86 Life and Death, Heredity and Evolution 



have to do with the production of disease. In bacteria 

 the knowledge of variation and heredity has passed through 

 the same series of stages that we noted in other organisms. 

 At first there seemed to be a mere chaos of diverse forms, 

 with no constancy or order; any kind of bacterium seemed 

 producible from any other, or from other organic or inor- 

 ganic sources. Then came a period of thorough study, with 

 development of precise technical methods. It was discov- 

 ered that there are a very great number of kinds of bacteria, 

 but that each remains true to its characters ; each is pro- 

 duced only by pre-existing individuals of the same race. 

 The differences between the races are often minute, mere 

 matters of a slight diversity in the chemical processes, in 

 the kind of sugar that is fermented by the particular race, 

 or the like. But each race remained true to its type, even 

 in these minute physiological details. This stage of knowl- 

 edge is the same as that on which is based the theory of 

 the constancy of genotypes in all sorts of organisms, a 

 theory that we sketched in Lecture 2. The constancy of 

 the races of bacteria has been set forth as one of the facts 

 opposed to the theory that organisms are undergoing evolu- 

 tionary changes. 



But in recent years a still more intensive study has 

 brought to light, in this group as in others, the actual 

 occurrence of hereditary changes, the production, from a 

 given race, of other races whose hereditary characters are 

 diverse from those of the parent race. In the bacteria, more 

 than in any other group of organisms, something has been 

 learned of the conditions which bring about these changes, 

 though knowledge on this point is still scanty. 



The bacteria present extreme difficulties for the critical 

 study of heredity and variation, owing to their minuteness. 

 To be certain of what the results mean it is necessary to 



