The last eleven years of Darwin\'s life were spent in enforcing and developing the principles already reached, and in enjoying the almost unchequered progress of the revolution he had so unconsciously to himself succeeded in inaugurating.

Only one year elapsed between the publication of the \'Descent of Man\' and that of its next important successor, the \'Expression of the Emotions.\' The occasion of this learned and bulky treatise in itself stands as an immortal proof of the conscientious way in which Darwin went to work to anticipate the slightest and most comparatively impertinent possible objections to his main theories. Sir Charles Bell, in one of the quaintly antiquated Bridgwater treatises—those marvellous monuments of sadly misplaced teleological ingenuity—had maintained that man was endowed with sundry small facial muscles solely for the sake of expressing his emotions. This view was so obviously opposed to the belief in the descent of man from some lower form, \'that,\' says Darwin, \'it was necessary for me to consider it;\' and so he did, in a lengthy work, where the whole subject is exhaustively treated, and[Pg 156] Bell\'s idea is completely pulverised by the apt allegation of analogous expressions in the animal world. In his old age Darwin grew, in fact, only the more ceaselessly and wonderfully industrious. In 1875, after three years of comparative silence, came the \'Insectivorous Plants,\' a work full of minute observation on the habits and manners of the sundew, the butterwort, the Venus\'s fly-catcher, and the various heterogeneous bog-haunting species known by the common name of pitcher plants. The bare mass and weight of the facts which Darwin had collected for the \'Origin of Species\' might well-nigh have stifled the very existence of that marvellous book: it was lucky that the premature publication of Wallace\'s paper compelled him to hurry on his \'brief abstract,\' for if he had waited to select and arrange the whole series of observations that he finally published in his various later justificatory volumes, we might have looked in vain for the great systematic and organising work, which would no doubt have been \'surcharged with its own weight, and strangled with its waste fertility.\' But the task that he himself best loved was to watch in minute detail the principles whose secret he had penetrated, and whose reserve he had broken, working themselves out before his very eyes, naked and not ashamed—to catch Act?on-like the undraped form of nature herself in the actual process of her inmost being. He could patiently observe the red and slimy hair-glands of the drosera closing slowly and remorselessly round the insect prey, and sucking from their bodies with sensitive tentacles the protoplasmic juices denied to its leaves by the poor and boggy soil, on which alone its scanty rootlets can[Pg 157] properly thrive. He could watch the butterwort curving round the edges of its wan green foliage upon the captured limbs of fly or aphis. He could note how the serried mass of finger-like processes in the utricles of the bladderwort slowly absorb organic matter from the larva of a gnat, or the minute water-insects entangled within its living and almost animated lobster-pot. He could track the long line of treacherous honey-glands by which the sarracenia entices flies into the festering manure-wells of its sticky pitchers. The minuteness and skill of all his observations on these lesser problems of natural selection inevitably inspired faith among outsiders in the cautious judgment of the observer and experimenter; and day by day throughout his later years the evidence of the popular acceptance of his doctrine, and of the dying away of the general ridicule with which it was first received by the unlearned public, was very gratifying to the great naturalist.

A year later, in 1876, came the \'Effects of Cross and Self Fertilisation in the Vegetable Kingdom.\' So far as regarded the world of plants, especially with respect to its higher divisions, this work was of immense theoretical importance; and it also cast a wonderful side-light upon the nature of that strange distinction of sex which occurs both in the vegetable and animal kingdom, and in each is the concomitant—one might almost say the necessary concomitant—of high development and complex organisation. The great result attained by Darwin in his long and toilsome series of experiments on this interesting subject was the splendid proof of the law that cross-fertilisation produces finer and healthier offspring, while continuous self-fertilisation[Pg 158] tends in the long run to degradation, degeneration, and final extinction.

