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.
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.
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?
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.
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!