_Absence of Indigenous Mammalia and Amphibia._--As in all other oceanic islands, we find here no truly indigenous mammalia, for though there is a mouse of the American genus Hesperomys, which differs somewhat from any known species, we can hardly consider this to be indigenous; first, because these creatures have been little studied in South America, and there may yet be many undescribed species, and in the second place because even had it been introduced by some European or native vessel, there is ample time in two or three hundred years for the very different conditions to have established a marked diversity in the characters of the species. This is the more probable because there is also a true rat of the Old World genus Mus, which is said to differ slightly from any known species; and as this genus is not a native of the American continents we are sure that it must have been recently introduced into the Galapagos. There can be little doubt therefore that the islands are completely dest.i.tute of truly indigenous mammalia; and frogs and toads, the only tropical representatives of the Amphibia, are equally unknown.
_Reptiles._--Reptiles, however, which at first sight appear as unsuited as mammals to pa.s.s over a wide expanse of ocean, abound in the Galapagos, though the species are not very numerous. They consist of land-tortoises, lizards and snakes. The tortoises consist of two peculiar species, _Testudo microphyes_, found in most of the islands, and _T. {279} abingdonii_ recently discovered on Abingdon Island, as well as one extinct species, _T.
ephippium_, found on Indefatigable Island. These are all of very large size, like the gigantic tortoises of the Mascarene Islands, from which, however, they differ in structural characters; and Dr. Gunther believes that they have been originally derived from the American continent.[60]
Considering the well known tenacity of life of these animals, and the large number of allied forms which have aquatic or sub-aquatic habits, it is not a very extravagant supposition that some ancestral form, carried out to sea by a flood, was once or twice safely drifted as far as the Galapagos, and thus originated the races which now inhabit them.
The lizards are five in number; a peculiar species of gecko, _Phyllodactylus galapagensis_, and four species of the American family Iguanidae. Two of these are distinct species of the genus Tropidurus, the other two being large, and so very distinct as to be cla.s.sed in peculiar genera. One of these is aquatic and found in all the islands, swimming in the sea at some distance from the sh.o.r.e and feeding on seaweed; the other is terrestrial, and is confined to the four central islands. These last were originally described as _Amblyrhynchus cristatus_ by Mr. Bell, and _A.
subcristatus_ by Gray; they were afterwards placed in two other genera Trachycephalus and Oreocephalus (_see_ Brit. Mus. Catalogue of Lizards), while in a recent paper by Dr. Steindachner, the marine species is again cla.s.sed as Amblyrhynchus, while the terrestrial form is placed in another genus Conolophus, both genera being peculiar to the Galapagos.
How these lizards reached the islands we cannot tell. The fact that they all belong to American genera or families indicates their derivation from that continent, while their being all distinct species is a proof that their arrival took place at a remote epoch, under conditions perhaps somewhat different from any which now prevail. It is certain that animals of this order have some means of crossing the sea not possessed by any other land vertebrates, {280} since they are found in a considerable number of islands which possess no mammals nor any other land reptiles; but what those means are has not yet been positively ascertained.
It is unusual for oceanic islands to possess snakes, and it is therefore somewhat of an anomaly that two species are found in the Galapagos. Both are closely allied to South American forms, and one is hardly different from a Chilian snake, so that they indicate a more recent origin than in the case of the lizards. Snakes it is known can survive a long time at sea, since a living boa-constrictor once reached the island of St. Vincent from the coast of South America, a distance of two hundred miles by the shortest route. Snakes often frequent trees, and might thus be conveyed long distances if carried out to sea on a tree uprooted by a flood such as often occurs in tropical climates and especially during earthquakes. To some such accident we may perhaps attribute the presence of these creatures in the Galapagos, and that it is a very rare one is indicated by the fact that only two species have as yet succeeded in obtaining a footing there.
_Birds._--We now come to the birds, whose presence here may not seem so remarkable, but which yet present features of interest not exceeded by any other group. About seventy species of birds have now been obtained on these islands, and of these forty-one are peculiar to them. But all the species found elsewhere, except one, belong to the aquatic tribes or the waders which are pre-eminently wanderers, yet even of these eight are peculiar.
