Physiological Foundations of Behavior

As regards the effects of excitation on the nervous tissues the facts are equally clear. Memory, the possibility of learning, the development of habit, all show clearly enough that excitation brings about exceedingly persistent changes somewhere in the nervous system. Moreover, the changes in form, size and protoplasmic structure of nerve cells in relation to excitation, fatigue and recovery are also effects of excitation which are much less readily reversible than the dynamic changes immediately concerned in the process. Probably excitation is as nearly completely reversible in the nerve fiber as anywhere, but it may at least be questioned whether even here the

passage of excitation does not leave some more or less persistent effeet beyond that of the brief refractory period. It is highly probable that excitation is necessary for the maintenance of the fully developed nerve fiber, as it is for muscle, and that the absence of excitation is a factor in the degeneration following separation of the nerve fiber from the cell body. But whatever the situation as regards the nerve fiber, it is certain that in nervous tissue in general certain effects of excitation may be highly persistent.

Turning to the question of the persistence of effects of excitation and transmission in protoplasms generally, it is evident that, from the viewpoint of this book, this question is closely connected with the question of the origin and nature of the physiological gradients. Postponing general discussion to the following section, certain facts may be noted here, viz., that in certain plants polarity, in certain others symmetry is usually determined by light (pp. 58, 115), that polarity may be electrically determined in certain animals (pp. 111, 115), and that in general the physiological gradients which represent physiological axes in their simplest known terms apparently originate as reactions to differentials in external factors which alter primarily the rate of living. In the following section we shall see how closely such gradients are related physiologically to excitation-transmission gradients.

There are of course great differences in the permanency of the reeord, 7. e., the degree of irreversibility of the effects of excitation in different protoplasms. In some simple forms, e. g., Ameba, they apparently are very largely and very rapidly reversible under ordinary conditions, but one of the features of progressive evolution is an increase in the persistence and stability of such protoplasmic records, and this stability or irreversibility attains the most striking manifestations in the phenomena of memory in the higher animals and man (Herrick, ’24, Chaps. V, XX).

It was shown in Chapter VII that physiological axes and directions in which pattern and order arise in individual development appear in their simplest known terms and in the earliest stages as quantitative gradients in physiological condition. Although various theories of polarity and Symmetry postulate ‘3 stereochemical or other molecular structure and orientation as the basis of these phe nomena, there is no actual evidence that such structure and orienta-

tion exists in protoplasm generally, in eggs or in the early stages of embryonic development. According to the evidence, physiological axes in their simplest terms and their earliest distinguishable stages are physiological gradients. In Chapter VIII it was shown that these gradients are fundamental factors in axiate localization and differentiation, that when they are obliterated experimentally, axiate differentiation fails to occur and that when new gradients are experimentally established they determine new and corresponding axial relations.

Such gradients exhibit in general the same physiological characteristics as excitation-transmission gradients except for the fact that they are more or less persistent according to the nature of the protoplasm in which they appear. They involve differences in rate of the fundamental metabolic reactions, and differences in electric potential similar to those observed in excitation and transmission, and in many cases differences in permeability of membranes have been demonstrated at different levels of these gradients and such differences are probably present in all.

In Chapter IX evidence was presented to show that the physiological gradients, even though in some cases they persist from one generation of individuals to another, are not inherent in protoplasm, but originate in a reaction of protoplasm to a local or differential impact upon it of an external factor. Moreover, this relation between the protoplasmic reaction which gives rise to the gradient and the external factor is not specific, qualitative or material in nature, but rather quantitative and dynamic.

From all these facts only one conclusion appears possible: the physiological gradient, which according to the evidence at hand, represents the physiological axis in its simplest terms, exhibits essentially the same dynamic characteristics as the excitation-transmission gradient and originates, like the latter, as. a reaction or response to a local or differential action from without. The facts indicate also that the essential feature of such action is not its specific character, but its quantitative effect upon the rate of dynamic change in the protoplasmic system. This statement may appear at first glance to conflict with some of the evidence presented in Chapter 1X. For example, it was pointed out there (pp. 117-119), that differences in oxygen supply or accessibility or differences of exposure permitting different rates of CO, discharge are probably in many cases essential or important factors in determining new axial gradients. The objection may be raised that such relations between protoplasm and environment are material

