Darwin himself was aware that his theory was contradictory, inconsistent and unrealistic and expressed his concerns in his articles and letters. For instance, in a letter to his close friend Asa Gray, a Harvard biology professor, he wrote that his theory of evolution was only a speculation: I am quite conscious that my speculations run quite beyond the bounds of true science. (N.C. Gillespie, Charles Darwin and the Problem of Creation, 1979, p. 2.) After Darwin, many scientists confirmed that the theory of evolution has no value and that it is only speculative. One of the branches of science that confirmed this was paleontology. All fossils collected so far demonstrate that evolution has never occurred with any of them. One fossil displaying this fact is the 50-million-year-old elm leaf fossil pictured.