This celebrated book, first published in 1798, had attempted to describe the forces which ensure the multiplication, or check the increase of population. The proposition laid down by Malthus was that population tends to vary with the means of subsistence. He had studied his problem from a social or political point of view, but the same principle was seen by Darwin to apply to all living creatures. Two forces are seen everywhere in conflict: (a) the luxuriant powers of reproduction possessed by and exercised by each species; (b) the difficulties and obstacles by which the species tend to be eliminated. The contest between the powers of reproduction and those of elimination--this "over-production" and "crowding-out"--is what was afterwards termed the "struggle for existence." "NATURAL SELECTION."--Darwin's momentous theory was that this struggle, proceeding for untold ages, had resulted in the continual formation of new species. Granted that the numerous offspring of any individual member of a species tend to vary, those variations survive which happen to be best fitted to cope with the environment. These in their turn leave offspring, the variations and the selections are repeated, and so on _ad infinitum_; and the result is that entirely new species are formed by a long process of insignificant changes. This, briefly put, is the celebrated theory of "Natural Selection." The habit of scientific caution was characteristic of Darwin, who at first would not write down "even the briefest sketch" of his hypothesis, but devoted nearly twenty years to the accumulation of evidential _data_. His friends continually warned him that he would be forestalled, and this actually occurred, as is well known, in 1858, when the book which was to give the new theory to the world was already half written. The naturalist, Alfred Russell Wallace, on a collecting expedition in the East Indies, "in a flash of insight" while sick with fever, found the same solution of the mystery that had puzzled biologists so long. Wallace's letter to Darwin, containing the abstract of his theory, came "like a bolt from the blue." The behaviour of the two men was worthy of the highest traditions of scientific research. The matter was put into the hands of Lyell, and Wallace's paper, together with certain extracts from Darwin's unpublished notes, were read before the Linnean Society, and the preparation of Darwin's book was hurried on. In November, 1859, _The Origin of Species_ was published.