MIGRATION OF BIRDS. 277 
some advantage.* Dr. A. R. Wallace offers a probable expla- 
nation of the manner in which the habit had its origin as 
follows : f ‘It appears to me probable that here as in many 
other cases ‘ survival of the fittest ’ will be found to have had 
a powerful influence. Let us suppose that in any species of 
migratory bird breeding can, as a rule, be only safely accom- 
plished in a given area ; and further, that during a greater part 
of the rest of the year sufficient food cannot be obtained in 
that area. It will follow that those birds which do not leave 
the breeding area at the proper season will suffer and ulti- 
mately become extinct, which will also be the fate of those 
which do not leave the feeding area at the proper time. Now, 
if we suppose that the two areas were (for some remote ancestor 
of the existing species) coincident, but by geological and 
climatic changes gradually diverged from each other, we 
can easily understand how the habit of incipient and partial 
migration at the proper seasons would at last become heredi- 
tary, and so fixed as to be what we term an instinct. It will 
probably be found that every gradation still exists in various 
parts of the world, from a complete coincidence to a complete 
separation of the breeding and subsistence areas ; and when 
the natural history of a sufficient number of species in all parts 
of the world is thoroughly worked out, we may find every link 
between species which never leave a restricted area in which 
they breed and live the whole year round to those other cases 
in which the two areas are absolutely separated. The actual 
causes that determine the exact time, year by year, at 
which certain species migrate will, of course, be difficult to 
determine.” 
The next problem to consider is the route by which migrants 
find their way between their summer and winter quarters. It 
must not be supposed that each bird moves indiscriminately 
southwards in autumn and northwards in spring. Indeed, 
it is by no means the case that the winter quarters are always 
southwards of the summer breeding grounds. For instance, 
owing to the gulf stream, the British Isles in winter have 
a milder climate than parts of Central Europe which lie 

* A.P. Taverner. ‘‘ Auk,” Vol. XXI., 1904, p. 322. 
t ‘* Nature,” Vol. X., p. 459. 
12 6(5)20 
