CROSSING OF SPECIES AND VARIETIES 125 



inability of the male element, even if it readies the ovum, to 

 fertilise the latter; or, again, in the early destruction of the 

 embryo, if an embryo is formed. Such an early death of the 

 embryo is a very frequent cause of sterility in first crosses ; and this 

 is not surprising when we remember that a hybrid has, as it were, 

 only half of the nature and constitution of its mother, and that 

 it may therefore, before birth, " as long as it is nourished within 

 its mother's womb or within the egg or seed produced by the 

 mother, be exposed to conditions in some degree unsuitable, and 

 consequently be hable to perish at an early period, more especi- 

 ally as all very young beings are eminently sensitive to injurious 

 or unnatural conditions of life." But, after all, as Darwin 

 remarks, " the cause more probably lies in some imperfection in 

 the original act of impregnation, causing the embryo to be im- 

 perfectly developed, rather than in the conditions to which it is 

 subsequently exposed."^ 



It must nevertheless be remarked that there is a great difference, 

 as regards consequences, between the crossing of two distinct 

 species and the crossing of two varieties of the same species. We 

 have already mentioned that domestication, in the case of species 

 long adapted to it, tends to eliminate sterihty in hybrids ; and, 

 consequently, the external conditions of a species long domesti- 

 cated may be changed very considerably without the reproductive 

 power of the species being thereby affected. On the other hand, 

 wild species, when taken from their natural conditions of life 

 and placed under domestication, will not breed, the most con- 

 spicuous example of this being the elephant. There is much 

 analogy between the sterihty arising from hybridism and the 

 sterihty arising from a radical change in the environment of a 

 wild animal, such as is effected when an animal is suddenly 

 placed under domestication. In either case, it is the reproduc- 

 tive organs which are affected and rendered impotent. We have 

 seen that such a result is, in the case of hybrids, not in the least 

 1 The Origin of Species, pp. 388, 389. 



