Physiological Foundations of Behavior

The gamete and the developing embryo are acted upon by, and react to factors external to themselves at all periods of their existence and whatever mechanisms or structures are present at any time are functioning. Material exchange is always going on and we know that excitation by various forms of energy and the transmission of such excitation may occur in these stages as well as in the fully developed individual. We know also that even within the range of the normal many differences between individuals result from differences in external conditions within the standardized range. And finally, we know that when we subject these stages to external conditions outside the standard range, 7. ¢., “abnormal” conditions, reaction of some sort, with equilibration leading either to a normal or an abnormal result, or destruction of the system and death may result. In fact, we can find no evidence for the hereditary predetermination of a normal or typical course of development, except in so far as each protoplasm possesses a certain hereditary constitution and as the conditions under which the individual develops are more or less exactly determined or standardized by the hereditary mechanisms of the parent and the species. The hereditary constitution of the protoplasm determines certain possibilities which are realized only in relation to external factors, and the hereditary mechanism of the parent or species, as noted above, determines position of gonads, method of egg deposition, etc. So far predetermination occurs, but such predetermination is indirect and physiological, rather than direct, that is, it determines, on the one hand, the possibilities of reaction and on the other, the range of conditions likely to be met.

From this viewpoint development is not fundamentally different from the so-called functional later stages. Mechanism and function of some sort are present at all stages and the mechanism and function at any stage constitute the structural and physiological basis for what follows, but what actually does follow in a particular case is determined by the mechanism functioning in relation to particular conditions. Adjustment, equilibration, is going on at all stages, but the sort and degree of equilibration at any stage is determined by the mechanisms characteristic of that stage and the conditions to _ which it is subjected. In the earlier stages of development and in

the simpler organisms equilibrations involving the larger features of form and structure are in general more extensive and more readily induced than in later stages and in the higher forms, because in the former function is less narrowly and exactly limited by morphological structure than in the latter. Except as regards the earliest stages, which are really stages of dedifferentiation and disappearance of gamete and zygote structure (Child, 715 b, Chaps. XIII-XV), we find that in general processes of structural differentiation are more conspicuous features of the earlier than of the later stages of development. The larger, more general features of structure appear first and the details are, so to speak, filled in later. As structure develops, it determines and limits function more and more definitely, therefore its stability increases and the specialized functions of organs are dependent upon its existence but both structure and function of some sort are present at all stages.

Development as regulation. — It is evident from what has been said that development itself may be regarded as a complex regulation resulting from the action of various factors. Among these fertilization is usually essential, but various others, e. g., the isolation of the gametes from the parent body, the altered nutritive and respiratory conditions, the physico-chemical constitution of the external medium to which the developing individual is exposed, or its relation to the parent body, may be concerned in determining the course and results of development, according to the species. Assuming that the normal hereditary constitution of the protoplasm is present, whether the course and result of development are what we know as normal depends simply upon what I have called the standardization of these environmental conditions through the hereditary mecbanisms of the species, 7. e., the determination in one way or another, and to a greater or less degree through heredity, of the conditions to which the developing individual shall be subjected. With the progress of evolution

this standardization becomes in general more complete, that is, the parent plays a more and more important part in determining the conditions which its developing offspring shall meet, and the regulatory mechanisms of the protoplasms become more delicate and varied. Normal development of a normal protoplasm is then a complex series of regulations or equilibrations to conditions within a standard range and abnormal development is a similar reaction to conditions transcending this range. Certain abnormalities, however, are hereditary and in such case must have a certain basis in the protoplasmic constitution. With these we are not concerned here except to point out that the expermimental work of recent years in genetics has shown that at least some such hereditary abnormalities originate in the action of abnormal factors upon the reproductive cells at some time. Such cases are equilibrations with persistent effect and it is a pertinent question which has no necessary connection with Lamarckian theory, whether all existing hereditary characteristics of organisms are not in the final analysis of such origin.

