As currently conceived, the biological effects of temperature extremes on terrestrial organisms are tacitly limited to chiefly the survival of individual adults, or again individually, destruction of young. A review of the literature pertaining to high temperatures relat ive to gametogenesis indicates the existence of a third possibility that involves multigeneration reproductive success or failure and thus operates at the populational rather than the individual level. Investigations of the reproductive consequences of above normal temperatures and the involvement of gametogenesis have been wholly random in their selection. Both plants and animals appear to be in seme way subject to these heat effects. Excessively high but individually non-lethal temperatures may induce total aspermia or a heightened mutation rate. Apparently there are no observations that indicate a similar susceptibility in female gamete formation. Heat effects inflicted during male gametogenesis in the scrotal mammals furnish indubitable evidence that even somatically optimum temperatures will produce sterility. Somewhat similar susceptibility appears to occur in plants and animals other than the scrotal mammals. High but nonsterilizing heat may induce mutation rates up to a Q10 of 5+ times normal. As yet the literature provides no clear evidence of instances in which gametogenic and somatic temperature requirements or tolerances are identical in males although females appear to be isothermic in this respect. Because of the latter condition it is therefore expected that instances of male isothermy will be found. The narrow margin between gametogenic heat damage and optimal somatic temperatures is such that local gonadel and total somatic thermoregulation becomes critical in terms of normal numerical and qualitative multigeneration success. This subtly operating factor may, inter alia, influence the body habitus and surface-to-volume ratios chiefly in behalf of effective heat dissipation rather than energy conservation, and hence may call for a reversal of some current views on the nature of the Bergman-Allen-Schmidt principles.