All organisms are endowed with the faculty of re-creating their own initial form of existence.
In words similar to these Alexander Goette, it seems to me, has given the shortest and the best expression of the fact of inheritance. Indeed, if the initial form in all its essentials is re-created, it follows from the principle of univocality, that, ceteris paribus, it will behave again as it did when last it existed.
By the fact of inheritance life becomes a rhythmic phenomenon, that is to say, a phenomenon, or better, a chain of phenomena, whose single links reappear at constant intervals, if the outer conditions are not changed.
It was first stated by Gustav Jaeger, and afterwards worked out into a regular theory by Weismann, that there is a continuity of material underlying inheritance. Taken in its literal meaning this statement is obviously self-evident, though none the less important on that account. For as all life is manifested on bodies, that is, on matter, and as the development of all offspring starts from parts of the parent bodies, that is, from the matter or material of the parents, it follows that in some sense there is a sort of continuity of material as long as there is life—at least in the forms we know of. The theory of the continuity of “germ-plasm” therefore would be true, even if germ-cells were produced by any and every part of the organism. That, as we know, is not actually the case: germ-cells, at least in the higher animals and in plants, are produced at certain specific localities of the organism only, and it is with regard to this fact that the so-called theory of the “continuity of germ-plasm” acquires its narrower and proper sense. There are distinct and specific lines of cell-lineage in ontogenesis, so the theory states, along which the continuity of germ-protoplasm is kept up, which, in other words, lead from one egg to the other, whilst almost all other lines of cell-lineage end in “somatic” cells, which are doomed to death. What has been stated here is a fact in many cases of descriptive embryology, though it can hardly be said to be more than that. But I regard it as very important that the fact of the continuity of some material as one of the foundations of inheritance has clearly been stated.
The important problem now presents itself: What is the material, the matter, which is handed down from generation to generation as the basis of inheritance? Weismann, as we know, regarded it as a very complicated structure, part of which by its disintegration became the foundation of individual embryology. We have disproved, on the authority of many facts, the latter part of this assumption; but of course the first part of it may turn out to be true in spite of this. We have no means at present to enable us to say a priori anything positive or negative about the important question of the nature of that matter, the continuity of which in inheritance is in some sense a self-evident fact, and we therefore shall postpone the answer until a later point of our analytical discussion.
One particular topic with regard to “material continuity” may, however, be stated before we proceed to any other discussion. The “continuity” must not be understood in the sense that, in a given species, exactly the same community of atoms, or electrons, or what you will, went through the whole line of generations. This is impossible for the fact of metabolism, i.e. dissimilation and assimilation. “The same” material, in the strict meaning of the word, only connects two generations in every case, and the material sameness that runs through all of them relates only to quality in general but not to a particular amount of matter as such.
So we see again (compare page 117) that the concept of the same implies a certain problem in biology, a problem that is to be discussed later.
We now begin the analytical study of inheritance, placing it upon as broad a basis 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.2
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 cell3 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 (G. Wolff), and though connected also with the events in regeneration proper,4 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. We, therefore, 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,5 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.
After we had established the concept of the harmonious-equipotential system in a former chapter, we went on to study the phenomena of the differentiation of it, and in particular the problem of the localisation of all differentiations. Our new concept of the complex-equipotential system is to lead us to an analysis of a different kind: we shall pay special attention to the origin, to the genesis of our complex systems that show equipotentiality.
If we review the process of ontogenesis, we are able to trace back every complex system to a very small group of cells, and this small group of cells again to one single cell. So in plants the cambium may be shown to have originated in a sort of tissue-rudiment, established at a very early period, and the ovary may be demonstrated to be the outcome of a group of but a few cells, constituting the first visible “Anlage” of the reproductive organs. At the end then, or from another point of view at the beginning, a single cellular element represents the very primordial egg-cell. The primordial egg-cell has undergone a long line of consecutive divisions; the single eggs are the last result of them.
We now proceed to some considerations which have a certain logical similarity to those which inaugurated our analysis of the differentiation of the harmonious-equipotential systems, though the facts in question are very different.
Viewed by itself without any kind of prepossessions, as it might be by anyone who faces a new problem with the single postulate of introducing new natural entities—to use the scholastic phrase—as little as possible, the development of the single egg might be regarded as proceeding on the foundation of a very complicated sort of machine, exhibiting a different kind of construction in the three chief dimensions of space, as does also the organism which is to be its result.
