So matters stood when Darwin’s Origin of Species carried the argument for evolution a long step further. Accepting fully the views of Lamarck, Darwin attempted, by his doctrine of Natural Selection, first to reinforce those views, secondly to explain much that they could not be made to cover. It is plain that _if we assume the existence of a severe competition for livelihood among the members of a species_, any favourable variations of structure or instinct which may occur among certain members of the species will give their type an advantage over the normal type in the struggle for existence. They will, on the average, live longer and produce more offspring. Ultimately, as the struggle for life is always most severe among nearly related organisms, which seek a living from the same sources, the less perfectly equipped type will be extinguished, and so on, until a species exhibiting the most complete form of adaptation has been evolved. The variations on which Natural Selection has to work are produced, according to Darwin, not only by the exercise of particular organs as in Lamarck’s theory, but also and more potently by “innate variations” originating from unascertained causes in the reproductive cells. Variations, it is indisputable, are always occurring; probably no two members of any species exactly resemble each other. Among low and primitive organisms, such as the _Foraminifera_, Dr. W. B. Carpenter (I quote from A. R. Wallace’s Darwinism) found, on careful examination, the range of variation so great that characteristics typical not merely of species but of genera and even of orders were liable to vary,[55] while at the other end of evolution, in man, to give only one instance, Mr. J. Wood is stated by Darwin to have observed no less than five hundred and fifty-eight variations in the muscular structure of thirty-six subjects examined.[56] The cause of these variations is often quite obscure, but it is certain that some kinds of them are capable of arising as the natural response of the organism to changed conditions of food or habitat Conditions such as these, affecting the whole constitution of the organism, have been proved capable of affecting the reproductive cells, and thus of giving rise to hereditary characteristics. Natural Selection, then, by preserving and encouraging the better fitted as opposed to the less fit, acts as a spur to the Lamarckian principles of development by exercise of function, while it also lays hold of and intensifies all kinds of other favourable variations occurring either casually or in consequence of change of habitat, and weeds out the types in which such variations happen to be unfavourable. According to Darwin, therefore, given (1) constant variations of structure arising from use, disuse, or from other known or unknown causes, (2) the capacity to transmit by inheritance these variations _whether innate or acquired_, (3) a constant struggle for