S4 FILARIA AND THE MOSQUITO 



times present in enormous numbers — hundreds in every 

 drop of blood. These parasites Lewis had found in 

 more than one instance in association with elephantiasis 

 or elephantoid diseases. On my return to China in 

 1870 I endeavoured to ascertain if these parasites 

 occurred also in China. I discovered that they w^ere 

 present in some districts in 10 per cent, of the popula- 

 tion ; in other districts they were present in 50 per 

 cent. ; w^hile in other places they w^ere not found at 

 all. One thing w^as certain — that this little organism 

 was not a mature animal. It showed no evidence of 

 growth while hi the blood, or of any organs such as 

 would lead 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 in. to 4 in. 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 f 

 another, and if such a passage w^ere 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 absorbed the blood of the human body, and 

 with the blood the little wriggling parasite which it 



