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.