Chapter XXIV: Summary and Conclusion (3)

[57] In the _Geological Magazine_, April, 1880, Mr. Searles V. Wood adduces what he considers to be the "conclusive objection" to Dr. Croll's excentricity theory, which is, that during the last glacial epoch Europe and North America were glaciated very much in proportion to their respective climates now, which are generally admitted to be due to the distribution of oceanic currents. But Dr. Croll admits his theory "to be baseless unless there was a complete diversion of the warm ocean currents from the hemisphere glaciated," in which case there ought to be no difference in the extent of glaciation in Europe and North America. Whether or not this is a correct statement of Dr. Croll's theory, the above objection certainly does not apply to the views here advocated; but as I also hold the "excentricity theory" in a modified form, it may be as well to show why it does not apply. In the first place I do not believe that the Gulf Stream was "completely diverted" during the glacial epoch, but that it was diminished in force, and (as described at p. 144) _partly_ diverted southward. A portion of its influence would, however, still remain to cause a difference between the climates of the two sides of the Atlantic; and to this must be added two other causes--the far greater penetration of warm sea-water into the European than into the North American continent, and the proximity to America of the enormous ice-producing mass of Greenland. We have thus three distinct causes, all combining to produce a more severe winter climate on the west than on the east of the Atlantic during the glacial epoch, and though the first of these--the Gulf Stream--was not nearly so powerful as it is now, neither is the difference indicated by the ice-extension in the two countries so great as the present difference of winter-temperature, which is the essential point to be considered. The ice-sheet of the United States is usually supposed to have extended about ten, or, at most, twelve, degrees further south than it did in Western Europe, whereas we must go twenty degrees further south in the former country to obtain the same mean winter-temperature we find in the latter, as may be seen by examining any map of winter isothermals. This difference very fairly corresponds to the difference of conditions existing during the glacial epoch and the present time, so far as we are able to estimate them, and it certainly affords no grounds of objection to the theory by which the glaciation is here explained.

[58] Dr. Croll objects to this argument, and adduces the case of Greenland as showing that ice may accumulate far from sea. But the width of Greenland is small compared with that of the supposed Antarctic ice-cap. (_Climate and Cosmology_, p. 78.)

[59] The recent extensive glaciation of New Zealand is generally imputed by the local geologists to a greater elevation of the land; but I cannot help believing that the high phase of excentricity which caused our own glacial epoch was at all events an assisting cause. This is rendered more probable if taken in connection with the following very definite statement of glacial markings in South Africa. Captain Aylward in his _Transvaal of To-day_ (p. 171) says:--"It will be interesting to geologists and others to learn that the entire country, from the summits of the Quathlamba to the junction of the Vaal and Orange rivers, shows marks of having been swept over, and that at no very distant period, by vast masses of ice from east to west. The striations are plainly visible, scarring the older rocks, and marking the hill-sides--getting lower and lower and less visible as, descending from the mountains, the kopjies (small hills) stand wider apart; but wherever the hills narrow towards each other, again showing how the vast ice-fields were checked, thrown up, and raised against their Eastern extremities."

This passage is evidently written by a person familiar with the phenomena of glaciation, and as Captain Aylward's preface is dated from Edinburgh, he has probably seen similar markings in Scotland. The country described consists of the most extensive and lofty plateau in South Africa, rising to a mountain knot with peaks more than 10,000 feet high, thus offering an appropriate area for the condensation of vapour and the accumulation of snow. At present, however, the mountains do not reach the snow-line, and there is no proof that they have been much higher in recent times, since the coast of Natal is now said to be rising. It is evident that no slight elevation would now lead to the accumulation of snow and ice in these mountains, situated as they are between 27deg and 30deg S. Lat.; since the Andes, which in 32deg S. Lat. reach 23,300 feet high, and in 28deg S. Lat. 20,000, with far more extensive plateaus, produce no ice-fields. We cannot, therefore, believe that a few thousand feet of additional elevation, even if it occurred so recently as indicated by the presence of striations, would have produced the remarkable amount of glaciation above described; while from the analogy of the northern hemisphere, we may well believe that it was mainly due to the same high excentricity that led to the glaciation of Western and Central Europe, and Eastern North America.

These observations confirm those of Mr. G. W. Stow, who, in a paper published in the _Quarterly Journal of the Geological Society_ (Vol. XXVII. p. 539), describes similar phenomena in the same mountains, and also mounds and ridges of unstratified clay packed with angular boulders; while further south the Stormberg mountains are said to be similarly glaciated, with immense accumulations of morainic matter in all the valleys. We have here most of the surface phenomena characteristic of a glaciated country, only a few degrees south of the tropic; and taken in connection with the indications of recent glaciation in New Zealand, and those discovered by Dr. R. von Lendenfeld in the Australian Alps between 6,000 and 7,000 feet elevation (_Nature_, Vol. XXXII. p. 69), we can hardly doubt the occurrence of some general and wide-spread cause of glaciation in the southern hemisphere at a period so recent that the superficial phenomena are almost as well preserved as in Europe. Other geologists however deny that there are any distinct indications of glacial action in South Africa; but the recent discovery by Dr. J. W. Gregory, F.G.S., of the former extension of glaciers on Mount Kenya 5,000 feet below their present limits, renders probable the former glaciation of the South African Highlands.

[60] The astronomical facts connected with the motions and appearance of the planet are taken from a paper by Mr. Edward Carpenter, M.A., in the _Geological Magazine_ of March, 1877, entitled, "Evidence Afforded by Mars on the Subject of Glacial Periods," but I arrive at somewhat different conclusions from those of the writer of the paper.

[61] In an article in _Nature_ of Jan. 1, 1880, the Rev. T. W. Webb states that in 1877 the pole of Mars (? the south pole) was, according to Schiaparelli, entirely free of snow. He remarks also on the regular contour of the supposed snows of Mars as offering a great contrast to ours, and also the strongly marked dark border which has often been observed. On the whole Mr. Webb seems to be of opinion that there can be no really close resemblance between the physical condition of the Earth and Mars, and that any arguments founded on such supposed similarity are therefore untrustworthy.

[62] _London, Edinburgh and Dublin Philosophical Magazine_, Vol. XXXVI., pp. 144-150 (1868).

