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ABSTRACT The horizontal temperature zones of the earth tend to restrict the latitudinal ranges of species but allow the possibility of exceedingly broad longitudinal dispersals. In the Tropical Zone, biodiversity on the continental shelves is not homogeneous but is concentrated in two conspicuous peaks, one in the Indo-Pacific Ocean and the other in the Atlantic. The Indo-Pacific biodiversity peak is located within a relatively small area called the East Indies Triangle. The Atlantic peak is located in the southern Caribbean Sea. Evidence that has been accumulated over the years indicates that each area functions as a centre of origin and evolutionary radiation. What are the causes of these concentrations and their present functions? A newly published model indicates a positive relationship between environmental temperature and the rate of speciation. While this helps to explain the generally high tropical diversity, and the negative relationship between diversity and latitude, it does not provide a reason for the longitudinal concentrations. But, other new research serves to substantiate previous indications of a positive relationship between speciation rate and species diversity. The existence of this positive feedback, together with some contributory factors, provides the reason why concentrations occur. The evolutionary radiation probably begins when the build-up of species diversity reaches a critical level. The warm-temperate biotas are derived from the tropics. Their northern longitudinal relationships tend to be minor but, in the southern hemisphere, the West Wind Drift is an important dispersal mechanism for both warm-temperate and cold-temperate species. The cold-temperate biotas peaked in two areas, the North Pacific and the Antarctic; each has developed into a centre of origin. The continuous dispersal of well-adapted species from the centres helps peripheral communities maintain diversity.