The Project Gutenberg eBook of Physical Geography

121. It is in the space comprised between two of the eastern tributaries of the Tigris, the Khaus and the Great Zab, or Abou Selman of the Arabs, that the extensive ruins of Koyunjik, Khorsabad, and especially of Nimroud, are situated, the last of which have been so satisfactorily identified with the capital of Assyria—the ancient Nineveh—by our enterprising and talented countryman Mr. Layard, to whose exertions, under circumstances of peculiar difficulty, surrounded by every privation, our national Museum is indebted for that magnificent collection of Assyrian monuments which at this moment forms the admiration of the British public. It is to be hoped that our Government will follow up the researches commenced by Mr. Layard, and that several of the gigantic sculptures removed by him, with such perseverance and labour, to Bussorah, will ere long be added to the riches of the British Museum.

See Mr. Layard’s work on “Nineveh and its Remains,” 2 vols. 8vo., and his illustrated work in folio—the former one of the most interesting narratives ever published on the antiquities of Central Asia.

123. [Lieutenant W. F. Lynch, of the United States Navy, has recently published an interesting and valuable narrative of an expedition to the Dead Sea and River Jordan. According to his measurements and surveys, the level of the Dead Sea is 1,316·7 feet below that of the Mediterranean. The city of Jerusalem is 2,610·5 feet above the latter, and 3,927·24 feet above the former sea. The greatest depth of the Dead Sea is 1308 feet. Lieutenant Lynch states the density of the water of the Dead Sea to be 1·13, that of distilled water being 1.]

124. The water of Lake Eltonsk contains chloride of calcium.

125. The water of the Dead Sea, according to Lieutenant Lynch, contains 26·42 per cent. of saline ingredients, one of which is chloride of magnesium.

126. Professor Schoenbein of Basle attributes the peculiar smell, when bodies are struck by lightning, to a principle existing in the atmosphere, which he calls ozone, liberated by the decomposing action of electricity, and possessing the same electrical characters as bromine, chlorine, and iodine. He ascribes the luminous appearance of the ocean to the action of that principle on the animal matter it contains.

127. Annales des Sciences Géologiques, par M. Rivière, 1842.

128. The mean of any number of unequal quantities is equal to their sum divided by their number: thus the mean temperature of the air at any place during a year is equal to the sum of the mean temperature of each month divided by 12. This method, however, will only give an approximate value; therefore, to ascertain the mean annual temperature at any place accurately, the mean of a number of years must be taken.

129. Lines drawn on a map or globe through all places where the mean annual temperature is the same are isothermal lines.

130. For example, Professor Dove has found that the mean temperature of December, January, and February, at Toronto in Canada, added to the mean temperature of the same months at Hobart Town in Van Diemen’s Land, exceeds the sum of the mean temperature of June, July, and August, at the same places, added together, by 22°·7 of Fahrenheit. Similar results, though varying in amount, were obtained for many corresponding places in the two hemispheres, which establishes the law given in the text.

131. In the same manner as isothermal lines are supposed to pass through all parts of the globe where the mean temperature of the air is the same, so the isogeothermal lines are supposed to pass through all places where the mean heat of the ground is the same: the isotherial lines are supposed to be drawn through all places having the same mean summer temperature; and the isochimenal lines pass through all places where the mean winter temperature is the same. The practice of representing to the eye these lines on a map or terrestrial globe is of the greatest use in following and understanding the complicated phenomena of temperature and magnetism.

132. If the heights above the earth increase by equal quantities, as a foot or a mile, the densities of the strata of air, or the heights of the barometer which are proportional to them, will decrease in geometrical progression: for example, if the height of the barometer at the level of the sea be 29·922 inches, it will be 14·961 inches at the height of 18,000 feet, or one-half as great; it will be one-fourth as great at the height of 36,000 feet, one-eighth at the height of 54,000 feet, and so on.

