1903.] NATURAL SCIENCES OF PHILADELPHIA. 15 



Jn the case of species represented in both areas but by distinct sub- 

 species, sometimes it is the humid form that is present and sometimes 

 the arid form (in a few cases there are intermediates). For instance, 

 we find the typical coast form of Cyanocitta, but the typical arid race of 

 Oreortyx. And in the case of distinct species, as a result of eastward 

 and westward invasion respectively, the ranges of the separate repre- 

 sentatives here overlap; for example, Parus rufescens and Parus gam- 

 beli. We find that as a rule two faunae (of the same zone) are occupied 

 each by a different subspecies of the same species, or a different species 

 of the same genus ; and that a single genus is seldom represented in one 

 fauna by more than one species. (If it is, then it seems to have come 

 through the invasion of one of the congeneric forms from an adjacent 

 fauna, as with the two chickadees just mentioned.) This seems to me a 

 strong argument in favor of the theory that isolation (either by long 

 distance or intervention of barriers) has been an absolutely essential 

 condition to the differentiation of species. A fauna is a certain assem- 

 blage of animals occupying a given uniform area; but the propor- 

 tionate composition in both species and individuals is constantly 

 changing, as influenced by neighboring faunae, as well as endemic 

 factors. 



