Fig. 3.—Skeleton of a Radiolarian Very Greatly Enlarged.
II
THE EARLIEST KNOWN VERTEBRATES
"We are the ancients of the earth
And in the morning of the times."
There is a universal, and perfectly natural, desire for information, which in ourselves we term thirst for knowledge and in others call curiosity, that makes mankind desire to know how everything began and causes much speculation as to how it all will end. This may take the form of a wish to know how a millionaire made his first ten cents, or it may lead to the questions—What is the oldest animal? or, What is the first known member of the great group of backboned animals at whose head man has placed himself? and, What did this, our primeval and many-times-removed ancestor, look like? The question is one that has ever been full of interest for naturalists, and Nature has been interrogated in various ways in the hope that she might be persuaded to yield a satisfactory answer. The most direct way has been that of tracing back the history of animal life by means of fossil remains, but beyond a certain point this method cannot go, since, for reasons stated in various places in these pages, the soft bodies of primitive animals are not preserved. To supplement this work, the embryologist has studied the early stages of animals, as their development throws a side-light on their past history. And, finally, there is the study of the varied forms of invertebrates, some of which have proved to be like vertebrates in part of their structure, while others have been revealed as vertebrates in disguise. So far these various methods have yielded various answers, or the replies, like those of the Delphic Oracle, have been variously interpreted so that vertebrates are considered by some to have descended from the worms, while others have found their beginnings in some animal allied to the King Crab.
Every student of genealogy knows only too well how difficult a matter it is to trace a family pedigree back a few centuries, how soon the family names become changed, the line of descent obscure, and how soon gaps appear whose filling in requires much patient research. How much more difficult must it be, then, to trace the pedigree of a race that extends, not over centuries, but thousands of centuries; how wide must be some of the gaps, how very different may the founders of the family be from their descendants! The words old and ancient that we use so often in speaking of fossils appeal to us somewhat vaguely, for we speak of the ancient civilizations of Greece and Rome, and call a family old that can show a pedigree running back four or five hundred years, when such as these are but affairs of yesterday compared with even recent fossils.
Perhaps we may better appreciate the meaning of these words by recalling that, since the dawn of vertebrate life, sufficient of the earth's surface has been worn away and washed into the sea to form, were the strata piled directly one upon the other, fifteen or twenty miles of rock. This, of course, is the sum total of sedimentary rocks, for such a thickness as this is not to be found at any one locality; because, during the various ups and downs that this world of ours has met with, those portions that chanced to be out of water would receive no deposit of mud or sand, and hence bear no corresponding stratum of rock. The reader may think that there is a great deal of difference between fifteen and twenty miles, but this liberal margin is due to the difficulty of measuring the thickness of the rocks, and in Europe the sum of the measurable strata is much greater than in North America.