Let us consider for a moment the discovery of the cause of malaria. This discovery, due to the Englishman, Ross, in connection with birds, and to the Italian, Grassi, in connection with man, consists in having found out that the plasmodium of malaria, which produces the malady, is inoculated in man and in the various animals subject to it, by a special kind of mosquito. Let us inquire what was the state of science prior to this discovery. In 1880 Laveran had described an animal micro-organism, which preyed upon the red corpuscles of the blood, producing an attack of fever with the cycle of its existence. Subsequent studies confirmed and elucidated this fact, and the _plasmodium malariae_ became a matter of common knowledge. It was known that animal micro-organisms, unlike vegetable micro-organisms, after a cycle of life in which reproduction takes place by scission--that is, by subdivision of a single body into several other bodies equal to the first, give place to _sexual forms_, masculine and feminine, which are separate, and incapable of scission, but are designed for _fusion into one another_, after which the organism recommences its cycle of scissions until it again reaches the sexual forms. Laveran had found that in the blood of sufferers who recover spontaneously from malarial fever there are a great number of corpuscles which have no longer the rounded forms of the plasmodia, but are crescent-shaped and rayed. He took these to be transformations of the plasmodia, "modified in form" and "incapable of producing disease," and pronounced them to be "degenerate" organisms, almost as if they had been deformed and exhausted by the "excess of work" they had previously performed. These organisms were described as "Laveran's degenerative forms." After the discovery of the transmission of malaria in 1900, Laveran's "degenerative forms" were recognized as the sexual individuals of the reproductive cycle: individuals which were incapable of conjugation in the blood of man, and could only produce new organisms in the body of the mosquito. We may well wonder: Why did not Laveran simply recognize those sexual forms, and why did he not seek for the period of conjugation in the plasmodia, which were animal micro-organisms? If he had borne in mind the complete cycle of the protozoa, he would have recognized them. But evidently Morel's theories of the degeneration of man had made a much livelier impression on his imagination; and his leap from these remote theories to his interpretation of the plasmodia seemed an achievement of "genius." It may be said that this "feat of genius," this visionary generalization, prevented Laveran from seeing the truth. A form of _arrogance_ and _levity_ is apparent in such errors.