Popular Science Monthly/Volume 85/July 1914/Facts and Factors of Development II
|FACTS AND FACTORS OF DEVELOPMENT|
II. Development of the Mind
THE development of the mind parallels that of the body: whatever the ultimate relations of the mind and body may be, there can be no reasonable doubt that the two develop together from the germ. It is a curious fact that many people who are seriously disturbed by scientific teachings as to the evolution, or gradual development of the human race, accept with equanimity the universal observation as to the development of the human individual,—mind as well as body. The animal ancestry of the race is surely no more disturbing to philosophical and religious beliefs than the germinal origin of the individual, and yet the latter is a fact of universal observation which can not be relegated to the domain of hypothesis or theory, and which can not be successfully denied. If we admit the fact of the development of the entire individual, surely it matters little to our philosophical or religious beliefs to admit the development or evolution of the race.
The origin of the mind, or rather of the soul, is a topic upon which there has been much speculation by philosophers and theologians. One of the earliest hypotheses was that which is known as transmigration or metempsychosis. This doctrine probably reached its greatest development in ancient India, where it formed an important part of Buddhistic belief; it was also a part of the religion of ancient Egypt; it was embodied in the philosophies of Pythagoras and Plato. According to these teachings, the number of souls is a constant one; souls are neither made nor destroyed, but at birth a soul which had once tenanted another body enters into the new body. This doctrine was generally repudiated by the Fathers of the Christian Church. Jerome and others adopted the view that God creates a new soul for each body that is generated, and that every soul is thus a special divine creation. This has become the prevailing view of the Christian Church and is known as creationism, On the other hand Tertullian taught that souls of children are generated from the souls of parents as bodies are from bodies. This doctrine, which is known as traducianism, has been defended by certain modern theologians, but has been formally condemned by the Roman Catholic Church.
Traducianism undoubtedly comes nearer the scientific teachings as to the development of the mind than does either of the other doctrines named, but it is based upon the prevalent but erroneous belief that the bodies of the parents generate the body of the child, and that correspondingly the souls of the parents generate the soul of the child. Now we know that the child comes from germ cells which are not made by the bodies of the parents, but which have arisen by the division of antecedent germ cells. Every cell comes from a preexisting cell by a process of division, and every germ cell comes from a preexisting germ cell. Consequently it is not possible to hold that the body generates germ cells, nor that the soul generates souls. The only possible scientific position is that the mind (or soul) as well as the body develops from the germ.
No fact in human experience is more certain than that the mind develops by gradual and natural processes from a simple condition which can scarcely be called mind at all; no fact in human experience is fraught with greater practical and philosophical significance than this; and yet no fact is more generally disregarded. We know that the greatest men of the race were once babies, embryos, germ cells, and that the greatest minds in human history were once the minds of babies, embryos and germ cells, and yet this stupendous fact has had but little influence on our beliefs as to the nature of man and of mind. We rarely think of Plato and Aristotle, of Shakespeare and Newton, of Pasteur and Darwin, except in their full epiphany, and yet we know that when each of these was a child he "thought as a child and spake as a child," and when he was a germ cell he behaved as a germ cell.
The development of the mind from the activities of the germ cells is certainly most wonderful and mysterious, but probably no more so than the development of the complicated body of the adult animal from the structures of the germ. Both belong to the same order of phenomena and there is no more reason for supposing that the mind is supernaturally created than that the body is. Indeed, we know that the mind is formed by a process of development, and the stages of this development are fairly well known. There is nowhere in the entire course of mental development a sudden appearance of psychical process, but rather a gradual development of these from simpler and simpler beginnings. No detailed study has been made of the reactions of human germ cells and embryos, but there is every reason to believe that these reactions are simpler in the embryo and germ cell than in the infant, and they are generally similar to the reactions of the germ cells and embryos of other animals and to the behavior of many lower organisms.
A few years ago such a statement would have been branded as "materialism" and promptly rejected without examination by those who are frightened by names. But the general spread of the scientific spirit is shown not only by the growing regard for evidence, but also by the decreasing power of epithets. "Materialism," like many another ghost, fades away into thin air or at least loses many of its terrors, when closely scrutinized. But the statement that mind develops from the germ cells is not an affirmation of materialism, for while it identifies the origin of the entire individual, mind and body, with the development of the germ, it does not assert that "matter" is the cause of "mind" either in the germ or in the adult. It must not be forgotten that germ cells are living things and that we go no further in associating the beginnings of mind with the beginnings of body in the germ than we do in associating mind and body in the adult. It is just as materialistic to hold that the mind of the mature man is associated with his body as it is to hold that the beginnings of mind in the germ are associated with the beginnings of the body, and both of these tenets are incontrovertable.
