He noted, for example, that certain fossils of supposedly extinct species closely resembled living species in the same geographical area. In the Galapagos Islands, off the coast of Ecuador, he also observed that each island supported its own form of tortoise, mockingbird, and finch; the various forms were closely related but differed in structure and eating habits from island to island. Both observations raised the question, for Darwin, of possible links between distinct but similar species. After returning to England in 1836, Darwin began recording his ideas about changeability of species in his Notebooks on the Transmutation of Species. Darwin's explanation for how organisms evolved was brought into sharp focus after he read An Essay on the Principle of Population, by the British economist Thomas Robert Malthus, who explained how human populations remain in balance. Malthus argued that any increase in the availability of food for basic human survival could not match the geometrical rate of population growth. The latter, therefore, had to be checked by natural limitations such as famine and disease, or by social actions such as war. Darwin immediately applied Malthus's argument to animals and plants, and by 1838 he had arrived at a sketch of a theory of evolution through natural selection. .
For the next two decades he worked on his theory. Origin of Species was not published until 1859 and this together with the long delay of the publishing of Descent of Man made everyone think that the words were not his alone. It was only in the Descent of Man published in 1871 that Darwin explained his theory of how natural selection worked. (Stevens).
Natural Selection is the process by which environmental effects lead to varying degrees of reproductive success among individuals of a population of organisms with different hereditary characters, or traits. The characters that inhibit reproductive success decrease in frequency from generation to generation.
