CHAPTEK III 

 ANCESTRY OF HOMOIOTHERMIO ANIMALS 



The general structure of the genealogical tree of ver- 

 tebrates is now fairly well established. Since Cambrian 

 times fishes gave rise to amphibians and amphibians to 

 reptiles. From early reptiles came the birds and 

 mammals. 



Although some fishes, notably the tunnies (Boulenger, 

 1910), after activity may show a body temperature some- 

 what above that of the water in which they live, there 

 are none which maintain a high and constant tempera- 

 ture comparable to that of birds and mammals and there 

 is no evidence that any fishes have been able to do so in 

 the past. Amphibians and reptiles, both aquatic and 

 terrestrial, also may have body temperatures above the 

 medium in which they live, but are not homoiothermic 

 and apparently have never been so. 



In speaking of the dinosaurs, Lull (1924) says: ''That 

 their activity, which implies increased metabolism, raised 

 the bodily temperature during the time of such activity, 

 it is, I think, safe to assume — an analogy is seen in the 

 tuna among the bony fishes today — ^but that they pos- 

 sessed a mechanism for the maintenance of a constant 

 bodily temperature irrespective of external conditions, 

 as with birds and mammals, is sustained by no evidence 

 thus far oifered. In fact, their entire lack of heat-retain- 

 ing clothing, such as the feathers of the bird or the hair 

 of the mammal, negatives such a possibility." The same 



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