Russel Wallace : Alfred Russell Wallace (sic)
'When we speak of oceans, continents, and islands, we seem always to have known all about them, whereas we have only lately found out what they really are. Till recently it was believed that land and sea are and have ever been immoveable, and that fossils were the productions of nature, formed where found; the natural conclusion, when it was accepted that land had always been land and sea always sea, being, that all shells found on inland mountains were monstrosities of nature. But when the science of geology arose, and it was believed that fossil shells found on high mountains had once been alive, it followed that the mountains must have been at some period under the sea, and geologists went to the opposite extreme of believing that all seas have been land, and all continents have been oceans. This we may call the geological view; it is the natural result of geological teaching, and, as such, is still held by many eminent men of science. Of late years, however, it has been ascertained that, though most of the present land has once been under the sea, still most of the sea has by no means once been land. Hence many eminent geologists now believe that the distribution of land and sea has always been more or less what it still is, and that though nearly all the land has at some time been under sea, it has never been all submerged at the same time. This view--that of the permanence of continents and oceans--may be called the modern view; and it is only by it that the continuity of life, observed in the study of the geographical distribution of animals, is made possible.
'Passing on to the main features of the earth's surface necessary to a true comprehension of its past history, we have first to consider the great depth of the sea compared to the height of the land. Humboldt calculated that the average height of the land on the globe is 1000 ft. above sea level; it has lately been discovered, from the Challenger observations, that the mean depth of the sea is about 14500 ft. Although the land rises to heights of from 20 to 30 thousand feet in mountain ranges, by far the larger portion of the continents consists of low lands, i.e., land of less than 1000 feet elevation. On the other hand while the greatest depth of the sea does not probably much exceed 20 or 30 thousand feet, the greater part of the sea is of immense depth. Hence we see that the [[p. 2]] former opinion that the sea bottom was like the land, with great depths here and there like the mountains on the land, is untenable, and that the distinctive feature of the sea bottom is its comparatively uniform great depth. The disproportion between the mean depth of the sea and the mean height of the land is so great, that (the areas of land and sea being in the proportion of 2 to 5) if all the land were submerged, the average height of the sea would only be increased by 300 ft. Now we have absolute knowledge that in all geological time no such total submergence has ever taken place; for had it so happened there would not have been that continuity of life which we observe through all time. The mere fact of the comparatively enormous depth of the sea points strongly to the belief that sea and land have not interchanged much, the land having been always more or less land, and the oceanic areas always ocean. Again, it is found that the line of 1000 fathoms depth of the sea, closely follows the present outline of the land, with the 2000 fathoms line generally close to it, so that there are no traces of submergence of former continents.
'Another series of facts supporting this view of permanence is found in the geological formations of continents and islands. In every continent one finds examples of the same series of formations following each other in belts, but though these marine deposits prove that the land must have been under water, they by no means prove that all the continent, where they exist, was under water at one time. In fact they rather shew that there must have been dry land close by from which the particles of the rocks were carried, and that land not for off, for pebbles are rarely carried further than a mile from the shore, while sand always sinks to the bottom within 15 miles, and particles just visible to the eye within 100 or 150 miles from the shore. Hence we see that all the materials of future land were deposited near land; and though the presence of sandstone proves that the spot where it is found was once under water, it also proves that there was land close by above water from which the sediment composing the bed was carried. More than this, the order of the series--pebbles, sand, and mud,--proves the direction in which the land must have lain.
'So universally is this the case that in beds of all ages, even back to the oldest primary, Professor Geikie has pointed out evidence of the existence of land, not only from the deposits themselves, but from ripple marks, shore marks, wind marks, worm tracks, and the tracks of creeping animals.
'The deep sea dredgings made by the officers of the Challenger, have revealed the existence over the ocean bed of 'ooze,' a fine impalpable mud believed to have been formed entirely of the shells and hard parts of marine animals. The ooze is usually whitish down to 2000 fathoms, red from 2 to 3 thousand fathoms, but below that grey, the reason of the change being the chemical action of the superabundant oxygen at great depths. It is the [[p. 3]] belief of all the Challenger staff, that no sedimentary rock exists which can be identified with the deep sea ooze. Thus we have conclusive proof that the ocean must have ever been ocean.
'Darwin long ago pointed out that oceanic islands are either volcanic or coralline, containing no fragment of sedimentary rock such as that found on continents. Such islands cannot be remnants of a former continent, else it is certain that fragments of sedimentary rock would have been preserved in some of them. This argument for a long time stood alone, but now a large body of evidence points to the certainty of the permanence of land and water.
'To summarise the various arguments, we have--
'(1) The great depth of the ocean compared with the height of the land.
'(2) The great depth and general flatness of the ocean bed.
