Illustration of four breeds of sheep (Ovis aries) from plates in Jardine's 'The Naturalist's Library' series, specifically 'The Natural History of the Ruminating Animals, Goats, Sheep, Wild and Domestic Cattle &C' published by W.H Lizars 1836. Charles Darwin understood the human selection, that created these various forms of sheep, to be heuristically analogous to the natural selection that changed and originated new species over geological time. The idea formed the opening for his book 'The Origin of Species' (1859) and was further dealt with by his subsequent 'The Variation of Animals and Plants Under Domestication' published by John Murray 1868.