Individuality in Organisms

The transmission of excitations is one of the characteristic features of living protoplasm, and undoubtedly occurs to a greater or less extent in all protoplasm. In its simplest form it is perhaps little more than a spreading or irradiation to a greater or less distance of the change produced at the point of origin, but in its most ^ It should perhaps be noted that from the standpoint of current physico-chemical theory transmission itself may be regarded as molecular, atomic, ionic, or electronic transportation. Nevertheless, the differences between such transportation and the transportation in mass of substances is sufficient to warrant the distinction made.

highly specialized form, the nerve impulse, it probably differs more or less widely from the initial change. The second point of importance in connection with such transmission is the existence oi a decrement in intensity or energy of the change in the course of its transmission. Apparently a part of its energy is used in overcoming a resistance or inertia or in producing other changes which play no part in further transmission. The existence of this decrement, which may be called the transmission-decrement, determines that at a greater or less distance from its point of origin the transmitted change becomes inappreciable or ineffective, and transmission does not proceed farther. In Fig. i the intensity of excitation or the amount of increase in metabolic activity is indicated diagrammatically for different distances from the point of origin in a by the bands of different width concentric at a. The limit of effectiveness of transmission depends on the intensity or energy of the original change produced at a and, secondly, upon the character of the protoplasm. The higher the conductivity of the surrounding protoplasm — in other words, the less its resistance or the greater its sensitiveness to the transmitted change — the greater the distance to which the change will be transmitted before becoming ineffective, and vice versa. In the existence of this transmission-decrement the resemblance to the transmission of waves in water and to various other forms of physical transmission, such as electrical transmission, is also apparent. A decrement in velocity of transmission accompanies decrement in intensity, at least in certain forms of transmission, but is not of primary importance in the present consideration.

If the external factor acts only momentarily at a, the increase in rate of reaction at a is usually only momentary or of short duration, and a sooner or later returns to or approaches its original condition, perhaps in some cases with a gradually disappearing rhythm of increase and decrease in rate. The transmitted change consists in this case of a wave or a series of successively decreasing waves of change. It is probable that even the occurrence and passage of such momentary changes as these in a substratum so sensitive and so intimately associated with the reactions as protoplasm produce changes which persist for a longer or shorter time after the metabolic change has disappeared, but such changes are usually slight or inappreciable. If, however, the external factor continues to act on a for a sufficiently long time, or if it acts intermittently with sufficient and not too great frequency or intensity, it produces sooner or later more or less permanent changes in the protoplasm, which are most marked in the region a and decrease with the transmission-decrement. The exact nature of these changes is not certainly known, but their effect is to increase the reactive capacity, to alter the protoplasm so that in the absence of external stimuli, or with a given intensity of external stimulus, a rate or intensity of chemical reaction exists higher than the rate under similar conditions before the change. In the terms generally employed, the irritability of the protoplasm is increased.

Since this change is greatest in the region a, Fig. j.. where the excitation is greatest, and decreases with increasing distance from this region, the result of continued or frequently repeated excitation is the establishment of a gradient in the condition of the protoplasm which constitutes a more or less permanent material substratum for a persistent metabolic gradient independent of the local external stimulus. In short the effect of the local action of an external factor on protoplasm may sooner or later result in the establishment of a metabolic gradient, or the material basis for such a gradient, which persists for a longer or shorter time after the external factor has ceased to act. As a matter of fact, such gradients, once established, often persist throughout the life of the individual. These gradients may be directly visible in a graded structure of the protoplasm as well as in differences in rate of reaction, or they may appear only or chiefly in the differences in rate, according to the nature of the protoplasm. There is considerable evidence to show that when once established to a certain degree they tend to persist and even to become more marked, because the rate and extent of further changes in the protoplasm at different levels of the gradient are determined by the differences in rate of reaction at these different levels. As a rapidly flowing stream quickly removes from its channel obstacles which a slowly flowing stream removes only slowly or not at all, so the changes in protoplasm which make a higher rate of reaction possible are more rapid and more extensive with a high than with a low rate of reaction. If these considerations are correct, and there are, as will appear, many facts to support them, it is evident that a persistent metabolic gradient associated with a material gradient in the protoplasmic substratum may arise as the result of the local or differential

