No. 520] MENDELIAN PHENOMENA 



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based on the same pair of chromosomes. Several other 

 similar cases are known. So that in order to prove that 

 anything smaller than a chromosome is responsible for a 

 Mendelian character difference it must be shown that we 

 can get into a single individual more independent Men- 

 delian character pairs than it has pairs of chromosomes. 

 Both Baur and Shull, in private correspondence with the 

 writer, have admitted the justice of this contention, and 

 have promised to put it to the test. 



Under the present hypothesis what has heretofore been 

 called a pair of Mendelian unit characters must have a 

 new name. For instance, in the red and white pea cross, 

 if our assumption regarding the cause of the Mendelian 

 phenomena is correct, we are not dealing with a pair of 

 characters at all in the de Vriesian sense. Red is as- 

 sumed to be a generalized function of the cell, and white 

 likewise, but the difference between the red and the white 

 concerns a single cell organ, and it is this difference with 

 which we are dealing. Dr. McDougal, in conversation 

 with the writer on this point a few days ago, suggested 

 that in discussing this hypothesis we use the term * 1 char- 

 acter differential" instead of character pair, and the sug- 

 gestion seems to be a good one. We do, however, need 

 a few terms, which I shall now proceed to introduce 

 Under our hypothesis each of the organs of the cell is 

 supposed to have various functions in development. One 

 function may relate to the development of a particular 

 character, and another to a very different character. 

 Furthermore, several or all the organs in the cell may 

 each have a function relating to the same character. 



But not only are the organs of the cell supposed to play 

 their part in development, but various organs, tissues and 

 substances produced in the organism during development 

 may also have functions which play a part in the develop- 

 ment of various portions of the organism. For instance, 

 hairs normally develop only in the skin. Hence the skin 

 itself influences the development of hair. It is supposed 

 that certain substances, called hormones, may materially 

 affect the development of parts of the organism widely 



