Physiological Foundations of Behavior

Reflex action, as the term is commonly used, implies the presence of a more or less differentiated sensory apparatus of some sort, the receptor, a differentiated nervous path, the conductor, and a differentiated motor or other organ as the effector, Herrick (’22, Chap. IV and ’24, Chap. XVII), and various other authorities also include a central nervous organ, the adjustor, in the reflex mechanism. The reflex arc is then a rather highly specialized structural and functional mechanism which accomplishes a definite reaction of a particular kind involving certain definite organs. Reflex behavior is in general conspicuously and immediately regulatory in character. The burned or pricked finger is reflexly withdrawn from the source of disturbance. Irritation of the eye by intense light, by dust, etc., brings about closure of the lid and increased secretion of the lachrymal glands. Reflex equilibration of the body follows disturbance of normal position and so on. Consciousness, intelligence, reason are not concerned in such reactions. In nature, however, the reflex arc is not usually if ever, an independent reaction system but is correlated with other arcs. In the nervous systems of the higher animals and man these correlations and combinations constitute an apparatus of regulation, or of equilibration of almost inconceivable complexity, delicacy and range of action (Herrick, ’24, Chaps. XVII, XIX).

It is a self-evident fact that the reflex arcs and the reflex behavior of any organism are dependent upon the course of development in that organism. They are consequences and expressions of all that has gone before. The receptor and effector connections of each reflex are, the interrelations of different ares, whatever their adaptive evolutionary significance, must all have a physiological basis in the developmental processes and are evidently outgrowths of the general organismic pattern. In fact, the physiological continuity in the individual between the physiological or metabolic gradient and the reflex are is evident (Child, ’21.a, Chap. XIII). The physiological gradient is

the general physiological foundation on which the reflex arc develops. If we consider development in its functional, rather than in its structural aspects, it appears that the gradient is the primitive and generalized excitation arc out of which the various reflex arcs develop by specialization of function and differentiation of structure. In short, the physiology of development of the reflex arc has its starting point in the excitability of protoplasm, the differential action of environmental factors upon it and the resulting physiological gradient or gradients.

It is perhaps necessary to emphasize again the fact that this continuity with which we are concerned here is strictly physiological continuity in individual development, not hereditary continuity. The possibilities of reflex pattern in any particular protoplasm are of course given in the hereditary constitution of that protoplasm, but the actual reflex pattern appears only in the development of the individual, 7. e., as a behavior pattern in the broad sense in that protoplasm. And the evidence indicates that the physiological gradient is the primary mechanism of organismic integration, equilibration and regulation. If it is true, as I have endeavored to show, that the physiological gradient in each case represents the behavior of a protoplasm of a certain hereditary constitution in response to an external differential, it becomes clear at once that this behavior is the physiological factor which initiates the realization of the hereditary possibilities, that is to say, the development of the germ plasm into individual organisms. From the physiological viewpoint, then, the reflex ares and the reflex behavior of any particular organism, like other characteristics of the individual, are determined by this primary behavior and from the viewpoint of heredity, by the hereditary constitution of the protoplasm. Here, as elsewhere, heredity determines the possibilities in each case and behavior in the broad sense determines the realization of possibilities in each individual.

Elsewhere (Child, ’21a) I have maintained that it is impossible to account physiologically for the origin and development of the nervous system except in some such terms as these. If the individual is not from the beginning of its development a behavior pattern we cannot escape some form of dualism or vitalism. The reflex is the unit reaction (Sherrington) or the physiological unit (Parker) on which nervous function and integration are built up, and if physiological continuity between the reflex and the primitive behavior of protoplasm, as shown in the gradients resulting from differential excitation, does not exist, the problems of the origin and development of

_ the nervous system and of reflex behavior must, I believe, be regarded The relations between reflex, trial and error, and tropic behavior require brief consideration. Trial reactions and tropisms occur both in organisms without and those with structurally differentiated nervous systems. In the former there are, strictly speaking, no reflex ares, but temporary or permanent excitation arcs or excitation gradients may be present and so be factors in the reactions. In forms possessing nervous systems, reflex arcs may play a part in both trial reactions and in tropisms. The behavior of the decapitated frog when a drop of acid or other irritating substance is placed on one hind leg and the other held is a beautiful case of reflex trial reaction. Failure to remove the source of irritation by reflex movements of the other leg is followed by various other movements, until, if the irritation is sufficient, the greater part of the muscular system of the animal is involved. In organisms with bilateral sense organs and a nervous system a tropic orientation must depend upon a reflex mechanism. In fact, tropisms in general, at least in animals, are often regarded as reflex behavior.

