use of in the process of building, becomes so firmly established as to form the sole highway to and from the nest, so likewise, when the impulse to seek isolation repeats itself, the bird is constrained to seek the neighbourhood wherein it had experienced the enjoyment of breeding or of birth. Thus the little that is added one year becomes the basis for further additions in the next, and new centres of distribution are continually being formed from which expansion proceeds anew. Now as the range gradually extends into regions where the climate alternates and food at certain seasons is consequently scarce, the distance between the customary area of association and that of reproduction must perforce widen. The question then arises: How will the young that have no experience find their way to regions wherein they can endure? The forces which may have been organised to subserve the end in view are three: (1) Acquired experience, (2) tradition, (3) the gregarious instinct. The pioneer that carries the range a little further forward starts from a base where it has associated with companions and found food plentiful; and when the impulse to live in society again asserts itself, it not only repeats its former experience but hands on the habit thus acquired to those of the next generation that happen to accompany it. Granting, however, that by successive increments in the distance traversed, traditional guidance may in time accomplish much, it cannot account for all the known facts, it cannot at any rate explain the fact that in some cases the inexperienced offspring finds its way to the food area without guidance. Something, therefore, _is_ inherited. And my suggestion is this: That the gregarious instinct, the ancient origin of which we can infer from its manifestation in so many and diverse forms of life, supplies the material upon which evolution works; that variations of the initial impulse, at first slight and not in themselves of selection value, in so far as they coincide in direction with modifications of procedure due to experience or tradition, are preserved; and that, in the process of time, they are so accumulated as to form a specific congenital endowment determining a definite mode of behaviour. FOOTNOTES: [1] June 1915, R. M. Barrington. [2] _Dictionary of Birds_, p. 556. [3] _Social Psychology._ [4] _Manual of Psychology._ [5] _Ibis_, April 1918. [6] _Zoologist_, 1912, p. 327. INDEX Acquired experience, 300 Adjustments, transitory, of distribution, 275 Alarm notes, 119 Arrival, advantages and disadvantages of late, 33-44 Assemblies in winter, 262, 263 Assembly grounds, 173 Attainment of reproduction, 171 Barrington, R. M., on the sex of migrants, 25 Battle between two male Cuckoos, 82 ---- between two Moor-Hens, 86, 92, 93, 94 ---- ---- Pied Wagtails, 86 ---- ---- Raven and Buzzard, 217 ---- ---- Raven and Peregrine, 216 law of, 13,19 Behaviour routine, 262