No. o4.3] OifZGAV OF UNIT CHABACTERS 



269 



modification from the wild £/. asiwws of northern Africa), 

 the domestic horse is not a pnre strain and is certainly 

 polyphyletic, having in its blood that of several races, 

 such as the Arab and the Forest or Norse horse, animals 

 which have specific distinctness although they still inter- 

 breed. 39 To this mixed strain or polyphyletic heredity of 

 the horse, are probably attributable many of the allometric 

 variations in the bones of the skull and in the enamel 

 pattern of the teeth of the mule in some of which we 

 observe a nearer approach to the ass type than in others. 

 If we could cross the ass with a pure horse race like the 

 Steppe or Prjevalsky horse we should probably obtain 

 more precise results. Another disturbing feature in the 

 comparisons and indices given below is that we do not 

 know the exact structure of the skull of either of the 

 parents from which the mule skulls examined were 

 derived. 



Despite these sources of fluctuation and of error, the 

 general results obtained are fairly positive and definite. 



The first point of interest in the segregation of unit 

 characters in the mule is that connected with the three 

 germ inal layers, namely, the epiblast, mesoblast and hypo- 

 blast. All the characters of epiblastic origin appear to be 

 derived from the sire, namely, the epidermal derivatives, 

 the distribution of the hair, especially in the mane and 

 tail, the hoofs, etc., are those of the ass, although the color 

 pattern, as in the "calico" mules described by Kwart, 

 may be derived from the mare. The nervous system and 

 psychic tendencies, all of epiblastic origin are also 

 derived from the ass, including minor psychic character- 

 istics, such as aversion to water. Still more striking, 

 perhaps, is the fact that the enamel pattern of the grind- 

 ing teeth, again of epiblastic origin, is mainly that of the 

 ass, although, as shown below, there are some inter- 

 mediate and some distinctive horse-like characters in the 



