mature and reproduce themselves. In like manner peculiar conditions of existence not unfrequently produce similar changes without the co-operation of man; e.g., that one kind of frog passes through the tadpole state in the egg and jumps from the egg as a ready-made frog. In all these cases the co-operation of inner formative causes with the conditions of existence is obvious, and it cannot be denied that natural selection plays the decisive part in some of them, though in the transformation of the axolotl, which suddenly changes from a water-creature into an aircreature, there can be no question of natural selection or the struggle for existence. From the standpoint of onesided Darwinism the thing can only be explained by bringing the whole transformation under the notion of variation, and perhaps making the removal into another climate the occasion of the variation. In wild nature the new form would now have to undergo the struggle for existence, and to fix itself by breeding in before the process of forming a species would be completed. But it is very easily seen that such an extension of the notion of variation really includes everything that the champions of the law of development can require; for nobody will believe that this change is an accidental one, compared with which any other conceivable change might just as well have occurred; but we see that here a movement was made in, as it were, a predescribed course.76 The whole difficulty of understanding lies in rightly apprehending the notion of the law of development. The word sounds somewhat suspicious to many men of science, much as if we spoke of a plan of creation,' implying a succession of repeated interferences of supernatural forces. There is, however, not the least reason in the inner causes,' of which we are here speaking, to presuppose any mystical assistance to the wonted course of natural forces. So that the law of development' also, according to which organisms rise in a definite gradation, 76 Haeckel, N. Schöpfungsgesch., 4 Aufl., S. 315 f., E.T. 354. can be nothing else than the co-operation, conceived as a unity, of the universal laws of Nature in order to produce the phenomenon of development. Kölliker's 'law of development,' just as well as the numerous laws of formation which Haeckel propounds, is, logically considered, primarily only a so-called 'empirical law,' i.e., a collection, drawn from experience, of certain rules in natural phenomena, whose ultimate causes we do not yet know. We may, however, attempt to form a picture to ourselves of the true natural causes which underlie the law of development, even were it only to show that there is not the slightest occasion to take refuge in a mystical conception. Haeckel has expressed the idea that his plastid theory is to be reduced to a carbon theory, i.e., that we are to seek in carbon-of course in some way as yet completely obscure to us-for the cause of the peculiar movements which we observe in protoplasm, and which we regard as the elements of all vital phenomena. This idea does not carry us very far, but we may here employ it as a point of connexion in order to explain our idea of the nature of the law of development. If we look somewhat closely into the chemistry of carbon compounds, we find that there already exists a complete theory for the formation of organic acids, which we may very well compare with a law of development. The 'plan' of this whole development lies predescribed in the doctrine of the 'quantivalence' of atoms; and as by a fixed principle of substitution any given organic acid can, as it were, be developed onward into another, we have a possibility, running, as it seems, to infinity, of ever more complicated and ever more manifold formations before us, which, despite their enormous multitude, follow only a narrow and predescribed course. What can or can not arise is determined in advance by certain hypothetical properties of the molecules.77 77 Weihrich, Ansichten d. neueren to Kolbe's theory, on which an atom Chemie: Mainz, 1872, S. 43 f., refers of hydrogen can be replaced by me We might here break off, and simply compare the plan, known in its main features, of all possible organic substances, as an illustration, with the as yet unknown plan of all possible animal forms. We will go, however, a step farther, and refer to the connexion between the form of crystals and the mode of composition of the crystallised matter. That a similar connexion exists between matter and form even in organisms is no new idea. The analogy is obvious, and has often been employed for many purposes. That this brings us back finally to peculiarities of the molecules is very natural. For our purpose it is quite indifferent whether the form is brought into combination with a definite animal material, which has a definite position in the genealogical tree of materials, or whether it is regarded as the result of a co-operation of all the materials present in an animal body; and both may at bottom come to the same thing. It is enough to admit any kind of connexion between form and matter, and we have before us the law of development of organisms in the most palpable shape as the law of substitution of carbon compounds. Whether this be so or not, in any case this illustration will suffice to show that we need not conceive the law of development as anything supernatural or mystical, and thus the chief obstacle to the recognition of its importance will be removed. The law of development gives the possible forms; natural selection from their enormous multitude chooses the actual forms; but it can summon forth nothing that is not contained in the plan of organisms, and the mere principle of utility becomes impotent if a modification of the animal is required of it which is thyl, CH. The methyl itself contains hydrogen, for each atom of which an atom of methyl may be substituted. By such substitutions formic acid is turned into acetic acid, acetic acid into propionic acid, this into butyric acid, and so on. Of course the general idea developed in the text is independent of this special theory; but this latter shows very well what may be conceived as a law of development, so far as the more complex formations are imagined as successively arising out of the more simple. against the law of development. But this does not touch Darwin, since he chooses only what is useful amongst the spontaneously occurring variations. His doctrine is only completed in so far as we must assume that the circle of possible variations is determined by a universal law of development. We might now suppose that the assumption of such a law of development renders the theory of natural selection superfluous, since the multitude of forms must be produced in course of time without any selection. Such a view overlooks, in the first place, the enormous importance of the competition for existence, which is not a theory, but a demonstrated fact. At the same time we must maintain that the law of development, no matter what we imagine to lie behind it, is at all events not a dæmonically working power producing unconditionally the pure forms answering its requirements. If even in crystallisation, where the conditions are so much simpler, we discover the most manifold irregularities, so that the crystal of theory is strictly only an ideal, we shall easily understand in the case of organisms, that the law of development cannot prevent perturbations and malformations of all kinds, mixed forms by the side of pure ones, imperfections beside the type, although it exercises its influence upon all the forms that But if even the pure forms, according to the law of development, run into infinity, the possible number becomes very much greater through the modified forms, and yet it remains always a mere fraction of what is conceivable. Everything cannot come from everything, as even the ancient Materialists understood. Amongst this luxuriant multitude of forms comes now the struggle for existence, ordering and sifting, and establishes the equilibrium described above, which we recognised as the maximum of simultaneously possible life. Whether those forms to which natural selection finally leads, and which it renders stable, are finally at the same time the purest types according to the law of development, may remain. occur. undetermined; but at all events, we shall assume that the stability of species is the greater the more often this coincidence is attained, A more serious question which here presents itself is whether, on the assumption of a mechanically working law of development, the apparently like primitive forms of organisms, from which we deduce all living forms, are to be considered as really constituted alike or not? In putting this question, we do not wish to shake that law which the most influential representatives of the doctrine of descent declare so extremely important-the law of the agreement of ontogeny' and 'phylogeny,' as Haeckel says, or the doctrine that in each creature the stadia of its prehistory are summarily repeated in the history of its own development, especially in foetal life. We will, in the first place, only remark that this law is indeed of great heuristic importance to the theorists of the doctrine of descent, but that its necessity is precisely from the standpoint of pure Darwinism difficult to understand. Of advantage in the struggle for existence from traversing these stadia there can be no question, and the principle of heredity is not so unconditionally valid that it could explain this correspondence. It can hardly be, then, but that there are chemical and physical causes present which render it necessary to traverse these stadia, and in this there is already involved the recognition of the law of development as we conceive it. If now it is asked whether the forms which look the same or like in the first stadia of development are also really constituted alike, we may infer the contrary simply from the fact that they produce a different result. If, e.g., the embryo of the dog has a striking likeness to that of man in the fourth week, yet from the one is produced a dog and from the other a human being. It might be supposed that this not unimportant difference was only gradually developed through the one of the two like embryos being constantly nourished by the juices of a |