Physiological gradients as regulatory mechanisms. — The conception of the physiological gradients and their significance in individual development is merely an attempt based on many lines of evidence to throw some light on the physiological character of the general regulatory mechanisms and processes which underlie development. The mechanism of excitation and transmission and the excitation-transmission gradient are regulatory mechanisms of fundamental significance in life, and the establishment of a more or less permanent physiological gradient as the basis of a physiological axis is a further regulatory modification of the protoplasm in reaction to an external differential. As already pointed out (pp. 133-137), once the gradient is established in a protoplasm, it tends under ordinary conditions to attain a certain height, slope and length characteristic for such protoplasm and changing as the condition of the protoplasm changes. The further course of developmental events in relation to it consists in a complex series of regulations involving form, structure, constitution and function of the various parts concerned. These represent the reactions of the hereditary constitution of each particular protoplasm to the physiological gradients established in that protoplasm and within the range of normal] development they approach the condition characteristic of the mature normal individual of the species. From this viewpoint the physiological gradient is merely the primary regulatory mechanism in the development of the axiate

individual and probably also in the development of the surface-interior organism (pp. 93-94). Under conditions outside the standard or normal range the course of development and the end attained may be altered in one way or another. Such alterations appear in the development of new individuals from isolated blastomeres of an embryo or from isolated pieces of the body of a mature individual. In Planaria, for example, the course and results of the development of isolated pieces differ widely according to age, nutrition, and physiological condition of the animals from which the pieces are taken and also according to size of piece, region of body represented, temperature and chemical constitution of the external medium (Child,’ 11 b, ¢, d,’12a,’14 e, 16 b, ’20a, ’21¢). Moreover, the work of many investigators has demonstrated that the course and results of embryonic development may be very greatly altered by changes in environmental factors, and I have endeavored to show that changes in the gradients are fundamental factors in such alterations (Child, 716 d,’17d,’21c). It has been shown further that physiological gradients can be obliterated and new gradients established experimentally (see Chaps. VIII, IX) with corresponding changes in the course and results of development. All such changes represent regulations or equilibrations of the protoplasmic system to the conditions to which it is subjected. The fact that many of them differ widely from the normal means merely that under different conditions different equilibria are attained or approached.

The relation of dominance and subordination between regions and parts of the organism, which is the most general and primitive sort of physiological relation in the organism is, as I have pointed out, primarily an expression of a physiological gradient and therefore a feature of the regulatory mechanism which that gradient represents (see Chapter X). As regards the special functions of the organs of the individual at any stage, we recognize clearly that the unity and integration of the individual are maintained by the interrelation and equilibration of such function, in short, by regulation. The various regulatory mechanisms of the physiologists, e. g., mechanisms regulating transport of water, salts and carbohydrates in plants, blood-flow, respiration, heat production, heat loss, secretion, excretion, neutrality, etc., are cases in point. But as noted above, under conditions transcending the normal range, the working of these mechanisms is altered and may lead to abnormal results, or even to death. Here as elsewhere the normal represents merely a certain range of constitution and con-

ditions more or less standardized by evolution and heredity. The physiological unity of the organism is in fact dependent upon the existence of regulatory mechanisms of one kind or another and the evidence indicates that the general basis of such mechanisms is on the one hand the hereditary constitution of each particular protoplasm which determines the character of its reaction to any conditions and, on the other, the primary regulatory mechanism of the individual organism, the physiological gradient. The fact that each protoplasm possesses a certain hereditary constitution implies the potential existence in it of certain protoplasmic regulatory mechanisms of physical and chemical character, but only as these are integrated and ordered on a large scale by the establishment of a differential of some sort, involving different molar regions of the protoplasm do they become of organismic significance (pp. 6-12, 43). The physiological gradient represents, I believe, the most general and primitive form of such differential and therefore constitutes the primary organismic regulatory mechanism. The gradient or gradients established in a protoplasm initiate a series of regulations involving form, structure and function, and these constitute development. The structure of any given moment or stage is a factor in determining and limiting the character of the function occurring in it and the function of any given moment alters the structure in which it occurs.

We are accustomed to speak of development as ending with maturity. Actually of course, development never ends while life continues, for structural and functional equilibrations are always going on and leaving their records in the protoplasm. So far as its developmental aspects are concerned, then, life may be regarded as a continuous series of regulations, or in the words of Spencer as “continuous adjustment of internal relations to external relations. ”’

In the past biologists have often drawn a sharp distinction between development as a process of construction of a machine and behavior as the functioning or working of the machine after construction is completed. But if the position taken in the preceding section is correct, development represents behavior as truly as any activities of the mature organism, and we have to distinguish not between development and behavior, but between different aspects and phases of developmental and other forms of behavior.

