expanded into a book of the same name. Shermer commented that this anticipated ideas about design in nature and directed evolution that would arise from religious traditions throughout the 20th century. Assessment of Wallace's role in history of evolutionary theory In many accounts of the development of evolutionary theory, Wallace is mentioned only in passing as simply being the stimulus to the publication of Darwin's own theory. In reality, Wallace developed his own distinct evolutionary views which diverged from Darwin's, and was considered by many (especially Darwin) to be a leading thinker on evolution in his day, whose ideas could not be ignored. One historian of science has pointed out that, through both private correspondence and published works, Darwin and Wallace exchanged knowledge and stimulated each other's ideas and theories over an extended period. Wallace is the most-cited naturalist in Darwin's Descent of Man, occasionally in strong disagreement. Darwin and Wallace agreed on the importance of natural selection, and some of the factors responsible for it: competition between species and geographical isolation. But Wallace believed that evolution had a purpose ("teleology") in maintaining species' fitness to their environment, whereas Darwin hesitated to attribute any purpose to a random natural process. Scientific discoveries since the 19th century support Darwin's viewpoint, by identifying additional mechanisms and triggers such as mutations triggered by environmental radiation or mutagenic chemicals. Wallace remained an ardent defender of natural selection for the rest of his life. By the 1880s, evolution was widely accepted in scientific circles, but natural selection less so. Wallace's 1889 Darwinism was a response to the scientific critics of natural selection. Of all Wallace's books, it is the most cited by scholarly publications. Other scientific contributions Biogeography and ecology In 1872, at the urging of many of his friends, including Darwin, Philip Sclater, and Alfred Newton, Wallace began research for a general review of the geographic distribution of animals. Initial progress was slow, in part because classification systems for many types of animals were in flux. He resumed the work in earnest in 1874 after the publication of a number of new works on classification. Extending the system developed by Sclater for birds—which divided the earth into six separate geographic regions for describing species distribution—to cover mammals, reptiles and insects as well, Wallace created the basis for the zoogeographic regions in use today. He discussed the factors then known to influence the current and