impossible.[210] Still more important than these observations themselves is the great truth he derives from them—since rediscovered and worked out in detail by Von Baer—that in the development of each individual the generic characters make their appearance before the specific characters.[211] Nor is this a mere accidental or isolated remark, but, as we shall show in the next chapter, intimately connected with one of the philosopher’s metaphysical theories. Although not an evolutionist, he has made other contributions to biology, the importance of which has been first realised in the light of the evolution theory. Thus he notices the antagonism between individuation and reproduction;[212] the connexion of increased size with increased vitality;[213] the connexion of greater mobility,[214] and of greater intelligence,[215] with increased complexity of structure; the physiological division of labour in the higher animals;[216] the formation of heterogeneous organs out of homogeneous tissues;[217] the tendency towards greater centralisation in the higher organisms[218]—a remark connected with his two great anatomical discoveries, the central position of the heart in the vascular system, and the possession of a backbone by all red-blooded animals;[219] the resemblance of animal intelligence to a rudimentary human intelligence, especially as manifested in children;[220] and, finally, he attempts to trace a continuous series of gradations connecting the inorganic with the organic world, plants with animals, and the lower animals with man.[221] The last mentioned principle gives one more illustration of the distinction between Aristotle’s system and that of the evolutionist, properly so called. The continuity recognised by the former only obtains among a number of coexisting types; it is a purely logical or ideal arrangement, facilitating the acquisition and retention of knowledge, but adding nothing to its real content. The continuity of the latter implies a causal connexion between successive types evolved from each other by the action of mechanical forces. Moreover, our modern theory, while accounting for whatever is true in Aristotle’s conception, serves, at the same time, to correct its exaggeration. The totality of existing species only imperfectly fill up the interval between the highest human life and the inorganic matter from which we assume it to be derived, because they are collaterally, and not lineally, related. Probably no one of them corresponds to any less developed stage of another, although some have preserved, with more constancy than others, the features of a common parent. In diverging from a single stock (if we accept the monogenetic hypothesis,) they have become separated by considerable spaces, which the innumerable multitude of extinct species alone could fill up.