CuAP. VIII. SUMMARY. 205 



cicntly alike to be considered as varieties, and their mongrel 

 ofl'spring, are very generally, but not, as is so often stated, 

 invariably fertile. Nor is this almost universal and perfect 

 fertility surprising, Avhen we remember liow liable we are to 

 argue in a circle with respect to varieties in a state of nature ; 

 and when we remember that the greater number of varieties 

 have been jiroduced under domestication by the selection of 

 mere external difl'crences, and that they have not been long 

 exposed to uniform conditions of life. It should also be espe- 

 cially kept in mind, that long-continued domestication tends to 

 eliminate sterility, and is therefore little likely to induce this 

 same cjuality. Independently of the question of fertility, in all 

 other respects there is the closest general resemblance between 

 hybrids and mongrels, in their variability, in their power of 

 absorbing each other by repeated crosses, and in their inheri- 

 tance of characters from both parent-forms. Finally, then, 

 although we are profoundly ignorant of the precise cause of 

 the sterility of first crosses and of h^'brids, the facts given in 

 this chapter do not seem to me opposed to the belief that vari- 

 eties and species are not fundamentally different. 



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