ich it was
reared or had previously reared offspring. What is the nature of these
changes and of the impulse which is first brought into functional
activity; whence comes the stimulus; and what directs the bird on its
journey--these are all different aspects of one great problem, the
problem of migration. I do not propose to discuss all these various
aspects, for indeed I have no suggestions to offer which are in the
least likely to be helpful, but I seek rather to ascertain whether the
phenomena which we have explored bear any relation to the problem as a
whole; whether, that is to say, the competition for territory and all
that appertains to it can have supplied the conditions under which, in
the process of time, this complex and definite mode of behaviour has
evolved.

We are sometimes told that we must seek the origin of migration in the
physical changes that have occurred in the ancient history of the
earth--in glacial conditions which gradually forced birds to the south,
or in the "stability of the water and mobility of the land" which
brought about a gradual separation of the feeding area from the breeding
area--and which continued for a sufficient length of time to lead to the
formation of an instinct, and that the instinct persists because it is
serviceable in promoting the welfare of the race. But when we consider
the lapse of time, and the changes that must have occurred in the
character of the bird population--the appearance of new forms and the
disappearance of the old, the ebb and flow of a given species in a given
area--and bear in mind that, notwithstanding this, the migratory
instinct, if not stronger, is assuredly no less strong, and the volume
of migration, if not greater, is assuredly no less; in short, that the
whole phenomenon is progressive rather than retrogressive, we shall find
the view that the instinct owes its origin to conditions which no longer
exist, receives but little encouragement.

I doubt not that, throughout the ages, geological changes have been an
important factor in directing or limiting the scope of migration, and
moreover are so still; just as climatic changes and the relative
abundance or scarcity of enemies have influenced the course of its
evolution. These are all contributory factors operating in the external
environment. But there are, besides, internal factors which form part of
the inherited constitution of the bird, and, being passed on from
generation to generation,