Some Salient Points in the Science of the Earth

by Sir John William Dawson

Those smaller portions of land separated from the continental masses, the islands properly so called, present, as might be expected, many peculiar features. Wallace divides them into two classes, though he admits that these pass into each other. Continental islands are those in the vicinity of continents. They consist of ancient as well as modern rock formations, and contain animals which indicate a former continental connection. Some of these are separated from the nearest mainland only by shallow seas or straits, and may be assumed to have become islands only in recent geological times. Others are divided from the nearest continent by very deep-water, so that they have probably been longer severed from the mainland. These contain more peculiar assemblages of animals and plants than the islands of the former class. Oceanic islands are more remote from the continents. They consist mostly of rocks belonging to the modern geological periods, and contain no animals of those classes which can migrate only by land. Such islands may be assumed never to have been connected with any continent. The study of the indigenous population of these various classes of islands affords many curious and interesting results, which Wallace has collected with vast industry and care, and which, on the whole, he explains in a judicious manner and in accordance with the facts of geology. When, however, he maintains that evolution of the Darwinian type is "the key to distribution," he departs widely from any basis of scientific fact. This becomes apparent when we consider the following results, which appear everywhere in the discussion of the various insular faunas and floras:—(1) None of these islands, however remote, can be affirmed to have been peopled by the spontaneous evolution of the higher animals or plants from lower forms. Their population is in every case not autochthonous, but derived. (2) Even in those which are most distant from the continents, and may be supposed to have been colonized in very ancient times, there is no evidence of any very important modification of their inhabitants. (3) While the facts point to the origin of most forms of terrestrial life in the Palearctic and Nearctic regions, they afford no information as to the manner or cause of their origination. In short, so far is evolution from being a key to distribution, that the whole question would become much more simple if this element were omitted altogether. A few examples may be useful to illustrate this, as well as the actual explanation of the phenomena afforded by legitimate science.

The Azores are situated in a warm temperate latitude about 900 miles west of Portugal, and separated from it by a sea 2,500 fathoms in depth. The islands themselves are almost wholly volcanic, and the oldest rocks known in them are of late Miocene age. There is no probability that these islands have ever been connected with Europe or Africa, nor is there at present any certainty that they have been joined to one another, or have formed part of any larger insular tract. In these islands there is only one indigenous mammal, a bat, which is identical with a European species, and no doubt reached the islands by flight. There is no indigenous reptile, amphibian, or fresh-water fish. Of birds there are, exclusive of waterfowl, which may be regarded as visitors, twenty-two land birds; but of these, four are regarded as merely accidental stragglers, so that only eighteen are permanent residents. Of these birds fifteen are common European or African species, which must have flown to the islands, or have been drifted thither in storms. Of the remaining three, two are found also in Madeira and the Canaries, and therefore may reasonably be supposed to have been derived from Africa. One only is regarded as peculiar to the Azores, and this is a bullfinch, so nearly related to the European bullfinch that it may be regarded as merely a local variety. Wallace accounts for these facts by supposing that the Azores were depopulated by the cold of the Glacial age, and that all these birds have arrived since that time. There is, however, little probability in such a supposition. He further supposes that fresh supplies of stray birds from the mainland, arriving from time to time, have kept up the identity of the species. Instead of evolution assisting him, he has thus somewhat to strain the facts to agree with that hypothesis. Similar explanations are given for the still more remarkable fact that the land plants of the Azores are almost wholly identical with European and African forms. The insects and the land snails are, however, held to indicate the evolution of a certain number of new specific forms on the islands. The beetles number no less than 212 species, though nearly half of them are supposed to have been introduced by man. Of the whole number 175 are European, 19 are found in Madeira and the Canaries, 3 are American. Fourteen remain to be accounted for, though most of these are closely allied to European and other species; but a few are quite distinct from any elsewhere known. Wallace, however, very truly remarks that our knowledge of the continental beetles is not complete; that the species in question are small and obscure; that they may be survivors of the Glacial period, and may thus represent species now extinct on the mainland; and that for these reasons it may not be irrational to suppose that these peculiar insects either still inhabit, or did once inhabit, some part of the continents, and may be portions of "ancient and widespread groups," once widely diffused, but now restricted to a few insular spots. Among the land snails, if anywhere, we should find evidence either of autochthonous evolution or of specific change. These animals have existed on the earth since the Carboniferous period, and, notwithstanding their proverbial slowness and sedentary habits, they have contrived to colonize every habitable spot of land on the globe—that is, unless in some of these places they have originated de novo. In the Azores there are sixty-nine species of land snails, of which no less than thirty-two, or nearly one-half, are peculiar, though nearly all are closely allied to European types. What, then, is the origin of these thirty-two species, admitting for the sake of argument that they are really distinct, and not merely varietal forms, though it is well known that in this group species are often unduly multiplied. Three suppositions are possible, (1) These snails may have originated in the islands themselves, either by creation or evolution from lower forms; say, from sea snails. (2) They may have been modified from modern continental species. (3) They may be unmodified descendants of species of Miocene or Pliocene age now existing on the continents only as fossils. As the islands appear to have existed since Miocene times, it is no more improbable that species of that or the Pliocene age should have found their way to them than that modern species should; and as we know only a fraction of the Tertiary species of Europe or Africa, it is not likely that we shall be able to identify all of these early visitors. Unfortunately no Miocene or Pliocene deposits holding remains of land snails are known in the Azores themselves, so that this kind of evidence fails us. In Madeira and Porto Santo, however, where there are numerous modern snails, there are Pliocene beds holding remains of these animals. In Madeira there are, according to Lyell, 36 Pliocene species, and in Porto Santo 35, and of these only eight are extinct. Thus we can prove that many of the peculiar species of these islands have remained unchanged since Pliocene times. While differing from modern European shells, several of these species are very near to European Miocene species. Thus we seem to have evidence in the Madeira group, not of modification, but of unchanged survival of Tertiary species long since extinct in Europe. May we not infer that the same was the case in the Azores? These results are certainly very striking when we consider how long the Azores must have existed as islands, how very rarely animals, and especially pairs of animals, must have reached them, and how complete has been the isolation of these animals, and how peculiar the conditions to which they have been subjected in their island retreat.

