We are sometimes told that we must seek the origin of migration in the physical changes that have occurred in the ancient history of the earth--in glacial conditions which gradually forced birds to the south, or in the "stability of the water and mobility of the land" which brought about a gradual separation of the feeding area from the breeding area--and which continued for a sufficient length of time to lead to the formation of an instinct, and that the instinct persists because it is serviceable in promoting the welfare of the race. But when we consider the lapse of time, and the changes that must have occurred in the character of the bird population--the appearance of new forms and the disappearance of the old, the ebb and flow of a given species in a given area--and bear in mind that, notwithstanding this, the migratory instinct, if not stronger, is assuredly no less strong, and the volume of migration, if not greater, is assuredly no less; in short, that the whole phenomenon is progressive rather than retrogressive, we shall find the view that the instinct owes its origin to conditions which no longer exist, receives but little encouragement. I doubt not that, throughout the ages, geological changes have been an important factor in directing or limiting the scope of migration, and moreover are so still; just as climatic changes and the relative abundance or scarcity of enemies have influenced the course of its evolution. These are all contributory factors operating in the external environment. But there are, besides, internal factors which form part of the inherited constitution of the bird, and, being passed on from generation to generation, afford the conditions under which migration is constantly being renewed. It is, I believe, in this field of organic change and relationship that the conditions of origin must be sought. Just as the moth in passing from the rudimentary to the perfect condition runs through a series of changes, each one of which is marked by a typical behaviour response adjusted to meet some particular circumstance in the external environment, so the annual history of a bird displays an ordered routine, each phase of which can be observed to correspond with one of the successive changes in the environment. In almost every direction, we find that this routine is characterised, in broad outline, by great uniformity; so much so that, providing we know the history of one species, we can forecast with no small degree of certainty the general course of behaviour of other members of the family. But only the _general_ course. There is endless variation in just the particular way in which the behaviour is adapted to meet the needs of particular species--the major details may be said to be specific, the minor details varietal.