20 Darwin, and after Darwin. 



superimposed upon, forms of Segregation that have 

 been previously induced ; for when Negative Segre- 

 gation arises [i.e. isolation due to mutual sterility], 

 and the varieties of a species become less and less 

 fertile with one another, the complete infertility that 

 has existed between them and some other species 

 does not disappear, nor does the Positive Segregation 

 cease [i. e. any other form of isolation previously 

 existing]. ... In the second place, whenever 

 Segregation is directly produced by some quality of 

 the organism, variations that possess the endowment 

 in a superior degree will have a larger share in pro- 

 ducing the segregated forms of the next generation, 

 and accordingly the segregative endowment of the 

 next generation will be greater than that of the 

 present generation ; and so with each successive 

 generation the segregation will become increasingly 

 complete." And to this it may be added, in the 

 third place, that where the segregation (isolation) is 

 due to the external conditions of life under which 

 the organism is placed, or where it is due to natural 

 selection simultaneously operating in divergent lines 

 of evolution, the same remarks apply. Hence it 

 follows that discriminate isolation is, in all its forms, 

 cumulative. 



4. The next point to be noted is, that the cumu- 

 lative divergence of type thus induced can take 

 place only in as many different lines as there are 

 different cases of isolation. This is a point which 

 Mr. Gulick has not expressly noticed ; but it is one 

 that ought to be clearly recognized. Seeing that 

 isolation secures the breeding of similar forms by 

 exclusion (immediate or eventual) of those which are 



