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THE EVOLUTION OF
ANIMAL INTELLIGENCE

BY

S. J. I HOLMES, Ph. D.

ASSISTAa»-«lOFESSOB OF ZOOLOGT IN
THE UXIVEBSITY OF WISCONSIN.

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1911

NEW YORK

HENRY HOLT AND COMPANY

1911

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5425

GOPTKIOHT, 1911
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HENRY HOLT AND COMPANY

CAMKLOT PRKSS, 444-46 PEARL STREET, NEW YORK

PREFACE

The investigations of recent years have added so much to
our knowledge of the activities of animals that any adequate
account of the whole field of animal behavior would require
several volumes. The present work is confined to certain
topics which bear upon the evolution of animal inteWigence,
and our treatment of even this part of the subject has been
of necessity fragmentary. It has been our aim to give a
fairly clear conception of the activities upon which intelli-
gence is based, to show how intelligence is'related to these
activities, and to sketch the general course of the evolution of
intelligence in the animal kingdom. No effort has been
made to deal with all the classes of animals in which intelli-
gence is manifested, and some groups which were not
essential to the development of our theme have received
little attention.

I wish to express my thanks to Prof. L. J. Cole and Prof.
H. B. Torrey for their helpful criticisms of the manuscript of
this book before it went to press. To Prof. L. T. Hobhouse,
Prof. E * L. Thorndike and Dr. L. Witmer I am indebted for
permission to reproduce the figures which are attributed to
these writers in the text, and I am also indebted to the
Macmillan Company for kindly allowing me to reproduce
figures 15 and 16 from their publications. My greatest debt
is to mv wife for helo and encouragement in many ways.

S. J. H.

m

CONTENTS

I. Introduction 1

II. Reflex Acno:, 11

III. The Tropisms 18

IV. The Behavior of Protozoa 63

V. Instinct 91

VI. The Evolution of Instinct » . 115

VII. The Non-intelligent Modifications of Behavior . . . 139

VIII. Pleasure, Pain, and the Beginnings of Intelligence . 164

IX. Priliitive Types of Intelligence in Crustaceans and

Molluscs 180

X. Intelligence in Insects 191

XI. Intelligence in the Lower Vertebrates 218

* XII. The Intelligence of Mammals 232

XIII. The Mental Life OF Apes AND Monkey.=; 260

Index 283

THE EVOLUTION OF ANIMAL
INTELLIGENCE

CHAPTER I
INTRODUCTION

"If the doctrine of Evolution is true, the inevitable implication
is that Mind can be understood only by obser\dng how Mind is
evolved." — Herbert Spencer, Principles of Psychology.

*' AUe instinktiven Triebe und zweckbewussten Willensausserungen
dienen entweder der Erhaltung des eigen Lebens oder der Erzeugung
und Pflege der Nachkommenschaft." — Schneider, Der Thierische
Wille.

"Si la^psychologie humaine est legitime, la psychologie compar^e
Test aussi, pour les memes raisons." — Claparede.

In the last two decades the science of animal psychology
has had an unprecedented development. Previous to this
time and subsequent to the appearance of the Origin of
Species students of the animal mind were concerned in
bringing forward proofs of the similarity of the mental
processes of men and animals and in tracing the gradual
unfolding of the mental powers in passing from lower to
higher forms. The followers of Darwin in their efforts to
show that evolution is applicable in the mental as well as the
physical world were naturally prone to minimize the gulf
separating the human and the animal mind which the tradi-
tional psychology had formerly taught was a wide and im-
passible one.

1

2 INTRODUCTION

Hence we find among writers of the period mentioned a
tendency to interpret the actions of animals as the outcome
of highly developed psychical faculties, and to accept rather
too uncritically stories indicative of great sagacity on the
part of animals with the end of showing that the brute
creation does not stand so very far below us after all. The
works of Romanes on Mental Evolution in Animals and
Animal Intelligence are typical of the period and its tenden-
cies toward anthropomorphism, which is now the bete noir
of every dabbler in the subject of animal behavior. While
Romanes has been criticized with a certain amount of
justice for basing conclusions on anecdotes from more or
less questionable sources he has the merit of bringing to-
gether a large number of facts regarding animal intelHgence
and of giving an able and lucid account of the general course
of mental evolution, and there is no doubt that had his
life extended into the period of more careful and accurate
experimentation he would have been among the first to
appreciate and encourage the newer animal psychology.

The advent of experimental psychology with its various
appliances for observing and recording the manifestations
of mental phenomena, the study of the functions of the
nervous system by the experimental methods of the phy-
siologists, and the growing vogue of methods of experimental
analysis in biology in general could not fail to have a marked
influence on the study of the mental life of animals. After
the scientific world had comfortably settled down in the
belief in continuous mental evolution, and the subject had
mainly lost its controversial interest, attention became more
strongly directed to the analysis of animal behavior with the
end of explaining why animals act as they do. Then came
the period of reflexes and tropisms and the analysis of com-
plex behavior into simpler processes, with which we are now

INTRODUCTION 3

struggling. Animal psychology blossomed into an experi-
mental science, proud of her new growth and often looking
back with something of scorn upon the days of her infancy.
The position of animal psychology among the sciences is,
however, a somewhat peculiar one. Concerning the con-
scious life of animals as distinguished from the objective
facts of behavior — the life which falls within the pecuHar
province of psychology as distinguished from physiology —
our knowledge rests upon an insecure foundation. We have
no means of cognizing directly the conscious states of any
creature besides ourselves and what we know of the psy-
chology of our fellow human beings is based upon what we
find taking place in our own minds. We infer conscious-
ness in other beings because we are conscious ourselves,
and we judge of the mental states in the minds of others,
such as joy, sorrow, anger, or fear, from certain physiolog-
ical manifestations which are like the accompanying mani-
festation of these mental states in ourselves. With beings
much like ourselves our inferences may be fairly accurate.
When thrown amid the people of other nations or races
our judgments are more apt to be erroneous. And when
we try to infer what goes on in the mind of a cat or a
dog the difficulties are very greatly increased. We may
feel con\dnced that the sensations of these animals and
even the basic emotions, such as fear, anger, etc., are
very similar to our own, but the difficulty of ascertaining
what sort of intellectual life one of these creatm-es enjoys
is evinced by the very different interpretations of the
subject wliich have been made by competent psychologists.
If we try to imagine what sort of psychic states are as-
sociated with the supra-oesophageal ganglion of an ant or
a crayfish, analogy almost completely fails us. However
much we may learn about the behavior of these creatures,

4 INTRODUCTION

we could never feel much confidence in any conclusions as
to the kind of conscious states which make up their mental
life. We may be asked, in what way do we know these
animals have conscious states at all? May they not be as
Descartes has considered all animals below man, merely
unconscious automata? The question throws us back upon
what criterion we adopt of consciousness and upon this
subject the opinions of psychologists present us with no end
of differences.

The criterion of consciousness which is perhaps most
widely adopted at the present time is the ability to learn by
experience, or associative memory. Among those who have
adopted this standpoint may be mentioned Bethe, Loeb
and Bohn. Romanes and Lloyd Morgan, while they
recognize that ability to learn is indicative of the presence of
consciousness, hesitate to draw the conclusion that con-
sciousness is absent in those animals which are devoid of
associative memory. In his work on Animal Intelligence
Romanes states that "because a lowly organized animal
does not learn by its own individual experience, we may
not therefore conclude that in performing its natural or
ancestral adaptations to appropriate stimuli consciousness,
or the mind element, is wholly absent; we can only say that
this element, if present, reveals no evidence of the fact.^'
Lloyd Morgan writes likewise in a guarded manner : " When
we see that a chick, for example, pecks at first at any small
object, it is difficult to say, on these grounds, whether it is
a sentient animal or an unconscious automaton; and if it
continued to behave in a similar fashion throughout life,
our difficulty would still remain. But when we see that
some objects are rejected while others are selected, we infer
that consciousness in some way guides its behavior. The
chick has profited by experience. But even this is only

INTRODUCTION 5

a criterion of what we may term effective consciousness.
There may be sentience which is merely an accompaniment
of organic action without any guiding influence on subse-
quent modes of behavior. In that case it is not effective;
and whether it is present or not we have no means of ascer-
taining."

It may be argued that since learning in us is a conscious
process that therefore all animals that learn are likewise
conscious. The inference may be probable, though by no
means necessary; but it would afford no ground for denying
consciousness in animals in which learning does not occur.
If I am pricked by a needle I am acutely conscious. The
feeling of pain is aroused very directly, and it is difficult to see
how it can be dependent in any way on associative memory.
If my memory should fail me to the extent that I would
straightway forget the experience every time I was pricked
would not the pricking arouse the same painful sensation
as before? Would not sound waves produce the sensation
of hearing and retinal stimulation the sensation of light in
the absence of any power of recalling similar sensations
received in the past? Unless it can be shown that there is
some relation of dependence of consciousness upon associa-
tive memory in ourselves there is little ground for setting
up associative memory as a criterion of consciousness in
animals. The criterion reverses the obvious relation of
the phenomena. Instead of consciousness being dependent
upon associative memory, associative memory implies the
previous existence of consciousness. How can the exist-
ence of anything be dependent on the association of its
elements, if these elements can exist only on the condition
that they are associated? The criterion makes the exist-
ence of the more simple and general dependent on the exist-
ence of a special function within its own realm.

6 INTRODUCTION

One reason that has led to the adoption of the criterion
of associative memory is the evidence afforded that the animal
at this stage is guided by experiences of pleasure and pain.
We repeat acts that bring us pleasure and avoid those that
bring pain. Where in animals certain responses that have
been made once tend to be performed more readily on sub-
sequent occasions and other acts which are followed by
avoiding reactions are discontinued, it is natural to infer
that pleasure and pain accompany these different modes
of behavior. Pleasure and pain have very commonly been
spoken of as agents of reinforcement and inhibition. W^hen
the pleasure-pain response appears on the scene conscious-
ness is commonly assumed to take a guiding hand in the
determination of behavior. The advent of this type of
behavior marks a critical period in the evolution of the
animal mind and we shall consider it more closely in a sub-
sequent chapter. There we shall attempt to show that
this type of reaction does not involve the injection of any
radically new element into its course of evolution. If we
adhere to the doctrine of psycho-physical parallelism in any
of its forms we cannot speak, in strictness, of pleasure and
pain as agents in accommodation: they are only the signs
of a certain kind of adjustment. This adjustment according
to the theory of parallelism has its physiological explanation
without calling upon the interference of psychical states.
We might ask broadly: If psychical states do not as such
interfere with the course of physical phenomena, how can
we adopt any kind of behavior as a criterion of conscious-
ness? I doubt if either Bethe of Loeb is willing to defend
the view that consciousness is an agent in directing physical
phenomena, and to accept the logical consequences of such
a position. It can be shown, I believe, that this view creates
many serious difficulties without giving us any real aid in

INTRODUCTION 7

explaining one. But a discussion of this question in all its
bearings would carry us too far.

If there is no infallible test for the determination of the
existence of consciousness in animals we are by no means led,
as some physiologists would have us believe, to the denial
of the possibility of a comparative psychology. If we can
explain the behavior of lower organisms in terms of phy-
siological laws without assuming any role of conscious states
in determining their acts, and were able to extend the same
kind of explanation to higher forms until ultimately the
whole sentient creation were embraced in our system, the
rejection of comparative psychology would logically lead,
as Claparede thas well shown, to the denial of human psy-
chology as well. On the other hand, starting in with the
assumption that human psychology is a legitimate subject
matter for a science, there is no logical ground for refusing
to draw inferences concerning the mental aspects of the
behavior of apes, and other higher mammals; and if we
can infer something of the mental life of the animals near-
est ourselves we are warranted in extending our psycho-
logical inferences as far down in the scale as analogy per-
mits us to go. How far analogy warrants us in going
is capable of but a very indefinite answer. When we
come to ants and spiders everyone may perhaps be allowed
to have his own opinion; and there may be a more than
sufficient compensation for our inabihty to reach conclu-
sive proofs of our views regarding the psychic life of such
creatures in having a perennial source of contention with
which comparative psychologists may occupy themselves
whenever they come together.

The existence of mind in the lower animals has something
more than a theoretical interest. In our experiments on
these creatures it would be unfortunate if we were mistaken

8 INTRODUCTION

in regarding them as unconscious automata and should
proceed to cut them up in a ruthless fashion on that as-
sumption. It may be, as Norman has attempted to show,
that pain sensations in the lower invertebrates are not
acute at most, and may be absent entirely, but not much
would be lost by giving the animals the benefit of the
doubt so far as is consistent with attaining the objects of
experimental work.

The scientific instinct for continuity tempts us to ascribe
some form of sentiency to all living forms. Certainly it is
not possible to disprove the existence of consciousness of
some sort in any organism. In the lowest animals it may
be nothing more than a very dull form of sensibility. If we
would avoid the assumption of an absolute beginning of
consciousness we may hold that something akin to con-
sciousness, but very much more primitive than any of the
forms of it with which we are acquainted, exists in connec-
tion with all inorganic processes. Such a position affords
a consistent viewpoint and has been accepted by many as
enabling us to conceive the whole process of psychic evolu-
tion as one without any breaks or discontinuities anywhere
in the series.

Our task in the present volume is to sketch briefly the
evolution of behavior from its simplest manifestations
in the reflex actions of the lowest organisms to its more
elaborate expressions in the higher mammals. No attempt
has been made to give a survey of behavior in all the groups
of the animal kingdom. Beginning with simple reflex action
we have described those forms of behavior which are com-
monly called tropisms and which are by many regarded as a
comparatively direct outcome of reflex nritability. After
an account of behavior of the lowest members of the ani-
mal kingdom we have given a general account of instinct,

INTRODUCTION 9

the analysis of instinct into simpler processes, and the
different views of the origin and evolution of instinct.
Considerable space is devoted to the transition between in-
stinct and intelligence, and the evolution along the lines
of plasticity and ready modifiabihty of behavior which has
prepared the way for the appearance of inteUigence. This
is followed by a general and confessedly incomplete sketch
of the gradual evolution of inteUigence in the animal king-
dom, and a discussion of some of the evidences for the ex-
istence in the higher animals of a certain power of making
inferences which some modem psychologists have claimed
is not found* in any sentient creature below man.

A comparative survey of the actions of animals shows us
that behavior is very much the same sort of thing wherever
found. It is one of the many valuable contributions of the
great pioneer in genetic psychology, Herbert Spencer, to
have shown the fundamental unity of biological and psycho-
logical processes in all their varied manifestations. Instinct,
memory, volition and reason are all parts of that general
process of adjustment of the organism to its environment,
in which life in all its stages essentially consists. As we
pass from lower to higher forms we have an increase in the
complexity and perfection of the adjustments; the corre-
spondence increases in space and in time, in deiiniteness and
in generality, but everywhere it is "the adjustment of
internal relations to external relations." This conception
of mental life marks an important advance upon the psy-
chology current in Spencer's early years; it set aside the
arbitrary distinctions of the so-called "faculties" and pre-
pared the way for a clearer insight into the gradual unfold-
ing of mind which the labors of genetic psychologists are
slowly giving us.

10 INTRODUCTION

BIBLIOGRAPHY

BoHN, G. La Naissance de rintelligence. Paris, 1909.
Claparede, E. Les animaux sont ils-conscients? Rev. Philos.,

51, 24, '01. La psychologie compar^e est-elle legitime? 1. c.

Arch, de Psych., 5, 13, '05.
Huxley, T. H. On the Hypothesis that Animals are Automata

and its History. '74.
Loeb, J. Comparative Physiology of the Brain and Comparative

Psychology. N. Y., '04.
Marshall, H. R. Instinct and Reason. N. Y., '98.
Morgan, C. Lloyd. An Introduction to Comparative Psychology.

London, '94. Animal Behaviour. London, '00.
Romanes, G. J. Animal Intelligence. N. Y., '83. Mental Evo-
lution in Animals. London, '85.
Spencer, H. Principles of Psychology, 3d. ed. N. Y., '94.
TiTCHENER, E. B. Were the Earliest Organic Movements Conscious

or Unconscious? Pop. Sci. Mon., 60, 458, '02.
Washburn, M. F. The Animal Mind. N. Y., '09.
Wasmann, E. Instinkt und Intelligenz im Tierreich, 3d. ed.

Freiburg, i. B., '05.

CHAPTER II.

REFLEX ACTION.

"Pure reflexes are admirably adapted to certain ends. They are
reactions which have long proved advantageous in the phylum of
which the existent individual is a representative embodiment.
Perfected during the course of ages, they have during that course
attained a stability, a certainty, and an ease of performance beside
which the stabtlity and facility of the most ingrained habit acquired
during an individual life is presumably small." — Sherrington,
The Integrative Action of the Nervous System.

The term reflex action is used in a wide and somewhat
indefinite sense, but it is applied in general to responses of the
organism which are evoked directly upon the application of
a stimulus. In a typical reflex we have a sensory or afferent
impulse set up in some sense organ and proceeding toward
a ganglion or larger nerve center. From the latter an efferent
impulse passes outward along a motor nerve, causing a
muscular contraction. Frequently the impulse is reflected
back near its point of origin, more or less after the fashion
in which light may be reflected back to its source from a
mirror and the reaction occurs in the stimulated part. It is
from analogy with the reflection of light that the term re-
flex action is chosen, although the analogy, it must be con-
fessed, is rather loose, and in some respects misleading.
When one involuntarily winks the eyes after they are
suddenly illuminated by a glare of light, impulses are set
up in the retina which proceed along the optic nerve to the
brain. Here they traverse certain established connections
among the fibers, by means of which they become directed
outward along the motor nerves to the muscles of the eye-

11

12 REFLEX ACTION

lids causing them to contract. This process takes place in
a small fraction of a second after stimulation, and occurs
not only without the action of the will, but even in spite
of efforts to prevent it.

Another reflex act, the so-called tendon reflex, may readily
be observed by striking the large tendon which is inserted
into the patella just below the knee. If the leg is hanging
loosely a light blow on the tendon will cause the foot to be
suddenly extended. Here the impulses set up by the blow
proceed to the spinal cord, and are returned along motor
nerves to the extensor muscles of the leg causing them to
contract. If the nervous centers of the spinal cord are im-
paired, as occurs in certain diseased conditions, this reflex
can no longer be evoked.

In a frog these spinal reflexes may readily take place if the
spinal cord is cut across a short distance in front of the center
which supplies nerves to the parts concerned. A stimulus
supplied to the hind leg of such a frog causes the leg to be
withdrawn. If in a frog whose brain has been destroyed the
side of the body is stimulated by acid there may be produced,
besides the twitching of the muscles near the stimulated part,
a movement forward of the hind foot of the same side which
often succeeds in wiping away the acid. If the stimulus is
applied near the middle of the body both hind feet may
be brought forward to remove it. And if the stimulus be
applied to one side and the leg of the same side be held, the
opposite leg may be brought into play, especially if the
stimulus is a strong one, and may succeed in removing the
offending substance. The accuracy of these movements,
especially in the crossed reflex, is exaggerated in most
accounts, but sufficient exactness is frequently attained to
effect the removal of the irritating material.

Reflex action may occur in various degrees of complexity.

REFLEX ACTION 13

In the simplest cases there is movement in the single part
affected, as when a single tentacle of a polyp is withdrawn
upon being stimulated. In other cases a single stimulus
may produce combined movements of several parts, as when
the various appendages of a brainless crayfish are set in mo-
tion when but one of them is stimulated. A stimulus on the
right side of a frog may not only cause coordinated action in
the various muscles of the hind leg, but the right fore leg
may be apphed to the stimulated part as well. And if
the animal is rendered especially responsive by the injection
of a solution of strychnine, contraction may take place in
most of the muscles of the body.

One factor which is concerned in the complexity of the
response is the strength of the stimulus. A Hght stimulus
apphed to the tentacle of a jelly fish usually causes a contrac-
tion of that tentacle only, but if the stimulus is stronger,
the neighboring tentacles will also contract, and if still
stronger, movements may be set up in the umbrella or disk
which cause the animal to swim away. Complex responses
are also produced if a stimulus affects many sense organs at
once.

A further method of comphcation occurs in the so-called
chain reflexes in which one action serves as the stimulus
for a second, and this for a third, and so on. A frog with
its cerebral hemispheres removed will respond to the move-
ments of a fly in its vicinity by snapping at it. This com-
pHcated reflex causes the fly to be seized by the jaws; the
stimuli caused by the juices of the fly acting on the taste
buds of the frog's mouth set up the swallowing reflex, which
involves complex and coordinated movements of the tongue
and throat. When in the stomach the fly reflexly excites
this organ to activity, resulting finally in the extrusion of the
digested mass into the intestine, which in turn is reflexly

14 REFLEX ACTION

excited to the performance of peristaltic movements and
other functions.

Similarly a brainless crayfish will seize a bit of food in
its small chela, pass it forward to the maxillipeds, which
cooperate to bring it between the mandibles where it is
chewed, after which it is swallowed, passing into the
stomach, where it sets in action the gastric mill resulting
in the further trituration of the food.

Reflex actions may be inhibited in various ways. If the
spinal cord of a frog is severed, and a stimulus applied to the
anterior cut end at the same time the hind toe is pinched,
the withdrawal of the leg which occurs regularly under nor-
mal conditions may be prevented from taking place. Im-
pulses passing down the cord block or inhibit impulses tend-
ing to pass down the motor nerves to the leg. Strong stimuli
applied to the optic lobes or certain other parts of the brain
of a frog produce the same effect. Reflex acts in animals
generally take place more readily and more uniformly if the
brain be destroyed, indicating that the brain acts as a con-
stant inhibitory organ upon the lower nerve centers. Stim-
uli applied to other parts of the body may also inhibit
reflexes. If one toe of a decapitated frog is strongly stimu-
lated by being held in dilute sulphuric acid and the other
toe stimulated by an electric current, the withdrawal of the
latter will be delayed or entirely checked.

To designate as reflex action the direct responses of the
lowest animals which are devoid of a nervous system is to ex-
tend somewhat the original meaning of the term. These re-
sponses, however, are now commonly spoken of as ^'reflexes"
and they are very similar to the simple reflex acts of higher
forms in which a nervous arc forms the pathway of the
impulses. To a certain extent all living matter conducts
stimuli and hence performs the function of nervous tissue.

REFLEX ACTION 15

In the Protozoa this transmission may take place with great
readiness, although there are not known to be any specialized
pathways which the impulses follow. If the bell-animalcule
Vorticella is touched, the long flexible stalk will quickly
contract. A Stentor when stimulated contracts with great
suddenness, and Paramoecium when it encounters a stimulus
of any kind performs a stereotyped motor reflex which con-
sists in reversing the stroke of the cilia, swimming backward,
and turning toward the aboral side. This response is shown
even by small pieces of the infusorian in much the same way.
Most of the protozoa react to stimuli by some form of motor
reflex. Even plants show reflexes, as in the drooping of the
leaves of the sensitive plant, the curling of the tentacles of the
sundew, the movements of the Venus* fly trap, and many
other less manifest exhibitions of irritabihty.

A conspicuous trait of reflexes is their adaptiveness. The
winking of the eye, the withdrawal of the frog's leg, the re-
moval of acid from the side of the body, the snapping and
swallowing of food are all actions which, in one way or an-
other, secure the welfare of the individual. But the pur-
posiveness is like that of a machine which is constructed to
do a particular thing; it is the result of conformation and
arrangement of its parts. By pressing a lever a machine
may be made to perform a very simple act or produce an
elaborate piece of music, according to the nature of the
mechanism. Given the proper organization an animal may
likewise perform acts of great complexity and adaptiveness
in response to certain kinds of stimulation; the purposive-
ness of the behavior results not from choice, but from a sort
of mechanical necessity. The explanation of this adap-
tiveness, like the explanation of the adaptiveness of a
machine to perform a certain kind .of work, lies in the
causes that led to the production of a certain kind of

16 REFLEX ACTION

structure. For a solution of this problem we are naturally
led to the study of the evolution of organic life.

RHYTHMICAL ACTIVITY

Organisms perform many actions repeatedly which appar-
ently go on by themselves independently of outer stimulation.
Such actions may be viewed as responses, but the stimuli
causing them are internal instead of external. The beating
of the heart and the regular movements of respiration are
familiar examples of such rythmical activity.

In the horse-shoe crab Limulus the abdominal appendages
Which bear the gills execute a regular to and fro movement
which will occur as well if the nerve cord is cut between the
thorax and the abdomen. Even if the nerve cord is cut
between the several abdominal ganglia the appendages will
still beat regularly, although all coordination of their
movements is destroyed. Among medusae there is a regular
contraction of the disk or bell during swimming which
usually disappears if the marginal nerve ring is removed,
but Loeb has shown that if the jelly fish Gonionemus, after
being operated on in this manner, is placed in sea water
devoid of calcium salts the rhythmical pulsations will
reappear.

Among the Protozoa rhythmical movements are shown in
the regular swa5dng of Stentor, and the periodical contrac-
tions of Vorticella; and Loxophyllum, as is described in a
later chapter, shows a rhythmical alternation of movements
which are performed in much the same way by very small
pieces into which the inf usorian may be divided.

BIBLIOGRAPHY

Bbthe, a. Das Centralnervensystem von Carcinus moenas. Arch,
f. mik. Anat., 50 and 51, '97. AUgemeine Anatomic und Physi-
ologic des Nervensystems. Leipzig, '03.

RHYMTHMICAL ACTIVITY 17

LoEB, J. Comparative Physiology of the Brain and Comparative

Psychology. N. Y., '00.
LuKAS, F. Psychologic der niedersten Tiere. Wien and Leipzig,

'05.
ScHRADER, M. Zur Physiologio des Froschgehirns. Pfltiger's Archiv,

41, 75, '87.
Sherrington, C. S. The Integrative Action of the Nervous System.

N. Y., '03.
Uexkxjll, J. von. Umwelt und Innenwelt der Tiere. Berlin, '09.

CHAPTER III
THE TROPISMS

"The understanding of complicated phenomena depends upon an
analysis by which they are resolved into their simple elementary
compounds." — Loeb, Physiology of the Brain.

"On s'est fait beaucoup d'idees fausses sur les tropismes; les
litterateurs, les philosophes; les savants meme, ont dissert^ sur eux
d'une facon tout a fait fantaisiste, les uns raillant, les autres louant
sans reserve, ce qu'ils n'avaient pas compris." — Georges Bohn.

THE TROPISMS IN GENERAL

Certain stimuli exercise a directive effect upon the move-
ments of animals causing them to go toward or away from
the source of stimulation. Such movements are commonly
called tropisms. The flight of a moth toward the candle,
the gathering of male moths around a box containing a
female, the movements of protozoans away from regions of
unusual heat or cold may be taken as illustrations of such
directed movements.

No attempt will be made to formulate an accurate defini-
tion of the word tropism because usage has not established
the meaning of the term with sufficient precision to render
this possible. There is considerable difference of opinion
in regard to the explanation of tropisms, and the kinds of
behavior to which the term should be applied. In what
follows phenomena will be described which have been com-
monly classed as tropisms however diverse they may be in
character and causation.

The study of tropisms, which has attracted a large share
of attention in recent years, was given a great stimulus by

18

THE TROPISMS IN GENERAL 19

Loeb through his work "Der HeUotropismus der Tiere'^ and
various later papers. The ''HeHotropismus der Tiere"
was a pioneer work; but perhaps its chief value Ues not so
much in the number of interesting facts and records of
ingenious experiments it contains as in the general theory-
there advanced, which has been the means of stimulating
many investigators to enter the field.

The book emphasized the idea that it is the business of
the investigator not merely to describe the facts of behavior
and marvel over their wonderful nature, but to try to ex-
plain them. It presented him with a theory which if true
would carry us a step forward in the analysis of many
kinds of behavior, especially in the lower organisms. The
book shows the influence of Sachs, whose conception of the
mechanism of tropisms in plants forms the nucleus of Loeb^s
theory. Loeb attempted to show that the phenomena of
Heliotropism were essentially similar in plants and animals.
The orientation of the body to the direction of the rays, the
fact that the more refrangible rays are the more effective in
producing the heliotropic response, and the dependence of
light reactions upon light intensity and temperature are
shown by plants and animals in much the same way. All
of these effects are supposed to rest upon certain funda-
mental properties of living substance common to plants and
animals. This conclusion was naturally a somewhat start-
ling one, especially when the light reactions of highly de-
veloped animals which were commonly supposed to be due to
psychic causes were placed in the same category with the
turning of leaves and stems toward the sunlight.

The prospect of finding a mechanical explanation of the
behavior of organisms is always an alluring one. In
this case it proved to be especially so because the general-
ization included such apparently diverse phenomena, and

20 THE TROPISMS

could be applied with only a slight modification to other
kinds of tropisms. "These tropisms," says Loeb, ''are
identical for animals and plants. The explanation of them
depends first upon the specific irritability of certain elements
of the body-surface, and second, upon the relation of sym-
metry of the body. Symmetrical elements at the surface of
the body have the same irritability; unsymmetrical elements
have a different irritability. Those nearer the oral pole
possess an irritabiUty greater than that of those near the
aboral pole. These circumstances force an animal to orient
itself in such a way that symmetrical points on the surface
of the body are stimulated equally. In this way the animals
are led without will of their own either toward the source of
stimulus or away from it. Thus there remains nothing for
the ganglion cell to do but to conduct the stimulus, and this
may be accomplished by protoplasm in any form."

If, for instance, a worm were near a bit of food we might
suppose that the substances diffusing from the food would
strike one side of the body of the worm causing the muscles
there to contract more strongly than the opposite ones.
The worm would turn toward the food until both sides
of the body were equally affected, when it would proceed
directly toward the source of stimulation. Negative re-
sponses receive an essentially similar interpretation. The
point that is emphasized by the theory is that choice or
volition on the part of the animal is excluded; the actions of
the creature are supposed to be mechanically determined.
It is "forced" to go toward or away from the source
of stimulation as the strength of the stimulus and the
organization of its body determine. The moth flies to the
candle, not because it is drawn by curiosity, as suggested
by Romanes, or from any other conscious motive, but
because it is compelled to orient its body so that sym-

THE TROPISMS IN GENERAL 21

metrical points receive equal amounts of stimulation.
The orientation of animals to heat, chemical substances,
gravity, the electric current and currents of air and water
is naturally explained in much the same way. Animal
instincts may be analyzed into simple tropisms or may
be conceived to be developed from them as a basis.

The tropism theory of Loeb and his followers has met with
a certain amount of opposition, especially by Jennings,
who showed that in many of the so-called tropisms of the
lower organisms there is no definite orientation produced.
In the gathering of Paramoecia in weak acid, for instance,
the organisms are not forced into line with the diffusion
currents and compelled to swim toward the chemical, but
individuals which swim into the acid by chance remain
there; whenever they attempt to pass from the dilute acid
to water they reverse their course, and thus are kept con-
fined to one region. While the chemotactic grouping of
P^tramoecia depends upon a definite reflex, it is produced in a
manner quite different from Loeb's scheme of orientation.
Many of the so-called tropisms of the infusoria and other
asymmetrical forms were found to take place in accordance
with the method followed by Paramoecium.

In some cases where orientation is effected it may take
place more or less indirectly by the selection of random
movements. In the earthworm and in the larvae of blow
flies which are negatively phototactic it has been shown by
the writer that movements which bring the animal toward
the light are checked or reversed and only those which hap-
pen to direct the animal away from the light are followed up.
Whatever immediate orienting tendency the light may have
in these cases is relatively unimportant as compared with
the element of selection of favorable chance movements in
directing the animal away from the light. The tropism

22 THE TROPISMS

does not as in Paramoecium occur independently of orienta-
tion, but the orientation is only indirectly produced instead
of being the result of appropriate direct reflexes.

The movements of organisms toward or away from certain
stimulations, even where they are quite directly caused,
may be brought about in a variety of ways. They are by
no means always the result of a forced orientation. Never-
theless such movements are commonly described as tropisms.
Whether or not the tropic response is effected in one or the
other of the ways we have described it may be just as much
the result of reflex action and just as little the outcome of
intelligence and volition.

CHEMOTAXIS

Movements toward or away from chemicals are naturally
widespread. Many kinds of bacteria are markedly che-
motactic. Englemann found that Bacterium termo gathers
near the margin of the cover-glass where there is more
oxygen than near the center. If green algae are present the
bacteria congregate around them as long as they are exposed
to sunlight and are therefore giving off oxygen. If the slide
is kept in the dark so that no more oxygen is produced and
the oxygen present becomes evenly diffused the bacteria
become uniformly scattered over the field. Englemann
in an ingenious experiment also showed that if a spectrum
were thrown on a long thread of the alga Cladophora the
bacteria would congregate most abundantly in the red end
of the spectrum where most oxygen is given off. The ex-
periment affords a delicate test of the amount of oxygen
evolved under the influence of the various kinds of light.

Bacterium termo and other species were found by Pfeffer
to be attracted to various salts if they were employed in
weak solution, as well as a variety of other substances of the

CHEMOTAXIS

23

most diverse chemical constitution. Many compounds were
found to exercise a repellent effect, especially in strong con-
centration. Amoeba reacts negatively to most chemicals
that affect it. ^\Tiere it comes in contact with the diffusing
chemical the ectoplasm contracts; currents beginning in the
stimulated region gradually extend through the body;
pseudopods are thrust out opposite the stimulated part
and the Amoeba begins to crawl away. Positive chemo-
taxis probably plays a part in the food taking of Amoeba,
although its r61e has not been clearly worked out. In the

FiQ. 1. — A. B. C. Sucessive phases of the reaction of Amoeba to a
chemical (represented by the dotted area).

amoeboid plasmodia of myxomycetes, which usually live upon
decaying wood, Stahl has found that extract of bark pro-
duces a positive response.

Chemotactic reactions are as a rule pronounced among
the infusoria. Paramcecia and some other forms collect in
weak acids, while they are generally negative to stronger acids
and to alkahes. If a drop of strong carbon dioxide solution
is placed in the midst of a lot of Paramcecia the infusorians
will form a ring about it. Being negative to the stronger
acid at the center of the drop and positive to the weaker
concentration that surrounds it they naturally collect in a

24 THE TROPISMS

band or ring whose diameter varies with the size and strength
of the drop. As the solution diffuses and becomes diluted
at the center, the ring widens and extends toward the
center and after a time closes in to form a soHd group. If
two species of protozoa having a different attunement to
carbon dioxide are used they may form two rings, one out-
side of the other.

The tendency of Paramoecium to form groups may be
explained as due, in part at least, to their positive chemo-
taxis to carbon dioxide. If water containing Paramcecia
is allowed to stand undisturbed for some time most of the

individuals will be found in
groups or clusters as if they
were drawn together by their
social proclivities or by some
object of common interest. But
Fig. 2 —Showing the positive the cause of the association is

chemotaxis of Paramoecium to i • i *

a drop of a weak solution of car- much Simpler. As Paramcicea

bon dioxide. . «. , j* • j xv

give off carbon dioxide there
is a greater quantity of it in a region where several
Paramcecia happen to be associated. Positive chemotaxis
to this substance tends to keep the cluster together as well
as to retain other individuals which happen to come into
the region, until finally most of the Paramcecia in the vicinity
are assembled in the group. Other infusorians, such as
Oxytricha and Loxocephalus, form associations which are
not due to carbon dioxide or to any other acids since they
show no positive chemotaxis to these substances. There
is probably some substance which produces the grouping,
although it has not yet been identified.

The chemotaxis of the lower organisms is to a certain
extent related to the welfare of the individuals, although in
only a general sort of way. The negative reaction to strong

CHEMOTAXIS 25

chemicals is a purposive trait, although many forms do not
react away from solutions which are decidedly injurious.
According to Massart Polytoma uveUa may not be repelled
even by the strongest chemicals. Other organisms while
perfectly able to swim away, do not seem to be prepared for
meeting such a situation as the presence of injurious chem-
icals by any appropriate response. If specimens of Loxo-
phyllum meleagris and Paramoecium are placed together
under a cover-glass and a small drop of acid applied to one
edge the Paramoecia will avoid the acid, but the Loxophylla
move about helplessly, many of them swimming directly
into the strong acid, others which may be overtaken by the
acid showing no more tendency to swim away from it than
toward it. The result is that nearly all of the Paramoecia
in the region of the diffusing acid escape, while nearly all
of the Loxophylla perish. y^.'vAr v

The general tendency of Paramoecium to collect in all kinds
of weak acid and to react negatively to alkalies is prob-
ably of little service to the organism. Bacteria show a posi-
tive reaction to various substances which they never encoun-
ter under normal conditions and whose presence can scarcely
be of any benefit to them. On the other hand, the positive
reactions of several species to oxygen, as well as the negative
reactions of certain anaerobic species, are doubtless beneficial
modes of response. It is not improbable that in many
cases the response to substances that are not beneficial is
the incidental result of the organization which causes posi-
tive responses to substances that are beneficial. For
instance, it may be advantageous to Paramoecium to be con-
stituted so as to react positively to weak solutions of carbon
dioxide, and its positive response to other weak acids may
depend upon the same peculiarity. While much of the
chemotactic behavior of lower organisms may be indifferent

26 THE TROPISMS

as regards utility or may be positively injurious, a consider-
able amount of adaptiveness is shown, especially to sub-
stances which are habitually encountered. The situation
is much as would be expected if starting with reactions which
primarily showed no relation to utility organisms came to
have their irritability modified and directed along service-
able lines to the extent, and only to the extent that was
necessary to insure survival under the conditions to which
they were naturally exposed.

Chemotaxis is shown by parts of organisms, especially
the free cells such as spermatozoa, antherozoids and leuco-
c\i;es. Pfeffer showed that the spermatozoids of ferns
would collect in capillary tubes containing a dilute solution
of mahc acid and he concluded that the presence of this
substance in the archegonia is the means of drawing the
spermatozoids to the egg cell and thus effecting fertilization.
A similar role is supposed to be played by cane sugar in
mosses. The white blood cells which have the property of
crawling about much like Amoeba are attracted by certain
substances, and often gather in large numbers where there
is a bacterial infection. If a small tube containing a cultiu"e
of Staphylococcus aureus in agar is placed in one of the lymph
spaces of a frog the white corpuscles will migrate into it in
large numbers. A tube with the same culture medium,
but without the bacteria will not be invaded, showing that
it is the presence of some substance given off by the bacteria
which causes the leucoc>i;es to enter the tube.

The chemotactic movements of Amcsba and related
forms differ markedly from the scheme followed by the in-
fusoria and many flagellates. In Amoeba the part directly
affected is the first to act and there is a more or less definitely
directed series of movements away from the chemical.
In the positive reactions of Paramcecium there is no orienta-

CHEMOTAXIS 27

tion of the body, no seeking of the stimulus and no attra<.-tion
in any sense. The Paramoecia as they happen to approach
a drop of dilute chemical to which they are positiveiy
chemotactic, s\\im into it without interruption, but when
they encounter the other side of the drop and experience
the transition from weak acid to water they give the motor
reflex or avoiding reaction and change their course. As
often as the Paramoecium encounters this boundary it
reacts in the same way. It behaves as if caught in a trap
which presents no obstacle to its entrance, but effectually
prevents its exit. Other Paramoecia sv^imming by chance
into the drop are also caught there and soon a collection is
formed which may ultimately include all the individuals of
the region. In the negative response the method is much
the same only it is the transition from weak to strong acid
which causes the stimulation. The avoiding reaction is
given in the same way in each case. All of the chemotactic
responses take place without any orientation of the body,
by a method of trial and error. ;

In the flagellate Chilomonas Jennings has shown that
chemotactic grouping takes place just as in ParamcBcium.
In the bacterium Spirillum he finds much the same method
employed. Spirillum S'wims by the movement of flagella
at one or both ends of the body. \Mien stimulated it
simply reverses its direction of movement. Positive and
negative reactions take place in exactly the same way.
What things are avoided and what are sought depends
upon what acts as a stimulus causing the reversing reaction.
Apparently these simple organisms have no power of
orienting their bodies; they simply move back and forth,
relying on chance to get them into a favorable situation.
When such a situation has been reached the organisms
remain relatively quiet. In several cases among the flagel-

/

28 THE TROPISMS

lates and in spermatozoids and swarm spores it has been
supposed that there is a direct turning toward the chemical.
\ This may occur in some forms and more especiallly the
\ symmetrical ones, but it has not yet been established.
Analogy with the effect of light on Euglena which may call
forth either the motor reflex or a gradual swerving of its
course toward the stimulated side makes the existence of a
parallel response to chemical stimulation more or less
probable.

The coelenterates show scarcely any behavior which can be
described as chemotactic although, reaction to chemicals is of
course general. If a Hydra is locally stimulated by a strong
chemical which is allowed to diffuse against the body from
a fine capillary tube there is contraction of the muscles in the
stimulated area causing the body to bend toward the chem-
ical. This is certainly not a teleological response; rather
the reverse of one. A still stronger stimulus however will
produce a general contraction of the body which enables the
animal to avoid more serious injury. In planarians both
positive and negative reactions may be induced. The com-
mon fresh- water Planaria maculata may be made to follow a
piece of meat around in any desired direction. The substances
diffusing from the meat seem to stimulate the worm to turn
its head directly toward the food. Strong chemicals, on
the other hand, cause it to turn directly away. Earth-
worms, according to Darwin, find food that is buried under
the earth through the sense of smell. Disagreeable sub-
stances cause contraction and writhing about, which may
bring the worm out of the region of the offending stimulus.
A strong chemical applied to one side of the body may cause
the animal to turn toward the other side, but the avoidance
of such stimuli is mainly effected by general and random
movements.

GEOTAXIS 29

Crustaceans are able to detect food at some distance and
direct themselves toward it, but one may question if such
phenomena should be classed under the head of true chemo-
taxis. Similarly with the olfactory reactions of ants.
These insects follow a scent track with considerable accuracy.
In this way they may be guided to food discovered by other
ants and find their way back to the nest. The tendency to
follow these tracks is doubtless instinctive, and so also is the
action of a dog in following the trail of a rabbit. The be-
havior of the dog is on a much higher level than a mere
tropism, and it is probable that the behavior of the ant is
also, but to a less degree. But where to draw the line
between such actions and chemotaxis proper is perhaps
capable of only an arbitrary decision.

GEOTAXIS

Gravity exercises a directive effect upon the position and
movements of many animals while in other forms it has little
orienting power. The protozoans Euglena and Chlamy-
domonas commonly swim upward in the dark as well as in
the light, but this reaction is checked at a low temperature
of 5° or 6° C. Massart studied the geotactic movements of a
number of unicellular plants and animals by placing them in
a capillary tube open at either end so as to render the supply
of oxygen the same above and below. He found that the
sense of the geotactic response varies in allied species of
the same genus, as for instance. Spirillum some of which
are positive and some negative under the same conditions.
The sense of the response could be changed in some cases by
temperature. Thus Chromulina, which is negative at 15®
to 20° C., becomes positive at a temperature of 5° to 7° C.

The geotaxis of Paramoecium is quite variable and often
quite feeble. Generally there is a tendency to swim up-

30 THE TROPISMS

ward, but Miss Moore has shown that lack of food, changes
of temperature and other factors may cause a reversal of
the response. The infusorian Spirostomum has the peculiar
trait of attaching itself to the bottom by means of a mucous
thread at the posterior end and orienting itself in a vertical
position.

Among the Ccelenterates as in the Protozoa there are
many forms in which geotaxis seems entirely absent. The
fresh water Hydra will attach itself to the bottom or side
of an aquarium or hang downward from the surface film
with apparently equal readiness. Many anemones are
equally indifferent to their position, but Sagartia, according
to Torrey, will bend upward if attached to the side of an
aquarium and slowly migrate to the top. Loeb found that
if Cerianthus, which lives with the lower part of its body
buried in the sand, is placed in an inverted position in a test-
tube the foot will curve downward and the bending will
gradually continue until the animal finally straightens out
into an upright position.

In jellyfish which generally swim with the oral side down-
ward orientation has been attributed to the statocysts which
occur on the margin of the umbrella. In Gonionemus,
however, Murbach has shown that orientation is unimpaired
after the destruction of all these organs, so they cannot be
the exclusive seat of the reaction to gravity. The orienta-
tion is not one which is passively assumed for, as Murbach
has shown, specimens which have been killed float with the
oral surface upward.

In the Ctenophores the statocyst which is located at the
aboral pole of the body is a more essential organ of equilib-
rium, for as Verworn has shown the removal of this organ
is followed by loss of orienation to gravity. In some
Ccelenterates — Antennularia (Loeb), Sertulariella (Driesch),

GEOTAXIS 31

Cormyorpha (Torrey) — geotropism is manifested in the
direction of growth much as in the higher plants.

Among the worms geotactic migrations are manifested by
Convoluta and were found by Bohn to vary with the tide
as is described in another connection (Chapter VII). Des-
truction of the statoHths in this form was found to destroy
its reaction to gravity. The sea-cucumber Cucumaria
cucumis is described by Loeb as having a marked tendency
to crawl upward. The upward migration occurs as well in
inverted glass cylinders, showing that it is not the supply
of oxygen which attracts the animal to the upper surface.

A tendency to vertical migration is shown by many free
swimming Crustacea. In the copepod Labidocera Parker
observed that the females showed a marked negative geo-
taxis in the dark, but when exposed to light their geotaxis
became positive. Whether they were illuminated from above
or below did not alter their response. Although negatively
phototactic they nevertheless swim downward when light
falls on them from below.

Many crustaceans have statocjsts which are especially
concerned in the maintenance of equilibrium. In Mysis
they are located in the inner branch of the caudal appendages.
Delage found that if these organs were destroyed the animals
had difficulty in maintaining their equilibrium, which was
further increased by the destruction of the eyes. In the
decapod Crustacea the statocysts are situated in the basal
joint of the antennules. Destruction of these organs is
followed by a variable degree of loss of equilibrium in differ-
ent species.

The most instructive and ingenious experiments in the
functions of the statocysts were performed by Ki-eidl on the
prawn Palaemon. In moulting, the lining of the statocysts
is cast off and the statoliths or small particles of sand they

32 THE TROPISMS

contain are consequently ejected. The prawns replace
these by small bodies which they pick up with their chelae and
put in the statocyst. Kreidl placed several Palaemons after
moulting in a dish containing some small bits of iron, which
the prawns put into their statocysts in place of the usual
sand. If now a strong magnet was brought near, the bits
of iron would be brought toward it and exercise a pull
analagous to the vertical pull of gravity. Kreidl found that
by changing the position of the magnet the Palaemons could
be induced to orient their bodies in any desired position.

When in the web many kinds of orbweaving spiders hang
with their heads down. Mosquitoes, on the other hand,
when resting on a vertical wall usually have the reverse
orientation. Other insects when under rocks or boards
tend to hang down from the upper surface rather than rest
on the lower one. Lady beetles, according to Loeb, when
placed in a dark box uniformly crawl upward and come to
rest at the highest point. Cockroaches prefer to rest on the
vertical sides of a box rather than the top or bottom, but they
do not seem to have a marked tendency to crawl upward to
the highest point. A tendency to crawl upward is not un-
common in insect larvae, especially those which feed on
plants, a trait which is naturally of service in leading them
to their food.

The tendency to maintain a certain orientation with
respect to gravity is more common than the bent toward
upward or downward migration. This is especially true
of higher animals, and among the vertebrates orientation is
practically the only response to this force. The inner ear
in vertebrates plays an important part in the maintenance
of the normal position, but a description of the numerous
experiments on this subject would carry us too far. The
presence of a specific sense organ for the maintenance of

THIGMOTAXIS OR STEREOTROPISM 33

equilibrium is by no means general. Such organs occur in
medusae, ctenophores, turbellaria, Crustacea, mollusks and
vertebrates, and have been evolved along many independent
lines of descent. They may be absent in many species of
all these groups, excepting the ctenophores and vertebrates,
without entailing any loss of the sense of equilibrium.
Geotropic irritability in many cases seems to be quite
generally distributed throughout the body. Where special
organs of equilibration have been evolved orientation is
generally only partially dependent upon them. In but
few cases is orientation to gravity entirely destroyed when
these organs are removed.

THIGMOTAXIS OR STEREOTROPISM

The lives of all animals are spent in more or less per-
manent contact with solid objects and reactions to the
stimuli thus afforded are universal. Contact of various
kinds means food, enemies, shelter and many other things
of interest to the organism or its posterity. Any animal in
order to stand the least chance of survival must be endowed
with the power to react to contact with various solid ob-
jects in an adaptive manner. There are such varied modes
of reaction to contact that the Hmitation of our descriptive
terms is a matter of unusual difficulty. A contact reaction
by a pseudopod of an Amoeba and the heels of a mule are
very different kinds of behavior although, on reflection, it
is evident that they possess certain features in common.
One class of reactions to contact has been termed by Loeb
stereotropism, which he defines as " the peculiarity, possessed
by some animals, of orienting their bodies in a definite way
toward the surface of other solid bodies," and he distinguishes
stereotropism from other responses, such as locomotor
movements, which follow the application of a contact stimu-

34 THE TROPISMS

lus. Verworn coined the word thigmotaxis, which is practi-
cally synonymous with stereotropism, in order to designate

"all those cases of barotaxis in which

the phenomena are caused by the more or less strong contact
of living substance with solid bodies," the term barotaxis
embracing reactions which are "called forth by pressure
acting unequally on different sides" of the body. Rheotaxis
caused by currents of air or water, geotaxis which according
to Verworn is produced by differences in pressure on differ-
ent parts and thigmotaxis are all classed by this writer as
special cases of barotaxis.

Thigmotaxis may be positive or negative, the difference
depending in many cases on the strength of the stimulus.
This is shown by the reaction of the rhizopod Orbitolites.

^ ^%^^

Fig. 3. — Showing the positive reaction of Amoeba to contact with a
solid object. (After Jennings.)

When quiet this form sends out long and very fine filament-
ous pseudopodia. When these are brought in gentle con-
tact with another object the protoplasm flows out along the
filament and finally draws the whole body toward the object.
If the pseudopod is cut or pressed upon by a needle the
protoplasm flows centrally and the pseudopod is withdrawn.
Amoeba when floating in the water usually sends long pseu-
dopods out in various directions. If one of these strikes
the bottom it commonly adheres to it; the endoplasm
streams into it from the main body and finally the whole
Amoeba begins to crawl in the direction of the attached

THIGMOTAXIS OR STEREOTROPISM 35

pseudopod. Such a reaction is of evident service to the
Amoeba in leading it mto contact with objects where it
secures food and protection. The process of food taking
itself is probably dependent to a certain extent upon a
thigmotactic motion. Strong contact such as striking a
pseudopod with a needle causes the reverse or withdrawing
reaction. By repeated application of the stimulus to one
side Amoeba may be driven about in any desired direction.

Paramoecia were found by Jennings to react to contact
by collecting against solid objects. Often bits of substance
are fairly coated by these infusorians. The cilia in contact
with the object cease beating and stand out at right angles
to the body while the cilia of the oral groove and sometimes
also in other parts of the body, continue their movement.
Among the hypotrichous infusoria the tendency to keep in
contact with solids is very strong. In these forms the cilia
on the imder side of the body are metamorphosed into cirri
which are employed in running about over objects much in
the same way as an insect uses its legs. An Oxytricha is
described by Verworn as happening to get upon the round
egg of the clam Anodonta. It ran about for hours unable
to leave the surface since the egg rested on the bottom at
only one point.

The infusorian Loxophyllum is usually engaged in gliding
about on its right side. It is apparently indifferent as
regards orientation to gravity, as it will move about on the
under side of a surface film with its right side uppermost
as well as on the bottom. "WTien turned over on its left
side it rights itself in several different ways. Even small
pieces show the same tendency to keep the right side in
contact with solids and they will also right themselves
when turned over. Much of the righting movements of the
lower animals consists in the effort to keep a certain side in

36 THE TROPISMS

contact with a solid rather than in any response to gravity.
This is notably the case in the fresh-water Hydra. In
Planaria there is an effort to keep the ventral surface in
contact with some object whatever position in relation to
gravity this may involve. Planaria maculata shows a
modification of the thigmotactic response which Pearl
has called goniotaxis. When placed in a dish these animals
form groups in the angle between the bottom and sides.
The amount of surface in contact with the solid is not
appreciably greater than when the Planarian is on a flat
surface, but what is sought by the animal is a situation such
that the body becomes bent at an angle.

Thigmotaxis is a very common trait among worms in
general. The effort to get into holes or crevices, or to work
in under rocks, by which these forms secure protection from
their various enemies, is to a large extent a manifestation of
this type of reaction. Maxwell showed that if specimens of
Nereis, which are usually found in burrows in the sand
near the seashore, are placed in a dish with a number of
glass tubes just large enough for them to enter, they will
crawl into the tubes and remain there even when exposed
to direct sunlight which is strong enough to kill them.

Most amphipods and many isopods are strongly thigmo-
tactic and tend to collect in crevices between rocks or among
masses of seaweed where they secure contact over a consider-
able surface of the body. Among insects the instinct to
creep into crevices is a common trait. Earwigs if given an
opportunity to wedge themselves under a glass plate will
remain there, in spite of their negative phototaxis, even
when exposed to strong Hght. The moth Amphipyra, al-
though positive in its reaction to light, will cease its photo-
tactic activities if given an opportunity to crawl under a
glass plate, where it will remain quiet (Loeb).

