The Doctrine of Evolution: Its Basis and Its Scope

by Henry Edward Crampton

So it is with the nervous systems of man and other animals, and with their functions. The nervous system of the human organism comprises identical organs with the same arrangements that are found in other primates and in lower vertebrates as well; the differences in structure are differences in the degree of the complexity of certain parts, notably of the cerebrum. Therefore the evolution of human mentality, which depends upon a human type of brain as a physical basis, is already demonstrated with the proof that the human brain and nervous system have evolved. It is true that an invariable and necessary connection between mind and matter is implied in the foregoing statement, and this is something which demands further consideration at a later point. But just how the human mind is produced by or depends upon the brain, is of far less importance for us at this time than the obvious fact that mental performance requires active nervous tissues. So far investigation has been unable to discover a valid reason for a belief in the existence of mental phenomena, as such, apart from some kind of material basis. And while we may prefer to restrict the use of the word mind to the series of nervous processes going on in the human organ of thought, in so far as these processes are carried on by the peculiar tissues of the nervous system they cannot be finally distinguished from the functional products or accompaniments of the same kind of active tissues and organs in lower creatures. Thus the subject of mental evolution becomes much clarified at the outset by understanding that nervous processes and nervous systems evolve together.

In the direct treatment of the facts and principles of mental evolution we can use exactly the same classification and subdivisions of the materials of study as heretofore, because psychological data are the correlates of material organic systems, and also because the former, being natural phenomena, are subject to the methods of analysis which can be employed for any series of objects that have undergone evolution. Separating the matter of fact from the question as to the method, and recalling the main bodies of evidence as to the reality of evolution, we may establish four sections of the subject before us: these are (1) the anatomy, (2) the embryology, and (3) "palæontology" of mind, and (4) an inquiry into the way nature deals with the psychical characteristics of organisms in accomplishing their evolution. To specify more particularly, it is possible in the first place to compare the activities belonging to the category of mental and nervous operations, displayed by man and other organisms, and the results form the subject of comparative descriptive psychology; the second division, namely, developmental or genetic psychology, deals with the sequence of events in the life of a single individual by which the infantile and adolescent types of mind become adult intellectuality; in the third place, in speaking of the palæontology of mind, the phrase is used to refer to the varied and changing mental abilities of human races in historic and prehistoric times as they may be demonstrated and determined by the evidences of the culture of such earlier epochs. In considering the matter of method, the questions are whether variation, inheritance, and selection are as real in the world of mental phenomena as they are in the material world, and whether the laws are the same or similar in the two cases. We shall learn how the results of such studies prove with convincing clearness, first, that the contents of the individual mind and of the minds of various human races are truly the products of natural evolution, and second, that the human mind differs only in degree from that of lower organisms, and not in kind or fundamental nature.

* * * * *

When the operations of human mental life are examined, they include what are called processes of reason as apparently distinctive elements. The lower mammalia exhibit a simpler order of "mentality" denoted intelligence, while the nervous processes of still simpler forms are called instinctive and reflex activities. These are the terms of the comparative array of psychology which are to be separately examined and classified, and to be brought into an evolutionary sequence if common-sense directs us to do so.

Let us begin our comparative study with an example of the simplest animals that consist of only a single cell, such as the little protozoon Amoeba. We have become familiar with this organism as one that carries on all of the vital functions within the limits of a single structural unit; it is a mass of protoplasm enclosing a nucleus, and as a biological individual it must perform all of the eight tasks that are essential for life. It does not possess a digestive tract, but it does digest; it does not have breathing organs, but it does respire; and it is particularly noteworthy that it must coordinate the different activities of its parts, and maintain definite relations with the environment, even though its coordination and sensation are not accomplished by any special parts that would deserve the name of elementary nervous organs. Its many activities are simple responses to stimuli that reach it from without, and its reactions to such stimuli are called reflex processes. Should the light become too strong, it will slowly crawl to a shady place; should the water in which it lives become warmer, it responds by displaying greater activity. It exhibits, in a word, the property of irritability—that is, simply the power of receiving and reacting to stimuli; and being only a single cell this property is held in common by all of its parts.

