The author was associated with the original discovery and description of these supposed earliest traces of life; and has since, in the intervals of other work, devoted much time to further exploration and research, the results of which have been published from time to time in the form of scientific papers. He has also given attention to the later discoveries which have tended to fill up the gap between the Laurentian fossil and its oldest known successors. In 1875 he endeavoured to sum up in a popular form what was then known, in a little volume named "The Dawn of Life," which has long been out of print; and in 1893 the matter was referred to in a chapter of his work "Salient Points in the Science of the Earth." In 1895 he was invited to present the subject to a large and intelligent audience in a course of lectures delivered in the Lowell Institute, Boston; and the success which attended these lectures has induced him to reproduce them in the present work, in the hope that inquiries into the Dawn of Life may prove as fascinating to general readers as to those who prosecute them as a matter of serious work, and that their presentation in this form may stimulate further research in a field which is destined in the coming years to add new and important domains to the knowledge of life in the early history of the earth. Hypotheses respecting the introduction and development of life are sufficiently plentiful; but the most scientific method of dealing with such questions is that of searching carefully for the earliest remains of living beings which have been preserved to us in the rocky storehouses of the earth. There are many earnest labourers in this difficult field, and it will be the object of the writer in the following pages to do justice to their work as far as known to him, as well as to state his own results. J. W. D. CONTENTS I PAGE The Chain of Life Traced Backward in Geological Time 3 II Life in the Early Cambrian 17 III Pre-Cambrian Life 47 IV Foundations of the Continents, and their General Testimony as to Life 79 V Probabilities as to Laurentian Life, and Conditions of its Preservation 107 VI The History of a Discovery 125 VII The Dawn of Life 147 VIII Contemporaries of Eozoon 193 IX Difficulties and Objections 221