Physiology. 13 



the bacteria are filtered out by passing through the inter- 

 vening soil layers. As a consequence spring water as it 

 issues from the soil is relatively poor in bacteria, but 

 quickly becomes contaminated after it reaches the sur- 

 face. Some of the highly infectious diseases, such as 

 cholera and typhoid fever are often transmitted by means 

 of a contaminated water-supply. 



21. Saprophytic bacteria. If bacteria are consid- 

 ered from the manner in which they live, they may be 

 divided into two very unequal divisions, known as sapro- 

 phytes and parasites. Those belonging to the first class 

 subsist on dead organic matter, while the parasitic forms 

 are able to thrive in living tissues of either vegetable or 

 animal nature. The great majority of different forms 

 belong to the first class, and it is undoubtedly true that 

 this condition is more fundamental than the parasitic 

 method of life. The saprophytes find their food- supply 

 in the vegetable and animal matter that has already ceased 

 to live. They are the organisms concerned in the tear- 

 ing down processes of nature and their beneficent func- 

 tion in the world about them is in their scavenger char- 

 acter as they make way with the offal and debris of 

 organic life. 



22. Parasitic bacteria. There is no sharp line sep- 

 arating the parasitic bacteria from those that live on 

 lifeless matter. In all probability these forms that are 

 now able to thrive in living tissue came originally from 

 ancestors of a saprophytic type. Some, as the tubercle 

 bacillus, have gradually adapted themselves to the more 

 restricted parasitic method of growth, just as many para- 

 sitic plants have been produced, and as a result of this 

 specialization, they have often lost the power to thrive 

 under conditions generally favorable to saprophytic forms. 



In parasitism a marked variation in degree is to be 



