THE MICROBES OF HUMAN DISEASES. 157 



or Schizophyta, there is a very remarkable dimorphism 

 of mode and habitat. In Beggiatoa of sulphurous 

 waters, for instance, and in Cladothrix, which forms a 

 whitish pellicle on the surface of putrefying liquids, 

 Zopf has found, under certain conditions, all the forms 

 designated as Micrococcus, Bacillus, Leptothrix, and 

 Bacterium; that is, microbes strictly so called, in- 

 cluding those which are the producing agents of 

 contagious diseases. 



Where these algae are found in water or on a damp 

 soil, conditions of existence favourable to their develop- 

 ment, there they live and multiply. But when the 

 soil dries up, when a river returns to its bed after 

 a flood, or a marsh disappears in consequence of the 

 evaporation of its waters, all these algae give forth 

 dormant spores, destined to ensure their propagation. 

 We have described the formation of these spores by 

 concentration of the protoplasm in the interior of 

 each cell ; in this form their volume is very small, and 

 they are extremely light, so that as soon as they are 

 desiccated, and then only, these spores are carried 

 away by the slightest breeze and borne to great dis- 

 tances. These are termed air-germs. 



When these moving germs encounter a favourable 

 medium, at once moist and warm, such as the human 

 mouth or lungs, they fasten there and are immediately 

 developed, first in the form of Micrococcws, then of 

 that of Bacterium, Bacillus, or Leptothrix, according 

 to the species to which the spore in question belongs. 



