Darwin's most important contribution was to note two mechanisms for evolution, random mutation and natural selection. In contrast with Aristotle, Darwin did not think that mutations per se had any purpose. They were completely accidental. However, within a species, of the hundreds of different minute variations, some would contribute to survival of a creature, leading those organisms possessing a specific traits to survive and breed more efficiently than those lacking that specific trait, and eventually, over generations, this differential survival would lead to a gradual shift in the nature of the species. In Aristotle, on the other hand, individuals and species would adapt to suit their environments and to become more efficient, just as when we use a muscle it becomes stronger, so when giraffes constantly reach for high branches, their necks would become longer and they would pass on this characteristic to their young.