ABORTIVE OR RUDIMENTARY ORGANS
_The abortive organs of naturalists._
Parts of structure are said to be "abortive," or when in a still lower
state of development "rudimentary{486}," when the same reasoning power,
which convinces us that in some cases similar parts are beautifully
adapted to certain ends, declares that in others they are absolutely
useless. Thus the rhinoceros, the whale{487}, etc., have, when young,
small but properly formed teeth, which never protrude from the jaws;
certain bones, and even the entire extremities are represented by mere
little cylinders or points of bone, often soldered to other bones: many
beetles have exceedingly minute but regularly formed wings lying under
their wing-cases{488}, which latter are united never to be opened: many
plants have, instead of stamens, mere filaments or little knobs; petals
are reduced to scales, and whole flowers to buds, which (as in the
feather hyacinth) never expand. Similar instances are almost
innumerable, and are justly considered wonderful: probably not one
organic being exists in which some part does not bear the stamp of
inutility; for what can be clearer{489}, as far as our reasoning powers
can reach, than that teeth are for eating, extremities for locomotion,
wings for flight, stamens and the entire flower for reproduction; yet
for these clear ends the parts in question are manifestly unfit.
Abortive organs are often said to be mere representatives (a
metaphorical expression) of similar parts in other organic beings; but
in some cases they are more than representatives, for they seem to be
the actual organ not fully grown or developed; thus the existence of
mammae in the male vertebrata is one of the oftenest adduced cases of
abortion; but we know that these organs in man (and in the bull) have
performed their proper function and secreted milk: the cow has normally
four mammae and two abortive ones, but these latter in some instances are
largely developed and even (??) give milk{490}. Again in flowers, the
representatives of stamens and pistils can be traced to be really these
parts not developed; Koelreuter has shown by crossing a diaecious plant (a
Cucubalus) having a rudimentary pistil{491} with another species having
this organ perfect, that in the hybrid offspring the rudimentary part is
more developed, though still remaining abortive; now this shows how
intimately related in nature the mere rudiment and the fully developed
pistil must be.
{486} In the _Origin_, Ed. i. p. 450, vi. p. 619, the author does
not lay stress on any distinction in meaning between the terms
_abortive_ and _rudimentary_ organs.
{487} _Origin_, Ed. i. p. 450, vi. p. 619.
{488} _Ibid._
{489} This argument occurs in _Origin_, Ed. i. p. 451, vi. p. 619.
{490} _Origin_, Ed. i. p. 451, vi. p. 619, on male mammae. In the
_Origin_ he speaks certainly of the abortive mammae of the cow
giving milk,--a point which is here queried.
{491} _Origin_, Ed. i. p. 451, vi. p. 620.
Abortive organs, which must be considered as useless as far as their
ordinary and normal purpose is concerned, are sometimes adapted to other
ends{492}: thus the marsupial bones, which properly serve to support the
young in the mother's pouch, are present in the male and serve as the
fulcrum for muscles connected only with male functions: in the male of
the marigold flower the pistil is abortive for its proper end of being
impregnated, but serves to sweep the pollen out of the anthers{493}
ready to be borne by insects to the perfect pistils in the other
florets. It is likely in many cases, yet unknown to us, that abortive
organs perform some useful function; but in other cases, for instance in
that of teeth embedded in the solid jaw-bone, or of mere knobs, the
rudiments of stamens and pistils, the boldest imagination will hardly
venture to ascribe to them any function. Abortive parts, even when
wholly useless to the individual species, are of great signification in
the system of nature; for they are often found to be of very high
importance in a natural classification{494}; thus the presence and
position of entire abortive flowers, in the grasses, cannot be
overlooked in attempting to arrange them according to their true
affinities. This corroborates a statement in a previous chapter, viz.
that the physiological importance of a part is no index of its
importance in classification. Finally, abortive organs often are only
developed, proportionally with other parts, in the embryonic or young
state of each species{495}; this again, especially considering the
classificatory importance of abortive organs, is evidently part of the
law (stated in the last chapter) that the higher affinities of organisms
are often best seen in the stages towards maturity, through which the
embryo passes. On the ordinary view of individual creations, I think
that scarcely any class of facts in natural history are more wonderful
or less capable of receiving explanation.
