well this is funny. My first thought was INTJ because I thought he had the "beating the dead horse" writing style (getting really deep on every single point and reiterating them from multiple perspectives) that Ni users tend to have.
Not having read much on Darwin's biography, I think Jung calls him an extraverted thinker. Seeing how Jung uses those terms he seems to consider Darwin an ENTP.
Alfred Wallace was the ENTP...he co discovered evolution. Darwin was sooooo different. Very J, yet very intuitive....INTJ is the best bet (and ive studied this subject a lot). Remember, it was basically known as "natural history" to study this stuff back then...not super Ti theoretical like the Theory of Relativity. We are talking about tons of stupid little notes (again "Natural history"), that then basically were ingested --> 500 page abstract to then "tease out" the theory of natural selection. Its Ni gone wild!
What makes everyone think Darwin was an introvert? I'd heard his ability to stay on track and attend classes was woeful. He jumped on a ship and explored the world meeting new races and seeing new habitats. He was driven by what was around him more than anything. He wasn't looking inwards for ideas.
Freude, schöner Götterfunken Tochter aus Elysium, Wir betreten feuertrunken, Himmlische, dein Heiligtum! Deine Zauber binden wieder Was die Mode streng geteilt; Alle Menschen werden Brüder, Wo dein sanfter Flügel weilt.