South American species, probably introduced in the floating timber, though they may also have come with living plants, which are often brought from Bahia. Two species, however, are peculiar, and one is closely allied to a Brazilian species, so that it must have been introduced by natural agencies before the settlement of the island; the other is of a genus confined to Madagascar.

Now it is a suggestive fact that the Mozambique current, bending round the Cape of Good Hope to the Equator, is one of the sources of the Gulf-stream; so that it is not impossible that a tree, carried down by a flooded. river on the west coast of Madagascar, might ultimately reach the Azores. That it should convey living larvæ or pupa of Elaters may also not be impossible; and if such a log reached the Azores but once in ten thousand years, and but one log in a thousand should convey living Elaters, we should still, if the calculations of geologists have any approximate value whatever, be far within the epoch of existing genera, and even of most existing species. relation so isolated and extraordinary as that between a single insect of the Azores and those of Madagascar, may well be due to a concurrence of events as rare and improbable as this seems to be.

A

The Azores, and in a less degree the Madeiras, appear to me to teach us this important lesson in the laws of distribution of birds and insects, that it has been determined neither by the direction of ocean currents nor by that of the most prevalent winds, but almost wholly by more exceptional causes such as storms and hurricanes, which still continue to bring immigrants from the nearest lands.

Mr. Murray's argument for a land-connection between the various Atlantic islands, from the Azores to the Cape de Verdes, and even to St. Helena, has perhaps more to be said for it; but I do not think that the facts require anything beyond the extension of each group into one or more considerable islands. Such an extension is perhaps indicated by the comparatively elevated submarine bank on which each group stands; and it is evident that more extended land-surfaces would not only bring the groups nearer to each other, but, by offering a much greater length

of opposing coasts, would greatly facilitate the migration and accidental transmission of individuals.

Another point of great importance to which attention is called in Mr. Murray's essay, is the long-persisting identity of form which seems to be a characteristic of insects, and which is thought to allow ample time for those revolutions in geography to which he so constantly appeals. But this antiquity and persistence of insect-forms will have allowed equal time for the action of a most powerful agent of distribution, which is too hastily dismissed. I allude to those changes of climate, which within a period so recent as the Miocene, have at one time clothed the now inhospitable regions of North Greenland, Spitzbergen and other Arctic lands, with forests and evergreens and flowering shrubs, and at another have covered the Northern United States and Central Europe with a mantle of ice like that which at present envelopes Greenland. These vast climatic changes must have afforded ample facilities for insect migrations,-between the eastern and western hemispheres, when the arctic regions were inhabited by a temperate flora and fauna,-between the northern and the southern, when the animals and plants of either hemisphere were driven towards the Equator by the glaciation of their native regions, and when a portion would cross that barrier, either along the elevated lands or by transmission over narrow seas. This cause is admitted by our best botanists to be amply sufficient to account for the presence of European genera and species of plants on the Andes, in Chili, Patagonia and Terra del Fuego, in New Zealand, and in the Australian Alps. The relations of South America, Australia, and other southern lands to each other, are still more marked, and probably more deep-seated, and seem to imply either a greater extension or the existence of intermediate lands at some former period, but not an actual continuity with one another.

I believe that the curious and suggestive facts which I have disinterred from that bulky and little-read volume, the Insecta Maderensia, may be of some use to students of the geographical distribution of animals.

CHAPTER XIII

EVOLUTION AND THE DISTRIBUTION OF ANIMALS 1

IN the Nineteenth Century of December 1878, Dr. P. L. Sclater called attention to the subject of the geographical distribution of animals in its bearing on the theory of evolution, and gave numerous special cases in which the actual distribution of particular species and groups is very difficult to explain on that theory without making assumptions which, in his opinion, the evidence at our disposal does not warrant. Difficulties of this nature are so numerous, and many of them seem to him so weighty, that in order to explain them, he is led to question, what is almost an axiom with evolutionists, that identity of structure is, without exception, an indication of descent from a common parent. Similar doubts, though not stated in exactly the same terms, were felt by the late Professor Mivart 2; and it therefore becomes a matter of interest to examine a little more closely into the alleged difficulties, in order to see whether they are not really explicable on the principle of descent with modification, only calling to our aid such general assumptions as are fully warranted by what we actually know of the migrations and extinctions of living things, and of the past changes in the physical condition of the earth and its inhabitants.

As Dr. Sclater's article gives an excellent summary of the nature and meaning of zoological distribution, and of

1 This chapter first appeared in the Nineteenth Century of February 1879, under the title Animals and their Native Countries."

2 Genesis of Species, Chap. III.

the main general conclusions arrived at by naturalists, our purpose will be best attained by proceeding at once to consider his special cases of difficulty; and in doing so we shall have occasion to discuss, as fully as may be required, the general principles and particular illustrations needed to elucidate them.

We have first the case of the Little Blue Magpie of Spain (Cyanopica Cooki), which has a very close ally in the extreme parts of Eastern Asia and Japan (C. cyaneus), while there is nothing closely allied to these in all the intervening regions or in any other part of the world. This is said to be an infringement of the canon as to the continuity of specific areas, and as such to require explanation. Before proceeding further, it will be well to inquire into the value of this canon of continuity, and whether it is so clear and well established that infringements of it are altogether exceptions to the usual course of nature. So far from this being the case, I believe it will be found that, between the complete continuity of the area occupied by a species or a genus and such wide discontinuity as occurs in the present example, there is every possible gradation; and further, that the instances of discontinuity are very numerous, while those of complete continuity are far less generally the rule than appears at first sight.

In order to understand the bearing of this class of phenomena on the theory of derivation by modification from an allied form, let me briefly indicate the probable course of a genus of animals from its birth or origin to its final extinction.

Genera are groups of species which agree among themselves, and differ from all other groups in the same family or order, by the possession of some structural peculiarities. We must therefore suppose a genus to have had its origin in some variation of structure which was useful to its possessors such as a modification of the bill, feet, or wings of a bird, or of the teeth, claws, or horns of a mammal. According to the theory of natural selection, the possessors of such a useful peculiarity would increase at the expense of their close allies who did not possess it,

and would soon form a distinct group of individuals breeding together and constituting a species-the first species of the new genus. This species having in time supplanted the parent species, and being better adapted than it for the battle of life, would almost certainly cover a wider area, and thus come into competition with several of the allied species of the old genus, some of which it would also probably supplant, and occupy the areas they formerly occupied. But as the old genus had been modified into distinct species (differing, perhaps, slightly in colour or habits in accordance with the varying physical conditions), so the now widespread species of the new genus would vary, and become modified in a somewhat analogous manner, forming a genus consisting of several species. Now, if the generic form thus produced was one of great inherent vigour and adaptability, and if the peculiarity of structure it possessed was of considerable importance, it would become what Mr. Darwin terms a dominant group: that is, it would spread widely over the earth under various modified forms suited to the various conditions it became subject to. At last it would reach its maximum of development, and cease to spread further, either owing to its inability to adapt itself to further changes of climate, &c., or, what is more likely, from its coming into competition with other dominant groups which had in like manner spread from some other centres.

Now, during all this time, which may be termed the period of growth of the genus, its area will have been almost necessarily continuous, and the areas occupied by its several species (also continuous) will probably overlap each other. But now commences its period of decay. Other groups of the same or allied families have given rise to varieties which have also become dominant species and genera, which, under the somewhat changed physical conditions that in time have come about, beat it in the battle of life, and force it to retire step by step from the vast area it had overrun. First one species and then another will dwindle away and become finally extinct, and by so doing will necessarily leave gaps in its area of distribution. This process going steadily on, the time will