224 Darwin, and after Darwin. 



set forth by Mr. Gulick : " The fundamental cause 

 of this seems to lie in the fact that no two portions of 

 a species possess exactly the same average characters ; 

 and, therefore, that the initial differences are for 

 ever reacting on the environment and on each other 

 in such a way as to ensure increasing divergence 

 in each generation, as long as the individuals of 

 the two groups are kept from intergenerating 1 ." In 

 other words, as soon as a portion of a species is 

 separated from the rest of that species, so that 

 breeding between the two portions is no longer 

 possible, the general average of characters in the 

 separated portion not being in all respects precisely 

 the same as it is in the other portion, the result of 

 in-breeding among all individuals of the separated 

 portion will eventually be different from that which 

 obtains in the other portion ; so that, after a number 

 of generations, the separated portion may become 

 a distinct species from the effect of isolation alone. 

 Even without the aid of isolation, any original dif- 

 ference of average characters may become, as it 

 were, magnified in successive generations, provided 

 that the divergence is not harmful to the individuals 

 .presenting it, and that it occurs in a sufficient pro- 

 portional number of individuals not to be immedi- 

 ately swamped by intercrossing. For, as Mr. Murphy 

 has pointed out, in accordance with Delboeufs law, 

 "if, in any species, a number of individuals, bearing 

 a ratio not infinitely small to the entire number of 

 births, are in every generation born with a particular 

 variation which is neither beneficial nor injurious, 



1 Divergent Evoluion through Cumulative Segregation, Linn. Journ. 

 Zoology, vol. xx. p. 215. 



