To what does this change lead? Let us suppose that there is an area inhabited by one species; that the number of inhabitants has reached the maximum that the means of sustenance will allow; and that the season of reproduction is drawing to a close. The position will then be as follows. All the available breeding ground is divided into territories; each territory is occupied by one unit, the family, and each individual is able to fend for itself; changes both internal and external begin to take place, the gregarious instinct comes into functional operation, and the supply of food diminishes--that roughly is the position. The internal factor operates so that the sight of this individual or the call of that, instead of evoking hostility as heretofore and keeping different units apart, proves now an irresistible attraction; so that in place of a number of individuals evenly dispersed over the whole of this area, a small number of flocks of various dimensions are stationed at certain points, which points are determined partly by experience, partly by the supply of food, and partly by accident. This implies for each individual some movement in some direction. But since the population of this imaginary area has reached the maximum, and the supply of food, though limited in distribution, is nevertheless plentiful, such movements will be irregular and will proceed in no definite direction. Now let us suppose that the breeding range extends and that fresh ground is occupied by pioneers. When reproduction and the rearing of broods are ended and the gregarious instinct becomes dominant, these pioneers, or at least some of them, will revisit the area wherein formerly they associated with companions. Their offspring, however, though they will have the inherited impulse and the innate tendency, will not have the experience; how then will they behave? There can be no doubt that some will accompany the older birds, and, being led by them, will share the experience of a former generation; nor any question that others will collect together in the neighbourhood of their birthplace and, if their impulse is satisfied, will remain there so long as food is to be found. Thus the gregarious instinct, working in close relation with acquired experience, will on the one hand lead to the formation of organised movements in certain directions, whilst on the other it will lead to the formation of new areas of association which will follow in the wake of the expansion.