INSECTS AND HUMAN DISEASE 117 



elephantiasis or elephantoid diseases. On my return to 

 China in 1876 I endeavoured to ascertain if these parasites 

 occurred also in China. I discovered that they were present 

 in some districts in ten per cent, of the population ; in other 

 districts they were present in fifty per cent. ; while in other 

 places they were not found at all. One thing was certain 

 that this little organism was not a mature animal. It 

 showed no evidence of growth while in the blood, or of 

 any organs such as would leave one to suppose that it 

 was capable of reproducing itself. The inference was 

 therefore that it was the young of some other animal. 

 For this I searched many times, and at last found such 

 to be the case, although my discovery had been anticipated 

 by Bancroft and by Lewis himself. 



" The parental worm was quite a big animal, from about 

 3 to 4 inches in length, of a thickness of a strand of 

 fishing gut. It lay in the lymphatic vessels. But between 

 this mature animal and its young, actively wriggling 

 progeny in the blood no intermediate form could be 

 discovered. The problem naturally suggested itself, How 

 does this parasite contrive to pass from one human being 

 to another ? 



" Now it occurred to me that if it could not pass by virtue 

 of its own effort from one human body to another, and if 

 such a passage were necessary, as it obviously is, some 

 other agent must intervene, and that that other agent must 

 be one which is capable of piercing the skin of the human 

 body, and also one which absorbs the blood of the human 

 body, and with the blood the little wriggling parasite 

 which it contains. Such translation was, in my opinion, a 

 first and necessary step for the parasite to take when it 

 would quit one human body and get into another. Now, 

 the agent which occurred to me as being the most likely 

 to effect the necessary step in the translation of the filaria 

 was the mosquito." 



Whilst in Amoy, Manson examined the blood of over a 



