Of the three groups of Atlantic islands belonging to this sub-region, the Canaries are nearest to the Continent, some of the islands being only about fifty miles from the coast of Africa. They are, however, separated from the mainland by a very deep channel (more than 5,000 feet), as shown on our general map. The islands extend over a length of 300 miles; they are very mountainous and wholly volcanic, and the celebrated peak of Teneriffe rises to a height of more than 12,000 feet. The small Madeira group is about 400 miles from the coast of Morocco and 600 from the southern extremity of Portugal; and there is a depth of more than 12,000 feet between it and the continent. The Azores are nearly 1,000 miles west of Lisbon. They are quite alone in mid-Atlantic, the most westerly islands being nearer Newfoundland than Europe, and are surrounded by ocean depths of from 12,000 to 18,000 feet. It will be convenient to take these islands first in order.
Azores.—Considering the remoteness of this group from every other land, it is surprising to find as many as fifty-three species of birds inhabiting or visiting the Azores; and still more to find that they are of Palæarctic genera and, with one exception, all of species found either in Europe, North Africa, Madeira, or the Canaries. The exception is a bullfinch peculiar to the islands, but closely allied to a European species. Of land birds there are twenty-two, belonging to twenty-one genera, all European. These genera are Cerchneis, Buteo, Asio, Strix, Turdus, Oriolus, Erithacus, Sylvia, Regulus, Saxicola, Motacilla, Plectrophanes, Fringilla, Pyrrhula, Serinus, Sturnus, Picus, Upupa, Columba, Caccabis, and Coturnix. Besides the bullfinch (Pyrrhula) other species show slight differences from their European allies, but not such as to render them more than varieties. The only truly indigenous mammal is a bat of a European species. Nine butterflies inhabit the Azores; eight of them are European species, one North American. Of beetles 212 have been collected, of which no less than 175 are European species; of the remainder, nineteen are found in the Canaries or Madeira, three in South America, while fourteen are peculiar to the islands.
Now these facts (for which we are indebted to Mr. Godman's Natural History of the Azores) are both unexpected and exceedingly instructive. In most other cases of remote Oceanic islands, a much larger proportion of the fauna is endemic, or consists of peculiar species and often of peculiar genera; as is well shown by the case of the Galapagos and Juan Fernandez, both much nearer to a continent and both containing peculiar genera and species of birds. Now we know that the cause and meaning of this difference is, that in the one case the original immigration is very remote and has never or very rarely been repeated, so that under the unchecked influence of new conditions of life the species have become modified; in the other case, either the original immigration has been recent, or if remote has been so frequently repeated that the new comers have kept up the purity of the stock, and have not allowed time for the new conditions to produce the effect we are sure they would in time produce if not counteracted. For Mr. Godman tells us that many of the birds are modified—instancing the gold-crested wren, blackcap, and rock dove—and he adds, that the modification all tends in one direction—to produce a more sombre plumage, a greater strength of feet and legs, and a more robust bill. We further find, that four of the land-birds, including the oriole, snow-bunting, and hoopoe, are not resident birds, but straggle accidentally to the islands by stress of weather; and we are told that every year some fresh birds are seen after violent storms. Add to this the fact, that the number of species diminishes in the group as we go from east to west, and that the islands are subject to fierce and frequent storms blowing from every point of the compass,—and we have all the facts requisite to enable us to understand how this remote archipelago has become stocked with animal life without ever probably being much nearer to Europe than it is now. For the islands are all volcanic, the only stratified rock that occurs being believed to be of Miocene date.
Madeira and the Canaries.—Coming next to Madeira, we find the number of genera of land birds has increased to twenty-eight, of which seventeen are identical with those of the Azores. Some of the commonest European birds—swallows, larks, sparrows, linnets, goldfinches, ravens, and partridges, are among the additions. A gold-crested warbler, Regulus Maderensis, and a pigeon, Columba Trocaz, are peculiar to Madeira.
In the Canaries we find that the birds have again very much increased, there being more than fifty genera of land birds; but the additions are wholly European in character, and almost all common European species. We find a few more peculiar species (five), while some others, including the wild canary, are common to all the Atlantic Islands or to the Canaries and Madeira. Here, too, the only indigenous mammalia are two European species of bats.
Land Shells.—The land shells of Madeira offer us an instructive contrast to the birds of the Atlantic Islands. About fifty-six species have been found in Madeira, and forty-two in the small adjacent island of Porto Santo, but only twelve are common to both, and all or almost all are distinct from their nearest allies in Europe and North Africa. Great numbers of fossil shells are also found in deposits of the Newer Pliocene period; and although these comprise many fresh species, the two faunas and that of the continent still remain almost as distinct from each other as before. It has been already stated (p. [31]) that the means by which land mollusca have been carried across arms of the sea are unknown, although several modes may be suggested; but it is evidently a rare event, requiring some concurrence of favourable conditions not always present. The diversity and specialization of the forms of these animals is, therefore, easily explained by the fact, that, once introduced they have been left to multiply under the influence of a variety of local conditions, which inevitably lead, in the course of ages, to the formation of new varieties and new species.
