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of forest, and where I at once obtained some of the rare birds and insects peculiar to Celebes. After about a month I returned to Macassar, and found that I could obtain a passage to the celebrated Aru Islands, where at least two species of birds of paradise are found, and which had never been visited by an English collector. This was a piece of good fortune I had not expected, and it was especially fortunate because the next six months would be wet in Celebes, while it would be the dry season in the Aru Islands. This journey was the most successful of any that I undertook, as is fully described in my book; and as no letters referring to it have been preserved, I shall say no more about it here. The illustration opposite is from a photograph of a native house in the island of Wokan, which was given me by the late Professor Moseley of the Challenger expedition, because it so closely resembles the hut in which I lived for a fortnight, and where I obtained my first King bird of paradise, that I feel sure it must be the same, especially as I saw no other like it. It is described at the beginning of chapter xxxi. of my “Malay Archipelago,” and will be of interest to such of my readers as possess that work. Several months later I arrived again at Macassar, and after arranging and despatching my Aru collections, I went to an estate a few days’ journey north, the property of a brother of my kind friend Mr. Mesman. I had a house built for me in a patch of forest where I lived with two Malay servants for three months making very interesting collections both of birds and insects; and I have rarely enjoyed myself so much as I did here. About the end of November I returned to Macassar, and in December embarked on the Dutch mail steamer for Amboyna, calling by the way at Timor and at Banda. At Amboyna I made the acquaintance of a German and a Hungarian doctor, both entomologists, and in a fortnight's visit to an estate in the interior surrounded by virgin forest I obtained some of the lovely birds and gorgeous insects which have made the island celebrated. The only letter I

possess which indicates something of my opinions and anticipations at this period of my travels is one to Bates, dated Amboyna, January 4, 1858, from which I will make a few extracts. The larger portion is occupied with remarks on the comparative riches of our respective regions in the various families of beetles, founded on a letter I had received from him a few months before, which, though very interesting to entomologists, are not suitable for reproduction here. I then touched on the subject of my paper referred to at the end of the last chapter. “To persons who have not thought much on the subject I fear my paper on the ‘Succession of Species’ will not appear so clear as it does to you. That paper is, of course, merely the announcement of the theory, not its development. I have prepared the plan and written portions of a work embracing the whole subject, and have endeavoured to prove in detail what I have as yet only indicated. It was the promulgation of Forbes's theory of “polarity’ which led me to write and publish, for I was annoyed to see such an ideal absurdity put forth, when such a simple hypothesis will explain all the facts. I have been much gratified by a letter from Darwin, in which he says that he agrees with “almost every word' of my paper. He is now preparing his great work on “Species and Varieties, for which he has been collecting materials twenty years. He may save me the trouble of writing more on my hypothesis, by proving that there is no difference in nature between the origin of species and of varieties; or he may give me trouble by arriving at another conclusion; but, at all events, his facts will be given for me to work upon. Your collections and my own will furnish most valuable material to illustrate and prove the universal applicability of the hypothesis. The connection between the succession of affinities and the geographical distribution of a group, worked out species by species, has never yet been shown as we shall be able to show it. “In this archipelago there are two distinct faunas rigidly circumscribed, which differ as much as do those of Africa and South America, and more than those of Europe and North America; yet there is nothing on the map or on the face of the islands to mark their limits. The boundary line passes between islands closer together than others belonging to the same group. I believe the western part to be a separated portion of continental Asia, while the eastern is a fragmentary prolongation of a former west Pacific continent. In mammalia and birds the distinction is marked by genera, families, and even orders confined to one region; in insects by a number of genera, and little groups of peculiar species, the families of insects having generally a very wide or universal distribution.” This letter proves that at this time I had not the least idea of the nature of Darwin's proposed work nor of the definite conclusions he had arrived at, nor had I myself any expectation of a complete solution of the great problem to which my paper was merely the prelude. Yet less than two months later that solution flashed upon me, and to a large extent marked out a different line of work from that which I had up to this time anticipated. I finished the letter after my arrival at Ternate (January 25, 1858), and made the following observation: “If you go to the Andes I think you will be disappointed, at least in the number of species, especially of Coleoptera. My experience here is that the low grounds are much the most productive, though the mountains generally produce a few striking and brilliant species.” This rather hasty generalization is, I am inclined still to think, a correct one, at all events as regards the individual collector. I doubt if there is any mountain station in the world where so many species of butterflies can be collected within a walk as at Para, or more beetles than at my station in Borneo and Bates' at Ega. Yet it may be the case that many areas of about a hundred miles square in the Andes and in the Himalayas actually contain a larger number of species than any similar area in the lowlands of the Amazon or of Borneo. In other parts of this letter I refer to the work I hoped to do myself in describing, cataloguing, and working out the distribution of my insects. I had in fact been bitten by the passion for species and their description, and if neither Darwin nor myself had hit upon “Natural Selection,” I might have spent the best years of my life in this comparatively profitless work. But the new ideas swept all this away. I have for the most part left others to describe my discoveries, and have devoted myself to the great generalizations which the laborious work of speciesdescribers had rendered possible. In this letter to Bates I enclosed a memorandum of my estimate of the number of distinct species of insects I had collected up to the time of writing—three years and a half, nearly one year of which had been lost in journeys, illnesses, and various delays. The totals were as follows:—

