There's a good wiki article on him, or google 'patrick matthew project' for the evidence that Darwin and Wallace deliberately stole the idea from him, To me the circumstantial evidence seems strong.

Darwin's prejudice was that evolution must always be gradual. PM realised that once a species was optimal for its environment it might change little, but when the environment changed it would change fast.

Darwin's error still gives false comfort to creationists today: 'Where are the missing links, the transitional forms'? We've found some now of course, but PM's reasoning that they'd be relatively rare was absolutely correct.