"Wallace's contribution and Darwin's indebtedness to him"

I feel it important to point out an important fact... precedence -the significance and importance of being the first to publish in scientific journals.

Darwin had been working on what would become The Origin of Species and Descent of Man for decades. He also knew that those who pride themselves as being special and above the rest of the world would not swallow it well, if at all. After all, these same folks used the bible to justify slavery and white supremacy.

Wallace had similar conclusions as Darwin and actually sent a draft paper to Darwin for his review and comments. Darwin had the bitter realization that after all his work, he was about to be scooped. To his credit, Wallace respected Darwin's contributions and acknowledged that his conclusions did not pre-date those of Darwin. The compromise of both presenting at the same time preserved Darwin's claim to prescedence and acknowledged Wallace's own work.

A fitting compromise in my opinion. Darwin's work was not just earlier, but was far more comprehensive in scope. It lacked only the mechanism for inheritance... and that's another story.