29. _A. wattersi._}
PITTIDae (Pittas).
30. _Pitta oreas._ Allied to _P. cyanoptera_ of Malaya and S. China.
PICIDae (Woodp.e.c.k.e.rs).
31. _Picus insularis._ Allied to _P. leuconotus_ of j.a.pan and Siberia.
MEGALaeMIDae.
32. _Megalaema nuchalis._ Allied to _M. oortii_ of Sumatra and _M.
faber_ of Hainan. No allies in China.
CAPRIMULGIDae (Goatsuckers).
33. _Caprimulgus stictomus._ A sub-species of _C. monticolus_ of India and China.
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COLUMBIDae (Pigeons).
34. _Treron formosae._ Allied to Malayan species.
35. _Sphenocercus sororius._ Allied to Malay species and to _S.
sieboldi_ of j.a.pan. No allies of these two birds inhabit China.
36. _Chalcophaps formosana._ Allied to the Indian species which extends to Tena.s.serim and Hainan.
TETRAONIDae (Grouse and Partridges).
37. _Orcoperdix crudigularis._ A peculiar genus of partridges.
38. _Bambusicola sonorivox._ Allied to the Chinese _B. thoracica_.
39. _Arcoturnix rostrata._ Allied to the Chinese _A. blakistonii_.
PHASIANIDae (Pheasants).
40. _Phasia.n.u.s formosa.n.u.s._ Allied to _P. torquatus_ of China.
41. _Euplocamus swinhoii._ A very peculiar and beautiful species allied to the tropical fire-backed pheasants, and to the silver pheasant of North China.
STRIGIDae (Owls).
42. _Athene pardalota._ Closely allied to a Chinese species.
43. _Lempigius hambroekii._ Allied to a Chinese species.
This list exhibits to us the marvellous fact that more than half the peculiar species of Formosan birds have their nearest allies in such remote regions as the Himalayas, South India, the Malay Islands, or j.a.pan, rather than in the adjacent parts of the Asiatic continent. Fourteen species have Himalayan allies, and six of these belong to genera which are unknown in China. One has its nearest ally in the Nilgherries, and five in the Malay Islands; and of these six, four belong to genera which are not Chinese. Two have their only near allies in j.a.pan. Perhaps more curious still are those cases in which, though the genus is Chinese, the nearest allied species is to be sought for in some remote region. Thus we have the Formosan babbler (_Garrulax ruficeps_) not allied to the species found in South China, but to one inhabiting North India and East Thibet; while the black bulbul (_Hypsipetes nigerrimus_), is not allied to the Chinese species but to an a.s.samese form.
In the same category as the above we must place eight species not peculiar to Formosa, but which are Indian or Malayan rather than Chinese, so that they offer examples of discontinuous distribution somewhat a.n.a.logous to what {407} we found to occur in j.a.pan. These are enumerated in the following list.
SPECIES OF BIRDS COMMON TO FORMOSA AND INDIA OR MALAYA, BUT NOT FOUND IN CHINA.
1. _Siphia superciliaris._ The Rufous-breasted Flycatcher of the S. E.
Himalayas.
2. _Halcyon coromanda._ The Great Red Kingfisher of India, Malaya, and j.a.pan.
3. _Palumbus pulchricollis._ The Darjeeling Wood-pigeon of the S. E.
Himalayas.
4. _Turnix dussumieri._ The larger b.u.t.ton-quail of India.
5. _Spizaetus nipalensis._ The Spotted Hawk-eagle of Nepal and a.s.sam.
6. _Lophospiza trivirgata._ The Crested Gos-hawk of the Malay Islands.
7. _Bulaca newarensis._ The Brown Wood-owl of the Himalayas.
8. _Strix candida._ The Gra.s.s-owl of India and Malaya.
The most interesting of the above are the pigeon and the flycatcher, both of which are, so far as yet known, strictly confined to the Himalayan mountains and Formosa. They thus afford examples of discontinuous specific distribution exactly parallel to that of the great spotted kingfisher, already referred to as found only in the Himalayas and j.a.pan.
