12 ECONOMIC MAMMALOGY 



try, lambs and calves, or entail much expense in the effort to prevent 

 their depredations. Prairie-dogs in an uninhabited region would do no 

 harm to mankind, but on cattle ranges and about cultivated fields they 

 are often very destructive. Weasels are very useful in the destruction of 

 rodents, but may be very destructive to poultry unless the henhouses 

 are so constructed as to exclude them. 



A few deer in the woods and pastures are harmless and pleasant to 

 see, but it is quite conceivable that if their enemies were all destroyed 

 and they were for a few years free from disease and the effects of 

 disastrous storms, they might become so abundant as to be destructive 

 to forests, gardens and farm crops. Indeed, deer have locally become 

 harmful in some places. Even some highly useful animals must some- 

 times give way to a certain extent before the encroachment of exten- 

 sive agriculture and other activities of advancing civilization. To the 

 Indians and white explorers on the Great Plains of the West the bison 

 was a blessing, but the large herds migrating through cultivated fields 

 of the thickly-settled portions of that region as they are now would be a 

 calamity. Dogs are useful in their proper places, pleasant companions 

 and faithful friends, but occasionally one develops the sheep-killing 

 habit, and it must then be controlled or disposed of. 



It is quite possible that, as with birds, no species of mammal may 

 be considered, under all circumstances, either wholly beneficial or 

 entirely harmful. Certainly the great majority of species are bene- 

 ficial in some ways and harmful in others. The first problem of 

 economic mammalogy is to place in the balance the beneficial and in- 

 jurious habits of each species, and thus to ascertain whether it is more 

 or less useful than harmful. This process requires an intimate knowl- 

 edge of the habits of many other organisms. For example, an in- 

 sectivorous mammal may take some useful species of insects and some 

 harmful ones, in which case its economic status must be based upon an 

 exact knowledge of the proportions of the various groups of insects 

 in the mammal's food, and an equally exact knowledge of the habits 

 of the various groups of insects themselves. However, it is possible that 

 the greatest value of insectivorous mammals lies in their work in keep- 

 ing the whole insect tribe in check, rather than their destruction of 

 particular kinds of insects. 1 The same idea may be applied also to 

 mammals that destroy mice and other injurious animals. 



1 This subject is discussed more at length in Henderson, The practical value of 

 birds, pp. 42-45, 74-75, 1927, with footnote references to other publications. 



