HEREDITY OF RACEHORSE STAMINA 79 



the filial generation. Latency may thus be the result 

 of a minimum number of the chromosomes carrying 

 the factors for the dark fibre. The gametic formula, 

 RR, would therefore represent a horse in which the R 

 factor was present in a maximum number of chromo- 

 somes which can carry the character, and the formula 

 PP, in a strict sense, a horse in which R was not 

 present in any chromosome. The dark fibres, how- 

 ever, would appear to be never entirely absent from 

 the tissues of the thoroughbred, but certain individuals 

 carry so few that they are practically no use to them 

 as aids to stamina. 



The epistatic and hypostatic theory is thoroughly 

 in accordance with the conception of the recapitula- 

 tion process. During foetal life and foalhood the 

 two muscle characters — red and pale — are morpho- 

 logically indistinguishable. At a year old the differ- 

 ence is still imperceptible, and even at two years of 

 age the physiology of the dark fibre is imperfect. It 

 rarely reaches the full potentiality before the bearer 

 is four years of age, and sometimes not till he is six. 



In collecting the data for this paper I have received 

 valuable assistance from my son, H. C. Robertson. 

 I have also to thank the Editor of " The Sportsman " 

 for allowing me to reproduce the time-distance table 

 (Table II.) which originally appeared in one of my 

 communications to that paper. 



The statistics for this paper were brought up to June 30th, 1911. 

 Whilst the material was in the printer's hands, the extra data for the 

 latter part of 1911 became available, but too late to be incorporated in 

 the tables. I have gone carefully through the extra figures and find that 

 thej- make no material alteration in the results, except that the increased 



