HEREDITY OF RACEHORSE STAMINA 53 



It will be apparent that here, as in other Mendelian 

 characters, it is possible considerable variations in 

 the relative proportions of the two kinds of fibre may 

 exist. 



The variation in the amount of black pigment on 

 the limbs of bay horses may offer an analogy compar- 

 able with the variation of the specific properties of 

 the dark fibre. In certain bays black pigment is 

 absent on the limbs, in others it is scanty, whilst a 

 third variety shows intense pigmentation. All, how- 

 ever, behave in the same manner in relation to the 

 recessive character, chestnut, which, in itself, has a 

 wide range of tint. Individual horses may be ar- 

 ranged in such a manner as to form a graduated 

 series, commencing with very light yellow chestnut 

 with flaxen mane and tail, and yellow almost albinis- 

 tic limbs (reference is not made to white stockings, 

 which are truly albinistic) and ending in dark bay 

 with intensely melanistic mane, tail, and limbs. At 

 times it is difficult to decide whether a horse is a 

 rufous bay or a chestnut, and still more difficult to 

 distinguish between a dark chocolate chestnut and 

 a black. It will be readily understood that in the 

 characters of muscle fibre, of which the heterozygous 

 form is evidently a mosaic* of both characters, it is 

 not possible to fix a hard and fast distinguishing line 

 between the muscles of the three types of race- 

 horses, namely, sprinters, intermediates, and stayers. f 



*Mr. Robertson tells me that in the horse, unlike the rabbit, the red 

 fibres are not found in one muscle and the pale in another, but that they 

 both occur in varying proportions in the same muscle. — [Editor.] 



t See footnote, page 43. 



