t to supply them with the
necessary food with the necessary rapidity, they would run the risk of
losing their offspring and the species would not endure; on the other
hand, cliffs upon which the Guillemot can rear its young are limited,
but the supply of food presents no difficulty, and consequently the
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 tide recedes at the
base, and which on this account are denied to the Guillemot, but
manifestly it cannot be allowed so much space as the Bunting, for then
comparatively few individuals would attain to reproduction.

Again, the Reed-Warbler inhabits swamps overgrown with the common reed,
and in such places insect life is abundant just at the time when the
young are hatched. But these swamps cover a comparatively small acreage
in the breeding range of the bird, and if each pair were to attempt to
establish dominion over an area equal, let us say, to that of the
Willow-Warbler, the species would have but a poor chance in the stru