LECTURE IX

NATURAL SELECTION, AND THE ANTIQUITY OF LIFE

IN the "Origin of Species" Darwin says that the sudden appearance of species belonging to several of the main divisions of the animal kingdom in the lowest known fossiliferous rocks is at present inexplicable, and may be truly urged as a valid objection to his views.

If his theory be true, he says that "it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed, as long as, or probably far longer than, the whole interval from the Cambrian age to the present day, and that during these vast periods the world swarmed with living creatures. Here," he says, "we encounter a formidable objection; for it seems doubtful whether the earth, in a fit state for the habitation of living creatures, has lasted long enough. To the question why we do not find such fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system I can give no satisfactory answer."

On its geological side this difficulty is even greater than it was in Darwin's day, for we now know that the fauna of the Lower Cambrian was rich and varied; that most of the modern types of animal life were represented in the oldest fauna which has been discovered, and that all its types have modern representatives. The paleontological side of the subject has been ably summed up by Walcott in an interesting memoir on the oldest fauna which is known to us from fossils, and his collection of one hundred and forty-one American species from the Lower Cambrian is distributed over most of the marine groups of the animal kingdom, and, except for the absence of the remains of vertebrated animals, the whole province of animal life is almost as completely covered

by these one hundred and forty-one species as it could be by a collection from the bottom of the modern ocean. Four of the American species are sponges, two are hydrozoa, nine are actinozoa, twenty-nine are brachiopods, three are lamellibranchs, thirteen are gasteropods, fifteen are pteropods, eight are crustacea, fifty-one are trilobites, and trails and burrows show the existence of at least six species of bottom forms, probably worms or crustacea. The most notable characteristic of this fauna is the completeness with which these few species outline the whole fauna of the modern sea-floor. Far from showing us the simple unspecialized ancestors of modern animals, they are most intensely modern themselves in the zoological sense, and they belong to the same order of nature as that which prevails at the present day.

The fossiliferous beds of the Lower Cambrian rest upon beds which are miles in vertical thickness, and are identical in all their physical features with those which contain this fauna. They prove beyond question that the waters in which they were laid down were as fit for supporting life at the beginning as at the end of the enormous lapse of time which they represent, and that all the conditions have since been equally favorable for the preservation and the discovery of fossils. Modern discovery has brought the difficulty which Darwin points out into clearer view, but geologists are no more prepared than he was to give a satisfactory solution, although I shall now try to show that the study of living animals in their relations to the world around them does help us, and that comparative anatomy and comparative embryology and the study of the habits and affinities of organisms tell us of times more ancient than the oldest fossils, and give a more perfect record of the early history of life than paleontology.

While the history of life as told by fossils has been slow and gradual, it has not been uniform, for we have evidence of the occurrence of several periods when modification was comparatively. rapid.

We are living in a period of intellectual progress, and among terrestrial animals cunning now counts for more than size or strength, and fossils show that, while the average size of mammals has diminished since the Middle Tertiary, the size of their brains has increased more than one hundred per cent; that the

brain of a modern mammal is more than twice as large, compared with its body, as the brain of its ancestors in the Middle Tertiary. Measured in years the Middle Tertiary is very remote, but it is very modern compared with the whole history of the fossiliferous rocks, although more of brain development has been effected in this short time than in all preceding time from the beginning.

The later paleozoic and early secondary fossils mark another period of rapid change, when the fitness of the land for animal life, and the presence of land plants, brought about the evolution of terrestrial animals.

I shall give reasons for seeing, in the Lower Cambrian, another period of rapid change, when a new factor-the discovery of the bottom of the ocean-began to act in the modification of species, and I shall try to show that, while animal life was abundant long before, the evolution of animals likely to be preserved as fossils took place with comparative rapidity, and that the zoölogical features of the Lower Cambrian are of such a character as to indicate that it is a decided and unmistakable approximation to the primitive fauna of the bottom, beyond which life was represented only by minute and simple surface animals not likely to be preserved as fossils.

