ISOLATION 287 



being restricted, not merely to the same island but 

 actually to the same valley. Moreover, the differ- 

 ences between varieties in the various valleys increase 

 in proportion to the distance which separates the val- 

 leys. Thus it was possible for Gulick to estimate 

 roughly the amount of divergence between the occu- 

 pants of any two given valleys b} r measuring the 

 number of miles between them. To Wallace's re- 

 mark that this fact might be due to the direct influence 

 of life conditions, Gulick answers that no actual differ- 

 ence was observable between valleys and that life con- 

 ditions could not account for the regular progression 

 of variations. 



In this case, the action of isolation seems to have 

 been perfectly fortuitous. A certain number of indi- 

 viduals similar to the others, were separated from the 

 rest of their species b} 7 - a geographical barrier, and 

 from the mere fact of isolation, new characters ap- 

 peared in them ; and this undoubtedly occurred without 

 the help of any environmental influence, for whenever 

 any number of individuals are divided up at haphaz- 

 ard into two groups, the average extent of individual 

 divergence is never the same in both groups. This is 

 the starting point of variation. Whenever the influ- 

 ence of the environment is superadded, divergences 

 grow more and more noticeable. 



Besides this first form of isolation, or geographical 

 isolation, which segregates a heterogeneous group of 



