life of the individual, for biological interpretation there is only one end, the prospective value of which is the continuance of the race. We may say that the latter phase is the more important of the two because it is directly concerned with reproduction. But we shall make a great mistake if we attach peculiar importance to one phase, or to one mode of behaviour within that phase, or to one action within that mode of behaviour; for if there is one thing certain it is that the whole is an inter-related whole in which each part depends for its success upon that which precedes it.
In that phase in which the territory is the central feature of the situation, the struggle for existence is in operation in its acutest form; all the congenital and acquired capacities of the bird—pugnacity, song, capacity to utilise in later phases the experience gained in prior phases, all these are organised to subserve an end—a proximate end—which in its simplest terms may be described as "isolation." Isolation is then the first step in the process of reproduction, and any individual that fails to make it good, fails to procreate its kind. But isolation implies separation, and the degree of separation varies in different species, from the few square feet of cliff required by the Guillemot to the few square miles of barren moor over which the Peregrine exercises dominion. One species must occupy sufficient ground to enable it to secure food for its young; another requires sufficient, but no more, upon which to deposit its egg; and