Nothing brings home more vividly to the zoölogist a picture of the diversity of the Lower Cambrian fauna and of its intimate relation to the fauna on the bottom of the modern ocean than the thought that he would have found on the old Cambrian shore the same opportunity to study the embryology and anatomy of pteropods and gasteropods and lamellibranchs, of crustacea and medusæ, echinoderms and brachiopods, that he now has at a marine laboratory; that his studies would have followed the same lines then that they do now, and that most of the record of the past which they make known to him would have been ancient history then. Most of the great types of animal life show by their embryology that they run back to simple and minute ancestors which lived at the surface of the ocean, and that the common meeting point must be projected back to a still more remote time, before these ancestors had become differentiated from each other.

After we have traced each great line of modern animals as far backward as we can through the study of fossils, we still find these

lines distinctly laid down. The Lower Cambrian crustacea, for example, are as distinct from the Lower Cambrian echinoderms or pteropods or lamellibranchs or brachipods as they are from those of the present day, but zoology gives us evidence that the early steps in the establishment of these great lines were taken under conditions which were essentially different from those which have prevailed, without any essential change, from the time of the oldest fossils to the present day, and that most of the great lines of descent were represented in the remote past by ancestors, which, living a different sort of life, differed essentially, in structure as well as in habits, from the representatives of the same types which are known to us as fossils.

In the echinoderms we have a well-defined type represented by abundant fossils, very rich in living forms, very diversified in its modifications, and therefore well fitted for use as an illustration. This great stem contains many classes and orders, all constructed on the same plan, which is sharply isolated and quite unlike the plan of structure in any other group of animals. All through the series of fossiliferous rocks echinoderms are found, and their plan of structure is always the same. Paleontology gives us most valuable evidence regarding the course of evolution within the limits of a class, as in the crinoids or the echinoids; but we appeal to it in vain for light upon the organization of the primitive echinoderm or for connecting links between the classes. To our questions on these subjects, and on the relation of the echinoderms to other animals, paleontology is silent, and throws them back upon us as unsolved riddles.

The zoologist unhesitatingly projects his imagination, held in check only by the laws of scientific thought, into the dark period before the times of the oldest fossils, and he feels absolutely certain of the past existence of a stem from which the classes of echinoderms have inherited the fundamental plan of their structure. He affirms with equal confidence that the structural changes which have separated this ancient type from the classes which we know from fossils are very much more profound and extensive than all the changes which each class has undergone from the earliest paleozoic times to the present day. He is also disposed to assume, but, as I shall show, with much less reason, that the amount of

change which structure has undergone is an index to the length of time which the change has required, and that the period which is covered by the fossiliferous rocks is only an inconsiderable part of that which has been consumed in the evolution of the echinoderms.

The zoologist does not check the flight of his scientific imagination here, however, for he trusts implicitly to the embryological evidence which teaches him that still farther back in the past all echinoderms were represented by a minute floating animal which was not an echinoderm at all in any sense except the ancestral one, although it was distinguished by features which natural selection has converted, under the influence of modern conditions, into the structure. of echinoderms. He finds in the embryology of modern echinoderms phenomena which can bear no interpretation but this, and he unhesitatingly assumes that they are an inheritance which has been handed down from generation to generation through all the ages from the prehistoric times of zoology. Other groups tell the same story with equal clearness. A lingula is still living in the sand-bars and mud-flats of the Chesapeake Bay under conditions which have not effected any essential change in its structure since the time of the Lower Cambrian. Who can look at a living lingula without being overwhelmed by the effort to grasp its immeasurable antiquity; by the thought that while it has passed through all the chances and changes of geological history, the structure which fitted it for life on the earliest paleozoic bottom is still adapted for a life on the sands of the modern sea-floor?

The everlasting hills are the type of venerable antiquity; but lingula has seen the continents grow up, and has maintained its integrity unmoved by the convulsions which have given the crust of the earth its present form.

As measured by the time-standards of the zoologist lingula itself is modern, for its life history still holds locked up in its embryology the record, repeated in the development of each individual, of a structure and a habit of life which were lost in the unknown past at the time of the Lower Cambrian, and it tells us vaguely but unmistakably of life at the surface of the primitive ocean at a time when it was represented by minute and simple floating

ancestors.