In the excito-motor group of behavior phenomena, motor changes in position in space are a conspicuous feature. These may consist of locomotion of the organism as a whole, of movement of some organ or part through change in direction, rate or character of growth, contraction, etc., or of movements of substances in consequence of excitatory changes in permeability of membranes, etc., e. g., in secretion. Since such behavior is usually the most conspicuous feature of the reaction of organisms to environment in the more advanced stages of development, we may for convenience distinguish it as excito-motor behavior from developmental behavior. From the present viewpoint, however, such behavior is essentially one aspect or form of developmental behavior, characteristic of the more advanced stages after morphological mechanisms have differentiated, and developmental behavior in general is merely one aspect of life. Life itself, in so far as it consists in reaction or response is behavior (Chaps. I, XII, XIV). Jennings says for example:

“ Behavior is merely a collective name for the most obvious and most easily studied of the processes of the organism, and it is clear that these processes are closely connected with, and are indeed outgrowths from the more recondite internal processes.”’ (Jennings ‘06, p. 339). Moreover, if behavior means reaction or response to some external factor we are forced to conclude that behavior occurs, not only in living organisms, but in all non-living things. In short the universe considered as a series of changes causally determined by the relations between its constituent parts is behavior.

But, employing the term for the present in its common biological sense as referring primarily to excito-motor reactions, it is evident that in the behavior of living organisms the capacities of the individual for adjustment, equilibration or regulation are even more conspicuous and striking factors than in its other activities. In general, behavior appears to be directed toward the end of adjustment and the most delicate, most rapid and most complete adjustments are accomplished in this way. That is to say, behavior generally, though not always, leads to results which are useful to the organism in one way or another and enable it to exist, to maintain itself, to propagate its kind and particularly in the higher, more strictly psychic forms of behavior to profit by past experience and to act more or less intelligently when brought into relation to new collocations of external factors.

Even the simpler organisms behave, in general, in ways that favor their maintenance. In the absence of food, activities appear which tend to provide it. Motile forms in a region of insufficient oxygen attempt to escape. Forms normally living in darkness or weak light tend to move away from strong light, those needing light move from darkness or weak light into stronger. All such reactions are not only physiologically speaking equilibrations, but are also regulations in the stricter sense that they are useful. In fact, as Jennings puts it, “behavior is adjustment or regulation” (Jennings, 05 b, p. 474).

As in other fields of biological activity, so in the behavior of organisms in the stricter sense, we do, however, find many reactions which are not useful, and which may even be injurious or lead to death. The preference of fishes for water containing certain injurious drugs (p. 220), the reaction of the moth to the flame, are cases in point. Again, the reactions of which the organism is capable may not suffice to meet the conditions to which it is exposed and death results in spite of them. In water with low oxygen content, for example, Planaria shows first a distinct negative reaction to gravity. Such a reaction would ordinarily bring it toward the water surface and into levels of higher oxygen content, but in a closed vessel without air the reaction, although useless, occurs. In general such reactions are observed under external conditions well outside the standardized range which we call normal, in other words, under conditions to which the evolutionary process of standardization has not been applied. Physiologically they are of the same character as the useful forms of behavior, but they are not useful and may be injurious or deadly. The regulatory mechanisms are at work, but are inadequate, and any equilibrium attained or approached is so far from the normal as to be useless or injurious. In some cases of this sort the reaction of the organism undergoes modification in the course of time and a useful reaction takes the place of the original useless or injurious reaction, but in other cases no useful reaction is possible and injury or death is inevitable.

