The theory of evolution by natural selection is now scientific orthodoxy, but when it was unveiled it caused a storm of controversy, from fellow scientists as well as religious people. They criticised it for being short on evidence and long on assertion and Darwin, being the honest scientist that he was, agreed with them. He knew that his theory was riddled with 'difficulties', but he entrusted future generations to complete his work and prove the essential truth of his vision, which is what scientists have been doing for the past 150 years.