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.