Darwinism, concept of evolution developed in the mid-19th cent. by Charles Robert Darwin. Darwin's meticulously documented observations led him to question the then current belief in special creation of each species. After years of studying and correlating the voluminous notes he had made as naturalist on H.M.S. Beagle, he was prompted by the submission (1858) of an almost identical theory by A. R. Wallace to present his evidence for the descent of all life from a common ancestral origin; his monumental Origin of Species was published in 1859. Darwin observed (as had Malthus) that although all organisms tend to reproduce in a geometrically increasing ratio, the numbers of a given species remain more or less constant. From this he deduced that there is a continuing struggle for existence, for survival. He pointed out the existence of variations—differences among members of the same species—and suggested that the variations that prove helpful to a plant or an animal in its struggle for existence better enable it to survive and reproduce. These favorable variations are thus transmitted to the offspring of the survivors and spread to the entire species over successive generations. This process he called the principle of natural selection (the expression "survival of the fittest" was later coined by Herbert Spencer). In the same way, sexual selection (factors influencing the choice of mates among animals) also plays a part. In developing his theory that the origin and diversification of species results from gradual accumulation of individual modifications, Darwin was greatly influenced by Sir Charles Lyell's treatment of the doctrine of uniformitarianism. Darwin's evidence for evolution rested on the data of comparative anatomy, especially the study of homologous structures in different species and of rudimentary (vestigial) organs; of the recapitulation of past racial history in individual embryonic development; of geographical distribution, extensively documented by Wallace; of the immense variety in forms of plants and animals (to the degree that often one species is not distinct from another); and, to a lesser degree, of paleontology. As originally formulated, Darwinism did not distinguish between acquired characteristics, which are not transmissible by heredity, and genetic variations, which are inheritable. Modern knowledge of heredity—especially the concept of mutation, which provides an explanation of how variations may arise—has supplemented and modified the theory, but in its basic outline Darwinism is now universally accepted by scientists.