Island Life/XII
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-Shells 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 temperate zone, and they differ less than six degrees in latitude, 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 from any other land, and the depth of the ocean around them, make them typical examples of the class. 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.
| 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.[102]—The great feature of oceanic islands—the absence of all indigenous land-mammalia and amphibia—is well shown in this 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 passed over such a wide expanse of ocean unassisted by man.
Let us now see what animals are believed to have reached the group by natural means, and thus constitute its indigenous fauna. These consist of birds, insects, and land-shells, 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 | (Turdus merula) |
| 5. | Robin | (Erythacus rubecula) |
| 6. | Blackcap | (Sylvia atricapilla) |
|
7. | Gold-crest | (Regulus cristatus) |
| 8. | Wheatear | (Saxicola œnanthe) |
| 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 Woodpecker | (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. Godman'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. Godman 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 Madeira it must have travelled quite as far as from Portugal to the islands. Mr. Godman 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 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 latitude 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 birds accounts in a great measure for their identity 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 passed 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-shells. The butterflies, 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 constitute new genera. The following list of these peculiar species will be interesting:—
| Carabidæ. | |
| Anchomenus aptinoides | Allied to a species from the Canaries. |
| Bembidium hesperus | Allied to the European B. lætum. |
| Dytiscidæ. | |
| Agabus godmanni | Allied to the European A. dispar. |
| Colydiidæ. | |
| Tarphius wollastoni | A genus almost peculiar to the Atlantic islands. |
|
Elateridæ. | |
| Heteroderes azoricus | Allied to a Brazilian species. |
| Elastrus dolosus | Belongs to a peculiar Madagascar genus! |
| Melyridæ. | |
| Attalus miniaticollis | Allied to a Canarian species. |
| Rhyncophora. | |
| Phlæophagus variabilis | Allied to European and Atlantic species. |
| Acalles droueti | A Mediterranean and Atlantic genus. |
| Laparocerus azoricus | Allied to Madeiran species. |
| Asynonychun godmansi | 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. vulcanus of Madeira. |
| Staphylinidæ. | |
| Xenomma melanocephala | Allied to X. filiforme from the Canaries. |
This greater amount of speciality in the beetles than in the birds may be due to two causes. In the first place many of these small insects have no doubt survived the glacial epoch, and may, in that case, represent very ancient forms which have become extinct in their native country; and in the second place, insects have many more chances of reaching remote islands than birds, for not only may they be carried by gales of wind, but sometimes, in the egg or larva state or even as perfect insects, they may be drifted safely for weeks over the ocean, buried in the light stems of plants or in the solid wood of trees in which many of them undergo their transformations. Thus we may explain the presence of three common South American species (two elaters and a longicorn), all wood-eaters, and therefore liable to be occasionally brought in floating timber by the Gulf Stream. But insects are also immensely more numerous in species than are land-birds, and their transmission would be in most cases quite involuntary, and not dependent on their own powers of flight as with birds; and thus the chances against the same species being frequently carried to the same island would be considerable. If we add to this the dependence of so many insects on local conditions of climate and vegetation, and their liability to be destroyed by insectivorous birds, we shall see that, although there may be a greater probability of insects as a whole reaching the islands, the chance against any particular species arriving there, or against the same species arriving frequently, is much greater than in the case of birds. The result is, that (as compared with Britain for example) the birds are, proportionately, much more numerous than the beetles, while the peculiar species of beetles are much more numerous than among birds, both facts being quite in accordance with what we know of the habits of the two groups. We may also remark, that the small size and obscure characters of many of the beetles renders it probable that species now supposed to be peculiar, really inhabit some parts of Europe or North Africa.
It is interesting to note that the two families which are pre-eminently wood, root, or seed eaters, are those which present the greatest amount of speciality. The two Elateridæ alone exhibit remote affinities, the one with a Brazilian the other with a Madagascar group; while the only peculiar genera belong to the Rhyncophora, but are allied to European forms. These last almost certainly form a portion of the more ancient fauna of the islands which migrated to them in pre-glacial times, while the Brazilian elater appears to be the solitary example of a living insect brought by the Gulf Stream to these remote shores. The elater, having its nearest living ally in Madagascar (Elastrus dolosus), cannot be held to indicate any independent communication between these distant islands; but is more probably a relic of a once more widespread type which has only been able to maintain itself in these localities. Mr. Crotch states that there are some species of beetles common to Madagascar and the Canary Islands, while there are several genera, common to Madagascar and South America, and some to Madagascar and Australia. The clue to these apparent anomalies is found in other genera being common to Madagascar, Africa, and South America, while others are Asiatic or Australian. Madagascar, in fact, has insect relations with every part of the globe, and the only rational explanation of such facts is, that they are indications of very ancient and once widespread groups, maintaining themselves only in a few widely separated portions of what was at one time or another the area of their distribution.