'(3) The nature of the sedimentary rocks, proving the proximity of land from which the sediment was derived, such rocks occurring through all geological time.
'(4) Over all the continents beds are found containing remains of birds and beasts, many of them of large size, such as would require large areas of continent for their support.
'(5) Absence of stratified deposits from oceanic islands.
'(6) The wide-spread oceanic ooze distinct from all known sedimentary rocks.
'These arguments combine to prove that the continents have always occupied very much their present position, being subject to undulatory movements, which have caused the submergence of different parts at different times. Thus, while some parts of the existing continents were below, there must at the same time have been other lands above the surface of the sea, the proportion of land to water being somewhat variable, but approximately the same as now.
'We now come to the actual distribution of land on the globe. The first thing we note is the great disproportion in extent of the sea and land, and that the land is massed together in one hemisphere, the continents all clustering round the North Pole. This is well illustrated by the fact that it is possible to pass from any point on one continent to any point on another without ever being out of sight of land, the intervening seas being both narrow and of no great depth. This points to the probability that in past time there may have been continuous land, affording passage for all land animals from continent to continent. In this connection we note the great similarity of life in all continents, a similarity which increases as we go back in time, the last separation having occurred at a comparatively recent period.
'We now pass to the characteristics of islands, and the animal and vegetable remains known to be found in them. We can study the past natural history of the world more easily on islands than in continents, on account of their smaller extent and long continued isolation.
[[p. 4]] 'Islands, according to Darwin, consist of two radically distinct classes: (1) continental, (2) oceanic.
'The oceanic islands have never formed part of continents, though not necessarily very distant from them, but are the result either of volcanic upheaval or coralline formation. They are always surrounded by deep sea, that is, sea of more than 1000 fathoms depth. Of these coralline islands we never can find the foundations, but probably they are founded on old volcanic surfaces worn away by sea action. These exhibit great peculiarities in their natural history, in that they have neither mammals nor amphibia, but contain a small number of reptiles, birds, and other classes, and these are very often distinct from anything found elsewhere in the world.
'Continental islands may be subdivided into Recent and Ancient.
'Recent continental islands have been separated in recent times so that their geological features are the same, and their natural history nearly the same as those of continents. Examples of these are England, separated by a sea 22 miles wide from Europe, and Borneo, separated by 100 miles from Asia. Though they contain the same animals as the continents near them, the number is smaller and the varieties fewer.
'Ancient continental islands are of older date and are always separated by deeper sea, thus while the recent ones are always within the 100 fathoms line, the ancient are outside the 100 but within the 1000 fathoms line. In regard to their natural history, though they contain many forms identical with those of the continent, they contain also many which are totally distinct, so that there must have been a great change in the forms of life, and this change implies great lapse of time. Geologically they always agree with the continents, and exhibit a fair sample of the continental geology.
Lecture II. Recent Continental Islands.
'Long before the era of geological investigation, it was suggested that England must have been once joined to the continent. As early as 1605, Verstegan, in his 'Antiquities of the English Nation,' pointed out that England must have formed part of the continent from the presence of wolves, which would not have been introduced by man, any more than it was probable that man would ever carry foxes into the Isle of Wight, thus proving that as lately as 250 years ago there were no foxes in that island. No one, he says, would have imported wolves, therefore 'these wicked beasts did of themselves pass over.' This argument, from the absence of noxious animals, is sound and of wide application. For the further we go back the more abundant we find noxious animals, whereas, if man had brought them over there would have been less in ancient than in modern times. In the time of pre-historic man we find the [[p. 5]] greatest number: in addition to those at present existing or recently exterminated were the hyæna, bear, lion, reindeer, bison, rhinoceros, and others. These and other facts give us a clue to the physical history of the island.
'Great Britain is the most typical instance in the world of a recent continental island. Its geology is quite continental, containing a sample of almost all the rocks found in the continent, from the earliest to the latest. Physically we are separated by a bank less than 600 feet deep. We have much evidence to prove that Britain was till recently--that is to say, till the last glacial period--connected with the continent. Of this we have three proofs.
'(1) The existence round our coast of numerous submerged forests--along the south coast in Devonshire, in Wales at Holyhead, and on the east coast in various places. In these we have deposits of wood, twigs, and fruits, with many roots embedded in the soil, which proves that the trees were not drifted into their present position.
'In Glamorganshire we have further evidence of elevation in the presence of caves midway in the sea cliffs. These were explored by many naturalists, Dr. Buckland among them. In one of these caves were found very numerous antlers of deer, proving it to have been a den into which hyænas and bears dragged the deer which formed their prey. We are compelled to suppose that the land stretched seaward from the present cliffs, but has since been submerged and washed away.
