THE GEOLOGICAL RECORD. 101 



some primitive reptile allied to the Crocodilia and Dino- 

 sauria. The avian branch has undergone comparatively 

 little pruning. Quickly supplanting the Pterosauria 

 after the Jurassic epoch, they radiated along all sorts of 

 adaptive lines, most of which survive to the present day. 

 The beautifully adapted organisation of birds, with warm 

 blood, efficient lungs, sharp senses, quick movements, 

 and light feathers, has secured them a supremacy in the 

 air which has hardly been challenged even by the 

 mammals. 



The Mammalia, that class of vertebrates to which we 

 ourselves belong, arose earlier than the birds, probably 

 from some primitive reptilian stock in Permian times. 

 Indeed, the Theromorph reptiles of the Trias so nearly 

 approach the mammalian type of structure in the char- 

 acter of the skull, palate, lower jaw, and other important 

 points, that they are now generally held to have been, 

 if not the ancestors themselves, at all events closely 

 allied to them. Quite independently of birds, and on 

 different lines of specialisation, the Mammalia have 

 acquired a four-chambered heart, completely separating 

 the arterial blood from the venous, and a se]f- regulating 

 mechanism, keeping the blood at a constant high tem- 

 perature, independent of that of the surrounding environ- 

 ment (p. 41). Of very adaptive build, the mammals 

 soon diverged from the primitive ancestral egg-laying 

 type now almost extinct, but still preserved in the 

 archaic Monotremes living in Australia, the famous 

 Ornithorhynchus, and Echidna. Adopting the advan- 

 tageous method of nourishing their young during early 

 life in the mother's womb, the placental mammals spread 

 rapidly over the earth, ousting the lower reptilian type 

 of organisation, and diverging in various adaptive direc- 

 tions, they became the dominant group in Eocene times 

 (Fig. 6). The mammals have, however, suffered severely 

 in the struggle. Large groups have vanished altogether, 

 while others are on the verge of extinction. The Mar- 



