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HARDLY any branch of natural history has been so neglected as that which treats of the various modes by which the different classes of organisms have become dispersed over the surface of the globe. Scattered observations have indeed been made by many writers, but Lyell and Darwin were the first to gather together the existing evidence on the subject, or to test by actual experiment the effects of exposure to salt water on the vitality of seeds and land-shells. Owing to this neglect the idea has arisen that seas of very moderate width serve as complete barriers to the dispersal of most living things; and it has been thought necessary to postulate great and often repeated geographical mutations, and even to bridge across the widest and deepest oceans, in order to account for the actual distribution of mammals or reptiles, of plants, insects, or terrestrial mollusca.

It was Darwin who first taught us that these assumptions of vast and repeated changes in the distribution of sea and land were at once inadmissible and unnecessary. By his original and masterly investigation


into the phenomena presented by oceanic islands he showed that these islands had never been connected with the continents, as had been almost invariably assumed by previous writers, and, consequently, that their entire fauna and flora must have originated from such species as could, in the course of ages, have reached the islands by natural means of dispersal. Hence the importance of studying what are the means of dispersal of the various groups, and why it is that, with the two absolute exceptions of mammals and amphibia, none of the larger groups of animals or plants are invariably absent from this class of islands. As a corollary from his investigation he was led to conclude that the great oceans were, broadly speaking, permanent features of the earth's surface, and that it was scientifically inadmissible to bridge them over in various directions and at various geological epochs in order to provide a short and easy road for the passage of beetles or snakes, snails or frogs, and thus save us the trouble of solving the problem of their actual distribution by less obvious and also by less heroic means.

Having myself devoted some time and research with the object of showing that almost every anomaly in the distribution of animals and plants may be explained by a careful consideration of the various means of dispersal which organisms possess, combined with the climatic and geographical changes which are known to have occurred during later geological times, and taking into account the known distribution of the several groups at remote epochs as proved by the discovery of fossils in regions far removed from the lands now inhabited by their living representatives, I am especially interested in Mr. Kew's attempt to bring together all that is known of the means of dispersal of one of the groups as to which such information was most needed. He has devoted to the task much labour and research, and has brought together a mass of information of great value. Many of the facts he adduces are so curious and interesting that they will attract the attention of many classes of readers and thus lead, it is to be hoped, to the accumulation of facts which are still required to complete our knowledge of this important subject.

I heartily congratulate the author on his choice of so useful and interesting an inquiry for his first work, and on the systematic and accurate manner in which he has marshalled the facts he has collected. Many books of far greater pretension, even though they should contain descriptions of scores of new species and work out their internal structure with the greatest accuracy, may yet be of less interest to the philosophical naturalist than this unpretending little volume. In its pages we are afforded a glimpse of what seem at first sight to be but trifles and accidents in nature's workshop, but which are really the tools with which she produces some of her most striking results. It is owing to such trifling occurrences as the occasional attachment of a living shell to a beetle's leg, or the conveyance of seeds in the mud adhering to a bird's foot, that many remote islands have become stocked with life, and the range of species extended or modified over the earth ; while through changes of the organic environment thus effected even the origination or the extinction of species may have been brought about.



THE distribution of fresh-water and land shells has been looked upon as presenting special difficulties on the heory of single birth-places or “centres of creation." Mr. Darwin, in characteristic letters, in 1856-7, spoke of being driven mad by land molluscs, and fresh-water kinds, he said, had been a horrid incubus. In the

Origin of Species," however, he was able to suggest several possible means of dispersal both for terrestrial and aquatic groups, and speaking of the latter he remarked that many facts throwing light on the subject would doubtless be discovered. Quite a number of facts of the kind indicated have been recorded since the publication of the "Origin," and a collection of these together with some hitherto unpublished items—both as regards fresh-water and land shells—is now given. In compiling this, and the essays on subsidiary subjects also given, the writer has received constant help in the way of notes, references, &c., from a number of friends and correspondents whose courteous co-operation is

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“ Life and Letters," vol. ii. pp. 85, 93.
Origin,” 1859, p. 385, and see ed. 6, pp. 344, 353.

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