The Descent of Man (1871) ranks next in wide importance to The Origin of Species. It is the application in detail of the same principles to the human race. That the application was inevitable was already evident in the earlier book; and it was this that brought upon the Origin the most virulent abuse. Just because it is so inevitable, The Descent of Man has not the unique interest of The Origin of Species. Once we are familiar with the view that all the species of animals have been produced by the accumulation of minute variations, there is no surprise in the idea that man and all his powers may have been so produced likewise. Nevertheless, Darwin differs on this point from the man who shares with him the honour of discovering the theory of evolution. Mr. Wallace, while arguing with Darwin that man has been evolved out of some lower form, holds that ‘natural selection could only have endowed the savage with a brain a little superior to that of an ape,’ and that in the higher human faculties there is evidence of the working of a supernatural power. The position is a strange one. If the whole creation moves harmoniously through all its grades by the action of one law, it will need overwhelming evidence to show that just at the end this law is superseded by another altogether unlike it. Either the supernatural governs the whole of life, or its introduction to explain one stage is gratuitous.
After The Descent of Man came The Expression of the Emotions in Man and Animals (1872); and that again was followed by Insectivorous Plants (1875). The former was originally intended merely to form a chapter in the Descent; but the materials grew, and the result is one of the most readable of books. The Insectivorous Plants embodies one of the most remarkable of Darwin’s discoveries. Its richness is due to the patience and skill with which the facts were accumulated. Sixteen years passed between the time when Darwin first noticed that plants lived on insects and the appearance of the book. In the interval he had done many things; but, whenever he had leisure, he was always adding to his store of facts relating to this class of plants; and, as he justly says, ‘a man after a long interval can criticise his own work, almost as well as if it were that of another person.’
Later, Darwin wrote on the fertilisation of plants, in order to demonstrate the importance of cross-fertilisation; on the forms of flowers; and on the movements of plants,—the last a kind of extension and generalisation of the book on climbing plants, endeavouring to co-ordinate all the movements of plants as variations of an inherent tendency of the parts to a revolving motion. The theory has not been accepted by botanists. Last of all, in 1881, appeared the monograph on The Formation of Vegetable Mould through the Action of Worms. This book is just the expansion and completion of a paper read by Darwin to a scientific society as far back as 1837. All that time the subject dwelt in his mind; and when at last leisure permitted, he developed it into what is perhaps the most purely delightful of all his books. In greatness it does not come into competition with some of them at all; but the familiarity of the phenomena, the care with which they are examined, the skill of the arrangement and the charm of finding meaning in what had been so meaningless, have made the volume one of the most widely read of all Darwin’s works.
That which distinguishes Darwin from other naturalists is the combination of extraordinary speculative power with great knowledge of detail and unlimited patience. These qualities have been combined in others as well, but never, within the field of natural history, in the same degree. More commonly they are found separate. The ordinary type of naturalist is the man who knows an immense number of facts about plants and animals, and who rests content with that knowledge. He may be master of everything about the great subject of scarabees, but it scarcely occurs to him to explain the scarabees themselves, still less to use them in explaining other creatures. On the other hand, the opposite type, the type which speculates only without first laying the foundation of fact, is likewise common enough. How ineffectual this is may be seen from the history of earlier speculations on evolution. The Vestiges of Creation and the theory of Lamarck are superseded, not so much because of deficiency in speculative power, as because the theories are not sufficiently buttressed by facts. Even though Darwin’s own theory should ultimately be, in one sense, as dead as that of Chambers, it will always remain one of the landmarks of thought.
Undoubtedly Darwin’s intellect was fundamentally speculative. We have seen how in the book on Variation under Domestication his affection clung to Pangenesis, perhaps the most questionable part of its contents. He was restless under the sense of an unexplained fact, and thankful for even a provisional explanation. He notes the effect upon him of the discovery that science cannot remain content with facts alone. Geologising with Sedgwick in North Wales, he heard about a tropical shell which had been picked up in a neighbouring quarry. ‘I told Sedgwick of the fact, and he at once said (no doubt truly) that it must have been thrown away by some one into the pit; but then added, if really embedded there it would be the greatest misfortune to geology, as it would overthrow all that we know about the superficial deposits of the Midland Counties.... I was then utterly astonished at Sedgwick not being delighted at so wonderful a fact as a tropical shell being found near the surface in the middle of England. Nothing before had ever made me thoroughly realise, though I had read various scientific books, that science consists in grouping facts so that general laws or conclusions can be drawn from them.’ It is this conception that he kept steadily before his eyes, and his glory lies in his success in drawing general laws from his facts.
Alfred Russel Wallace
(1822).
The work of the other evolutionists, so far as it is not technical rather than literary, is almost accounted for when Darwin’s is described. With respect to one indeed, Mr. Alfred Russel Wallace, an inevitable injustice is done whatever course be pursued. He is the co-discoverer with Darwin of the scheme of evolution associated with the name of the latter; and though the fame has gone to the elder man, it seems clear that if not Darwin then Mr. Wallace was destined to stir the mind of the age with this great conception. Mr. Wallace has been an extensive traveller; he published, in 1853, a volume of Travels on the Amazon, giving an account of journeys in that region during part of which he was the companion of Mr. Henry Walter Bates, whose Naturalist on the Amazon (1863) is well known as one of the most interesting and valuable books of travel and natural history in the language. It was however his observations in the Malay Archipelago that led Mr. Wallace to the theory of evolution, and perhaps he is best known by his book, The Malay Archipelago (1869).