'(2) In sinking wells in Holland, an old land surface has been found 50 feet below the present surface. In Scotland, in the mining districts, numerous pits and borings have been made in search of minerals. After passing through a superficial accumulation of gravel and sand, an old land surface is reached with river channels, at a depth of from 100 to 260 feet. This proves that there must have been a submergence of between 3 and 4 hundred feet at a comparatively recent period. This old surface is covered by boulder clay, which could only have been deposited there during the glacial epoch.
'(3) In dredging in the mouth of the English Channel, shallow-water species of shells have been found at a depth of ninety fathoms. Arctic shells, also characteristic shallow-water species, have been found at about the same depth off the Shetland Islands; all under circumstances which preclude the possibility of their having been drifted into their present position.
'These proofs go to shew that our islands must have sunk some 90 or 100 fathoms, and we may therefore assume that they once formed part of the continent. The natural result of this is, that the animals are almost identical with those of the continent, though there are minute differences.
'As the subject is of some importance, we will devote some time to consider the main features of the natural history of these islands as compared with that of the continent.
[[p. 6]] 'Of land mammalia we have 28 native species, besides 4 which have been exterminated by man, viz., the wolf, bear, wild ox, and wild boar. Pre-historically there were many more, as the mammoth, rhinoceros, horse, reindeer, glutton, hyæna, &c.; so that altogether we had at least 50 distinct species. The reason of their extinction was the submergence of the British Islands during the glacial epoch, till they consisted of a small archipelago of islands, formed of the high lands of Wales, Cumberland, Scotland, and probably England south of the Thames. Thus, on the flanks of Snowdon, at a height of 13 or 14 hundred feet, we find sea shells identical with those now found on the coast of Norway, and there are indications that the submergence was at one time much deeper. There seems to have been more than one such submergence, the last taking place in the latter part of the glacial epoch. Afterwards the land rose till it was connected with the continent, and it was then that the existing animals crossed. That the union however was short, is proved by the paucity of our species of the smaller and more sluggish families as compared with the continent. When land rises from the sea, it must take some time before vegetation is sufficient to support animal life, and at first only the stronger and more abundant species will take up their abode. We find we have all the larger, and very few of the smaller species. Thus taking the rodents, which comprise the rabbits, hares, squirrels, &c., while the continent has 32, England has but 13. This is rendered clearer by reference to Ireland, which is more remote from the continent, and is separated by a deeper sea, which is a proof that it was separated earlier. Consequently mammalia and reptiles are much fewer in Ireland, for while we have 32 mammalia, Ireland has only 20. The difference is greater in reptiles and amphibia: thus Belgium has 22, England 13, and Ireland only 4. We thus find the relative numbers of these animals easily explained.
'Among the mammalia, reptiles, and amphibia, it is the opinion of the best naturalists that we have in Britain neither a peculiar species nor distinguishable variety. In birds the case is different: some say we have four distinct species; but we have certainly one, the red grouse. There is nothing at all like it on the continent: the nearest to it being the willow grouse, an inhabitant of Scandinavia. These though closely allied are very different: in summer our bird is whiter, theirs browner; while in winter ours is darker and theirs whiter. The explanation is of course that the colouring is in both cases protective: ours would be rendered conspicuous by a white winter plumage, because we have not snow on the ground all the winter; while the converse would hold in Scandinavia. Of others the British coal tit and the long-tailed tit are regarded by some as good species, by others only as varieties.
'It has long been known that some rivers and lakes possess peculiar kinds of trout and charr, but how far these were distinct from each other in different parts of the island, and from continental species, was not known till Dr. Günther compared them. He arrives [[p. 7]] at the result that 15 varieties of fish are quite peculiar to Great Britain; some being confined to one lake, and some to two or more.
'It is interesting to notice that 6 out of these are natives of Ireland, where the two chief causes of specific change,--isolation and altered conditions--are both more powerful than in Britain, for we have seen that Ireland has been longer separated from the continent, and there the insular condition of climate is carried to an extreme.
'Among remarkable instances of limited distribution, we have the Loch Stenis trout confined to a small group of lakes in Orkney, the Welsh charr confined to the Llanberis lakes, and the vendace confined to small lakes at Loch Maben in Scotland.
