The ol' "everybody does it" excuse again.
Just to clarify--all scientists are engaged in leveraging the work of others, and, lately, in breakneck races to be first to major milestones in the process of said leveraging, so as to get the credit, and the resulting academic cheese.
Darwin was cudgled by his friends into publishing before he really intended to, because another avid collector with enough of a scientific background to make his way to the principles of evolution through variation and selection, was hot on his tail, and he was substantially preceeded in in the neighborhood of his speculations by some of the leading lights of the previous generation of scientists.
Like any good Popperian, Darwin was solving a problem with a rather detailed pedagree that had already been voiced by the scientists that preceeded him.
At least you agree he didn't ORIGINate the idea.
Indeed not--he only solved a problem set, whose solution would have been published, probably within the year, by someone else with a less compulsive need to gather up the devastating evidence quite so encyclopeiacly.