102 INSECTS AND HUMAN WELFARE 



the induced natural control of insect pests. This method was 

 devised as a direct result of an understanding of the biologi- 

 cal environment of insects, and has been applied with the 

 greatest success to the control of imported pests. With cer- 

 tain modifications it has also proved of use in dealing with 

 native insects. Its possibilities have been by no means fully 

 explored, although it is continually receiving more attention. 

 There is no doubt that this work will continue to increase, in 

 our attempt to modify permanently the conditions that are 

 otherwise met by insect species imported into new regions. 



The rapid and continual influx of new insect pests into 

 countries like our own, that enjoy extensive commercial inter- 

 course with other parts of the globe, will undoubtedly con- 

 tinue in the future and the specific problems of the ento- 

 mologist will become more international in their nature. 



Practically all of the barriers to the dispersal of small 

 animals like insects, that existed when living things reached 

 their present state on the earth, have either been removed or 

 made less certain in their action. Mountain ranges and 

 deserts have been bridged by highways over which a con- 

 tinuous stream of humanity and materials is passing, bringing 

 with it everything that can make the comfortable journey. 

 Oceans, the most effective natural barriers, are crossed in all 

 directions with cargoes containing many things not listed on 

 the purser's books. In short, there is endless opportunity for 

 small living things to migrate to all quarters of the globe. 

 Even tropical insects find in the human communities of cooler 

 lands, opportunity to live under climatic conditions which 

 they could not otherwise withstand. 



Such migrations of insect life have, of course, been in prog- 

 ress for many years, and their economic significance has 

 long been realized. Gradually, in various parts of the world, 

 legislative measures have been enacted, aiming at a restric- 

 tion of the opportunity for noxious insects, weeds, and for 

 animal and plant diseases to gain entrance to new regions. 

 These are naturally modeled after the quarantine measures 

 taken to prevent the spread of human diseases. They have 



