It is true of all plant and animal life; it is true of man as an individual; it is true of his mind as well as of his body; it is true of society as an aggregation of individuals.

As men have evolved from a lower to a higher, a simple to a complex state, so they are still evolving and rising "on stepping stones of their dead selves to higher things." Natural selection, or the survival of the fittest, is one of the processes by which evolution takes place.

According to this law, only the fittest survive in the struggle for life.

Darwin was led to this discovery on reading Malthus's thesis regarding the disproportion between the rates of increase in population and food, and the consequent struggle for existence.
All living organisms require food and space.

The power of multiplication in plants and animals is so great that food or space is sooner or later entrenched upon, and then commences this inevitable struggle for existence.