struggle for existence becomes a struggle
for the means of subsistence, and that anything in the inherited
constitution of the bird which can be organised to subserve the
biological end in view becomes of selection value. So long as food can
always be procured in the new areas of association, the individuals that
behave in accordance with ancestral routine gain thereby no particular
advantage; but directly the breeding range extends into regions where
the supply fluctuates, traditional experience becomes a factor in
survival, and those individuals that come under its influence will, on
the average, be more likely to endure and so to procreate their kind and
maintain the tradition. Let it once be granted that there is an innate
capacity to retain in later phases of routine the experience gained in
earlier phases, and it is difficult to see how traditional guidance can
be refused recognition as a factor in the developing situation. But only
_a_ factor, and by no means the most important one; for observation has
shown that the young are capable of performing the return journey
without guidance. Something therefore _is_ inherited, some impulse
which comes into functional activity at a specified time, and leads the
bird to set forth in a given direction.

There are no grounds for supposing that the experience of one generation
forms any part of the hereditary equipment of subsequent generations. In
what direction then are we to look for the congenital factor? What is
given is an inherited tendency to co-operation and mutual help, and an
innate capacity to make use of the results of experience. The inherited
tendency, as we have seen, leads on the one hand to the formation of new
areas of association, whilst on the other, since it is the means of
bringing isolated individuals into contact, it leads to experience being
handed on from generation to generation, which, in its turn, results in
a certain amount of backward movement along the line of expansion. It
forms part of the hereditary equipment of many species, and is
serviceable in promoting the welfare of the individual. Moreover, there
is reason to believe that its origin dates back to an early period in
the evolution of the higher forms of life; and if in the subsequent
course of evolution it could have been so organised as to serve a double
purpose, so much the more reason would there have been for its survival.
In what does the instinct consist? Is it merely that the s