74 THE LAW OF BIRTHS AND DEATHS 



and rather closely inter-bred animals have become that 

 not far from one-third of the mares failed to produce 

 living offspring. Thus during 1866, 807 male colts and 

 816 female colts were born, and 743 mares failed to produce 

 offspring. During 1867, 876 males and 702 females were 

 born, and 794 mares failed." * 



As the cases of the mares which slipped their foals 

 are not separated from those where the mare proved 

 barren, the actual degree of sterility is not clear, though 

 it was apparently very large. This failure to distinguish 

 between failure of instinct, failure of ovulation, failure 

 of fertilisation, and cases of miscarriage impairs the 

 value of much of the evidence collected by Darwin, 

 and illustrates the necessity of a thorough analysis. 



Perhaps the most conspicuous attribute of race-horses 

 is a very high development of nervous energy. This 

 they owe, partly to selection and inheritance, for they 

 have been bred for speed for many generations, and speed 

 is dependent rather upon nervous energy than muscular 

 strength ; partly, also, to the system of training and 

 feeding. They are regularly exercised, which stimulates 

 the production of nervous energy, and fed chiefly on 

 oats, which have about the highest proteid content of 

 any vegetable food. Of course, when the mares are taken 

 for breeding this regimen is usually abandoned, and they 

 are generally turned out to grass. Nevertheless, much 

 of the effect must remain. 



Another writer speaking of race-horses says : "At all 

 events it is found in practice, that though the majority 

 of maiden mares will become stinted while at work, yet 

 that a large number require a run at grass before they 

 become in foal." 2 



1 Descent of Man, chap. viii. 



* The Horse in the Stable and in the Field, Stonehenge, chap, xi. 



