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Upon closer analysis, however, it is found that this theory, like those before discussed, is open to serious objections. First, some of our summer residents that migrate south for the winter do not stop in equatorial regions, where they might find the periods of day and night about equally divided, but push on beyond, some penetrating as far south as Patagonia. Also it might be asked: "If the lengthening day is the stimulating factor, why should our summer birds wintering in the Tropics ever start northward, as in their winter quarters the variation in the length of day from winter to summer is imperceptible?" Like all the other theories advanced, this also, as at present understood, is subject to unanswered criticism.

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The theory of continental drift postulates an original northern land mass, called Laurasia, and a southern one, called Gondwana. According to this concept, each eventually broke into several segments which eventually became the present continents. It is further assumed that occasionally Laurasia and Gondwana drifted close to one another or were at times in actual contact. On the basis of this geological theory, Wolfson (1940) has attempted to explain the migrations of some species of birds from one hemisphere to the other, as, for example, the Greenland wheatear, Arctic tern, and several shore birds (turnstone, sanderling, knot, golden plover, and others). Acceptance of this hypothesis requires abandonment of the belief that the development of migration was the result of useful ends that were served thereby, and in its place, to give approval to the idea that migration was merely "the natural consequence of an inherent behavior pattern responding to the drifting of continental masses."