The Origin of Species

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.