THE 



M A M M ALIA. 



INTRODUCTION. 



T 



HE shortest definition of this important 

 class of the animal kingdom, to which man 

 himself belongs, is as follows: vertebrates 

 with hair and with milk-glands for the first nour- 

 ishment of the young. It is likewise a definition 

 that gives the only absolutely exclusive characters 

 of the class, for no other group of vertebrates has 

 hair or milk-glands. 



General Characters of the Vertebrata. — The 

 mammals are in the first place true vertebrates, for, 

 like all the other animals belonging to this sub- 

 kingdom, fishes, amphibians, reptiles, and birds, 

 they have an internal skeleton which forms the axis 

 of the body, and which is composed of a column of 

 separate vertebra;. This vertebral column is crowned 

 by the skull, which contains the brain and the 

 principal organs of sense, and it has in addition 

 two pairs of lateral appendages, namely the fore- 

 and the hind-limbs. This skeleton, which in the 

 higher vertebrates is always formed of bones, is 

 enveloped by muscles Avhich are attached to its 

 individual parts, and these parts thus perform the 

 function of levers on which the muscles act in con- 

 tracting. In most other animals the muscles are 

 attached to the more or less hard skin or external 

 covering. But the skeleton also forms the axis of 

 the body, which is so divided by it that we have on 

 the back (namely in a tube behind or above the 

 solid centres of the vertebra;) the central nervous 

 system, that is, the brain and the spinal column, 

 and on the abdominal side the other organs, those 

 serving for the development of the animal, namely 

 the alimentary system and the organs of respiration 



and reproduction. Moreover, the skeleton also 

 forms a number of cavities, in which the organs are 

 sometimes so much embedded as to lie in perfectly 

 closed capsules, as is the case, for example, with 

 the brain. 



None of the Vertebrata have more than two 

 pairs of limbs, a fore and a hind pair; and this 

 also is an essential point of distinction as compared 

 with other animals, for in every other sub-kingdom 

 the limbs or motor appendages, if present at all, 

 are alwaj's found in greater number. These limbs 

 may be only in a rudimentary condition or even 

 altogether absent, but their number can never be 

 increased. 



Different Types of the Vertebrata. — If now we 

 take a glance at the different types of vertebrates 

 with the view of comparing them together, we 

 observe at once a sharp line of demarcation in the 

 terminations of the limbs. In the fishes the limb 

 ends in a considerable number of rays, while in all 

 other living vertebrates the terminal portion of the 

 limbs never has more than five fingers or toes, or, 

 to use the general term employed by men of science, 

 digits. The fishes accordingly are man)'-fingered ; 

 amphibians, reptiles, birds, and mammals, on the 

 other hand, five-fingered (or five-toed). The num- 

 ber of these fingers or toes may be reduced in the 

 process of development, but every limb in all 

 members of the classes named has always five 

 digits to begin with. This is a fact of great im- 

 portance. We may deduce from it that vertebrates 

 with a smaller number of digits must be descended 



from five-fins-ered or five-toed ancestors, and that 

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