133. A very ingenious little instrument, called the Aneroid Barometer, has been lately invented in France; which, at the same time that it forms an exact and very portable weather-glass, in the common acceptation of that term, may be employed with considerable accuracy in ascertaining differences of level. Although not to be compared, as an instrument of precision, with the ordinary mercurial barometer, it is infinitely more portable, and gives with promptitude and accuracy small differences of level.

A friend of the author’s has recently tested it in the latter respect on some of our railways, and found that observations made with it carefully will give, on a line of 200 miles in extent, the relative levels of the different stations within a few feet. The observations can be made in a couple of minutes. The gentleman in question writes to us, that he considers the Aneroid Barometer will prove a very useful instrument to the geological and the botanical traveller.

See, for a description of this instrument, a pamphlet recently published at 84, Strand, by Mr. E. J. Dent, on the Construction and Uses of the Aneroid Barometer. London, 1849.

134. The moon’s orbit is very much elongated, so that her distance from the earth varies considerably, and consequently her attractive force. Moreover, her attraction varies with the rotation of the earth, which brings her twice in 24 hours in the meridian of any place, once in the superior and once in the inferior meridian; but her action on the atmosphere is much inferior to that of the heat of the sun.

135. Mr. Pentland has, however, found in the Peru-Bolivian Andes, at elevations between 11,000 and 14,000 feet, the horary oscillations of the barometer as regular, and nearly as extensive, as on the level of the sea in the same latitude.

136. Lieutenant Maury, of the United States Navy, is led to believe that there is a region within the limit of the N.E. trade-winds, in the Atlantic, in which the prevailing winds are from the south and west: this region is somewhat in the shape of a wedge, with its base towards the coast of Africa, between the equator and 10° N. lat., and between the meridians of 10° and 25° W. long. In this space, in which the law of the trade-winds is reversed, there are great atmospheric disturbances, violent squalls, sudden gusts of wind, thunder, storms, heavy rains, baffling airs, and calms.

137. In the northern hemisphere, a north wind sets out with a less rotatory motion than the places have at which it successively arrives, consequently it veers through all the points of the compass from N. to N.E. and E. If a south wind should now spring up, it would gradually veer from S. to S.W. and W., because its rotatory velocity would be greater than that of the places it successively comes to. The combination of the two would cause a vane to veer from E. to S.E. and S.; but the rotation of the earth would now cause the south wind to veer round from S. to S.W. and W.; and should a north wind now arise, its combination with the west wind would bring the vane round from W. to N.W. and N. again. At the Greenwich Observatory the wind makes five gyrations in that direction in the course of a year. In Europe it is the contention of the N.E. and S.W. winds which causes the rotation of the wind, and the principal changes of weather, the S.W. being warm and moist, the N.E. cold and dry, except where it comes over the German Ocean.

138. In all hurricanes hitherto observed, the sinking of the mercury, and the increase of the wind, have been more or less regularly progressive till within three or four hours’ sail of the centre of the storm; and in one class they have continued so even to the centre; while in another class, and by far the most terrible, the depression of the mercury has been sudden and excessive when within that distance of the centre, and the violence of the tempest far beyond the average. When a ship is within 50 or 60 miles of the centre, the storm has the mastery, and seamanship is of little avail. Rules for avoiding this calamity, and for managing a ship when involved in a hurricane, are fully explained in the “Sailor’s Horn-Book for the Laws of Storms,” by H. Piddington, Esq., President of the Marine Courts of Inquiry at Calcutta. The following approximate table is given by him, to serve as a guide till better data shall be obtained:—

Average fall of the barometer per hour.	Distance of a ship from the centre of the storm, in miles.
From 0·020 to 0·060	From 250 to 150
From 0·060 to 0·080	From 150 to 100
From 0·080 to 0·120	From 100 to 80
From 0·120 to 0·150	From 80 to 50

The rate of fall per hour doubles after the storm has lasted six hours, and within three hours of the centre of the hurricane the mercury will fall four times as fast, if it be of the violent class.

