THE GROWTH OF GROUPS 153 



under the name of myiasis. If for the sake of discussion 

 we regard the attacks of the fly as disease, we shall 

 then see in myiasis an example of a disease which may 

 originate sporadically, as the result of a harmless organism 

 becoming vicious, on different occasions, in separate 

 places. The sporadic occurrence of the disease is not 

 due to a particular vicious strain of organisms which is 

 carried about from place to place, as we commonly assume 

 disease to be carried, but to the sporadic appearance of 

 a vicious habit among a widespread group of organisms 

 which is usually harmless. The habit may be regarded 

 as a varietal character of Lucilia, which appears occa- 

 sionally in that genus but not in Musca or in Calliphora. 

 It is possible that the harmful character of certain other 

 organic agents of disease may arise as a sudden change 

 of habit, in a widespread and harmless organism. From 

 our knowledge of the mutation of organisms in general 

 we might expect similar changes to occur among the 

 protozoa and bacteria ; and if such expectations were 

 realized, various anomalies in the mode of occurrence and 

 distribution of certain diseases would be explained. On 

 the other hand, there is of course no doubt that many 

 diseases are due to well-established strains of permanently 

 harmful organisms and when diseases such as these 

 break out in a new spot, we may be sure that the parti- 

 cular organism has been carried to that spot from else- 

 where. But with the facts of mutation before us, it is 

 not safe to assume the eternal stability of pathogenic 

 organisms. We can lay down no rule for disease in 

 general, the many kinds must be considered separately. 



The foregoing argument depends on the assumption 

 that the habit of depositing eggs in the human nostrils or 



