WHAT IS A MAMMAL? 



I 



is beyond our complete comprehension. We may be certain 

 that if typical well-specialized mammals lived in early 

 Tertiary times we must look very much deeper into the past to 

 find the very first mammal-like quadrupeds. Palaeontology 

 has a remarkable record of mammal history from the Eocene 

 to the Recent, by no means complete, but full enough to reveal 

 much data bearing on the appearance and development of the 

 orders, families, and genera of present-day mammals. 



Classification of Mammals 



Our modem system of the classification of mammals is based 

 upon the external and internal structure, anatomy, of living 

 mammals, and the internal structure of the fossil mammals of 

 bygone epochs. As a rule, only the hard parts of mammals 

 are preserved as fossils, and we have no certain knowledge of 

 the external appearance of these mammals, drawing conclu- 

 sions as to what they looked like alive only by analogy. For 

 purposes of classification, most external data are of importance 

 only in separating species and subspecies, since the truly 

 fundamental characters of relationship are to be found under 

 the hair and skin. The general scheme of this classification 

 is to start with the large groups of mammals all possessing 

 certain important characters. These large groups are in turn 

 split up into smaller groups on the basis of common characters 

 within each smaller group and certain differences in structure 

 which distinguish one group from all the other groups. By a 

 series of such reductions of the larger groups, eventually the 

 scheme arrives at a very small group, which includes only one 

 unit, the species or subspecies, as the case may be, which 

 differs in some character or characters from all the other unit 

 groups, but is related through the larger groupings to many 

 other mammals. 



We have then the largest group within the Animal Kingdom, 

 which includes all the mammals and excludes all other quad- 

 rupeds, and this is called the Class Mammalia. For the sake 

 of example, let us start down through the lesser groups toward 

 some particular species, the Star-nosed Mole, and we encounter 

 successively the Subclass Eutheria (all the mammals except 

 the Monotremes or egg-laying mammals), the Order Insec- 

 tivora (mammals of small size, primitive structure, and special- 



