22 ADAPTATION AND DISEASE 



to a period subsequent to the dispersal of peoples over the 

 habitable globe. 



The New Diseases op the Gbeat War 



But to return to the main point, that of the evolution of 

 new infections. No one can fail to be impressed with the fact 

 that while among the millions of our troops engaged, three of 

 the terrible plagues of former campaigns have been virtually- 

 suppressed — small-pox, enteric, or typhoid, and typhus — other 

 and less fatal epidemics have manifested themselves, and some 

 of these are hitherto undescribed and unrecognized conditions. 

 Either they are new diseases, or old diseases assuming a new 

 form under changed conditions of environment. There has 

 been some doubt as to whether all the cases called Trench fever 

 are of the same nature, whether under the name we deal with 

 one or more than one disorder, but this has during the last few 

 months been dissipated, as the symptomatology of the disease 

 has become clearly defined ; to the best of my knowledge here 

 is a disease, or group of diseases, that is new to us. The same 

 is true of " Trench shin." 



The New Diseases of Animals 



Nor is our evidence confined to man and human infections. 

 It is, for example, difficult to believe that a disease leading to 

 so great a mortality, and with symptoms and lesions so character- 

 istic as are those of Hog Cholera, could have occurred in previous 

 generations without being observed and described. But so far 

 as I can discover this disease was unknown until the 'sixties, 

 when it made its appearance in the United States, and thence 

 it has spread over the civilized — and hog-eating — world. 



How, then, are we to picture to ourselves the evolution of an 

 infective disease ? Are there any data which permit us to 

 deduce reasonably sure conclusions as to the nature of the 

 process ? I believe that there are, and that the observations 

 of the last few years, and more particularly certain observations 

 made here in London, have given the key. 



