No. 545] ORIGIN OF UNIT CHARACTERS 



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change is an allometron. In mammals rectigradations 

 are comparatively few; allometrons comprise the vast 

 nnmber of changes in the hard parts. In the origin of 

 cusp and horn rudiments rectigradations are parallel 

 (see Fig. 3), in the changing proportions of a skull 

 allometrons are divergent (Figs. 1, 3). 



Granting, without at present considering the evidence, 28 

 that these rectigradations and allometrons arise con- 

 tinuously through entirely unknown laws, that they are 

 blastic or germinal characters, the question' arises, do 

 they become separable as unit or alternating characters 

 in heredity. 



In general, paleontology furnishes quite as strong 

 proof as Mendelism or experimental zoology as to the 

 individuality, separablencss, and integrity of single char- 

 acters in evolution. But, whether both rectigradations 

 and allometrons are separable in heredity can only be 

 demonstrated through experiments on cross breeding or 

 hybridizing. 



The special object of this Harvey lecture is to show 

 that certain at least of the rectigradations and allome- 

 trons observed in mammals are separable in heredity, 

 that they split up into larger and smaller groups or 

 units, some into partially blending units, others into ab- 

 solutely distinct or non-blending unit; finally that at 

 least in the first cross they exhibit dominance. 



The very important remaining question whether, like 

 the quality of "tallness" or " shortness" in Mendel's 

 classic experiments on the pea, these allometrons con- 

 tinue to split into dominants and recessives in later 

 crosses, has not been investigated but is probably ca- 

 pable of investigation in mammals which do not become 

 sterile in the first hybrid generation. 



Five examples of the continuous evolution of recti- 

 gradations and allometrons may be cited, namely: 



