But the young Cuckoo deserts this country many weeks after its parents, and there is no reason to suppose that it lives in society when eventually its destination is reached; and the young Falcon passes to the south, and is certainly not gregarious--how then can we explain their behaviour in terms of something which they show no signs of possessing? I do not wish to make light of a difficulty which admittedly, at first sight, is a grave objection to the view that the gregarious instinct has been operative in the manner here claimed for it. It must, however, be borne in mind that this instinct, though originally developed to serve the purpose of mutual protection, supplies the material upon which evolution works when the extension of the breeding range creates a situation requiring readjustment on the part of the organism to new conditions of life; and that those variations which can be so modified as to be in useful relation to the new environmental circumstances are seized upon by natural selection and, being transmitted, form the foundation of a specific inherited response, no longer dependent upon, though operating in close relation with the primitive response whence originally it sprang. Thus the primordial instinct becomes so organised as to serve a secondary purpose, that of rendering secure a means of access to a certain food supply. In the course of evolution species were bound to arise which, owing to some peculiar conditions, derived greater advantage from living solitary than from living in society. Does it then follow, because such species manifest no inclination to live in society, that the instinct never has played any part in their lives? Or because the primary purpose has lapsed, does it follow that the secondary no longer exists? Let me recapitulate the principal considerations which I have discussed in this chapter. Though I have been advancing a theory, and though I have taken much for granted, yet it will, I think, be admitted that both the theory and what has been taken for granted rest on observational grounds. As our starting-point we have a bird whose inherited nature alternates according to the season, and in whose nature we can distinguish two contra-phases--the one to live in society, the other to live solitary. While both have their part to play in furthering the 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.