Another example may be cited that shows even more clearly that the internal factor regulating the growth in the new part is probably some sort of tension. I refer to the development of the tail of fundulus from an oblique cut, or of the bilobed tail of stenopus from a cross cut. The assumption of the typical form that leads to the holding in check of the growth in certain regions, as compared with others, can be best understood, I think, as due to some sort of tension established in the different parts, that regulates the growth in those regions.
It is evident that whatever factor will serve to explain the preceding cases must also be expected to apply to the development of the whole embryo from parts of the egg or blastula, if the position that I have taken is correct, namely, that these phenomena belong to the same general group. Does the tension hypothesis make clearer the development of a whole embryo from a part of an egg? This means, can we think of the readjustment that takes place as due to the establishment of a characteristic equilibrium that expresses itself in the tensions of the different regions? There is, so far as I can see, no difficulty in supposing that the organization is at bottom a system of this kind; indeed, it seems to me that from this point of view we can get a better appreciation of the organization and of the series of changes that take place in it during development. The example that Driesch has chosen as a typical one of vitalistic action, namely, the proportionate development of a part of the archenteron of the half-embryo, seems to me to be likewise a case to which we can apply the tension view.
In these, as well as in all other cases, we must think of the tensions as existing, not only in one direction, but in the three dimensions of space, and of all combinations of these. The material in which the tensions exist must be thought of as labile, so that a change in one region involves a rearrangement in many cases of the entire system. The new rearrangement appears to take place on the foundations of the old system.
It may appear that this idea of a system of tensions is too vague, that it fails to point out how the reorganization takes place, and that it gives not much more than the facts do themselves. There is a certain amount of truth in these objections which I fully appreciate, but something further can be said on these points. The view is vague in so far as we cannot picture to ourselves in a mechanical way just how such a system could bring about the suppression of growth in one region and allow the maximum amount in another region. But this is asking too much, since the hypothesis can only claim, at present, to furnish a means by which we can at least imagine what sort of a process is involved, and cannot give the details of the process itself. It can be shown experimentally that if the phenomenon is one of tension certain results should follow that are observed to take place, as when by keeping the shorter half of the planarian from reuniting to the larger half, or by breaking the union if it has been formed, a head develops also at the posterior cross-cut. In the second place, although we cannot understand how the rearrangement of the tensions in a piece takes place, yet from a causal point of view we can see how a change in one region of a labile system may produce, by means of a change of tension, a complete rearrangement of the parts throughout. It can even be claimed for the tension hypothesis that it at least becomes easier for us to see how such a change could take place, because it represents the organization as the expression of a system under tension, and hence, if the material is sufficiently flexible, a readjustment will probably take place when the system is changed in any region. It enables us to see how the organization of the egg may be divided by every cell division, and yet after the reunion of the cells the original equilibrium be established. We may perhaps claim, therefore, that in these respects the hypothesis does give us something more than do the facts; and, inasmuch as it brings a large number of phenomena under a common point of view, the idea may be worth further consideration.
In conclusion, I may add that the hypothesis is, I hope, also a legitimate one, in the sense that being within reach of an experimental proof or disproof, it may serve at least as a working hypothesis. Perhaps more fundamental than the idea that a system of tensions exists throughout the organization is the conception that the organization is itself a system of interrelated parts, and not a homogeneous substance or a mass composed of a large number of repeated parts, or rather, despite the presence of smaller, repeated units, the organization is not the result of their interaction, but of their regular arrangement as parts of a whole structure. If, then, this interrelation of the different parts of the structure can be looked upon as the result of a system of tensions, we can at least form a better idea as to how a piece of a whole can readjust itself into a new whole of smaller size. And it is this possibility of rearrangement or regulation that is one of the most characteristic properties of living things.
