Charles Darwin.

The origin of species online

. (page 45 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 45 of 50)
Font size
QR-code for this ebook

ever, be proved true only in those cases in which the ancient state of the
progenitor of the group has not been wholly obliterated, either by successive
variations having supervened at a very early period of growth, or by such
variations having been inherited at an earlier age than that at which they first
appeared. It should also be borne in mind, that the law may be true, but yet,
owing to the geological record not extending far enough back in time, may
remain for a long period, or forever, incapable of demonstration. The law
will not strictly hold good in those cases in which an ancient form became
adapted in its larval state to some special line of life, and transmitted the
same larval state to a whole group of descendants; for such larvae will not
resemble any still more ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which are second
to none in importance, are explained on the principle of variations in the
many descendants from some one ancient progenitor, having appeared at a
not very early period of life, and having been inherited at a corresponding
period. Embryology rises greatly in interest, when we look at the embryo as
a picture, more or less obscured, of the progenitor, either in its adult or larval
state, of all the members of the same great class.


Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It would
be impossible to name one of the higher animals in which some part or
other is not in a rudimentary condition. In the mammalia, for instance, the
male possesses rudimentary mammae; in snakes one lobe of the lungs is


rudimentary; in birds the "bastard-wing" may safely be considered as a
rudimentary digit, and in some species the whole wing is so far rudimentary
that it cannot be used for flight. What can be more curious than the presence
of teeth in foetal whales, which when grown up have not a tooth in their
heads; or the teeth, which never cut through the gums, in the upper jaws
of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to the
same identical species, which have either full-sized and perfect wings, or
mere rudiments of membrane, which not rarely lie under wing-covers firmly
soldered together; and in these cases it is impossible to doubt that the rudi-
ments represent wings. Rudimentary organs sometimes retain their potential-
ity: this occasionally occurs with the mammae of male mammals, which
have been known to become well developed and to secrete milk. So again
in the udders in the genus Bos, there are normally four developed and two
rudimentary teats; but the latter in our domestic cows sometimes become
well developed and yield milk. In regard to plants, the petals are some-
times rudimentary, and sometimes well developed in the individuals of the
same species. In certain plants having separated sexes, Kolreuter found that
by crossing a species, in which the male flowers included a rudiment of a
pistil, with an hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much increased in size; and
this clearly shows that the rudimentary and perfect pistils are essentially
alike in nature. An animal may possess various parts in a perfect state, and
yet they may in one sense be rudimentary, for they are useless: thus the
tadpole of the common salamander or water-newt, as Mr. G. H. Lewes re-
marks, "has gills, and passes its existence in the water; but the Salamandra
atra, which lives high up among the mountains, brings forth its young full-
formed. This animal never lives in the water. Yet if we open a gravid female,
we find tadpoles inside her with exquisitely feathered gills ; and when placed
in water they swim about like the tadpoles of the water-newt. Obviously
this aquatic organization has no reference to the future life of the animal,
nor has it any adaptation to its embryonic condition; it has solely reference
to ancestral adaptations, it repeats a phase in the development of its

An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to allow
the pollen tubes to reach the ovules within the ovarium. The pistil consists
of a stigma supported on a style; but in some Compositae, the male florets,
which of course cannot be fecundated, have a rudimentary pistil, for it is
not crowned with a stigma; but the style remains well developed and is
clothed in the usual manner with hairs, which serve to brush the pollen
out of the surrounding and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a distinct one: in cer-
tain fishes the swim-bladder seems to be rudimentary for its proper function


