1910.] THROUGH HYBRIDIZATION. 287 



unit-characters are " differentials," as this was recognized by ]\Iendel 

 himself and has been common knowledge to all investigators of 

 Alendelian heredity since. 



The length of hair in guinea-pigs and rabbits, the stature of 

 peas, sweet peas, beans, etc., the length of styles in Primula and 

 (Enotliera, density of the heads in wheat and barley, and in fact 

 practically all other characters with which Mendelian investigators 

 have worked, have been so obviously differentials that it is impos- 

 sible to assume that any iSIendelian has ever meant anything else by 

 the expression, " unit-character." 



This being true, the contention of Riddle that even in the absence 

 of a given unit-character there is not a complete absence of the 

 particular manifestation in which the essence of that unit-character 

 consisted, or in other words, that the unit-character is simply a 

 phase or " strength " of some " rather general protoplasmic power," 

 is not likely to seriously disturb the IMendelians, since that is a fact 

 with which they have long been familiar. 



It appears to me that the unnecessary shifting of the terminology 

 of clearly distinguishable empirical phenomena is undesirable. The 

 unit-characters are real things capable of repeated demonstration. 

 They are still dift'erential characters, and possess the capacity to 

 behave as units, entering into various combinations with other unit- 

 characters and capable of reextraction from them, or of being 

 absent altogether, regardless of the manner in which their behavior 

 is explained. The genes, on the other hand, — the ultimate organs 

 of the protoplasm or conditions of the protoplasmic substance upon 

 whose existence depends the capacity to give certain series of re- 

 actions, or to pass through certain cycles of ontogenetic develop- 

 ment, — are purely inferential. Their nature is not yet capable of 

 demonstration. They are " unknown gods " to whom each new 

 prophet may appropriately apply a new name whenever he ascribes 

 to them new attributes. 



While the ultimate nature of the genes lies wholly beyond the 

 powers of present-day analysis, and there is nothing therefore to 

 warrant a departure from the prevailing attitude of suspended judg- 

 ment, the more intimately the unit-characters themselves are studied, 

 the better will be the basis provided for an understanding of their 



