Individuality in Organisms

preted and harmonized. If the metabolic rate in the organ or part in question is sufficiently hi^^h, it is capable of undergoing its characteristic development and differentiation without nervous stimulation, assuming of course that its other relations as a part of the individual are not fundamentally altered; but when its intrinsic metabolic rate falls below a certain level its development does not occur, or is incomplete, or it undergoes atrophy unless its rate is further increased by nervous stimulation. In the case of striated muscle during the earlier stages of development the intrinsic metabolic rate is high enough to permit without nervous stimulation the accumulation of structural material and the characteristic course of differentiation determined by other correlative conditions, but as differentiation and senescence progress the metabolic rate falls, and finally the muscle is not even able to maintain itself in the absence of the accelerating influence of nervous stimulation upon its metabolic rate, because when its rate falls below a certain level it does not replace its losses by new muscle substance. In the regeneration of the amphibian leg and other cases where the influence of the nervous system is in dispute, the relations are without doubt essentially the same.

There is no reason to believe that the nerve impulse is anything more than an acceleration of metabolism. The appearance of the nervous system does not constitute the addition of something new to the organism; il is merely the visible expression of relations already existing and, as the facts indicate, of the relations which constitute the foundation and starting-point of individuation. The question whether metabolic gradients involving different metabolic processes may exist at the same time in the same protoplasm must at least be raised. So far as gradients depending on transmission are concerned, this question is really the question whether different sorts of changes or excitations may be transmitted through the same protoplasm and whether different metabolic effects result. Any answer to this question at present is little more than a guess. It is perhaps conceivable that at least in undifferentiated or slightly differentiated protoplasm some degree of difference in the character of the transmitted change may exist under different conditions of excitation, etc. If such differences do exist, they must of course be important factors in development and differentiation, but they merely complicate and do not alter fundamentally the character of unity and order in the individual. At present there seems to be no real evidence that they exist.

In chaps, iv and v, I have pointed out that the inhibition or retardation of new individuation by the dominant region of an individual occurs when the original gradient is sufficiently fixed in the protoplasm, or the metabolic rate at the levels concerned is sufficiently high to prevent the establishment of a gradient in another direction or to obliterate more or less completely or prevent the further development of a gradient in another direction. In Tubularia the inhibiting influence of the apical region on the development of a hydranth at the basal end of a piece is apparently simply the obhterating effect of the original gradient on the gradient in the

opposite direction. If the latter attains a sufTicicntly high rate it interferes with or obhterates the other and the hydranth develops, though partial inhibition may be evident in its shortness and slow development. In the case of a lateral bud of a plant, the development of which is inhibited by the main growing tip, the relation is probably the same. As long as the bud is within the range of dominance of the growing tip its own gradient from apex to base is more or less completely obliterated by a gradient from base to apex determined by the main growing tip. This may in time alter the protoplasmic gradient in the bud determined in the earlier stages of its individuation so that it becomes incapable of development or develops only into a short branch, a spine, or some other rudimentary structure. It is interesting to note that Mogk in his studies of plant correlation finds that when the axillary shoots of a seedling are allowed to grow until they attain dominance over the main shoot (see pp. 152, 153), the latter often dies and the death gradient is in the reverse direction from that of death from lack of water or other conditions in an uninhibited shoot. Leaves and roots probably represent partially inhibited gradients under certain conditions, and some of the specialized outgrowths on the animal body, such as appendages, may perhaps in some cases represent somewhat similar relations, though I know of no definite evidence bearing on this point.

So far as the evidence goes, it indicates that all inhibition of this sort is a matter of interference between gradients in opposite or nearly opposite directions, the one gradient reducing, obhterating, or even reversing the other. This interference is in certain respects analogous to physical interference in the transmission of water waves, sound waves, light waves, etc., but the protoplasmic substratum in the organism represents a factor not concerned in physical interference in nonsolid media. Undoubtedly a gradient which is originally dynamic becomes more or less stably fixed or established in the protoplasm as a gradient in irritability, structure, or differentiation, because the effects of the transmitted excitations modify the protoplasmic condition and this modification may become more or less persistent. Temporary inhibition may result from temporary interference between metabolic gradients, but for permanent or long-enduring inhibition the protoplasmic condition determined by one gradient must be reduced or obliterated or its direction reversed by the action on the protoplasm of another gradient. In the cases of obliteration or reversal of the axial gradients by other gradients this factor undoubtedly plays a more or less important part, and the increasing stability of the protoplasmic substratum with the progress of individual development and evolution^ determines that such obliteration and reversal occur much more readily in the lower than in the higher organisms. Since conduction in the nerve is apparently associated with an axial gradient, it is at least an interesting question whether nervous inhibition may not be fundamentally a similar relation of gradients, either in different neurons or in the innervated organ. The mechanism of nervous inhibition is still obscure, but if the nervous

