138 GENETICS AND EUGENICS 



snout (as in Berkshires). It is probable that they are similar 

 in genetic character. Black among swine is dominant over 

 red, as in cattle, horses and rodents. But the dominance of 

 black is commonly imperfect or complicated by the presence 

 of a spotting factor in the red breeds known as Tamworth 

 and Duroc-Jersey. (See Figs. 86-93.) 



A curious morphological variation, syndactylism, is a domi- 

 nant unit-character. In this variation the normal two hoofs 

 of each foot have completely fused together and the foot has 

 a single hoof like a "mule." Hence the variety is called 

 "mule-footed." A breed having this characteristic has been 

 established in the United States. Although the hoofs are 

 fused the bones proximal to the toe retain their original 

 paired character. (See Figs. 94 and 95.) 



Sheep. In sheep ordinary white fleece is dominant over 

 black fleece, the latter occasionally cropping out in flocks as 

 a recessive, as indicated in the old saying "every flock has 

 its black sheep." Black sheep breed true inter se. Black is 

 probably not a reversionary variation but a loss variation of 

 a pattern factor found in wild sheep and similar to the bay 

 pattern of horses. Wild sheep are white or whitish except at 

 the extremities where the pigmentation is heavier. In some 

 breeds of sheep the skin and wool of the extremities is dark, 

 similar to the coat of Himalayan rabbits, and white spotting 

 may affect these pigmented regions just as it does the coat 

 of Himalayan rabbits. (See Figs. 96-100.) Hornlessness is 

 a variation from the original horned condition of wild sheep 

 which is dominant in females but recessive in males, a matter 

 deserving further consideration in connection with the sub- 

 ject of heredity as affected by sex. (See Figs. 96-104.) 



Dogs. By Darwin and most other students of the origin 

 of dogs, the conclusion has been reached that dogs are de- 

 scended from several different wild species of wolves inde- 

 pendently domesticated in different parts of the world. 

 These, it was thought, having been subsequently inter- 

 crossed have produced a highly variable stock from which 

 selection has isolated the genetically diverse modern breeds. 



