account for their being almost entirely destitute of the remains of terrestrial plants.

-

Present unequal distribution of land and sea. Without dwelling longer on the proofs with which geology supplies us of former changes in physical geography, it is not too much to say that every spot which is now dry land has been sea at some former period, and every part of the space now covered by the deepest ocean has been land. The present distribution of land and water encourages us to believe that almost every conceivable transformation in the external form of the earth's crust may have been gone through. In one epoch the land may have been chiefly equatorial, in another for the most part polar and circumpolar. At one period most of it-may have been north of the line, in another south of it; or at one time all in the west, at another the whole of it in the east. In illustration of this point, it may be well to state that there is now just twice as much land in the eastern as there is in the western hemisphere; and even assuming the existence of an antarctic continent, more than twice as much land north of the equator as south of it. But what is most singular, as showing the capricious distribution of the land in the present state of the earth's crust, we find it possible so to divide the globe into two equal parts, that one hemisphere shall contain as much land as water, while the other is so oceanic that the sea is to the land very nearly as 8 to 1.* This is shown by projecting the hemispheres on the plane of the horizon of a point in lat. 52° N. and in long. 6° W. of Greenwich. The point alluded to is situated in St. George's Channel, about midway between Pembroke and Wexford, and the eye of the observer is supposed to be so placed above it as to see from thence one half of the globe. In such a position he would behold at one view the greatest possible quantity of land, or, if transferred to the opposite or antipodal point, the greatest possible quantity of water.

In previous editions I used, in illustration of the same subject, a map projected for me by the late Mr. James Gardner on the horizon of London, for he regarded that metropolis

The exact proportion of land to sea, as calculated by Mr. Saunders, is 1 to

VOL. I.

S

1106 in the Land Hemisphere, and 1 to 7.988 in the Water Hemisphere.

Fig. 11.

Fig. 12.

MAP SHOWING THE PRESENT UNEQUAL DISTRIBUTION OF LAND AND WATER ON THE SURFACE OF THE GLOBE.

Fig. 11. Here a point in St. George's Channel, midway between Pembroke and Wexford, is taken as a centre, and we behold the greatest quantity of water existing in one hemisphere.

Fig. 12. Here the centre is the antipodal point to that taken in Fig. 11, and we see the greatest quantity of land existing in one hemisphere.

as the centre of the Land hemisphere. The maps now presented to the reader have been executed by Mr. Trelawny Saunders, who has so divided the globe as to add to the Land hemisphere part of S. America, including a portion of the Peruvian coast, while an equivalent area of the China Sea is transferred to the Water hemisphere. Intimately connected with the excess of land in the one hemisphere as compared to that in the other is the fact that, even allowing for the antarctic continent as expressed in the map, only one-thirteenth part of the dry land has any land diametrically opposite to it. Thus, in fig. 12 the land shaded black between the China Sea and Lake Baikal answers to that portion of S. America and Tierra del Fuego which is antipodal to it. Farther north, a part of the continent of Asia, extending along the arctic sea, as well as a large tract of Greenland and other arctic lands shaded in the same manner, are antipodal to the antarctic continent. The dark spots in South America represent tracts antipodal to Java, Borneo, the Celebes and Philippines, a part of Sumatra, and the Malay peninsula. The specks in Africa bear a similar relation to the islands in the Pacific Ocean, and the dark patches in Spain and Morocco mark those countries as partially antipodal to New Zealand.

The limits of the supposed antarctic continent have been drawn with reference to the known position of Victoria, Wilkes', Enderby's, and Graham's Lands, and the points where Ross, Weddell, and other navigators were stopped by the ice; but in order not to exaggerate the proportion of dry land in the unexplored area I have assumed one-eighth of it to be sea. This reduction has been made by extending the basin of the ocean somewhat nearer the pole than the points to which our navigators have yet penetrated, both between Graham's and Enderby's Lands and between the latter and Termination Land, in the former of which regions the ships were usually stopped by pack-ice before reaching the 70th, and in the other the 65th degree of latitude. On the other hand, I have thought it safer not to represent all the unexplored area at the N. Pole as sea; and have therefore given oneeighth of it as land, which has been done by introducing several supposed islands in the open sea said to exist off the

260 GEOGRAPHICAL CHANGES CAUSING FLUCTUATIONS [CH. XII.

Russian coast, and, according to Morton, off the N.W. of Greenland.

Former geographical changes which may have caused the fluctuations in climate revealed to us by geology.—Having now shown the reader that there have been endless changes in the form of the earth's crust in geological times, whereby the position as well as the height and depth of the land and sea has been made to vary incessantly, and that on these geographical conditions the temperature of the atmosphere and of the ocean in any given region and at any given period must mainly depend, I shall next proceed to speculate on the nature of the changes which, if assumed, might account for the leading facts revealed to us by geology as explained in the last two chapters.

In order that our speculations may be confined within the strict limits of analogy, I shall assume, 1st, That the proportion of dry land to sea continues always the same. 2ndly, That the volume of the land rising above the level of the sea is a constant quantity; and not only that its mean, but that its extreme height, is liable only to trifling variations. 3rdly, That on the whole and in spite of local changes, both the mean and extreme depth of the sea are invariable; and 4thly, That the grouping together of the land in continents is a necessary part of the economy of nature. I think it consistent with due caution to make this last assumption, because it is possible that the laws which govern the subterranean forces, and which act simultaneously along certain lines, cannot but produce, at every epoch, continuous mountainchains; so that the subdivision of the whole land into innumerable islands may be precluded.

If it be objected, that the maximum of elevation of land and depth of sea are probably not constant, nor the gathering together of all the land in certain parts, nor even perhaps the relative extent of land and water, I reply, that the arguments about to be adduced will be strengthened if, in these peculiarities of the surface, there be considerable deviations from the present type. If, for example, all other circumstances being the same, the land is at one time more divided into islands than at another, a greater uniformity of climate

might be produced, the mean temperature remaining unaltered; or if, at another era, there were mountains higher than the Himalaya, these, more especially when placed in high latitudes, would cause a greater excess of cold. Or, if we suppose that at certain periods no chain of hills in the world rose beyond the height of 10,000 feet, a greater heat might then have prevailed than is compatible with the existence of mountains thrice that elevation.

Since I first proposed in 1830 to account for the more genial climates of former times, by showing that there is now an excess of land in polar regions, Mr. Hopkins has made some important calculations to prove that, by reasoning on data supplied by the isothermal maps of Dove, we may infer that a great alteration in climate would be brought about in the northern hemisphere by what every geologist must regard as slight alterations in geography. If, said he, we assume; 1st, the diversion of the Gulf-stream from its present northerly course; 2ndly, the depression of the existing land of Northern and Western Europe to the amount of no more than 500 feet; and 3rdly, a cold current from the North, sweeping over the submerged area, the effect would be, that both on Snowdon and the lower mountains of the West of Ireland the snow-line would descend to within 1,000 feet of the sea-level, and glaciers reach the sea.* Now everyone who is aware of the rising and sinking of land, of which we have proofs since the present species of animals and plants were in existence, or since the commencement of the Glacial epoch, will be prepared to concede that, without violating probability, we may imagine far more important changes to have occurred since the older Pliocene period than those above suggested. Even if we admit that the Glacial period began as far back as the close of the Newer Pliocene era, when 5 in 100 of the mollusca were of different species from those now living, we might still fairly speculate on the lapse of a period more than ten times as long since the older Pliocene deposits were formed, for in these more than half the shells belong to extinct species.

* Quarterly Journ. Geol. Soc. 1852.