xii MAMMALS OF AMERICA 



relations for the thousands of varied forms about us. Evolution is a constant factor in the 

 animal world. No matter how biologists may differ about the modes of evolution, nowadays 

 none of them will refuse to admit that changes have taken place in the different animal 

 structures. Evolution may be either progressive or retrogressive, resulting in specialization 

 or degeneration. 



Evolution is expressed in a multitude of ways. The hair, the structure that on the 

 Rabbit is so soft and flexible, through specialization becomes the quill of the Porcupine, a 

 complex structure very little like the hair from which it has been evolved. This is called 

 specialization through change in form. The difference in the character of teeth, such as 

 the flat grinding tooth of a cow and the sharp shearing tooth of a cat, is a specialization of 

 this sort. Another type of specialization results in the loss of parts. This is illustrated by 

 the evolution of the Horse from a little five-toed animal of an older geological epoch into 

 a one-toed animal of today. Specialization that results in the increase or addition of parts 

 is seen among the bats, where many complex structures on the nose and in the ear occur. 

 Minor changes in Mammals are to be seen on every hand. Such changes are those in the 

 colors of the pelage to suit the environment, the increase or decrease in size of a species to 

 suit the food supply, and the growth of special areas of hair, like the tufts on the ears of 

 the Lynx. 



Just exactly how all these changes are brought about is a fruitful source of argument 

 among students of biolog\', but two main factors are currently recognized, namely, heredity 

 and the effect of environment. Heredity might be construed as the result of an earlier 

 environment which then would make environment the main factor. Certainly a close 

 connection can almost always be established between an animal's structure and its environ- 

 ment. As a result of the competition among animals, bringing into operation the principle 

 of "survival of the fittest," the Mammals have spread out into ever\^ conceivable environment, 

 every possible economic niche opened up to them by nature. We find Mammals of different 

 orders occupying the same niche; but generally when a Mammal has thoroughly mastered 

 its environment, its advantages are such that an intruder cannot meet it in active com- 

 petition and survive the encounter. "Adaptive radiation " is the name given to this spread- 

 ing out of a certain type of Mammal life. Rodents, for example, have become specialized 

 to the point that we find them living successfully as typical terrestrial animals (the Chip- 

 munk), as semi-subterranean (the Ground Squirrel), as wholly subterranean (the Pocket 

 Gopher), as aquatic (the Beaver), as arboreal (the Gray Squirrel), and finally as semi-aerial 

 (the Flying Squirrel) . In this same order, the method of locomotion has passed from the 

 typical running on all fours to swimming, to crawling in restricted burrows, to leaping 

 kangaroo fashion with the hind legs alone, and to gliding like an aeroplane. 



The distribution of Mammals has become ver>' general because of this adaptability 

 to environment. Mammals becoming accustomed to cold climates have extended their 

 ranges until some species are found under the Arctic Circle and on the highest mountain 

 peaks where intense cold reigns. On the other hand, desert Mammals have pressed into 

 the heart of the most arid, hopeless wastes of sand. Food seems to be the only requirement 

 necessary, lack of water and cover seeming to have little effect. As a rule, the largest and 

 best-developed fauna, however, is to be found where conditions are most suitable. 



REQUIREMENTS FOR STUDY 



The requirements for the study of Mammals are primarily few, and most of us already 

 possess them. The average child has a keen interest in the life about him and ordinarily 

 in the process of growing up acquires considerable knowledge of the commoner animals 

 of his vicinity. Given the interest, a sharp eye and an acute ear are material aids of first 

 importance. Observations often need to be interpreted, the inter]3retation not infrequently 

 calling for the skill and experience of the zoologist. Many things to be noted, however, 