We come next to a simple many-celled animal like the polyp Hydra, or a jellyfish. In such an animal the body is composed of numerous cells which are not all alike either in their make-up or in their functions. Some of them are concerned primarily with digestion, others with protection, while still others are exempt from these tasks and as sense-cells they devote all their energies to the reception of stimuli from without, or, beneath the outer sheet of cells of the two-layered body, they conduct impulses from one part of the animal to another, and thus serve as coordinating members of the community. For the first time, then, a nervous system as such is set apart and specialized to devote itself to the two tasks of sensation and coordination that are performed by nervous systems throughout the entire range of organisms higher in the scale. But the activities of Hydra, like those of Amoeba, are reflex and mechanical,—that is to say, given similar stimuli and similar physiological states of the animal, the reactions will be the same. A little water-crustacean like Daphnia may swim against the tentacles of Hydra; it is stung to death by the minute cell-batteries which the animal possesses, and then in a mechanical way the tentacles transport the food to the mouth, through which it is passed inward to the digestive cavity. There is nothing that can be called "mentality" throughout these processes, but the series of activities is much more complex than in Amoeba because the whole organism is constructed more elaborately, and because the special and peculiar mechanism directing the activities has advanced to a far higher condition.

Passing to the jointed animals like worms and insects, we find nervous mechanisms that are still more intricate, and with their advance in structural respects there is a corresponding and correlated progress in their functions. Because the whole organism has developed more highly differentiated groups of organs to perform the several biological tasks, such as eating and respiring and moving, it is necessary for the nervous structures concerned with the direction of these actions to become more efficient. An earthworm avoids the light of day and digs its burrow and seeks its food by wonderfully coördinated activities of its muscles and other parts, which are controlled by a double chain of ganglia along its ventral side, connected with a similar pair of grouped nerve-cells above the anterior part of the digestive tract. The ganglia of each segment exercise immediate supervision over the structures of their respective territory, while they pass on impulses to other ganglia so that movements involving many segments can be properly adjusted. Everything an earthworm does is controlled by the cells grouped in these ganglia, or scattered along the intervening connecting cords. We speak of its acts as instinctive, employing a term which seems to indicate a different kind of operation carried on by the nervous system, but a moment's thought will show that an instinctive act is simply a complex group of reflex acts. The physical basis and ultimate unit is a cell, and the functional unit is likewise a cell act; therefore the seeming difference proves to be one merely of degree and not of kind. The greater complexity of the worm's nervous system as compared with that of Hydra gives to the whole mechanism a plasticity that diverts the attention from the mechanical nature of the entire instinctive act and of its basic cell elements.

The instinct, like the elementary reflex, is determined by heredity. Because a certain configuration of the cells and fibers making up a nervous system is inherited as well as the characters of the constituent elements themselves, a worm or an insect is enabled to act as it does. A butterfly does not have to learn how to fly, for it flies instinctively. When it emerges from its chrysalis with its complete adult series of wings and muscles, it has also the nervous mechanism by which these parts are mechanically controlled. A ground-wasp deposits its eggs in a small burrow in which it places also a caterpillar or a grasshopper paralyzed by stinging, so that when the larva is hatched from an egg it finds an ample supply of fresh food provided by a complex series of its mother's acts that seem to be directed by conscious maternal solicitude. When the larva passes through the later stages of development and makes its way to the open air as a fully formed adult, it in its turn may go through the same course of action as its parent, but it is clear that it cannot have any remembrance of its mother's work or any personal knowledge of the value of burying its own eggs in a chamber with a living prisoner to serve as food. It was an egg when its parent did these things; as a parent itself it does not remain on watch to see how beneficial or fruitless its acts may be. A mechanism produced by nature's methods, the ground-wasp behaves as it is capable of working with its inherited structure and its inherited instinctive powers of coördination and sensation.

The complex lives of communal insects like ants and bees bring us to the level of mentality where an understanding of causes and effects seems to be the guide for conduct. Nevertheless the facts do not warrant the assumption that reason and intelligence play any part in the mental life of these creatures, as they do in the lives of man and the apes. Because we ourselves can see the utility of the definite and peculiar behavior of the queen and the worker, there is no logical necessity for assuming an identical form of knowledge as a possession of these insects. Many investigators have dealt with these fascinating subjects, and they are almost unanimous in the conclusion that the instinct of an insect is a mechanical and hereditary synthesis of combined reflex acts.

The lower orders of psychological processes play a far larger part in the lives of the higher animals than we are wont to believe. A pointer and sheep dog possess different qualifications in the way of instincts that make them useful to man in different ways. A bulldog or a game-cock does not reason out its course of action during a contest, but like a mechanism when the spring is released, it acts promptly and with effect. A ball flashing past the human eye causes the lids to close unconsciously, and it is not always possible to inhibit this instinctive mechanical act by the exercise of the will. An examination of the workings of the human body reveals manifold activities of an even lower or reflex nature, like the movements of the viscera and the adjustments in respect to the amount of supplies of blood sent to different parts of the body as local needs arise. Directed always by specific portions of the nervous system, such reflex actions play their part in human life without any effort on the part of reason and so-called will, and without coming into consciousness except indirectly and subsequently.