Colonel James Capper discovered the rotatory motions of storms, and W. C. Redfield, Esq., of New York, was the first who determined their laws. Colonel Reid, Governor of Barbadoes, and Dr. Thom, of the 86th regiment, have also written on the subject.

139. The four subordinate forms of clouds are the cirro-stratus, composed of little bands of filaments, more compact than the cirrus, forming horizontal strata, which seem to be numerous thin clouds when in the zenith, and at the horizon a long narrow band. The cumulo-stratus consists of the summer-cloud, like snowy mountains heaped on one another, which at sunrise have a black or bluish tint at the horizon, and pass into the nimbus, or rain-cloud, which has a uniform grey tint, fringed at the edges; and the fourth is the cirro-cumulus, a combination of filaments and heaped-up cumuli or summer-clouds.

140. The reader is referred to the chart of the distribution of rain in the Physical Atlas of Alexander Keith Johnston, Esq., where the value of the practice referred to in note p. 27 is shown.

141. The reader is referred to the “Connection of the Physical Sciences” for an account of Dr. Dalton’s theory of definite proportions, and the relative weight of atoms.

142. The reader is referred to the 18th section of the “Physical Sciences” for reflection, refraction, and absorption of light, and to the 19th section for the constitution of the solar light and colours.

143. For the cause of mirage, see the “Connection of the Physical Sciences.”

144. For phenomena and theory of polarized light, see section 21, “Connection of the Physical Sciences.”

145. Every substance, whether solid or fluid, has its own polarizing angle.

146. The reader is referred to a plate in “Johnston’s Physical Atlas” showing the phenomena of the polarization of the atmosphere.

147. See sections 28 and 29 of the “Connection of the Physical Sciences:” on Electricity.

148. Sound travels at the rate of 1120 feet in a second in air at the temperature of 62° of Fahrenheit; so if that number be multiplied by the seconds elapsed between the flash of lightning and the thunder, the result will be the distance in feet at which the stroke took place.

149. Colonel Sabine’s Notes to “Kosmos.”

150. The foci are all of different intensities; that in the South Atlantic, discovered by M. Erman, has the least intensity of the four, and the other in the southern hemisphere, discovered by Sir James Ross, has the greatest; taking 1 as the unit at the magnetic equator in Peru, their intensities are as 2·071 and 0·706. In the northern hemisphere the American focus is more intense than that in Siberia, which is moving from west to east, while the minor focus in the southern hemisphere is moving from east to west.

151. The author is indebted to the admirable and profound investigations of Colonel Sabine for almost all she knows on the subject of terrestrial magnetism. In these, and in his notes on the English translation of “Kosmos,” the reader will find all that is most interesting on the subject. In his own works there are plates of the course of the different magnetic lines mentioned in the text.

152. At St. Helena, the north end of the needle reaches its eastern extreme in May, June, July, and August, and nearly at the same hours it reaches its western extreme in November, December, January, and February. The passage from one to the other takes place at, or soon after, the equinoxes in March and April, September and October.—Colonel Sabine’s Notes to “Kosmos.”

153. The sporules or seeds of the fungi are so minute that M. Freis counted above ten millions in a single plant of the recticularia maxima: they were so subtile that they were like smoke.

154. The solar spectrum, or coloured image of the sun, formed by passing a sunbeam through a prism, is composed of a variety of invisible as well as visible rays. The chemical rays are most abundant beyond the violet end of the spectrum, and decrease through the violet, blue, and green, to the yellow, where they cease. The rays of heat are in excess a little beyond the red end, and gradually decrease towards the violet end. Besides these there are two insulated spots at a considerable distance from the red, where the heat is a maximum. Were the rays of heat visible, they would exhibit differences as distinct as the coloured rays, so varied are their properties according to their position in the spectrum. There are also peculiar rays which produce phosphorescence, others whose properties are not quite made out, and probably many undiscovered influences; for time has not yet fully revealed the sublimity of that creation, when God said, “Let there be light—and there was light.”