The true land-birds are forty-two in number, and all but one are entirely confined to the Galapagos; while three-fourths of them present such peculiarities that they are cla.s.sed in distinct genera. All are allied to birds inhabiting tropical America, some very closely; while one--the common American rice-bird which ranges over the whole northern and part of the southern continents--is the only land-bird identical with those of the mainland. The following is a list of these land-birds taken from Mr.
Salvin"s memoir in the _Transactions of the Zoological Society_ for the year 1876, to which are added nine species collected in 1888 and {281} described by Mr. Ridgway in the _Proceedings of the U.S. National Museum_ (XII. p. 101) and some additional species obtained in 1889.
t.u.r.dIDae.
1. Nesomimus trifasciatus } This and the two allied species 2. ,, melanotus } are related to a Peruvian bird 3. ,, parvulus } _Mimus longicaudus_.
4. ,, macdonaldi (Ridg.) 5. ,, personatus (Ridg.)
MNIOTILTIDae.
6. Dendroeca aureola { Closely allied to the wide-ranging { _D. aestiva_.
HIRUNDINIDae.
7. Progne concolor { Allied to _P. purpurea_ of North { and South America.
COEREBIDae.
8. Certhidea olivacea } A peculiar genus allied to the 9. ,, fusca } Andean genus Conirostrum.
10. ,, cinerascens }
FRINGILLIDae.
11. Geospiza magnirostris 12. ,, strenua 13. ,, dubia A distinct genus, but allied to the 14. ,, fortis South American genus Guiraca.
15. ,, nebulosa 16. ,, fuliginosa 17. ,, parvula 18. ,, dentirostris 19. ,, conirostris (Ridg.) 20. ,, media (Ridg.) 21. ,, difficilis (Sharpe) 22. Cactornis scandens 23. ,, a.s.similis 24. ,, abingdoni 25. ,, pallida A genus allied to the last.
26. ,, brevirostris (Ridg.) 27. ,, hypoleuca (Ridg.) A very peculiar genus allied to 28. Camarhynchus psittaculus Neorhynchus of the west coast 29. ,, cra.s.sirostris of Peru.
30. ,, variegatus 31. ,, prosthemelas 32. ,, habeli 33. ,, townsendi (Ridg.) 34. ,, pauper (Ridg.) {282}
ICTERIDae.
35. Dolichonyx oryzivorus Ranges from Canada to Paraguay.
TYRANNIDae.
36. Pyrocephalus na.n.u.s 37. P. minimus (Ridg.) Allied to _P. rubincus_ of Ecuador.
38. Myiarchus magnirostris Allied to West Indian species.
COLUMBIDae.
39. Zenaida galapagensis { A peculiar species of a S.
{ American genus.
FALCONIDae.
40. Buteo galapagensis A buzzard of peculiar coloration.
STRIGIDae.
