Читать книгу Animals before man in North America - Frederic A. Lucas - Страница 9

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Below the worms are the Cœlenterata, including a large number of lowly but beautiful animals, such as the jelly-fish, Portuguese man-of-war, sea-anemones, and the coral-forming polyps so persistently and wrongly called insects. As may be inferred from the names, the vast majority of these are marine; most of them have a long pedigree, dating back to the beginning of the recorded history of life, and the corals have played an important part in building up the land, and are still industriously at work constructing barrier reefs and coral islets.

Then come the Porifera, or sponges, for so long a time bandied about between the animal and vegetable kingdoms, but now definitely located in the former. This division contains several orders, which, with one exception, have an interior framework of glassy or horny fibers, and it is this framework that forms the sponge of commerce, all the living matter having been removed. Sometimes the former existence of sponges is revealed merely by the presence of spicules, or portions of the skeleton; sometimes these have held together and retained the general outline of the entire sponge. A living sponge is really an assemblage of numerous individuals disposed about a common cavity, provided with one or more openings through which the water passes, and, small as are the individuals, yet their combined action causes such a current that on a calm day the presence of a large sponge may be revealed by the motion of the water. Huxley has compared a sponge to a “kind of subaqueous city, where the people are arranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along.”

Last and least are the single-celled organisms grouped under the name of Protozoa, making up in number of individuals what they lack in size. Some, like the Infusoria, are soft, and when dead leave behind no trace of their existence, but others, belonging to the class Rhizopoda, form beautiful shells of carbonate of lime, or still more beautiful geometrical skeletons of flint. The Foraminifera of to-day dwell in the depths of the ocean, and in some localities their minute shells accumulate to form the most important constituent of the soft ooze. In other places the flinty skeletons of the Radiolarians predominate, and in the past they have existed in such numbers as to form considerable deposits of radiolarian limestone. It gives one a vivid idea of time and number to try and imagine how long it must have taken and how many individuals it must have required for their microscopic shells to form a bed of rock a foot thick and even a mile square.

As with vertebrates, all orders of invertebrates now living are represented by fossils, except where they have been too soft and small for preservation, while several important orders occur only as fossils, and several more are even now verging on extinction.

Owing to their hard coverings, the shell-bearing mollusks are naturally the most abundant of fossils, and many thousand species are known. They play a most important part in defining the limits of groups of rocks and in identifying the individual beds, some species being found through a number of strata, while again others are confined to and characteristic of a single layer of rock. And this furnishes a hint of the intimate connection there is between the divisions of life and time and of the manner in which the latter are defined.

There is a ceaseless warfare waged by water against the land; the sea hurls its waves against the coast, rivers cut their channels through earth and rocks, and every rain washes something of the earth into the sea, directly or indirectly as the case may be. This warfare began when the first ridge of rock peered above the level of the primeval ocean, and has been carried on without a moment’s intermission ever since, the results of the conflict being the formation of beds of mud or sand that later hardened into rock. Into the mud and sand sank not only the remains of animals that dwelt in the lake or ocean where the beds were being deposited, but those of creatures that lived upon the adjacent land and perished along the shore or were swept down by rivers. Hence the layers of rock contain the vestiges of the plants and animals that lived at the time they were being formed, and these fossils serve to identify the strata in which they are found. So rocks above those which show no traces of living things are arranged or classified according to the fossils they contain, each layer or stratum being termed a formation or stage. Now the life of the globe has been ever varying with the movements of its crust, some plants and animals dying out and others arising to take their places, so that at no two periods of time were the living beings just the same. Certain kinds of animals will be found in a number of layers of rock and then disappear, or be present in greatly reduced numbers, while from time to time new plants and animals make their appearance. And while these changes have in the main been slow, at some periods they took place much more rapidly than at others, causing very marked differences between the animals found in various beds of rock, and these differences are used as boundary marks to distinguish the divisions of geological time.

Any well-defined stratum, or bed of rock, which is shown by its structure and the fossils it contains to have been the result of the uninterrupted deposit of sediment, is termed a Formation, or Stage, and it is easy to see that this may vary greatly in thickness. According to the extent of the resemblances between the animals they contain, Formations are combined in Series, the Series in Systems, while these in turn are united in Groups; such, at least, is the classification and such the names adopted by the International Geological Congress. And as these assemblages of rocks necessarily represent the time that elapsed while they were being formed, the various divisions just named are made to correspond to divisions of time known, counting from the smallest to the largest, as Age, Epoch, Period, and Era. This may be shown as follows:

Divisions of rocks.		Divisions of time.
	Group	Paleozoic		Era.
	System	Upper Silurian		Period.
	Series	Niagara		Epoch.
	Stage or Formation	Medina		Age.

For example, the Medina sandstone, well known as a building stone in western New York, is the Medina stage of the Niagara series of the Upper Silurian system of the Paleozoic group; and while all this may seem very technical and uninteresting, it is merely an aid to the proper locating of specimens, and it is just as necessary to exactly locate animals as it is to locate cities. For example: To say that Rochester is in the United States would mean very little, but to say that Rochester is in Monroe County, New York, would show just where it is situated. Similarly, to say that remains of the great dinosaur Triceratops occur in the Laramie sandstone is to convey the information that Triceratops is one of a group of extinct reptiles and tell at just what period it lived.

One thing must be borne in mind, and that is that periods of geological time have no exact equivalent in years. Our own standard for the measurement of time is the period required for the earth to make a complete revolution about the sun; but divisions of geological time have no such fixed standard, being records of the changes that have taken place among plants and animals rather than actual measurement of the lapse of years; hence these divisions may be, and are, of very unequal length.

The number of years represented by any given group of rocks is computed by estimating the time that would be required to wear away and deposit in the shape of mud or sand sufficient of the earth’s surface to form the beds under consideration. But as the rate of wear varies greatly, not to say enormously, according to the material, rainfall, and elevation of the surface acted on, there can be no fixed rate of wear, and it is not surprising to find that estimates vary immensely. For this reason it is quite impossible to give a very accurate or satisfactory answer to the oft-asked question, How many years ago did this creature live?