system is really the final expression of the primitive dominance in the individual, it is conceivable that the highly specialized nervous inhibition may have something in common with the primitive form of inhibition in the lower animals and plants. The data of reconstitution discussed in chaps, iv and v show very clearly that new metabolic gradients arise in relation to various external factors: in Tuhularia the cut end (pp. 132-37); in Corymorpha the difference between a free surface and one in contact (pp. 142-46); in Harenactis difference in the character of a wound determining more or less growth of new tissue and so the localization of a new apical region. As regards the plants, the evidence from adventitious buds (pp. 83-86) also indicates that the axes of such buds arise anew, slight differences in metabolic rate between different cells apparently often determining whether a new individual shall arise in one place or another. As regards various plants, we know that certain of the minor axes, and in some cases the major axis, are determined by the differential action of light. I believe we are justified in saying that whenever a new metabolic gradient of sufhciently high rate is established by an external factor a new individuation occurs.

It is of course easy to assume, as is often done, that polarity and symmetry are self-determined in the indi\ddual, and that these self-determined relations are simply altered and modified by external factors. But the evidence for self-determination is lacking, and theevidence for external determination is abundant and highly conclusive. The assumption of self-determined polarity and symmetry in protoplasm is simply superfluous, and the burden of proof is upon its supporters.

Of course the metaboUc gradients present in one individual may persist in the parts when that individual divides, so that in such cases the axial relations of the new individual are predetermined. This is the case in fission in Planaria (pp. 92-96) and in many other forms. Apparently also the gradient in a reproductive body, e.g., many eggs, is often determined by its relations of attachment, nutrition, etc., to the parent body. In pieces of Tubularia, Corymorpha, Planaria, and many other forms, the original polarity gradually disappears as the length of the isolated piece decreases until it becomes practically apolar, and new polarities arise in relation to conditions at the cut ends (pp. 97-101). This fact indicates that polarity is rather a matter of relation of parts than a fundamental property of protoplasm, for in fractions of the axis below a certain length it disappears.

In nature a particular kind of individual show^s certain characteristic axial relations; it is radially or bilaterally symmetrical, or a combination in a characteristic way of radial and bilateral arrangements. But the characteristic axial relations are not invariable; they appear regularly merely because events follow the same course in successive generations. In plants the axial relations can be altered in many ways and by many external factors. Bilateral symmetry may be transformed into radial or radial into bilateral, the position of branches may be altered from alternate to opposite or to whorled, and so on. The bilateral tentacle groups on the rings

in Harenactis (Fig. 82, p. 147) show that the radial arrangement characteristic of the animals in nature is not invariably determined in the protoplasm, but is only one of various possibilities, which may or may ncjt be realized according to conditions. If my conception of the relation between the metabolic gradient and dominance is correct, then of course the origin of a new gradient is the origin of a new dominance, and if such a gradient is uninhibited by gradients in other directions, and if its metabolic rate is hi^^^h enough, it becomes the major axis of an individual and its region of highest rate the dominant apical rcgUm.

The belief that qualitative differences of some sort in the fundamental constitution of the organism must underlie the morphological and physiological differences which arise during development in different parts of the individual has been so widespread among biologists that any attempt at even a statement of the problem of differentiation in anything like quantitative terms is sure to meet with serious objection and criticism in some quarters. Nevertheless, the simplest and most satisfactory, and, I believe, the only adequate, interpretation of the data of reconstitution which have been discussed in preceding chapters is that the starting- {)(jint of differentiation is in differences in metabolic rate. The attempt to interpret these facts on any other basis very soon becomes involved, either in the barren assumptions of the hypotheses which simply postulate an invisible organization to account for a visible, or else

in the equally barren neo-vitalistic assumptions of some non-mechanistic controlling or determining principle, entelechy, or whatever we please to call it. The head of Planaria will serve to illustrate the point. I have shown that a series of different forms of head occur in reconstitution, ranging from the normal to the headless condition (pp. 106-8). These different forms represent various degrees of inhibition and they result, not only from the inhibitory influence of other parts (pp. 108-14), but can be produced experimentally by a great variety of conditions. In a lot of similar pieces from animals in similar physiological condition a decrease in head-frequency or a shift toward the headless condition can be induced by low temperature, narcotics, carbon dioxide, etc., although in certain cases, as we have seen (pp. 11 2-13), the results are complicated by the metabolic relations between the headforming region and other parts of the piece. On the other hand, conditions which accelerate metaboUsm, such as high temperature or increased motor activity, increase the head-frequency or shift it toward the normal end of the series. We cannot believe that differences in temperature or motor activity alter the fundamental "organization" in the head-forming region, but it is a fact that such conditions according to their degree may determine any or all of the various kinds of head between the normal and headless extremes.

