the community of the life-purpose, and this idea makes very clear to us the relativity of the antithesis of unity and multeity. In the vegetable world many regard as a unit not only the cell and the whole plant, but also the branch, the shoot, the leaf, the bud. For practical reasons we may choose the single shoot which can lead an independent existence as an individual; then the single cell is only a part of this, and the plant is a colony. The difference, however, is relative. If the single cell of a higher plant cannot lead an independent existence, but must remain surrounded by other cells, neither can the offshoot without being rooted either in the plant or in the ground. All life is possible only in connexion with the natural environment; and the idea of an independent life in the whole oak-tree is just as much an abstraction as in the smallest fragment of a fallen leaf. Our modern Aristotelians lay great stress upon this, that the organic part can only arise and only exist in the organism. But there is not much to be done with the mystical dominion of the whole over the part. The separated plant-cell carries on its cell-life in fact longer than the separated heart of the frog beats. If no fresh sap comes to the cell it dies, as in the like case the whole tree dies; the shorter or longer duration depends upon the circumstances, not upon the nature of the thing. Rather should we lay stress upon this, that plants do not collect together externally from cells, that the single cells do not form themselves directly from the nutritive element and so accede to the whole, but that they always arise in other cells by means of their division. In fact, the Aristotelian principle that the whole is before the part applies for the most part, as far as we can see, to the organic world; but the circumstance that Nature so proceeds as a rule by no means entitles us to attribute an absolute universality to this principle. The mere fact even of inoculation is enough to confine it to the narrow limits of ordinary empirical principles. In the last century experiments were very

62

popular in the transfusion of blood from one animal to another, and at least partially they succeeded. In more recent times organic parts have been actually transferred from one body to another and brought to live, and yet our experimentation on this aspect of vital conditions has scarcely begun. Nay, in the lower plants we find, in fact, the fusion of two cells as well as the division, and in the lower animals the fusion of two individuals has been ascertained. The Radiopods, the descendants of the Vorticella, frequently approach each other, embrace each other, and there arises at the point of contact first flattening and then perfect fusion. A similar process of copulation occurs with the Gregarines, and even in the case of a worm, the Diplozoon, Siebold found that it arises through the fusion of two Diporpæ.63

Relative unity occurs amongst the lower animals very remarkably in those polyps which possess a common stem, on which there appears by gemmation a mass of creatures, which in a certain sense are to be regarded as independent, but in another sense only as organs of the entire stem. We are led to the supposition that in these beings even the voluntary movements are partly general, partly special in their nature; that the sensations of all these semi-independent stems stand related to each other, and yet have their separate operation too. Vogt is quite right when he calls the controversy as to the individuality of these beings a controversy as to the Kaiser's beard. "There occur gradual transitions. The individualisation step by step increases." 64

62 As is well known, these experiments have very recently been taken up again, and have repeatedly produced favourable results.

63 Comp. Vogt, Bilder aus d. Thierl., S. 124-142. The recent discoveries on this head are briefly put together in Gegenbaur, Grundz. d. vergl. Anatomie, Leipz. 1870, S. 110 ff. Here we only call attention to the fact

that (S. 112) in Actinosphaerium three individuals can unite in this manner. Comp. moreover, for the whole question, Haeckel's theory of individuality in the 'Generelle Morphologie,' i. 265 ff.

64 One of the most remarkable facts in this subject is the colonial nervous system in the Bryozoa: cf. Gegenbaur, l.c., S. 19 f.

So far in our first edition. We come back now to the notion of Species, and must first make some remarks which rest not so much on modern discoveries and observations, as on a more exact survey of the whole field, and of the principles of the struggle for existence. The first remark is this, that the notion of species, on more accurate inspection, reveals itself as a product of those times in which the attention of mankind was chiefly directed to the large or more highly organised creatures, and in which the microscope and all the infinite fulness of the lower animal and vegetable worlds were yet unknown. This becomes still plainer if we take into account, besides species, the genera, orders, and classes, which even in Linné's time appeared to embrace so admirably the entire animal world. Nowadays the whole network covers only the upper part of the animal series, and the lower we descend the more is the inquirer puzzled. A crowd of fresh marks appears, now agreeing, now crossing each other, to require even in the narrowest field a multiplicity of divisions and subdivisions, with which at the higher end of the series it was possible comfortably to embrace, e.g., the whole 'typus' of the vertebrates. While, however, on the one side downwards the wealth of forms becomes so great that no logical network of ideas suffices to embrace it, on the other side the oldfashioned criterion of common descent here becomes utterly inconceivable. When, therefore, Haeckel, in his 'Philosophy of the Sponges,' 65 develops twelve different, partly natural, partly artificial systems merely from the narrower and wider view of the notion of species, we must descry in this neither an untrustworthy playing with marks nor an isolated anomaly. If man had commenced his study of natural beings with the lower animals, the idea of species, by many held so sacred, would probably never have arisen. The view which we must now form

