THE INFLUENCE OF PHYSICAL CONDITIONS IN THE 

 GENESIS OF SPECIES." 



By Joel A. Allen. 



Among biologists who accoi)t the modern theory of evohition as 

 the only reasonable hypothesis available for the explanation of the 

 diversity of structure among organized beings, there is a wide differ- 

 ence of opinion as to wliat are the leading causes of differentiation. 

 The doctrine of natural selection, or the survival of the fittest, has 

 recently been brought prominently forward as the key to this complex 

 problem, and is upheld by a large class of enthusiastic adherents, 

 who accept it as the full solution of the whole question. By others 

 the conditions of environment are believed to be far more influential 

 in effecting a certain class of modifications, at least, than the neces- 

 sarily precarious influence of natural selection, which must take its 

 origin in isolated instances of variation in favorable directions, and 

 depend for its continuance upon these fortuitous advantages being 

 inherited by the descendants of the favored individuals in which they 

 originate. The modifying influence of conditions resulting from 

 geographic or climatic causes was long since noticed, and for nearly a 

 century has been considered by many writers as explanatory of much 

 of the diversity existing not only in the human race but among ani- 

 mals. It has, however, remained until recently vaguely grounded, 

 being based more in conjecture than on observed facts. Scarcely, 

 indeed, have two decades passed since the real nature and extent of 

 geographical variation among animals, and even as yet among only a 

 few species, began to receive careful attention, while only within the 

 last fifteen years has any attempt been made to correlate the ob- 

 served differences with the climatic or geographical conditions of 

 habitat. Only within recent years have the differences in general 

 size, and in the relative size of different parts, been ascertained by 

 careful measurement, and the differences in the character of the tegu- 

 mentary covering (as the pelage in mammals) and in color, in indi- 



« Roiiriiitod, with note and hrackotod additions by llic antlior. from the Kadi- 

 cal Review, 1, 1877, pp. 108-140. 



375 



