ur,
manifestations of cyclical change leading to definite biological
consequences. I now propose to inquire whether those consequences are
such as might, in the course of time, give rise to the seasonal change
of abode.

We are apt to think of migration in terms of the Warbler that enlivens
our hedgerows in the spring after travelling hundreds of miles from the
south, or of the Redwing that comes from the far north and seeks its
food during the winter on the meadows, or perhaps of the American Golden
Plover that each year covers a vast expanse of ocean in its journey from
its breeding ground. The length of the distance strikes the imagination
and constrains us to focus attention upon the extremes.

But migration is of much wider significance than is here represented. I
sit beside the River Severn in April and watch Swallows, Tree-Pipits,
and Yellow Wagtails passing in twos and threes, in small parties, or it
may be in small flocks; and I observe that while some establish
themselves in the neighbourhood, others pass on. Or I watch
Herring-Gulls returning to the breeding station at Bolt Head, an endless
stream of individuals coming from the east as far as eye can reach;
following them for some miles inland I see them still, first as specks
upon the horizon, then passing beside me as they beat their way slowly
against the strong south-westerly winds, and finally disappearing from
view in the direction of the cliffs. Or again, I watch Buntings and
Finches deserting the flock and seeking stations in the marsh, or
amongst the furze-bushes on the common, or in the spinneys. In each case
the proximate end of the behaviour is alike--wherein then lies the
difference? Only in the distance which separates the territory from the
area in which the birds formerly associated. And intermediate between
the extremes, I doubt not, if we had a sufficient body of observations,
that we should find numerous gradations, the lesser merging step by step
into the greater. Is the Swallow a migrant and the Herring-Gull not; is
the Tree-Pipit a migrant and the Bunting not; must a bird cross many
miles of sea or of land before it can be considered a migrant; is the
length of the distance traversed a criterion of migration? Surely not.
The distance traversed is merely a collateral consequence of the process
as a whole.

The annual life-history of a bird presents, as we have seen, two
distinct phases--the one in which the individual dominates the
si