These cases of useless or injurious behavior, like the useless or injurious forms occurring under certain conditions in the regulation of form and development (pp. 219-220) are of interest as showing something of the limitations of organismic mechanisms, but they do not alter the fact that in the main the behavior of organisms is both equilibratory and useful, 7. e., regulatory in the stricter sense. The potentialities of behavior of each organism are of course given in the hereditary constitution of its protoplasm in what Jennings terms its action system, in its simplest terms, but the actual behavior pattern is realized only in behavior, 7. e., in the reaction of

this protoplasm to external factors. As already pointed out, this does not mean that light is essential to the development of the behavior mechanisms in which the eye is concerned, or sound to those involving the ear. It means merely what I have endeavored to show in the preceding section, viz., that protoplasm is not autonomous and that the individual from its beginning represents a series of reactions to external factors of a particular protoplasm with certain potentialities. The behavior of the mature organism represents merely the most advanced and most conspicuous features of this reaction series and is dependent upon the various reactions of earlier stages. In his discussion of regulation in behavior and in other fields Jennings (’06 a, Chap. XXI) reaches essentially this conclusion.

As regards the methods by which regulation is accomplished through behavior, it is evident that they must depend on the nature of the behavior mechanisms present in each organism. Moreover, if it is true that behavior in the ordinary biological sense is an expression of the structural and functional mechanisms which make up the organism, it is also evident that the behavior mechanisms present in any case must depend upon the fundamental pattern of the organism concerned. In most plants, for example, in the absence of special motor mechanisms, behavior reactions are usually accomplished through growth, turgor changes, etc., while in motile organisms movement of the organism is commonly concerned in such reactions. In Chapter VI it was pointed out that an organism without a permanent physiological axis must necessarily accomplish its behavior reactions in ways different from those which are characteristic of axiate organisms. Again, the method by which the result is accomplished must differ in certain respects in radial and in bilateral animals. And going one step farther it is also clear that the method of reaction must depend upon the degree to which permanent structural and functional mechanisms have developed in relation to the general organismic pattern.

A brief consideration of the simpler forms of motor behavior will serve to show something of their regulatory character, and of their dependence upon the kind and degree of development of organismic pattern and its mechanisms. The question whether there is a fundamental or primitive mechanism or method of motor behavior in motile organisms is one of very great interest, and in attempts to answer this question two widely different conceptions have arisen. One of these, the theory of reac-: tion by trial and error or, as commonly known-at present, the theory of trial, was formulated and developed with reference to the behayior of the simpler organisms by J ennings, the other, the theory of tropisms, with reference to motor reactions of animals, by Loeb.1

Trial and error reaction The work of Jennings has made the conception of reaction by trial so familiar that only a restatement of it is necessary: First, definite internal processes, é. g., respiration, digestion, transmission of excitation, often internal motor activity, etc., are occurring in organisms, and their sum total constitutes the physiological state of the organism at any given moment. Second, interference with these processes determines changes in behavior and varied movements result because motility is a feature of the action system of the organism. Third, these movements are purely random movements not directed with respect to the disturbance, but they subject the organism to different conditions and among these conditions some one may relieve the disturbance of the physiological state and then the changes in behavior cease. In short, the organism, excited by some external factor, moves about at random until it happens upon an environment in which the excitation no longer occurs.

According to this conception any orientation of the organism or of its direction of movement with respect to the external factor is purely a matter of chance and results from the fact that the organism, when disturbed, performs varied movements, some one of which may happen to bring it into a definite position or direction of movement with respect to the external factor and if this relieves the disturbance it continues. Although we need not suppose that in the simple organisms the disturbance constitutes pain and its removal pleasure, it is evident that the theory of reaction by trial as the fundamental behavior pattern leads us, as we enter the field of more strictly psychic behavior, to a pleasure-pain psychology.

The tropism theory. — The term tropism has been variously and loosely employed. Some regard as a tropism any reaction in which ‘Only a few references to the extensive bibliography relating to these two theories and their relative importance need be given here. For statements of the trial-and-error theory the experimental data on which it is based and discussion of the tropism theory from Jennings’ viewpoint see Jennings, 04, ’05, ’06, ’08 09, 10. The theory of tropisms and the data bearing upon it have been ‘dealt with in many special papers by Loeb and others and in the following general

