230 Trotter, Land-Bird Fauna of N. E. America. Vjulv 



Chat, a bird regarded as characteristic of the Carolinian fauna, 

 has made its appearance as a breeder, while the Chewink, a species 

 of decidedly austral range, rarely going beyond the transition zone, 

 has likewise invaded these localities. Mr. Richard C. Harlow, 1 

 records the Tufted Titmouse and the Yellow- breasted Chat in the 

 decidedly Canadian element of Pike Co., Pennsylvania. Un- 

 doubtedly the conditions incident to 'second growth' are largely a 

 determining factor in this invasion of new territory, for the opening 

 up of a tract of country to more sunlight would certainly bring 

 about an environment not unlike the typical Carolinian region 

 further south. Still the individuals of these species must have the 

 tendency to move northward farther than their apparent faunal 

 limit, otherwise they would not find these favorable spots in new 

 territory. Most likely they invade the region by way of the river 

 valleys, spreading out into the surrounding districts. 



Dr. Merriam has accumulated a vast amount of evidence to 

 show the control exerted by temperature in the distribution of 

 living organisms. 2 But is not this temperature relation more 

 apparent than real, a temporary adjustment to the environing 

 conditions which the temperature brings about rather than a hard 

 and fast relation between temperature and the organism direct ? 

 The whole question is recondite, but it seems hardly possible for 

 such closely related species as, for example, the Wood Thrush, the 

 Veery, and the Gray-cheeked Thrush to be so profoundly in- 

 fluenced by temperature alone as to have their northward breeding 

 ranges so widely different. Rather it seems to me each form 

 represents either a pioneer or a laggard movement, as the case 

 may be, in a general tendency of various species of birds to spread 

 gradually northward into a region of new environing conditions 

 which has been opened to them since the Glacial Period. The 

 Canadian fauna, barring the more or less circumpolar forms, 

 thus represents an advance group of species that spread into 

 northerly breeding grounds at a probably early day after the dis- 

 appearance of glacial conditions; the Alleghanian fauna that of 

 species that spread at a later date and are still spreading into new 



1 ' Summer Birds of Western Pike county, Pennsylvania.' Cassinia, 1906, pp. 16-25. 



2 'Laws of Temperature Control of the Geographical Distribution of Terrestrial 



Animals and Plants.' National Geographic Magazine, Vol. VI, 1894. pp. 229-238. 