155. Professor Quetelet is desirous that the periodical phenomena of vegetation should be observed at a number of places, in order to establish a comparison between the periods at which they take place; and for that purpose he gives a list of the commonest plants, as lilac, laburnum, elder, birch, oak, horse-chestnut, peach, pear, crocus, daisy, &c., which he himself observes annually at Brussels.

156. Dandelion opens at five or six in the morning, and shuts at nine in the evening; the goat’s-beard wakes at three in the morning, and shuts at five or six in the afternoon. The orange-coloured Escholtzia is so sensitive that it closes during the passage of a cloud. “The marigold that goes to bed wi’ the sun, and with him rises weeping,” with many more, are instances of the sleep of plants.

157. M. de Candolle established 20 botanical regions, and Professor Schow the same number; but Professor Martius, of Munich, has divided the vegetation of the globe into 51 provinces, namely, 5 in Europe, 11 in Africa, 13 in Asia, 3 in New Holland, 4 in North and 8 in South America, besides Central America, the Antillas, the Antarctic Lands, New Zealand, Van Diemen’s Land, New Guinea, and Polynesia. To these, other divisions might be added, as the Galapagos, which is so strongly defined.

Baron Humboldt gives the following concise view of the distribution of plants, both as to height and latitude:—

The equatorial zone is the region of palms and bananas.

The tropical zone is the region of tree-ferns and figs.

The subtropical zone, that of myrtles and laurels.

The warm temperate zone, that of evergreen trees.

The cold temperate zone, that of European or deciduous trees.

The subarctic zone, that of pines.

The arctic zone, that of rhododendrons.

The polar zone, that of alpine plants.

Upper Limit of Trees on Mountains.—The upper limit of trees is distinguished by the Escalloniæ, on the Andes of Quito, at the height of 11,500 feet above the level of the sea.

In tropical Mexico, the upper limit of trees, at the height of 12,789 feet, is distinguished by the Pinus occidentalis.

In the temperate zone the limit of trees is marked by the Quercus Semicarpifolia, at 11,500 feet, on the south side of the Himalaya, and by the Betula Alba, on the north side, at the height of 14,000 feet: the same birch forms the limit on the Caucasus, at the elevation of 6394 feet. On the Pyrenees and Alps the limit is marked by the Coniferæ or pine tribe: on the Pyrenees by the Pinus uncinata, at the height of 10,870 feet; on the south side of the Alps by the larch, at the elevation of 6700 feet; and by the Pinus abies, at 5883 feet, on the north.

In Lapland, the Betula Alba forms the upper limit of trees, at the height of only 1918 feet.

The upper Limit of Shrubs.—In the Andes of Quito the Bejarias are the shrubs that attain the greatest height, and terminate at 13,420 feet above the sea-level.

The juniper, Salix, and Ribes, or currant tribe, form the upper limit of Shrubs on the south side of the Himalaya, at the height of 11,500 feet. The tama, or Genista versicolor, a species of broom, flourishes at the height of 17,000 feet on the north side, and vegetation is prolonged to nearly 18,000 feet.

The Rhododendron forms the upper limit of shrubs on the Caucasus, at 8825 feet; in the Pyrenees it grows to 8312 feet; in the Alps to 7480 feet; and in Lapland it forms the upper limit of shrubs at an elevation of 3000 above the Arctic Ocean.

158. The British flora contains at least 3000 species.

159. The plants with which the Chinese give flavour to tea are the Olea fragrans, Chloranthus inconspicuus, Gardenia florida, Aglaia odorata, Mogorium sambac, Vitex spicata, Camellia sasanqua, Camellia odorifera, Illicium anisatum, Magnolia yulan, Rosa indica odoratissima, turmeric, oil of Bixa orellana, and the root of the Florentine iris.

