In granting his permission for the translation of his little book, Dr. Fritz Müller kindly offered to send some emendations and additions to certain parts of it. His notes included many corrections of printers' errors, some of which would have proved unintelligible without his aid, some small additions and notes which have been inserted in their proper places, and two longer pieces, one forming a footnote near the close of Chapter 11, the other at the end of Chapter 12, describing the probable mode of evolution of the Rhizocephala from the Cirripedia. Of the execution of the translation I will say but little. My chief object in this, as in other cases, has been to furnish, as nearly as possible, a literal version of the original, regarding mere elegance of expression as of secondary importance in a scientific work. As much of Dr. Müller’s German does not submit itself to such treatment very readily, I must beg his and the reader’s indulgence for any imperfections arising from this cause. W.S.D. LONDON, 15_th February_, 1869. AUTHOR’S PREFACE It is not the purpose of the following pages to discuss once more the arguments deduced for and against Darwin’s theory of the origin of species, or to weigh them one against the other. Their object is simply to indicate a few facts favourable to this theory, collected upon the same South American ground, on which, as Darwin tells us, the idea first occurred to him of devoting his attention to “the origin of species,—that mystery of mysteries.” It is only by the accumulation of new and valuable material that the controversy will gradually be brought into a state fit for final decision, and this appears to be for the present of more importance than a repeated analysis of what is already before us. Moreover, it is but fair to leave it to Darwin himself at first to beat off the attacks of his opponents from the splendid structure which he has raised with such a master-hand. F.M. DESTERRO, 7_th September_, 1863. HISTORY OF CRUSTACEA CHAPTER I. INTRODUCTORY. When I had read Charles Darwin's book ‘On the Origin of Species,’ it seemed to me that there was one mode, and that perhaps the most certain, of testing the correctness of the views developed in it, namely, to attempt to apply them as specially as possible to some particular group of animals. such an attempt to establish a genealogical tree, whether for the families of a class, the genera of a large family, or for the species of an extensive genus, and to produce pictures as complete and intelligible as possible of the common ancestors of the various smaller and larger circles, might furnish a result in three different ways.