Chap. VIII. 
SEXUAL SELECTION. 
319 
the fertility of each species will tend to increase, from 
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 food, it is 
extremely probable that those individuals which from 
