Darwin worked mostly on birds and he gradually understood how they adapt to different types of food through
selection of individuals with better-suited bill shapes. Then they differentiated into species because individuals with certain bill shapes had offspring only with individuals with similar bill shapes (even if interracial cross-breeds happened, they probably did not survive due to having intermediary bill shapes, not well suited to eat food typical for race of either parent).
So he understood that evolution works through selection (better adapted individuals have more offspring) and endogamy (closed gene pools).
But I don't think Darwin understood for example how evolutionary leaps work, or that he was even aware that such events happen.
On evolutionary leaps / evolutionary jumps :
http://sciencenordic.com/evolution-giant-leaps-or-half-measures
Evolution by leaps / jumps is a lot closer to creationism than the old theory about gradual process at a constant but always very slow pace.
Of course not all evolution happens by leaps, but new findings suggest that a lot of crucial parts of evolution did take place like this.