It seems to me that the mind is related to the body as function is to structure; there are those who maintain that structure is the cause of function, that the real problem in evolution or development is the transformation of one structure into another, and that the functions which go with certain structures are merely incidental results; on the other hand are those who maintain that function is the cause of structure and that the problem of evolution or development is the change which takes place in functions and habits, these changes causing corresponding transformations of structure. Among adherents of the former view may be classed many morphologists and Neo-Darwinians, among proponents of the latter, many physiologists and Neo-Lamarckians. It seems to me that the defenders of each of these views fail to recognize the essential unity of the entire organism, structure as well as function; that neither of these is the cause of the other, though each may modify or condition the other, but that they are two aspects of one common thing, viz., organization. In the same way I think that the body or brain is not the cause of mind, nor mind the cause of body or brain, but that both are inherent in one common organization or individuality.
In asserting that the mind develops from the germ as the body does, no attempt is made to explain the fundamental properties of body or mind. As the structures of the body may be traced back to certain fundamental structures of the germ cell, so the characteristics of the mind may be traced back to certain fundamental properties and activities of the germ. Many of the psychical processes may be traced back in their development to properties of sensitivity, reflex motions, and persistence of the effects of stimuli. All organisms manifest these properties and for aught we know to the contrary they may be original and necessary characteristics of living things. In the simplest protoplasm we find organization, that is, structure and function, and in germinal protoplasm we find the elements of the mind as well as of the body, and the problem of the ultimate relation of the two is the same whether we consider the organism in its germinal or in its adult stage.
In some way the mind as well as the body develops out of the germ. What are the germinal bases of mind? What are the psychical anlagen in embryos and how do they develop? In this case, even more than in the development of the body, we are compelled to rely upon the comparison of human development with that of other animals, but the great principle of the oneness of life, as respects its fundamental processes, has never yet failed to hold true and will not fail us here. In the study of the psychical processes of organisms other than ourselves we are compelled to rely upon a study of their activities, their reactions to stimuli, since we can not approach the subject in any other way. The reactions and behavior of organisms under normal and experimental conditions give the only insight which we can get into their psychical processes—and this applies to men no less than to protozoa.
1. Sensitivity.—The most fundamental phenomenon in the behavior of organisms is irritability or sensitivity, which is the capacity of receiving and responding to stimuli: this is one of the fundamental properties of all protoplasm. But living matter is not equally sensitive to all stimuli, nor to all strengths of the same stimulus. Many of the simplest unicellular plants and animals show that they are differentially sensitive; they often move toward weak light and away from strong light, away from extremes of heat and cold, into certain chemical substances and away from others—in short, all organisms, even the simplest, may respond differently to different kinds of stimuli or to different degrees of the same stimulus. This is what is known as differential sensitivity
Fig. 17. Distribution of Bacteria in the Spectrum. The largest group is in the ultra-red at the left; the next largest group is in the yellow-orange close to the line D. (From Jennings, after Engelmann.)
(Figs. 17, 18, 19.) On the other hand, many organisms respond in the same way to different stimuli and this may be taken to indicate generally that they are not differentially sensitive to such stimuli; it is not to be concluded that because organisms respond differently to certain stimuli they are therefore capable of distinguishing between all kinds of stimuli, for this is certainly not true. Even in adult men the capacity of distinguishing between different kinds of stimuli is far from perfect.
Egg cells and spermatozoa show this property of sensitivity. The egg is generally incapable of locomotion, and since the results of stimulation must usually be detected by movements it is not easy to determine to what extent the egg is sensitive; but though the egg lacks the power of locomotion, it possesses in a marked degree the power of intra-cellular movement of the cell contents. When a spermatozoon comes into contact with the surface of the egg the cortical protoplasm of the egg flows toward that point and may form a cone or protoplasmic prominence into which the sperm is received (Figs. 4, 5, E C). It is an interesting fact that the same sort of response follows when a frog's egg is pricked by a needle, thus showing that in this case the egg does not distinguish between the prick of the needle and that of the spermatozoon. The spermatozoon is usually a locomotor cell and it responds differently to certain stimuli, just as many bacteria and protozoa do; spermatozoa are strongly stimulated by weak alkalies and alcohol, they gather in certain chemical substances and not in others, they collect in great numbers around fertilizable egg cells, etc.
