Vor. II, Pr. IT] LOOMIS—A REVIEW OF THE TUBINARES 29 
Thus equipped, a young Black-vented Shearwater reared on 
Natividad Island, Lower California, would have to learn when 
and where to go if he took part in the exodus-migration lead- 
ing northward to the latitude of Vancouver Island, British 
Columbia. Return-migration would be a simpler task, for the 
young shearwater would have less to learn. 
Some writers, clouding the issue, have separated return- 
migration from exodus-migration, and have held that desire 
for procreation and special physiological demands as to tem- 
perature during reproduction are paramount causes. Desire 
for procreation does not prompt sedentary species to migra- 
tion; nor does a waning desire prompt birds-of-the-year to 
exodus-migration; nor do special temperature requirements 
prompt the transequatorial migrants that leave regions having 
a temperature similar to the temperature at the breeding sta- 
tions. Above and behind the alleged physiological incentives 
are paramount causes. Without previous experience, the young 
Sooty Shearwater performs an exodus-migration to the North 
Temperate Zone and a return-migration to the South Temper- 
ate Zone. 
It is not in return-migration, but in exodus-migration in a 
direction opposite to the Equator, in early exodus-migration in 
warm temperate regions, and in transequatorial migration that 
we find the supreme test for all explanations of the cause of 
bird migration. 
Education 
Guidance by Old Birds.—Arrested migration has been the 
chief stumbling-block in the way of philosophers who have 
sought to interpret bird migration. Most reports on migration 
relate not to migrating birds, but to birds that have halted by 
the way. No real conception of migratory movements can be 
gained unless the birds are observed actually im transitu. View- 
ing arrested exodus-migration from very different bird-watch- 
ing stations, some have reached the conclusion that the young- 
of-the-year migrate earlier than the adults, and others that the 
adults migrate earlier than the young-of-the-year.* When 
Proc. Acad. Nat. Sci. Phila., 1896, p. 110; Dwight, Ann. N. Y. Acad. Sci., 1900, 
v. 13, pp. 127, 129; Clarke, Studies in Bird Migration, 1912, v. 1, pp. 26, 142, 196, 
308, v. 2, p. 55. 
