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or detritus, but of a lake dammed up by glacial ice. Darwin was ashamed of his arguments and conclusions about Glen Roy. “My error,” he said, “has been a good lesson to me never to trust in science to the principle of exclusion.”” But it is inevitable that apparently definite views should receive just such shocks upon the introduction of a great new principle in a science. The facts that find their explanation under the single newly discovered cause are necessarily referred, before the advent of the new hypothesis, to very various and unrelated CauSeS. On the question of the origin of species there were really two hypotheses, creation and descent, when Darwin took hold of it; and he adopted the process of exclusion in treating them. The evidence was all in favor of descent by natural selection and opposed to creation. But he was himself emphatic in the declaration that the origin of species by natural selection was not demonstrated. Belief in it must be based on general considerations, – that natural selection is an actually existing cause, and that it explains a host of facts and brings them under one point of view. One hypothesis 1 Life and Letters, Vol. I. p. 57.

was excluded and the other adopted. And the one that was accepted was based, not on direct proof, but on one of the most magnificent series of deductions that the world has ever seen. The further discussion of this subject will be deferred for the present. In the following chapters Darwin's method of treating the problems that presented themselves to his mind will be analyzed in some detail, and the closing chapter will deal with the logical history of the principle of natural selection.



HE starting points of many of Darwin's

o researches were furnished him by other intelligent men. In many cases these men not only were in possession of the facts, but had hit upon their true explanation. With the facts thrust upon them, with enough reasoning ability to pursue them, they gave away their heritage, – luckily to one who knew its value. In his frank but modest analysis of his own mental qualities he said of himself, as already quoted, “I think I am superior to the common run of men in noticing things which easily escape attention, and in observing them carefully. . . . From my earliest youth I have had the strongest desire to understand or explain whatever I observed, . . . that is, to group all facts under some general laws.” There can be no doubt that his great interest in apparently little things, and his efforts to make the most of them, were due to his conviction that important things were hidden behind them, that they were illustrations of general laws. Lawson, Vice-Governor of the Galapagos Islands at the time of Darwin's visit, knew that the tortoises of the different islands differed from one another, and even declared to Darwin that he could tell from which island any tortoise came." He had the time, and the material lay at his feet; but he left it for Darwin to make the Galapagos Islands famous as illustrations of the local variations of species. Darwin himself had to have the evidence thrust upon him from several directions before he grasped its significance, but his greater appreciation of the nature and value of the facts made him their master. After his return from the Beagle voyage, Mr. Wedgwood of Maer Hall suggested to him that the apparent sinking of superficial bodies, ashes, marl, cinders, etc., into the earth is due to the action of earth-worms.” Both the facts and the theory were ready at hand. To the one man they were probably interesting as intellectual playthings; to the other they became the starting point for a

* Life and Letters, Vol. I. p. 83.

* Naturalist's Voyage around the World, pp. 393-398. * The Formation of Vegetable Mould, etc., p. 3.

long investigation. Darwin read one of his first papers, “On the Formation of Vegetable Mould,” before the Geological Society of London, November 1, 1837; and on the same apparently insignificant subject he published the last book of his life. While collecting in the Chonos Archipelago, Southern Chile, he found numbers of an insignificant little cirriped, none more than one tenth of an inch long, embedded in the shell of Concholepas peruviana." The zoological material of his trip was turned over to specialists for description, he furnishing the field notes and editing the publications. Much zoological information was thus given to the world, but none of all that material ever served as a starting point for a great investigation. The little abnormal cirriped was left to Darwin himself; probably because it was too small an affair to be taken charge of by others. In his hands it became the germ of a monograph on the Cirripedia, which is still the classical literature of the group. To determine its position he studied the structure of as many genera as possible. Dr. J. E. Gray, who had already collected a large amount of material for a mon1 A Monograph of the Cirripedia, Vol. I, Preface, p. 5;

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