What is the theme in On the Origin of Species by Charles Darwin?

Struggle for existence is the main theme of this book. Crucial to Darwin's theory of natural selection is the idea that far more plants and animals are produced than can survive. Plants and animals produce in very great numbers and would, within a short period, create very large populations. This applies both to those organisms that reproduce quickly, such as some species of insects, and to those organisms which reproduce slowly, such as elephants or humans. This is due to what Darwin calls the geometrical rate of reproduction.