Each of these later works of Darwin's consists, as a rule, of an expansion of some single chapter or paragraph in the 'Origin of Species;' or, to speak more correctly, of an arrangement of the materials collected and the experiments designed for that particular portion of the great projected encyclopædia of evolutionism, of which the 'Origin of Species' itself was but a brief anticipatory summary or rough outline. Thus, the book on Orchids, published in 1862, is already foreshadowed in a part of the chapter on the Difficulties of the Theory of Natural Selection; the 'Movements and Habits of Climbing Plants' (1865) is briefly summarised by anticipation in the long section on Modes of Transition; the 'Variation of Animals and Plants under Domestication' (1868) consists of the vast array of _pièces justificatives_ for the first chapter of the 'Origin of Species;' and the germ of the 'Cross and Self Fertilisation' (1876) is to be seen in the passage 'On the Intercrossing of Individuals' in Chapter IV. of the same work. It was well indeed that Darwin began by publishing the shorter and more manageable abstract; the half, as the wise Greek proverb shrewdly remarks, is often more than the whole; and a world that eagerly devoured the first great deliverance of the Darwinian principle, might have stood aghast had it been asked to swallow it piecemeal in such gigantic treatises as those with which its author afterwards sought thrice to vanquish all his foes and thrice to slay the slain.