First, then, we can understand how the tiger-beetles (Cicindelidæ) are absent; since they are insects which have a short weak flight, but yet to whom flight is necessary. If a few had been blown over to Madeira, they would soon have become exterminated. The same thing applies to the Melolonthidæ, Cetoniidæ, Eumolpidæ, and Galerucidæ,—all flower and foliage-haunting insects, yet bulky and of comparatively feeble powers of flight. Again, all the large genera abundant in South Europe, which have been mentioned above as absent from Madeira, are wholly apterous (or without wings), and thus their absence is a most significant fact; for it proves that in the case of all insects of moderate size, flight was essential to their reaching the island, which could not have been the case had there been a land connection. There are, however, one or two curious exceptions to the absence of these wholly apterous European genera in Madeira, and as in each case the reason of their being exceptions can be pointed out, they are eminently exceptions that prove the rule. Two of the apterous species common to Europe and Madeira are found always in ants' nests; and as ants, when winged, fly in great swarms and are carried by the wind to great distances, they may have conveyed the minute eggs of these very small beetles. Two European species of Blaps occur in Madeira, but these are house beetles, and are admitted to have been introduced by man. There are also three species of Meloe, of which two are European and one peculiar. These are large, sluggish, wingless insects, but they have a most extraordinary and exceptional metamorphosis, the larvae in the first state being minute active insects parasitic on bees, and thus easily conveyed across the ocean. This case is most suggestive, as it accounts for what would be otherwise a difficult anomaly. Another case, not quite so easily explained, is that of the genus Acalles, which is very abundant in all the Atlantic islands and also occurs in South Europe, but is always apterous. It is however closely allied to another genus, Cryptorhynchus, which is apterous in some species, winged in others. We may therefore well suppose that the ancestors of Acalles were once in the same condition, and that some of the winged forms reached Madeira, the genus having since become wholly apterous.
We may look at this curious subject in another way. The Coleoptera of Madeira may be divided into those which are found also in Europe or the other islands, and those which are peculiar to it. On the theory of introduction by accidental immigration across the sea, the latter must be the more ancient, since they have had time to become modified; while the former are comparatively recent, and their introduction may be supposed to be now going on. The peculiar influence of Madeira in aborting the wings should, therefore, have acted on the ancient and changed forms much more powerfully than on the recent and unchanged forms. On carefully comparing the two sets of insects (omitting those which have almost certainly been introduced by man) we find, that out of 263 species which have a wide range, only 14 are apterous; while the other class, consisting of 393 species, has no less than 178 apterous; or about 5 per cent in the one case, and 45 per cent in the other.[[8]] On the theory of a land connection as the main agent in introducing the fauna, both groups must have been introduced at or about the same time, and why one set should have lost their wings and the other not, is quite inexplicable.
Taking all these singular facts, in connection with the total absence of all truly indigenous terrestrial mammalia and reptiles from these islands—even from the extensive group of the Canaries so comparatively near to the continent, we are forced to reject the theory of a land connection as quite untenable; and this view becomes almost demonstrated by the case of the Azores, which being so much further off, and surrounded by such a vast expanse of deep ocean, could only have been connected with Europe at a far remoter epoch, and ought therefore to exhibit to us a fauna composed almost entirely of peculiar forms both of birds and insects. Yet, so far from this being the case, the facts are exactly the reverse. Far more of the birds and insects are identical with those of Europe than in the other islands, and this difference is clearly traced to the more tempestuous atmosphere, which is shown to be even now annually bringing fresh immigrants (both birds and insects) to its shores. We here see nature actually at work; and if the case of Madeira rendered her mode of action probable, that of the Azores may be said to demonstrate it.
Mr. Wollaston has objected to this view that "storms and hurricanes" are somewhat rare in the latitude of Madeira and the Canaries; but this little affects the question, since the time allowed for such operations is so ample. If but one very violent storm happened in a century, and ten such storms recurred before a single species of insect was introduced into Madeira, that would be more than sufficient to people it, as we now find it, with a varied fauna. But he also adds the important information that the ordinary winds blow almost uninterruptedly from the north-east, so that there would be always a chance of a little stronger wind than usual bringing insect, or larva, or egg, attached to leaves or twigs. Neither Mr. Wollaston, Mr. Crotch, Mr. A. Murray, nor any other naturalist who upholds the land-connection theory, has attempted to account for the fact of the absence of so many extensive groups of insects that ought to be present, as well as of all small mammalia and reptiles.
