258 DARWINISM TO-DAY. 



in illustration of the latter is the occurrence together, in the Siski- 

 you Mountains of northern California, of the brown Parus rufescens 

 of the wet coastal fauna and the gray Parus gambeli of the arid 

 Sierran fauna. (See Anderson and Grinnell, Proc. Ac. Nat. Sc., 

 Phila., 1903, p. 13.) The Siskiyou Mountains occupy a line of 

 mergence between the two faunae, and the two respectively repre- 

 sentative chickadees have evidently extended their ranges toward 

 each other until now over this one small area they occupy com- 

 mon ground. Several parallel cases could be cited; their signifi- 

 cance seems obvious. 



"We come now to consider the origin of the races of Parus 

 rufescens. In a species of recent arrival into a new region (by 

 invasion from a neighbouring faunal area), as it adapts itself better 

 and better to its new surroundings, granted the absence of closely- 

 related or sharply-competing forms, its numbers will rapidly 

 increase. This means that there will be increased competition within 

 the species itself, on account of limited food supply. The alterna- 

 tive results are either starvation for less vigorous individuals 

 during recurring seasons of unusual food scarcity, or dissemination 

 over a large area. In a way the first might be considered as bene- 

 ficial in the long run, as doubtless leading to the elimination of 

 the weaker; such a process evidently does take place to a greater 

 or less degree all the time, and is important for the betterment of 

 the race. But as a matter of observation Nature first resorts to 

 all sorts of devices to ensure the spreading of individuals over all 

 inhabitable regions; in other words, the extremest intra-competi- 

 tion does not ensue until after further dissemination is impossible. 

 In birds we find a trait evidently developed on purpose to bring 

 about scattering of individuals. This is the autumnal 'mad im- 

 pulse' which occurs just after the complete annual moult, when 

 both birds-of-the-year and adults are in the best physical condition, 

 and just before the stress of winter food shortage. Even in the 

 most sedentary of birds, in which no other trace of a migratory 

 instinct is discernible, this fall season of unrest is plainly in evi- 

 dence. I may suggest, not unreasonably, that autumnal migration 

 may have had its origin in such a trait as this, the return move- 

 ment in the spring becoming a necessary sequence. (See Loomis, 

 Proc. Cal. Acad. Sc., 3d series, Zoology, II, Dec., 1900, 352.) It 

 is a matter of abundant observation that autumn is the season when 

 we find the most unlooked-for stragglers far out of their normal 

 range and when sober, stay-at-home birds, like Pipilo crissalis and 

 the chickadees, wander far from the native haunts where they so 

 closely confine themselves the rest of the year. It is also the expe- 

 rience of collectors that the greatest number of these stragglers 



