VARIATION 199 



is to be found in the case of the " mule-footed " hog. In 

 this instance, the split hoof has united into one solid 

 hoof like the horse, and this variation is strongly inherited. 

 Meristic variations are often transmitted by heredity. 



190. Functional variations. In our discussion of 

 variation up to this point, we have chiefly concerned 

 ourselves with such changes as are exhibited in the form 

 of animals. We have now to consider those important 

 modifications which affect the performance of the indi- 

 vidual. An animal may retain the same form as the 

 average of the race but be vastly more efficient in the 

 performance of some one or more of the physiological 

 functions. This type of variation is one of the most 

 important to the animal-breeder. The value of many of 

 the domestic animals depends entirely upon their func- 

 tional development. The chief advantage possessed by 

 some domesticated animals over their wild progenitors 

 is due to their higher functional efficiency. This is illus- 

 trated by the domestic cow, the improved breeds of wool 

 sheep, and the trotting horse. Such differences in form 

 between the wild and domestic sorts as are present are 

 chiefly significant as indicating the correlation which exists 

 between function and form. The changed form, if it ex- 

 ists, is incidental and the result of functional influence. 



The valuable variation which the breeder has em- 

 phasized in his selection has been the ability of the animal 

 to produce economically and largely some valued animal 

 product. The breeder did not in the beginning consciously 

 select variations in form. It is true, however, that the 

 efficient performance of animals is to a certain extent 

 correlated with the form, and it follows, therefore, that 

 we may within certain rather narrow limits rely upon 

 the external form as an indication of functional efficiency. 



