THE COMING OF EVOLUTION 197 



One of the most striking developments of nineteenth- 

 century science, and one which, by increasing our 

 Microbes and direct control of the environment, has 

 Disease, had a marked influence on our ideas of 

 the relative positions of man and of " nature," was 

 the growth in knowledge of the origin and causes of 

 microbic disease in plants, in animals and in mankind. 

 About 1838 Schwann discovered that the yeast 

 present in fermentation consists of minute living 

 vegetable cells, and that the chemical changes which 

 go on in the fermenting liquor are due in some way 

 to the action of the life of these cells. Schwann also 

 perceived that putrefaction was a similar process, and 

 showed that it too did not occur if precautions were 

 taken to destroy by heat all existing living cells in 

 contact with the substance examined, and to preserve 

 it thereafter from contact with all air save what had 

 passed through red-hot tubes. Thus both fermenta- 

 tion and putrefaction were proved to be due to the 

 action of living micro-organisms. 



These results were extended about 1855 by Louis 

 Pasteur (1822-1895), who disproved the old idea of 

 spontaneous generation in any known case, and showed 

 that the presence of bacteria could always be traced 

 to the entrance of germs from outside, or the growth 

 of those already present. Pasteur thus showed that 

 certain diseases, such as chicken-cholera and the 

 silk- worm disease, were caused by specific microbes; 

 and gradually the germs characteristic of other 

 diseases, many of them prevalent among mankind, 

 have been discovered and their life-history traced. 

 Though chronologically much of the results of 

 Pasteur's discoveries have only been reached in 