But could such a theory—irrespective of all the experimental facts which contradict it—could such a theory stand before the one fact, that there occurs a genesis of that complex-equipotential system, of which our one single egg forms a part? Can you imagine a very complicated machine, differing in the three dimensions of space, to be divided hundreds and hundreds of times and in spite of that to remain always the same whole? You may reply that during the period of cell-divisions there is still no machine, that the machine is established only after all the divisions are complete. Good; but what then constructs this machine in the definitive cells of our systems, say in the eggs? Another sort of machine perhaps? That could hardly be said to be of much use. Or that entelechy of which we have spoken? Then you would recur to our first proof of vitalism and would burden entelechy with a specific performance, that is, with the construction of the hypothetic machine which you are postulating in every single egg. But of course you would break the bounds of physics and chemistry even then.
It seems to me that it is more simple, and, so to say, more natural, not to recur to our first proof of life-autonomy in order to keep to the “machine theory” in this new branch of inquiry, but to consider facts as they offer themselves to analysis.
But then, indeed, we are entitled to draw an independent second proof of the autonomy of life from our analysis of the genesis of systems of the complex-equipotential type. We say it is a mere absurdity to assume that a complicated machine, typically different in the three dimensions of space, could be divided many, many times, and in spite of that always be the whole: therefore there cannot exist any sort of machine as the starting-point and basis of development.
Let us again apply the name entelechy to that which lies at the very beginning of all individual morphogenesis.
Entelechy thus proves to be also that which may be said to lie at the very root of inheritance,6 or at least of the outcome of inheritance; the individual formation of the next generation is shown not to be performed by a machine but by a natural agent per se.
Our second vitalistic argument is not quite without importance for the first one.
People have said occasionally that my first proof of vitalism had not sufficiently considered the fact that an harmonious-equipotential system is full of nuclei, and that these nuclei might be machines.
We have now shown that this is impossible.
But let me add another remark: even if the nuclei were “machines”, this would not suffice for a mechanistic explanation of the differentiation of an harmonious system. For the nuclei, as the experiments show, are all of the same type in such a system. Their totality as such is a sum, though (presumably) a sum of machines. But for a mechanistic explanation of harmonious differentiation we should be forced to consider the system in question as one great super-machine, embracing ex hypothesi many small machines, all of them alike. And just this super-machine, relating to the totality of the system, cannot exist. Thus even the view that the single nuclei are machines would not help us, quite apart from the fact that this view is wrong.
But what about the material continuity appearing in inheritance, which we have said to be almost self-evident, as life is only known to exist on material bodies? Is there not, in fact, a serious contradiction in admitting at the same time entelechy on the one side and a sort of material condition on the other as the basis of all that leads to and from inheritance? Not at all, so it seems to me.
Let us try to comprehend what is meant by the statement that entelechy and something material are at work in inheritance at the same time. Entelechy has ruled the individual morphogenesis of the generation which is regarded as being the starting-point for inheritance, and will rule also the morphogenesis of the generation which is to follow; entelechy determines the egg to be what it is, and the morphogenesis starting from this egg to be what it is also. Entelechy, at present, is not much more for us than a mere word, to signify the autonomous, the irreducible of all that happens in morphogenesis with respect to order, in the one generation and in the next. But may not the material continuity which exists in inheritance account perhaps for the material elements which are to be ordered? In such a way, indeed, I hope we shall be able to reconcile entelechy and the material basis of heredity. May it not be that there exist some “means” for morphogenesis, which are handed down from generation to generation always controlled by entelechy, and which constitute the real significance of the continuity of matter during inheritance?
Discoveries of the last decades show that such means of a material character, though not the foundation of that order of processes which is inherited, are nevertheless among the most necessary conditions for the accomplishment of inheritance in general. It is scarcely necessary to remind you that for very many years all concrete research on heredity proper—that is, the actual comparison of the various specific characters in the generations of the grandfather, the father, and the child—was due to Galton. You may also be aware that, in spite of Galton’s inestimable services, it was not till 1900 that one of the important principles concerned in inheritance was found independently by de Vries, Correns, and Tschermak, and that this principle happened to be one that had been discovered already, stated with the utmost clearness and precision by the Augustinian monk, Gregor Mendel, as early as 1865, though it had been completely forgotten ever since.
