In the first edition the principle was raised of individual bodies being alone the object of Natural History, and that in the next place they are to be arranged according to the combination of their organs or component parts, and by no means after the division or mere form of a single organ; that, for example, a special organ or anatomical system lies as the basis of each Vegetable and Animal class, and that there must be therefore as many classes, and no more, as there are cardinal organs present upon which to found them. On that account it was absolutely necessary first of all to find out these cardinal organs, and determine their rank; and, in so doing, it was shown that organs and classes are at bottom of one kind, and that the development by stages or degrees of the embryo is the antetype of that of the classes; furthermore, that each class takes its starting-point from below, and consequently that the classes do not stand simply one above the other, but fall into a series of mutually parallel ranks. Now it is this which, along with the doctrine of the infusorio-vesicular form of the organic mass, and that touching the signification of parts, as to how e. g. the blossom is the repetition of the vegetable axis or trunk, the cephalic bones that of the vertebræ, the feet of the branchiæ, and the maxillæ in turn of the feet, appears to me the cardinal point attained in my Philosophy of Nature; more especially, because it was these very doctrines which were first of all, i. e. before all the others, comprehended and almost universally adopted. The inorganic matters and activities pass, however, parallel also to the anatomical formations and functions; and that this is the case too with the spiritual or psychical functions the philosophy of the future will probably be in the condition to point out. The reader will not expect to find that the serial arrangement of Plants and Animals, with their parallelism, has been in every instance thoroughly attained. The present is but a sample of how we are to proceed in our desire of obtaining a Natural system. With such an attempt one has something to change every year, and I have in the present translation made some alterations in respect to the Mollusca and Fishes. In this sense then it is my wish that the book may be regarded, and accordingly received with its due amount of indulgence. LORENZ OKEN. PREFACE.