Other oceanic islands present great varieties of conditions, but leading to similar conclusions. Some, as the Bermudas, seem to have been settled in very modern times with animals and plants nearly all identical with those of neighbouring countries, though even here it would appear that there are some indigenous species which would indicate a greater age or more extended lands, now submerged.[186] Others, like St. Helena, are occupied apparently with old settlers, which may have come to them in early Tertiary, or even in Secondary periods, which have long since become extinct on the continents, and whose nearest analogues are now widely scattered over the world. Islands are therefore places of survival of old species—special preserves for forms of life lost to the continents. One of the most curious of these is Celebes, which seems to be a surviving fragment of Miocene Asia, which, though so near to that continent, has been sufficiently isolated to preserve its old population during all the vast lapse of time between the middle Tertiary and the present period. This is a fact which gives to the oceanic islands the greatest geological interest, and induces us to look into their actual fauna and flora for the representatives of species known on the mainland only as fossils. It is thus that we look to the marsupials of Australia as the nearest analogues of those of the Jurassic of Europe, and that we find in the strange Barramunda (ceratodus) of its rivers the only survivor of a group of fishes once widely distributed, but which has long since perished elsewhere.

[186] Heilprin mentions eleven marine mollusks supposed to be peculiar to the islands, and eight species of land shells, as well as a few Crustaceans hitherto found only in the Pacific. The comparisons are, however, admitted to be incomplete.

Perhaps one of the most interesting examples of this is furnished by the Galapagos Islands, an example the more remarkable that no one who has read in Darwin's fascinating "Journal" the description of these islands, can have failed to perceive that the peculiarities of this strange Archipelago must have been prominent among the facts which first planted in his mind the germ of that theory of the origin of species which has since grown to such gigantic dimensions. It is curious also to reflect that had the bearing of geological history on the facts of distribution been as well known forty years ago as it is now, the reasoning of the great naturalist on this and similar cases might have taken an entirely different direction.

The Galapagos are placed exactly on the equator, and therefore out of reach of even the suspicion of having been visited by the glacial cold, though from their isolation in the ocean, and the effects of the currents flowing along the American coast, their climate is not extremely hot. They are 600 miles west of South America, and the separating ocean is in some parts 3,000 fathoms deep. The largest of the islands is 75 miles in length, and some of the hills attain an elevation of about 4,000 feet, so that there are considerable varieties of station and climate. So far as is known they are wholly volcanic, and they may be regarded as the summits of submerged mountains not unlike in structure to the Andes of the mainland. Their exact geological age is unknown, but there is no improbability in supposing that they may have existed with more or less of extension since the Secondary or Mesozoic period. In any case their fauna is in some respects a survival of that age. Lyell has truly remarked, "In the fauna of the Galapagos Islands we have a state of things very analogous to that of the Secondary period."

