CHAPTER VII 

 ECONOMIC PROBLEMS 



THE earliest annals of human history are full of 

 the records of disease outbreaks in man or in 

 his domestic animals, and also of ' plagues ' Ox animals 

 such as field-mice, locusts, and beetles. These were 

 specially well-marked manifestations of the fluctua- 

 tions in population which we have seen to be almost 

 universal among wild animals. In later years a new 

 menace began to be added to this one. Originally 

 the world had been split up by natural barriers into 

 fairly well-limited zoogeographical areas. But the 

 invention by man of better and better means of 

 transport has had the unintended result of spreading 

 round the world large numbers of animals whose 

 arrival has often been the start of serious new pests 

 or diseases. Massey (1933) has drawn attention to 

 the new possibilities for the spread of yellow fever 

 consequent on the development of air travel in 

 tropical regions, especially in Africa. Yellow fever 

 is a virus disease of man which attacks also certain 

 monkeys, and is transmitted by the bite of mos- 

 quitoes which are specific hosts for the virus. Yellow 

 fever used to be indigenous to West Africa, where it 

 is comparatively mild. It spread to other regions, 

 notably Central and South America. Although we 

 have almost come to think of ' yellow jack ' as a 

 vanishing disease, it is now realized that its spread 

 into East Africa and from there to Asia has been in 

 the past prevented mainly by transport barriers. 



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