one species had wings
developed in a 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, sec-
ondary sexual characters, used by Hunter, relates to characters which are
attached to one sex, but are not directly connected with the act of reproduc-
tion. The rule applies to males and females; but more rarely to the females,
as they seldom offer remarkable secondary sexual characters. 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; I particularly attended to
Mr. Waterhouse' s remark, while investigating this order, and I am fully
convinced that the rule almost always holds good. I shall, in a future work,
give a list of all the more remarkable cases. I will here give only 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 distinct 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 the same species is so great that it is no exaggeration to
state that the varieties of the same species differ more from each other in
the characters derived from these important organs, than do the species
belonging to other distinct genera.


As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them; and
the rule certainly seems to hold good in this class. I cannot make out that
it applies to plants, and this would have seriously 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 a species, the fair presumption is that it is of high importance
to that species: nevertheless it is in this case eminently liable to variation.
Why should this be so? On the view that each species has been inde-
pendently created, with all its parts as we now see them, I can see no ex-
planation. But on the view that groups of species are descended from some
other species and have been modified through natural selection, I think we
can obtain some light. First let me make some preliminary remarks. If, in
our domestic animals, 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 uniform character; and the breed
may be said to be degenerating. 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 case; for in such cases natural
selection either has not or cannot have come into full play, and thus the
organization is left in a fluctuating condition. But what here more par-
ticularly concerns us is, that those points in our domestic animals, which
at the present time are undergoing rapid change by continued selection, are
also eminently liable to variation. Look at the individuals of the same breed
of the pigeon, and see what a prodigious amount of difference there is in
the beaks of tumblers, in the beaks and wattle of carriers, in the carriage
and tail of fantails, etc., these being the points now mainly attended to by
English fanciers. Even in the same sub-breed, as in that of the short-faced
tumbler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be a con-
stant struggle going on between, on the one hand, the tendency to reversion
to a less perfect state, as well as an innate tendency to new variations, 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 com-
pletely as to breed a bird as coarse as a common tumbler pigeon from a
good short-faced strain. But as long as selection is rapidly going on, much
variability in the parts undergoing modification may always be expected.

Now let us turn to nature. When a part has been developed in an extraor-
dinary 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 several species branched
off from the common progenitor of the genus. This period will seldom be
remote in any extreme degree, as species 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 con-


tinually 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, still expect to find more variability in such
parts than in other parts of the organization 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 see no reason to doubt. Hence, when an organ, however
abnormal it may be, has been transmitted in approximately the same condi-
tion to many modified descendants, as in the case of the wing of the bat,
it must have existed, according to our theory, for an immense period in
nearly the same state; and thus it has come not to be 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 discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more variable
than generic. To explain by a simple example what is meant: if in a large
genus of plants some species had blue flowers and some had red, the color
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 color would become a generic character, and its variation
would be a more unusual circumstance. I have chosen this example because
the explanation which most naturalists would advance is not here ap-
plicable, 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
point in the chapter on Classification. It would be almost superfluous to
adduce evidence in support of the statement, that ordinary specific charac-
ters are more variable than generic; but with respect to important char-
acters, I have repeatedly noticed in works on natural history, that when an
author remarks with surprise that some important organ or part, which is
generally very constant throughout a large group of species, differs con-
siderably in closely allied species, it is often variable in the individuals of
the same 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 Saint-Hilaire apparently entertains no doubt, that the more an
organ normally differs in the different species of the same group, the more
subject it is to anomalies in the individuals.

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 that species are only
strongly marked and fixed varieties, we might expect often to find them still
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 allied genera,
are called generic characters; and these characters may be attributed to
inheritance from a common progenitor, for it can rarely have happened
that natural selection will have modified several distinct species, fitted to
more or less widely different habits, in exactly the same manner: and as
those so-called generic characters have been inherited from before the
period when the several 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 since the period
when the species branched off from a common progenitor, it is probable
that they should still often be in some degree variable — at least more vari-
able than those parts of the organization which have for a very long period
remained constant.


I think it will be admitted by naturalists, without my entering on details,
that secondary sexual characters are highly variable. It will also be ad-
mitted that species of the same group differ from each other more widely in
their secondary sexual characters, than in other parts of their organization:
compare, for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly dis-
played, with the amount of difference between the females. The cause of
the original variability of these characters is not manifest: but we can see
why they should not have been rendered as constant and uniform as others,
for they are 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 favored 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 have
succeeded in giving to the species of the same group a greater amount of
difference in these than in other respects.

It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts of
the organization in which the species of the same genus differ from each
other. Of this fact I will give in illustration the two first instances 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 num-
ber of joints in the tarsi is a character 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 the fossorial hymenoptera, the neuration of the wings is a charac-
ter of- the highest importance, because common to large groups; but in
<:ertain genera the neuration differs in the different species, and likewise in
the two sexes of the same species. Sir J. Lubbock has recently remarked,
that several minute crustaceans offer excellent illustrations of this law. "In
Pontella, for instance, the sexual characters are afforded mainly by the
anterior antennae and by the fifth pair of legs : the specific differences also are
principally given by these organs." This relation has a clear meaning on
my view: I look at all the species of the same genus as having as certainly
descended from a common progenitor, as have the two sexes of any one
species. Consequently, whatever part of the structure of the common pro-
genitor, 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 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 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 are possessed by all the species; 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 slight
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 their difference in closely allied species;
that secondary sexual and ordinary specific differences are generally dis-
played in the same parts of the organization, — are all principles closely
connected together. All being mainly due to the species of the same group
being the descendants of a common progenitor, from whom they have in-
herited 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 ac-


cumulated by natural and sexual selection, and having been thus adapted

Online LibraryCharles DarwinThe origin of species → online text (page 14 of 50)