d flow of energy into
all the specific modes of behaviour connected with reproduction, amongst
which those directly concerned in the securing and defence of the
territory will receive their share; so that instead of a progressive
weakening of just those elements in the situation which make for
success, the level of their efficiency will be maintained as a result of
such reinforcement. But the female becomes intolerant of her own sex
when she has discovered a male ready to breed, and, later, assists her
mate in resisting intrusion; and by raising her emotional tone, he may
be the means of furthering more strenuous behaviour on her part. Each
member of the pair would in this way contribute towards the energy of
behaviour of its mate, and hence add indirectly to the security of the
territory.

It may be well to illustrate the foregoing remarks. Suppose that there
is a small piece of woodland barely sufficient to hold three pairs of
Willow-Warblers, and suppose that the male and female in the middle
territory did not respond to one another's influence quite as readily as
the adjoining males and females, what would be the result? The emotional
tone of the central pair would stand at a lower level of intensity; and,
since their congenital dispositions would lack the necessary
reinforcement, the birds would tend to become less and less punctilious
in keeping their boundaries intact, whereas the adjoining pairs, always
on the alert and meeting with little opposition, would encroach more and
more and gradually extend their dominion. And so, by the time the young
were hatched, the parents would be in occupation of an area too limited
in extent to insure the necessarily rapid supply of food, and would be
compelled to intrude upon the adjoining ground. But knowing how routine
becomes ingrained in the life of the individual, knowing that for weeks
this pair had submitted to their neighbours, can we believe that they
would be capable of asserting their authority and that the young would
be properly cared for? Or suppose that different pairs of Kittiwake
Gulls on the crowded ledges, or different pairs of Puffins in the
crowded burrows, varied in like manner, would they all have equal
chances of rearing their offspring? The struggle for reproduction is
nowhere more severe than amongst the cliff-breeding sea birds; it is not
for nothing that one sees Kittiwake Gulls, locked together, fall into
the water hundreds of feet below and strugg