all these directions the territory would be equally preempted, but 
rather toward the north where there are fewer birds. Thus mi¬ 
gration from the tropics might have had its origin. The direct 
result of such a movement would be that those individuals that 
were forced to become explorers in search of an adequate food 
supply would come to a halt only when compelled to do so by lack 
of food or when harrassed by superior competitors or, finally, by 
the demands of that period in their life cycle when the physiolog¬ 
ical impulse to nest-building can be no longer delayed. 
Taverner explains the fall migration in the same way. That 
is, an overpopulation occurs in the nesting region at the north. 
The old birds drive away the young ones, or the first nestlings to 
mature become better established than those hatched later, driving 
the latter out. These being thus forced to migrate, on account of 
unfavorable conditions in the north find relief only by moving 
south and this constitutes the fall migration. This theory as¬ 
sumes that it is among weaker birds, those unable to hold their 
own, that the wonderful and complex habit of migration has de¬ 
veloped, a habit demanding apparently far greater qualities of 
courage, persistence and resourcefulness than would be required 
by competition for a livelihood with their fellows in a neighbor¬ 
hood already familiar to them. 
Ancestral-Habit All of the theories thus far mentioned to ex- 
Theories plain migration, namely, instinct, homesickness, 
dispersal, quality-of-nestling food, safe nesting sites, vacuum and 
overpopulation, seek to find an explanation in factors now opera¬ 
tive. It is possible that a key to the puzzle may be found by re¬ 
garding the performance as an inheritance of habit whose origin 
depends upon factors which have now ceased to act. 
eraser’s Theory f^e most reccnt theories embracing 
this point of view was proposed in Germany by 
Graser in 1904 and is based upon the supposition that the ances¬ 
tors of modern birds, living in Tertiary times were very vigorous 
flyers who passed freely from one Tertiary island to another across 
immense stretches of water in order to find food and nesting sites. 
As the widespread tropical environment of the Tertiary times gave 
place to modern climatic conditions with changing seasons, and, 
as the present distribution of land and water gradually developed 
from the immense Tertiary seas with their numerous islands, birds 
more a’nd more found suitable conditions of life in restricted areas 
wandering less and less until finally these ancestral wanderings 
have become limited to the regular fall and spring migrations, 
while many species are practically stationary. The logical conclu¬ 
sion of Graser’s theory is that birds are constantly becoming less 
