Page:Popular Science Monthly Volume 52.djvu/568

550 are of a permanent character, begin with a broad foundation of observation. The work that begins with theory and afterward seeks for verification through observation may be brilliant and attractive, but its results are rarely of lasting value. Mr. Walcott's scientific work has followed the normal and conservative course, beginning with the prolonged collection of specimens and other facts, following with generalization, chiefly in the field of the correlation of formations, and leaving largely to the future conclusions as to sequence and genesis.

His paleontological studies have been of two classes: biological, viewing fossil organisms as members of the animal kingdom, and stratigraphical, viewing associated fossils or faunas as the representatives of contemporaneous life and the labels of synchronous formations. His biological labors include the description of a considerable number of new families, genera, and species of Palæozoic invertebrates, and an elaborate study of the structure and organization of trilobites, which has served to give them, for the first time, a definite and unquestioned position in the systematic scheme of animal forms.

The trilobites were dominant forms in early Palæozoic time, and continued, with diminishing numbers and importance, until the Carboniferous period. Exhibiting a considerable range of differentiation, they have been of great service for the classification of terranes, and the nomenclature of the Cambrian horizons has been based upon them. Nevertheless, their systematic affinities were long in doubt, because they were known only through imperfect specimens, exhibiting the dorsal armor, but showing no trace of appendages for locomotion, respiration, etc. From time to time the discovery of legs and other members had been announced and subsequently disproved, and geologists had become so skeptical as to the possibility of their determination that when traces of a leg were actually discovered by E. Billings, in 1870, little credit was given to the announcement. Here was an important biological blank to be filled, and Mr. Walcott, at the suggestion of Louis Agassiz, undertook to fill it. The examination of thousands of trilobite specimens perfect as to the carapace revealed but a few traces of organs, and it was found that those traces all came from a certain layer of Trenton limestone only a few inches in thickness. That layer was carefully quarried over a considerable area, even though it became necessary to remove several feet of superjacent strata. Several thousand complete tests were obtained from it, and two hundred and seventy of these were found to contain some of the missing members in greater or less perfection.To such specimens elaborate study was given, chiefly by means of translucent thin sections, such as are employed by the petrographer. With their aid, and through prolonged labor and study, Mr. Walcott