of giving buoyancy, but has become converted into a nascent breathing organ
or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have reason
to suppose that they were formerly more highly developed, ought not to be
considered as rudimentary. They may be in a nascent condition, and in
progress toward further development. Rudimentary organs, on the other
hand, are either quite useless, such as teeth which never cut through the
gums, or almost useless, such as the wings of an ostrich, which serve merely
as sails. As organs in this condition would formerly, when still less developed,
have been of even less use than at present, they cannot formerly have been
produced through variation and natural selection, which acts solely by the
preservation of useful modifications. They have been partially retained by
the power of inheritance, and relate to a former state of things. It is, how-
ever, often difficult to distinguish between rudimentary and nascent organs;
for we can judge only by analogy whether a part is capable of further de-
velopment, in which case alone it deserves to be called nascent. Organs in
this condition will always be somewhat rare; for beings thus provided will
commonly have been supplanted by their successors with the same organ in
a more perfect state, and consequently will have become long ago extinct.
The wing of the penguin is of high service, acting as a fin; it may, therefore,
represent the nascent state of the wing; not that I believe this to be the
case; it is more probably a reduced organ, modified for a new function;
the wing of the Apteryx, on the other hand, is quite useless, and is truly
rudimentary. Owen considers the simple filamentary limbs of the Lepidosiren
as the "beginnings of organs which attain full functional development in
higher vertebrates"; but, according to the view lately advocated by Dr.
Giinther, they are probably remnants, consisting of the persistent axis of a
fin, with the lateral rays or branches aborted. The mamniary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a
cow, as in a nascent condition. The ovigerous frena of certain cirripedes,
which have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.

Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In closely
allied species, also, the extent to which the same organ has been reduced
occasionally differs much. This latter fact is well exemplified in the state
of the wings of female moths belonging to the same family. Rudimentary
organs may be utterly aborted; and this implies, that in certain animals or
plants, parts are entirely absent which analogy would lead us to expect to
find in them, and which are occasionally found in monstrous individuals.
Thus in most of the Scrophulariaceae the fifth stamen is utterly aborted;
yet we may conclude that a fifth stamen once existed, for a rudiment of it
is found in many species of the family, and this rudiment occasionally be-
comes perfectly developed, as may sometimes be seen in the common snap-
dragon. In tracing the homologies of any part in difTerent members of the
same class, nothing is more common, or, in order fully to understand the


relations of the parts, more useful than the discovery of rudiments. This
is well shown in the drawings given by Owen of the leg bones of the horse,
ox and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterward wholly disappear. It is also, I believe, a universal rule, that a
rudimentary part is of greater size in the embryo relatively to the adjoining
parts, than in the adult; so that the organ at this early age is less rudimen-
tary, or even cannot be said to be in any degree rudimentary. Hence rudi-
mentary organs in the adult are often said to have retained their embryonic

I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment; for the
same reasoning power which tells us that most parts and organs are ex-
quisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs are imperfect and useless. In works
on natural history, rudimentary organs are generally said to have been cre-
ated "for the sake of symmetry," or in order "to complete the scheme of
nature." But this is not an explanation, merely a restatement of the fact. Nor
is it consistent with itself: thus the boa-constrictor has rudiments of hind
limbs and of a pelvis, and if it be said that these bones have been retained
"to complete the scheme of nature," why, as Professor Weismann asks, have
they not been retained by other snakes, which do not possess even a vestige
of these same bones? What would be thought of an astronomer who main-
tained that the satellites revolve in elliptic courses round their planets "for
the sake of symmetry," because the planets thus revolve round the sun? An
eminent physiologist accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or matter injurious
to the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed of mere cellular
tissue, can thus act? Can we suppose that rudimentary teeth, which are sub-
sequently absorbed, are beneficial to the rapidly growing embryonic calf by
removing matter so precious as phosphate of lime? When a man's fingers
have been amputated, imperfect nails have been known to appear on the
stumps, and I could as soon believe that these vestiges of nails are developed
in order to excrete horny matter, as that the rudirrientary nails on the fin
of the manatee have been developed for this same purpose.

On the view of descent with modification, the origin of rudimentary
organs is comparatively simple; and we can understand to a large extent
the laws governing their imperfect development. We have plenty of cases
of rudimentary organs in our domestic productions, as the stump of a tail
in tailless breeds, the vestige of an ear in earless breeds of sheep — the re-
appearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals — and the state of the whole
flower in the cauliflower. We often see rudiments of various parts in monsters ;
but I doubt whether any of these cases throw light on the origin of rudi-


mentary organs in a state of nature, further than by showing that rudiments
can be produced; for the balance of evidence clearly indicates that species
under nature do not undergo great and abrupt changes. But we learn from
the study of our domestic productions that the disuse of parts leads to their
reduced size; and that the result is inherited.

