THE STORY OF THE BACTERIA. 47 



and queried in a quizzing way how long it 

 would be before somebody would be trying to 

 modify the colors of some of the bacteria by 

 the well-known horticultural methods. His 

 idea was a clever one, but he was behind the 

 times, for already a German bacteriologist 

 had, starting with a deep purple-forming spe- 

 cies of bacteria, and selecting and replanting 

 the lighter-colored colonies, at last obtained 

 cultures which were nearly white, but were in 

 other respects essentially the same. Thus the 

 great and far-reaching principles of natural 

 selection, in accordance with which life, slowly 

 emerging from its primeval simplicity, at last 

 came to be manifested in that grand scale of 

 living beings at the top of which man stands 

 supreme, are still to be traced way down among 

 the invisible organisms which typify the earliest 

 and simplest expression of life. 



But certain of these color-forming bacteria 

 are sometimes very disagreeable intruders 

 upon domestic life. Occasionally, without 

 warning, the milk of a particular dairy sud- 

 denly develops a very uncanny deep-blue 

 color, which, like an epidemic, spreads to all 



