Read no further if you want a definite answer to this question. It is a sort of detective story with clues scattered around. The circumstances surrounding the question however are so interesting since they involve two of the most important scientific publications of the 19th century.
The truly ground-breaking studies of Gregor Mendel were read before the Society for the Study of Natural Science of Brunn in 1865 entitled Versuche uber Pflanzen-Hybriden (Experiments in Plant Hybridization). Mendel ordered 40 reprints of his paper to send to famous European scientists; Darwin by then was certainly one of the most famous. Darwin's book on Origin of Species had been out for 6 years and was already in its 3rd edition. It had been translated into German, French, Dutch, Spanish, Polish and Russian.
Mendel had of course read and studied the Origin of Species in the German translation, Uber die Entstehung der Arten as soon as the second edition appeared in 1863. In his personal copy, he made many notes in the margin with his small and careful handwriting with double underlines of some of the text and even interspersed with the occasional exclamation mark. He bought most of Darwin's other works and studied them carefully making frequent annotations. So it would be natural for him to send Darwin, as an eminent English naturalist, one of his 40 reprints.
Of the 40 reprints of Mendel's article records exist that one was sent to each of the following scientists: von Marilaun, Kerner, Beijerinck, Boveri, Schleiden, and the Swiss botanist Karl Wilhelm von Nageli, now working in Munich. The last exchanged letters with Mendel over 7 years on the topic. More copies of the reprint were to be found in learned societies around Europe including the Royal Society, the Linnaean Society and the Greenwich Observatory …