MANIFESTED IN THE ANIMAL KINGDOM 83 



and careful selection. This selection is still going on, 

 and we hear of 200-egg birds and 250-egg birds and so 

 on. Yet these highly prolific birds are still subject to 

 the operation of the principle. 



The potential degree of fertility is inherited. 



The actual degree of fertility is decided by the direct 

 action of the environment. 



" In Europe close confinement has a marked effect 

 on the fertility of the domestic fowl : it has been found 

 in France that with fowls allowed considerable freedom 

 only 20 per cent, of the eggs failed ; when allowed 

 less freedom, 40 per cent, failed ; and in close confine- 

 ment sixty out of the hundred were not hatched." * 



The question of importance from our present point 

 of view is not the number of eggs laid but the number 

 which are fertile. In the case of eggs which fail to hatch 

 the whole sequence of reproductive phenomena is carried 

 through except the union of the sperm cell with the ovum. 

 There is no reason whatever to doubt that the sperm cells 

 actually come into contact with the ovum. The fact 

 that the egg is successfully laid shows that there is no 

 obstruction to the passage of the sperm cells up the female 

 ducts. Presumably, therefore, sterility is due to the 

 effect of lack of exercise through the medium of the 

 nervous system on the condition of the ovum, which, 

 as a consequence, fails to admit the sperm cell. One 

 of the principal effects of exercise is the expenditure of 

 nervous energy. 



If the reader sits on a fence and watches a number of 

 fowls pecking and scratching for hours in a farmyard 

 or field, he cannot fail to be struck by the immense amount 

 of energy expended. And if, as is generally the case, 

 such fowls are only partially fed and are expected to 



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



