For existence is an essential condition for natural selection). There is a fundamental difference among these influences: on the Greek conception (and arguably also in Darwin), strife and contest are positive, generating in Heraclitus a cosmic sense of justice, in Greek culture generally pushing humans towards greatness, and in Darwinian thought, the struggle is a crucial feature of the mechanism of natural selection and the development of species. This struggle for limited resources is not the chief characteristic of the overall 'economy' of life.