and qualitative in that they involve specific chemical substances. This is of course true but it is equally true that these relations also possess a quantitative and dynamic aspect. Within certain limits the supply or accessibility of oxygen is an essential factor in determining the rate of fundamental protoplasmic activities, in other words, the rate of living. In general, decrease in oxygen supply below a certain level retards, and increase accelerates the rate of living. Similarly the accumulation of CO, in protoplasm above a certain limit re- — tards and increase of its rate of discharge accelerates the rate of living. As regards some of the specialized mechanisms of higher animals and as regards anerobic organisms these statements require qualification, or may not be true, but, so far as we know, they hold good for the less highly specialized erobic protoplasms, and it is in such protoplasms, if anywhere, that these conditions are effective in determining new axial gradients. It has never been maintained that axial gradients may not be determined and established by material relations between protoplasm and environment, but it is maintained that the essential factor of such relations as regards gradient determination, is not their specific material character, but their quantitative dynamic effect upon the protoplasm. There is then as regards this point no conflict between evidence and conclusion.

Again, it may be pointed out that axial gradients are normally or usually determined in different ways in different protoplasms, in some cases by light, in some probably by oxygen, in some perhaps by gravity, etc. This is true, but this means only that the normal or usual factor is the factor to which the protoplasm is normally or usually subjected, or which is most effective under natural conditions. In experiment the gradients may be determined by other factors than those which are normally effective. In the egg of the alga, Fucus, for example, the gradient is usually determined by differential illumination (pp. 58-61) but as Hurd (19, ’20) has shown, polarities, 7. e., the gradients of eggs lying close together are determined with reference to each other. In such cases a material relation perhaps to oxygen or CO» is apparently the effective factor instead of light. In hydroids the axial gradients may be determined by difference in exposure to water, oxygen or CO, or both, being probably the effective factors, but as Lund (’21) has found, the gradients may also be determined by the electric current. Under certain experimental conditions the polar gradient in the sea anemone, Harenactis, may be determined by local injury (pp. 119-121), and while it is not known how the gradient is normally determined it is certainly in some other

way than this. And finally, it has been shown that axial gradients y be altered in the same directions or even obliterated by different chemical agents without relation to the chemical COD ion of such agents (pp. 104-107). In the light of the evidence the only possible conclusion seems to be that any external factor which is able to alter the rate of the fundamental changes concerned in the life of 2 protoplasm may be effective in | determining, altering or obliterating the axial gradient or gradients in the protoplasn. | Summing up, the important conclusions from this brief review _ are these: The physiological and axial gradients are primarily quan- | titative gradients in specific protoplasms; they represent physiolog- _ jeal axes and directions of order and pattern in development, in their simplest known terms; they show all the physiological characteristics of excitation-transmission gradients except that they are | usually more persistent than the latter. They originate through the local or differential action, continued for a certain length of time, of external factors which alter the rate of changes fundamentally concerned in life. In short, whatever the character of the determining factor in a particular protoplasm or under particular conditions, the physiological gradients apparently constitute essen- | fially more or less fixed or permanent excitation-transmission gra- | dients and become the basis of the further local and regional differ- | ences which constitute axiate development. If these conclusions are correct, the chief difference between the axial gradient and the excitation-transmission gradient is the relative fixity or permanency of the former as compared with the latter.

In a particular protoplasm this difference is undoubtedly depend- _ ent upon the difference in the period of local or differential exposure " to the external factor in the two cases. An excitation-transmission | gratient may arise from a momentary or very brief exposure, but F the determination of a physiological gradient requires a relatively dong time. We have as yet few exact data on this point, but in "the case of the Fucus egg, for example, it is known that the differential illumination must continue for at least several hours, the length of time being dependent within certain Inmits on intensity and on wave length (Winkler, 00 b, Hurd, ’20). In hydroids the period of differential exposure to water (oxygen or CO.) has not been determined, but is at least several hours. As regards animal eggs we have no data, but the period may be days or perhaps weeks if the local action or differential is slight.

