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By speciation is meant the fixation of discontinuity among organisms. Discontinuity is maintained by isolating mechanisms that prevent the interbreeding of carriers of different adaptive complexes of genes. A theory is suggested according to which the development of isolating mechanisms follows, rather than accompanies, that of the adaptive complexes themselves. The development of physiological isolation takes place principally along the geographical boundaries separating the distribution areas of the incipient species. Some evidence for and against this theory is discussed.
The American Naturalist © 1940 The University of Chicago Press