Xo. 545] ORIGIN OF UXIT C U A II AC IE US 



207 



of zebras either blend with or are dominated by the cor- 

 responding characters in their horse and ass mates. 

 Thus, as influencing dominance or prepotency, the value 

 which a character has attained in the past struggle for 

 existence seems to count for something. In zebras and 

 in horses certain physical and mental traits are more 

 highly heritable than others. Among the characteristics 

 which are often handed down unblended in zebra-horse 

 hybrids and to a less extent in zebra-ass hybrids are the 

 size of the ears, the form of the hoofs, the massiveness of 

 the jaws ; while among psychic characters are transmitted 

 the extreme caution, the wonderful alertness and 

 quickness. 



The new results brought forward in this Harvey lecture 

 from the comparison of the skull and teeth of the horse, 

 ass and mule on the whole strengthen the theory of unit in- 

 heritance both in rectigradations and in allometrons. The 

 measure of unit character inheritance as contrasted with 

 blended inheritance is very precisely brought out in the 

 detailed study of the twenty-two characters which are 

 examined below. Before discussing these characters in 

 detail it is interesting to point out that the ancestors of 

 the horse and the ass have probably been separated for 

 at least 500,000 years. In the meantime the horse has 

 become extremely dolichocephalic, the ass has remained 

 comparatively mesocephalic ; the horse has a relatively 

 long, the ass a relatively short face; the horse has highly 

 complex, the ass has somewhat simpler grinding teeth; 

 the horse exhibits advanced adaptation to grazing habits 

 and has become habituated to a forest and plains life in 

 comparatively fertile countries, while the wild ass is by 

 preference a browsing animal, finding its food in exces- 

 sively arid countries where there is a marked dearth of 

 water and water courses. The physical and psychical 

 divergences in these two animals have developed over an 

 enormously long period of time. Every single tooth and 

 bone of the horse and ass show differences both in recti- 

 gradation and in allometric evolution. 