65 Die Kalkschwämme, eine Mono- 4 Abschn.; Philosophie der Kalkgraphie in 2 Bdn., Berl. 1872, 1 Bd., schwämme, S. 476 ff.

of the whole series of organisms is no longer that of a ladder in regular and intelligible succession from the lowest to the highest, but we have an enormous substructure to the whole system, which is still in continuous movement, and from this arise upwards the ever more firmly marked and clearly sundered forms of the higher plants and animals.

With this connects itself a second remark, which mainly applies to the higher organic forms. If, namely, we presuppose that these forms have in the course of long spaces of time so formed and marked themselves off from one another as we now see them before us, it necessarily follows from this that they must in general possess a high degree of stability, and that varieties and intermediate forms can no longer easily arise in free nature so long as the relative life-conditions of the species do not change with climate, cultivation, and other circumstances. For if we start from a condition of variability, and have the struggle for existence at work for long spaces of time, the best adapted forms must necessarily keep the ground; and, in fact, not only those which are best adapted in themselves, but also the best adapted combination of those species which, in the competition with each other, enable, as it were, the maximum of life to be maintained. Amongst the animals, for example, the hunger and the strength of the lion will bring themselves into a kind of equilibrium with the rapidity of the gazelle, with a simultaneous adaptation of both species to all other competitors for existence. This relation agrees with Fechner's 'principle of decreasing variability,' but is, as we conceive it, a simple consequence of the principles of the theory of evolution and the struggle for existence, while Fechner tries to develop a priori an entirely universal cosmical principle of this kind.66

66 Fechner's principle of the tendency to stability has a certain similarity with the way in which Zöllner,

by the aid of Schopenhauer's philosophy and the mechanical principle of the least compulsion, tries to de

The consequences of this pretty obvious remark have not always been sufficiently kept in view. Otherwise, for example, the transitional forms which evolution postulates would not have caused so much difficulty. We may regard the influence of man as a variation of the natural conditions which make existence possible for certain forms, that in a state of nature would probably soon disappear again before the older forms which had maintained themselves in the struggle for existence. As it is, however, we see how man, in the case, for instance, of pigeons and dogs, in the course of a few generations reaches new forms, which, so long as they are kept under the same protecting conditions, very speedily attain the purity and exclusiveness of a separate species, and are only in deference to theory to be called 'varieties.' 67 And this by no means happens only in the case of 'artificial' selection, which strives after a definite model, but also in the case of unconscious' selection,68 i.e., in the case of a procedure which brings a variety to the ever greater perfection and persistence of a new type, through the simple effort to keep the race pure and to develop a peculiarity, so that for the rest Nature here strives freely, as it were, after a definite model, where a halt is made. If this is once attained, it may then maintain itself unaltered for any length of time.

duce that every system of atomic vibrations in a given space has the tendency to let the number of collisions (and thus of sensation and pain) fall, to a minimum.

In the principle of tendency to stability Fechner finds at the same time the reconciliation of causation and of teleology, since on this principle the earth must necessarily approximate to a condition in which "everything harmonises as well as possible" (Einige Ideen, &c., S. 88 ff.). Fechner's idea, as well as Zöllner's, are at present but boldly hazarded metaphysical notions, which as yet entirely lack proof and demonstration. If we limit ourselves, on the contrary,

to the relative adaptation of organisms to the conditions of existence in a given extended period, then the tendency to stability follows immediately from the principle of the struggle for existence.

67 Comp. Darwin, The Variation of Animals and Plants under Domestication, i. 32. Here it is shown that the domesticated pigeons, although they all descend from a single wild species, number more than a hundred and fifty kinds, and must be divided into at least five new classes if they are to be dealt with on the same principles as the wild classes.

68 Darwin, loc. cit., i. 214.