210 



MIGRATION 



ogists are primarily naturalists, and experiments awaited the growing 

 interest of physiologists and ecologists in the problem, which in turn 

 was an outcome of the extraordinary advances in the field of vitamins 

 and hormones. Nevertheless, though observation will always have its 

 peculiar values, it should henceforth be more fully informed through 

 experiment and be content to recognize that conclusive and objective 

 results are the province of the latter (Baldwin and Kendeigh, 1932). 

 The literature of migration is too extensive to be read by anyone 

 other than the specialist, but a knowledge of its general content is 



Fig. 51. — ^Distribution and migration of the scarlet tanager. During the breed- 

 ing season indi\iclual scarlet tanagers may be 1,900 miles apart in an east-and- 

 west line across the breeding range. In migration, however, the lines converge 

 until in southern Central America they are not more than 100 miles apart. 



(After Lincoln, 1935.) 



essential to securing an adequate historical background and the proper 

 perspective for the future of experimentation. For this, the sources 

 are time-consuming, and it is preferable to turn to compendia and 

 summaries for the most part. These are well exemplified by the 

 following: Cooke, 1885, 1910, 1913; Gatke, 1895; Dixon, 1895; 

 Whitlock, 1897; Taverner, 1904; Walter, 1908; Hcnshaw, 1910, 1921; 

 Clarke, 1912; Coward, 1912; Cathelin, 1920; Lucanus, 1922; Thom- 

 son, 1926, 1936; Wetmore, 1926; Grinnell, 1931; Heape, 1931; Rowan, 

 1931, 1932; Chapman, 1932; Lincoln, 1935. \ 



The divergent views concerning migration have centered about 

 three major questions, namely, the origin of the behavior, the factors 