The movements of fertilized egg cells, cleavage cells, and early embryonic cells are usually limited to flowing movements within the
Fig. 18, a, b, c. Repulsion of Spirilla by Common Salt, a, condition immediately after adding crystals; b and c, later stages in the reaction.
x y z, repulsion of Spirilla by distilled water. The upper drop consists of sea water containing Spirilla, the lower drop of distilled water. At x these have just been united by a narrow neck; at y and z, the bacteria have retreated before the distilled water. (From Jennings, after Massart.)
individual cells. These movements, which are of a complicated nature, are of the greatest significance in the differentiation of the egg into the embryo; they are caused chiefly by internal stimuli and by non-localized external ones. Modifications of the external stimuli often lead to modifications of these intra-cellular movements and to abnormal types of cleavage and development—in short, these movements show that the fertilized egg is differentially sensitive.
In the further course of development particular portions of the embryo become especially sensitive to some kinds of stimuli, while other portions become sensitive to others. In this way the different sense organs, each especially sensitive to one particular kind of stimulus, arise from the generalized sensitivity of the oosperm, and thus general sensitivity, which is a property of all protoplasm, becomes differential sensitivity and special senses in the process of embryonic differentiation. Such sensitivity is the basis of all psychic processes: sensations are the elements of the mind.
3. Tropisms, Reflexes, Instincts.—All the responses of germ cells, and of the simplest organisms, to stimuli are in the nature of tropisms or reflexes, that is, relatively simple, automatic responses. Such tropisms or reflexes are seen in the movements of bacteria, protozoa and
Fig. 19. Reactions of Paramecium to Heat and Cold. At a the infusoria are uniformly distributed in a trough, both ends of which have a temperature of 19°; at b the infusoria are shown collected at the cooler end of the trough; at c they have collected at the warmer end of the trough. (From Jennings, after Mendelssohn.)
many higher animals and plants as well as in movements of spermatozoa, the movements of the protoplasm in egg cells and embryonic cells, the movements of cells and cell masses in the formation of the gastrula, alimentary canal, nervous system and other organs. Indeed the entire process of development, whether accompanied by visible movements or not, may be regarded as a series of automatic responses to stimuli.
When the embryo becomes differentiated to such an extent as to have specialized organs for producing movement its capacity for making responsive movements to stimuli becomes much increased. If the responses of animals and plants to stimuli are of such a sort that the organism turns or moves toward or away from a source of stimulus they are termed tropisms; if the responses are very complicated, one response calling forth another and involving many reflexes, as is frequently the case in animals, they are known as instincts. In the embryo the rhythmic contractions of heart, amnion and intestine are early manifestations of reflex motions. These appear chiefly in the involuntary muscles before nervous connections are formed, the protoplasm of the muscle cells probably responding directly to the chemical stimulus of certain salts in the body fluids, as Loeb has shown. Reflexes which appear later are the random movements of the voluntary muscles of limbs and body, which are called forth by nerve impulses. Tropisms are manifested only by organisms capable of considerable free movement and hence are absent in the foetus though present in many free living larvæ. Some instincts are present immediately after birth, such as the instinct of sucking or crying, though these are so simple when compared with some instincts which develop later that they might be classed as reflexes; it is doubtful whether any of the activities before birth could properly be designated as instincts. Reflexes, tropisms and instincts have had a phylogenetic as well as an ontogenetic origin, and consequently we might expect that they would in general make for the preservation of the species, and as a matter of fact we usually find that they are remarkably adapted to this end. For instance the instincts of the human infant to grasp objects, to suck things which it can get into its mouth, to cry when in pain, are complicated reflexes which have survived in the course of evolution probably because they serve a useful purpose.
Very much has been written on the nature and origin of instincts, but the best available evidence strongly favors the view that instincts are complex reflexes, which, like the structures of an organism, have been built up, both ontogenetically and phylogenetically, under the stress of the elimination of the unfit, so that they are usually adaptive.
