Well, here's a fine how-d'you do.
The Image Base of the Fine Arts Museums of San Francisco, a massive art database which I would have normally been inclined to trust implicitly, has this image:
John Collier, artist
English, 1850 -
G. Sidney hunt, 19th - 20th century
but, at Dr. Matthew Eddy's History of Science HPSM MA Module Resource Webpage, we see the same image - this time with the notation:
Charles Darwin
Naturalist and Gentleman
By John Collier, 1883
National Portrait Gallery
and it is, in fact, no other than Charles Darwin, himself, there portrayed.