Shall we ever be able to understand that? Or will organic systematics always remain empirical classification? We cannot answer this question. If we could, indeed, we should have what we desire! As simple relations of space are certainly not the central point of any problematic rational organic systematics even of the future, the question arises, whether there could be found any principle of another type in the realm of synthetic *a priori* judgments which could allow an inherent sort of evolution of latent diversities, as do all judgments about spatial symmetry. At the end of the second course of these lectures, which is to be delivered next summer, we shall be able to say a few more words about this important point. The concept of what is called “a type,” due almost wholly to Cuvier and Goethe, is the most important of all that classification has given to us. Hardly second in importance is the discovery of the “correlation of parts,” as a sort of connection which has the character of necessity without being immediately based upon causality. Rádl seems to be the only modern author who has laid some stress on this topic. The harmony which we have discovered in development is also part of this correlation. When, later on, we come to discuss analytically our well established entelechy as the ultimate basis of individual organisation, we shall be able to gain more satisfactory ideas with respect to the meaning of the non-causal but necessary connection, embraced in the concepts of type and of correlation of parts. The type is a sort of irreducible arrangement of different parts; the correlation deals with the degree and the quality of what may be called the actual make of the parts, in relation to one another: all ruminants, for instance, are cloven-footed, the so-called dental formulae are characteristic of whole groups of mammals. Of course all such statements are empirical and have their limits: but it is important that they are possible.[142] [142] Recent years have created the beginnings of a systematics based on chemical differences of metabolism and its products: such differences in fact have been found to go hand in hand with diversities of the type in some cases (v. Bunge, Przibram, etc.). It has been the chief result of comparative embryology to show that the type as such is more clearly expressed in developmental stages than it is in the adults, and that therefore the embryological stages of different groups may be very much more similar to each other than are the adults: that is the truth contained in the so-called “biogenetisches Grundgesetz.” But the specific differences of the species are not wanting in any case of ontogeny, in spite of such similarities in different groups during development.