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THE PRESENT EVOLUTION OF MAN—PHYSICAL
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attack of every such disease for a greater or lesser length of time confers immunity against subsequent attacks, are known to all; but the actual cause of the recovery and the subsequent immunity is still in dispute. Evidently the body of a man who has had cholera, for instance, and is immune, differs in some important respect from his body before he had the disease, and was not immune; but what is the difference? Why is it that the cholera microbes are able to flourish in him during one period and not during the other? The most elaborate microscopic, the most delicate chemical examinations furnish no information. Judged by them the tissues have undergone no change.
"There has for long been the utmost anxiety on the part of physicians and others to obtain some explanation of these remarkable facts. Klebs and Pasteur explained them on the assumption, that during the first attack of a disease some material that was essential for the nutrition of the pathogenic organisms, that had by this time been found to be associated with some of these diseases, had been used up, and the supply being cut off, the organisms were no longer able to exist in the body, and exhibit the characteristic evidences of their presence. This substance must have been present in exceeding small amount, as no alterations in the composition of the blood, or other fluids of the body, could be determined by any methods of chemical analysis that could be applied. Then came the theory advanced by Chauveau and others: that just as micro-organisms, when growing in artificial media, produced excretory products, the presence of which was inconsistent with the continued life of the organism; so in the body, bacteria, during the course of the disease, gave rise to some material which might act deleteriously on their own protoplasm, and which, remaining in the body for a considerable length of time, interfered with the growth of any similar organisms that might in future be intro-