'The reason of the extreme local modification in fishes is obvious. It is due to the extreme difficulty with which fishes can pass from one river basin to another. The manner in which fishes are able to do this is not clearly known. It may occasionally occur by means of waterspouts, which have often taken up living fishes, and their eggs may have been carried with greater ease. Darwin suggests that their ova may be carried on weeds attached to the feet of aquatic birds flying across from lake to lake. Again, probably some of these fishes once inhabited the streams which flow into or out of the lakes, as well as the lakes themselves, and in this case may have been carried over a watershed owing to the damming up of the bottom of the valley by ice, as must have often happened during the glacial epoch. This damming up by ice explains the parallel roads of Glenroy in Scotland. These peculiarities in the species of birds and fish must have been established since the glacial epoch, and there is reason to believe that the change is still going on and not yet completed. Thus sportsmen in the south of Scandinavia find that the willow grouse is not wholly white in winter, while in the higher parts of Scotland remarkable white specimens are occasionally found. In the case of fishes which are carried from lake to lake by means of rare accidents, peculiarities are formed by isolation, and the necessity of adopting new conditions of life. If transmission were not rare but fresh members of the original stock were constantly arriving there could be no variation. Rarity in the mode of transmission is absolutely necessary to form a new species.
'With respect to insects, entomologists say that there are no species peculiar to Britain; some few which have hitherto only been found there most likely existing on the Continent, though hitherto overlooked. But on looking into the question I do not think that this is likely to be the case. Of Lepidoptera there are 70 species or varieties distinct from any found on the Continent: the most conspicuous is the great copper butterfly, which is now extinct, though formerly in great plenty in parts of the Cambridgeshire fens.
'Of the smaller moths the list of peculiar British species includes some which seem by their habits adapted to a mild insular climate such as ours, while they would not be able to survive the greater [[p. 8]] rigour of a continental winter. These feed in mines made in green leaves during the winter. It is therefore quite possible that these species may never be found on the continent. This mildness of our climate is also illustrated by two other species which are found in Britain and not on the parts of the continent opposite, but in the extreme south of Europe, on the shores of the Mediterranean.
'Of Coleoptera some 70 or 80 species present differences. Entomologists think these will all be some day found on the continent: but there is an argument which throws doubt on this opinion, namely, that some species are found only on special islands off our coasts, and not on the mainland. Thus the small island known as Lundy Island, situated in the Bristol Channel, contains two minute beetles not found elsewhere either in Britain or the continent. The British Isles consist of about 1000 separate islands, very few of which have been thoroughly searched by entomologists; and if this small rock in the Bristol Channel contains two distinct species, it is at least probable that many others will also present peculiar forms. This argument also shows the extreme probability that there exist in Britain peculiar forms of insects which do not exist on the continent.
'The Isle of Man has been searched by a resident entomologist, and he points out the existence of some five or six varieties of English Lepidoptera, which may be distinguished by much darker colouring. Of the few butterflies in the island the small tortoise-shell is the only one presenting any variety. In this the specimens are all of remarkably small size, not larger than half that of the average English specimens.
'In the Shetland Islands there is also a moth,1 one of the commonest in the islands, quite unlike in colour its representative on the mainland.
'In the land and freshwater shells and caddis flies there are a few peculiarities in the British species.
'The same thing occurs in plants to a small degree. There are two or three not found on the Continent. Some, which occur only in Ireland and some of the western islands of Scotland, are only found elsewhere in America.
'In mosses we find twenty-six peculiar species, not the least like anything found on the Continent; some have their nearest allies in the Andes, and others in the Himalaya, or in New Zealand. This is explained by the extreme ease with which spores of mosses can be carried by the wind and sea; and it further indicates that the absence of a moss does not prove that its spores have never reached the spot, but that the conditions are not suitable for its growth.
'Looking at all the facts we find undoubted peculiarities in the birds, fishes, insects, and plants of Britain.
'Turning next to another recent continental island, namely, [[p. 9]] Formosa in the China Seas, we note that it is only 100 miles distant from the mainland, and like our own island within the hundred fathom line. It differs, however, from our islands in the number of peculiar species of the higher animals. For while we have none peculiar, Formosa, out of a total of thirty-five species has fourteen peculiar, including a monkey, a pig, and an antelope; while out of 128 birds no less than forty-two are distinct. This proportion of peculiar species is the highest to be met with in any island on the globe which ranks as a recent continental island. Some of the peculiar birds show nearest affinity with those of other parts of Asia, and not with China, belonging, some to Himalayan, and some to Malayan types. This points to a more ancient separation, since there has been time both for a new distribution of existing species, and for the development of species entirely new.
'Other recent Continental islands are Japan, the Falkland Islands, and Iceland, all of which are within the hundred fathoms line.
Lecture III. Ancient Continental Islands.
'We will treat this evening of two of the most remarkable islands in the world, Madagascar and New Zealand, which being the most ancient, offer the greatest number of peculiarities.
