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 struggle for existence. So that, in a case of this description, the supply of food and the comparative scarcity of breeding stations have been factors of like importance in the evolution of the territory. Finally we were led to inquire as to how it comes about that the extent of the area occupied by each individual is adapted to the circumstances in which the individual finds itself; and we came to the conclusion that the movements of the bird, subsequent to the initial act of establishing itself in a position, are regulated and defined by the law of habit formation. For example, the Warbler, in response to its inherited nature, takes up a position in an appropriate situation. It then proceeds to search for food; it makes short journeys first in this direction and then in that; it repeats these journeys, and gradually forms a habit which compels it to remain within more or less well-defined boundaries. But the actual distance that it traverses on the occasion of its first attempt must be determined by the relative abundance or scarcity of the particular kind of insect life which it requires. So that, although habit defines and in some measure helps to determine the boundaries of the territory, it is clear that in the last resort they must depend upon the nature of the conditions in the external environment.