104 INSECTS AND HUMAN WELFARE 



entirely, and they may be dependent also upon the ease and 

 rapidity with which some species become established in iso- 

 lated localities. 



So far as the interchange of insect pests between Europe 

 and America is concerned, the New World has decidedly 

 played the losing game. Whether this has a biological basis 

 is doubtful, for undoubtedly the more thrifty and painstak- 

 ing European peoples have scrutinized more carefully the 

 things which have been received into their domain. On the 

 other hand it is quite possible that the fauna of the palaearctic 

 region may impose a more severe struggle for existence upon 

 newcomers from our region than occurs in the inverse di- 

 rection. At any rate America seems so far to have received 

 more undesirable insects from abroad than has Europe. 



The most promising outlook which the entomologist can 

 enjoy is the decreasing prevalence of several of the more im- 

 portant insect-borne diseases. Through his efforts, coupled 

 with those of medical investigators and sanitarians, really 

 wonderful progress has been made in the reduction and re- 

 striction of malaria, yellow fever, plague, and typhus fever. 

 The future will undoubtedly see a still greater activity along 

 this line. One must not suppose, however, that the medical 

 entomologist need not expect to receive set-backs at any 

 time, or to have new problems thrust before him. Certain 

 disease-bearing insects may be expected to extend their 

 range in the same way that other insects are migrating. On 

 account of their close association with man, many of the in- 

 sect carriers of human diseases are already very widespread, 

 but many unpleasant surprises are doubtless now hidden in 

 little known parts of the world, from whence they may spread 

 without warning. 