RHEOTAXIS 37

RHEOTAXIS

Under the head of rheotaxis are included the movements
of animals as directed by currents. This response is shown
even among very low forms. The plasmodium of the slime
mould Aethalium when placed on a filter paper along which
a current of water is passing slowly creeps opposite the
direction of the flow. It is not improbable that spermatozoa
migrate up the oviduct on account of the current which
the beating of the cilia causes to flow toward the uterus.

Among fishes orientation to currents is a common phenom-
enon. Many fishes have the instinct to head up stream
and swim against the current. Lyon has shown that this
is due in large measure to reactions to movements of objects
in the visual field. In his experiments he used an aquarium
with a glass bottom ''so supported that the bottom was
freely accessible. Close along the bottom, beneath the glass,
could be drawn a long piece of white cloth with black
strips painted across it. This would give the impression of a
moving bottom. Fish (Fundulus) placed in the aquarium
oriented themselves with the head in the direction of the
moving bottom and swam along it to the end of the aquarium.
Reversing the movement of the bottom reversed the orien-
tation and movement of the fish."

The orientation of animals to currents has been explained
as due to differences in pressure produced by the current in
different parts of the body. It is obvious that such differences
can be produced only when the current moves past the animal.
Where the animal is entirely immersed and is carried along
passively the effect, as Lyon points out, is the same as if
it were in quiet water. It is like a man in a balloon carried
by the wind. Moving at the same rate as the air about him
he becomes conscious of motion only when he can see objects
on the earth passing by beneath.

38 THE TROPISMS

In another experiment by Lyon fish were placed in a
long bottle which was corked. When the bottle was pulled
through the water the fish immediately swam opposite the
direction of movement. If the bottle was allowed to flow
down stream the fish would swim to the up stream end;
if it was pulled up stream the fish would swim to the down
stream end. It is evident that rhfiotaxis takes place through
orientation to objects in the field of vision. In young
lobsters Hadley has shown that rheotaxis is a sight response
much as in fishes. In later stages when the lobsters keep
j closer to the bottom rheotaxis is gradually lost.

Blinded fish may orient themselves to currents so long as
they are in contact with the bottom of the stream, but when
they leave the bottom they lose entirely their rheotactic
response, unless different parts of the stream with which
they come in contact have different velocities.

Many insects tend to fly against the wind (anemotropism).
This also is probably a sight reflex, since the insect receives
pressure from the air only when flying against it. May
flies, according to Radl, often hover over one spot, slowly
rising and sinking, but keeping their bodies facing the wind
and their long fore-legs stretched forw^ard. The same trait
is shown by many insects, especially flies, even w^hen there is
no perceptible breeze. Often a group of flies will keep
hovering near one, and will move as one's body moves always
keeping away at about the same distance. Apparently
v»'e have to do here with an effort to maintain a certain
relation to the visual field which so largely determines the
rheotactic responses of fishes.

It is ob\dous that under the head of rheotaxis phenomena
have been included which are quite unlike in their causation,
some of them being reactions to pressure differences, others
presenting interesting points of similarity to phototaxis.

COMPENSATORY MOVEMENTS 39

COMPENSATORY MOVEMENTS

A class of phenomena ha\ing certain relations to rheotaxis
are the so-called compensatory motions of animals. These
may easily be illustrated by a common frog. If a frog be
slowly rotated about a vertical axis it will turn its head
and may begin to walk opposite the direction of movement.
If it is tilted downward in front, the head will be raised,
while if it is inclined upward, the head will be lowered.
Various combined motions will be responded to by move-
ments which tend to keep the head in the same position as
before.

A pigeon when slowly rotated on a horizontal turntable
turns its head opposite the direction of movement until
it reaches a certain angle with the body when it is suddenly
jerked back to its original position. It immediately repeats
the previous movement until its head reaches again the
maximum angle when it is jerked back again as before.
If the head is held during rotations, compensatory motions
follow^ed by regular jerking back movements are performed
by the eyes. Mammals show similar movements, and mice
and several other forms run around on the turntable opposite
the direction of rotation. An interesting form of compen-
satory movement is shown by the common domestic fowl.
Hold an individual in the hands and move it slowly back
and forth, up and down, or side wise. If the fowl is not
carried too far the head will keep in almost exactly the same
position, the neck being often stretched out to its extreme
length before the head follows the movements of the body.

It was formerly supposed that in vertebrate animals
compensatory motions were dependent on the semicircular
canals, but it was found that these motions still persisted
after the semicircular canals were plugged up or extirpated,
or after the nerves supplying them were cut. The otoHths

40 THE TROPISMS

are not necessary organs for these movements (Lyon).

In insects compensatory motions are easily observed.
If flies, beetles and many other forms are rotated on a tm-n-
table they generally walk around opposite the direction of
rotation. This reaction apparently depends upon the eyes
since it is no longer performed if the eyes are blackened over.
If robber flies or dragon flies are held in the hand and rotated
the head will show reverse movements much as in the frog.
In the crayfish and other higher crustaceans compensatory
motions are shown by the eye stalks. If a specimen is
rotated slowly about its long axis the eye stalks will move
opposite the direction of rotation. Similar movements
follow upon rotation about a vertical axis. Blacking over
the eyes of the crayfish causes a diminution of compensatory
motions upon rotation in a vertical plane, but produces
little or no effect on movements of the eye stalks upon rota-
tion about a dorso- ventral axis (Lyon).

In many cases the effort to keep a constant visual field is
an important element in determining compensatory motions,
but it is not the sole cause of such motions in all forms.
Sight is probably of more importance than any other single
factor, but static organs in some cases seem to play a part
also. In the dog-fish, however, Lyon has shown that com-
pensatory movements of the eyes regularly occur, although
somewhat weakened, in specimens in which both the optic
and the auditory nerves have been cut.

PHOTOTAXIS

Of the numerous papers on tropisms a greater number
have been devoted to phototaxis than to any other subject.
Only a very brief discussion of the results therefore can be
attempted here. Investigation has shown that reactions
to light do not fall under any one general scheme of explana-

PHOTOTAXIS 41

tion. Loeb early distinguished phototaxis or heliotropism
from mere reaction to differences in the intensity of light
(Unterschiedsempfindlichkeit) where there are no definitely
directed movements and subsequently appUed the term
photokinesis to the latter phenomenon. The tube-dwelling
annelid Serpula may be made to suddenly draw back if
a shadow is thrown upon its expanded gills. If the light
be suddenly increased no reaction occurs. In the bivalve
Psammobia, Nagel found that sudden increase of light caused
a retraction of the extended siphons; while many other
species (Cardium, Mactra, Solen) would give a similar
reaction to shadows. In this connection may be mentioned
the reactions to shadows of the large leech Qepsine which
is parasitic on turtles. If a shadow is thrown upon a lot
of hungry leeches in a dish of water they will raise up and
extend the anterior part of the body and sway it about in
various directions. The function of this response is to enable
the leeches to attach themselves to any passing turtle and
thereby secure their food.

Animals may form collections in shaded localities not
because they are negatively oriented by the rays of light,
but because light stimulates them to general activity, and
when they happen to come into a place where they are less
stimulated they become relatively quiet. Loeb found that
if fresh w^ater planarians are placed in a round dish in front
of a window they collect at the sides of the dish which are
more or less shaded instead of at the side farthest away from
the window where they would naturally assemble if they were
guided solely by negative phototaxis. In bright light these
animals are active and when they wander into a shaded
spot they move more slowly and thus tend to collect there,
Parker, however, has shown that Planaria maculata is
oriented to a certain extent by the rays of light, and Walter's

42 THE TROPISMS

extended studies prove that both phototaxis and photokine-
sis are factors which, in varying degrees, determine the
behavior of this species.

Positive phototaxis is often combined with photokinesis.
In the large amphipod Talorchestia there is a marked photo-
taxis. Specimens placed in a glass dish before the window
or an artificial light keep hopping toward the light and strug-
gling to get as near it as possible for hours at a time. Yet if
individuals chance to get into a shaded region where they are
less strongly stimulated they often remain there. Among
positively phototactic insects similar behavior is not uncom-
mon. Many insects are quite spasmodic in their photo-
taxis. They may run about in apparent unawareness of
light and then suddenly become seized with an impulse to
go toward it. These insects frequently keep in shaded
localities most of the time and would ordinarily be thought
to be negatively phototactic while in reality they only
manifest a tendency to rest in shaded places into which
they happen to wander. The proclivity to crawl under
objects is commonly also, in part at least, a manifestation of
positive thigmotaxis.

Among animals which regularly go toward or away
from the light there are considerable variations in the method
employed. WTiile in many forms there is a direct orientation
to the rays, in others the orientation is only brought about
indirectly. Larv2e of blow flies commonly crawl away
from the light, but by a method which I have elsewhere re-
ferred to as the selection of random movements. To quote
from my previous account, "^\Tien strong light is thrown
on a fly larva from in front, the anterior end of the creature
is drawn back, turned toward one side, and extended again.
Often the head is moved back and forth several times before
it is set down. Then it may set the head down when it is

PHOTOTAXIS 43

turned away from the light and pull the body around. If
the head in moving to and fro comes into strong light it is
often retracted and extended again in some other direction,
or it may be swung back without being withdrawn. If a
strong light is thrown upon a larva from one side it may
swing the head either toward or away from the light. If
the head is swung toward the light, it may be withdrawn or
flexed in the opposite direction, or, more rarely, moved to-
ward the hght still more. If it is turned away from the light
the larva usually follows up the movement by locomotion.
Frequently the lai'va deviates considerably from a straight
path, but as it continually throws the anterior end of the
body about and most frequently follows up the movement
which brings it away from the stimulus, its general direction
of locomotion is away from the light. In very strong illum-
ination the extension of the anterior part of the body away
from the light is followed by a retraction, since in whatever
direction it may extend it receives a strong stimulus and the
larva wTithes about helplessly for some time. Sooner or
later, however, it follows up the right movements."

The earthworm was found to orient itself in a similar
manner. " If a strong light is held in front of the worm it at
first responds by a vigorous contraction of the anterior part
of the body; it then swings the head from side to side, or
draws it back and forth several times, and extends again.
If in doing so it encounters a strong stimulus from the light
a second time it draws back and tries once more. If it
turns away from the light and then extends the head, it
may follow this up by the regular movements of loco-
motion. As the worm extends the head in crawling it
moves about from side to side, and if it happens to turn it
toward the light it usually withdraws it and bends in a
different direction. If it bends away from the hght and

44 THE TROPISMS

extends, movements of locomotion follow which bring the
animal farther away from the som-ce of stimulus."

While orientation in the earthworm is ordinarily brought
about mainly by the selection of random movements, light
has a certain power to cause the worm to turn directly away
from it, but this is only a minor factor under usual conditions.
Harper has shown that if the earthworm PerichoBta her-
mudensis is exposed to very strong light it turns directly
away from it, while in weaker Hght orientation is brought
about by the method just described. In the leeches Clepsine
and NepheHs negative orientation is effected in part directly,
and in part by following up those random movements
which bring respite from the stimulus.

Among the protozoa the light reactions of Stentor cendeus
have been found to occur in much the same way as the reactions
of ParamcBcium and other infusoria to chemicals. Accord-
ing to Mast the anterior end of Stentor is much more sen-
sitive to photic stimuli than other parts of the body. If a
a Stentor swims into a more highly illuminated region if gives
the avoiding reaction, swimming back, turning to the aboral
side and going ahead in a new direction. If light is coming
into a dish from one side the Stentor gradually gets oriented
to the direction of the rays. If it turns toward the light
it gives the avoiding reaction and keeps on repeating the action
until it becomes pointed away from the light, when it swims
away in a fairly direct path. The infusorian could not be
seen to turn directly away from the light; it swims away from
the light because if it starts to swim in any other direction
its course is checked and its direction of swimming changed.

Euglena vindis, like Stentor, was found by Jennings to
give a motor reflex upon strong illumination. The anterior
end of the body which contains the red eye-spot is more
sensitive than other regions and if Euglena passes into a

PHOTOTAXIS

45

region of higher light intensity it is apt to reverse its move-
ment and change its course. In this way it may avoid
regions of strong illumination. In weak light, however,
Euglena is positively phototactic. It swims in a spiral,
but in a fairly direct path toward the light.
If during its progress the light is carried to
one side Euglena gradually turns until it is
oriented to the direction of the rays. Al-
though Jennings attempts to explain the
orientation as a result of a modified motor
reflex, I cannot see but that the method em-
ployed is one of direct orientation. Depar-
tures from the line are corrected, not as in
the earthworm, by a lot of undirected move-
ments until the right one is hit upon, but by
an appropriate turn in the right direction.

Among a very large number or organisms
in various phyla of the animal kingdom we
find that there is a fairly definite and direct
orientation to the rays of light. Deviations
from the path to or from the light are checked
by a movement which brings the animal into
line again. In lower forms this movement
is doubtless an involuntary one based upon
the property of responding to a localized
light stimulus by a direct reflex. Depar-
ture from the line of orientation subjects
the animal to unequal stimulation on the two sides and
the unequal motor activity thus produced brings the animal
back into line again so that both sides are equally stimulated
and locomotion takes place either toward or away from the
light in the direction of the rays. The course of the animal is,
as it were, automatically regulated. The mechanics of the

B:

Fig. 4. — Euglena
viridis.

46 THE TROPISMS

process varies greatly in different animals. Orientation may
be effected by the action of the cilia, flagella, muscles of the
body wall, legs and wings; and the way in which special
parts function to bring about orientation may be funda-
mentally different in different forms. In some cases the
whole body is sensitive to light, in other cases only certain
regions, and in many forms the response is entirely depend-
ent upon the eyes.

There has been more or less controversy as to whether
the phototactic reaction is due to the direction of the rays
or to the differences in light intensity. A good part of this
discussion has been based on misunderstandings. If an
animal is oblique to the rays of light it naturally becomes
stimulated with unequal degree of intensity on its two sides,
which probably causes the orienting movements. There is
little doubt that direction of light in the majority of cases,
if not in all, causes orientation in this manner; it is possible,
however, that in more or less transparent animals the direc-
tion the rays take through the tissues may determine
orientation in a way analogous to the directive effect of the
galvanic current, but at present we have no convincing evi-
dence that such is the case.

It was shown by Loeb in all the animals he subjected to
the experiment that it is the rays toward the violet end of the
spectrum that are the most potent in producing the photo-
tactic response. Red light has comparatively little orienting
power and many phototactic animals react to it as to dark-
ness. I have found, however, that the amphipod Orchestia
agilis when brought into a phototactic dark room illumin-
ated by light through ordinary red glass is markedly positive.
Specimens in a dark box held so as to get the red light re-
flected from one wall still showed a positive response although
the light was so dim I could scarcely see their movements.

PHOTOTAXIS 47

WTiile some exceptions to the above rule have been pointed
out by Minkiewicz and Bohn, it is one of very general
validity and affords a striking paralleUsm to the phototactic
movements of plants.

The phototactic response in animals may be modified or
reversed by a variety of agents. Whether animals are^
positive or negative often depends upon the intensity of
the light. This is well illustrated by the reactions of Volvox.
This form consists of an almost spherical colony of cells
each of which is furnished with a pair of flagella which serve
as organs of locomotion. One axis of the colony is some-
what longer than the others and upon this axis Volvox
commonly rotates while swimming. AMien exposed to a
moderate Hght the colony swims toward it in a nearly
straight line; when it happens to get out of orientation it
turns directly back into line again. If it reaches a point of
too great intensity its movements become slower, its orien-
tation less precise and it may stop or swim about slowly in
various directions. If exposed to hght above the optimum
it orients itself in the reverse direction and swims away.
Many flagellate protozoa and the swarm spores of many
algse are similarly positive in weak light and negative in
strong. Among higher organisms reversals following change
of light intensity are less common although there are several
cases. The earthworm Allolobophora joetida which is com-
monly negative shows a slight positive reaction in exceed-
ingly weak light (Adams). A great many positive forms
remain positive in as strong light as has been brought to
bear upon them and most negative species remain negative
in as weak light as they respond to in any definite way.

The sense of the phototactic response is sometimes
changed by exposure to light or darkness. Groom and
Loeb found that the larvae of Balanus are positive in the

48 THE TROPISMS

morning even to strong light, but later in the day they be-
come negative in light of less intensity. This apparently
determines the periodic depth migration of these forms, for
they are usually found at the surface of the sea in the morning
and in the afternoon in deeper water. If a culture is placed
at any time in darkness for some hours, the larvse when
first exposed to light are positive, but later become negative,
the change taking place more quickly the more intense the
light. Similar effects of exposure have been found in other
forms. A curious instance of the effect of previous exposure
is afforded, however, by the amphipod Orchestia agilis.
If specimens are subjected to strong light they are markedly
positive and remain so for a long time. If they are suddenly
transferred to light of weaker intensity they immediately
show a decided negative reaction. This, however, is only
temporary; within a few minutes or half an hour they all
become positive again. If now they are exposed to still
feebler light they again become negative. I have performed
this experiment repeatedly and have found that these
changes occur in the most decided and striking manner.

In some cases positive phototaxis may be actually in-
creased, up to a certain hmit, with the length of the exposure.
Animals whose first responses may be hesitating or indefinite
get, as it were, "warmed up" to the work, and finally be-
come almost violent in their efforts to reach the light. The
water scorpion Ranatra after having been kept in the dark
for several hours is commonly negative; then it shows
spasmodic fits of a positive response which grow longer and
stronger until the creature chases wildly after the light
and becomes wrought up into the highest pitch of excitement.
I have found that fiddler crabs, which when first exposed
to artificial light show signs of alarm and run away, gradually
become more and more strongly positive and after a time

PHOTOTAXIS 49

become apparently oblivious to any other stimulus except
the light which they slavishly follow.

The effect of temperature on phototaxis is by no means
uniform. Loeb found that certain marine copepods and the
larvse of Polygordius which are positively phototactic may
be rendered negative when subjected to a higher tempera-
ture. Strasburger found, on the other hand, that the
swarm spores of many algse become positive at a higher
temperature and negative at a lower. The flagellate Chro-
mulina, according to Massart, is positive at20°C. but negative
at 5° C. Orchestia agilis has been found by the writer to
become strongly negative if dipped into water, but if the
w^ater is heated nearly to the point of producing death the
reaction becomes positive.

The concentration and chemical nature of the mediimi
also influence phototaxis. It was found by Loeb that
negative specimens of Polygordius and certain copepods were
rendered positive by increasing the salt content of the sea
water, while the addition of fresh water rendered specimens
negative which previously showed a positive response.
Larvse of Palsemonetes which are normally positive become
negative if the sea water is diluted with half its volume of
distilled water (Lyon).

Certain infusoria, Stentor viridis and Paramcedum bur-
saria, which contain chlorophyll go toward the light only
when the supply of oxygen is insufficient (Engelmann),
but whether the response is phototactic or photopathic is
uncertain; at all events it seems to be an adaptation to
lack of oxygen, for in the light oxygen becomes produced as
in plants by the chlorophyll in the organism. The amphipod
Jassa which is usually negative becomes markedly positive
in foul sea w^ater. Carbon dioxide and other acids were found
bv Loeb to cause positive phototaxis in Gammarus and Cy-

50 THE TROPISMS

clops. In the latter a weakly alkaline condition seems to
induce a negative reaction. The experiments of Mr.
Jackson on Hyalella, a fresh water amphipod which is
ordinarily negative in its response, have shown that a variety
of substances cause a positive reaction. Mrs. Michener* in
experiments on a number of diverse forms finds that if the
response is negative it may be made positive by various
chemicals, although normally positive forms are rarely if
ever rendered negative. A wide range of chemicals was
experimented with and a positive response was evoked by
acids, salts, alkalies and various other substances, as in the
experiments of Mr. Jackson, showing that there is no definite
relation between the nature of the response and the class of
chemical compounds employed as stimuli.

The effect of contact stimuli on phototaxis affords one of
the most curious phenomena connected with the modi-
fication of the responses of organisms to light. In many
cases contact causes an immediate reversal of the response.
Miss Towle found that negative specimens of the ostracod
Cypridopsis were rendered positive by being picked up in a
pipette and dropped out again. Negative specimens when
colliding with one end of a dish straightway became positive
and swam to the other end. Yerkes obtained similar re-
sults in another ostracod, Cypris. Daphnia, which is ordi-
narily positive, may be made negative for a very short time
by repeatedly picking it up in a pipette. The copepod Temora
longicornisy which is usually negative, was found to become
temporarily positive by being shaken (Pouchet). Positive
specimens of Labidocera, on the other hand, may be made
temporarily negative by handling them in a pipette (Parker).

Possibly alHed to these effects is the curious reversal of
phototaxis which occurs in certain terrestrial amphipods when
thrown into water. Having observed that the terrestrial

♦Experiments unpublished.

PHOTOTAXIS 51

Amphipoda are usually positively phototactic while the
aquatic species are negative it occurred to me to try the effect
of throwing terrestrial forms into water. Specimens of
Orchestia agilis which were very strongly positive were
employed. As soon as they were in the water they all
immediately became strongly negative and remained so
for days in various intensities of light. That the trans-
formation is not due to change of temperature was shown
by the fact that the same result was obtained whether the
water was warmer or colder than the air, or at the same
temperature. It is not improbable that it is the stimulus
of contact afforded by the water that caused the reversal
of the response.

In the water scorpion Ranatra contact stimuli were found
to have a very marked effect on the insect's reaction to light.
Handling these creatures throws them into a death feint
which entirely inhibits, for a time, all phototactic response.
It frequently happens that on recovery from the feint there
is a negative response which later changes to positive.
If specimens which are swimming at the end of a dish nearest
the light are simply picked up by the breathing tube and
dropped back into the water they immediately begin
swimming with equal vigor in the other direction. They
may be caused to reverse their phototaxis in this way
repeatedly. The same effect can be produced if they are
seized or stroked while under the water. Specimens which
are making frantic efforts to go toward the light when in
the air may be caused to become negative by simply dropping
them into water. When taken out they usually show a
marked negative response for some minutes, but later be-
come positive.

The method by which animals orient themselves to hght
naturally varies with their nervous and muscular organiza-

52

THE TROPISMS

tion and the nature of their locomotor organs. There are
comparatively few cases in which light orients an animal
by causing directly a greater contraction of the muscles
of the side most affected. The observations of Mast on the
planulse of Eudendrium indicate that these forms may be
oriented in this way. Eudendrium planulse are cone-
shaped organisms with the wide end in front, and having
the body covered by cilia by means of which they swin
through the water. According to Mast, *4f the ray direc-
tion is but slightly changed after the planulse are oriented

Fig. 5. — The water scorpion Ranatra, showing the different attitudes
assumed according as the light falls upon it from in front or from behind.
The arrows indicate the direction of the rays.

they do not turn directly toward the source of light in its
new position, but merely swing the anterior end a little far-
ther toward it each time. In the meantime the body grad-
ually turns so as to become oriented again. If however the
direction of the rays is changed to such an extent that the
sides of the organism become fully exposed, they with very
few exceptions appear to turn toward the light at once. In
this process they swing the anterior end laterally until it
nearly if not quite faces the source of light."
The larvae of Arenicola according to Lillie orient them-

PHOTOTAXIS 53

selves to light by bending directly toward the more stimu-
lated side. These larvae swim by means of two bands of cilia
around the body, and for a short time after hatching are
markedly positive, swimming toward the Hght in nearly
straight lines. If while the larvae are swimming toward the
Kght the position of the light is changed they bend toward
the light until oriented in the direction of the rays. Certain
salts were found by Lillie to inhibit muscular action while
they did not interfere with the action of the cilia. LarvaB
in solutions of these salts would continue swimming, but their
phototaxis was entirely destroyed. Mast who has studied
the orientation of the same form finds that " by using two
sources of Hght so situated that the rays cross at right angles
in the region where the specimen is located, and then alter-
nately intercepting the light from each of the two sources,
it can be seen clearly that the larva, by muscular movement,
turns its anterior end toward the source of Hght directly.
There is no trial in this process. It is an asynametrical
response to an asymmetrical stimulation."

light has a certain orienting effect on Planaria maculata,
even in specimens devoid of eyes although it is masked by a
large amount of random activity. Planaria turns directly
away from strong mechanical and chemical stimuH, as shown
by Pearl, by lengthening the side stimulated instead of by
contracting the opposite side. The effect of the stimulus
is upon the muscles in the vicinity of the stimulated point.
There is no ventral nerve cord with its numerous cross
connectives such as we find in annelids and there seems to be
no mechanism by which an impulse set up on one side can
be transmitted more strongly to the muscles of the opposite
side of the body than to any other region. The turning away
is therefore effected in planarians in a very different way
than in annelids, probably by the contraction of the dorso-

54 THE TROPISMS

ventral muscles near the stimulated point. It is not im-
probable that the negative phototaxis of an eyeless Planaria,
like its turning away from strong stimuli in general, is due
to the local effect of light in the region on which it impinges.

In the earthworm or leech turning away from strong light
is accomplished in all probability by the contraction of the
longitudinal muscles on the opposite side of the body, the
result being in the nature of a crossed reflex which is so com-
mon in animals with an axial central nervous system. In the
crustacean Eubranchipus which usually swims on its back
there is a marked positive phototaxis if the animals are sub-
jected to a rather small source of light. In ordinary day-
light before a window they pay Httle heed to the Hght, but
if taken into a dark room and exposed to light from an
electric bulb they will swim toward it and follow it about
in any direction. If they are obhque to the rays they
bend the tail suddenly toward the more illuminated side
one or more times until they become oriented.

In most Crustacea as in most insects orientation is affected
through the unequal action of the appendages on the two
sides of the body. In a form which is positively phototactic
light entering one eye sets up impulses which passing into
the brain and nerve cord, cause, directly or indirectly, move-
ments predominantly of flexion of the legs of the same side
and of extension of the appendages of the opposite side of
the body. If this is a sort of mechanical reflex process we
should expect that, in a positively phototactic form, if one
eye were destroyed or blackened over, the animal would
move continually toward the normal side. This experi-
ment of blackening over one eye was tried by the writer on
the large sand flea, Talorchestia, and it was found that the
specimens no longer went straight toward the light, but
performed circus movements toward the side of the un-

PHOTOTAXIS 55

blackened eye. The same experiment was tried on several
postively phototactic insects with the same results. In the
small sand flea Orchestia agilis which is sometimes positive
and sometimes negative in its reactions to light blackening
over one eye causes circus movements toward the normal
side when it is positive and in the reverse direction when
it is negative. Since throwing this form into water changes
its phototaxis from positive to negative it can be made to
go around in a circle in either direction according to the
medium in which it is placed. Similar circus movements
after one eye was blackened over have been observed by
Parker in Vanessa, by Hadley in the lobster, and by Rddl
in various kinds of arthropods.

Not all phototactic insects, however, perform circus
movements when blinded on one side. If the honey bee,
for instance, which is strongly positive, is treated in this
way it usually follows the light almost as directly as when in
a normal condition. In the water scorpion Ranatra I have
found that while in some specimens there is a strong tendency
to perform circus movements toward the normal side,
others go toward the light in a direct line. In some instances
specimens which at first performed circus movements came
after a number of trials gradually to straighten their path
toward the fight until finally they followed it in a straight
line. Back swimmers, Notonecta, which at first performed
circus movements and approached the light only after much
wasted effort, were found to straighten their course and
follow the fight as well as if they had use of both eyes. These
facts indicate that phototaxis may fall to a certain extent
under the pleasure-pain type of beha^dor which wdU be con-
sidered in a later chapter. Light in some animals is foUowed
much as an object of interest is pursued by a higher animal.
If a creature learns to go to the light an element of satisfac-

56 THE TROPISMS

tion or pleasure is probably associated with the phototactic
response.

There are a few cases in which positively phototactic
species swim backward instead of forward toward the light.
An instance of this kind was found by Lyon in Palsemonetes.
Hadley found that larval lobsters swim with the head pointed
away from the light whether they are postive or negative
in their reaction. In the pycnogonid Anoplodactylus,
Cole observed that in crawling toward the light the anterior
end was in advance, but in swimming toward the light the
animals moved approximately backward.

Fiddler crabs form an exception to most phototactic
animals in that in going toward the light the body is oriented
sidewise instead of with its longitudinal axis in the direction
of the rays. These animals are strongly positive and, like
Ranatra, their response becomes stronger with longer ex-
posure. They are timid animals and often a sudden move-
ment of the light will send them scuttling away in alarm,
but after following the light for some time they become
more oblivious to other stimuli and slavishy follow it with
less show of fear. Orienting movements are different from
those used in ordinary locomotion which is not the case with
most forms. Other animals may orient themselves by walk-
ing faster, so to speak, on one side than on the other, but
with lateral orientation special movements have to be per-
formed in order to change the direction of locomotion.
Here again it is difficult to apply the theory of forced
orientation in its usual form, and we are led to conclude that
the reaction to hght is to a certain extent one of the pleasure-
pain type.

According to Rddl phototaxis has an intimate relation to
vision. In a number of experiments Cole has found that
reactions to light are influenced by the extent to which the

PHOTOTAXIS 57

eyes are capable of forming images. If animals are stimu-
lated by two sources of light of the same degree of intensity
but of different area, the forms without eyes or in which the
eyes produce no image turn practically as often to the one
source of light as the other while animals with eyes producing
a distinct image will, if positively phototactic, generally
turn toward the light of larger area. With the perfection
of the organs of vision the primitive phototactic tenden-
cies of animals may become modified so as to afford the
basis for the reactions to special objects of the visual field
which we find in the more highly developed instincts.

In many animals there is a strong reflex tendency to keep,
so to speak, in statu quo with the visual field. This tendency
accounts for many cases of so-called rheotropism as is shown
by the experiments of Hadley on the lobster and Lyon on
fishes. The same tendency is, as we have seen, manifested
also in compensatory motions. Radl has shown that in
Daphnia and some other forms in which the eyes are mov-
able there is an effort on the part of the eyes to become
oriented to the rays of light. In the vertebrates in which
the eyes are freely movable the same tendency is more or
less pronounced. This trait may be connected with the
involuntary tendency of animals to follow the movements
of objects which cross their field of vision. Vision in the
lower animals is concerned much more with the movements
than with the form of objects. It may be possible to trace
more accurately than has been done the relation between
phototaxis, compensatory motions of the head and body,
and the involuntary tendency to follow moving objects
with the eyes. This field of investigation has been but
little explored, but it is one which promises to be fruitful
of significant results.

68

THE TROPISMS

THERMOTAXIS

Almost all organisms which are free to move go awa}'' from
regions in which the temperature is injuriously high, and
many, although a less number, withdraw from regions which
are colder than a certain optimum. If the water near an
Amoeba is heated by a hot needle the animal will contract
on the side nearest the needle, send out pseudopods in some
other direction and crawl away. Paramoecium and other
infusoria form groups in a region of optimum temperature by
the same method employed in reacting to chemicals. When
a Paramoecium swims into a region above the optimum it
gives the motor reflex and goes in another direction. When it
encounters a region below a certain temperature it behaves

Fig. 6. — Reactions of Paramoecia to heat and cold. One end of the
slide is heated to 35° C. while the other end is kept on ice. The Par-
amoecia gather in an intermediate zone d c.

in the same way. If Paramoecia are placed in a trough one
end of which is heated to 35° C. while the other is placed upon
ice the infusorians will form a band near the middle where
the temperature ranges from 24° to 28° C. Previous ex-
posure to higher or lower temperatures than those usually
experienced changes the optimum to a considerable degree.
Most organisms above the protozoa turn directly away
from hot or cold objects. Sometimes the animal may escape
undue stimulation by making a number of random move-

ELECTROTAXIS 59

ments, trusting to luck to get it into a more favorable situa-
tion. A comparatively large element of random movement
may be combined with a tendency to turn directly away
from the stimulus, as is apparently the case in planarians.
The subject of orientation to heat rays has been little
studied. With aquatic organisms it would be difficult
to separate the reaction to heat rays and reaction to warmer
or colder regions of the water. Of the effect of radiant
heat alone on the movements of terrestrial organisms we
have as yet very inadequate knowledge.

ELECTROTAXIS

Reactions to the electric current form no part of the be-
havior of animals under natural conditions. Nevertheless,
many forms respond to the electric current in a very precise
manner. An attempt to review the rather extensive
literature on this subject would lead to a considerable
amount of technical discussion, and an adequate treatment
would require much more space than can be devoted
to it here. The subject is one, however, which has an
important bearing on the general theory of tropisms, and
we shall therefore describe briefly some typical cases of
electrotactic response.

An Amceha proteus placed in the pathway of a weak electric
current assumes an elongated form and creeps toward the
cathode. The protoplasm toward the end nearest the anode
apparently contracts and vdih a stronger current may
undergo a granular disintegration. Pseudopods are put out
on the side toward the cathode and cause the flow of the
protoplasm in that direction.

When Paramoecium is subjected to the action of the cur-
rent it s^dms with a fairly accurate orientation of the body
toward the cathode. If two poles of a galvanic battery are

60 THE TROPISMS

inserted in the two sides of a drop of water containing
Paramoecia the infusorians will soon all collect around the
negative pole. In solutions of sodium chloride and certain
other salts Paramoecium frequently swims backward
toward the anode, and Bancroft has found that true anodal
electrotaxis, in which the infusorians swim forward toward
the anode, may occur for a short time in solutions of sodium
chloride, sodium carbonate and other salts if the Para-
moecia are washed in distilled water before being placed in
the solutions.

Electrotaxis has been observed in coelenterates, worms,
moUusks, Crustacea, the larvae of insects and in fishes, tad-
poles and salamanders. In general the reaction of worms
andTmollusks is negative and the reaction of Crustacea
positive. In tadpoles which orient their bodies parallel
with the direction of the current the direction of orientation
is dependent upon the strength of the current.

Orientation to the electric current is apparently brought
about by the polar effect of the current upon the tissues of
the body. In Paramoecium there is a direct orientation pro-
duced by the unequal beat of the cilia on the two sides. In
cathodal orientation the forward phase of the stroke of the
cilia is accentuated where the current leaves the body so that
there may be a reversal of the effective stroke; where the
current enters, the backward stroke continues so the organism
naturally swings into line with the current. In higher organ-
isms there is a distinctly evident polar effect upon the
musculature of the body which can be seen to produce the
orientation of the organism. The effect of the electric
current on animals generally is to produce movements
which are in a certain sense "forced,^' and the orientation
so brought about affords an excellent example of a typical
tropism as conceived by Dr. Loeb.

ELECTROTAXIS 61

BIBLIOGRAPHY
Bancroft, F. W. The Control of Galvanotropism in Paramcecium

by Chemical Substances. Univ. of Calif. Pubs. Physiology, 3,

21, '06.
BoHN, G. Th^orie nouvelle du phototropisme. C. R. Ac. Sci.

Paris, 139, 890, '04. La Naissance de I'lntelligence. Paris, '09.
Cole, L, J. Notes on the Habits of Pycnogonids, Biol. Bull., 2,

195, '01. An Experimental Study of the Image-forming Power

of Various Types of Eyes. Proc. Am. Ac. Arts and Sci., 42,

335, '07.
Davenport, C. B. Experimental Morphology. N. Y., '97-99.
Harper, E. H. Reactions to Light and Mechanical Stimulation in

the Earthworm, Perichceta hermiLdensis. Biol. Bull., 10, 17, '05.
Holmes, S. J. Phototaxis in the Amphipoda. Am. Jour. Physiol.,

5, 211, '01. The Selection of Random Movements as a Factor

in Phototaxis. Jour. Comp. Neur. Psych. 15, 98, '05. The

Reactions of Ranatra to Light, 1. c, 15, 305, '05. Phototaxis

in Fiddler Crabs and its Relation to Theories of Orientation.

1. c, 18, 493, '08.
Holt, E. B., and Lee, F. S. The Theory of Phototactic Response.

Am. Jour. Physiol., 4, 460, '01.
Jennings, H. S. Contributions to the Study of the Behavior of

Lower Organisms. Carnegie Inst. Pubs. Washington, '04.

Behavior of Lower Organisms. N. Y., '06.
LoEB, J. Der Heliotropismus der Thiere und seine Uebereinstim-

mung mit dem Heliotropismus der Pflanzen, Wtirzburg, 90.
. Studies in General Physiology, Chicago, '05. The Dynamics

of Living Matter. N. Y. '06.
Mast, S. O. Light Reactions in Lower Organisms, 1, Sientor asruleus.

Jour. Exp. ZooL, 3, 359, '06. Light Reactions in Lower Organ-
isms, II, Volvox globator. Jour. Comp. Neur. Psych., 17, 99, '07.

Light and the Behavior of Organisms. N. Y., '11.
Mendelssohn, M. Ueber den Thermotropismus einzelliger Organis-

men. Pfluger's Archiv, 60, 1, '95.
MiNKiEwicz, C. Sur le chromotropisme et son inversion artificielle.

C. R. Ac. Sci. Paris, 143, 785, '06.
Nagel, W. a. Der Lichtsinn augenloser Thiere, Jena, '96.
NuEL, J. P. La Vision. Paris, '04.
Parker, G. H. The Phototropism of the Mourning Cloak Butterfly.

Vanessa antiopa. Mark Anniversary Volume, 453, '03.
Putter, A. Studien tiber Thigmotaxis bei Protisten. Arch. f.

Anat. u. Physiol, physiol. Abth. Suppl. 243, '00.

62 THE TROPISMS

Radl, E. Untersuchungen ttber den Phototropismus der Tiere.

Leipzig, '03.
Strasburger, E. Die Wirkung des Lichtes und der Warme auf

Schwarmsporen. Jen. Zeit. n. F. 12, 551, 78.
ToRELLE, E. The Response of the Frog to Light. Am. Jour.

Physiol., 9, 466, '03.
Verworn, M. General Physiology, N. Y., '99.

Walter, H. E. The Reactions of Planarians to Light. Jour.
Exp. ZooL, 5, 35 and 1 17, '07.

CHAPTER IV
THE BEHAVIOR OF PROTOZOA

" One of the first lessons which the study of animal behaviour, in
its organic aspect, should impress upon our minds is, that living cells
may react to stimuli in a manner which we perceive to be subservi-
ent to a biological end, and yet react without conscious purpose-
that is automatically." — C. Lloyd Morgan, Animal Behaviour.

The student of the evolution of mind naturally looks with
interest to the behavior of those organisms which lie nearest
to the root of the tree of life. What mental powers are
evinced by the lowest animals, or whether, indeed, the lowest
animals exhibit any mental powers at all are questions of
fundamental importance to comparative psychology. Never-
theless with all the theoretic interest and importance attach-
ing to the study of the powers and performances of these
low forms it is somewhat surprising that until quite recently
the subject attracted few serious investigators.

Binet in his book on the Psychic Life of Micro-organisms
makes one of the first thorough-going attempts to estimate

e extent of the protozoan mind. Among the psychic
operations which he claims are manifested are:

1. "Perception of the external object;

2. '' The choice made between a number of objects;

3. '*The perception of their position in space;

4. " Movements calculated, either to approach the body
and seize it, or to flee from it."

Choice is manifested according to Binet in the selection of
food. Many species live on a few kinds of food and refuse
others. This choice cannot be explained as due to physical

63

64 THE BEHAVIOR OF PROTOZOA

causes; "it is one of the most incomprehensible of phe-
nomena; it is exceedingly difficult to explain it without resort
to anthropomorphism." Protozoa not only perceive ex-
ternal objects but ''they also indicate, by their movements,
an exact knowledge of the position occupied by these bodies.
It might be said that they invariably possess a sense of
position in space. The possession of this sense is absolutely
indispensible to them, for it does not suffice them to know
of the presence of an exterior body in order to approach it
and seize it; they must furthermore know its position, so as
to direct their movements accordingly.

"The simplest form of a sense of localization is met with in
the Amoeba which, when it closes about a nutritive particle,
always emits its pseudopods at precisely that part of its body
where the foreign substance caused the irritation. The
most complicated instance of localization is met with in the
Didinium, which we have so often cited; the Didinium knows
exactly the position of the prey it follows, for it takes aim
at the object of its pursuit like a marksman, and trans-
pierces it with its nettle-like darts."

Instinct, memory, fear and a certain degree of intelligence
are among the psychic endowments with which Binet credits
the protozoa. A good sample of his interpretation of
protozoan behavior is the following: "The Bodo caudatus
is a voracious Flagellate possessed of extraordinary audacity;
it combines in troops to attack animalcula one hundred
times as large as itself, as the Colpods for instance, which
are veritable giants when placed alongside of the Bodo,
Like a horse attacked by a pack of wolves, the Colpod is
soon rendered powerless; twenty, thirty, forty Bodos throw
themselves upon him, eviscerate and devour him completely
(Stein).

"All these facts are of primary importance and interest,

THE BEHAVIOR OF PROTOZOA 65

but it is plain that their interpretation presents difRculties.
It may be asked whether the Bodos combine designedly in
groups of ten or twenty, understanding that they are more
powerful when united than when divided. But it is more
probable that voluntary combinations for purposes of attack
do not take place among these organisms; that would be to
grant them a high mental capacity. We may more readily
admit that the meeting of a number of Bodos happens by
chance; when one of them begins an attack upon a Colopod,
the other animalcula lurking in the vicinity dash into the
combat to profit by a favorable opportunity."

More recent investigations have shown that the behavior
of protozoa gives no evidence of the high psychic develop-
ment assumed by Binet. There has been a strong tendency
on the part of certain investigators to explain the behavior
of these low forms as due in large measure to comparatively
simple physical and chemical factors. Others contend that
the phenomena are much more complex and at present defy
analysis into physical and chemical processes, while a few
go further and maintain that we must assume some super-
physical agency, a vital principle, or entelechy of some
sort, to explain the results.

Much attention has been bestowed upon the activities of
Amoeba, which is generally assumed to be one of the most
primitive of the protozoa. Amoeba is a jelly-like organism
with a clear, outer, relatively dense layer of protoplasm, the
ectosarc, surrounding a more fluid, granular substance, the
endosarc, which contains the nucleus. The Amoeba com-
monly moves by sending forth blunt processes or pseudopodia
from the side of the body; material flows into the pseudopod
which may increase greatly in size; then other pseudopods
are put forth on the same side, the posterior part of the body
being pulled, in the meantime, in the direction in which the

66 THE BEHAVIOR OF PROTOZOA

pseudopods are protruded. The details of the movements
vary considerably in different species. In Amoeba verrv^
cosa Jennings describes the locomotion as '*in many
respects comparable to rolling, the upper surface continually
passing forward and rolling under the anterior end so as to
form the under side/' This is shown by placing in the
water particles of soot some of which adhere to the surface
of the animal; if a single particle be watched it will be seen
to pass around, as the Amoeba progresses, like an object on
the surface of a rolling cylinder. The anterior edge of the
Amoeba is thin and fiat and adheres closely to the substance
while the thicker posterior edge is free. The movement is
like the rolling of a contractile sac wdth semifluid contents.
Amceha Umax has a similar method of locomotion but has
few pseudopods. Bellinger who has studied the locomotion
of AmxBba proteus describes a method of locomotion quite
different from that found by Jennings. Dellinger hit upon
the device of observing Amoeba from the side, by means
of a horizontal miscroscope. The Amoeba sends out a
pseudopod free into the water, which is then bent down and
attached to the substrate; the posterior end is now raised
and pulled forward; then another pseudopod is pushed out
and attached like the first and the body pulled forward again.
During the progress the Amoeba is attached only at a few
points on short improvised feet which are drawn in as the
Amoeba passes over them.

In a small unidentified species of Amoeba the method of
locomotion, as observed by the writer, differs from all the
foregoing ones. There are broad ectoplasmic pseudopods
put out at the clear anterior end, commonly first on one side
and then on the other; the endoplasm flows into them and
the hinder part of the body which contains the contractile
vacuole is drawn forward. There is no rolling ; particles on the

THE BEHAVIOR OF PROTOZOA 67

surface of the hinder half of the animal may undergo little
change of position for a long time.

There are various ways of imitating the movements of
Amoeba by drops of oil or other fluids subjected to changes
of surface tension. If a drop of mercury is placed in dilute
nitric acid and a piece of potassium bichromate placed near
it the drop of mercury will bulge out toward the bichromate
and may surround it. The bichromate as it diffuses against
the mercury causes a diminution of surface tension at the
region of contact. The stronger contraction of the rest of
the surface film forces the mercury to protude at the weakest
point; producing an out-pushing resembling the pseudopod
of the Amoeba. It has been contended that variations in
surface tension account in great measure for the movements
of Amoeba and other Rhizopods much as in inorganic fluids.
There is certainly a striking analogy between the phenomena
in the two cases, but the studies of Jennings have shown
that explanation of the phenomena is not quite so simple.
Jennings argues that the currents in Amoeba are not like
those occurring in drops of fluid which are changing the
surface tension because in the Amoeba there are no lateral
return currents which are present in the mo\dng fluid and
hence the surface tension theory cannot account for the
Amoeba's changes of form. It must not be forgotten that
we are dealing with substances of quite different consistency
one of which has a cortical layer of considerable thickness and
density which is entirely absent in the other, and that the
behavior of internal currents may be affected by this factor;
but aside from this, there are many facts which seem to in-
dicate that the ectoplasm behaves more like a muscular layer
than one whose contraction is entirely due to the surface film.
Contracting pseudopods may become wrinkled, which could
not occur if mere surface film contraction were responsible

68 THE BEHAVIOR OF PROTOZOA

for the withdrawal. Sometimes pseudopods move back and
forth as if they possessed a certain degree of flexibility and
rigidity. The rolling motion of certain species offers another
difficulty, so it cannot be said that, at present, the surface
tension theory suffices to explain the movements of the
organism. That it may play a part in the process is not
improbable. It must not be forgotten that there are many
sm'faces within the mere outer layer, such as alveolar walls,
etc., where surface tension may be a potent agent. In
forms a little higher than Amoeba we meet with a more or
less fibrillar ectoplasm whose contraction occurs much as
in an ordinary muscle fiber. It is not improbable that the
fundamental features of contraction in the specialized ecto-
plasm of flagellates and infusorians are the same as in the
unspecialized ectoplasm of the rhizopods on the one hand
and in the more highly specialized muscular tissue of higher
animals on the other. The cause of muscular contraction
is one of the most obscure problems of physiology and until
it is solved we shall probably be unable to explain the mech-
anism of the movements of the simplest animals.

In taking food Amoeba has been described as flowing
around an object and engulfing it in its endoplasm where it
undergoes digestion. Surface tension has been supposed
to play a part in the process. A fine splinter of glass brought
against a drop of water will be quickly drawn in through the
contraction of the surface film. A drop of chloroform
may be made to draw in a glass splinter covered wifh
shellac; after the shellac is dissolved the glass splinter will
be extruded, through the force of surface tension, as from a
drop of mercury, thus simulating both the ingestion of food
and the defecation of the undigested residue (Rhumbler).
The observations of Jennings on food taking in Amceha
proteus show that the protoplasm does not flow around the

THE BEHAVOIR OF PROTOZOA 69

object as if drawn by surface tenison; a small pseudopod is
put out on either side of the food; these processes extend
and curve around it until they meet, and then the object
is drawn into the endoplasm. In Amoeba proteus and limax
"there is no adherence between the protoplasm and the
food body," although there is adherence in Amceba verrucosa
whose ectoplasm seems to be more adhesive to all kinds of
objects.

Amoeba like higher animals may follow its food. Jennings
describes an Amoeba attempting to engulf a spherical cyst of
Euglena. As the Amoeba came in contact with it the cyst
rolled away; the Amoeba followed; the cyst continued to
be pushed ahead now one way and now another and the
Amoeba changed its course accordingly. After the cyst had

Fio. 7. — Reaction of Amoeba to a strong mechanical stimulus. The
arrows indicate the direction of the currents.

been rolled against an obstacle and the Amoeba was about
to succeed in capturing it, a large infusorian appeared on the
scene and swept it away.

The reactions of Amoeba to stimuli may be either positive
or negative. To a strong mechanical stimulus such as con-
tact with the point of a needle Amoeba reacts by crawling
away. Should the stimulus be applied at the anterior end
the stimulated part stops, there is a local contraction of the
ectoplasm and the granules begin to stream away from the
point of contact. This stream meets the stream proceeding
in the general direction of locomotion and the two combine

70 THE BEHAVIOR OF PROTOZOA

to form a stream which issues in another direction. Stimu-
lation of the posterior end causes the animal to quicken its
pace. Stimulating a pseudopod causes it to be withdrawn;
and by repeated stimulation the animal may be driven
about at will.

To the weaker stimuli which are received by coming into
gentle contact with solid objects, Amoeba often reacts
positively. If, when floating about in the water, one pseu-
dopod comes in contact with a solid it adheres to it; there
is a flow of granules toward the point of contact, from the
rest of the body; this pseudopod enlarges, the others contract
until most of the body has flowed into the attached pseudo-
pod, when the Amoeba crawls along the surface of the object.
The utility of the negative response to strong mechanical
stimuli is obvious since it enables the organism to avoid
injurious agencies. The positive reaction tends to keep
the Amoeba in contact with solid objects where most of its
food is secured and where it receives protection. Probably
also it plays a part in determining the behavior toward food.

To injurious chemicals Amoeba reacts much as to strong
mechanical stimuli; it creeps away from regions of higher
temperature, it reacts negatively to light. All these responses
are purposive in that they are adapted to the preservation
of the organism. Simple as Amoeba apparently is it manages
to cope very effectively with the conditions of its existence.
One might conceivably construct a machine which would
run of itself, gather the food needed to supply the energy
used in its workings, avoid automatically contact with
obstacles which would impair its running, move away from
regions too hot or too cold for its efficient operation, protect
itself by producing coverings in unfavorable situations, and
guide itself into the most favorable regions for its mainte-
nance; but what a wonderfully complicated mechanism it

THE BEHAVIOR OF PROTOZOA 71

would have to be! Yet a simple, apparently almost struc-
tureless mass of jelly does all this and more. And if our
mechanism had the property of repairing its own injuries and
producing other pieces of mechanism like itself, its structural
arrangements would be almost if not quite beyond our power
to conceive. One cannot, therefore, but look with a feeling of
admiration and w^onder at so comparatively simple a creature
as Amoeba, which is capable of performing so much.

But the story does not end even here. In addition to aU
the adaptive properties mentioned Amoeba has the power
of modifying its behavior to suit new conditions. Toward
water different from that in which it is immersed Amoeba
reacts negatively, but after remaining in such water for
some time it resumes its usual activities. "Wlien first ex-
posed to bright light it withdraws its pseudopods and re-
mains quiet, but after continued exposure it adapts its be-
havior to the new conditions and again becomes active.
After a short period of starvation Amoeba moves about
more actively than usual, whereas when it is gorged with
food it becomes relatively sluggish. The condition of the
animal determines its behavior in other ways and the changes
of behavior are usually advantageous to the organism. The
behavior of Amoeba is essentially like that of higher animals;
it avoids things which are injurious; it seeks things which
are beneficial and it adapts its behavior to new conditions.
Life is very much the same sort of thing whether in an
Amoeba or a man.

The chief rival of Amoeba in the attentions of the com-
parative psychologist is Paramoecium. This is a cigar-
shaped infusorian rounded in front and pointed at the
posterior end, and covered by a uniform coating of cilia.
It has an oblique oral groove leading posteriorly into a gullet
into which are swept small particles which serve for food.

72

THE BEHAVIOR OF PROTOZOA

By the action of its cilia Paramoecium swims through the
water in a spiral course, rotating on its long axis to the left
and keeping the oral side facing the center of the spiral.
At times Paramoecium may swim backward, by reversing
the effective stroke of the cilia, although the direction of
rotation is unchanged.

Paramoecium is a common organism in vegetable infu-
sions, where it subsists upon bac-
teria which are swept by cilia into
its gullet. There seems to be little
power of choice as to what sub-
stances are taken in; particles of
India ink and carmine are swept in
and swallowed in the same way as
its normal food, and the organism
selects its food only by swimming
elsewhere when the materials swept
in are unsuitable. Metalnikow,
however, states that when Par-
amoecia are fed with carmine for
fifteen days they gradually take
in less of this substance and finally
refuse it. Metalnikow's experiments
were repeated by Schaeffer who
failed to confirm these results when

Fig. 8.— Paramoecium cau- the carmine was kept in a condi-
datum. (After Kellogg.) ,. i .i . •. ii i ti

tion such that it could be readily

ingested. Occasional individuals, however, failed to take
in any of the substance, but examination showed that they
were deformed or had recently divided so that the oral ap-
paratus was not capable of sweeping in food. After thirty-
three days the Paramoecia failed to show any appreciable
diminution in the carmine ingested.

THE BEHAVIOR OF PROTOZOA 73

Paramoecium is a very restless organism, swimming
actively much of the time. Occasionally it remains quiet
when in contact with solid objects. When manifesting its
so-called thigmotactic response the cilia in contact with
the solid remain stiff and immobile as if anchoring the animal
to the spot, while the cilia over the rest of the body keep
moving, although with diminished vigor. If bits of cotton
wool are placed in the water Paramoecia are more apt to
come to rest, owing to the greater opportunity afforded of

Fig. 9. — Successive stages of the motor reflex of Paramoecium. (After

Jennings.)

securing contact stimuli. This trait keeps the Paramoecia
among bacterial scums and in other situations where they
may obtain their food.

