32 Strttctural Characters : Aiiimals [ch. 



Animals. Structural Characters, 



Man. 



A considerable number of diseases and malformations 

 have been shown to behave usually as dominants. A few 

 conditions may be said, more doubtfully, to behave as 

 recesslves. The subject of human inheritance is discussed 

 in Chap. xii. Of normal characteristics, eye-colour is the 

 only one yet studied (Hurst, i6i)^ sufficiently to justify a 

 positive statement as to the existence of a Mendellan system 

 of descent. 



Cattle. 



37. Absence of horns in polled breeds of Cattle Is 

 dominant to the presence of horns (R.E.C. 19; SpIUman, 



246). 



In sheep the Inheritance of horns Is sex-limited {g.v.), 

 and from evidence given me by Mr E. P. Boys-Smith I 

 suspect that this is true in the case of Goats also. 



Horse. 



38. There is little doubt that the gait known as 

 "pacing" is recessive to the ordinary trotting gait In the 

 American trotters. Trotters bred together may produce 

 pacers, but hitherto I have found no authentic instance of 

 genuine natural pacers, when mated together, producing 

 trotters. Correspondents have sent me word of several 

 apparent exceptions to this rule, but all on inquiry have 

 proved to be erroneous. In the pacing gait the two legs of 

 the same side of the body are moved together or nearly so, 

 while in trotting the foreleg of one side moves almost with 

 the hind leg of the other. Horses may be trained with 

 more or less success to adopt either gait, but the distinction 

 between natural pacers and natural trotters is a fairly sharp 

 one (16). The physiological nature of the difference is 

 quite obscure, but presumably it is of nervous origin. 



Mouse. 



39. From time to time mice are found hairless, with the 

 skin thrown up into corrugated folds. Experimenting with 

 such mice Mr Archibald Campbell found the condition to 



* See also Davenport (107). 