Land-shells of the Azores.—Like the insects and birds, the land-shells of these islands have a generally European aspect, but with a larger proportion of peculiar species. This was to be expected, because the means by which molluscs are carried over the sea are far less numerous and varied than in the case of insects;[103] and we may therefore conclude that their introduction is a very rare event, and that a species once arrived remains for long periods undisturbed by new arrivals, and is therefore more likely to become modified by the new conditions, and then fixed as a distinct type. Out of the sixty-nine known species, thirty-seven are common to Europe or the other Atlantic islands, while thirty-two are peculiar, though almost all are distinctly allied to European types. The majority of these shells, especially the peculiar forms, are very small, and many of them may date back to beyond the glacial epoch. The eggs of these would be exceedingly minute, and might occasionally be carried on leaves or other materials during gales of exceptional violence and duration, while others might be conveyed with the earth that often sticks to the feet of birds. There are also, probably, other unknown means of conveyance; but however this may be, the general character of the land-molluscs is such as to confirm the conclusions we have arrived at from a study of the birds and insects,—that these islands have never been connected with a continent, and have been peopled with living things by such forms only as in some way or other have been able to reach them across many hundred miles of ocean.
The Flora of the Azores.—The flowering-plants of the Azores have been studied by one of our first botanists, Mr. H. C. Watson, who has himself visited the islands and made extensive collections; and he has given a complete catalogue of the species in Mr. Godman's volume. As our object in the present work is to trace the past history of the more important islands by means of the forms of life that inhabit them, and as for this purpose plants are sometimes of more value than any class of animals, it will be well to take advantage of the valuable materials here available, in order to ascertain how far the evidence derived from the two organic kingdoms agrees in character; and also to obtain some general results which may be of service in our discussion of more difficult and more complex problems.
There are in the Azores 480 known species of flowering-plants and ferns, of which no less than 440 are found also in Europe, Madeira, or the Canary Islands; while forty are peculiar to the Azores, but are more or less closely allied to European species. As botanists are no less prone than zoologists to invoke former land-connections and continental extensions to account for the wide dispersal of objects of their study, it will be well to examine somewhat closely what these facts really imply.
The Dispersal of Seeds.—The seeds of plants are liable to be dispersed by a greater variety of agents than any other organisms, while their tenacity of life, under varying conditions of heat and cold, drought and moisture, is also exceptionally great. They have also an advantage, in that the great majority of flowering plants have the sexes united in the same individual, so that a single seed in a state fit to germinate may easily stock a whole island. The dispersal of seeds has been studied by Sir Joseph Hooker, Mr. Darwin, and many other writers, who have made it sufficiently clear that they are in many cases liable to be carried enormous distances. An immense number are specially adapted to be carried by the wind, through the possession of down or hairs, or membranous wings or processes; while others are so minute, and produced in such profusion, that it is difficult to place a limit to the distance they might be carried by gales of wind or hurricanes. Another class of somewhat heavier seeds or dry fruits are capable of being exposed for a long time to sea-water without injury. Mr. Darwin made many experiments on this point, and he found that many seeds, especially of Atriplex, Beta, oats, Capsicum, and the potato, grew after 100 days' immersion, while a large number survived fifty days. But he also found that most of them sink after a few days' immersion, and this would certainly prevent them being floated to very great distances. It is very possible, however, that dried branches or flower-heads containing seeds would float longer, while it is quite certain that many tropical seeds do float for enormous distances, as witness the double cocoa-nuts which cross the Indian ocean from the Seychelle Islands to the coast of Sumatra, and the West Indian beans which frequently reach the west coast of Scotland. There is therefore ample evidence of the possibility of seeds being conveyed across the sea for great distances by winds and surface currents.[104]
Birds as Seed-carriers.—The great variety of fruits that are eaten by birds afford a means of plant-dispersal in the fact that seeds often pass through the bodies of birds in a state well-fitted for germination; and such seeds may occasionally be carried long distances by this means. Of the twenty-two land-birds found in the Azores, half are, more or less, fruit-eaters, and these may have been the means of introducing many plants into the islands.
Birds also frequently have small portions of earth on their feet; and Mr. Darwin has shown by actual experiment that almost all such earth contains seeds. Thus in nine grains of earth on the leg of a woodcock a seed of the toad-rush was found which germinated; while a wounded red-legged partridge had a ball of earth weighing six and a half ounces adhering to its leg, and from this earth Mr. Darwin raised no less than eighty-two separate plants of about five distinct species. Still more remarkable was the experiment with six and three-quarter ounces of mud from the edge of a little pond, which, carefully treated under glass, produced 537 distinct plants! This is equal to a seed for every six grains of mud, and when we consider how many birds frequent the edges of ponds in search of food, or come there to drink, it is evident that great numbers of seeds may be dispersed by this means.