So far as we can determine, protoplasmic changes involved in the brief or momentary excitation of protoplasm may be rapidly and completely reversible, though it is evident that some effects, however slight, may persist for a longer or shorter time, even in such cases. But it is also evident that with the continued, or frequently repeated local, or differential action of the external factor the persistent or irreversible changes in the protoplasm become appreciaable. These changes are in general such as to perpetuate the gradient determined by the external factor. Just what they are physicochemically is not known, but biologically they constitute an equilibration, a regulation in the protoplasmic system, initiated by the external factor. Since the continued local or differential action of the external factor constitutes an alteration in the conditions to which the protoplasm is subjected, the equilibration in the protoplasmic system results in altered conditions in that system, that is, in the gradual determination of a gradation in physiological condition, involving both structural and dynamic features. In other words, the local or differential action of the external factor gradually develops a more or less permanent record in the protoplasm and this record constitutes the beginnings of the physiological axes and of axiate pattern.

The permanence of such record differs very greatly in different protoplasms. In Ameba each pseudopod is a gradient for the time being (Hyman, ’17), but does not become a permanent axis, though in the “hmax” forms (Fig. 13), it may persist for a considerable period. In such cases the lack of physical and probably of chemical stability in the protoplasm determines that each new gradient is sooner or later obliterated by another or others. In axiate organisms, however, such gradients, having passed what may be called the critical point in their development, become the protoplasmic foundations of physiological axes and directions of order. Moreover, they not only persist as real regional physiological differences after the external factors which determined them have ceased to act, but they undergo a, progressive development until they attain a certain steepness or slope dependent upon the hereditary constitution of the protoplasm in which they appear. Experimentally we find that the physiological differences, at different levels as indicated by susceptibility, permanganate reduction, permeability, ete., are in general slight in earlier stages of development and undergo increase up to a certain point. This means that after a physiological gradient is once established, its later slope or steep-

ness and so its length, and in fact all its physiological characteristics are dependent primarily on the constitution of the protoplasm in which it exists and only secondarily on external factors. The physiological gradients of different species must differ in certain respects as the protoplasms differ, and in a particular protoplasm very different external differentials may give rise to gradients which are similar. When once the physiological differential has become persistent in the protoplasm, the protoplasmic constitution is the primary factor in determining further changes.

This viewpoint eliminates the difficulties involved in accounting for the high degree of constancy in individual development within the species in terms of physiological gradients determined by external factors variable in degree or different in kind. The external factor initiates, and the hereditary constitution of the protoplasm determines the general course of development of the physiological gradient. There is then, I think, no escape from the conclusion that the axial gradient is essentially an excitation-transmission gradient which has become more or less persistent through the occurrence of irreversible or slowly reversible protoplasmic changes. If such gradients represent the foundations in specific protoplasms of the pattern of the individual, this pattern is fundamentally an excitation-transmission pattern, originating in a reaction to an external factor. Such reaction is as truly behavior as the growth reactions of a plant or the motor reactions of Paramecium, an insect or a human being. It differs in character from these reactions because the mechanism is different. The gradient involves no mechanism beyond the protoplasmic mechanism, but the other reactions mentioned involve organismic mechanisms of various degrees of development and complication (Herrick, ’23, Chap. XIX). The various lines of evidence point to the conclusion suggested in Chapter V, viz., that excitation and its transmission are the primary factors of organismic integration in protoplasm. We know that excitation and transmission constitute the physiological basis of organismic excito-motor behavior (Chap. XI). If this is true, the physiological origin of the organism as an individual and of excito-motor behavior is the same. In view of the facts, the assertion that the individual organism represents a behavior pattern in a protoplasm of specific hereditary constitution is not a fanciful speculation, but a conclusion based on many different lines of experimental and observational evidence and involving no assumptions not based on such evidence.

It is perhaps necessary to repeat here that physiological dominance or control is a relation of organismic magnitude. That is to say, it consists not in a relation between individual molecules, colloid particles or any other single component parts of the protoplasmic system F but rather in the control of one region, cell or cell mass by another. In fact, physiological dominance and subordination are relations between parts each of which constitutes a living protoplasmic system with all the complexity of constitution and of physico-chemical relations among its parts that the word “living”’ implies.

