MANIFESTED IN THE ANIMAL KINGDOM 77 



cart-horse. With the effect of old age on the degree 

 of fertility there will be occasion to deal later. 



Another writer says : "In the mare barrenness is 

 equally due to a variety of causes. In a number of 

 breeding studs the proportion of sterile mares has varied 

 from 20 to 40 per cent." Among a number of other 

 causes specified are " high condition (plethora) with pro- 

 fuse secretion and excitement, low condition, with imper- 

 fect maturation of the ova and lack of sexual desire." * 

 Here we have both aspects of the principle clearly set 

 forth. The same writer says again : " An irritable womb, 

 with frequent straining, and the ejection of a profuse 

 secretion, may sometimes be corrected by a restricted 

 diet and full but well-regulated work. Even fatigue 

 will act beneficially in some cases ; hence the practice 

 of the Arab riding his mare to exhaustion just before 

 service." 



This last fact is particularly significant. The Arabs 

 have been famous horse-breeders for centuries. Probably 

 the custom originally arose through the more or less 

 accidental observation that the mare was more readily 

 impregnated when fatigued. This observation would 

 be confirmed by subsequent experience. Now the most 

 obvious effect of fatigue is to exhaust the nervous energy 

 of the animal. Doubtless there are other effects, but 

 the fact fits in with the theory in a most striking way. 



Discussing the best period for sending the mare to the 

 horse, a writer says : " As, however, mares are very uncer- 

 tain animals, he will do well to take advantage of the 

 first opportunity after March, as by putting off the visit 

 to the horse he may be disappointed altogether, or the 

 foal may be dropped so late that winter sets in before 

 he has acquired strength to bear it. These remarks 

 1 The Diseases of the Horse, United States Department of Agriculture. 



