n. The
initial movement in their case will be in the direction from which
outward expansion has all along taken place. Within a comparatively
short distance they will reach districts where the species is plentiful,
and here, associating with others that have some traditional experience,
they will be guided by them and will find themselves in regions where
food is plentiful. Hence in each generation those will survive that,
owing to some congenital variation of their instinct, seek satisfaction
for their impulse in a direction which brings them under the influence
of tradition. And though at first but slight and not in themselves of
survival value, such variations, since they coincide with modifications
of behaviour due to acquired experience, will be preserved and in the
process of time so accumulated as to be capable of determining the
direction and extent of the movement.

But the young Cuckoo deserts this country many weeks after its parents,
and there is no reason to suppose that it lives in society when
eventually its destination is reached; and the young Falcon passes to
the south, and is certainly not gregarious--how then can we explain
their behaviour in terms of something which they show no signs of
possessing? I do not wish to make light of a difficulty which
admittedly, at first sight, is a grave objection to the view that the
gregarious instinct has been operative in the manner here claimed for
it. It must, however, be borne in mind that this instinct, though
originally developed to serve the purpose of mutual protection, supplies
the material upon which evolution works when the extension of the
breeding range creates a situation requiring readjustment on the part of
the organism to new conditions of life; and that those variations which
can be so modified as to be in useful relation to the new environmental
circumstances are seized upon by natural selection and, being
transmitted, form the foundation of a specific inherited response, no
longer dependent upon, though operating in close relation with the
primitive response whence originally it sprang. Thus the primordial
instinct becomes so organised as to serve a secondary purpose, that of
rendering secure a means of access to a certain food supply. In the
course of evolution species were bound to arise which, owing to some
peculiar conditions, derived greater advantage from living solitary than
from living in society. Does it then follow, because such speci