41. Asio galapagensis } Hardly distinct from the widespread } _A. brachyotus._
42. Strix punctatissima Allied to _S. flammea_ but quite distinct.
We have here every gradation of difference from perfect ident.i.ty with the continental species to genera so distinct that it is difficult to determine with what forms they are most nearly allied; and it is interesting to note that this diversity bears a distinct relation to the probabilities of, and facilities for, migration to the islands. The excessively abundant rice-bird, which breeds in Canada and swarms over the whole United States, migrating to the West Indies and South America, visiting the distant Bermudas almost every year, and extending its range as far as Paraguay, is the only species of land-bird which remains completely unchanged in the Galapagos; and we may therefore conclude that some stragglers of the migrating host reach the islands sufficiently often to keep up the purity of the breed. Next, we have the almost cosmopolite short-eared owl (_As...o...b..achyotus_), which ranges from China to Ireland, and from Greenland to the Straits of Magellan, and of this the Galapagos bird is probably only one of the numerous varieties. The little wood warbler (_Dendroeca aureola_) is closely allied to a species which {283} ranges over the whole of North America and as far south as New Grenada. It has also been occasionally met with in Bermuda, an indication that it has considerable powers of flight and endurance. The more distinct _species_--as the tyrant fly-catchers (Pyrocephalus and Myiarchus), the ground-dove (Zenaida), and the buzzard (Buteo), are all allied to non-migratory species peculiar to tropical America, and of a more restricted range; while the distinct _genera_ are allied to South American groups of thrushes, finches, and sugar-birds which have usually restricted ranges, and whose habits are such as not to render them likely to be carried out to sea. The remote ancestral forms of these birds which, owing to some exceptional causes, reached the Galapagos, have thus remained uninfluenced by later migrations, and have, in consequence, been developed into a variety of distinct types adapted to the peculiar conditions of existence under which they have been placed. Sometimes the different species thus formed are confined to one or two of the islands only, as the three species of Certhidea, which are divided between the islands but do not appear ever to occur together. _Nesomimus parvulus_ is confined to Albemarle Island, and _N. trifasciatus_ to Charles Island; _Cactornis pallida_ to Indefatigable Island, _C. brevirostris_ to Chatham Island, and _C. abingdoni_ to Abingdon Island.
Now all these phenomena are strictly consistent with the theory of the peopling of the islands by accidental migrations, if we only allow them to have existed for a sufficiently long period; and the fact that volcanic action has ceased on many of the islands, as well as their great extent, would certainly indicate a considerable antiquity.
The great difference presented by the birds of these islands as compared with those of the equally remote Azores and Bermudas, is sufficiently explained by the difference of climatal conditions. At the Galapagos there are none of those periodic storms, gales, and hurricanes which prevail in the North Atlantic, and which every year carry some straggling birds of Europe or North America to the former islands; while, at the same time, the majority of the tropical American birds are {284} nonmigratory, and thus afford none of the opportunities presented by the countless hosts of migrants which pa.s.s annually northward and southward along the European, and especially along the North American coasts. It is strictly in accordance with these different conditions that we find in one case an almost perfect ident.i.ty with, and in the other an almost equally complete diversity from, the continental species of birds.
_Insects and Land-sh.e.l.ls._--The other groups of land-animals add little of importance to the facts already referred to. The insects are very scanty; the most plentiful group, the beetles, only furnishing about forty species belonging to thirty-two genera and nineteen families. The species are almost all peculiar, as are some of the genera. They are mostly small and obscure insects, allied either to American or to world-wide groups. The Carabidae and the Heteromera are the most abundant groups, the former furnishing six and the latter nine species.[61]
{285}
The land-sh.e.l.ls are not abundant--about twenty in all, most of them peculiar species, but not otherwise remarkable. The observation of Captain Collnet, quoted by Mr. Darwin in his _Journal_, that drift-wood, bamboos, canes, and the nuts of a palm, are often washed on the south-eastern sh.o.r.es of the islands, furnishes an excellent clue to the manner in which many of the insects and land-sh.e.l.ls may have reached the Galapagos. Whirlwinds also have been known to carry quant.i.ties of leaves and other vegetable _debris_ to great heights in the air, and these might be then carried away by strong upper currents and dropped at great distances, and with them small insects and mollusca, or their eggs. We must also remember that volcanic islands are subject to subsidence as well as elevation; and it is quite possible that during the long period the Galapagos have existed some islands may have intervened between them and the coast, and have served as stepping-stones by which the pa.s.sage to them of various organisms would be greatly facilitated. Sunken banks, the relics of such islands, are known to exist in many parts of the ocean, and countless others, no doubt, remain undiscovered.
_The Keeling Islands as Ill.u.s.trating the Manner in which Oceanic Islands are Peopled._--That such causes as have been here adduced are those by which oceanic islands have been peopled, is further shown by the condition of equally remote islands which we know are of comparatively recent origin.
