Fig. 14.—An “Harmonious-equipotential System” of whatever kind.According to the “machine-theory” of life this system ought to possess a certain unknown very complicated machinein its completeness:(a) in its total length,and (b) in each of the equal volumesv,v1,v2,v3and so on,and (c) in each of the unequal volumesw,x,y, and so on,and (d) in every imaginable volume, no matter of what size.Therefore the “machine-theory” of life is absurd.
Fig. 14.—An “Harmonious-equipotential System” of whatever kind.
According to the “machine-theory” of life this system ought to possess a certain unknown very complicated machinein its completeness:(a) in its total length,and (b) in each of the equal volumesv,v1,v2,v3and so on,and (c) in each of the unequal volumesw,x,y, and so on,and (d) in every imaginable volume, no matter of what size.Therefore the “machine-theory” of life is absurd.
But we have forgotten, I see, that in our operation the absolute amount of substance taken away from the system was also left to our choice. From this feature it follows that not only all the differentVn, all of the same size, must possess the hypothetic machine in its completeness, but that all amounts of the valuesVn-n,nbeing variable, must possess the totality of the machine also: and all valuesVn-n, with their variablen, may again overlap each other.
Here we are led to real absurdities!
But what is the conclusion of our rather wild considerations?
It seems to me that there is only one conclusion possible. If we are going to explain what happens in our harmonious-equipotential systems by the aid of causality based upon the constellation of single physical or chemical factors and events, theremustbe some such thing as a machine. Now the assumption of the existence of a machine proves to be absolutely absurd in the light of the experimental facts.Therefore there can be neither any sort of a machine nor any sort of causality based upon constellation underlying the differentiation of harmonious-equipotential systems.
For a machine, typical with regard to the three chief dimensions of space, cannot remain itself if you remove parts of it or if yourearrange64its parts at will.
Here we see that our long and careful study of morphogenesis has been worth while: it has afforded us a result of the very first importance.
The Autonomy of Morphogenesis Proved
No kind of causality based upon the constellations of single physical and chemical acts can account for organic individual development; this development is not to be explained by any hypothesis about configuration of physical and chemical agents. Therefore there must be something else which is to be regarded as the sufficient reason of individual form-production. We now have got the answer to our question, what our constantEconsists in. It is not the resulting action of a constellation. It is not only a short expression for a more complicated state of affairs, it expressesa true element of nature. Life, at least morphogenesis, is not a specialised arrangement of inorganic events; biology, therefore, is not applied physics and chemistry: life is something apart, and biology is an independent science.
All our results at present, indeed, are negative in their form; our evidence was throughout what is calledper exclusionem, or indirect or apagogic. There were excluded from a certain number of possibilities all except one; a disjunctive proposition was stated in the form:Eis either this, or that, or the other, and it was shown that it could not be any of all these except one, therefore it was proved to be that one. Indeed, I do not see how natural science could argue otherwise; no science dealing with inorganic phenomena does; something new and elemental must always be introduced whenever what is known of other elemental facts is proved to be unable to explain the facts in a new field of investigation.
We shall not hesitate to call by its proper name what we believe we have proved about morphogenetic phenomena.What we have proved to be true has always been calledvitalism, and so it may be called in our days again. But if you think a new and less ambitious term to be better for it, let us style it the doctrine of theautonomy of life, as proved at least in the field of morphogenesis. I know very well that the word “autonomy” usually means the faculty ofgivinglaws to oneself, and that in this sense it is applied with regard to a community of men; but in our phrase autonomy is to signify thebeing subjectedto laws peculiar to the phenomena in question. This meaning is etymologically defensible, and besides that I perhaps may remind you of a certain chapter of Professor Ward’s Gifford Lectures, in which he holds the view that, psychologically and epistemologically, there is more than a mere verbal relation between the civil and the natural “law.”
Vitalism then, or the autonomy of life, has been proved by us indirectly, and cannot be proved otherwise so long as we follow the lines of ordinary scientific reasoning. There can indeed be a sort of direct proof of vitalism, but now is not the time to develop this proof, for it is not of the purely scientific character, not so naïve as our present arguments are, if you choose to say so. An important part of our lectures next summer will be devoted to this direct proof.
“Entelechy”
But shall we not give a name to our vitalistic or autonomous factorE, concerned in morphogenesis? Indeed we will, and it was not without design that we chose the letterEto represent it provisionally. The great father of systematic philosophy, Aristotle, as many of you willknow, is also to be regarded as the founder of theoretical biology. Moreover, he is the first vitalist in history, for his theoretical biology is throughout vitalism; and a very conscious vitalism indeed, for it grew up in permanent opposition to the dogmatic mechanism maintained by the school of Democritus.
Let us then borrow our terminology from Aristotle, and let that factor in life phenomena which we have shown to be a factor of true autonomy be calledEntelechy, though without identifying our doctrine with what Aristotle meant by the wordέντελέχεια. We shall use this word only as a sign of our admiration for his great genius; his word is to be a mould which we have filled and shall fill with new contents. The etymology of the wordἐντελέχειαallows us such liberties, for indeed we have shown that there is at work a something in life phenomena “which bears the end in itself,”ὃ ἔχει ἐν ἑαυτᾣ τὸ τέλος.
Our concept of entelechy marks the end of our analysis of individual morphogenesis. Morphogenesis, we have learned, is “epigenesis” not only in the descriptive but also in the theoretical sense: manifoldness in space is produced where no manifoldness was, real “evolutio” is limited to rather insignificant topics. But was there nothing “manifold” previous to morphogenesis? Nothing certainly of anextensivecharacter, but there was something else: there was entelechy, and thus we may provisionally call entelechy an “intensive manifoldness.” That then is our result: not evolutio, but epigenesis—“epigenesis vitalistica.”
