58i 
Whatever force there may be in this argument, it is 
scarcely consistent with the principles laid down by Mr. 
Spencer; for, if we assume that the race is so adapted 
to external conditions that further change is not required 
with reference to them, it does not follow that the struggle 
with enemies will naturally afford a special opportunity 
for the action of Natural Selection. For Mr. Spencer lays 
down two different definitions of the survival of the fittest, 
according to one of which a species survives by virtue 
of its enormous fertility. Now we have already pointed 
out that a great output of life is a necessary condition of 
the action of Natural Selection, and that if a great output 
of life is sufficient to secure the survival of a species 
without any modification, then one of the conditions 
necessary for the action of Natural Selection prevents it 
from coming on the scene. But let us, for the sake of 
argument, assume that a great output of life may act in 
two ways,—either by leading up to Natural Selection or 
by dispensing with its action. In which case is the output 
of life most likely to act to the exclusion of Natural 
Selection ? A great output of life, accompanied with a 
corresponding output of “ fortuitous ” variations, would 
affect nothing in adapting a species to a new locality 
apart from Natural Selection. But a great output of life 
would preserve the species from extinction through the 
attack of enemies. Where there is a fixed balance be 
tween species living in the same locality, there is a 
correlation between the output of life and the destruc 
tion to which a species is liable. 
Nor is it true to say that a great output of life will lead 
to the transmutation of species by Natural Selection in 
consequence of the competition between individuals of 
the same species, for the normal population of a district 
is correlated to the number of enemies ; while an excep-