150 THE GROWTH OF GROUPS 



believe it to be different among those bacteria and pro- 

 tozoa that are the factors of disease. 



The aspect of disease as the interaction of particular 

 organisms is, comparatively speaking, a new idea, scarcely 

 thirty years old, and even now it is usual to regard the 

 various kinds of disease as fixed for all time, just as 

 formerly we regarded the various kinds of animals from 

 the same point of view. If disease is the interaction of 

 particular organisms, there must be an evolution of disease 

 just as there is an evolution of organisms. Although the 

 physician may be willing to admit this in a general way, 

 yet he does not regard it as of any practical importance. 

 Thus he may say to himself " appreciable evolutionary 

 change can only be accomplished in the course of cen- 

 turies, enough for me to know of things that can be seen 

 in my lifetime." This seems to be a sound enough con- 

 clusion for practical men, but the premise is not sound, 

 for there is no doubt that appreciable change may occur 

 suddenly in the succession of organisms, and the fact 

 should be remembered by the investigators of disease. 

 The sudden appearance in an organism of new habit, 

 which becomes henceforth permanent in its descendants, 

 is not uncommon. Such a habit may or may not affect 

 the lives of other animals or plants. In the well-known 

 case of the parrot Nestor, we have an example of a habit 

 which is harmful to another animal and indirectly so to 

 man. Nestor is a small parrot, common in the South 

 Island of New Zealand, which became notorious by 

 acquiring the habit of feeding upon the flesh of living 

 sheep. Clinging to the wool of the sheep's back, it 

 bites through the skin and devours enough of the tissues 

 to cause the death of the animal. It is said that it aims, 



