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