The principles of caffeine and theine are, in all respects, identical.

160. Davis on China.

162. Dr. J. D. Hooker.

163. The Euphorbia and Borreria are the distinguishing features of the low grounds in the Galapagos islands; while the Scleria, croton, and Cordia mark the high grounds. Compositæ and Campanulaceæ distinguish St. Helena and Juan Fernandez. The prevailing plants in the Sandwich group are the Goodeniaceæ and Lobeliaceæ; and in New Zealand ferns and club-mosses prevail, almost to the exclusion of the grasses.—Dr. J. D. Hooker.

164. Of 2891 species of flower-bearing plants in the United States of North America, there are 385 found also in northern and temperate Europe.

165. In the basin of Titicaca in Peru-Bolivia, Mr. Pentland has seen a variety of maize ripen as high as 12,800 feet.

166. Dr. Weddell, a very distinguished botanist, who has recently returned from an exploration of the districts of the Andes which furnish the Peruvian bark of commerce, has discovered several new species of Cinchona, the total number of which, according to his beautiful monography, now amounts to 21.

167. Professor Martius, of Munich, in his great work on Palms, has described 500, accompanied with excellent coloured plates. It is supposed that the number of species throughout the world amounts to 1000.

168. There are innumerable points of analogy between the vegetation of the Brazils, equinoctial Africa, and India: but the number of species common to these three continents is very small.

169. Dr. J. D. Hooker.

170. The cosmopolite ulvæ are the Enteromorpha, Codium, &c.

171. Dr. J. D. Hooker has divided the marine vegetation into ten provinces:—the Northern Ocean, from the pole to the 60th parallel of north latitude;—the North Atlantic, between the 60th and 40th parallels, which is the province of the delessariæ and fucus proper;—the Mediterranean, which is a sub-region of the warmer temperate zone of the Atlantic, lying between the 40th and 23d northern parallels;—the tropical Atlantic, in which sargassum, rhodomelia, corallinia, and siphinea abound;—the antarctic American region, from Chile to Cape Horn, the Falkland Islands, and the whole circumpolar ocean south of the 50th southern parallel;—the Australian and New Zealand province, which is very peculiar, being characterized, among other generic forms, by cystoseiriæ and fuceæ;—the Indian Ocean and the Red Sea;—and the last, which comprises the Japan and China Seas. There are several undetermined botanical marine provinces in the Pacific and elsewhere.

172. The British flowering sea-weeds are the Zostera and Zanichellia.

173. The vegetation at different depths in the Egean Sea is as distinctly marked as that at different heights on the declivity of a mountain. The coast plants are the Padina pavonia and Dictyota dichotoma. A greater depth is characterized by the vividly green and elegant fronds of the Caulerpa prolifera, probably the prasium of the ancients; associated with it are the curious sponge-like Codium Bursa, and four or five others. The Codium flabelliforme, and the rare and curious vegetable net called Microdictyon umbilicatum, characterize depths of 30 fathoms. The Dictyomenia, with stiff purple corkscrew-like fronds, and some others, go as low as 50 fathoms, beyond which no flexible sea-weeds have been found. The coral-like Millepora polymorpha take their place, and range to the depth of 100 fathoms, beyond which there is no trace of vegetable life, unless some of the minute and microscopic infusorial bodies living there be regarded as plants.—“Travels in Lycia,” by Lieutenant Spratt, R. N., and Professor E. Forbes.

174. The notocanthus and macrourus are the deep-water fish in the arctic regions; they also inhabit the seas of New Zealand. The Pacific fish that enter the Atlantic are some of the mackerel tribe, sharks, and lophobranches. The genera most prevalent in the southern hemisphere are the notothemia, borichthys, and harpagifer. The same species of these genera are found in the seas of the Falkland Islands, Cape Horn, the Auckland Islands, and Kerguelen’s Land.—Dr. Richardson.