The principal feature of the behavior of Paramoecia is
what Jennings has called the "motor-reflex" or "avoiding
reaction.'^ It consists of swimming backward by reversal
of the action of the cilia, turning to the aboral side and then
going ahead again. This is the stereotyped response which
Paramoecium gives in essentially the same way when en-
countering almost any kind of stimulus. If stimulated by
a fine needle on the aboral side it will back off and turn to-
ward instead of away from the stimulating object. Even

74 THE BEHAVIOR OF PROTOZOA

when the posterior end is stimulated the Paramoecium may
swim back directly against the needle. A slight stimulus
at the posterior end often causes the infusorian to accelerate
its swimming, but, with this exception, the nature of the
response seems to bear no relation to the part of the body
to which the stimulus is applied. The anterior end of the
body is the most sensitive region, although stimuli at that
point do not call forth any different kind of response, but
only evoke the usual motor reflex with greater readiness.

The motor reflex or avoiding reaction has the effect of
getting the animal away from inj urious stimulations. Often,
however, at first it may bring it into a worse situation than
before; the Paramoecium may back into an injurious chem-
ical, or turn toward a mechanical or other stimulus which
affects its aboral side; if so, the motor reflex is repeated until
the organism makes its escape from the unfavorable situa-
tion. The method of adjustment may be clumsy and in-
direct; had Paramoecium the power as higher organisms
have, of turning directly away from the stimulus, its reac-
tions would probably be more effective; but its restless
activity and quick movements ai-e a partial compensation
for defects in precision of response.

The motor reflex may be carried out in varying degrees of
completeness. The duration of the backward swimming
and the amount of turning to the aboral side are subject
to much variation. The two phases of the response may
be greatly prolonged in a solution of potassium iodide. The
Paramoecium swims backward for several minutes; then
spins around toward the aboral side for a considerable time
and finally swims forward. In ordinary water the distance
the Paramoecium swims backward is increased with a
stronger stimulus and the aboral rotation may continue until
the infusorian describes several complete circles. With

THE BEHAVIOR OF PROTOZOA 75

slight stimulations the first phase of the reaction may result
m a momentary stopping of the course or even simply a
slackening of speed, and the aboral turning may manifest
itself in swinging the anterior side around in a little larger
spiral than before. In fact this aboral turm'ng may be re-
garded as but an accentuation of the process which in ordi-
nary swimming keeps the aboral side facing the outside of the
spiral. Sometimes, as in a dilute solution of sodium chlor-
ide, the Paramcecium stops swimming forward and turns
around aborally, with its posterior end keeping nearly in
one spot and its anterior end describing a wide circle.

Paramcecium, as we have described in a previous chapter,
reacts negatively to both hot and cold water.

There seems to be no orientation
here to the heat rays; when Para-
mcecium swims to a colder or a
warmer region it gives the motor
reflex and goes in another direc-
tion. Ordinary light has little

effect upon Paramcecium but Her- „

. . Fig. 10. — Course of Paramoe-

t el has shown that it gives a nega- cium in a drop of dilute

.• , chemical.

tive response to very power-
ful ultra-violet rays. Its reactions to gravity are not deci-
ded and are influenced by various factors as is mentioned in
the foregoing section on Geotaxis.

The behavior of Paramcecium is made up of a very limited
number of stereotyped modes of response. It reacts to all
sorts of stimuli by the same motor reflex carried out in
various degrees of vigor according to the strength of the
stimulus. But while very machine-like its behavior is at
the same time highly plastic and adaptive. Conditions
which are unfavorable act as stimuli, and the animal keeps
on reacting until it gets into a region which is more favorable

76 THE BEHAVIOR OF PROTOZOA

to its life. While we commonly speak of the positive reac-
tions of the organism, strictly speaking there are, with the
exception of its thigmotaxis, no positive reactions. If
Paramoecia collect in weak acid it is not because the acid
attracts them in any way; they are not stimulated even
when they accidentally enter the acid; they react only
when they pass from the acid into the water. It is not
their positive reaction to the acid but their negative reaction
to the water that causes what appears to be a positive
chemotaxis. The same is of course true of their reactions
to various other stimuli. Their life is one of continual
avoidance. Only when conditions are favorable is there
cessation of movement. If we do not wish to attribute to
such creatures the power of choice we must admit that the
method of behavior secures it the same advantages that
choice affords.

The behavior of Paramoecium, like that of every other
organism, is modified by changes of its internal condition.
Strong induction shocks render Paramoecium insensitive to
weaker shocks and if individuals are kept for a time at
a temperature higher than normal a higher temperature is
required to cause the avoiding reaction. Individuals that
have been kept without food become restless while well fed
ones are more sluggish and more apt to come to rest against
solid objects. Reactions to gravity are influenced by food
and other conditions and thigmotaxis is markedly affected by
temperature. Behavior may be modified by repetition of the
same contact stimulus as described in the following quota-
tion from Jennings: "If a bit of filter paper is placed in a
preparation of Paramoecia, the following behavior may often
be observed. An individual swims against it, gives the avoid-
ing reaction in a slightly marked way, swimming backward a
little; then it swims forward again, jerks back a shorter

THE BEHAVIOR OF PROTOZOA 77

distance, then settles against the paper and remains. After
remaining a few seconds, it may move to another position,
still remaining in contact with the paper. Then it may
leave the paper and go on its way. All this may happen
without the slightest evident change in the outer conditions.
So far as can be seen, the Paramoecium first responds to the
solid by the avoiding reaction, later by the positive contact
reaction, and still later suspends the contact reaction, all with-
out any change in external conditions. The changes inducing
the change in reaction must then be within the animal."

The behavior of Paramoecium is quite typical for infusor-
ians in general, but different forms present some interesting
modifications. Wliat Maupas has called the ^ 'hunter ciliates"
show a more highly developed behavior in taking food, as
they not only exhibit a power of selection of certain kinds of
food, but have a remarkable power of engulfing large objects.
One of those whose habits are the best known is Didinium
nasutum (Stein). The body is in the shape of a barrel;
at the middle of the anterior end is a small projection where
the mouth is located. The mouth, which is usually kept
closed, leads to a pharynx lined with chitinous rods which
can act as a sort of "seizing organ," in the capture of prey.
This organ can be protruded and withdrawn, and may be
opened out to a remarkable degree.

When Didinium in the course of its swimming comes in
contact with a Paramoecium or other small organism, the
seizing organ is shot forth into the body of the prey. The
seizing organ is then drawn in and the mouth spreads open
to receive the prey which is gradually pulled into the body
of the Didinium.

The swallowing capacity of Didinium is almost incredible.
Mast observed one of these infusorians which swallowed a
Paramoecium ten times as large as its captor, the latter

78 THE BEHAVIOR OF PROTOZOA

appearing as if forming a mere film over its engulfed victim.
Nothing seems too large for Didinia to attack. Their
powers of digestion seem equal to their voracity; according
to Mast a Didinium will digest an ordinary Paramoecium
once every three hours. According to Balbiani Didinium
actively pursues its prey and when sufficiently near *'it
begins by casting at it a quantity of bacillary corpuscles
which constitute its pharyngeal armature." Coming to
closer quarters it thrusts forth its prehensible apparatus
into its victim and drags it back toward its mouth. This
behavior is referred to by Binet as a ''most complicated
instance of localization '^ involving a precise knowledge of
the position of the prey at which the Didinium takes a
definite aim.

We have here an instance of how easily one may be de-
ceived in interpreting the behavior of lower organisms.
There is no evidence that Didinium pursues its prey like a
hunter. Mast has shown that it does not discharge tri-
chocysts at a distance; in fact it has none to discharge; the
loose trichocysts seen when Didinium attacks Paramoecium
and which Balbiani thought were shot out by the hunter in-
fusorian were derived entirely from the organism attacked.
According to Jennings, Dininium reacts in much the same
way "not only to objects which may serve as food, but to
all sorts of solid bodies. In other words, the process is one
of trial of all sorts of conditions. On coming in contact
with a solid, Didinium 'tries' to pierce and swallow it.
If this succeeds, well and good; if it does not, something
else is 'tried.' In a culture containing many specimens of
Didinium, the author has seen dozens of individuals reacting
in this way to the bottom and sides of the glass vessel, ap-
parently making persevering efforts to pierce the glass.
Others 'try' water plants, or masses of small algae, about

THE BEHAVIOR OF PROTOZOA 79

which many specimens gather at times. Of course they get
no food in this way." They make attempts on various other
organisms with which they come into contact, but often
fail on account of the toughness or lai-ge size of the organism
attacked.

What seems at first to be choice of prey is really some-
thing quite different. The infusorian swallows what it
can and not what it will. ^'The apparent choice of food/'
says Mast, "is due to the fact that the seizing organ will
adhere to some organisms and not to others. The Didinia
come in contact with all sorts of objects in their random
swimming and attempt to swallow all those to which the
seizing organ will adhere." After all Didinium like Para-
moecium is a pretty simple sort of a creature in its be-
havior. It has but a few simple tricks which it tries
over and over again. Its going gunning with its armory
of projectiles, its accurate sense of position and its marks-
manship are all creations of the observer's fancy.

A protozoan exhibiting somewhat more complex behavior
than is shown by Paramoecium is the large cihate Loxophyl-
lum meUagris. This organism usually glides along the
surface of objects by means of cilia on the side of the body,
but at times it may swim in a spiral manner through the
water. If strongly stimulated while swimming it may
back off and turn in another direction much as Paramoecium
does, but the organism seems to be averse to swimming and
generally takes advantage of the first opportunity to glide
along over some solid object or the surface film. These
gliding movements are carried on in a more or less rythmical
way most of the time. The body is narrowed and elongated
and swims forward for a short distance, then there is a con-
traction resulting in making the body shorter and broader
and at the same time a reversal of the effective beat of the

80 THE BEHAVIOR OF PROTOZOA

cilia which causes a backward movement. This is followed
by a turning to one side, the oral in this case, after which
Loxophyllum swims forward in a different direction. The
performance is repeated time after time, and the infusorian
is kept circling about in the same region, for, it may be,
several hours. In this species there is a regular association
between the elongation of the body and the backward strokes
of the cilia, and between the contraction of the body and the
reversed beat of the cilia, an association which is preserved
even in the movements of small fragments of the body.
If Loxophyllum is cut into several pieces these pieces may
swim in a spiral course or glide over the surface of objects
much like the entire organism. When they go forward
they become narrow and elongated and when they swim
backward they become shorter and wider, and they perform
these movements in regular alternation, at the same time
circling slowly toward the oral side. The factors which
determine the peculiar traits of behavior in this species seem
to be present in all parts of the body, for no matter how minute
the fragments into which the organism is divided the action
of the parts, so far as physical conditions permit, is the same.
Habitual contact with solid objects seems to have been the
cause of the development in Loxophyllum of some features of
behavior not found in other swimming infusoria. There
are small bendings of the sensitive anterior end in various
directions as if it were feeling its way along. There are
undulations of the margin and bendings and twistings
of the whole body. The food-taking movements as described
by Oelzelt-Newin are quite complex and involve a number
of coordinated acts. The organism glides over its food
and when the large slit-like mouth is in the proper position
the lips, which are usually tightly closed, open and begin a
series of spreading movements which result in engulfing

THE BEHAVIOR OF PROTOZOA

81

the object. Then there are righting movements which are
brought into play when Loxophyllum is turned over on
its left side. There is not, as we might be led to expect,
but a single stereotyped method of righting. The organism
rights itself by a number of very different methods which
present an indefinite number of modifications. One cannot
but wonder when watching the varied movements of this
graceful and supple infusorian that a single cell is capable
of such behavior.

One of the most highly developed
types of behavior which has been
carefully studied in the protozoa is
exhibited by the large infusorian
Stentor. There are several species
of this genus, but all are trumpet-
shaped, with a mouth situated at
the bottom of a depression at one
side of the anterior end. The oral
end of the organism is surrounded
by a zone of membranellse which at
one end descends in a spiral course
toward the mouth. The whole sur-
face of the body is covered by uni-
form cilia, with the exception of a
small area of naked protoplasm at
the small end or foot, by means of
which Stentor is able to attach itself
to foreign objects. \Mien free in the
water Stentor is able to swim by
the action of its cilia and membranellse. Like Paramoe-
cium it follows a spiral course and when stimulated it may
perform the motor reflex, backing off by reversing the
beat of its cilia, turning to the aboral side and then going

Fig. 11.-
morphus.

-Stentor poly-
( After Stein.)

82 THE BEHAVIOR OF PROTOZOA

ahead in another direction. To a degree unusual among
infusoria Stentor has the power of changing the form of
its body. It may extend into the form of a very long slender
trumpet, or contract almost into a sphere. The ability
to undergo these changes is due to the presence of numerous
contractile threads or myonemes which extend for the most
part longitudinally just beneath the outer layer of ecto-
plasm. Food taking in Stentor is accomplished with the aid
of the cilia at the anterior end of the body and the membran-
ellae leading to the oral opening. The currents set up by
the beating of these organs carry bodies to the mouth
which has the power of taking in comparatively large ob-
jects, for one often sees rotifers, diatoms, algae, and various
protozoans in the endoplasm of the animal.

While swimming freely in the water the behavior of
Stentor is in general similar to that of Paramoecium. The
spiral swimming and the motor reflex in response to chemical,
mechanical, thermal and electrical stimuli are much the
same* in both organisms. Stentors as a rule react to light
which has little effect on Paramoecia. In Stentor cceruleuSf
which is negatively phototactic, sudden illumination evokes
the motor reflex which after one or more trials enables the
animal to reach a more shaded region. The anterior end is
the region most sensitive to light and when the organism
is pointed toward the light it gives the motor reflex, and
swims in a different direction. If still pointing obliquely
toward the light it may repeat the motor reflex and continue
to do so until its anterior end is directed away from the
source of stimulation, when the Stentor swims off in the
direction of the rays.

When Stentor is attached it exhibits several peculiar types
of activity. It may contract or extend the body, and it
often sways about in various directions in a more or less

THE BEHAVIOR OF PROTOZOA 83

rhythmical manner. To contact stimuli Stentor responds
in a variety of ways dependent on the strength of the
stimuli and the number of times they have been repeated.
A moderately strong stimulus causes the Stentor to contract
violently, but after a number of repetitions the contractions
diminish in vigor and finally disappear. Jennings found
that when Stentor is stimulated by a quantity of fine par-
ticles of India ink or carmine which were poured upon the
disk by a capillary pipette, a regular series of responses
was given. Frequently the Stentor would not respond at
first, but would sweep the particles into its gullet con-
tinuously. Sooner or later, however, the organism would
respond by bending to the aboral side. This may be
repeated several times, but if it fails to afford relief from
the stimulation another reaction is "tried." There is a
sudden reversal of the action of the cilia and the par-
ticles are then thrown off the disk. The response is but
momentary, however, and then the usual movements are
resumed. If these two reactions are fruitless the Stentor
contracts strongly, thus drawing its body out of the re-
gion of the impending particles. After a little it slowly
extends again, and if the particles still fall on the disk the
contraction may be repeated. If the stimuli still come
after a number of such attempts to avoid them the Sten-
tor makes several violent contractions in quick succession
and breaks loose from its attachment and swims away.
We have a series of reactions to the same external stimulus.
If one reaction is unsuccessful another is tried until the
organism finally obtains relief. These reactions are all of an
adaptive character, so we can say that the creature is pro-
vided with a number of ways of meeting a given situation.
The external stimulus remaining the same, the particular
reaction that is given obviously depends upon the condition

84 THE BEHAVIOR OF PROTOZOA

of the organism. For the physiological state A there is one
reaction, for state B another, and so on. As in a higher
animal the behavior of Stentor depends upon its previous
history. In a sense the organism may be said to profit by
experience, although it cannot be said to learn, because
there is no formation of new associations such as occurs
in the learning of higher forms.

What are these internal physiological states to which the
adaptive changes of behavior in Stentor are due? We
have here two kinds of modification, as Jennings has pointed
out, the failure to respond to a stimulus which at first evoked
a reaction, and the replacement of one reaction by another.
In regard to the first modification Jennings remarks that
"It seems improbable that the change of behavior is due
to fatigue, since the change occurs after but a single stimula-
tion and a single contraction. It could hardly be supposed
that these would fatigue the animal to such an extent as to
prevent further contraction. And if we use stronger
stimuli, we find that the animal continues to contract suc-
cessively every time the stimulus is applied, for an hour or
more." Even after the animal has ceased to contract
strongly it may respond to a stimulus by bending to one side,
thus showing, according to Jennings, that the failure to
contract is not due to "a fatigue of the perceptive power,
for the bending into a new position shows that the stimulus
is perceived, though the reaction differs from the first one.'*
Fatigue is usually associated in our minds with a condition
indicative of exhaustion, and of such Stentor certainly gives
no evidence after a few stimulations, but that there may be
a slight degree of essentially the same state which when carried
further we designate as fatigue, seems to me a possibility
not ruled out by the experiments. It is possible, however,
that the change in question may be due to something simi-

THE BEHAVIOR OF PROTOZOA 85

lar to the phenomenon of acclimatization to chemicals and
other agencies which is apparently a fundamental charac-
teristic of living organisms.

The replacement of reactions by Stentor probably con-
sists to a considerable degree in variations in the vigor and
completeness of a single reaction. A gentle contraction may,
owing to the bodily peculiarities of the animal, involve a
turning to the aboral side, as its easiest channel of expres-
sion. With stronger contraction this would naturally be
obscured, so that an apparently new response may result
from the heightened irritability caused by the preceding
stimuli. The succession of contractions resulting in the
separation of the organism from its attachment naturally
falls under the same interpretation, and is analagous to
what takes place in an excised heart which, upon being
given a single stimulus, may contract once or several times
according to its condition of irritability. The successive
contractile phenomena in Stentor are more or less analagous to
those of summation of stimuli in an excised muscle. The re-
versal of the beat of the cilia is a separate reaction, although
it may have a certain relation to the phenomenon of con-
traction. We are not justified in assuming that Stentor
passes through a number of discrete internal states each
of which has a correspondingly discrete motor response.
The number of physiological states as in every organism
is unlimited and the behavior of the animal shows us a
series of reactions differing for the most part in degree of
vigor rather than in kind, like the motor reflex in Paramoe-
cium, which may be carried out in various degrees of com-
pleteness from a momentary slowing of speed to a prolonged
backward swimming followed by numerous rotations toward
the aboral side.

Schaeffer has carried on a series of careful experiments in

86 THE BEHAVIOR OF PROTOZOA

which it was shown that when Stentor was offered either
alternately or at the same time nutritious objects such as
Phacus or Euglena and such substances as starch grains,
powdered carmine or India ink, fine sand or sulphur, the
former would be swept into the gullet and ingested, while
the latter would usually be rejected. Conditions of hunger
or satiety influence the selection of food. Very hungry
Stentors may ingest indigestible particles of carmine or
India ink, but when better fed the discrimination is more
precise and only digestible material is taken in. Hungry
Stentors differ from well fed ones also in the greater extension
of the body and the greater activity of the membranellae,
but they are less responsive to mechanical stimuli.

The behavior of Protozoa, as we have seen, is influenced
by their previous activity as well as by changes of external
conditions. That behavior should be modified by these
things is of course inevitable, for no organism is ever twice
the same, and the life of every organism is one of constant
adjustment to the external world. How far these changes
indicate the presence of mind is a question about which
there is much dispute. These changes are to a considerable
degree of an adaptive nature, but the same may be said of
many purely physiological processes occurring in our bodies.
There are some phenomena described which have been
interpreted as the acquirement of habit and even as learning
by experience, but the observations on this score scarcely
justify, in the opinion of the writer, the interpretations
that have been placed upon them. Mr. Stevenson Smith
has performed some experiments which lead him to the
conclusion that Paramoecium is able to acquire advantageous
habits. He placed a Paramoecium in a fine tube containing
a small amount of water. The inner diameter of the tube
was less than the length of the Paramoecium, so that the

THE BEHAVIOR OF PROTOZOA 87

creature had great difficulty in reversing its course, having
to bend its body in the form of a U to get around. If the
diameter of the tube is not too small the time which it takes
the Paramcecium to turn, says Smith, "may gradually be
shortened and a most surprising aptitude of turning be
developed. ... I have found a reduction of turning
time, after the animals have been in the tube for twelve
hours or more, from four to five minutes to a second or two,
which is the minimum time in which the turns can be
made." Although Paramoecia kept without food for twelve
hours would diminish sufficiently in size to enable them to
turn within the tube with much greater ease — a fact which
Mr. Smith apparently has not considered — Day and Bentley,
who have repeated Smith's experiments, have found that the
greater faciHty in turning is acquired within a few minutes.
It should be borne in mind that Paramcecium is an organ-
ism which takes in and excretes water many times more
rapidly than even the specialized organs of excretion of
higher animals, and that the abnormal conditions resulting
from confinement within a very small amount of water
may possibly cause a certain change in size within a short
time. I have often observed a marked shrinkage in Para-
moecia when they are placed in a medium of somewhat
higher osmotic pressure. In abnormal conditions Paramoecia
become more plump and the body seems softer and more
flexible, and it is also possible that, since Paramcecium is
endowed with a certain degree of contractility, the stimuli
encountered through its frequent efforts to turn within the
tube might cause a shortening of the body which would cer-
tainly occur in a more marked way in a Stentor and many
other infusorians under these conditions. The experiment
has its practical drawbacks as a means of testing habit for-
mation or "learning," since the changes of size, form and con-

88 THE BEHAVIOR OF PROTOZOA

sistency which the animals might undergo would so greatly
influence the result. The fact described by Day and Bentley
that Paramoecia which had acquired a facility of turning
still showed the effects of their experience after having been
placed for an interval of twenty minutes in their culture
medium may well be due to the persistence of the purely
physiological or pathological effects of their previous con-
finement.

Smith devised another experiment to show the modifica-
tion of the reaction of Paramoecium to changes of temperature.
A tube containing several Paramoecia was so arranged that
either end could be heated or cooled at will. A\Tien one end
was heated the Paramoecia would dart about at random, and
when they swam into the cooler water they would often turn
back to the hot water again. After the temperature of the
two ends of the tube was reversed the animals would dart
about much as before, reaching the cool water only after a
number of trials. With repeated reversals, however, the
movements of the Paramoecia became "slower and more
regulated" and they would "seldom turn more than once
toward the cold water before swimming in that direction.^'
According to Smith, Paramoecium does not give evidence of
the possession of associative memory, but he concludes that
"its behavior may be modified to show the results of practice,
both in a reduction of the time involved in performing a
movement and in the increase of the suitability of the move-
ment to accomplish the appropriate result." The modified
reaction to temperature, I believe, may be accounted for not
so much through the effect of practice in the performance
of an act, but as a consequence of a general diminution of
excitability. Take a few drops of a Paramoecium culture
and place them on a slide. For a time the animals scurry
about in the greatest haste and confusion, and frequently

THE BEHAVIOR OF PROTOZOA 89

give the motor reflex with no apparent cause. After a time
tkeir actions become slower and more sober. Have we any-
thing essentially different in the experiments of Mr. Smith?
Possibly so, but I cannot convince myself of it from the
results described.

Hodge and Aikins in their study of the daily life of a
Vorticella observed that one individual, after having en-
gulfed yeast cells for some time, refused them and persisted
in so doing for several hours. ^^Hiat this fact signifies can-
not be decided from the single observation reported; there
are a number of possibilities, and the correct interpretation
can be made only after carefully planned experiments.

There have been few systematic investigations with the
end of testing the educability of the protozoa, and while
granting the possibility that future work may compel U3
to modify our conclusion, it may be said that, thus far, there
is no unmistakable evidence that the protozoa are capable
of forming true habits or of learning by association.

BIBLIOGRAPHY

BiNET, A. The Psychic Life of Micro-organisms. Chicago, '94.
Day, L. M. and Bentley, M. A Note on Learning in Paramoecium.

Jour. An. Behavior, 1, 67, '11.
Dellinger, 0. P. Locomotion of Amoebae and allied forms. Jour.

Exp. Zool., 3, 337, '06.
GiBBS, D. and Bellinger, 0. P. The Daily Life of Amoeba proteus.

Am. Jour. Psych., 19, 232, '08.
Hodge, C. F. and Aikins, H. A. The Daily Life of a Protozoan.

Am. Jour. Psych., 6, 524, '95.
Holmes. S. J. The Behavior of Loxophyllum, etc. Jour. Exp.

ZooL, 4, 306, '07. Rhythmical Activity in Infusoria. Biol.

Bull., 13, 306, '07.
Jennings, H. S. Contributions to the Study of the Behavior of

Lower Organisms. Carnegie Inst. Pubs. Wash., '04. The

Behavior of Paramoecium. Additional Features and General

Relations. Jour. Comp. Neur. Psych. 14, 441, '04. Behavior

of Lower Organisms. N. Y., '06.

90 THE BEHAVIOR OF PROTOZOA

Mast, S. 0. The Reactions of Didinium nasutum (Stein) with

Special Reference to the Feeding Habits and the Functions of the

Trichocysts. Biol. Bull., 16, 91, '09.
Oelzelt-Newin, a. Beobachtungen liber das Leben der Proto2oen

Zeit. f. Psych, und Physiol, der Sinnesorgane, 41, 349, '06.
Prowazek, S. von. Einfiihrung in die Physiologie der Einzelligen

(Protozoen). Leipzig and Berlin, '10.
PtJTTER, A. Die Reizbeantwortungen der ciliaten Infusorien. Zeit.

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Rhumbler, L. Physikalische Analyse von Lebenserscheinungen der

Zelle. I, Bewegung, Nahrungsaufnahme, Defakation, Vacuolen-

Pulsation und Gehausebau bei lobosen Rhizopoden. Arch. f.

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ScHAEFFER, A. A. Selection of Food in Stentor. Jour. Exp.

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Smith, 9. The Limits of Educability in Paramoecium. Jour.

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Verworn, M. Psycho-physiologische Protistenstudien. Experi-

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Watkins, G. p. Psychical Life in Protozoa. Am. Jour. Psych.,

11, 166, '00.

CHAPTER V
INSTINCT

"L'instinct sait tout, dans les voies invariables qui lui ont 6iA
trac^es; il ignore tout, en dehors de les voies. Inspirations sublimes
de science, inconsequences ^tonnantes de stupidity, sont k la fois son
partage, suivant que I'animal agit dans des conditions normales ou
dans des conditions accidentelles." — ^Fabre. Souvenirs EntomO'
logigues. T. I.

"We are governed by instinct, as well as cats and goats." —
Voltaire. Philosophical Dictionary.

While it may not be necessary to define a term so well known
as instinct, it may not be without interest to quote the follow-
ing definitions which have been given by various writers:

" We may call the instincts of animals those faculties implanted in
them by the Creator, by which, independent of instruction, observa-
tion or experience, and without a knowledge of the end in view, they
are all ahke impelled to the performance of certain actions tending to
the well being of the individual and the preservation of the species." —
KiRBY AND Spence, Introduction to Entomology, 1858.

"A propensity prior to experience and independent of instruction."
— ^Paley, Natural Theology.

"An action, which we ourselves require experience to enable us to
perform, when performed by an animal, more especially a very
young one, without experience, and when performed by many
individuals in the same way, without their knowing for what
purpose it is performed, is usually said to be instinctive. But I
could show that none of these characters are universal." — Darwin,
Origin of Species.

Instinct is "compound reflex action." — Herbert Spencer,
Principles of Psychology.

" Instinct is a general term comprising all those faculties of mind
which lead to the performance of actions that are adaptive in char-
acter, but pursued without necessary knowledge of the relation be-

91

92 INSTINCT

tween the means employed and the end attained." — Romanes,
Article Instinct, Encyclopedia Britannica.

Instinct — "Purposeful action without consciousness of the pur-
pose."— Von Hartmann, The Philosophy of the Unconscious.

''Instinct is inherited faculty, especially is inherited habit." —
EiMER, Organic Evolution.

" Qu'est-ce que I'instinct? Un mot." — G. Bohn.

"L'Instinct n'est rien." — Condillac.

The above definitions show how differently instinct has
been conceived as regards its causation, although as to the
kind of behavior to which the term is applied there is in
general a broad basis of agreement. Some modern writers
would have us discard the term instinct entirely on account
of its vagueness and because, as commonly used, it carries
with it certain connotations of which they do not approve.
''Instinct," says Bohn, "is a legacy of the past, the middle
ages, the theologians and the metaphysicians" — a word
which does not stand for any well-defined reality. Consign
it therefore to the dust bin, and describe behavior in other
and more scientific terms. Instinct is a word whose con-
notation very naturally has varied according to the scientific
and philosophical views of the writers who have employed it,
but if we were to reject terms generally on this ground our
language, even in science, would undergo an embarrassing
amount of modification. There are few scientific terms,
especially in psychology, which we should be willing to accept
to-day with the meanings they had a hundred years ago.
Stripped of its older metaphysical implications, which
need not annoy us, and used to designate certain types of
behavior, the term is a very useful one. It may not be pos-
sible to define it with precision. It is also difficult to define
a child, a shrub or a tree. We know with a fair degree of
clearness what is meant by the statement that nest building
in birds and comb making in bees are instinctive. The

INSTINCT 93

same idea may be expressed by the use of "brand-new"
scientific terms which have never been soiled by theologians
and metaphysicians, but the need for a general term for
kinds of behavior commonly classed as instinctive would
still remain, and despite the efforts of a few comparative
psychologists, the word instinct will I think continue in
reputable use.

Illustrations of instinct abound everywhere and a very
few will suffice. A flesh fly when first emerging from its
pupa case is very soon ready for performing the various
functions of its life. It guides itself accurately in flight,
and deftly escapes its would be captors by quick and appro-
priate movements. It is drawn by the sense of smell to
suitable objects for food. It avoids various kinds of injur-
ious stimuli. It recognizes out of a vast number of animate
objects the opposite sex of its own species. When ready
to deposit its eggs it selects, out of a great variety of materials
the proper substances to afford food for its future larvae.
Its acts are unguided by previous experiences; they are not
prompted by reflection or thought; they result from a blind
impulse urging the insect to discharge its energies in certain
specific ways without knowing why. An organism of the
degree of psychic development of a flesh fly may modify its
acts to a certain degree through the effects of experience,
but as a matter of fact such modification plays but a small
part in the creature's life.

Some years ago the writer studied the behavior of a species
of amphipod, Amphithoe longiniaiia, and compared the
activities of the adult with those of the newly hatched young.
Amphithoe lives in tubular nests which are usually lodged
among sea weed. The nests are somewhat longer than the
animal and are spun of a w^b-like material into which bits
of sea weed are often incorporated which help to conceal

94 INSTINCT

the occupant. In its nest Amphithoe lies in wait for prey,
ready to dart out upon any small creature which touches
the ends of its long antennae.

The activities of the adult Amphithoe, with the exception of
those concerned in reproduction, are almost exactly parallelled
by those of its young. I have taken the eggs from the mater-
nal brood pouch shortly before hatching and kept them
isolated in individual dishes. For some time after emerging
from the egg the young were weak and had imperfect control
of their movements, which were jerky and irregular. Soon the
minute creatures could crawl and swim much like the adults,
and the next day they began constructing nests which were
the same in shape as those formed by their parents. The
attitudes in the nest, the waving of the antennae, the beating
of the swimmerets, the restless movements of the legs and
mouth parts, springing after food, belligerency toward
passers by, the little unobtrusive signs of timidity, the
reversal of position in the nest on the approach of danger
and the general behavior outside of the nest were, on the
next day after hatching, almost exactly the same as in older
individuals. The only differences in behavior were due to
the feebleness of the young and their imperfect control of
their movements. The young are hatched with all the
instincts necessary fully to equip them for the business of
life. No experience is necessary to teach them what is ad-
vantageous for them to do.

It is this skill and apparent foresight exhibited in instinc-
tive behavior that gives rise to a popular notion that animals
are somehow mysteriously endowed with a knowledge of
those things which are necessary for their life. It is some-
times asserted that young ducklings will make for the nearest
water before they have gained any experience of the neighbor-
hood and in the absence of any signs by which the presence

INSTINCT 95

of water might be indicated; the ducklings just know
instinctively where water is to be found. A friend of mine
once urged as a fact of which our boasted scientific theories
of instinct give no explanation the circumstance that the
burrowing mammals of California, before an unusually
rainy season, would leave their holes near the gulches and
migrate to the hillsides. Long before any indications were
furnished to the weather bureau the instinct of these animals
was said to warn them of the danger of floods. Some time
previous to the conversation the mammals were said to
have emigrated from the lower parts of the vaUeys, and
therefore a rainy winter was predicted. It so turned out,
however, that the season in question proved to be an unusu-
ally dry one, and the wonderful instinct of the burrowing
mammals gave them a false alarm. The migration of the
mammals may or may not have taken place as reported,
but the episode illustrates a very prevalent misconception of
the nature and possibihties of instinct. Instincts frequently
have a relation to future events, but that they involve a
mysterious knowledge of things unpredictable by human
reason still awaits proof.

With all their wonderful adaptiveness instincts are far
from ideally perfect Much of Mark Twain's remarks on
the futility and imbecility, the wasted effort and labor at
cross purposes shown in the behavior of ants may easily be
verified by any observer. Flesh flies will deposit their
eggs on the carrion plant {Stapelia hirsuta) whose odor
resembles that of decaying meat on which the eggs are
usually laid. The domestic hen will sometimes attempt to
hatch out corn cobs or other inanimate objects, and her
maternal instincts will lead her to foster ducklings as readily
as her own kin. Sometimes animals devour their own
eggs or young, as I have several times observed in cray-

96 INSTINCT

fishes and spiders; and in some centipedes the males will uni-
formly devour the eggs if they are not concealed by the
female. Darwin states that in a South American species of
Molothrus the instincts for securing proper care of the eggs
are so imperfect that numerous eggs are simply dropped on
the ground and abandoned.

Ma^ instincts are at first not clearly defined. The
young chick pecks at all sorts of small objects of good and
bad taste alike. The young lamb will follow any sort of
moving object of a certain size as well as its own mother.
It is said to suck indefi-nitely at a piece of wool unless guided
by some fortunate circumstance to the proper fount of
nutriment. Young terns and many other young birds will not
at first distinguish their parents, but will cuddle under one's
hand in perfect confidence and contentment. They react
in much the same way to a great variety of large moving
objects. In most cases these mean the parent birds, and
the instinct of the young becomes directed to their parents
because the latter were the first living objects coming
within their experience. Foster mothers of various kinds
are adopted by many .young birds and mammals as readily
as members of their own species.

While many instincts exist in a completely developed state
when the animal first enters upon active life, others are
manifested only when it has reached a certain degree of
maturity. Such have been called by Lloyd Morgan "de-
ferred^ instincts." Instinctive fear in birds may not appear
at first, but only after several days. Young nestling terns
which for a short time after hatching will cuddle contentedly
under one's hands, behave very differently before they are a
third grown. They then scuttle away in wildest alarm
upon one's approach and hide by crouching down in the grass,
where they will lie perfectly quiet. The instinct of feigning

INSTINCT 97

death which does not occur in the young fledgling now ap-
pears on the scene; the young birds will allow themselves to
be handled and pulled about without betraying a sign of hfe,
and will even suffer their tail or wing feathers to be pulled
out one by one without a wince. After a time, as if the
bird recognized the futility of the ruse, the death feint is
discontinued with a surprising suddenness to be followed
by violent struggles, screams and pecking at its captor in
its effort to make its escape. Later when the birds are able
to fly the crouching and death feigning instincts disappear.

With birds which are hatched in a helpless and almost feath-
erless condition most of the instincts of the species, with the
exception of opening the mouth for food upon the appearance
of a large moving object in their vicinity, are in abeyance.
In such forms running, pecking, flight, etc., are none the
less instinctive; they are simply kept from appearing on
account of immaturity. The young of the mound-building
bird will take wing when first hatched from the egg, but young
swallows are not able to fly imtil after some weeks. That
previous experience is not necessary to enable them to ac-
complish this feat is shown by the fact that young nestlings
kept where they had no opportunity to use their wings
until they were of the proper age for flight, were able
to fly at their first attempt with perfect ease. That young
birds are taught to fly by their parents is a popular myth.
The instinct to fly is there in every case; its appearance is
merely deferred, like the mating and nest building instincts,
until the bird reaches a certain degree of development.
Similarly with the running and swimming of mammals.
A young puppy placed in the water will flounder helplessly
and soon drown, but if an older dog is thrown into the water,
though he may never have been in the water before, he will
swim toward the shore.

7

98 INSTINCT

With animals which go through a profound metamorphosis
in the course of their development we find correspondingly
great changes in instinctive behavior. Nothing could be,
more dissimilar than the instincts of the stealthy dragon-
fly nymph which prowls among the debris at the bottom of
- ■ ., ' ^

ponds and streams for its food; and the graceful and rapid
darting of the full fledged dragon-fly as it chases its prey
through the air. The transition between these two stages
is very abrupt. When ready for its final moult the dragon-
fly nymph crawls upon the stem of some plant or upon a
stone, its skin splits down the back, and out comes the imago,
which needs only to dry its wings a little to be ready for its
vita nuova in the world of sunshine. The difference between
the behavior of the crawling, gnawing caterpillar and the
active honey sucking butterfly; of the helpless wriggling
grub and the honey bee; of the free swimming larva and the
worm that burrows in the sand of the seashore are instances,
out of thousands that might be given, of the great differences
in instinctive behavior at different periods of life in forms
which undergo marked metamorphoses in structure.

Where animals are hatched in the form of the adult we find
little change in instinct. The young trap door spider, accord-
ing to Moggridge, constructs its tiny tubular dwelling with its
ingeniously fitted trap door in almost a perfect miniature of
the adult nest. Montgomery finds that the young of the
orbweavers Epeira scolpetaria and E. mdrmorea spin, at their
very first attempt, a diminutive web of the same degree of
perfection as that of the full grown spider. Here, as in the
case of the amphipod previously described, the young closely
resemble the older individuals. Where the change of form
is greater, as in insects with a gradual or incomplete meta-
morphosis, there is a gradual change of instinct. The be-
havior of the tadpole graduates insensibly into the very

INSTINCT 99

different behavior of the frog. But with abrupt structural
changes such as occur in insects with complete metamor-
phosis the changes in instinct at successive periods is equally
great and often more striking.

The transitoriness of many instincts has been illustrated
in some of the cases referred to, in which the instincts of
larval life are superseded by those of a later period. The
same trait is commonly manifested in the behavior of higher
forms. Here the instinct may be fostered and continued \
by habit, and if it does not become aroused by the appropri- ,
ate objects soon fades away. According to Spaulding, "A
chicken that has not heard the call of the mother until
eight or ten days old then hears it as if it heard it not. I
regret to find that on this point my notes are not so full as
I could wish, or as they might have been. There is, however,
an account of one chicken that could not be returned to the
mother when ten days old. The hen followed it and tried
to entice it in every way; still it continually left her and ran
to the house or to any person of whom it caught sight. This
it persisted in doing, though beaten back with a small branch
dozens of times, and, indeed, cruelly maltreated." If
calves are prevented from sucking for some time after
birth they frequently although not invariably lose the
instinct to suck, and may then be safely returned to the
mother. There is obviously an adaptiveness in this transi-
toriness of instinct in higher forms. WTiere the instinct
finds no proper object to call it forth it is rather of advantage
to the animal to be rid of it; the ground is in a measure
cleared for the development of new adaptations.

Modern literature on animal behavior has much to say
regarding the kinship of instinct and reflex action. Both
are based on inherited organization; both consist to a con-
siderable degree of purposive actions in relation to outer

100 INSTINCT

objects, although without a knowledge of the end they sub-
serve. There is so complete a gradation of responses
between simple reflexes and complex instincts that it be-
comes an arbitrary matter where the line is drawn between
them. In ourselves coughing, sneezing, winking, hiccough-
ing, swallowing, vomiting, jerking back when tickled or
painfully stimulated are commonly set down as reflexes.
Sucking, biting, chewing, spitting out, making a face over
disagreeable objects, grasping with fingers and toes, carrying
objects to the mouth, etc., are usually classed as human
instincts (Preyer). These acts are manifested by the human
infant at a very early period and in much the same way by
different individuals, and there can be no doubt that their
relation to the inherited organization is the same as in the
lower animals. Chewing, spitting out and making a face
over a disagreeable taste are little more complex than the
reflexes of swallowing and coughing. If not performed
involuntarily, there is at least a strong involuntary pro-
clivity to their performance which would express itself in
action if not suppressed by an effort of the will. Swallowing,
coughing and sneezing are likewise capable of voluntary
suppression, so that we cannot separate these activities
sharply on the basis of their relation to the will any more
than on the ground of complexity.

In man the gradation from the simple to the more com-
plex manifestations of instinct is not so obvious owing to the
fact that human instincts are so closely interwoven with
habits and the workings of intelligence; but in lower forms
where intelligence is reduced to a minimum the relation is
shown very clearly. In an animal such as the crayfish the
relation of instinct and reflex action may be studied very
advantageously by the experimental method. The cray-
fish has a number of well defined instinctive reactions such.

INSTINCT 101

as locomotion by walking and swimming, darting back upon
the approach of danger, seeking dark and protected situ-
ations, rearing up when threatened and holding the claws
in a position for defence, withdrawing movements, moving
toward certain odors and feeling about with the chelae for
food, seizing food in the chelae and passing it to the mouth,
chewing and swallowing, rejecting objects from the mouth,
and a number of others. The crayfish may form associa-
tions to a limited degree, but if it had to rely entirely on its
congenital endowment of instincts it would probably get
through the world almost as successfully as it does with its
modicum of intelligence.

The relation of instincts and reflexes in the crayfish has
been studied with considerable thoroughness by means of
operations on the nervous system. The nervous mechanism
of the crayfish consists of a brain which gives branches to
the eyes, first and second antennae, and anterior part of the
thorax; a ventral nerve cord consisting of a double chain of
ganglia which is connected with the brain by commissures
passing around the esophagus; and a small visceral system.
The ganglia of the ventral nerve cord are connected by
cross commissures as wxll as the longitudinal ones which
form the larger part of the nerve chain, and they give off
nerves which are distributed to the segments in which
they lie. Typically there is a pair of ganglia in each seg-
ment of the body, but anteriorly the ganglia belonging to
the segments bearing the mouth parts have become fused
into a single sub-esophageal ganglion which supplies these
appendages. The brain may be regarded as a nerve center
homologous with the ganglia of the ventral nerve cord, but
like the subesophageal ganglion, it is formed of more than
one pair of ganglia which have been fused together.

Experiments on the crayfish show very clearly that the

102 INSTINCT

instincts of the animal are by no means monopolized by the
brain, but that the various ganglia of the ventral nerve cord
are the controlling centers of many activities. If we cut
the commissures connecting the brain with the chain of
ventral ganglia, the reactions of the eyes and antennse take
place in the usual way. If the eye stalk is stimulated it is
vathdrawn; if the antennules or antennse are touched they
are drawn back, but there is no reaction from the legs. On
the other hand, if a leg is seized an effort is made to with-
draw it; if this is not successful other legs may be employed
to push against one's hand or the chelipeds may reach over
and pinch the offender. The crayfish can walk in the usual
manner, and when placed in the water it can swim as well
as a normal individual. Its movements are more restless
than before, owing to the lack of inhibitory impulses which
under ordinary circumstances are issued from the brain.
Food is seized by the chelipeds, passed to the mouth parts,
chewed and swallowed; if stones or other innutritions objects
are presented to the mouth parts they are at once rejected,
showing that connection with the brain is by no means
necessary for the proper discrimination of food.

If the commissures are cut farther back, between the sub-
esophageal and the first thoracic ganglia, power of moving
the legs still remains, although locomotion is somewhat
impeded. If a leg is seized it is withdrawn or defended by
the other appendages. Pieces of meat or paper given to the
chelipeds are passed from one to the other and pressed
between the mouth parts where they may be seized and
swallowed. Often the mouth parts are tardy in responding,
and the chelipeds may vainly persist for hours in pressing
the object against them. Sometimes bits of food are torn
to pieces and then offered to the mouth parts. Stones or
other hard objects are not passed to the mouth, and if seized

INSTINCT 103

are soon rejected. The legs are almost incessantly engaged
in cleaning movements, picking at one another, and at the
abdomen and its appendages.

With the nerve cord cut between the first and second
thoracic ganglia the responses of the parts in front of the cut
are little affected, but those of the last four pairs of thoracic
legs are much reduced in vigor. The chelse still perform
defensive movements, but the feeding movements no longer
occur. Cutting the nerve cord further back interferes still
more with the power of coordinated locomotion, although
the withdrawing and defensive movements still persist. In
fact, any pair of legs will perform these movements if the
cord is cut both in front of and behind the ganglion supply-
ing these legs with nerves.

Each ganglion is a reflex center regulating the movements
of the appendages of the segment in which it lies. Into it
impulses pass from the appendages and are sent out to
muscles which effect the withdrawing or the defensive acts
in response to the outer stimulus. When several ganglia
are joined together, the impulses from the various append-
ages are coordinated; instead of a single adaptive reflex, we
have a complex cooperative response, such as occurs in
walking, cleaning movements and mutual defense^of ap-
pendages which are seized. What a wonderful combination
and coordination of impulses ! From the simple reflex of the
isolated segmental ganglion to the complex behavior of the
brainless crayfish, and from this to the still more complex
behavior of the normal animal there is a regular gradation.
Nowhere can we draw a sharp line between reflex behavior
on the one hand and instinct on the other. Both are based
on a wonderfully complex and beautifully organized nervous
mechanism.

Among the most remarkable of the instincts of crusta-

104 INSTINCT

ceans is the peculiar trait shown by several species of spider
crabs of decking themselves out with a covering of seaweed,
sponges, hydroids, etc., so that they are quite effectually
concealed in their natural surroundings. Spider crabs
kept in aquaria have been seen by several observers to snip
off bits of seaweed with their pincers, reach over their backs
and stick them down among the stiff curved hairs which
occur on the upper surface of the carapace. The seaweeds
and sessile animals thus transplanted often grow, so that
the back of the crab comes to be a veritable botanical and
zoological garden on a small scale. There are few instinc-
tive acts which appear to be more the result of deliberate
intention, yet the disguisement is not only instinctive, but,
as Minkiewicz has shown, it is performed by crabs in which
the brain is entirely cut off from connection with the legs by
cutting the esophageal commissures that lead to the large
ventral ganglion. The sense of wonder with which instinct
was formerly regarded, and which the reflex theory might
seem to rudely destroy, is a sentiment which can scarcely
fail to arise when we contemplate the organization which
makes possible the blind performance of such remarkable acts.
Among the insects, as in the Crustacea, the seat of many
instincts is in the ventral ganglia rather than in the brain.
A decapitated fly is able to walk and fly and go through with
elaborate movements of cleaning the wings, legs and body.
In experiments made by the writer on the water scorpion,
Ranatra, it was found that after decapitation the insects
were able to walk and swim almost as well as before. In
fact, they became much more restless and would walk about
for hours without *coming to rest; and when they finally
became quiet they could be aroused by the slightest stimuli.
When placed on their backs they would readily right them-
selves. If the tip of the breathing tube was seized while the

INSTINCT 105

insect was swimming its efforts to swim away would be
performed with much greater vigor. If it does not effect
its escape by this method it has recourse to a remarkably
neat and apparently intelligent device. The hind legs are
thrown back as far as possible, whereby they are enabled
to grasp the breathing tube a short distance behind the
body; then by exerting a pull they bend the body ven-
trally. This soon places the second pair of legs so that the
offending object can be reached, when all four legs are em-
ployed to push the body away, which is very frequently
accomplished. The behavior of a decapitated Ranatra in
this situation certainly affords an excellent simulation, not
only of purposive action, but also of considerable ingenuity
in its accomplishment.

The reactions of the brainless frog form the stock illus-
trations of reflex action. The withdrawal of a foot when
pinched is one of the simplest of these. When a drop of
acid is placed on one side of the body the hind foot of that
side is brought forward to wipe it off. With a somewhat
stronger stimulus the fore leg of the same side may be
moved back to the irritated spot. If the acid is placed on
the middle of the posterior part of the back both hind legs
are employed to remove it. We have here reflexes of a
higher degree of complexity involving the coordinated move-
ments of many muscles. If a frog with the greater part of
its brain removed is taken in the hands it uses both hind
legs to push against the hands, and at the same time inflates
the lungs with air, causing the body to swell so that it more
readily slips from the grasp. The use of the hind limbs
and the swelling of the body may be regarded as two com-
plex refle es excited by the same cause and which cooperate
to enable the animal to effect its escape, but the behavior
may equally well be described as an instinctive reaction.

106 INSTINCT

The clasping of the female frog by the male during the
breeding season affords a typical example of instinctive
behavior; nevertheless, it occurs in entire independence of
the higher nerve centers. "The Abbe Spallanzani showed
tliat a male frog may have its head cut off during copulation
without ceasing to cling tenaciously to the female. Goltz
went still further and cut off the head of a male, then cut the
body through between the third and fourth vertebrae, and
removed the viscera from the body cavity; the section of the
frog that remained after these operations consisted of the
first three vertebrae, the pectoral girdle and the fore legs.
Yet when the skin of the inner surfaces of the fore legs was
rubbed with the finger this segment would show the same
clasping efforts as a normal male frog."

Nearly all the characteristic responses of the frog will take
place in individuals deprived of the cerebral hemispheres
which are the part of the brain usually considered as the
seat of intelligence and volition. Such a frog, if given suffi-
cient time to recover from the shock of the operation, will
leap about and swim spontaneously, snap at insects which
come within range, bury itself in the mud on the approach
of winter, and in many other ways behave in a normal
ranine manner.

Beginning with the anterior part of the brain and destroy-
ing successively the parts of the central nervous system
lying behind it, we cause, one after the other, the various
instinctive acts of the frog to disappear, until we have left
only the reflexes of the posterior part of the spinal cord. We
reduce the frog to a more and more simple type of reflex
mechanism, but we cannot say where the animal ceases to
be more than a reflex mechanism of a complicated kind.
The behavior of the frog is almost entirely made up of in-
stinctive and reflex acts, many of which have their seat

INSTINCT 107

either in the spinal cord or the lower centers of the brain.
It thus resembles the segmental reflexes which constitute
much of the behavior of worms and arthropods. In the
frog, as Schrader has observed, the central nervous system
''can be divided into a series of sections each of which is capa-
ble of performing an independent function.'' Its mode
of action is hence of the same fundamental kind that we
find in the nervous systems of the lower articulate animals.

The relation of reflex action to instinct which is disclosed
through operations on the nervous system is shown also
by a study of the gradual development of instinct through
the animal kingdom. The behavior of the protozoa, as we
have seen, consists mostly of rather simple stereotyped
activities which have all the directness of the simple reflex
acts of higher forms. The behavior of the lower Metazoa
falls largely within the same general type. Tracing the
evolution of behavior upward we find a gradual increase in
the number, complexity and perfection of reflex acts.
Where instinct may be said to begin is an arbitrary matter.
If instinct be, as Spencer defines it, "compound reflex ac-
tion," it begins of course where reflex action passes from the
simple to the compound, but this point is not so easy to
mark as theoretically it might appear. It is commonly said
that in reflex action only a part of the body responds, as in
winking the eye, or jerking back the foot, whereas in instinc-
tive behavior there is a response by the organism as a whole.
This distinction is at times difficult to draw, and it is not
consistently adopted by most writers, but it is perhaps as
useful a distinction as can be made.

While instinct is most intimately related in its nature and
origin to reflex action it would be an error to regard it as con-
sisting of nothing but direct responses to external stimuli.
The animal is not merely a machine responding to the

108 INSTINCT

various influences from the environment which affect it.
It possesses a native fund of impulse which causes it to act
in more or less definite ways independently of the stimula-
tions of the outer world. Several modern writers have
over-emphasized the element of responsiveness in instinct,
as if an animal were like an instrument played upon by
outer forces and had its actions fatally determined by the
action of those forces on its own inner mechanism. Other
writers have treated instinct as determined by a sort of
internal impulsion. The latter conception is implied in the
German word "Trieb," or driving force, and in Paley's
''propensity prior to experience." Lloyd Morgan says in
speaking of instinct: "Initiated by an external stimulus
or group of stimuli, it is at any rate in many cases, deter-
mined also in greater degree than reflex action by an internal
factor which causes uneasiness or distress, more or less
marked, if it do not find its normal instinctive satisfactions.
Take, for example, the before mentioned instinct of the
great water-beetle to leave the pond and burrow in the bank
when the time for pupation is at hand. There is something
more here than a local response to an external stimulus;
something more, it would seem, than mere reflex action.
There are activities affecting the whole behavior of the
organism, and there seem to be internal promptings of some
kind due to organic conditions whose seat is in the body of
the developing larva. Or take the migration of birds, their
nest-building instincts, the activities involved in the rearing
of their young; there is surely, it may be said, something in
all this which may be distinguished, even if the line of de-
markation be hard to draw, from reflex action. We cannot
say more, however, than that the one is a more fully cor-
porate act than the other.''
It is without question that internal states form the prompt-

INSTINCT 109

ings of many instinctive acts. Hunger drives the lioness
to seek for prey; sexual impulses lead to the search for mates;
and a bird in confinement may become uneasy when the
time for migration arrives. The same thing is even more
conspicuously illustrated in the instinct of play. A lamb
may frisk about from sheer good feeling. A kitten may
crouch and spring as if upon a mouse when there is no ex-
ternal object to excite its action. And where the play
activities are associated with external objects, the latter
serve only to awaken the stored energy of impulse which
may be nearly ready to discharge on its own account. The
play impulse which may sometimes vent itself in random
movements usually takes fairly definite channels of expres-
sion which are quite characteristic of particular species of
animals. The energies of the young animal tend to dis-
charge themselves in movements similar to those which form
the regular behavior of the adult, and a certain degree of
proficiency is reached in those activities which form the
more serious occupations of later life. But the promptings
to such behavior are due mainly or wholly to internal
impulses.

