1900.] on Malaria and Mosquitoes. 307 



That this actually happens could, fortunately, be proved without 

 any difficulty. As I had now been studying the parasites of birds 

 for some months, I possessed a number of birds of different species, 

 the blood of which I had examined from time to time (by pricking 

 the toes with a fine needle). Some of them were infected, and some, 

 of course, were not. Out of 111 wild sparrows examined by me in 

 Calcutta, I found H. relicta — the parasite which I had just culti- 

 vated in Culex fatigans — in 15, or 13*5 per cent. As a rule, non- 

 infected birds were released ; but I generally kept a few to use for 

 the control experiments mentioned above, and the blood of these 

 birds had consequently been examined on several occasions, and had 

 always been found free from parasites. At the end of June I pos- 

 sessed five of these healthy control birds — four sparrows and one 

 weaver-bird. All of them were now carefully examined again and 

 found to be healthy. They were placed in their cages within 

 mosquito-nets, and at the same time a large stock of old infected 

 mosquitoes were released within the same nets. By " old infected 

 mosquitoes " I mean mosquitoes which had been previously fed re- 

 peatedly on infected birds, and many of which on dissection had been 

 shown to have very large numbers of blasts in their salivary glands. 

 Next morning, numbers of these infected gnats were found gorged 

 with blood, proving that they had indeed bitten the healthy birds 

 during the night. The operation was repeated on several succeeding 

 nights, until each bird had probably been bitten by at least a dozen 

 of the mosquitoes. On July 9, the blood of the birds was examined 

 again. I scarcely expected, any result so complete and decisive. 

 Every one of the five birds was now found to contain parasites — and 

 not merely to contain them, but to possess such immense numbers of 

 them as I had never before seen in auy bird (with H. relicta) in 

 India. While wild sparrows in Calcutta seldom contain more than 

 one parasite in every field of the microscope, those which I had just 

 succeeded in infecting contained, ten, fifteen, twenty and even more 

 in each field — a fact due probably to the infecting gnats having been 

 previously fed over and over again on infected birds, a thing which 

 can rarely happen in nature. 



The experiment was repeated many times — generally on two or 

 three healthy birds put together. But I now .improved on the 

 original experiment by also employing controls in the following 

 manner. A stock of wild sparrows would be examined, and the 

 infected birds eliminated. The remainder would then be kept apart, 

 and at night would be carefully secluded from the bites of gnats by 

 being placed within mosquito nets. These constituted my stock of 

 healthy birds. From time to time two or three of these would be 

 separated, examined again to ensure their being absolutely free from 

 parasites, and then subjected to the bites of " old infected mosquitoes," 

 and, of course, kept apart afterwards for daily study. Thus my stock 

 of healthy birds was also my stock of control birds. Until they were 

 bitten by gnats, I found that they never became infected (except in a 



