The geographical distribution of cold-blooded vertebrates.

"Darlington's most important contribution to science was his theory of the Old World tropical origin of dominant vertebrate groups. He first sketched out this formulation—which would influence research in zoogeography for a generation—in The Quarterly Review of Biology of 1948, then presented it in full dress in his 1957 text, Zoogeography: The Geographical Distribution of Animals." (E. O.Wilson)