[63] _Climate and Time in their Geological Relations_, p. 341.

[64] _Nature_, Vol. XXI., p. 345, "The Interior of Greenland."

[65] Prof. J. W. Judd says: "In the case of the Alps I know of no glacial phenomena which are not capable of being explained, like those of New Zealand, by a great extension of the area of the tracts above the snow-line which would collect more ample supplies for the glaciers protruded into surrounding plains. And when we survey the grand panoramas of ridges, pinnacles, and peaks produced for the most part by sub-aerial action, we may well be prepared to admit that before the intervening ravines and valleys were excavated, the glaciers shed from the elevated plateaux must have been of vastly greater magnitude than at present." (Contributions to the Study of Volcanoes, _Geological Magazine_, 1876, p. 536.) Professor Judd applies these remarks to the last as well as to previous glacial periods in the Alps; but surely there has been no such extensive alteration and lowering of the surface of the country since the erratic blocks were deposited on the Jura and the great moraines formed in North Italy, as this theory would imply. We can hardly suppose wide areas to have been lowered thousands of feet by denudation, and yet have left other adjacent areas apparently untouched; and it is even very doubtful whether such an extension of the snow-fields would alone suffice for the effects which were certainly produced.

[66] _Geological Magazine_, 1876, p. 392.

[67] Colonel Fielden thinks that these trees have all been brought down by rivers, and have been stranded on shores which have been recently elevated. See _Trans. of Norfolk Nat. Hist. Soc., Vol. III._, 1880.

[68] _Geological Magazine_, 1876, "Geology of Spitzbergen," p. 267.

[69] The preceding account is mostly derived from Professor Heer's great work _Flora Fossilis Arctica_.

[70] _Geological Magazine_, 1875, p. 531.

[71] _Geological Magazine_, 1876, p. 266. In his recent work--_Climate and Cosmology_ (pp. 164, 172)--the late Dr. Croll has appealed to the imperfection of the geological record as a reply to these arguments; in this case, as it appears to me, a very unsuccessful one.

[72] It is interesting to observe that the Cretaceous flora of the United States (that of the Dakota group), indicates a somewhat cooler climate than that of the following Eocene period. Mr. De Rance (in the geological appendix to Capt. Sir G. Nares's _Narrative of a Voyage to the Polar Sea_) remarks as follows: "In the overlying American Eocenes occur types of plants occurring in the European Miocenes and still living, proving the truth of Professor Lesquereux's postulate, that the plant types appear in America a stage in advance of their advent in Europe. These plants point to a far higher mean temperature than those of the Dakota group, to a dense atmosphere of vapour, and a luxuriance of ferns and palms." This is very important as adding further proof to the view that the climates of former periods are not due to any general refrigeration, but to causes which were subject to change and alternation in former ages as now.

[73] Mr. S. B. J. Skertchley informs me that he has himself observed thick Tertiary deposits, consisting of clays and anhydrous gypsum, at Berenice on the borders of Egypt and Nubia, at a height of about 600 feet above the sea-level; but these may have been of fresh-water origin.

[74] By referring to our map of the Indian Ocean showing the submarine banks indicating ancient islands (Chap. XIX.), it will be evident that the south-east trade-winds--then exceptionally powerful--would cause a vast body of water to enter the deep Arabian Sea.

[75] In his recently published _Lectures on Physical Geography_, Professor Haughton calculates, that more than half the solar heat of the torrid zone is carried to the temperate zones by ocean currents. The Gulf Stream itself carries one-twelfth of the total amount, but it is probable that a very small fraction of this quantity of heat reaches the polar seas owing to the wide area over which the current spreads in the North Atlantic. The corresponding stream of the Indian Ocean in Miocene times would have been fully equal to the Gulf Stream in heating power, while, owing to its being so much more concentrated, a large proportion of its heat may have reached the polar area. But the Arctic Ocean occupies less than one-tenth of the area of the tropical seas; so that, whatever proportion of the heat of the tropical zone was conveyed to it, would, by being concentrated into one-tenth of the surface, produce an enormously increased effect. Taking this into consideration, we can hardly doubt that the opening of a sufficient passage from the Indian Ocean to the Arctic seas would produce the effects above indicated.

[76] For an account of the resemblances and differences of the mammalia of the two continents during the Tertiary epoch, see my _Geographical Distribution of Animals_, Vol. I. pp. 140-156.

[77] Professor Haughton has made an elaborate calculation of the difference between existing climates and those of Miocene times, for all the places where a Miocene flora has been discovered, by means of the actual range of corresponding species and genera of plants. Although this method is open to the objection that the ranges of plants and animals are not determined by temperature only, yet the results may be approximately correct, and are very interesting. The following table which summarizes these results is taken from his Lectures on Physical Geography (p. 344):--

_______________________________________________________________________
| | | Present | Miocene | |
| |Latitude.|Temperature.|Temperature.|Difference.|
|_____________________|_________|____________|____________|___________|
| 1. Switzerland | 47d.00 | 53d.6 F | 69d.8 F | 16d.2 F |
| 2. Dantzig | 54d.21 | 45d.7 ,, | 62d.6 ,, | 16d.9 ,, |
| 3. Iceland | 65d.30 | 35d.6 ,, | 48d.2 ,, | 12d.6 ,, |
| 4. Mackenzie River | 65d.00 | 19d.4 ,, | 48d.2 ,, | 28d.8 ,, |
| 5. Disco (Greenland)| 70d.00 | 19d.6 ,, | 55d.6 ,, | 36d.0 ,, |
| 6. Spitzbergen | 78d.00 | 16d.5 ,, | 51d.8 ,, | 35d.3 ,, |
| 7. Grinnell Land | 81d.44 | 1d.7 ,, | 42d.3 ,, | 44d.0 ,, |
|_____________________|_________|____________|____________|___________|

It is interesting to note that Iceland, which is now exposed to the full influence of the Gulf Stream, was only 12deg.6 F. warmer in Miocene times, while Mackenzie River, now totally removed from its influence was 28deg warmer. This, as well as, the greater increase of temperature as we go northward and the polar area becomes more limited, is quite in accordance with the view of the causes which brought about the Miocene climate which is here advocated.

