288 RELATION OF TERRITORY TO MIGRATION 



extent. New colonies will thus come into 

 being ; and as the unlimited increase of the 

 population over limited areas gradually reintro- 

 duces 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 impos- 

 sible 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 observa- 

 tion 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 



