^°l'w^^] Grinnell, Chestnut-backed Chickadee. 37 1 



ation. Endemic animals must adaptively respond or else be exter- 

 minated or restricted to the places where faunal change is slowest. 

 The possibility at once presents itself of Parus pre-hudsonicus 

 having been already native of the coast before the latter became 

 faunally distinct from the interior. But in either case the original 

 populating of the region must have been through invasion from 

 elsewhere, as effected by shifting climatic conditions. 



At any rate a center of distribution must have arisen in the new 

 region of different faunal conditions. Just as quick as the new 

 colony began to reproduce fast enough to furnish a return flow of 

 individuals the immigration of individuals bearing the inherited 

 stock characters from the parent region would be checked. This 

 would mean that the new colony would become a new center of 

 differentiation because of the isolation thus afforded. (As to what 

 brings about the acquisition or change of innate characters, whether 

 by natural selection or some other more direct cause, we need not 

 here try to discuss.) 



As the dissemination of individuals to prevent congestion of 

 population will be continually away from the centers of distribu- 

 tion, it follows that the characters newly acquired at the centers 

 where the rate of differentiation is greatest will be constantly car- 

 ried away from those centers. If the region of intermediate faunal 

 conditions were narrow, as in the present case, individuals bearing 

 the inherited characters impressed by their separate areas of differ- 

 entiation would from generation to generation invade toward each 

 other until intermediates would be swamped, or there might be an 

 unfit strip left between where neither would flourish. This might 

 be bridged over by hybrids for a while. But the specific charac- 

 ters becoming strengthened by time would make hybridization less 

 and less likely to take place, and there would result the two dis- 

 tinct species as we now know them. 



In the case of Parus rufescens and Parus hudsonkus there seems 

 to be now a narrow hiatus between the two. At least I can find 

 no record of the two species having been found in the same local- 

 ity. The narrowness of the region of intermediate faunal condi- 

 tions may therefore be considered as the reason why we do not 

 find connecting links between hudsojiicus and rufescens at the pres- 

 ent time. For the amount of difference between these two chicka 



