PREFACE was affirmed a few years ago, by one of the most eminent of living biologists, that it is no time to discuss the origin IT of the Mollusca or of Dicotyledons, while we are not even sure how it came to pass that rimula o bconica has in twenty-five years produced its abundant new forms almost under our eyes. To this statement I venture to demur. I yield to none in my admiration for the results achieved by the analytical methods introduced by Mendel, and I do not doubt the possibility that the d ct experimental study of variations and their inheritance may eventually play a large part in bringing the tangled . problems of evolution into the full daylight for which we all hope. But this is no reason for condemning those countless uncharted routes which may lead, even if circuitously, to the same goal. Any step towards the solution of the essentially historical problems of Botany-for example those concerned with the origin and development of such morphological groups as the Dicotyledons, or of such biological groups as the Aquatic Angiosperms-must necessarily contribute some mite to our conceptions of the course of evolution. These less direct I methods of approaching the central problem of biology may perhaps, at the best, bring only a faint illumination to bear upon it, but in the deep obscurity involving all evolutionary thought at the present time, we cannot afford to despise the feeblest rush-light even the glimmering of a glow-worm may at least enable us to read the compass, and learn in which direction to expect the dawn. I approached the study of Water Plants with the hope that the consideration of this limited group might impart some degree of precision to my own misty ideasof evolutionary processes, Botanists seem to be universally agreed that the Aquatic Angiosperms are derived from terrestrial ancestors, and have adopted the water habit at various times subsequent to their first appearan as Flowering Plants. The. hydrophytes thus present the great advantage to the student, that they form a group for whose history there is a generally accepted foundation. Throughout the present study I have constantly borne phylogenetic questions in mind, and the first three Parts of this book may be regarded as a clearing of the ground for the more theoretic considerations concerning the evolutionary history of water plants to which the Fourth Part is mainly devoted. In that section of the book, and sporadically in the earlier chapters, I have set down such speculations as have been borni in upon me in the course of a study of water plants with which I have been occupied more or less continuously for the last ten years. The literature relating to Aquatic Angiosperms has now grown to such formidable proportions that I have felt the necessity of trying to provide some clue to the labyrinth. With this end in view I have given a bibliography of the principal sources, which includes a brief indication of the nature and scope of each work, with page numbers showing where it is cited in the text. For the convenience of those seeking information about any particular plant, I have indexed the families and genera named in the titles enumerated, and in the notes regarding the contents of each memoir...