Again, how does either an "organization" or an entelechy aid us in interpreting the structures formed on rings in Ilarenactis (pp. 146-49) ? Here results range from various bilateral arrangements of parts to the characteristic radial symmetry, and from single tentacles to normal animals. Either the plan of or^^^anization or the purpose of entelechy must be very (lilTcrcnt in different tentacle groups on such rings. \Vc know, however, that the pieces will not form rings except under certain experimental conditions, and that when they do not they undergo reconstitution in the usual way to animals of the usual form. Evidently the development of these structures on the rings results from certain experimental conditions, but if simple experimental conditions can alter the fundamental axial relations in the individual, what is the necessity of the postulated organization, or entelechy, or other similar principle ? And does not the obhteration in Corymorpha of the original axial relations and the establishment of new relations in their place, by means of experimental conditions whose action upon metabolism is primarily quantitative (pp. 142-46), indicate that the axes themselves are primarily quantitative relations? Similarly the fact that the localization of experimental reproduction may be determined as a resultant of dilTerent axes or by a minor axis in the absence of the major axis (pp. 163-68) forces us to the conclusion that the different axes are fundamentally identical and therefore represent quantitative relations.

Moreover, the conception of the organic axis as a metabolic gradient enables us not only to interpret, but to control and to predict. In recent work on the oligochete annelids, by Dr. Hyman, it has been possible on the basis of the metabolic axial gradient to predict and control experimental results, and this is possible among the flatworms to an even greater degree. As regards the manner in which physiological and morj^hological specialization results from difference in metabolic rate there are various possibilities. In a physico-chemical complex like living protoplasm a change in temperature of a certain amount alters the rate of chemical reaction to a certain degree, but it also alters many other conditions in protoplasm, e.g., osmotic conditions, surface-tension, aggregate condition of colloids, etc., and it alters some in a greater, others in a less, degree. In such a case the change in each particular process or condition in the living protoplasm may be quantitative, but since different factors are altered in different degree the total change may determine qualitative differences in the reactions or their products. Changes of this sort may result, not merely from differences in temperature, but from other primarily quantitative changes. In fact, it is very doubtful whether we can alter metabolic rate to any great extent without bringing such changes in quality somewhere in the complex.

Elsewhere I have called attention to various facts which have as yet received but little attention, but which indicate that a relation exists between morphological structure and metabolic rate.^ Structural features which are stable with a certain metaboHc rate are eliminated when the rate increases, while decrease in rate may determine the addition of new structural substances, and so on. Metabolic rate is apparently a factor, though of course by no means the only one, in determining what substance or substances accumulate in the living cell as structural substratum, and the structural substratum is an important factor in determining the character of the reactions which occur in it.

The lack of specificity in the action oi a great variety ot experimental conditions upon development and morphology has often been noted. For example, the aberrations or abnormalities in development, or m(jre properly the partial inhibitions of development produced by low temperature, various narcotics and poisons, and many other conditions are essentially the same. The reason for the lack of specificity undoubtedly lies in the fact that the action of these various substances and conditions is primarily quantitative, yet a greater or less degree of cUfferentiation, various differences in form and arrangement, and even the presence or absence of specific organs may be determined by their action.

The results of the quantitative changes in living protoplasm in a particular case must of course depend upon its specific constitution. The kind of specialization or differentiation which arises at a particular level of a metabolic gradient must depend upon this constitution, and the developmental and morphological resemblances between different forms must of course depend in general upon similarities of constitution. The development of the region of highest metabolic rate in the major gradient as a growing tip in plants and as a central nervous system or brain in animals must result from differences in constitution and dynamic processes in the plant and animal protoplasm, but growing tii)s in general and central nervous systems in general have certain common characteristics.

We must, I beheve, conclude that the conception of the metabolic gradient, a gradient primarily quantitative^ originating in and primarily determined by the dominant region, as the basis of physiological and morphological order, of "organization," specialization, and differentiation in the organic individual, not only presents no fundamental difficulties, but is supported by a great body of experimental and observational evidence from various biological fields.