The element of internally initiated impulse in instinct is
not confined to higher forms. It is probably coextensive
with animal life. Amid all the stereotyped responses of
the Protozoa we have a large element of activity determined
by internal factors. The almost constant swimming of
many infusorians, and the regular rhythmical activity of
others are, like the beating of the heart and other organic
rhythms, the result of causes within the organism.

It is of course difficult in many cases to ascertain whether
activity results, perhaps indirectly, from outer stimulations
or from internal changes. In a great many cases the
organism needs but a slight provocation to discharge its

no INSTINCT

energies in instinctive acts. When the spider spins its web,
when the wasp digs a hole and stores it with a certain kind
i of prey for its young, and when the bird builds its nest, and
/ the beaver its dam there is of course response to certain
features of the environment; there is also an innate propen-
sity for the organic machinery to work in certain ways,
much as a piece of clock work runs in a particular fashion
after it has been wound up and set going.

Activity which is internally initiated is not fundament-
ally different from activity which we commonly call reflex;
the stimuli by which it is evoked are internal instead of
external; they result in many cases from the rhythms of
organic functions, chance discharges of nervous energy
due to various physiological changes, and various other
factors. Such activities are to a high degree characteristic
of particular species and are doubtless as rigidly determined
by organization as are the direct responses to external
stimuli.

The nature of the instinctive act that may be performed
in a given situation is notoriously dependent upon the
internal condition of the animal. The same stimulus may
evoke in different states quite contrary impulses. The
sight and smell of food may arouse an animal to vigorous
efforts to secure it, or produce feelings of aversion and
movements of avoidance, according to the creature's state
of hunger or satiety. The sexual behavior of animals is
dependent to a very marked degree upon internal conditions
which are correlated with the production and maturation
of the sex cells. Salmon begin their up-stream migrations,
the male frog develops his tendency to clasp the female;
birds herald the advent of the breeding season with court-
ship and song, and the males of many mammals show at
this time an unusual degree of belligerency. The change

INSTINCT 111

in instinctive behavior during the breeding season may be
due to the production of internal secretions which influence
the irritabiUty of certain parts of the nervous system, but,
however caused, it is, like the varying responses to food,
water, etc., pretty closely subservient to the needs of the
species.

This dependence of behavior upon internal conditions
naturally increases the range of its possible adaptations.
Animals are often endowed with adaptive responses cor-
responding to this, that, or the other internal state. Pre-
vious exercise and many other factors change these internal
states, so that what an animal may do in a given situation
is not to be inferred from the external conditions alone. If
one response does not suit the animal tries another, and so
on. The condition of the animal is changed after one or
more reactions and this change produces a different reaction
to the stimulus.

According to Whitman, the leech Clepsine when it is
stimulated may roll into a ball, hug the bottom, or crawl
away. "If the leech has eggs it will not roll up, but if it
has no eggs, or if it has young, it may adopt either mode of
escape, while if it has eggs it has no choice but to remain
quiet over them. The act of rolling up into a passive ball
may be performed (a) under compulsion, as when it is her
last resort in self defense; (b) under a milder provocation,
as one of three courses of behavior, as when the resting place
is turned up to light, and the choice is offered between
remaining quiet in place, creeping away at leisure, or rolling
into a ball and dropping to the bottom; (c) or finally, under
no special external stimulus, but rather from internal motive,
the normal demand for rest and seclusion, presumably very
strong in Clepsine after gorging itself with the blood of
its turtle host." ''The differential reaction," says Lloyd

112 INSTINCT

Morgan, "according as the animal has eggs or not suggests
intelligence; but," he adds — and this seems to me to be a
more probable conclusion — that "it may be instinct varying
according to the conditions of stimulation."

Varied activity under unfavorable conditions charac-
terizes alike the behavior of the Protozoa and the most
highly evolved animals. While not involving intelligence
it performs, in a measure, the function of intelligence, as it
gives the animal greater opportunities for making favorable
adjustments. ^^ Nature,^ says James, ^^ implants contrary
impulses to act in many classes of things, and leaves it to
slight alterations of the conditions of the individual case to
decide which impulse shall carry the day. Thus greediness
and suspicion, curiosity and timidity, coyness and desire,
bashfulness and vanity, sociability and pugnacity seem to
shoot over into each other as quickly, and to remain in as
unstable equilibrium, in the higher birds and mammals as
in man. They are all impulses, completely blind at first,
and productive of motor reactions of a rigorously deter-
minate sort. Each of them, tlien, is an instinct, as instincts
are commonly defined. But they contradict each other —
'experience' in each particular opportunity of application
usually deciding the issue. The animal that exhibits them
loses the instinctive demeanor and appears to lead a life of
hesitation and choice, an intellectual life, not, howevevy
because lie has no instincts — ^rather because he has so many
of them that they block each other's path."

At its first appearance intelligence is able to modify but
slightly the course of instinctive behavior. It is a faculty
which, as Hobhouse remarks, " arises within the sphere of
instinct " and is devoted to the task of enabling the instinc-
tive proclivities of the animal to work themselves out more
effectively. The close connection of intelligence and instinct

INSTINCT 113

is shown by the fact that animals are so exceedingly stupid
in everything not closely related to their instinctive in-
terests. A cat pays not the least attention to a multitude
of things going on around her, but the sight of a canary or
the noise made by a gnawing mouse puts her on the qui-vive.
Only a few objects have meaning; the rest do not form a
part of what might be called her effective environment;
to her they are non-existent.

Not only in lower forms, but in the higher members of the
animal kingdom as well, intelligence may be said to be the
handmaid of instinct. Animals profit by experience in
order to live their life along the lines marked out for them
by their instinctive make-up, and whatever pleasure or
satisfaction their lives may bring is attained by following
their instinctive bent. "Why," asks James, in a significant
passage, "do the various animals do what seem to us such
strange things in the presence of such outlandish stimuli?
Why does the hen, for example, submit herself to the tedium
of incubating such a fearfully uninteresting set of objects as
a nestful of eggs, unless she have some prophetic inkling
of the result? The only answer is ad hominem. We can
only interpret the instincts of brutes by what we know of
instincts in ourselves. Why do men always lie down, when
they can, on soft beds rather than on hard floors? Why
do they sit around the stove on a cold day? . . . Why
does the maiden interest the youth so that everything about
her seems more important and significant than anything
else in the world? Nothing more can be said than that these
are human ways, and that every creature likes its own ways,
and takes to following them as a matter of course. Science
may come and consider these ways and find that most of
them are useful. But it is not for the sake of their utility
that they are followed, but because in following them we

114 INSTINCT

feel that it is the only appropriate and natural thing to
do. . . . And so, probably, does each animal feel about the
particular things it tends to do in the presence of particular
objects. ... To the lion it is the lioness which is made to
be loved; to the bear, the she-bear. To the broody hen the
notion would probably seem preposterous that there should
be a creature in the world to whom a nestful of eggs was not
the utterly fascinating and precious, never-to-be-too-much-
sat-upon-object which it is to her."

With increasing intelligence attention becomes devoted
to things which are more remotely connected with instinctive
interests, but even in ourselves instinct, in one form or
another, supplies the basis of most of our springs of action.
With us, as with the lower animals, self-preservation and
the perpetuation of the stock afford, broadly interpreted,
the main business of life.

BIBLIOGRAPHY
Darwin, C. Origin of Species, 1859. Posthumous essay on Instinct

published in Romanes* Mental Evolution in Animals.
HoBHOUSE, L. T. Mind in Evolution, '01.

Hudson, W. H. The Naturalist in the La Plata. London, '95.
James, W. Principles of Psychology, 2 vols. '90.
Morgan, C. L. Habit and Instinct, London, '96. Animal Behaviour,

London, '00.
Reimarus, H. S. AUgemeine Betrachtungen liber die Triebe der

Thiere. Hamburg, 1773, 3d ed.
Romanes, G. J. Animal Intelligence, N. Y., '83. Mental Evolution

in Animals, London, '85.
Schneider, G. H. Der thierische Wille, Leipzig, '80.
Schneider, K. C. Vorlesungen iiber Tierpsychologie, '09.
Spaulding, D. a. Instinct, With Original Observations on Young

Animals. Macmillan's Mag. 27, 282, '73. Reprinted in Pop.

Sci. Mon. 61, 126, '02.
Spencer, H. Principles of Psychology, London, 1855.
Wasmann, E. Instinkt und Intelligenz im Tierreich, 3d ed.,

Freiburg, i. B.. '05. Translation of 2nd ed. St Louis, '03.
Ziegler, H. E. Der Begriff des Instinktes einstund jetzt. Jena, '10.

CHAPTER VI
THE EVOLUTION OF INSTINCT

"The primary roots of instincts reach back to the constitutional
properties of protoplasm, and their evolution runs, in general, parallel
with organogeny. As the genesis of organs takes its departure from
the elementary structure of protoplasm, so does the genesis of in-
stincts proceed from the fundamental functions of protoplasm." —
Whitman, Animal Behavior.

" Instinct precedes intelligence both in ontogeny and in phylogeny,
and it has furnished all the structural foundations employed by
intelligence." — Ibid.

" It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species under its present
conditions of life. . . . If it can be shown that instincts do vary
ever so little, then I can see no difficulty in natural selection preserv-
ing and continually accumulating variations of instinct to any extent
that was profitable. It is thus, as I believe, that all the most complex
and wonderful instincts have originated." — Darwin, Origin of
Species.

Efforts to explain the origin of instinct by gradual evolu-
tion were made from time to time before Darwin applied
his theory of natural selection to the solution of the problem.
The most noteworthy theory was Lamarck's doctrine that
instinct is inherited habit. It is well knowTi that actions
frequently performed come in course of time to be performed
automatically and unconsciously, as is illustrated by the
familiar example of learning to play the piano. Granting
that the modifications produced by habit are inherited, it
is evident that the repetition of an action generation after
generation would produce a congenital proclivity to its
performance which might in time develop into a true in-
stinct. Since habits are so frequently the result of intel-

115

116 THE EVOLUTION OF INSTINCT

ligent experience, instinct was conceived by some writers as
due to the gradual automatizing of such experience by
frequent repetition; in the words of G. H. Lewes, instinct
is "lapsed intelligence," a view which makes intelligence
first in order of appearance and instinct a secondary result
of a sort of psychic degeneration.

As Whitman has urged, according to the doctrine of
Lewes, "we should expect to find the lowest animals free
from instinct and possessed of pure intelligence. In the
higher forms we should expect to see intelligence lapsing
more and more into pure instinct." As every student of
animal behavior now knows, we find just the reverse.
Among low forms behavior is all but exclusively of the reflex
type. Passing up the animal series we find intelligence
gradually growing upon instinctive foundations. '^In higher
forms not a single case of intelligence lapsing into instinct
is known. In forms that give indubitable evidence of
intelligence we do not see conscious reflection crystallizing
into instinct, but we do find instinct coming more and more
under -the sway of intelligence."

Herbert Spencer, who was keenly alive to the difficulties
of the theory of lapsed intelligence as an explanation of the
origin of instinct in general, put forward an ingenious specu-
lation in which he attempted to derive instinct from reflex
action and the inheritance of acquired associations between
reflexes. In order to illustrate how an instinct might arise
he takes a low aquatic creature with rudimentary eyes.
"Sensitive as such eyes are only to marked changes in the
quantity of light, they can be affected by opaque bodies
moving in the surrounding water, only when such bodies
approach close to them. But bodies carried by their motion
very near to the organism will, by their further motion, be
brought in contact with it. . . . In its earliest forms sight is.

THE EVOLUTION OF INSTINCT 117

as before said, little more than anticipatory touch; visual im-
pressions are, in all these creatures, habitually followed by
tactual ones. But tactual impressions are, in all these crea-
tures, habitually followed by contractions. . . . From the
zoophytes upward touch and contraction form an habitual
sequence; and hence, in creatures whose incipient vision
amounts to little more than anticipatory touch, there con-
stantly occurs the succession — a visual impression, a tactual
impression, a contraction." This habitual association will
link the two responses so that a contraction will follow
immediately upon the visual stimulus. The effect of such
experiences accumulated by heredity generation after
generation is to establish a new congenital response which
is of value to the species. With increased powder of sensory
discrimination the visual stimuli produced by smaller objects
whose contact does not cause a protective contraction, but
rather the activities of food taking, may in a similar manner
become associated with movements of prehension, thus
enabling the animal to react in different ways to objects at a
distance. In this way Spencer supposes instincts to have
been built up by growing out of simple reflex acts instead of
being the outcome of a lapsed intelligence. Spencer's
conception is more congruous with the general doctrine of
psychic evolution and does not involve the assumption that
the lower we go in the animal kingdom the more purely
intelligent the actions of animals become.

The theory of Spencer, which was put forward in 1855, is
based entirely on the assumption of the transmission of
acquired characters like so many other of his psychological
speculations. After the "Origin of Species" was published
Spencer accepted the theory of natural selection, but as-
signed to it a subordinate rdle, especially in the evolution
of mind.

lis THE EVOLUTION OF INSTINCT

The Lamarckian theory of the origin of instinct has in
more or less modified forms enjoyed a wide popularity among
writers on genetic psychology. The veteran psychologist
Wundt in his discussion of theories of instinct in his Human
and Animal Psychology assumes unquestioningly the trans-
mission of acquired characters, practically ignoring the
doctrine of natural selection, and even representing that
Darwin ''explains instinct as inherited habit"!

One of the most extreme positions is that of Eimer who
rejects the theory of natiu*al selection and adopts the pure
Lamarckian standpoint. One of his arguments in favor of
this theory is drawn from the instincts of the mason wasp,
Odynerus parietum. This species provisions its nest with
larvae which it paralyzes by stinging them in the ventral
ganglia. After collecting several larvae and storing them
in a hole in the ground, the wasp lays an egg on the store of
food, seals up the hole with clay, and then begins the con-
struction of another nest. " What a wonderful contrivance " !
exclaims Eimer, ''What calculation on the part of the animal
must have been necessary to discover it! The larvae of the
wasp require animal food. Dead food enclosed in the cell
would soon putrefy; living active animals would disturb the
egg, and accordingly the wasp paralyzes grubs and packs
them like sacks of meal one after another in the cell. How
did she arrive at this habit? At the beginning she prob-
ably killed larvae by stinging them anywhere and then
placed them in the cell. The bad results of this showed
themselves; the larvae putrefied before they could serve
as food for the larval wasps. In the meantime the mother-
w\isp discovered that those larvae which she had stung in
particular parts of the body were motionless but still alive,
and then she concluded that larvae stung in this particular
way could be kept for a longer time unchanged as living

THE EVOLUTION OF INSTINCT

119

motionless food In this case it is absolutely im-
possible that the animal has arrived at its habit otherwise
than by reflection upon the facts of experience."

The careful studies of the Peckhams on the instincts of the
solitary wasps have shown that many of the assumptions
upon which Eimer rests his argument are erroneous. In
the first place the Peckhams found that the insects stored as
food were by no means uniformly paralyzed and that in

Fig. 12. — Th£ wasp Ammophila stinging a caterpillar. (After Peckham. )

most nests several caterpillars died. Even where all were
dead the wasp larvae fed upon them, so that it is open to
question if much is gained b}^ having the prey in a paralyzed
condition. The Peckhams conclude that "the primary
purpose of the stinging is to overcome resistance and to
prevent the escape of the victims, and that incidentally some

120 THE EVOLUTION OF INSTINCT

of them are killed and others paralyzed.'' In Ammophila
'^ the prey may be stung so slightly that it can rear and strug-
gle violently or so severely that it dies almost at once, and
in neither case is a break made in the generation of Ammo-
philes, since in the former, the egg or larva is so firmly fas-
tened as to keep its hold, while in the latter the dead and
decomposing caterpillar is eaten without dissatisfaction or
injury."

An important fact which Eimer has apparently failed to
consider is that in most of the solitary wasps the nest with
its provision and egg is abandoned as soon as completed and
never seen again, the wasp dying before its progeny emerges.
The same is true among the more primitive bees, such as
Osmia, where elaborate provision is made for the larva
which only emerges the following year. The typical con-
dition among the solitary species of both bees and wasps is
one in which the parent never sees its own offspring, never
has an opportunity of watching the results of its own experi-
ments. Unless gifted with a truly preternatural intelli-
gence what means has a solitary wasp or bee which never
sees the larva or pupa of its own species of knowing what
conditions of food and habitation are the most suitable for
its progeny? Far from being the only possible explanation
of the origin of the remarkable instinct he has cited, the
doctrine of Eimer is improbable to the point of absurdity.

Romanes who has treated the evolution of instinct at
length in his Mental Evolution in Animals regards both
natural selection and use inheritance as important factors.
Instincts due to the first factor he calls primary, while
those due to the latter are called secondary. Several in-
stances are cited in which it is claimed there is strong proof
that certain instincts have been produced through the ac-
cumulated effects of experience. Other instances are given

THE EVOLUTION OF INSTINCT 121

which point to a blended origin of instinct. The cases
adduced by Romanes have been critically analyzed by
Lloyd Morgan and Whitman, who have shown that they are
of very doubtful value as evidence, and we need not repeat
the arguments of these writers. One class of cases adduced
by Romanes we shall mention since it shows how easily the
facts may be misinterpreted. Wildness and tameness among
birds are apparently inherited instincts. Most readers are
familiar with the statement that birds on little frequented
islands and in recently explored regions betray at first no
marked fear of man and may frequently be knocked over
with clubs, whereas in places where they have been hunted
for several generations they become very wild, the fear of
man being shown by the young birds as well as the old.
This fact is supposed to be explainable only on the assump-
tion that the painful experiences inflicted by man have
become associated with the appearance of the human form,
and that this association is transmitted to the young birds,
giving them an innate fear of man before they have had any
experience with their persecutor. Those who have adopted
this explanation have failed to consider the important r6le
of imitation in the behavior of young birds. Fear is un-
doubtedly instinctive, and it may be aroused by large moving
objects or unfamiliar appearances of any kindj but in general
it may be said that a sort of tradition determines, to a very
large extent, the objects by which fear is awakened. The
excellent observations of Hudson on fear in birds have con-
vinced him that the "fear of particular enemies is in nearly
all cases — ^for I will not say all — ^the result of experience and
tradition." Young birds have a marked proclivity to flee
from objects at which their parents take alarm^ and to scuiTy
away upon hearing the parental danger signal. Habits of
fear in regard to particular enemies are rapidly acquired and

122 THE EVOLUTION OF INSTINCT

passed on. Hudson gives among other illustrations of this
fact an account of some English sparrows which he was
accustomed to feed from a window. "The bread and seed
were thrown on to a low roof just outside the window, and I
noticed that the young birds when first able to fly were al-
ways brought by the parents to this feeding place, and that
after two or three visits they would begin to come of their
own accord. At such times they would venture quite close
to me, showing as little suspicion as young chickens. The
adults, however, although much less shy than birds of other
species, were extremely suspicious, snatching up the bread
and flying away; or, if they remained, hopping about in a
startled manner, craning their necks to view me, and mak-
ing so many gestures and motions, and little chirps of alarm,
that presently the young would become infected with fear.
The lesson was taught them in a surprisingly short time;
then suspicion was seen to increase from day to day, and
about a week later they were scarcely to be distinguished
in behavior from the adults. It is plain that, with these
little birds, fear of man is an associate feeling, and that,
unless it had been taught them, his presence would trouble
them as little as does that of a horse, sheep, or cow.'^

The large Rhea of South America which is used for food
and must have been hunted by savages for a very long period
should certainly show a strong innate fear of man, but Hud-
son found that the young captured just after hatching would
follow him about in perfect confidence. WTien he would
imitate the danger note of the parents the young would rush
to him in great terror, although no animal was in sight.
"If," says Hudson, "I had caused a person to dress in white
or yellow clothes for several consecutive days, and had he
shown himself to the birds, I have no doubt that they
would soon have acquired a habit of running in terror from

THE EVOLUTION OF INSTINCT 123

him, even without the warning cry, and that the fear of a
person in white or yellow would have continued all their
lives."

Birds have been pursued by hawks for countless gener-
ations, but there is, according to Lloyd Morgan, no evidence
that they have an instinctive fear of hawks, any more than
of large moving objects in general which are seen in the air.
Such fear is readily taught the young, and different species of
hawks, as Hudson has shown, come to inspire different
degrees of fear according to their varied powers of harm.

While instinctive fear of particular enemies may exist in
certain animals, the evidence that it has in any case been
recently acquired is entirely inadequate. There seems to
be more evidence that wildness has been partially lost in
the young of some domesticated animals. Darwin states
that "hardly any animal is more difficult to tame than the
young of the wild rabbit; scarcely any animal is tamer than
the young of the domestic rabbit ^'; and also that the young
of the tame duck are more tame than those of the wild duck
— a statement which is confirmed by the observations of
Dr. Rae. The evidence that the diminution of wildness is
due to the disuse of the instinct is not, however, sufficient.
The differences may have been due, in part at least, to dif-
ferences among the wild ancestry of the species, or they may
have arisen through selection, perhaps unconsciously,during
domestication. The wilder individuals would be more apt
to escape or fare ill than the tamer ones, and there would
therefore be a certain tendency for selection to diminish the
wild instinct. It would require more careful study and
comparison of the instincts of the young of domesticated
and of wild species than has yet been made to furnish ade-
quate evidence for the loss of fear through disuse.

We shall not enter, at any great length, into a discussion of

124 THE EVOLUTION OF INSTINCT

the supposed influence of the transmission of acquired
characters in the evolution of instinct. The fundamental
question is, of course, a biological one, and in whatever way
it is decided the psychologist will have to shape his theories
accordingly. Opinion among biologists has been setting
rather strongly against neo-Lamarckism, and the same
tendency is evinced among many writers on animal beha-
vior, such as Lloyd Morgan, Forel, Groos, Whitman, Bald-
win, and others. The strong resemblance between habits
and instincts which has so often been commented upon,
naturally disposes one to regard the latter as in some way
derived from the former; nevertheless, it is especially in the
field of instinct that the Lamarckian theory, which at first
seems so plausible, is found upon critical examination to
reveal its inadequacy.

In the insects, where we find so many striking examples
of almost pure instinct, there are numerous highly complex
instinctive acts which are performed only once in the life-
time of the individual. The larva of the promethus moth,
for instance, when nearly ready to pupate, spins an elabo-
rate cocoon in which it passes the winter. It lines the
cocoon with a loose mass of silken threads which will lie
next to its body; the outer layer is a firm resistant coat which
is admirably adapted to keep out cold and moisture, and at
one end, with apparent foresight, there is left an opening
filled with loose silk through which the moth may push its
way when emerging from the pupal case. Still more re-
markable provision is apparently manifested in the way in
which the cocoon is attached; it is usually spun against
a leaf by which it is partly enclosed, and to guard against
falling when the leaf breaks off, a strand of web is spun
along the petiole to the twig. How could such a cocoon
spinning instinct have arisen? Was it by reflection upon

THE EVOLUTION OF INSTINCT 125

the results of experiment? To anyone who has made any-
first hand study of lepidopteran psychology the supposition
that the ingenious mechanical devices shown in the cocoon
were hit upon as the result of a series of experiments is
about as probable as that a cat would be able to comprehend
the differential calculus. Yet we read such phrases as "such
calculation," "what wonderful contrivance," applied to such
performances by some of the foremost students of animal
psychology a couple of decades ago. How absurd they all
seem now ! To be able to improve a habit there must be an
opportunity for repeating the action. CaterpOlars might
be supposed, by the mere act of once constructing a cocoon
to have adapted their organization to this operation, and
we might suppose that this modification affects the germ
cells so that the next generation spins with somewhat
greater facility. But caterpillars which constructed their
cocoons badly would have no opportunity to improve, and
their bad methods would be handed on, and confirmed
more and more in their badness.

After the moth emerges from the cocoon she soon deposits
her eggs upon a species of plant which affords suitable food
for the larvae. How did the moth come to have this instinct?
Can any one believe that the moth watched the results of
laying eggs on different kinds of plants and formed the
habit of ovipositing on those upon which the larvae happened
to thrive?

Many of the most complex instincts of insects are in re-
lation to constructing some sort of protective dwelling for
the winter and in making provision for their progeny, and
in neither of these cases is there, in most forms, room for im-
provement through profiting by failures. Protective dwel-
lings are usually made but once, and in providing for the
young there is usually no opportunity for the parent to

126 THE EVOLUTION OF INSTINCT

observe the effect of its operations, so that if it makes
mistakes it will go on doing so indefinitely. What we know of
insect psychology renders it entirely out of the question to
suppose that insects would be able to reflect upon their errors
and mend their ways had they every opportunity to observe
their deleterious effects. The offspring, instead of the par-
ents, have to take all the consequences of the mistakes, and
they if they survived would certainly not have enough wit,
if they had the desire, to make things any better for the
next generation.

As was first pointed out by Darwin, an important objec-
tion to the Lamarckian theory of instinct is afforded by the
instincts of worker bees and ants. The instincts of these
workers are among the most wonderful in the whole animal
kingdom, and they are much more varied and highly devel-
oped than in the males and fertile females. As Darwin
remarks ** peculiar habits confined to the workers or sterile
females, however long they might be followed, could not
possibly affect the males and fertile females, which alone
can have descendants. I am surprised that no one has
hitherto advanced this demonstrative case of neuter insects
against the well-known doctrine of inherited habit advanced
by Lamark."

In the hive bee the activities of the queen, after the nuptial
flight, are almost entirely confined to laying eggs; she takes
no part in the household duties of the hive or in the care of
offspring. The drone's sole function in life is to impregnate
the queen; he takes no part in the work of the community.
Gathering honey, making comb, caring for the young,
keeping the hive clean, etc., are the result of instincts in
the worker of which neither of the parents shows the least
trace.

This apparently crucial argument against Lamarckism

THE EVOLUTION OF INSTINCT 127

has been emphasized by Weismann and others, but, as
Spencer has ably shown, the case is not so easily disposed of.
According to Spencer the fact that certain structures and
instincts occur in the workers which are not found in the
fertile insects is not because the workers have acquired them
since their separation as a distinct caste, but because the
fertile insects have lost them. The worker caste is produced
by lack of sufficient nutrition. There is a stunting of growth,
a failure on the part of the reproductive organs to develop,
and in ants an atrophy of the wings and wing muscles. In
the early stages of the evolution of the social state the fertile
female possessed all the instincts for making the nest,
gathering food and caring for the young, as is now done in the
more primitive social conmiunities of bees and wasps. In so-
cial wasps, as a rule, the occupants of cells richly supplied with
food emerge as fully developed females; where the food supply
is limited the females are smaller and sterile, and with varied
amounts of food various intermediate gradations of size are
produced from the largest females to the smallest workers. The
smaller individuals which on account of the partial atrophy
of their reproductive systems take little or no part in repro-
duction busy themselves with building cells and storing them
with food, and in taking care of the young. The fertile
females live with the rest of the community and share its
labors, but in the following year they scatter and form the
nuclei of new colonies. At first the queen starts the nest,
rears and cares for the young, and performs all the tasks in-
cidental to her small household economy. When the young
emerge they cooperate in the work of the queen. As the
conmiunity grows the queen gradually \sdthdraws herself
from the labors shared by the workers and devotes herself
more and more to laying eggs.
Among bees there are numerous gradations between the

128 THE EVOLUTION OF INSTINCT

solitary forms and the highly organized social state repre-
sented by the hive bee. The various grades of social life
which these insects present have been described in a very
interesting little treatise by Buttel-Reepen on '^Die stam-
msgeschichtliche Entstehung des Bienenstaates.'^ The series
of forms which Buttel-Reepen describes indicates very
clearly that there has been a gradual specialization of
function in the queen from a condition in which she performed
all the duties of the household to one in which her functions
are confined practically to reproduction. A sort of division
of labor has come about so that the workers and the queen
together perform the labors formerly accomplished by
the queen alone. Herbert Spencer is therefore in the main
right in his contention that the fact that the neuters have
instincts not found in the queen is due to the queen^s having
lost them in specializing in the direction of increased capacity
for reproduction. It is possible to maintain, therefore, that
the transmission of acquired characters may have moulded
the instincts of the worker caste up to that point in social
evolution at which they came to be practically sterile.

The differences between the queen and worker are
occasioned by differences in food. With a diminution of
the food supply and a consequent arrest of development
of the organs of reproduction there is a suppression of the
reproductive instincts, but the fostering and household
instincts are retained. The worker, we might say, halts at
a phyletically older period. The queen with a richer food
supply develops beyond to a stage in which what might
have become worker characters are suppressed in the interest,
perhaps of greater reproductive efficiency. The condition
might be compared to what is found in certain parasitic
isopods in which the adult female after having passed through
active and highly organized stages of metamorphosis comes

THE EVOLUTION OF INSTINCT 129

to degenerate into an almost shapeless mass devoted entirely
to feeding and the production of eggs. If now the develop-
ment of some of the females could be checked, by insuffi-
cient nutrition at an earlier period of metamorphosis we
should have a caste of sterile forms more highly organized in
structure and of more varied instincts than the fertile females.
It would certainly be an error to conclude that the superiority
of the sterile individuals was due to their having independ-
ently acquired, since the differentiation of the caste, charac-
ters which were not a part of the legacy of their parents.
This is, I think, a fair statement of the neo-Lamarckian
side of the question. Granting that the theory is otherwise
acceptable the Lamarckian doctrine is still open to serious ob-
jections when applied to the instincts of neuter insects.
It would compel us to assume that all the adaptive struc-
tures and instincts of the worker bee were but the survival of
a more primitive period of social evolution before the complete
separation of the fertile and sterile castes, when presumably
they might have been evolved through the inherited effects
of habit. And not only should we have to assume that there
had been no improvement in the non-reproductive activities
since the queen had ceased performing them, but the theory
would lead us to expect a certain amount of deterioration
through disuse. It certainly cannot be supposed that
during a period long enough for the queen to lose her wax
glands, pollen basket, and all her instincts for gathering
honey, making comb, and caring for the young, the structures
and instincts of the workers should have suffered no degenera-
tion. Instead of this there has undoubtedly been a decided
improvement. Nowhere in social bees do we find so high a
degree of structural adaptation, such varied activity, such
perfect workmanship in the construction of comb with its
varied cells for queen, drone and worker, and such complex

130 THE EVOLUTION OF INSTINCT

social life as among the workers of the hive bee. If so con-
siderable a portion of this remarkable evolution has occurred
since the separation of castes excluded the Lamarckian factor
from playing any helpful part, we might conclude that the
Lamarckian theory is not necessary in order to account for
the earlier stages of social progress.

In the hive bee we not only have the instincts of the more
primitive forms carried to a high degree of perfection and
specialization, but we have new instincts which must have
arisen since the separation of the worker caste, since they
have a direct relation to this very separation. Such, for
instance, are shown in the remarkable behavior of bees
when deprived of their queen. When the queen is removed
from the hive and there is no other soon to hatch, the workers
destroy a number of old cells and construct a new queen
cell, and then proceed to feed the young grub that is en-
closed on liberal quantities of royal jelly by means of which
it is caused to develop into a new queen. In this case of
the instincts which lead the bees to regulate the supply
of queens the theory of inherited effects of experience,
as Spencer has advocated it, cannot apply. The instinct
cannot have antedated the separation of the castes because
it is based upon the existence of caste differences.

Other difficulties are presented by the differentiations
within the neuter class among ants and termites where there
occur two or more kinds of sterile insects adapted by struc-
ture and instinctive endowment for different functions in
the community. In many cases the evidence clearly
points to the evolution of new adaptive characters in the
worker caste, instead of merely the degeneration of the
fertile insects. And where this has occurred the Lamarckian
theory fails to explain the facts.

In the chapter on Instinct in the Origin of Species Darwin

THE EVOLUTION OF INSTINCT 131

attempted to show that the main factor to which instincts
owe their origin is natural selection. The necessity for an
appeal to a previous intelligence was swept away, and while
Darwin did not deny that the inheritance of acquired char-
acters played a part in the development of instinct, he
ascribed to this factor a very subordinate rdle, and, as we
have seen, pointed out some very serious objections which
beset the theory.

Darwin's theory assumes that instincts vary. Animals
which are endowed with congenital variations of instinct
which are advantageous to them will, other things equal,
survive; those which have injurious variations will tend to
perish. Fortunate variations of behavior may thus be
accumulated along useful lines and build up complex and
highly adaptive modes of behavior. That, as the theory
requires, instincts, like corporeal structm-es, are subject to
congenital variations we have abundant evidence. We should
of course expect that variations in instinct would follow
variations in structure, but we should scarcely expect from
this standpoint to find instincts so variable as they are.
Apparently a slight structural variation may produce a
variation in instinctive behavior that seems out of all pro-
portion to the cause.

Some of the most careful investigations of variations of
instinct have been made by the Peckhams in their classical
studies on the instincts of solitary wasps. Concerning
Anmiophila which stores its nest with caterpillars which
it paralyzes by stinging them in the ventral ganglia, the
Peckhams remark: "In the three captures that came
under our observation, all the caterpillars being of the same
species and almost exactly of the same size, three different
methods were employed. In the first seven stings were
given at the extremities, the middle segments being left

132 THE EVOLUTION OF INSTINCT

untouched, and no malaxation was practised. In the
second seven stings again, but given in the anterior and
middle segments, followed by slight malaxation. In the
third only one sting was given, but the malaxation was
prolonged and severe/' The severity of the stinging as
indicated by the conditions of the caterpillars stored in the
nest also varies. Of fifteen caterpillars which were stung
by Ammophila umaria the Peckhams found that '' some of
them lived only three days, others a little longer, while
still others showed signs of life at the end of two weeks.''
And in summing up their observations on the stinging in-
stinct these writers state that "out of forty-five species of
our solitary wasps about one-third kill their prey outright.
Of those that remain there is not a single species in which
the sting is given with invariable accuracy. To judge from
results they scarcely sting twice alike, since the victims of
the same wasp may be killed at once or may live from one
day to six weeks, or perhaps ultimately recover. Even the
caterpillars of Ammophila, the most distinguished surgeon,
live anywhere from two to forty days."

Contrary to the conclusions of Fabre, who contended that
the instincts of Ammophila are practically undeviating, the
Peckhams remark that "the one preeminent, unmistakable
and ever present fact is variability. Variability in every
particular — ^in the shape of the nest and the manner of dig-
ging it, in the condition of the nest (whether closed or open)
when left temporarily, in the method of stinging the prey,
in the degree of malaxation, in the manner of carrying the
victim, in the way of closing the nest, and last, and most
important of all, in the condition produced in the victims by
stinging."

Among certain species of ants, Polyergus rufescens and
several species of Formica and Lasius, whose larvse ordinarily

THE EVOLUTION OF INSTINCT 133

spin a cocoon, there are frequently individuals which fail to
spin, and apparently they suffer no injury from this lack of
covering. It is said, by many writers, that there is great
variation in the pugnacity of ants, some individuals attacking
all sorts of enemies with the greatest ferocity, while others
are so cowardly that they flee on the least intimation of
danger. In honey bees some forms have the instinct of
making drone cells greatly exaggerated and there are great
variations in the pugnacity of different stocks.

Careful studies which have been made of the behavior of
the crayfish, the earthworm, Hydra and various protozoa
have revealed a surprising degree of individual variability.
In the field of tropisms striking variations are frequent
among animals subjected to the same external conditions.
Passing to higher organisms we find numerous records of
the variation of instinct, especially in birds and mammals.
Variations in the nest building of birds are common, and
numerous instances have been compiled by Darwin in his
posthumous essay on instinct. Beclistein states that in the
nightingale some individuals show an inherited tendency
to sing in the daytime instead of at night. There are many
instances in which birds have apparently lost the instinct
of migration. Certain breeds of domestic fowl have lost the
instinct to incubate their eggs and among other breeds this
instinct is notoriously variable.

Trainers of animals frequently remark upon the striking
differences which are presented not only in aptitude for learn-
ing but in the habits and disposition of different individuals.
Yerkes finds in the dancing mouse marked differences in
sensitiveness to visual, auditory, tactual and olfactory
stimuli in individuals of the same age and sex; striking differ-
ences also occur in their general habits and in docility.
The peculiar dancing proclivity of this variety is of uncertain

134 THE EVOLUTION OF INSTINCT

origin, but it is a suggestive fact that the same trait in vary-
ing degrees occasionally crops out in other varieties of mice,
and Haake has described a similar curious variation in a
species of shrew. Hereditary peculiarities of movement
have been described many times both in man and in animals.
Darwin quotes an account communicated by the Rev. W.
Darwin Fox of a terrier which when begging moved her
paws in a very peculiar manner very different from that of
other dogs; "her puppy, which never could have seen her
mother beg, now when full grown performs the same peculiar
movement exactly in the same way. Another peculiar he-
reditary variation is reported in a letter by Dr. Huggins to
Darwin, of an English mastiff which when first taken out,
at the age of six weeks, from the house where he was born
started back in alarm at the first butcher shop he had seen.
Later when the endeavor was made to get him past the
butcher shop he threw himself down and could not be induced
by coaxing or threats to pass the shop. On enquiry it
was found that the same peculiai- antipathy was possessed by
the father of the dog, by the grandfather and by two others
of the latter's descendants

Illustrations of variations in instinctive behavior might
be multiplied almost indefinitely, but what has been said will
perhaps suffice to give some indication of the prevalence
of such variation throughout the animal kingdom. So far
as can be determined, variations of instinct have little regard
to utility; they may be of service to their possessors, or,
like Huggins' case of the inborn aversion of a dog to butcher
shops, of no particular value, or positively injurious as in
the occasional deterioration of the instinct of incubation.
As instincts are no less important than corporeal structures
in the struggle for existence, we can readily conceive how
useful variations may be accumulated by natural selection,

THE EVOLUTION OF INSTINCT 135

and in the case of many instincts it is not difficult to picture
to one's self how natural selection may have brought them to
their present state of perfection. To find, as we can in many
cases, instincts in various grades of development in alhed
species of animals, while it shows the general course of their
evolution, tells us little of their method of evolution. That
instincts are variable, that they are often imperfect, that their
general course of development has been along adaptive lines,
and that, as Darwin emphasized, they are always primarily
of value to the species possessing them and only incidentally
of service to others, are facts indicative of the potency
of natural selection in the evolution of instinctive be-
havior.

Evidence for the potency of selection is furnished by the
study of the striking modifications of instinct which have
taken place in animals under domestication. While only
rarely has the attempt been made to modify instincts along
particular lines by selection, yet they have doubtless been
modified as the incidental result of selection on other lines.
A sort of unconscious selection has probably played a part
in modifying especially the emotional characteristics of
animals. Ugly and vicious dispositions in dogs, for instance,
would tend to be eliminated, and the qualities of affection,
fidelity and other traits which commend the animals to the
good graces of their keepers would be fostered. The
useful instincts of the pointer and the setter, however they
may have made their beginning, have certainly been devel-
oped to a considerable degree by continued selection. The
curious instincts of tumbling and pouting which, as Wliitman
has shown, have their basis in traits of behavior found in
pigeons in general, have been developed by the efforts of
fanciers to an almost monstrous degree. The Indian sub-
breed of tumblers which has been bred for at least two-hundred

136 THE EVOLUTION OF INSTINCT

and fifty years tumbles even when on the ground and con-
tinues to do so until taken up.

Whether or not other factors have piayed a part in the
evolution of behavior we shall not here discuss. Certainly
their claims cannot be said, at present, to rest upon a very
satisfactory basis. If it may savor of dogmatism to contend
for the all-sufficiency of natural selection, it is but an ex-
hibition of folly to reject as of little worth the only hypothe-
sis by which we can account for much of the evolution along
adaptive lines which has taken place.

If we reject tho Lamarckian theory it is still possible to
conceive how the activities of organisms may after all have
a guiding influence upon the course of evolution. It is a
somewhat striking coincidence that three writers, J. Mark
Baldwin, H. F. Osborn, and C. Lloyd Morgan, put forward
independently, and at nearly the same time, a theory to
explain how this guiding influence might take place with-
out having recourse to the Lamarckian factor. There is
considerable evidence from fossil forms, the structural
adaptations of living organisms, and the interrelations of
structure and behavior, which indicates that evolution has
proceeded along lines corresponding to the modifications
produced in the individual by its own activities. There can
be no doubt that the adaptations acquired through these
activities have frequently enabled the organism to survive
in the struggle for existence. This power of individual ac-
commodation, like other characteristics of the organism, is
subject to a certain degree of congenital variability. It fol-
lows that those congenital variations which enable the organ-
ism to. acquire adaptive modifications with greater readiness
will be preserved, and consequently variations in the direc-
tion of these acquired modifications will accumulate. Were
it not for the adaptations acquired through the organism's ac-

THE EVOLUTION OF INSTINCT 137

tivity congenital variations in certain directions might never
have risen to the point at which they would be of service.
Unless supplemented by acquired modifications they may
never have attained a selective value. We might say then
that it is the activities of the organism which to a consider-
able degree determine the selective value of its congenital
variations. As Baldwin remarks, "Congenital variations,
on the one hand, are kept ahve and made effective by their
use for adjustments in the life of the individual; and, on the
other hand, adaptations become congenital by further pro-
gress and refinement of variation in the same lines of function
as those which their acquisition by the individual called
into play. But there is no need in either case to assume
the Lamarckian factor. In cases of conscious adaptation,
we reach a point of view which gives to organic evolution a
sort of intelligent du'ection after all; for of all the variations
tending in the direction of an adaptation, but inadequate
to its complete performance, only those will be supplemented
and kept alive which the intelligence ratifies and uses.''

It is in the evolution of instinct, if anywhere, that the factor
of organic selection would appear to be especially potent.
The practical outcome of its operation is much the same
as if the effects of experience were actually inherited, and we
are thus enabled to explain, in terms of selection, phenomena
which at first appear to furnish strong evidence for the
transmission of acquired characters.

While we may recognize the value of the theory of orgamc
selection, it is not clear that we should regard it as a new
factor in evolution. It cannot, in strictness, be regarded as
a compromise between Neo-Darwinism and Neo-Lamarckism.
Rather it is one of the ways in which natiu*al selection may
be conceived to act. It consists of the selection of those
congenital variations which facilitate the acquirement of

138 ' THE EVOLUTION OF INSTINCT

adaptations. It is still natural selection; but the theory-
explains how, through natural selection, acquired modifica-
tions may determine the direction of evolution.

BIBLIOGRAPHY

Baldwin, J. M. Development and Evolution. N. Y., '02.

Ball, W. P. Neuter Insects and Lamarkism, Nat. Sci., 4, 91, '94.

Cunningham, J. T. Neuter Insects and Darwinism, Nat. Sci., 4,
281, '94.

Darwin, C. Origin of Species, '59.

EiMER, 0. E. Organic Evolution, '90.

Lewes, G. H. Problems of Life and Mind, London, '74-'80.

Morgan, C. L. Animal Life and Intelligence, '91. Habit and
Instinct, London, '96. Animal Behaviour, London, 'GO.

Romanes, G. J. Mental Evolution in Animals, London, '85.

Spencer, H. Principles of Psychology, 1855. A rejoinder to
Professor Weismann, Contemporary Rev., 64, 893, '93.

Weismann, a. The All-sufficiency of Natural Selection. Con-
temporary Rev., '64, 309 and 596, '93. The Evolution Theory,

2 vols., '04.

Whitman, C. O. Animal Behavior, Wood's Hole Biological

Lectures for 1898, '99.

Wundt, W. Lectures on Human and Animal Psychology, '94.

CHAPTER VII

THE NON-INTELLIGENT MODIFICATIONS OF
BEHAVIOR

" Both elements, automatism and plasticity, are found in different
proportions with all animals from the highest to the lowest." —
Wasmann, Psychology of Ants and of Higher Animals.

While there is in all organisms a certain measure of useless
if not positively injurious activity, the organic mechanism
is a self regulating one, and meets varied conditions of
life with appropriate changes of response. These adaptive
changes in relation to different circumstances are found even
in the lowest organisms. With the evolution of life they be-
come more varied and more specialized and contribute to the
development of intelligence which may be regarded as but
one species of the comparatively large genus of adaptive
variation of behavior. In accordance with a common
usage the term intelligence is here restricted to those forms
of behavior based upon the formation of associations.
As Spencer has shown, intelligence is a part of the general
process of adjustment which makes up the behavior of an
animal. To a certain extent behavior is stereotyped and
to a certain extent it is plastic. Both kinds of behavior are
necessary in varying degrees in the life of all animals. In
this chapter we shall consider some of the plastic features
of behavior in which the element of association is not involved.

ACTION OF COMBINED STIMULI

Reaction to any given stimulus is often interfered with by
a tendency to react to other stimuli received at the same

139

140 MODIFICATIONS OF BEHAVIOR

time. An electric current which would cause a Paramoecium
if swimming freely in the water to turn to the cathode, will
usually produce no movement in a specimen in contact
with a solid. The reactions of Paramoecium to chemical,
thermal and mechanical stimuli are also greatly influenced
by the same factor. Earthworms, as Darwin has observed,
usually fail to react to light when mating or feeding.
A striking illustration of the influence of the substratum
in reactions to light is afforded by the leech Branchellion.
When in a dish of water the leech reacts to a passing shadow
by raising its anterior end and swaying it about. If the
leech is in contact with its host, the torpedo, it pays not the
slightest heed to passing shadows. In Ranatra the positive
response to light is checked when the animal is feeding or
cleaning itself and is quickly resumed when these operations
are completed. And in fiddler crabs the positive reaction
to light may be overcome by fear of an approaching object,
although with longer exposure to light the phototactic re-
sponse becomes the more potent one. Contact in many lower
animals inhibits, as we have seen in a previous chapter, re-
actions to light and many other stimuli, and in higher forms
it may profoundly modify behavior in relation to enemies.
To a considerable degree these changes of behavior are the
results of the simple interference of reactions; but stimuli
may act in such a way that they produce a marked physio-
logical change in the organism, as when a contact stimulus
brings about the death feint, and the lack of responsiveness
is due then to the induced condition rather than to an an-
tagonistic movement.

DIMINUTION OF REACTION TO REPEATED STIMULATION

Modifications of reaction due to the simultaneous reception
of other stimuli are closely affiliated with modifications due

DIMINUTION OF REACTION 141

to previous stimuli. A single stimulus when repeated with
sufficient frequency sooner or later brings about a change
of response. The most usual modification is a gradual
cessation of the reaction. Light mechanical stimulation of
the anterior end of Loxophyllum causes at first a ready
response; several repetitions in close succession diminish
the responsiveness so that a much stronger stimulus often
fails to produce any noticeable effect. In a few seconds,
however, recovery is apparently complete and the animal
is as responsive as before. In Stentor, as we have seen,
after a few light contact stimuli no further contraction takes
place. Hydra, according to Wagner, if subjected to a
slight mechanical stimulus such as is caused by tapping on
the object on which it rests, usually contracts completely.
"As the tapping continues this state of contraction is
maintained for several seconds, sometimes even from one-
half to one minute; but sooner or later, in spite of continuous
stimulation, the Hydra slowly expands. When it has reached
its normal state of expansion it remains in that position
as long as the stimulus is not increased, and even when it
is slightly increased. ... If the interval between the
stimuli is considerably increased so as to allow the Hydra
to expand fully after each contraction, the tap being given
the moment expansion ceases, the result is a different one.
There is in this case no change in the reaction after repeated
stimulation. . . . Recovery from the acclimatizing effect
must, therefore, be very rapid. *'

Walter found that a planarian, if lightly jarred during
its gliding movements, would halt momentarily and then
continue its course. If the stimulus was repeated at inter-
vals of a second the worm would halt with less and less
certainty and finally glide along undisturbed. The effect
of the stimulation was very evanescent for after a minute

142 MODIFICATIONS OF BEHAVIOR

the behavior of the worm was the same as before. The
writer has found that mosquito larvae which descend quickly
from the surface of the water when a shadow passes over
them, cease to react after a number of trials but become
as responsive as ever after a short rest.

Many forms which react by contraction to a sudden increase
or decrease of illumination show similar modifications. The
leech Clepsine, when a shadow is cast upon it, raises and
elongates the anterior end of the body, but after a number of
such reactions shadows produce no result. Mrs. Yerkes in
experimenting on a- tube dwelling annelid Hydroides found
that shadows which at first caused the worm to retract into
its tube, would, if repeated at brief intervals, produce no
response. With longer intervals the response was more
regular. Hargitt in studying the same form arranged a
pendulum so that it would throw a shadow on the worm
at regular intervals and found that "with the full second
movement there was more or less constant reaction with
each passing shadow. With the half second movement it
was found that, after the first few beats, a considerable
portion of the worms failed to respond at all, and with
the quarter second beats almost all the colonies became
indifferent to the presence of the passing shadows." A
condition which Hargitt considers akin to fatigue is finally
produced, in which the organism becomes comparatively
irresponsive to light. Walter in his studies on the reactions
of planarians to light finds that " when worms were placed
in a field of non-directive light, parts of which were of two
different intensities, the number of wigwag responses
made at the critical line separating the two intensities
grew less after the animals had repeatedly crossed the line.
At first the new condition of sharply contrasted light in-
tensities in the worm's field of locomotion called out a large

DIMINUTION OF REACTION 143

percentage of wigwag responses. Later, however, by
repeated experiences, the worm became familiar with this
feature of its environment and made fewer wigwag motions,''
the percentage in a specimen of Planaria gonocephala be-
coming gradually reduced from 84 per cent, in the first
twenty-five crossings to 32 per cent, in the fourth twenty-
five.

There are many bivalve molluscs w^hich retract their
extended siphons or close their shells upon being stimulated
by shadows, or in some cases when subjected to increased
illumination, but after one or more repetitions of the stimulus,
the number varying greatly in different species, the response
is no longer made. Gradual cessation of response to a given
stimulus may be due to (1) a fatigue of the motor apparatus,
(2) to a dulling of the sensibility of receptors, or (3) perhaps
to other states of the organism. With the first of these is
not improbably to be classed the gradual diminution of the
duration of death feint, which maaiy animals show in succes-
sive feints. The death feigning attitude is one in which
the muscles are usually rigid and the condition naturally
involves more or less fatigue. As the feints decrease in
length the body also becomes more relaxed and the attitude
of the body less constant and characteristic. In lower
forms the death feint but slowly wears itself out. In higher
ones feigning is carried out only once or a very few times be-
fore the animal refuses to feign longer. Hence in many cases
the factor of intelligence may come into play, but whether
this will explain all cases where the response is given up sud-
denly appears doubtful.

A considerable part of the cases of cessation of reaction to a
given stimulus are probably due, as Bohn contends, to a
''fatigue sensorielle'' or dulling of sensibility. This sup-
position is supported by the fact that the central apparatus

144 MODIFICATIONS OF BEHAVIOR

is much more susceptible to the effects of fatigue than the
afferent or efferent nerves. It is not a fatal objection to the
theory of fatigue that the response falls off very quickly.
An extreme sensitiveness may result from a certain con-
dition of balance which a slight chemical change might
overthrow without rendering the organism insensitive to
stronger stimuli. A very high degree of sensibility is as a
rule very easily affected. In ourselves the ability to detect
a faint odor or taste is exhausted by a very few trials
and in lower organisms where sensitivity is often exceed-
ingly acute, there are, as we might expect, much greater
fluctuations. It is not improbable that in many cases
something analogous to fatigue may take place in the
central apparatus in the pathway between the afferent and
the efferent impulses. The animal might thus, without hav-
ing either its receptors or its motor apparatus appreciably
affected, fail to respond in the same way, if at all, to stimuli
which at first brought about a reaction.

At times the response to a given stimulus may be increased
instead of diminished with repeated application. Statke-
witsch found that Paramoecia would often fail to respond to
weak induction shocks, but where several were given the re-
sults were cumulative and the animals swam toward the
cathode. Planaria after a period of rest are apparently in a
condition of lowered tonus and fail to react in the usual
manner to stimuli but if the stimuli are repeated the re-
actions appear and for a time may increase in vigor. An
analogous phenomenon is presented in the responses to light
of Ranatra and fiddler crabs which become more and more
energetic with longer exposure to the stimulus. An inter-
esting instance is afforded in the reaction of the tentacle
of Cerianthus to repeated tactile stimuli. Bohn found
that the tentacles when stimulated became flexed toward

DIFFERENT KINDS OF RESPONSE 145

the mouth. Early in the morning repeated stimulation
causes the tentacles to respond with increased energy, later,
after a short period of increase of responsiveness, the ten-
tacles react less vigorously, and toward evening they be-
come insensible after a few stimulations.

Very similar phenomena w^ere found to occur in the annelid
Serpula when stimulated by shadows. The responses were
variable; some individuals would show an increase of re-
sponsiveness followed by a decrease, while others would show
a diminution from the beginning. Bohn has attempted to
subsume the variations of responsiveness to a given stimulus
under one general *'law" resting on a physico-chemical
basis. All stimulation according to him is followed at first
by an increase and then by a decrease of reaction. In some
cases the initial increase is so slight as not to attract atten-
tion and we apparently get a falling off from the start. In
others the continued increase of responsiveness has not
been followed long enough to discover the diminution which
must come sooner or later. We may grant the latter part
of Bohn's contention, but the first is much more difficult to
substantiate.

