28 Lectures on Bacteria. [$ iv. 



are considering. Full expression was given to this view by 

 Billroth (6) in 1874 in a lengthy publication, in which he includes 

 all the forms which he had examined, and they were many and 

 various, in one species which he names Coccobacteria septica. 

 N'ageli (7) and his school have supported the same views since 

 1877. Nageli indeed expresses his opinion on the one hand 

 with circumspection and reserve, saying that he finds no necessity 

 for separating the thousands of Bacterium-forms which have come 

 under his observation even into two species, but that it would be 

 rash to speak decidedly on a subject that is so imperfectly ex- 

 plored. On the other hand he goes so far as to say : If my view 

 is correct, the same species in the course of generations assumes 

 a variety of morphologically and physiologically dissimilar forms 

 one after another, which in the course of years and decades 

 of years at one time turn milk sour, at another give rise to 

 butyric acid in sauerkraut, or to ropiness in wine, or to putrefac- 

 tion in albumen, or decompose urine, or impart a red stain to 

 food-material containing starch, or produce typhus, relapsing 

 fever, cholera, or malarial fever. 



In presence of this statement of opinion our practical interests 

 require that we should obtain a decided answer to the question 

 of species which we are here considering, for it certainly is not 

 a matter of indifference in medical practice, for example, 

 whether a Bacterium which is everywhere present in sour milk 

 or in other objects of food, but without being injurious to health, 

 is capable or not of being changed at any moment into a form 

 which produces typhus or cholera. The scientific interest cer- 

 tainly demands that the question should be set at rest. 



It may safely be maintained that continued investigation has 

 at length arrived at a decision and it is this, that there is no 

 difference as regards the existence of species and their deter- 

 mination between this and any other portion of the domain of 

 Natural History. 



Species may be distinguished provided we follow the course 

 of development with sufficient attention. Some which are 



