reason to believe that its origin dates back to an early period in the evolution of the higher forms of life; and if in" the subsequent course of evolution it could have been so organised as to serve a double purpose, so much the more reason would there have been for its survival. In what does the instinct consist? Is it merely that the sight of this individual or the call of that proves at some particular moment an irresistible attraction, or does the appropriate organic condition give rise, as is generally supposed, to some preceding state of uneasiness? In the former case, the temporarily isolated individual or colony would have but little chance of sharing in the benefits which mutual association confers upon the associates; in the latter, the feeling of discomfort would lead to restlessness, and would thus bring the bird into touch with the environing circumstances under which instinctive behaviour could run its further course. So that it is probable that the movements of each individual, prior to its becoming a unit in the flock, are not accidental but are determined in some measure by racial preparation.
Now if the fundamental assumption of the doctrine of the struggle for existence be true, the gregarious instinct will not be quite alike in all the members of different broods, nor even in each member of the same brood; that is, variation will occur in all possible directions. And we shall not. I think, exceed the limits of probability if we assume that different individuals