DIFFERENT KINDS OF RESPONSE TO A GIVEN STIMULUS

Qualitative variations in the reactions to repeated stimula-
tions are common. We have seen that they occur in an
organism as low in the scale as Stentor. Hydra, according
to Wagner, when repeatedly stimulated by a capillary glass
rod at sufficiently long intervals will, if it does not in time
ignore the stimuli entirely, come to respond in a new way;
it bends to one side until the tentacles touch the bottom,
then loosens the foot and attaches itself in another locality.
Striking changes in the response to a given stimulus are
furnished by the reactions of sea anemones. The anemone

10

146 MODIFICATIONS OP BEHAVIOR

Aiptasia when a drop of water falls on its disk suddenly
contracts. To several subsequent drops there is no further
response. Then if the drops continue to fall the animal
contracts still further and draws in its disk. If stimulated
lightly by a rod the anemone contracts strongly. If, when
it subsequently extends, it is again stimulated it responds in
the same way, and continues to do so during a number of
trials. Finally the anemone as it extends bends over in a
new direction, and if the stimulus persists this reaction is
repeated several times; then another direction of extension
is tried and finally if the stimulus is not avoided the animal
releases its foothold and crawls into a new locality.

Anemones have various methods of getting rid of foreign
bodies on the disk, or even morsels of food, if much food has
already been taken. The procedure in Stoichactis as
described by Jennings is as follows: ''The tentacles bearing
the debris or the rejected food body collapse, becoming thin
and slender, and lying flat against the disk. At the same time
the disk surface in this region begins to stretch, separating
the collapsed tentacles widely. A^ a result the waste mass is
left on a smooth, exposed surface, the tentacles here having
practically disappeared — though under usual conditions they
form a close investment almost completely hiding the surface
of the disk. Thus the waste mass is fully exposed to the
action of waves or currents, and the slightest disturbance
in the water washes it off. Under natural conditions this
must usually result in an immediate removal of the debris.
If this does not occur at once, often the region on which
the debris is resting begins to swell, and becomes a strongly
convex, smooth elevation, thus rendeiing the washing away
of the mass still easier.

" But the process may go much farther. If the debris is
not removed in the way just described, new reactions set in.

DIFFERENT KINDS OF RESPONSE 147

If the mass is nearer one edge of the disk this edge usually
begins to sink, while at the same time the tentacles between
the edge and the waste object collapse and practically
efface themselves. Thus a smooth, sloping surface is pro-
duced and the waste mass slides -off the disk. If this does
not occur at once, after a little time the region l3dng behind
the mass (between it and the center of the disk) begins to
swell, producing a high, rounded elevation, with tentacles
plump and swollen. The waste mass is now on a steep
slope, and is bound soon to slide down and over the edge.
Sometimes by a continuation of this process the entire disk
comes to take a strongly inclined position, with the side
bearing the debris below. Often one portion of the edge of
the disk after another is lowered in this way, till all the w^aste
matter has been removed. The disk then resumes its
horizontal position, with nearly flat or slightly concave
surface." If the edge near which the foreign body is placed
cannot be lowered the part external to the body may be
raised while the surface toward the opposite edge is depressed
so that the object may be rolled off the disk in another
direction.

In Planaria, according to Pearl, repeated mechanical
stimulation of the anterior end causes the worm to turn
farther away at each succeeding stimulus, without at first
causing any movements of locomotion. After a time the
worm jerks back vigorously, bends the body strongly to one
side and then extends usually toward the source of stimulus.
Stronger stimuli soon alter the general physiological condi-
tion of the organism; "the animal becomes 'stirred up'
generally, moves about with increased rapidity, its sensitive-
ness to stimuli becomes diminished, and it will give only the
negative response to stimulation of the anterior end. . . .
One may get totally different appearances from an individual

148 MODIFICATIONS OF BEHAVIOR

which has been 'stirred up' from what are seen in the case
of one which is in the normal condition/'

In the earthworm repeated stimuU give rise to a great
variety of reactions which have been classified by Jennings
as follows:

'' (a) The state of rest, in which the worm does not
react readily to slight stimuli, such as a touch with the tip
of a glass rod.

''(b) A state of moderate activity, in which a touch at the
anterior end causes movement backward; at the posterior
end movement forward, while lateral stimuli (in the anterior
region) cause turning away from the side stimulated.

"(c) A state of excitement, after repeated stimuli, in
which the animal persists in the direction of movement once
begun, merely stopping for a few seconds when stimulated
at the end which is advancing.

"(d) A state of greater excitement, in which stimuli merely
cause the animal to hasten its movements in the direction in
which it has started, without regard to the localization of
the stimulus.

"(e) A state of still greater excitement, after long-con-
tinued and intense stimulation. Now the worm responds
to a stimulus at the anterior end, that would in a resting
worm cause only a comparatively slight reaction, by a rapid
'right-about-face.' The body is suddenly doubled at its
middle, so that the anterior and posterior halves become
parallel, with the two ends pointing in the same direction,
then the posterior half is quickly whipped about, so that
the whole worm is again straight, but is facing the opposite
direction from that in which it was pointed before the
reaction. . . .

" (f) A state of still more intense excitement, after repeated
strong stimulation that is of such a character as to actually

DIFFERENT KINDS OF RESPONSE 149

injure the tissues. The worm now responds to a repetition
of the stimulus (and often when the new stimulus is only
slight) by lifting the anterior fourth of the body into a
vertical position, and waving it about in a frantic manner.
-This behavior is usually alternated with the right-about-face
reactions, and with persistent rapid crawling forward and
backward. ..." These reactions do not exhaust the
list but are simply more typical features of an almost endless
variety of modifications.

The causes of the change of response to a given stimulus
may be many. The factor of fatigue is doubtless an im-
portant one, especially in the nervous centers. An afferent
fiber may be connected with several motor pathways, so
that slight variations in the ease with which impulses may
travel along certain lines may determine the direction of
motor discharge. A very slight degree of fatigue of any
pathway might then be a cause of a change of response after
one or a very few repetitions. An increase of transmitting
function as a result of previous exercise may also play its
part, so that we may say that the factors which produce an
increase or decrease of responsiveness also effect the replace-
ment of one response by another of a different kind. In the
one case they modify the intensity of the reaction, in the
other they act so as to switch off the action to other lines.

The fluctuations of tonus here and there in the nervous and
muscular systems afford, as Uexkiill has shown, a mechanism
by which the responses of the organism may be varied to an
indefinite degree. Every act of an organism alters the dis-
tribution of tonus in its body, and the way in which the organ-
ism responds to the next stimulation is naturally affected
by this circumstance. Then variations in assimilation,
respiration, excretion and other functions are constantly inter-
fering directly or indirectly with the outcome. There is

150 MODIFICATIONS OF BEHAVIOR

therefore no occasion for wonder if the organism responds in
different ways to a given stimulus. The wonder would be
if it always responded in the same way.

INFLUENCE OF INTERNAL FACTORS ON BEHAVIOR

That behavior of organisms in general should be greatly
influenced by their internal condition is obvious. Hunger
and repletion influence the activity of animals, even in the
lowest forms. Even the white blood corpuscles when they
have ingested a number of bacteria refuse to take in any
more. Paramoecium, while it may not reject food swept
into its gullet, behaves differently when well fed, and Stentor,
as we have seen in a previous chapter, takes in objects
when it is hungry which at other times are rejected. Hy-
dras when hungry eagerly take in food, but they are quite
indifferent to it when well fed. If starved for some time
they become more active, extend and contract the tentacles
and body, and move about in various directions as if to
increase their chances of coming in contact with food. The
jelly-fish Gonionemus when hungry swims about actively,
frequently coming to the surface and settling slowly through
the water, with its tentacles extended to catch its food. After
a hearty meal the jelly-fish is more frequently at rest and has
its tentacles contracted.

The effect of hunger on the movements of sea anemones
is very striking. When starving, anemones will often take
in such things as filter-paper and stones which they reject
under ordinary conditions, as is well shown in the experiments
of Torrey on Sagartia. Weak food stimuli produced by
giving the anemones filter-paper soaked with dilute crab
juice produce at first the food reaction, but in a little while
the animal no longer responds (Nagel, Parker). If meat
is offered the food taking activities continue for a long time.

INFLUENCE OF INTERNAL FACTORS 151

but eventually become weaker and may later be replaced by
movements of rejection. That it is not the mere filling of the
digestive cavity alone that determines the change of be-
havior is showTi in an experiment of Jennings on Aiptasia,
in which the digestive cavity was filled with filter-paper.
When so filled that pieces of paper were repeatedly disgorged
the anemones continued to take in new pieces. The meta-
bolic conditions of the organism are therefore a determining
factor in its behavior toward food.

To a certain extent though, the food taking is influenced
by the previous local stimulations of particular parts of the
body. Nagel and Parker have found in Adamsia and
Metridium that the tentacles of a certain part of the body
after being stimulated several times with filter-paper soaked
in meat juice will refuse to carry it toward the mouth. If
now the soaked paper is offered to other tentacles they give
the usual food taking reaction. Similai' behavior was ob-
served by Jennings in Aiptasia, who found also that if one set
of tentacles had carried several pieces of meat to the mouth
until they refused to carry it longer, the other tentacles,
while they might still carry the meat, would cease to respond
much sooner than they would if the animal had not already
received food. The animal according to Jennings "is a
unit so far as hunger and satiety are concerned."

The effects of hunger and satiety on the behavior of higher
forms are so general and so familiar that we need not pursue
the subject further. As regards food animals in general are
self regulating mechanisms, and if under certain conditions
an injurious quantity of food is devoured, or material which
is unwholesome is selected, the exceptional behavior only
serves to emphasize the rule that the food taking of animals
is on the whole pretty adequately adjusted to their needs.
In higher forms this function is conscious and voluntary,

152 MODIFICATIONS OF BEHAVIOR

but carry the process down the scale of life and it begins to
take on the character of other organic regulations such as we
find in the tissue cells of our own bodies; for even there the
intake and assimilation of nutriment is regulated as in free
organisms. In none of the animals we have described is
there evidence of intelligence in the selection of food. The
choice made is more readily explained in terms of what
Bohn calls differential sensibility. The organism is so con-
structed that it responds to certain kinds of stimuli in one
way and to other kinds of stimuli in a different way. But it
also has the faculty of responding to the same stimulus in
different ways at different times. Here it may be assumed
that the different response is the effect of changes of internal
conditions which alter the irritability of the animals. The
stimuli set up by the presence of food in the digestive tract,
the secretory activity of the cells, the processes of absorption
and assimilation going on throughout the body afford a
complex of influences affecting the neuro-muscular mechan-
ism and naturally modifying its action.

In the sea anemones, contact tends to set in operation
one or the other of two mechanisms — one involved in taking
in food, the other in its rejection. These mechanisms are
mutually inhibitory and often there is a struggle between
them resulting in a hesitation or vacillation in the response
of the organism. Internal conditions act as a sort of brake
on one or the other mechanism and thus change the nature
of the response to a particular external stimulus.

This type of behavior which has often been assumed to
indicate consciousness if not intelligence is not anything
which cannot be accounted for on the basis of reflex action.
It is a type of behavior which is wide spread, in fact probably
coextensive with animal life. If it is a criterion of intelli-
gence we must assume that all animals are intelligent and

HABITS 153

not only that, but that the cells of our bodies are intelligent
likewise.

The food taking activities of animals are admirably adapted
to illustrate the essential unity of behavior throughout the
animal kingdom. These activities have their roots in the
fundamental processes of organic life. Intelligent food
getting is based on fundamental instinctive tendencies,
in fact is but a way in which these tendencies are shaped
in order a little more effectually to reach the goal. The
still hunt of the hungry lion, the expanded tentacles of the
hungry anemone or jelly-fish, the restless activity of the
starving protozoan are all expressions of a common state,
and are dictated by a common need.

HABITS

Even very primitive animals may acquire ways of acting
or responding to stimuH through the effects of their previous
experience. These habits, if we may call them such, are
usually not very permanent and tend to wear away after
the determining cause is removed. Jennings has observed
that specimens of the anemone Aiptasia annulata which
live in crevices among the rocks where in expanding they
have to bend their bodies in an irregular way stiU retain their
irregular movements in expanding after they have been
removed from their original habitats. This led him to make
some observations on normal anemones subjected to re-
peated stimulations. In one case "an individual attached
to a plane horizontal glass surface was bent in extension
far over to the left. Stimulating it repeatedly, it con-
tracted at each stimulation, then bent, in extending, again
to the left. This continued for fifteen stimulations, one
succeeding another as soon as the animal had become fully
extended. At the next contraction the animal turned and

154 MODIFICATIONS OF BEHAVIOR

bent over to the right. Now when stimulated it contracted
as before, then bent regularly, in extending, over to the
right. It seemed to have acquired a new habit — bending
to the right instead of to the left.'^ Observation showed
that the bend of the body persisted in the contracted as well
as the extended state, and that all parts extended propor-
tionately and thus led to a repetition of the previous action.
The habit of action in this case had a persistent structural
basis in the bend of the body. If now the anemone was
caused to contract very strongly in all parts so that it was
no longer bent to one side in the contracted state it would
lose its habit of bending when it subsequently expanded.
The role of the nervous system in this case is a very doubtful
one and it is not improbable that the so-called habit is
merely a result of purely mechanical factors. The same
may also be true of the habit of irregular bendings acquired
as a result of living in crevices between the rocks, which
might be compared to the difficulty experienced by a person
whose body and limbs have been confined for a long time
in any one position in making new movements.

Jennings has performed numerous experiments on the
formation of righting habits in the starfish Asterias forrei.
Starfish when placed on their backs have several methods
of turning over. Different individuals have their personal
peculiarities in this as in other respects, and in most speci-
mens there is a tendency to use one particular ray or set of
rays upon which to turn. These differences as Moore
has pointed out may be due to inequalities in the size of
^'Vy the arms, injuries to certain arms, or any initial twist an
arm may have had due to its previous position. Before
any attempt to train a starfish was made the animal was
put through a set of tests to determine the rays most com-
monly employed in the righting reaction. After this the

RHYTHMS IN BEHAVIOR 155

starfish was prevented from using these rays and was al-
lowed to turn only on those rays which it was least prone
to employ. If, after a number of trials, the starfish came
to employ the latter most frequently it would be fair to
conclude that the animal had acquired a new habit of turn-
ing. After one or two days' training most of the individ-
uals experimented on came to turn upon rays which they
were at first disinclined to use. The effect of the training
was not manifest, however, after twenty-four hours. In
specimens "trained" for a week or more the habit formed
persists, according to Jennings, for twenty-four hours or
even several days. The data obtained on this subject
were not extensive, and when we consider several sources
of error involved, not entirely convincing; further work
would be necessary before we could safely conclude that
starfish form lasting habits.

How shall habit formation in the starfish be interpreted?
Moore has sho\sTi that irritation of an arm inhibits its ac-
tivity and causes the starfish to use other arms in the right-
ing movements; and the operation of handling the animal
during its "lessons" so as to prevent a certain arm from
being used might produce a similar effect. There is no evi-
dence that there is any element of association involved, and
it is not quite clear whether the basis of the habit lies in
the nervous system or in other parts of the organization.

RHYTHMS IN BEHAVIOR

Georges Bohn has the merit of having discovered that
the rhythms of activity which are produced in many animals
by regularly recurring external conditions such as the alter-
nation of day and night and the periods of low and high tide
may persist for some time after the animals are withdrawn
from the direct influence of the outer periodic changes.

156 MODIFICATIONS OF BEHAVIOR

Convoluta roscoffensis, a small green turbellarian worm
which lives in sandy beeches overflowed by the tide, makes
periodical depth migrations in the sand. At low tide the
worms come to the surface where they form a green coat
upon the shore. When the tide comes in the worms de-
scend and thus avoid the shock of the waves. Bohn found
that the same periodic migrations occurred in specimens
which were taken from the shore and kept in an aquarium.
If the worms were placed in a tube of sand a green ring could
be seen to rise and descend synchronously with the ebb
and rise of the tide. These rhythms persisted for several
days after the worms were removed from the beach. Keeble
has compared the vertical movements of Convolutas kept
in the laboratory with the movements of specimens on the
beach, and states that ^'for eight successive tides the
animals in the laboratory maintain their rhythm, synchron-
ous with the ebb and flow of the waters over the roscoffensis
zone: then, though the rhythmic movement up and down
may continue, its temporal periodicity loses precision, and,
finally, the rhythm is worn down."

A parallel phenomenon was discovered by Bohn in the
diatom Pleurosigma. When the sea retired these diatoms
were observed to form a brown scum over the sand. When
the tide came in the diatoms descended. Placed in an
aquarium they performed regular migrations for several
days in accordance with the tidal rhythms. This periodic-
ity failed to manifest itself in darkness and, therefore,
according to Bohn, depends upon variations in phototaxis
instead of the response to gravity. Bohn has reported
that a curious tidal rhythm occurs in the small gastropod,
Littorina rudis, which lives upon the rocks where it is only
occasionally wet by the waves. At low tide these molluscs
withdraw into their shells and remain inactive. When

RHYTHMS IN BEHAVIOR 157

the spray from the incoming tide begins to moisten them
they emerge and begin crawling about. Their phototaxis
changes with the tide, becoming negative when the tide
is high and positive when it is low. Bohn placed Littorina
in an aquarium and found that for several days they under-
went changes in their phototactic responses parallel with
those of specimens upon the rocks. Similar experiments
have been made by Morse on a species of Littorina on the
New England coast, but he failed to obtain any evidence
of a tidal rhythm. And more recently Haseman has in-
vestigated the tidal rhythms of several species of Littorina,
none of which showed any rythmical movements independ-
ent of the direct influence of the tides.

Among actinians Bohn has found in many cases daily
rhythms due to the alternation of Hght and darkness super-
posed upon tidal rhythms. Both of these rhythms are
influenced greatly by the nature of the habitat in which the
anemones live. Individuals situated rather high upon the
rocks and living therefore under strongly contrasted con-
ditions in high and in low tide exhibit tidal rhythms to a
marked degree; whereas those which hve at depths in which
they are little affected by the waves show little or no in-
fluence of the tide. Alternation of day and night affects
anemones to a greater or less degree in all habitats; its
influence is compHcated by many factors, such as degree of
exposure to the sun, depth of water, shock of waves, tem-
perature, purity of the water, and various other causes.
The daily rhythms differ greatly in anemones from different
local situations, according to the influences to which the
animals are adapted. These rhythms with their various
characteristics peculiar to different habitats were found to
persist for several days in aquaria, but they gradually wore
away.

158 MODIFICATIONS OF BEHAVIOR

Among higher invertebrates tidal rhythms have been
observed by Drzewina in the hermit crab Clibanarius miS'
anthropus. Specimens were collected at various times and
kept in aquaria one end of which was shaded. Nearly all
of the individuals manifested a positive phototaxis when the
tide was high, and became negative when the tide was low.
These regular changes persisted in the lot which was kept
longest, for three weeks.

We have in these periodic variations of behavior habits
of action in relation to different influences of the environ-
ment which have been acquired by the experience of the
organism. These habits have probably not been acquired
tlirough intelligence in most cases and certainly not in the case
of the diatoms, and it does not seem improbable that all of
them may be dependent upon some general modifications
of the organism as a whole rather than upon merely the
mechanism of response to stimuH.

HABITS VARIOUSLY CAUSED

Habits may be due to a variety of causes. They may
result from the repeated displacements of given structures.
They may depend upon artificially induced organic rhythms.
They may arise as the outcome of intelligently formed acts.
What closely simulates a temporary habit may be the physi-
ological effect of the summation of stimuli. An action
system put in operation one or more times by a given stimu-
lus may, up to a certain point, become increasingly respon-
sive to that stimulus. There is an increase of the tonus of
the parts concerned. If in righting itself a starfish comes
to employ a particular ray, that ray as a result of its exercise
may respond with increased readiness. Whether the tem-
porary habits formed in the starfish studied by Jennings
were due in part at least to an increase of tonus as the result

DECEPTIVE APPEARANCES OF INTELLIGENCE 159

of previous activity I do not pretend to say; but the case is
cited as an illustration of a possible way in which a certain
class of habits may be interpreted.

Habit formation may be shown in the action of individual
organs of our bodies. The stomach according to the re-
searches of Pawlow readily acquires habits, and the influence
of habit in the functioning of the intestines is generally
familiar. Between such habit formation and the increase
of an organ through action there is much in common, and
between these phenomena and the adaptive changes of an
organ in relation to its condition of stimulation there is
doubtless a fundamental kinship. \Miat Roux has called
the "overcompensation of what is used" is a principle which
probably manifests itself in all these cases.

DECEPTIVE APPEARANCES OF INTELLIGEWCE

There are a great many cases which have been adduced as
indicating intelligence and even a simple form or reasoning
which may be explained like the phenomena we have de-
scribed. It is frequently difficult to distinguish acts which
are instinctive from those which an animal has learned to
perform, and it is well in general to be guided by the prin-
ciple enunciated by Lloyd Morgan, which is a sort of special
case of the law of parsimony — namely, that " In no case may
we interpret an action as the outcome of the exercise of a
higher psychical faculty, if it can be interpreted as the out-
come of one which stands lower in the psychological scale.''
One of the cases most suggestive of the power of forming
associations in the Coelenterates is recorded by Fleure and
Walton whose account is as follows:

"We have given a specimen of Actinia a scrap of filter-
paper about once in twenty-four hours, placing it on the
same tentacles each time. As a general rule the fragment was

160 MODIFICATIONS OF BEHAVIOR

carried by the tentacles to the mouth and there swallowed,
to be ejected as inedible after a longer or shorter period.
After a few days, the number varying in different individuals
from two to five days, the fragment is no longer swallowed
and, in about an other two days, the tentacles will no longer
take hold of it. This procedure is more regular in the case
of pellets of paper than in the case of India rubber, in which
results were very variable. The results seem to indicate a
certain amount of persistence of impressions, when the latter
are received several times in succession at short intervals.

"The first impression which persists is one in the mouth
region leading to refusal of the pellet. As this is strengthened
the sequence is further abbreviated, an inhibitory stimulus
would seem to proceed from the mouth to the tentacles
preventing them from gripping, when the stimulation due to
contact with the filter-paper has passed from them to the
mouth.

" A further point of interest is that what does persist seems
to remain a property of the tentacles affected, and of that
part of the mouth directly related to them. It does not
appear to be a possession of the entire animal, for other
tentacles, on the opposite side for instance, can be tricked
subsequently, at any rate once or twice, before they too
exhibit the inhibitory reaction." The effects of the ex-
perience were found to be lost after from six to ten days.

Whether or not we have in this instance the formation of
a new association, as the behavior of the anemone seems
to indicate, is not entirely certain. It is possible that the
seat of the change of behavior is in the tentacles alone.
Allabach has shown that after the tentacles of Metridium
have responded to a stimulus a few times their production
of mucus becomes much diminished and this probably
affects their subsequent activity. If this factor would

DECEPTIVE APPEARANCES OF INTELLIGENCE 161

modify the irritability of the tentacles for some time it
might explain the change of behavior. This may not be
the true explanation of the phenomenon, but it will serve
to show how careful we must be, in studying the behavior
of lower organisms, about inferring the presence of associa-
tive memory. There have been almost no studies of the
power of association in the Ccelenterates, where the various
possibilities of error have been carefully excluded.

Darwin in his work on earthworms attributes a certain
degree of intelligence to these creatures on account of their
pecuHar habits of plugging up their burrows with dead leaves.
The worms pull in the leaves of the linden by their tips, while
the leaves of the rhododendron which are smaller at the base
are pulled in by the petiole. Pine needles which frequently
occur in pairs with a common base are not seized by the
small end, which would cause difficulity in getting both
needles into the hole, but by the enlargement at the basal end.
Darwin gave the worms triangles of paper and found that
they usually seized these by the most acute angle in carrying
them to their burrows. The conclusions of Darwin that the
behavior of the earthworms indicates a certain degree of
intelligence was a very natural one. Hanel, however,
who has repeated and verified Darwin^s experiments and
performed a number of others, finds no ground for assuming
any intelligence in the earthworm and ascribes the behavior
of the animal to a series of more or less complex reflexes in
relation to the form and chemical nature of the objects
drawn in. There is no evidence of profiting by experience
in the earthworm's behavior and, however complex the acts
performed, there is nothing that is thus far known that
precludes us from considering them as belonging entirely
to the reflex type.

11

162 MODIFICATIONS OF BEHAVIOR

BIBLIOGRAPHY

BoHN, G. Sur les mouvements oscillatoires des Convoluta roscof-

fends. C. r. Ac. Sci., Paris, 137, 576, '03.

Les Convoluta roscoffensis et la theorie des causes actuelles.

Bull. Mus. Hist. Nat., 9, 352, '03.

Intervention des influences pass^es dans les mouvements actuels

d'un animal. C. r. Soc. Biol., Paris, 56, 789, '04.

Periodicity vital des animaux soumis aux oscillations du niveau

des hautes mers. C. r. Ac. Sci., Paris, 139, 610, '04.

Oscillations des animaux littoraux synchrones des mouvements

de la mar^e. Ibid.. 139, 646, '04.

Mouvements de manege en rapport avec les mouvements de la

mar^e. C. r. Soc. Biol., Paris, 57, 297, '04.

Les causes actuelles et les causes pass6es. Rev. Scientif., 3, 353

and 389, '05.

Le rhythme nycth^m^ral chez les actinies. C. r. Soc. Biol.,

Paris., 62, 473, '07.

La persistance du rhythme des marges chez I'Actinia equina.

Ibid., 61, 661, '06.

Introduction k la psychologie des animaux k symmetrie

rayonn^e.' 1, Les 6tats physiologiques des actinies. Bull. Inst.

G^n. Psych., 7, 81 and 135, '07.

De I'acquisition des habitudes chez les 6toiles de mer. C. r.

Soc. Biol., Paris, 64, 277, 532, 633, '08.

Introduction k la psychologie des animaux k symmetrie rayon-

n^e. 2, Les essais et erreurs chez les 6toiles de mer et les

ophiures. Bull. Inst. Gen. Psych., 8, 21, '08.

La naissance de I'intelligence, Paris, '09.
Darwin, C. The Formation of Vegetable Mould through the Action

of Worms, with Observations on Their Habits. N. Y., '83.
Fleure, H. J., and Walton, C, L. Notes on the Habits of Some

Sea-Anemones. Zool. Anz., 31, 212, '07.
Ghinst, van der. Quelques observations sur les actinies. Bull.

Inst. G^n. Psych. Paris, 6, 267, '06.
Glaser, 0. C. Movement and Problem Solving in Ophiura brevi-

spina. Jour. Exp. Zool., 4, 203, '07.
Hargitt, C. W. Experiments on the Behavior of Tubicolous

Annelids. Jour. Exp. Zool., 3, 295, '06.

Further Observations on the Behavior of Tubicolous Annelids.

Ibid., 7, 157, '09.

DECEPTIVE APPEARANCES OF INTELLIGENCE 163

Jennings, H. S. Contributions to the Study of the Behavior of

Lower Organisms. Carnegie Inst. Pubs., Wash., *04.

Modifiability in Behavior. 1. Behavior of Sea Anemones.

Jour. Exp. ZooL, 2, 447, '05.

The Method of Regulation in Behavior and in Other Fields,

Ibid., 2, 473, '05.

Modifiability in Behavior. 2. Factors Determining Direction

and Character of Movement in the Earthworm. Ibid., 3,

435, '06.

Behavior of Lower Organisms, N. Y., '06.

Behavior of the Starfish, Asterias forrei de Loriol. Univ. of

Cahf. Pubs. Zool. 4, 53, '07.
Kbeble, F. Plant Animals. A Study in S5rmbiosis. Cam-
bridge, '10.
Pearl, R. The Movements and Reactions of Fresh-Water Plan-

arians. Quart. Jour. Mic. Sci., 46, 509, '03.
Pbeyer, W. Ueber die Bewegungen der Seesterne. Mitth. a. d.

zool. Stat, zu Neapel, 7, 27, and 191, '86.
Uexkull, J. VON. Studien uber den Tonus. Zeit. f. Biol., 44, 269,

'03; 46, 1, '04; 46, 372, '04; 49, 307, '07; 50, 168, '08.

Umwelt und Innenwelt der Tiere, BerHn, '09.
Washburn, M. F. The Animal Mind, N. Y., '09.
Yerkes, a. W. Modifiability of Behavior in Hydroides dic^Uhus,

Jour. Comp. Neur. Psych., 16, 441, '06.

CHAPTER VIII

PLEASURE, PAIN, AND THE BEGINNINGS OF
INTELLIGENCE^

"Apprehensio sensitiva non attingit ad communem rationem
boni, sed ad aliquod bonum particulare, quod est delectabile. Et
ideo secundum appetitum sensitivum, qui est in animalibus, opera-
tiones quaeruntur propter delectionem." — Thomas Aquinas, Summa
Theologica.

Psychologists nowadays with comparatively few exceptions
agree in regarding intelligence not as a faculty standing in
sharp contrast to instinct, as was formerly taught, but as one
resting on a foundation of instinct, and gradually growing
out of behavior of the purely instinctive type. The term
intelHgence is used here in the wider sense as embracing all
those forms of profiting by experience through the formation
of associations. It therefore includes psychic activity rang-
ing from simple associative memory to complex trains of
reasoning. What distinguishes intelligence from instinct is
that in the latter the connections between acts are based
upon hereditary organization, whereas in the former they
are established through experience. The apparently new
thing involved in intelligent behavior is the power of form-
ing associations. So far as we can judge of the psychic states
of an animal from its behavior, animal intelligence in its first
manifestations consists in repeating acts which bring pleasure
and in avoiding things which cause pain, and a discussion of
the transition from instinct to intelligence naturally involves

^ This chapter is taken with some modifications from an article of the
same title contributed by the writer to the Journal of Comparative Neu-
rology and Psychology. I am indebted to the editor for permission to
reprint a considerable part of the article here.

164

BEGINNINGS OF INTELLIGENCE 165

a consideration of the role of pleasure and pain as agents of
accommodation.

The tendency of animals to repeat acts which result in
pleasure and to discontinue or inhibit acts which bring them
pain is a fundamental feature of behavior on the utility of
which it would be superfluous to comment. But why do
animals behave in this fortunate manner, and how did they
come to acquire the faculty of so behaving? To our ordinary
plain w^ay of thinking it appears sufficient to say that a dog
eats meat because he likes it, and that he runs away from the
whip to avoid its painful incidence upon his integument.
These acts are such natural and obvious things to do under
the circumstances that to inquire why the animal does what
it likes and avoids what is disagreeable may seem a sort
of philosophic quibble which only a mind "debauched by
learning" would think of indulging in. But a little con-
sideration will show that we have here a real and very
knotty problem, or rather set of problems, of the greatest
importance to the student of genetic psychology.

There are few better illustrations of the modification of
behavior through experiences of pleasure and pain than that
afforded by the behavior of young chicks, which has been
so well studied by Lloyd Morgan. A young chick when
first hatched has the instinct to peck at all sorts of objects
of about a certain size. If an object is a little too large the
chick may hesitate. Should it venture to peck at the object
and derive a pleasant taste from it the hesitation in the pres-
ence of similar objects becomes reduced and will finally
disappear. If the chick in the course of its pecking seizes
a caterpillar having a nauseous taste it is much less apt to
seize a similar caterpillar a second time. The painful or
unpleasant experience it derives in some way inhibits further
action toward that class of objects.

166 BEGINNINGS OF INTELLIGENCE

We have in this modification of instincts through the
pleasurable or painful effects they produce the beginning of
intelligence. The pecking, swallowing, and avoidance of
certain objects are purely instinctive acts based on the
chick's inherited organization. After its first experiences
with pleasant or nasty caterpillars the chick is a different
creature; it has learned by experience; and henceforth its
acts, which at first were in a general way adaptive, become
more perfectly adapted to its needs as the result of its learn-
ing. Instinct supplied the impetus to action and in a measure
determined the direction of action, but intelligence refines
upon the instinctive behavior and effects a closer adjustment
to the environment.

In lower forms associations are formed as a rule with great
slowness. Behavior is almost entirely instinctive, and the
organism can be made to deviate from its stereotyped
methods of action only with difficulty. It is probable that
in low forms where associations of only the simplest kind can
be established there is no association of ideas involved;
and in fact there is no conclusive evidence of the existence
of ideas even in animals quite high in the scale. Most animal
learning consists in forming associations between certain
sense experiences and certain actions which bring pleasure
or pain. A common way of teaching an animal a trick is to
try in various ways to induce it to perform the desired
action and then to reward it by food or some other means
of giving it pleasure. In this way the connection between
the situation and the act is reinforced, and the act follows
more readily when the animal is placed a second time under
the same conditions.

Consider the case of a cat placed in a box which can be
opened by pressing down a lever or pulling a string, as in the
experiments of Thorndike. If the cat is hungry and food

BEGINNINGS OF INTELLIGENCE 167

is placed outside, the animal will probably make vigorous
efforts to escape by clawing and biting in various parts of
the enclosure, which are the usual instinctive methods em-
ployed in similar situations. If the right movement is hit
upon and the cat gets out and secures food, it will probably
make its escape more readily than before when placed in
the box a second time. After a number of trials the cat
will come to make the right movements for escaping very
soon after being placed in the box and its useless random
movements will be discontinued. The connection between
the perception of the mechanism of escape in the box and
the act necessary to gain its liberty comes to be more and
more firmly established in the cat's brain with repeated
experiences. The cat perceives a number of things in the
box and performs a number of different acts, but out of
all these, associations are formed only between certain
stimuli and those responses to them which bring pleasure
to the animal.

Pleasure and pain have apparently a fundamental con-
nection with the development of intelligent responses out
of instinctive activity. Were there not something to clinch
or strengthen the connection between certain stimuli and
the appropriate responses to them the organism might per-
form random movements till doomsday without being a
whit better off. It is a problem therefore of fundamental
importance to ascertain in what the mechanism of this
ability to profit by experience essentially consists. It is
not mere habit, not the mere making more permeable certain
preformed connections in the brain. One act would then be
just as apt to be followed up as another. Whether an act
tends to be followed or not depends on what it brings to the
organism. Apparently we have to do with a selective

168 BEGINNINGS OF INTELLIGENCE

agency which preserves or repeats certain activities and re-
jects others on the basis of their results.

The importance of random movements lies in the fact that
they offer opportunities for making favorable adjustments.
For the development of intelligence they play a role similar
to that of variations in the process of evolution. The
animal that does the most exploration is the one most likely to
hit upon new advantageous adjustments. In the same way
intelligent adjustments, as James has contended, are favored
by a multiplicity of instincts, especially if these instincts are
of a contrary or conflicting nature, for now one and now
another instinctive tendency may be reinforced in different
conditions to which it may be adapted. Instinctive fear
may be modified through experience so that it is no longer
attached to objects that are found to be harmless, while it may
be intensified in relation to other objects that are found
to be sources of injury. Where there is hesitation between
the exercise of two instincts such as the tendency to pursue
an animal as prey, and the instinctive fear which that animal
may awaken, experience may quickly point out which
proclivity is the more advantageous to follow. The pleasure-
pain reaction enables an animal to select, so to speak out of
its stock of instinctive endowments those responses which
are best adapted to the particular situations that confront
it. It is a means of adapting instincts to new or inconstant
conditions and thus of effecting a closer adaptation to the
environment than that which would be possible by following
purely congenital modes of response. The development of
the pleasure-pain reaction marks one of the most important
steps in the evolution of behavior, for the entire super-
structure of intelligence in all its stages is based upon it,
and it is not surprising that many writers regard it as an
index of the beginning of consciousness, a point where a new

BEGINNINGS OF INTELLIGENCE 169

entity is somehow mysteriously injected into the miiverse.

It is a general rule that what is pleasant is beneficial and
what is painful is injurious, and, therefore, by following
its desires and aversions an animal is guided in a tolerably
safe course. Eating when hungry, drinking when thirsty,
seeking warmth when cold, exercise when in a state of
vigor, and rest when fatigued, all bring a state of satisfac-
tion or pleasure. On the other hand, eating and drinking
after a certain stage of repletion has been reached, or at-
taining too great a degree of warmth may be positively
painful, the pain being correlated with carr)dng on these
activities until they become injurious to the organism.

But it is well known that this correlation is not an absolute
one. With complex creatures like ourselves with a multitude
of different propensities and interests it is not infrequent
that the pursuit of what is agreeable leads to all sorts of
unfortunate consequences, even of a purely physiological
nature. In the lower animals where pleasure is a safer
guide than among ourselves, what is pleasant is not always
what is organically good. Poisonous articles may be eaten
with apparent relish and alcoholic liquors are readily im-
bibed even by such primitive creatures as bees and wasps
upon their very first acquaintance with these intoxicants.
But aside from exceptional cases, pleasure in the animal
world is a sufficiently good index of what is beneficial that
under conditions which ordinarily present themselves it
seldom leads to injurious courses of action.

The relation between the pleasant and the beneficial is,
however, probably not a primary one, and it is not improb-
able that it represents a connection established by natural
selection, as was first maintained by Herbert Spencer.

" If the states of consciousness which a creature endeavors
to maintain are the correlatives of injiuious actions and if

170 BEGINNINGS OF INTELLIGENCE

the states of consciousness which it endeavors to expel are
the correlatives of beneficial actions^ it must quickly disap-
pear through persistence in* the injurious and avoidance
of the beneficial. In other words, those races of beings only
can have survived in which, on the average, agreeable or
desired feelings went along with activities conducive to the
maintenance of life, while disagreeable and habitually-
avoided feelings went along with activities directly or in-
directly destructive of life; and there must ever have been,
other things equal, the most useful and long-continued
survivals among races in which these adjustments of feelings
to actions were the best, tending ever to bring about perfect
adjustment/'

This explanation which has become widely accepted
leaves a fundamental question unanswered. It does not
explain why certain acts are stamped in and certain others
stamped out. Of the mechanism of this process, which is
the real problem involved in the pleasure-pain reaction,
we are as ignorant as before. The explanation means that
animals which took pleasure in following acts that brought
them benefit were preserved and those that did not behave in
this manner were eliminated. But why does an animal tend
to repeat an act that brings it pleasure and avoid one that
produces pain? It seems so natural for creatures to behave
in this way that the existence of any problem here is usually
unsuspected, but this is the problem that confronts us when
we endeavor to obtain a clear understanding of the way
in which intelligence develops out of instinct.

In the pleasure-pain response we have two problems of a
quite different nature: (1) the problem of how behavior is
modified by its results, and (2) the problem of why pleasure
is associated with certain physiological activities, such as
securing movements, and pain with others, such as avoiding

BEGINNINGS OF INTELLIGENCE 171

movements. The latter problem is one whose solution
appears hopeless. If we accept the doctrine of psycho-
physical parallelism in any of its forms, we must deny that
psychic states are, strictly speaking, the causes of physical
changes. Why then should pleasure be connected with one
kind of activity and pain with another? Why not just the
reverse? This problem is, I believe, insoluble, because it is
a question of the relation of the physical and the psychical;
it is of essentially the same nature as the question why one
kind of retinal stimulation produces a sensation of red and
another a sensation of green. Physical and psychical states
are correlated in particular ways; this we accept as a matter
of observed connection. But why a certain kind of brain
vibration is associated with a state of consciousness we call
a sensation of red instead of some other state is a question
upon which we may intend our minds indefinitely without
the least profit. If w^e adopt any other theory of the relation
of mind and body we are in no way better off. If we have
to do with a preordained connection of pleasure with certain
physiological activities and pain with certain others, this
connection is no more intelligible if we admit the interaction
of psychical and physical states than it is under the theory
of parallelism. We can only say that such is the observed
relation of the phenomena.

What is feasible to attempt to solve is the problem of the
adaptive modification of behavior which we may say is the
objective side of the pleasure- pain process. We are dealing
with a series of physiological reactions and how they come
to be modified. We may assume that psychical states enter
into the chain of causes and effects that make up an animal's
behavior, but it is not clear that such an assumption throws
the least light upon our problem, and it is open to serious
objections on both scientific and metaphysical grounds. We

172 BEGINNINGS OF INTELLIGENCE

shall therefore consider the question purely from a phy-
siological standpoint. Viewed objectively we find that
in an animal's behavior certain acts when once performed
tend to be performed with greater readiness under similar
conditions a second time, while other acts once performed
tend under similar conditions to be inhibited. This prob-
lem of learning, Baldwin observes "is the most urgent,
difficult and neglected question in the new genetic psychol-
ogy." Spencer with his characteristic insight into funda-
mental problems has grappled with it and has attempted
to give a physiological explanation. Pleasure, according
to Spencer, is the concomitant of heightened nervous dis-
charge; pain the concomitant of lessened discharge. In an
animal with a diffuse discharge of its nervous energy result-
ing in random movements, some of these movements bring
a heightened nervous discharge with its psychic accompani-
ment of pleasure. This tends to reinforce the movement that
brought the increase of nervous energy and to cause it to be
repeated. Responses resulting in pain tend on account of the
diminution of nervous discharge that follows to be discon-
tinued, and in this way the organism is kept repeating certain
acts and avoiding others. "Along with the concentrated
discharge to particular muscles,'' says Spencer, "the gang-
lionic plexuses inevitably carry off a certain diffused dis-
charge to the muscles at large, and this diffused discharge
produces on them very variable results. Suppose, now,
that in putting out its head to seize prey scarcely within
reach, a creature has repeatedly failed. Suppose that along
with the group of motor actions approximately adapted to
seize prey at this distance, the diffused discharge is, on
some occasion, so distributed throughout the muscular
system as to cause a slight forward movement of the body.
Success will occur instead of failure; and after success will

BEGINNINGS OF INTELLIGENCE 173

immediately come certain pleasurable sensations with an
accompanying draught of nervous energy toward the
organs employed in eating, etc. That is to say, the lines
of nervous communication through which the diffused dis-
charge happened in this case to pass, have opened a new
way to certain wide channels of escape; and, consequently,
they have suddenly become lines through which a large
quantity of molecular movements that were followed by
success are likely to be repeated; what was at first an acci-
dental combination of motions will now be a combination
having considerable probability/'

Bain's view of learning is much like that of Spencer.
'^We suppose movements spontaneously begun, and acci-
dentally causing pleasure; we then assume that with the
pleasure there will be an increase of vital energy, in which
increase the fortunate movements will share, and thereby
increase the pleasure. Or, on the other hand, we suppose
the spontaneous movements to give pain, and assume that,
with the pain, there will be a decrease of energy, extending
to the movements that cause the evil, and thereby providing
a remedy. A few repetitions of the fortuitous concurrence
of pleasure and a certain movement, will lead to the forging
of an acquired connection, under the law of retentiveness
or contiguity, so that, at an after time, the pleasure or its
idea shall evoke the proper movement at once."

The theories of Bain and Spencer are (Jiscussed in de-
tail by Baldwin, who, while differing from these writers in
certain points which need not here be dwelt upon, adopts
essentially the same view as regards the mechanism of re-
inforcement and inhibition. With Bain and Spencer,
Baldwin assumes that " the pleasure resulting from the first
accidentally adaptive movement, issues in a heightened
nervous discharge toward the organs which made the move-

174 BEGINNINGS OF INTELLIGENCE

ment, a discharge which finds its way to the same channels
as before, and so makes it likely that the same movement
will be repeated, the external conditions remaining the same.
Pleasure and pain can be agents of accommo-
dation and development only if the one pleasure, carry
with it the phenomenon of 'motor excess,' and the other,
pain, the reverse — probably some form of inhibition or of
antagonistic contraction."

The theories of Spencer, Bain and Baldwin are physio-
logical since they attempt to explain the modifications of
behavior, not through the influence of certain psychic states,
but as the effect of the physiological conditions of which these
states are the concomitants. The theories are all open to the
objection that pleasure is by no means the constant con-
comitant of heightened nervous discharge. Laughing and
crying are very similar in their physiological expression,
though they go along with very different psychic states,
A child who burns his hands and writhes about in agony
certainly manifests a heightened nervous discharge, but he
shows no tendency to put his hands again into the fire.
Another outreaching movement of the child brings his hands
toward a pleasant degree of warmth. The movement
tends to be repeated. The nervous discharge in the first
case is much greater than in the second, but in both cases
it goes to the arm, though along somewhat different nerves.
It is obvious, I think, that we cannot account for the differ-
ence between the responses to pleasurable and painful
stimuli on the basis of any quantitative difference in the
discharges to the part affected. It is a matter of nervous
connection rather than quantity of nervous energy.

Pain-giving stimuli, owing to the arrangement of an animal's
reflex arcs, are generally followed by a withdrawing move-
ment of the part stimulated, but that there is a tendency

BEGINNINGS OF INTELLIGENCE 175

for the "increased energy of the pleasure process" to flow
"into the channels of the movement associated with pleasm-e"
(that iS; I take it, the movement which brings pleasm-e)
is by no means evident. There is, I think, no primary ten-
dency, as Spencer and Bain seem to think, for the nervous
discharge to take the direction of the organ from which the
pleasure is derived. Animals, it is true, move so as to
bring an organ which is pleasantly stimulated again under
the action of the stimulus, but this is often due to the dis-
charge going mainly to a quite different part of the body,
such as distant appendages, instead of to the part directly
affected.

The theory of heightened nervous discharge as expounded
by Spencer, Bain and Baldwin, fails to give us, I think, the
desired explanation of the acquirement of individual ac-
commodations, and one naturally turns to other theories of
the psycho-physiology of pleasure and pain for light. Here,
however, we are led into a veritable quagmire of psychological
speculation, for there are few fields in which there are so
many and so fundamental differences of opinion among
competent psychologists. The physiological concomitants
of pleasure and pain have afforded a subject of numerous
laboratory studies and almost no end of theories. There
is good evidence that pain sensations are produced by
the stimulation of specific nerves, but as regards the
physiological states accompan3dng pleasure and pain the
results of experiments as well as opinions based on them
are very discordant. And it is difficult to see how most
of the pleasure-pain theories would help us to explain
the mechanism of accommodation even if they were
established, so that the outlook for the solution of the
problem as it is commonly formulated does not seem, at
present, an encouraging one.

176 BEGINNINGS OF INTELLIGENCE

A new point of view in regard to our problem has been
presented by Hobhouse in his Mind in Evolution. To
illustrate this view let us recur to our chick. When a nasty
caterpillar is seen for the first time the visual stimulus sets
up a pecking reaction. This is followed by the stimulus of a
bad taste which sets up various rejection movements, such
as ejection of the food and wiping the bill. The order of
events is:

Stimulus. . . . pecking. . . . bad taste. . . .rejection.

When the same kind of caterpillar is met with a second
time the stimulus tends to elicit the rejection movements
with which it has been associated instead of the movements
of pecking. Is not the inhibition due to the fact that the
stimulus has become associated with a response which is
incongruous with the first? Movements of rejection and
avoidance are incompatible with those of pecking and swal-
lowing and it may therefore be unnecessary to look to any
peculiarity of the physiological correlates of pain for an
explanation of the inhibition of the original reaction. The
stimulus becomes coupled with a new reflex arc; nervous en-
ergy is drained off in a new channel, and the future behavior
becomes changed. If the taste is a very bad one, a great deal
of energy is involved and the connection with the rejec-
tion response made very permeable and the rejection move-
ment easily set up. If a person is confronted with a sight
of some nauseating medicine he has recently taken, avoid-
ing or rejection movements are set up, such as making a
face, or even retching movements of the stomach. Is it
not these movements or attempts at movements that really
inhibit the taking of the medicine? This is evinced by the
chick described by Lloyd Morgan, which after an experience
with a nasty caterpillar approached one a second time,
but stopped and wiped its bill and went away as if it actually

BEGINNINGS OF INTELLIGENCE 177

repeated its first experience. Of coulee inhibition of the
original response does not always involve contrary move-
ments, but there may be impulses to such movements which
do not issue in action. The principal feature in the modi-
fication of action through painful experiences is the assimi-
lation of impulses incongruous with the original one.

In the reinforcement or stamping in of a reaction to a
particular stimulus that brings pleasure, it certainly seems
as if pleasure or its physiological correlate in some way
serves to cement more firmly the association between the
stimulus and the response. Let us consider, however,
the case in which the chick pecks at a caterpillar which
has a good taste. The presence of the caterpillar in the
mouth excites the swallowing reflexes; in the presence of a
similar caterpillar the pecking response is made more readily
than before, and whatever hesitation there may have been at
first disappears. Is not the difference from the pain-re-
sponse due to the fact that there is an organic incompatibOity
between the first and second responses in the pain response,
while there is an organic congruity or mutual reinforcement
of these responses in the other? Pecking and swallowing
form the normal elements of a chain reflex; when one part
of the system is excited it tends to excite the rest, to increase
the general tonus of all parts concerned in the reaction.

According to the view here presented, whether a particular
response to a stimulus tends to be repeated more readily
or discontinued, depends not upon the peculiar physiological
state which may be produced in the brain, but upon the
kind of responses which the stimuli brought by the act call
forth. If an outreaching reaction becomes coupled with a
withdrawing response the result is inhibition. If the re-
action, on the other hand, brings stimuli which produce
congruent reactions the association formed with these

12

178 BEGINNINGS OF INTELLIGENCE

latter reinforces the first reaction. The pleasure-pain
response then resolves itself into the formation of associa-
tions. Withdrawing and defensive responses are usually
initiated by pain giving stimuli, and the instinctive or
random movement which brings a painful stimulus is in-
hibited under similar conditions in the future, not because
of the pain of its physiological correlate, but because it
comes to be associated with a withdrawing or defensive,
and hence an incongruous or inhibitory reaction. Pleasure
and pain thus interpreted have no mysterious power of
stamping in or stamping out certain associations. Whether
the result is reinforcement or inhibition depends on the
way in which a reaction and the secondary responses re-
sulting from the situation in which the organism is thereby
brought, happen to harmonize.

The step from instinct to intelligence viewed as a physio-
logical process involves, therefore, no essentially new
element beyond the well known physiological properties of
the nervous system, and we are not committed to any par-
ticular hypothesis as to the physiological accompaniments
of pleasure and pain, or pleasantness and unpleasantness,
in order to understand how behavior may become adaptively
modified. How far the interpretation given will enable us
to explain the development of intelligence I do not pretend
to say. It may break down in attempts to apply it to
higher forms of learning, but it affords a useful working
hypothesis and takes us a way, I think, toward the solution
of our problem.

BIBLIOGRAPHY
Bain, A. The Sensations and the Intellect, 3d. ed., *94.

The Emotions and the Will, 4th. ed., '99.
Baldwin, J. M. Mental Development in the Child and in the Race.

Methods and Processes, 2nd. ed., N. Y., '97.

Development and Evolution, N. Y., '02.

BEGINNINGS OF INTELLIGENCE 179

HoBHOUSE, L. T. Mind in Evolution, '01.

Holmes, IS. J. Pleasure, Pain and the Beginnings of Intelligence.

Jour. Comp. Neur. Psych., 20, 145, '10.

The Beginnings of Intelligence, Science, 33, 473, '11.
Marshall, H. R. Pleasure, Pain and -Esthetics, N. Y., '94,
Spencer, H. Principles of Psychology, 1855.

CHAPTER IX

PRIMITIVE TYPES OF INTELLIGENCE IN
CRUSTACEANS AND MOLLUSCS

It is scarcely possible at present to fix, even with the
rudest approximation, the point where intelligence makes
its first appearance in the course of evolution. There is
little doubt that the step from instinct to intelligence has
been made, not once merely, but several times. The in-
telligence of the higher Mollusca had, in all probability, an
origin independent from that of the arthropods, and the in-
telligence of the vertebrates was probably developed in-
dependently of that of the other groups. Among the arthro-
pods themselves it is not likely that the intelligence manifested
by the arachnids had a common origin with that of the
insects, and within both of these large groups intelligence
may have been independently developed out of behavior of
the purely instinctive type.

Intelligence grows out of the complexity and perfection
of the nervous mechanism, and along whatever line organiza-
tion reaches a certain degree of development intelligence
appears on the scene. From what has been said in previous
pages we are prepared to appreciate the fact that intelligence
is not an entirely new power unrelated to the other activities
of organic life, but a process growing out of other organic
functions and having the same end as these other functions;
it is, as Spencer has so well emphasized, but a higher phase
of those processes of adjustment and regulation which make
up the life of the animal.

180

PRIMITIVE TYPES OF INTELLIGENCE 181

The criterion of intelligence which we have adopted — the
power of forming associations — is one which is accepted by
a considerable number of comparative psychologists. Un-
fortunately the term intelligence is used in a variety of senses
by different vsTiters — a fact which is in part responsible for
the different expressions of opinion as to where in the animal
kingdom intelligence makes its beginning. The acute
Father Wasmann wdll have none of intelligence in any
animal below man, but as he defines it, the term connotes
the power of reasoning by the use of general concepts.
The controversy which has arisen over this employment of
the term is a matter for the lexicographer instead of the
psychologist, and so long as a WTiter makes his peace with
the dictionary we have no quarrel with him. We prefer,
however, to employ the term in its more widely accepted
meaning.

As stated in a previous chapter, there is no evidence that
there is any power of forming associations in the Protozoa.
In the Coelenterata behavior, although of the reflex type,
is often higlily plastic and capable of being modified in
many ways as the result of previous experience; but while
intelligence has often been claimed for these forms, there is,
in the opinion of the wTiter, no case in which the formation
of associations is satisfactorily proven. The same state-
ment may also be risked for that large and miscellaneous
assortment of animals grouped under the term Vermes.
The behavior of Echinoderms is certainly complex and plastic
to a remarkable degree, but even in this group the power
of forming associations is very doubtful. Preyer, who has
made a very thorough study of the behavior of the starfish,
claims to have discovered indubitable indications of intelli-
gent action, but the later studies of Jennings andGlaseronthe
behavior of starfish and ophiurans failed to confirm Preyer's

182 PRIMITIVE TYPES OF INTELLIGENCE

conclusions. A starfish may possibly acquire habits of a
certain kind, but it is not proven that it is able to form
associations.

