ight of this
individual or the call of that proves at some particular moment an
irresistible attraction, or does the appropriate organic condition give
rise, as is generally supposed, to some preceding state of uneasiness?
In the former case, the temporarily isolated individual or colony would
have but little chance of sharing in the benefits which mutual
association confers upon the associates; in the latter, the feeling of
discomfort would lead to restlessness, and would thus bring the bird
into touch with the environing circumstances under which instinctive
behaviour could run its further course. So that it is probable that the
movements of each individual, prior to its becoming a unit in the flock,
are not accidental but are determined in some measure by racial
preparation.

Now if the fundamental assumption of the doctrine of the struggle for
existence be true, the gregarious instinct will not be quite alike in
all the members of different broods, nor even in each member of the same
brood; that is, variation will occur in all possible directions. And we
shall not, I think, exceed the limits of probability if we assume that
different individuals vary in the persistency with which they strive to
attain their unknown end, and in the direction in which they travel in
pursuit of it. So that in each generation they will fall into three
classes: (1) those which are inert, (2) those which wander along the
line of expansion, (3) those which wander in other directions. If then
the struggle for life at this particular juncture in the evolution of
the breeding range is a struggle for the means of subsistence, the
members of these three classes will not be in a like satisfactory
position so far as the competition for food is concerned. Those in the
first class--_i.e._, those in which the activity feelings are weak--will
neither gain the benefits which arise from mutual help, nor will they
have much prospect of enduring through the season of scarcity. Those in
the third class will, it is true, derive some assistance one from
another, and so be in a better position to discover what food may be
available; but inasmuch as they will remain in regions where the climate
alternates and the supply of food is liable to fall below the minimum
required, the chances are that a high percentage will fail in the
struggle for existence. We come now to those in the second class, and it
is upon them that I wish more particularly to focus attentio