175. The Chinese fresh-water fish are cyprinidæ, ophicephali, and siluridæ—genera which agree closely with those in India, though the species are different.

176. The carnivorous Cetacea, with two remarkable exceptions, inhabit the ocean—the Delphinus Inca, of the Amazons and its affluents; and the D. Gangeticus, of the Ganges.

177. Captain Scoresby’s “Arctic Voyages.”

178. One of the most celebrated species of this division is the crocodile of the Nile, which probably is to be met with in the western branch of that river, the Bahr-el-Abiad, as high as 4000 feet. Immense numbers of this species, of every size and age, are found embalmed in the catacombs of the ancient Egyptians, which are perfectly identical with the existing species, and offering another proof of the important fact first announced by Cuvier, from his examination of the mummies of the ibis, that no animal, in its wild state, had presented the least change, within the longest historical periods.

179. Mr. Pentland informs me that crocodiles are found in some of the rivers of Bolivia at a much greater elevation.

180. Animals of a gigantic size, and allied to the lizard family, formerly inhabited the latitudes of Britain. A monster (the Mosasaurus) much surpassing the largest living crocodile is found in our Sussex chalk-beds; and an animal allied to the Iguana, the iguanodon of Mantell, is of frequent occurrence in the strata upon which the chalk reposes in the weald of Sussex, the Isle of Wight, &c. Some bones of the iguanodon would indicate an animal more than 50 feet long.

181. Petrel, from St. Peter.

182. In some parts of the earth the same conditions which regulated the distribution of the ancient fauna and flora still prevail. The flora of the carbonaceous epoch is perfectly similar to that of New Zealand, where ferns and club-mosses are so abundant; and the fauna of that ancient period had been representative of that which recently prevailed in these islands, since foot-prints of colossal birds have been discovered in the red sandstone of Connecticut.

The age of reptiles of the Wealden and other secondary periods is representative of the fauna of the Galapagos islands, which chiefly consists of tortoises and creatures of the lizard or crocodile family; and the cycadaceous plants and marsupial animals of the oolite are representative of the flora and fauna of Australia.

The colossal birds which prevailed in New Zealand, almost to the entire exclusion of reptiles and quadrupeds, lasted to a very late period; they differed in the structure of the beak and skull from every class of birds, recent or fossil.

183. Perhaps no quadruped in the wild state will be found to have so wide a vertical range of habitat as this animal. It is found in the plains of Tartary, in the valley of the Tigris, at a very few feet above the sea-level, and in the most elevated plains of the Himalaya, at elevations exceeding 15,300 feet.

184. It is by no means certain that the wild Ass of the three countries mentioned in the text belongs to the same species. The Kiang of Tibet appears to be the same as the Dziggetai (Equus Hemonus) of Pallas, which is met with throughout central Asia; but the species found in the Run of Cutch is of a different colour and form: whilst the one neighs like a horse, the other brays like an ass; in one the striped colour of the zebra family exists in the young, and not in the second.

185. The attention of the scientific world in France has been recently directed to the advantages that might arise from the naturalization of the Llama tribe in Europe, and especially of its two most useful species, the Llama and the Alpaca. M. J. Geoffroy St. Hilaire, a zoologist of some note, but rather carried away by theoretical views in a branch of science where observation, and observation alone, ought to be our guide, and ignorant perhaps of what had been done in England on the same subject, where the experiment had long since been tried, and with very inadequate success, has presented lately some papers to the Academy of Sciences on this subject. We cannot imagine, even if the naturalization of the Llama on a large scale was possible, what benefit could arise from it to our agriculturists. The wool of the llama is coarse, and so infinitely inferior to the commonest qualities of sheep’s wool, that in its native country it is seldom used for any other purpose than the manufacture of ropes, of a rough carpeting and packing-cloth, and for the coarsest apparel of the poor Indian. As to its use as a beast of burden, whilst the llama eats as much as the ass, it does not carry more than one half what he can, and can scarcely travel one half of the same daily distance; besides, the female llama is useless in this respect. The flesh of the llama, as above stated, is greatly below that of all our domestic animals, even of the Italian buffalo.