Some General Remarks on Vitalism
We now shall leave entelechy where it stands: next summer we shall turn back to it and shall make its full logical and ontological analysis our chief study. At present we are satisfied with having proved its existence in nature, with having laid some of the foundations of a doctrine to be based upon it. I hope that these foundations will evince themselves strong: that isall-important.65It indeed has been the fault of all vitalism in the past that it rested on weak foundations. Therefore the discussion of the basis underlying our doctrine of the autonomy of life is to occupy us still a considerable time. We shall devote to it two more of this year’s lectures and three of the next; we shall examine all sorts of phenomena of life in order to find out if there are any further proofs of vitalism, independent perhaps, of what we way call ourfirst proof, which is based upon the analysis of thedifferentiation of harmonious-equipotential systems. We shall find some more independent proofs; and besides that we shall find many kinds of phenomena upon which future times perhaps may erect more of such independent proofs.
For we shall be chary of bestowing the name “proof” except on what is a proof indeed, of course according to our critical conviction. Vitalistic views in biology have arisenin rather numerous forms during the last fifteen years, especially in Germany—though in very strong contrast to the so-called official German biology—but I can only admit that one of all the arguments of “neo-vitalism” has proved its statements. I refer to the theory of “morphaesthesia” as developed by Noll, which we shall study briefly in the next lecture. I cannot concede that Reinke or Schneider or Pauly have really proved what they believe, and I cannot even allow to the most original thinker in this field, Gustav Wolff, that he has given a real demonstration of his views. He states that the existence of so-called “primary purposefulness,” that is, the existence of adaptive processes, which cannot be imagined to have arisen on Darwinian principles, is able to prove vitalism; but I say that it only proves teleology, which is a broader concept than vitalism.
The possibility of a machine at the root of the phenomena in question always has to be excluded in order that vitalism may be proved, and I cannot grant that the necessity of such an exclusion has been actually shown by any of my fellow-combatants against so-called mechanism, exceptNoll.66
The Logic of our First Proof of Vitalism
Let us devote the end of our present lecture to an account of the logical means by which it has been possible to develop what we hope will be regarded as a trueproofof life autonomy.
Firstly, we have looked upon the phenomena ofmorphogenesis without any prepossessions; we may say that we have fully surrendered ourselves to them; we have not attacked them with any sort of dogmatism except the inherent dogmatism of all reasoning. But this dogmatism, if it may be called so, does not postulate that the results of the inorganic doctrines must hold for the organic world, but only that both the inorganic and the organic must be subject to certain most general principles.
By studying life as a given phenomenon, by fully devoting ourselves to our problem, we not only have analysed into its last elements what was given to us as our subject, but we also, more actively, have created new combinations out of those elements: and it was from the discussion of these positive constructions that our argument for vitalism was derived.
We have analysed morphogenesis into elementary processes, means, potency, formative stimulus, just as the physicist analyses mechanics into time, velocity, mass, and force; we have then rearranged our elements into “systems”—the equipotential systems, the harmonious-equipotential system in particular, just as the physicist composes his elements into the concepts of momentum or of kinetic energy or of work. And finally, we have discussed our compositions and have obtained our result, just as the physicist gets his ultimate results by discussing work and kinetic energy and momentum.
Of course the comparison is by no means intended to show that mechanics and biology are sciences of the same kind. In my opinion, they are not so at all; but nevertheless there do exist similarities of a logical kind between them.
And it is not the formal, logical character alone whichallows us to compare biology with other natural sciences: there is still something more, there is one kind of assumption or postulate, or whatever you may choose to call it, without which all science whatever would be altogetherimpossible. I refer to the concept ofuniversality. All concepts about nature which are gained by positive construction out of elements resulting from analysis, claim to be ofuniversal validity; without that claim there could indeed be no science.
Of course this is no place for a lecture on methodology, and it therefore must suffice to make one remark with special regard to our purpose, which we should like to emphasise. Our concept of the harmonious-equipotential system—say rather, our concept of the prospective potency itself—presumes the understanding that indeedallblastomeres andallstems ofTubularia, including those upon which we havenotcarried out our experiments, will behave like those we have experimented with; and those concepts also presume that a certain germ of Echinus,A, the blastomeres of which were not separated, would have given two whole larvae, if separation had taken place, while another germ,B, which actually gave us two larvae after separation, would only have given one without it. Without this presumption the concept of “potency” is meaningless, and, indeed, every assumption of a “faculty” or a “possibility” would be meaningless in the whole area of science.
But this presumption can never be proved; it can only be postulated. It therefore is only with this postulate that our first proof of vitalism holds; but this restriction applies toeverylaw of nature.
I cannot force you to agree with this postulate: but if you decline you are practically saying that there exists a sort of pre-established harmony between the scientific object and the scientist, the scientist always getting into his hands such objects only as have been predestinated from the very beginning to develop two larvae instead of one, and so on.
Of course, if that is so, no proof of natural laws is possible at all; but nature under such views would seem to be really dæmonic.
And so, I hope, you will grant me the postulate of the universality of scientific concepts—the only “hypothesis” which we need for our argument.
Our next studies on the physiology of form will be devoted in the first place to some additional remarks about our harmonious-equipotential systems themselves, and about some other kinds of morphogenetic “systems” which show a certain sort of relationship with them. For it is of the greatest importance that we should become as familiar as possible with all those facts in the physiology of form upon the analysis of which are to be based almost all of the future theories that we shall have to develop in biology proper and philosophical. Our discussions, so far as they relate to questions of actual fact, will contain only one other topic of the same importance.
But though it is designed to complete and to deepen our analysis, the present considerations may yet be said to mark a point of rest in the whole of our discussions: we have followed one single line of argumentation from the beginning until now; this line or this stream of thought, as you might call it, is now to break into different branches for a while, as if it had entered from a rocky defile into a plain. It seems to me that such a short rest will be not unconducive to a right understanding of all we have made out; and such a full and real conceiving again, such a realisingof our problems of morphogenesis and their solutions, will be the best preparation for the philosophical part of these lectures.
