in the Galapagos Islands. The problems which occurred to him were: "Why are the animals of the latest geological epoch in South America similar in facies to those which exist in the same region at the present day, and yet specifically and generically distinct?" And, "Why are the animals and plants of the Galapagos Archipelago so like those of South America, and yet different from them? Why are those of the several islets more or less different from one another?"

These problems were only explicable on the assumption of modification, and in order to explain the cause of these modifications he turned to the only certainly known examples of descent with modification-viz., those presented by domestic animals and cultivated plants. The details of these he worked up and experimented upon in a much more thorough manner than his predecessors, especially in regard to pigeons. He soon perceived "that selection was the keystone of the main success in making useful races of animals and plants"; but says: "how selection could be applied to organisms living in a state of nature remained for some time a mystery to me."

"In October 1838," he writes, "that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement, Malthus on Population,' and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observations of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work; but I was so anxious to avoid prejudice that I determined not for some time to write even the briefest sketch of it. In June 1842, I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil, in thirtyfive pages, and this was enlarged during the summer of 1844 into one of 230 pages."

Not till 1858 was the theory published, and then only on pressure of the strongest character being brought to bear on him. On June 18th, 1858, Darwin, having convinced himself and accumulated the evidence and proofs he wanted, was at last at work on the book, when he received, most unexpectedly, Wallace's MS. from Ternate, in which the Theory of Natural Selection was set forth clearly and decisively, almost in Darwin's own words. Darwin wished to publish Wallace's paper without reference to his own work; but at the urgent solicitation of Lyell and Hooker he consented to allow extracts from his own MS. of 1844, together with a letter to Asa Gray of 1857, to be read before the Linnean Society on July Ist, 1858.

Darwin was at this time forty-nine years of age, and Wallace thirty-five. On Darwin's part this publication was the result of twenty-one years of deliberate work; and of views formed nineteen years beforehand, and actually written out in MS. of 230 pages fourteen years previously. The dual authorship of the "Theory," and its simultaneous announcement from opposite sides of the world, were causes for sincere congratulation. It was fortunate for Darwin, in causing him to publish his views more speedily, and in a more condensed and attractive form than he originally purposed, and as leaving him at liberty for further work. It was also fortunate for Wallace in securing cordial and sympathetic recognition in the most gratifying manner of his independent discovery. Finally, it was fortunate for the world, and a lesson for all time to come, of how an emergency, involving the tenderest susceptibilities of scientific reputation, can be treated so as to redound to the infinite and lasting credit of all concerned.

With regard to Wallace, it is interesting to know the immediate causes which suggested the theory, especially in view of Darwin's history, and I cannot do better than quote Wallace's own words:

"In February 1858 I was suffering from a rather severe attack of intermittent fever at Ternate, in the Moluccas; and one day, while lying on my bed during the cold fit, wrapped in blankets, though the thermometer was at 88° Fahr., the problem again presented itself to me, and something led me to think of the 'positive checks' described by Malthus in his 'Essay on Population,' a work I had read several years before, and which had made a deep and permanent impression on my mind. These checks-war, disease, famine, and the likemust, it occurred to me, act on animals as well as man. Then I thought of the enormously rapid multiplication of animals, causing these checks to be much more effective in them than in the case of man; and while pondering vaguely on this fact there suddenly flashed upon me the idea of the survival of the fittest-that the individuals removed by these checks must be on the whole inferior to those that survived. In the two hours that elapsed before my ague fit was over, I had thought out almost the whole of the theory; and the same evening I sketched the draft of my paper, and in the two succeeding evenings wrote it out in full, and sent it by the next post to Mr. Darwin."

Thus we see that Malthus was the turning-point with both authors; and all historically inclined will place this famous essay on their shelves, alongside the works of Darwin and Wallace. To me it has always been a striking fact, that with both authors, and perfectly independently, the turning-point in the argument should have been the application to the whole animal world of principles already established and accepted in regard to man. This is curiously significant in view of the objection, so often and so ignorantly brought against Darwinism, that it is degrading, because it applies to man laws governing the structure and habits of animals.

From 1846 to 1854 Darwin devoted himself to a monograph on Barnacles, consisting of four large volumes, two on recent and two on fossil species: a heavy task which he was led to undertake, largely, through "a sense of presumption in accumulating facts and speculating on the subject of variation without having worked out my due share of species." "No one," he says, "has a right to examine the question of species who has not minutely described many."

Subsequent to the publication of the "Origin of Species" much time was taken up by successive editions of the work, and much lost through illhealth. Darwin himself described his books as the "milestones of my life"; and they fall into two great groups-those completing the "Origin of Species," and those on more or less independent lines, such as the great Botanical Series.

I. WORKS COMPLETING THE "ORIGIN OF SPECIES."

This work was itself an "abstract," and this was the title he proposed to give it, having originally designed it to be of much greater length. However, yielding to the publishers, he produced it in its present forın.

In 1868 appeared "Variations of Animals and Plants under Domestication," in which a detailed account was given of artificial breeding; and this was intended to be followed by two other works dealing with variation, heredity, embryology, geographical distribution, &c., in similar detail; but these were never written.

The detailed application of the theory to man was inevitable; and in the first edition of the "Origin of Species" he says: "In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by graduation. Light will be thrown on the