action of an external factor on a morphologically and physiologically homogeneous living mass. The formation of metabolic gradients in another way is possible, at least in single cells. If, for example, inactive substances of different weight from the active protoplasm are present in the cell, and if the position of the cell with respect to the force of gravity remains unchanged for a sufhciently long time, the inactive substances and active protoplasm may be more or less definitely localized in different parts of the cell and so a metabolic gradient may result. Again, it is conceivable that continued intake of nutrition at some particular point of the cell surface might load that portion of the cell with inactive reserve substances and so give rise to a gradient. To what extent the origin of metabolic gradients is due to such factors as this is still a question. In the frog's egg gravity undoubtedly contributes to intensify the existing gradient by bringing about a further separation of the heavier yolk granules and the lighter protoplasm, but it is not responsible for the origin of the gradient. Unquestionably the primary factor in the origin of these persistent metabolic-protoplasmic gradients is in most cases at least a reaction-gradient, and the persistent or permanent gradient in the protoplasmic substratum is secondary.

Such metabolic gradients are, I believe, the simplest expression of physiological unity and order in living protoplasm, and at the same time they are the simplest and primary form of the organic axes of so-called polarity and symmetry and the starting-point of the mysterious ''organization." They are factors in determining the direction of growth and differentiation and so are the basis of the geometrical space relations and the sequences in time which arise during the development of the individual. They may then be called axial gradients. The region of highest rate in such a gradient is the apical, the region of lowest rate the basal region of the axis which represents the general direction and course of the gradient.

Other factors besides actual rate of metabolic reactions are doubtless concerned in the formation and establishment of these gradients in protoplasm, but these are associated either with the rate of reaction or its change, or with the character of the protoplasm in which the reaction occurs and have to do rather with particular cases than with the gradient in general. The intensity of reaction, for example, is probably such a factor. A sudden or very rapid increase in rate on excitation is probably more effective in producing transmitted changes than a gradual increase, and it is also probable that in protoplasm with a high reactionintensity excitations are transmitted to greater distances than where the reaction-intensity is low. Excitation and transmission are undoubtedly also correlated with the physiological stability and physico-chemical constitution of the protoplasm. Such factors as these may play a part in determining length, slope, or other characteristics of the gradient, but the primary factor in its production appears to be rate of reaction.

In a metabolic gradient a relation of dominance and subordination exists between the level of highest and the levels of lower metabolic rate. A brief consideration will show that this relation is a simple and necessary result of the differences in rate of reaction. In the first place, the apical region (a, Fig. i) is the chief factor in determining the rate of reaction at other levels, for in the varying conditions of a natural environment it responds more rapidly or with a higher rate of reaction than other levels of the gradient to external exciting conditions, and it is also more sensitive and may react to conditions which produce no reaction at lower levels of the gradient. With every such increase of metabolic rate in response to external exciting factors a gradually decreasing wave of change spreads from this region of highest rate, as in the original excitation which gave rise to the gradient, though the intensity, velocity, and limit of effectiveness of the transmitted change may be much greater than in the original transmission.

This change transmitted from the apical region a plays the chief part in determining the metabolic condition at other levels, because a is the region of highest metabolic rate and the changes transmitted from it are more intense than those from other levels and because the establishment of the protoplasmic gradient makes conduction in this direction more effective than in any other. Consequently the region a dominates or controls other regions within a certain distance and to a greater or less degree by influencing, through the changes transmitted from it, their metabolic rate. Dominance or control of one part over another in the organism is fundamentally, I beheve, a matter of difference in metabolic rate, the region of higher rate being dominant.

If, after such a gradient is established, some other region, such as b, Fig. 2, undergoes excitation at the same time as a and by an external factor of the same intensity as that acting at a, the response will be less rapid and less intense than that of a, and, as indicated in Fig. 2, the transmitted change will be weaker, perhaps less rapid, and its limit of effectiveness less than that arising from a. The influence of the region h must then be less than that of a in determining the metabolic rate in other regions and a remains the dominant region.

Fig. 2. — Diagram illustrating origin of major and minor gradients in a simple case: a, apical region of major gradient; h, apical region of minor gradient. If, however, the region h or any other region is sufficiently intensely or sufficiently often locally excited independently of a, a persistent gradient may arise with relation to h without destroying that related to a. In such a case two dominant regions, a and Z>, exist, but a may still dominate 6 to a greater or less extent unless the reactive capacity or irritability of h becomes equal

to that of a. Every other point in the mass, so far as it is within the Hmit of effectiveness of both a and h, will be subordinate to both to a greater or less degree, and its metabolic condition will be the resultant of its position in the two gradients. Such an organism possesses not only a polar axis or gradient, but an axis or gradient of symmetry as well, and in the same way other gradients and other relations of dominance and subordination may arise. Obviously it is possible for various gradients to exist simultaneously in a living mass, and their relations may be very different in different cases, as are the relations between the different axes in organisms. Interference between different gradients in opposite or nearly opposite directions, or obliteration of one gradient by another of higher rate of reaction, undoubtedly occurs, as following chapters will show, but their relations need not be considered here.

