Cyrus Thomas (July 27, 1825–1910) was a U.S. ethnologist and entomologist prominent in the late 19th century and noted for his studies of the natural history of the American West. Thomas was born in Kingsport, Tennessee. He studied law and was a lawyer early on in his career up until 1865, serving as county clerk of Jackson County, Illinois in the early 1850s. In 1865 he entered the ministry of the Evangelical Lutheran Church. Four years later in 1869 he joined the expedition of Ferdinand Vandeveer Hayden, who had organized a scientific corps for the exploration of the Rocky Mountains. He was the agricultural statistician and entomologist on the Hayden Geological Survey of 1871 that led to the creation of Yellowstone National Park in 1872[1]. In 1873 he was appointed a professor of natural science at Southern Illinois University. In 1879 he was appointed state entomologist of Illinois. Thomas was a leading early student of archaeology of Native Americans. In the 1890s he played a crucial role in debunking the previously common theory that the Mound Builders were a separate lost race rather than ancestors of more recent American Indians. Though he debunked the theory of the "lost race", he also concluded that the mounds were built after Europeans made contact with the Native Americans, a view now long-since disproved following the advent of scientific dating techniques. He also made early studies of Maya hieroglyphics. Thomas also wrote on climatology, a new field in the 19th century. He was a leading proponent of the now-debunked theory known as Rain follows the plow, which stated that increased population and cultivation of the Great Plains would render the land lush and fertile.