2 LAND MAMMALS IN THE WESTERN HEMISPHERE 



is clothed with hair ; a character found in no other animals. 

 In a few mammals the skin is naked, and in still fewer there 

 is a -partial covering of scales. The list of characters common 

 to all mammals, which distinguish them from other animals, 

 might be indefinitely extended, for it includes all the organs 

 and tissues of the body, the skeletal, muscular, digestive, ner- 

 vous, circulatory, and reproductive systems, but the two or 

 three more obvious or significant features above selected will 

 suffice for the purposes of definition. 



While the structural plan is the same throughout the entire 

 class, there is among mammals a wonderful variety of form, 

 size, appearance, and adaptation to special habits. It is as 

 though a musician had taken a single theme and developed it 

 into endless variations, preserving an unmistakable unity 

 through all the changes. Most mammals are terrestrial, living, 

 that is to say, not only on the land, but on the ground, and 

 are herbivorous in habit, subsisting chiefly or exclusively upon 

 vegetable substances, but there are many departures from this 

 mode of life. It should be explained, however, that the term 

 terrestrial is frequently used in a more comprehensive sense for 

 all land mammals, as distinguished from those that are aquatic 

 or marine. Monkeys, Squirrels, Sloths and Opossums are 

 examples of the numerous arboreal mammals, whose structure 

 is modified to fit them for living and sleeping in the trees, 

 and in some, such as the Sloths, the modification is carried so 

 far that the creature is almost helpless on the ground. An- 

 other mode of existence is the burrowing or fossorial, the animal 

 living partly or mostly, or even entirely underground, a typical 

 instance of which is the Mole. The Beaver, Muskrat and Otter, 

 to mention only a few forms, are aquatic and spend most of 

 their life in fresh waters, though perfectly able to move about 

 on the land. Marine mammals, such as the Seals and Whales, 

 have a greatly modified structure which adapts them to life in 

 the sea. 



Within the limits of each of these categories we may note 