[78] The objection has been made, that the long polar night would of itself be fatal to the existence of such a luxuriant vegetation as we know to have existed as far as 80deg N. Lat., and that there must have been some alteration of the position of the pole, or diminution of the obliquity of the ecliptic, to permit such plants as magnolias and large-leaved maples to flourish. But there appears to be really no valid grounds for such an objection. Not only are numbers of Alpine and Arctic evergreens deeply buried in the snow for many months without injury, but a variety of tropical and sub-tropical plants are preserved in the hot-houses of St. Petersburg and other northern cities, which are closely matted during winter, and are thus exposed to as much darkness as the night of the Arctic regions. We have besides no proof that any of the Arctic trees or large shrubs were evergreens, and the darkness would certainly not be prejudical to deciduous plants. With a suitable temperature there is nothing to prevent a luxuriant vegetation up to the pole, and the long continued day is known to be highly favourable to the development of foliage, which in the same species is larger and better developed in Norway than in the south of England.

[79] _Geological Magazine_, 1873, p. 320.

[80] _Geological Magazine_, 1877, p. 137.

[81] _Manual of Geology_, 2nd Ed. p. 525. See also letter in _Nature_, Vol. XXIII. p. 410.

[82] _Nature_, Vol. XVIII. (July, 1878), p. 268.

[83] "On the Comparative Value of certain Geological Ages considered as items of Geological Time." (_Proceedings of the Royal Society_, 1874, p. 334.)

[84] _Trans. Royal Society of Edinburgh_, Vol. XXIII. p. 161. _Quarterly Journal of Science_, 1877. (Croll on the "Probable Origin and Age of the Sun.")

[85] _Philosophical Magazine_, April, 1853.

[86] It has usually been the practice to take the amount of denudation in the Mississippi valley, or one foot in six thousand years, as a measure of the rate of denudation in Europe, from an idea apparently of being on the "safe side," and of not over-estimating the rate of change. But this appears to me a most unphilosophical mode of proceeding and unworthy of scientific inquiry. What should we think of astronomers if they always took the lowest estimates of planetary or stellar distances, instead of the mean results of observation, "in order to be on the safe side!"? As if error in one direction were any worse than error in another. Yet this is what geologists do systematically. Whenever any calculations are made involving the antiquity of man, it is those that give the _lowest_ results that are always taken, for no reason apparently except that there was, for so long a time, a prejudice, both popular and scientific, against the great antiquity of man; and now that a means has been found of measuring the rate of denudation, they take the slowest rate instead of the mean rate, apparently only because there is now a scientific prejudice in favour of extremely slow geological change. I take the mean of the whole; and as this is almost exactly the same as the mean of the three great European rivers--the Rhone, Danube, and Po--I cannot believe that this will not be nearer the truth for Europe than taking one North American river as the standard.

[87] "On the Height of the Land and the Depth of the Ocean," in the _Scottish Geographical Magazine_, 1888.

[88] These figures are merely used to give an idea of the rate at which denudation is actually going on now; but if no elevatory forces were at work, the rate of denudation would certainly diminish as the mountains were lowered and the slope of the ground everywhere rendered flatter. This would follow not only from the diminished power of rain and rivers, but because the climate would become more uniform, the rainfall probably less, and no rocky peaks would be left to be fractured and broken up by the action of frosts. It is certain, however, that no continent has ever remained long subject to the influences of denudation alone, for, as we have seen in our sixth chapter, elevation and depression have always been going on in one part or other of the surface.

[89] The following statement of the depths at which the Palaeozoic formations have been reached in various localities in and round London was given by Mr. H. B. Woodward in his address to the Norwich Geological Society in 1879:--

_Deep Wells through the Tertiary and Cretaceous Formations._

Harwich at 1,022 feet reached Carboniferous Rock.
Kentish Town ,, 1,114 ,, ,, Old Red Sandstone.
Tottenham Court Road ,, 1,064 ,, ,, Devonian.
Blackwall ,, 1,004 ,, ,, Devonian or Old Red Sandstone.
Ware ,, 800 ,, ,, Silurian (Wenlock Shale).

We thus find that over a wide area, extending from London to Ware and Harwich, the whole of the formations from the Oolite to the Permian are wanting, the Cretaceous resting on the Carboniferous or older Palaeozoic rocks; and the same deficiency extends across to Belgium, where the Tertiary beds are found resting on Carboniferous at a depth of less than 400 feet.

[90] _Geological Magazine_, Vol. VIII., March, 1871.

[91] Mr. C. Lloyd Morgan has well illustrated this point by comparing the generally tilted-up strata denuded on their edges, to a library in which a fire had acted on the exposed edges of the books, destroying a great mass of literature but leaving a portion of each book in its place, which portion represents the thickness but not the size of the book. (_Geological Magazine_, 1878, p. 161.)

[92] Professor J. Young thinks it highly probable that--"the Lower Greensand is contemporaneous with part of the Chalk, so were parts of the Wealden; nay, even of the Purbeck a portion must have been forming while the Cretaceous sea was gradually deepening southward and westward." Yet these deposits are always arranged successively, and their several thicknesses added together to obtain the total thickness of the formations of the country. (See Presidential Address, Sect. C. British Association, 1876.)

[93] Mr. John Murray in his more careful estimate makes it about 51-1/2 millions.

[94] As by far the larger portion of the denuded matter of the globe passes to the sea through comparatively few great rivers, the deposits must often be confined to very limited areas. Thus the denudation of the vast Mississippi basin must be almost all deposited in a limited portion of the Gulf of Mexico, that of the Nile within a small area of the Eastern Mediterranean, and that of the great rivers of China--the Hoang Ho and Yang-tse-kiang, in a small portion of the Eastern Sea. Enormous lengths of coast, like those of Western America and Eastern Africa, receive very scanty deposits; so that thirty miles in width along the whole of the coasts of the globe will probably give an area greater than that of the area of _average_ deposit, and certainly greater than that of _maximum_ deposit, which is the basis on which I have here made my estimates. In the case of the Mississippi, it is stated by Count Pourtales that along the plateau between the mouth of the river and the southern extremity of Florida for two hundred and fifty miles in width the bottom consists of clay with some sand and but few Rhizopods; but beyond this distance the soundings brought up either Rhizopod shells alone, or these mixed with coral sand, Nullipores, and other calcareous organisms (Dana's _Manual of Geology_, 2nd Ed. p. 671). It is probable, therefore, that a large proportion of the entire mass of sediment brought down by the Mississippi is deposited on the limited area above indicated.

