Darwin proposed the concept of natural selection being the mechanism by which new species arise from pre-existing ones.
It states that there is 'struggle for existence' within a population and variation exists within all population. The continuous competition between individuals for environmental resources creates a 'struggle for existence' and this makes sure that certain organisms would fail to survive or reproduce.
This would eliminate less suited organisms and better-adapted organisms would survive and pass on their traits to the next generation leading to evolution.