e so great, neither will natural enemies be so
plentiful; and since the offspring, guided by prior experience, return
to the neighbourhood of their birthplace, the advantages thus gained
will be shared by the succeeding generation. It follows, then, that the
range of a species will not always be continuous, will not, that is to
say, proceed by a series of successive steps, but that sometimes in this
direction and at other times in that, the chain of territories will be
interrupted and different individuals separated by distances of greater
or lesser extent. New colonies will thus come into being; and as the
unlimited increase of the population over limited areas gradually
reintroduces into them the struggle for territory, new centres of
distribution, where the process will repeat itself and from which
expansion will proceed afresh, will be formed. Hence, though it is
clearly impossible for the progeny of one pair of Yellow Buntings to
overspread the whole of the 46 square miles, it is by no means
impossible for the limits of their range to exceed even those limits
within the eleven years.

To sum up our knowledge regarding this phase. Of the organic condition
which renders the impulse responsive to stimulation we know very little;
and though certain facts of observation seem to indicate the direction
in which the stimulus is to be found, we must here again confess to much
ignorance. So far as can be seen, however, the impulse to seek isolation
with its correlative territory, leads to constant modification in the
breeding range of most species. The occupation of the small space of
ground which each individual requires, the extent of which has been
gradually adjusted to suit the needs of different species, results in
expansion not only in one direction but in every direction, and not only
in one season but in every season. And if there were no complications in
the external world this expansion would proceed, as we have seen, with
astonishing rapidity. But complications, some of which are favourable
and others unfavourable, are numerous, and it is difficult to estimate
their importance or to indicate their precise effect; the former,
however, accelerate the rate of expansion, whilst the latter retard it.
Those individuals that wander outwards and seek territory on the
outskirts of the range we have called "pioneers." They will have
advantages over others that, wandering inwards, seek isolation in
congested districts, and w