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Show 222 THE DESCENT OF MAN. PART I. there is in hybrids between lessened fertility and vitality: other analogous cases could be added. Even if it should hereafter be proved that all the races of men were perfectly fertile together, he who was inclined from other reasons to rank them as distinct species, might with justice argue that fertility and sterility are not safe criterions of specific distinctness. We know that these qualities are easily affected by changed conditions of life or by close inter-breeding, and that they are governed by highly complex laws, for instance that of the unequal fertility of reciprocal crosses between the same two species. With forms which must be ranked as undoubted species, a perfect series exists from those which are absolutely sterile when crossed, to those which are almost or quite fertile. TJ:te degrees of sterility do not coincide strictly with the deO'rees of difference in external structure or habits of lif~ Man in many respects may be compared with those animals whic~ have long been domesticated, and a large body of evidence can be advanced in favour of the Pallasian doctrine 13 that domestication tends to eliminate the 13 'The Variation of An~mals and Plants under Domestication,' vol. ii. P· 109. I rna~ here remm~ the reader that the sterility of specie~ wh~n crossed 1.s not a spemally-acquired quality; but, like the incapaClt! of ~ertam trees to be grafted together, is incidental on other acqmred differences. ~he nature of these differences is unknown, but they relate more espeCially to the reproductive system, and much less ~o external structu~e or to ordinary differences in constitution. One Important eleme~t m the sterility of crossed species apparently lies in one or both havmg been long habituated to fixed conditions. for we kno":" that changed conditions have a special influence on th~ reproductive system, and we ~ave good reason to believe (as before remarked) that ~~e fiuc~uatt?g conditions of domestication tend to eliminate that sterility whiCh 1s so general with species in a natural state when crossed. It has elsewhere been shewn by me (ibid 1 .. 185 d ' 0 . . f s . ' . vo . ll. p. ' an . ngm o pemes, 5th edit. p. 317) that the sterility of crossed specws has not been acquired through natural selection: we can see that when two forms have already been rendered very sterile, it is scarcely CHAP. VII. THE RACES OF MAN. 223 sterilitv which is so general a result of the crossing of ·es" 1·n a state of nature. From these several con-spec! ' h .c t f t" siderations, it may be justly urged that t ~ per1ec . er I-r t of the intercrossed races of man, 1f established, ~!uld not absolutely preclude us from ranking them as distinct species. Independently of fertility, the character of the off-spring from a cross has sometimes been thought to afford evidence whether the parent-forms ought to be mnked as species or varieties; but after carefully studying the evidence, I have come to the conclusion that .no general rules of this kind ?a~ be trusted. Thus. with mankind the offspring of d1stmct races resemble .m. all respects the offspring of true species and of ;anet~es. 'rhis is shewn, for instance, by the manner m whiCh the characters of both parents are blended, and by one form absorbing another through repeat~d cross?s. In this latter case the progeny both of crossed spemes and varieties retain for a long period a tendency. to revert to their ancestors, especially to that one whiCh is prepotent in transmission. vVhe_n any character has suddenly appeared in a race or spemes as the result of a possible that their sterility should ~e a~gn;te?ted by the pres~rva;io~li~r urvival of the more and more stenle mdiv1duals; for as t e s ~n Y ~ncreases fewer and fewer offspring will be produced from whiCh to breed, and at last only single individuals will be p:~duced, at ~e rarest intervals. But there is even a higher grade of stenlity than th~. B?th Gartner and Kolreuter have proved that in genera of ~!ants ~ncludmg numerous species a series can be formed from spemes which when crossed yield few~r and fewer seeds, to species which never pro~uce a · le seed but yet are affected by the pollen of the other spemes, for ~~eg germ en' swells. It is here manifestly impossible .to select the more sterile individuals, which have already ceased t~ Yield seeds; so that the acme of sterility, when the gennen alone 1s affected, cannot be ained throu.,.h selection. This acme, and no doubt the oth~r grades ~f sterility ;re the incidental results of certain unkno~ d1ff~rences in the con;titution of the reproductive system of the spec1es which are crossed. I |
