What was Darwin's opinion of Jews?
I've read quite a bit of Darwin's writings, including letters and notebooks, not just books published in his lifetime. I don't recall ever coming across the word "Jew".
Let's check the online database at The Darwin Correspondence Project. This is a very good index to all of Darwin's letters (both from and to).
Turns out there is an index term "human group: Jews". You can find the hits on this page:
http://darwin.lib.cam.ac.uk/perl/nav?class=human_group;term=Jews;dmode=dr
None of these letters happen (yet) to be available in full text. The database gives a summary of each letter.
The hits amount to two letters (one from and one to Darwin) which touch upon the heredity effect, or lack thereof, of circumcision. (Remember that the inheritence of aquired characteristics was still an issue at the time, and Darwin allowed for the "effects of use and disuse" as a subsidiary mechanism to natural selection.)
Finally there's a letter from Darwin to one Baruch Jakob Placzek (1835–1922) expressing an interest in reading his book. Comment in the database concerning the letter:
A learned Jew in Poland [Napthali Lewy?] has published a volume showing that evolution is an ancient belief.
Don't have time to google and figure out exactly what that means just now.