Many seeds have hispid awns, hooks, or prickles which readily attach them to the feathers of birds, and a great number of aquatic birds nest inland on the ground; and as these are pre-eminently wanderers, they must often aid in the dispersal of such plants.[105]
Facilities for Dispersal of Azorean Plants.—Now in the course of very long periods of time the various causes here enumerated would be sufficient to stock the remotest islands with vegetation, and a considerable part of the Azorean flora appears well adapted to be so conveyed. Of the 439 flowering-plants in Mr. Watson's list, I find that about forty-five belong to genera that have either pappus or winged seeds; sixty-five to such as have very minute seeds; thirty have fleshy fruits such as are greedily eaten by birds; several have hispid seeds; and eighty-four are glumaceous plants, which are all probably well-adapted for being carried partly by winds and partly by currents, as well as by some of the other causes mentioned. On the other hand we have a very suggestive fact in the absence from the Azores of most of the trees and shrubs with large and heavy fruits, however common they may be in Europe. Such are oaks, chestnuts, hazels, apples, beeches, alders, and firs; while the only trees or large shrubs are the Portugal laurel, myrtle, laurestinus, elder, Laurus canariensis, Myrica faya, and a doubtfully peculiar juniper—all small berry-bearers, and therefore likely to have been conveyed by one or other of the modes suggested above.
There can be little doubt that the truly indigenous flora of the islands is far more scanty than the number of plants recorded would imply, because a large but unknown proportion of the species are certainly importations, voluntary or involuntary, by man. As, however, the general character of the whole flora is that of the south-western peninsula of Europe, and as most of the introduced plants have come from the same country, it is almost impossible now to separate them, and Mr. Watson has not attempted to do so. The whole flora contains representatives of eighty natural orders and 250 genera: and even if we suppose that one-half the species only are truly indigenous, there will still remain a wonderfully rich and varied flora to have been carried, by the various natural means above indicated, over 900 miles of ocean, more especially as the large proportion of species identical with those of Europe shows that their introduction has been comparatively recent, and that it is, probably (as in the case of the birds) still going on. We may therefore feel sure that we have here by no means reached the limit of distance to which plants can be conveyed by natural means across the ocean; and this conclusion will be of great value to us in investigating other cases where the evidence at our command is less complete, and the indications of origin more obscure or conflicting.
Of the forty species which are considered to be peculiar to the islands, all are allied to European plants except six, whose nearest affinities are in the Canaries or Madeira. Two of the Compositæ are considered to be distinct genera, but in this order generic divisions rest on slight technical distinctions; and the Campanula vidalii is very distinct from any other known species. With these exceptions, most of the peculiar Azorean species are closely allied to European plants, and are in several cases little more than varieties of them. While therefore we may believe that the larger part of the existing flora reached the islands since the glacial epoch, a portion of it may be more ancient, as there is no doubt that a majority of the species could withstand some lowering of temperature; while in such a warm latitude and surrounded with sea, there would always be many sunny and sheltered spots in which even tender plants might flourish.
Important Deduction from the Peculiarities of the Azorean Fauna and Flora.—There is one conclusion to be drawn from the almost wholly European character of the Azorean fauna and flora which deserves special attention, namely, that the peopling of remote islands is not due so much to ordinary or normal, as to extraordinary and exceptional causes. These islands lie in the course of the south-westerly return trades and also of the Gulf Stream, and we should therefore naturally expect that American birds, insects, and plants would preponderate if they were conveyed by the regular winds and currents, which are both such as to prevent European species from reaching the islands. But the violent storms to which the Azores are liable blow from all points of the compass; and it is evidently to these, combined with the greater proximity and more favourable situation of the coasts of Europe and North Africa, that the presence of a fauna and flora so decidedly European is to be traced.
The other North Atlantic Islands—Madeira, the Canaries, and the Cape de Verdes—present analogous phenomena to those of the Azores, but with some peculiarities dependent on their more southern position, their richer vegetation, and perhaps their greater antiquity. These have been sufficiently discussed in my Geographical Distribution of Animals (Vol. I. pp. 208-215); and as we are now dealing with what may be termed typical examples of oceanic islands, for the purpose of illustrating the laws, and solving the problems presented by the dispersal of animals, we will pass on to other cases which have been less fully discussed in that work.
BERMUDA.
The Bermudas are a small group of low islands formed of coral, and blown coral-sand consolidated into rock. They are situated in 32° N. Lat., about 700 miles from North Carolina, and somewhat farther from the Bahama Islands, and are thus rather more favourably placed for receiving immigrants from America and its islands than the Azores are with respect to Europe. There are about 100 islands and islets in all, but their total area does not exceed fifty square miles. They are surrounded by reefs, some at a distance of thirty miles from the main group; and the discovery of a layer of earth with remains of cedar-trees forty-eight feet below the present high-water mark shows that the islands have once been more extensive and probably included the whole area now occupied by shoals and reefs.[106] Immediately beyond these reefs, however, extends a very deep ocean, while about 450 miles distant in a south-east direction, the deepest part of the North Atlantic is reached, where soundings of 3,825 and 3,875 fathoms have been obtained. It is clear therefore that these islands are typically oceanic.
