The number of offspring produced by an organism in a given time. In higher animals, fecundity generally focuses on female offspring, because only they will produce offspring. Normally all organisms, assuming they reach reproductive age, are sufficiently fecund to replace themselves several times over. Darwin noted this, together with the fact that population numbers nevertheless tended to remain fairly constant: these observations led him to formulate his theory of evolution by natural selection. Compare fertility. See also reproduction rate.