CHAPTER I 



INTRODUCTORY 



THE Mammalia form a group of vertebrated aninials which 

 roughly correspond with what are termed in popular language 

 " quadrupeds," or with the still more vernacular terms of " beasts " 

 or " animals." The name " Mammal " is derived from the most 

 salient characteristic of the group, i.e. the possession of teats ; 

 but if the term were used in an absolutely strict etymological sense, 

 it could not include the Monotremes, which, though they have 

 mammary glands, have not fully-differentiated teats (see p. 16). 

 There are, however, as will be seen shortly, other characters which 

 necessitate the inclusion of these egg-laying quadrupeds within 

 the class Mammalia. 



The Mammalia are unquestionably the highest of the Verte- 

 brata. This statement, however, though generally acceptable, 

 needs some explanation and justification. " Highest " implies 

 perfection, or, at any rate, relative perfection. It might be said 

 with perfect truth that a serpent is in its way an example of 

 perfection of structure : not incommoded with limbs it can slip 

 rapidly through the grass, swim like a fish, climb like a monkey, 

 and dart upon its prey with rapidity and accuracy. It is an 

 example of an extremely specialised reptile, the loss of the limbs 

 being the most obvious way in which it is specialised from 

 more generalised reptilian types. Specialisation in fact is often 

 synonymous with degradation, and, this being the case, implies 

 a restricted life. On the other hand, simplification is not always 

 to be read as degeneration. The lower jaw, for instance, of 

 mammals has fewer bones in it than that of reptiles, and is more 

 concisely articulated to the skull ; this implies greater efficiency 



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