Professor Dana further remarks: "Over interior oceanic basins as well as off a coast in quiet depths, fifteen or twenty fathoms and beyond, the deposits are mostly of fine silt, fitted for making fine argillaceous rocks, as shales or slates. When, however, the depth of the ocean falls off below a hundred fathoms, the deposition of silt in our existing oceans mostly ceases, unless in the case of a great bank along the border of a continent."

[95] From the same data Professor Haughton estimates a minimum of 200 million years for the duration of geological time; but he arrives at this conclusion by supposing the products of denudation to be uniformly spread over the _whole sea-bottom_ instead of over a narrow belt near the coasts, a supposition entirely opposed to all the known facts, and which had been shown by Dr. Croll, five years previously, to be altogether erroneous. (See _Nature_, Vol. XVIII., p. 268, where Professor Haughton's paper is given as read before the Royal Society.)

[96] See _Geological Magazine_ for 1877, p. 1.

[97] In his reply to Sir W. Thomson, Professor Huxley _assumed_ one foot in a thousand years as a not improbable rate of deposition. The above estimate indicates a far higher rate; and this follows from the well-ascertained fact, that the area of deposition is many times smaller than the area of denudation.

[98] Dr. Croll and Sir Archibald Geikie have shown that marine denudation is very small in amount as compared with sub-aerial, since it acts only locally on the _edge_ of the land, whereas the latter acts over every foot of the _surface_. Mr. W. T. Blanford argues that the difference is still greater in tropical than in temperate latitudes, and arrives at the conclusion that--"If over British India the effects of marine to those of fresh-water denudation in removing the rocks of the country be estimated at 1 to 100, I believe that the result of marine action will be greatly overstated" (_Geology and Zoology of Abyssinia_, p. 158, note). Now, as our estimate of the rate of sub-aerial denudation cannot pretend to any precise accuracy, we are justified in neglecting marine denudation altogether, especially as we have no method of estimating it for the whole earth with any approach to correctness.

[99] Agassiz appears to have been the first to suggest that the principal epochs of life extermination were epochs of cold; and Dana thinks that two at least such epochs may be recognised, at the close of the Palaeozoic and of the Cretaceous periods--to which we may add the last glacial epoch.

[100] This view was, I believe, first put forth by myself in a paper read before the Geological Section of the British Association in 1869, and subsequently in an article in _Nature_, Vol. I. p. 454. It was also stated by Mr. S. B. J. Skertchley in his _Physical System of the Universe_, p. 363 (1878); but we both founded it on what I now consider the erroneous doctrine that actual glacial epochs recurred each 10,500 years during periods of high excentricity.

[101] Explication d'une seconde edition de la _Carte Geologique de la Terre_ (1875), p. 64.

[102] For most of the facts as to the zoology and botany of these islands, I am indebted to Mr. Godman's valuable work--_Natural History of the Azores or Western Islands_, by Frederick Du Cane Godman, F.L.S., F.Z.S., &c., London, 1870.

[103] See Chap. V. p. 78.

[104] Some of Mr. Darwin's experiments are very interesting and suggestive. Ripe hazel-nuts sank immediately, but when dried they floated for ninety days, and afterwards germinated. An asparagus-plant with ripe berries, when dried, floated for eighty-five days, and the seeds afterwards germinated. Out of ninety-four dried plants experimented with, eighteen floated for more than a month, and some for three months, and their powers of germination seem never to have been wholly destroyed. Now, as oceanic currents vary from thirty to sixty miles a day, such plants under the most favourable conditions might be carried 90 X 60 = 5,400 miles! But even half of this is ample to enable them to reach any oceanic island, and we must remember that till completely water-logged they might be driven along at a much greater rate by the wind. Mr. Darwin calculates the distance by the average time of flotation to be 924 miles; but in such a case as this we are entitled to take the extreme cases, because such countless thousands of plants and seeds must be carried out to sea annually that the extreme cases in a single experiment with only ninety-four plants, must happen hundreds or thousands of times and with hundreds or thousands of species, naturally, and thus afford ample opportunities for successful migration. (See _Origin of Species_, 6th Edition, p. 325.)

[105] The following remarks, kindly communicated to me by Mr. H. N. Moseley, naturalist to the _Challenger_, throw much light on the agency of birds in the distribution of plants:--"Grisebach (_Veg. der Erde_, Vol. II. p. 496) lays much stress on the wide ranging of the albatross (Diomedea) across the equator from Cape Horn to the Kurile Islands, and thinks that the presence of the same plants in Arctic and Antarctic regions may be accounted for, possibly, by this fact. I was much struck at Marion Island of the Prince Edward group, by observing that the great albatross breeds in the midst of a dense, low herbage, and constructs its nest of a mound of turf and herbage. Some of the indigenous plants, _e.g._ Acaena, have flower-heads which stick like burrs to feathers, &c., and seem specially adapted for transposition by birds. Besides the albatrosses, various species of Procellaria and Puffinus, birds which range over immense distances may, I think, have played a great part in the distribution of plants, and especially account, in some measure, for the otherwise difficult fact (when occurring in the tropics), that widely distant islands have similar mountain plants. The Procellaria and Puffinus in nesting, burrow in the ground, as far as I have seen choosing often places where the vegetation is the thickest. The birds in burrowing get their feathers covered with vegetable mould, which must include spores, and often seeds. In high latitudes the birds often burrow near the sea-level, as at Tristan d'Acunha or Kerguelen's Land, but in the tropics they choose the mountains for their nesting-place (Finsch and Hartlaub, _Orn. der Viti- und Tonga-Inseln_, 1867, Einleitung, p. xviii.). Thus, _Puffinus megasi_ nests at the top of the Korobasa basaga mountain, Viti Levu, fifty miles from the sea. A Procellaria breeds in like manner in the high mountains of Jamaica, I believe at 7,000 feet. Peale describes the same habit of _Procellaria rostrata_ at Tahiti, and I saw the burrows myself amidst a dense growth of fern, &c., at 4,400 feet elevation in that island. Phaethon has a similar habit. It nests at the crater of Kilauea, Hawaii, at 4,000 feet elevation, and also high up in Tahiti. In order to account for the transportation of the plants, it is not of course necessary that the same species of Procellaria or Diomedea should now range between the distant points where the plants occur. The ancestor of the now differing species might have carried the seeds. The range of the genus is sufficient."

