THE PRESENT EVOLUTION OF MAN — PHYSICAL 207 



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 dis- 

 pute. 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 differ- 

 ence ? 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 explan- 

 ation 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 com- 

 position 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- 