HARMONIOUS-EQUIPOTENTIAL SYSTEMS FORMED BY WANDERING CELLS
All of the harmonious-equipotential systems which we have studied so far were the bases of histological differentiation; that is to say, the processes of their differentiation consisted in specifically localised elements of theirs becoming differentin situ. Now we know at least one type of systems which also may be called harmonious-equipotential, but the differentiation of which does not simply relate to elements at a fixed place. An additional phenomenon enters here into the sphere of the others. The elements not only become different where they are, but a specific changing of locality, a specific kind of wandering, goes hand-in-hand with differences relating to the prospective value to be attained. I am speaking of the formation of the larval skeleton of our well-known Echinus. We know that the mesenchyme cells, which have left the blastoderm and are arranged in a sort of ring of bilateral structure, are the starting-point of this skeleton: it indeed originates in a sort of secretive process on the part of the cells; the cells are moving about and are secreting carbonate of lime during their wandering. The experiments now have shown, as we know, that a whole, though smaller, skeleton may also be formed, if only a half or a quarter of the mesenchyme cells are present, as happens to be the case in all experiments with isolatedblastomeres of the two or four-cell stage of cleavage. It is clear that in these cases the performance of each single cell must be different from what it is in the normal case, and that the same sort of differences in the morphogenetic performances appears again, if the two- and the four-cell stage are compared with each other. And there are still some other phenomena showing the possibility of different performances being carried out by the individual cells. Peter has shown that the number of mesenchyme cells may vary enormously under certain conditions; but, in spite of that, the skeleton always will be complete. It may be said that this line of research is only of a relative value to our own questions, as, of course, variability relates to different individuals: but it seems to me that it adds a very good supplementary instance to what the experiment on the individual itself has established.
We should only be repeating ourselves if we were to analyse again what happens here as the expression of the harmonious-equipotentiality itself. But indeed there occurs something new in this instance: the single mesenchyme cell not only has to perform in each case that single act of specific secretion which the case requires, but it also has to wander to the right place in order to perform it; there must be some order, not only about the acts of secretion after wandering, but also in the migrations themselves. If undisturbed ontogeny alone were possible, and if therefore a theory like that of Weismann were in place, we might say perhaps that each mesenchyme-cell is specified not only as to its performance in secretion, but also with regard to its chemotactical irritability, the latter being typically localised, so that its effect becomes typical, thanksto the typical arrangement of all the cells with respect to each other. But that is certainly not the case. Now, you may ask yourselves if you could imagine any sort of a machine, which consists of many parts, but not even of an absolutely fixed number, all of which are equal in their faculties, but all of which in each single case, in spite of their potential equality, not only produce together a certain typical totality, but also arrange themselves typically inorderto produce this totality. Weareindeed familiar with certain occurrences in nature where such curious facts are observed, but I doubt if you would speak of “machines” in these cases. The mesenchyme-cells, in fact, behave just as a number of workmen would do who are to construct, say, a bridge. All of themcando every single act, all of them alsocanassume every single position: the result always is to be a perfect bridge; and it is to be a perfect bridge even if some of the workmen become sick or are killed by an accident. The “prospective values” of the single workman change in such a case.
I well know that it is only an analogy which I am offering to you. The mesenchyme-cells have not “learned,” have no “experience.” All that is to occupy us next summer. But in spite of it, there is truth in the analogy; and perhaps you will prefer it to the merely abstract consideration.
ON CERTAIN COMBINED TYPES OF MORPHOGENETIC SYSTEMS
For the sake of completeness it may be remarked, only by the way, that the type of the proper harmonious-equipotential system may go hand in hand with anothertype of “systems” which play a part in morphogenesis; a type which we have shortly mentioned already and which will be studied fully a few chapters later. We know that there are equipotential systems with complex potencies: that is to say, systems which may produce a whole organism equally well from any one of their elements; we know the cambium of Phanerogams to be such a system. Now it is easily understood that the germ of our Echinus, say in the stage of two or four or eight cleavage cells, is not only an harmonious-equipotential system, but a complex-equipotential system too. Not only may there arise a whole organism out of 2/4 or 3/4; or 3/8, 4/8, 5/8, 6/8, 7/8 of its elements, in which cases the harmonious rôle of the single element with regard to its single performance in a totality is variable, but there may also arise four whole single larvae out of the four cells of the four-cell stage, or eight single whole larvae out of the eight-cellstage.67In these cases, of course, each of the four or eight elements has performed not a part of the totality, changing with its “position,” but the totality itself. With respect to these possible performances the “systems” present in the four or eight-cell stages of cleavage must be called complex-equipotential ones.
We propose to give the name ofmixed-equipotential systemsto all those equipotential systems which, at the same time, may be regarded as belonging to the harmonious or to the complex type. It is not only among cleavage-stages that they are to be found; you may also find them very clearly exhibited in our ascidianClavellinafor instance.We know already that the branchial apparatus of this form is typically harmonious-equipotential, but it is complex-equipotential too, for it also may regenerate what is wanting in the proper way, by a budding from the wound; and the same is true of many other cases, the flatwormPlanariafor instance.
Another type of systems, which might be said to be of a higher degree, is exhibited in some very strange phenomena of regeneration. It was first shown most clearly by some experiments of Godlewski’s that a whole tail may be regenerated from a wound inflicted on the body of a newt, even if this wound involves section of only a portion of the body-diameter. Section of the whole of the body-diameter of course would cause the formation of the whole tail also; but it was found that even an incomplete cross-section of the body is capable of performing the whole on a smaller scale. The series of possible cross-sections which are all capable of regeneration would have to be called a system of the complex type in this case; but, now we learn that everysinglecross-section is of the harmonious type, we must speak ofcomplex-harmonious systems. What we have described is not the only instance of our new type of morphogenetic systems. Some other instances had been discovered a few years earlier, though nobody had pointed out their true significance. In the flatwormPlanariaa partial cross-section is also capable of forming a whole structure, say a head, and all cases of so-called “super-regeneration” after the infliction of a complicated wound probably belong here also.
You may say that our two additions to the theory ofsystems are merely formal, and indeed I am prepared to concede that we shall not learn anything altogether new from their discussion: their analysis would lead either to what was our “first proof” of the autonomy of life-phenomena or to what will be our “second” one. But the mere descriptions of the facts discovered here will interest you, I think, and will fill your minds with more vivid pictures of the various aspects of form-autonomy.
