afford the conditions under which migration is
constantly being renewed. It is, I believe, in this field of organic
change and relationship that the conditions of origin must be sought.

Just as the moth in passing from the rudimentary to the perfect
condition runs through a series of changes, each one of which is marked
by a typical behaviour response adjusted to meet some particular
circumstance in the external environment, so the annual history of a
bird displays an ordered routine, each phase of which can be observed to
correspond with one of the successive changes in the environment. In
almost every direction, we find that this routine is characterised, in
broad outline, by great uniformity; so much so that, providing we know
the history of one species, we can forecast with no small degree of
certainty the general course of behaviour of other members of the
family. But only the _general_ course. There is endless variation in
just the particular way in which the behaviour is adapted to meet the
needs of particular species--the major details may be said to be
specific, the minor details varietal.

Now it is that part of the behaviour routine which has reference to the
relationship between one bird and another upon which, for the time
being, I wish to dwell; for the interest that A displays in B is by no
means always the same--it changes according to the season, and this
change can be observed to be uniform throughout a wide range of species.

In winter, in whatsoever direction we turn, we observe not only that
different individuals but that different species also collect together
in flocks. And since food at that season is not always easy to obtain,
and, moreover, is only to be found in certain situations, which are
limited both in number and extent, it would seem that such assemblages
are in the main determined by accident. No doubt the abundance or the
scarcity of food does determine the movements of birds, and hence to
that extent may be held to account for the flocks. But we shall but
deceive ourselves if we think that it is the sole or even the principal
reason, or that the situation is in no wise affected by internal
factors. The behaviour of the individual in relation to the flock bears
ample testimony to the presence of a gregarious impulse which derives
satisfaction from the fact of close association.

As an illustration, let us take a bird whose movements are easily
watched, and in whose hereditary const