enjoyment, but all that can be definitely asserted is—that past experience somehow becomes ingrained in the life of the individual and determines present behaviour. What, however, is of importance to us at the moment is not the ad hoc nature of the bird, but the biological consequences to which the behaviour leads. For if, on the average, individuals return to their former haunts, it follows that the annual dispersion will not be merely a repetition in this season of that which had occurred in a previous one, but that the little added this year will become the basis for further additions in the next. The innate ability is handed down from generation to generation, and, in so far as it contributes to success, is fostered and developed by selection; and the modifications of behaviour to which it leads, since the results of prior process in the parent persist as the basis and starting-point of subsequent process in the offspring may in a sense also be said to be handed down.
(3) The conditions in the external world may be organic or inorganic. By organic I mean the conditions which depend upon the number of competitors or enemies by which a bird is surrounded. The competitors may include other species which require a similar environment; and the enemies, species which prey upon it, or animals which take its young or its eggs. They vary in different seasons, in different districts, and in nature and extent—the success of one species leads to the failure of another, and the
