166 CRITIQUES AND ADDRESSES. [vn. 



the elimination of the whiter and smoother-haired. In 

 fact, by the operation of causes precisely similar to those 

 which, in the famous instance cited by Mr. Darwin, 

 have given rise to a race of black pigs in the forests 

 of Louisiana, a negro stock would eventually people 

 the region. 



Again, how often, by such physical changes, must a 

 stock have been isolated from all others for innumerable 

 generations, and have found ample time for the hereditary 

 hardening of its special peculiarities into the enduring 

 characters of a persistent modification. 



Nor, if it be true that the physiological difference of 

 species may be produced by variation and natural selec- 

 tion, as Mr. Darwin supposes, would it be at all astonish- 

 ing if, in some of these separated stocks, the process of 

 differentiation should have gone so far as to give rise to 

 the phenomena of hybridity. In the face of the over- 

 whelming evidence in favour of the unity of the origin 

 of mankind afforded by anatomical considerations, satis- 

 factory proof of the existence of any degree of sterility 

 in the unions of members of two of the "persistent 

 modifications " of mankind, might well be appealed to 

 by Mr. Darwin as crucial evidence of the truth of his 

 views regarding the origin of species in general. 