While dealing with our harmonious-equipotential systems as the starting-points of processes of restitution,e.g.inTubularia,Clavellina, the flatworms, and other instances, we always have regarded cross-sections of the body as constituting the elements of equipotentiality. Now cross-sections, of course, are by no means simple in themselves, but are made up of very different tissues, which are derivates of all three of the original germ layers—ectoderm, mesoderm, and endoderm. Owing to this composite character of the cross-sections, taken as elements of harmonious systems, a special phenomenon of morphogenesis is presented to us, which teaches somewhat more than the mere concept of harmonious-equipotentiality can express. If composite elements concerned in morphogenesis result in one whole organisation in spite of the development of the single tissues of these elements going on independently, then there must be a sort of correspondence or reciprocity of the harmonious development among these tissue constituents themselves; otherwise a proportionate form could not be the final result. We may conveniently speak of areciprocity of harmonyas existing between the single tissues or germ layers which constitute many harmonious-equipotential systems, and there can belittle doubt that we have here an important feature with regard to generalmorphogenesis.68
A few other groups of morphogenetic facts may find their proper place here, though they are not properly to be regarded as additions to the theory of harmonious systems but as forming a sort of appendix to it.
THE “MORPHAESTHESIA” OF NOLL69
We may briefly mention that group of botanical phenomena, by which the botanist Noll has been led to the concept of what he calls “morphaesthesia,” or the “feeling” for form; a concept, the full discussion of which would lead to almost the same conclusions as our analysis of the harmonious systems has done. In the Siphoneae, a well-known order of marine algae with a very complicated organisation as to their exterior form, the protoplasm which contains the nuclei is in a constant state of circulation round the whole body, the latter not being divided by proper cell-walls. On account of this constant movement it is certainly impossible to refer morphogenetic localisation to definite performances of the nuclei. Nor can any sortof structure in the outer protoplasmic layer, which is fixed, be responsible for it, for there is no such structure there: hence there must be a sort of feeling on the part of the plant for its relative body localities, and on account of this feeling morphogenesis occurs. This “feeling” is styled “morphaesthesia” by Noll, and to it he tries to refer all sorts of different botanicalform-phenomena,70for instance what is called “autotropism,” that is, the fact that branches of plants always try to reassume their proper angle with regard to their orientation on the main axis, if this orientation has been disturbed. It may be an open question if this particular application of the theory is right: certainly there seems to be much truth in the establishment of the concept of morphaesthesia, and we only have to object to its psychological name. But that may be done in a more general form on a later occasion.
RESTITUTIONS OF THE SECOND ORDER
In the hydroid polypTubularia, already familiar to us as being a most typical representative of the harmonious-equipotential systems, a very interesting phenomenon has beendiscovered71, almost unparalleled at present but nevertheless of a general importance, a phenomenon that we may call a restitution of a restitution, or a restitution of the second order. You know that the first appearance of the new head ofTubularia, after an operation, consists in theformation of two rings of red lines, inside the stem, these rings being the primordia of the new tentacles. I removed the terminal ring by a second operation soon after it had arisen, disturbing in this way the process of restitution itself: and then the process of restitution itself became regulated. The organism indeed changed its course of morphogenesis, which was serving the purposes of a restitution, in order to attain its purpose in spite of the new disturbance which had occurred. For instance, it sometimes formed two rings out of the one that was left to it, or it behaved in a different way. As this difference of morphogenetic procedure is a problem by itself, to be discussed farther on, we shall postpone a fuller description of this case of a restitution of the second degree.
At present I do not see any way of proving independently the autonomy of life by a discussion of these phenomena; their analysis, I think, would again lead us to our problem of localisation and to nothing else; at least in such an exact form of reasoning as we demand.
ON THE “EQUIFINALITY” OF RESTITUTIONS72
I have told you already thatTubulariain the phenomena of the regulation of restitutions offers us a second problem of a great general importance, the problem of theEquifinality of Restitutions. There indeed may occur restitutions, starting from one and the same initial state and leading to one and the same end, but using very different means, following very different ways in the different individuals of one and the same species, taken from the same locality, or even colony.
Imagine that you have a piece of paper before you and wish to sketch a landscape. After drawing for some time you notice that you have miscalculated the scale with regard to the size of the paper, and that it will not be possible to bring upon the paper the whole of the landscape you want. What then can you do? You either may finish what you have begun to draw, and may afterwards carefully join a new piece of paper to the original one and use that for the rest of the drawing; or you may rub out all you have drawn and begin drawing to a new scale; or lastly, instead of continuing as you began, or erasing altogether, you may compromise as best you can by drawing here, and erasing there, and so you may complete the sketch by changing a little, according to your fancy, the proportions as they exist in nature.
This is precisely analogous to the behaviour of ourTubularia.Tubulariaalso may behave in three different ways, if, as I described to you, the terminal one of its two newly arisen rings of tentacle primordia is removed again. It may complete what is left, say the basal tentacle ring, then put forth from the horny skeleton (the “perisarc”) the new head as far as it is ready, and finally complete this head by a regular process of budding regeneration. But it also may behave differently. It may “erase” by a process of retro-differentiation all that has been left of what had already been formed, and then may formde novothe totality of the primordia of a new head. Or, lastly, it may remove a part of the middle of the one ring of tentacle rudiments which was left, and may use this one ring for the formation of two, which, of course, will not be quite in the normal relations of place with regard to each other andto the whole, but will be regulated afterwards by processes of growth. Thus, indeed, there is a sort of equifinality of restitution: one starting-point, one end, but three different means and ways.
It would, of course, contradict the principle of univocality, as we shall see more fully later on, to assume that there actually are different ways of regulation whilst all the conditions and stimuli are the same. We are obliged to assume, on the contrary, that this is not the case, that there are certain differences in the constellation, say of the general conditions of age or of metabolism, which are responsible for any given individual choosing one process of restitution instead of another; but even then the phenomenon of equifinality remains very striking.