{492} The case of rudimentary organs adapted to new purposes is
discussed in the _Origin_, Ed. i. p. 451, vi. p. 620.
{493} This is here stated on the authority of Sprengel; see also
_Origin_, Ed. i. p. 452, vi. p. 621.
{494} _Origin_, Ed. i. p. 455, vi. p. 627. In the margin R. Brown's
name is given apparently as the authority for the fact.
{495} _Origin_, Ed. i. p. 455, vi. p. 626.
_The abortive organs of physiologists._
Physiologists and medical men apply the term "abortive" in a somewhat
different sense from naturalists; and their application is probably the
primary one; namely, to parts, which from accident or disease before
birth are not developed or do not grow{496}: thus, when a young animal
is born with a little stump in the place of a finger or of the whole
extremity, or with a little button instead of a head, or with a mere
bead of bony matter instead of a tooth, or with a stump instead of a
tail, these parts are said to be aborted. Naturalists on the other hand,
as we have seen, apply this term to parts not stunted during the growth
of the embryo, but which are as regularly produced in successive
generations as any other most essential parts of the structure of the
individual: naturalists, therefore, use this term in a metaphorical
sense. These two classes of facts, however, blend into each other{497};
by parts accidentally aborted, during the embryonic life of one
individual, becoming hereditary in the succeeding generations: thus a
cat or dog, born with a stump instead of a tail, tends to transmit
stumps to their offspring; and so it is with stumps representing the
extremities; and so again with flowers, with defective and rudimentary
parts, which are annually produced in new flower-buds and even in
successive seedlings. The strong hereditary tendency to reproduce every
either congenital or slowly acquired structure, whether useful or
injurious to the individual, has been shown in the first part; so that
we need feel no surprise at these truly abortive parts becoming
hereditary. A curious instance of the force of hereditariness is
sometimes seen in two little loose hanging horns, quite useless as far
as the function of a horn is concerned, which are produced in hornless
races of our domestic cattle{498}. Now I believe no real distinction can
be drawn between a stump representing a tail or a horn or the
extremities; or a short shrivelled stamen without any pollen; or a
dimple in a petal representing a nectary, when such rudiments are
regularly reproduced in a race or family, and the true abortive organs
of naturalists. And if we had reason to believe (which I think we have
not) that all abortive organs had been at some period _suddenly_
produced during the embryonic life of an individual, and afterwards
become inherited, we should at once have a simple explanation of the
origin of abortive and rudimentary organs{499}. In the same manner as
during changes of pronunciation certain letters in a word may become
useless{500} in pronouncing it, but yet may aid us in searching for its
derivation, so we can see that rudimentary organs, no longer useful to
the individual, may be of high importance in ascertaining its descent,
that is, its true classification in the natural system.
{496} _Origin_, Ed. i. p. 454, vi. p. 625.
{497} In the _Origin_, Ed. i. p. 454, vi. p. 625, the author in
referring to semi-monstrous variations adds "But I doubt whether
any of these cases throw light on the origin of rudimentary organs
in a state of nature." In 1844 he was clearly more inclined to an
opposite opinion.
{498} _Origin_, Ed. i. p. 454, vi. p. 625.
{499} See _Origin_, Ed. i. p. 454, vi. p. 625. The author there
discusses monstrosities in relation to rudimentary organs, and
comes to the conclusion that disuse is of more importance, giving
as a reason his doubt "whether species under nature ever undergo
abrupt changes." It seems to me that in the _Origin_ he gives more
weight to the "Lamarckian factor" than he did in 1844. Huxley took
the opposite view, see the Introduction.
{500} _Origin_, Ed. i. p. 455, vi. p. 627.
