No. 544] ORIGIN OF UNIT CHARACTERS 



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covery of Mendel (1865) and by the negative results of 

 experiments on fluctuating or quantitative variation. 



From the prevalence of discontinuity in heredity, the 

 separateness of "unit characters" as they appear in the 

 body and the equally sharp separableness of their com- 

 plex of "factors," "determiners" or "genes" in the 

 germ has arisen the theoretical assumption of the dis- 

 continuity of origin of all characters in the germ. We 

 shall now show that this assumption is a non-sequitur. 



First, however, the truly marvelous and epoch-making 

 Mendelian discoveries require our especial examination 

 in their bearing on the problem of continuity and discon- 

 tinuity. We have reviewed 14 the contributions of Allen, 

 Bateson, Castle, Cannon, Cuenot, Darbishire, Davenport, 

 Durham, Farrabee, v. Gruita, Haacke, Hagedoorn, Har- 

 mon, Hurst, Laughlin, Morgan, Pearson, Plate, Punnett, 

 and Bosenoff. This review covers unit characters only 

 as observed in mammals, to which none the less the prin- 

 ciples discovered by Mendel in the common garden pea 

 {Pisum sativum) apply with striking uniformity. 



The prevailing field of the researches of these talented 

 investigators in mammals has been in color characters, 

 chemical in essence, in various species of rodents, chiefly 

 mice and guinea pigs, also in Ungulates, such as horses 

 and cattle, the latter studied less by experiment than from 

 stud books. Hair form in rodents and in man and skin 

 pigment have also been exactly investigated. The most 

 striking general result is the principle of antithesis of 

 characters which mutually exclude each other, as typified 

 by the antithesis of Mendel's "tallness" and "short- 

 ness" in peas. 



The second great feature is that when these antithetic 

 characters meet in the germ cells, one dominates over the 

 other ; this dominance is a sort of perpetual prepotency. 

 "Prepotency," observes Darbishire, "is an attribute of 

 individuals and capricious in its appearance. . . . What- 

 ever be the nature of this power ... it is clear that it 



