tuation, the other in which it is subordinated to the welfare of the
community. Let us take these two phases separately and endeavour to see
how they may have influenced the seasonal movements; and first let us
take the more important of the two, namely that one which is directly
concerned in the continuance of the race.

In this phase we must consider the three factors to which allusion has
already been made:--(1) the internal impulse, (2) the innate ability to
return to the former breeding ground, (3) the conditions in the external
environment. These three work in close relation and, as I shall
endeavour to show, lead to important results.

(1) If there were nothing in the inherited nature beyond an impulse to
seek the breeding ground, if, that is to say, when the appropriate
locality were reached, the bird took no further interest in the
developing situation, the attainment of reproduction would become
largely a matter of chance. A male in a congested district, having no
incentive to seek fresh ground, would remain inactive until a female
happened to cross its path and stimulate its sexual impulse, when its
activity would take another form. Hence some districts would be
over-populated, whilst others would remain unexplored. But the system of
reproduction does not consist merely of a search for the breeding
ground, and of the discharge of the sexual function; it is a much more
complex business, yet withal more complete. Nothing is left to chance;
the end is attained step by step; and each successive stage marks the
appearance of some specific factor which contributes towards the success
of the whole. We start with the appropriate organic condition under
which, when adequate stimulation is provided, the disposition to secure
a territory comes into functional activity. Within the field of this
disposition we can distinguish certain specific impulses. In sequential
order we have the impulse to seek the breeding ground; the appropriate
situation which gives rise to an impulse to dwell in it; and the act of
establishment which supplies the condition under which the impulse to
drive away intruders is rendered susceptible to stimulation. Grouping
these impulses, for the convenience of treatment, under one general
heading, I speak of an impulse to seek isolation. It implies some kind
of action with some kind of change as its correlated effect; and from it
there flows a ceaseless energy directed towards a definite end which fo