As regards the choice of topics, I have given prominence to discoveries of facts only when they could be shown to have promoted the development of the science; on the other hand, I have made it my chief object to discover the first dawning of scientific ideas and to follow them as they developed into comprehensive theories, for in this lies, to my mind, the true history of a science. But the task of the historian of Botany, as thus conceived, is a very difficult one, for it is only with great labour that he succeeds in picking the real thread of scientific thought out of an incredible chaos of empirical material. It has always been the chief hindrance to a more rapid advance in botany, that the majority of writers simply collected facts, or if they attempted to apply them to theoretical purposes, did so very imperfectly. I have therefore singled out those men as the true heroes of our story who not only established new facts, but gave birth to fruitful thoughts and made a speculative use of empirical material. From this point of view I have taken ideas only incidentally thrown out for nothing more than they were originally; for scientific merit belongs only to the man who clearly recognises the theoretical importance of an idea, and endeavours to make use of it for the promotion of his science. For this reason I ascribe little value, for instance, to certain utterances of earlier writers, whom it is the fashion at present to put forward as the first founders of the theory of descent; for it is an indubitable fact that the theory of descent had no scientific value before the appearance of Darwin’s book in 1859, and that it was Darwin who gave it that value. Here, as in other cases, it appears to me only true and just to abstain from assigning to earlier writers merits to which probably, if they were alive, they would themselves lay no claim. J. SACHS. WÜRZBURG, _July 22, 1875_. THE AUTHOR’S PREFACE _To the English translation of the History of Botany of Julius von Sachs._