| Note.— | The light tint | indicates | sea | less than 1,000 fathoms deep. |
| The dark tint | " | " | more than 1,000 fathoms deep. | |
| The figures show the depth in fathoms. | ||||
Soundings were taken by the Challenger in four different directions around Bermuda, and always showed a rapid deepening of the sea to about 2,500 fathoms. This was so remarkable, that in his reports to the Admiralty, Captain Nares spoke of Bermuda as "a solitary peak rising abruptly from a base only 120 miles in diameter;" and in another place as "an isolated peak rising abruptly from a very small base." These expressions show that Bermuda is looked upon as a typical example of an "oceanic peak"; and on examining the series of official reports of the Challenger soundings, I can find no similar case, although some coasts, both of continents and islands, descend more abruptly. In order to show, therefore, what is the real character of this peak, I have drawn a section of it on a true scale from the soundings taken in a north and south direction where the descent is steepest. It will be seen that the slope is on both sides very easy, being 1 in 16 on the south, and 1 in 19 on the north. The portion nearest the islands will slope more rapidly, perhaps reaching in places 1 in 10; but even this is not steeper than many country roads in hilly countries, while the remainder would be a hardly perceptible slope. Although generally very low, some parts of these islands rise to 250 feet above the sea-level, consisting of various kinds of limestone rock, sometimes soft and friable, but often very hard and even crystalline. It consists of beds which sometimes dip as much as 30°, and which also show great contortions, so that at first sight the islands appear to exhibit on a small scale the phenomena of a disturbed Palæozoic district. It has however long been known that these rocks are all due to the wind, which blows up the fine calcareous sand, the product of the disintegration of coral, shells, serpulæ, and other organisms, forming sand-hills forty and fifty feet high, which move gradually along, overwhelming the lower tracts of land behind them. These are consolidated by the percolation of rain-water, which dissolves some of the lime from the more porous tracts and deposits it lower down, filling every fissure with stalagmite.
The Red Clay of Bermuda.—Besides the calcareous rocks there is found in many parts of the islands a layer of red earth or clay, containing about thirty per cent. of oxide of iron. This very closely resembles, both in colour and chemical composition, the red clay of the ocean floor, found widely spread in the Atlantic at depths of from 2,300 to 3,150 fathoms, and occurring abundantly all round Bermuda. It appears, therefore, at first sight, as if the ocean bed itself has been here raised to the surface, and a portion of its covering of red clay preserved; and this is the view adopted by Mr. Jones in his paper on the "Botany of Bermuda." He says, after giving the analysis: "This analysis tends to convince us that the deep chocolate-coloured red clay of the islands found in the lower levels, and from high-water mark some distance into the sea, originally came from the ocean floor, and that when by volcanic agency the Bermuda column was raised from the depths of the sea, its summit, most probably broken in outline, appeared above the surface covered with this red mud, which in the course of ages has but slightly changed its composition, and yet possesses sufficient evidence to prove its identity with that now lying contiguous to the base of the Bermuda column." But in his Guide to Bermuda Mr. Jones tells us that this same red earth has been found, two feet thick, under coral rock at a depth of forty-two feet below low-water mark, and that it "rested on a bed of compact calcareous sandstone." Now it is quite certain that this "calcareous sandstone" was never formed at the bottom of the deep ocean 700 miles from land; and the occurrence of the red earth at different levels upon coralline sand rock is therefore more probably due to some process of decomposition of the rock itself, or of the minute organisms which abound in the blown sand.[107]
Zoology of Bermuda.—As might be expected from their extreme isolation, these islands possess no indigenous terrestrial mammalia, frogs, or snakes.[108] There is however one lizard, which Professor Cope considers to be distinct from any American species, and which he has named Plestiodon (Eumeces) longirostris. It is said to be most nearly allied to Eumeces quinquelineatus of the south-eastern States, from which it differs in having nearly ten more rows of scales, the tail thicker, and the muzzle longer. In colour it is ashy brown above, greenish blue beneath, with a white line black-margined on the sides, and it seems to be tolerably abundant in the islands. This lizard is especially interesting as being the only vertebrate animal which exhibits any peculiarity.
Birds.—Notwithstanding its small size, low altitude and remote position, a great number of birds visit Bermuda annually, some in large numbers, others only as accidental stragglers. Altogether, over 180 species have been recorded, rather more than half being wading and swimming birds, whose presence is not so much to be wondered at as they are great wanderers; while about eighty-five are land birds, many of which would hardly be supposed capable of flying so great a distance. Of the 180 species, however, about thirty have only been seen once, and a great many more are very rare; but about twenty species of land birds are recorded as tolerably frequent visitors, and nearly half these appear to come every year.
There are only eleven species which are permanent residents on the island—eight land, and three water birds, and of these one has been almost certainly introduced. These resident birds are as follows:—
- 1. Galeoscoptes carolinensis. (The Cat bird.) Migrates along the east coast of the United States.