It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more and
more complete reduction of a part, until at last it became rudimentary —
as in the case of the eyes of animals inhabiting dark caverns, and of the
wings of birds inhabiting oceanic islands, which have seldom been forced
by beasts of prey to take flight, and have ultimately lost the power of flying.
Again, an organ, useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and exposed islands ;
and in this case natural selection will have aided in reducing the organ,
until it was rendered harmless and rudimentary.

Any change in structure and function, which can be efTected by small
stages, is within the power of natural selection; so that an organ rendered,
through changed habits of life, useless or injurious for one purpose, might
be modified and used for another purpose. An organ might, also, be retained
for one alone of its former functions. Organs, originally formed by the aid
of natural selection, when rendered useless may well be variable, for their
variations can no longer be checked by natural selection. All this agrees well
with what we see under nature. Moreover, at whatever period of life either
disuse or selection reduces an organ, and this will generally be when the
being has come to maturity and has to exert its full powers of action, the
principle of inheritance at corresponding ages will tend to reproduce the
organ in its reduced state at the same mature age, but will seldom affect
it in the embryo. Thus we can understand the greater size of rudimentary
organs in the embryo relatively to the adjoining parts, and their lesser
relative size in the adult. If, for instance, the digit of an adult animal was
used less and less during many generations, owing to some change of habits,
or if an organ or gland was less and less functionally exercised, we may infer
that it would become reduced in size in the adult descendants of this animal,
but would retain nearly its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it be
finally quite obliterated? It is scarcely possible that disuse can go on pro-
ducing any further effect after the organ has once been rendered function-
less. Some additional explanation is here requisite which I cannot give. If,
for instance, it could be proved that every part of the organization tends to
vary in a greater degree toward diminution than toward augmentation of
size, then we should be able to understand how an organ which has become
useless would be rendered, independently of the effects of disuse, rudimen-
tary, and would at last be wholly suppressed ; for the variations toward dimin-
ished size would no longer be checked by natural selection. The principle of


the economy of growth, explained in a former chapter, by which the materials
forming any part, if not useful to the possessor, are saved as far as is possible,
will perhaps come into play in rendering a useless part rudimentary. But
this principle will almost necessarily be confined to the earlier stages of the
process of reduction; for we cannot suppose that a minute papilla, for in-
stance, representing in a male flower the pistil of the female flower, and
formed merely of cellular tissue, would be further reduced or absorbed for
the sake of economizing nutriment.

Finally, as rudimentary organs, by whatever steps they may have been de-
graded into their present useless condition, are the record of a former state
of things, and have been retained solely through the power of inheritance —
we can understand, on the genealogical view of classification, how it is
that systematists, in placing organisms in their proper places in the natural
system, have often found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary
organs may be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which serve as a clew
for its derivation. On the view of desce.nt with modification, we may con-
clude that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far from presenting a strange difficulty, as they
assuredly do on the old doctrine of creation, might even have been antici-
pated in accordance with the views here explained.


In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups — that the nature
of the relationships by which all living and extinct organisms are united by
complex, radiating, and circuitous lines of affinities into a few grand classes
— the rules followed and the difficulties encountered by naturalists in their
classifications — the value set upon characters, if constant and prevalent,
whether of high or of the most trifling importance, or, as with rudimentary
organs, of no importance — the wide opposition in value between analogical
or adaptive characters, and characters of true affinity; and other such rules;
— all naturally follow if we admit the common parentage of allied forms, to-
gether with their modification through variation and natural selection, with
the contingencies of extinction and divergence of character. In considering
this view of classification, it should be borne in mind that the element of
descent has been universally used in ranking together the sexes, ages, dimor-
phic forms, and acknowledged varieties of the same species, however much
they may differ from each other in structure. If we extend the use of this ele-
ment of descent — the one certainly known cause of similarity in organic
beings — ^we shall understand what is meant by the Natural System: it is
genealogical in its attempted arrangement, with the grades of acquired dif-
ference marked by the terms, varieties, species, genera, families, orders, and


On this same view of descent with modification, most of the great facts
in Morphology become intelligible — whether we look to the same pattern
displayed by the different species of the same class in their homologous
organsj to whatever purpose applied; or to the serial and lateral homologies
in each individual animal and plant.