'Madagascar is separated from Africa by a channel 250 miles wide and of considerable depth, there being a depression in the middle which exceeds 1000 fathoms. The island is larger than the whole of the British Islands, being 1000 miles in length. Its physical features are, briefly, a long central granitic plateau flanked by secondary rocks. Passing on to its natural history, we find it contains no less than 66 species of mammals--a certain proof that the island once formed part of a continent; but whereas it contains none of the giraffes, zebras, baboons, antelopes, lions, rhinoceroses, leopards, hyænas, &c., so characteristic of Africa, it has distinct forms of lemurs quite peculiar and characteristic. These are generally regarded as lowly organised apes, and are here most abundant. All its mammals are of small size, and the proportion of Insectivora is large, there being 12 species. These are of low organization, of ancient type, and of nocturnal and subterranean habits, and it is in Madagascar alone that they form an important class. There are Insectivora in Africa, but they are not closely allied to those of Madagascar, the latter having their closest allies in Cuba and Hayti. There are also a few representatives of the Carnivora, and a peculiar river hog, which occurring also in Africa, gives one more proof of a former connection with that continent. There is also a small species of hippopotamus, which, though now extinct, has been found in a semi-fossil state.
'Among the birds there are no less than 35 genera entirely peculiar to the island, many of which are very isolated and very difficult to class in any family of living birds, while others have African and Asian affinities. Reptiles are as numerous in Madagascar as in Africa; but again we find groups in the one unknown [[p. 10]] in the other, while there are three groups of snakes and some lizards found there which exist elsewhere in South America alone.
'Hence we see that the fauna of Madagascar is largely composed of small and weak types, which, though showing a decided relation to the African fauna, bears a striking resemblance in several cases to that of South America. This relationship has been explained by some naturalists by a hypothetical continent which once formed a connection between Madagascar and South America, after which, by a rise in the sea bed, there was a temporary connection with Africa. This however was shown in the first lecture to be impossible, and though it might account for these particular facts, it fails to account for many others.
'For a true explanation, we must look to changes in the distribution of animals effected during long periods of time.
'Recent geological discoveries in rocks of Miocene age reveal the existence at that time of giraffes, hippopotami, lions, antelopes, and zebras, in the south of France, Greece, and the Himalayas--in fact nearly all the types of the larger animals now characteristic of Africa must then have existed over the whole of Southern Europe. The presence of the Nummulitic limestone, a marine rock of Eocene age, proves that a little earlier Africa was completely cut off from Europe and Asia by a sea stretching from the Mediterranean to the Bay of Bengal. Geologists believe that Africa at this time contained none of these larger mammals, but only more lowly organized creatures such as insectivora, lemurs, &c. But it is certain these latter must also once have existed in Europe, as a fossil lemur has been found in the south of France.
'About the latter part of the Miocene period, a connection was made between Europe and Africa. Now we know that about this time the Northern Hemisphere was gradually getting colder, hence all the larger mammals migrated to Africa. But Madagascar must have been separated before this influx occurred, and the lowly organized animals which have been exterminated in Africa are there preserved. Thus we account for the main features of the natural history of this island: but we have also to explain the connection with America. The discovery of a fossil lemur in France, shows that at the period when the climate of Europe was tropical there existed in Europe animals now found in Madagascar. At this time there was probably a land connection between Europe and North America by way of Iceland, which enabled many animals to pass from one continent to the other.
'It is a universal rule that the more highly organized animals expel those of lower organization, so that as time goes on the latter are able only to survive when protected by subterranean or nocturnal habits, or by living in dense forests or in islands into which the more highly organized cannot penetrate. Hence when driven from the two Continents these animals have taken refuge in Madagascar and the West Indies, which afford, accordingly, a sample of the life of the world in the early Tertiary period.
[[p. 11]] 'The flora of Madagascar presents similar characteristics: it is extremely luxuriant, and possesses some curious and peculiar plants such as the "Traveller's Tree" and "Lattice-Leaf Plant." Also in Madagascar there are a few plants which are identical with those of tropical America, or belong to the same genera.
'New Zealand consists of a pair of islands which, though now separate, were recently connected. It is in some respects anomalous, containing no land mammal,2 in which it resembles oceanic islands, but its geological structure being similar to that of the Continent, proves it to be strictly continental. Besides this it has a group of wingless birds--now represented only by the Apteryx, a curious bird entirely destitute of wings, standing about eighteen inches high. Quite recently, however, there were many gigantic kinds, some of which must have been exterminated by the Maoris, as proved by remains found in caves and surface deposits, consisting of bones, eggs, chicks, and fragments with feathers attached. Native legends render it quite certain that they have been hunted down. These birds certainly could not have passed over the sea, and hence prove connection with the continent of Australia.
'The existence of the remains of seven or eight different species of these giant birds in the south island is very remarkable, because we never find in a limited district several species of any of the larger animals coexisting. They probably once inhabited several distinct islands which became afterwards united, and the species were driven together as the area grew smaller by subsidence.
