When I read about the modern evolutionary synthesis, the text said that it showed both ideas to be compatible, once thought to be contradictory. Would it be because Mendel described an element of "randomness" in character transmission, as opposed to the mechanism of natural selection originally described by Darwin, or is it something else? I do not yet have a big knowledge on evolution, so pardon any mistakes.
It's thought that Mendel's communications were anti-Darwinist, and some argue that his original paper was written specifically as a refutation of Darwin's descent with modification. Mendel believed species were created by God and not Darwinian evolution. Because of this, when he found a law defining constant probabilities of heredity, he saw it as more of an argument for the consitency of genetic information and thus species. To him this was in contrast to the ability of heritable traits to vary slightly in progeny, leading to selective advantages. Source I paraphrased: http://jhered.oxfordjournals.org/content/87/3/205.full.pdf