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resulted in depression at one point and elevation at another, land has become ocean and ocean land. And, in the almost unimaginable period of time which has passed since the earth first shrank from its hypothetical condition of nebulous vapor to be a ball of land covered with water, such changes have occurred over and over again. They have, however, mostly taken place slowly and gradually. The principal seat of great

Fig. 175. --Restoration of the skeleton in probable normal position of Dimorphodon macronyr. (After Seeley.)

change is in the regions of mountain chains, which, in most cases, are simply the remains of old folds or wrinkles in the crust of the earth. When an aquatic animal dies, it sinks to the bottom of the lake or ocean, unless, of course, its flesh is eaten by some other animal. Even then its hard parts will probably find their way to the bottom. There the remains will soon be covered by the always dropping sediment. They are on the way to become fossils. Some land animals also might, after death, get carried by a river to the lake or ocean, and find their way to the bottom, where they, too, will become fossils, or they may die on the banks of the lake or ocean and their bodies may get buried in the soft mud of the shores. Or, again, they are often trodden in the mire about salt springs or submerged in quicksands. It is obvious that aquatic animals are far more likely to be preserved as fossils than land animals. This inference is strikingly proved by fossil remains. Of all the thousands and thousands of kinds of extinct insects, mostly land animals, comparatively few specimens are known as fossils. On the other hand, the shell-bearing mollusks and crustaceans are represented in almost all rock deposits which contain any kind of fossil remains. It is obvious that any portion of the earth's surface covered by stratified rocks must have been at some time under water, the bottom of a lake or ocean. If now this portion shows a series of layers or strata of different kinds of sedimentary rocks, it is evident that it must have been under water several times, or at least under different conditions. It is also evident that fossils found in this portion of the earth will contain remains of only those animals which were living at the various times this portion of the earth was under water. Of the animals which lived on it when it was land there will be no trace, except, possibly, a few land or fresh-water forms, which might be swept into the sea or might be preserved in the mud of ponds.

[graphic]

Fig. 176.-Restoration of Dimorphodon macronyr. (After Seeley.)

That is, instead of finding in the stratified rocks of any portion of the earth remains of all the animals which have lived on that portion since the earth began, we shall find, at best, only remains of a few kinds of those animals which have lived on this portion of the earth when it was covered by the ocean or by a great lake. Thus, the great body of fossil remains of animals reveal only a broken and incomplete history of the animal life of the past. But the record, so far as it goes, is an absolutely truthful one,

[graphic]

and when the many deposits of fossils in all parts of the different continents are examined and compared, it is possible to state numerous general truths in regard to past life and the succession of animals in time. The science of extinct life is known as paleontology.

The study of paleontology has revealed much of the history of the earth and its inhabitants from the first rise of the land from the sea till the present era. This whole stretch of time

-how long no one can guess—is divided into eras or ages; these ages usually into lesser divisions called periods, and the periods into shorter lengths of time called epochs. Each epoch is more or less sharply distinguished from every other by the different species of animals and plants which lived while its rocks were being deposited. In the earth's crust, where it has not been distorted by foldings and breaks, the oldest stratified rocks lie at the bottom of the series, and the newest at the top. The fossils found in the lowest or oldest rocks represent, therefore, the oldest or earliest animals, those in the upper or newest rocks the newest or latest animals.

An examination of a whole series of strata and their fossils shows that what we call the most specialized or most highly organized animals did not exist in the earliest epochs of the earth's history, but that the animals of these epochs were all of the simpler or lower kinds. For example, in the earlier stratified rocks there are no fossil remains of the backboned or vertebrate animals. When the vertebrates do appear, through several geological epochs they are fishes only, members of the lowest group of backboned animals. More than this, they represent generalized types of fishes which lack many of the special adaptations to marine life that modern fishes show. For this reason they bear a greater resemblance to the earlier reptiles than do the fishes of to-day; in other words, they were a generalized type, showing the beginnings of characters of their own and other types. It is always through generalized types that great classes of animals approach each other.

In a later epoch the batrachians or amphibians appeared; in a still later period, the reptiles; and last of all, the birds and the mammals, the last being the highest of the backboned animals. The following table gives the names and succession of the various geological periods, and indicates briefly some of the kinds of animals living in each. In each of these di

ERAS OR
PERIODS.

AGES OR SYSTEMS.

ANIMALS ESPECIALLY CHARACTERISTIC

OF THE ERA OR AGE.

Cenozoic.

Era of Mammals.

Quaternary or Pleis

| Man; mammals, mostly of spetocene (age of man and insects).......

cies still living Tertiary: Pliocene,

Mammals abundant; belonging to Miocene, Eocene....

numerous extinct families and orders.

Cretaceous..

Mesozoic.

Era of Reptiles.

Jurassic......

Birdlike reptiles; flying reptiles;

toothed birds; first snakes; bony fishes abound; sharks again

numerous. First birds; giant reptiles; ammo

nites; clams and snails abun

dant. First mammals (a marsupial);

sharks reduced to few forms; bony fishes appear.

Triassic ......

[blocks in formation]

(Earliest of true reptiles. Am

phibians; lung fishes; fringe fins: first cray fishes; insects abundant; spiders; fresh-wa

ter mussels. ( First amphibian (froglike ani

mals); sharks; ostracophores; first land shells (snails); mol

lusks abundant; first crabs. | First truly terrestrial or air

breathing animals; first insects; corals abundant; mailed

fishes. | First known fishes, ostraco

phores, mailed and with cartilaginous skeleton; brachio

pods; trilobites, mollusks, etc. Invertebrates only.

Silurian (age of inver-
tebrates) .....

Ordovician or Lower

Silurian .....
Cambrian.....

Archean. - - -

Algonkian. Lauren- Simple marine invertebrates.
tian..........

visions of geological time some one class of animals was especially numerous in species, and was evidently the dominant group of animals through that period. The different ages are therefore spoken of in terms of the prevailing life. Thus, the “Silurian Age" is known as the age or era of invertebrates; the “Devonian,” as the age of fishes. In the same way we have the “Reptilian Age," the “Mammalian Age,” according

to the great class of animals predominating at that time. Of course, in each of the later epochs there lived animals representing the principal classes or groups in all of the preceding ones, as well as the animals of that particular group which may have first appeared in this epoch, or was its dominant group.

Fig. 177.-Restoration of Dimorphodon macronyr, showing probable wings.

(After Seeley.)

In the study of fossils not only is it necessary for us to consider the actual forms and structures and the species they represent, but we should so far as possible reconstruct the conditions under which the organisms were alive, and the threads of genealogy which connect those of one period with those which precede or follow them. By such studies as these we are brought close to a consideration of the method of creation, and to a knowledge not only of the origin of species but to the causes underlying the divergence of the great trunks of animal and plant life.

“In youth,” says Dr. A. S. Packard, "the older naturalists of the present generation were taught the doctrine of creation by sudden, cataclysmal, mechanical creative acts, and those to whose lot it fell to come into contact with the ultimate facts and principles of the new biology had to unlearn this view, and gradually to work out a larger, more profound, wider reaching and more philosophic conception of creation."

An early paleontologist, Dr. A. Gaudry, utters these suggestive words:

“We cannot refrain from looking with curious admiration upon the innumerable creatures that have become preserved to us from

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