278 TOWARDS THE HUMAN FORM 



they had had time to modify themselves ; and, since we find 

 practically all the types of the present day at the beginning 

 of the Tertiary, it is extremely probable that they had already 

 been achieved in the Cretaceous, and that owing to one of 

 those peculiar chances so frequent in palaeontology, we are 

 acquainted only with the abnormal types of this epoch. 



The Mammals had evolved side by side with the Birds. In 

 the Trias they were already represented by the genera Droma- 

 therium and Microconodon. To these were added during the 

 Jurassic, many Marsupials with special dentition, 1 and during 

 the Cretaceous further new genera of the same type, together 

 with Plagiaulax, provided with teeth of a new type. None of 

 these appear to be of great importance in the fauna of the time 

 at the outset, nor is it till the seasons become marked that they 

 become more considerable. By the end of the Cretaceous 

 Period, however, the seasonal cycle is accentuated. Birds 

 and Mammals were not affected by this modification in the 

 climate which, as we saw, reacted so profoundly on the Insects. 

 Their blood was at a constant temperature, and they main- 

 tained the same activity all through the year. And since 

 Birds sit on their eggs and Mammals are viviparous, the young 

 of both were able to avoid, like the full grown creatures, the 

 vicissitudes inseparable from variations in the temperature. 

 With Reptiles, however, it is quite otherwise. 



All existing Reptiles have a body-heat which changes accord- 

 ing to external variations in temperature. An excess of heat 

 or cold benumbs and can kill them, and they take no care of 

 their progeny, which are more exposed than themselves to 

 extremes of heat and cold. There is no reason for believing 

 that it was otherwise with the great Reptiles of the past. 

 The minute size of their brains indicates that they were 

 extremely unintelligent creatures, and their organization was 

 no higher than that, for instance, of their contemporary the 

 Crocodile. As in the Crocodile, the arterial and venous blood 

 was probably mingled. But even if they had been more perfect 

 in this respect they would have been little better off. Inner 

 heat is a function of activity, and the unintelligent Reptiles 



1 The Triconodonts : Amphilistes, Phascolotherium, Triconodon ; and the 

 Trituberculates : Amblotherium, Dryolesles, Amphitherium of the Jurassic ; 

 and Pedromdys, Dielphops, and Crinolestes of the Cretaceous. 



