The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century. Darwin's "On the Origin of Species", published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine natural selection producing the variation seen within genera and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the London specimen of "Archaeopteryx" in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the class of the highly derived birds, and that of the more primitive reptiles. In a letter to Darwin, the palaeontologist Hugh Falconer wrote: