Task: Question Generation
Input (English): In the United States, botanist Asa Gray an American colleague of Darwin negotiated with a Boston publisher for publication of an authorised American version, but learnt that two New York publishing firms were already planning to exploit the absence of international copyright to print Origin. Darwin was delighted by the popularity of the book, and asked Gray to keep any profits. Gray managed to negotiate a 5% royalty with Appleton's of New York, who got their edition out in mid January 1860, and the other two withdrew. In a May letter, Darwin mentioned a print run of 2,500 copies, but it is not clear if this referred to the first printing only as there were four that year.
Output (English): What was the print run that Darwin mentioned for On the Origin of Species?