CHAPTER II 



STRUCTURE AND PRESENT DISTRIBUTION OF THE MAMMALIA 



External Form. It would be quite impossible for any one to 

 confuse any other quadrupedal animal with a mammal. The 

 body of a reptile is, as it were, slung between its limbs, like the 

 body of an eighteenth century chariot between its four wheels ; 

 in the mammal the body is raised entirely above, and is 

 supported by, the four limbs. The axes of these limbs too, as a 

 general rule, are parallel with the vertical axis of the body of 

 their possessor. There is thus a greater perfection of the 

 relations of the limbs to the trunk from the point of view of a 

 terrestrial creature, which has to use those limbs for rapid move- 

 ment. The same perfection in these relations is to be seen, it 

 should be observed, in such running forms among the lower 

 Vertebrata as the Birds and the Dinosaurs, where the actual 

 angulation ' of the limbs is as in the purely running Mam- 

 malia. These relations are of course absolutely lost in the 

 aquatic Cetacea, and not marked in various burrowing creatures. 

 The way in which the fore- and hind-limbs are angulated is 

 considerably different in the two cases. In the latter, which 

 are most used and, as it were, push on the anterior part of 

 the body, the femur has its lower end directed forwards, the 

 tibia and the fibula project backwards at the lower end, while 

 the ankle and foot are again inclined in the same direction as 

 the femur. With the fore - limbs there is not this regular 

 alternation. The humerus is directed backwards, the fore-arm 

 forwards, and the hand still more forwards. This angulation 

 seems to facilitate movement, inasmuch as it is seen in even the 

 Amphibia and the lower Eeptiles, in which, however, the differ- 

 ences between the fore- and hind*-limbs are less marked, indicat- 



