70 Darwin, and after Darwin. 



certainly would they have become fused by inter- 

 crossing with one another, had it not been for the 

 barrier of sterility imposed by the primary distinc- 

 tion. Or rather, I should say, had it not been 

 for the original occurrence of this barrier, these now 

 closely-allied species could never have become species. 

 Again, that dominant species should be rich in varie- 

 ties is what might have been expected ; for the 

 greater the number of individuals in a species, the 

 greater is the chance of variations taking place in 

 all parts of the organic type, and particularly in the 

 reproductive system, seeing that this system is the 

 most sensitive to small changes in the conditions 

 of life, and that the greater the number of indi- 

 viduals composing a specific type, the more certainty 

 there is of some of them encountering such 

 changes. Hence, the richness of dominant species 

 in varieties is, I believe, mainly due to the greater 

 opportunity which such species afford of some degree 

 of cross-infertility arising between their constituent 

 members. 



Here is another general fact, also first noticed by 

 Darwin, and one which he experiences some difficulty 

 in explaining on the theory of natural selection. He 

 says : 



In travelling from north to south over a continent, we generally 

 meet at successive intervals with closely-allied or representative 

 species, evidently filling the same place in the economy of the 

 land. These representative species often meet and interlock, 

 and as one becomes rarer and rarer, the other becomes more and 

 more frequent, till the one replaces the other. But if we com- 

 pare these species where they intermingle, they are generally as 

 absolutely distinct from each other in every detail of structure as 

 are specimens taken from the metropolis of each. ... In the 



