THE GROWTH OF GROUPS 155 



for the special tendency of the green-bottles to become 

 harmful. Indeed there can be little doubt as to the 

 reality of this tendency. If it be granted, then we may 

 draw the conclusion that the sporadic occurrence of 

 myiasis is not due to the dissemination of a harmful 

 organism but to an occasional and local change in the 

 character of a harmless one. A way of escape from 

 this conclusion may be tried thus. One may say, " I 

 will grant that the green-bottles as a class have the 

 harmful tendency, but the tendency is in all of them at 

 all times. The infrequency of its appearance is explained 

 by the fact that the flies seldom enjoy the opportunity of 

 exercising it." But this argument appears unsound, for 

 if every green-bottle possessed the tendency, myiasis 

 would be far more common than it is. It would be 

 liable to occur in all parts of the world, wherever men lie 

 about in the daytime in a state of deep sleep or intoxica- 

 tion. Moreover there would be far more recorded cases 

 than one of an outbreak among sheep, 



In the foregoing only muscid flies are referred to. 

 The Sarcophagidcz and especially the (Estridce are better 

 known for their habit of depositing their eggs in living 

 tissues. The habit is established in those families. 



It may seem unjustifiable to speak of harmful instincts, 

 such as those of Nestor and Lucilia, as character-units. 

 It is indeed assumptive since the Mutation Theory has 

 not as yet met with general approval. This theory may 

 be regarded as a statement that every hereditable attribute 

 of every living thing, be it total size, or size of a part in 

 proportion to the whole, be it a peculiarity of form or of 

 colour, a change in the rate of reproduction, or a peculiar 

 mode of action such as we call instinct, that every such 