_Comparison of the Faunas of Hainan, Formosa, and j.a.pan._--The island of Hainan on the extreme south of China, and only separated from the mainland by a strait fifteen miles wide, appears to have considerable similarity to Formosa, inasmuch as it possesses seventeen peculiar land-birds (out of 130 obtained by Mr. Swinhoe), two of which are close allies of Formosan species, while two others are identical. We also find four species whose nearest allies are in the Himalayas. Our knowledge of this island and of the adjacent coast of China is not yet sufficient to enable us to form an accurate judgment of its relations, but it seems probable that it was separated from the continent at, approximately, the same epoch as Formosa and j.a.pan, and that the special features of each of these islands are mainly due to their geographical position. Formosa, being more completely isolated than either of the others, possesses a larger proportion of peculiar species of birds, while its tropical situation and lofty mountain ranges {408} have enabled it to preserve an unusual number of Himalayan and Malayan forms. j.a.pan, almost equally isolated towards the south, and having a much greater variety of climate as well as a much larger area, possesses about an equal number of mammalia with Formosa, and an even larger proportion of peculiar species. Its birds, however, though more numerous are less peculiar; and this is probably due to the large number of species which migrate northwards in summer, and find it easy to enter j.a.pan through the Kurile Isles or Saghalien.[96] j.a.pan too, is largely peopled by those northern types which have an unusually wide range, and which, being almost all migratory, are accustomed to cross over seas of moderate extent. The regular or occasional influx of these species prevents the formation of special insular races, such as are almost always produced when a portion of the population of a species remains for a considerable time completely isolated. We thus have explained the curious fact, that while the mammalia of the two islands are almost equally peculiar, (those of j.a.pan being most so in the present state of our knowledge), the birds of Formosa show a far greater number of peculiar species than those of j.a.pan.
_General Remarks on Recent Continental Islands._--We have now briefly sketched the zoological peculiarities of an ill.u.s.trative series of recent continental islands, commencing with one of the most recent--Great Britain--in which the process of formation of peculiar species has only just commenced, and terminating with Formosa, probably one of the most ancient of the series, and which accordingly presents us with a very large proportion of peculiar species, not only in its mammalia, which have no means of crossing the wide strait which separates it from the mainland, but also in its birds, many of which are quite able to cross over.
Here, too, we obtain a glimpse of the way in which {409} species die out and are replaced by others, which quite agrees with what the theory of evolution a.s.sures us must have occurred. On a continent, the process of extinction will generally take effect on the circ.u.mference of the area of distribution, because it is there that the species comes into contact with such adverse conditions or competing forms as prevent it from advancing further. A very slight change will evidently turn the scale and cause the species to contract its range, and this usually goes on till it is reduced to a very restricted area, and finally becomes extinct. It may conceivably happen (and almost certainly has sometimes happened) that the process of restriction of range by adverse conditions may act in one direction only, and over a limited district, so as ultimately to divide the specific area into two separated parts, in each of which a portion of the species will continue to maintain itself. We have seen that there is reason to believe that this has occurred in a very few cases both in North America and in Northern Asia. (_See_ pp. 65-68.) But the same thing has certainly occurred in a considerable number of cases, only it has resulted in the divided areas being occupied by _representative forms_ instead of by the very same species. The cause of this is very easy to understand. We have already shown that there is a large amount of local variation in a considerable number of species, and we may be sure that were it not for the constant intermingling and intercrossing of the individuals inhabiting adjacent localities this tendency to local variation in adaptation to slightly different conditions, would soon form distinct races. But as soon as the area is divided into two portions the intercrossing is stopped, and the usual result is that two closely allied races, cla.s.sed as representative species, become formed. Such pairs of allied species on the two sides of a continent, or in two detached areas, are very numerous; and their existence is only explicable on the supposition that they are descendants of a parent form which once occupied an area comprising that of both of them,--that this area then became discontinuous,--and, lastly, that, as a consequence of the discontinuity, the two sections of the parent species became segregated into distinct races or new species. {410}
Now, when the division of the area leaves one portion of the species in an island, a similar modification of the species, either in the island or in the continent, occurs, resulting in closely-allied but distinct forms; and such forms are, as we have seen, highly characteristic of island-faunas.
