LATITUDINAL VARIATIONS IN ORGANIC DIVERSITY

? ? A NIMAL life is, on the whole, far more abundant and varied within -^-the tropics than in any other part of the globe, and a great num ber of peculiar groups are found there which never extend into temperate regions. Endless eccentricities of form and extreme richness of color are its most prominent features, and these are manifested in the highest de gree in those equatorial lands where the vegetation acquires its greatest beauty and its fullest development." Thus wrote A. R. Wallace (1878). His remarks apply equally to the vegetable and animal kingdoms, to the terrestrial realm, and to at least the surficial parts of the oceans. Surely this correlation of floral and faunal diversity with latitude is one of the most imposing biogeographic features on earth. On the one hand, its existence poses large-scale problems in evolution. On the other, it offers a potential tool to the geologist-paleontologist who attempts to wring patterns of earth history out of the fossil record. The following pages serve to review some previously known latitudinal gradients in organic diversity, to describe quantitatively gradients in molluscan diversity along North American shores, and to inquire into the origin of these patterns.