MANIFESTED IN THE ANIMAL KINGDOM 75 



The difference between a horse at work and a horse 

 at grass amounts to this, that the former obtains regular 

 exercise and is fed on a diet of corn. The exercise, if 

 regular and not excessive, will, coupled with a diet which 

 is very rich in proteids, and very nutritious in proportion 

 to its bulk, stimulate the animal's system to a high develop- 

 ment of nervous energy. The horse at grass feeds on 

 the most bulky and innutritious of foods, a food con- 

 sisting mostly of water and waste products and deficient 

 in proteids. In order to satisfy its bodily needs it must 

 be eating nearly all day. This bulky and relatively in- 

 nutritious food entails a disproportionate amount of 

 work on the part of the digestive system. With the 

 corn-fed horse probably much more energy is expended, 

 but far more is produced, and the animal, if properly 

 managed, will invariably be in high condition. With 

 the grass-fed horse, though little is expended, except 

 in the disproportionate amount of digestive work, little 

 is produced. The difference in the amount of energy 

 produced by the corn-fed and the grass-fed horse is too 

 notorious to need emphasising. Thus we see that condi- 

 tions which lead to a high development of nervous energy 

 are apt to cause sterility, whilst conditions the main effect 

 of which is to reduce the amount of nervous energy lead 

 to increased fertility. 



Darwin mentions a fact which at first sight seems 

 inconsistent with those detailed above. He says that 

 " mares which have been brought up on dry food in the 

 stable are often infertile when first turned out to grass." l 

 Though Darwin does not give the fact on the authority 

 of his own observation, yet it is probably correct. On 

 the hypothesis under consideration it is susceptible of a 

 very simple explanation. We have seen that the amount 



1 Variation of Animals and Plants under Domestication, chap. xvi. 



