in an ungainly way, bumps up against
its companions as if it had not full control over its movements. Its
behaviour produces irritation which is expressed by a vocal outburst,
and followed by actions the meaning of which is clear. Moreover, the
call is taken up by other individuals and sweeps over part of the flock
as does the greeting. But the nature of the cry is entirely different
from that which greets the arrival of a companion--humanly speaking it
is a passionate and impatient utterance, the height of displeasure. The
arrival, then, acts as a stimulus to something in the inherited
constitution which is expressed in, and presumably is satisfied by, this
vocal outburst; and, since the bird that arrives joins also in the
chorus, there is reason to think that the impulse which determines its
movements is similar to that which is temporarily aroused in the flock.

Apart, however, from the evidence derived from the affective aspect of
the operation of the instinct, the general course of behaviour lends
support to the view that the assemblies are determined by internal
factors, and are not merely the outcome of circumstances in the external
environment. Observe, for example, the manner in which the flock is
built up. Single individuals are content to rest alone so long as no
assembly is in sight, but they are drawn towards their companions
directly the opportunity arises, just as surely as the smaller
aggregation is drawn towards the flock; and so, as the flock increases,
it gradually absorbs all the lesser flocks and smaller parties, for the
greater the flock the greater the attraction seems to be; and different
individuals appear to gain some satisfaction from being in close bodily
contact with one another.

When the Curlew flies to that part of the mud-flat which is first
exposed by the receding tide, and there associates with others, it does
not then do so because it has any interest in its fellows, nor because
they serve as an attraction, but because it is constrained by hunger--in
other words, the association is determined by accident. But when, during
periods of repose, it sees a flock, flies to it, and takes up a position
in the midst of it, it does so not because suitable accommodation is
lacking--not therefore because of external constraint--but because it
derives some pleasure from satisfying something in its organic complex.
We speak of this behaviour and of the emotion which characterises it as
the _gregar