Darwin backs up his works by several other naturalists. He uses some that agree and some that disagree and he attacks the disagreements head on. This gives people reason to agree with Darwin's views because they are presented with arguments and then the arguments are disassembled.
One of the major aspects of Darwin's On the Origin of Species which has been fought for and against is the Struggle for Existence. The Struggle for Existence is a survival of the fittest and only the beneficial mutations will survive. This struggle goes on all the time, everywhere in the world. Species evolve and will continue to evolve for as long as the world exists.