Butterflies ... --- --- --- ... 62o species
Moths --- --- --- --- ... 2000 , ,
Beetles --- --- --- --- ... 37OO },
Bees, wasps, etc. ... --- --- ... 750 on
Flies... --- --- --- --- ... 660 m
Bugs, cicadas, etc.... --- --- ... 500 or
Locusts, etc. --- --- --- ... 160 m
Dragonflies, etc. ... --- --- ... I IO 2,
Earwigs, etc. --- --- --- --- 40 x
Total --- --- ... 854o species of Insects.

It was while waiting at Ternate in order to get ready for my next journey, and to decide where I should go, that the idea already referred to occurred to me. It has been shown how, for the preceding eight or nine years, the great problem of the origin of species had been continually pondered over, and how my varied observations and study had been made use of to lay the foundation for its full discussion and elucidation. My paper written at Sarawak rendered it certain to my mind that the change had taken place by natural succession and descent—one species becoming changed either slowly or rapidly into another. But the exact process of the change and the causes which led to it were absolutely unknown and appeared almost inconceivable. The great difficulty was to understand how, if one species was gradually changed into another, there continued to be so many quite distinct species, so many which differed from their nearest

allies by slight yet perfectly definite and constant characters. One would expect that if it was a law of nature that species were continually changing so as to become in time new and distinct species, the world would be full of an inextricable mixture of various slightly different forms, so that the well-defined and constant species we see would not exist. Again, not only are species, as a rule, separated from each other by distinct external characters, but they almost always differ also to some degree in their food, in the places they frequent, in their habits and instincts, and all these characters are quite as definite and constant as are the external characters. The problem then was, not only how and why do species change, but how and why do they change into new and welldefined species, distinguished from each other in so many ways; why and how do they become so exactly adapted to distinct modes of life; and why do all the intermediate grades die out (as geology shows they have died out) and leave only clearly defined and well-marked species, genera, and higher groups of animals. Now, the new idea or principle which Darwin had arrived at twenty years before, and which occurred to me at this time, answers all these questions and solves all these difficulties, and it is because it does so, and also because it is in itself self-evident and absolutely certain, that it has been accepted by the whole scientific world as affording a true solution of the great problem of the origin of species. At the time in question I was suffering from a sharp attack of intermittent fever, and every day during the cold and succeeding hot fits had to lie down for several hours, during which time I had nothing to do but to think over any subjects then particularly interesting me. One day something brought to my recollection Malthus's “Principles of Population,” which I had read about twelve years before. I thought of his clear exposition of “the positive checks to increase”— disease, accidents, war, and famine—which keep down the population of savage races to so much lower an average than that of more civilized peoples. It then occurred to me that these causes or their equivalents are continually acting in the

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