CHAPTER XLI 



THE EVOLUTION OF THE MAMMALIA 



In considering the evolution of the mammals it is necessary 

 to revert to the Theromorph reptiles, in the more highly 

 developed members of which, such as Cynognathus, it was 

 found that the following characters were present. The skull 

 had two occipital condyles, a false palate, heterodont teeth in 

 sockets with the mammalian method of replacement. The 

 dentary was large, the remaining membrane-bones of the lower 

 jaw were small, and the jaw articulation was beginning to be 

 taken on by means of the squamosal ; the quadrate was loose 

 and small. The limb girdles were of the mammalian type, 

 and the limbs were long and supported the body clean off the 

 ground. These characters point unmistakably to the fact 

 that the mammals were derived from ancestors which were 

 Theromorph reptiles. 



The dominant factor in mammalian evolution appears to 

 have been the development of the brain along the lines of 

 increase in size of the roof of the cerebral hemispheres, and the 

 formation of a special area of cerebral cortex called neopallium, 

 which was no longer under the dominance of the fibres coming 

 from the olfactory centres. The neopallium became an organ 

 for the retention of past sensations and for the delicate co- 

 ordination of the activities of the body of the animal, which 

 thus became capable of more efficient response to external sets 

 of circumstances, and capable of profiting by experience. It 

 enabled the animal to improve the speed and precision of its 

 method of locomotion with the help of the long and delicately 

 formed limbs ; and the fact that the skin lost its hard horny 

 scales and became supple and covered with hairs, enabled it to 



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