orientation of the body or of direction of movement with respect to the external factor occurs, whether by trial and error or otherwise. Others reserve the name “tropism” for those reactions in which direct, immediate, or “forced”’ orientation occurs in consequence of a- symmetric action of the external factor upon an organism with some sort of axiate organization. Both Webster’s New International and the Standard Dictionaries define tropism as the inherent or innate tendency of organisms to react in a definite manner to an external stimulus. Loeb, who is chiefly responsible for the theory of tropisms as applied to motor reactions, apparently regards a tropism as a forced orientation of an organism based upon a symmetrical mechanism of some sort, 7. e., a reaction directly determined through the agency of some known or postulated physiological mechanism in consequence of the unequal action of the external factor upon the two sides of the unoriented body (Loeb, ’18 d). In the case of the motile animal with its axis at an angle to the direction of action of the external factor, the unequal action of this factor upon the side of the body turned toward, and that turned away from it, forces the organism directly to change its position or direction of movement until both sides are affected alike, that is, until it is moving either directly toward, or directly away from the source of the external action. Some organisms are positively, others negatively tropic to particular factors and often the tropism characteristic of particular conditions is reversible under others. The growth reactions of the plants leading to orientation with respect to external factors and known as tropisms constituted the starting point of Loeb’s theory of tropisms as applied to motile organisms, and Loeb considers the tropism as the fundamental form of reaction of the motile organism.

The theory has been repeatedly and severely criticised and many of the criticisms remain unanswered. Jennings extends the conception of tropism to any reaction in which orientation occurs, whether directly or by trial. According to this view of the tropism, it is not the most primitive form of excitomotor behavior, but results, either from trial, or from the presence of a specialized mechanism which permits immediate orientation. In an animal such as an insect with bilaterally localized sensory and motor organs, e. g., eyes and appendages, a tropic reaction must involve at least a highly complex reflex mechanism and if such a mechanism exists, the tropism in the insect is certainly far from being a primitive form of reaction.

The two reaction patterns contrasted. — It is obvious that the trial-and-error method of reaction to any particular factor depends on the absence of mechanisms which provide for immediate, direct, or in the higher animals and man, conscious and intelligent orientation. The tropism theory, on the other hand, postulates the existence in all reacting organisms of a mechanism so related to the action of externa] factors that it makes possible direct or forced orientation. In short, although Loeb holds that the tropism itself is not regulatory, adaptive or useful, it is evident that the mechanisms postulated as the basis of the tropisms imply a far greater degree of evolutionary standardization, 7. e., of adaptation, than do the mechanisms of trial and error.

In the case of Ameba the interpretation of directed movement, e. g., with respect to light, in terms of Loeb’s theory of tropisms requires the postulation of a photoreceptor mechanism of remarkable delicacy and one in which the chemical or other action of light is very directly related to amoeboid movement. If such a mechanism exists, it is certainly a regulatory mechanism, useful to the animal, and therefore a feature in evolutionary standardization, that is, an adaptation. Again, in the tropic orientation of axiate organisms, an axiate pattern must be present, and Loeb (’18 d) apparently regards symmetry as an essential factor in tropic behavior. In fact, it seems evident that the tropisms, as conceived by Loeb, require a greater degree of development of organismic pattern and mechanism than do the trial reactions. If pure tropisms exist they certainly represent a more advanced stage in the development of behavior than the trial reactions.

Obviously both methods of reaction, so far as they are really different, represent processes of regulation or equilibration. Whether the reaction is by trial or tropic, it normally represents an approach toward an equilibrium after disturbance of the previously existing condition. Obviously also both sorts of reaction represent a result, an expression of the structural and functional mechanisms which make up the organismic pattern and which are themselves a consequence of developmental behavior.

Attention may be briefly called to the relation between the motor reactions and the axial gradients which I have called the primary, regulatory and behavior mechanisms of the organism as an individual. The anaxiate Ameba does not turn its body about in orientation but develops a temporary regional differential, in other words, it acquires a new temporary behavior pattern of axiate character (see pp. 57-59). In the axiate organism this pattern is already established and has determined various mechanisms, and radial or bilateral pattern and mechanisms are also present. Fixity and complex-

ity of mechanism are far advanced in these forms, but, as pointed out in Chap. VI, even such organisms may develop new excito-motor behavior patterns, either for the time being, or as a more or less permanent habit. These new patterns apparently originate, like the axial gradients, in regional differentials in activity, determined by some factor external to the protoplasm concerned. If they persist for any considerable length of time, they may become important factors in further reactions.