60 THE GREAT AGE OF THE EARTH. millions of years, rain must have fallen upon its surface to effect the present degree of denudation. The formation of coal and of coral, of stalactites and stalagmites, furnish indisputable evidence of the lapse of ages upon ages of time. "Men are in the habit of measuring the greatness and the wisdom of the universe by the duration and the profit which it promises to their own race; but the past history of the earth already shows what an insignificant moment the duration of the existence of our race upon it constitutes. A Nineveh vessel, a Roman sword awakens in us the conception of gray antiquity. What the museums of Europe show of the remains of Egypt and Assyria we gaze upon with silent astonishment, and despair of being able to carry our thoughts back to a period so remote. Still must the human race have existed for ages and multiplied itself before the pyramids of Nineuch could have been erected. We estimate the duration of human history at 6000 years; but immeasurable as this time may appear to us, what is it in comparison with the time during which the earth carried successive series of rank plants and mighty animals and no men; during which in our neighborhood the amber tree bloomed and dropped its costly gum on the earth and in the sea; when in Siberia, Europe and North America groves of tropical palms flourished; where gigantic lizards, and after them elephants, whose mighty remains we still find buried in the earth, found a home? * And the time during which the earth generated organic beings is again small when we compare it with the ages during which the world was a ball of fused rocks. For the duration of its cooling from 2000° to 200° centigrade, the experiments of Bishop upon basalt show about 350 millions of years necessary. And with regard to the time during which the first nebulous mass condensed into our planetary system our most daring conjectures must cease. The history of man, therefore, is * * GRADATIONAL EVOLUTION OF FORM. 61 but a short ripple in the ocean of time" (Helmholtz). It is only necessary to appreciate the great age of the earth to understand the might of the slow and silent changes in perpetual operation, and to realise the truth of the remark that "changes which are rare in time are frequent in eternity." It was this revalation of the order, instead of disorder, observed in the history of the development of the inorganic world that enabled palæontologists to disclose in fossils the Gradational Evolution of Forms of Life. It is only within the past twenty years that proof of this fact has received its highest confirmation in the accumulation of evidence of the existence of man in ages antedating any historical record. It may be stated as now beyond question that "not only man, but, what is more to the purpose, intelligent man, existed at times when the whole physical conformation of the country was totally different from that which characterises it now." And the fact itself that the difference is so slight between the most ancient remains of human forms as yet discovered-the celebrated Neanderthal skull for instance-and the forms now upon the surface of the earth, is most conclusive evidence of the time required to effect any very great change in the physical conformation and constitution of our earth. The horse, which now exists is the same in all essential regards as the horse of the period of time referred to, but is a very different animal from the horse of a much more ancient period. It was the recognition of the eras of time necessary to effect radical changes in the structure of animal forms under the slow agencies of natural selection that forced upon scientists the conviction that no series of sudden catastrophes or convulsions have marked or marred the even course of nature. As Mr. Huxley has remarked: "Catastrophic paleontologists are now practically extinct." It may 62 GRADATIONAL EVOLUTION OF FORM. not be said, indeed, that our record is in all respects complete. When we consider the disturbing elevations and depressions to which all parts of the earth have been repeatedly subjected, the very small extent of surface as yet explored-not the one-thousandth part of the whole -the metamorphic changes which heat has induced in the lowest strata of rock, and when we recall the perishability of intermediate forms, and of all except the hardest parts of all forms, we cease to wonder at the "missing links." Many of the specimens still preserved are in sadly mutilated state, like the wounded in the broken ranks at the roll call after battle. But the losses are compensated in some degree by the skill of the interpreters. Some of you are doubtless familiar with the story of Zadig in the Romances of Voltaire. He was a youth who had "chiefly studied the properties of plants and animals, and soon acquired a sagacity that made him discover a thousand differences, when other men see nothing but uniformity." The light and long furrows impressed upon the sand between the marks of the paws, revealed to him that the animal escaped was a bitch recently whelped; other side traces told of the hanging ears of a spaniel, and the slighter impression of one of the paws showed that the animal was a little lame. Thus also he was able to divine the height of a runaway horse, the length of his tail, the character of his shoes and bit, in a manner to astound his questioners, and, as has often happened since, under similar circumstances, to render him liable to persecution for sorcery. But how much more incredible and incomprehensible to the unlearned is the more definite and extensive knowledge afforded by a footprint to the paleontologist or comparative anatomist? "Whoso sees merely the print of a cleft foot," wrote Cuvier, "may conclude that the animal which left this impression ruminated, and this conclusion is as certain as any other in physics or morals. This footprint GRADATIONAL EVOLUTION OF FORM. 63 alone yields to him who observes it, the form of the teeth, the form of the jaws, the form of the vertebræ, the forms of all the bones of the legs, of the thighs, of the shoulders and of the pelvis of the animal which has passed by; it is asurer mark than all those of Zadig." A fossil fragment of a lower jaw directly attached to a piece of skull would enable the paleontologist as positively to assert that the animal of which the fragment was part had two occipital condyles with an ossified basi-occipital bone, had also red corpuscles in its blood and breasts to suckle its young. Thus the excavation of a few fossil bones of the leg in our country completed the line of descent of the horse, the discovery of a couple of small back teeth (of a predatory marsupial) in the Trias formation established the existence of mammals at that early period of time, and the imperfect impression from the Jura of a fossil bird (the archæopteryx) with a lizard's tail confirmed the conjecture previously made that birds were developed from lizards. Thus from a fragment of bone or a tooth, from a feather or a footprint, has been deciphered, as from ancient hieroglyphics, the gradational development of animal life. -For my part, said Mr. Darwin, in speaking of the imperfection of the geological record, for my part, following out Lyell's metaphor, I look at the geological record as a history of the world imperfectly kept and written in a changing dialect; of this history we possess the latest volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life which are entombed in our consecutive formations and which falsely appear to us to have been abruptly introduced. On this view the difficulties discussed are greatly diminished or even disappear. 64 THE EVOLUTION OF FORMS OF LIFE. LECTURE IV: THE EVOLUTION OF FORMS OF LIFE. CONTENTS. Comparative Anatomy-Of the Eye and the Ear-Order of Development-Jean Lamarck-Wilhelm von Goethe-The Intermaxillary Process-Erasmus Darwin-Anatomical Resemblances-The Hand and its Homologues-Comparative Embryology-Ernst Haeckel— The Rudimentary Organs-Bone Rudiments-Muscle RudimentsRudiments from the Digestive System-Other Rudiments-Explanations of Rudiments. To-day we approach the development of life from the standpoint of Comparative Anatomy. The most striking feature in a general survey of the forms of life is their almost infinite diversity. The mind is fairly confused in its attempt to review the vast procession of animated beings in constant defile about us, having nothing more in common, apparently, than motion or growth, reproduction or assimilation, the grosser phenomena of life. Humboldt estimated, many years ago, that there were 56,000 species of plants and 51,700 species of animals. We know now that species are numberless because they are mutable. It is comparative anatomy alone that reveals to us the similarity of structure prevading all these forms, notwithstanding their great diversity in external appearance. By the study of comparative anatomy we are thus enabled to group the forms of life into species, genera and tribes, to single out elements or types of structure from which all the various forms are modeled, however much they vary to superficial inspection, and to trace the points of resemblance to and |