256 DARWINISM TO-DAY. 



other American chickadees is the colour of the back, which is an 

 intense rusty-brown, approaching chestnut. It is of common note 

 that the most evident effects of similar climatic conditions on other 

 animals is a corresponding intensification of browns, especially 

 dorsally. We may, therefore, consider the chestnut-backed chicka- 

 dee, as indicated by its chief specific character, to be a product 

 exclusively of the peculiar isohumic area to which we find it 

 confined. 



"Parus rufescens, from Sitka to Monterey, has a chestnut-coloured 

 back. And from Sitka to Point Arena, between which we find 

 the extremest humidity, another conspicuous character is uniform, 

 the colour of the sides, which are also deep rusty brown. But from 

 Point Arena south to San Francisco Bay (Marin District), these 

 lateral-brown areas suddenly weaken to pale-rusty; while from 

 San Francisco south past Monterey (Santa Cruz District), adult 

 "birds have the sides pure smoke-gray without a trace of rusty. 



"The species thus presents geographic variation within itself, and 

 three distinguishable forms have been named, respectively, the chest- 

 nut-sided chickadee (Parus rufescens rufescens), the Marin chick- 

 adee (Parus rufescens neglectus), and the Santa Cruz chickadee 

 (Parus rufescens barlowi). But all three sub-species are unmis- 

 takably the chestnut-backed chickadee (Parus rufescens). . . . 



"As has already been asserted, Parus rufescens doubtless arose 

 as a geographical race of Parus pre-hudsonicus [the hypothetical 

 common ancestor of the present species, Parus hudsonicus, occupy- 

 ing the interior of Alaska and British Columbia east to Labrador 

 and Nova Scotia, and Parus rufescens]. It is now called a 'species' 

 because intermediates have dropped out; in other words, the 

 divarication is now wholly complete and there are two separate 

 twigs. The area of intermediate faunal conditions between the 

 humid coast belt and the arid interior region of British Columbia 

 and Alaska is very narrow, consisting, in places personally 

 traversed by me, of but a few miles over a mountain ridge. This 

 very narrowness of the area of faunal mergence probably accounts 

 for the lack of intermediates at the present day between hudsonicus 

 and rufescens. 



"In the case of Parus rufescens and Parus hudsonicus, 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 

 locality. The narrowness of the region of intermediate faunal 

 conditions may therefore be considered as the reason why we do 

 not find connecting links between hudsonicus and rufescens at the 

 present time. For the amount of difference between these two 

 chickadees does not strike me as any greater than, for instance, 



