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the model, to depart from which will but sow hindrances in the path of scientific progress.
The order to which the butterwort and the bladderworts belong also afforded valuable results. The leaf of the butterwort bears glandular hairs, and its margins curve inwards when excited by contact of various bodies, especially living insects, and, at the same time, these are caught in the viscid secretion of the glands, and their juices absorbed by the plant. The bladderworts are even more remarkably constructed, for they have a portion of their leaves developed into subaqueous bladders, with a narrow entrance beneath, defended by a complex valve, which facilitates the entrance of water insects or crustaceans, but prevents their exit. The whole interior of the bladder is lined with transparent fourbranched protoplasmic hairs, but nevertheless the bladderwort is unlike the preceding plants in having no power of digesting its prey, however long it may remain in captivity. Yet there is no doubt that the imprisoned creatures do decay in their watery cell, and that the hairs just described absorb the products of their decay.
Such is a brief account of Darwin's work on “ Insectivorous Plants." With his characteristic expressions he acknowledges the valuable aid given him by Professor Burdon-Sanderson, and by his son, Mr. Francis Darwin. The former was enabled to give the first brief account of the process of digestion in these plants, as observed by Darwin, in a lecture before the Royal Institution, in June, 1874, and Dr. (now Sir Joseph) Hooker called general public notice to the subject of Carnivorous Plants in his lecture before the British Association at
Belfast in the same year: so that a thoroughly awakened attention was given to this new work from Darwin's pen. The public and the scientific world learnt to appreciate yet more keenly his varied talent, his long patience, his reserve of power; and thence dated very definitely a general appreciation of the fundamental unity of the animal and plant kingdoms, seeing that the salient faculties of digestion, of purposive locomotion, of rapid communication and consentaneous action were no longer restricted to animals, but were possessed in a high degree by plants also. Eager followers soon brought forward further proofs of unity of functions in the two kingdoms, and of reciprocal combinations between them, and now no one in the slightest degree acquainted with modern biology doubts that life is at bottom one phenomenon, shared equally and manifested in essentially the same modes by the living substance of plant and animal alike.
Following "Insectivorous Plants" came "The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom," in 1876. Darwin had led the way in the study of this subject by his book on Orchids, and his lead had been excellently followed by Hildebrand, Hermann Müller, Sir John Lubbock, and others. The path having been indicated, it had appeared comparatively easy for botanists to follow it up. But there yet remained a region of experimental inquiry which it required Darwin's patience and ingenuity to master and to expound conclusively. Although it might be practically granted that natural selection developed a process because advantage was gained by it, was it possible to demonstrate that
flowers cross-fertilised bear more and larger seeds, which produce healthier offspring than those fertilised from their own pollen? This Darwin set himself exhaustively to do. For more than a dozen years after his book on orchids appeared, unwearied experiments on plants were progressing, and nature was being questioned acutely, untiringly. Competitive germination was carried on. The two classes of seeds were placed on damp sand in a warm room. As often as a pair germinated at the same time, they were planted on opposite sides of the same pot, with a partition between. Besides these pairs of competitors, others were planted in beds, so that the descendants of the crossed and self-fertilised flowers might compete. The resulting seeds were carefully compared, and their produce again compared. Species were selected from widely distinct families, inhabiting various countries. From a large number of plants, when insects were quite excluded by a thin net covering the plant, few or no seeds were produced. The extent of transport of pollen by insects was unveiled, and the relation between the structure, odour, and conspicuousness of flowers, the visits of insects, and the advantages of cross-fertilisation was shown. "We certainly," says Darwin, "owe the beauty and odour of our flowers, and the storage of a large supply of honey, to the existence of insects." The multitude of facts gathered about insects could only have been discovered and rightly appreciated by one who was a true entomologist as well as a botanist.
In the last chapter of the book the author discusses with remarkable power the causes of the phenomena he
has discovered. He believes that the favourable effects of crossing are due to the parents having been subjected to diverse conditions; but what the precise benefit is, or how it can operate so as to render the offspring more healthy and vigorous, he cannot discern. "And so it is," he observes, "with many other facts, which are so obscure that we stand in awe before the mystery of life." So it is. The man who probably understood nature better than any man who has ever lived, who had not only asked her multitudinous questions, but to whom very many answers had been undoubtedly vouchsafed in response to his persevering, humble, diligent, acute questioning, acknowledges that he knows little; that much remains a mystery. But from all we know of him, from his books, his letters, his friends, his was the joy of a soul in sympathy with the master power of the universe. He marched continually on the confines of the unknown, and to him was granted the felicity of largely extending the boundaries of the known.
Again, in 1877, a new work proceeded from Darwin's pen, "The Different Forms of Flowers in Plants of the same Species," dedicated to Professor Asa Gray. It gathered up the contents of numerous papers read before the Linnean Society, with later additions, and showed conclusively how many plants possess distinctive forms of flowers in the same species, adapted to, and in some cases absolutely necessitating, reciprocal fertilisation through the visits of insects. It gave evidence of all the well-known Darwinian characteristics of long-continued labour, thought, and experiment.
In 1880 "The Power of Movement in Plants" was ex
emplified in a fresh volume, in which the veteran was materially assisted by his son, Mr. Francis Darwin. Its object was to describe and connect together several large classes of movements, common to almost all plants. The surprising fact was established, that all the parts or organs of plants, whilst they continue to grow, are continually revolving, or circumnutating as Darwin called it. This movement commences even before the young seedling has broken through the ground. The combination of this with the effects of gravity and light explains countless phenomena in the life of plants. The tip of the rootlet is thus enabled to penetrate the ground, and it is proved to be more sensitive than the most delicate tendril. Movement goes on through all stages of life. Every growing shoot of a great tree is continually describing small ellipses; the tip of every rootlet endeavours to do the same. The changes of position of leaves and of climbing plants, and the sleep of leaves are all brought under this great principle of circuminutation. It is impossible in reading the book not to be struck with the great resemblance between the movements of plants and many of the actions performed unconsciously by the lower animals. "With plants an astonishingly small stimulus suffices, and, even with allied plants, one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals, and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves. Yet plants do not