526 Malaria 



tions, and remembering that the same was true of the filarial 

 worms and their embryos, Manson came to the conclusion 

 that they must be taken out of the blood by some suctorial 

 insect. The one naturally first considered was the mosquito, 

 which was known to abound wherever malaria prevailed. 

 Examining mosquitoes that had been permitted to distend 

 themselves with the blood containing the parasites, Manson 

 found that in the stomach of the insect the peculiar phenom- 

 enon known as " flagellation," long before observed by Lav- 

 eran, took place in the parasites, giving rise to long, slender, 

 lashing, and, finally, free-swimming filaments. These, he 

 conjectured, might be the form in which the parasites left 

 the mosquito to infect the swamp water, with which human 

 infection eventually was brought about. Here Manson 

 failed, but while he was investigating he explained the 

 whole matter to Major Ronald Ross, who was soon to go to 

 India, and whom he advised to make the matter a subject for 

 study when he arrived at his destination. Ross* accepted 

 the opportunity that soon presented itself, and, after a most 

 painstaking investigation, the details of which are given in a 

 paper which can be found in the International Medical An- 

 nual, f 1890, made the second great discovery in the parasit- 

 ology of malarial ever. He found that, as Manson thought, 

 the mosquito is the definitive host of the parasite, but that the 

 matter is much less simple than was imagined, for the organ- 

 isms taken up by the mosquito undergo a complicated life 

 cycle requiring about a fortnight for completion, after which, 

 not the water into which the mosquito might fall and into 

 which its contained organisms might escape, but the mosquito 

 itself becomes the agent of infection. In other words, the 

 parasites taken up by the mosquito, after the completion of 

 the necessary developmental cycle, are returned by the mos- 

 quito to new human beings, who thus become infected. Thus 

 it was shown that malaria is not a miasmatic disease at all, 

 but that it is an infectious disease whose parasites divide their 

 life cycle between man and the mosquito, each becoming 

 infected by the other. The only role of the swamp is to 

 furnish the mosquitoes, and since these are only more numer- 

 ous, where swamps are numerous, but may occur without 

 swamps, the not infrequent occurrence of malarial fevers 

 apart from swamps is also explained. Ross further discov- 



* "Indian Medical Gazette," xxxui, 14, 133, 401, 448. 

 t E. B. Treat & Co., New York. 