We do not intend to deny the existence of intelligence in
the groups mentioned; we think it not improbable that
intelligence of a primitive sort may be discovered, at least
in the more highly developed members of these divisions;
but at the present time we can only grant the Scotch verdict
of "not proven."

In the Arthropoda instinctive activity is frequently re-
presented as reaching its culmination, and some investigators
have gone so far as to assert that the behavior of these animals
is made up entirely of instincts and reflexes. This opinion
is in part based on a priori deductions from the organization
of the nervous system and it is held to chiefly by morpholo-
gists and physiologists whose observation of the behavior of
animals is limited and warped by preconceptions.

Bethe, who has done a large amount of thorough and
valuable work on the anatomy and physiology of the nervous
system of arthropods, and who has very successfully employed
the results of these investigations in the analysis of normal
behavior, was led to the somewhat extreme position of
denying, not only associative memory, but consciousness
as well, in all the arthropods. The complex behavior of
these forms, according to him, can be analyzed in terms of
reflex action, and there is consequently no ground for assum-
ing any psychic elements whatsoever in these animals.

At the close of his important memoir on the nervous
system of the crab Carcinus mcenas, there are described a
few experiments which convinced Bethe that this animal
is unable to profit by experience. Bethe placed a crab in an
aquarium containing a devil fish, Eledone, which took up
its station in a dark corner. The crab when placed in the

PRIMITIVE TYPES OF INTELLIGENCE 183

aquarium quickly rushed, in obedience to its proclivity to
shun the light, into the dark comer where it got into difficul-
ties with the de\dl fish. Bethe now interfered and freed the
crab from its captor and put it back in the aquarium. Back
the crab went again into the arms of the enemy. Five
successive times it repeated the performance (and another
individual did the same six times) without learning to avoid
the retreat of the devil fish.

In another experiment a piece of meat was placed in an
aquarium containing some hungry crabs, and the hand of
the experimenter was held over the meat. ^Mienever a
crab seized the food the creature was maltreated and driven
away; it was thought that if the crab were capable of learning
it would come to associate the sight of the experimenter's
hand with the painful experience following the seizure of
the meat and keep at a distance. After several such ex-
periences it went after the meat as at first, and Bethe
concluded that the creatures were nothing but "reflex
machines," without a glimmer of intelligence.

These few experiments by which the intelligence of the
crab is summarily disposed of, form an almost amusing
contrast to the long, detailed and exhaustive work on the
anatomy and physiology of the nervous system. The
experiments are obviously inadequate, not only because
they are much too few in number, but because they do not
afford the best opportunities for bringing out whatever
power of forming associations a crab may possess. In the
first experiment, granting that the crabs were not more
afraid of Bethe than of the devil fish, as they had apparently
as much reason for being, it would have been necessary for
the crab to inhibit a strong instinct before it could manifest
any tendency it may have acquired to avoid the dark comer
with its sinister occupant. A crab when afraid makes for a

184 PRIMITIVE TYPES OF INTELLIGENCE

dark hole if there is any within reach; and with its whole
energies bent upon getting away from the large creature
into whose hands it is taken, what wonder if the devil fish,
if it would otherwise be remembered, should be temporarily
forgotten.

The second experiment likewise is one which involves the
conquest of a strong instinctive proclivity and it includes
also too small a number of trials to be in any way con-
vincing.

The later experiments of Yerkes on Carcinus were more
fortunate in yielding positive results. The crabs were placed
in a box one end of which led to an aquarium. The end
nearest the aquarium was divided so as to afford a right and
a wrong path to the water. With successive trials the crab
came to learn, although with extreme slowness, to choose
the right path. Other simple labyrinths were employed
and the crab in each case succeeded, after a sufficient number
of trials, in learning the way to the water. The work of
Cowles on Ocypoda yielded confirmatory results, although
the idiosyncrasies of the animal caused the results to be
somewhat less clearly defined.

Experiments similar to those on Carcinus were performed
by Yerkes and Huggins on the crayfish. A simple labyrinth
was constructed consisting of a box having a small compart-
ment at one end, and an opening at the other leading to an
aquarium. From the open end a median partition extended
back a short distance, and one of the passages so formed
was closed with a glass plate. The crayfish liberated from
the small compartment was provided with a choice of two
paths only one of which would lead it to the water; and the
endeavor was made to ascertain if the crayfish, after a number
of trials, would unerringly choose the right path. The
crayfish used were put through a number of preliminary ex-

PRIMITIVE TYPES OF INTELLIGENCE 185

periments with both passages open to determine if they had
any tendency to go toward the right or the left, and after
it was shown that either path was chosen with equal readi-
ness, the glass plate was inserted and the animals put again
into the box. In the first experiment the crayfish took the
correct path in 50 per cent, of the trials, and during the
subsequent trials the percentage of correct choices gradually
rose until in the final ten trials it reached 90 per cent. The
improvement was very gradual, as is indicated by the follow-
ing series of percentages of successful trials for each set of
ten trips: 50, 60, 75.8, 83.3, 76.6, 90. Although slowly
acquired, the habit of following the right path was not
forgotten after an interval of two weeks.

Further experiments in which the box was thoroughly
washed out after each trip to eliminate any guiding influence
of odor or moisture gave similar results. If after a cray-
fish had learned to turn to the right, the right pathway was
closed, the animal, after a number of trials, would turn to the
left. One specimen which had come to turn to the left in
the first fifty trials, turned to the left uniformly thereafter
in the following fifty trips. The left side was then blocked
by the glass and in the next fifty trips forty were successful.
Finally the crayfish made but one error in fifty trials.
Further changes in the position of the glass plate were made,
but the crayfish after a number of trials adjusted itself to
the new condition.

The docility of the crayfish is sho\\m also by some experi-
ments of the writer in which the animals were trained to come
for food. To quote from a previous paper: "At first I
would very slowly bring a piece of meat held in a fine forceps
near the antennules. After the movements of the antennules
and mouth parts, the grasping movements of the chelipeds
would result in securing the meat. After some trials I would

ISG PRIMITIVE TYPES OF IXTF.LLIGENCE

not allow Uie meat to be pulled away from the forceps until
the craj'fish struggled awhile to secure it ; at the same time I
moved my hand about so as to accustom the animal to my
movanoits. There is a strag^ between the instinct to
flee &om a large moving object and the instinct to secure a
SKVQiy morsel which has been sdied. With careful manage-
ment the latter instinct may be made to predominate over
the form^ and gradually the fear of one's movements be-
comes much reduced. The crayfish finally came to associate
thea{^xrDach of my hand with being fed, and would rear up and
hold out its larger chdae much as in the ordinary posture for
defoise. . . . One individual would greet me as I entered
my room in the morning by raising up its chelipeds and
coming toward me, and it woidd follow me about as I went
from one side of its enclosure to the other. When fed,
bowevo*, it would manifest no further interest in my move-
moits."

The alHlity of hermit ^rahs to form associations has been
prov^i by the experimaits of Spaulding on Pagurus longi^
carpus. Several specimens <tf this active species were placed in
«i aquarium supplied with running water. A dai^ screen was
made so that it could be placed in the middle of the aquarium
leaving only a narrow sKt on either side through which the
oabs could pass from one compartment to the other. As
the hermits are positivdy |rfiototactic they tend to keep
in the lifter half ol the aquarium, and to make the lighting
of the two parts as diffaent as possible one side of the aquar-
ium was covered with heavy dark paper. When the screen
was put in the aquarium, and all the crabs placed behind it,
they quickly made for the openings at the sides of the parti-
tion and went out into the light. Every day the screen
was insoted for a ffvea interval in the aquarium, and a
piece of fish put behind it. The number of crabs entering

PRIMITIVE TYPES OF INTELLIGENCE 187

the darkened part after the food was noted and the length of
time required for the crab to enter. On the first day only
three out of the thirty crabs used availed themselves of the
food. The number gradually increased on succeeding days,
and the average time of their response decreased iitil; on
the eighth day, all the crabs but one entered the dark en-
closure, and most of them entered with little delay.

"After a few days of this treatment immediately upon
the insertion of the screen the crabs became most agitated,
some hurrying and scurrying about, others making almost
directly for the openings.'^ An association was evidently
formed between the appearance of the screen and the ex-
perience of being fed, and this association led the animals to
act counter to their natural proclivity to seek the light.

The experiments of Drzewina on Pachygrapsus, in which
it was shown that the crabs which were compelled to go
through a certain opening in order to get nearer the light
gradually learned the way and arrived more quickly in the
compartment of their enclosure which was most illuminated,
show a similar power of forming associations.

In the Mollusca we meet with indications of intelligence of
a primitive sort in the movements of gasteropods such as the
limpets, which have the faculty of making considerable
journeys from their accustomed stations on the rocks and
returning to their orginal position. Bethe has studied the
homing of limpets and has come to the conclusion that these
animals simply follow their own slimy trails and are guided
back to their resting place by a kind of chemotaxis. No
intelligence is required by the limpets; they simply obey
a blind tropism.

Lloyd Morgan experimented with limpets by removing
them for some distance from their scars on the rock and
noting how many found their way back within a given time.

188 PRIMITIVE TYPES OF INTELLIGENCE

The results of his experiment are summed up in the following
table:

No. removed

Distance in

inches

Number

returned

In 2 tides

Je

I 4 tides

La1

25

6

21

..

21

12

13

5

..

21

18

10

6

2

36

24

1

1

3

While the majority found their way back when removed but
a few inches, only one returned from a distance of two feet.
The return of the limpets was watched in many cases and
found to be ''fairly, but not quite direct." When the
limpets reach their scar " they twist and turn about so as to
fit down in the normal position, which is constant. When
they come up the wrong way round they rotate pretty
rapidly through the 180 degrees to get into position."

According to Bohn, Patellas when they have remained for
some time in a certain position, whether horizontal, vertical
or oblique, orient themselves in a similar position after they
have been removed from their original habitat. Even if
they are allowed to remain on their particular scars and the
rock to which they adhere is turned in another position they
become uneasy and crawl around until they find a niche in
which they may lie in their previous orientation.

It is among the active and highly organized cephalopods
with their large and complex nervous centers and highly
developed organs of vision that we naturally expect to find
the highest degree of psychic development. Unfortunately
we have very few observations on this head. Schneider
relates that a young octopus which be observed in the
Naples aquarium made an attack on a hermit crab living
within a shell upon which were several anemones. In
approaching the crab the octopus was stung by the net-
tling cells of the anemones and quickly withdrew. There-

PRIMITIVE TYPES OF INTELLIGENCE 189

after it carefully avoided further contact with the crab.
Older octopi, according to Schneider, contrive to extract the
hermits from their shells without being stung. The ob-
servations of Schneider on the young of the octopus were
verified by von Uexkull in Eledone which also learned to
avoid a torpedo from which it had received an electric
shock.

Kollmann gives an acount of an octopus which was placed
in an aquarium with a large lobster and several other
animals. The octopus manoeuvred constantly in order to
seize the lobster, but the latter was on the alert and usually
made its escape, and on one occasion inflicted a severe cut
on its adversary's arm. The lobster was finally seized in an
unwary moment and surrounded by the long and powerful
arms of its captor. It was liberated by an attendant and
placed in an adjoining aquarium separated from the first
by a cement partition which projected about 2 cm. above the
surface of the water. The octopus then, although the lob-
ster was out of its sight, made a sudden spring over the
partition and soon caught and overcame its prey.

It is difficult to estimate the psychic aspect of a single
act such as this. According to Kollmann it shows that the
octopus has the power of representing the absent lobster and
of remembering where it was placed; but it is not safe to go
farther than to say that a certain amount of intelligence
was probably involved.

BIBLIOGRAPHY

Bethe, a. Das Nerv^ensystem von Carcinus moenas I, Arch. f.

mik. Anat. 50, 460 and 549, '97; II, I.e. 51, 447, '97.
Drzewina, a. Les reactions adaptives des Crabes. Bull. Inst.

G^n. Psych. 8, 235, '08.
Kollmann, J. Aus dem Leben der Cephalopoden, Vierteljahrschr.

wiss. Philos. 1, '77.

190 PRIMITIVE TYPES OF INTELLIGENCE

Morgan, C. L. Animal Behaviour, London, '00.

Schneider, G. H. Der thierische Wille, '80.

Spaulding, E. G. An Establishment of Association in Hermit Crabs,
Eupagurus longicarpus. Jour. Comp. Neur. Psych. 14, 49, '04.

XJEXKtJLL, J. VON. Physiologische Untersuchungen an Eledone mos-
chata. Zeit. fur Biol. Vols. 28, 30, 31, '92-'95.

WiLLCOX, M. A. Homing of Fissurella and Siphonaria. Science,
n. s. 22, 90, '05.

Yerkes, R. M. Habit Formation in the Green Crab, Carcinus gran-
ulatus. Biol. Bull. 3, 241, '02.

Yerkes, R. M., and Huggins, G. E. Habit Formation in the Craw-
fish, Camharus ajffinis. Harvard Psych. Studies 1, 565, '03.

Yung, E. La psychologie de I'escargot. C. r. et Trav. Soc. Helv.
Sci. Nat. '93, 127.

CHAPTER X
INTELLIGENCE IN INSECTS

"Si tons les actes instinctifs des Insectes portaient constamment
I'empreinte 4\adeiit d'une necessity aveugle, il y aurait beaucoup
moins k admirer en eux qu'on ne le fait communemeiit. Ce qui excite
surtout notre surprise, c'est que frequemment ils s'accommodent aux
circonstances, et que leurs actes prennent alors une telle apparence
de raison, qu'il faut y regarder de pres pour ne pas les attribuer k
une veritable combinasion d'idees. — Lacordaire, Introduction d
VEniomologie.

In the insects manifestations of intelligent behavior are
much more common and more striking than in the Crustacea
and molluscs. It is a general rule that the degree of intel-
ligence in these forms runs parallel with the degree of com-
plexity and perfection of their instincts and with the degree
of development of the nervous system and sense organs.
Among primitive groups of insects the intelligence manifested
is very slight, while it reaches its culmination in the hymenop-
tera whose instincts have long been objects of wonder and
admiration.

The power of associating certain appearances with food
might be expected to occur among the earliest manifestations
of intelligence, and we find many illustrations of this ability
even among the more primitive insects. Miss Sondheim
kept a damsel fly larva in a dish of water, where it was
frequently fed. At first the larva scuttled away in fear
whenever Miss Sondheim approached, but after a time its
timidity was overcome. Later it became so tame that it
would take flies out of her hand, and came toward her
whenever she approached. Finally it would come out of the

191

192 INTELLIGENCE IN INSECTS

water and climb upon her hand in order to get the food.
Another larva which was worked with failed to profit in
the least from repeated efforts to train it. Forel similarly
trained a large water beetle, which at first fled upon his
approach, to come toward him for food. The beetle came
to eat when on the table, whereas naturally it feeds only in
the water, but it still retained its old method of turning over
on its back before eating, which it did very clumsily when
out of its natural element. Lubbock trained a wasp to
come for food, and finally it would allow itself to be taken
into the hand and stroked, whereas at first it would show
strong resentment at attempts of this kind. Very similar
results were obtained by Adlerz in a species of sand wasp.

Mr. J. Wodsedalek has recently made an extended study
of the formation of associations in the May-fly nymph
Heptagenia interpunctata (Say) , which is very common near
the University of Wisconsin, where the work was carried
on. Although negatively phototatic, the nymphs were
trained to go toward a stone (to which they had a strong
propensity to cling), at increasingly great distances against
the rays of light, until finally they would go toward it at a
distance much greater than they could be induced to do at
first. They were also trained to come for food, and by
repeated stirring up, several lots of nymphs came to be so
afraid that whenever the observer approached they would
scurry about with every appearance of great alarm.
Nymphs placed in other dishes where they were not dis-
turbed showed practically no signs of fear. It is of inter-
est to find in these primitive insects that behavior is
modified in the two ways which in higher forms we should
have little hesitation in regarding as indicative of pleasure
and pain.

There are several instances of the "training" of ants.

INTELLIGENCE IN INSECTS 193

Formica rufibarbis is a very pugnacious species and the odor
of one's hand readily proyokes it to a fight. Wasmann
gradually trained a worker of this species, offering it honey
on the end of a needle, and after it came to accept the food
without hesitation, placing the honey on his finger where
it came to be accepted with no manifestation of fear or
hostility.

Insects, like higher animals, learn to avoid injurious sub-
stances which they at first attempted to use for food.
Renter placed near a nest of ants some syrup containing
poison. The ants partook of the syrup eagerly, but soon
ejected it from their stomachs; after a little they came to
avoid the syrup although numbers of them w^ere commonly
near it.

The ability of insects to find their way back to their nest
or home is developed in many cases to a very remarkable
degree. Bethe, possessed of the idea that insects are reflex
machines incapable of learning by experience, explains this
power in the case of ants as an instance of chemotaxis; but
in the bees and wasps which find their way back from consid-
erable distances through the air, where scent tracks would
not persist, he is driven to assume some mysterious power,
acting in a manner analagous to magnetic force, which
guides these insects to their goal. Ants have the instinct
to follow the scent tracks left by their feet in going from
the nest, but as Cornetz has shown, they generally do not
follow these at all closely, and usually return by a much
more direct course than the irregular path w^hich is commonly
taken in their outgoing journeys for food. The power of
return exhibited by bees and wasps is sho\\Ti pretty clearly
by the experiments of Lubbock, Buttel-Reepen, the Peck-
hams, Wagner and others to depend upon the individual
experience of these insects. The homing of insects takes

13

194

INTELLIGENCE IN INSECTS

place in essentially the same way as the homing of carrier
pigeons, and involves an acquaintance with the locality
gained by previous exploration. The Peckhams found that
solitary wasps before their first depai'ture from the nest
make elaborate "locality studies/' circling around the nest
in wider and wider courses and at the same time flying higher
and higher in the air. Speaking of an Ammophila which
for some time previously had been exploring a garden in

Fig. 13. — A locality study of the wasp Sphex. (After Peckham.)

search of a place to dig a nest, the Peckhams say, " At last
a spot is selected and she begins to dig, but two or three
times before the work is completed she goes away for a
short flight. When it is done, and covered over, she flies
away, but returns again and again within the next few hours
to look at the spot and, perhaps, to make some little altera-
tion in her arrangements. From this time on, until the
caterpillars are stored and the egg laid, she visits her nest
several times a day, so that she becomes perfectly familiar
with the neighborhood, and it is not surprising, after all,
that she is able to carry her prey from any point in her

INTELLIGENCE IN INSECTS 195

territory in a nearly direct line to her hole — we say nearly
direct, for there was almost invariably some slight mistake
in the direction which made a Httle looking about necessary
before the exact spot was found.

"After days passed in flying about the garden — going up
Bean Street and down Onion Avenue, time and time again —
one would think that any formal study of the precise locality
of a nest might be omitted, but it was not so with our wasps.
They made repeated and detailed studies of the surroundings
of their nests. Moreover, when their prey was laid down
for a moment on the w^ay home, they felt the necessity of
noting the place carefully before leaving it." Similar
" locality studies '' varying more or less in character, are made
by many other genera of wasps, but after a number of
flights from home the preliminary circling about becomes
gradually reduced, and finally the insects fly away in a
straight line.

The role of visual memory is shown in the way in which
wasps are disconcerted by a change in the region about their
nests made during their absence. ''Aporus faciatus,'^ say
the Peckhams, "entirely lost her way when we broke off
the leaf that covered her nest, but found it, without trouble,
when the missing object was replaced. All the species of
Cerceris were extremely annoyed if we placed any new object
near their nesting places. Our Ammophila refused to make
use of her burrow after we had drawn some deep lines in the
dust before it. The same annoyance is exhibited when
there is any change made near the spot upon which the prey
of the wasp, whatever it may be, is deposited temporarily."
Even a slight change so disconcerted the wasps that they were
obliged to hunt for a long time before recovering their prey.

Belt has recorded a very interesting case of a wasp, Polistes
camifex, which had caught a caterpillar too large to be carried

196 INTELLIGENCE IN INSECTS

to its nest. After chewing the caterpillar for some time
the wasp bit it in two and rolled up one of the parts in order to
carry it away. As if it had in mind retm*ning for the other
piece the wasp circled about, forming larger and larger
circles, then departed for a distance, but returned as if to
get another look at the situation and flew away. In less
than two minutes the wasp again appeared on the scene,
having probably disposed of its burden in its nest. It had
difficulty in finding the remainder of the caterpillar and re-
turned repeatedly to the same seed pods near which the prey
was located. Whenever in flying about it came near the
pods it would alight and continue the search on foot. Its
persevering efforts were rewarded by the discovery of the
remainder of the caterpillar; then it seized its prey eagerly,
and '' as if there was nothing more to come back for, flew
straight to its nest without taking any further note of the
locality.'*

Similar behavior is shown in the trial flight of bees. When
young bees, or bees which have been carried into a new place,
make their first excursions from the hive they circle about as
they rise through the air before venturing very far away. Only
after having flown back and forth from the hive several
times do they finally come to dispense with their preliminary
movements of exploration. If young bees are removed
from the hive, even for only a short distance, before they
have made their trial flight, they fail to find their way back.
On the other hand, old bees may be removed for a long
distance and almost all succeed in returning. The distance
from which bees are able to find their way home depends upon
the character of their surroundings, and particularly upon
the distances they have been in the habit of going for
honey. Romanes took bees from a hive which was situated
near the sea, carried them in a boat a short distance from

INTELLIGENCE IN INSECTS 197

the shore and set them free. That they might be easily
recognized the bees were previously made to walk in bird
lime and the hive was carefully watched for their return,
but none found their way home. Another lot of bees was
then liberated on the shore not far from the hive. In this
region there were no flowers, consequently it was one not
frequented by the bees, and they were no more successful
in finding their way back than in the first experiment. A
number of bees were next carried inland where they had been
in the habit of foraging for honey, and liberated; nearly all
quickly found their way back to the hive.

Buttel-Reepen found that if a hive is carried into a new
locality a considerable distance from its original situation
and concealed among shrubbery or between buildings so
that it cannot be seen from a distance, and the old bees are
removed before they have made a trial flight, they usually
fail to return even if they have been taken but one-hundred
feet from the hive. If the hive is removed to a short distance
of a few rods, numbers of the bees return to the original
position of the hive and fly about as if in search of the hive,
although the latter may be in plain sight. The memory
of bees for the position of an object is apparently better than
their memory of the object itself. In this the insect mind
acts rather differently from that of higher animals for which
the object, wherever situated, is the thing that usually
determines action.

Insects seem chained down t^ topographical relations and
free themselves from their guiding influences only with
difficulty. This is illustrated by Fabre's experiments on the
mason bee, Chalcidoma muraria. During the absence of the
bee Fabre removed her nest the distance of one met».
The bee returned to the old locahty of the nest, but failed
to discover her own. TMien another nest was placed in the

198 INTELLIGENCE IN INSECTS

position of the old one the bee would work upon it as if
unaware of the substitution. The same trait is shown in
the interesting experiments of Turner on the homing of bur-
rowing bees. Melissodes, the first form worked with, digs
holes in the ground and makes excursions from the nest at
quite regular intervals. During the bee's absence a rect-
angular piece of white paper with a hole in the center was
placed over the nest so that the hole of the paper coincided
with that of the burrow. The bee when returning circled
about the nest, hovered over the paper, and then circled
about again; after repeating such performances for two min-
utes she entered the nest. On her next return she hovered
about for half a minute and made her entrance. During
her next absence a hole was made in the ground about four
inches away and covered by the white paper as before,
while a piece of watermelon rind with a hole in the center
was placed over the burrow. When the bee returned she
hovered over the melon rind and circled about for a minute
as if appreciating that things were not quite as they should
be and then entered her nest. The melon rind was then
removed and a rectangular piece of white paper was arched
over the nest so as to form a covering open at either end.
The piece of paper with the hole in the center was left where
it was in the preceding experiment. When the bee returned
she circled around for about a minute and then went into
the hole in the middle of this paper. The insect was deceived,
but only temporarily, for she quickly came out of the artificial
hole, entered one end of the tent-like covering, and found
her hole. The arrangements were left undisturbed during
the next three flights of the bee, and the insect foimd her
nest with little loss of time, as she did also when the tent
was turned at right angles to its previous position.
In another experiment the tent was removed for a short

INTELLIGENCE IN INSECTS 199

distance and a black piece of paper with a hole in the center
was placed over the nest so that the hole in the paper lay
directly over the opening of the nest. The bee returned and
entered her nest after hovering for but a few seconds over the
black paper. Finally all the accessories were swept away
and the region around the nest covered uniformly with
green grass leaving the opening uncovered. On her return
the bee was disconcerted, circled about the nest for about
two minutes and finally entered it.

The hole of another species of burrowing bees was found
near one of a series of bricks which formed the border of a
walk. Near the hole was the cover of a bottle. During the
absence of the bee Turner punched holes of the same diameter
as the bee's nest and bearing the same relation to the other
bricks as the nest did to the brick near it. The top of the
bottle was placed near one of these artificial holes. On her
return the bee alighted some distance away and came along
the series of bricks until she encountered the hole near the
bottle cover when she immediately plunged into it. She
quickly recognized her error, withdrew, and soon found her
own hole. During her second absence holes were punched
in front of several more bricks on either side of the nest, but
the bee on her return once more entered the hole near the
bottle cover. She emerged, hovered over the spot, and
again entered the same hole, but soon came out and found
her own nest. Evidently the bottle cap served as a land-
mark indicating the position of her nest. The environment
of the different holes was so similar that a change in the
position of this one object changed the principal feature of
the local topography.

There is no support here for Bethe's theory of a mysteri-
ous force, the assumption of a dead reckoning process, or
the '^kinaesthetic reflex" of Pieron. The results are only

200 INTELLIGENCE IN INSECTS

explicable on the assumption that the bee has a memory of
space relations and guides her flight accordingly. This con-
clusion is supported by the results of Turner's experiments
on the homing of the mud dauber, which are in principle
the same as the foregoing, although differing much in detail.
In bees and wasps the memory of locality is shown by their
returning repeatedly to the same spot for food. Forel in
experimenting on the power of vision and the formation of
associations in bees made use of variously colored paper
flowers on each of which he placed a drop of honey. The
artificial flowers were placed among some Dahlias which the
bees were visiting. A red paper flower was brought near a
bee resting on a Dahlia, but the bee was at first so occupied
in gathering honey that she could be induced to visit the
red flower only when the honey was brought within reach
of her proboscis. The bee's back was then marked
with red paint in order that she could be distinguished
from other bees. When the bee returned from the hive
she went straight for the artificial red flower, then to a blue
artificial flower with a yellow center and finally back to the
first. Another bee which visited a white artificial flower
was painted yellow. On her return from the hive she flew
to the same artificial flower and then visited two others and
did not return to the Dahlias. Later the bulk of the bees
w^hich for a long time had, with few exceptions, ignored the
artificial flowers seemed to have their attention directed to
them by other visitors and threw themselves upon the arte-
facts in swarms. After they had devoured the honey the
bees began to go back to the Dahlias, but when colored
artificial flowers devoid of honey and hence lacking an
attractive odor were placed among the plants, many bees
began to visit them and examine them carefully as if they
expected to find honey there. These facts, as Forel rightly

INTELLIGENCE IN INSECTS 201

concludes, ''can only be explained by an association of space
form and color memories with memories of taste."

We are certainly justified in concluding that insects are
not mere reflex machines incapable of learning by experience.
They can form associations very quickly in many cases.
They give evidence of memory. They have a remarkable
ability for retaining impressions of topographical relations.
We may not be compelled to admit that they have ideas,
but it must be granted, I think, that a wasp which after
cutting a caterpillar in two and carrying away one part,
came back and searched diligently for the remainder, re-
tained somehow an impression of the missing part and its
location. If out of sight it was not out of mind. The hunt-
ing of the wasp is instinctive and when we see a wasp
flitting about here and there in a feverish search for prey
we cannot assume that it carries in its mind an image of the
object of its pursuit. But the case is different with a wasp
which has secured its prey and comes back to find it. The
prey and its position are represented by some sort of "en-
gram" in the nervous center of the wasp, which is formed
by the various stimuli, optical, olfactory and tactual, which
resulted from the encounter. If the wasp does not have an
idea of its prey it has something which plays a role similar
to that of ideas in ourselves. As the wasp when it has
disposed of the second moiety of the caterpillar no longer
returns, its mental content is evidently changed by having
carried the part to its nest. If there is something represent-
ing "part-of-caterpillar-among-leaves" that leads the wasp
on its hunt, we may conclude that there is also something
corresponding to " part-of-caterpillar-now-in-nest " which
prevents further search.

I realize that one is on treacherous ground in trying to
interpret the workings of the insect mind. Forel, whose

202 INTELLIGENCE IN INSECTS

judgments on animal psychology are usually conservative,
attributes to insects an ''ability to instinctively draw in-
ferences from analogy/' Some of the facts adduced are the
following: After bees had been trained to come to artificial
flowers of a certain color for honey they deserted the Dahlias
upon which they had been working and began to work
upon other artefacts of different colors and in various
positions. The bee may be supposed, according to Forel,
if we interpret him correctly, to go through with a mental
process corresponding to "This appearance means honey;
therefore this other similar appearance likewise means honey;
I will investigate it." The behavior of the bee may indicate
a step toward rational procedm*e, but we are hardly
justified in assuming that any act of comparison between
similar flowers takes place in the insect's mind. A certain
appearance has been associated with the act of sucking
honey. This association leads the bee to visit the same
artificial flower again; or we may say that this object tends
to set in action the honey-getting activities. If the same
object causes the return of the bee we do not appeal to any
inference from analogy. If now a similar object provokes
the visit of the bee, it may mean simply that the stimulus
is sufficiently like the first to set the honey-getting activities
in motion. The bee gets a different perception from the
second object, but it does not necessarily recognize that it is
different from and at the same time similar to the first.
What appears in many cases to be reasoning from analogy,
involving judgments of likeness, is really based on nothing
more than lack of discrimination. While granting that a
simple act of inference may be performed by the bee, the
facts do not, I think, require us to conclude that it actually
is performed.

Another case involving a decided approach to reason,

INTELLIGENCE IN INSECTS

203

according to another prominent student of conservative
judgment, Professor Lloyd Morgan, is furnished by the
observations of the Peckhams on a soUtary wasp Ammophila
which, after filUng up its hole with dirt even with the surface
of the ground, picked up a small pebble in her mandibles and

Fig. 14.

-The wasp Ammophila using a stone to pound down the dirt
in its hole. (After Peckham.)

used it to pound in the loose dirt placed in the hole, '' Before
we could recover from our astonishment at this performance,"
write the Peckhams, "she had dropped her stone and was
bringing more earth, and in a moment we saw her pick
up the pebble and again pound the earth into place with it.
Once more the whole process was repeated, and then the
little creature flew away." According to Morgan, "here

204 INTELLIGENCE IN INSECTS

we have intelligent behavior rising to a level to which some
would apply the term rational. For the act may be held to
afford evidence of the perception of the relation of the means
employed to an end to be attained, and some general con-
ception of purpose." Truly a ''tool using animal!" But
in estimating the psychic aspect of the performance we
must bear in mind that the act is one which borders closely
upon the normal instinctive behavior of the insect. The
seizure of pebbles in the mandibles and the packing in of
dirt are parts of the instinctive process of filling up the hole.
The wasp combines two features of its hole-filling instinct in
a rather unusual w^ay. Does she really perceive the relation
of means to end? I am not so sure that she does.

Many readers who peruse this chapter will miss the
wonderful accounts of insect ingenuity which they may
have expected to find. The literature of insect behavior
contains these in abundance. Run through the files of
Nature, Science Gossip, Der Zoologische Garten, La Nature,
The Zoologist, the older numbers of the Annals and Magazine
of Natural History, and The American Naturalist, the
various entomological journals, and works of travellers
w^ith a leaning toward natural history; and peruse the many
volumes that have been written on the instincts and intelli-
gence and reasoning power of animals, and you will encounter
an enormous mass of material, some of it carefully recorded,
much of it not, more of it vitiated by anthropomorphic
interpretation which one cannot help feeling has biased the
observer in his account of the facts, but the data that can be
employed in drawing conclusions regarding the degree of
intelligence shown by the forms observed is disappointingly
small. Animals are observed overcoming obstacles or meet-
ing unusual situations, and the occurrence is straightway
recorded as an illustration of sagacity, something for which

INTELLIGENCE IN INSECTS 205

*'mere instinct" will not account. In most cases there
has been no previous study of the animars behavior with a
view to ascertaining the nature and limitations of its instinc-
tive performances. Ants are observed to build bridges over
water or other substances which they are desirous of crossing,
by bringing grains of sand or bits of earth and dropping them
until they can effect a passage. The ants are then credited
with ingenuity, reason, imagination and other mental
qualities which human beings would employ to overcome a
similar difficulty. And taken by themselves the facts
seem to justify such conclusions. Light on the matter,
however, is thrown, as in so many other cases, by a study of
the creature's instincts, which show^s that the apparent feat
of engineering is the result of an instinctive propensity of the
ant, slightly modified perhaps to meet the particular occasion.
Wasmann, in an instructive experiment, placed on the
nest of Formica sanguinea a watch glass filled wdth water
in the center of which was a sort of island on w^hich were
isolated a few pupae. The ants brought sand and threw
it into the watch glass until they had formed a passage way
to the pupse, which were then carried away. Ingenuity,
surely, one is tempted to say! But the next experiment
inspires caution. A watch glass with no island and no pupse
was placed in the nest. This was filled like the previous one.
At least one important factor in the ants' activities is the
instinct to cover offending objects which cannot be removed
— an instinct analagous to that of bees, which leads them to
cover over with propolis objects too large to remove from the
hive. If a dog performed similar actions in endeavoring to
reach an object otherwise inaccessible we should be justified
in attributing to the animal a considerable degree of intelli-
gence. Some intelligence may have been involved in the
behavior of the ants, but not necessarily more than a very

206 INTELLIGENCE IN INSECTS

little. The celebrated instance adduced by Leuckart of
ants bringing grains of sand to cover a streak of tobacco
solution across their trail may be explained in a similar way.

Another instance. Kirby and Spence record a case
communicated by a German artist, whom they were assured
was "a man of strict veracity.'^ A dung beetle, having
made a pellet for the reception of its eggs, found that it was
unable to roll the pellet out of a depression into which it
had fallen. The beetle then repaired to a dung heap near by
and returned with three companions, with whose assistance
the ball was rolled out, after which the three beetles took
their departure. This is one of the evidences from which
insects are considered to be "able to communicate and
receive information, which, in whatever way effected,
would be impracticable if they were devoid of reason.''

Blanchard in his ''Metamorphoses, Moeurs et Instincts
des Insectes," gives an account of a very similar performance,
which the author considers to evince '' une intelligence de la
situation vraiment ^tonnante, et une facility de communica-
tion entre les individus de la m^me esp^ces, plus surprenante
encore.'' Here again we must take into consideration the
normal instinctive behavior of these insects. Frequently
two or more beetles are found rolling the same ball. As
Fabre has shown in his careful studies of the sacred scarab,
an allied beetle with similar habits, such associations are
dependent on quite different motives than the altruistic
desire of rendering assistance. The helpful comrades turn
out to be bent on getting the ball for themselves. Sometimes
they abandon the task voluntarily; often they wage a com-
bat with the original owner. The succoring of a comrade in
distress is only an appearance which a fuller study of the
habits of these insects places in a quite different light.

These cases illustrate a common source of erroneous

INTELLIGENCE IN INSECTS 207

conclusions in the study of animal intelligence, one which
is responsible for a great multitude of stories of doubtful
value. Observations may have been recorded faithfully
and accurately, but where they have not been made by in-
vestigators thoroughly acquainted with the general behavior
of the forms observed, mistaken interpretations are almost
certain to arise. On the other hand, one is tortured by the
feeling that our experimental methods often fail to give us a
true measure of an animaPs possible attainments, and that
it is just in meeting exceptional situations which occur in
the animal's natural course of life that the highest manifesta-
tion of its intelligence is reached.

A factor which markedly affects the behavior of many
insects, especially the social ones, is the influence of numbers.
Small stocks of bees, according to Buttel-Reepen, lose their
spirit and allow themselves to become the prey of moths
and robber bees which are not so easily tolerated by larger
stocks. They work with less vigor and fight with less
courage, as if conscious of the fact that in numbers there is
strength and that their number is small. Forel says of ants
that "the courage of each ant grows in proportion to the
number of her comrades or friends and diminishes in just
the proportion that she is isolated. . . . The same
worker ant, which in the midst of her associates, is ready
to face death ten times over, when alone and twenty steps
away from her nest, becomes cowardly, avoids the least
danger, and seeks safety in flight from an ant much weaker
than herself." In regard to Formica sanguinea Wasmann
states that "if a numerous population inhabits a rotten
fir stump, on the surface of which we find some of the
ants running about, a gentle kick will at once call forth a
whole army ready for the fray. In a moment the whole
surface of the stump is covered with thousands of ants

208 INTELLIGENCE IN INSECTS

furiously hurrying to and fro. But if the colony is weak,
the same kick, which at other times calls forth an army,
will have the contrary effect. The ants which just before
were running about the surface disappear through the^ en-
trances of the nest as if by magic, and deathlike quiet
succeeds.^'

Wagner in his valuable studies on the behavior of bumble
bees, has observed that the bees of well populated nests
are much more pugnacious than those from nests containing
few members. In regard to the household activities of
bumble bees Wagner remarks that "Wenn eine Hummel
allein mit der Ausbesserung des Nestes beschaftigt ist, so
erscheint ihre Tatigkeit, obwohl wie stets geschaftig, doch
wenig energisch und intensiv. Kaum hat sich jedoch der
ersten Hummel eine zweite angeschlossen, so nimmt bei
gleichen sonstigen Bedingungen die Energie der Arbeit zu
und wachst bei jedem neu hinzutretenden Mitarbeiter mit
voller Augenscheinlichkeit. Gans dieselbe Erscheinung habe
ich auch bei Grabwespen beobachtet.^' The "brooding^'
of the cells, gathering food, and various other activities
increase in vigor with increase in numbers, so that the popula-
tion of the large nests seems larger and that of the small
nests smaller than it is in reality.

The influence of numbers upon the industry and courage
of social insects has been adduced as a proof of a high degree
of intelligence. That a populous ant colony will make an
attack upon a neighboring nest, while a colony with few
members shows a disinclination for warfare, may give the
appearance of a conscious realization of strength and a
power of reflecting upon the probable issue of an encounter.
As Wagner has pointed out, the explanation of such pheno-
mena may lie in the effect upon the individual of various
stimuli caused by its associates. The ant or bee commonly

INTELLIGENCE IN INSECTS 209

receives from other members of its community numerous
olfactory, tactile, and perhaps auditory stimulations to
which it instinctively responds. These stimulations pro-
duce a condition of general nervous irritability which
spurs the insects on to activity and makes them ready to
combine in an attack upon an enemy. Whether or not
similar effects, as Wagner contends, are produced in solitary
insects — and I am not convinced that they are, from the
evidence adduced — it is undoubtedly true that social
insects are dependent upon the stimuli received from the
cooperation of others to a remarkable degree. The confusion
produced by the loss of a queen and the gradual languishing
of a swarm in which no queen can be supplied, show how
sensitive* are bees to changes in their social environment.
Among bees, ants and termites signs of anger by one in-
dividual may awaken the whole community to a high pitch
of excitement. Each individual then serves to arouse the
others, and the larger the community the greater the mass
effect.

Closely associated in many cases with the influence of
numbers is the effect of imitation. The activities of insects
not only arouse the energies of their fellows, but they also
direct their efforts and in this manner secure cooperation
toward a common end. Ants keep together in their forag-
ing expeditions and often follow the " scouts'' which act
as leaders, guiding the expedition to the nest to be pillaged.
"In artificial nests," says Wheeler, "one usually sees a
particular activity started by one or a few workers, which
have more initiative or respond more quickly to a change
of conditions than the great bulk of the colony. The move-
ments of such individuals attract the attention of others in
their immediate neighborhood and these forthwith proceed to
imitate their more alert companions. Then the activity

14

210 INTELLIGENCE IN INSECTS

spreads like a conflagration till it has seized on most or all
the members of a community." It is in their attacks upon
enemies that imitation in ants is especially marked. An
attack by an ant is the signal for others to join in the fray.
Sometimes a strange ant may be tolerated in a nest until
it happens to arouse the animosity of one of the members,
when various others fall to and help to dispatch the intruder.

Wasmann states that several beetles of the species Dinarda
dentata were received as guests in a nest of Formica sanguinea
and had lived there for some time and propagated. He
then placed in the nest a specimen of an allied species of
Dinarda which was attacked and killed. This aroused the
killing propensities of the other ants which fell upon the
Dinarda dentatas, and the guests which had been kept for
so long met their fate. That ants in their treatment of
aphids are influenced by imitation is indicated by the fact,
signalized by Forel and Adlerz, that Formica sanguinea,
which very rarely make use of honey dew as food, readily
adopts the custom of its slaves upon perceiving them solicit
the aphids for their sweet exudation. Slave making ants
readily adopt guests which are received in a friendly manner
by their slaves, although they would otherwise be apt to
attack them, and slaves in turn are disposed to be friendly
to the guests which they perceive to be tolerated by their
masters.

In treating of insect intelligence we shall find it instructive
to consider its failures as well as its exceptional manifesta-
tions. As Forel has remarked insects are exceedingly stupid
as regards everything not closely related to their instinctive
interests. But even when the latter are involved, they
usually fail to make the simplest and most obvious inferences.
A striking case is furnished by the Amazon slave-making
ant, Polyergus rufescens, which on account of the remarkable

INTELLIGENCE IN INSECTS 211

military tactics displayed in its frequent warlike expeditions
against other ants might be expected to rank among the
most intelligent of insects. Polyergus is dependent upon
its slaves for food, having almost completely lost the in-
stincts for food taking from its long habituation to parasitic
habits. There is no physical peculiarity which prevents
these ants from getting their own food, and occasionally they
take liquid food when by chance the mouth parts are brought
in contact with it. As Lubbock, Wasmann and others
have shown the Amazons if deprived of their slaves will
starve to death in the midst of plenty without making
the least effort to secure food of their own accord. As
Wasmann observes, ''their hunger does not compel them
like other animals to seek for food themselves, but only to
beg food of other ants by taps of their feelers. The sensitive
perception of food placed immediately before them, in spite
of their feeling of hunger, does no longer excite in them the
natural impulse of tasting it.'' We should naturally expect
that a creature possessing the rudiments of intelligence would
be able to associate the appearance and odor of food with
the act of feeding. Possibly the Amazons might, if skillfully
managed, be taught to form this association, but that they
do not do so under the stress of starvation shows how poor
in resources is the emmet mind.

To the same purport we may cite the following quotation
concerning ants from Sir John Lubbock: "In order to test
their intelligence, it has always seemed to me that there was
no better way than to ascertain some object that they would
clearly desire, and then to interpose some obstacle which a
little ingenuity would enable them to overcome. Following
up, then, the preceding observations, I placed some larvae
in a cup which I put on a slip of glass surrounded by water,
but accessible to the ants by one pathway in which was a

212 INTELLIGENCE IN INSECTS

bridge consisting of a strip of paper 2/3 inch long and 1/3
inch wide. Having then put a Lasius niger from one of
my nests to these larvae, she began carrying them off, and
by degrees a number of friends came to help her. I then,
when about twenty-five ants were so engaged, moved the
little paper bridge slightly, so as to leave a chasm, just so
wide that the ants could not reach across. They came and
tried hard to do so; but it did not occur to them to push the
paper bridge, though the distance was only about 1/3 inch,
and they might easily have done so. After tr3dng for about
a quarter of an hour, they gave up the attempt and returned
home. This I repeated several times.

"Then, thinking that paper was a substance to which
they were not accustomed, I tried the same with a bit of
straw 1 inch long and 1/8 inch wide. The result was the
same. I repeated this more than once.

"Again I suspended some honey over a nest of Lasius
flavus at a height of about 1/2 inch, and accessible only by
a paper bridge more than 10 feet long. Under the glass I
then placed a small heap of earth. The ants soon swarmed
over the earth on to the glass, and began feeding on the honey.
I then removed a little of the earth, so that there was an
interval of about 1/3 of an inch between the glass and the
earth; but, though the distance was so small, they would not
jump down, but preferred to go round by the long bridge.
They tried in vain to stretch up from the earth to the glass,
which, however, was just out of their reach, though they
could touch it with their antennae; but it did not occur to
them to heap the earth up a little, though if they had moved
only half a dozen particles of earth they would have secured
for themselves direct access to the food. This, however,
never occurred to them. At length they gave up all attempts
to reach up to the glass, and went round by the paper bridge.

INTELLIGENCE IN INSECTS 213

I left the arrangement for several weeks, but they continued
to go round by the long paper bridge."

The facts above stated should render us suspicious of
conclusions regarding the high degree of intelligence which
ants have been supposed to manifest in certain of their
activities, especially in their powers of communication, in
their military manceuvers, and in the keeping of slaves and
guests. A foraging ant finds some sugar, or a dead insect too
heavy to carry to the nest, and she goes home, communicates
by means of striking with the antennae with other ants, and
then returns with several companions to her prize. Or it may
be that some of the "scouts" of a marauding species discover
a nest of a species preyed upon, and after visiting her
own nest and making her report, guides an expedition of
warriors to the habitation of the enemy. The older writers
on ants and some of the modem ones have made much of
their power of communication, and in reading their accounts
one might almost be led to believe that ants have a language
with a large vocabulary, and hold elaborate dissertations
on the food discovered, the whereabouts of their enemies,
their strength, and the most feasible way in which to conduct
an attack. That some power of communication exists has
been abundantly shown, but for the most part it consists
of signs instinctively made under certain conditions and
which are instinctively responded to by other ants. In
spite of the valuable investigations of several of the foremost
m3Tmecologists our knowledge of ant "language" is very
imperfect. Among the actions which have been considered
to be involved in communication are striking with the anten-
nae, butting with the head, opening the jaws, beating the
floor with the abdomen, and the production of sounds by
various kinds of apparatus for stridulation. What the
particular things may be which are signified by these various

214 INTELLIGENCE IN INSECTS

acts — ^if they do signify particular things — ^has not been
discovered. The general upshot of a long series of experi-
ments by Lubbock is that an ant, while having the power
of leading others to food, is unable to inform its comrades
as to the whereabouts of food so that they may reach it by
themselves. Lubbock allowed ants to take food and go back
to the nest; when these ants returned accompanied by several
companions the former were caught in order to discover if
their companions would then be able to find the food alone.
This they rarely succeeded in doing, although they would
scurry about in various directions as if seeking something.
These facts seem to show that the communication of the
location of food or other desirable objects is, as Romanes
has expressed it, "in the nature of some sign amounting to
no more than a 'follow me.^ ^^ While ants may not be able
to talk about things in their sign language, they apparently
express their different feelings and inclinations in ways which
are intelligible to other ants. Wasmann has compiled a
sort of vocabulary of signs made by the antennae — a "Worter-
buch der Fiihlersprache,'^ which is about as extensive as
Mr. Garner's language of apes. According to the vigor
and frequency of the strokes of the antennae, and the part
of the body stroked, the ant which is addressed may be im-
portuned for food, warned of danger, or induced to cooperate
with the communicant in various activities. Naturally one
is inclined to be skeptical regarding what seems like many
of the romantic tales of animal psychology, but Wasmann
is expressing little more than the general opinion among
students of ant life regarding the powers of communication
possessed by these insects, and the conclusions of so careful
and experienced a myrmecologist and one withal so little to
be suspected of a tendency to "humanizing the brute''
are deserving of the most careful consideration.

INTELLIGENCE IN INSECTS 215

The power of communication among bees is very limited.
As the result of a long series of experiments on honey bees
Lubbock has concluded that these insects do not lead one
another to places where they find food. After numerous
observations and experiments on bumble bees Wagner has
come to a similar conclusion for these forms. In both
hive bees and bumble bees the angry hum of a few bees is
taken up by others and a sort of communication of anger
spreads through the group. Similarly a note of distress, the
''heulen," of the hive bee which frequently follows upon
the loss of the queen, spreads from the bees which first
discover the loss. Another note is predominant in swarming
time, which sometimes evokes swarming activities in neigh-
boring hives (Buttel-Reepen). In bumble bees where the
language of sound is apparently more simple, the hum of the
wings, according to Wagner, serves solely as a warning of the
presence of danger — " die Huromeln mit Hilfe ihrer Flugel
nur von drohender Gefahr und von nichts anderem Kunde
geben konnen." All of the varieties of sound which the
bumble bees make with the aid of their wings have not the
least effect upon their comrades, with the single exception of
the peculiar note emitted in time of danger which serves
most efficiently to arouse other inhabitants of the nest.
There is nothing in the communication of ants or bees that
calls for the exercise of much intelligence. Their language,
like the language of animals everywhere, consists solely of
instinctively made and instinctively recognized signs.

BIBLIOGRAPHY

Bethe, a. Dtirfen wir den Ameisen und Bienen psychiche Qualitaten
zuschreiben? Pfltiger's Archiv., 70, 15, '98.
Noch einmal iiber die psychiche Qualitaten der Ameisen, I.e. 79,
39, '00.

216 INTELLIGENCE IN INSECTS

Die Heimkehrfahigkeit der Ameisen und Bienen, zum Teil nach

neuen Versuchen. Biol. Cent., 22, 193 and 234, '02.
Buttel-Rbepen H. von. Sind die Bienen Reflex-Machinen?

Biol. Cent. 20, 97, 177, 209, '00. Trans, by M. H. Geisler,

A. I. Root Co., '07.

Zur Psychobiologie der Hummeln. Biol. Cent. 27, 579, 604, '07.
CoRNETZ, V. Trajets de Fourmis et Retours au Nid. Mem. Inst.

G^n. Psych. No. 2, '10.

Le Sentiment Topographique chez les Fourmis. Rev. des Idees,

Dec, '09.

Un Regie de Constance dans les Trajets Loint^ins de la Fourmi

Exploratrice. Ibid, Dec. '10.
Emery, C. Intelligenz und Instinkt der Tiere. Biol. Cent., 13,

151, '93.
EscHERicH, K. Die Ameise, '06.

Die Termiten oder weissen Ameisen. Leipzig, '09.
Fabre, J. H. Souvenirs Entomologiques, 10 vols. Paris, '79-'04.
FoREL, A. The Senses of Insects (Translation.) London, '08.

Ants and Some Other Insects. (Trans, by W. M. Wheeler.)

Monist, 14, 33, '04.

Le M^moire du Temps et 1' Association des Souvenirs chez les

Abeilles. Bull. Inst. G4n. Psych. 6, 258, '06.
KiRBY. W. and Spence, W. An Introduction to Entomology,

4 vols. 1st. ed. 1815, 7th. ed. 1856.
Lubbock, Sir J. Ants, Bees and Wasps, N. Y., '83.

On the Senses, Instincts and Intelligence of Animals, with Special

Reference to Insects. N. Y., '88.
McCooK, H. C. Ant Communities, N. Y. '09.

Peckham, G. W. and E. G. On Instincts and Habits of the Soli-
tary Wasps. Wis. Geol. and Nat. Hist. Survey, Bull. No. 2,
' '98. Wasps, Social and Solitary. Boston, '05.
SoNDHEiM, M. Wahrnehmungsvermogen einer Libellenlarve. Biol.

Cent. 21, 317, '01.
Turner, C. H. Do Ants Form Practical Judgments? Biol. Bull.

13, 333, '07.

The Homing of Ants. Jour. Comp. Neur. Psych. 17, 367, '07.

The Homing of the Mud-dauber. Biol. Bull. 15, 215, '08.

The Homing of the Burrowing Bees (Anthophoridse) . Ibid,

15, 247, '08.

Experiments on Color-vision of the Honey-bee. Ibid. 19,

257, '10.

INTELLIGENCE IN INSECTS 217

Wagner, M. Psychobiologische Untersuchungen an Hummeln.

Zoologica, 46, '07.
Wasmann, E. Die psychiche Fahigkeiten der Ameisen. Zoologica,

Heft. 26, '99.

Comparative Studies in the Pyschology of Ants and of Higher

Animals (Trans, from 2nd. ed.), St. Louis, '05.
Wheeler, W. M. Ants, N. Y., '10.

CHAPTER XI
INTELLIGENCE IN THE LOWER VERTEBRATES

''To the minnow every cranny and pebble, every quality and
accident of its little native creek may have become familiar; but
does the minnow understand the ocean tides?" — Thomas Carlyle.

If an extra mundane observer were ignorant of the evolu-
tion of the vertebrates beyond the Silurian or the Devonian
epochs it is doubtful if he would pick out these animals as
the ones destined to surpass all others in psychic develop-
ment. The numerous species of highly organized cepha-
lopods that thronged the seas, the trilobites with their
highly developed organs of vision, the gigantic eurypterids
that crawled over the bottom of the shallow oceans, the
crustaceans, the terrestrial arachnids and the rapidly
evolving group of insects might all have been regarded as
having as much promise of future psychic development as the
back-boned "winners of life's race." And most of the
branches of the vertebrate tree really developed no further
than their invertebrate competitors. From among the
diverging branches of this phylum one only contained the
stock that led to the mammals and culminated in man.