Such are the Keeling or Cocos Islands in the Indian Ocean, situated about the same distance from Sumatra as the Galapagos from South America, but mere coral reefs, supporting abundance of cocoa-nut palms as their chief vegetation. These islands were visited by Mr. {286} Darwin, and their natural history carefully examined. The only mammals are rats, brought by a wrecked vessel and said by Mr. Waterhouse to be common English rats, "but smaller and more brightly coloured;" so that we have here an ill.u.s.tration of how soon a difference of race is established under a constant and uniform difference of conditions. There are no true land-birds, but there are snipes and rails, both apparently common Malayan species. Reptiles are represented by one small lizard, but no account of this is given in the _Zoology of the Voyage of the Beagle_, and we may therefore conclude that it was an introduced species. Of insects, careful collecting only produced thirteen species belonging to eight distinct orders. The only beetle was a small Elater, the Orthoptera were a Gryllus and a Blatta; and there were two flies, two ants, and two small moths, one a Diopaea which swarms everywhere in the eastern tropics in gra.s.sy places. All these insects were no doubt brought either by winds, by floating timber (which reaches the islands abundantly), or by clinging to the feathers of aquatic or wading birds; and we only require more time to introduce a greater variety of species, and a better soil and more varied vegetation, to enable them to live and multiply, in order to give these islands a fauna and flora equal to that of the Bermudas. Of wild plants there were only twenty species, belonging to nineteen genera and to no less than sixteen natural families, while all were common tropical sh.o.r.e plants.[62] These islands are thus evidently stocked by waifs and strays brought by the winds and waves; but their scanty vegetation is mainly due to unfavourable conditions--the barren coral rock and sand, of which they are wholly composed, together with exposure to sea-air, being suitable to a very limited number of species which soon monopolise the surface. With more variety of soil and aspect a greater variety of plants would establish themselves, and these would favour the preservation and increase of more insects, birds, and {287} other animals, as we find to be the case in many small and remote islands.[63]
_Flora of the Galapagos._--The plants of these islands are so much more numerous than the known animals, even including the insects, they have been so carefully studied by eminent botanists, and their relations throw so much light on the past history of the group, that no apology is needed for giving a brief outline of the peculiarities and affinities of the flora.
The statements we shall make on this subject will be taken from the Memoir of Sir Joseph Hooker in the _Linnaean Transactions_ for 1851, founded on Mr.
Darwin"s collections, and a later paper by N. J. Andersson in the _Linnaea_ of 1861, embodying more recent discoveries. {288}
The total number of flowering plants known at the latter date was 332, of which 174 were peculiar to the islands, while 158 were common to other countries.[64] Of these latter about twenty have been introduced by man, while the remainder are all natives of some part of America, though about a third part are species of wide range extending into both hemispheres. Of those confined to America, forty-two are found in both the northern and southern continents, twenty-one are confined to South America, while twenty are found only in North America, the West Indies, or Mexico. This equality of North American and South American species in the Galapagos is a fact of great significance in connection with the observation of Sir Joseph Hooker that the _peculiar_ species are allied to the plants of temperate America or to those of the high Andes, while the non-peculiar species are mostly such as inhabit the hotter regions of the tropics near the level of the sea. He also observes that the seeds of this latter cla.s.s of Galapagos plants often have special means of transport, or belong to groups whose seeds are known to stand long voyages and to possess great vitality. Mr.
Bentham also, in his elaborate account of the Compositae,[65] remarks on the decided Central American or Mexican affinities of the Galapagos species, so that we may consider this to be a thoroughly well-established fact.
The most prevalent families of plants in the Galapagos are the Compositae (40 sp.), Gramineae (32 sp.), Leguminosae (30 sp.), and Euphorbiaceae (29 sp.). Of the Compositae most of the species, except such as are common weeds or sh.o.r.e plants, are peculiar, but there are only two peculiar genera, allied to Mexican forms and not very distinct; while the genus Lipochaeta, represented here by a single species, is only found elsewhere in the Sandwich Islands though it has American affinities.
