The single most important question darwin was never able to answer was:

The most important question he couldn't answer is in fact how do organisms evolve. He knew that they evolve over a period of time but didn't know how it occurs and many after couldn't answer it either. Some even stipulated that evolution occurs through use or disuse of organs and the body develops around that, but they couldn't fully understand how it worked.

A, i have to keep writing because the website doesn't allow me to post one lettered responses