NATURAL HISTORY 



II. MAMMALS 



IN an earlier chapter we have dealt with the evolution of 

 animals in general, their haunts or habitats, their everyday 

 functions, their behaviour, and what we have called the 

 dawn of mind. Here we select one class, that of Mammals, and, 

 presupposing what has gone before, we shall discuss them in 

 the main from one point of view how they are suited to the 

 particular conditions of their life. 



Origin of Mammals 



The genealogical tree of animals splits at the top into Birds 

 and Mammals, and these are on quite different lines of evolution. 

 They are not related to one another, except to this extent, that 

 they have a common ancestry among the extinct Reptiles, as 

 we have already seen. For just as Birds sprang from some un- 

 certain stock of bipedal Dinosaurs, so Mammals must be traced 

 back to another extinct Reptilian stock the Cynodonts. These 

 Cynodonts (also known as Therapsids) occur as Triassic fossils 

 in Africa and North America; though they were genuine rep- 

 tiles they had -very mammal-like skulls (see figure facing p. 

 454). Thus the teeth may be distinguished as incisors, canines, 

 and molars, as in a dog; hence the name Cynodont, "dog- toothed." 



The Earliest Mammals 



The earliest mammals were small creatures, the largest no 

 bigger than a rat. The teeth of some of them indicate insect- 



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