THE GROWTH OF GROUPS 159 



opinion that monstrosities and all obvious variations are 

 somewhat akin to disease : just as a naturalist who had 

 studied pathology might express the opinion that the 

 manner of disease was somewhat akin to the manner of 

 variation. There is indeed a certain resemblance. If, for 

 the sake of discussion, we regard each case that might 

 come before the general physician as a separate event, and 

 attempt to classify those various events ; we notice at once 

 that some fall naturally into classes, while others do not. 

 Diseases fall into natural classes, but not injuries. We 

 may of course speak of cuts, bruises, or burns, but these 

 classes are only set up for convenience. No two injuries 

 are alike, in each case the destructive agent is different, in 

 kind or degree, in duration or point of application. But, 

 on the other hand, if we see two cases of disease, small-pox, 

 for example, in widely separate parts of the world, they 

 are obviously similar, the separate events called cases of 

 small-pox form a class or species. Moreover a species of 

 disease may be divided naturally into sub-species. The 

 method of this classification is much the same as that 

 which the naturalist applies to organisms. We may call 

 it for the moment natural classification. In many cases 

 the similarity of the method of classification is the out- 

 come of the fact that the disease is the effect of one 

 organism upon another, both organisms being liable to 

 vary in the manner common to living things, that is to 

 say the specific group of the host and the specific group 

 of the parasite are composite, each contains numerous 

 sub-specific groups. 



But if now we set aside the parasitic diseases and 

 regard some of those that are not due to parasitism, still 

 we find that even they may be submitted to natural 