If the dynamic conception of the organic individual is correct, the starting-point hes, not in a certain organization, but in a certain reaction system. This is a protoplasm of specific constitution with a corresponding metabolic specificity, or one may say that this specificity is the expression of a specific constellation of conditions and that this in turn has been determined by the specific constellation of factors external to itself to which each organism, individual, or part has been subjected in the past. It is this reaction system, not an organization, which constitutes the basis of inheritance, and it is in this system that differences in metabolic rate initiate the process of organization. We may for convenience regard the embryonic or undifferentiated cell of the species as representing this fundamental reaction system, although even there the system is doubtless not reduced to its lowest terms. The developmental changes in this system fall into two groups, the self-determined' changes

^ It is perhaps desirable to indicate just what is meant by selfdetermination in this connection. All that the word is intended to imply here is that the region of highest metabohc rate may undergo certain progressive changes, which are derermined by its own constitution and by continued metabolism in it. The*^e changes may in time make this region different structurally and physiologically from what it was originally, even though it is independent of other parts.

characteristic of the dominant region and the correlatively determined changes characteristic of subordinate regions. It is a very significant fact that the self-determined changes in animals always result, where they proceed far enough, in the development of a nervous system. Of course as a matter of fact the changes which occur in the development of a central nervous system are not all aljsolutely self-determined, for if they were all cells of the nervous system would be alike. We may say, however, that in the animal the nervous system or its apical portion represents more nearly than any other part of the body the result of self-determined progressive changes in the fundamental reaction system of the species, while other parts represent the result of changes determined by correlation and dependence^ From this point of view the animal organism is fundamentally nervous system; all other parts represent lower levels of metaboHsm and independence. The central nervous system represents more nearly than any other part of the individual the product of the fundamental reaction system at its highest level. The cephalic nervous system is, so to speak, the organism at its best.

In the plant, however, the self-determining dominant region remains, at least during growth, in an unditTerentiated or relatively undifferentiated condition as the growing tip, and growth and cell division arc its chii'f activities. In consequence of this condition its dominance over other regions is slight, the degree of individuation in the plant remains low, and the life of the plant remains simple and narrowly limited in character. This difference between animals and plants, in iluone the development of the dominant region into the

central nervous system, the most stable structure physiologically of the body, and in the other its persistence indefinitely as an embryonic cell or a group of cells, must be an expression of the fundamental difference between the two groups of organisms. Evidently this difference is primarily a difference in relation between the protoplasmic substratun^ and the metabolic reactions. Stable morphological structure and differentiation in the plant consist largely in the deposition of carbohydrates and other non-proteid substances within or about the cells, while in the animal morphological differentiation very generally has its origin and foundation in the accumulation and specialization of protoplasm itself. Apparently the protoplasmic substratum of the plant is much less stable physiologically than that of the animal. The plant seems to be incapable or almost incapable of synthesizing proteid molecules which are physiologically stable where the metabolic rate is high. The protoplasm of the plant cell is certainly much more directly and intimately involved in the chemical reactions of metabolism than that of most animal cells; consequently in regions of high metabolic rate no persistent protoplasmic structure like that of the animal cell can arise, because there is no accumulation of relatively stable substances in the cell. In regions where the metabolic rate is lower, substances may accumulate in the cell as structure which with a higher metabolic rate would be decomposed. In the plant, therefore, morphological differentiation increases with increasing distance from the growing tip and decreasing metabolic rate, while in the animal differentiation begins and is most stable in the apical region — the region of highest reaction rate —

and progresses from this to other parts. Animal mctab- oHsm evidently synthesizes highly stable molecules, even where metaboHc acti\qty is most intense. In the plant the whole substratum may apparently be mobilized to some extent when the metabolic rate is high, and only as the rate becomes lower do substances accumulate as structure. In nearly all if not all animals, on the other hand, certain protoplasmic substances are relatively more stable under the existing metabolic conditions than in the plant and therefore accumulate, and a progressive structural development and dilTerentiation occur even when the metabolic rate is highest. In the animals the morphological structure which develops in the region of highest metabolic rate is physiologically the most stable structure of the body, because the less stable substances are decomposed in the intense metabolic activity and so do not form permanent constituents of the substratum. In regions of lower metabolic rate substances accumulate which are readily removed by an increase in metabolic rate. These parts may therefore undergo dedifferentiation and rcdillerentiation. The head-region, however, or more specifically, the central nervous system, is almost or quite incapable of dedifferentiation under ordinary conditions, because its structure has developed under conditions of more intense metabolic activity than any other part of the body and is therefore more stable. If the metabolic rate could be increased sufficiently above the rate in the developing nervous system without bringing about death, doubtless dedifferentiation of the nervous system would occur to some extent. To refer brietly to the analogy between the organism and the flowing

stream which I have used elsewhere/ the plant is somewhat like a stream flowing in an alluvial channel, capable of shifting and removing previous structural deposits, and, when its rate is highest, of holding all its sediment in suspension. The animal, on the other hand, represents a condition like that in the stream when deposition of sediment is going on and giving rise to stable structure, even where the rate of flow is highest. In such a stream the most stable structure develops where the rate of flow is highest, while the structure developed with a low rate of flow is readily altered or eliminated by an increase in rate.

