No. 567] EFFECT OF DISTRIBUTION ON SPECIATION 153 



lated to their difference in mode of life, already shown 

 in an incipient manner, would soon lead to their generic 

 differentiation. 



It is not argued that under a given set of ecologic con- 

 ditions, only one type could be produced, nor that accord- 

 ing to the idea of some zoologists, as set forth and refuted 

 by Grinnell and Swarth (1913), should individuals of one 

 geographic race be transplanted into the region of a dif- 

 ferent geographic race, the first race would assume within 

 a few generations all the characters of the second race. 

 Whether the changes due to the influence of the environ- 

 ment be looked upon as the results of natural selection 

 and adaptation, or merely as the results of a stimulus to 

 the germ plasm, the new type would not necessarily be 

 always the same, this, however, depending upon the num- 

 ber of potential responses in the type, and, as excellently 

 shown by Buthven (1909) in his study of evolution in the 

 genus Thamnophis, upon the modifications previously 

 undergone by the type we are dealing with. 



It is very evident that there are many variations in 

 animals which seem to fall into neither the extrinsic nor 

 intrinsic category, but which are neutral and vary inde- 

 pendently of climate or habits, and may be inherited phy- 

 logenetic tendencies. It is very largely due to these 

 neutral variations, frequently to be ascribed to ortho- 

 genetic evolution, tending in different directions in dif- 

 ferent places, and given an opportunity to diverge by iso- 

 lation, that different species may be produced to occupy 

 regions of similar climatic and environmental conditions, 

 and different genera may be found occupying the same 

 ecologic niches. 



To choose an example in the same family quoted before, 

 we may cite the case of Tamias in eastern North America, 

 and Eutamias in western North America. In this case the 

 characters separating the genera are not clearly related 

 to their mode of life, the chief difference being the loss of 

 one small premolar in Tamias, and its retention in Euta- 

 mias. The extent of divergence of these neutral varia- 



