Island Life Part 8

_Oceanic Islands._--The total absence of warm-blooded terrestrial animals in an island otherwise well suited to maintain them, is held to prove that such island is no mere fragment of any existing or submerged continent, but one that has been actually produced in mid-ocean. It is true that if a continental island were to be completely submerged for a single day and then again elevated, its higher terrestrial animals would be all destroyed, and if it were situated at a considerable distance from land it would be reduced to the same zoological condition as an oceanic island. But such a complete submergence and re-elevation appears never to have taken place, for there is no single island on the globe which has the physical and geological features of a continental, combined with the zoological features of an oceanic island. It is true that some of the coral-islands may be formed upon submerged lands {245} of a continental character, but we have no proof of this; and even if it were so, the existing islands are to all intents and purposes oceanic.

We will now pa.s.s on to a consideration of some of the more interesting examples of these three cla.s.ses, beginning with oceanic islands.

All the animals which now inhabit such oceanic islands must either themselves have reached them by crossing the ocean, or be the descendants of ancestors who did so. Let us then see what are, in fact, the animal and vegetable inhabitants of these islands, and how far their presence can be accounted for. We will begin with the Azores, or Western Islands, because they have been thoroughly well explored by naturalists, and in their peculiarities afford us an important clue to some of the most efficient means of distribution among several cla.s.ses of animals.

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CHAPTER XII

OCEANIC ISLANDS:--THE AZORES AND BERMUDA

THE AZORES, OR WESTERN ISLANDS

Position and Physical Features--Chief Zoological Features of the Azores--Birds--Origin of the Azorean Bird Fauna--Insects of the Azores--Land-Sh.e.l.ls of the Azores--The Flora of the Azores--The Dispersal of Seeds--Birds as Seed-Carriers--Facilities for Dispersal of Azorean Plants--Important Deduction from the Peculiarities of the Azorean Fauna and Flora.

BERMUDA

Position and Physical Features--The Red Clay of Bermuda--Zoology of Bermuda--Birds of Bermuda--Comparison of the Bird Faunas of Bermuda and the Azores--Insects of Bermuda--Land Mollusca--Flora of Bermuda--Concluding Remarks on the Azores and Bermuda.

We will commence our investigation into the phenomena presented by oceanic islands, with two groups of the North Atlantic, in which the facts are of a comparatively simple nature and such as to afford us a valuable clue to a solution of the more difficult problems we shall have to deal with further on. The Azores and Bermuda offer great contrasts in physical features, but striking similarities in geographical position. The one is volcanic, the other coralline; but both are surrounded by a wide expanse of ocean of enormous depth, the one being about as far from Europe as the other is from America. Both are situated in the {247} temperate zone, and they differ less than six degrees in lat.i.tude, yet the vegetation of the one is wholly temperate, while that of the other is almost tropical. The productions of the one are related to Europe, as those of the other are to America, but they present instructive differences; and both afford evidence of the highest value as to the means of dispersal of various groups of organisms across a wide expanse of ocean.

THE AZORES, OR WESTERN ISLANDS.

These islands, nine in number, form a widely scattered group, situated between 37 and 39 40' N. Lat. and stretching in a south-east and north-west direction over a distance of nearly 400 miles. The largest of the islands, San Miguel, is about forty miles long, and is one of the nearest to Europe, being rather under 900 miles from the coast of Portugal, from which it is separated by an ocean 2,500 fathoms deep. The depth between the islands does not seem to be known, but the 1,000 fathom line encloses the whole group pretty closely, while a depth of about 1,800 fathoms is reached within 300 miles in all directions. These great depths render it in the highest degree improbable that the Azores have ever been united with the European continent; while their being wholly volcanic is equally opposed to the view of their having formed part of an extensive Atlantis including Madeira and the Canaries. The only exception to their volcanic structure is the occurrence in one small island only (Santa Maria) of some marine deposits of Upper Miocene age--a fact which proves some alterations of level, and perhaps a greater extension of this island at some former period, but in no way indicates a former union of the islands, or any greater extension of the whole group. It proves, however, that the group is of considerable antiquity, since it must date back to Miocene times; and this fact may be of importance in considering the origin and peculiar features of the fauna and flora. It thus appears that in all physical features the Azores correspond strictly with our physical definition of "oceanic islands," while their great distance {248} from any other land, and the depth of the ocean around them, make them typical examples of the cla.s.s. We should therefore expect them to be equally typical in their fauna and flora; and this is the case as regards the most important characteristics, although in some points of detail they present exceptional phenomena.

[Ill.u.s.tration: OUTLINE MAP OF THE AZORES.]

NOTE.-- The light tint shows where the sea is less than 1,000 fathoms deep.

The dark tint " " " more than 1,000 fathoms deep.

The figures show depths in fathoms.

