r
us, who can perceive its prospective value, is isolation in an
appropriate environment. The emphasis here is on "isolation," for it
involves competition, and there cannot be competition without some
change in the relative positions occupied by different individuals; so
that in each recurring season there will be not only a re-arrangement of
ground formerly occupied but an arrangement of ground formerly deserted.

(2) That the older birds return to the locality wherein they had
formerly reared offspring, and the younger to the neighbourhood of their
birthplace, was always deemed probable. But in recent years evidence
which cannot be rebutted has been supplied by the marking of birds. This
evidence, details of which can be found in the summary of results
published annually by Mr. Witherby in _British Birds_, demonstrates that
the adult frequently returns not only to the same locality in which it
formerly bred, but even to the same station; that it does so year after
year; that this mode of behaviour is not peculiar to one sex; and that
many of the young breed in the locality in which they were reared. Such
being well-established facts, we can infer the existence of an innate
ability to revisit the place wherein the enjoyment of breeding, or of
birth, had formerly been experienced. Of its nature we know little or
nothing. It would almost seem as if there must be some recollection of
past enjoyment, but all that can be definitely asserted is--that past
experience somehow becomes ingrained in the life of the individual and
determines present behaviour. What, however, is of importance to us at
the moment is not the _ad hoc_ nature of the bird, but the biological
consequences to which the behaviour leads. For if, on the average,
individuals return to their former haunts, it follows that the annual
dispersion will not be merely a repetition in this season of that which
had occurred in a previous one, but that the little added this year will
become the basis for further additions in the next. The innate ability
is handed down from generation to generation, and, in so far as it
contributes to success, is fostered and developed by selection; and the
modifications of behaviour to which it leads, since the results of prior
process in the parent persist as the basis and starting-point of
subsequent process in the offspring may in a sense also be said to be
handed down.

(3) The conditions in the external world may be organic or in