100 GENETICS AND EUGENICS 



no one familiar with Mendelian phenomena would venture 

 to classify the anatomical parts or physiological processes of 

 an organism as unit-characters in heredity merely because 

 they are distinct anatomical parts or distinct physiological 

 processes. 



The head, the hand, the stomach, stomach-digestion, — 

 these are not unit-characters so far as any one knows. But if 

 a race without hands were to arise and this should Mendelize 

 in crosses with normal races, then we should speak of a unit- 

 character or unit-factor for " hands," loss of which or varia- 

 tion in which had produced the abnormal race. But in so 

 doing we should refer not to the hand as an anatomical part J 

 of the body nor to the thousand and one factors concerned 

 in its production but merely to one hypothetical factor to 

 which we assign the failure of the hand to develop in a 

 particular case. It is immaterial whether we call this a imit- 

 character or unit-factor or use both terms inter-changeably, 

 but it would be a mistake to suppose that they refer to differ- 

 ent things or that one is less abstract than the other. Histori- 

 cally the term unit-character has priority, though factor seems 

 better to express the abstract and purely hypothetical nature 

 of the conception involved. The application of the term 

 unit-character at first to certain agencies which were later 

 found to be complex led to the coining of a new term (unit- 

 factor) to apply to the newly recognized simpler agencies. If 

 this process were to be continued indefinitely we should have 

 to invent a new set of terms for every step in advance in 

 Mendelian analysis. It seems better to discard earlier and 

 imperfect analyses as knowledge advances but not to multi- 

 ply technical terms needlessly when no new conception is 

 involved. 



Parental and filial generations. The manifestation of Men- 

 delian characters is often very different in successive gener- 

 ations, for which reason it is necessary to have a convenient 

 means of designating the different generations concerned. 

 The significant generation from which reckoning should be 



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