MICROBES AND PROTISTA. 7 



to rise, and from this point of view they are pro- 

 fitable in industry and commerce. 



But in addition to these useful microbes, there are 

 others which are injurious to us, while they fulfil 

 the physiological destiny marked out for them by 

 nature. Such are the microbes which produce dis- 

 eases in wine ; most of the changes in alimentary and 

 industrial substances ; and, finally, a large number of 

 the diseases to which men and domestic animals are 

 subject. The germs of these diseases, which are only 

 the spores or seeds of these microbes, float in the air 

 we breathe and in the water we drink, and thus 

 penetrate into the interior of our bodies. 



Hence we see the importance of becoming acquainted 

 with these microbes. Their study concerns the agri- 

 culturist, the manufacturer, the physician, the pro- 

 fessor of hygiene, and, indeed, we may say that it 

 concerns all, whatever our profession or social position 

 may be, since there is not a single day, nor a single 

 instant, of our lives in which we cannot be said 

 to come in contact with microbes. They are, in 

 fact, the invisible agents of life and death, and this 

 will appear more plainly from the special study we 

 are about to make of the more important among 

 them. 



Since it is easier to know and observe beings which 

 are visible to the naked eye, we shall first speak of 

 fungi that is, of the larger microbes, with whose 

 habits and organization we are also best acquainted. 



