Chap. VIII. FERTILITY OF MONGRELS. 269 



at first appears. It can, in the first place, be clearly- 

 shown that mere external dissimilarity between two spe- 

 cies does not determine their greater or lesser degree of 

 sterility when crossed ; and we may apply the same rule 

 to domestic varieties. In the second place, some emi- 

 nent naturalists believe that a long course of domesti- 

 cation tends to eliminate sterility in the successive 

 generations of hybrids, which were at first only slightly 

 sterile ; and if this be so, we surely ought not to expect 

 to find sterility both appearing and disappearing under 

 nearly the same conditions of life. Lastly, and this 

 seems to me by far the most important consideration, 

 new races of animals and plants are produced under 

 domestication by man's methodical and unconscious 

 power of selection, for his own use and pleasure : he 

 neither wishes to select, nor could select, slight differ- 

 ences in the reproductive system, or other constitutional 

 differences correlated with the reproductive system. 

 He supplies his several varieties with the same food ; 

 treats them in nearly the same manner, and does not 

 wish to alter their general habits of life. Nature acts 

 uniformly and slowly during vast periods of time on the 

 whole organisation, in any way which may be for each 

 creature's own good ; and thus she may, either directly, 

 or more probably indirectly, through correlation, modify 

 the reproductive system in the several descendants from 

 any one species. Seeing this difference in the process 

 of selection, as carried on by man and nature, we need 

 not be surprised at some difference in the result. 



I have as yet spoken as if the varieties of the same 

 species were invariably fertile when intercrossed. But 

 it seems to me impossible to resist the evidence of the 

 existence of a certain amount of sterility in the few 

 following cases, which I will briefly abstract. The evi- 

 dence is at least as good as that from which we believe 



