SUPPLEMENTAL HISTORY OP MAN, 179 



tain the causes which promote the tendency to vary- 

 in mankind, yet that such tendency does clearly 

 exist in other animals, and therefore, by analogy, 

 that it exists also in man, and is sufficient to ac- 

 count for, if not to explain, the phenomenon in 

 question. 



It has been already stated from Cuvier, that 

 varieties in zoology consist of such differences as 

 arise between the descendants from one common 

 stock. The difference between immediate colla- 

 teral relations are those of individual description, 

 but when these become hereditary, then may they 

 properly be called varieties. These hereditary 

 varieties always propagate with each other, while 

 animals specifically different, seldom do, and when 

 instances occur, the hybrid offspring is not prolific ; 

 or, at least, not beyond one or two descents. That 

 the natural barrier between the species has been 

 evermore effectually destroyed, so as by the con- 

 nexion of two to produce a third, distinct from either 

 and permanently prolific, has never been satisfac- 

 torily proved. 



The integrity of all the individuals of mankind, as 

 a single species, may be tried and proved by this 

 test. The whole human race is prolific by promis- 

 cuous intermixture. 



It must not be forgotten also, that, although there 

 are black races and white races, there are indivi- 

 duals of almost every tint leading from one of these 

 extremes of colour to the other; although there are 

 races with a facial line nearly vertical, and others 



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