It has long been known that restitution in general does not always follow the same lines of morphogenesis as are taken by ontogeny, and it was this feature that once led Roux to point out that the adult forms of organisms seem to be more constant than their modes of origin. But, comparing ontogeny with restitution in general, we see that only the ends are the same, not the points of starting; the latter are normal or non-typical in ontogeny, atypical in restitution. In the new discoveries of an equifinality of restitutions we have thesamestarting-point, which is decidedly non-typical but atypical,i.e.dependent on our arbitrary choice, leading bydifferentways always to thesameend.
There may be many who will regard the fact of equifinality as a proof of vitalism. I should not like to argue in this easy way; I indeed prefer to include part of the phenomena of equifinality in our first proofof autonomy, and part in the second one, which is to follow.
Another important phenomenon of the equifinality of regulation was discovered by Morgan. A species of the flatwormPlanariawas found to restore its totality out of small pieces either by regeneration proper, if the pieces were fed, or by a sort of rearrangement of material, on the basis of its harmonious-equipotentiality, if they were kept fasting. It is important to note that here we see one of the conditions determining the choice of the way to restoration, as we also do in the well-known equifinal restitutions of the root in plants, where the behaviour of the organism depends on the distance of the operation-wound from thetip.73InTubulariathe actual stage of restitution that has been already reached by the stem when the second operation takes place, may account for the specification of its future organogenesis, but this is not at all clearly ascertained at present.
Clavellinaalso shows equifinality in its restitution, as has already been shortly mentioned. The isolated branchial apparatus may restitute itself by retro-differentiation to an indifferent stage followed by renovation; or it may regenerate the intestine-sac in the proper way. Nothing is known here about the conditions, except perhaps that young individuals seem more apt to follow the first of these two ways, older ones the second; but there are exceptions to this rule.
The discussion of other instances of equifinality, thoughimportant in themselves, would not disclose anything fundamentally new, and so we may close the subject with the remark that nothing can show better than the fact of the equifinality of restitutions how absolutely inadequate all our scientific conceptions are when confronted with the actual phenomena of life itself. By analysis we have found differences of potencies, according as they are simple or complex; by analysis we have found differences of “systems,” differences of means, and indeed we were glad to be able to formulate these differences as strictly as possible: but now we see how, in defiance of our discriminations, one and the same species of animals behaves now like one sort of our “systems,” and now like the other; how it uses now one sort of “potencies,” now another.
But even if it is granted that, in the presence of such phenomena of life, our endeavour seems to be like a child’s play on the shores of the ocean, I do not see any other way for us to go, so long, at least, as our goal is human science—that is, a study of facts as demanded by our mental organisation.
REMARKS ON “RETRO-DIFFERENTIATION”
We shall finish this part of our studies by mentioning a little more explicitly one fundamental fact which has already entered incidentally into our considerations, viz.retro-orback-differentiation.74We know that it occurs inClavellinaand inTubularia; we may add that it also happens inHydra, and that in the flatwormPlanariathe pharynx, if it is too large for a piece that is cut out,may be differentiated back and be replaced by a new pharynx, which is smaller.
It is not death and sloughing of parts that occurs in thesecases,75but a real process of active morphogenesis; not, however, a process consisting in the production of visible manifoldness, but the opposite. Loeb was the first to lay much stress upon this topic, and indeed, there may appear a very strange problem in its wake: the problem, whetherallmorphogenesis might be capable perhaps of going backwards under certain conditions.
It is important to note that inmost76cases retro-differentiation occurs in the service of restitution: it goes on wherever restitution requires it. This fact alone would show that not very much could be explained here by the discovery of modern chemistry, important as it is, that one and the same “ferment” or “enzyme” may affect both the composition and the decomposition of the same compound. We could regard what is called “catalysis” solely as an agent in the service of entelechy. But this point also will become clearer in another part of the work.
We have finished our long account of individual morphogenesis proper. If we look back upon the way we have traversed, and upon those topics in particular which have yielded us the most important general results, the material for the higher analysis which is to follow, it must strike us, I think, that all these results relate to regulations. In fact, it is “secondary” form-regulations, according to our terminology, that we have been studying under the names of equifinality, back-differentiation, restitution of the second order, and so on, and our harmonious-equipotential systems have figured most largely in processes of secondary form-regulations also. But even where that has not been the case, as in the analysis of the potencies of the germ in development proper, form-regulations of the other type have been our subject, regulations of the primary or immanent kind, the connection of normal morphogenetic events being regulatory in itself. It was not the phenomenon of organic regulation as such that afforded us the possibility of establishing our proof of the autonomy of morphogenesis: that possibility was afforded us by the analysis of the distribution of potencies; but upon thisdistribution regulation is based, and thus we may be said to have studied some types of regulation more or less indirectly when analysing potencies.
It therefore seems to me that we shall have hopes of a successful issue to our inquiries, if we now, on passing to what is called the physiology of the vegetative functions, proceed to focus our attention on the concept of regulation as such. And that is what we shall do: on our way through the whole field of physiology, we shall always stop at any occurrence that has any sort of regulatory aspect, and shall always ask ourselves what this feature has to teach us.
But let us first try to give a proper definition of our concept. We shall understand by “regulation” any occurrence or group of occurrences on a living organism which takes place after any disturbance of its organisation or normal functional state, and which leads to a reappearance of this organisation or this state, or at least to a certain approach thereto. Organisation is disturbed by any actual removal of parts; the functional state may be altered by any change among the parts of the organism on the one hand, by any change of the conditions of the medium on the other; for physiological functioning is in permanent interaction with the medium. It is a consequence of what we have said that any removal of parts also changes the functional state of the organism, but nevertheless organisation is more than a mere sum of reactions in functional life. All regulations of disturbances of organisation may be calledrestitutions, while to regulations of functional disturbances we shall apply the nameadaptations. It is withadaptationsthat we have to deal in the following.
Let us begin our studies of adaptations in a field which may justly be called a connecting link between morphogenesis and physiology proper, not yet wholly separated from the science of the organic form, morphology.
Morphological adaptationis a well-established fact, and I need only mention the striking differences between the land and water form of amphibious plants, or the differences between the same species of plants in the Alps and in the plains, or the very different aspect of the arms of an athlete and of an ascetic, to recall to your memory what is meant by this term.
Morphological adaptation is no part of individual morphogenesis proper, but occurs at the end of it; at least it never occurs previous to the full individual life of an organism, previous to its true functional life; for it relates to the functions of the complete organism.
THE LIMITS OF THE CONCEPT OF ADAPTATION
It is especially, though by no means exclusively, among plants that morphological adaptation assumes its most marked forms; and this topic, indeed, may very easily be understood if we remember that plant-life is in the very closest permanent dependence on the medium, and that this medium is liable to many changes and variations of all kinds. In order to elucidate our problem, it therefore seems convenient to restrict our considerations for a whileto the study of plants. There exist very many external formative stimuli in the morphogenesis of vegetation: would it then be possible to regard every effect of such an external formative stimulus as a real morphological adaptation? No; for that would not meet the point. The generalharmonyof form is indeed concerned if gravity forces roots to shoot forth below at a spot where they can enter the ground, or if light induces branches and leaves to originate at places where they can obtain it for assimilation; but gravity and light themselves are mere formative stimuli—of the localising type—in these instances, for they relate only to the individual production of form, not to the functioning of already existing form. We therefore are warned not to confuse the effects of formative stimuli from without with real adaptive effects until we have fully analysed the particular case.
We have drawn a sharp line between causes and means of morphogenesis, applying the term “means” to those conditions of the morphogenetic process which relate neither to the specificity nor to the localisation of its constituents, though they are necessary for the accomplishment of the process in the most thorough manner. Would it be possible to connect our new concept of an adaptation with our well-established concept of a means of morphogenesis in such a way that we might speak of a morphological “adaptation” whenever any specific feature about morphogenesis proves to be immediately dependent for its success on some specific means, though it does not owe its localisation to that means as its “cause”? It seems to me that such a view would also fall wide of the mark. It is well known, for instance, that the flowers of many plants never fully develop in thedark; light is necessary for their morphogenesis. Is, therefore, their growth in the presence of light to be called a morphological “adaptation” to light? Certainly not: they simplycannotoriginate without light, because they require it for some reason. It is precisely here that our conception of light as a “means” of morphogenesis is most fully justified. There aremany77such cases; and there are still others of an apparently different type, but proving the same. All pathological forms produced in plants by animal parasites or by parasitic fungi could hardly be called adaptations, but must be attributed to some abnormality of means or of stimuli. It may be that the organism reacts as well as possible in these cases, and that if it reacted otherwise it would die—we know absolutely nothing about this question. But even then there would only be some sort of regulationinthe process of pathological morphogenesis, butthe processitself could hardly be called adaptive.
So far we have only learned what is not to be regarded as morphological adaptation. No response to external formative stimuli is in itself an example of adaptation, nor are processes dependent for their existence on any kind of condition or means to be called, simply because they are dependent on them, adaptations to those agents. What then, after all, is a morphological adaptation?
Let us remember what the word adaptation is really to mean in our discussions: a state of functioning is adapted—a state of functioning must therefore have been disturbed; but as functioning itself, at least in plants, certainly stands in close relations to the medium, it follows that all adaptations are in the last resort connected with those factors of the medium which affect functioning. In being correctives to the disturbances of functioning they become correctives to the disturbing factors themselves.
But again, the question seems to arise whether these factors of the medium, when they provoke an adaptation by some change that is followed by functional disturbance, do so in the capacity of “causes” or of “means,” and so it might seem that we have not gained very much so far by our analysis. The reproach, however, would not be quite justified, it seems to me: we indeed have gained a new sort of analytical concept, in the realm of causal concepts in general, by clearly stating the point that adaptations are related directly to functionality, and only indirectly, through functionality, to external changes. By the aid of this logical formulation we now are entitled to apply the term “cause,” in our restricted sense of the word, to every change of the medium which is followed by any sort of adaptation in regardto itself. Our definition stated that a “cause” is any one of the sum of necessary factors from without that accounts either for the localisationorforthe specificationof the effect, and the definition holds very well in this case. Indeed, the specification of the effect is determinedbythe outside factor in every case of an adaptationtoit, by the merefactof its being a specific adaptation to this specific factor.
We must not forget that in this chapter we are not studying real individual morphogenesis as the realisationof what has been inherited, but that at present we regard morphogenesis proper as an accomplished fact. Morphogenesis proper has laid the general lines of organisation; and now adaptation during the functional life, so to speak, imposes a second kind of organisation upon the first. It is for that reason that the meaning of the word “cause” is now becoming a little different from what it was before.
In order to study a little more in detail what has been discovered about morphological adaptation in animals and plants, let us separate our materials into two groups, one of them embracing adaptations with regard to functional changes from without, the other adaptations to those functional changes which come from the very nature of functioning. Almost all of our previous general considerations have applied to the former group, with which we shall now proceed to deal.
ADAPTATIONS TO FUNCTIONAL CHANGES FROM WITHOUT78
The differences between plants grown in very dry air, very moist air, and water, respectively, are most distinctly seen in all the tissues that assist in what is called transpiration, that is, the exchange of water-vapour between the plant and the medium, but especially in the epidermis and the conductive fibres, both of which are much stronger in plants grown in the dry. Indeed, it seems from experiments that transpiration is the most essential factor to which “adaptation” occurs in amphibious plants, thoughthe changes of the mechanical conditions according to the medium also seem to have some sort of structural effect. If plants stand very deeply in water, the conditions of illumination, so important for assimilation in plants, may have been altered, and therefore much of the structural change can be attributed also to them. It is unimportant in our general question what is due to one of these factors and what to the other. That there is a real sort of adaptation cannot be doubtful; and the same is true, as experimental observations of the last few years have shown, with regard to the structural differences between so-called sun-leaves and shade-leaves of plants grown in the air: it has been actually shown here that the functional life of the former goes on better in the sun, of the latter better in the shade.
