HYBRIDISM 241 



European dogs, the explanation which will occur to 

 every one, and probably the true one, is that these 

 dogs have descended from several aboriginally distinct 

 species. Nevertheless the perfect fertility of so many 

 domestic varieties, differing widely from each other 

 in appearance, for instance of the pigeon or of the 

 cabbage, is a remarkable fact ; more especially when 

 we reflect how many species there are, which, though 

 resembling each other most closely, are utterly sterile 

 when intercrossed. Several considerations, however, 

 render the fertility of domestic varieties less remark- 

 able than at first appears. It can, in the first place, be 

 clearly shown that mere external dissimilarity between 

 two species 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 eminent naturalists believe that a long course of 

 domestication tends to eliminate sterility in the suc- 

 cessive 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 dis- 

 appearing 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 pro- 

 duced 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 differences in the reproductive system, or other 

 constitutional differences correlated with the repro- 

 ductive 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 repro- 

 ductive system in the several descendants from 

 any one species. Seeing this difference in the pro- 

 cess of selection, as carried on by man and nature, 



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