INTRODUCTION 5 



species are large and conspicuous. A very large proportion are small, 

 secretive species, many of them nocturnal in habit, seldom seen by any- 

 one except special students of the subject. To the great majority of 

 men and women a mouse is a mouse and a rat is a rat ; but the natural- 

 ist recognizes hundreds of species of mice and rats, divided into cer- 

 tain very definite groups such as wood rats, meadow mice, deer mice, 

 pocket mice and so on. Most people in the United States are familiar 

 with the chipmunks, but are surprised to learn that there are many 

 species and varieties of them, not simply one kind. 



Though the small species are less conspicuous and less generally 

 known than the larger ones, yet economically some of them are vastly 

 more important. The economic effect of destroying all the bears in the 

 world would not be very great, and the economic effect of destroying 

 all the elephants in the world would, so far as we can foretell, be 

 rather local, but the economic results of destroying all the mice, all 

 of the gophers, or all of the rabbits in the w r orld, would be very pro- 

 nounced and widespread. Yet one cannot say with certainty that any 

 species can be destroyed without affecting many others in one way or 

 another. If a species that preys upon certain other species suddenly be- 

 comes extinct, the latter, released from the attacks of one of its enemies, 

 may multiply enormously and continue to do so until further in- 

 crease is checked by the limit of its food supply. On the other hand, 

 if a species that furnishes the food supply of another species becomes 

 extinct, the surviving species would be in danger of starvation, unless 

 it turns its attention to some other source of food, and thus affects 

 other species upon which it begins to prey. 



As all nature is complex, so economic mammalogy comprises a great 

 mass of interwoven phenomena. The interrelations of organisms and 

 their interactions and influence one upon another are complicated. 

 Through ages of adjustment and re-adjustment each species has, under 

 natural conditions, a more or less definite niche in the whole environ- 

 ment, influencing and influenced by all the species around it. This 

 adjustment has been much disturbed by the rapid advance of civiliza- 

 tion, which has within a short time carried into vast stretches of new 

 territory stock-raising and agriculture on a large scale, with the de- 

 struction of forests, the draining of swamps, the introduction of plants 

 and animals of various species from other regions ; the transformation 

 of large arid areas into cultivated fields, orchards and pastures by sys- 

 tems of irrigation ; the destruction of cover and decrease of food sup- 



