nge extends and that fresh ground
is occupied by pioneers. When reproduction and the rearing of broods are
ended and the gregarious instinct becomes dominant, these pioneers, or
at least some of them, will revisit the area wherein formerly they
associated with companions. Their offspring, however, though they will
have the inherited impulse and the innate tendency, will not have the
experience; how then will they behave? There can be no doubt that some
will accompany the older birds, and, being led by them, will share the
experience of a former generation; nor any question that others will
collect together in the neighbourhood of their birthplace and, if their
impulse is satisfied, will remain there so long as food is to be found.
Thus the gregarious instinct, working in close relation with acquired
experience, will on the one hand lead to the formation of organised
movements in certain directions, whilst on the other it will lead to the
formation of new areas of association which will follow in the wake of
the expansion.

We have assumed, in the imaginary case which we have just taken, that
the conditions in the external world are such as enable the birds to
endure throughout the year--in short, that there are no complications
regarding the supply of food. But we must bear in mind that so long as
conditions are favourable during the period of reproduction, which is of
short duration, the breeding range can continue to expand, and that
therefore, in the course of centuries, regions will come to be occupied
wherein, owing to alternations of climate or physical changes in the
surface of the earth, food will be impossible, or at any rate difficult
to obtain at certain seasons. Hence there will come a time when the area
of association ceases to follow in the wake of the expansion, and the
breeding area begins to diverge from the subsistence area.

How, then, is the gulf between these two areas to be bridged? We can of
course say that those individuals which, in virtue of some slight
variation of hereditary tendency, return to regions where food is
plentiful will survive; whilst others, less well endowed, will perish.
We can state the position in some such general terms, and doubtless
there would be truth in the statement, but it does not carry us far; we
wish to know more of the nature of the tendency, and of the manner in
which it has evolved. Well now, in this new situation which arises, two
things are apparent--that the