gists who believe with Weismann that there is a sharp distinction between the germ plasm and the somatic or body plasm, and that this germ plasm is limited to the germ cells and germ-cell producing tracts, the regeneration of a nearly whole body or even a considerable part of a body from a region which does not include a germ cell presents a serious obstacle. But

before this obstacle can be considered as one rendering the germ plasm theory absolutely untenable, it is necessary to prove what the regenerated parts are composed of. Are they composed simply of repeated similar cells, all of one tissue type, or do they include other kinds of cells or tissues than those particular kinds from which the regenerated part springs? It is, of course, admitted that many, indeed most cells of the body, can reproduce other cells like themselves. Now is it a fact that regenerated parts are composed of different kinds of cells? As a matter of fact this has been proved to be so by observation and by experiment. Numerous instances are known in which body cells arising originally from one germ layer have produced in the course of regeneration not only cells like themselves, but others which in normal development could only arise from another germ layer. So it is plain that the study of regeneration has already done much to modify our former conceptions of the factors and conditions of development.

FIG. 172.-Regeneration of the blastula and gastrulæ of sea urchins; line indicates where the blastula or gastrula was cut in half; the smaller figures show results of the regeneration of the two halves of each.

CHAPTER XIV

PALEONTOLOGY

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 pinguis) 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 Dimorphodon 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.)

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.