8 TROPICAL MEDICINE AND HYGIENE 



as mechanical carriers. Thus the ordinary house-fly 

 may, after alighting on the excreta, of a typhoid patient, 

 carry the bacilli to human food, such as milk, directly. 

 In other instances, blood-sucking insects take up bacteria, 

 such as those of plague and leprosy, and may, in the 

 former case at least, infect other animals. 



The bacteria present in the water in which larvae live 

 are taken up by such larvae, and in some instances, e.g., 

 Bacillus pyocyaneus, the bacteria continue to live during 

 the various stages of development of the larvae and may 

 be widely distributed by the adult insect or imago. 

 The conveyance of helminthes and of vegetable organisms 

 by insects will be considered in Part II and Part III. 



ORIGIN OF PARASITES. 



The question is sometimes raised as to the origin 

 of parasites, and particularly of such parasites as are 

 found only in so recent (geologically) a development 

 as man. No direct genealogy can be drawn up for these 

 parasites ; they must be derived from pre-existing non- 

 parasitic forms which gradually became parasitic during 

 one, probably the sexual, cycle, and later parasitic through- 

 out their entire cycle. Possibly, this change first took 

 place in birds or bats, and by development from them 

 those parasites, which are now parasitic in man only, 

 were developed. The intervening links are lost and any 

 explanation can be merely hypothetical. 



