MANIFESTED IN THE ANIMAL KINGDOM 91 



numbers of sperm cells. Many of these must have come 

 into contact with the egg cells. Why then is conception 

 under such conditions so infrequent ? There seems no 

 escape from the conclusion that the result is due to the 

 effects of high feeding and lack of exercise, through the 

 medium of the nervous system, on the condition of the germ 

 cells. And if human beings are subject to the same bio- 

 logical laws as the lower animals, then a simitar combina- 

 tion of causes should be followed in their case by similar 

 effects. 



Again : " Although so many of the Felidae breed 

 readily in the Zoological Gardens, yet conception by no 

 means always follows union ; in the nine-year report, 

 various species were specified which were observed to 

 couple seventy-three times, and no doubt this must have 

 passed many times unnoticed ; yet from the seventy- 

 three unions only fifteen births ensued." * 



We have already seen that horses which have been 

 brought up on dry food in the stable, and no doubt 

 systematically worked during that period, are apt to be 

 infertile at first when turned out to grass. An analogous 

 phenomenon is reported by Darwin of wild animals in 

 captivity. " Those animals which usually breed freely 

 under confinement, rarely breed, as I was assured, within 

 a year or two of their first importation." 2 When mares 

 are turned out to grass they are able to obtain plenty of 

 exercise in the field, and the change of diet is from one 

 of very high feeding value to one of very low feeding 

 value. In spite of this the animal's system will continue, 

 for a time, to produce energy from its own resources on 

 the same scale as before, and the abatement will be 

 gradual. The same phenomenon in the case of wild animals 



1 Variation oj Animals and Plants under Domestication, chap, xviii. 

 * Ibid, 