A comparative anatomist looking back upon the course of
evolution might have said: The vertebrates were obviously
the forms with the most promising psychological future.
Many of these ancient forms doubtless possessed a cerebral
cortex, a sort of appendix to the central nervous system,
whose especial business it is to take care of the establishment
of associations. The opportunity was open to them through
the increase in size and complexity of the association centers

218

INTELLIGENCE IN LOWER VERTEBRATES 219

for a career of an almost imlimited mental development.
The molluscs and the arthropods, on the other hand, have a
different sort of a brain. They have no cerebral cortex ; there-
fore they cannot form associations and consequently they are
but more or less complicated "reflex machines." So with
certain of the vertebrates, the bony fish whose cerebral
cortex is represented by a membrane of non-nervous tissue
over the basal ganglionic centers, they too must be chained
down to the routine life of reflexes and instincts, with no
power of learning, no ability to profit by experience. So
our comparative anatomist might have argued and so indeed
some comparative anatomists have argued. This contention
as we have seen is far from justified in the arthropods, and
we shall see that it is equally groundless as regards the
vertebrates with no cerebral cortex.

Every angler can doubtless furnish evidence of the learning
of fishes. In trout streams that have been much frequented
the fish become much more wary of the bait than at first,
and some of the old, experienced fishes can be induced only
with great difficulty to take the line. On the other hand,
certain fish will allow themselves to be caught and hauled
out of the water repeatedly without conquering their pro-
pensity to dart at the bait.

No one can read very much in comparative psychology
without frequently encountering the story of Mobius' pike
— a story which the professor was fond of repeating in
his lectures and which came to be looked forward to as a
regular annual event by his students. This celebrated pike
was kept in a part of an aquarium separated by a glass plate
from an adjoining part which contained several minnows.
The pike made frequent dashes for the minnows and each
time received a bump against the glass plate. After about
three months of attempts to catch the minnows the pike

220 INTELLIGENCE IN LOWER VERTEBRATES

became convinced that his efforts were fruitless and they
were given up. The glass partition was then taken away.
The pike which had come to associate darting at the minnows
with bumps on its nose left the minnows unmolested there-
after, being apparently unaware of the removal of the im-
pediment to catching its prey. The interpretation of the
experiment of Mobius was questioned by Bateson, whose
experiments on the intelligence of fishes gave in general
negative results, but it was confirmed by the investigations of
Triplett on the educability of the perch. Triplett used an
aquarium divided in the middle by a glass plate; in one com-
partment he placed a male and a female perch for a period
of thirty minutes three times a week, after which they were
removed and fed on w^orms. Soon after the two perch were
put in one compartment some minnows were introduced
into the other. The perch immediately began the pursuit
and frequently butted their heads against the glass, especially
when the minnows drew near. The efforts of the fish,
especially the female, were very vigorous, but near the end
of the period both had given up the chase. In the next
trial the perch vigorously pursued the minnows, but with
somewhat less energy than in the first experiment. For a
month the behavior of the perch continued much the same.
The minnows and perch were then kept in the two parts
of the aquarium as in the experiments of Mobius and remained
there a week. The perch seldom collided with the partition,
although they watched the minnows frequently. When
angle worms were placed on the other side of the partition
the perch dashed violently against the glass for some time
in their efforts to reach them. They had learned to avoid the
minnows, but the suggestion aroused by the worms on which
thev were regularly fed proved too strong for them. After
the perch had ceased their futile efforts to catch the minnows

INTELLIGENCE IN LOWER VERTEBRATES 221

one of the latter was placed in the compartment of the larger
fish. It was treated at first much as before, but after a little
" the perch became more demonstrative toward it, but re-
strained themselves from striking, so long as it quietly-
avoided them/' Allien the minnow made a quick movement
the perch were more apt to dart after it and on one occa-
sion it had to be rescued. After a time the pursuit of the
minnows became less eager and although they were followed
the instinct to dart at them was generally inhibited.

The formation of an association in relation to the glass
partition was shown by the fact that when the partition was
removed the fish behaved much as if it still remained. "The
male swam out to the place, stopped, made little bumps
forward as if expectng to strike the usual obstruction, and
was plainly at a loss. He then turned and swam down as
if following the glass.'' Often the fish would swim to the
mark where the plate had been and then turn back.

These experiments showed that the natural instinct of the
perch to charge after the minnow was inhibited, although not
completely so, and that the fish came to associate the appear-
ance of a certain region of the aquarium with the experience
of being bumped. Pieron found that a cyprinoid fish,
Carassius auratus, would snap at live worms placed in a glass,
tube, but the number of attacks in a given time gradually
grew less on the following days. The glass tube was placed
in the water for twenty-seven minutes each time. In the
first experiment the fish made 117 attacks, on the three
successive days the attacks were 58, 38, and 25 in number.
Several days later the number of attacks had diminished
still more. The tube was then left in the aquarium and the
fish soon came to ignore it entirely, and even refused to eat
the worms when they were placed free in the water. When
the number of attacks on the tube diminished they could be

222 INTELLIGENCE IN LOWER VERTEBRATES

augmented if the tube was placed in a different part of the
aquarium or by feeding the fish with a few of the worms.
That fish readily associate certain stimuli with food has
been shown by several observers. Herrick found that
catfish which at first would seize pieces of cotton which were
brought in contact with its barbels would gradually cease
to react to them. After a few trials the fish would make a
movement toward the cotton, but it was soon checked.
Finally the cotton could be rolled over the barbels or other
parts of the body without eliciting any response. The
fish probably recognized the cotton thi-ough the sense of
touch as it made little difference if, at any time, red cotton
was substituted for white. Washburn and Bentley in their
experiments on the creek chub showed that this fish is able
to associate different colors such as red and green with food.
The chub in one set of experiments was fed out of a pair of
red forceps which were let into the water alongside of a green
pair. Although a meal worm was placed in the red pair the
fish often snapped at the green forceps first, but after a num-
ber of trials the green forceps were ignored. When the fish
had been given a number of worms the forceps were put in
empty. The fish continued to snap at the red pair and
avoid the green regardless of their relative positions. Red
and green forceps of different degrees of brightness were
used and precautions were taken to eliminate the in-
fluence of odor, but in the last 40 experiments in which no
food was placed in either forceps and the relative position
of the forceps exchanged after each trial the fish snapped at
the red pair every time. It is probable, therefore, that the
chub is able to distinguish colors as opposed to mere differ-
ences in brightness — although the proof of this is not en-
tirely conclusive — and it is very evident that it is able to
form associations with a fair degree of rapidity.

INTELLIGENCE IN LOWER VERTEBRATES 223

Many fish can easily be trained to come toward one for
food. I have trained goldfish so that they would swim
toward my hand to be fed, when I put it over the aquarium,
and I find upon enquiry that many others have done the
same. A sunfish which I kept for several months would
take bits of snail meat out of my fingers; whenever my
hand was brought near the water the sunfish would approach
and hold its mouth near the surface in readiness to snap
at the food. I gradually accustomed the fish to jump out
of the water for the food to a distance of nearly three inches.
I would often put my hand near the water as if I were
holding a piece of meat. The sunfish would almost invari-
ably come to the surface in a position of readiness for a
leap, but it would not jump at my empty fingers. A piece
of dark colored snail meat not much larger than a pin's
head or a small black mark on the end of my finger would be
sufficient, however, to cause a jump. I noticed also that
the fish came to be very attentive to my general movements,
and whenever I drew near the aquarium it would be found
pointed toward me with its head near the glass; and if I
walked around the aquarium the fish would faithfully follow.

Reighard in an investigation of warning coloration in fishes
fed a colony of snappers with small fishes of the genus
Atherina. Some of the Atherinas were stained red and
others were unstained, some of the red ones were rendered
unpalatable by putting in their mouths parts of the ten-
tacles of a large jelly fish, which were plentifully supplied
with nettling cells. Red Atherinas were readily taken
when thrown into the water, but those which had been made
unpalatable with the tentacles of the jelly fish, while snapped
up at first, were generally avoided after a few trials. There-
after the snappers avoided the red Atherinas which had not
been rendered unpalatable. They had come to associate the

224 INTELLIGENCE IN LOWER VERTEBRATES

red color with the irritation caused by the tentacles so that
the red came to be a "warning color." Subsequently un-
colored Atherinas were given them which were for the most
part instantly taken. Red Atherinas were offered after
an interval of eight days when they were very sparingly
taken and then refused entirely, and again after an interval
of twenty days when they remained entirely untouched.

There are a number of instances of fishes coming to be fed
when a bell was rung or some other sound made, but it is
probable as Kreidl has shown in one instance that the fish
associated the food with the appearance of the person
making the sound rather than with the sound itself.

The memory of topographical relations seems to be well
developed in certain fishes as in certain insects. It is mani-
fested most clearly in those forms which have a more or
less fijced habitation or which build a nest for their eggs.
A good illustration of this faculty is afforded by a species
of Goby studied by Mile. Goldsmith. This species, Gohius
minutus, is commonly found in tide pools under a shell of
some bivalve mollusc where it may lie half buried in the
sand. That the fish recognizes its shell by sight is shown
by the fact that when Mile. Goldsmith drove a specimen
from under its shell and placed the aquarium where it was
kept in the dark the fish did not succeed in finding its shell
during twenty-four hours: when light was admitted it dis-
covered the shell at once. The experiment was repeated
many times with different individuals, with the same result.

The ability of Gobius to learn a certain path to its shell
is shown in the following experiment : A goby was placed in
an aquarium divided in the middle by a glass plate which
left a narrow passage way at one end. The shell of the
goby was placed in one compartment C and the fish was
driven through the passage way into the adjoining part of

INTELLIGENCE IN LOWER VERTEBRATES 225

the aquarium. In its endeavors to reach its nest the goby
swam against the glass partition ten times and then found
the passage. It was then driven from its shell into the
adjoining compartment again. It now rammed its head
against the partition six times, then found the passage and
entered its shell. In the next trial the fish made only three
or four attempts to go through the glass plate, and after
this it found the passage way at once. In a short time the
fish had learned the way to its nest and thereafter followed
it with little hesitation.

After this set of trials the goby, now left to itself, began
to explore the aquarium of its own initiative, by making
excursions farther and farther from the nest, keeping close
to the outer wall and returning each time by the same route
by which it set out. When it arrived at the partition it
bumped against it, but after two or three trials it stopped
a little short of the barrier and continued to do so in its
subsequent excursions.

After the habit of avoiding the glass plate had become
well established the plate was removed. The goby never-
theless continued to follow the old path in its journeys to
and from the nest. Even when the fish had happened two
or three times to cross the place where the partition had
been, it still persisted in taking the old round-about course
from one part of the aquarium to the other. Finally after
the fish had crossed directly several times the old habit
became gradually broken up and the fish paid no attention
to the place where the partition had been located. In one
interesting experiment Mile. Goldsmith filled the shell of a
goby with mastic and placed it in its old position. The
goby came to its shell and endeavored to get under it, trying
first its accustomed point of entrance, and then making
attempts to dig under it in various other places. It dug

15

226 INTELLIGENCE IN LOWER VERTEBRATE^

each time in a new place as if bearing in mind its failures
at other points. After a time the goby seemed to become
discouraged and left the shell, but returned at times and
resumed its attempts to enter. After an hour and a half
the goby gave up its efforts entirely. The next day the
mastic was removed and the shell placed in its original
position. After several hours the goby had not entered
the shell and swam by it for a long time without giving it
the least attention. The shell was then removed to another
part of the aquarium. As soon as the goby perceived the
shell it quickly made for it and installed itself under it as
if it had discovered a new shell instead of the old one. With
the goby as with the mason bee, Chalcidoma, the place
which a thing occupies is its chief recognition mark. The
same shell in a new place was for the goby a new object,
with promise of being a suitable domicile which, it had come
to recognize, the shell in the old place was not.

In this connection the experiment of Lloyd Morgan on
the behavior of a male stikleback is of interest. "A nest
had been built on a round glass bell jar which stood near
a window. Some aquatic vegetation grew in the tank,
and the nest was built on the window side. An experiment
was made by turning the large bell jar through a right angle.
The male stickleback searched for its nest in the old direc-
tion on the window side, that is to say, the same position in
reference to the incidence of the light. The search was, of
course, fruitless, and a new nest was begun in this position.
Presently the old nest was discovered, and was then vig-
orously destroyed in just the same way as the nest of a rival
is pulled to pieces and scattered. Here a new incidence of
light and new direction of shadows seemed to have com-
pletely transformed the visual situation."

The Amphibia, notwithstanding the fact that they have

INTELLIGENCE IN LOWER VERTEBRATES 227

a cerebral cortex, which is lacking in the bony fishes, cannot
be said to surpass the latter in the development of intelli-
gence. The tailed amphibians are notoriously sluggish and
give every appe^ance of extreme stupidity. Nevertheless
they show a remarkably acute and delicate sensibility and a
power of very rapid action on occasions which is beHed by
their general appearance and customary slow movements.
Our knowledge of their intelligence is slight. Few have
attempted to educate such apparently unpromising subjects
and neither Brehm, Buchner nor Romanes gives any re-
markable cases illustrative of their sagacity. Professor
Whitman, in his interesting account of the behavior of
Necturus, considers that this animal has a certain amount of
intelligence which is involved in its learning by experience
how to direct its movements, but he assumes nothing be-
yond this. Pieron has observed that if worms enclosed in
a glass tube are introduced into an aquarium wdth specimens
of Triton, the amphibians make a large number of attempts
to seize the worms before showing any noticeable falling
off in the number of their fruitless efforts; fish under similar
circumstances learn to avoid the worms much more quickly.
In another experiment a quadrangular flask containing
several worms was placed on its side for a half hour a day
in an aquarium, containing six tritons. Sometimes the
animals found the entrance by chance and ate some of the
worms. With the exception of one individual which seemed
to learn the way fairly quickly there was no marked increase
in the facility with which the tritons entered the flask. The
experiment, while not thorough-going, indicates that asso-
ciations in these animals are usually formed but slowly.

Among the Anura there have been several studies on the
intelligence of frogs and toads. These animals are capable
of forming simple associations, but their powers are very

228 INTELLIGENCE IN LOWER VERTEBRATES

limited. Abbot placed a large fly on a piece of thin mica
surrounded by needles so that the frog would be pricked
in its efforts to seize it. This did not prevent the creature
from snapping at the fly indefinitely. "In one instance
the frog, which had been fasting for seventy-two hours,
continued to snap at the needle protected fly until it had
entirely skinned its upper jaw.'^ This convinced Abbot
that "the wits of a frog were too limited to be demon-
strated." Knauer found that frogs would snap at worms on
the other side of a glass plate and persist in doing so at inter-
vals all day without learning that their attempts were futile.

On the other hand, several cases are recorded of frogs as
well as toads being trained to come for food. Toads often
occupy for a long time a particular retreat to which they
return after making their nocturnal tours, thus showing
that they have a memory for location. Yerkes in his stud-
ies of habit formation in frogs finds that frogs learn very
slowly. In the experiments performed the frogs were placed
in a long box furnished with a tank filled with water at one
end and divided so that the frog would have to take a cer-
tain course from the point of entrance to get into the tank.
The labyrinth employed was very simple, but it took from
50 to 100 trials for a frog to learn to avoid the closed pas-
sages and to reach the water by the most direct route. The
associations once formed were found to persist for over a
month.

Schaeffer^s recent experiments on habit formation in
frogs have shown that frogs may learn to avoid disagree-
able objects after a very few trials. When offered hairy
caterpillars the frogs would eagerly snap at them and then
quickly eject them from the mouth. After from four to
seven such experiences the frogs came to leave the cater-
pillars alone.

INTELLIGENCE IN LOWER VERTEBRATES 229

In another set of experiments frogs were offered earth-
worms which had been dipped in chemicals. These worms
were frequently snapped at and swallowed. The diet pro-
duced in some cases symptoms of uneasiness and some of
the frogs would avoid eating earthworms for several days
afterward, although they would partake freely of other
kinds of food.

An apparatus was arranged so that the frogs would re-
ceive an electric shock every time they snapped at a worm.
These frogs would avoid food altogether for a few days
after the shock. ' Whether an association was formed in
this case, or whether the result is due simply to the per-
sistance of the effect of the strong stimulus is uncertain.
That the frogs learn to avoid certain kinds of food more
quickly than they learn to follow a particular path may,
as Schaeffer suggests, be due to the fact that the discrim-
ination of food is so common an experience in the course
of a frog's daily life. The greater severity of the penalty
for error may be also a factor.

WTiat has been written on intelligence of reptiles is for
the most part in the form of scattered, casual observations.
We have several records of the taming of different reptiles,
of their following their keepers, their distinguishing between
different persons, and their coming when called. Delboeuf
kept two lizards in captivity and in time they became quite
attached to their keeper. They would run to him from
across the room when called and crawl upon his body in the
hope of being fed. Each showed jealousy if any attention
were paid to the other.

Gilbert White in his Natural History of Selboume gives
an account of a tortoise which distinguished between dif-
ferent persons and became much attached to an old lady so
that "whenever the good old lady came in sight who had

230 INTELLIGENCE IN LOWER VERTEBRATES

waited on it for more than thirty years, it always hobbled
with awkward alacrity toward its benefactress, while to
strangers it was altogether inattentive. '* Jesse writes of a
young alligator which followed its master about the house
like a dog, " scrambling up the stairs after him, and showing
much affection and docility/'

The formation of habits in turtles has been studied by
Yerkes. A simple labyrinth was employed through which
the turtle was left to find its way. Fifty trials were made,
six or eight being given each day, and the time recorded
which the turtle required to make its escape. The way was
learned with a fair degree of rapidity, the time taken in
successive trips being shortened rapidly at first and then
more slowly.

In other experiments turtles learned not to fall off a board
after a number of trials. More recently habit formation
has been studied in the turtle Chrysemis by Cast eel, who
found that the animals learn to discriminate between colors
and to distinguish different series of parallel lines of the
same size, but with the lines of different width. Learning
was slow, since on the average " 183 trials were necessary to
establish discrimination." As in the frog what was ac-
quired was not soon forgotten; one specimen showed "per-
fect memory" for a line pattern two weeks after it had
been learned.

In general it may be said that the intelligence of reptiles
is on a higher level than in fishes and amphibians. The
subject is one upon which we have little well established
information, and it affords an interesting field for future
investigation.

BIBLIOGRAPHY

Bateson, W. On the Sense Organs and Perceptions of Fishes.
Jour. Mar. Biol. Ass. United Kingdom, 1, 225, '87.

INTELLIGENCE IN LOWER VERTEBRATES 231

Casteel, D. B. The Discriminative Ability of the Painted Turtle.

Jour. An. Behavior, 1, 1, '11.
Delboeuf, J. Quelques Reflexions Generales k propos de la Psy-

chologie des Lezards. Rev. Scient. 4, 805, '95.
Edinger, L. Haben die Fische ein Gedachtniss? Allg. Zeitung.

'99. (Trans, in Smithsonian Rep., '99, 375.)
Goldsmith, M. R^cherches sur la Psychologie de quelques Poissons

Littoraux. Bull. Inst. Gen. Psych., Paris, 5, 51, '05.
Gurley, R. R. The Habits of Fishes. Am. Jour. Psych., 13,
408, '02.
Herrick, C. J. The Organ and Sense of Taste in Fishes. BuU.

U. S. Fish Comm., 22, 237, '03.
Holmes, S. J. The Biology of the Frog, N. Y. '06.
Landois, H. Haben die Fische Gedachtniss? Zool. Garten, 38,

124, '97.
Lecaillon, a. Sur quelques Faits Relatifs k I'Ethologie et la

Psycho-Physiologie des Batraciens. Bull. Inst. G6n. Psych.,

8, 142, '08.
MObius, K. Die Bewegung der Thiere und ihr psychischer Horizont.

Schrift. d. naturwiss. Ver. f. Schleswig-Holstein, 1, 113, '73.
Reighard, J. An Experimental Field-study of Warning Coloration

in Coral-reef Fishes. Carnegie Inst. Pubs. 103, '09.
ScHAEFFER, A. A. Habit Formation in Frogs. Jour. An. Behavior,

1, 309, '11.
Thorndike, E. L. a Note on the Psychology of Fishes, Am. Nat.,

33, 923, '99.
Triplett, N. B. The Educability of the Perch. Am. Jour. Psych.,

12, 354, '01.
Washburn, M. F. and Bentley, I. M. The Estabhshment of an

Association Involving Color Discrimination in the Creek Chub,

Semotilus atromaculatus. Jour. Comp. Neur. Psych., 16, 113, '06.
Yerkes, R. M. The Instincts, Habits and Reactions of the Frog.

I. Associative Processes of the Green Frog. Harvard Psych.

Studies, 1, 579, '03.

Space Perceptions in Tortoises. Jour. Comp. Neur. Psych.,

14, 17, '04.

CHAPTER XII
THE INTELLIGENCE OF MAMMALS

"Le premiere partie de cet ouvrage d^montre que les betes sont
capable de quelques connoissances. Ce sentiment est celui du
vulgaire: il n'est combattu que par des philosophes, c'est k dire,
par des hommes qui d' ordinaire aiment mieux une absurdity qu'ils
imaginent, qu'une v^rite qui tout le monde adopt." — Condillac,
Train des Animaux.

"The power of association over brutes is very evident in all the
tricks which they are taught; and the whole nature of each brute,
which has been brought up among others of the same species, is a
compound of instinct, his own observations and experiences, and
imitation of those of his own species." — Hartley, Observations on
Man.

" Animals pass from one imagination to another by the connection
which they have felt before; for example, when his master takes a
stick, the dog fears a whipping. And in many instances children
with the rest of mankind proceed nowise differently in their passages
from thought to thought." — Leibnitz, New Essays Concerning
Human Understanding.

Until quite recently most of our knowledge of the psy-
chology of mammals, as of other animals, was obtained
simply by watching them. In this way has been accumu-
lated a large fund of information concerning their instincts
and habits, and to a certain extent their general intelligence.
But in this as in other fields of investigation, the method of
experiment has come to be indispensable when the attempt
is made to study the phenomena analytically. There is of
course no especial magic in the experimental method; it is
simply a means of improving the conditions of observation.
And in animal psychology especially, the method may have
drawbacks which counterbalance some of its advantages.

232

THE INTELLIGENCE OF MAMMALS 233

The labyrinth and puzzle box devices of our psychological
laboratories have the advantage of enabling us to study
animal behavior under strictly controlled conditions, and
they readily yield results in a form capable of easy tabula-
tion, but they have been criticized with a certain measure
of justice on the ground that the artificial conditions to
which the animals are exposed make them appear more
stupid than they really are. A fox in its wary prowlings
for prey or in its attempts to outwit its pursuers may mani-
fest a considerably higher degree of intelligence than it
would show if confined to a box from which it had to liber-
ate itself by raising levers and pulling bolts. In wild an-
imals especially there is a falling off of spirit and initiative
when they are placed in an artificial environment. The
lion of the desert is a very different creature from the lion
of the circus; the keenness and alertness with which the one
conducts his hunt for prey, stand in marked contrast with
the melancholy and reluctant performances of the other
when under the trainer's whip. Other animals such as
raccoons and some monkeys take to a life of confinement
much more readily, and therefore afford particulai'ly valu-
able subjects for experiment. Those experiments are of
the most value which stimulate to the greatest extent the
free exercise of an animal's faculties, and in order to secure
this result an animal's instinctive interests and promptings
should be given, so far as possible, free play.

The experiments of the last few years have given us a
more just estimate of the nature and limitations of the
intelligence of the higher animals than we formerly possessed.
Animal psychologists have come to scrutinize their results
much more closely and to be much more cautious in their
statements regarding what goes on in the mind of the animal
studied. The interpretation of what mental processes are

234 THE INTELLIGENCE OF MAMMALS

involved in most animal performances is a matter of much
difficulty. We may be guided on the one hand by analogy
with ourselveS; which leads us to infer that actions similar
to our own are accompanied by similar mental states; and
by the law of parsimony on the other, which forbids us to
assume the existence of higher mental qualities if the phe-
nomena can be explained in terms of simpler mental pro-
cesses. In the imperfect state of our knowledge these two
guides often lead to opposed conclusions. If we applied the
principle of Morgan to the psychology of our fellow human
beings we should be continually led astray. So in our inter-
pretations of the psychology of the higher animals we may
very frequently be "missing it^' more or less widely in our
adherence to this principle. The antecedent probability
in favor of not giving the animal the benefit of the doubt
diminishes as we ascend the scale of psychic life. We may
suspect that our interpretations "fall short/' but our opin-
ions cannot be said to rest on a secure basis until we are
able to support them by experimental proof. The principle
of Morgan affords a check to the natural tendency to "an-
thropomorphism^' which is a common human failing; it
throws the burden of proof on whomsoever attempts to es-
tablish the existence in animals of higher faculties, and if
the positive conclusions to which it permits us to come fall
short of the truth we can at least rely on them so far as
they go.

As a typical instance of the workings of the animal mind
we may cite the performances of Professor Lloyd Morgan's
dog, Toby, which had learned how to open a gate that led
out of his master's yard. The gate was fastened by a latch,
but swung open by itself if the latch was raised. Whenever
the dog desired to make his escape he put his head between
the bars, lifted the latch and went out. Such an act- might

THE INTELLIGENCE OF MAMMALS 235

of course have been the result of the dog's studymg the
hinges, latch and general make-up of the gate, and conclud-
ing that if the latch were raised the gate would be free to
swing open. Such a course would be a very natural one
for a human being, but few would consider that a dog
would be likely to follow it. The dog might, however,
learn to open the gate by watching someone do it and then
imitating him. In this case the dog might be thought to
conclude that "since a man lifted the latch and went out,
therefore, I can lift the latch and go out." As a matter of
fact the dog learned to make his escape in neither of these
ways. His method of learning the trick which was watched
from the beginning was as follows: Being placed in the
yard from which he was anxious to escape Toby poked his
head between the bars of the fence in various places and by
chance placed it under the latch and raised it, when the
gate swung open and he scampered out on the street. The
method pursued was one of trial and error. The fortunate
movement which effected the dog's liberation was associated
with the perception of the latch, but the association was not
perfect at first. Only after ten or twelve experiences, in
which the number of times the dog poked his nose through
wrong places gradually diminished, did he learn to go
directly to the right place, and raise the latch.

The experiments of Thorndike have convinced him that
the intelligence of animals is limited to the type that we
have just considered. He holds that animals do not draw
inferences and that, barring the apes, it is doubtful if they
have ideas. Thorndike's experiments were among the
first systematic attempts to get at the nature and limitation
of animal intelligence by means of experimental methods.
Through his pronounced spirit of iconoclasm toward anec-
dotal psychology and anthropomorphism Thorndike was

236

THE INTELLIGENCE OF MAMMALS

led to adopt an extreme position which has not been justi-
fied by future experiments and which was not consistent
with many of his own results. Though extreme, the reac-
tion against the method of writers who based their deduc-
tions concerning animal intelligence on stories and casual
observations was none the less wholesome, as it succeeded
in stimulating the study of animal intelligence by more
careful and critical methods than those formerly employed.

FiQ. 15. — ^Puzzle box used in the experiments of Thorndike on cats.
(After Thorndike.)

In most of Thorndike^s experiments boxes were employed
from which animals could escape by raising a lever, pulling
a cord, or by some such simple device. In some cases vari-
ous combinations of these devices were used. A hungry
cat or dog was confined in the box and food was placed on
the outside so that it could be seen. The animal in its
efforts to get out and obtain the food would usually begin
by biting and clawing the bars of the box. Sooner or later
a lucky movement would raise a lever or pull a cord so that
the door of the box would open and allow the animal to get
the food. After this the animal would be put into the box

THE INTELLIGENCE OF MAMMALS 237

a second time, when it usually recommenced its bitings and
clawingS; but it generally effected its escape more quickly
than at first. The experiment was then repeated until the
animal had perfectly mastered the means of escape. The
time taken to escape was recorded and was found to di-
minish, as a rule, with successive trials.

In one set of experiments a cat was liberated from the
box whenever she licked herself, and in another set when-
ever she scratched herself. Although there could be no
perceptible relation between the means employed and the
end achieved, the cats learned to make the appropriate
movement after being put in the box, although their asso-
ciation curves showed a gradual descent. There is a curious
tendency for the act to be performed less and less vigorously.
"The licking degenerates into a mere quick turn of the head
with one or two motions up and down with tongue extended.
Instead of a hearty scratch, the cat waves its paw up and
down rapidly for an instant."

Previous experience is a factor which influences the quick-
ness of forming associations. "After getting out of six or
eight boxes by different sorts of acts the cat's general tend-
ency to claw at loose objects within the box is strengthened
and its tendency to squeeze through holes and bite bars is
weakened; accordingly it will learn associations along the
general line of the old much more quickly. Further, its
tendency to pay attention to what it is doing gets strength-
ened, and this is something which may properly be called
a change in degree of intelligence." None of the acts per-
formed by cats and dogs in his numerous experiments
exhibits, according to Thorndike, any power of reasoning
and usually no association of ideas. A point upon which
Thorndike lays especial stress is the gradual descent of the
curves representing the times required in forming the asso-

238 THE INTELLIGENCE OF MAMMALS

elation. If there were any element of inference involved
there ought to be, according to him, a sudden vertical de-
scent of the time curve. " Where the act resulting from the
impulse is very simple, very obvious, and very clearly de-
fined, a simple experience may make the association per-
fect, and we may have an abrupt descent in the time curve
without needing to suppose inference. But if in a complex
act, a series of acts or an ill-defined act one found a sudden
consummation in the associative process, one might very
well claim that reason was at work. Now, the scores of
cases recorded show no such phenomena. The cat does not
look over the situation, much less think it over, and then
decide what to do. It bursts out at once into the activities
which instinct and experience have settled on as suitable
reactions to the situation, ^confinement wlien hungry with
food outside.' It does not ever in the course of its successes
realize that such an act brings food and therefore decide to
do it and thenceforth do it immediately from decision
instead of from impulse. The one impulse, out of many
accidental ones, which leads to pleasure, becomes strength-
ened and stamped in thereby, and more and more firmly
associated w^ith the sense-impression of that box's interior.
Accordingly it is sooner and sooner fulfilled. Futile im-
pulses are gradually stamped out. The gradual slope of
the time curve, then, shows the absence of reasoning. They
represent the wearing smooth of a path in the brain, not the
decisions of a rational consciousness."

Even ideas are unnecessary, according to Thorndike, to
account for most feats of animal intelligence. The cat,
which after having made a lucky movement and escaped
from the box and got some fish, might be supposed to asso-
ciate the appearance of the mechanism of escape with the
idea of the pleasure resulting from eating the food. But,

THE INTELLIGENCE OF MAMMALS 239

according to Thorndike, the learning of the cat may be
accounted for more simply. The sight of the means of
escape in the box, instead of calling up the idea of the pre-
viously experienced good taste of the fish, has become
associated simply with the movement necessary to effect an
escape. The consciousness involved in the acts consists
therefore of immediate sense impressions and the impulses
to action with which they have been associated; the animals
"have no images or memories at all, no ideas to associate."

The learning of the animal, according to this view, is on
a level with the semi-conscious perfection of many of our
own activities, such as catching a ball and playing tennis,
where certain perceptions come to call forth very quickly
the appropriate motor response without the intervention
of ideas. We do not reason about our movements in such
cases but the right ones come to be performed as the result
of stamping in the movements that brought success. Few
of us have paid particular attention to the movements of
the tongue in chewing food, but a little observation directed
to the subject cannot fail to impress one with the deftness
with which this member, otherwise so unruly, avoids being
bitten, and with the efficient way in which it helps to masti-
cate the food. The tongue probably performs many of its
movements instinctively, but it has doubtless learned a good
deal by experience, without being consciously directed.

Thorndike's theory of the nature of animal intelligence
has the principle of parsimony in its favor, but its author
has endeavored to give it further support by a number of
experiments designed to test the presence of ideas in the
animal's mind. One of these was as follows: A cat was
made to go through a door into a box where she was shut in.
By pulling a loop the cat gets out and eats fish. After being
put in through the door a number of times and fed when-

240 THE INTELLIGENCE OF MAMMALS

ever she made her escape, the cat came to enter the box of
her own accord. Does she associate the idea of being in
with the idea of eating fish and enter accordingly? Thorn-
dike endeavored to answer this question by dropping cats
into the box through a door in the top, and then feeding
them as before when they got out. The cats had the same
opportunity of associating the idea of being in the box with
the idea of eating fish, but the element lacking was the
impulse to walk in through the door. All of these cats,
three in number, failed to enter the box after fifty, sixty
and seventy-five trials respectively. "Either a cat cannot
connect ideas, representations, at all," says Thorndike,
*^or she has not the power of progressing from the thought of
being in to the act of going in. . . . The impulse is the sine
qua non of the association. The second cat has everything
else, but cannot supply that."

In several other experiments cats and dogs were placed
in a box, and the experimenter would take the paw of the
animal and make the movement necessary to open the box,
after which the animal would be allowed to go out and get
food. This was repeated ten or fifteen times and then the
animal was left to its own devices. After numerous ex-
periments of this sort with various kinds of boxes it was
found that the animals uniformly failed to profit by this
mode of instruction. None of the animals which failed to
get out of a box of their own accord succeeded in escaping
after having been several times put through the necessary
movements. They had the opportunity to associate the
idea of certain movements with escaping and getting food,
at least provided they paid attention to what was being
done with them. But the animaPs own impulse to do the
act was lacking. ''The animal cannot form an association
leading to an act unless the particular impulse to that act

THE INTELLIGENCE OF MAMMALS 241

is present as an element of the association; he cannot supply
it from a general stock. The groundwork of animal asso-
ciations is not the association of ideas, but the association
of idea or sense-impression with impulse.'*

Notwithstanding his general attitude of opposition to the ^
doctrine of association of ideas in animals, Thorndike re-
cords a few experiments which led him to the conclusion
that in certain cases such associations may occur. In one
case a hungry cat was placed in a box and the experimenter
sat about eight feet away from it. At intervals of about
two minutes he would say, "I must feed those cats." Ten
seconds afterward he would take a piece of fish, go to the
box and hold it so that the cat was compelled to climb up
the front of the box to obtain it. Would the cat after a
number of trials come to associate (A) the sound of the
words with (B) the sense impression of the experimenter's
movements in taking the fish and walking to the box, and
climb up (C) before it had experienced the second term (B)
of the association? If so Thorndike concludes that the
action of the cat " is to be explained by the presence through
association, of the idea (B)." The possibility is left open
"that (A) was associated directly with the impulse to (C),
although that impulse was removed from it by ten seconds
of time." But Thorndike thinks this is '' highly improbable,
unless the neurosis of (A), and with it the psychosis, con-
tinues until the impulse to (C) appears. But if it does so
continue during the ten seconds, and thus get directly linked
to (C), we have exactly a representation, an image, a memory
in the mind for eight or ten seconds." Leaving out of ques-
tion the existence of an idea of (A), during the interval,
why should we say that it is "highly improbable" that
(A) should become directly linked with (C)? The implica-
tion of the argument is that the time separating the two

242 THE INTELLIGENCE OF MAMMALS

events is too great. But if (A) can be associated with (B)
over an interval of ten seconds why can it not be directly
associated with the impulse (C) over an interval but slightly
greater? Ten seconds after (A) the cat sees (B), eleven
seconds after she performs (C). That the cat had pre-
viously associated (B) and (C) does not necessarily play
any part in the process. The experiment proves only that
some sort of a neurosis persists from (A) during ten or more
seconds of time; but it fails to afford any proof of the r6le of
the idea (B) in the process. It shows the persistence of
impressions, not the association of ideas. Indirectly it
may support the theory of association. If the neurosis of
(A) persists and is accompanied by consciousness in the
form of an idea of (A), ideas of (B) probably occur also, and
if ideas occur why may they not become associated as well
as impressions and impulses? Such ideas may be more like
after-images in ourselves, but no sharp line can be drawn
between the latter and ideas properly so called. While
such an experiment as the one described may not prove the
association of ideas it may serve to make association more
probable by showing the persistence of impressions.

The experiments of L. W. Cole on the raccoon yielded
better evidence of the existence of ideas than the investiga-
tions of Thorndike, owing perhaps to the greater degree of
intelligence of the animals employed. Raccoons learned to
get out of a box with seven fastenings consisting of two
buttons, two loops, a thumb latch, a treadle and a hook.
They were first put into boxes with one or two of these
devices, and when these were learned others were added until
the above combination was reached, which seemed to be
about the limit of a raccoon^s learning capacity. The rac-
coons in attacking the fastenings did not take them up in
any constant order in successive trials, but they showed a

THE INTELLIGENCE OF MAMMALS 243

good memory for any combination they may have learned.
Three raccoons which had learned to open the box with
seven fastenings were put into the box again after an interval
of 147 days. Only one individual succeeded in undoing all
of the seven fastenings and escaping. His times in four
successive trials were 34, 28, 131, and 182 seconds. The
other two worked most of the fastenings, but generally
failed to undo a horizontal lock. Da\ds found that rac-
coons remembered how to undo the fastenings of a puzzle
box for over a year.

One significant feature of the raccoons' method of attack
on their problems is that they employ different means of
accomplishing the same result. They worked fastenings
with either the right or the left paw or with both together.
"All of the raccoons turned a button once or twice with
the nose in early trials, then settled down to w^orking it with
the paw." This looks very much as if there were something
besides the sensori-motor associations assumed by Thorn-
dike. We cannot say that there is nothing but the asso-
ciation of the sight of a certain object with a particular
impulse to movement, if in effecting a certain change an
entirely different organ is substituted for the one pre-
viously employed. When an animal moves with its paw a
fastening it formerly moved with its nose it gives evidence
of being guided by an idea of what it is setting out to
accomplish.

Evidence for the existence of ideas was derived also from
other sources. It was found that raccoons were able to
associate being in a box with getting food after they came
out, so that when they were dropped in, as in the experi-
ment with the cats, they came after a while to go into the
box of their own accord. The motor impulse to enter the
box was not in these cases an element in the association

244 THE INTELLIGENCE OF MAMMALS

that was formed. Apparently the thought of bemg in the
box made the raccoons go in.

Cole found that the raccoons were aided in learning the
mechanism of escape from the box if they were put through
the appropriate movements. Not only was the average
time required for such animals to escape from the box short-
ened as compared with the time required by the individuals
which were unaided, but several animals which failed entirely
to make their escape succeeded in getting out after having
been several times put through the act. This result may
be indicative of the existence of ideas, but not entirely con-
clusive, as it might be explained possibly as the result of the
animaFs attention having been limited to a certain part of
the box. This might direct his efforts toward the spot
where he would be more likely to hit upon the fortunate
movement. Raccoons which had come to climb upon a
box and enter through a hole in the top after having several
times dropped through the hole would dodge in through the
door if it were left open after their exit. As in the cases
where the raccoons learned to escape by themselves, they
used different paws to undo various fastenings, although
one fore paw was almost always used when the experimenter
guided their actions. Whether or not the raccoon re-
peats the action he is put through depends much upon his
convenience. If it is the easiest way he will continue as he
was taught; if not he is apt to substitute some other method.

If a raccoon has learned to undo a latch, turn a button, or
pull a loop, he attacks the same object when it is placed in ^
different part of the enclosure. Davis, however, attained
a different result with some of his raccoons, which would
often spend considerable time in vain clawing about where
the fastening had been placed. Probably these different
results depend upon differences in the previous experiences

THE INTELLIGENCE OF MAMMALS 245

of the animals, or their differences in temperament and
general intelligence, which Davis finds are very marked.
In one case a loop was left lying upon the top of the box.
When seen by the raccoon it was clawed back into the box
and then pulled. Is this merely the blind association of
perception with motor impulse? I think not.

Cole found that if food was put in a box and the place
where the raccoon had formed the habit of entering was
closed, the animal would attempt to enter the box at other
places. In one experiment a tight box was used with a
hole through the top. After the raccoon had entered
through the hole and obtained food a number of times the
hole was closed and an opening made under one side of the
box. At first the animal attempted to enter in the usual
way, but finding his passage barred soon left it; after a
second attempt he discovered the side opening, entered and
secured the food. The side opening was then closed and a
wire cylinder eighteen inches high placed on end over the
original opening at the top of the box. Thirty seconds
after the raccoon was released he had climbed up the outside
and down the inside of the cylinder and entered the hole.
The piece of apple in the box could not be seen, nor smelled,
according to Cole, for " the room was full of the odor of apple.''
The animal apparently retained an image of the apple in the
box and realized that if he could not reach it in one way he
might in another.

Such behavior, in which an animal's activity seems to be
directed to achieving a certain end, is not uncommon. A
case described by Hobhouse is suggestive. A dog was
held at the back of a house with which he was unfamiliar
and saw his master enter by the back door and appear at a
window in the same side of the house. "After trying to
follow his master through the back — unsuccessfully, because

246 THE INTELLIGENCE OF MAMMALS

the door is shut — he makes off round two corners to the
front door, and so into the dining-room. He had never
been in this room before, but has once been from the back
into the house by the front door. The experiment is once
repeated, and the dog remembers this route five days later.
On arriving at the house on this occasion, he is taken through
a side door into the dining-room, and then out at the back.
He first finds his way in through the front as mentioned, and
then for a further trial both front and back door are shut.
The dog goes to and fro from one door to the other, and then
suddenly goes right off around the house, and in by the side
door — a route which he had never taken before. There may
have been an element of chance in this success, but, on the
whole we seem to have a series of acts dictated by the desire
to find the master operating on the remembrance of the
modes of entrance.'^

The ability of animals such as horses to find their way
back for miles over a road which they have only followed
once is indicative of something more than mere sensori-
motor association. I well remember a horse we once
owned whose memory for the proper turns in the road he
had taken in going away from home I had often tested and
found to be almost infallible. Like many other horses he
was a much more willing traveller when homeward bound,
but whether he was influenced by an idea of hay and oats
and rest to be enjoyed at the end of his journey it might be
hazardous to say. If the homing of the animal were due
to a blind sensori-motor association, we should have to
assume that the sight of particular objects along his course
came to be associated with particular movements; object
A, for instance, with a slight turn to the right, and object
B with a slight turn to the left, and so on. If the animal
passed over the road again in the same direction we might

THE INTELLIGENCE OF MAMMALS 247

suppose that these various objects, by association, called
forth muscular impulses similar to those originally given,
thereby causing the animal to keep the same path as before.
But on the return the objects are not only met with in a
different order, the visual fields of the animal present in
many respects a quite different aspect, and the muscular
impulses given at the various turns are just the reverse of
those given as various landmarks were encountered during
the outgoing journey. Must we not assume that in the
animal's mind, as in our own, there is a consciousness of the
general space relation of objects seen along the journey and
the animaFs own changes of position in relation to those
objects? Does not the horse feel that he is going away
farther and farther from his bam, although the latter cannot
be seen, and does he not in some way keep the general
topographical situation in mind amid all the panoramic
scene-shif tings in his field of vision? If the horse does not
have clearly defined ideas he seems to possess some con-
sciousness of unperceived objects and their relation to
present ones. Horses and other animals commonly find
their way home over routes very different from the ones on
which they set out, even in places, such as woods, where they
cannot perceive the general landscape. We may say of
course that the animal is guided by a "sense of direction,''
but if we ascribe this faculty to the animal in any but a
mystical sense, we can scarcely escape assuming that there is
something that fulfils the function of a representative con-
sciousness of the absent elements of the situation. The
animal which makes for home by a new route cannot be said
to be guided by sensori-motor impulses in relation to certain
trees or rocks, because these objects have not been asso-
ciated with any impulses in the animaPs experience. If we
examine our own mental experience in such a situation it

248 THE INTELLIGENCE OF MAMMALS

will be evident that we have a consciousness of an outlying
region beyond our immediate sphere of perception which
stands in a definite relation to the latter, and that the object
of our search lies in a certain direction. When the per-
ceptual and the ideal content of our minds get disjoined we
have the horrible consciousness of being lost. If the ideal
content were absent and we had nothing to depend upon
but a store of sensori-motor associations would we be able
to get back to our starting place? I think not. And with-
out this content I fancy the horse would be as helpless as
ourselves.

Whether animals draw inferences of a simple sort is a
subject we shall dwell upon further in the chapter on the
mental life of apes and monkeys. It is not at all likely
that animals have any power of abstract or conceptual
reasoning; as Morgan remarks they probably "do not think
the therefore," but mental action essentially inferential in
character may not involve any processes of a complicated
kind. As Binet has attempted to make clear in his work
on the Psychology of Reasoning, there is a fundamental
similarity between reason and simple perception. The
shape and color of an orange recall the sensations of odor,
taste, touch and muscular movements which we have ex-
perienced in connection with such visual impressions in the
past. These various states are combined in a percept
which seems to us a simple object. The visual impression
has assimilated various other attributes, and we therefore
tend to act differently toward such appearances on the
basis of this association. The sweet taste may cause us to
reach out for the orange and we might justify our procedure
by a process of reasoning about the relation of the various
attributes of the object. But we certainly do not do so
before the sweetness of the orange is borne in upon us.

THE INTELLIGENCE OF MAMMALS 249

The sweetness occurs to us very quickly and directly, and
with it, if we are hungry, the impulse to seize, and eat the
orange. In this, as in most affairs of life, we arrive at the
conclusion first and reason about it afterward. Our con-
scious reasoning is a process of reviewing and verification
which can usually be dispensed with, rather than one of
discovery. The animal that reaches for an object which it
has learned is good to eat gets along without the retrospec-
tive review. He may go through with mental processes of
various degrees of complexity. The visual sensation may
call up directly the impulse to seize and eat the orange. It
may call up along with the latter the taste and other attri-
butes of the orange. It may caU up the taste and other
attributes which in turn arouse the impulses to seize and
eat. The various mental steps may be present in different
degrees of vividness. The relation of different states may
be attended to; the animal may finally come to "think the
therefore, '* and so on.

"Inference," says Hobhouse, "is one function, from the
simplest case quoted by Mr. Morgan of the chick, up to the
highest elaboration of experience by the human intellect.
The differences are differences in articulateness on the one
side, and comprehensiveness on the other." There is good
reason to believe that animals profit by the association of
ideas, and that they do certain acts, not for their own sake,
but as a means to an ulterior end which is kept in mind.
If we do not choose to designate the mental operations
involved in such behavior by the term reason we must at
least admit that they are on the road to it.

How far along animals like dogs, raccoons and elephants
may be on the highway toward reason properly so-called
it is impossible at present to state. I have read critically
a good many stories of animal intelligence which have left

250 THE INTELLIGENCE OF MAMMALS

in my mind a general conviction that animals are capable
of drawing simple conclusions, but there are alternate pos-
sibilities of interpretation in many cases, and incompleteness
of data in so many others that I am unable to present any
number of instances in which inference of a very articulate
kind is indubitably shown. The subject is one on which
we need more experiments performed by investigators
acquainted with the animal's previous history and keenly
alive to the various possible psychological interpretations
which may be put upon an animal's behavior. The diffi-
culties and pitfalls of the subject are far beyond the realiza-
tion of most of the contributors to our data on comparative
psychology. There is a large amount of material too care-
fully recorded to be cavalierly rejected as worthless, but too
incomplete to be accepted as entirely conclusive on the sub-
ject of animal inference; it will doubtless prove of great
value in suggesting lines for future work.

A case in point is the following account of two dogs,
contributed by Mr. Stone to Romanes' "Animal Intelli-
gence." " One of them, the larger, had a bone, and when he
had left it the smaller dog went to take it, the larger one
growled, and the other retired to a corner. Shortly after-
ward the larger dog went out, but the other did not appear
to notice this, and at any rate did not move. A few minutes
later the large dog was heard to bark out of doors; the little
dog then, without a moment's hesitation, went straight to
the bone and took it. It thus appears evident that she
reasoned — 'That dog is barking out of doors, therefore he
is not in this room, therefore it is safe for me to take the
bone.' The action was so rapid as to be clearly a conse-
quence of the other dog's barking."

The behavior described will not appear to anyone familiar
with dogs as anything improbable. The doubtful feature

THE INTELLIGENCE OF MAMMALS 251

is the syllogism in the dog's mind. The dog probably did
not reason the matter out in the explicit fashion indicated
by the words employed by Mr. Stone, and perhaps the writer
would not insist that he did. The bark may have made the
small dog aware that the large dog was out of the room and
that it was safe for him to seize the bone. He may not have
thought that therefore he could safely seize the bone, but
the large dog being away the small one just went after the
bone because he was no longer restrained. It is possible,
though less likely, that the bark simply served to direct the
attention of the small dog from whatever object it was
bestowed upon, and that it was then directed to the bone
which was seized because the large dog was out of the way.
Or it may be that when the small dog betook himself to the
corner he fell half asleep and was brought to himself by the
bark of the other dog. How varied are the interpretations
that can be made of the contents of the dog^s mind! We
may feel convinced from our general knowledge of dog
behavior and the special circumstances of the case that
there was something in the small dog's mind corresponding
to ''large dog outside, I can now get the bone;" but our con-
viction does not constitute proof. And so it goes with story
after story.

While the proof of the existence of explicit inference may
be difficult though by no means impossible, there is a singular
lack of conclusiveness in the arguments sometimes employed
to prove its absence. It is argued by Thomdike that the
gradual descent of the time curves of learning in his experi-
ments showed the absence of reasoning. But when we
examine these curves it becomes apparent that their shape,
in a considerable proportion of cases, is far from gradual,
as in most of the figures on pp. 18, 20 and 24. Small finds
in the curves of learning of the rat that there is, as a rule, a

252 THE INTELLIGENCE OF MAMMALS

marked drop after the first success, and a similar phenom-
enon is remarked by Hobhouse. Still less indicative of the
gradual stamping in process are the results of the experi-
ments of Davis and L. W. Cole on the raccoon. Davis
found that the time curves of two children in learning the
fastenings of a box were very similar to those of the raccoon,
and Lindley in his Study of Puzzles obtains very similar
results wdth young children.

When Thorndike speaks of reasoning he evidently has in
mind a fairly typical process of ratiocination, and not the
simple process of the inferential type described above,
which we should most naturally expect in the animal mind.
A man would find his way out of a puzzle box after once
discovering the proper method, but granting that a cat has
a certain power of inference it would not be surprising that,
with her hazy consciousness of the situation, feeble and fluc-
tuating attention and indefinite memory of just what she
did before, several trials would be required to enable her
to solve her problem. It is quite easy to convict a cat of
Btupidity by showing that she cannot reason in human
fashion. Whether she is capable of mental action of a
primitive inferential type is a quite different question.

We are apt to overestimate the importance of the ability
to reason as if it were the chief thing of value in intelligent
behavior. There are other mental traits which may enable
an animal to get what it wants better than an increment of
reasoning power. General activity, power of attention,
interest, quickness of forming associations, delicacy of dis-
crimination, duration of memory and the ability to form
complex associations are all of the utmost importance in
many situations in an animal's life. We frequently meet
people who, when they are compelled to exercise their reason-
ing powers, act as if the effort were a painful one and as if

THE INTELLIGENCE OF MAMMALS 253

they were groping in a sort of intellectual fog. We are often
amazed at the obvious fallacies they fall into, like Mrs.
Eddy's famous method of proving propositions by inversion;
but the same people may exhibit an unusual degree of
keenness and practical sense in the ordinary affairs of life.
In animals these intellectual faculties of a lower order are
developed in different ways according to the habits of the
species. Give a fox greater power of inferential thinking,
but decrease his alertness, curiosity, suspiciousness, and
quickness of perception, and he might fall a victim to the
hunter while his mind was occupied on some other subject.
In the development of these various intellectual qualities \
there has been an enormous progress from the stupid mar- \
supials to the apes and monkeys, during which the founda-
tions for the superstructure of reason were broadly laid.
Just as the various non-intelligent modifications of behavior
facilitate the development of intelligence, so do the diverse ;
manifestations of intelligence prepare the way for the advent
of reason.

The question as to whether animals imitate acts from which
they see that other animals derive an advantage has an
important bearing on our views of their psychic development.
The word imitation is employed in a very wide sense by
some writers such as Tarde and Baldwin, but it is more
commonly used to designate the performance of an act after
perceiving the act performed by another creature. In
imitation of this type we usually distinguish two kinds,
the instinctive, and the reflective or rational. In the first
the perception of another animal performing an act forms
the stimulus which sets off an innate tendency to perform
a similar act. In fishes which run in schools the turning
about of one individual may cause all the others to turn;
each individual has an innate proclivity to follow the

254 THE INTELLIGENCE OF MAMMALS

movements of the others, and by vu*tue of this trait the
fishes keep together and escape common dangers. A
similar kind of imitation is met with even in insects. Ants,
according to Wasmann, not infrequently imitate one an-
other's acts, and other observers have remarked upon the
same proclivity in bees and termites.

Imitation plays an especially important role in the life
of birds. According to Lloyd Morgan, "If one of a group
of chicks learn by casual experience to drink from a tin of
of water, others will run and peck at the water, and thus
learn to drink. A hen teaches her little ones to pick up
grain and other food by pecking on the ground and dropping
suitable materials before them, while they seemingly imitate
her action in seizing the grain. One may make chicks and
pheasants peck by simulating the action of a hen with a
pencil point or pair of fine forceps. According to Mr.
Peal the Assamese find that the young jungle pheasants will
perish if their pecking responses are not thus artifically
stimulated; and Professor Clay pole tells me that this is
also the case with young ostriches hatched in an incubator.''
Chicks avoid objects which they perceive arouse alarm in
others, and if they see other chicks pecking at things of
which they stand a little in awe they frequently muster up
courage and follow their companions.