It is very important to emphasise this point, as the adaptive character of all sorts of structural differences in plants dependent on light and on moisture has lately been denied, on the supposition that there is only a stopping of organogenesis in the case of the more simple, a continuance in the case of the more complicated modification, but nothing else. Indeed, all morphological adaptation has been conceived as only consisting in differences dependent upon the absence or the presence of necessary means or causes of development, and as offering no problem of its own. We have gained the right position from which to oppose this argument, it seems to me, in our formula that all adaptations do relatenotdirectlytothe agents of the medium, but to changes of functional states inducedbythose agents; that adaptations onlyare“adaptations” by being correctives to the functional state.
There simplyisan “adaptation” of structure insucha sense in all the cases we have mentioned. We can say neither more nor less. Granted that one of the outside factors which comes into account is merely a necessary “means”: then why is the histological consequence of the presence of the means an actual adaptation to it as far as its relation to functioning is concerned—why is the consequence of its absence also an adaptation to this absence in its relation to functioning? Why, to complete the series, is the degree of the consequence of its presence an adaptation to the degree of its presence?
All these relationships, which are so many facts, have been absolutely overlooked by those who have been pleased to deny morphological adaptation to functional changes from without.
To do full justice to them we may speak of “primary” regulative adaptations in all the cases mentioned above, applying the word “primary,” just as was done with regard to restitutions, to the fact that there is some sort of regulationinthe normal connection of processes. We reserve the title of “secondary adaptations” for cases such as those described, for instance, byVöchting,79where not merely one and the same tissue originates adaptively with regard to the degree of its normal functioning, but wherea profound disturbance of all functioning connections, due to the removal of portions of the organisation, is followed by histological changes at absolutely abnormal localities; that is, where a real change of thekindof functioning is the consequence of the adaptation. It, of course, will be found very difficult to discriminate such phenomena from real restitutions, though logically there exists a very sharp line between them.
A few more concrete instances may now close this account of adaptation to functional changes coming from without. Though almost all the adaptive characters in the aquatic forms of amphibious plants represent a less complicated state of organisation than the corresponding structures in their terrestrial forms, and therefore have wrongly been regarded as simply due to a stopping of morphogenesis for want of necessary means, yet there are a few of them that are positive complications in comparison with the land-forms: the so-called aërenchyme, especially well developed in the water-form ofJussiaeais such an instance. This tissue stands in the direct service of respiration, which is more difficult to be accomplished under water than ordinarily, and represents a true adaptation to the altered function.
Among animals there is only one well-studied instance of our first type of adaptive morphological characters.Salamandra atra, the black salamander, a species which only inhabits regions at least two thousand feet above sea-level, does not bring forth its young until metamorphosis has taken place. The larvae, however, may be removed from the mother’s body at an earlier stage and forced to complete their development in water. Under these circumstances,as was shown in an excellent memoir byKammerer,80they will change the whole histological type of their gills and skin in order to meet the new functional conditions. The change of the conditions of functioning is very severe here, for whereas the gills had served for nutrition and respiration in the uterus—by a process of endosmosis—they now serve for respiration only, and, of course, are surrounded by quite an abnormal chemical medium.
TRUE FUNCTIONAL ADAPTATION81
But all other cases of morphological adaptation among animals, and several in the vegetable kingdom too, belong to our second group of these phenomena, which in our analytical discussion we have called adaptations to functional changes that result from the very nature of functioning, and which we shall now call by their ordinary name, “functional adaptation.”
It was Roux who first saw the importance of this kind of organic regulation and thought it well to give it a distinguishing name.By functioning the organisation of organic tissues becomes better adapted for functioning.These words describe better than any others what happens. It is well known that the muscles get stronger and stronger the more they are used, and that the same holds for glands, for connective tissue, etc. But in these cases only quantitative changes come into account. We meet with functional adaptations of a much more complicated and importantkind, when for instance, as shown byBabák,82the intestine of tadpoles changes enormously in length and thickness according as they receive animal or vegetable food, being nearly twice as long in the second case. Besides this the so-called mechanical adaptations are of the greatest interest.
It has long been known, especially from the discoveries of Schwendener, Julius Wolff, and Roux, that all tissues whose function it is to resist mechanical pressure or mechanical tension possess a minute histological structure specially suitable to their requirements. This is most markedly exhibited in the stem of plants, in the tail of the dolphin, in the arrangements of the lime lamellae in all bones of vertebrates. All these structures, indeed, are such as an engineer would have made them who knew the sort of mechanical conditions they would be called upon to encounter. Of course all these sorts of mechanically adapted structures are far from being “mechanically explained,” as the verbal expression might perhaps be taken to indicate, and as indeed has sometimes been the opinion of uncritical authors. The structures existformechanics, notbyit. And, on the other hand, all these structures, which we have called mechanically “adapted” ones, are far from being mechanical “adaptations,” in our meaning of the word, simply because they are “adapted.” Many of them indeed exist previous to any functioning, they are for the most part truly inherited, if for once we may make use of that ambiguous word.
But, the merely descriptive facts of mechanical adaptedness having been ascertained, there have now been discovered real mechanical processes of adaptations also. They occur among the statical tissues of plants, though not in that very high degree which sometimes has been assumed to exist; they also occur in a very high perfection in the connective tissue, in the muscles and in the bone tissue of vertebrates. Here indeed it has proved possible to change the specific structure of the tissue by changing the mechanical conditions which were to be withstood, and it is in cases of healing of broken bones that these phenomena have acquired a very great importance, both theoretically and practically: the new joints also, which may arise by force of circumstances, correspond mechanically to their newly created mechanical function.
So far a short review of the facts of “functionelle Anpassung.” They seem to prove that there does exist a morphological adaptation to functional changes which result from the very nature of functioning. In fact, the actual state of all functioning tissue, the intensity of its state of existence, if you care to say so, may be said to be due to the functioning itself: the so-called atrophy by inactivity being only one extreme of a very long line ofcorrespondences.83
We now, of course, have to ask ourselves if any more intimate analysis of these facts is possible, and indeed we easily discover that here also, as in the first of our groups of morphological adaptations, there are always single definite agents of the medium, which might be called “causes” or “means” of the adaptive effects, the word “medium” beingtaken as embracing everything that is external to the reacting cells. But of course also here the demonstration of single formative agents does not detract in the least from the adaptive character of the reaction itself. So we may say, perhaps, that localised pressure is the formative stimulus for the secretion of skeleton substance at a particular point of the bone tissue, or of the fibres of the connective tissue; the merely quantitative adaptations of muscles might even allow of a still more simpleexplanation.84But adaptations remain adaptations in spite of that; even if they only deserve the name of “primary” regulations.
