CONDITIONS NECESSARY TO INVESTIGATION. 23 



within the culture, owing to there having been a commingling of 

 species; for if the latter has been the case, one of the members in 

 this community usually flourishes at the expense of its neighbour, and 

 an alteration in the general type of growth is at once apparent in the 

 artificial cultivation. 



By a course of constant re-inoculation through a series of carefully 

 prepared media, any tendency to such an alteration becomes much 

 more prominent ; and when a line of demarcation at length becomes 

 clearly visible between two mingled species, the same system allows 

 of their ready separation, and the cultivation of each apart as a single 

 species, such cultures being then termed "pure cultivations." 



In certain favourable media, it is possible, by merely noting the 

 salient features, such as the form and general appearance of the 

 growth, to state with certainty not only the fact that it is a pure culti- 

 vation, or otherwise, of a micro-organism, but also to place it at once 

 in one or other of the classes into which the lower fungi are divided, 

 and to describe broadly the physiological and pathogenic functions 

 with which such an organism is likely to be endowed. 



As Koch points out, these macroscopic appearances and modes of 

 growth must, in the present state of our knowledge of the morpho- 

 logical and physiological distinctions, be looked upon as of even 

 greater value than microscopic and chemical characteristics. Hence 

 the enormous importance attached to the study of these macroscopic 

 appearances, especially in the case of micrococci, whose microscopi- 

 cal differentiation is in many instances absolutely impossible. 



(c.) In the third place, it must be possible, by the inoculation of the 

 organism thus isolated into the system of an animal liable to the 

 disease, to reproduce it, and the disease so reproduced must have 

 all the symptoms of the original complaint, or at least a sufficient 

 number of them to give it the specific character. 



(d.) And, fourthly, in the tissues of the animal thus attacked, there 

 must be a recognition of the same foreign elements in the same 

 relation to the tissues of their host as was observed in the original 

 case. 