But islands also favour the occasional preservation of the unchanged species--a phenomenon which very rarely occurs in continents. This is probably due to the absence of compet.i.tion in islands, so that the parent species there maintains itself unchanged, while the continental portion, by the force of that compet.i.tion, is driven back to some remote mountain area, where it also obtains a comparative freedom from compet.i.tion. Thus may be explained the curious fact, that the species common to Formosa and India are generally confined to limited areas in the Himalayas, or in other cases are found only in remote islands, as j.a.pan or Hainan.
The distribution and affinities of the animals of continental islands thus throws much light on that obscure subject--the decay and extinction of species; while the numerous and delicate gradations in the modification of the continental species, from perfect ident.i.ty, through slight varieties, local forms, and insular races, to well-defined species and even distinct genera, afford an overwhelming ma.s.s of evidence in favour of the theory of "descent with modification."
We shall now pa.s.s on to another cla.s.s of islands, which, though originally forming parts of continents, were separated from them at very remote epochs. This antiquity is clearly manifested in their existing faunas, which present many peculiarities, and offer some most curious problems to the student of distribution.
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CHAPTER XIX
ANCIENT CONTINENTAL ISLANDS: THE MADAGASCAR GROUP
Remarks on Ancient Continental Islands--Physical Features of Madagascar--Biological Features of Madagascar--Mammalia--Reptiles--Relation of Madagascar to Africa--Early History of Africa and Madagascar--Anomalies of Distribution and How to Explain Them--The Birds of Madagascar as Indicating a Supposed Lemurian Continent--Submerged Islands between Madagascar and India--Concluding Remarks on "Lemuria"--The Mascarene Islands--The Comoro Islands--The Seych.e.l.les Archipelago--Birds of the Seych.e.l.les--Reptiles and Amphibia--Freshwater Fishes--Land Sh.e.l.ls--Mauritius, Bourbon, and Rodriguez--Birds--Extinct Birds and their Probable Origin--Reptiles--Flora of Madagascar and the Mascarene Islands--Curious Relations of Mascarene Plants--Endemic Genera of Mauritius and Seych.e.l.les--Fragmentary Character of the Mascarene Flora--Flora of Madagascar Allied to that of South Africa--Preponderance of Ferns in the Mascarene Flora--Concluding Remarks on the Madagascar Group.
We have now to consider the phenomena presented by a very distinct cla.s.s of islands--those which, although once forming part of a continent, have been separated from it at a remote epoch when its animal forms were very unlike what they are now. Such islands preserve to us the record of a by-gone world,--of a period when many of the higher types had not yet come into existence and when the distribution of others was very different from what prevails at the present day. The problem presented by these ancient islands is often complicated by the changes they themselves have undergone since the period of their separation. A partial subsidence will have led to the {412} extinction of some of the types that were originally preserved, and may leave the ancient fauna in a very fragmentary state; while subsequent elevations may have brought it so near to the continent that some immigration even of mammalia may have taken place. If these elevations and subsidences occurred several times over, though never to such an extent as again to unite the island with the continent, it is evident that a very complex result might be produced; for besides the relics of the ancient fauna, we might have successive immigrations from surrounding lands reaching down to the era of existing species. Bearing in mind these possible changes, we shall generally be able to arrive at a fair conjectural solution of the phenomena of distribution presented by these ancient islands.
Undoubtedly the most interesting of such islands, and that which exhibits their chief peculiarities in the greatest perfection, is Madagascar, and we shall therefore enter somewhat fully into its biological and physical history.
_Physical Features of Madagascar._--This great island is situated about 250 miles from the east coast of Africa, and extends from 12 to 25 S. Lat.
It is almost exactly 1,000 miles long, with an extreme width of 360 and an average width of more than 260 miles. A lofty granitic plateau, from eighty to 160 miles wide and from 3,000 to 5,000 feet high, occupies its central portion, on which rise peaks and domes of basalt and granite to a height of nearly 9,000 feet; and there are also numerous extinct volcanic cones and craters. All round the island, but especially developed on the south and west, are plains of a few hundred feet elevation, formed of rocks which are shown by their fossils to be of Jura.s.sic age, or at all events to belong to somewhere near the middle portion of the Secondary period. The higher granitic plateau consists of bare undulating moors, while the lower Secondary plains are more or less wooded; and there is here also a continuous belt of dense forest, varying from six or eight to fifty miles wide, encircling the whole island, usually at about thirty miles distance from the coast but in the north-east coming down to the sea-sh.o.r.e. {413}