Like other oceanic islands, the Galapagos have no indigenous mammals, with the doubtful exception of a South American mouse; but, unlike most others, they are rich in reptiles. At the head of these stand several species of gigantic tortoises. This group of animals, so far as known, commenced its existence in the Eocene Tertiary; and in this and the Miocene period still more gigantic species existed on the continents. It has been supposed that at some such early date they reached the Galapagos from South America. Another group of Galapagan reptiles, perhaps still more remarkable, is that of iguana-like lizards of the genus Amblyrhyncus, which are vegetable feeders,—one of them browsing on marine weeds. They recall the great iguana-like reptiles of the European Wealden, and stand remote from all modern types. There are also snakes of two species, but these are South American forms, and may have drifted to the islands in comparatively recent times on floating trees. The birds are a curious assemblage. A few are common American species, like the rice bird. Others are quaint and peculiar creatures, allied to South American birds, but probably representing forms long since extinct on the continent. The bird fauna, as Wallace remarks, indicates that some of these animals are old residents, others more recent arrivals; and it is probable that they have arrived at various times since the early Tertiary. He assumes that the earlier arrivals have been modified in the islands "into a variety of distinct types"; but the only evidence of this is that some of the species are closely related to each other. It is more likely that they represent to our modern eyes the unmodified descendants of continental birds of the early Tertiary. Darwin remarks that they are remarkably sombre in colouring for equatorial birds; but perhaps their ancestors came from a cooler climate, and have not been able to don a tropical garb; or perhaps they belong to a far-back age, when the vegetable kingdom also was less rich in colouring than it is at present, and the birds were in harmony with it. This, indeed, seems still to be the character of the Galapagos plants, which Darwin says have "a wretched, weedy appearance," without gay flowers, though later visitors have expressed a more favourable opinion.

These plants are in themselves very remarkable, for they are largely peculiar species, and are in many cases confined to particular islands, having apparently been unable to cross from one island to another, though in some way able to reach the group. The explanation is that they resemble North American plants, and came to the Galapagos at a time when a wide strait separated North and South America, allowing the equatorial current to pass through, and drift plants to the Galapagos, where they have been imprisoned ever since. This was probably in Miocene times, and when we know more of the Miocene flora of the southern part of North America we may hope to recover some of the ancestors of the Galapagos plants. In the meantime their probable origin and antiquity, as stated by Wallace, render unnecessary any hypothesis of modification.

Before leaving this subject, it is proper to observe that on the continents themselves there are many remarkable cases of isolation of species, which help us better to understand the conditions of insular areas. The "variable hare" of the Scottish highlands, and of the extreme north of Europe, appears again in the Alps, the Pyrenees, and the Caucasus, being in these mountains separated by a thousand miles of apparently impassable country from its northern haunts. It no doubt extended itself over the intervening plains at a time when Europe was colder than at present. Another curious case is that of the marsh-tit of Europe. This little bird is found throughout south-western Europe. It reappears in China, but is not known anywhere between. In Siberia and northern Europe there is, however, a species or distinct race which connects these isolated patches. In this case, if the Siberian species is truly distinct, we have a remarkable case of isolation and of the permanence of identical characters for a long time; for in that case this bird must be a survivor of the Pliocene or Miocene time. On the other hand, if, as is perhaps more likely, the marsh-tit is only a local variety of the Siberian species, we have an illustration of the local recurrence of this form when the conditions are favourable, even though separated by a great space and long time.

The study of fossils gives us the true meaning of such facts, and causes us to cease to wonder at any case of local repetition of species, however widely separated. The "big trees" of California constitute a remarkable example. There are at present two very distinct species of these trees, both found only in limited areas of the western part of North America. Fossil trees of the same genus (Sequoia) occur as far back as the Cretaceous age; but in this age ten or more species are known. Nor are they confined to America, but occur in various parts of the Eurasian continent as well. Two of the Lower Cretaceous species are so near to the two modern ones that even an unbeliever in evolution may suppose them to be possible ancestors; the remaining eight are distinct, but some of them intermediate in their characters. In the Tertiary period, intervening between the Cretaceous and the modern, fourteen species of Sequoia are believed to have been recognised, and they appear to have existed abundantly all over the northern hemisphere. Thus we know that these remarkable Californian giants are the last remnant of a once widely distributed genus, originating, as far as known, in the Cretaceous age. Now had a grove of Sequoias, however small, survived anywhere in Europe or Asia, and had we no knowledge of the fossil forms, we might have been quite at a loss to account for their peculiar distribution. The fossil remains of the Tertiary rocks, both animal and vegetable, present us with many instances of this kind.