Accumulated theoretical and factual materials were brought by Charles Darwin (1809–1882) into a coherent system, later called Darwinism. Darwin found that hereditary variability and natural selection are the main factors driving evolution. In his theory, Darwin proceeded from the existence of two types of variability — definite and indefinite (hereditary). In the case when the current environmental conditions equally affect the change in all or most of the individuals, there is a certain variability, for example, the relationship between climate and the thickness of the skin or coat. A certain variability in the absence of an active factor, as a rule, is not hereditarily fixed in the next generation (that is, it is purely adaptive). Indefinite changes occur in individual individuals also under the influence of the external environment, but are random and hereditary in nature. If the resulting undefined changes are useful for a given species, then in the process of natural selection they are fixed, giving in the subsequent beginning to a new species. For example, if within a group of plants of the same species, individual plants with signs of cold tolerance have arisen under the influence of random causes, then when they fall into a colder climate, it is cold-resistant plants that survive, giving rise to new cold-resistant plants. Due to the continuous action of natural selection, animals or plants that are in different habitats, adapt to local conditions, change in different directions (in accordance with these conditions) and will diverge in their characteristics, or diverge. This divergence should lead to the formation of new forms, which in turn will also diverge, so that more or fewer new forms (species or varieties) arise from one initial form.
Darwin took the concept of a species already firmly established in biology and spoke of evolution as the origin of species. However, Darwin's theory only indicated the main factors of evolution and therefore gave only a general description of the process.
The writings of C. Darwin were highly appreciated by K. Marx and F. Engels. They noted that the evolutionary doctrine substantiated the possibility of using the historical method in application to nature, struck at religious ideas about the constancy and immutability of everything that exists, and on idealistic and metaphysical views on the development of the organic world. The writings of C. Darwin were named by F. Engels one of the greatest achievements of natural science of the 19th century.