No. 580] OIUGIX OF SIXGLE CHARACTERS 



221 



developed gradually or suddenly within the highly varied 

 horses of this genus. It appears (Fig. 5) more or less 

 fully developed in all of the many known species of Pleis- 

 tocene horses of America as described by Leidy, Gidley 

 and Hay. It lies near the surface of the crown, and in 

 much-worn teeth it disappears because the fold is seldom 

 continued down into the lower half of the crown. It is 

 entirely absent in the grinding teeth of the domesticated 

 ass (E. asinus), yet it is present in the kiang (E. hiang). 



The complete germinal separability of the " pli cabal- 

 lin" as a hereditary character is demonstrated by its 

 absence in the grinders of the mule, the cross between 

 E. caballus $ and E. asinus d"; these grinders of the mule 

 hybrid also prove that rectigradations are distinct from 

 allometrons, because the rectigradations of the maternal 

 horse molar are not inherited in the hybrid while the 

 allometrons are inherited, namely, the elongated propor- 

 tions of the maternal horse molar. I am preparing to 

 investigate the grinding teeth of the hinny, the cross 

 between the male horse and the female ass, to ascertain 

 whether the same contrast in heredity prevails here. I 

 suspect not because the hinny appears to have the shorter 

 head of the ass rather than the very long horse-like head 

 of the mule. 



The germinal separability of allometrons or propor- 

 tional characters of mammals is also observed, but it 

 appears to be less complete than that of rectigradations. 

 This is demonstrated not only in the grinding teeth but 

 in the skull of the mule hybrid, in which the majority of 

 the head proportions present the same indices as in the 

 horse, while the minority of the head proportions present 

 a blend between the indices of the horse and ass. Again 

 in Homo sapiens the allometrons are in the first genera- 

 tion completely separated; in intermarriage of dolicho- 

 cephalic and brachycephalic individuals the children 

 do not form a blend of their parents but inherit either 

 the pure dolichocephalic or pure brachycephalic head 

 form. Prolonged interbreeding and intermixture be- 

 tween long-headed and broad-headed human races ap- 