- 2. Sialia sialis. (The Blue bird.) Migrates along the east coast.
- 3. Vireo novæboracensis. (The White-eyed green Tit.) Migrates along the east coast.
- 4. Passer domesticus. (The English Sparrow.) ? Introduced.
- 5. Corvus americanus. (The American Crow.) Common over all North America.
- 6. Cardinalis virginianus. (The Cardinal bird.) Migrates from Carolina southward.
- 7. Chamœpelia passerina. (The ground Dove.) Louisiana, W. Indies, and Mexico.
- 8. Ortyx virginianus. (The American Quail.) New England to Florida.
- 9. Ardea herodias. (The Great Blue Heron.) All North America.
- 10. Gailinula galeata. (The Florida Gallinule.) Temperate and tropical North America.
- 11. Phäeton flavirostris. (The Tropic Bird.)
It will be seen that these are all very common North American birds, and most of them are constant visitors from the mainland, so that however long they may have inhabited the islands there has been no chance for them to have acquired any distinctive characters owing to the want of isolation.
Among the most regular visitants which are not resident, are the common N. American kingfisher (Ceryle alcyon), the night-hawk (Chordeiles virginianus), the wood wagtail (Siurus novæboracensis), the snow-bunting (Plectrophanes nivalis), and the wide-ranging rice-bird (Dolichonyx oryzivora), all very common and widespread in North America.
Comparison of the Bird-faunas of Bermuda and the Azores.—The bird-fauna of Bermuda thus differs from that of the Azores, in the much smaller number of resident species, and the presence of several regular migrants. This is due, first, to the small area and little varied surface of these islands, as well as to their limited flora and small supply of insects not affording conditions suitable for the residence of many species all the year round; and, secondly, to the peculiarity of the climate of North America, which causes a much larger number of its birds to be migratory than in Europe. The Northern United States and Canada, with a sunny climate, luxuriant vegetation, and abundant insect-life during the summer, supply food and shelter to an immense number of insectivorous and frugivorous birds; so that during the breeding season Canada is actually richer in bird-life than Florida. But as the severe winter comes on all these are obliged to migrate southward, some to Carolina, Georgia, and Florida, others as far as the West Indies, Mexico, or even to Guatemala and South America.
Every spring and autumn, therefore a vast multitude of birds, belonging to more than a hundred distinct species, migrate northward or southward in Eastern America. A large proportion of these pass along the Atlantic coast, and it has been observed that many of them fly some distance out to sea, passing straight across bays from headland to headland by the shortest route.
Now as the time of these migrations is the season of storms, especially the autumnal one, which nearly coincides with the hurricanes of the West Indies and the northerly gales of the coast of America, the migrating birds are very liable to be carried out to sea. Sometimes they may, as Mr. Jones suggests, be carried up by local whirlwinds to a great height, where meeting with a westerly or north-westerly gale, they are rapidly driven sea-ward. The great majority no doubt perish, but some reach the Bermudas and form one of its most striking autumnal features. In October, Mr. Jones tells us, the sportsman enjoys more shooting than at any other time. The violent revolving gales, which occur almost weekly, bring numbers of birds of many species from the American continent, the different members of the duck tribe forming no inconsiderable portion of the whole; while the Canada goose, and even the ponderous American swan, have been seen amidst the migratory host. With these come also such delicate birds as the American robin (Turdus migratorius), the yellow-rumped warbler (Dendrœca coronata), the pine warbler (Dendrœca pinus), the wood wagtail (Siurus novæboracensis), the summer red bird (Pyranga æstiva), the snow-bunting (Plectrophanes nivalis), the red-poll (Ægiothus linarius), the king bird (Tyrannus carolinensis), and many others. It is no doubt in consequence of this repeated immigration that none of the Bermuda birds have acquired any special peculiarity constituting even a distinct variety; for the few species that are resident and breed in the islands are continually crossed by individual immigrants of the same species from the mainland.
Four European birds also have occurred in Bermuda;—the wheatear (Saxicola œnanthe), which visits Iceland and Lapland and sometimes the northern United States; the skylark (Alauda arvensis), but this was probably an imported bird or an escape from some ship; the land-rail (Crex pratensis), which also wanders to Greenland and the United States; and the common snipe (Scolopax gallinago), which occurs not unfrequently in Greenland but has not yet been noticed in North America. It is however so like the American snipe (S. wilsoni), that a straggler might easily be overlooked.
Two small bats of N. American species also occasionally reach the island, while two others from the West Indies have more rarely occurred, and these are the only wild mammalia except rats and mice.
