90 ANIMAL LIFE AND HUMAN PROGEESS 



film of parafiin ; or it will disappear if the pools are drained, 

 or filled up, or poisoned. But is it not a fine instance of the 

 web of hfe that the importation of top-minnows into infested 

 waters is one of the surest ways of getting rid of rnosquitoes 

 and thus of malaria ? Eleven species of Indian fishes are of 

 proved value as mosquito-destroyers, and some might well 

 be used in city tanks. 



Yellow fever is spread by another mosquito {Stegomyia 

 fasciata) ; and the blood- worm (Filaria bancrofti), which causes 

 elephantiasis, is carried by yet another.. The yoimg worms 

 from the blood of a victim undergo changes in the interior of 

 the mosquito's body, and the resulting forms pass to the 

 proboscis, by which they are deposited on man's skin, into 

 which they bore. 



One of the important things to imderstand about parasites 

 is that they very often do little or no harm to their wonted 

 host ; it is when they are passed into a new host that they 

 spell disaster. Thus the microscopic animals (trypanosomes) 

 which cause sleeping sickness in man, and its analogue in 

 cattle and horses, seem to be at home in antelopes and reed- 

 bucks, which are accustomed to them. When the tsetse 

 flies sucking the infected blood are themselves infected, and 

 transfer the trypanosomes by and by to man and his stock, 

 then the mischief is done. For the blood of the new hosts 

 has not the necessary anti-toxin to the formidable trypano- 

 some. The whole riddle is not yet read, but we need it 

 only as an illustration of a general fact, that the tsetse fly 

 links antelope or some other wild reservoir of trypanosomes 

 to man, and that the damage done is due to altering the 

 existing arrangement of the threads in the web of life. 

 That alterations have been necessary and will continue to 

 be necessary as civilisation proceeds cannot be gainsaid ; the 

 point is to realise what all such changes are likely to involve, 

 and to bring p11 available science to bear on any proposed 

 change, so that distant as well as immediate consequences 

 may be, if possible, anticipated. 



It may here be mentioned that the widespread belief that 

 house-flies sometimes bite is due to the occasional presence 