_Chief Zoological Features of the Azores._[50]--The great feature of oceanic islands--the absence of all indigenous land-mammalia and amphibia--is well shown in this {249} group; and it is even carried further, so as to include all terrestrial vertebrata, there being no snake, lizard, frog, or fresh-water fish, although the islands are sufficiently extensive, possess a mild and equable climate, and are in every way adapted to support all these groups. On the other hand, flying creatures, as birds and insects, are abundant; and there is also one flying mammal--a small European bat. It is true that rabbits, weasels, rats and mice, and a small lizard peculiar to Madeira and Teneriffe, are now found wild in the Azores, but there is good reason to believe that these have all been introduced by human agency. The same may be said of the gold-fish and eels now found in some of the lakes, there being not a single fresh-water fish which is truly indigenous to the islands. When we consider that the nearest part of the group is about 900 miles from Portugal, and more than 550 miles from Madeira, it is not surprising that none of these terrestrial animals can have pa.s.sed over such a wide expanse of ocean una.s.sisted by man.

Let us now see what animals are believed to have reached the group by natural means, and thus const.i.tute its indigenous fauna. These consist of birds, insects, and land-sh.e.l.ls, each of which must be considered separately.

_Birds._--Fifty-three species of birds have been observed at the Azores, but the larger proportion (thirty-one) are either aquatic or waders--birds of great powers of flight, whose presence in the remotest islands is by no means remarkable. Of these two groups twenty are residents, breeding in the islands, while eleven are stragglers only visiting the islands occasionally, and all are common European species. The land-birds, twenty-two in number, are more interesting, four only being stragglers, while eighteen are permanent residents. The following is a list of these resident land-birds:--

1. Common Buzzard (_Buteo vulgaris_) 2. Long-eared Owl (_Asio otus_) 3. Barn Owl (_Strix flammea_) 4. Blackbird (_t.u.r.dus merula_) 5. Robin (_Erythacus rubecula_) 6. Blackcap (_Sylvia atricapilla_) {250} 7. Gold-crest (_Regulus cristatus_) 8. Wheatear (_Saxicola oenanthe_) 9. Grey Wagtail (_Motacilla sulphurea_) 10. Atlantic Chaffinch (_Fringilla tintillon_) 11. Azorean Bullfinch (_Pyrrhula murina_) 12. Canary (_Serinus canarius_) 13. Common Starling (_Sturnus vulgaris_) 14. Lesser Spotted Woodp.e.c.k.e.r (_Dryobates minor_) 15. Wood-pigeon (_Columba palumbus_) 16. Rock Dove (_Columba livia_) 17. Red-legged Partridge (_Caccabis rufa_) 18. Common Quail (_Coturnix communis_)

All the above-named birds are common in Europe and North Africa except three--the Atlantic chaffinch and the canary which inhabit Madeira and the Canary Islands, and the Azorean bullfinch, which is peculiar to the islands we are considering.

_Origin of the Azorean Bird-fauna._--The questions we have now before us are--how did these eighteen species of birds first reach the Azores, and how are we to explain the presence of a single peculiar species while all the rest are identical with European birds? In order to answer them, let us first see what stragglers now actually visit the Azores from the nearest continents. The four species given in Mr. G.o.dman's list are the kestrel, the oriole, the snow-bunting, and the hoopoe; but he also tells us that there are certainly others, and adds: "Scarcely a storm occurs in spring or autumn without bringing one or more species foreign to the islands; and I have frequently been told that swallows, larks, grebes, and other species not referred to here, are not uncommonly seen at those seasons of the year."

We have, therefore, every reason to believe that the birds which are now residents originated as stragglers, which occasionally found a haven in these remote islands when driven out to sea by storms. Some of them, no doubt, still often arrive from the continent, but these cannot easily be distinguished as new arrivals among those which are permanent inhabitants.

Many facts mentioned by Mr. G.o.dman show that this is the case. A barn-owl, much exhausted, flew on board a whaling-ship when 500 miles S.W. of the Azores; and even if it had come from {251} Madeira it must have travelled quite as far as from Portugal to the islands. Mr. G.o.dman also shot a single specimen of the wheatear in Flores after a strong gale of wind, and as no one on the island knew the bird, it was almost certainly a recent arrival.

Subsequently a few were found breeding in the old crater of Corvo, a small adjacent island; and as the species is not found in any other island of the group, we may infer that this bird is a recent immigrant in process of establishing itself.

Another fact which is almost conclusive in favour of the bird-population having arrived as stragglers is, that they are most abundant in the islands nearest to Europe and Africa. The Azores consist of three divisions--an eastern, consisting of two islands, St. Michael's and St. Mary's; a central of five, Terceira, Graciosa, St. George's, Pico, and Fayal; and a western of two, Flores and Corvo. Now had the whole group once been united to the continent, or even formed parts of one extensive Atlantic island, we should certainly expect the central group, which is more compact and has a much larger area than all the rest, to have the greatest number and variety of birds. But the fact that birds are most numerous in the eastern group, and diminish as we go westward, is entirely opposed to this theory, while it is strictly in accordance with the view that they are all stragglers from Europe, Africa, or the other Atlantic islands. Omitting oceanic wanderers, and including all birds which have probably arrived involuntarily, the numbers are found to be forty species in the eastern group, thirty-six in the central, and twenty-nine in the western.