The fundamental differences in behavior between plant and animal are of course associated with this difference. Since the plant is to a large extent incapable of developing morphological colloid structures, such as nerve and muscle, its reactions to external factors are limited very largely to growth reactions, instead of being motor reactions like those in most animals. The low degree of individuation and physiological efficiency in the plant as compared with the animal must also depend on this low degree of physiological stabihty in the protoplasmic substratum.

The occurrence of reproduction in consequence of physiological isolation of parts under experimental conditions makes it highly probable that at least many of the processes of agamic reproduction in nature are like- ^ Child, "The Regulatory Processes in Organisms," Jour, of Morphol.,XXll, 191 1. wise the result of physiological isolation. Elsewhere I have endeavored to show that physiological isolation is a fundamental factor in asexual reproduction in both plants and animals, and that reproduction results from physiological isolation because the isolated part loses to a greater or less extent its differentiation as a part, becomes physiologically younger, and undergoes a new individuation.' In chap, iv above it was also pointed out that agamic reproduction in Tubularia and Planaria is readily interpreted as the result of physiological isolation. Moreover, in the discussion of the data of experimental reproduction we have seen that physiological isolation and reproduction may result, not only from increase in size beyond the range of dominance, but also from decrease in the range of dominance in consequence of decrease in metabolic rate in the dominant region, from decrease in conductivity in the path of transmission, and finally from a decrease in receptivity of a subordinate part, brought about by the action of local factors, which determine the establishment of new gradients in it or make it otherwise more independent. Undoubtedly all these different forms of physiological isolation occur in nature, and in many reproductive processes more than one of them are probably concerned. Reproduction in consequence of increase in size is one of the commonest forms of reproduction in organic individuals from the single cell to complex organisms among both animals and plants. Reproduction also occurs very commonly under conditions unfavorable t»)

^ Child, "Die physiologische Isolation von Teilen des OrKanismus." Vortrage tend Aujsatze iiher Entunckelungsmechanik, H, XI. iqii; Senescence and Rejuvenescence, 1915, PP- 228. growth or active life; that is, under conditions which undoubtedly decrease metabolic rate and so decrease the range of dominance. Under such conditions unicellular forms often fragment into a number of small individuals, and some of the simple plants break up into their constituent cells, which then grow and divide to form small individuals, even under the same conditions which made impossible the persistence of the original larger individual. Other plants give rise to adventitious buds, sometimes in great numbers, under such conditions, while still others break up into quiescent forms, and so on. In my study of senescence and rejuvenescence I have pointed out that the decrease in metabolic rate with advancing senescence in the lower animals and plants often leads automatically by decreasing dominance to physiological isolation of parts and so to rejuvenescence and reproduction of new individuals. ' Reproduction under depressing conditions has often been interpreted in a teleological way as an attempt of the organism to avoid extinction by producing new individuals, some of which might succeed in finding favorable conditions for continued existence. As a matter of fact, however, such reproduction is merely the expression of physiological weakness; the individual can no longer maintain itself as a unity in its original size, and as the original unity disappears, new unities arise as local metabolic conditions determine.

Regarding the part played by changes in the conductivity of the path of transmission in bringing about physiological isolation and reproduction in nature, we know little. It is undoubtedly a fact that the increase in conductivity during development of the individual brings about an extension of dominance ami so inhibits or retards physiological isolation (see pp. 149-51), and it is probable that sooner or later with advancing senescence a decrease in conductivity occurs in at least some cases. It is also probable that decrease in conductivity occurs in the lower organisms under external conditions which decrease metabolic rate in the organism in general. Such changes, where they occur, may j)lay a part in determining physiological isolation and reproduction.

Local external conditions undoubtedly assist in the physiological isolation of subordinate parts in man\' cases. In various plants local conditions very favorable to metabolic activity and growth may dctennine the development of buds in spite of the inhibiting influence of the dominant region. We have seen how in pieces of Tuhularia stem the presence of the wound at the basal end assists in establishing the new gradient, even in spite of the presence of the old (see pp. 132-37). This is a good case of physiological isolation by the action of local factors.