Summing up, the reflex, strictly speaking, is a specialized behavior pattern depending on the presence of certain morphological mechanisms; but it is physiologically a development from the primary organismic behavior mechanism, the excitation gradient. Both trial and error reactions and tropisms are integration patterns of the behavior mechanisms present in the organisms concerned and in organisms possessing reflex arcs either trial reaction or tropism may consist of reflexes. The question of the rdle of the reflex are in conscious intelligent behavior of man and the higher animals is outside the limits of this consideration (see the concluding chapters of Herrick,’24).

While regulation is in physiological terms equilibration accomplished in one way or another, from the viewpoint of evolution it is in general adaptive. The protoplasmic constitution which makes possible certain equilibrations favoring maintenance of the system in response to external disturbance is able to survive and persist under conditions which determine the destruction of some other protoplasm that does not possess the same possibilities of equilibration. The latter protoplasm, on the other hand, may be able to undergo equilibration to certain other disturbing conditions which destroy the former. Different protoplasmic constitutions present different

possibilities of equilibration and every change in protoplasmic constitution provides new possibilities. The mechanisms of equilibration are the physicochemical changes which make up life in their interactions and interrelations. In the organism all the mechanisms of physiological correlation are concerned. Whatever value we may assign to natural selection with respect to morphological characters, we cannot deny its importance with respect to these mechanisms of

equilibration. Its action is inevitable. A protoplasm or organism which cannot equilibrate under certain conditions in such a way as to persist perishes, while one which is able to equilibrate survives. Going one step farther it is evident that in the course of evolution the useless or injurious equilibrations must in the main disappear, since the organisms in which they occur must be more or less rapidly weeded If the conception of the individual organism which has been developed in the preceding chapters and elsewhere (Child, ’15 ¢, ’21 a) in some measure approximates the truth, we are forced to the conclusion that evolution is primarily concerned, not with morphological characters, but with these physiological mechanisms of equilibration and integration. In other words, the organism has evolved primarily, not as a morphological structure, but as a behavior mechanism in the broadest sense. The morphological features are secondary products of the behavior mechanism.

Evolution, as suggested at various points in this chapter, has been a process of standardization, of the behavior mechanisms of organisms, and through these behavior mechanisms, of the conditions which they are likely to meet in nature. In this way we attain again the conception of the normal in organisms and the normal in environmental factors. Within the range of these norms the mechanisms of behavior, that is, of equilibration or regulation, are in the main useful or adaptive; they tend toward persistence of the individual or maintenance of the species, or often both. Such adaptation is not dependent upon some mysterious purposive character of the mechanisms of equilibration, but appears to be primarily a matter of selection of protoplasmic constitutions, of action systems with their mechanisms.

It is difficult not to believe that the natural selection of mechanisms of equilibration or regulation is an important factor in evolution but its action does not exclude or conflict with the action of various other factors. Changes in protoplasmic constitution may conceivably occur suddenly as mutations, or gradually as lesser variations. Certain ‘courses of change may be inherent in certain protoplasmic constituirreversible differentiation may occur in the evolutionary history of a protoplasm, and, as Herrick (’20) has pointed out, may appear as

