THE BIOLOGY OF BACTERIA 29 



— is due to their poisonous products, or toxins, as they 

 have been termed. These may be of the nature of fer- 

 ments, and they become diffused throughout the body, 

 whether the bacteria themselves occur locally or generally. 

 They may bring about very slight and even imperceptible 

 changes during the course of the disease, or they may kill 

 the patient in a few hours. Latterly bacteriologists have 

 come to understand that it is not so much the presence 

 of organisms which is injurious to man and other animals, 

 as it is their products which cause the mischief ; and the 

 amount of toxic product bears no known proportion to the 

 degree of invasion by the bacteria. The various and widely 

 differing modes of action in bacteria are therefore dependent 

 upon these three elements: the tissues or medium, the bac- 

 teria, and the products of the bacteria ; and in ^11 organismal 

 processes these three elements act and react upon each other. 



A word may be said here respecting the much-discussed 

 question of species in bacteria. A species may be defined as 

 " a group of individuals which, however many characters they 

 share with other individuals, agree in presenting one or more 

 characters of a peculiar and hereditary kind with some certain 

 degree of distinctness." ^ Now, as regards bacteria, there is 

 no doubt that separate species occur and tend to remain as 

 separate species. It is true, there are many variations, due in 

 large measure to the medium in which the organisms are 

 growing, — variations of age, adaptation, nutrition, etc., — yet 

 the different species tend to remain distinct. Involution 

 forms occur frequently, and degeneration invariably modifies 

 the normal appearance. But because of the occurrence of 

 these morphological and even pathological differences it must 

 not be argued that the demarcation of species is wholly 

 arbitrary. 



Means of Sterilisation. As this term occurs frequently in 

 even a book of this untechnical nature, and as it is expressive 



^ G. J. Romanes, Darwin and After Darwin ^ vol. ii., 231. 