Here as elsewhere, however, Darwin\'s principle does not spring spontaneous, like Athene from the head of Zeus, a goddess full-formed, uncaused, inexplicable: it arises gradually by a slow process of development and modification from the previous investigations of earlier biologists. At the close of the last century, in the terrible year of upheaval 1793, a quiet German botanist, Christian Konrad Sprengel by name, published at Berlin his long unheeded but intensely interesting work on the \'Fertilisation of Flowers.\' In the summer of 1789, while all Europe was ablaze with the news that the Bastille had been stormed, and a new era of humanity begun, the calm and peaceful Pomeranian observer was noting in his own garden the curious fact that many flowers are incapable of being fertilised without the assistance of flying insects, which carry pollen from the stamens of one blossom to the sensitive surface or ovary of the next. Hence he concluded that the secretion of honey or nectar in flowers, the contrivances by which it is protected from rain, the bright hues or lines of the corolla, and the sweet perfume distilled by the blossoms, are all so many cunning devices of nature to ensure fertilisation by the insect-visitors. Moreover, Sprengel observed that many flowers are of one sex only, and that in several others the sexes do not mature simultaneously; \'so that,\' said he, \'nature seems to intend that no flower shall be fertilised by means of its own pollen.\' Indeed, in some instances, as he showed by experiments upon the yellow day lily, plants impregnated from their own stamens cannot be made to set[Pg 159] seed at all. \'So near,\' says his able successor, Hermann Müller, \'was Sprengel to the distinct recognition of the fact that self-fertilisation leads to worse results than cross-fertilisation, and that all the arrangements which favour insect-visits are of value to the plant itself, simply because the insect-visitors effect cross-fertilisation!\' As in most other anticipatory cases, however, it must be here remarked that Sprengel\'s idea was wholly teleological: he conceived of nature as animated by a direct informing principle, which deliberately aimed at a particular result; whereas Darwin rather came to the conclusion that cross-fertilisation as a matter of fact does actually produce beneficial results, and that therefore those plants which varied most in the direction of arrangements for favouring insect-visits were likely to be exceptionally fortunate in the struggle for existence against competitors otherwise arranged. It is just the usual Darwinian substitution of an efficient for a final cause.

Even before Sprengel, K?lreuter had recognised, in 1761, that self-fertilisation was avoided in nature; and his observations and experiments on intercrossing and on hybridism were largely relied upon by Darwin himself, to whom they suggested at an early period many fruitful lines of original investigation. In 1799, again, Andrew Knight, following up the same line of thought in England as Sprengel in Germany, declared as the result of his close experiments upon the garden pea, that no plant ever fertilises itself for a perpetuity of generations. But Knight\'s law, not being brought into causal connection with any great fundamental principle of nature, was almost entirely overlooked by the[Pg 160] scientific world until the publication of Darwin\'s \'Origin of Species,\' half a century later. The same neglect also overtook Sprengel\'s immensely interesting and curious work on fertilisation of flowers. The world, in fact, was not yet ready for the separate treatment of functional problems connected with the interrelations of organic beings; so Knight and Sprengel were laid aside unnoticed on the dusty top bookshelves of public libraries, while the dry classificatory and systematic biology of the moment had it ail its own way for the time being on the centre reading-tables. So many separate and independent strands of thought does it ultimately require to make up the grand final generalisation which the outer world attributes in its totality to the one supreme organising intelligence.

But in the \'Origin of Species\' itself Darwin reiterated and emphasised Knight\'s law as a general and all-pervading principle of nature, placing it at the same time on broader and surer biological foundations by affiliating it intimately upon his own great illuminating and unifying doctrine of natural selection. He also soon after rescued from oblivion Sprengel\'s curious and fairy-like book, showing in full detail in his work on orchids the wonderful contrivances by which flowers seek to attract and to secure the assistance of insects for the impregnation of their embryo seeds. In the \'Variation of Animals and Plants under Domestication,\' he further showed that breeding in-and-in diminishes the strength and productiveness of the offspring; while crossing with another stock produces, on the contrary, the best possible physical results in both directions. And now at last, in the \'Effects of[Pg 161] Cross and Self fertilisation,\' he proved by careful and frequently repeated experiments that a constant infusion of fresh blood (so to speak) is essential to the production of the healthiest offspring. In the words of his own emphatic summing up, \'Nature abhors perpetual self-fertilisation.\'

