628 
Insects and Disease 
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 single case 
in which I think I had overlooked the parasites on the first occasion), although 
large numbers of healthy birds were kept in this manner. The results in 
the case of the sparrows which were subjected to the bite of the infected gnats 
were different, indeed. Out of 28 of these, dealt with from time to time, 
no less than 22, or 79 per cent., became infected in from five to eight days. 
And, as in the first experiment, all the infected birds finally contained very 
numerous parasites. 
“It was most interesting to watch the gradual development of the parasitic 
invasion in these birds; and this development presented such constant 
characters that, apart from other reasons, it was quite impossible to doubt 
that the infection was really caused by mosquitoes. The course of events 
was always as follows: The blood would remain entirely free from parasites 
for four, five, six, or even seven days. Next day one or perhaps two parasites 
would be found in a whole specimen. The following day it was invariably 
observed that the number of organisms had largely increased; and this 
increase continued until in a few days immense numbers were present—so 
that, finally, I often observed as many as seven distinct parasites contained 
within a single corpuscle! Later on many of the birds died; and their 
organs were found to be loaded with the characteristic melanin of malarial 
fever. 
“I also succeeded in infecting on a second trial one of the six sparrows 
which had escaped the first experiment; and also a crow and four weaver- 
birds; and, lastly, gave a new and more copious infection to four sparrows 
which had previously contained only a few parasites. 
“These experiments completed the original and fundamental observa¬ 
tions on the life-history of the Haemamcebidae in mosquitoes. The parasites 
had been carried from the vertebrate host into the gnat, and had finally 
been carried back from the gnat to the vertebrate host. The theories of 
King, Laveran, Koch, and Bignami, and the great induction of Manson, 
were justified by the event: and I have given a detailed historical and critical 
account of these theories, and of my own difficulties, in the hope of bringing 
conviction to those who might perhaps otherwise think the story to be too 
wonderful for credence.” 
Since Ross’s work, a host of new observations and facts have been made 
known by various investigators. All of these studies only add to the cer¬ 
tainty that the malaria parasite depends absolutely upon mosquitoes for 
its full development and for its dissemination. Many of these observations 
and experiments have to do with actual tests of malaria prevention. Con- 
