evident. Use and disuse of parts have caused the greater development or the reduction of these parts. Homologous parts tend to vary in the same manner, and when adjacent, tend to cohere. Organic beings low in the scale are more variable than those standing higher. Rudimentary organs are variable.

The existence of lowly organized forms is briefly discussed :

"In some cases lowly organized forms appear to have been preserved to the present day from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of favourable variations arising. In some cases, variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate. In some few cases there has been what we must call retrogression of organization. But the main cause lies in the fact that under very simple conditions of life, a high organization would be of no service-possibly would be of actual disservice, as being of a more delicate nature and more liable to be put out of order and injured."

The difficulties of the theory are frankly considered. The chief of these are the absence of transitions, the existence of instincts and organs of extreme complexity, and the fact that species when crossed are either sterile or produce sterile offspring, whereas varieties when crossed breed with unimpaired fertility. It is shown that instincts, however originally acquired, are inherited, and are variable; hence they are liable to be improved and specialized by natural selection.

Another great objection to the acceptance of the theory of evolution is the absence of intermediate links among fossil forms, but Darwin shows that the geological record is necessarily very imperfect, and the intermediate links, which do exist among fossil forms,

cannot be accounted for at all on any other hypothesis than that of descent with modification.

The phenomena of geographical distribution are next reviewed, and the impossibility of accounting for them on any other hypothesis than that of evolution insisted upon. Finally, the classification of organisms, the facts of embryology and morphology are reviewed, and all shown to support the theory. The arguments drawn from these various departments of biological science are not essentially different in Darwin's hands from what they were when treated by his predecessors. But since the time of Lamarck, knowledge in these departments had enormously increased, especially in that of embryology. Darwin presented the mass of evidence with a force derived from the breadth and diversity of his knowledge; and it may be said that these arguments give the momentum to his demonstration, while the point of the instrument is furnished by his theory of the method of evolution.

Darwin considered the "Origin of Species" as only a general sketch of this theory and the facts which support it. He originally intended to treat each one of its chapters in greater detail, and so expand it into a volume. This plan was never carried out to the letter; the volumes on the variation of animals and plants under domestication appeared, but the proposed works on variation of organisms in a state of nature, on the struggle for existence and natural selection were never written. Their place was supplied by the succession of works, describing beautiful and elaborate researches into the most diverse classes of biological phenomena, which will ever remain among the classics of the literature of science. Some of these volumes are monuments rather of bibliographical than empirical

inquiry they all have a close relation to the question of organic evolution, but they do not describe investigations into the process of modification, into the causes of variation or of heredity. They enlarge the boundaries of knowledge, and bring into the field of mental vision new phenomena to be explained by the general theory of biology. Darwin acknowledged that his explanation of organic evolution was not exhaustive, but he devoted himself to the application of the theory he had adopted rather than to the deeper investigation of the premises on which it was based. He had no other choice; he was not a specialist in physiological studies, and even if he had been, the physiology of his day could have given him little guidance. In its present state physiology can say little definite on the fundamental problems of heredity and variation, they remain to be attacked by the physiology of the future.

The two volumes on the "Variation of Animals and Plants under Domestication" were published in 1868. In this case, the original intention to publish a detailed examination of each class of facts embraced in the theory of natural selection was completely carried out; we have a study as exhaustive as the wonderful working power of Darwin could make it, of the process of modification which can be actually traced in the history of cultivated forms of life. In the introduction the whole theory of evolution is restated in outline. The links in the argument are very much what they were before. Man unintentionally exposes his animals and plants to various conditions of life, and variability supervenes, which he cannot prevent or check. The variations are inherited, and by selecting certain variations to propagate from, man is able to modify species in almost any direction he pleases. Similarly, variations

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undoubtedly occur in a state of nature, and only those in which the variation is an improvement in relation to the conditions of life, being able to survive, natural selection produces modification and divergence. The first volume is simply and solely a collection of facts; it describes the variations which have occurred in cultivated organisms, and the effects which have been produced by selecting the variations. In the second volume we have a study of the phenomena of inheritance, selection, the causes of variation, and the laws of variation. In the chapter on the causes of variation, it is distinctly explained that "variations of all kinds and degrees are directly or indirectly caused by the conditions of life to which each being, and more especially its ancestors, have been exposed." Use and disuse are shown to be important causes of modification, by increasing or decreasing an organ; the structure of an organ changes when the use of it, that is to say, its function, changes.

In this work, Darwin attempted to penetrate the mystery of the process of heredity by the light of imagination. It contains his hypothesis of pangenesis. Here for once he deserted his usual method of proceeding always from the known to the unknown, from the familiar to the apparently inexplicable. His gemmules have no relation to any known facts concerning the structure or functions of organisms. The reproductive elements are known to be unspecialized cells which are nourished, like the rest of the cells in the organism, by the circulating fluids. We know that the reproductive cells have the property of going through the same series of changes as those exhibited by the parent, and we know that this property must have been impressed upon them in some way or other while in

the body of the parent. But our knowledge is not made clearer by a hypothesis of gemmules which build up the reproductive cells. It is certain that the cells are not built up by composition from smaller units, unless the supposed units are the molecules of the substances by which the reproductive cells are nourished.

The "Fertilization of Orchids" preceded "Animals and Plants under Domestication," although it has less immediate connection with the doctrine of evolution. To the general reader it is perhaps the most attractive of all Darwin's works. The investigation of the way in which cross-fertilization is effected in the orchids might have been carried out without a belief in evolution. The point of view from which Darwin regards the subject is easily indicated. It is a fact, though we do not know exactly why, that cross-fertilization is an advantage to bisexual plants, and every peculiarity in orchidaceous flowers, the simplest as well as the most brizarre and apparently monstrous, has some definite relation to the agencies by which cross-fertilization is effected, that is to say, to the structure and habits of insects, which carry the pollen from one plant to another. Thus in plants originally of the regular form, slight variations arose, by which cross-fertilization was favoured; the offspring of these therefore survived, and were more vigorous than the seedlings of the unmodified flowers, and natural selection thus accumulated the variations until the results we now see were produced. In this work an important principle in the theory of modification is enunciated and illustrated, the principle of change of function, on which so much stress has been laid by Dr. Dohrn, in his inquiries into the probable history of the vertebrate organization :— "The regular course of events seems to be that a part which