Insects of Bermuda.—Insects appear to be very scarce; but it is evident from the lists given by Mr. Jones, and more recently by Professor Heilprin, that only the more conspicuous species have been yet collected. These comprise nineteen beetles, eleven bees and wasps, twenty-six butterflies and moths, nine flies, and the same number of Hemiptera, Orthoptera, and Neuroptera respectively. All appear to be common North American or West Indian species; but until some competent entomological collector visits the islands it is impossible to say whether there are or are not any peculiar species.[109]
Land Mollusca.—The land-shells of the Bermudas are somewhat more interesting, as they appear to be the only group of animals except reptiles in which there are any peculiar species. The following list was kindly furnished me by Mr. Thomas Bland of New York, who has made a special study of the terrestrial molluscs of the West Indian Islands, from which those of the Bermudas have undoubtedly been derived. The nomenclature has been corrected in accordance with the list given in Professor Heilprin's work on the islands. The species which are peculiar to the islands are indicated by italics.
| List of the Land-Shells of Bermuda. | ||
| 1. | Succinea fulgens. (Lea.) | Also in Cuba. |
| 2. | ,, Bermudensis. (Pfeiffer.) | ,, Barbadoes (?) |
| 3. | ,, margarita. (Pfr.) | ,, Haiti. |
| 4. | Pœcilozonites Bermudensis. (Pfr.) | A peculiar form, which, according to Mr. Binney, "cannot be placed in any recognised genus." A larger sub-fossil variety also occurs, named H. Nelsoni, by Mr. Bland, and which appears sufficiently distinct to be classed as another species. |
| 5. | ,, circumfirmatas (Redfield.) | |
| 6. | ,, discrepans. (Pfr.) | |
| 7. | ,, Reinianus. (Pfr.) | |
| 8. | Patula (Thysanophora) hypolepta (Shuttleworth.) | |
| 9. | ,, vortex. (Pfr.) | Southern Florida and West Indies. |
| 10. | Helix microdonta. (Desh.) | Bahama Islands, Florida, Texas. |
| 11. | ,, appressa. (Say.) | Virginia and adjacent states; perhaps introduced into Bermuda. |
|
12. | ,, pulchella. (Müll.) | Europe; very close to H. minuta (Say) of the United States. Introduced into Bermuda (?) |
| 13. | ,, ventricosa. (Drap.) | Azores, Canary Islands, and South Europe. |
| 14. | Bulimulus nitidulus. (Pfr.) | Cuba, Haiti, &c. |
| 15. | Stenogyra octona. (Ch.) | West Indies and South America. |
| 16. | Stenogyra decollata (Linn.) | A South European species. Introduced. |
| 17. | Cœcilianella acicula. (Müll.) | Florida, New Jersey, and Europe. |
| 18. | Pupa pellucida. (Pfr.) | West Indies, and Yucatan. |
| 19. | ,, Barbadensis. (Pfr.) | Barbadoes (?) |
| 20. | ,, Jamaicensis. (C. B. Ad.) | Jamaica. |
| 21. | Helicina convexa. (Pfr.) | Barbuda.[110] |
Mr. Bland indicates only four species as certainly peculiar to Bermuda, and another sub-fossil species; while one or two of the remainder are indicated as doubtfully identical with those of other countries. We have thus about one-fifth of the land-shells peculiar, while almost all the other productions of the islands are identical with those of the adjacent continent and islands. This corresponds, however, with what occurs generally in islands at some distance from continents. In the Azores only one land-bird is peculiar out of eighteen resident species; the beetles show about one-eighth of the probably non-introduced species as peculiar; the plants about one-twentieth; while the land-shells have about half the species peculiar. This difference is well explained by the much greater difficulty of transmission over wide seas, in the case of land-shells, than of any other terrestrial organisms. It thus happens that when a species has once been conveyed it may remain isolated for unknown ages, and has time to become modified by local conditions unchecked by the introduction of other individuals of the original type.
Flora of Bermuda.—Unfortunately no good account of the plants of these islands has yet been published. Mr. Jones, in his paper "On the Vegetation of the Bermudas" gives a list of no less than 480 species of flowering plants; but this number includes all the culinary plants, fruit-trees, and garden flowers, as well as all the ornamental trees and shrubs from various parts of the world which have been introduced, mixed up with the European and American weeds that have come with agricultural or garden seeds, and the really indigenous plants, in one undistinguished series. It appears too, that the late Governor, Major-General Lefroy, "has sown and distributed throughout the islands packets of seeds from Kew, representing no less than 600 species, principally of trees and shrubs suited to sandy coast soils"—so that it will be more than ever difficult in future years to distinguish the indigenous from the introduced vegetation.
From the researches of Dr. Rein and Mr. Moseley there appear to be about 250 flowering plants in a wild state, and of these Mr. Moseley thinks less than half are indigenous. The majority are tropical and West Indian, while others are common to the Southern States of North America; the former class having been largely brought by means of the Gulf Stream, the latter by the agency of birds or by winds. Mr. Jones tells us that the currents bring numberless objects animate and inanimate from the Carribean Sea, including the seeds of trees, shrubs, and other plants, which are continually cast ashore and sometimes vegetate. The soap-berry tree (Sapindus saponaria) has been actually observed to originate in this way.
