Darwin's theory also known as the theory of natural selection is based on the principle of survival of the fittest. The variations in the organisms and the individuals who are better adapted to the environmental conditions and struggle for existence survive in comparison to those who fail to get adapted to the environment. Charles Darwin failed to explain the origin of the variation and mechanism of speciation thus limited his theory.
Branching descent and natural selection are the two key concepts of Darwinian theory of evolution.