Guillemots and Razorbills return at intervals to the breeding stations early in the season, and these visits are repeated with growing frequency until the birds are finally established. I have witnessed these periodic returns during March in the south of England, and during April in the north-west of Ireland, and I am informed that in the latter district such visits may occur as early as February. Gätke, who had ample opportunity of observing the birds in Heligoland, puts their return at an even earlier date. "They visit their breeding places," he says, "in flocks of thousands at the New Year, often even as early as December, as though they wanted to make sure of their former haunts being well preserved and ready for their reception." Such visits, however, are irregular in occurrence; the birds arrive, and, after spending a short time upon the ledges, disappear. And since there is not the same evidence in their coming and going of that method which we observe in the periodical returns of the Bunting or the Finch, it may be thought that needless importance is being attached to an episode in their lives which is quite intelligible in terms of a feeble response determined by a dawning organic change. While it may be quite intelligible in such terms it is not thereby explained; for every response must have as its antecedent an inherited connection in the nervous system determined on biological grounds. Besides, these early periodic returns conform in general to the type of behaviour displayed by other species, the males of which return to their breeding grounds many weeks before the real business of reproduction begins. Are we then justified in regarding them as accidents of the developing situation? Are we not rather bound to admit that they have some definite biological end to serve? [Illustration: Competition for territory is seldom more severe than amongst cliff-breeding sea birds, and the efforts of individual Razorbills to secure positions on the crowded ledges lead to desperate struggles. Emery Walker ph.sc.] These examples show that the males of many species reverse their mode of life at the commencement of the breeding season and proceed to isolate themselves, each one in a definitely delimited area. There are three ways in which we may attempt to interpret this particular mode of male behaviour. We may regard it as an accidental circumstance, nowise influencing the course of subsequent procedure; or, appealing to the law of habit formation, we may regard it as an individual acquirement; or again, we may invest it with a deeper significance and seek its origin in some specific congenital disposition determined on purely biological grounds.