469 
1921.] Bird-Migration by the Marking Method. 
that migration is a far too complex and also a far too regular 
phenomenon to be created anew each season merely under 
stress of circumstances ; moreover, it is known that migration 
begins before the need is in the least pressing. The more or 
less indefinite wanderings of some sea-fowl, the irregular 
dispersals of some other birds, and the late “ weather move¬ 
ments” that occur in severe seasons may be attributed 
to immediate causes, but a deeper seated origin—not 
necessarily identical for every species—there niust surely be, 
for the highly developed habit of some of our more typical 
migrants. 
A little consideration will show how speculation regarding 
this origin is rendered futile for lack of a certain kind of 
fact. For instance, there is the perhaps rather far-fetched 
theory that the migrational habit was established by some 
great meteorological change in the distant past—-say by a 
Glacial Epoch, as has been suggested, which drove the birds 
resident in northern latitudes towards the Equator, and made 
them form there a second home : to this they would annually 
return, it is supposed, after the cessation of the unfavourable 
conditions had allowed them to re-colonise their original more 
northerly area as a summer home, the individuals continuing 
to use the routes followed by the species at the time of the 
first great movement. Then there is the more recent 
theory (cf. Pycraft, History of Birds, 1910, p. 100) that the 
migrational habit arose from the gradual northward spread 
of a species from a supposed original southern area in search 
of fresh feeding and breeding grounds, the birds withdrawing 
to this original area each winter. Without discussing these 
theories, it may be noted how their proof or disproof would 
necessarily rest on a knowledge of the facts concerning the 
relation of particular summer-quarters to the corresponding 
winter-quarters, and of the routes connecting them. Thus it 
is often suggested ihat the members of a species summering 
farthest north winter farthest south, and that those mid¬ 
way are more or less stationary (cf. Swallow, Section X.) ; 
but the observer only sees a general southward movement, 
and typical individuals must be singled out for study before 
