xii INTRODUCTION 



astonished at the great changes which have taken piace in it 

 and in the science of microbiology in general. It would take 

 him some time to overtake and realise the progress attained. 

 And yet in spite of all that has been done there remains still 

 much work for the future. Many scourges still await a remedy. 

 In the case of tuberculosis, although extraordinary advances 

 have been made in its study, the final solution of the problem 

 is still reserved for the future. The great question of cancer, 

 so important and so difficult, has been hardly more than opened. 

 There remain to be discovered the microbes of many diseases, 

 e.g., hydrophobia, scarlatina, and measles, which are perhaps 

 filtrable micro-organisms, invisible with the best microscopes. 



The field of infectious diseases is extending wider and wider 

 with the progress of microbiology. We find that certain dis- 

 eases thought to be diseases of metabolism are beginning to be 

 classed in this group. Arterio-sclerosis, an affection so wide- 

 spread and so apt to cut short our existence, results from the 

 activity of our intestinal flora. Perhaps before long it will be 

 possible to explain diabetes, gout, and rheumatism by the 

 injurious activity of some variety of microbe. 



Even in those problems of hygiene which affect society in 

 general microbiology is taking the predominant role. The 

 grand problem of a rational food supply, which used to be 

 thought capable of solution by inventions of chemistry and 

 physics, will necessarily have to be studied by microbiological 

 methods, in view of the fact that the intestinal bacteria play 

 one of the most important parts in everything that concerns 

 nutrition. It is not sufficient to state the nutritive value of a 

 food in terms of the calories which it contains ; it will still be 

 necessary to define precisely its relations to the intestinal flora 

 from the point of view of the production of microbic poisons. 



M. Gustave le Bon asked me to put together in a little 

 volume for his Library a summary of what is known about 

 microbes and toxins. I advised him to apply rather to one of 

 my young colleagues, and I indicated in particular Dr. Burnet. 

 I am happy to find that I could not have chosen better. In 

 spite of the great difficulties there are in attempting to describe 



