The volume on the variation of animals and plants contained also Darwin's one solitary contribution to the pure speculative philosophy of life--his 'Provisional Hypothesis of Pangenesis,' by which he strove to account on philosophical principles for the general facts of physical and mental heredity. Not to mince matters, it was his one conspicuous failure, and is now pretty universally admitted as such. Let not the love of the biographer deceive us; Darwin was here attempting a task _ultra vires_. As already observed, his mind, vast as it was, leaned rather to the concrete than to the abstract side: he lacked the distinctively metaphysical and speculative twist. Strange to say, too, his abortive theory appeared some years later than Herbert Spencer's magnificent all-sided conception of 'Physiological Units,' put forth expressly to meet the self-same difficulty. But while Darwin's hypothesis is rudely materialistic, Herbert Spencer's is built up by an acute and subtle analytical perception of all the analogous facts in universal nature. It is a singular instance of a crude and essentially unphilosophic conception endeavouring to replace a finished and delicate philosophical idea. Earlier still, in 1862, Darwin had published his wonderful and fascinating book on the 'Fertilisation of Orchids.' It is delightful to contemplate the picture of the unruffled naturalist, in the midst of that universal storm of ecclesiastical obloquy and scientific enthusiasm which he had roused throughout Europe, sitting down calmly in his Kentish conservatory to watch the behaviour of catasetums and masdevallias, and to work out the details of his chosen subject, with that marvellous patience of which he was so great a master, in the pettiest minutiæ of fertilisation as displayed by a single highly developed family of plants. Whoever wishes to learn the full profundity of Darwin's researches, into every point that he set himself to investigate, cannot do better than turn for a while to the consideration of that exquisite treatise on one of the quaintest fairylands of science. He will there learn by what an extraordinary wealth of cunning devices natural selection has ensured the due conveyance of the fecundating pollen from stamens to stigmas within the limits of a single group of vegetable organisms. Here the fertilising mass is gummed automatically between the eyes of the exploring bee, and then bent round by the drying of its stalk so as to come in contact with the stigmatic surface. There the pollen club is jerked out elastically by a sensitive fibre, and actually flung by its irritable antennæ at the unconscious head of the fertilising insect. In one case, the lip of the flower secretes moisture and forms a sort of cold bath, which wets the wings of the bees, so compelling them to creep out of the bucket by a passage close to the anthers and stigma; in another case, the honey is concealed at the bottom of so long a tube that only the proper fertilising moth