He was considered the 19th century's leading expert on the geographical distribution of animal species and is sometimes called the "father of biogeography". Wallace was one of the leading evolutionary thinkers of the 19th century and made many other contributions to the development of evolutionary theory besides being co-discoverer of natural selection. These included the concepts of warning colouration in animals, and reinforcement (sometimes known as the Wallace effect), a hypothesis on how natural selection could contribute to speciation by encouraging the development of barriers against hybridisation. Wallace's 1904 book "Man's Place in the Universe" was the first serious attempt by a biologist to evaluate the likelihood of life on other planets. He was also one of the first scientists to write a serious exploration of the subject of whether there was life on Mars.