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origin of man and his history." In 1871 the "Descent of Man" was published, and in 1872 "The Expression of the Emotions," which was originally intended to be only a chapter in the "Descent of Man."

II. THE SERIES OF BOTANICAL WORKS.

These dealt with the development of special questions and problems arising in direct connection with the "Origin of Species."

In 1862 appeared the "Various Contrivances by which Orchids are Fertilised by Insects," of which Professor Huxley says: "Whether we regard its theoretical significance, the excellence of the observations, and the ingenuity of the reasonings which it records, or the prodigious mass of subsequent investigation of which it has been the parent, it has no superior in point of importance."

From the first, Darwin was convinced that no theory could be satisfactory which did not explain the way in which mechanisms, involving adaptation of structure and function to the performance of certain operations, had come about. In 1793 Sprengel had established the fact that in a large number of cases a flower is a piece of mechanism, the object of which is to convert insect visitors into agents of fertilisation. What Sprengel did not do was to show that plants provided with such flowers gained any advantage thereby. Darwin worked at this subject for many years, from 1839 onwards, and showed cross-fertilisation to be favourable, and in many cases essential, to the fertility of the plant and the vigour of the offspring; and that all mechanisms favouring cross, and hindering self-fertilisation, give an advantage, and are hence preserved and improved through Natural Selection.

Orchids form an excellent group for the study of these mechanisms of cross-fertilisation, extraordinary modifications of which are found. The flowers are large and conspicuous, and many of them of singular form. The method of cross-fertilisation in Orchis mascula we considered in detail in a former lecture.* We must remember that all these details worked out by Darwin before he had himself seen any insects visit the particular orchids which he described, the necessary confirmation being supplied in 1873 by Hermann Müller in his work on the "Fertilisation of Flowers."

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In fine, an extraordinary diversity of devices, of marvellous interest, for ensuring cross-fertilisation, is met with among orchids. In some the pollen cases explode on being touched, and shoot their pollen at the insect. Some flowers are adapted for particular insects, and failure to become acclimatised is now recognised in many cases as being due, not to the plants being unable to live, but to the absence of the proper insects which alone can effect fertilisation, these as a rule being bees and butterflies. This subject may well be described as the Romance of Natural History, and Darwin says: "I never was more interested in any subject in all my life than in this of orchids."

* See p. 146.

In 1876 appeared "Cross and Self-Fertilisation in Plants," the outcome of a great series of laborious and difficult experiments on the fertilisation of plants, which occupied him for eleven years. The book on orchids showed how perfect are the means for ensuring cross-fertilisation; the new book demonstrated how important are the results. The investigations of which this book was the outcome were commenced with a simple experiment made for quite another purpose. Darwin raised two large beds, close together, of cross-fertilised and selffertilised seedlings of Linaria vulgaris, the common toad-flax, and his attention was aroused by the fact that the cross-fertilised plants, when fully grown, were plainly taller and more vigorous than the selffertilised ones.

In 1877 the "Forms of Flowers" was written, being the development of a short paper read before the Linnean Society in 1862 on the two forms, or dimorphic conditions, of Primula: one of which has short stamens situated in the middle of the tube of the corolla, and a long style, the stigma of which is on a level with the open flower. The other form has long stamens reaching to the centre of the flower, while the style is short and the stigma half-way down the corolla, at the same level as the stamens of the other form. (See Fig. 36.) Darwin showed that these flowers are barren if insects are prevented from visiting them, and further, that each form is almost sterile when fertilised by its own pollen, but each is fertile when fertilised with the pollen of the other. By referring to the figures it will be seen

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that insects visiting the flowers will carry pollen from the long anthers of the short-styled form to the stigma of the long-styled form, but would not reach the stigma of the short-styled form. Darwin showed that in this way more seeds were produced than by any other of the four possible unions. Thus,

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although both forms are hermaphrodite, they bear the same relation to each other as do the two sexes of an ordinary animal.

A few more botanical works must be mentioned, published in 1875, each of which was the outcome of many years of laborious observations and experiments; each breaking new ground and giving results of great importance and interest.

In "Climbing Plants," the spontaneous revolutions of tendrils and of the stems of climbing plants were investigated, and the causes producing them reduced to simple laws.

"Insectivorous Plants" was a typical piece of Darwinian work. "In 1860," he says, "I was idling and resting near Hartfield, where two species of Drosera abound, and I noticed that numerous insects had been entrapped by the leaves. I carried home some plants, and on giving them some insects, saw the movements of the tentacles, and this made me think it possible that the insects were caught for some special purpose. Fortunately, a crucial test occurred to me, that of placing a large number of leaves in various nitrogenous and nonnitrogenous fluids of equal density; and as soon as I found that the former alone excited energetic movements, it was obvious that here was a fine new field for investigation."

These researches showed that the plants secreted a digestive fluid like that of animals, and that insects were actually used as food.

The "Power of Movement in Plants," a tough piece of work, was published in 1880. "In accordance with the principles of Evolution it was impossible to account for climbing plants having been developed in so many widely different groups, unless all kinds of plants possess some slight power of movement of an analogous kind."

We come now to the last of his books, a singularly interesting and most characteristic piece of work. On the Ist of November 1837-i.e., about a year after his return from the voyage of the BeagleDarwin read a paper before the Geological Society

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