[106] _Nature_, Vol. VI. p. 262, "Recent Observations in the Bermudas," by Mr. J. Matthew Jones.

[107] "The late Sir C. Wyville Thomson was of opinion that the 'red earth' which largely forms the soil of Bermuda had an organic origin, as well as the 'red clay' which the _Challenger_ discovered in all the greater depths of the ocean basins. He regarded the red earth and red clay as an ash left behind after the gradual removal of the lime by water charged with carbonic acid. This ash he regarded as a constituent part of the shells of Foraminifera, skeletons of Corals, and Molluscs, [_vide_ _Voyage of the Challenger_, Atlantic, Vol. I. p. 316]. This theory does not seem to be in any way tenable. Analysis of carefully selected shells of Foraminifera, Heteropods, and Pteropods, did not show the slightest trace of alumina, and none has as yet been discovered in coral skeletons. It is most probable that a large part of the clayey matter found in red clay and the red earth of Bermuda is derived from the disintegration of pumice, which is continually found floating on the surface of the sea. [See Murray, "On the Distribution of Volcanic Debris Over the Floor of the Ocean;" _Proc. Roy. Soc. Edin._ Vol. IX. pp. 247-261. 1876-1877.] The naturalists of the _Challenger_ found it among the floating masses of gulf weed, and it is frequently picked up on the reefs of Bermuda and other coral islands. The red earth contains a good many fragments of magnetite, augite, felspar, and glassy fragments, and when a large quantity of the rock of Bermuda is dissolved away with acid, a small number of fragments are also met with. These mineral particles most probably came originally from the pumice which had been cast up on the island for long ages (for it is known that these minerals are present in pumice), although possibly some of them may have come from the volcanic rock, which is believed to form the nucleus of the island." _The Voyage of H.M.S. Challenger_, Narrative of the Cruise, Vol. I. 1885, pp. 141-142.

[108] Four bats occur rarely, two being N. American, and two West Indian Species. _The Bermuda Islands_, by Angelo Heilprin, Philadelphia, 1889.

[109] Fourteen species of Spiders were collected by Prof. A. Heilprin, all American or cosmopolitan species except one, _Lycosa atlantica_, which Dr. Marx of Washington describes as new and as peculiar to the islands. (Heilprin's _The Bermudas_, p. 93.)

[110] Mr. Theo. D. A. Cockerell informs me that there are two slugs in Bermuda of which specimens exist in the British Museum,--_Amalia gagates_ Drap. common in Europe, and _Agriolimax campestris_ of the United States. Both may therefore have been introduced by human agency. Also _Vaginulus Morelete var. schivelyae_ which seems to be a variety of a Mexican species; perhaps imported.

[111] "Notes on the Vegetation of Bermuda," by H. N. Moseley. (_Journal of the Linnean Society_, Vol. XIV., _Botany_, p. 317.)

[112] _Gigantic Land Tortoises Living and Extinct in the Collection of the British Museum._ By A. C. L. G. Guenther, F.R.S. 1877.

[113] The following list of the beetles yet known from the Galapagos shows their scanty proportions and accidental character; the forty species belonging to thirty-three genera and eighteen families. It is taken from Mr. Waterhouse's enumeration in the _Proceedings of the Zoological Society_ for 1877 (p. 81), with a few additions collected by the U. S. Fish Commission Steamer _Albatross_, and published by the U. S. National Museum in 1889.

CARABIDAE. ELATERIDAE.
Feronia calathoides. Physorhinus galapagoensis
,, insularis. HETEROMERA.
,, galapagoensis. Allecula n. s.
Amblygnathus obscuricornis. Stomion helopoides.
Solenophorus galapagoensis. ,, laevigatum.
Notaphus galapagoensis. Ammophorus obscurus.
DYTISCIDAE. ,, cooksoni.
Eunectes occidentalis. ,, bifoveatus.
Acilius incisus. Pedonoeces galapagoensis.
Copelatus galapagoensis. ,, pubescens.
PALPICORNES. Phaleria manicata.
Tropisternus lateralis. CURCULIONIDAE.
Philhydrus sp. Otiorhynchus cuneiformis.
STAPHYLINIDAE. Anchonus galapagoensis.
Creophilus villosus. LONGICORNIA.
NECROPHAGA. Mallodou sp.
Acribis serrativentris. Eburia amabilis.
Phalacrus darwinii. ANTHRIBIDAE.
Dermestes vulpinus. Ormiscus variegatus.
MALACODERMS. PHYTOPHAGA
Ablechrus darwinii. Diabrotica limbata.
Corynetes rufipes. Docema galapagoensis.
Bostrichus unciniatus. Longitarsus lunatus.
Tetrapriocerca sp. SECURIPALPES.
LAMELLICORNES. Scymuns galapagoensis.
Copris lugubris.
Oryctes galapagoensis.

[114] Mr. H. O. Forbes, who visited these islands in 1878, increased the number of wild plants to thirty-six, and these belonged to twenty-six natural orders.

