CHAPTER I 
INTRODUCTORY 
THE Mammalia form a group of vertebrated animals 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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