No. 572] SHORTER ARTICLES AND DISCUSSION 503 



genes are involved. 2 This is all there is to the matter. We need 

 not dwell, therefore, at length on Professor Castle's statement 

 that here is another case of an erroneous conclusion reached in 

 consequence of using small letters for "absent" characters, 

 except to remark that Dexter did not use small letters for absent 

 characters, and that the erroneous conclusion has been drawn by 

 Professor Castle himself. 



T. H. Morgan 



Columbia University 



Professor Morgan has called my attention to the fact that in 

 criticizing a single point in Mr. Dexter 's review I have given the 

 impression, to some at least, that 1 regarded Dexter's views as 

 erroneous. Such was not my intention, and I wish to correct the 

 impression, if I may. I do not for a moment question the reality 

 of "unit-character" inheritance or indorse the idea of "the or- 

 ganism as a whole" as the only inheritance unit. I agree here en- 

 tirely with the view which I understand Dexter to hold. If Xa- 

 bours has encountered nothing but simple allelomorphs among his 

 grasshoppers (which I neither assert nor deny), this by no means 

 proves that only simple allelomorphs exist even among said grass- 

 hoppers. An organism which seems to have only one variable 

 "gene" may nevertheless possess any number of other genes 

 which are not varying so far as we can discover, and in which con- 

 sequently all zygotes are homozygous and all gametes similar to 

 each other. 



It is only in Dexter's discussion of the significance of the ex- 

 ceptional "BE I" individual that I should dissent from any part 

 of his excellent review. Nabours's explanation of this case, ac- 

 cording to Dexter, is essentially that of "non-disjunction," in- 

 stead of which Dexter himself offers the explanation of "link- 

 age," and proposes a repetition of the experiment to decide 

 between them. Now I do not question for a moment the genuine- 

 ness of either "non-disjunction" or "linkage," as they occur for 

 example in Drosophila. Through the kindness of Professor Mor- 

 gan I have been able to demonstrate both these phenomena re- 

 peatedly to classes in genetics in the course of their laboratory 

 work upon Drosophila. The point which I wished to make in com- 



2 Crossing over would not take place if the factors in question were allelo- 

 morphic. If the case is one of non-disjunction the subsequent generation 

 would also give a different kind of result from that of linkage. (See 

 Bridges, J&ur. Exp. Zool, 1913.) 



