144 



THE EVOLUTION OF SEX. 



demarcation of a species is the vague line which marks the physiological range 

 of natural and successful intercrossing. Domesticated breeds are usually 

 fertilizable mutually, and their progeny is fertile ; we regard them as mere 

 varieties. Nearly related species, however, only rarely admit of being crossed 

 even when under man's control ; and species-hybrids tend to be themselves 

 sterile. In structural characters two varieties of dog may seem more widely 

 separate than two nearly allied species, yet the varieties of dog may be inter- 

 crossed, while this very rarely occurs with the two species. The difference in 

 the reproductive elements must often be greater than the structural differences 

 of the adults would suggest. Hertwig has experimented of late with the 

 artificial hybridization of echinoderms, and Bom with that of amphibians. Both 

 emphasize the difficulties of the process, and the varying degrees of success 

 that may result. In three cases Born brought about reciprocal hybridization 

 but this is by no means always the case. Sometimes real fertilization took 

 place, but nothing followed ; in other cases the ova segmented ; in a few the 

 larval stages were reached ; and in two cases metamorphosis was survived. 

 The hybridization is the more readily effected the nearer the elements are to 

 perfect maturity. Sometimes the success seemed greatly to depend on the 

 concentration of the sperm-fluid, — the more dilute this was, the fewer sperms 

 were there to overcome the difficulties of effecting entrance to the ovum. 



There is no doubt that at least many species-hybrids tend to sterility, but 

 this is exhibited in varying degrees. The male mules are always sterile, but 

 the females may be successfully impregnated by horse or ass. In many cases 

 the hybrids are not fertile with one another, but remain so with the parent form 

 In a few cases the reproductive functions seem for a time at least to be 

 exaggerated rather than diminished as the result of crossing. It seems also 

 certain that while variety-hybrids are usually fertile, their constitution is more or 

 less unstable. They are often very variable, and apt to die out, as has been 

 repeatedly observed in the human species. The illnatured saying, "God 

 made the white man, God made the black man, the devil made the mulatto " 

 refers to the frequently inconvenient variability of those variety-hybrids. 



Brooks has laid considerable emphasis on the variability of hybrids in 

 connection with his theory of heredity. " Hybrids and mongrels," he says, 

 "are highly variable, as we should expect from the fact that many of the cells 

 of their bodies must be placed under unnatural conditions, and must therefore 

 have a tendency to throw off gemmules." "Hybrids, from forms which have 

 been long cultivated or domesticated, are more variable than those from wild 

 species or varieties, and the children of hybrids are more variable than the 

 hybrids themselves." "But domesticated animals and plants live under 

 unnatural conditions, and they are therefore more prolific of gemmules than 

 wild species ; and as the body of a male hybrid is a new thing, the cells will be 

 much more likely than those of the pure parent to throw off gemmules. The 

 fact that variation is due to the male influence, and that the action upon the 

 male parent of unnatural or changed conditions results in the variability of the 

 child, is well shown by crossing the hybrid with the pure species ; for when the 

 male hybrid is crossed with a pure female, the children are much more 

 variable than those born from a hybrid mother by a pure father." 



When we regard the male as katabolic, his variability becomes intelligible ; 

 while in hybridization, which means the sexual union of organisms with a more 

 than usually divergent life-experience, the reproductive elements which are 

 intermingled have probably a corresponding divergence in chemical 

 constitution. 



