experience, they will be guided by them and will find themselves in regions where food is plentiful. Hence in each generation those will survive that, owing to some congenital variation of their instinct, seek satisfaction for their impulse in a direction which brings them under the influence of tradition. And though at first but slight and not in themselves of survival value, such variations, since they coincide with modifications of behaviour due to acquired experience, will be preserved and in the process of time so accumulated as to be capable of determining the direction and extent of the movement.
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 thie 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
