172 THE LAW OF BIRTHS AND DEATHS 



any application to the variation in the degree of sexual 

 fertility under the direct influence of the environment. 

 The evidence relating to this part of the subject has 

 already been fully discussed, and need not be repeated here. 



The formula which has been given as expressing Spencer's 

 real meaning may fairly be applied to asexual reproduc- 

 tion. For we have seen that asexual reproduction is 

 favoured by abundant nutrition. But there is no inverse 

 proportion, as the amount devoted to individuation is 

 not necessarily decreased when the amount devoted to 

 reproduction increases. It is when the organism is 

 possessed of an abundance of nutrition over and above 

 the amount necessary to maintain the individual more 

 than can profitably be assimilated without further growth 

 that asexual reproduction is most abundant. In fact, 

 the amount devoted to reproduction appears to be at 

 its highest when the amount devoted to individuation is 

 also at its highest, and the organism has more nutriment 

 than it can utilise for its own purposes. Therefore we 

 may justly say that the degree of asexual fertility is 

 directly proportional to the surplus of nutrition over and 

 above the cost of individuation. 



The reader may perhaps ask : Does not Spencer provide 

 a very large body of evidence in support of his hypothesis ? 

 He does. But it consists of facts drawn from asexual 

 reproduction, the truth about which has been set forth 

 above ; of evidence drawn from the comparative fertility 

 of different species, to which we have seen that that 

 formula is inadequate as an expression of the facts ; 

 and of evidence drawn from the relative fertility of wild 

 and domesticated varieties, which we have seen will be 

 modified by the selection of the latter. None of this 

 evidence has any bearing on the variation of the degree 

 of sexual fertility under the direct influence of the environ- 

 ment. When all this is subtracted, the amount of evidence 