Whatever the factors of evolution, there can be little doubt that it is primarily concerned with mechanisms of equilibration, integration or regulation, in short with behavior mechanisms in the broad - sense. Morphological characters are the products or the records in _ protoplasm of the action of the behavior mechanisms and so are of secondary importance. If this is true the organism, not only in its _ life as an individual, from the beginning of development on, but in

its evolution, is fundamentally a system of behavior mechanisms. Evolution has standardized for every species both these mechanisms and the range of conditions to which they are likely to be exposed. The result is on the one hand what we call the normal in nature and on the other the capacities of the organism for “return to normal,” “adjustment of internal relations to external relations,” or regulation. This chapter is a consideration of certain aspects of life as a series of equilibrations or regulations. The mechanisms of the living organism are from the beginning on behavior mechanisms, and excitomotor behavior is not something distinct from these mechanisms and superadded at some stage, but it is the outgrowth, the consequence, and for the individual the culmination of organismic integration and the most advanced expression of organismic pattern. The physiological gradient as the primary behavior mechanism of axiate type and the primary factor of axiate pattern and integration constitutes the general physiological foundation on which the axiate behavior patterns are built up. From this viewpoint life is, physiologically speaking, behavior, equilibration, regulation, and evolution is a process of standardization of the behavior mechanisms of protoplasms and of the range of environmental conditions to which they are likely to be subjected.

In an earlier discussion of regulation (Child, ’11 f) attention was called to the analogy in certain respects between the living organism and the flowing stream. Both are within a certain range equilibrating systems and in both structure and function are similarly interrelated. The stream like the organism is always approaching equilibrium, but if it attains equilibrium it ceases to flow and is “dead.” From the morphology of its banks and channel we can determine that such a ‘“‘dead stream” was once a flowing stream, but this morphology is merely the record of its past activity. As the stream flows structural and functional equilibration, in short, regulation, is always taking place and always making the stream different in some way from what it was previously. We cannot, I believe, adequately conceive the living organism except as such a system in which equilibration between the parts and between the whole and its environment is continu-- ously going on and continuously changing the organism. We are forced to believe that the organism is not left absolutely unchanged by anything that has any relation to it. Life is change, modification, progression.

We reach the conclusion then that the organism does not, strictly speaking, represent the maintenence of a certain equilibrium, in spite of external disturbance, but rather a continuous alteration and equilibration in reaction to external factors. It is not a “closed system” maintaining itself against the rest of the world, but a system open at every point and in continuous and necessary relation to environment and the same is true for its parts in their relations with each other. Only through the fact that it is an open system are devel- - opment and evolution possible. Evolution is a process of standardization of the potentialities of behavior, of regulation, and of its mechanisms and through this means, of the environmental conditions to which the individual is likely to be subjected.

Even though we accept the conclusion of the preceding chapter that life is a series of modifications of protoplasmic and organismic patterns, it is evident that both in the individual and in evolution different components and features of pattern exhibit different degrees of fixity or modifiability. This is particularly true for excito-motor - behavior, but it is also true for development, and for the structural and functional mechanisms of the mature organisms. Moreover, different species show wide differences in modifiability of pattern. In some, even the more fundamental components of pattern may be readily and greatly modified, while in others they are fixed or stable to a high degree. Again the stability of a particular component of pattern may differ widely at different stages of development. It may be readily modifiable at one stage and highly stable at another.

It is evident that our interpretation of fixity and modifiability must depend upon our conception of organismic pattern in general (see particularly Chaps. III, XII, XIII). Preformistic theories, for example, lead us to a conception of a normal pattern, which is predeter- _mined and innate, that is to say inherited, while modifications of that pattern represent effects of individual reactions to environmental factors. From the physiological viewpoint, however, the normal pattern is no more and no less preformed, predetermined, or hereditary than any modifications or abnormal patterns. All organismic patterns are hereditary in that they represent potentialities of a certain protoplasmic constitution. On the other hand, the evidence forces us to the conclusion that all are likewise behavior patterns, that is, their realization is not an autonomous action of a protoplasm, but the reaction of a protoplasm of specific constitution to an environment. If this is true, fixity and modifiability of pattern are to be interpreted physiologically, not in terms of a predetermined normal and departures from it, but rather in terms of the physicochemical constitution of each particular protoplasm and the effects of external factors upon it. The normal in any field of biology represents merely a certain range of pattern among the potentialities of the protoplasmic system, and its significance is not physiological but evolutionary. Physio-

