MIGRATION 213

manage to cross the wide stretches of water and come
back year by year to the same hedgerow is a mystery
that man as yet has vainly striven to fathom.
Wallace’s theory of migration is that in past times
there was a more equable climate than now, and that
as the seasons became more distinct the birds had to
fly further and further ; those that did not fly at the
right time in the right direction would, as the necessity
for migration grew more imperative, tend to disappear,
and so the habit became instinctive. A bird always
breeds in the coldest climate he visits, and always
crosses the sea at its shallowest portion, where there
once was a chain of islands. But it is really remark-
able how many survive the journey, considering that
the migration frequently takes place in the darkness
of the night, and that many of the birds, particularly.
the smaller ones, are such poor flyers under ordinary
circumstances. This, of course, refers to the birds that
‘go foreign ;’ the home migration of the native birds
affording but little difficulty of explanation. Birds
may be migrants in one country and residents in
another. There is the robin, for instance, which is a
migrant in Germany ; and in such cases the northern
colony is always reinforced by the visitors during the
summer. Some birds visit a country twice during the
year, on their way to and from the termini of their
journeys; and these journeys do not extend in all
cases to the equator or near it. The fieldfare, for ex-
ample, finds England quite far enough south for its
comfort, and returns, as a rule, to the icy north to
breed. One bird, the snow bunting, has been found
nesting within eight degrees of the North Pole. Some