I have dwelt at some length upon those portions of the 'Origin of Species' which deal in detail with the theory of natural selection, the chief contribution which Darwin made to the evolutionary movement, because it is impossible otherwise fully to understand the great gulf which separates his evolutionism from the earlier evolutionism of Lamarck and his followers. But it is impracticable here to give any idea of the immense wealth of example and illustration which Darwin brought to the elucidation of every part of his complex problem. In order to gain a full conception of this side of his nature, we must turn to the original treatise itself, and still more to the subsequent volumes in which the ground-work of observations and experiments on which he based his theory was more fully detailed for the specialist public. The remainder of Darwin's epoch-making work deals, strictly speaking, rather with the general theory of 'descent with modification' than with the special doctrine of natural selection. It restates and reinforces, by the light of the new additional concept, and with fuller facts and later knowledge, the four great arguments already known in favour of organic evolution as a whole, the argument from Geological Succession, the argument from Geographical Distribution, the argument from Embryological Development, and the argument from Classificatory Affinities. Each of these we may briefly summarise. The geological record is confessedly imperfect. At the time when Darwin first published the 'Origin of Species,' it had disclosed to our view comparatively few intermediate or transitional forms between the chief great classes of plants or animals; since that time, in singular confirmation of the Darwinian hypothesis, it has disclosed an immense number of such connecting types, amongst which may be more particularly noticed the 'missing links' between the birds and reptiles, the ancestors of the horses, the camels, and the pigs, and the common progenitor of the ruminants and the pachyderms, two great groups classed by Cuvier as distinct orders--all of which instances were incorporated by Darwin in later editions of his 'Origin of Species.' But, apart from these special and newly discovered cases, the whole general course of geological history 'agrees admirably with the theory of descent with modification through variation and natural selection.' The simpler animals of early times are followed by the more complex and more specialised animals of later geological periods. As each main group of animals appears upon the stage of life, it appears in a very central and 'generalised' form; as time goes on, we find its various members differing more and more widely from one another, and assuming more and more specialised adaptive forms. And in each country it is found, as a rule, that the extinct animals of the later formations bear a close general resemblance and relationship to the animals which now inhabit the same regions.