3. Memory.—Another general characteristic of protoplasm is the capacity of storing up or registering the effects of previous stimuli. A single stimulus may produce changes in an organism which persist for a longer or shorter time, and if a second stimulus occurs while the effect of a previous one still persists, the response to the second stimulus may be very different from that to the first. Macfarlane found that if the sensitive hairs on the leaf of Dionea, the Venus fly-trap (Fig. 20, SH), be stroked once, no visible response is called forth, but if they be stroked a second time within three minutes the leaf instantly closes. If a longer period than three minutes elapses after the first stimulus and before the second no visible response follows, i.e., two successive stimuli are necessary to cause the leaves to close, and the two must not be more than three minutes apart; the effects of the first stimulus are in some way stored or registered in the leaf for this brief time. This kind of phenomenon is widespread among living things and is known as "summation of stimuli." In all such cases the effects of a former stimulus are in some way stored up for a longer or shorter time in the protoplasm. It is possible that this is the result of the formation of some chemical substance which remains in the protoplasm for a certain time, during which time the effects
Fig. 20. Dionæa muscupula (Venus' Flytrap). Three leaves showing marginal teeth and sensitive hairs (SH). The leaf at the left is fully expanded, the one at the right is closed.
of the stimulus are said to persist, or it may be due to some physical change in the protoplasm analogous to the "set" in metals which have been subjected to mechanical strain.
Probably of a similar character is the persistence of the effects of repeated stimuli and responses on any organ of a higher animal. A muscle which has contracted many times in a definite way ultimately becomes "trained" so that it responds more rapidly and more accurately than an untrained muscle; and the nervous mechanism through which the stimulus is transmitted also becomes trained in the same way. Indeed such training is probably chiefly a training of the nervous mechanism. The skill of the pianist, of the tennis player, of the person who has learned the difficult art of standing or walking, or the still more difficult art of talking, is probably due to the persistence in muscles and nerves of the effects of many previous activities. All such phenomena were called by Hering, "organic memory," to indicate that this persistence of the effects of previous activities in muscles and other organs is akin to that persistence of the effects of previous experiences in the nervous mechanism which we commonly call memory. It seems probable that this ability of protoplasm in general to preserve for a time the effects of former stimuli is fundamentally of the same nature as the much greater power of nerve cells to preserve such effects for much longer periods and in complex associations, a faculty which is known as associative memory. The embryos, and indeed even the germ cells of higher animals, may safely be assumed to be endowed with protoplasmic and organic memory, out of which, in all probability, develop associative and conscious memory in the mature organism.
4. Intellect, Reason.—Even the intellect and reason which so strongly characterize man have had a development from relatively simple beginnings. All children come gradually to an age of intelligence and reason. In its simpler forms at least reason may be defined as the power of predicting future events and of reaching conclusions regarding unexperienced phenomena under the influence of past experience. In the absence of individual experience young children have none of this power, but it comes gradually as a result of remembering past experiences and of fitting such experiences into new conditions. Young infants and many lower animals lack the power of reason, though their behavior is frequently of such a sort as to suggest that they are reasoning. Even the lowest animals avoid injurious substances and conditions and find beneficial ones; more complex animals learn to move objects, solve problems, and find their way through labyrinths in the shortest and most economical way; but this apparently intelligent and purposive behavior has been shown to be due to the general elimination of all sorts of useless activities, and to the persistence of the useful ones.
The ciliated infusorian, Paramecium, moves by the beating of cilia which are arranged in such a way that they drive the animal forward in a spiral course. However, when it is strongly irritated, the normal forward movement is reversed; the cilia beat forward instead of backward and the animal is driven backward for some distance (Figs. 21, 1, 2, 3); it then stands nearly still merely rolling over and swerving toward the aboral side and finally it goes ahead again, usually on a new course (Fig. 21, 3, 4, 5, 6). These movements seem to be conditioned rather rigidly by the organization of the animal: they are more or less fixed and mechanical in character though to a certain extent they may be modified by experience or physiological states. Paramecium behaves as it does in virtue of its constitution, just as an egg develops in a particular way because of its particular organization.
But although limited in its behavior to these relatively simple motor reactions, Paramecium does many things which seem to show intelligence and purpose. It avoids many injurious substances, such as strong salts or acids and it collects in non-injurious or beneficial substances, such as weak acids, masses of bacteria upon which it feeds, etc. It avoids extremes of heat and cold and if one end of a dish containing paramecia is heated and the other end is cooled by ice, the paramecia collect in the region somewhere between these two extremes (Fig. 19). Jennings, by studying carefully the behavior of single individuals, established the fact that this apparently intelligent action is due to differential sensitivity and to the single motor reaction of the animal. If in the course of its swimming a Paramecium comes into contact with an irritating substance or condition, it backs a short distance, swerves toward its aboral side, and goes ahead in a new path; if it again comes in contact with the irritating
Fig. 21. Diagram of the Avoiding Reaction of Paramecium. A is a solid object or other source of stimulation. 1-6, successive positions occupied by the animal. The rotation on the long axis is not shown. (After Jennings.)
conditions this reaction is repeated, and so on indefinitely until finally a path is found in which the source of irritation is avoided altogether. In short, Paramecium continually tries its environment, and backs away from irritating substances or conditions. Its apparently intelligent reactions are thus explained as due to a process of "trial and error."
