104 ON VARIABILITY AND ADAPTATION 



has become more or less endemic, namely, diphtheria, and 

 observe the series of progressive cases that is to be made out 

 from the simple sore throat associated with the mildest con- 

 stitutional effects, through slight cases of croup to those terribly 

 malignant attacks in which the manifestation of the disease 

 and death are almost synchronous. 



If the micro-organisms of disease have constant attributes, 

 if the species of bacteria are fixed, their characters unvarying, 

 then all these differences in the nature and intensity of the 

 symptoms produced by any one micro-organism at diverse 

 periods and in diverse individuals can only be due to subtle 

 meteorological changes, to differences in the power of resistance 

 to microbic invasions displayed by the individual, and to the 

 dose, if I may so term it, of infective material incepted by the 

 individual. Now, without doubt, all these factors are of very 

 great importance, but at the same time it cannot be denied that 

 they do but vaguely advance our knowledge. Up to the present 

 the study of atmospheric influences as affecting zymotic diseases, 

 while impressing upon us one or two main truths, has, in other 

 respects, involved us in a maze of contradictions, and while the 

 last few years have greatly extended our knowledge of the 

 nature of the conflict between the organism and the microbe, 

 clinical experience is constantly bringing before us the para- 

 doxical phenomenon of disease, in its severest form, attacking 

 individuals in whom one would hold the constitutional powers 

 of resistance to be at their highest. So, too, difficulties in the 

 way of exact determination are insuperable, clinically, with 

 reference to the " dosage " of infective material. 



But if, while granting freely that all the above-mentioned 

 factors play each a part, it can be shown that modifications of 

 the powers of micro-organisms can be induced, both within the 

 organism and outside it — alterations in appearance, in the 

 ferments and other products of their vital activity and (among 

 pathogenic microbes) in virulence ; alterations, not temporary, 

 but continuing through generations ; alterations that are capable 

 of being demonstrated in the test-tube and under the micro- 

 scope — then we reach one stage further in actual advance in 

 our knowledge of infectious disease, and from this advance gain 

 a firmer standpoint from which to appreciate the other factors. 



Largely influenced by the admirable researches of the German 



