Sorry, but I don't remember that being part of the theory at all. Can you provide a reference for that?
Biology is supposed to be "objective," and limited to a range of observed events. The scientist is supposed to be a disinterested observer, but what is obsaerved requires explanation to be meaningful and every scientist necessarily "adds to" his observation as he observes. Part and parcel of Darwinism is the culture in which Darwin was nurtured. It is this which prevents his science from being objective. So far as science alone is concern, he was most concerned to reject catastropism, the geological equivalent of revolution , in favor of gradualism-- a idea most congenial to Victorian Englishmen.