1914] Grinnt 11: Mammals and Birds of the Colorado Vallt y 99 



occupants in other regions, occupied here? Probably not. for the inter- 

 vention of barriers has doubtless prevented the invasion of types 

 which, if they could have once gotten there, would have thriven and 

 assumed a place as endemic elements in the fauna. Sporadic incur- 

 sion, as of migrants among birds, and strays among both birds and 

 mammals, do not appear to the writer to figure in such a process. 

 Rather must it be a progressive invasion of the species en masse, 

 acquiring, it may be, adaptive modifications as it proceeds. In other 

 words, the conquering of the land is the combined result of the 

 facilities offered by it plus the relative amenability of each species 

 concerned. 



The twelve riparian species and subspecies peculiar to the Colorado 

 fauna vary much in degree of difference from their near relatives 

 which occupy adjacent differentiation areas. These varying degrees 

 of difference might be interpreted as measures of the periods of time 

 elapsed since the entrance into the region of each of the types 

 involved. That this conclusion is poorly grounded is evident upon 

 consideration of the various other elements which must figure in the 

 process of species formation. Among these may be suggested: degree 

 of isolation, divergence of homologous associational conditions in the 

 new region from those in the ancestral, and inherent susceptibility 

 to adaptive modification in each of the species concerned. 



In the problem of the origin of the riparian portion of the Colorado 

 fauna we seem to have to do with an accentuated kind of isolation. 

 For, as already asserted, there is such a thing as more and less isolation. 

 In the region here considered, possessing extreme associational con- 

 trast, we find the ordinary geographic, or more properly speaking, 

 physiographic, isolation coupled with associational isolation. In con- 

 sequence of this extra favorable contingency, differentiation of species 

 may have progressed with particular celerity, with such distinct forms 

 to show for it as Pipilo abcrii, Vermivora luciae, Toxostoma crissale, 

 and Procyon pallidus. 



The axiom has presented itself in this connection that the more 

 restricted a species is associationally, that is, the more confined to a 

 narrow range of associational conditions, the more subject it is to the 

 important factor of isolation ; hence the more liable to give rise to 

 new incipient strains in different parts of its general range. 



An assertion which seems at first glance opposed to the above is : 

 that the less restricted a species is associationally, that is, the more 

 widely adaptable to varying conditions, the more numerous the chances 



