In 1859, when Charles Darwin's "On the Origin of Species" was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as, "... the most obvious and gravest objection which can be urged against my theory," but explained it by relating it to the extreme imperfection of the geological record. He noted the limited collections available at that time, but described the available information as showing patterns that followed from his theory of descent with modification through natural selection. Indeed, "Archaeopteryx" was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian dinosaurs and birds. Many more transitional fossils have been discovered since then, and there is now abundant evidence of how all classes of vertebrates are related, including many transitional fossils. Specific examples of class-level transitions are: tetrapods and fish, birds and dinosaurs, and mammals and "mammal-like reptiles".