The only species of flowering plant peculiar to Bermuda is Carex Bermudiana (Hemsley), which is said to be allied to a species found only in St. Helena; but there are some local forms of continental species, among which are Sisyrinchium Bermudianum and a variety of Rhus toxicodendron. There are, however, two ferns—an Adiantum and a Nephrodium, which are unknown from any other locality. The juniper, which is so conspicuous a feature of the islands, is said to be a West Indian species (Juniperus barbadensis) found in Jamaica and the Bahamas, not the North American red cedar; but there seems to be still some doubt about this common plant.
Mr. Moseley, who visited Bermuda in the Challenger, has well explained the probable origin of the vegetation. The large number of West Indian plants is no doubt due to the Gulf Stream and constant surface drift of warm water in this direction, while others have been brought by the annual cyclones which sweep over the intervening ocean. The great number of American migratory birds, including large flocks of the American golden plover, with ducks and other aquatic species, no doubt occasionally bring seeds, either in the mud attached to their feet or in their stomachs.[111] As these causes are either constantly in action or recur annually, it is not surprising that almost all the species should be unchanged owing to the frequent intercrossing of freshly-arrived specimens. If a competent botanist were thoroughly to explore Bermuda, eliminate the species introduced by human agency, and investigate the source from whence the others were derived and the mode by which they had reached so remote an island, we should obtain important information as to the dispersal of plants, which might afford us a clue to the solution of many difficult problems in their geographical distribution.
Concluding Remarks.—The two groups of islands we have now been considering furnish us with some most instructive facts as to the power of many groups of organisms to pass over from 700 to 900 miles of open sea. There is no doubt whatever that all the indigenous species have thus reached these islands, and in many cases the process may be seen going on from year to year. We find that, as regards birds, migratory habits and the liability to be caught by violent storms are the conditions which determine the island-population. In both islands the land-birds are almost exclusively migrants; and in both, the non-migratory groups—wrens, tits, creepers, and nuthatches—are absent; while the number of annual visitors is greater in proportion as the migratory habits and prevalence of storms afford more efficient means for their introduction.
We find also, that these great distances do not prevent the immigration of some insects of most of the orders, and especially of a considerable number and variety of beetles; while even land-shells are fairly represented in both islands, the large proportion of peculiar species clearly indicating that, as we might expect, individuals of this group of organisms arrive only at long and irregular intervals.
Plants are represented by a considerable variety of orders and genera, most of which show some special adaptation for dispersal by wind or water, or through the medium of birds; and there is no reason to doubt that besides the species that have actually established themselves, many others must have reached the islands, but were either not suited to the climate and other physical conditions, or did not find the insects necessary to their fertilisation, and were therefore unable to maintain themselves.
If now we consider the extreme remoteness and isolation of these islands, their small area and comparatively recent origin, and that, notwithstanding all these disadvantages, they have acquired a very considerable and varied flora and fauna, we shall, I think, be convinced, that with a larger area and greater antiquity, mere separation from a continent by many hundred miles of sea would not prevent a country from acquiring a very luxuriant and varied flora, and a fauna also rich and peculiar as regards all classes except terrestrial mammals, amphibia, and some groups of reptiles. This conclusion will be of great importance in those cases where the evidence as to the exact origin of the fauna and flora of an island is less clear and satisfactory than in the case of the Azores and Bermuda.
102 ↑ For most of the facts as to the zoology and botany of these islands, I am indebted to Mr. Godman's valuable work—Natural History of the Azores or Western Islands, by Frederick Du Cane Godman, F.L.S., F.Z.S., &c., London, 1870.
103 ↑ See Chap. V. p. 78.
104 ↑ Some of Mr. Darwin's experiments are very interesting and suggestive. Ripe hazel-nuts sank immediately, but when dried they floated for ninety days, and afterwards germinated. An asparagus-plant with ripe berries, when dried, floated for eighty-five days, and the seeds afterwards germinated. Out of ninety-four dried plants experimented with, eighteen floated for more than a month, and some for three months, and their powers of germination seem never to have been wholly destroyed. Now, as oceanic currents vary from thirty to sixty miles a day, such plants under the most favourable conditions might be carried 90 X 60 = 5,400 miles! But even half of this is ample to enable them to reach any oceanic island, and we must remember that till completely water-logged they might be driven along at a much greater rate by the wind. Mr. Darwin calculates the distance by the average time of flotation to be 924 miles; but in such a case as this we are entitled to take the extreme cases, because such countless thousands of plants and seeds must be carried out to sea annually that the extreme cases in a single experiment with only ninety-four plants, must happen hundreds or thousands of times and with hundreds or thousands of species, naturally, and thus afford ample opportunities for successful migration. (See Origin of Species, 6th Edition, p. 325.)