Coleoptera.—The beetles of Madeira and the Canaries have been so carefully collected and examined by Mr. T. V. Wollaston, and those of the Azores described and compared by Mr. Crotch, and they illustrate so many curious points in geographical distribution, that it is necessary to give some account of them. No less than 1,480 species of beetles have been obtained from the Canaries and Madeira, only 360 of which are European, the remainder being peculiar to the islands. The Canaries are inhabited by a little over 1,000 species, Madeira by about 700, while 240 are common to both; but it is believed that many of these have been introduced by man. In the Azores, 212 species have been obtained, of which 175 are European; showing, as in the birds, as closer resemblance to the European fauna than in the other islands which, although nearer to the continent, offer more shelter and are situated in a less tempestuous zone. Of the non-European species in the Azores, 19 are found also in the other groups of islands, 14 are peculiar, while 3 are American. Of the European species, 132 are found also in the other Atlantic islands, while 43 have reached the Azores only. This is interesting as showing to how great an extent the same insects reach all the islands, notwithstanding the difference of latitude and position; and it becomes of great theoretical importance, when we find how many extensive families and genera are altogether absent.
The Madeira group has been more thoroughly explored than any other, and its comparatively remote situation, combined with its luxuriant vegetation, have been favourable to the development and increase of the peculiar forms which characterize all the Atlantic islands in a more or less marked degree. A consideration of some of its peculiarities will, therefore, best serve to show the bearing of the facts presented by the insect fauna of the Atlantic islands, on the general laws of distribution. The 711 species of beetles now known from the Madeira group, belong to 236 genera; and no less than 44 of these genera are not European but are peculiar to the Atlantic islands. Most of them are, however, closely allied to European genera, of which they are evidently modifications. A most curious general feature presented by the Madeiran beetles, is the total absence of many whole families and large genera abundant in South Europe. Such are the Cicindelidæ, or tiger beetles; the Melolonthidæ, or chafers; the Cetoniidæ, or rose-chafers; the Eumolpidæ and Galerucidæ, large families of Phytophagous, or leaf-eating beetles; and also the extensive groups of Elateridæ and Buprestidæ, which are each represented by but one minute species. Of extensive genera abundant in South Europe, but wholly absent in Madeira, are Carabus, Rhizotrogus, Lampyris, and other genera of Malacoderms; Otiorhynchus, Brachycerus, and 20 other genera of Curculionidæ, comprising more than 300 South European and North African species; Pimelia, Tentyra, Blaps, and 18 other genera of Heteromera, comprising about 550 species in South Europe and North Africa; and Timarcha, containing 44 South European and North African species.
Another most remarkable feature of the Madeiran Coleoptera is the unusual prevalence of apterous or wingless insects. This is especially the case with groups which are confined to the Atlantic islands, many of which consist wholly of wingless species; but it also affects the others, no less than twenty-two genera which are usually or sometimes winged in Europe, having only wingless species in Madeira; and even the same species which is winged in Europe becomes, in at least three cases, wingless in Madeira, without any other perceptible change having taken place. But there is another most curious fact noticed by Mr. Wollaston; that those species which possess wings in Madeira, often have them rather larger than their allies in Europe. These two facts were connected by Mr. Darwin, who suggested that flying insects are much more exposed to be blown out to sea and lost, than those which do not fly (and Mr. Wollaston had himself supposed that the "stormy atmosphere" of Madeira had something to do with the matter); so that the most frequent fliers would be continually weeded out, while the more sluggish individuals, who either could not or would not fly, remained to continue the race; and this process going on from generation to generation, would, on the well-ascertained principles of selection and abortion by disuse, in time lead to the entire loss of wings by those insects to whom wings were not a necessity. But those whose wings were essential to their existence would be acted upon in another way. All these must fly to obtain their food or provide for their offspring, and those that flew best would be best able to battle with the storms, and keep themselves safe, and thus those with the longest and most powerful wings would be preserved. If however all the individuals of the species were too weak on the wing to resist the storms, they would soon become extinct.[[7]]
Now this explanation of the facts is not only simple and probable in itself, but it also serves to explain in a remarkable manner some of the peculiarities and deficiencies of the Madeiran insect fauna, in harmony with the view (supported by the distribution of the birds and land shells, and in particular by the immigrant birds and insects of the Azores) that all the insects have been derived from the continent or from other islands, by immigration across the ocean, in various ways and during a long period. These deficiencies are, on the other hand, quite inconsistent with the theory (still held by some entomologists) that a land communication is absolutely necessary to account for the origin of the Madeiran fauna.