DOUBLEDAY'S THEORY 179 



developing the maximum degree of fertility has been 

 exceeded that such a formula has any application. 



Doubleday was not unaware of this. He conducted a 

 series of experiments to ascertain the effects of various 

 kinds of manures on plants and their fertility. As a 

 result he came to the conclusion that, " whatever might 

 be the principle of manure, an overdose of it invariably 

 induced sterility in the plant." Thus he admits in this 

 passage that it is only an overdose which induces sterility. 

 Unfortunately, he only embodied half the truth in his 

 formula. What resulted was this : whilst Doubleday 

 insisted upon one half of the truth and ignored the other 

 half, Spencer, Darwin, and the rest of his critics were 

 careful to insist upon that half of the truth which he 

 had left out of his formula, and to ignore as far as possible 

 the half upon which he insisted. It was, of course, quite 

 easy under the circumstances to quote a hundred facts 

 which apparently gave a flat contradiction to Doubleday's 

 formula. 



The second defect is that, as stated, the formula gives 

 too great a weight to one factor in the environment to 

 the exclusion of the rest. There are innumerable other 

 factors in the environment which affect fertility besides 

 food, and these must act through the medium of some 

 factor in the organism which is not specified in the formula. 

 Although Doubleday seemed to recognise that other factors 

 affect the degree of fertility, yet he deals almost exclusively 

 with the effects of variations in the food supply. Food 

 has unquestionably a great effect upon the degree of 

 fertility, especially among the lower animals and plants, 

 but the figures previously given showing the enormous 

 proportion of completely infertile marriages among the 

 intellectual classes seem to show that cerebral develop- 

 ment resulting from intellectual and athletic exercises 

 is of still greater influence in producing sterility among 



