422 THE AMEBIC AN NATURALIST [Vol. XLIX 



the loss of handing and in certain environments there is 

 a return to the normal banding to the extent that the -fly 

 can not be distinguished somatically from a normal 

 banded fly. 6 My contention is that since we know noth- 

 ing of the nature of the change in the germ-plasm that 

 leads to the appearance of a new or the loss of an old 

 character, any assumption that is based on the nature of 

 that change involves the Mendelian interpretation in un- 

 necessary implications. We need only assume that some 

 change has occurred, as the result indicates ; my formulas 

 give the same results as do those of presence and ab- 

 sence and serve the purpose of briefly indicating a change, 

 the machinery involved, and the necessary consequences. 



OTHEE TYPES OF ABNOEMAL ABDOMEN 



Irregularities in the arrangement of the rings of the 

 abdomen are not uncommon in Drosophila. Sometimes 

 they appear to have been caused by injury to the 

 larvae or pupae, but still other abnormalities are inherited 

 in the sense that they occur in certain stocks in more or 

 less definite percentages. Several times I have bred ab- 

 normal types: some of them have failed to reappear; 

 others have reappeared in a certain percentage of cases. 

 Two stocks of the latter kind may be referred to here. 

 My main purpose in describing them is to anticipate the 

 possible confusion that might arise if some one finding 

 these or similar ones should suppose them to be the same 

 types as those described as abnormal abdomen in this 

 paper. 



The six drawings in Fig. 2, a-f , represent some of the 

 characteristic types of a certain stock. The failure of the 

 third abdominal ring to extend across the middle line, as 