As to the Alpaca, it is very doubtful if, living as it does in an extremely dry, elevated, equable, and clear atmosphere, it would ever become accustomed to the damp and variable climate of our northern latitudes, or to that of the great European chains of mountains, the Alps and the Pyrenees, and if it did, that its wool would not be greatly deteriorated. As to the vicuña, it is purely a wild species, and has hitherto resisted all the efforts of the aborigines, the most patient and docile of the human race, to render it prolific in its own climate and in domesticity.

It appears, therefore, that the domestication of the several species of Auchenia in Europe would be a costly and useless experiment, on the large scale on which it is proposed to try it; indeed, this will appear evident when it is known that in the Peru-Bolivian Andes the llama and alpaca are daily disappearing to make room for the more useful and profitable breed of the common European sheep, whilst, as a beast of burden, the ass is everywhere taking its place.

Connected with this subject, a very singular fact, and, if well established, a very curious one, has been announced by M. Geoffroy St. Hilaire, on the authority of one of our countrymen, Dr. Weddel, recently returned from South America, that a cross-breed between the Alpaca and the Vicuña had been obtained, and that the mules from this cross-breed were capable of reproducing this newly-created species, the wool of which is represented to be of a valuable quality. Now, if there exists in zoological science a fact clearly established, it is this: that within historical periods no new species of vertebrate animal has been created—in fact, the great law of the immutability of species. The remains of the several wild animals which have been buried for more than 30 centuries in the catacombs of Egypt, and in the ruins of Nineveh, are perfectly identical with those now existing in the most minute details of their anatomical structure. We have examined, in the case referred to, the evidence adduced by M. Geoffroy St. Hilaire in support of his favourite doctrine, and we do not by any means consider it sufficient to shake the conclusions arrived at by all the great zoologists of past and present times—by the Cuviers, the Humboldts, and the Owens of our own period—on the impossibility of the production of a new species of animals, or the immutability of species in the animal creation. Contradictions to this law we know have been brought forward by writers of the theoretical school of naturalists, to support favourite theories of their authors; but we believe such dangerous doctrines are founded on the vagaries of a school which have ever placed in natural history observation in the back, and the dreams of imagination in the foreground.

186. There are 8 families, 14 genera, and 123 species of marsupial animals, amounting to about one-twelfth of all the mammalia. The opossum is American; the seven other families are inhabitants of Australia and the Indian Archipelago.

Of the Didelphidæ or opossum family there are 21 species, all inhabitants of America; the Virginian opossum is about the size of a cat, the other species are not larger than rats or mice. A pretty kind in Surinam, the D. dorsigera, is so named because it carries its young on its back, which hold on by their prehensile tails twisted round that of the mother: another species is aquatic, and in its habits resembles the otter.

The Dasyuridæ and Phalangers are nocturnal: the Dasyuridæ and wombats burrow.

187. Sir Charles Lyell estimates the number of existing species of animals and vegetables, independent of the infusoria, to be between one and two millions, which must surely be under the mark, considering the enormous quantity of animal life in the ocean, to the amount of which we have not even an approximation. If the microscopic and infusorial existence be taken into the account, the surface of the globe may be viewed as one mass of animal life—perpetually dying—perpetually renewed. A drop of stagnant water is a world within itself, an epitome of the earth and its successive geological races. A variety of microscopic creatures appear, and die; in a few days a new set succeeds; these vanish and give place to a third set, of different kinds from the preceding; and the débris of all remain at the bottom of the glass. The extinction of these creatures takes place without any apparent cause, unless a greater degree of putrescence of the water be to them what the mighty geological catastrophes were to beings of higher organization—the introduction of the new is not more mysterious in one case than in the other.