The so-called “rule of Mendel” is based upon experiments with hybrids, that is, with the offspring of parents belonging to different varieties; but it relates not to the characters of the generation resulting immediately from hybridisation, the “first” generation of hybrids, as we shall call it, but to the characters of that generation which is the result of crossing the hybrids with each other, provided that this leads to any offspring at all. There are many cases indeed, both amongst animals and plants, where the offspring of the hybrids, or in other terms the “second” generation, is found to consist of individuals of three different types—the mixed7 type of the hybrids themselves, and the two pure types of the grandparents. Whenever the individuals of the “second” generation are separated into these three different types, hybrids are said to “split”. It is the fact of this splitting on the one hand, and on the other hand a certain statement about the numbers of individuals in the three different types of the “second” generation, that give its real importance to Mendel’s rule.
From the fact of the splitting of hybrids in the second generation most important consequences may be drawn for the theory of inheritance; the split individuals, if crossed with each other, always give an offspring which remains pure; there is no further splitting and no other change whatever. The germ-cells produced by the split individuals of the second generation may therefore be said to be “pure”, as pure as were those of the grandparents. But that is as much as to say that the pureness of the germ-cells has been preserved in spite of their passing through the “impure” generation of the hybrids, and from this fact it follows again that the union of characters in the hybrids must have been such as to permit pure separation: in fact, the germ-cells produced by Mendelian hybrids may hypothetically be regarded as being pure themselves.
We have not yet considered one feature of all experiments in hybridisation, which indeed seems to be the most important of all for the theory of inheritance, if taken together with the fact of the pureness of the germs. The rule of Mendel always relates to one single character of the species or varieties concerned in hybridisation, and if it deals with more than one character, it regards every one of them separately; indeed, the rule holds for every one of them irrespective of the others. We cannot study here how this most important fact of the independence of the single characters of a species with regard to inheritance leads to the production of new races, by an abnormal mixture of those characters. We only take advantage of the fact theoretically, and in doing so, I believe, we can hardly escape the conclusion that the independence of the single characters in inheritance, taken together with the pureness of the germ-cells in the most simple form of hybrids, proves that there occurs in inheritance a sort of handing over of single and separate morphogenetic agents which relate to the single morphogenetic characters of the adult.
Mendelism and Cytology
Modern cytology now strongly supports this view.
The discoveries of Rauber, Boveri, and Herbst had shown that it is the nucleus of the germ-cells that plays the most important part in inheritance, the protoplasm being only responsible for certain properties of the offspring that are of minor importance.8 Herbst, e.g., was able to show that hybrids of echinoderms become the more of the “maternal” type the more there is chromatin in the fertilised egg derived from the mother.9
But it is the modern theory of maturation and fertilisation in particular that has supported the Mendelian law most strongly: in maturation half of the chromosomes are eliminated from the egg and the spermia; fertilisation restores the total number again. Then separation follows again in the next maturation, etc., etc.
But the individuality of the chromosomes is preserved throughout the line of these processes and, on the other hand, the separation of the chromosomes in maturation is quite ad libitum and does not relate to the very same chromosome groups that had been united by the preceding fertilisation.
Let me give an example.
Let us assume that we have ripe eggs and ripe spermiae, of different varieties. Each of them contains n chromosomes, which we will call A, B, C… in the egg, and a, b, c… in the spermia. Fertilisation occurs and the number of chromosomes is now A, B, C… + a, b, c… = 2n in each egg.
And now we take account only of the chromosomes called A and a.
The next maturation then will lead to a separation again of the chromosomes, and thus we get what we will briefly call A-eggs, a-eggs, A-spermiae and a-spermiae.
The next fertilisation then gives us fertilised eggs of the types: AA, Aa, aA, aa, that is, ¼ pure A’s, ¼ pure a’s, and ½ Aa’s, i.e. mixed eggs.
But these numbers, ¼, ¼, ½, are just the same numbers that occur in Mendelian “splitting”. Thus there is a correspondence among two kinds of splitting, the cytological and the Mendelian, and we are therefore fully entitled to regard the first of these splittings as the foundation of the second.
So much for the more general outlines of the relations between Mendelism and cytology.
If, now, we introduce the modern name of genes for the ultimate material units transported in propagation from one generation to the next, we are entitled to say that inheritance has as its material basis the uniting and splitting of genes.10
But the genes as material entities, of whatever kind, cannot, by themselves, account for inheritance.
