The species with eyes appears suddenly, capriciously as it were, and it is this species which changes the environment by creating its visible aspect. The eye does not come into being because it is needed. Just the contrary; because the eye appears it can henceforth be applied as a serviceable instrument. Each species builds up its stock of useful habits by selecting among, and taking advantage of, the innumerable useless actions which a living being performs out of sheer exuberance.