logically it represents merely the realization of certain possibilities of the system in relation to certain conditions, but from the evolutionary viewpoint it represents a standardization of both the protoplasmic system and the conditions to which it is subjected (Chap. XIII). Moreover, a distinction is to be made between the modifiability of pattern in organisms through the direct action of external factors and the modifiability through internal factors, changes in physiological state (Jennings). In the case of excito-motor behavior modifiability through changes in external factors is so obvious and characteristic that it is commonly taken for granted and the student of animal behavior is chiefly concerned with modifiability in its relation to internal physiological conditions or physiological states. Physiological states do not, however, arise or change autonomously so far as we know, but in the final analysis are related to external factors. To the “behaviorist” the point of chief interest is of course the modifiability of behavior by experience, the ability of the organism to learn, and it is evident that such learning can occur only when some sort of record or effect of a previous reaction to an external factor persists in the protoplasm (Herrick, ’24, Chap. XX).

Development as modification— The development of the individual is itself a series of modifications of pattern (see Chap. XIII). Moreover, if the physiological conception of organismic pattern is correct, protoplasm does not give rise autonomously to such pattern, but only as it is modified by external factors. According to the conception of the physiological gradient, organismic pattern originates in the local or differential alteration in rate of the fundamental physiological activities of protoplasm by some external factor. Such alteration amounts essentially to local or differential excitation or inhibition. This of course constitutes a modification of the preéxisting pattern, and if this modification persists, it becomes the basis of a physiological axis and of the developmental changes occurring in relation to that axis. New gradients arising in particular regions during the course of development initiate new series of modifications and the appearance of local qualitative differences affords a basis for chemical or transportative correlation and so for further modifications. In fact, this process of development of an individual from an egg, a spore, an isolated piece, or any other reproductive element, involves a modifiability so great that many biologists are still not quite able to accept the evidence of their senses, even when reinforced by the experi-

~ mental method. They find it necessary or desirable to postulate at ; - least the rudiments of the pattern as a property of the germ plasm, but they either ignore the problem of the origin of this primary pattern or offer only speculative solutions of it. Physiologically considered, the development of individual pattern is the realization of new patterns out of the potentialities of preéxisting patterns through behavior. In the development of the social institution which we call a state new patterns, new mechanisms, which existed only as potentialities of the system, 7. ¢., as ideas, or possible ideas in the minds of men, become real working patterns and mechanisms. We are accus-

~ tomed to say that the human mind has “created” these new patterns and mechanisms. In the development of organisms an essentially similar “creation” occurs and there is no reason so far to believe that Driesch’s entelechy or Bergson’s élan vital is any more necessary for such creation than for the creation of a new chemical compound out of the reacting substances, or for the creation of the features involved in the development and differentiation of a flowing stream and its banks and bed.

Under the usual standardized range of conditions the order and course of the modifications constituting development are relatively constant in each particular protoplasm and from this fact the idea has arisen of the normal in development as something fixed or predetermined, at least to a relatively high degree (pp. 215-219). From this viewpoint, modifiability represents the capacity of the organism to depart from this fixed or predetermined course of events in reaction to external factors. The normal may include a certain range of modifiability, but beyond this, modifiability leads to “abnormal” results. In the preceding chapter I attempted to show that the real significance of the normal in biology is not physiological but historical or evolutionary. Physiologically normal and abnormal represent merely different degrees or ranges of modification and may be determined by different energies, intensities, or periods of action of the same factor.

Changes in modifiability with progress of development. — When we examine modifiability or developmental pattern more closely we see that it is apparently related in certain ways to stage of development of the individual and of evolution of the species or group. These relations may be briefly stated as follows: modifiability of the morphological pattern and of the special functional expression of each structural mechanism decreases in general with the progress of development and of evolution. By this statement we mean merely