_Abortion from gradual disuse._
There seems to be some probability that continued disuse of any part or
organ, and the selection of individuals with such parts slightly less
developed, would in the course of ages produce in organic beings under
domesticity races with such parts abortive. We have every reason to
believe that every part and organ in an individual becomes fully
developed only with exercise of its functions; that it becomes developed
in a somewhat lesser degree with less exercise; and if forcibly
precluded from all action, such part will often become atrophied. Every
peculiarity, let it be remembered, tends, especially where both parents
have it, to be inherited. The less power of flight in the common duck
compared with the wild, must be partly attributed to disuse{501} during
successive generations, and as the wing is properly adapted to flight,
we must consider our domestic duck in the first stage towards the state
of the Apteryx, in which the wings are so curiously abortive. Some
naturalists have attributed (and possibly with truth) the falling ears
so characteristic of most domestic dogs, some rabbits, oxen, cats,
goats, horses, &c., &c., as the effects of the lesser use of the muscles
of these flexible parts during successive generations of inactive life;
and muscles, which cannot perform their functions, must be considered
verging towards abortion. In flowers, again, we see the gradual abortion
during successive seedlings (though this is more properly a conversion)
of stamens into imperfect petals, and finally into perfect petals. When
the eye is blinded in early life the optic nerve sometimes becomes
atrophied; may we not believe that where this organ, as is the case with
the subterranean mole-like Tuco-tuco <_Ctenomys_>{502}, is frequently
impaired and lost, that in the course of generations the whole organ
might become abortive, as it normally is in some burrowing quadrupeds
having nearly similar habits with the Tuco-tuco?
{501} _Origin_, Ed. i. p. 11, vi. p. 13, where drooping-ears of
domestic animals are also given.
{502} _Origin_, Ed. i. p. 137, vi. p. 170.
In as far then as it is admitted as probable that the effects of disuse
(together with occasional true and sudden abortions during the embryonic
period) would cause a part to be less developed, and finally to become
abortive and useless; then during the infinitely numerous changes of
habits in the many descendants from a common stock, we might fairly have
expected that cases of organs becom<ing> abortive would have been numerous.
The preservation of the stump of the tail, as usually happens when an
animal is born tailless, we can only explain by the strength of the
hereditary principle and by the period in embryo when affected{503}: but
on the theory of disuse gradually obliterating a part, we can see,
according to the principles explained in the last chapter (viz. of
hereditariness at corresponding periods of life{504}, together with the
use and disuse of the part in question not being brought into play in
early or embryonic life), that organs or parts would tend not to be
utterly obliterated, but to be reduced to that state in which they
existed in early embryonic life. Owen often speaks of a part in a
full-grown animal being in an "embryonic condition." Moreover we can
thus see why abortive organs are most developed at an early period of
life. Again, by gradual selection, we can see how an organ rendered
abortive in its primary use might be converted to other purposes; a
duck's wing might come to serve for a fin, as does that of the penguin;
an abortive bone might come to serve, by the slow increment and change
of place in the muscular fibres, as a fulcrum for a new series of
muscles; the pistil{505} of the marigold might become abortive as a
reproductive part, but be continued in its function of sweeping the
pollen out of the anthers; for if in this latter respect the abortion
had not been checked by selection, the species must have become extinct
from the pollen remaining enclosed in the capsules of the anthers.
{503} These words seem to have been inserted as an afterthought.
{504} _Origin_, Ed. i. p. 444, vi. p. 611.
{505} This and similar cases occur in the _Origin_, Ed. i. p. 452,
vi. p. 621.
Finally then I must repeat that these wonderful facts of organs formed
with traces of exquisite care, but now either absolutely useless or
adapted to ends wholly different from their ordinary end, being present
and forming part of the structure of almost every inhabitant of this
world, both in long-past and present times--being best developed and
often only discoverable at a very early embryonic period, and being full
of signification in arranging the long series of organic beings in a
natural system--these wonderful facts not only receive a simple
explanation on the theory of long-continued selection of many species
from a few common parent-stocks, but necessarily follow from this
theory. If this theory be rejected, these facts remain quite
inexplicable; without indeed we rank as an explanation such loose
metaphors as that of De Candolle's{506}, in which the kingdom of nature
is compared to a well-covered table, and the abortive organs are
considered as put in for the sake of symmetry!
{506} The metaphor of the dishes is given in the Essay of 1842, p.
47, note 3.{Note 173}