[115] Juan Fernandez is a good example of a small island which, with time and favourable conditions, has acquired a tolerably rich and highly peculiar flora and fauna. It is situated in 34deg S. Lat., 400 miles from the coast of Chile, and so far as facilities for the transport of living organisms are concerned is by no means in a favourable position, for the ocean-currents come from the south-west in a direction where there is no land but the Antarctic continent, and the prevalent winds are also westerly. No doubt, however, there are occasional storms, and there may have been intermediate islands, but its chief advantages are its antiquity, its varied surface, and its favourable soil and climate, offering many chances for the preservation and increase of whatever plants and animals have chanced to reach it. The island consists of basalt, greenstone, and other ancient rocks, and though only about twelve miles long its mountains are three thousand feet high. Enjoying a moist and temperate climate it is especially adapted to the growth of ferns, which are very abundant; and as the spores of these plants are as fine as dust, and very easily carried for enormous distances by winds, it is not surprising that there are nearly fifty species on the island, while the remote period when it first received its vegetation may be indicated by the fact that nearly half the species are quite peculiar; while of 102 species of flowering plants seventy are peculiar, and there are ten peculiar genera. The same general character pervades the fauna. For so small an island it is rich, containing four true land-birds, about fifty species of insects, and twenty of land-shells. Almost all these belong to South American genera, and a large proportion are South American species; but several of the insects, half the birds, and the whole of the land-shells are peculiar. This seems to indicate that the means of transmission were formerly greater than they are now, and that in the case of land-shells none have been introduced for so long a period that all have become modified into distinct forms, or have been preserved on the island while they have become extinct on the continent. For a detailed examination of the causes which have led to the modification of the humming birds of Juan Fernandez see the chapter on Humming Birds in the author's _Natural Selection and Tropical Nature_, p. 324; while a general account of the fauna of the island is given in his _Geographical Distribution of Animals_, Vol. II. p. 49.

[116] No additions appear to have been made to this flora down to 1885, when Mr. Hemsley published his _Report on the Present State of our Knowledge of Insular Floras_.

[117] _Journal of the Linnean Society_, Vol. XIII., "Botany," p. 556.

[118] _Geographical Distribution of Animals_, Vol. II. p. 81.

[119] _St. Helena: a Physical, Historical, and Topographical Description of the Island, &c._ By John Charles Melliss, F.G.S., &c. London: 1875.

[120] Mr. Marsh in his interesting work entitled _The Earth as Modified by Human Action_ (p. 51), thus remarks on the effect of browsing quadrupeds in destroying and checking woody vegetation.--"I am convinced that forests would soon cover many parts of the Arabian and African deserts if man and domestic animals, especially the goat and the camel, were banished from them. The hard palate and tongue, and strong teeth and jaws of this latter quadruped enable him to break off and masticate tough and thorny branches as large as the finger. He is particularly fond of the smaller twigs, leaves, and seed-pods of the _Sont_ and other acacias, which, like the American robinia, thrive well on dry and sandy soils, and he spares no tree the branches of which are within his reach, except, if I remember right, the tamarisk that produces manna. Young trees sprout plentifully around the springs and along the winter water-courses of the desert, and these are just the halting stations of the caravans and their routes of travel. In the shade of these trees annual grasses and perennial shrubs shoot up, but are mown down by the hungry cattle of the Bedouin as fast as they grow. A few years of undisturbed vegetation would suffice to cover such points with groves, and these would gradually extend themselves over soils where now scarcely any green thing but the bitter colocynth and the poisonous foxglove is ever seen."

[121] _Coleoptera Sanctae Helenae_, 1877; _Testacea Atlantica_, 1878.

[122] On Petermann's map of Africa, in _Stieler's Hand-Atlas_ (1879), the Island of Ascension is shown as seated on a much larger and shallower submarine bank than St. Helena. The 1,000 fathom line round Ascension encloses an oval space 170 miles long by 70 wide, and even the 300 fathom line, one over 60 miles long; and it is therefore probable that a much larger island once occupied this site. Now Ascension is nearly equidistant between St. Helena and Liberia, and such an island might have served as an intermediate station through which many of the immigrants to St. Helena passed. As the distances are hardly greater than in the case of the Azores, this removes whatever difficulty may have been felt of the possibility of _any_ organisms reaching so remote an island. The present island of Ascension is probably only the summit of a huge volcanic mass, and any remnant of the original fauna and flora it might have preserved may have been destroyed by great volcanic eruptions. Mr. Darwin collected some masses of tufa which were found to be mainly organic, containing, besides remains of fresh-water infusoria, the siliceous tissue of plants! In the light of the great extent of the submarine bank on which the island stands, Mr. Darwin's remark, that--"we may feel sure, that at some former epoch, the climate and productions of Ascension were very different from what they are now,"--has received a striking confirmation. (See _Naturalist's Voyage Round the World_, p. 495.)

[123] "Notes on the Classification, History, and Geographical Distribution of Compositae."--_Journal of the Linnean Society_, Vol. XIII. p. 563 (1873).

[124] The Melhaniae comprise the two finest timber trees of St. Helena, now almost extinct, the redwood and native ebony.

[125] _Journal of the Linnean Society_, 1873, p. 496. "On Diversity of Evolution under one set of External Conditions." _Proceedings of the Zoological Society of London_, 1873, p. 80. "On the Classification of the Achitinellidae."

[126] "Memoirs on the Coleoptera of the Hawaiian Islands." By the Rev. T. Blackburn, B.A., and Dr. D. Sharp. _Scientific Transactions of the Royal Dublin Society._ Vol. III. Series II. 1885.

[127] See Hildebrand's _Flora of the Hawaiian Islands_, Introduction, p. xiv.

[128] _Flora of the Hawaiian Islands_, by W. Hildebrand, M.D., annotated and published after the author's death by W. F. Hildebrand, 1888.

[129] These are obtained from Hildebrand's _Flora_ supplemented by Mr. Bentham's paper in the _Journal of the Linnean Society_.

[130] Among the curious features of the Hawaiian flora is the extraordinary development of what are usually herbaceous plants into shrubs or trees. Three species of Viola are shrubs from three to five feet high. A shrubby Silene is nearly as tall; and an allied endemic genus, Schiedea, has numerous shrubby species. _Geranium arboreum_ is sometimes twelve feet high. The endemic Compositae are mostly shrubs, while several are trees reaching twenty or thirty feet in height. The numerous Lobeliaceae, all endemic, are mostly shrubs or trees, often resembling palms or yuccas in habit, and sometimes twenty-five or thirty feet high. The only native genus of Primulaceae--Lysimachia--consists mainly of shrubs; and even a plantain has a woody stem sometimes six feet high.

[131] _Geological Magazine_, 1870, p. 155.

[132] _Transactions of the Edinburgh Geological Society_, Vol. I. p. 330.

[133] _Quarterly Journal of Geological Society_, 1850, p. 96.

[134] _British Association Report_, Dundee, 1867, p. 431.

