habit, but one now not absolutely necessary to the bird's welfare, we can see why it should be, as it frequently is, so greatly influenced by surrounding circumstances and conditions.
Taking the movement from its proper starting-point, which we assume to be the movement from south to north, in the three spring months, we must now look for sufficient causes to induce the undertaking of such long journeys. These causes are suggested by the two principal objects effected, on their arrival at their northern destination, viz., rearing of their young, and procuring suitable and sufficient food for both themselves and offspring. If migration is for these two purposes only, then it should prove to be the case that food was not sufficiently abundant in the south for both its resident and migratory birds. This certainly could not have been the case, and we believe, therefore, that migratory movements, at the outset, were to a very limited extent only; a few birds at a time seeking to avoid their enemies, and have undisturbed possession of a locality, by pushing out from their accustomed haunts, for, comparatively speaking, a few miles. The young of such pioneer birds would naturally leave the neighborhood of their nest, and return to their parents' usual haunt with them; but, on the return of another breeding-season, they would themselves seek a resting-place near where they themselves were reared, and the older birds would go to the same nest or nesting-place that a year ago they occupied. This is precisely what occurs now, year after year. Now, as birds increased, century after century, the limits of this northward movement would be extended, until it became in time the journey of thousands of miles that it now is.
Assuming, then, that migration arose for the dual purpose of safe nidification and a certainty of sufficient food, we are met by the ugly question, "Why do not all the southern birds come-north?" If, when the whole avi-fauna was concentrated at the south, there was any struggle whatever for favorable feeding or breeding grounds, then, naturally, the weaker would go to the wall, or, in other words, would be driven beyond the limits of their accustomed habitat. These weaker birds, taken together, having once formed the habit of visiting certain localities at stated times for given purposes, or periodically were forced to do so, would vary in their methods of reaching these localities, in their choice of regions wherein to remain, and the length of their annual visit, just in proportion as their habits generally varied from those of both other species of the same family and from species of other families. For instance, to avoid a common enemy, a number of species might have gradually learned to migrate at night; while others, although forced to migrate, had not this same enemy to contend with. In this way, the habit of nocturnal migration would long ago have been formed, and it would, by inheritance, be continued by their descendants, even after the enemy had been long extinct.
Having reached the northern summer homes, and, free from molesta-
