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 w-ere bitten 

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

 in which 1 think 1 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 dilTerent, 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 Ha;mama;bida; in moscjuitoes. 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 stor)^ 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- 



