But we thus far have reached only negative results. Is the question necessarily to remain at this point, which could hardly be said to be very satisfying; or could we perhaps get better, that is, positive results about inheritance by a change of our analytic methods? Let us try to analyse the facts that occur in inheritance instead of beginning with hypotheses which claim to be complete explanations. Perhaps we shall gain, if but small, yet certainly fixed results by an analysis which goes from the facts to the theory and not from the theory to the facts.
Let the discussions that are to follow be placed upon a basis as broad as possible.
Our studies of morphogenetic restitution have shown us that besides the harmonious-equipotential systems another and widely different type of morphogenetic “systems” (i.e. unities consisting of elements equal in morphogenetic faculty) may also be the basis of restitution processes. Whilst in the harmonious system the morphogenetic acts performed by every single element in any actual case are single acts, the totality of all the single acts together forming the harmonious whole, in the other type of systems now to be examined, complex acts, that is, acts which consist of a manifoldness in space and in time, can be performed by each single element, and actually are performed by one or the other of them. We therefore have given the title of “complex-equipotential systems” to the systems in question, as all our denominations are based on the concept of the prospective morphogenetic potency, that is of the possible fate of the elements.
The cambium of the Phanerogams may be regarded as the very type of a complex-equipotential system, promoting restitution of form. It runs through the whole stem of our trees, in the form of a hollow tube, placed between the inner and the outer cell-layers of the stem, and either branch or root may originate from any single one of its cells, just as circumstances require. We might call the cambium a system of the “complex” type of course, even if every one of its constituents were able to form only a root or only a branch by way of restitution. But in fact one and the same element can form both of these complex-structures; it depends only on its relative position in the actual part of the stem isolated for the purposes of experiment, what will be accomplished in every case. Here we have a state of affairs, which we shall encounter again when studying regeneration in animals: every element of the system may be said to contain potencies for the “ideal whole,” though this ideal whole will never be realised in its proper wholeness.[124]
But there is no need to recur to the “ideal whole” in many other cases of adventitious restitution in plants. On isolated leaves of the well-known begonia, a whole plant, containing all the essential parts, may arise from any single cell[125] of the epidermis, at least along the veins, and in some liverworts it has been shown by Vöchting, that almost every cell of the whole is able to reproduce the plant, as is also the case in many algae.
In the animal kingdom it is chiefly and almost solely the phenomena of regeneration proper which offer typical instances of our systems, since adventitious restitution, though occurring for instance in the restitution of the lens of vertebrates from the iris, and though connected also with the events in regeneration proper,[126] is of but secondary importance in animal restitution, at least, if compared with restitution in plants. If we study the regeneration of a leg in the common newt, we find that it may take place from every section, the point of amputation being quite at our choice. Without regarding here the exact order of the regeneration phenomena, which is almost unknown at present, we in any case can say without any doubt that the line of consecutive possible cross-sections forms a complex-morphogenetic system, as every one of them is able to give rise to a complex organ, viz. the foot and part of the leg. It is an open question whether this complex system is to be called “equipotential” or not. It indeed seems to be inequipotential at the first glance, for each single section has to form a different organogenetic totality, namely, always that specific totality which had been cut off; but if we assume hypothetically that the real “Anlage” which is produced immediately by the cells of the wounded surface is the very same for all of them, and that it is the actual state of organisation which determines to what result this Anlage is to lead,[127] we may say that the series of consecutive cross-sections of a newt’s leg does form a morphogenetic system of the complex-equipotential type, promoting secondary regulations of form.
Now all these difficulties vanish, if we consider the regeneration of animals, such for instance as many worms of the annelid class or our familiar ascidian Clavellina, in which regeneration in both directions is possible. The wound at the posterior end of the one half which results from the operation forms a posterior body half, the wound at the anterior end of the other half forms an anterior one. Again, it is the ideal whole which we meet here: each section of the body indeed may be said to contain the potencies for the production of the totality, though actually this totality is always realised by the addition of two partial organisations. The title of complex-equipotential systems thus seems to be fully justified as applied to the systems which are the basis of regeneration: each section of the regenerating body may in fact produce the same complex whole, or may, if we prefer to say so, at least prepare the ground for that complex Anlage, out of which the complex totality is actually to arise, in the same manner.
It often occurs in science, that in rather strange and abnormal conditions something becomes apparent which might have been found everywhere, which is lying before our eyes quite obviously. Are we not in just such a condition at present? In order to study the complex-equipotential systems, we turn to the phenomena of regeneration and of restitution in general; we occasionally even introduce hypotheses to render our materials more convenient for our purposes; and all the time there is one sort of complex-equipotential system in the body of every living being, which only needs to be mentioned in order to be understood as such, and which indeed requires no kind of preliminary discussion. The system of the propagation cells, in other words the sexual organ, is the clearest type of a complex-equipotential system which exists. Take the ovary of our sea-urchin for instance, and there you have a morphogenetic system every element of which is equally capable of performing the same complex morphogenetic course—the production of the whole individual.
Further on we shall deal exclusively with this variety of our systems, and in doing so we shall be brought back to our problem of heredity. But it had its uses to place our concept of the complex-equipotential system upon such a broad basis: we at once gave a large range of validity to all that is to follow—which, indeed, does not apply to inheritance alone, though its significance in a theory of heredity may be called its most important consequence.
The Second Proof of Life-Autonomy. Entelechy at the Bottom of Inheritance