Page:Popular Science Monthly Volume 73.djvu/116

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By Professor T. D. A. COCKERELL

IN attempting to trace the evolution of plants and animals, the naturalist finds himself continually regretting what is called "the imperfection of the geological record." Of all the creatures which have lived and died upon the earth, only a very small proportion have left any record in the rocks; and since the remains are widely scattered and belong to very diverse periods of time, anything like a complete consecutive series is usually unattainable. It is somewhat as though the student of languages of some future age might be obliged to depend for his knowledge of the English tongue upon small fragments of the pages of Webster's Dictionary, perhaps about an inch square for each page. He would gather his precious scraps together, and by diligently comparing them, would readily deduce a number of things about the construction of the language. He would feel able to restore, in some measure, a certain proportion of the missing words, forming derivatives according to the rules he had been able to ascertain. But how he would long for a single complete page!—for a single series actually presenting to him the different modifications and amplifications of some root in all their richness and variety.

Each year witnesses an increasing number of paleontological discoveries, so that the incomplete series in our museums are gradually becoming more complete and more representative of the actual course of evolution. In some well-known instances, such as those of the horse and elephant groups, the successive stages are now so well known that it is not very difficult for imagination to supply the connecting links; but in others the record is either a total blank or a miserable scrap merely sufficient to awaken curiosity. Take, for instance, the butterflies. According to Dr. D. Sharp, the living species of butterflies known to science number about 13,000, while it is not impossible that 30,000 or even 40,000 actually exist. Butterflies form such a large and varied group, spread over nearly every part of the world where vegetation grows, that it is certain that they have a long history behind them, and that the total number of forms which have existed must run into the hundreds of thousands. Yet the actual number of fossil butterflies so far discovered is only twenty-two! Even this meager figure is in a sense an exaggeration, inasmuch as many—indeed most—of the