112 CALIFORNIA ACADEMY OF SCIENCES. 



great assumption: they say that it must be the change 

 of organism which causes the sterility — it must be the 

 secondary distinctions which determine the primary. 

 But the contrary proposition is surely at least probable, 

 namely, that it is the sterility which, by preventing inter- 

 crossing with parent forms, has determined the sec- 

 ondary distinctions — or, rather, that it has been the 

 original condition to the operation of the modifying 

 causes in all cases where free intercrossing has not 

 been otherwise prevented." The author then proceeds 

 to show that it is not logical to suppose that any one of 

 so infinite a number of changes could have the same 

 effect in modifying the reproductive system. Further- 

 more, these changes constantly occur among domesti- 

 cated animals without affecting the fertility of the 

 breed. Darwin has shown that among wild species the 

 ratio between structural affinity and the degree of ster- 

 ility is not always constant, some very distinct species 

 being found to hybridize with facility; but this is not in 

 accord with the view that the sterility is universally due 

 to structural differences. "Mr. Darwin further shows 

 that, ' independently of the question of fertility, in 

 all other respects there is the closest general resemblance 

 between hybrids and mongrels.' Clearly, this fact 

 implies that natural selection and artificial selection run 

 perfectly parallel in all other respects, save in the one 

 respect of reacting on the reproductive system, where, 

 according to the views against which I am arguing, 

 they must be regarded as differing, not only constantly 

 but also profoundly. Lastly, Mr. Darwin concedes — or 

 rather insists — that ' the primary cause of the sterility 

 of crossed species is confined to differences in their 

 sexual elements.' A general fact which assuredly proves 

 that the primary specific distinction is one with which 

 the organism as a whole is not concerned: it is merely a 