The behavior of worms, star-fishes, crustaceans, mollusks, as well as of fishes, frogs, reptiles, birds and mammals, have been studied and in all cases it is found that their method of responding to stimuli is not at first really purposive and intelligent but by the gradual elimination of useless responses and the preservation (or remembering) of useful ones the behavior may come to be purposive and intelligent.
Thorndike found that when dogs, cats and monkeys were confined in cages which could be opened from the inside by turning a button, or pressing upon a lever, or pulling a cord, they at first clawed around all sides of the cage until by chance they happened to operate the mechanism which opened the door. Thereafter they gradually learned by experience, that is, by trial and error, and finally by trial and success, just where and how to claw in order to get out at once. When a dog has learned to turn a button at once and open a door we say he is intelligent, and if he can learn to apply his knowledge of any particular cage to other and different cages, a thing which Thorndike denies, we should be justified in saying that he reasons, though in this case intelligence and reason are founded upon memory of many past experiences, of many trials and errors and of a few trials and successes.
There is every evidence that human beings arrive at intelligence and reason by the same process, a process of many trials and errors and a few trials and successes, a remembering of these past experiences and an application of them to new conditions. A baby grasps for things which are out of its reach, until it has learned by experience to appreciate distances; it tests all sorts of pleasant and unpleasant things until it has learned to avoid the latter and seek the former; it experiments with its own body until it has learned what it can do and what it can not do. Is not this learning by experience akin to the same process in the dog and more remotely to the trial and error of the earthworm or the adaptive reflexes of Paramecium? Is not intelligence and reason in all of us, and upon all subjects, based upon the same processes of trial and error, memory of past experiences and application of this to new conditions? Surely this is true in all experimental and scientific work. Indeed the scientific method is the method of trial and error, and finally trial and success—the method recommended by St. Paul to "try all things and hold fast that which is good."
In Paramecium the reflex type of behavior is relatively complete; there is no associative memory and no ability to learn by experience. In the earthworm associative memory is but slightly developed and the animal learns but little by experience and can make no application of past experiences to new conditions. In the dog associative memory is well developed; the animal learns by experience and can, to a limited extent, apply such memory of past experiences to new conditions. In adult man all of these processes are fully developed and particularly the last, viz., the ability to reason. But in his development the human individual passes through the more primitive stages of intelligence, represented by the lower animals named; the germ cells and embryo represent only the stages of reflex behavior, to these trial and error and associative memory are added in the infant and young child, and to these the application of past experience to new conditions, or reason, is added in later years.
5. Will.—Another characteristic, which many persons regard as the supreme psychical faculty, is the will. This faculty also undergoes development and from relatively simple beginnings. The will of the child has developed out of something which is far less perfect in the infant and embryo than in the child. Observations and experiments on lower animals and on human beings, as well as introspective study of our own activities, appear to justify the following conclusions:
(1.) Every activity of an organism is a response to one or more stimuli, external or internal in origin. These stimuli are in the main, if not entirely, energy changes outside or inside the organism. In lower organisms as well as in the germ cells and embryos of higher animals the possible number of responses are few and prescribed owing to their relative simplicity, and the response follows the stimulus directly. In more complex organisms the number of possible responses to a stimulus is greatly increased, and the visible response may be the end of a long series of internal changes which are started by the original stimulus.
(2.) The response to a stimulus may be modified or inhibited in the following ways:
(a) Through conflicting stimuli and changed physiological states (due to fatigue, hunger, etc.). Many stimuli may reach the organism at the same time and if they conflict they may nullify one another or the organism may respond to the strongest stimulus and disregard the weaker ones. When an organism has begun to respond to one stimulus it is not easily diverted to another. Jennings found that the attached infusorian, Stentor, which usually responds to strong stimuli by closing up, may, when repeatedly stimulated, loosen its attachment and swim away, thus responding in a wholly new manner when its physiological state has been changed by repeated stimuli and responses. Whitman found that leeches of the genus Clepsine prefer shade to bright light, and other things being equal they always seek the under sides of stones and shaded places; but if a turtle from which they normally suck blood is put into an aquarium with the leeches, they at once leave the shade and attach themselves to the turtle. They prefer shade to bright light but they prefer their food to the shade. The tendency to remain concealed is inhibited by the stronger stimulus of hunger. On the other hand he found that the salamander, Necturus, is so timid that it will not take food, even though starving, until by gradual stages and gentle treatment its timidity can be overcome to a certain extent. Here fear is at first a stronger stimulus than hunger and unless the stimulus of fear can be reduced the animal will starve to death in the presence of the most tempting food.