The physiological dominance of one part over another is certainly not a constant, unchanging relation, but depends upon the metabolic rate in the dominant apical region and the conductivity of other regions. The metabolic rate in the dominant region is also not constant, but must fluctuate with changes in external conditions. With a slight rise in metabolic rate in the dominant regions its influence on other regions is slight and does not extend far, but when the increase is great the degree of dominance is greater and extends to a greater distance. Certainly in the primitive individual these relations must be regarded as constantly undergoing change in degree and extent, though under the usual conditions they must also show a general average. So for each level there will be a general average of the effect of the transmitted

change upon it, and the gradient will be further intensified by the fact that slight transmitted changes do not reach the more remote parts at all while they do affect the parts nearer the dominant region. It is this general average which determines the more conspicuous and lasting effects at different levels. The continued existence of a metabolic gradient of this kind undoubtedly determines an increase in the conductivity of the protoplasm for the transmitted excitation. Many facts indicate that within certain limits the occurrence and repetition of transmission increase the conductivity, and in all animals except the simplest a nervous system which possesses a very high degree of conductivity develops in relation to the primary gradients. In most organisms there is therefore an .extension of dominance during development, the transmitted changes become effective through a greater distance, and their limit of effectiveness, w^hich of course determines the range of dominance, becomes farther and farther removed from the point of origin. This extension of dominance, however, is itself limited by the changes known as senescence, which become evident in a general decrease in reaction rate. These relations of parts, dependent in the final analysis on differences in metabolic rate, constitute, as I believe, the foundation of unity and order in the organic individual, the startingpoint of physiological individuation.

If this conclusion is correct, the organic individual, as a living entity possessing some degree of physiological — not merely physical — unity and order, consists in its simplest forms of one or more gradients in part of a cell, a cell, or a cell mass of specific physico-chemical constitution. The process of individuation is the process of establishment of the gradient or gradients as a more or less persistent condition, and the degree of individuation depends upon the permanency of the gradient, the metabolic rate in the dominant region, the conductivity of the protoplasm, and probably on other factors as well.

From this point of view the assumption of a mysterious, self-determined organization in the protoplasm, the cell or the cell mass as the basis of physiological individuality becomes entirely unnecessary. The origin of physiological individuality is to be found, not in living protoplasm alone, but in the relations between living protoplasm and the external world. In view of the fact that the organic individual after its formation is far from independent of its environment, it is difficult to see why we should assume that it is independent and self-determining in its origin.

It must not be supposed, however, that every new individual originates in the manner described above. When the axial gradient is once established in a cell or an organism, it may simply persist through the process of cell division or other forms of reproduction so that the unity and order of the new individual represent ■simply the unity and order of the parent or a part of it. In such cases the basis of individuality is inherited from the parent. In nature we find both possibilities realized : physiological individuality may arise de novo through 1 the relation between living protoplasm and its environment, or it may be inherited from previously existing individuals. To put it more concretely, an axial gradient cannot arise in the first instance independently of conditions external to the mass of protoplasm concerned.

but, once established, it may persist through many generations. The question at once arises whether a quantitative gradient, such as has been described, constitutes an adequate basis for the physiological specialization and structural differentiation which arise in relation to the axes of the individual and in the higher organisms become very complex. Organs showing very definite qualitative differences in chemical constitution and metabolism and great differences in functional activity develop in the organism. Qualitative specific differences of some sort are commonly believed to be necessary as a starting-point for such complexity, hence the usual theoretical assumption of some sort of underlying organization as the basis of organic individuality. Some of the facts bearing upon this question will be considered in later chapters; here attention may be called to three points: first, it is a familiar fact of chemistry that purely quantitative differences may bring about the formation of qualitatively different products from the same reacting substances, and in a complex physico-chemical system, such as living protoplasm, the possibilities for the origin of qualitative from quantitative differences is very much greater than in the simple chemical reaction in the test tube; secondly, it is by no means clear what is quantitative and what is qualitative in organic structure and form, or in metabolism, for many structural difterences which are ordinarily considered as qualitative prove on analysis to depend on quantitative differences in certain constituents of the complex; and, thirdly, morphological differences usually regarded as qualitative can unquestionably be produced

and controlled experimentally by metabolic changes which are primarily quantitative. The morphology of the channel of a rapidly flowing stream is very different from that of a stream which flows slowly, and there can be little doubt that in the organism substances which are decomposed and transformed or eliminated with a high rate of reaction remain and accumulate in the protoplasm and may form characteristic morphological features when the rate of reaction is low.

