A search for "Mendel" returns the following:
Focke, Wilhelm Olbers. 1881. Die Pflanzen-Mischlinge: ein Beitrag zur Biologie der Gewächse . Berlin: Gebrüder Borntraeger. [Mendel’s work mentioned on pp. 108 -111 but in Darwin’s copy these pages are uncut]
Hoffmann, Hermann. 1869. Untersuchungen zur Bestimmung des Werthes von Species und Varietät, ein Beitrag zur Kritik der Darwinischen Hypothese . Giessen: J. Ricker’sche Buchhandlung. [with quotation from Mendel 1865 on p. 52. No Darwin annotations are on this page.][Darwin Pamphlet Collection reviews]
Context: Charles Darwin published "On the Origin of Species" in 1859. The book contains a long appendix with unanswered questions, many of which would have been answered by the discovery of genetics, but the connection between natural selection and genetics wasn't really made until around the 1930s. However, the seminal paper on genetics, Gregor Mendel's "Experiments on Plant Hybridization", was published in 1866, so could in theory have been read by Charles Darwin (who died in 1882). As it was, Mendel's work languished in relative obscurity until the 20th century. I find that one of history's great "what ifs": what if work on genetics had begun nearly a century earlier (like what if Babbage's Analytical Engine had started the computer revolution nearly a century earlier)?