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.
Passing by many interesting members of the psychological series of intergrading forms, we reach the familiar animals like the cat and dog and horse which display what is called intelligence. This is the power to learn by experience, and to improve the quality and promptitude of reactions to stimuli. In certain respects intelligence seems to differ from instinct, inasmuch as it involves a response to stimuli that may be altered and quickened by repeated experience, but in ultimate analysis the two forms of psychological processes are fundamentally alike.
A single example chosen from Thorndike's extensive investigation will serve to bring out the primary characteristics of intelligence. A cat was placed in a latticed cage provided with a door that could be opened from within when a catch was pressed down, and meat was put in a dish outside the door where the cat could see it. At first, the animal escaped from the cage by freeing the door during its aimless scrambling about the catch, but as trial after trial was made, the time necessary for the cat to make its way out was shortened, until after seventy-five or one hundred trials, the animal immediately opened the door and seized the food.
In mechanical terms, the connection between "scrambling about the door" and "freedom to get the meat" became established by numerous repetitions until the originally disconnected elements were physiologically associated and made inseparable. When animals like horses and seals and dogs are trained for the circus, it is by exactly the same method, for training consists merely in the establishment of a psychological sequence so that the performance of one series of acts leads mechanically to others. Thus we learn that the psychological property called intelligence is the ability to establish wide relations between numerous activities which are themselves of a more or less complex nature; and we find also that because these elements are ultimately nerve-cell and sense-cell reflexes, an intelligent response is quite as machine-like as any and all of its elements. A difference in degree of complexity and extent is the only thing that places intelligence apart from instinct and reflex action, for the units are the same in all cases,--so far as science knows.
The apes are of the greatest value in providing the transition from the grade of intelligence to the human level where reason is found. Whether or not a chimpanzee can reason at all is less important than the fact that its total "mental" powers are lower than those of man, and higher than those of inferior mammalia. Apes are far more susceptible to training than cats and dogs, because their improved nervous mechanism enables them to establish a psychological sequence with greater facility. If we are to judge by the facts at hand, these creatures possess a low order of mentality, like, but by no means equivalent to, that of man.
At the end of the comparative scale, we reach the human mind which is characterized by its ability to perceive and recognize far wider relations than those which are involved in intelligence. Human consciousness is the stream of thoughts and feelings which constitute the immediate contents of mind. In our own case, we know both the activities we perform and some of the internal phenomena with which such activities are connected. Then we are impelled to compare the objective phenomena of action with the behavior of other men and of lower organisms, and if their behavior does not coincide with our own we are justified in believing that its direction lacks some of the elements we know about in our own case. This is the method of comparative psychology, which establishes the conclusion that reason is the more complex term of a series to which reflex action, instinct, and intelligence directly lead.
Were we to study in detail the psychology of adult human beings, we would find only more truly that instinct and intelligence play a large part in our everyday mental life, and more certainly that even the highest reasoning powers we possess are only more complex in nature than the nervous processes of lower mammals and invertebrates. Just as the nervous systems advance in physical or structural respects, so must their activities become more and more complex until the result is human faculty.
We must now briefly consider what may be called the "comparative anthropology" of mind which deals with the various degrees of mental ability displayed by different human races; this subject follows inevitably upon the comparison of the human mind viewed as a single type with the psychological processes of lower animals. When we reviewed the diverse characteristics of human races--the protrusion of the jaws, greater or lesser stature, and the like--it appeared that so-called "lower" races could be distinguished which differed from the "higher" races in the direction of the apes; the question immediately arises whether similar distinctions and relations are discoverable on the basis of mental traits. But in the present case there are not so many well-substantiated differentia at the disposal of the student, and it does not appear so clearly that the "higher" races are furthest from the lower primates and lower mammalia as regards their mental processes.
What facts there are, however, prove to be highly significant, and they materially amplify our conception of human faculty as a product of evolution. The essential point is that the intellectual attainments of various races are by no means the same. The calculus is a mental product of the white race only; gunpowder and printing from movable type were independently invented by the Caucasian and Mongolian races; but the American Indian and the Negro never originated them. Human faculty, to employ the most general term for all that distinguishes man from the brutes, proves to be a very varied thing when we draw comparisons between and among races with independent lines of ancestry and heredity occupying widely separated areas.
Should we analyze it, we find it to be composed of three constituents; namely, the physical elements of the brain, the degree to which the observational or perceptual and higher elements cooperate in building up the conceptions peculiar to the type, and the materials with which the physical mechanism deals, in the way of environmental, educational, and social "grist for the mental mill." Many anthropologists accord too great an importance to the third constituent of human faculty, I believe, and they are therefore led to deny that races differ in mental respects to so large a degree as the thoroughgoing evolutionist would contend. They hold that differences in such things as powers of observation are due to training: that, for example, an American Indian or a South Sea Islander sees certain things in his environment more quickly than a white man only because these are the things which the experiences of his earlier life have accustomed him to look for and to find. This may be granted, and it may also be admitted that children of so-called "lower" races can be educated side by side with the youth of white races without noticeably falling behind, up to a certain point when, at the age of adolescence, in the classic case of the Australian natives, other factors prove to be obstacles to further progress. We must also recognize that the character of the environment of a race determines to a large extent the mode of life of the people; a forest-dwelling Indian of the interior is a hunter as well as a warrior, while a South Sea Islander is a navigator and a fisherman.
But the fact remains that the inhabitants of similar countries have reached markedly different grades of intellectual and cultural life. Anglo-Saxon dominance must be referred ultimately to Anglo-Saxon heredity and not to the peculiarities of the land. Although adaptation is no less necessary for men as individuals and as social groups than it is for all other living things, I believe that it is to diversity in constitutional endowments, however these may have arisen, that we must attribute the superiority of some races over others.
The question is not whether a savage race can or cannot adopt the higher conceptions of a civilized people; the fact is that they have not actually become civilized by themselves. Thus, while evolution in mental respects has not resulted in the loss of plasticity in the case of the brain and the nervous system as a whole, wherefore the activities of these organs still remain capable of individual and racial modifications that are impossible in the case of the skeleton and in the color and shape of the eye, it remains true that races do differ intellectually, and that their differences are marks of a mental evolution quite as definite as their physical natural histories of change.