How, then, can the theory apply to a species that breeds under such conditions? What part can the territory play in furthering the life of the individual when large numbers of nests are built closely together in the same tree? There is much evidence to show that mutual protection is a necessary condition of the Rook's existence; many cases are on record of rookeries being destroyed by Carrion-Crows, Hooded Crows, and Ravens. For instance, Mr. Ward Fowler records a case in which a pair of Crows attacked a small rookery, ransacked the nests, and destroyed the eggs, with the result that not a single pair of Rooks was left in the settlement. Each Rook must therefore secure a position within the precincts of the community if it is to have a chance of success in the attainment of reproduction. But every locality cannot supply sufficient trees of the right kind, appropriately situated and in suitable relation to the food supply, in which numbers of nests can be built in close proximity; so that if more than one community were to attempt to establish itself in a limited area, the supply of food or the supply of trees might become a pressing problem. Each community must therefore be prepared to defend its own interests, and each must be regarded as one unit and the area occupied as one territory within which are included a number of lesser territories. The individual may fail to establish itself within a community, but, even if it succeeds, the community may fail to establish the rights of communal ownership; hence it has to face a twofold possibility of failure, and if it lacked the inherited nature which leads the Guillemot to secure a position upon the ledge, or the Bunting to obtain a position in the marsh, the chances are that it would fail in the attainment of reproduction. The question now arises as to how it comes about that the area occupied by each individual conforms in broad outline to that which has proved beneficial for the welfare of the species as a whole. We shall find that up to a point the answer is a simple one. No one could study the behaviour of animals without observing the important part that habit plays in the life of the individual; an action performed to-day is liable to be repeated to-morrow and the following day until it becomes ingrained in the life of the individual. This must not be taken to mean, however, that a particular action has to be performed for many days in succession before it becomes definitely fixed; if only it is repeated a number of times, even within the space of a few hours, it will acquire sufficient strength for its continuance; but continued repetition gives increased fixity, and, as time goes by, it becomes increasingly difficult for the creature to make a change unless the character of the situation necessitates readjustment.