'There are other birds of New Zealand somewhat allied to those of Australia, just as those of Madagascar are to Africa, while the greater number of those which occur in Australia are not found in New Zealand, and some found in New Zealand are not found in Australia. For instance, the starling family is found in New Zealand, but is entirely unrepresented in Australia.
'The reptiles are as peculiar in their distribution as the birds; for instance, there are in Australia 36 genera of lizards, of which many inhabit the temperate regions, yet not one of them is found in temperate New Zealand.
'The flora of New Zealand is again remarkable by the absence of the most characteristic Australian plants. Thus in Australia the most characteristic and abundant genera--the Eucalyptus or gum tree, which comprises more than 100 species, and the Acacia which embraces more than 300 species, as well as many others which are found over the whole of Australia, are without a single representative in New Zealand.
'Yet of the plants in New Zealand more than half are allied to those of Australia, and also show affinities with those of the Pacific Islands. Another curious point is that the flora as a whole is rather tropical than temperate, the plants of temperate Australia being entirely absent.
[[p. 12]] 'For the explanation of these facts we must refer to the shape of the sea bottom, and to the geology of Australia. The thousand fathom line shows that Australia is joined to New Zealand by a narrow submerged bank which connects it with the north-east portion of Australia. This accounts for the absence in New Zealand of the fauna and flora of temperate Australia. Again, the geological structure shows that beds of Cretaceous age extend across Australia from north to south, in the longitude of the Gulf of Carpentaria, dividing the island into two parts, a larger western and a much smaller eastern portion. A connection existed between New Zealand and the eastern half of Australia, when it was separated from the western island by sea, extending over these Cretaceous beds. The marsupial forms now inhabiting Australia must about the end of the secondary period have entered the Western Australian island from Asia, and the peculiar Australian flora must also have been developed in the western island, and could not reach the eastern island till they became united in the Tertiary period, when all connection with New Zealand had been severed. Prior to this the large wingless birds, similar to those now found in New Zealand, must have had a very wide range, as is proved by the fossil remains of one being found in England, and the existence of surviving species of cassowary in New Guinea, New Britain and Ceram, and the Rhea in South America. These birds must then have inhabited Australia and New Zealand as well as some of the Pacific Islands, which were then connected with New Zealand. The occurrence of so large a number of recently extinct species in one island proves that New Zealand must have been once more extensive than at present. Probably but one or two species existed there originally, but on the breaking up of the continent into an archipelago each island would acquire its own particular forms, and on the elevation of the land again these have all been crowded together into one island.
'Celebes affords an instructive example of how a narrow strip of sea is sufficient to keep out immigrants. Its fauna and flora consist of peculiar types related both to India, Australia, and Africa, shewing that it must have been connected with Asia at the early period when Madagascar was connected with Africa, and has ever since remained separate. Though numerous islands have risen around it, there has never been connection with any of them. Its relations are more nearly with India than with Borneo, which is separated from it only by a narrow belt of sea, about 50 miles wide. These islands are extremely interesting, as preserving types of the ancient world which have survived here, through never having had to compete with higher organisms.
'As a further illustration of the small power of these lowly organized creatures to compete with higher forms, we may notice that animals and plants introduced by man from England into New Zealand are rapidly exterminating the aboriginal types.
Lecture IV. Oceanic Islands.
'There is an old tradition that a continent once existed in the [[p. 13]] Atlantic, between Europe and America, which was known by the name of Atlantis. The presence of the Azores was supposed to confirm the existence of this hypothetical continent. This view was however exploded by Darwin, who showed that the Azores were of oceanic origin.
'The Azores consist of a large group of islands situated in the Atlantic at a distance of 900 miles from Spain, the nearest part of Europe, the sea between being two or three thousand fathoms deep. They are entirely volcanic and of large extent, the extreme islands being 400 miles apart.
'Though large, they possess neither land mammals nor amphibia, nor indeed is there any form of land vertebrate animal, reptile, or fish, although the climate and physical conditions are admirably suited to their support. On the other hand, of flying species, as birds and insects, we have representatives in all the islands.
'Of land birds there are twenty-two kinds, eighteen being residents, and four being only stragglers. It is certain that others do come, as the natives all agree that after great storms (and they are frequent) strange birds are seen. Captains of vessels have seen birds settling on their ships when 500 miles from the Azores, and a thousand miles from any continent. The Azores may be divided into three groups, the eastern, central, and western. Of these, the eastern contains most species, the central next, and the western group fewest of all. As the most easterly, and not the largest island has the greatest number of birds, this supports the view that they have been blown over from Europe, and are not indigenous. If the islands had ever been connected with Europe the central group would have had most species, and this is not the case.
