THE GROWTH OF GROUPS 151 



particularly and successfully, at the kidneys of the sheep ; 

 but many people would demand substantial evidence 

 before believing that Nestor has an inborn knowledge 

 of mammalian anatomy, especially those of us who 

 know the difficulty of gauging the exact position of 

 internal organs from the outside. But indeed the sceptic 

 might be quite wrong. Thus, he might think, "It is 

 impossible for one of the lower animals to know what 

 man does not know ; " but there are many facts to prove 

 the unwisdom of such a thought. It is idle to discuss 

 whether the lower animals have knowledge or not, at 

 any rate they often behave as though they had very exact 

 knowledge. But this is a digression. This peculiar 

 propensity of Nestor became so common that rewards 

 were offered for the destruction of the bird. The habit 

 became a custom among the parrots. From my point 

 of view, this fact need not be explained, but it should be 

 remembered. A certain kind of organism suddenly 

 became harmful to another kind. This is the fact, and 

 the inference is that similar events probably occurred 

 in the past and will occur in future. 



It may be said that this is all beside the mark, parrots 

 are not a disease of sheep any more than tigers are a 

 disease of man, but the discussion is general and concerns 

 all those organisms that are harmful to others. Para- 

 sitic organisms harmful to others are numberless. We 

 do not know how the harmful habits were acquired, 

 their origin is lost history ; but it is important to re- 

 member that in this one case where the method of origin 

 of a harmful habit was witnessed, that habit was acquired 

 suddenly. Although it is not safe to infer that all para- 

 sitism is of sudden occurrence, yet we know that it may 



