" Granting, with the doctrine of evolution, that all the complex assemblages of 

 existing animal forms are modified derivatives from previously existing forms, and 

 that these are ultimately to be traced back to some common ancestor, it must of 

 necessity follow that any given fauna will depend for the degree of its peculiarity, 

 wliether great or small, upon the amount of modification, relative to any other 

 fauna, which it will have undergone. And this modification can be effected in two 

 ways : by inherent modification of the individual types composing the fauna, and 

 by intermixture with, or immigration from, contiguous or neighbouring faunas. In 

 both cases, manifestly, isolation or its opposite, union of habitation, will constitute 

 the governing factor in determining the amount of variation." — HEiLPPaN. 



