own, but in some way or another they make their way into the salivary glands of the insect and accumulate in the cells which secrete the saliva. Thence the blasts pass into the salivary duct and down the grooved proboscis of the insect ([Fig. 3], No. 4). The next time the mosquito has a meal off a man, some of these blasts will be washed into the man’s blood by the saliva which causes the irritation set up by a mosquito’s bite. It is known that when an infected insect bites a healthy man malaria ensues; and though the blasts have not hitherto been seen to enter the blood-corpuscles, they certainly give rise to the disease, and it can hardly be doubted that they force their way into the red corpuscles and form the young amœbulæ which we described at the beginning of this article.
The appended scheme will perhaps make clear the very diverse phases of the somewhat polymorphic organisms. Those stages which occur in the blood of man are printed in ordinary type, but those which occur in the mosquito are in italics:
The foregoing account of this varied and romantic life-history is no hypothetical one. With the exception that, so far as we know, no one has yet seen the blasts enter the corpuscles and become amœbulæ, every stage in the story has been verified over and over again by competent observers, and their observations are now accepted by all whose opinion in such matters has weight. Further, the facts here recorded are not peculiar to parasites in man. Allied forms of Protozoa attack other vertebrates, and, in fact, the first hæmatozoön whose life-history was thoroughly worked out by Ross was the Hæmamœba (Proteosoma) relicta, which causes a malaria-like disease in birds, and is conveyed from one bird to another by means of the common gnat, Culex pipiens. Again, the parasite which causes so much loss to stock-owners, the Texas fever organism, Pyrosoma bigeminum, is, thanks to the researches of Smith and Kilborne, now known to be conveyed from one ox to another by the cattle-tick, Boöphilus bovis. Thus, however strange the life-history of the malarial parasite may seem to the unscientific, it is very much what might have been expected by zoologists who have worked on allied organisms, and it is vouched for in its main features by the most expert workers in England, France, America, Italy, and Germany. The whole literature of the subject of transmission of disease by insects has been ably sifted and brought together by Dr. Nuttall in a monograph whose title is mentioned in the Bibliography.
For two years and a half Major Ross dissected mosquitoes, looking for traces of the malaria organism and finding none, but at last found what he sought in a species of mosquito that had hitherto escaped his attention. This means that, like most other parasites, the Hæmamœbidæ will develop in one kind of animal and in one kind only. If taken up by another kind they are simply digested. The mosquito with the