Abbildungen der Seite

recent members of the tertiary strata. From these circumstances, it is a received opinion, among certain geologists, that the animals which were first created were of an exceedingly simple structure, and that they gradually became more complex in their frame.

Although it be true, that in the lower strata there is a large proportion of the remains of animals which possess an apparently simple structure, nothing can be more unsound than to found upon such observations a doctrine such as we have above stated. What we have at one time called simple, has again and again been afterwards found to be exceedingly the reverse, so that the term is really nothing more than an expression of our ignorance, a statement of the limit beyond which we have not yet been able to advance. The animalculæ called Infusoria, are living creatures, found in stagnant waters, so wonderfully minute, that they are invisible to the naked eye,-a collection of many thousand individuals occupying no greater space than the tenth part of an inch. For a long time after they were discovered by means of the microscope, they were thought to be little more than specks of animal matter endowed with locomotive powers, but the ingenious researches of Ehrenberg, a philosopher of Berlin, who employed a very powerful instrument, laid open to our wondering sight a new creation. That distinguished naturalist has shown, that these animalculæ are provided with limbs and organs, and with a system of vessels and nerves; and even figures of their teeth accompany

his curious memoir. Thus, the lowest member in the supposed graduated scale of animal structure, in place of being a simple body, is probably a very complicated piece of mechanism. Besides, corals and shells, though of most frequent occurrence, are not the only animal remains found in the lower strata, for recent observations have discovered in these rocks, the vertebræ or joints of the backbone of fishes, as well as other parts belonging to them, and even impressions of entire fish have been met with. Now, one single undoubted specimen of an animal of that description, found in such a

situation, is as conclusive as ten thousand would be in overthrowing the whole doctrine, that there has been a gradual development of structure in animal life, as we ascend from the lowest to the uppermost strata.

A most curious circumstance, connected with fossils, is, the unequivocal evidence they afford of there having been formerly a completely different state of our planet with regard to climates, from that which now exists. Throughout all the strata, from the lowest member of the secondary series, up to the last layer lying immediately beneath that which, in geological language, is termed a formation of the recent period, we find, in our northern latitudes, numerous remains of animals and plants belonging to genera, which are now known to exist only in tropical climates. In the most northern part of Asiatic Siberia, at the mouth of the River Lena, which flows into the Arctic Ocean, in the 70th degree of latitude, there are vast accumulations of the bones of an extinct species of elephant, and these in such a state of preservation, that a great part of the ivory used in St. Petersburg, is brought from thence. Indeed the quantity is so great, that a Russian natural, ist has stated it as his belief, that the number of elephants now living on the globe, must be greatly inferior to those which occur in a fossil state in those parts of Siberia. The entire carcase of one of those animals was found enclosed in a mass of ice, where it must have remained incased for thousands of years; and yet, from the preservative quality of the ice, the flesh was in such a state, that, when it was disentombed by the accidental breaking up of the mass, it was devoured by the wolves and other wild animals. Moreover, it was thickly covered with hair, of which the existing species of elephants are nearly destitute; thus proving that it was of a species adapted to a cold climate. Then, as to plants, specimens of rocks have been brought from Melville Island, the remote northern land discovered in our late polar expeditions, some of which contain, imbedded in the stone, portions of plants belonging to an order now known to

exist only in the warmest parts of the equatorial regions. The greatest degree of heat seems to have existed during the deposition of the inferior beds of the secondary strata ; and it appears also, from the nature of the fossil plants found in these strata, that there must have existed, at the same time, a very considerable degree of moisture in the atmosphere. The heat seems to have gradually diminished, so that at last, during the deposition of the most recent of the tertiary strata, the climate of the northern hemisphere does not appear to have been very different from what it is now.

