leads to pairing, is free to develop in the female earlier than would be the case if there were nothing in the external environment to attract the pair to a particular spot; and the longer the period over which the process of pairing can be spread, the greater chance will females have of discovering mates, the less severe will the competition tend to become, and, consequently, the smaller the percentage of individuals that fail to obtain suitable partners. In these ways the territory has been serviceable alike to a number of species. But much as the questions of mutual discovery and regular distribution may have influenced the course of its development, there can, I think, be little doubt that, on the one hand, the supply of the necessary accommodation for rearing offspring, and on the other, the necessity for an adequate supply of food in close proximity to the nest, have been the main determining factors, and have led to a wide divergence in its function. At the one extreme the function is to insure a plentiful supply of food for the young; at the other, to insure a station suitable for rearing offspring. I took the Bunting and the Guillemot as types of the two extremes. The young of the former species are born in a very helpless state. They are susceptible to cold and unable to withstand prolonged exposure, and therefore it is essential that there should be an ample supply of food, upon which the parents can draw liberally, in the vicinity of the nest. But the nest is placed in a variety of situations, and accommodation in this respect may be said to be unlimited. The young of the latter species are not so susceptible to exposure, and moreover there is always an abundance of food in the waters beneath the cliff; but ledges of rock, upon which the egg can be securely deposited and the young successfully reared, are limited both in number and extent. The position then is as follows: there are situations in plenty in which hosts of Buntings can build their nests but the supply of food is a difficulty, and if the respective areas of different individuals were insufficient 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