Many of the skeletal features of Archaeopteryx, particularly the feathers,can be difficult to see to the untrained eye. This scientific drawing labels the subtle features present on the Thermopolis Specimen. In 1859, Charles Darwin published On the Origin of Species, presenting his hypothesis for the mechanisms of evolution. In his manuscript, Darwin theorized in the existence of "transitional forms," a prehistoric animal seemingly caught in the midst of evolving into a new species, but none existed at the time his book was published. Four years later, the first Archaeopteryx was discovered in Germany. The Thermopolis Specimen itself was discovered near Eichstatt in Germany. Since it is part of the collection of the Wyoming Dinosaur Center - in Thermopolis, WY - it is referred to as the Thermopolis Specimen.