Page:Territory in Bird Life by Henry Eliot Howard (London, John Murray edition).djvu/268

210 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 offering, 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