154 THE EVOLUTION OF SEX. 



physiological range of natural and successful intercrossing. Domesti- 

 cated breeds are usually fertilisable 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 intercrossed, while this very 

 rarely occurs with the two species. The difference in the reproductive 

 elements must often Ije greater than the structural differences of the 

 atlults would suggest. Hertwig has experimented of late with the artificial 

 hybridisation of echinoderms, and Born with that of amphibians. Both 

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

 that may result. In three cases Born brought about reciprocal hybridisa- 

 tion, but this is by no means always the case. Sometimes real fertilisation 

 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 hybridisation 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, l)ut 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 con- 

 stitution 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 ill- 

 natured 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 oftgemmules." " 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 intel- 

 ligible; while in hybridisation, which means the sexual union of organisms 