_Origin of the Galapagos Flora._--These facts are explained by the past history of the American continent, its {289} separation at various epochs by arms of the sea uniting the two oceans across what is now Central America (the last separation being of recent date, as shown by the considerable number of identical species of fishes on both sides of the isthmus), and the influence of the glacial epoch in driving the temperate American flora southward along the mountain plateaus.[66] At the time when the two oceans were united a portion of the Gulf Stream may have been diverted into the Pacific, giving rise to a current, some part of which would almost certainly have reached the Galapagos, and this may have helped to bring about that singular a.s.semblage of West Indian and Mexican plants now found there. And as we now believe that the duration of the last glacial epoch in its successive phases was much longer than the time which has elapsed since it finally pa.s.sed away, while throughout the Miocene epoch the snow-line would often be lowered during periods of high excentricity, we are enabled to comprehend the nature of the causes which may have led to the islands being stocked with those north tropical or mountain types which are so characteristic a feature of that portion of the Galapagos flora which consists of peculiar species.
On the whole, the flora agrees with the fauna in indicating a moderately remote origin, great isolation, and changes of conditions affording facilities for the introduction of organisms from various parts of the American coast, and even from the West Indian Islands and Gulf of Mexico.
As in the case of the birds, the several islands differ considerably in their native plants, many species being limited to one or two islands only, while others extend to several. This is, of course, what might be expected on any theory of their origin; because, even if the whole of the islands had once been united and afterwards separated, long continued isolation would often lead to the differentiation of species, while the varied conditions to be found upon islands differing in size and alt.i.tude as well as in luxuriance of vegetation, would often lead to the extinction of a species on one island and its preservation on another. If the several islands had been equally well {290} explored, it might be interesting to see whether, as in the case of the Azores, the number of species diminished in those more remote from the coast; but unfortunately our knowledge of the productions of the various islands of the group is exceedingly unequal, and, except in those cases in which representative species inhabit distinct islands, we have no certainty on the subject. All the more interesting problems in geographical distribution, however, arise from the relation of the fauna and flora of the group as a whole to those of the surrounding continents, and we shall therefore for the most part confine ourselves to this aspect of the question in our discussion of the phenomena presented by oceanic or continental islands.
_Concluding Remarks._--The Galapagos offer an instructive contrast with the Azores, showing how a difference of conditions that might be thought unimportant may yet produce very striking results in the forms of life.
Although the Galapagos are much nearer a continent than the Azores, the number of species of plants common to the continent is much less in the former case than in the latter, and this is still more prominent a characteristic of the insect and the bird faunas. This difference has been shown to depend, almost entirely, on the one archipelago being situated in a stormy, the other in a calm portion of the ocean; and it demonstrates the preponderating importance of the atmosphere as an agent in the dispersal of birds, insects, and plants. Yet ocean-currents and surface-drifts are undoubtedly efficient carriers of plants, and, with plants, of insects and sh.e.l.ls, especially in the tropics; and it is probably to this agency that we may impute the recent introduction of a number of common Peruvian and Chilian littoral species, and also of several West Indian types at a more remote period when the Isthmus of Panama was submerged.
In the case of these islands we see the importance of taking account of past conditions of sea and land and past changes of climate, in order to explain the relations of the peculiar or endemic species of their fauna and flora; and we may even see an indication of the effects of climatal changes in the northern hemisphere, in the north {291} temperate or alpine affinities of many of the plants, and even of some of the birds. The relation between the migratory habits of the birds and the amount of difference from continental types is strikingly accordant with the fact that it is almost exclusively migratory birds that annually reach the Azores and Bermuda; while the corresponding fact that the seeds of those plants, which are common to the Galapagos and the adjacent continent, have all--as Sir Joseph Hooker states--some special means of dispersal, is equally intelligible. The reason why the Galapagos possess four times as many peculiar species of plants as the Azores is clearly a result of the less constant introduction of seeds, owing to the absence of storms; the greater antiquity of the group, allowing more time for specific change; and the influence of cold epochs and of alterations of sea and land, in bringing somewhat different sets of plants at different times within the influence of such modified winds and currents as might convey them to the islands.