The immediate result of these new statements and this fresh rationale of Knight's law was to bring down Sprengel forthwith from the top shelf, where he had languished ingloriously for seventy years, and to set a whole school of ardent botanical observers working hard in the lines he had laid down upon the mutual correlations of insects and flowers. A vast literature sprang up at once upon this enchanting and long-neglected subject, the most eminent workers in the rediscovered field being Delpino in Italy, Hildebrand and Hermann Müller in Germany, Axel in Sweden, Lubbock in England, and Fritz Müller in tropical South America. Darwin found the question, in fact, almost taken out of his hands before he had time himself to treat of it; for Hildebrand's chief work was published as early as 1867, while Axel's appeared in 1869, both of them several years earlier than Darwin's own final essay on the subject in the 'Effects of Cross and Self Fertilisation.' No statement, perhaps, could more clearly mark the enormous impetus given to researches in this direction than the fact that D'Arcy Thompson, in his appendix to Müller's splendid work on the 'Fertilisation of Flowers,' has collected a list of no less than eight hundred and fourteen separate works or important papers bearing on that special department of botany, almost all of them subsequent in date to the first publication of the 'Origin of Species.' So widely did the Darwinian wave extend, and so profoundly did it affect every minute point of biological and psychological investigation.
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.