Vol. XXI 

 1904 



J Grinnell, Chestnut-backed Chickadee. 3^9 



gradual response to the new set of environmental factors, a geo- 

 graphical race became dififerentiated which might have then been 

 properly called Pariis pre-hndsotiicus rufescens. 



Unfortunately this process, which I believe to be constantly 

 going on among all animals, is so slow that its actual operation 

 under natural conditions has so far defied direct observation and 

 measurement during a man's lifetime. But it seems quite logical 

 to consider the natural process identical with that under ' arti- 

 ficial ' conditions, where the rate is readily perceptible. 



We seem warranted in considering all observed living forms^ 

 including ' species,' and completely isolated (insular) as well as 

 intergrading ' races ' as just a momentary glimpse, so to speak, of 

 a tree-like branchwork slowly rising through time, some of the 

 limbs ramifying freely and rapidly, others growing slenderly with- 

 out offshoots, but all advancing continually, though changing in 

 outward appearance at different rates ; only we at our brief glance 

 can see but a horizontal section, that is, only the set of tips of this 

 otherwise ancestral tree. 



Accepting this standpoint as the most reasonable hypothesis yet 

 presented, and moreover not at variance with our facts, I feel justi- 

 fied in judging of the methods of ramification and progress through 

 time from observation of the existing set of ' tips ' (rr species and 

 subspecies). Among these, from the nature of the case, we should 

 be able to recognize various stages in the process of species forma- 

 tion, and from these judiciously selected steps demonstrate the 

 completed stairway which leads up from the very incipiency of 

 differentiation (as impossible of ultimate detection by us as the 

 vanishing point) to the complete separation of two distinct species. 

 The steps are of course really infinite in number, like the points 

 in a geometrical line ; the transition proceeds gradually without a 

 break. 



In tracing the hypothetical lines of development of the chick- 

 adees, I do not feel guilty of bold speculation ; for I am only 

 attempting to express in a selected case what is to me clearly 

 evidenced from a survey of bird races in general. 



As has already been asserted, Panis ru/escens doubtless arose 

 as a geographical race of Parus pre-hudsonicus. It is now called 

 a ' species ' because intermediates have dropped out ; in other 



