Insects and Disease 
627 
wonderful as that interpretation is. The blast must evidently pass down 
the ducts of the salivary gland into the wound made by the proboscis of 
the insect, and thus cause infection in a fresh vertebrate host. 
“ That this actually happens could, fortunately, be proved without difficulty. 
As I had now been studying the parasites of birds for some months, I possess 
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 in wild sparrows 
examined by me in Calcutta, I had found H. relicta —the parasite which I 
had just cultivated in Culex fatigans —in 15, or 13.5 per cent. As a rule, 
non-infected birds were released; but I generally kept a few 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 possessed five of these healthy control birds— 
four sparrows and one weaver-bird. All of them were now carefully examined 
again and found 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 repeatedly fed on infected birds, 
and many of which on dissection had been shown to have a very large number 
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 any bird (with H. relicta) in India. While wild sparrows 
in Calcutta seldom contain more than one parasite in every field of the micro¬ 
scope, those which I had just succeeded in infecting contained ten, fifteen, 
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 now I 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 excluded 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 insure their being absolutely free from 
