FERTILITY 603 



sexual constitution of the species which are crossed ; or on 

 their elective affinity." Sixthly, cross sterihty between species 

 may depend possibly in certain cases upon distinct causes, 

 such as deterioration due to unnatural conditions to which the 

 hybrid embryo may be exposed in the uterus, egg, or seed of 

 the mother. " Seventhly, hybrids and mongrels present, with 

 the one great exception of fertihty, the most striking accordance 

 in all other respects ; namely, in the laws of their resemblance 

 to their two parents, in their tendency to reversion, in their 

 variabihty, and in being absorbed through repeated crosses by 

 either parent form." It is obvious, however, that this last 

 conclusion requires some modification in the hght of recent 

 MendeUan research. 



Darwin maintains further that the cross fertility which 

 exists between the different varieties of various species of 

 domesticated animals, in spite of their great divergence in 

 external appearance, is the direct eiiect of domestication which 

 ehminates the tendency towards mutual sterility. In this way 

 " the domesticated descendants of species, which in their natural 

 state would have been in some degree sterile when, crossed, 

 become perfectly fertile together." Both Darwin and Wallace 

 lay stress upon the apparent existence of a parallelism between 

 crossing and change of conditions in so far as these afEect the 

 power to reproduce. " Shght changes of conditions and a sKght 

 amount of crossing, are beneficial ; while extreme changes,' and 

 crosses between individuals too far removed in structure or 

 constitution, are injurious." ^ Furthermore, domestic animals 

 are less susceptible to the influences of changed conditions of 

 existence than wild animals, a fact which finds a parallel in the 

 absence of sterihty between domesticated varieties of the same 

 species. 



Wallace has cited several cases in which it has been shown 

 that hybrids between distinct species are fertile inter se. Such 

 cases are the hybrids between the domestic and Chinese geese, 

 those between the Indian humped and common cattle, and the 

 various hybrids between the different species of the genus Cants. 

 A recently recorded case of a fertile hybrid between a hon 

 and a jaguar may also be cited. These and other observations 

 ' Wallace (A. B.), Darwinism, London, 1897. 



