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.
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.
* * * * *
In my own view the strongest and most impressive evidence bearing upon the
great problem before us is provided by the series of transformations by
which the human intellect develops during an individual life. Mind has an
embryology no less significant than that of the skull or of any other
element of the body; and its investigation leads to the evolutionary
interpretation quite as surely as the study of the various grades of adult
psychology constituting the anatomical sequence, which we have reviewed
previously. When in the earlier part of the book we dealt with embryology
in general, we learned how the changes which take place when an organism
develops from an egg demonstrate the actuality of true organic
transformation without the necessity of concluding or inferring that this
process might occur. It is not superfluous to insist again that the
essential fact in evolution is the alteration of one organic
characteristic into another type; must we not recognize at the very outset
that mental transformation is as real as physical development?
In the first instance we might concern ourselves with the physical basis
of mind and its history. In the earliest stages of human embryology no
nervous system whatsoever is present, and it is unreasonable to suppose
that there is anything going on which corresponds to human thought. A
little later a cellular tube is established as a primitive nerve axis,
which at first is nearly uniform throughout its entire length and displays
no differentiation into brain and spinal cord. Before long an enlargement
of the anterior end expands and develops into a primitive three-parted
brain. It is not yet a real brain, however, and it is entirely incapable
of functioning in such a way as to justify the use of the word _mental_
for the results of its operations. We know that it is only in the cerebral
hemisphere of the adult brain that the processes of true human
consciousness go on. But it is not until long after the three-parted stage
that the cerebral hemispheres make their appearance therefore we cannot
speak of mind as present when the cell and tissue basis of mind is not
present. When, now, the cerebral hemispheres do appear, they are small
bean-shaped structures no larger relatively than those of a fish. Later
they enlarge so as to attain the relative size of the cerebral hemispheres
of an amphibian, and still later they are like those of a reptilian brain.
Continuing to enlarge, they begin to fold so that the total surface is
increased without very much addition to their bulk. At this time the
cerebral hemispheres of the brain of the human embryo are like those of an
adult cat or dog. The process of general enlargement and of progressive
convolution are continued, and stages are reached and passed which
correspond with the monkey and ape conditions.
Nothing in human development is more impressive than the origin of the
cerebrum and its development by passing through successive stages which
are counterparts in the main of the adult brains of other and lower
animals. The alteration of a tissue-mechanism constructed in one way into
a tissue-mechanism of a more complex nature, provides the most conclusive
evidence of the reality of brain evolution, because the process of
transformation actually takes place.
But in the present connection we are more interested in the dynamic or
functional aspects of mental evolution, which it must be remembered are
inseparably bound up with the physical structures and their modifications.
After a human infant is born its activities are reflex and mechanical like
those of the adult members of lower groups. As it grows it performs
instinctive acts because its inherited nervous system operates in the
purely mechanical manner of a lower mammal's nervous system. For these
reasons an eminent psychologist has said that the mental ability of an
infant six months old is about that of a well-bred fox terrier. The same
infant at nine months displays an intelligence of a higher order equal to
that of a well-trained chimpanzee; it has become what it was not, and in
so far it has truly evolved in mental respects. At two years of age the
child is incapable of solving problems of the calculus, for its reasoning
powers are elementary and restricted, but these same powers change and
intensify so as to render the older mind quite capable of grasping the
highest of human conceptions and ideas. In my judgment the unbroken
transformation of a child's mind that exhibits only instinct and
intelligence into an adult's mind with its power of reasoning, is far more
conclusive as proof of mental evolution than the inference drawn from the
comparisons we have made above of the adult psychological phenomena of
man, ape, cat, and fish. It is surely natural for such mental
transformations to take place, for they do take place in the vast majority
of human beings; when they do not, in cases where the brain fails to
mature, we speak of unnatural or diseased minds.