Imagens das páginas



This much then we have gained, that we may assert without hesitation, that all the more perfect organic natures, such as fishes, amphibious animals, birds, mammals, and man at the head of the list were all formed upon one original type which varies only more or less in parts which are none the less permanent, and which still daily changes and modifies its form by propagation.—Goethe (1796).

In a suggestive sentence, Haeckel speaks of our knowledge of the line of descent in the history of any group of animals or plants as being derived from “three ancestral documentsmorphology, embryology, and paleontology.".

Of these three, paleontology is at once the most certain and the most incomplete. Each fossil animal is a record, absolutely authentic, so far as it goes, admitting of no doubt or question, but for the most part yielding only a very little of the truth involved in its existence.

For no animal whatever is preserved as a fossil except as the result of an unusual combination of circumstances. Only those parts which are themselves hard, calcareous, silicious, or horny, with rare exceptions, can retain their form in the rocks, and even these, shells, teeth, bones, and the like, are often crushed or distorted so that their actual form or nature may be open to question. In addition, only the minutest fraction of the sedimentary rocks of the earth has been laid bare by artificial excavation or by natural erosion, and thus opened to the inspection of man, and the number of fossils actually observed can be only the most trivial fraction of a fraction of the organisms actually existing and preserved.

With all this, the human race has in the past shown a singular lack of insight in the interpretation of animal remains found in the stone. As Lyell has graphically shown, it took one hundred and fifty years of dispute and argument to persuade even learned men that shells and teeth in the rocks were actual remains of actual animals, and another hundred and fifty years to demonstrate that the shell-bearing rocks were not masses of débris from Noah's flood. Nothing in the history of science is more tedious than the arguments directed against the first students of fossils, to show that these structures were mere sports of nature, whimsicalities of creation, or freaks developed in the fatty matter (materia pinquis) of the earth by the entangling influence of the revolving stars. Notwithstanding all these defects in material, and this stupidity of theory, the study of fossils has still gone on, and by its means we are able to delineate with large certainty the line of evolution of most groups of animals, and the nature of faunal relations in the different periods of geological time. If we had not already a theory of evolution by derivation of forms, we should be obliged to invent one in face of the facts of paleontology. In Huxley's words, “fossils are only animals and plants which have been dead rather longer than those which died yesterday.” Fossils are either actual remains of bones or other parts preserved intact in soil or rocks, or else, and more commonly, parts of the animals which have been turned into stone, or of which stony casts have been made. All such remains buried by natural causes are called fossils. The process by which they are sometimes changed from animal substance into stone is called petrifaction. Fossils may be of three kinds. In the case of recently extinct animals, bones or other parts of the body may become buried in the soil and lie there for a long time without any change of organic into inorganic matter. Thus fossil insects are found with the bodies preserved intact in amber, a fossil resin from some ancient and extinct pine tree. Over eight hundred species of extinct insects are known from amber fossils. The bones of the earliest members of the elephant family, the teeth of extinct sharks, the shells of extinct mollusks and fragments of buried logs, are also often found intact, still composed of their original matter. In the second kind of fossils the original or organic matter is gone, the organic form and organic structure being preserved

Fig. 173.-Remains of 19, morphodon from the Lias of Lyme Regis, showing skull, neck, and back, and some of the bones of the skeleton. (After Seeley, from a slab in the British Museum.)


and grind the exaved min

in mineral matter. That is, the organic matter has been slowly and exactly replaced by mineral. As each particle of organic substance passed away by decay, its place was taken by a particle of mineral matter. Such fossils are called petrifactions. This is beautifully shown in the case of petrified wood. We can cut and grind thin a bit of petrified wood, and see in it, with a microscope, the exact details of its original fine cellular structure. This substituted mineral matter may be one of several minerals, but usually it is silica (quartz) or carbonate of lime (limestone) or sulphide of iron (iron pyrites). In the case of animal parts which were originally partly organic and partly inorganic, as bones and teeth and shells, often only the organic matter is replaced by the petrifying mineral, although sometimes the old inorganic matter is also replaced. Finally, sometimes the organic matter and organic structure are both lost, only the original outline of form of the whole part being retained. This occurs when the organic matter imbedded in mud and clay decays away, leaving a hollow which is filled up by some mineral different from the matrix. In this case the fossil is simply a cast of the original organic remains.

Some traces even of the finest organisms occasionally appear.

"Conditions have sometimes permitted even the most delicate structures, such as insects' wings and the impressions of jellyfishes to become retained in the soft mud, which afterwards became solidified. Localities famous the world over for the beauty and delicacy of their fossil remains are the lithographic stone quarries of Bavaria and certain beds in France” (EASTMAN).

These deposits were perhaps formed in the clear, quiet waters of a coral lagoon.

Examination and study of the rocks of the earth reveal the fact that fossils, or the remains of animals and plants, are found in certain kinds of rocks only. They are not found in lava, because lava comes from volcanoes and rifts in the earth's crust, as a red-hot, viscous liquid, which cools to form a hard rock. No animal or plant caught in a lava stream will leave any trace. Furthermore, fossils are not found in granite, nor in ores of metals, nor in certain other of the common rocks.

Many rocks are, like lava, of igneous origin; others, like granite, although not originally in melted condition, have been so heated subsequent to their formation, that any traces of animal or plant remains in them have been obliterated. Fossils are found almost exclusively in rocks which have been formed by the slow deposition in water of sand, clay, mud, or lime. The sediment which is carried into a lake or ocean by the streams opening into it sinks slowly to the bottom of the lake or ocean and forms there a layer which gradually hardens under pressure to become rock. This is called sedimentary rock, or stratified rock, because it is composed of sediment, and sediment always arranges itself in layers or strata. In sedimentary

Fig. 174.–Restoration of the skeleton of Dimorphodon macronyr. (After Seeley.)

or stratified rocks fossils are found. The commonest rocks of this sort are limestone, sandstone, and shales. Limestone is formed chiefly of carbonate of lime; sandstone is cemented sand; and shales, or slaty rocks, are formed chiefly of clay.

The formation of sedimentary rocks has been going on since land first rose from the level of the sea; for water has always been wearing away rock and carrying it as sediment into rivers, and rivers have always been carrying the worn-off lime and sand and clay downward to lakes and oceans, at the bottoms of which the particles have been piled up in layers and have formed new rock strata. But geologists have shown that in the course of the earth's history there have been great changes in the position and extent of land and sea. Sea bottoms have been folded or upheaved to form dry land, while regions, once land, have sunk and been covered by lakes and seas. Again, through great foldings in the cooling crust of the earth, which

« AnteriorContinuar »