To account for the presence of one peculiar species--the bullfinch (which, however, does not differ from the common European bullfinch more than do some of the varieties of North American birds from their type-species) is not difficult; the wonder rather being that there are not more peculiar forms. In our third chapter we have seen how great is the amount of individual variation in birds, and how readily local varieties become established wherever the physical conditions are sufficiently distinct. Now we can hardly have a greater difference of conditions {252} than between the continent of Europe or North Africa, and a group of rocky islands in mid-Atlantic, situated in the full course of the Gulf Stream and with an excessively mild though stormy climate. We have every reason to believe that special modifications would soon become established in any animals completely isolated under such conditions. But they are not, as a rule, thus completely isolated, because, as we have seen, stragglers arrive at short intervals; and these, mixing with the residents, keep up the purity of the breed. It follows, that only those species which reach the Azores at very remote intervals will be likely to acquire well-marked distinctive characters; and this appears to have happened with the bullfinch alone, a bird which does not migrate, and is therefore less likely to be blown out to sea, more especially as it inhabits woody districts. A few other Azorean birds, however, exhibit slight differences from their European allies.

There is another reason for the very slight amount of peculiarity presented by the fauna of the Azores as compared with many other oceanic islands, dependent on its comparatively recent origin. The islands themselves may be of considerable antiquity, since a few small deposits, believed to be of Miocene age, have been found on them, but there can be little doubt that their present fauna, at all events as concerns the birds, had its origin since the date of the last glacial epoch. Even now icebergs reach the lat.i.tude of the Azores but a little to the west of them; and when we consider the proofs of extensive ice-action in North America and Europe, we can hardly doubt that these islands were at that time surrounded with pack-ice, while their own mountains, reaching 7,600 feet high in Pico, would almost certainly have been covered with perpetual snow and have sent down glaciers to the sea. They might then have had a climate almost as bad as that now endured by the Prince Edward Islands in the southern hemisphere, nearly ten degrees farther from the equator, where there are no land-birds whatever, although the distance from Africa is not much greater than that of the Azores from Europe, while the vegetation is limited to a few alpine plants and mosses. This recent origin of the {253} birds accounts in a great measure for their ident.i.ty with those of Europe, because, whatever change has occurred must have been effected in the islands themselves, and in a time limited to that which has elapsed since the glacial epoch pa.s.sed away.

_Insects of the Azores._--Having thus found no difficulty in accounting for the peculiarities presented by the birds of these islands, we have only to see how far the same general principles will apply to the insects and land-sh.e.l.ls. The b.u.t.terflies, moths, and hymenoptera, are few in number, and almost all seem to be common European species, whose presence is explained by the same causes as those which have introduced the birds.

Beetles, however, are more numerous, and have been better studied, and these present some features of interest. The total number of species yet known is 212, of which 175 are European; but out of these 101 are believed to have been introduced by human agency, leaving seventy-four really indigenous. Twenty-three of these indigenous species are not found in any of the other Atlantic islands, showing that they have been introduced directly from Europe by causes which have acted more powerfully here than farther south. Besides these there are thirty-six species not found in Europe, of which nineteen are natives of Madeira or the Canaries, three are American, and fourteen are altogether peculiar to the Azores. These latter are mostly allied to species found in Europe or in the other Atlantic islands, while one is allied to an American species, and two are so distinct as to const.i.tute new genera. The following list of these peculiar species will be interesting:--

CARABIDae.

_Anchomenus aptinoides_ Allied to a species from the Canaries.

_Bembidium hesperus_ Allied to the European _B. laetum_.

DYTISCIDae.

_Agabus G.o.dmanni_ Allied to the European _A. dispar_.

COLYDIIDae.

_Tarphius wollastoni_ A genus almost peculiar to the Atlantic islands.

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ELATERIDae.

_Heteroderes azoricus_ Allied to a Brazilian species.

_Elastrus dolosus_ Belongs to a peculiar Madagascar genus!

MELYRIDae.

_Attalus miniaticollis_ Allied to a Canarian species.

RHYNCOPHORA.

_Phlaeophagus variabilis_ Allied to European and Atlantic species.

_Acalles droueti_ A Mediterranean and Atlantic genus.

_Laparocerus azoricus_ Allied to Madeiran species.

_Asynonychun G.o.dmansi_ A peculiar genus, allied to _Brachyderes_, of the south of Europe.

_Neocnemis occidentalis_ A peculiar genus, allied to the European genus _Strophosomus_.

HETEROMERA.

_Helops azoricus_ Allied to _H. vulca.n.u.s_ of Madeira.

STAPHYLINIDae.

_Xenomma melanocephala_ Allied to _X. filiforme_ from the Canaries.