The Origin of Species : The Preservation of Favoured Races in the Struggle for Life

Darwin's theory of evolution is based on key facts and the inferences drawn from them, which biologist Ernst Mayr summarised as follows:
-Every species is fertile enough that if all offspring survived to reproduce, the population would grow (fact).
-Despite periodic fluctuations, populations remain roughly the same size.
-Resources such as food are limited and are relatively stable over time.
-A struggle for survival ensues (inference).
-Individuals in a population vary significantly from one another.
-Much of this variation is heritable.