The physiological basis of dominance. — In an excitation-transmission gradient the region of primary excitation exercises a certain dominance or control over other levels of the gradient, simply because its excitation initiates the gradient and constitutes the primary factor in determining for the time being the physiological condition of other regions within the gradient. As already suggested (pp. 49- 52) this is apparently the most primitive sort of physiological correlation, 7. e., correlation of organismic magnitude, between different regions of a mass of protoplasm or of cells. The physiological differences at the different levels of the gradient which make dominance and subordination possible are not preéxistent differences, but arise through the excitation itself and through its transmission. The facts indicate that the primary excitation and the differences in condition at the different levels of the gradient are fundamentally quantitative changes, and finally the relation between the external factor which gives rise to excitation and the excitation itself is primarily quantitative, not specific or qualitative. If these conclusions are correct, the physiological relation of dominance and subordination characteristic of an excitation-transmission gradient is a relation resulting from differences in rate of living which are initiated by the local or differential action from without and further determined by transmission. If we agree that all protoplasms are irritable or excitable, it is evident that all protoplasms subjected to the action of external factors are capable of giving rise to this relation of dominance and subordination.

Moreover, there can be no question that this relation is organismic in character (pp. 51-52) since it originates in physiological differences between regions and cells and constitutes, at least for the time being, a more or less definite physiological relation between them. Physiological differences between different regions and cells are the characteristic features of every individual organism. In their absence the individual organism does not exist.

According to this view, physiological dominance or control appears in its most primitive form in excitation and transmission. These processes afford a basis for physiological correlation and control even in the complete absence of any definite channels or other means of transportative or material correlation between the parts concerned, and this relation of dominance and subordination is the most general physiological relation between regions and parts in the individual

(Chap. IV). It constitutes the basis of “wholeness,”’ 7. e., the capacity for integrated harmonious action of parts. Granting the correctness of the argument, we cannot escape the conclusion that physiological integration and the wholeness of the individual originate in excitation and transmission. This does not necessarily mean that all individual organisms arise directly and only through excitation and its transmission. Specific material or chemical relations between protoplasm and environment may conceivably give rise to local or regional differences and relations in the protoplasm, and such differences and relations may become the basis of an individual organism (see pp. 53- 55). Even though the external factor in such a case is a-specific substance, its effect on protoplasmic condition is often as a whole quantitative rather than specific, e. 9., in case of many nutritive substances, and in any case quantitative effects constituting excita- ‘tion or inhibition, as well as qualitative effects, are involved. Un- prejudiced survey of the evidence indicates that the quantitative changes in living protoplasm are far more important in initiating individual pattern and integration than any specific, qualitative factors (Chaps. VII-IX). The latter unquestionably affect the course of differentiation, but at present there is no reason to believe that any individual organism originates in specific material differences without excitatory factors.

Dominance in the axial gradients. — In Chapter X evidence was presented to show that dominance and subordination are characteristic features of physiological axial gradients. The high end of the gradient is the dominant region. The degree and range of dominance depend on various factors. According to the preceding section of the present chapter, however the physiological gradient arises, it is essentially an excitation-transmission gradient more or less permanently fixed through the accumulation of relatively stable changes in the protoplasm.

It follows that dominance and subordination in the physiological axis are derived from the dominance and subordination of the excitation-transmission gradient. The high end of a physiological gradient differs from, and acts upon other levels in the same way as the region of primary excitation in an excitation gradient. The appearance of qualitative differences at different levels of an axial gradient affords a basis for specific material relations, “chemical correlation”’ between these levels, and out of these relations another sort of dominance and subordination may arise (pp. 44-47). But even in such cases, if the gradient persists, excitation-transmission relations may still persist in it. In any case the development of the individual consists, not only in the qualitative differentiations of different regions and parts, and the establishment of specific chemical relations between them, but also in the development of the dynamic relations of excitation and transmission. In the higher animals we see these two sorts of dominance, nervous and chemical, side by side, both highly developed and in some cases affording to some extent a double control of the same organ. According to the views advanced here, nervous dominance develops out of the more general and fundamental factor in physiological integration, while chemical dominance by means of hormones or other substances is of secondary origin (see Chap. V), though unquestionably of great importance, particularly among the higher forms.

