however, the conditions in the external world are constantly changing
according to the relative abundance or scarcity of enemies, the rise or
fall of rivals, the physical changes in the earth's surface, and the
alterations of climate, it is clear that isolation can only be obtained
with difficulty, and that the competition for it must be severe. Some
individuals therefore fail to breed, whilst others, perhaps because
their impulse is stronger, persevere and seek stations elsewhere. What
are their prospects of finding them? By extending the field of their
activities, they will wander into districts remote from the scene of
competition, districts where not only food is plentiful but where
enemies and rivals are scarce; and to these pioneers, if to any, success
in reproduction will most certainly be assured. But not only is it they
who will benefit; their offspring also, when the time comes for them to
take their part in the maintenance of the race, will share in the
success of their parents, for even though they may not escape
competition from individuals of closely related forms, they will meet
with but little from those of their own kind. Now species which live
throughout the year in the vicinity of their territory are comparatively
few, the majority are obliged to wander in search of food so soon as
reproduction is ended, and their behaviour is determined not only by its
abundance or scarcity, but also by the powerful gregarious impulse which
waxes in proportion as the instincts connected with reproduction wane.
If, then, when the sexual instinct again becomes predominant, the
experience of the former season nowise affects their movements, little
or no progress will be made in the expansion of the range. But just as a
certain entrance into the bush and pathway through it, when once made
use of in the process of building, becomes so firmly established as to
form the sole highway to and from the nest, so likewise, when the
impulse to seek isolation repeats itself, the bird is constrained to
seek the neighbourhood wherein it had experienced the enjoyment of
breeding or of birth. Thus the little that is added one year becomes the
basis for further additions in the next, and new centres of distribution
are continually being formed from which expansion proceeds anew.

Now as the range gradually extends into regions where the climate
alternates and food at certain seasons is consequently scarce, the
distance between the cust