The Making of Species 



both in first crosses and in hybrids, is an ex- 

 tremely general result ; but that it cannot, under 

 our present state of knowledge, be considered as 

 absolutely universal." 



Similarly Wallace writes : " Nevertheless, the 

 fact remains that most species which have 

 hitherto been crossed produce sterile hybrids, 

 as in the well-known case of the mule ; while 

 almost all domestic varieties, when crossed, 

 produce offspring which are perfectly fertile 

 among themselves." 



Darwin resorted to much ingenious argument 

 in his attempt to explain what he believed to 

 be the almost universal sterility of hybrids, as 

 opposed to mongrels or crosses between varieties. 

 He pointed out that changed conditions tend to 

 produce sterility, as is evidenced by the fact that 

 many creatures refuse to breed in confinement, 

 and believed that the crossing of distinct wild 

 species produced a similar effect on the sexual 

 organs. He expressed his belief that the early 

 death of the embryos is a very frequent cause of 

 sterility in first crosses. 



Wallace thus summarises Darwin's conclusions 

 as to the cause of the sterility of hybrids : " The 

 sterility or infertility of species with each other, 

 whether manifested in the difficulty of obtaining 

 first crosses between them or in the sterility of 

 the hybrids thus obtained, is not a constant or 

 necessary result of species difference, but is in- 



116 



