HEREDITY OF RACEHORSE STAMINA 51 



equal numbers, and the number of fillies which win at 

 two years of age slightly exceeds the number of 

 colts, at three years of age and upwards horses and 

 geldings which are successful outnumber the mares by 

 nearly two to one. The fillies are, in fact, quite as if 

 not more speedy than the colts, but some factor or 

 factors correlated with sex inhibit in certain, but 

 not in all, cases the physiological properties of a 

 mare's dark red muscle fibre. It is a matter of com- 

 mon knowledge that some fillies stay even better at 

 two years than later in life. 



The Types of Racehorses. 



In Table III. (page 81) the sprinters, symbolically 

 indicated by PP, represent a group of horses unable to 

 run at true racing pace more than seven furlongs; inter- 

 mediates, indicated by PR, a second group which 

 stayed from 8 to 1 1 furlongs ; and stayers, indicated 

 by RR, a third group which won at twelve furlongs 

 and upwards. This classification is to a certain 

 extent arbitrary, and possible sources of error lie in 

 the border lines between sprinters and intermediates, 

 and intermediates and stayers. It is probable that 

 the limit of the sprinters is fixed a little too high, and 

 that winners at seven furlongs on straight and rather 

 severe courses, such as Newmarket, Ascot, Newbury, 

 and Grosforth Park, are really intermediates. Then 

 it may be open to question whether twelve furlongs 

 provides a sufiiciently severe test to distinguish in all 

 cases between horses of PR and RR composition. 

 The Epsom twelve furlongs most certainly does not ; 