(b) Responses may also be modified through compulsory limitation of many possible responses to a particular one, and the consequent formation of a habit. This is the method of education employed in training all sorts of animals. Thus Jennings found that a starfish could be trained to turn itself over, when placed on its back, by means of one particular arm simply by persistently preventing the use of the other arms. Many responses of organisms are modified in a similar way, not only by artificial limitations, but also by natural ones.
(c) Responses which have become fixed and constant through natural selection or other means of limitation may become more varied and general when the compulsory limitation is relaxed. Behavior in the former case is fixed and instinctive, in the latter more varied and plastic. Thus Whitman found that the behavior of domesticated pigeons is more variable and their instincts less rigidly fixed than in wild species. If the eggs are removed to a little distance from the nest the wild passenger pigeon returns to the nest and sits down as if nothing had happened. She soon finds out, not by sight but by feeling, that something is missing, and she leaves the nest after a few minutes without heeding the eggs. The ring-neck pigeon also misses the eggs and sometimes rolls one of them back into the nest, but never attempts to recover more than one. The dove-cote pigeon generally tries to recover both eggs.
(d) Finally in all animals behavior is modified though previous experience, just as structure is also. Where several responses to a stimulus are possible and where experience has taught that one response is more satisfactory than another, action may be limited to this particular response, not by external compulsion, but by the internal impulse of experience and intelligence. This is what we know as conscious choice or will. Whitman says:
Freedom of action does not mean action without stimuli, but rather the introduction of the results of experience and intelligence as additional stimuli. The activities, which in lower animals are "cabined, cribbed, confined," reach in man their fullest and freest expression; but the enormous difference between the relatively fixed behavior of a protozoan or a germ cell and the relatively free activities of a mature man is bridged not only in the process of evolution, but also in the course of individual development.
6. Consciousness.—The most complex of all psychic phenomena, indeed the one which includes many if not all of the others, is consciousness. Like every other psychic process this has undergone development in each of us; we not only came out of a state of unconsciousness, but through several years we were gradually acquiring consciousness by a process of development. Whether consciousness is the sum of all the psychic faculties, or is a new product dependent upon the interaction of the other faculties, it must pass through many stages in the course of its development, stages which would commonly be counted as unconscious or subconscious states, and complete consciousness must depend upon the complete development and activity of the other faculties, particularly associative memory and intelligence. The question is sometimes asked whether germ cells, and indeed all living things, may not be conscious in some vague manner. One might as well ask whether water is present in hydrogen and oxygen. Doubtless the elements out of which consciousness develops are present in the germ cells, in the same sense that the elements of the other psychic processes or of the organs of the body are there present—not as a miniature of the adult condition, but rather in the form of elements or factors, which by a long series of combinations and transformations, due to interactions with one another and with the environment, give rise to the fully developed condition.
Finally there seems good reason for believing that the continuity of consciousness, the continuing sense of identity, is associated with the continuity of material substance, for in spite of frequent changes of the materials of which we are composed our sense of identity remains undisturbed. However, the continuity of protoplasmic and cellular organization generally remains undisturbed throughout life, and the continuity of consciousness is associated with this continuity of organization, especially in certain parts of the brain. It is an interesting fact that in man and in several other animals which may be assumed to have a sense of identity, the nerve cells, especially those of the brain, cease dividing at an early age, and these identical cells persist throughout the remainder of life. If nerve cells continued to divide throughout life, as epithelial cells do, there would be no such persistence of identical cells, and one is free to speculate that in such cases there would be no persistence of the sense of identity.
Organization includes both structure and function, and continuity of organization implies not only persistence of protoplasmic and cellular structures, but also persistence of functions, of sensitivity, reflexes, memory, instincts, intelligence and will; the continuity of consciousness is associated with the continuity of these activities, as well as with the structures of the body in general and of the brain in particular. It is well known that things which interrupt or destroy these functions or structures interrupt or destroy consciousness. Lack of oxygen, anesthetics, normal sleep cause in some way a temporary interruption of these functions and consequently temporary loss of consciousness; while certain injuries or diseases of the brain which bring about the destruction of certain centers or association tracts may cause permanent loss of consciousness.
