Having found this important clue, Darwin followed it up with characteristic perseverance. In his quest for more fossil bones he was indefatigable. Mr Francis Darwin tells us, "I have often heard him speak of the despair with which he had to break off the projecting extremity of a huge, partly excavated bone, when the boat waiting for him would wait no longer." ("L.L." I. page 276 (footnote).) Writing to Haeckel in 1864, Darwin says: "I shall never forget my astonishment when I dug out a gigantic piece of armour, like that of the living armadillo." (Haeckel, "History of Creation", Vol. I. page 134, London, 1876.)
In a letter to Henslow in 1834 Darwin says: "I have just got scent of some fossil bones...what they may be I do not know, but if gold or galloping will get them they shall be mine." ("M.L." I. page 15.)
Darwin also showed his sense of the importance of the discovery of these bones by his solicitude about their safe arrival and custody. From the Falkland Isles (March, 1834), he writes to Henslow: "I have been alarmed by your expression 'cleaning all the bones' as I am afraid the printed numbers will be lost: the reason I am so anxious they should not be, is, that a part were found in a gravel with recent shells, but others in a very different bed. Now with these latter there were bones of an Agouti, a genus of animals, I believe, peculiar to America, and it would be curious to prove that some one of the genus co-existed with the Megatherium: such and many other points depend on the numbers being carefully preserved." ("Extracts from Letters etc.", pages 13-14.) In the abstract of the notes read to the Geological Society in 1835, we read: "In the gravel of Patagonia he (Darwin) also found many bones of the Megatherium and of five or six other species of quadrupeds, among which he has detected the bones of a species of Agouti. He also met with several examples of the polygonal plates, etc." ("Proc. Geol. Soc." Vol. II. pages 211-212.)
Darwin's own recollections entirely bear out the conclusion that he fully recognised, WHILE IN SOUTH AMERICA, the wonderful significance of the resemblances between the extinct and recent mammalian faunas. He wrote in his "Autobiography": "During the voyage of the 'Beagle' I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos." ("L.L." I. page 82.)
The impression made on Darwin's mind by the discovery of these fossil bones, was doubtless deepened as, in his progress southward from Brazil to Patagonia, he found similar species of Edentate animals everywhere replacing one another among the living forms, while, whenever fossils occurred, they also were seen to belong to the same remarkable group of animals. (While Darwin was making these observations in South America, a similar generalisation to that at which he arrived was being reached, quite independently and almost simultaneously, with respect to the fossil and recent mammals of Australia. In the year 1831, Clift gave to Jameson a list of bones occurring in the caves and breccias of Australia, and in publishing this list the latter referred to the fact that the forms belonged to marsupials, similar to those of the existing Australian fauna. But he also stated that, as a skull had been identified (doubtless erroneously) as having belonged to a hippopotamus, other mammals than marsupials must have spread over the island in late Tertiary times. It is not necessary to point out that this paper was quite unknown to Darwin while in South America. Lyell first noticed it in the third edition of his "Principles", which was published in May, 1834 (see "Edinb. New Phil. Journ." Vol. X. (1831), pages 394-6, and Lyell's "Principles" (3rd edition), Vol. III. page 421). Darwin referred to this discovery in 1839 (see his "Journal", page 210.)
That the passage in Darwin's pocket-book for 1837 can only refer to an AWAKENING of Darwin's interest in the subject--probably resulting from a sight of the bones when they were being unpacked--I think there cannot be the smallest doubt; AND WE MAY THEREFORE CONFIDENTLY FIX UPON NOVEMBER, 1832, AS THE DATE AT WHICH DARWIN COMMENCED THAT LONG SERIES OF OBSERVATIONS AND REASONINGS WHICH EVENTUALLY CULMINATED IN THE PREPARATION OF THE "ORIGIN OF SPECIES". Equally certain is it, that it was his geological work that led Darwin into those paths of research which in the end conducted him to his great discoveries. I quite agree with the view expressed by Mr F. Darwin and Professor Seward, that Darwin, like Lyell, "thought it 'almost useless' to try to prove the truth of evolution until the cause of change was discovered" ("M.L." I. page 38.), and that possibly he may at times have vacillated in his opinions, but I believe there is evidence that, from the date mentioned, the "species question" was always more or less present in Darwin's mind. (Although we admit with Huxley that Darwin's training in comparative anatomy was very small, yet it may be remembered that he was a medical student for two years, and, if he hated the lectures, he enjoyed the society of naturalists. He had with him in the little "Beagle" library a fair number of zoological books, including works on Osteology by Cuvier, Desmarest and Lesson, as well as two French Encyclopaedias of Natural History. As a sportsman, he would obtain specimens of recent mammals in South America, and would thus have opportunities of studying their teeth and general anatomy. Keen observer, as he undoubtedly was, we need not then be surprised that he was able to make out the resemblances between the recent and fossil forms.)
It is clear that, as time went on, Darwin became more and more absorbed in his geological work. One very significant fact was that the once ardent sportsman, when he found that shooting the necessary game and zoological specimens interfered with his work with the hammer, gave up his gun to his servant. ("L.L." I. page 63.) There is clear evidence that Darwin gradually became aware how futile were his attempts to add to zoological knowledge by dissection and drawing, while he felt ever increasing satisfaction with his geological work.
The voyage fortunately extended to a much longer period (five years) than the two originally intended, but after being absent nearly three years, Darwin wrote to his sister in November, 1834, "Hurrah! hurrah! it is fixed that the 'Beagle' shall not go one mile south of Cape Tres Montes (about 200 miles south of Chiloe), and from that point to Valparaiso will be finished in about five months. We shall examine the Chonos Archipelago, entirely unknown, and the curious inland sea behind Chiloe. For me it is glorious. Cape Tres Montes is the most southern point where there is much geological interest, as there the modern beds end. The Captain then talks of crossing the Pacific; but I think we shall persuade him to finish the coast of Peru, where the climate is delightful, the country hideously sterile, but abounding with the highest interest to the geologist...I have long been grieved and most sorry at the interminable length of the voyage (though I never would have quitted it)...I could not make up my mind to return. I could not give up all the geological castles in the air I had been building up for the last two years." ("L.L." I. pages 257-58.)
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