that the range of external conditions under which these features of pattern persist, increases during development and evolution. If development and evolution are equilibrations following disturbances, this progressive stabilization of pattern is to be expected. S$ pecifically it must result from physical and chemical changes which are irreversible or only in part reversible under ordinary conditions. The accumulation of relatively stable molecules provides a more or less permanent structural basis for functional stability, and functional stability tends in its turn still further to increase structural stability. Considering, for example, the primary factor of axiate pattern, the “polar” gradient, we see that a simple organism like Ameba may acquire such a gradient or axis temporarily as a result of an external differential (pp. 57-59), but the nature of the Ameba protoplasm is such that the gradient disappears soon after the external factor ceases to act. In the egg of Fucus, however, differential exposure to light for a few hours determines a gradient which becomes the basis of the axiate pattern of the plant (pp. 58-61). Similarly in the hydroids we see gradients determined by external differentials persisting after the external factor has ceased to act and becoming a permanent or relatively permanent basis of pattern (pp. 115-123). Even in the fully developed hydroid, however, the stability of this fundamental factor in the pattern, viz., polarity, is not very great, 7. €., 18 dependent on a relatively narrow range of conditions. Old gradients may be obliterated and new gradients, and so new polarities, may be experimentally determined in various ways (pp. 119, 123).

In Planaria, where a greater degree of differentiation has occurred in relation to the axial gradients, with a nervous system of well-defined axiate pattern as its primary characteristic, the experimental obliteration of preéxisting axial gradients and the establishment of new gradients requires much more extreme conditions. In many of the higher animals we are able at present to obliterate the major axial gradients and determine new ones only in the earlier stages of development or not at all.

Again, the experimental alteration of slope of the axial gradient through differential susceptibility and the resulting alterations of form are brought about by less extreme conditions in the earlier stages of development and in the simpler organisms than in later stages and more highly differentiated forms (pp. 101-109). In the sea urchin, for example, a certain degree of differential inhibition applied during the early stages may greatly reduce or obliterate polarity as well as bilaterality, while the same degree of inhibition at a later stage may reduce

or obliterate bilaterality while polarity still persists (Child, ’16 d), and at still later stages may have but little effect on either polarity or As regards form-regulation, the reconstitution of individuals from isolated pieces, the regeneration of appendages, organs or tissues, we find in general that the simpler organisms are more capable of modifications along this line than the more highly specialized. In many plants every cell or almost every cell is capable of giving rise to a new individual. Among sponges and hydroids very small fragments are capable of forming complete new individuals (H. V. Wilson, 07, ‘11 b). Among the Turbellaria limitations in this reconstitutional capacity occur. In some species, ¢. g. Planaria, some cells at all levels of the body are sufficiently modifiable to give rise either to a head or a posterior end according as they lie at the anterior or posterior end of an isolated piece. In other forms such as Dendrocelum (FP. R. Lillie, 01) only the cells of the more anterior body levels are able to develop new heads, and in most of the rhabdoccels and the polyclads levels posterior to, or more than a very short distance posterior to the cephalic ganglia do not give rise to new heads, but all levels are capable of giving rise to posterior parts. Among the aquatic oligochetes a very similar antero-posterior decrease in modifiability of pattern in the regulation of pieces occurs (Hyman, ’16 a). In the later stages of development of the arthropods and vertebrates no cells of any level of the body are able under any known conditions to give rise to a new head, though appendages and various minor parts are regenerated in arthropods and the lower vertebrates, and tissue regeneration occurs even in the mammals and man.

Similarly the range and frequency of “abnormal” forms in form regulation are much greater in the simpler than in the more complex animals. With the determination of new gradients the occurrence of biaxial and multiaxial forms (‘axial heteromorphosis ”’) is readily controlled experimentally in the hydroids (pp. 116-118). Among the flatworms and oligochetes biaxial forms are of less general occurrence and in other groups can be produced only in the earlier stages of development if at all. In the adult Planaria a series of different degrees of differential inhibition of headdevelopment can be determined experimentally in a great variety of ways (Child, ’21 ¢). In fishes and amphibia somewhat similar modifications can be experimentally determined in early developmental stages, but not later (Stockard, ’07, 09, 710, ’11, ’21; Bel-

So-called self-differentiation and mosaic development. — Many of the special problems of the physiology of development are simply different aspects of this general problem of modifiability of pattern. — The apparent self-differentiation (Roux) or independence of certain parts, the correlative differentiation or dependence of others, the “mosaic” character of the early developmental stages of the annelids and mollusks and certain other forms (pp. 146, 244) and the plasticity of the early stages of echinoderms, medusz, ete., all represent widely different degrees of modifiability either of different parts of the same embryo or of different species.