This process of extinction Mr. Darwin has called “natural selection.”  Natural selection is not a force, but a process, resulting from the combined action of the forces of environment.  It is not a cause in any proper sense of the word, but a result of a myriad of interacting forces.  The combination of these forces in a process of natural selection leading directly to a moral and spiritual goal demands an explanation in some ultimate cause.  This explanation we have already tried to find.

It is a process of extinction.  It favors the fittest, but only by leaving them to enjoy the food and place formerly claimed, or still furnished, by the less fit.  In any advancing group, as the less fit are crowded out, and the better fitted gain more place and food and more rapid increase, the whole species becomes on an average better conformed.  More abundant nourishment and increased vigor seem also to be accompanied by increased variation.  And by the extinction of the less fit the probability is increased that more fit individuals will pair with one another and give rise to even fitter offspring, possessing perhaps new and still more valuable variations.

But if, of a group of weaker forms, those alone survive which adopt a parasitic life, those which in adult life move the least will survive and reproduce; there will result the survival of the least muscular and nervous.  This degeneration will continue until the species has sunken into equilibrium, so to speak, with its surroundings.  Here natural selection works for degeneration.  Sessile animals have had a similar history.  But these parasitic and sessile forms had already been hopelessly distanced in the race for life.  Their presence cannot impede the leaders; indeed their survival is necessary to directly or indirectly furnish food for the better conformed.  In the animal and plant world there is abundant room and advantage at the top.

Once more, natural selection works as a rule for the survival of individuals, only indirectly for that of organs composing, or of species including, these individuals.  It may work for the development of a trait or structure which, while of no immediate advantage to the individual, increases the probability of its rearing a larger number of fitter offspring.  Thus defence of the young by birds may be a disadvantage to the parent, but this is more than counterbalanced in the life of the species by the number of young coming to maturity and inheriting the trait.  Even here natural selection favors the survival of the trait indirectly by sparing the descendants of the individual possessing it.  Natural selection may always work on and through individuals without always working for their sole and selfish advantage.

In human society we find the selection of families, societies, nations, and civilizations going on, but mainly as the result of the survival of the fittest individuals.