Tucutuco, the blindness, though so common, cannot be a very serious evil; yet it appears strange that any animal should possess an organ frequently subject to be injured. Lamarck would have been delighted with this fact had he known it when speculating (probably with more truth than usual with him) on the gradually acquired blindness of the Aspalax, a gnawer living underground, and of the Proteus, a reptile living in dark caverns filled with water, in both of which animals the eye is in an almost rudimentary state. . . . .. No doubt Lamarck would have said that the Tucutuco is now passing into the state of the Aspalax and Proteus." Darwin himself in later years would have said much the same, only he would doubtless have sought to find how the Tucutuco derived some advantage, direct or indirect, from its imperfect vision. Yet his somewhat contemptuous parenthesis shows that at the period of his Journal he rejected altogether the general views of Lamarck, and when years afterwards he expounded his own theory of organic evolution in the Origin of Species," he referred to the "erroneous views and grounds of opinion of Lamarck." But whatever the young naturalist thought of Lamarck's system, his remarks on the Tucutuco afford only one of the many evidences that his interest in the question of modification versus constancy in organic structure was thoroughly aroused during his years of travel. His celebrated chapter on the Galapagos Islands shows us that the probability of mutability in species was presented to him much more distinctly by the facts of distribution than by examples of imperfect adaptation to conditions. The islands are evidently geologically new, raised within a comparatively recent period above the surface of the ocean. Hardly any of

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the species found in the Archipelago occur in any other part of the world, and there are many species which are confined to a single island. At the same time the species in almost all cases belong to American genera.

"It is probable that the islands of the Cape de Verd group resemble in their physical conditions far more closely the Galapagos Islands than these latter physically resemble the coast of America, yet the aboriginal inhabitants of the two groups are totally unlike, those of the Cape de Verd Islands bearing the impress of Africa as those of the Galapagos Archipelago are stamped with that of America." Another series of facts had also brought Darwin near to the problem of the origin of species, namely, the similarity between the fossil and living animals on the same continent. In America "there are fossil ant-eaters, armadilloes, tapirs, peccaries, guanacoes, opossums, and numerous South American gnawers, monkeys and other animals. This wonderful relationship in the same continent between the dead and the living will, I do not doubt, hereafter throw more light on the appearance of organic beings on our earth, and on their disappearance from it than any other class of facts." He mentions Buffon in this connection. "If Buffon had known of the gigantic sloth and armadillolike animals, and of the later pachydermata, he might have said with a greater semblance of truth that the creative force in America had lost its power rather than that it had never possessed greater power." little further on he shows that he had at the time of the publication of the Journal considered the subject of the struggle for existence :-"The supply of food on an average remains constant; yet the tendency in every animal to increase by propagation is geometrical, and

A

its surprising effects have nowhere been more astonishingly shown than in the case of the European animals run wild during the last few centuries in America."

But the greater proportion of his interest and energy was devoted during his voyage to the study of geology. His power and the bent of his mind were abundantly exhibited in this early work. He has the most masterly grasp of detail; but this faculty is treated not as an end in itself, but as a means of supporting some sweeping generalization, which explains and holds together all the separate phenomena. In the first paper he published after his return from his voyage, he unites all the results of his studies of volcanoes into a comprehensive view of their relation to the elevation of mountain chains. His great work on Coral Islands, published four years afterwards, traces all their features to the continuous subsidence of a portion of the earth's crust, in relation to the habits of coral polyps and the configuration of the portion of land gradually sinking below the sea. For ten years after his return to England, from 1836 to 1846, Darwin was almost entirely devoted to geology. Through these studies he became a firm believer in evolution as the true character of the history of the earth, and the immensity of time, which even small epochs of this history had occupied, was deeply impressed on his mind. We know from his own words that, in spite of his labours in geology, he had the problem of evolution in the world of living things continually in his thoughts. In his introduction to the "Origin of Species," we are told that though facts in the distribution of the organic beings in South America, and of the relation of the living to the fossil inhabitants of that continent, first led him to believe in the possibility of

the evolution of living beings through descent with modification, he was not satisfied with believing in the truth of the theory, and supporting it by the facts of embryology, geographical distribution, geological succession, and the analogy of domestic productions. What he required was a cause. Darwin had his own peculiar view of the nature of a cause. He sought some one universal principle which would explain all the details; at least he wanted to have some idea of how the divergence at present existing could have been produced. He rejected entirely, as we have seen, the views of Lamarck. In his letter to Hæckel explaining the course by which he reached his views, he says:—

"At an early period it seemed to me probable that allied species were descended from a common ancestor. But during several years I could not conceive how each form could have been modified so as to become so admirably adapted to its place in nature. I began, therefore, to study domesticated animals and cultivated plants, and after a time perceived that man's power of selecting and breeding from certain individuals was the most powerful of all means in the production of new races. Having attended to the habits of animals and their relations to the surrounding conditions, I was able to realize the severe struggle for existence to which all organisms are subjected; and my geological observations had allowed me to appreciate, to a certain extent, the duration of past geological periods. With my mind thus prepared, I fortunately happened to read Malthus's Essay on Population,' and the idea of natural selection through the struggle for existence at once occurred to me. Of all the subordinate points in the theory, the last which I understood was the cause of the tendency, in the descendants from a common progenitor, to diverge in character."

Darwin does not tell us at what period of his mental development he read Malthus. We know the date at which his first manuscript on the subject of natural selection was drawn up. This happened in the year 1839, and the essay was copied in 1844 when the copy

was read by Dr. Hooker, and communicated to Sir Charles Lyell. An extract from this manuscript was read before the Linnæan Society in 1858, and published in 1859. This extract develops Darwin's own personal view of the theory of natural selection, in the form in which he held it, with little if any modification, to the end of his life. In the following few sentences the central point of the theory is asserted.

"Now can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits, or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would also have a better chance. Yearly more are bred than can survive; the smallest grain in the balance in the long run must tell on which death shall fall and which shall survive. Let this work of selection on the one hand and death on the other go on for a thousand generations, who will pretend to affirm that it would produce no effect, when we remember what in a few years Bakewell effected in cattle and Western in sheep, by this identical principle of selection."

In this published extract, as in all his expositions of his views, Darwin does not neglect the question of the origin of variations, but he does not go into it very elaborately. In the published extract, only half a paragraph deals with this question.

"Let the internal conditions of a country alter," he says, "if in a small degree, the relative proportions of the inhabitants will, in most cases, simply be slightly changed; but let the number of inhabitants be small, as on an island, and free access to it from other countries be circumscribed, and let the change of conditions. be circumscribed (forming new stations), in such a case the original inhabitants must cease to be as perfectly adapted to the changed conditions as they were originally. It has been shown in a former part of this work that such changes of external conditions would, from their acting on the reproductive system, probably cause the