To endeavour to account for this wonderful change in the temperature of the northern latitudes, is one of the most difficult problems in the physical history of the globe, because it involves such a variety of considerations; and we know that the most important and extensive changes in the forms of organized bodies, are brought about by very nice, shades of difference in the circumstances of climate and soil under which they are placed. In the early stages of geology, many theories were started: the earth was said to have been originally in a highly heated state, and to have gradually cooled; and it was maintained that during the progress of cooling, the various changes in climate took place : according to another theory, the position of the axis of the earth was at one time different from what it is now, and was so directed, that the polar regions were exposed to a much more direct action of the solar rays. But the inventors of these theories did not trouble themselves much with inquiring, whether they were in harmony with the laws which regulate the motions of the heavenly bodies; and when they were subjected to the examination of the astronomer, they could not stand the test of his severe investigations. An ingenious theory has been lately proposed by Mr. Lyell. His theory is, that all the indications of the former prevalence of warmer climates, may be accounted for by a different distribution of land and water; and we know from geological appearances, that a very different proportion of superficial land and water must formerly

have existed in the northern hemisphere from that which we now find. It is not very easy to state the grounds of this theory in an abridged form; but the following explanation will perhaps convey an intelligible idea of it. Wherever there is a great expanse of water, like the sea, there is always a more uniform temperature in the adjoining countries throughout the year, less extremes of heat and cold. On the contrary, extensive tracts of land are liable to considerable vicissitudes; and hence the difference of an insular and continental climate in the same parallel of latitude. Moscow and Edinburgh are very nearly in the same latitude ;

but while at the latter place, there is neither extreme cold nor excessive heat, at Moscow, the cold in winter is sometimes so intense as to freeze quicksilver, and there are often days in summer as hot as at Naples. In like manner, the higher you ascend, the air becomes colder ; and thus in lofty mountains, such as Ætna, the sugar-cane grows at the foot, and the lichen, or moss of Iceland, at the summit. In the lofty mountains of South America there are regions of eternal snow under an equatorial sun.

If we suppose, therefore, extensive continents, lofty mountains, , and numerous islands to have existed in southern latitudes, where there is now a wide expanse of sea, and an ocean to have occupied the place of northern Europe and Asia, it will be readily conceived, from the principles above stated, that very different climates would exist in the northern hemisphere from what now prevail.

All the solid strata, most abundant in animal remains, are either limestones, or contain a large proportion of lime in their composition. Many thick beds of clay also abound in them ; but in that case, limestone, in some form or other, is generally associated with the clay. From this it has been inferred, and not without a strong semblance of probability, that animals have mainly contributed to the formation of many limestone strata, in the same way as we see them now at work forming vast limestone rocks in the coral reefs of the Pa

cific ocean.

A reef of this sort extends for three hundred and fifty miles along the east coast of New Holland; and between that country and New Guinea the coral formations have been found to extend, with very short intervals, throughout a distance of seven hundred miles. Of all the

forms of organized bodies, which are found in a fossil state, from the lowest stratum in which they occur, to those of most modern date, shells and corals constitute by far the greatest propor. tion. All the strata must have been deposited in seas or lakes; and it is therefore natural, that animals living in water should be most abundant. Besides, as shells and corals are not liable to decay, they remain, while the soft boneless animals, which inhabit them, perish entirely ; and fish-bones, being more perishable than shells, are comparatively rare.



We have said that shells are by far the most numerous class of fossils: they are found in all formations, from the lowest stratum in which animal remains have been seen, to the most recent deposits now in progress. To a person who has made conchology a special object of study, there appear many striking differences between those found in a fossil state, and such as now exist in our seas, lakes, and rivers ; but were we to describe, or give representations, of even remarkable fossil shells, a general reader would discover, in most of them, nothing so peculiar as to arrest his attention. There is, however, one, which is so different from any thing now living, and of such common occurrence, that we are induced to give it as a good example of an extinct

genus. It is called the Ammonite, or Cornu Ammonis, that is, Horn of Ammon, from its resemblance to those horns which are affixed to the head of the statues of Jupiter Ammon.

« ZurückWeiter »