'All the birds are common European species, excepting the bullfinch, which is slightly different. The ordinary theory that change of conditions produces changes in form is shown to be incorrect, unless the changed conditions be accompanied by isolation. In the case of the Azores so many birds are every year carried over by storms, that the local conditions have no time to produce change of species, since any incipient change is neutralized by crossing with the old Continental variety. This is confirmed by the bullfinch, which has become changed, being a sedentary bird, not by any means likely to be carried out to sea by storms, and thus in this case local conditions have had time to act.
'The insects which fly well are all common European species, but of those which have little power of flight one-third are peculiar species.
'The beetles are tolerably numerous, but the proportion to European species is smaller than in the case of birds. This may be accounted for by many of the beetles in the larval condition being attached to particular plants, and consequently even though transported they could not survive in a country where these plants are absent.
[[p. 14]] 'Of seventy land shells, one-half are peculiar. Here again the great difficulty of transmission keeps the local breeds pure.
'The plants are 480 in number, and forty are peculiar: this is a large proportion considering the facility with which their seeds can be carried. Some can float for weeks in salt water uninjured; others are eaten by birds, and pass through them with powers of germination unimpaired; while others have hooks by which they become fastened to the feathers of birds; and others are so light that they can float in the air. The large heavy-fruited plants, which are not eaten by birds, are entirely absent, just as we should expect. Thus there are no oaks, chestnuts, hazels, crabs, beeches, or pines found in the islands. On the other hand, the laurels, myrtles, laurustinus, elder, etc., whose fruits are small and eatable, are all found there.
'The whole fauna and flora of these islands owe their presence there to transmission across the sea; and since the prevalent winds are contrary, being westerly, it is clearly not these prevalent winds by which they have been carried, but by exceptionally strong storms, thus proving that the exceptional storm winds are more efficient for this purpose than the prevalent winds.
'Another group is that of Bermuda, situated nearly in the same position relatively to America that the Azores are to Europe. They are a little nearer, that is to say, six or seven hundred miles distant from the Continent, and the group is much smaller and the climate warmer. They are coralline in structure, and not volcanic, and are situated in the centre of a large curve of coast, so as to be especially liable to be reached by stragglers blown out to sea. There are no mammalia, and only one reptile, a lizard, somewhat allied to a species of South-East America.
'Of birds there are ten residents--eight land and two water birds--but an enormous number of visitors come every year, although the island is small and insignificant, having no mountain, and being at no point on its surface 100 feet above the sea. In spring and autumn the island is covered with geese, ducks, buntings, fly-catchers, etc., about 200 species in number. The explanation of this is found in the great migration which takes place twice a year. Owing to the magnificence of the summer in North America more birds breed in Canada than in Florida, great hosts flocking northward at the breeding season. They go by land, but in returning fly across the sea from headland to headland. In this way thousands are blown out to sea and perish, but some few succeed year after year in reaching Bermuda. This explains partly, at any rate, how the increase of birds is checked in spite of their rapid rate of breeding.
'The insects are few in number, and North American in type.
'The trees and plants can be clearly traced to two different sources: one West Indian, carried by the Gulf Stream, and the other from the United States, probably introduced by seeds attached to the plumage of birds. Plants are so constantly introduced by these [[p. 15]] means that there are no peculiarities. Hence we see that migration over 700 miles of sea is impossible for mammalia and amphibia, nearly so for reptiles, but comparatively easy for birds and plants.
'We now pass to the Galapagos archipelago--a remarkable group of islands situated in the Pacific Ocean, on the Equator, and about 400 miles west of South America.
'They are intermediate in size between the Azores and Bermuda, are entirely volcanic, and are surrounded by a deep sea. There is no reason to believe that they ever formed part of a continent, or were ever much nearer to the continent than they are at present. There is a wonderful difference between the physical condition of these islands and those we have just considered. They are situated in a calm sea, where high winds are very infrequent and storms unknown; and these conditions have a powerful influence on the animal and vegetable forms which occur in the islands.
'Reptiles are fairly represented, there being two species of tortoise existing and one extinct. They are gigantic in size, being three or four feet long. They are allied to American species, and it is thought that their ancestors must have floated over. There are five lizards allied to American species, one being marine, feeding entirely on sea-weed, and the only marine lizard now on the globe. Of all non-flying animals the lizards are the only ones which, either themselves or their eggs, have some peculiar means at present unknown, of conveyance over wide tracts of sea. There is hardly an island without a lizard.
'There are two snakes, both closely allied to South American species, and their presence on a remote island group is somewhat of an anomaly. It is quite possible in this case that they have been conveyed by man, and a few centuries may have sufficed to differentiate them from their American allies.