[135] The list of names was furnished to me by Dr. Guenther, and I have added the localities from the papers containing the original descriptions, and from Dr. Haughton's _British Freshwater Fishes_.

[136] See "The Virginia Colony of Helix nemoralis," T. D. A. Cockerell, in _The Nautilus_, Vol. III. No. 7, p. 73.

[137] I am indebted to Mr. Mitten for this curious fact.

[138] The following remarks by Dr. Richard Spruce, who has made a special study of mosses and especially of hepaticae, are of interest. "From what precedes, I conclude that no existing agency is capable of transporting the germs of our hepatics of tropical type from the torrid zone to Britain, and I venture to suppose that their existence at Killarney dates from the remote period when the vegetation of the whole northern hemisphere partook of a tropical character. If I am challenged to account for their survival through the last glacial period, I reply that, granting even the existence of a universal ice-cap down to the latitude of 40deg in America and 50deg in Europe, it is not to be assumed that the whole extent, even of land, was _perennially_ entombed 'in thrilling regions of thick-ribbed ice.' Towards the southern margin of the ice the climate was probably very similar to that of Greenland and the northern part of Norway at the present day. The summer sun would have great power, and on the borders of sheltered fjords the frozen snow would disappear completely, if only for a very short period, and I ask only for a month or two, not doubting the capacity of our hepatics to survive in a dormant state under the snow for at least ten months in the year. I have gathered mosses in the Pyrenees where the snow had barely left them on August 2nd; by September 25th they were re-covered with snow, and would not be again uncovered till the following year. The mosses of Killarney might even enjoy a longer summer than this; for the gulf-stream laves both sides of the south-western angle of Ireland, and its tepid waters would exert great melting power on the ice-bound coast, preventing at the same time any formation of ice in the sea itself." This passage is the conclusion of a very interesting discussion on the distribution of hepaticae in a paper on "A New Hepatic from Killarney," in the _Journal of Botany_, vol. 25, (Feb. 1887), pp. 33-82, in which many curious facts are given as to the habits and distribution of these curious and beautiful little plants.

[139] While these pages are passing through the press I am informed by my friend Mr. W. H. Beeby that in the Shetland Isles, where he has been collecting for five summers, he has found several plants new to the British flora, and a few altogether undescribed. Among these latter is a very distinct species of Hieracium (_H. Zetlandicum_), which is quite unknown in Scandinavia, and is almost certainly peculiar to the British Islands. Here we have another proof that entirely new species are still to be discovered in the remoter portions of our country.

[140] In the first edition of this work the numbers were 400 and 340, showing the great increase of our knowledge during the last ten years, chiefly owing to the researches of Mr. A. H. Everett in Sarawak and Mr. John Whitehead in North Borneo and the great mountain Kini Balu.

[141] These are Allocotops, Chlorocharis, Androphilus, and Ptilopyga, among the Timeliidae; Tricophoropsis and Oreoctistes among the Brachypodidae; Chlamydochoera among the Campophagidae.

[142] In a letter from Darwin he says:--"Hooker writes to me, 'Miguel has been telling me that the flora of Sumatra and Borneo are identical, and that of Java quite different.'"

[143] "On the Geology of Sumatra," by M. R. D. M. Verbeck. _Geological Magazine_, 1877.

[144] _Pitta megarhynchus_ (Banca) allied to _P. brachyurus_ (Borneo, Sumatra, Malacca); and _Pitta bangkanus_ (Banca) allied to _P. sordidus_ (Borneo and Sumatra).

[145] The following list of the mammalia of the Philippines and the Sulu Islands has been kindly furnished me by Mr. Everett.

QUADRUMANA.

1. Macacus cynomolgus.
2. Tarsius spectrum.

CARNIVORA.

3. Viverra tangalunga.
4. Paradoxurus philippinensis. Also in Palawan.
5. Felis bengalensis. In Negros Island.

UNGULATA.

6. Bubalus mindorensis. Peculiar species.
7. Cervus philippinus. Peculiar species.
8. " alfredi. Peculiar species.
9. " nigricans. Peculiar species.
10. " pseudaxis. Sulu only. Probably introduced.
11. Sus marchesi. Peculiar species.

RODENTIA.

12. Sciurus philippinensis. Peculiar species.
13. Sciurus cagos. Peculiar species.
14. " concinnus. Peculiar. Mindanao and Basilan.
15. Phlaeomys cummingi. Peculiar genus.
16. Mus ephippium.
17. " everetti. Peculiar species.

INSECTIVORA.

18. Crocidura luzoniensis. Peculiar species.
19. Crocidura edwardsiana. Peculiar species.
20. Dendrogale sp.
21. Galeopithecus philippinensis. Peculiar species.

CHIROPTERA.

22. Pteropus leucopterus.
23. " edulis.
24. " hypomelanus.
25. " jubatus.
26. Xantharpyia amplexicaule.
27. Cynopterus marginatus.
28. " jagorii. Peculiar species.
29. Carponycteris australis.
30. Rhinolophus luctus.
31. " philippinensis. Peculiar species.
32. Rhinolophus rufus. Peculiar species.
33. Hipposideros diadema.
34. " pygmaeus. Peculiar species.
35. Hipposideros larvatus.
36. " obscurus. Peculiar species.
37. Hipposideros coronatus. Peculiar species.
38. Hipposideros bicolor.
39. Megaderma spasma.
40. Vesperugo pachypus.
41. " tenuis.
42. Vesperugo abramus.
43. Nycticejus kuhlii.
44. Vespertilio macrotarsus. Peculiar species.
45. Vespertilio capaccinii.
46. Harpiocephalus cyclotis.
47. Kerivoula hardwickii.
48. Kerivoula pellucida. Peculiar species.
49. " jagorii. Peculiar species.
50. Miniopterus schreibersii.
51. " tristis. Peculiar species.
52. Emballonura monticola.
53. Taphyzous melanopogon.
54. Nyctinomus plicatus.

[146] Extracted from Messrs. Blakiston and Pryer's _Catalogue of Birds of Japan_ (_Ibis_, 1878, p. 209), with Mr. Seebohm's additions and corrections in his _Birds of the Japanese Empire_ 1890. Accidental stragglers are not reckoned as British birds.

