Impressively Epicurus advanced a form of natural selection to explain biological life and the appearance of design within his purposeless universe of material atomic compounds.
Aristotle advocated immanent teleology, that all natural substances have essences or natures, and their final causes are therefore built-in. Under this train of thought, a heart exists for the purpose of pumping blood, a hand exists for the purpose of grasping and, most famously, reproductive organs exist for the purpose of procreation. Aristotle even suggested a hand severed from the body is not truly a hand as it's purpose is lost.
Epicureans reject this theory. They agree organs and body parts are essential for tasks like pumping blood, but disagree that the same organs exist for the inherent purpose of that task. That is, something being useful or essential for X does not mean this something was made for the purpose of X. Instead, Epicurus accounted for the apparent design of biological organs and organisms with a theory of natural selection.
In the past the earth was more fertile than today with numerous forms of life growing from seeds upon the ground (Lucretius wrote of wombs fastening to the earth). Many of these new creatures lacked the bodily organs necessary for survival and either immediately died or were hunted to extinction by faster and stronger animals. Thus bodily organs appear to possess innate purposes and animals appear adapted to their environments only because animals lacking these organs and adaptions no longer exist.
It's interesting that Lucretius believed animals could have extraordinary variety in appearance, but yet denied mythological creatures like centaurs really existed. A seed he wrote could only grow into a single animal; thus a half-man, half-horse centaur is impossible. A man or a horse may grow from a single type of seed, but not both.
The Epicurean argument for natural selection is often compared to the theories of Charles Darwin, but we should not overstate their similarities. Epicurus lacked any concept of evolution, of species naturally adapting over time in response to changes in their environment.
Posted by Gavin Doyle