Chap. VIII. SEXUAL SELECTION. 319 



the more fertile pairs producing a larger number of off- 

 spring, and these from their mere number will have the 

 best chance of surviving, and will transmit their ten- 

 dency to greater fertility. The only check to a con- 

 tinued augmentation of fertility in each organism seems 

 to be either the expenditure of more power and the 

 greater risks run by the parents that produce a more 

 numerous progeny, or the contingency of very numerous 

 eggs and young being produced of smaller size, or less 

 vigorous, or subsequently not so well nurtured. To 

 strike a balance in any case between the disadvantages 

 which follow from the production of a numerous pro- 

 geny, and the advantages (such as the escape of at least 

 some individuals from various dangers) is quite beyond 

 our power of judgment. 



When an organism has once been rendered extremely 

 fertile, how its fertility can be reduced through natural 

 selection is not so clear as how this capacity was first 

 acquired. Yet it is obvious that if individuals of a 

 species, from a decrease of their natural enemies, were 

 habitually reared in larger numbers than could be sup- 

 ported, all the members would suffer. Nevertheless the 

 offspring from the less fertile parents would have no 

 direct advantage over the offspring from the more fer- 

 tile parents, when all were mingled together in the 

 same district. All the individuals would mutually tend 

 to starve each other. The offspring indeed of the less 

 fertile parents would lie under one great disadvantage, 

 for from the simple fact of being produced in smaller 

 numbers, they would be the most liable to extermina- 

 tion. Indirectly, however, they wOuld partake of one 

 great advantage ; for under the supposed condition of 

 severe competition, when all were pressed for foofl, it is 

 extremely probable that those individuals which from 

 some variation in their constitution produced fewer eggs 



