Whatever opinion may be held about the idea of evolution itself, or about its possible modes of operation, it cannot be denied that change with time is an all-pervading principle of the natural world. Nor can it be denied that both our knowledge of the facts of nature and our appreciation of them, are as much subject to this principle as anything else. In consequence of it understanding grows and theory changes but it is not always easy to relate the two and we must always be alert to the necessity of keeping them in step. The need for a fresh assessment more in keeping with the knowledge of the times has, indeed, become more and more pressing. Many particular problems require to be thought out again from the beginning, but the special need is for a new, objective, and sober consideration of the facts, and, above all, of the facts in some of those aspects of biology which have so far received less attention, and which are therefore less obscured by the patina of controversy. It is as a modest contribution to such a new judgment that this book, which embodies the thoughts and observations of many years, has been written, and its purpose should not, through any default or omission on the part of is author, be left open to misunderstanding. Its primary aim is to redirect attention to facts concerning one great and fundamentally important section of the living world which, at best, have become forgotten in the pursuit of other and more fashionable lines of enquiry, or, at worst, have never been fully realized at all. Its second purpose is to demonstrate that when these neglected facts are taken properly into account, some, at least, of the best-known speculations about organic evolution are seen to have a less general applicability than is usually claimed. Lastly, the book tries to summarize the particular conclusions to which a judicial consideration of the facts that it presents appears to lead. Because the book is intended to be a step towards a fresh and untrammelled outlook on some of the problems of evolution, it has been felt unnecessary, and indeed undesirable, to burden it with many references to what, with the best intentions in the world, could only be an invidious and meagre selection from the vast general literature of that subject. Instead, there have been appended to appropriate chapters short notes giving a few chosen sources from which the reader should be able to pursue their respective subjects rather further and, especially, to find more widely ranging bibliographies. The chapters, apart from the first and last, fall into three groups. The second and third form what may be looked upon as a back-ground to the flowering plants; the next three provide a review of that whole group, written in somewhat novel terms and with appropriate reference to the evolutionary problems which it presents as a whole, and the remaining five cover a small selection of the many more particular problems which make these plants of special interest in the study of evolution.
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