THE MAMMALS 233 



Some rabbits and hares in the far north don a white coat 

 in the winter season. 



218. Skeleton. As in other vertebrates, the external 

 form of mammals is dependent in large measure upon the 

 internal skeleton. This consists of relatively compact 

 bones, the cavities of which are filled with marrow. Those 

 forming the skull are firmly united, and, as in other verte- 

 brates, afford lodgment for several organs of special sense 

 and for the brain, which, like that of the birds, completely 

 fills the cavity in which it rests. The vertebral column to 

 which the skull is attached differs considerably in length, 

 but it invariably gives attachment to the ribs, and to the 

 basal girdles supporting one or two pairs of limbs. Gener- 

 ally speaking, the number of bones in the head and trunk 

 of all mammals is the same, so the variations we note in 

 the species about us, for example, are simply due to differ- 

 ences of shape and proportion. As we are aware, there is a 

 great dissimilarity between the length of the neck of man 

 and that of the giraffe, yet the number of bones in each 

 is precisely the same. On the other hand, the variations 

 occurring in the limbs are often due to the actual disap- 

 pearance of parts of the skeleton. Five digits in hand 

 and foot is the rule, and yet, as we well know, the horse 

 walks on the tip of its middle finger and toe, the others 

 being represented by small, very rudimentary, splint bones 

 attached far up the leg. The even-hoofed animals walk on 

 two digits, two smaller hoofed toes being often plainly 

 visible a short distance up the leg, as in the pig. In the 

 whales the hind limbs have completely disappeared, and in 

 the seals, where the ' fore limbs are modified, as in the 

 whales, into flippers, the hind limbs show many signs of 

 degeneration. 



219. Digestive system. Some mammals, such as man, 

 monkeys, and pigs, are omnivorous ; others, like the cud- 

 chewers and gnawers, are vegetarians ; and still others, 

 like the foxes, weasels, and bears, are carnivorous. In 



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