THEORETICAL CONCLUSIONS
We have stated in the analytical introduction to this chapter and elsewhere, that functional changes, which lead to morphological adaptations of both of our groups, may arise not only from changes of factors in the medium, but also from a removal of parts. As such removal is generally followed by restitution also, it is clear that restitutions and adaptations very often may go hand in hand, as is most strikingly shown in a fine series of experiments carried out by Vöchting, which we have already alluded to. Here again I should like to lay the greatest stress upon the fact that, in spite of such actual connections, restitutions and adaptations always have been separated from another theoretically, and that the forms are never to be resolved into sums of the latter. Such a view has been advocated by some recentauthors, especially by Klebs, Holmes, andChild:85it is refuted I think by the simple fact that the first phase of every process of restitution, be it regeneration proper or be it a sort of harmonious differentiation, goes on without functioning at all, and onlyforfuturefunctioning.86
And there has been advocated still another view in order to amplify the sphere of adaptation: all individual morphogenesis, not only restitution, is adaptation, it has been said. In its strictest form such an opinion of course would simply be nonsense: even specific adaptive structures, such as those of bones, we have seen to originate in ontogeny previous to all specific functions, though for the help of them, to say nothing of the processes of the mere outlining of organisation during cleavage and gastrulation. But they are “inherited” adaptations, it has been answered to such objections. To this remark we shall reply in another chapter. It is enough to state at present that thereisa certain kind of, so to speak, architectonic morphogenesis, both typical and restitutive, previous to specific functioning altogether.
If now we try to resume the most general results from the whole field of morphological adaptations, with the special purpose of obtaining new material for our furtherphilosophical analysis, we have reluctantly to confess that, at present at least, it does not seem possible to gather any new real proof of life-autonomy, of “vitalism,” from these facts, though of course also no proof against it.
We have stated that there is in every case of both our types of adaptive events a correspondence between the degree of the factor to which adaptation occurs, and the degree of the adaptive effect. We here may speak of anansweringbetween cause and effect with regard to adaptation, and so perhaps it may seem as if the concept of an “answering reaction” (“Antwortsreaktion”), which was introduced into science byGoltz87and which is to play a great part in our discussions of next summer, may come into account: but in our present cases “answering” only exists between a simple cause and a simple effect and relates almost only to quantity and locality. There is therefore lacking the most important feature, which, as will be seen, would have made the new concept of value.
We only, I believe, can state the fact that therearerelations between morphogenetic causes and effects whichareadaptations, that functional disturbances or changes are followed by single histogenetic reactions from the organism, which are compensations of its disturbed or changed functional state. We are speaking of facts here, of very strange ones indeed. But I feel unable to formulate a real proof against all sorts of mechanism out of these facts: theremightbe a machine, to which all is due in a pre-established way. Of course we should hardly regard such a machine as very probable, after we have seen that itcannotexist in other fields of morphogenesis. But we are searching for a new and independent proof; and that is indeed not to be foundhere.88
At present it must be taken as one of the fundamentalfactsof the organogenetic harmony, that the cells of functioning tissues do possess the faculty of reacting to factors which have changed the state of functioning, in a way which normalises this state histologically. And it is a fact also that even cells, which are not yet functioning but are in the so-called embryonic or indifferent condition contributing to the physiological completion of the tissue, react to factors embracing new functional conditions of the whole in a manner which leads to an adaptation of that whole to those conditions.
This is a very important point in almost all morphological adaptation, whether corresponding to functional changes from without or resulting from the very nature of functioning. In fact, such cells as have already finished their histogenesis are, as a rule, only capable of changing their size adaptively, but are not able to divide into daughter-cells or to change their histological qualities fundamentally; in technical terms, they can only assist “hypertrophy” but not “hyperplasia.” Any adaptive change of a tissue therefore, that implies an increase in the number of cellular elements or a real process of histogenesis, has to start from “indifferent” cells, that is to say, cells that arenot yetfunctioning in the form that is typical of the tissue in question; and, strange to say, these “embryonic” cells—i.e.the “cambium” in higher plants and many kinds of cells in animals—cando what the functional state requires. It is to be hoped that future investigations will lay a greater stress upon this very important feature of all adaptation.
It is but a step from morphological adaptations to adaptations in physiology proper. The only difference between regulations of the first type and those which occur in mere functioning is, that the resulting products of the regulation are of definite shape and therefore distinctly visible in the first case, while they are not distinctly visible as formed materials but are merely marked by changes in chemical or physical composition in the latter.
Metabolism, it must never be forgotten, is the general scheme within which all the processes of life in a given living organism go on; but metabolism means nothing else, at least if we use the word in its descriptive and unpretentious meaning, than change in the physical or chemical characteristics of the single constituents of that organism. In saying this, we affirm nothing about the physical or chemical nature of the actual processes leading to those physical or chemical characteristics, and by no meansare these “processes”a prioriregarded as being physical or chemicalthemselves: indeed, we have learned that in one large field, in the differentiation of our harmonious systems they certainly are not. Now, if the metabolism does not end in any change of visible form, then true physiological processes, or more particularly physiological regulations, are going on before us. But we are dealing with morphogenetic events or regulations, if the result of metabolism is marked by any change in the constituents of form. This however may depend on rather secondary differences as to the nature of regulation itself, and any kind of metabolism may really be of the regulatory type, whether we actually see its result as a constituent of form,e.g.owing to the production of some insoluble compound, or whether we do not.
I do not mean to say that these are the only differences between mere physiological activities or regulations and organogenesis proper, as an originating of typical form-combination; but if we regard, as we do in this chapter, the given organisation of a living being as a substratum of its functional life, morphological and physiological adaptations are indeed of almost the same logical order.