change will lead? Just as the consequences
which flow from the functioning of the former impulse are accessible to
observation, so likewise can we observe the change that is wrought by
the latter impulse. The process is a gradual one. Less and less
attention is paid by the individual to intruders, more and more is it
disposed to pass beyond its accustomed limits. Little by little,
accompanied by its young or without them, as the case may be, the bird
deserts its territory and wanders out into the wilderness. Here it
associates with others, and finds in them a new interest and, I doubt
not, a new enjoyment. All this we can observe as it takes place. But
just as there is an innate capacity to seek, in the spring, the place
where the pleasures of breeding had formerly been enjoyed, so we are
bound to infer the existence in the adult of an innate capacity to
revisit the former area of association; and this capacity will
strengthen and confirm the gregarious instinct and set the direction of
the general course of movement.

We have seen, then, that the interest displayed by one bird in another
changes with the seasons; we have seen that it is so modified as to be
in useful relation to different environmental circumstances; as far as
possible we have traced out the consequences, and have reached the
conclusion that the change of behaviour must, on the one hand, lead to
expansion, and on the other, to contraction; and we have seen that this
conclusion is in accord with the facts of observation--that is the
general result of our inquiry into the functioning of the two powerful
impulses, the impulse associated with the disposition to secure a
territory and the gregarious impulse.

The phenomenon of migration embraces a number of separate problems, each
one of which presents features of great interest and of still greater
difficulty. On some of these problems I do not intend to touch; I seek
only to ascertain whether the impulses that are concerned in the
securing of a territory, and in the search for society, bear any
relation to the problem as a whole. I hold that the origin of migration
is not to be found merely in conditions peculiar to a remote past, but
that the conditions inhere in the organic complex of the bird, and are
thus handed down from generation to generation. Starting with this
assumption I examined the behaviour which normally accompanies the
seasonal life-history of the individual, and found, in that behavio