210 TERRITORY AND REPRODUCTION 



smaller the area over which each individual 

 exercises dominion, the greater the number 

 that will attain to reproduction and the greater 

 prospect the species will have of survival. The 

 emphasis in the one case lies on the fact that 

 the area occupied must be sufficiently large ; 

 on the other, on its being just sufficient and 

 no more to accommodate the egg. Hence 

 the difference in the function at the opposite 

 extremes is brought about, not by modifications 

 of the instinctive behaviour which leads to the 

 establishment and defence of the territory, but 

 solely by modifications in the size of the area 

 occupied, in accordance with the conditions 

 prevailing in the external environment. No 

 doubt, if we had the life-histories of a sufficient 

 number of species worked out, we should find 

 that the gradations were complete from the 

 one extreme to the other. We are justified 

 in thinking that this must be so because in 

 many directions we can not only observe 

 differences in the size of the area occupied, but 

 can recognise a close correspondence between 

 those differences and the conditions of life of 

 the species. Thus the Herring-Gull occupies 

 a comparatively small area, though one which 

 is many times larger than that of the Guillemot. 

 It requires more space because it not only builds 

 a nest but rears four instead of a single offspring, 

 and it can be allowed more space because the 

 young remain in the nest until they are capable 

 of sustained flight, and consequently it can 

 make use of many miles of cliff from which the 



