Читать книгу The Migration of North American Birds (1935) - Frederick Charles Lincoln - Страница 7

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Migration has long since become a definite hereditary habit that recurs in annual cycles, probably because of physiological stimuli associated with the reproductive period. In seeking its origin it is necessary to study the history of the birds' occupation of their present ranges, and from the information available to consider what appear to be reasonable theories. The two now most commonly accepted are diametrically opposed to each other.