'There are fifty-seven birds, of which thirty-one are land birds, all belonging to American families, but the species except one are distinct. This is explained by the fact that the equatorial sea is the calmest in the world, and there is no chance of constant immigration from America, though the distance is not more than one-half the distance of the Azores from Europe. The one exception is the 'rice bird,' the most abundant of American birds, and the one with the widest range of migration.
'Those which are most nearly allied to American species are the insect-eating genera, such as warblers, thrushes, fly-catchers, &c., all of which habitually migrate in America. The other group consisting of fruit-eating birds which do not migrate, such as finches and sugar birds, got over long ago, and never having been reinforced, have been very much modified, especially by the increased strength of their bills adapting them to feed on the cacti of the island.
'The plants are also peculiar, more than one-half highly so. Some are allied to plants on the opposite coast of America, but most of them show affinity with plants of Mexico and the West Indies. The [[p. 16]] explanation of this is that in comparatively recent times the Isthmus of Panama did not exist, and the oceanic currents were in consequence so modified that plants could be carried across the sea from this direction more easily than from the opposite coast. A remarkable point about the flowers is, that though these islands are situated on the Equator there is a complete absence of the brilliancy of display we always associate with tropical vegetation, so much so that when Darwin visited the islands, and assiduously collected the plants, he was for a long time under the impression that he had come at the wrong season for flowers, but afterwards found that nearly every plant was in full flower. This proves that tropical conditions alone are not sufficient to produce brilliant colour which probably depends mainly on the advantage the flowers derive from the visits of insects, which are attracted by their brilliant colours. In the Galapagos, however, there are very few insects, no tropical island being so strikingly deficient. Till recently no butterfly was thought to exist, and now there is only one small species known and these facts help to prove that we owe the colour of flowers to the habitual visits of insects.
'These various facts in the natural history of oceanic islands serve to teach us that the most efficient agents in transporting life from the continents are exceptional storms and not the prevalent winds. It depends on the frequency of such storms whether the animals and plants of the island resemble those of the continent, or are distinct from them.
'Turning to St. Helena, we have an island in the South Atlantic Ocean removed further from a continent than any other island we have mentioned, being 1100 miles distant from Africa. Its structure is volcanic, and it once contained a very luxuriant vegetation, since destroyed by goats, which were introduced by man and allowed to run wild in the island. No doubt it must at that time have contained an abundant insect fauna, but these being dependent on the vegetation have become extinct with it. There are now very few insects, but all are highly peculiar. The most plentiful are the beetles, of which the great proportion are peculiar species and even peculiar genera. There are no aboriginal vertebrates, but a few land shells, and forty or fifty aboriginal flowering plants. In Ascension Island we have a stepping stone by which some forms may have reached St. Helena. It is now barren, but was at one time much more extensive, and was certainly inhabited by both plants and animals. If we notice that Ascension Island is about equidistant from the coast of Africa and from St. Helena, the distance in either case not being much greater than that of the Azores from Europe, we need not feel so much surprise that any creature should have reached an island so remote as St. Helena. But since there are no vertebrates in Ascension Island, we may regard this as indicating the limit of transmission of the higher animals. None of these latter have been able to reach St. Helena, and the lower animals only at long intervals; these are therefore isolated and highly peculiar.
[[p. 17]] 'As a general summary of the subject of these lectures we have: First, that sea and land are not perfectly interchangeable, but probably mark out the original elevation and depression of the cooling earth-crust, having remained nearly the same through all geological time. Though all continents have been submerged, they have never been wholly submerged at the same time, and thus we have a continuity of life and transmission of forms from the earliest epochs. These partial depressions and elevations have led to many successive isolations and subsequent connection of one part with another, these isolations being highly conducive to the development of distinct species. In the case of continental islands which have long been isolated, as Madagascar, New Zealand, and the West Indies, we can tell their relative age by the amount of modification observed in their forms of life. We learn that even a narrow sea keeps out a very large proportion of the larger animals, as in the case of Celebes, separated from Borneo, and as in the case of England, separated from Europe.
'Recent continental islands are distinguished by the general resemblance of their natural history to that of the continents, and may always be known by lying within the hundred fathoms line, just as the ancient continental islands are always within the thousand fathoms line.
'Oceanic islands show by their physical productions that they have never been connected with the continents, and offer incidental evidence that oceans have always been oceans, for otherwise at least some of them would have retained traces of sedimentary rocks identical with those of a continent.
'Finally, we learn from our study of the natural history of islands, that all the features of the distribution of animals and plants are explicable by their known powers of migration and transmission across the sea, taken in connection with the slow change of specific forms, and on the hypothesis that our continents and oceans have been always permanent in general position though ever fluctuating in extent and outline.'