152 THE GROWTH OF GROUPS 



be so. In many other cases it is difficult to imagine how 

 it can have been acquired gradually. 



The green-bottle fly is another example of an organism 

 which sometimes acquires a parasitic habit. This fly is 

 well known in many parts of the world ; it is shaped much 

 as the common house-fly, and is conspicuous owing to its 

 bright metallic colour. There are several species of 

 green-bottle flies, but, broadly speaking, they form a 

 well-marked generic group named Lucilia (using the name 

 in the broad sense to include Compsomyia, Chrysomyia, 

 and Pychnosoma, as the term Anopheles is used to include 

 several genera of mosquitoes). Lucilia is widely distri- 

 buted over the world, together with other muscid flies 

 such as the smaller house-fly, Musca, and the larger blue- 

 bottle or blow-fly, Cattiphora. The three kinds are much 

 alike in their breeding habits, all lay their eggs in putrid 

 matter. Now, there are many recorded cases of flies 

 depositing their eggs on the bodies of living warm- 

 blooded animals, especially in the nasal passages and 

 less frequently in the ears of man. The larvae hatch 

 out and do great damage to the tissues. Eggs are also 

 deposited upon ulcerated surfaces, but such cases are 

 seldom reported. 



The point for emphasis is this. It is not a random 

 habit of flies in general to deposit their eggs within the 

 nostrils of a sleeping man, but a special habit of the 

 green-bottle flies, a habit which breaks out from time 

 to time independently in various parts of the world, for 

 example in America and in India. The result of Nestor's 

 vicious habit is hardly worthy of a place in the category of 

 diseases, but the damage done by this fly seems to be 

 more so, since it is sometimes spoken of as a disease 



