468 
RECAPITULATION. 
Chap. XIY. 
sliall decrease, or finally become extinct. As the indi¬ 
viduals of the same species come in all respects into the 
closest competition with each other, the struggle will 
generally be most severe between them ; it will be almost 
equally severe between the varieties of the same species, 
and next in severity between the species of the same 
genus. But the struggle will often be very severe be¬ 
tween beings most remote in the scale of nature. The 
slightest advantage in one being, at any age or during 
any season, over those with which it comes into compe¬ 
tition, or better adaptation in however slight a degree to 
the surrounding physical conditions, will turn the balance. 
With animals having separated sexes there will in 
most cases be a struggle between the males for possession 
of the females. The most vigorous individuals, or those 
which have most successfully struggled with their condi¬ 
tions of life, will generally leave most progeny. But 
success will often depend on having special weapons or 
means of defence, or on the charms of the males; and 
the slightest advantage will lead to victory. 
As geology plainly proclaims that each land has 
undergone great physical changes, we might have ex¬ 
pected that organic beings would have varied under 
nature, in the same way as they generally have varied 
under the changed conditions of domestication. And 
if there be any variability under nature, it would be an 
unaccountable fact if natural selection had not come 
into play. It has often been asserted, but the assertion 
is quite incapable of proof, that the amount of variation 
under nature is a strictly limited quantity. Man, 
though acting on external characters alone and often 
capriciously, can produce within a short period a great 
result by adding up mere individual differences in his 
domestic productions; and every one admits that there 
are at least individual differences in species under 