CHAPTER XIV
GENERAL CONSIDERATIONS AND CONCLUSIONS
In the preceding chapters certain matters had to be taken for granted, since it was not possible, or desirable, at the time to discuss more fully some of the terms that are in common use, or to analyze more completely many of the phenomena. It was also not necessary to give the general point of view under which the phenomena were considered in their physical, chemical, or even causal connection. Little harm has, I trust, been done by relegating such questions to the final chapter. An attempt will now be made to give more explicit statements in regard to the use and meaning of such terms as “organization,” “polarity,” “factors,” “formative forces,” “vitalistic” and “mechanical principles,” “adaptation,” etc.
It will be found that the hypotheses that have been advanced to account for the phenomena of development and of regeneration may be roughly classified under two heads: first, those in which the organization is “explained” as the result of the collective action of smaller units; and second, those in which the organization is itself regarded as a single unit that controls the parts. Let us examine these points of view more in detail, in order to see what has been meant in each case by “the organization.”
A favorite method of biological speculation in the last forty years has been to refer the properties of the organism to invisible units, and to explain the action of the organism as the resultant of their behavior. The hypothesis of atoms and of molecules, by means of which the chemist accounts for his reactions, has proved so exceedingly fruitful as a working hypothesis that it has had, I think, a profound influence on the mind of many biologists, who have, consciously or unconsciously, attempted to apply a similar conception to the structure of living organisms. The discovery that all of the higher organisms are made up of smaller units, the cells, and that the lower organisms are single, isolated cells, comparable to those that make up the higher forms, has also drawn attention to the idea that the whole organism is the result of the action of its units. Furthermore, within the cells themselves units of a lower order have also been discovered, such, for instance, as the chromosomes, the chlorophyl bodies, etc., that repeat on a smaller scale some of the fundamental properties of the entire organism, as growth and division. It has been assumed that still farther down in the structure there are smaller units having the same properties, and the smallest of these are the ultimate units. The organism is looked upon as the result of the properties of these minute germs. The gemmules of Darwin furnish an example of an hypothesis of this sort; also the intracellular pangens of De Vries, the plasomes of Wiesner, the biophors of Weismann, the idiosomes of Hertwig, and the micellæ of Nägeli are other examples of this way of interpreting the organization. These elements are endowed by their inventors with certain properties, and these are of such a sort that they give the appearance of an explanation to organic phenomena. It is useless to object to these hypotheses that they are purely ideal, or fictitious, and that those properties have been assigned to the germs that will bring about the desired explanation, and have not been shown to be the real properties of the germs themselves. But apart from the arbitrariness of the process, it cannot be claimed that a single one of these creations has been shown to be true, or has even been accepted by zoologists as probable. A more serious objection to this point of view is that the most fundamental characteristics of the organism, those that concern growth, development, regeneration, etc., seem to involve in many cases the organism as a whole. So many examples of this have been given in the preceding pages, that it is not necessary to go over the ground again. It has been shown that a change in one part takes place in relation to all other parts, and it is this interconnection of the parts that is one of the chief peculiarities of the organism. In phenomena of this kind even the cells seem to play a secondary part, and if so, we can, I think, safely leave out of account the smaller units of which the protoplasm is supposed to be built up and we can neglect them, if for no other reason than this, that the argument that has called them into existence starts out with the cell as the highest unit. If the cell can be thrown out, most probably the units of which the cell itself is supposed to be made up can be safely disregarded also.
It may be objected that only through a knowledge of the minute structure of the organism can we hope to understand the behavior of the whole; but my point of view is not that there may not be a fundamental structure, but that this is not formed by a repetition of elements, which give to the whole its fundamental properties. It can be shown, I think, with some probability that the forming organism is of such a kind that we can better understand its action when we consider it as a whole and not simply as the sum of a vast number of smaller elements. To draw again a rough parallel; just as the properties of sugar are peculiar to the molecule and cannot be accounted for as the sum total of the properties of the atoms of carbon, hydrogen, and oxygen of which the molecule is made up, so the properties of the organism are connected with its whole organization and are not simply those of its individual cells, or lower units.