Chap. VII. THE EACES OF MAN. 223 



sterility which is so general a result of the crossing of 

 species in a state of nature. From these several con- 

 siderations, it may be justly urged that the perfect ferti- 

 lity of the intercrossed races of man, if established, 

 would not absolutely preclude us from ranking them as 

 distinct species. 



Independently of fertility, the character of the off- 

 spring from a cross has sometimes been thought to 

 afford evidence whether the parent-forms ought to be 

 ranked as species or varieties ; but after carefully study- 

 ing the evidence, I have come to the conclusion that no 

 general rules of this kind can be trusted. Thus with 

 mankind the offspring of distinct races resemble in all 

 respects the offspring of true species and of varieties. 

 This is shewn, for instance, by the manner in which 

 the characters of both parents are blended, and by 

 one form absorbing another through repeated crosses. 

 In this latter case the progeny both of crossed species 

 and varieties retain for a long period a tendency to 

 revert to their ancestors, especially to that one which 

 is prepotent in transmission. When any character has 

 suddenly appeared in a race or species as the result of a 



possible that their sterility should he augmented by the preservation or 

 survival of the more and more sterile individuals ; for as the sterility 

 increases fewer and fewer offspring will be produced i'rom which to 

 breed, and at last only single individuals will be produced, at the rarest 

 intervals. But there is even a higher grade of sterility than this. Both 

 Gartner and Kolreuter have proved that in genera of plants including 

 numerous species, a series can be formed from species which when 

 crossed yield fewer and fewer seeds, to species which never produce a 

 single seed, but yet are affected by the pollen of the other species, for 

 the germen swells. It is here manifestly impossible to select the more 

 sterile individuals, which have already ceased to yield seeds ; so tW_ . ^ 

 the acme of sterility, wheu the germen alone is affected, caunpif*ne\ P A > 

 gained through selection. This acme, and no doubt the olhei^i\&es 

 of sterility, are the incidental results of certain unknowm d^^iicO*^' ' ^< 

 in the constitution of the reproductive system of the species/ "wiucKarj^^.,-^ 

 crossed. / ^0 



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