Evolution 



the teeth of the reptile are all of a kind, and they may be replaced 

 almost any number of times during the animal's life, whereas 

 those of the mammal show differentiation according to their 

 respective functions, and are only once changed, that is, when 

 the milk teeth are replaced by the permanent set. The teeth of 

 the mammal are of four kinds : incisors, or chisel-like cutting teeth, 

 canines, which are especially well developed in carnivorous 

 species, and which are used in the tearing up of flesh, etc., and 

 two groups of grinders — ^^premolars, which are replaced during 

 the animal's life, and molars, which occur only as permanent teeth. 



The mammals in all probability arose from a particular group 

 of reptiles which flourished in the Permian and Triassic periods, 

 and which disappeared very shortly afterwards. These pass 

 under the name of the Theromorpha, and a typical specimen, 

 Pareiosaurus, is illustrated in Fig. 102. The skull of another, 

 illustrated in Fig. 103, shows distinct indications of a mammal- 

 like differentiation of the teeth. The Tritylodon, whose skull 

 is shown in Fig. 104, is an intermediate type between this group 

 and the mammals, some zoologists regarding it, as it were, as the 

 last reptile, others as the first of the mammals. 



As to how the special mammalian features arose, or what 

 special conditions called them into existence, we are of course 

 without definite knowledge, for neither hair nor mammary glands 

 are recognisable in fossils. But it seems likely that the warm 

 blood and the hairy covering evolved in correlation with one 

 another, and as adaptions to meet a gradual cooling of the 

 climate. It is certain at all events that the present-day mam- 

 mals (and also the birds) are far better adapted for a cold environ- 

 ment than the reptiles, which very easily get frozen to death ; 

 and it is also known that ice periods occurred in South Africa, 

 where many fossil Theromorpha and the Tritylodon are found, 

 at the time when these creatures existed ; both of which facts 

 support the theory indicated. 



At the present time the mammals are the highest and on the 

 whole the most successful of the vertebrate groups. They in- 

 clude the largest and strongest, the swiftest-footed and the most 



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