THE rHILOSOPIfV OF MIGRATION. 87 



gradual rise of temperature experienced as the 

 several zones of the earth are crossed from north to 

 south, may serve as some guiding influence, at- 

 traction, or gentle impetus to birds in search of 

 warmer lands in autumn ; just as the fall in temper- 

 ature experienced in a passage from the Tropics to 

 the Temperate or Arctic zones might act inversely 

 in spring. Birds migrate in spring to regions where 

 the temperature is suitable to the vital function of 

 Reproduction — and it is an axiom in ornithology 

 that birds in the warmest areas of their distribu- 

 tion or habitat select the coolest areas of temper- 

 ature in which to breed. In autumn, birds are 

 in quest of higher temperature, or rather retreating 

 from regions where the temperature is rapidly fall- 

 ing. That birds, like many other animals, may be 

 able to foretell the near approach of barometrical 

 disturbance, that this finely-adjusted faculty is of 

 vital importance to such mighty travellers, seems 

 so palpable that we can scarcely question its 

 truth, although in the present state of our know- 

 ledge we fail to see how or why. Mr. J. A. Allen 

 has remarked that in America the migrations of 

 various birds precede storms or sudden falls of 

 temperature often only by a few hours, thus avoid- 

 ing these disturbances by keeping ahead of them. 

 Great migratory movements have repeatedly been 

 observed coincidently with favourable barometric 

 conditions, and to cease with the reverse. We shall 

 have occasion to allude to the influences of Temper- 

 ature on Migration Flight again. 