In the first place, their community is most decidedly an aggregate. This is a fact. But even if we knew nothing about this fact, we should have to postulate it. For, according to our second vitalistic argument, there cannot be a structure of the type of a well-ordered machine of three dimensions, which divides indefinite times and remains what it has been.
Thus entelechy and genes are working together. Entelechy uses the genes as its means, and all order in morphogenesis is exclusively due to entelechy.
In finishing our chapter on inheritance, we at the same time have finished the first main part of this book—that part of it which has been devoted exclusively to the study of the morphogenesis of the individual, including the functioning of the adult individual form.
The chief result of the first main part of this book has been to prove that an autonomy of life phenomena exists at least in some departments of individual morphogenesis, and probably in all of them; the real starting-point of all morphogenesis cannot be regarded as a machine, nor can the real process of differentiation, in all cases where it is based upon systems of the harmonious-equipotential type. There cannot be any sort of machine in the cell from which the individual originates, because this cell, including both its protoplasm and its nucleus, has undergone a long series of divisions, all resulting in equal products, and because a machine cannot be divided and in spite of that remain what it was. There cannot be, on the other hand, any sort of machine as the real foundation of the whole of an harmonious system, including many cells and many nuclei, because the development of this system—goes on normally, even if its parts are rearranged or partly removed, and because a machine would never remain what it had been in such cases.
Once more we repeat, at this resting-point in our discussions, that both of our proofs of life-autonomy have been based upon a careful analysis of certain facts about the distribution of morphogenetic potencies in two classes of morphogenetic systems, and upon nothing else. To recall only one point, we have not said that regeneration, merely because it is a kind of restitution of the disturbed whole, compels us to admit that biological events happen in a specific and elemental manner, but, indeed, regeneration does prove vitalism, because it is founded upon the existence of certain complex-equipotential systems, the analysis of the genesis of which leads to the understanding of life-autonomy. This distinction, in fact, is of the greatest logical importance.
Driesch, Organ. Regul., 1901.
The “ideal whole” is also proved to exist, if any given “Anlage”, say of a branch, is forced to give origin to a root, as has really been observed in certain plants. This case, like many other less extreme cases of what might be called “compensatory heterotypy”, is best to be understood by the aid of the concept of “prospective potency”. It is very misleading to speak of a metamorphosis here. See my Organ. Regul. pp. 77, 78.
Winkler has discovered the important fact that the adventitious buds formed upon leaves may originate either from one single cell of the epidermis or from several cells together; a result that is very important with respect to the problem of the distribution of “potencies”. (Compare p. 111.)
The “regeneration” of the brain of annelids, for instance, is far better regarded as an adventitious formation than as regeneration proper: nothing indeed goes on here at the locality of the wound; a new brain is formed out of the ectoderm at a certain distance from it.
A full “analytical theory of regeneration” has been developed elsewhere (Organ. Regul. p. 44, etc.). I can only mention here that many different problems have to be studied by such a theory. The formation of the “Anlage” out of the body and the differentiation of it into the completely formed results of regeneration are two of them. The former embraces the question about the potencies not only of the regenerating body but of the elements of the “Anlage” also; the latter has to deal with the specific order of the single acts of regenerative processes. The cells of the “Anlage” have proved to be equipotential in the harmonious way (p. 92).
And, of course, at the root of every new starting of certain parts of morphogenesis also, as in regeneration and in adventitious budding; these processes, as we know, being also founded upon “complex-equipotential systems”, which have had their “genesis”.
For the sake of simplicity, I shall not deal here with those cases of hybridisation in which one quality is “recessive”, the other “dominant”, but only allude to the cases, less numerous though they be, where a real mixture of maternal and paternal qualities occurs. And I also omit all exceptions based upon a “binding” of properties, etc. For a full discussion, compare the works of Bateson, T. H. Morgan, etc.
Compare my studies on Echinoderm hybrids, Arch: Entw. Mech. vii., 1898.
Arch. Entw. Mech. xxvii. and xxxiv., and Sitzungsber. Acad. Heidelberg, 1913, No. 8.
We have omitted all details intentionally and only add that there are many genes in one chromosome, and that, in the state of “synapsis”, single genes may be interchanged among two corresponding chromosomes. See Morgan, etc.