et of ledge correspond to the tree or clump
of bushes which acts as a headquarters for the Warbler, and the
occupation of them is determined, as it is in the case of the Warbler,
by racial preparation. Since, however, the ledge is only made use of for
the immediate purpose of incubation and is in no way affected by
questions relating to food, there is no occasion for the bird to wander
along the ledge nor to encroach upon those adjoining. Hunger stimulates
the Warbler to search the surrounding trees, and so to extend its area;
but hunger takes the Guillemot down to the water, and hence the area
which it primarily occupied remains unmodified.

To sum up: the territory is useful in various ways, but not necessarily
in the same way for every species. Reproduction would always have
remained fortuitous, and the number of individuals that attained to it
would seldom have reached the possible maximum unless some provision had
been included in its system for insuring that the males and females
could meet at the proper moment and afterwards remain in touch with one
another, and that the number of pairs inhabiting a given area did not
exceed the available means of support. I have tried to show that the
inclusion of a disposition to secure a territory tends to remove these
difficulties. In the first place, the disposition which leads to its
occupation comes into functional activity (in the male) early in the
season; and so, by the time that the appropriate pairing condition
arises in the females, the process of acquiring territories is well
advanced, and the males being regularly distributed, each in its
respective position, are readily found by their prospective mates. The
behaviour of each sex is thus adjusted to further the end of mutual
discovery. Next, after mating has taken place, the position occupied by
the male acts as a headquarters to which the birds can always repair,
and becomes a bond of union which is serviceable in that it prevents any
possibility of their drifting apart. And in the third place, the males
become pugnacious and in this way secure for themselves areas which vary
in size according to the conditions of existence of the species, so that
there is no possibility of too many congregating in this locality, and
all the less likelihood of too few finding their way to that; and hence,
on the average, different pairs are distributed throughout all suitable
localities. Furthermore, owing to the fact of their havi