Island Life Part 9

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 {255} 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 Elateridae 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 sh.o.r.es. 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 {256} 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-sh.e.l.ls of the Azores._--Like the insects and birds, the land-sh.e.l.ls 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;[51] 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 sh.e.l.ls, 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. G.o.dman's volume. As our {257} 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 cla.s.s 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 p.r.o.ne 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 s.e.xes 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 cla.s.s 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, {258} Beta, oats, Capsic.u.m, 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 Seych.e.l.le 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.[52]

_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 pa.s.s 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 {259} nine grains of earth on the leg of a woodc.o.c.k 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 gla.s.s, 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 p.r.i.c.kles 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.[53]

{260}

_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, {261} 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 Compositae 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 lat.i.tude 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 {262} 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 compa.s.s; 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 a.n.a.logous 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 ill.u.s.trating the laws, and solving the problems presented by the dispersal of animals, we will pa.s.s 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.[54]

Immediately beyond these reefs, {263} 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.

[Ill.u.s.tration: MAP OF BERMUDA AND THE AMERICAN COAST.]

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 {264} 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 Palaeozoic district. It has however long been known that these rocks are all due to the wind, {265} which blows up the fine calcareous sand, the product of the disintegration of coral, sh.e.l.ls, serpulae, 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.

[Ill.u.s.tration: SECTION OF BERMUDA AND ADJACENT SEA BOTTOM.

The figures show the depth in fathoms at fifty-five miles north and forty-six miles south of the islands respectively.]

_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 a.n.a.lysis: "This a.n.a.lysis 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 ident.i.ty 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, {266} or of the minute organisms which abound in the blown sand.[55]

_Zoology of Bermuda._--As might be expected from their extreme isolation, these islands possess no indigenous terrestrial mammalia, frogs, or snakes.[56] 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 alt.i.tude and {267} 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 novaeboracensis._ (The White-eyed green t.i.t.) Migrates along the east coast.

4. _Pa.s.ser domesticus._ (The English Sparrow.) ? Introduced.

5. _Corvus america.n.u.s._ (The American Crow.) Common over all North America.

6. _Cardinalis virginia.n.u.s._ (The Cardinal bird.) Migrates from Carolina southward.

7. _Chamoepelia pa.s.serina._ (The ground Dove.) Louisiana, W. Indies, and Mexico.

8. _Ortyx virginia.n.u.s._ (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. _Phaeton 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_), {268} the night-hawk (_Chordeiles virginia.n.u.s_), the wood wagtail (_Siurus novaeboracensis_), 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 mult.i.tude of birds, belonging to more than a hundred distinct species, migrate northward or southward in Eastern America. A large proportion of these pa.s.s along the Atlantic coast, and it has been observed that many of them fly some distance out to sea, pa.s.sing 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 {269} 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 (_t.u.r.dus migratorius_), the yellow-rumped warbler (_Dendroeca coronata_), the pine warbler (_Dendroeca pinus_), the wood wagtail (_Siurus novaeboracensis_), the summer red bird (_Pyranga aestiva_), the snow-bunting (_Plectrophanes nivalis_), the red-poll (_aegiothus 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 const.i.tuting 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 oenanthe_), 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.