It must not, indeed, be thought that all the weeding out which goes on favours the fittest.

Accidents will always happen.

On the whole, however, the type that is most at home under the surrounding conditions, it may be because it is more complex, or it may be because it is of simpler organization, survives the rest.
Now to survive is to survive to breed.

If you live to eighty, and have no children, you do not survive in the biological sense; whereas your neighbour who died at forty may survive in a numerous progeny.

Natural selection is always in the last resort between individuals; because individuals are alone competent to breed.