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[Pg 162] 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 encyclop?dia 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.[Pg 163]

Yet, with each fresh manifestation of Darwin\'s inexhaustible resources, on the other hand, the opposition to his principles grew feebler and feebler, and the universality of their acceptance more and more pronounced, till at last, among biologists at least, not to be a Darwinian was equivalent to being hopelessly left behind by the general onward movement of the time. In 1874 Tyndall delivered his famous address at the Belfast meeting of the British Association; and in 1877, from the same presidential chair at Plymouth, Allen Thomson, long reputed a doubtful waverer, enforced his cordial adhesion to the Darwinian principles by his inaugural discourse on \'The Development of the Forms of Animal Life.\' A new generation of active workers, trained up from the first in the evolutionary school, like Romanes, Ray Lankester, Thistleton Dyer, Balfour, Sully, and Moggridge, had now risen gradually around the great master; and in every direction he could see the seed he had himself planted being watered and nourished in fresh soil by a hundred ardent and enthusiastic young disciples. Even in France, ever irresponsive to the touch of new ideas of alien origin, Colonel Moulinié\'s admirable and sympathetic translations were beginning to win over to the evolutionary creed many rising workers; while in Germany, Victor Carus\'s excellent versions had from the very first brought in the enthusiastic Teutonic biologists with a congenial \'swarmery\' to the camp of the Darwinians. Correspondents from every part of the world kept pressing fresh facts and fresh applications upon the founder of the faith; and Darwin saw his own work so fast being taken out of his hands by specialist disciples that he[Pg 164] abandoned entirely his original intention of publishing in detail the basis of his first book, and contented himself instead with tracing out minutely some minor portions of his contemplated task as specimens of evolutionary method.

In 1877, in pursuance of this changed purpose, Darwin published his book on \'Forms of Flowers,\' in which he dealt closely with the old problem of differently shaped blossoms on plants of the same species. It had long been known, to take a single example, that primroses existed in two forms, the pin-eyed and the thrum-eyed, of which the former has the pin-like summit of the pistil at the top of the tube, and the stamens concealed half way down its throat; while in the latter these relative positions are exactly reversed, the stamens answering in place to the pistil of the alternative form with geometrical accuracy. As early as 1862 Darwin had shown, in the \'Journal of the Linnean Society,\' that this curious arrangement owed its development to the greater security which it afforded for cross-fertilisation, because in this way each flower had to be impregnated with the pollen, from a totally distinct blossom, growing on a different individual plant. In a series of successive papers read before the same Society in the years between 1863 and 1868, he had extended a similar course of explanation to the multiform flowers of the flaxes, the loosestrifes, the featherfoil, the auricula, the buckbean, and several other well-known plants. At last, in 1877, he gathered together into one of the now familiar green-covered volumes the whole of his observations on this strange peculiarity, and proved by abundant illustration and[Pg 165] experiment that the diversity of form is always due through natural selection to the advantage gained by perfect security of cross-fertilisation, resulting as it invariably does in the production of the finest, strongest, and most successful seedlings. Any variation, however peculiar, which helps to ensure this constant infusion of fresh blood is certain to be favoured in the struggle for life, owing to the superior vitality of the stock it begets. But it is worthy of notice, as showing the extreme minuteness and exhaustiveness of Darwin\'s method on the small scale, side by side with his extraordinary and unusual power of rising to the very highest and grandest generalisations, that the volume which he devoted to the elucidation of this minor factor in the question of hereditary advantages runs to nearly as many pages as the last edition of the \'Origin of Species\' itself. So great was the wealth of observation and experiment which he could lavish upon the solution of a single, small, incidental problem.

Even fuller in minute original research was the work which Darwin published in 1880, on \'The Power of Movement in Plants,\' detailing the result of innumerable observations on the seemingly irresponsible yet almost purposive rotations of the growing rootlets and young stems of peas and climbers. Anyone who wi............