The next book, on the 'Expression of the Emotions in Man and Animals,' published in 1872, grew out of the 'Descent of Man,' just as the latter grew out of 'Variation under Domestication,' for it had been intended as a chapter in the natural history of man, but as before, as soon as he began to put his notes
together he saw that it would require a separate volume. His study of this subject originated in 1840, in his conviction that even 'the most complex ... shades of expression must all have a gradual and natural origin.' The 'Expression of the Emotions' had a large sale, 6,267 copies being disposed of on the day
of publication. No second edition of this booK on ' Expression ' has appeared; so large a reprint was made that it was not exhausted during the author's lifetime, and thus unfortunately his large collection of material for a new edition was never made use of. A postscript to the book on 'Expression,' under the
title * Biographical Sketcn of an Infant,' appeared in 'Mind' in 1877, to the publication of which Darwin was encouraged by the appearance of a similar paper by M. Inline. From this time forward his working hours were almost entirely given up to the study of plants. There are, however, some important exceptions to this statement. New editions of his works took up a certain amount of time; thus the 'Origin' had five editions between
1869 and 1872, when a sixth and stereotyped edition was published. Of these the second was little more than a reprint, whereas the third, fourth, and fifth contained much new matter. A second and largely corrected edition of the 'Descent of Man' appeared in 1874, and in 1876 a second edition of 'Variation and Domestication,' the fruit of much labour, was brought out. Second editions of his 'Geological Observations' and 'Coral
Reefs' appeared in 1874 and 1876. Two other books, not on a botanical subject, were written in his later life. One of these, the biography of his grandfather, Erasmus Darwin, appeared
in 1879. It was written as a 'preliminary notice' to the English version of Dr. E. Krause's 'Essay on the Work of Erasmus
Darwin.' Darwin had a strong feeling for his forbears, and found much enjoyment in this new work of biographical writing.
In 1881 his book on the 'Formation of Vegetable Mould through the action of Worms' appeared. It was, like so much of his books, the result of the expansion or completion of earlier work. His attention had been directed to the action of earthworms in 1837, while he was staying with his uncle, Josiah Wedgwood, in Sta^rdshire, and in 1888, as already stated, a short paper on the subject was published by the Geological Society. Before the publication of this book Darwin wrote : 'This is a subject of but small importance; and I know not whether it will interest many readers, but it has interested me. It is the completion of a short paper read before the Geological Society more than forty years ago, and has revived old geological thoughts.' Both his estimate of the value of the book and his expectations as to its general success were wide of the mark. Its value was at once recognised by scientific opinion, and it proved to be widely popular with the general public.
Botanical Work. — It has been well said that one great service rendered by Darwin to science was the revival of teleology in a rational form, a form in which it is no longer opposed to, but 'wedded to morphology.' The knowledge of the manner in which the structures of living beings have become adapted to their various encfa gives a vigour to the study of the form and organisation of animals and plants, the importance of which can hardly be exaggerated. And it was to a great extent by his special botanical work that he wrought this cnange; for it was in botany that he showed in practice how powerful, for the study of adaptive structure, are the means of research which the ' Origin of Species ' has placed in our hands. It was work of this nature which occupied his later years; the subject-matter varied, but whether he investigated the fertilisation of flowers, the twining of steins, the movements of leaves, or the natural history of insectivorous plants, the character of the work remained the same. One of Darwin's earliest references to a botanical subject occurs in the note-book of 1837-8, in which facts bearing on evolution were collected. 'Do not plants which have male and female organs together yet receive influence from other plants? Does not Lyell give some arguments about varieties being difficult to keep on account of pollen from other plants, because this may be applied to show [that] all plants do receive intermixture? ' It was especially his belief that intercrossing within the limits of each species played an important part in keeping specific forms constant that led him to pay attention to the fertilisation of plants. His interest in the subject was heightened by reading, in 1841, Christian Konrad Sprengel's book, 'Das entdeckte Geheimniss der Natur,' published in 1793. This remarkable work, in which much of the modern theory of fertilisation is given, first led him to see in what detail the structure of flowers is adapted t-o certain ends. Sprengel's book was overlooked and slighted by the naturalists of his own day in spite of its originality, and it was a satis-