[147] Mr. Swinhoe died in October, 1877, at the early age of forty-two. His writings on natural history are chiefly scattered through the volumes of the _Proceedings of the Zoological Society_ and _The Ibis_; the whole being summarised in his _Catalogue of the Mammals of South China and Formosa_ (_P. Z. S._, 1870, p. 615), and his _Catalogue of the Birds of China and its Islands_ (_P. Z. S._, 1871, p. 337).

[148] Captain Blakiston has shown that the northern island--Yezo--is much more temperate and less peculiar in its zoology than the central and southern islands. This is no doubt dependent chiefly on the considerable change of climate that occurs on passing the Tsu-garu strait.

[149] See Dr. J. E. Gray's "Revision of the Viverridae," in _Proc. Zool. Soc._ 1864, p. 507.

[150] Some of the Bats of Madagascar and East Africa are said to have their nearest allies in Australia. (See Dobson in _Nature_, Vol. XXX. p. 575.)

[151] This view was, I believe, first advanced by Professor Huxley in his "Anniversary Address to the Geological Society," in 1870. He says:--"In fact the Miocene mammalian fauna of Europe and the Himalayan regions contain, associated together, the types which are at present separately located in the South African and Indian provinces of Arctogaea. Now there is every reason to believe, on other grounds, that both Hindostan south of the Ganges, and Africa south of the Sahara, were separated by a wide sea from Europe and North Asia during the Middle and Upper Eocene epochs. Hence it becomes highly probable that the well-known similarities, and no less remarkable differences, between the present faunae of India and South Africa have arisen in some such fashion as the following: Some time during the Miocene epoch, the bottom of the nummulitic sea was upheaved and converted into dry land in the direction of a line extending from Abyssinia to the mouth of the Ganges. By this means the Dekkan on the one hand and South Africa on the other, became connected with the Miocene dry land and with one another. The Miocene mammals spread gradually over this intermediate dry land; and if the condition of its eastern and western ends offered as wide contrasts as the valleys of the Ganges and Arabia do now, many forms which made their way into Africa must have been different from those which reached the Dekkan, while others might pass into both these sub-provinces."

This question is fully discussed in my _Geographical Distribution of Animals_ (Vol. I., p. 285), where I expressed views somewhat different from those of Professor Huxley, and made some slight errors which are corrected in the present work. As I did not then refer to Professor Huxley's prior statement of the theory of Miocene immigration into Africa (which I had read but the reference to which I could not recall) I am happy to give his views here.

[152] The total number of Madagascar birds is 238, of which 129 are absolutely peculiar to the island, as are thirty-five of the genera. All the peculiar birds but two are land birds. These are the numbers given in M. Grandidier's great work on Madagascar.

[153] _The Ibis_, 1877, p. 334.

[154] In a paper read before the Geological Society in 1874, Mr. H. F. Blanford, from the similarity of the fossil plants and reptiles, supposed that India and South Africa had been connected by a continent, "and remained so connected with some short intervals from the Permian up to the end of the Miocene period," and Mr. Woodward expressed his satisfaction with "this further evidence derived from the fossil flora of the Mesozoic series of India in corroboration of the former existence of an old submerged continent--Lemuria."

Those who have read the preceding chapters of the present work will not need to have pointed out to them how utterly inconclusive is the fragmentary evidence derived from such remote periods (even if there were no evidence on the other side) as indicating geographical changes. The notion that a similarity in the productions of widely separated continents at any past epoch is only to be explained by the existence of a _direct_ land-connection, is entirely opposed to all that we know of the wide and varying distribution of _all_ types at different periods, as well as to the great powers of dispersal over moderate widths of ocean possessed by all animals except mammalia. It is no less opposed to what is now known of the general permanency of the great continental and oceanic areas; while in this particular case it is totally inconsistent (as has been shown above) with the actual facts of the distribution of animals.

[155] _Geographical Distribution of Animals_, Vol. I., pp. 272-292.

[156] The term "Mascarene" is used here in an extended sense, to include all the islands near Madagascar which resemble it in their animal and vegetable productions.

[157] For the birds of the Comoro Islands see _Proc. Zool. Soc._, 1877, p. 295, and 1879, p. 673.

[158] The following is a list of these peculiar birds. (See the _Ibis_, for 1867, p. 359; and 1879, p. 97.)

PASSERES.

_Ellisia seychelensis._
_Copsychus seychellarum._
_Hypsipetes crassirostris._
_Tchitrea corvina._
_Nectarinia dussumieri._
_Zosterops modesta._
" _semiflava._
_Foudia seychellarum._

PSITTACI.

_Coracopsis barklyi._
_Palaeornis wardi._

COLUMBAE.

_Alectoraenas pulcherrimus._
_Turtur rostratus._

ACCIPITRES.

_Tinnunculus gracilis._

[159] Specimens are recorded from West Africa in the _Proceedings of the Academy of Natural Science_, Philadelphia, 1857, p. 72, while specimens in the Paris Museum were brought by D'Orbigny from S. America. Dr. Wright's specimens from the Seychelles have, as he informs me, been determined to be the same species by Dr. Peters of Berlin.

[160] "Additional Notes on the Land-shells of the Seychelles Islands." By Geoffrey Nevill, C.M.Z.S. _Proc. Zool. Soc._ 1869, p. 61.

[161] In Maillard's _Notes sur l'Isle de Reunion_, a considerable number of mammalia are given as "wild," such as _Lemur mongoz_ and _Centetes setosus_, both Madagascar species, with such undoubtedly introduced animals as a wild cat, a hare, and several rats and mice. He also gives two species of frogs, seven lizards, and two snakes. The latter are both Indian species and certainly imported, as are most probably the frogs. Legouat, who resided some years in the island nearly two centuries ago, and who was a closer observer of nature, mentions numerous birds, large bats, land-tortoises, and lizards, but no other reptiles or venomous animals except scorpions. We may be pretty sure, therefore, that the land-mammalia, snakes, and frogs, now found wild, have all been introduced. Of lizards, on the other hand, there are several species, some peculiar to the island, others common to Africa and the other Mascarene Islands. The following list by Prof. Dumeril is given in Maillard's work:--