Opinions on Isolation. 127 



that are needed by natural selection in order to produce diver- 

 gence. Therefore, it appears to me that, under the circum- 

 stances supposed, natural selection alone could not produce 

 divergence ; the most it could do would be to change the whole 

 specific type in some one direction, and thus induce trans- 

 mutation of species in a linear series, each succeeding member 

 of which might supplant its parent form. But in order to 

 secure diversity, multiplication, or ramification of species, 

 it appears to me obvious that the primary condition required is 

 that of preventing intercrossing with parent forms at the origin 

 of each branch, whether the prevention be from the first 

 absolute, or only partial. 



Now for Mr. Gulick. a portion of whose more 

 lengthy discussion of the subject, however, is all that 

 I need quote 



Having found that the evolution of the fitted is secured through 

 the prevention of crossing between the better fitted and the less 

 fitted, can we believe that the evolution of a special race, 

 regularly transmitting a special kind of fitness, can be realized 

 without any prevention of crossing with other races that have 

 no power to transmit that special kind of fitness? Can we 

 suppose that any advantage, derived from new powers that 

 prevent severe competition with kindred, can be permanently 

 transmitted through succeeding generations to one small section 

 of the species while there is free crossing equally distributed 

 between all the families of the species ? Is it not apparent that 

 the terms of this supposition are inconsistent with the funda- 

 mental laws of heredity ? Does not inheritance follow the lines 

 of consanguinity ; and when consanguinity is widely diffused, 

 can inheritance be closely limited ? When there is free crossing 

 between the families of one species, will not any peculiarity 

 that appears in one family either be neutralized by crosses 

 with families possessing the opposite quality, or, being preserved 

 by natural selection, while the opposite quality is gradually 

 excluded, will not the new quality gradually extend to all the 

 branches of the species ; so that, in this way or in that, increas- 

 ing divergence of form will be prevented ? 



