SIX FIGURBS
The maintenance of a high, constant body temperature despite variation of the environmental temperature is the most striking group characteristic of the higher vertebrates ; birds and mammals. Among the monotremes, the marsupials and the xenarthra, however, there are mammals with body temperatures and capacities for thermal regulation intermediate to those of the poikilotherms and most homoiotherms. These mammals all possess primitive characters, both morphological and physiological, and we have the broad view of homoiotherism evolving along with the other characteristics which distinguished the higher groups.
It is of interest, therefore, to compare homoiothermism in representatives of different groups, particularly primitive ones. It is also of interest to compare related species which experience different thermal environments. By means of these two kinds of observations we may compare the taxonomic distribution of horrioiothermism with its lability with changing environment (i.e., within a given taxonomic group)
These observations were made, incidental t o a study of mammals in Northern Panama during the Hummer of 1941, and were supported in part by a grant from the American Philosophical Society to Dr. R. K. Endeis.
* T h e term homoiotherniism has been loosely applied so that it ma>-refer
(1) merely to "warm-bloodedness," ( 2 ) to the constancy of the body temperature under ordinary conditions, or ( 3 ) to its constancy in the face of thermal stress. It is also used broadly as a sort of summation of all of these, and it is in this sense that it is applied here. As knowledge of this function increases it will probably be necessary t o define more euplicit t e ~n i s .