or a few species of other animals. When there is a 

 scarcity of food animals, there will naturally be a short- 

 age of those that feed upon them. It is interesting to 

 note that when there is a sporadic increase of some 

 form, as oftimes happens with certain mice, there is 

 an immediate increase of its natural enemies, such as 

 owls, weasels and shrews. In the winter of 1926 there 

 was an abundance of snow^y ow^s in the state of Penn- 

 sylvania, wiiere this form is usually scarce. In this 

 case there was probably a scarcity of Arctic Hares, on 

 which these owls generally feed in their northern 

 haunts, and the birds moved southward to a place 

 where there was a greater w^inter activity of animals. 



The extermination of any form should not be un- 

 dertaken unless its habits are well-known, because, not 

 infrequently, when an animal is exterminated, the 

 equilibrium of nature is destroyed. When some ani- 

 mal, which is not seriously pestiferous, has its enemies 

 eliminated, it will increase with amazing rapidity and 

 will often become a serious menace. 



The greatest weapon that the Economic Zoologist 

 has in combating a certain pest, is some other animal 

 that will feed upon it or that will parasitize it. The 

 most effective checks on destructive animal forms are 

 always natural ones in the form of enemies. The rea- 

 son for the increase of pests w^hich have ibeen intro- 

 duced from other countries is that their natural enemies 

 were not imported with them. In many cases, such as 

 the Japanese Beetle and numerous others, the intro- 

 duced animals are more abundant and, therefore, many 

 times more destructive here, than in their native lands, 

 because of the absence of their natural enemies. 



L i S R /: i; 