On the principle of successive slight variations, not necessarily or generally
supervening at a very early period of life, and being inherited at a correspond-
ing period, we can understand the leading facts in embryology; namely, the
close resemblance in the individual embryo of the parts which are homologous,
and which when matured become widely different in structure and function ;
and the resemblance of the homologous parts or organs in allied though dis-
tinct species, though fitted in the adult state for habits as different as is pos-
sible. Larvae are active embryos, which have been specially modified in a
greater or less degree in relation to their habits of life, with their modifica-
tions inherited at a corresponding early age. On these same principles, and
bearing in mind that when organs are reduced in size, either from disuse or
through natural selection, it will generally be at that period of life when the
being has to provide for its own wants, and bearing in mind how strong is
the force of inheritance — the occurrence of rudimentary organs might even
have been anticipated. The importance of embryological characters and of
rudimentary organs in classification is ihtelligible, on the view that a natural
arrangement must be genealogical.

Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families, with which this world is peopled, are all descended,
each within its own class or group, from common parents, and have all been
modified in the course of descent, that I should without hesitation adopt this
view, even if it were unsupported by other facts or arguments.


Recapitulation and Conclusion

Recapitulation of the Objections to the Theory of Natural Selection — ^Recapitulation
of the General and Special Circumstances in its Favor — Causes of the General
Belief in the Immutability of Species — How far the Theory of Natural Selection
may be extended — Effects of its Adoption on the Study of Natural History —
Concluding Remarks.

As THIS whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do not
deny. I have endeavored to give to them their full force. Nothing at first
can appear more difficult to believe than that the more complex organs and
instincts have been perfected, not by means superior to, though analogous
with, human reason, but by the accumulation of innumerable slight varia-
tions, each good for the individual possessor. Nevertheless, this difficulty,
though appearing to our imagination insuperably great, cannot be consid-
ered real if we admit the following propositions, namely, that all parts of the
organization and instincts offer, at least, individual differences — that there
is a struggle for existence leading to the preservation of profitable deviations
of structure or instinct — and, lastly, that gradations in the state of perfection
of each organ may have existed, each good of its kind. The truth of these
propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely
cautious in saying that any organ or instinct, or any whole structure, could
not have arrived at its present state by many graduated steps. There are, it
must be admitted, cases of special difficulty opposed to the theory of natural
selection: and one of the most curious of these is the existence in the same
community of two or three defined castes of workers or sterile female ants;
but I have attempted to show how these difficulties can be mastered.

With respect to the almost universal sterility of species when first crossed,
which forms so remarkable a contrast with the almost universal fertility of
varieties when crossed, I must refer the reader to the recapitulation of the
facts given at the end of the ninth chapter, which seem to me conclusively
to show that this sterility is no more a special endowment than is the inca-
pacity of two distinct kinds of trees to be grafted together; but that it is inci-
dental on differences confined to the reproductive systems of the inter-crossed
species. We see the truth of this conclusion in the vast difference in the re-
sults of crossing the same two species reciprocally — that is, when one species
is first used as the father and then as the mother. Analogy from the consider-
ation of dimorphic and trimorphic plants clearly leads to the same conclu-



sion, for when the forms are illegitimately united, they yield few or no seed,
and their offspring are more or less sterile; and these forms belong to the
same undoubted species, and differ from each other in no respect except in
their reproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their mongrel
offspring, has been asserted by so many authors to be universal, this cannot
be considered as quite correct after the facts given on the high authority of
Gartner and Kolreuter. Most of the varieties which have been experimented
on have been produced under domestication; and as domestication (I do not
mean mere confinement) almost certainly tends to eliminate that sterility
which, judging from analogy, would have affected the parent-species if in-
tercrossed, we ought not to expect that domestication would likewise induce

Online LibraryCharles DarwinThe origin of species → online text (page 45 of 50)