105 ↑ The following remarks, kindly communicated to me by Mr. H. N. Moseley, naturalist to the Challenger, throw much light on the agency of birds in the distribution of plants:—"Grisebach (Veg. der Erde, Vol. II. p. 496) lays much stress on the wide ranging of the albatross (Diomedea) across the equator from Cape Horn to the Kurile Islands, and thinks that the presence of the same plants in Arctic and Antarctic regions may be accounted for, possibly, by this fact. I was much struck at Marion Island of the Prince Edward group, by observing that the great albatross breeds in the midst of a dense, low herbage, and constructs its nest of a mound of turf and herbage. Some of the indigenous plants, e.g. Acæna, have flower-heads which stick like burrs to feathers, &c., and seem specially adapted for transposition by birds. Besides the albatrosses, various species of Procellaria and Puffinus, birds which range over immense distances may, I think, have played a great part in the distribution of plants, and especially account, in some measure, for the otherwise difficult fact (when occurring in the tropics), that widely distant islands have similar mountain plants. The Procellaria and Puffinus in nesting, burrow in the ground, as far as I have seen choosing often places where the vegetation is the thickest. The birds in burrowing get their feathers covered with vegetable mould, which must include spores, and often seeds. In high latitudes the birds often burrow near the sea-level, as at Tristan d'Acunha or Kerguelen's Land, but in the tropics they choose the mountains for their nesting-place (Finsch and Hartlaub, Orn. der Viti- und Tonga-Inseln, 1867, Einleitung, p. xviii.). Thus, Puffinus megasi nests at the top of the Korobasa basaga mountain, Viti Levu, fifty miles from the sea. A Procellaria breeds in like manner in the high mountains of Jamaica, I believe at 7,000 feet. Peale describes the same habit of Procellaria rostrata at Tahiti, and I saw the burrows myself amidst a dense growth of fern, &c., at 4,400 feet elevation in that island. Phaethon has a similar habit. It nests at the crater of Kilauea, Hawaii, at 4,000 feet elevation, and also high up in Tahiti. In order to account for the transportation of the plants, it is not of course necessary that the same species of Procellaria or Diomedea should now range between the distant points where the plants occur. The ancestor of the now differing species might have carried the seeds. The range of the genus is sufficient."
106 ↑ Nature, Vol. VI. p. 262, "Recent Observations in the Bermudas," by Mr. J. Matthew Jones.
107 ↑ "The late Sir C. Wyville Thomson was of opinion that the 'red earth' which largely forms the soil of Bermuda had an organic origin, as well as the 'red clay' which the Challenger discovered in all the greater depths of the ocean basins. He regarded the red earth and red clay as an ash left behind after the gradual removal of the lime by water charged with carbonic acid. This ash he regarded as a constituent part of the shells of Foraminifera, skeletons of Corals, and Molluscs, [vide Voyage of the Challenger, Atlantic, Vol. I. p. 316]. This theory does not seem to be in any way tenable. Analysis of carefully selected shells of Foraminifera, Heteropods, and Pteropods, did not show the slightest trace of alumina, and none has as yet been discovered in coral skeletons. It is most probable that a large part of the clayey matter found in red clay and the red earth of Bermuda is derived from the disintegration of pumice, which is continually found floating on the surface of the sea. [See Murray, "On the Distribution of Volcanic Débris Over the Floor of the Ocean;" Proc. Roy. Soc. Edin. Vol. IX. pp. 247-261. 1876-1877.] The naturalists of the Challenger found it among the floating masses of gulf weed, and it is frequently picked up on the reefs of Bermuda and other coral islands. The red earth contains a good many fragments of magnetite, augite, felspar, and glassy fragments, and when a large quantity of the rock of Bermuda is dissolved away with acid, a small number of fragments are also met with. These mineral particles most probably came originally from the pumice which had been cast up on the island for long ages (for it is known that these minerals are present in pumice), although possibly some of them may have come from the volcanic rock, which is believed to form the nucleus of the island." The Voyage of H.M.S. Challenger, Narrative of the Cruise, Vol. I. 1885, pp. 141-142.
108 ↑ Four bats occur rarely, two being N. American, and two West Indian Species. The Bermuda Islands, by Angelo Heilprin, Philadelphia, 1889.
109 ↑ Fourteen species of Spiders were collected by Prof. A. Heilprin, all American or cosmopolitan species except one, Lycosa atlantica, which Dr. Marx of Washington describes as new and as peculiar to the islands. (Heilprin's The Bermudas, p. 93.)
110 ↑ Mr. Theo. D. A. Cockerell informs me that there are two slugs in Bermuda of which specimens exist in the British Museum,—Amalia gagates Drap. common in Europe, and Agriolimax campestris of the United States. Both may therefore have been introduced by human agency. Also Vaginulus Morelete var. schivelyæ which seems to be a variety of a Mexican species; perhaps imported.
111 ↑ "Notes on the Vegetation of Bermuda," by H. N. Moseley. (Journal of the Linnean Society, Vol. XIV., Botany, p. 317.)