Nervous dominance. — In “The Origin and Development of the Nervous System” attention was particularly called to the very intimate physiological relation of nervous structure and function to the axial gradients and the relations of dominance and subordination characteristic of them. We find that the central nervous system is definitely localized at the higher levels of the chief axial gradients and the later complications arising in it appear to correspond to the complications in the gradients during the course of development. The inference is that the nervous system in the individual develops out of the more primitive excitation-transmission relations characteristic of the gradients and that the functional relations of dominance and subordination in the nervous system originate physiologically in the more primitive dominance and subordination of the gradients. Morphologically the nervous system represents the further progress of differentiation of the higher levels of the gradients beyond the stage of quantitative differences. This conception enables us to recognize a physiological continuity between the primitive excitation-transmission gradient and the nervous system, and between, the behavior of

- protoplasm in general in response to local or differential action of an external factor and the nervous mechanisms and phenomena of behavior in the higher organisms (Herrick, ’24, Chaps. XVIII, XIX). The early localization and differentiation of the nervous system and its function as an organ of integration and of relation to the external world appear, in the light of this conception, as a physiological _ consequence of the establishment of physiological gradients in protoplasms: that is to say, where the nervous system develops at all it constitutes a fundamental aspect of the development and differentiation of physiological gradients.

The fact that nervous systems do not appear at all in some organisms and attain very different degrees of differentiation and complexity in others depends of course primarily on the differences in hereditary constitution of the different protoplasms. In some protoplasms differentiation in relation to the gradients is not permanent enough, or does not proceed far enough, to give rise to definite nervous structure and function. In such forms the gradients, the integration dependent upon them and the behavior of the organism all remain relatively simple and primitive, as, for example, in the plants. At the other extreme are the higher animals in which the nervous system is the first definitive organ to become morphologically distinguishable and in which it attains a high degree of permanency and complexity of structure and function. Such differences result primarily from differences in protoplasmic constitution which are hereditary. The physiological gradients are merely the factors which determine the realization as an individual organism of the hereditary potentialities of each particular protoplasm. The development of nervous structure and function in any particular organism represents certain aspects of behavior in the presence of certain physiological conditions, viz., the axial gradients, and in every case such behavior takes place in a protoplasmic system possessing a certain hereditary constitution. This constitution plays an essential part in determining the character of the behavior and of the record which behavior leaves in the protoplasm and therefore in determining the course and result of development in each particular case.

To sum up: the available facts force the inference that the simple excitation-transmission gradient is the physiological basis of the relation of dominance and subordination which constitutes the integrating factor in the individual organism. Such dominance represents the action upon other regions of the region most affected in a dynamic way by an external factor. This primitive excitatory reaction and the resulting transmissive relation constitute the physiological basis of the mechanisms of control which develop in each particular kind of organism. Dominance does not arise autonomously at the beginning, or in the course of development, but results from a reaction of the protoplasmic system to an external factor. The primitive dominance, arising momentarily in an excitation gradient, the dominance of the growing tip of the plant in development, of the apical region in a hydroid, of the head in Planaria, etc., of the receptor end of the reflex are and of the brain in the behavior of man, all apparently originate in and develop from physiological gradients. If true, that means that they all result from the behavior of a protoplasm under certain conditions. But as the mechanisms differ with the constitution of different protoplasms and different stages of development the results differ, and we have at the one extreme the evanescent excitation and transmission and at the other the dominance of the brain.

In earlier chapters attention has been called repeatedly to the relation between heredity and the individual which this physiological conception of the individual involves, but at the risk of some repetition, it is desirable to bring together here and summarize briefly the chief points in this relation. In the first place, this conception recognizes physiological continuity from the simple excitation gradient in protoplasm to the structural and functional relations of the higher animals. Every individual represents a special case of realization of certain hereditary potentialities and in any one individual only a small part of the potentialities are realized. When we bear these facts in mind, it is perfectly evident that the hereditary mechanism alone cannot account for all the special cases of realization, no two of which are exactly alike. Moreover, if we accept Morgan’s conclusion that “every cell inherits the whole germ plasm”’ (see pp. 22-26), it is obvious that heredity alone cannot account for the fact that in development different cells and cell groups become different. Biological theory sometimes seems to regard the hereditary mechanism as if it were autonomous. Roux’s often repeated assertion that the earlier stages of development are determined by heredity, the later stages by function, seems to regard the hereditary mechanism as working autonomously in the earlier stages. According to Weismann the distribution of the determinants during development is accomplished entirely by the mechanisms of heredity working independently of environ-

ment and the belief that hereditary mechanisms work autonomously apparently persists up to the present time (see pp. 21, 221). Even the most extreme predeterministic conceptions must of course admit the existence of certain respiratory and nutritive relations between the developing individual and the external world, but these are regarded as primarily sources of energy for the hereditary mechanism. Roux recognizes such factors as “realization factors” but apparently does not regard them as essential in determining individual pattern.