The development of all of these psychical faculties runs parallel with the development of bodily structures and apparently the method of development in the two cases is similar, viz., progressive differentiation of complex and specialized structures and functions from relatively simple and generalized beginnings. Indeed the entire organism—structure and function body and mind—is a unity, and the only justification for dealing with these constituents of the organism as if they were separate entities, whether they be regarded in their adult condition or in the course of their development, is to be found in the increased convenience and effectiveness of such separate treatment.
Development, like many other vital phenomena, may be considered from several different points of view, such as (1) physico-chemical events involved, (2) physiological processes, (3) morphological characters, (4) ecological correlations and adaptations, (5) psychological phenomena, (6) social and moral developments. All of these phases of development are correlated, indeed they are parts of one general process, and a complete account of this process must include them all. General considerations may lead us to the belief that each of the succeeding aspects of development named above may be causally explained in terms of the preceding ones, and hence all be reducible to physics and chemistry. But this is not now demonstrable and may not be true. Function and structure may be related causally, or they may be two aspects of one substance. The same is true of body and mind or of matter and energy. But even if each of these different phases in the development of personality may not be causally explained by the preceding ones, at least the principle of explanation employed for any aspect of development ought to be consistent and harmonious with that employed for any other aspect.
The phenomena of mental development in man and other animals may be summarized in the following table:
Development of Psychical Processes in Ontogeny and Phylogeny
|All Living Things, including Germ Cells and Embryos, show:||Mature Forms of Higher Animals show:|
|1. Differential Sensitivity =||1. Special Senses and Sensations =|
|Different Responses to Stimuli differing in Kind or Quantity.||Sensations are the Elements of Mind.|
|2. Reflex Motions =||2. Instincts (Inherited), Habits (Acquired) =|
|Relatively Simple, Automatic Responses.||Complex Reflexes, involving Nerve Centers.|
|3. Organic Memory =||3. Associative Memory =|
|Results of Previous Experience registered in General Protoplasm.||Results of Experience registered in Nerve Centers and Association Tracts.|
|4. Adaptive Responses =||4. Intelligence, Reason =|
|Results of Elimination of Useless Responses through Trial and Error.||Results of Trial and Error plus Associative Memory, i.e. Experience.|
|5. Varied Responses||5. Inhibition, Choice, Will|
|Dependent upon Conflicting Stimuli and Physiological States.||Dependent upon Associative Memory, Intelligence, Reason.|
|6. Identity =||6. Consciousness =|
|Continuity of Individual Organization.||Continuity of Memory, Intelligence, Reason, Will.|
Factors of Development
These are some of the facts of development—a very incomplete résumé of some of the stages through which a human being passes in the course of his development from the germ. What are the factors of development? By what processes is it possible to derive from a relatively simple germ cell the complexities of an adult animal? How can mind and consciousness develop out of the relatively simple psychical elements of the germ? These are some of the great problems of development—the greatest and most far-reaching theme which has ever occupied the minds of men.
Preformation.—When the mind is once lost in the mystery of this ever recurring miracle it is not surprising to find that there have been those who have refused to believe it possible and who have practically denied development altogether. The old doctrine of "evolution" as it was called by the scientists of the eighteenth century, or of preformation as we know it to-day held that all the organs or parts of the adult were present in the germ in a minute and transparent condition as the leaves and stem are present in a bud, or as the shoot and root of the little plant are present in the seed. In the case of animals it was generally impossible to see the parts of the future animal in the germ, but this was supposed to be due to the smaller size of the parts and to their greater transparency, and with poor microscopes and good imagination some observers thought they could see the little animals in the egg or sperm, and even the little man, or "homunculus," was described and figured as folded up in one or the other of the sex cells.
This doctrine of preformation was not only an attempt to solve the mystery of development, but it was also an attempt to avoid the theological difficulties supposed to be involved in the view that individuals are produced by a process of gradual development rather than by supernatural creation. If every individual of the race existed within the germ cells of the first parents, then in the creation of the first parents the entire race with its millions of individuals was created at once. Thus arose the theory of "emboitement," or "box in box," the absurdities of which contributed to the downfall of the entire doctrine of preformation, which, in the form in which it was held by many naturalists of the eighteenth century, is now only a curiosity of biological literature.
