No. 567] EFFECT OF DISTRIBUTION ON SPECIATION 151 



This leads us to a consideration of the factors involved 

 in the differentiation of genera as contrasted with the 

 differentiation of species. In general it may be said that 

 extrinsic modifications, i. e., those which are in some way 

 connected with changes in temperature, humidity, char- 

 acter of flora, food, and other environmental conditions, 

 and which usually affect such characters as color, size, 

 length of hair, etc., lead to differentiation of species and 

 subspecies primarily. On the other hand, intrinsic modi- 

 fications, i. e., those which are related directly or indi- 

 rectly to a change in the habits or mode of life of the 

 animal or the occupation of a new niche in nature, usually, 

 if not always, lead to generic or family differentiation, 

 since it is evident that changes fitting an animal to live 

 arboreally instead of terrestrially, for instance, are of 

 such a nature, that if they are perpetuated and carried to 

 perfection, will not stop at specific difference but will 

 become of generic importance. 



It might be argued that there are no modifications which 

 might not, if carried far enough, ultimately lead to generic 

 differentiation. This is possible, but very improbable, 

 because the modifications here alluded to as " extrinsic" 

 are of such a nature that in the varying climatic condi- 

 tions there are likely to be intermediate forms which make 

 the division of the more widely separated ones into genera 

 impracticable. In the case of our 4 'intrinsic" modifica- 

 tions, intermediate forms are not so likely to exist when 

 once the incipient changes leading to an altered mode of 

 life have reached a fair degree of perfection. 



As a concrete example of what is meant by extrinsic 

 and intrinsic modifications, let us take the squirrels of a 

 given region, say eastern North America. There are four 

 genera to be distinguished,— Sciurius, Tamias, Sciuro- 

 pterus and Arctomys. The genus Sciurus contains 

 strictly arboreal, mostly nut-eating, omnivorous forms. 

 Tamias includes forms which are terrestrial, diurnal, 

 dwelling in natural or artificial holes and crevices, and 

 with a device for carrying food in their cheeks. Seiuro- 