In this connection the question must be raised whether the transmitted change is always of the same sort and produces the same effect in a protoplasm of given constitution. It is impossible at present to give a definite answer to this question, but there seems to be no positive evidence to show that the qualitative character of the effect is determined by the character of the transmitted change, although it has often been assumed that this is the case. It is very probable that the chemical or physico-chemical character of the transmitted change differs more or less widely in plants and animals, and in embryonic protoplasm as compared with the fully developed meduUated nerve, but the effect in each case seems to be primarily excitatory and quantitative. It seems even possible that in passing through .different tissues the character of the transmitted change may differ more or less according to the constitution of the tissues, but its effect may still remain essentially quantitative.

If it should be demonstrated that the same protoplasm may transmit different kinds of excitations, then of course different processes of morphogenesis and differentiation might be determined by the specific character of the transmitted changes affecting different regions. The demonstration of such relations would of course compHcate our conception of the course of development, but would not necessarily alter our views concerning the fundamental principles of individuation. The problem of the nature of transmitted changes in protoplasm has been the subject of much experiment and discussion and is still by no means solved, but our knowledge concerning them is sufficient to permit us to formulate a working hypothesis of the organic individual in terms of these transmitted changes rather than in terms of transported chemical substances.

As soon as local differences in chemical constitution of the protoplasm arise, whether they result from differences in metabolic rate or from differences in character of the transmitted change, the relations commonly called chemical correlation, consisting in the production and transportation of different specific substances, begin to play a part, and from this point on these chemical relations are factors of great importance in determining the character of the different parts, until in the adult stage of the highest forms, man and the other mammals, the complexity of chemical correlation is bewildering, as the work of recent years on hormones and internal secretions has clearly demonstrated. From the point of view developed here chemical correlation is, however, a secondary factor, for the underlying order which determines the orderly character of chemical correlation consists in the quantitative gradients which arise in the living mass.

Since a transmission-decrement in energy or in intensity of the transmitted change exists, the change is effective only within a certain limit of distance which we may call its range, and since physiological dominance depends upon the transmitted change it is similarly limited in range. If physiological individuation depends upon dominance of this sort associated with the metabolic gradient determined by transmitted changes, the range of dominance must determine a physiological limit of size, which the individual cannot exceed without the physiological isolation of some part from the dominance which previously determined the individuality. As already pointed out, the range of the transmitted change and so the range of dominance varies with the rate of reaction in the dominant region and with the conductivity of the protoplasm; therefore the physiological size limit of the individual must vary with the same factors. Reproduction in its simplest asexual forms results from the physiological isolation^ of parts of the individual body in consequence of their coming to lie beyond the physiological limit of size. Such physiological isolation may result from: first, increase in size of the body of the individual by continued growth until some part of it is brought beyond the range of dominance; secondly, decrease in the range of dominance and limit of size by decrease in the rate of reaction in the dominant region; thirdly, decrease in the conductivity of the protoplasm for the transmitted changes; fourthly, the direct local action of some external factor on a subordinate part, increasing its rate of reaction to a sufficient degree to make it more or less independent of or insusceptible to the effects transmitted from the dominant