Epigenesis.—As opposed to this doctrine of preformation, which was founded largely on speculation, arose the theory of epigenesis, which was in its main features founded upon the direct observation of development, and which maintained that the germ contains none of the adult parts, but that it is absolutely simple and undifferentiated, and that from these simple beginnings the individual gradually becomes complex by a process of differentiation. We owe the theory of epigenesis, at least so far as its main features are concerned, to William Harvey, the discoverer of the circulation of the blood, and to Caspar Friederich Wolff, whose doctor's thesis published in 1759, and entitled "Theoria Generationis," marked the beginning of a great epoch in the study of development. Wolff demonstrated that adult parts are not present in the germ, either in animals or in plants, but that these parts gradually appear in the process of development. He held, erroneously, that the germ is absolutely simple, homogeneous and undifferentiated, and that differentiation and organization gradually appear in this undifferentiated substance. How to get differentiations out of non-differentiated material, heterogeneity out of homogeneity, was the great problem which confronted Wolff and his followers, and they were compelled to assume some extrinsic or environmental force, some vis formativia or spiritus rector, which could set in motion and direct the process of development.
The doctrine of preformation, by locating in the germ all the parts which would ever arise from it, practically denied development altogether; epigenesis recognized the fact of development, but attributed it to mysterious and purely hypothetical external forces; the one placed all emphasis upon the germ and its structures, the other upon outside forces and conditions.
Preformation and Epigenesis.—Modern students of development recognize that neither of these extreme views are true—adult parts are not present in the germ, nor is the latter homogeneous—but there are in germ cells many different structures and functions which are, however, very unlike those of the adult, and by the transformation and differentiation of this germinal organization the complicated organization of the adult arises. Development is not the unfolding of an infolded organism, nor the mere sorting of materials already present in the germ cells, though this does take place, but rather it consists in the formation of new materials and qualities—of new structures and functions—by the combination and interaction of the germinal elements present in the oosperm. In similar manner the combination and interaction of chemical elements yield new substances and qualities which are not to be observed in the elements themselves. Such new substances and qualities, whether in the organic or in the inorganic world, do not arise by the gradual unfolding of what was present from the beginning, but they are produced by a process of "creative synthesis."
Modern studies of germ cells have shown that they are much more complex than was formerly believed to be the case; they may even contain different "organ-forming substances" which in the course of development give rise to particular organs; these substances may be so placed in the egg as to foreshadow the polarity, symmetry and pattern of the embryo, but even the most highly organized egg is relatively simple as compared with the animal into which it ultimately develops. Increasing complexity, which is the essence of development, is caused by the combination and interaction of germinal substances under the influence of the environment. The organization of the oosperm may be compared to the arrangement of tubes and flasks in a complicated chemical operation; they stand in a definite relation to one another and each contains specific substances. The final result of the operation depends not merely upon the substances used, nor merely upon the way in which the apparatus is set up, but upon both of these things, as well as upon the environmental conditions represented by temperature, pressure, moisture or other extrinsic factors.
Heredity and Environment.—Unquestionably the factors, or causes, of development are to be found not merely in the germ but, also in the environment, not only in intrinsic but also in extrinsic forces; but it is equally certain that the directing and guiding factors of development are in the main intrinsic, and are present in the organization of the germ cells, while the environmental factors exercise chiefly a stimulating, inhibiting or modifying influence on development. In the same dish and under similar environmental conditions, one egg will develop into a worm, another into a sea urchin, another into a fish, and it is certain that the different fate of each egg is determined by conditions intrinsic in the egg itself, rather than by environmental conditions. We should look upon the germ as a living thing, and upon development as one of its functions. Just as the character of any function is determined by the organism, though it may be modified by environment, so the character of development is determined by heredity, i.e., by the organization of the germ cells, though the course and results of development may be modified by environmental conditions.
In conclusion, we have briefly reviewed in this lecture the well known fact that every living thing in the world has come into existence by a process of development; that the entire human personality, mind as well as body, has thus arisen; and that the factors of development may be classified as intrinsic in the organization of the germ cell, and extrinsic as represented in environmental forces and conditions. The intrinsic factors are those which are commonly called heredity, and they direct and guide development in the main; the extrinsic or environmental factors furnish the conditions in which development takes place and modify, more or less, its course.
- In Paramœcium, there is certainly no consciousness of trial and error, and probably no unconscious attempt on the part of the animal to attain certain ends. Its responses are reflexes or tropisms, which are determined by the nature of the animal, and the character of the stimulus. The fact that these responses are in the main self-preservative is due to the teleological organization of Paramœcium which has been evolved, according to current opinion, as the result of long ages of the elimination of the unfit. If, in the opinion of any one, the expression "trial and error" necessarily involves a striving after ends, it would be advisable to replace it in this ease by some such term as "useful or adaptive reactions."
- The little plant in the seed is itself the product of the development of a single cell, the ovule, in which no trace of a plant is present, but of course this fact was not known until after careful microscopical studies had been made of the earliest stages of development.