ill succeed where the latter fail; and since
there is in the young an innate ability to return to the district
wherein they were reared, the advantages so gained may be said to be
handed on from generation to generation.

Let us now turn to the contra-phase, and endeavour to ascertain whether
the gregarious instinct bears any relation to the seasonal desertion of
the breeding ground. The conclusion at which we have already arrived
regarding this instinct is that it forms part of the inherited nature of
most species; that its functioning is suppressed when a bird is actually
in occupation of a territory; and that it is serviceable in promoting
the welfare of the individual. We cannot of course observe the instinct.
What we observe, when reproduction is ended, is a change in the
relations of different individuals; instead of arousing mutual
hostility, they attract one another, from which we infer the existence
of something which determines their conduct, and this "something" we
speak of as an instinct.

To what does this change lead? Let us suppose that there is an area
inhabited by one species; that the number of inhabitants has reached the
maximum that the means of sustenance will allow; and that the season of
reproduction is drawing to a close. The position will then be as
follows. All the available breeding ground is divided into territories;
each territory is occupied by one unit, the family, and each individual
is able to fend for itself; changes both internal and external begin to
take place, the gregarious instinct comes into functional operation, and
the supply of food diminishes--that roughly is the position. The
internal factor operates so that the sight of this individual or the
call of that, instead of evoking hostility as heretofore and keeping
different units apart, proves now an irresistible attraction; so that in
place of a number of individuals evenly dispersed over the whole of this
area, a small number of flocks of various dimensions are stationed at
certain points, which points are determined partly by experience, partly
by the supply of food, and partly by accident. This implies for each
individual some movement in some direction. But since the population of
this imaginary area has reached the maximum, and the supply of food,
though limited in distribution, is nevertheless plentiful, such
movements will be irregular and will proceed in no definite direction.

Now let us suppose that the breeding ra