^ Child, "Die physiologische Isolation von Teilen des Organismus," Vortrdge und Aufsatze iiber Entmckeliingsmechanik, H, XI, 191 1. region. This change I have called decrease in receptivity of the subordinate part for the transmitted change. The effect of physiological isolation of a part is essentially the same as that of physical isolation. In the lower organisms where its physiological and morphological characteristics as a part are less stable than in the higher forms and it is able to respond to the altered conditions accompanying physiological isolation, it loses more or less completely its character as a part because the conditions which determined and maintained its specialization no longer act. Consequently it undergoes dedifferentiation to a greater or less degree and so approaches or returns to the undifferentiated or embryonic condition, and is then capable, if differences in metabolic rate in the direction of the original gradient or gradients still exist in it, or if conditions determine the origin of new gradients in it, of development into a new individual. I have shown that development and differentiation are in general accompanied by a decrease in metabolic rate which constitutes physiological senescence and that the dedifferentiation of isolated parts brings about rejuvenescence varying in degree with the degree of dedifferentiation.^ New individuals formed from physiologically or physically isolated parts of preexisting individuals may therefore be physiologically younger than the individuals from which they arose and so be capable of repeating the developmental history' and process of senescence. Asexual reproduction in general results from such physiological isolation of parts and their dedifferentiation and redifferentiation into individuals. In the higher animals physiological

'Child, Senescence and Rejuvenescence, 1915; particularly chaps, ii, iv, V, vi, viii, x, xv. isolation of parts probably does not occur except occasionally in embryonic stages, for with the evolution and development of the nervous system in the individual the transmission-decrement decreases and the effective range of transmission therefore increases until in the nerves of mammals the transmission-decrement is inappreciable under natural conditions in the lengths of nerve fiber available for experiment. In these forms the physiological limit of size of the individual determined by the range of dominance is very great and is never attained by the individual because growth is limited by the progress of differentiation in the course of development. In such organisms, then, physiological isolation does not occur except occasionally in embryonic stages before the nervous system has developed or under special conditions which limit the range of dominance or decrease the receptivity of subordinate parts.

Moreover, in the higher animals the degree and stability of specialization of parts of the body is so great that in most cases they do not respond to physiological or physical isolation by reproduction, but either die or remain largely unchanged. For these reasons asexual reproduction among the higher animals is rare and is limited to early developmental stages. Sexual or gametic reproduction which results from the union of the two gametes or sex cells, which are usually specialized and differentiated as egg and sperm, is somewhat more complex than asexual reproduction, but I have already endeavored to show that there is a fundamental physiological similarity in the two processes,' and I shall consider the question briefly in a later chapter.

This, then, is in brief the dynamic conception of the organic individual which has grown out of years of experimental investigation, observation, and analysis of facts already at hand. Its distinctive feature is the interpretation of physiological unity and order in terms of differences in rate of reaction and of transmitted changes, instead of in terms of a hypothetical organization and of transportation of chemical substances. Ac- cording to this conception the central nervous system in its relation to other parts is merely the final expression of relation which is the foundation and starting-point of organic individuation. This conception provides a working hypothesis based on a great variety of evidence and readily accessible to experimental and analytic investigation, and while it is manifestly far from being a complete solution of the problem of organic individuality, I believe that it throws some light on various characteristics of the organism the nature and significance of which have heretofore remained obscure.

It is perhaps necessary to point out that this dynamic individuality is not the only kind of individuality which exists in the organic world. Physical individuals of crystalline or crystalloid character, and perhaps physicochemical individuals of other sorts exist in organisms. It is not with these, however, that we are concerned, but with that sort of individuality which is distinctive of the living organism, which determines harmonious development and functional unity throughout the continuous dynamic change which constitutes life. Where this organic individuality makes its first appearance it is impossible to say. The cell or protoplast in its simplest terms usually shows some degree of such indi-

viduation, but it is probable that some real or apparent individuations which arise temporarily or are persistent in the cell approach more nearly the inorganic than the organic kind. Nevertheless, wherever a region of high metabohc rate arises in protoplasm, there some degree of organic individuation arises, at least for the time being, provided relations already existing do not interfere with or inhibit the establishment of a metabolic gradient.

According to the dynamic conception organic individuality results in the final analysis from the relations between living protoplasm and the world external to it. If we accept this view we should expect to find morphological structure and differentiation making their first appearance in the superficial regions of the protoplasmic mass. These are in more direct relation with the external world and therefore more irritable and with the establishment of a region of high metabolic rate a metabolic gradient must arise much more rapidly in the superficial than in other regions. The facts agree well with this view, for the first indications of individuation in the organism are very generally superficial and in many of the simpler forms, such as the infusoria among animals, orderly morphological differentiation is always limited to the superficial regions. The nervous system i§ also superficial in origin. In the plant cells also the superficial portions of the cytoplasm generally show a higher degree of stabihty than other regions and are apparently chiefly concerned in whatever morphological protoplasmic differentiation occurs. If organic individuality is self-determined there is no apparent reason for its appearance as a superficial phenomenon.