CHAPTER II. 



THE CLASS MAMMALIA — GENERAL CHARACTERISTICS — THE "DENTAL FORMULA" — 

 DIVISION INTO ORDERS. 



THE name Mammalia is derived from the Latin word mamma, 

 " a teat," and signifies animals that have teats. The possession 

 of these organs constitutes the most apparent and decisive dis- 

 tinction between the Mammalia and the other classes of animals. They 

 alone bring forth their young alive and suckle them with their milk. 

 The fishes cast their spawn upon the waters, careless of their future off- 

 spring. The reptile leaves its eggs to be hatched by the sun's ray, and 

 so far exercises some degree of forethought for its posterity. The bird 

 sits patiently on her nest till her fledglings appear from the egg, and then 

 tenderly watches over them, feeds them, and gathers them under her 

 wings. But the mammal brings forth her young alive, and nourishes 

 them at the living fountain of her breast. 



While all mammals possess teats, the number and position of them 

 vary. Some, like the cow, have them on the belly ; some, like man, have 

 them on the breast; some, like the sow, have them on both. In general, 

 the number of these teats corresponds to the number of young each 

 animal bears at a birth. 



The Mammalia vary greatly in size. How enormous is the difference 

 in this respect between the elephant and the mouse, or between the 

 whale and the bat, between the giraffe and the mole. Yet all are con- 

 structed on the same plan. In all the vertebrates, as we have said, the 

 skull consists of modifications and developments of parts corresponding 

 to four vertebrjE ; in all the mammalia the number of the neck vertebrae 

 are the same. Seven vertebrae form the neck of the giraffe as well as 

 that of the seemingly neckless whale. In birds, on the contrary, the 

 number of vertebra; increase with the length of the neck. The vertebrae 

 in the back vary in number from ten to twenty-three, the commonest 

 number being thirteen ; man, however, has onlv twelve. The vertebras 