Cape Verd Islands.—There is yet another group of Atlantic islands which is very little known, and which is usually considered to be altogether African—the Cape Verd Islands, situated between 300 and 400 miles west of Senegal, and a little to the south of the termination of the Sahara. The evidence that we possess as to the productions of these islands, shows that, like the preceding groups, they are truly oceanic, and have probably derived their fauna from the desert and the Canaries to the north-east of them rather than from the fertile and more truly Ethiopian districts of Senegal and Gambia to the east. There is a mingling of the two faunas, but the preponderance seems to be undoubtedly with the Palæarctic rather than with the Ethiopian. I owe to Mr. R. B. Sharpe of the British Museum, a MS. list of the birds of these islands, twenty-three species in all. Of these eight are of wide distribution and may be neglected. Seven are undoubted Palæarctic species, viz.:—Milvus ictinus, Sylvia atricapilla, S. conspicillata, Corvus corone, Passer salicarius, Certhilauda desertorum, Columba livia. Three are peculiar species, but of Palæarctic genera and affinities, viz.:—Calamoherpe brevipennis, Ammomanes cinctura, and Passer jagoensis. Against this we have to set two West African species, Estrilda cinerea and Numida meleagris, both of which were probably introduced by man; and three which are of Ethiopian genera and affinities, viz.:—Halcyon erythrorhyncha, closely allied to H. semicærulea of Arabia and North-east Africa, and therefore almost Palæarctic; Accipiter melanoleucus; and Pyrrhulauda nigriceps, an Ethiopian form; but the same species occurs in the Canaries.
The Coleoptera of these islands have been also collected by Mr. Wollaston, and he finds that they have generally the same European character as those of the Canaries and Madeira, several of the peculiar Atlantic genera, such as Acalles and Hegeter, occurring, while others are represented by new but closely allied genera. Out of 275 species 91 were found also in the Canaries and 81 in the Madeiran group; a wonderful amount of similarity when we consider the distance and isolation of these islands and their great diversity of climate and vegetation.
This connection of the four groups of Atlantic islands now referred to, receives further support from the occurrence of land-shells of the subgenus Leptaxis in all the groups, as well as in Majorca; and by another subgenus, Hemicycla, being common to the Canaries and Cape Verd islands. Combining these several classes of facts, we seem justified in extending the Mediterranean sub-region to include the Cape Verd Islands.
III.—The Siberian Sub-region, or Northern Asia.
This large and comparatively little-known subdivision of the Palæarctic region, extends from the Caspian Sea to Kamschatka and Behring's Straits, a distance of about 4,000 miles; and from the shores of the Arctic Ocean to the high Himalayas of Sikhim in North Latitude 29°, on the same parallel as Delhi. To the east of the Caspian Sea and the Ural Mountains is a great extent of lowland which is continued round the northern coast, becoming narrower as it approaches the East Cape. Beyond this, in a general E.N.E. direction, rise hills and uplands, soon becoming lofty mountains, which extend in an unbroken line from the Hindu Koosh, through the Thian Shan, Altai and Yablonoi Mountains, to the Stanovoi range in the north-eastern extremity of Asia. South of this region is a great central basin, which is almost wholly desert; beyond which again is the vast plateau of Thibet, with the Kuenlun, Karakorum, and Himalayan snow-capped ranges, forming the most extensive elevated district on the globe.
The superficial aspects of this vast territory, as determined by its vegetable covering, are very striking and well contrasted. A broad tract on the northern coast, varying from 150 to 300 and even 500 miles wide, is occupied by the Tundras or barrens, where nothing grows but mosses and the dwarfest Arctic plants, and where the ground is permanently frozen to a great depth. This tract has its greatest southern extension between the rivers Obi and Yenesi, where it reaches the parallel of 60° north latitude. Next to this comes a vast extent of northern forests, mostly of conifers in the more northern and lofty situations, while deciduous trees preponderate in the southern portions and in the more sheltered valleys. The greatest extension of this forest region is north of Lake Baikal, where it is more than 1,200 miles wide. These forests extend along the mountain ranges to join those of the Hindu Koosh. South of the forests the remainder of the sub-region consists of open pasture-lands and vast intervening deserts, of which the Gobi, and those of Turkestan between the Aral and Balkash lakes, are the most extensive. The former is nearly 1,000 miles long, with a width of from 200 to 350 miles, and is almost as complete a desert as the Sahara.