Chap. YU.] THE RACES OF MAN. 215 



the sterility which is so general a result of the crossing of 

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

 erations, it may be justly urged that the perfect fertility 

 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 offspring 

 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 studying the evidence, I have 

 come to the conclusion that no general rules of this kind 

 can be trusted. Thus with mankind the offspring of dis- 

 tinct races resemble in all respects the offspring of true 

 species and of varieties. This is shown, for instance, by 

 the manner in which the characters of both parents are 

 blended, and by one form absorbing another through re- 

 peated crosses. In this latter case the progeny both of 

 crossed species and varieties retain for a long period a ten- 

 dency 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 single act of variation, as. is general with monstrosi- 

 ties, 14 and this race is crossed with another not thus char- 

 acterized, the characters in question do not commonly ap- 



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. that the acme of sterility, when the germen 

 •alone is affected, cannot be gained through selection. This acme, and 

 no doubt the other grades of sterility, are the incidental results of certain 

 unknown differences in the constitution of the reproductive system of the 

 species which are crossed. 



14 ' The Variation of Animals,' etc., vol. ii. .p. 92. 



