REPORT ON THE GROUSE DISEASE. 333 



susceptibility and spread of the malady, but cannot be regarded 

 as the causa caiisans. 



The first theory supposes the existence in the diseased animals 

 of a hypothetical virus, which, by its multiplication within the 

 organism, sets up the symptoms and phenomena of the disease. 

 This virus, when finding access to a healthy bird, would here again 

 multiply and produce the same disorder. In many infectious 

 diseases the virus has been definitely shown to be some low form 

 of life, generally belonging to the group of species of Bacteria. 

 Some of these species have this great character, that they can live, 

 thrive, and multiply within the body (in the blood and tissues) of 

 certain species of animals, and by their chemical action, or other- 

 wise, therein produce a definite group of symptoms characteristic 

 of the particular disease. When they or their ofi"spring find access 

 to a new susceptible body, they again multiply herein, and set up 

 the same diseased state. Thus some of the infectious or com- 

 municable diseases have been pi'oved to be caused by a definite 

 species of microbes, differing in the different diseases. They are 

 known as pathogenic microbes, and they are distinguished from 

 other non-pathogenic species of microbes — though both are similar 

 in morphological respects — in this important particular, that the 

 latter have no disease-producing or pathogenic power — e.g., some 

 species of microbes associated with fermentative and putrefactive 

 processes. 



The methods generally employed for the study of pathogenic 

 mici'obes are these: — (i) The blood or tissues, or both, of an 

 animal affected with an infectious disease should, as a first condi- 

 tion, contain some definite form of microbes discoverable by the 

 microscope ; (2) these microbes, taken from the blood or tissues 

 of a diseased bod}', when transplanted on to various artificial 

 nutritive media, multiply thereon, and thus produce new crops ; 

 (3) the microbes of these artificial crops, or those taken directly 

 from the diseased tissues, when transplanted (b}'^ inoculation or 

 otherwise) into a healthy susceptible animal, set up the same 

 disease as that from which they are derived ; and (4) in the animal 

 thus infected the same species of microbes must be found to exist 

 in the diseased tissues. 



Accordingly (1) I made a careful microscopic study of the 

 blood and diseased tissues (liver and kidney) of Grouse dead of 

 the disease. Three birds were examined : — (a) the one sent from 



