After a consideration of all evidence, including the fact that no ultra-violet rays were used, it was concluded that the explanation lay in the increased exercise taken during the periods of increased light. A simple test whereby certain birds were forced by mechanical means to take more exercise, the light being so reduced that there was merely sufficient glow for them to see the advancing mechanism that forced them into movement, showed that the rate of development of the sexual organs exactly paralleled that in the birds that were exposed to extended periods of illumination in the outdoor aviaries. Other features in this experiment--such as the behavior of the birds themselves--also indicated that more activity due to increased light is the governing cause of the spring development of the sexual organs. If this development be accepted as a controlling cause of migration, then this experiment must be recognized as of great importance. 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. =Theory of continental drift= 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."