Birds learn to fear certain enemies, such as hawks, at
least in part, through their instinctive response to the signs
of alarm in other birds. Their instinct of following guides
them in their migration routes, in which, despite the con-
tention of Herr Gatke, their course is in all probability a
matter of tradition.

In contrast to the above cases which may be regarded as
instinctive responses to particular stimuli are those instances
in which an animal more or less deliberately copies the actions

THE INTELLIGENCE OF MAMMALS 255

of another with the end of securing some advantage which
the other animal enjoys. Imitation of this character
involves a species of inference, and its occurrence in animals
is less \\idespread than was commonly supposed some
years ago. Thorndike tested cats and dogs with the view
of ascertaining if they could learn to get out of a puzzle
box by seeing other cats and dogs get out. After witness-
ing a number of times the successful exits of the animals
which had learned the means of escape these cats and
dogs were unable to get out by themselves any more
quickly than other individuals which had not the ad-
vantage of seeing how the escape was effected. In all the
experiments performed the animals did not show the least
tendency to imitate the performances of their successful
comrades. The experiments of Cole and Davis on the rac-
coon yielded similar negative results.

Small, in his studies on the white rat, finds evidence of a
very simple kind of imitation, but no clear cases of imitation
of the inferential type. When one rat sees another digging
it is apt to dig also, and when one runs over a box it is apt
to be followed by others. Berry, who has made a more
thorough study of imitation in the white rat, found better
evidence of intelligent imitation. "When two rats were
put into the box together, one rat being trained to get out
of the box and the other untrained, at first they were in-
different to each other's presence, but as the untrained rat
observed that the other one was able to get out while he
was not, a gradual change took place. The untrained rat
began to watch the other closely; he followed him all about
the cage, standing up on his hind legs beside him at the string,
and pulling it after he had pulled it, etc. We also saw that
when he was put back the immediate vicinity of the loop
was the point of greatest interest for him, and that he tried

256 THE INTELLIGENCE OF MAMMALS

to get out by working at the spot where he had seen the
trained rat try."

It is a significant fact that the untrained rat manifested
little interest in the actions of the trained one until he found
the latter could make his escape from the cage. If a new
rat were put in the cage the untrained rat would follow him
also, but if the new rat did not get out his companion
would soon cease to follow and imitate him. It is impossible
to regard such imitation as this as a series of congenital
responses to the perception of particular movements. The
rat imitates with the aim of getting out of the cage and
apparently recognizes in the movements of his companion
the means of deliverance. Further evidence of intelligent
imitation is furnished by Berry^s work on imitation in
cats.

The experiments of Hobhouse yielded many indications
of imitation of the inferential type, although most of them
leave something to be desired in the way of conclusiveness.
Hutchinson in his valuable book on Dog Breaking says
that dogs may be taught tricks much more readily if they
see other dogs perform the tricks and obtain a reward
for it.

A general consideration of the literature on imitation in
animals justifies us, I think, in concluding that a certain
amount of intelligent imitation occurs in animals below the
monkeys, but it must be admitted that there is but a
small amount of reliable data upon which to base an
opinion.

We have spoken of instinctive and intelligent imitation
as if they constituted two discrete classes of behavior. It
is important, I believe, in studying this subject to recognize
the kinship and transitional stages between these two kinds
of imitative activity. Animals tend to imitate those acts

THE INTELLIGENCE OF MAMMALS 257

which are most closely related to their own instinctive
movements. There may be no awareness of any advantage to
be gained thereby, as in the f amihar imitation of sounds by
birds. Magpies, ravens, mocking-birds and especially parrots
attempt to repeat various sounds which they are accustomed
to hear. Their efforts, imperfect at first, improve with
repetition. Making a sound like one which is heard seems for
some reason to afford these birds an agreeable experience.
As Baldwin would say, hearing the sound is followed by an
effort to "reinstate the stimulus" and so gives rise to a
"circular reaction." Variations of its own notes which
approach the sounds heard become "stamped in," and the bird
gradually comes to imitate the sounds more closely, much
as we gradually improve upon the accuracy of our own
movements in playing ball or tennis. In this imitation we
need suppose no element of transferred association. Based
perhaps upon a primary instinct to utter notes in response
to the notes which it hears, which we often find in young
birds, the tendency of birds to imitate sounds involves but
the rudimentary form of intelligence required for the forma-
tion of simple associations.

There is comparatively little imitation which is based on
a cold calculation of the advantages derived from copying
another animal. The imitation which is shown in a certain
stage of the life of the child is intimately related to a certain
satisfaction derived from attaining conformity to copy.
This trait the child has in common with the bird. Probably
our own unconscious or half conscious imitation of the pro-
nunciation and mannerisms of the people we live with is a
phenomenon of a similar kind. How far this kind of
imitation occurs in the mammals has not been clearly
brought out, but there are many indications of its influence
in several of our common domesticated species.

17

258 THE INTELLIGENCE OF MAMMALS

BIBLIOGRAPHY

Allen, J. The Associative Processes of the Guinea Pig. Jour.

Comp. Neur. Psych., 14, 293, '04.
Ament, W. Ein Fall von Ueberlegung beim Hund. Arch. ges.

Psych., 6, 249, ^05.
Berry, C. S. The Imitative Tendencies of White Rats. Jour.

Comp. Neur. Psych., 16, 333, '06.

An Experimental Study of Imitation in Cats, 1. c. 18, 1, '08.
BosTok, F. The Training of Wild Animals, N. Y., '03.
Burroughs, J. Do Animals Think? Harpers Mon., 110, 354, '05.
Carr, H. and Watson, J. B. Orientation in the White Rat. Jour.

Comp. Neur. Psych. 18, 27, '08.
Cesaresco, E. M. The Psychology and Training of the Horse,

London, '06.
Cole, L. W. Concerning the Intelligence of Raccoons. Jour.

Comp. Neur. Psych., 17, 211, '07.
Davis, H. B. The Raccoon: A Study in Animal Intelligence. Am.

Jour. Psych., 18, 479, '07.
Freund, F. Der "kluge" Hans. Ein Beitrag zur Aufklarung.

BerHn, 04.
Hachet-Souplet, W. Examen Psychologique des Animaux.

Paris, '00.
Hobhouse, L. T. Mind in Evolution. London, '01.
Hutchinson, W. N. Dog Breaking, 6th. ed., London, '79.
Mills, T. W. The Nature and Development of Animal Intelligence,

N. Y., '98.

The Nature of Animal Intelligence and the Methods of Investi-
gating It. Psych. Rev., 6, 262, '99.
Pfungst, O. Das Pferd von Herr von Osten (Der kluge Hans),

Leipzig, '07. Translation, " Clever Hans, " N. Y., '11.
Small, W. S. An Experimental Study of the Mental Processes of

the Rat. Am. Jour. Psych., 11, 133, '99, and 1. c. 12, 206, '00.
Thompson-Seton, E. Life-Histories of Northern Animals, N. Y. '09.
Thorndike, E. L. Animal Intelligence. Psych. Rev. Monogr.

Suppl., Vol. 2, No. 4, '98.

Do Animals Reason? Pop. Sci. Mon., 55, 480, '99.

A Reply to " The Nature of Animal Intelligence and the Methods

of Investigating It." Psych. Rev., 6, 412, '99.
ToENJiES, H. Principles of Animal Understanding, N. Y., '06.
Watson, J. B. Animal Education, Chicago, '03.
Yerkes, R. M. The Dancing Mouse, N. Y., '07.

THE INTELLIGENCE OF MAMMALS 259

Yoakum, C. S. Some Experiments on the Behavior of Squirrels.

Jour. Comp. Neur. Psych., 19, 541, '09.
Zell, T. Das rechnende Pferd. Ein Gutachten Gber den " klugen

Hans " auf Gnind eigener Beobachtungen, Berlin, *04.

1st das Tier unvernunftig? Neue Einblick in die Tierseele.

Stattgart, Fr., '04.
ZuRN, F. A. Die intellektuellen Eigenschaften (Geist und Seele)

des Pferdes. Stuttgart, '99.

CHAPTER XIII
THE MENTAL LIFE OF APES AND MONKEYS

"L'Homme ne poss^de aucune aptitude psychique fondamentale
qui, k un moindre degr^, ne se manifeste chez certaines B^tes." —
Milne-Edwards, Legons sur la Physiologie, T. 14.

"If no organic being excepting man had possessed any mental
power, or if his powers had been of a wholly different nature from
those of the lower animals, then we should never have been able to con-
vince ourselves that our high faculties had been gradually developed.
But it can be shown that there is no fundamental difference of this
kind. We must also admit that there is a much wider interval in
mental power between one of the lowest fishes, as a lamprey or
lancelet, and one of the higher apes, than between an ape and man;
yet this interval is filled by numberless gradations." — Darwin
Descent of Man.

I have reserved for a separate chapter a consideration of
the mental powers of the animals most closely related to
ourselves. Our simian cousins have long enjoyed the reputa-
tion of attaining, next to man, the highest psychic develop-
ment of any members of the animal kingdom. Much has
been written concerning their powers and performances,
but their psychology has been investigated far less exten-
sively and thoroughly than the importance of the subject
demands. It is particularly unfortunate that we know so
little of the most anthropoid of the ape tribe. Until recently
it has been exceedingly difficult to keep the larger apes long in
captivity, and psychologists have had few opportunities to
subject them to systematic experimentation. Further diffi-
culties are encountered owing to the large size and strength
of these animals, especially when these characteristics are
combined, as is often the case, with an unreliable or intract-
able disposition.

260

MENTAL LIFE OF APES AND MONKEYS 261

It is probable that further study may reveal in the anthro-
poid apes a higher psychic development than is found among
the smaller members of the monkey tribe from which has
been derived most of our knowledge of the simian mind.
Our account therefore will have to do mainly with the smaller
species which are more commonly and easily kept in captivity
and which are more distantly related to the human family.

Thorndike has performed several series of experiments
with three Cebus monkeys by placing food in boxes which
could be opened by working one or more mechanical
devices. The times taken by the monkeys in learning how
to open the boxes were recorded, and it was found that
associations were formed much more quickly than in cats,
dogs and chicks. "Whereas the latter," says Thorndike,
"were practically unanimous save in the cases of the very
easiest performances, in showing a process of gradual
learning by a gradual elimination of unsuccessful movements,
and a gradual reinforcement of the successful one, these are
unanimous, save in the very hardest, in showing a process of
sudden acquisition by a rapid, often apparently instanta-
neous, abandonment of the unsuccessful movements and a
selection of the appropriate one which rivals in suddenness
the selections made by human beings in similar performances."
This fact, according to Thorndike, does not show that
monkeys reason or even have ideas. Their greater clearness
of vision, the greater number and precision of their move-
ments, and their greater curiosity are factors which, aside
from the superior development of their brains, give the
monkeys an advantage over cats and dogs, in acquiring new
associations. The monkey mind shows an advance over
that of the lower mammals in the greater number, deHcacy,
complexity and permanence of its associations, and the
readiness with which associations are formed, but in their

262 MENTAL LIFE OF APES AND MONKEYS

method of learning, according to Thomdike, " the monkeys do
not advance far beyond the generalized mammalian type."

Like his cats and dogs, Thordike's monkeys failed to learn
acts by being put through them. They also failed to
leam acts which they had seen the experimenter perform a
large number of times. Neither did they appear to imi-
tate one another. If one monkey had learned to get into
a puzzle box, other monkeys failed to leam to do so after
witnessing his successful performance. The traditional belief
in the propensity of monkeys to imitate, therefore, receives
no justification at Thorndike's hands.

The poor opinion of the monkey mind to which Thomdike
was led by his experiments is very different from the one
commonly held, and most people would be inclined to regard
the individuals he experimented with as rather sorry speci-
mens of the monkey family. Experiments on one species
form an inadequate basis upon which to base conclusions
regarding simian intelligence in general, and it is not sur-
prising that other investigators of the behavior of monkeys
should have obtained results more creditable to the mental
ability of these animals. Some very interesting investiga-
tions have been conducted by Hobhouse upon a rhesus
monkey which he called Jimmy, and a chimpanzee which
for reasons of his own he named the Professor. Jimmy was an
active creature of rather irritable temper and very fond of
baked potato, which proved an excellent incentive to the
overcoming of obstacles. The Professor, on the other hand,
was very timid and unsociable, and could be managed only
with difficulty.

Both of these monkeys showed a certain power of adapting
means to ends in employing one object in order to get another
within reach. The Professor when first received from the
zoological gardens had already acquired the habit of throw-

MENTAL LIFE OF APES AND MONKEYS 263

ing his rug over objects at some distance from his cage and
pulling them in. If a nut or piece of banana was placed be-
yond his reach outside his cage he would get his rug from his
bed, stuff it with considerable labor between the bars of his
cage, and throw it like a net over the desired object. He
was taught to substitute a stick for the rug and succeeded
in employing it to secure bits of food. The next day he
learned to use a short stick in order to reach a longer one
with which he could secure a piece of banana. " I put my
stick,*' says Hobhouse, "out of his reach, and a piece of
banana beyond it again, while I gave him a short stick. He
did not, however, use it until I first pushed the big stick about
with it. He then made an attempt to reach my stick with
the short one, but without success. I then gave him
rather a larger stick, with which he at once tried to reach
mine, but instead of getting hold of it he knocked it slant-
wise, so that one end w^as farther off from him than before,
and one end nearer. He now directed his stick to the
nearer end, pulled mine in, and with its aid reached the
banana."

The chimpanzee would use his stick in different ways
according to circumstances. "The banana was generally
given him inside a cigar box. He would reach out with
his stick at the box, and sweep it round by a radial motion,
so that in so doing he was not obeying the natural impulse
to draw it straight toward him, but merely was bringing
it to a point to which he could afterward go and get it. One
half of his cage, however, was covered with plate glass,
so that if, in describing a quarter circle he swept the box up
against the glass he could not reach it at once with his arm.
He would then alter the motion, and rake with the point of the
stick, drawing the box in in a straight line. "Wlien he had to
fish for a box close to the wall, he would take trouble to get

264 MENTAL LIFE OF APES AND MONKEYS

his stick in between the wall and the box, showing that he
was quite aware of the way in which he had to push it."

Jimmy, like the Professor, learned to use a stick with which
to draw in objects, and he also learned to use a short stick in
order to reach a larger one with which he could obtain
his food. **If stick and fruit were both too far from him,
he would try to help himself by pulling the whole board on
which they lay toward him; and he seemed quite clear as to
the necessity of getting the stick beyond the thing that he
was pulling. If it lay just on his side of the apple or nut
he would be careful not to knock the apple away, but to
get the stick well beyond it."

Jimmy^s resourcefulness in attaining his ends was often
quite remarkable. Given a child's skipping rope with
which to obtain a piece of bread placed outside his cage he
made a cast at it with the wooden handle and finally drew
it within reach. After this he would use a cord and then a
wire for a similar purpose. Says Hobhouse, '^I put a piece
of onion in a basket within reach of his stick. After first refus-
ing any effort, he tried to reach it with the stick, and failed.
In reaching toward it, he found the big box (one of Jack's
boxes, which he also used) lying across his chain, and pre-
venting his reaching forward. He threw the box off. Hav-
ing failed with the stick, he will not try it again, but makes
wild efforts to throw the rope. Then he actually rolls the
box at the food; then goes off and gets the dust sheet from
the chair and tries unskilfully to sweep at it; finally makes a
longer stretch, and just reaches it with his own claw."

Jimmy also learned to use a chair or a box in order to get
objects otherwise out of reach. An onion was placed on
top of a table and his chain was adjusted so that he could
reach the table, but not the onion. After he was allowed to
reach the latter by using a box which was placed so that he

MENTAL LIFE OF APES AND MONKEYS 265

could climb upon it the box was removed. At first Jimmy
did not try to move the box back, but began to explore it and
opened the lid upon which he climbed and reached the food.
The next time he tried to pull the table toward him.
After a number of different manoeuvers to get the food he
learned to drag the box or a chair toward the table and
by this means reach the object he was seeking.

In another experiment a box was placed between the
fastening of his chain and a piece of potato which was just
out of reach, but which could be reached if the chain did

Fig. 16. — "Jimmy" using a box in order to reach food on the table.
(After Hobhouse).

not pass around the box. Jimmy soon came back to the
box, moved it out of the way and got his potato. He re-
peated the performance two more times without hesitation.
When his chain was shortened by a heavy fender being
placed over it he would come back and lift off the obstacle.
He recognized, apparently, that the box and the fender pre-
vented him from going as far as he otherwise might.

Similar adjustment of means to ends was shown by the
monkey kept by Miss Romanes, which she describes as
follows: "His chain is fastened to the marble slab of a
washhandstand, placed on the floor against the wall. It is
too heavy for him to pull along by his chain without hurting
himself, so when he desires to do any mischief which is

266 MENTAL LIFE OF APES AND MONKEYS

beyond the reach of his chain he dehberately goes to the
marble and pushes an arm down between the upright part
of it and the wall, until he has moved the whole slab
sufficiently far from the wall to admit of his slipping down
behind the upright part himself. He then places his back
against the wall and his four hands against the upright
part of the marble, and pushes the slab as far as he can
stretch his long legs. He only does this, however, when he is
bent on mischief, as the fact of food being beyond the reach
of his chain does not furnish a strong enough inducement to
lead him to take so much exertion."

Another interesting incident in this connection is furnished
by Prof. Mobius. A chimpanzee, Molli, was confined in a
wooden cell. When someone from the outside was driving
a nail into the wall of her cell, and she saw the nail coming
through the board, she went to her drinking vessel, took it
in both hands and pounded the nail back again.

The use of implements for various purposes is, according
to the testimony of several writers, a not uncommon feature
of monkey behavior. Prof. Cope, in describing the perform-
ances of a Cebus capucinus says: "He then used the strap
in a novel way. He was accustomed to catch his food with
his hands when thrown to him. Sometimes it fell short
three or four feet. One day he seized his strap and took
pretty correct aim and finally drew the pieces to within
reach of his hand. This performance he constantly repeats,
hooking and pulling the articles to him in turns of the strap.
Sometimes he loses hold of the strap. If the poker is handed
him he uses it with some skill for the recovery of the strap.
After punishment the animal would only exert himself in
this way when not watched; as soon as an eye was directed
to him he would cease.''

Witmer relates that Peter, a trained chimpanzee which

MENTAL LIFE OF APES AND MONKEYS 267

he had for a time under observation, would use a hammer to
drive nails with, string beads on a cord, select the proper
key from a bunch and unlock a padlock, and strike a match,
light a cigarette, and smoke it with an expression of serene
contentment. Peter developed a remarkable expertness
in riding a bicycle, a feat which he was accustomed to per-
form during his public exhibitions.

Mr. Belt tells of a Cebus he observed which would use a
stick with which to draw objects toward him. One day
some bird skins were placed, as was thought, far beyond his
reach, but he took his swing and used it so as to bring the
skins near enough to be seized. He also secured some
jelly, which was set out to cool, in the same way.

Miss Romanes in her account of the behavior of a Cebus,
from which we have already quoted, states that "if a nut
or any object he wishes to get hold of is beyond the reach of
his chain, he puts out a stick to draw it toward him, or if that
does not succeed, he stands upright and throws a shawl back
over his head, holding it by the two comers so that it faUs
down his back; he throws it forward with all his strength, still
holding on by the corners; thus it goes out far in front of him
and covers the nut, which he then draws toward him by
pulling in the shawl." The same monkey when given a
hammer to crack a nut with used it effectively for that
purpose. We have the testimony of Cuvier to the effect
that an orang would pull a chair from one end of the room
to the other so that by standing upon it he could open a
latch, but how the act was originally learned we are not
informed.

The use of sticks and stones by apes and monkeys has
been described by several writers. Pechuel-Losche, who has
had excellent opportunities for observing baboons in their
native habitat, has come to the conclusion that the accounts

268 MENTAL LIFE OF APES AND MONKEYS

of these animals throwing stones at their pursuers rest upon
inaccurate observation; the stones, he thinks, were merely
knocked down unintentionally during the flight of these
animals up a declivity. Stories of apes throwing down
fruit and other objects from trees he thinks are based
on the fact that these things are simply dropped when the
animals are put to flight. Father Wasmann, who eagerly
adopts these conclusions, proceeds to admonish us that
the boasted intelligence of apes is entirely illusory. ''Had
apes themselves," he tells us ''but a trace of intelligence,
they would have invented, long ago even in their free state
of nature the use of a few simple means of defence, such
as branches and stones. But why did they not? The only
possible answer is: because they evidently have no intelli-
gence. Not the brain alone makes man an intelligent being,
but his spiritual soul, and this spiritual soul is wanting in
the highest apes as well as in the insects. '^

If the failure of apes to use weapons of defence is indicative
of lack of intelligence, we may fairly conclude that where
weapons are used there is evidence that intelligence exists.
Whether or not baboons use sticks and stones in the way
alleged, there is good evidence that other members of the
ape tribe sometimes employ them as a means of attack.
Miss Romanes says regarding her Cebus : " To-day a strange
person (a dressmaker) came into the room where he is tied
up, and I gave him a walnut that she might see him break
it with his hammer. The nut was a bad one, and the woman
laughed at his disappointed face. He then became very
angry, and threw at her everything he could lay hands
on; first the nut, then the hammer, then a coffee-pot which
he seized out of the grate, and, lastly all his own shawls.
He throws things with great force and precision by holding
them in both hands, and extending his long arms well back

MENTAL LIFE OF APES AND MONKEYS 269

over his head before projecting the missile, standing erect
ihe while." At a later period Miss Romanes says of him:
" When he throws things at people now he first runs up the
bars of the clothes-horse; he seems to have found out that
people do not care much for having things thrown at their
feet, and he is not strong enough to throw such heavy objects
as a poker or a hammer at peoples' heads; he therefore
mounts to a level with his enemy's head, and thus succeeds
in sending his missile to a greater height and also to a greater
distance." Darwin states in his "Descent of Man:" "As
I have repeatedly seen, a chimpanzee will throw any object
at hand at a person who offends him." And other well
attested observations could be quoted to the same effect.
Despite the contention of Wasmann we must admit, I
think, that the use of objects by apes as weapons of attack
rests upon a fair basis of testimony.

A certain degree of foresight seemed to be manifested
by Miss Romanes' monkey in the way in which he disen-
tangled his chain when it became wound around a clothes
horse which was given him to run upon. "He looks at it
intently and pulls it with his fingers this way and that, and
when he sees how the turns are taken, he deliberately goes
round and round the bars, first this way, then that, until
the chain is quite disentangled. He often carries his
chain grasped in his tail and held high over his back to keep
it from getting in the way of his feet."

Mr. Hobhouse's Jimmy was not so circumspect. "He
would, however, as a rule, undo a single twist by retracing
his steps, and sometimes would undo a more complicated one
by a developed form of the method of trial and error, which
consisted in this: that each time he felt the cord shortening
on him, he would go back the way he had come." This
method was usually effective, but Jimmy showed little

270 MENTAL LIFE OF APES AND MONKEYS

improvement in its application. He made but a moderate
success in disentangling his chain when it became twisted
around among the legs of a chair or table.

While monkeys are generally credited with unusual powers
of imitation, the experiments of recent years have shown
that imitation is far less frequent than was supposed. Thorn-
dike tried to find if monkeys would learn to enter a puzzle
box any more quickly after having witnessed a number of
times how he opened the various fastenings. Several
kinds of boxes were used, but the monkeys did not, in any
case, make sufficient progress to justify the conclusion that
they learned by imitation. Neither did monkeys which
failed to learn how to enter the puzzle boxes after several
trials imitate others which had learned to operate the fasten-
ings. No evidence of imitation was furnished by the general
behavior of these animals, but since two of the monkeys
were on very unfriendly terms and the third was exceedingly
timid their social relations were not such as to favor the
imitation of one another's acts.

Further experiments were carried on by Watson on four
monkeys, a baboon, a Cebus, and two rhesus monkeys.
Watson tested his animals by performing in their presence
a number of acts which resulted in securing food, such as
drawing in food with a rake or a cloth, getting it from a
bottle with a fork, and poking it out of a glass cylinder with
a stick. After witnessing his operations a number of times
there was no effort on the part of any of the monkeys to get
the food in the way they had every opportunity to see was
effective. Experiments with puzzle boxes in which the
monkeys were given a chance to imitate either the experi-
menter or a monkey w^hich had already learned the trick
gave the same negative results.

While they yielded no evidence of imitation in its higher

MENTAL LIFE OF AFES AND MONKEYS 271

Fig. 17. — Peter at dinner. (By permission of the Psychological Clinic.)

272 MENTAL LIFE OF APES AND MONKEYS

forms Watson's monkeys occasionally performed acts which
were '' suggestive of a low order of imitation/' One of them
found a hole in a window frame. '*This one would 'peek'
and then another one would push him aside and 'peek ' in turn.
This was observed several times." Hirschlaff in his account
of the chimpanzee Consul describes a number of cases in
which he considers that imitation was unmistakably shown,
but his observations are not described in sufficient detail to
enable one to judge of the correctness of the interpretation.
Kinnaman reports a few cases of imitation in two rhesus
monkeys which he studied. In one instance in which the
female pulled out a olug immediately after seeing a male do
it the evidence for imitation is not entirely conclusive, since
the female had pulled out the plug on previous occasions.
The second case is a more satisfactory one and is described
as follows: " Recalling that she had failed to work the bear-
down lever for opening the box. ... I placed it before
her. She rushed up, but missing the plug she sat dow^n.
The male passed her, pushed the lever down and procured
the food. When the box w^as set again she worked the lever
and took the food in the same way that he had done. She
manipulated this apparatus several times immediately
and 250 times later as a part of a combination lock. Besides
these, once when the male peeped under the bottom of one
of the trees the female came and peeped in the same manner."

Mr. Witmer states that the chimpanzee, Peter, twice copied
a W which was written on a blackboard, and learned to repeat
the word ''mama." It is unfortunate that more extended
and thorough experiments were not carried out with so prom-
ising a subject.

The experiments of Shepherd on imitation in rhesus
monkeys yielded for the most part negative results. In a
few cases, however, the monkeys showed evidence of learn-

MENTAL LIFE OF APES AND MONKEYS 273

ing by imitating the experimenter's movements. In one
such case a piece of banana was suspended by a string a
Uttle beyond the monkey's reach, and a pole was so ar-
ranged that when it was swung under the banana the
monkey could get upon it and reach the fruit. A monkey
which had proven himself incapable of solving the problem

Fig. 18. — Peter's efforts at copying letters on a blackboard, a. Two
superimposed letters drawn by Mr. Witmer; a.\ Peter's copy after the sec-
ond tracing : a-, Peter's second effort when told to make a W again. (By
permisson of the Psychological Clinic.)

alone was shown how to manipulate the pole. After wit-
nessing the experimenter push the pole under the banana
the monkey gradually learned to perform the act, although
it was some time before he manipulated the apparatus with
accuracy and dispatch.

The most extensive study of imitation in monkeys has
been made by Mr. Haggerty. While many of the experi-

18

274 MENTAL LIFE OF APES AND MONKEYS

ments gave negative results and many others leave us in
doubt as to the interpretation of the behavior described,
there are some in which the evidence points strongly to-
ward imitation. The monkeys were put in cages in which
there were mechanical devices by operating which they
could procure food. Each monkey was given five trials of
fifteen minutes each on successive days. If he did not
secure food by his unaided efforts he was allowed to see
another monkey operate the device and was then allowed
to try again. The lessons were kept up until 100 tests were
made before the monkey was dismissed as a hopeless failure.
In many cases monkeys which failed to operate the devices
alone did so after watching other monkeys work them one or
more times. The attention of the monkeys was usually
stimulated when they saw other monkeys obtain food by
working the devices. Very frequently the imitation was
not perfect at first, but the various features of the trick
were learned one after the other.

There is a possible doubt relative to the interpretation of
Mr. Haggerty's experiments. Since the monkey which
served as a model learned the trick to be copied it is possible
that the monkey which performed the trick after watching
him may have learned it at first hand also. The case for
imitation can be made out only by the accumulation of a
sufficient number of instances to rule out coincidences and
accidents. How far Mr. Haggerty has made out his case
can be judged only by a careful study of the details given
in his paper.

Whether apes and monkeys reason is a question whose
answer depends on the sense in which the term reason is
employed. If we define reason as the derivation of conclu-
sions through the comparison of concepts it is not improbable
that no animal below man employs this faculty. But this

MENTAL LIFE OF APES AND MONKEYS 275

is far from implying that animals cannot perform mental
operations which are essentially inferential in their nature.
Reason, as has been stated before, is not a faculty which
stands sharply marked off from other forms of mental
activity. Between simple perception on the one hand and
abstract ratiocination on the other there is a fundamental
kinship and the latter process may be connected with the
first by numerous intermediate stages. Monkeys, in all
probability, have the power of using ideas derived from their
experience as a means of reaching practical results. The
monkey which pulls a chair or stool into a certain position,
gets on it and secures food, manifests a certain power of
inference. He may not explicitly reason: "This fruit is
beyond my reach; if I had something to get upon I could
secure the fruit; this chair would serve my purpose and
moreover is movable; ergo, I will pull it up and get on it." In
all probability a man in a similar situation would not reason
as explicitly either. He would perceive that the object was
out of his reach; seeing a chair the idea would come of pulling
it up and getting on it, and the idea would forthwith issue
in the proper act. There would be no formal syllogism gone
through with. The process would ordinarily take place
so quickly that he would scarcely be conscious of the steps.
It is true that the man might think about the matter in a
very complex way, and employ a lot of abstract and general
ideas, but this process would be dispensed with under or-
dinary conditions, especially if he were in a hurry. The
man's mental operations, even in their simplest form, would
nevertheless be in the nature of an inference, and so far as
we can judge from appearances, the same statement is true
of the mind of the monkey. The latter cannot, in all proba-
bility, think the situation over as the man can in terms of a
formal syllogism, but he has the more immediately useful

276 MENTAL LIFE OF APES AND MONKEYS

faculty of arriving at the practical conclusion as to the kind
of action which the conditions call for.

Inferences of this simple practical character are of enor-
mous value to such creatures as monkeys. They suflBce
also for the greater part of our own conduct, at least in
certain walks of life. Within the sphere of rational pro-
cedure in ourselves there are obviously vast differences in
the complexity and abstractness of our mental operations,
and it might be said that the gap between the animal and the
human mind corresponds roughly with the difference be-
tween our more simple and our more complex and abstract
thinking. A similar gap is bridged over during the life
of every individual in passing from infancy to maturity,
and we may well conceive the mind of man to have arisen
from the animal mind by a similar process of continuous
development.

Father Wasmann is one of the few comparative psycholo-
gists of note who hold the human and the animal mind to be
fundamentally distinct. While the real basis of his opinion
may be his adherence to the traditional theology which he
represents, with its peculiar views as to man's relation
to nature, Wasmann has attempted to show that
animals have no real intelligence, no power of abstraction, and
no power of rational thought. Even should we grant all
this, we should not be compelled to call into play a miracu-
lous intervention to account for the distinctive attributes of
human thinking. Giving names to particular mental
faculties tends to exaggerate their distinctiveness, as was
done in the faculty psychology of former days. When we
say of a stage of mental evolution, here there is reason while
at a stage just preceding, reason does not occur, our statement
does not necessarily imply an abrupt break or sudden step
in the evolutionary series. As Hobhouse has well remarked:

MENTAL LIFE OF APES AND MONKEYS 277

" If we allow reason to the human species in general, and yet
restrict it to that species, it must be by identifying the term
reason arbitrarily with a certain grade in the development
of analysis. It would be true to say that abstract or ex-
plicitly general reasoning emerges in the level of intelligence
under consideration, but we have seen that abstractness
is only one side of generality, and that the generality of
human as opposed to animal reasoning is once more primarily
a matter of explicitness. At bottom the function of mind j
in this as in the lower stages is to organize life by the correla- [
tion of experiences. As in every stage of mental growth
what is new is that the work of the mind becomes on the
one hand more explicit or articulate, on the other, more
comprehensive in scope.'*

Wasmann's position is typical of the attitude which
formerly characterized to a greater extent than happily is
the case now the relation of theology to science. It is the aim
of science to explain phenomena in terms of natural laws.
Theology, on the other hand, has always been interested in
finding things which cannot be so explained, and in thus
compelling us to take refuge in some sort of supernatural
intervention. Gaps, barriers, discontinuities, mysterious
and inexplicable phenomena have always afforded her,
therefore, a peculiar satisfaction; but one by one they have
been obliterated or resolved, and there can be little doubt, I
believe, that the question of the continuous evolution of the
human mind will go the way of the others.

While there is little to indicate that the apes are able to
reason in an abstract way, it is obviously absiu*d to attempt
to account for such behavior as we have described on the
basis of blind sensori-motor association. It is not going too
far, I think, to say that there is good evidence in the apes and
monkeys for the existence of ideas and for a certain power

278 MENTAL LIFE OF APES AND MONKEYS

of grasping relations. A monkey which uses a stool to
enable him to reach an object, and which removes an obstacle
from his chain so that he may get a piece of food can scarcely
be said to be devoid of a certain power of representation.
Many features of the behavior of apes and monkeys mark
these animals as belonging to a distinctly higher psychic
level than cats and dogs. It is not likely that a cat or a dog,
with all due allowance for its physical disabilities, would
employ a tool with which to pull in a bit of food, much less
use a stick in order to get the tool for this purpose. The
monkey looks on the tool as a means to an end, and accord-
ingly goes after it. There is less evidence that objects have
such a meaning to cats and dogs. These animals might try
in various ways to reach food lying on a table; if a chau- or
box were at hand they would doubtless mount upon it in
order to get the food. But even after seeing chairs and
boxes pulled around by human beings any number of times
it probably would not occur to one of these creatures to pull
the chair or box into position for its own use.

The mind of the lower mammals is pretty closely chained to
its various objects of perception. It may have ideas, but
they are lacking in "articulateness.^' But the monkey
seems to be gifted with a certain degree of initiative; things
occur to him, and he apparently thinks about things in the
effort to attain a particular result. He shows a decided
approach to ourselves in many little ways of doing things.
He is not interested merely in the gratification of his appe-
tites; he is actuated by a sort of intellectual curiosity in re-
gard to objects. Miss Romanes' monkey would almost always
set an orange to spinning before eating it just for the fun
of seeing it go; he took great delight in breaking objects to
pieces and in overturning things; he removed a bell handle
from the mantel piece which involved unscrewing three

MENTAL LIFE OF APES AND MONKEYS 279

screws; and when he was chained in a new place he investi-
gated for hours the new fastenings of his chain. The
development of this intellectual interest — if we may call it
such — in regard to objects affords an opportunity for further
improvement; it leads not only to the acquisition of new
knowledge, but to training in discrimination and practical
judgment.

BIBLIOGRAPHY

Brehm, a. E. Thierleben, 76.

Cope, E. D. Note on Intelligence in Monkeys. Am. Jour. Sci. and

Arts (3), 4, 147, 72; also in Ann. Mag. Nat. Hist. (4), 10, 229,

72.
CuviER, F. Description d'un Orang-Outang, et Observations sur

ses Facult^s Intellectuelles. Ann. du Mus. Hist. Nat., 16, 1808.
Darwin, C. R. Descent of Man, N. Y., 74.
Garner, R. L. The Speech of Monkeys, N, Y., '92.

Apes and Monkeys, their Life and Language, Boston, '00.
Haggerty, M. E. Imitation in Monkeys. Journ. Comp. Neur.

Psych., 19, 337, '09. Imitation in Monkeys. Century Mag.

'09. 544.
Hartmann, R. Anthropoid Apes, N. Y., '85.
HiRscHLAFF, L. DcF Schimpausc Konsul; ein Beitrag zur ver-

gleichenden Psychologie. Zeit. f. pad. Psychol., 7, 1, '05.
HoBHOUSE, L. T. Mind in Evolution, London, '01.
KiNNAMAN, A. J. Mental Life of two Macacus Rhesus Monkeys in

Captivity. Am. Jour. Psych., 13, 98 and 173, '02.
MoBius, K. Zur Psychologie des Schimpanse. Zool. Garten, 8,

279, '67.
Romanes, G. J. Animal Intelligence, N. Y., '83.

On the Mental Faculties of the Bald Chimpanzee, Anthropithetms

calvus. Proc. Zool. Soc, London, '89, 316.
Shepherd, W. Some Mental Processes of the Rhesus Monkey.

Psychol. Rev. Monogr. Suppl. 12, No. 52, '10.
SoKOLOwsKY, A. Beobachtungen iiber die Psyche der Menschenaffen.

Frankfurt, '08.
Thorndike, E. L. The Mental Life of the Monkeys, Psychol. Rev.

Monogr. Suppl 3, No. 15, '01.

The Intelligence of Monkeys. Pop. Sci. Mon., 54, '01.

280 MENTAL LIFE OF APES AND MONKEYS

Wallace, A. R. The Malay Archipelago, 3d. ed., London, 72.
Watson, J. B. Imitation in Monkeys. Psych. Bull. 5, 169, '08.

Some Experiments Bearing upon Color Vision in Monkeys.

Jour. Comp. Neur. Psych., 19, 1, '09.
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Intelligent Imitation and Curiosity in a Monkey, 1. c. 3, 225, '10.

INDEX

INDEX

Abbott, C. C, intelligence of frogs,

228
Actinians, effect of hunger on, 150,
151
geotaxis, 222

tidal and daily rhythms, 157
varied reactions, 144-147, 159-
161
Adams, G. P., phototaxis of earth-
worm, 47
Adamsia, food taking, 151
Adlerz, G., imitation in ants, 210

intelligence of wasp, 192
^thalium, rheotaxis, 37
Aikins, H. A. See Hodge, 89
Aiptasia, varied reactions, 146
effect of hunger on, 151
habit formation, 153
Allabach, L. F., reactions of Metri-

dium, 160
Allen, J. ,258

Alligator, intelligence, 230
Allolobophora, positive phototaxis

in weak light, 47
Ament, W., 258
Ammophila, homing, 194

stinging prey, 119, 120, 131,

132
using stone as a tool, 203, 204
variability of instinct, 131, 132
Amoeba, adaptiveness of behavior,
70, 71
chemotaxis, 23, 26, 70
electrotaxis, 59
food taking, 64, 68, 69
imitation of activities, 67, 68
locomotion, 65-67
phototaxis, 70

reactions to contact, 33-35, 69,
70

Amoeba, thermotaxis, 58
Amphibia, intelligence, 226-229
Amphipods, phototaxis changed
by contact, 50, 51
thigmotaxis, 36. See also Or-
chestia and Talorchestia
Amphipyra, phototaxis, 36

thigmotaxis, 36
Amphithoe, behavior, 30, 94
Anemones. See Actinians
Anemotropsm in insects, 38
Anoplodactylus, phototaxis, 56
Antennularia, geotaxis, 30
Ants, chemotaxis, 29

imitation in, 209, 210, 254
influence of numbers on cour-
age, 207
intelligence, 192, 193, 205, 214
variability in cocooning, 132,
133
Apes, intelligence, 260 ff
Aporus, homing, 195
Aquinas, T., pleasure as a deter-
minant of action in ani-
mals, 164
Arenicola, phototaxis of larvae, 52,

53
Associative memory, 5 ff. See

also Intelligence
Asterias, habit formation, 154, 155,

158
Atherina, intelligence, 223, 224

Baboons, us» of weapons, 267, 270
Bacterium termo, chemotaxis, 22
Bain, A., 178, pleasure and pain in

accommodation, 173-175
Balanus, phototaxis of larva, 47
Balbiani, E. G., capture of food in

Didinium, 78

284

INDEX

Baldwin, J. M., 138, 178, imitation,
257
contra Lamarckism, 124
organic selection, 136 S
pleasure and pain in accommo-
dation, 173-175
problem of learning, 172
Ball, W. P., 138
Bancroft, F. W., 61, electrotaxis in

Paramoecium, 60
Barotaxis defined, 34
Bateson, W., 230, intelligence in

fishes, 220
Bees, communication, 215
homing, 193, 196 ff
influence of numbers on pug-
nacity, 207-209
intelligence, 193, 196-201, 207-

209, 215
and Lamarkism, 126 ff
remembering flowers, 200-202
Beetles, intelligence, 192, 206
Belt, T., intelligence of Cebus, 267

intelligence of wasp, 195
Bentley, I. M., intelligence of chub,

222
Bentley, M. See Day, 87
Berry, C. S., 258, imitation in cats,
256
imitation in white rat, 255,
256
Bethe, A., 16, 189, 215, conscious-
ness in animals, 4, 6
homing of ants and bees, 193,

199
homing of limpets, 187
lack of learning in Carcinus,
182, 183
Binet, A., 89, behavior of Protozoa,
63-65
capture of food in Didinium,

78
relation of reason to percep-
tion, 248
Birds, fear in, 96, 121-123
imitation in, 254, 257

Birds, intelligence, 165, 166, 17&-
178
migration, 108

variations in instinct, 133. See
also particular species
Blanchard, E., intelligence of

beetle, 206
Blow-flies, instincts, 93

phototaxis of larvae, 21, 42,
43
Bodo, capture of prey, 64, 65
Bohn, G., 10, 61, 162, conscious-
ness in animals, 4
definition of instinct, 92
differential sensibility, 152
fatigue sensorielle, 143
geotaxis in Convoluta, 31, 156
homing of limpet, 118
law of varied responsiveness,

145
phototaxis of Littorina, 156,
157; of Pleurosigma, 156
reactions of Cerianthus, 144,

145
reactions of animals to colored

lights, 47
rhythms of activity, 155 ff
tidal rhythms of Convoluta,
31, 156; of Pleurosigma,
156; of anemones, 158;
of Littorina, 156, 157
tropisms, 18
Bostok, F., 258

Branchellion, interference of reac-
tions, 140
Brehm, A., 279
Bumble-bees, 208, 209, 215
Burroughs, J., 258
Buttel-Reepen, H. von, 216, com-
munication in bees, 215
courage in bees, 207
homing in bees, 193, 197
evolution of the
bee community, 128

Carcinus, intelligence, 182-184

INDEX

285

Cardium, reactions to shadows, 41
Carr, H., and Watson, J. B., 258
Casteel, D. B., 231, intelligence of

Chry semis, 230
Cat, imitation, 256

intelligence, 166, 236-242, 255
Catfish, intelligence, 222
Cebus monkey, intelligence, 262,
264-270
imitation, 270
Centipedes, males eating eggs of

female, 96
Cerceris, homing, 195
Cerianthus, geotaxis, 30

reactions to repeated stimuli,
144, 145
Cesaresco, E. M., 258
Chemotaxis, 21-29, 70, 75, 76, 187
Chick, imitation, 254

intelligence, 165, 166, 176-178
transitory instincts, 99
Chilomonas, chemotaxis, 27
Chlamydomonas, geotaxis, 29
Chromulina, geotaxis, 29

phototaxis affected by tem-
perature, 49
Chrysemis, intelligence, 230
Chimpanzee, imitation, 272, 273
intelligence, 262-266, 270, 272,
273
Gadophora, attracting bacteria,

22
Claparede, E., 10, comparative

psychology, 1, 7
Gaypole, Prof., imitation in young

ostrich, 254
Clepsine, phototaxis, 44

reaction to shadows, 41, 42
varied behavior, 111, 142
Cockroaches, geotaxis, 32
Cole, L, J., 61, phototaxis of pyc-
nogonids, 56
phototaxis and image forming
power of eyes, 56, 57
Cole, L. W., 258, intelligence of
raccoons, 242-245, 255

Communication in ants, 213, 214

in bees, 215
Compensatory motions, 39 ff

in relation to phototaxis, 57
Condillac, instinct, 92

intelligence in animals, 232
Consciousness, criterion of, 3 ff
Convoluta, tidal rhythms in, 31,

156
Cope, E. D., 279, intelligence of a

Cebus, 266
Cometz, v., 216, homing of ants,

193
Corymorpha, geotaxis, 31
Cowles, R. P., learning in Ocypoda,

184
Crayfish, compensatory motions,
40
devouring their eggs and

young, 95, 96
instincts as related to reflexes,
100 ff

intelligence, 184-186
reflexes, 13
Ctenophores, geotaxis, 30

statocysts, 30, 33
Cucumaria, geotaxis, 31
Cunningham, J. T., 138
Cuvier, F., 279, intelligence of

orang, 267
Cypridopsis, phototaxis changed

by contact, 50
Cypris, phototaxis changed by
contact, 50

Daphnia, movements of eye, 57
phototaxis changed by con-
tact, 50
Darwin, C. R., 114, 138, 162, 279
chimpanzee throwing stones,

269
definition of instinct, 91
evolution of instinct, 115, 130 ff
fear in young ducks, 123

in young rabbits, 123
followers of, 1

286

INDEX

Darwin, C. R., imperfect instincts

of Molothrus, 96
intelligence of earthworms, 161
Lamarokian theory of instinct

inadequate, 126
mental evolution, 260
reactions of earthworms, 28,

140
Davenport, C. B., 61
Davis, H. B., 258, intelligence of

raccoons, 243, 255
Day, L. M. and Bentley, M.,

learning in Paramoecium,

87-89
Death-feigning. SeeFeigningDeath.
Delage, Y., equilibrium in Crus-
tacea, 31
Delboeuf, J., 231, intelligence ni

lizards, 229
Dellinger, O. P., 89, locomotion in

Amoeba, 66
Descartes, R., consciousness in

animals, 4
Didinium, capture of food, 64,

77-79
Dog, imitation in, 256
instinct to swim, 97
intelligence, 235-237, 240, 245,

246, 249-251, 255
Dragon fly, larval and adult

instincts compared, 98
Driesch, H., geotaxis in Sertulari-

ella, 30
Drzewina, A., 189, tidal rhythms

in phototaxis of hermit

crabs, 158
learning in Pachygrapsus, 187

Earthworms, phototaxis, 21, 43,
44, 54
intelligence, 161
interference of reactions, 140
reactions to chemicals, 28
varied reactions, 148-, 149

Earwigs, thigmotaxis, 36

Edinger, L., 231

Eimer, O. E., 38, definition of
instinct, 92

origin of instinct, 118, 119
Electrotaxis, 59 fit
Eledone, intelligence, 189
Emery, C, 216

Englemann, T. W., chemotaxis of
bacteria, 22

influence of oxygen on photo-
taxis, 49
Epeira, instincts of young, 98
Escherich, K., 216
Eubranchipus, photataxis, 54
Eudendriumplanulse, phototaxis,52
Euglena, geotaxis, 29

phototaxis, 28, 44, 45

Fabre, J. H., 216, constancy of
instinct, 132
intelUgence of beetles, 206
locality sense of mason bee, 197
nature of instinct, 91
Fatigue as cause of • change of
behavior, 84, 142-145, 149
Fear of hawks, not innate, 123
in fiddler crabs, 140
in Rhea, 122, 123
in sparrows, 122
in young birds in general, 96,

121-123
in young ducks, 123
in young rabbits, 123
in young terns, 96, 97
Feigning death, change of attitude
in through fatigue, 143
in young terns, 96,97
Fiddler crabs, interference of reac-
tions, 140
phototaxis, 48, 56, 140, 144
Fishes, intelligence, 219 ff
rheotaxis, 37, 38, 57
Fleure, H. J., and Walton, C. L.,
162, reactions of Actinia,
159, 160
Fly, behavior of decapitated speci-
mens, 104

INDEX

287

Fly, larvae, See Blow-fly.

Forel, A., 216

contra Lamarckism, 124
courage in ants, 207
imitation in ants, 210
intelligence of bees, 200-202
intelligence of water beetles,
192

Formica sanguinea, 205, 207, 210

Fox, Rev. D., variant instinct in
dog, 134

Freund, F., 258

Frog, instincts, 105-107
intelligence, 227-229
reflexes, 12-15, 105-107

Fundulus, rheotaxis, 37

Gamer, R. L., 279

language of apes, 214
Gatke, migration of birds, 254
Geotaxis, 29, 34

Verwom's theory of, 34
Ghinst, van der, 162
Gibbs, D. and Dellinger, O. P., 89
Glaser, O. C, 162, behavior of

ophiurans, 181
Goby, intelligence, 224-226
Goldfish, learning to come for

food, 223
Goldsmith, M., 231, intelligence of

of goby, 224-226
Goltz, F., clasping instinct of

brainless male frog, 106
Gonionemus, effect of hunger on

behavior, 150
geotaxis, 30
reactions, 16
Goniotaxis, defined, 36
Groom, T. F., and Loeb, J., pho-

totaxis of Balanus larvae,

47
Groos, K., contra Lamarckism, 124
Gurley, R. R., 231

Haake, AV., dancing instinct in
shrew, 134

Habits in anemones, 153, 157, 158
in birds, 121-123
in hermit crabs, 158, 186, 187
in organs, 159
Paramoecium, 86-88
in starfish, 154, 155, 158
various causes of, 153-159
Hachet-Souplet, P., 258
Hadley, P. B., rheotaxis of lobst€r,
38, 57
phototaxis of lobster, 55, 56
Haggerty, M. E., 279, imitation in

monkeys, 273, 274
Hanel, E., supposed intelligence of

earthworms, 161
Hargitt, C. W., 162, reactions of
Hydroides to shadows,
142
Harper, E. H., 61, photatxis in

Perichaeta, 44
Hartley, intelligence in animals,

232
Hartmann, R., 279
Hartmann, von, definition of in-
stinct, 92
Haseman, rhythms in Littorina,

157
Hawks, fear of, 123
Heliotropism, 19. See also Photo-
taxis.
Heptagenia, intelligence, 192
Hermit crabs, intelligence, 186, 187

tidal rhythms, 158
Herrick, C. J., 231, intelligence in

cat-fish, 222
Hertel, E., reactions of Paramoe-
cium to ultra violet rays,
75
Hirschlaff, L., 279, imitation in

chimpanzee Consul, 272
Hobhouse, L. T., 114, 179, 258, 259
imitation, 256
instinct and intelligence, 112
intelligence of dog, 254
intelligence in monkeys, 262-
266, 269-270, 276, 277

288

INDEX

Hobhouse, L. T., problem of learn-
ing, 176
reason in animals, 249, 276, 277

Hodge, C. F., and Aikins, H. A.,
change of behavior in
Vorticella, 89

Holmes, S. J., 61, 89, 179, 231;
behavior of Loxophyllum,
16, 25, 35, 79-81, 141
instincts of Amphithoe, 93, 94;

of young terns, 96, 97
learning in crayfish, 185, 186;

in sunfish, 223
phototaxis of blow-fly larvae,
42, 43; of earthworm, 43,
44, 54; of Eubranchipus,
54; of fiddler crabs, 48,
56, 140; of Jassa, 49; of
Notonecta, 55; of Or-
chestia, 46, 48, 49, 51, 55;
of Ranatra, 48, 51, 52, 55,
56, 140; of Talorchestia,
42, 54
reactions of mosquito larvae to
shadows, 142

Holt, E. B., and Lee, F. S., 61

Horse, locality memory, 246-248

Hudson, W. H., 114, fear in birds,
121-123

Huggins, G. E., learning in cray-
fish, 184

Huggins, Dr., variant instinct in a
dog, 134

Hutchinson, W. N., 258, imitation
in dogs, 256

Huxley, T. H., 10

Hyalella, change of phototaxis by
chemicals, 50

Hydra, geotaxis, 30

reactions to chemicals, 28
reactions to repeated stimula-
tion, 141, 145
thigmotaxis, 36

Hydroides, reaction to repeated
stimulation by shadows,
142

Imitation in ants, 209, 210, 254

in birds, 254, 257

in cats, 255

in fishes, 253

in monkeys, 262, 270 S

in raccoons, 255

in rats, 255

in dogs, 255, 256
Insects, compensatory motions, 40

geotaxis, 32

instincts, 93, 95, 98, 104, 105,
108, 110, 118-120, 124-
133, 140, 191 S

intelligence in, 191 fif

photokinesis, 42

phototaxis, 42, 43, 48, 51, 55

rheotaxis, 38

seat of instincts, 104

thigmotaxis, 36
Instinct, 108

analysis of, 21

deferred, 96, 97

defined, 92

dependence on internal con-
ditions, 110 ff

evolution of, 115 fiE

human, 100

illustrations of, 93, 95

imperfection of, 95, 96

and intelligence, 112

internally initiated, 107 ff

modified by experience, 166

popular conception of, 94, 95

and reflex action, 99 ff

transitoriness of, 99

variability, 131 ff
Intelligence, beginnings of, 164 ff

criterion of, 181

deceptive appearances of, 159

in amphibians, 227-229

in apes, 260

in birds (chick), 165, 166, 176-
178

in crustaceans, 182-187

in fishes, 219-227

in insects, 191 ff

INDEX

289

Intelligence in lower inverte-
brates, 63 S, 86 ff, 182

in mammals, 166, 167, 232 fF

in mollusca, 187-189

in monkeys, 260

in reptiles, 230
Isopods, thigmotaxis, 36

Jackson, H. H. T., phototaxis of

Hyalella, 50
James, W., 114, contradictory
instincts, 112

instinct as determinant of
action, 113

instinct and intelligence, 168
Jassa, change of phototaxis, 49
Jelly-fish, reflexes, 13
Jennings, H. S., 34, 61, 89, 163;
Aiptasia, food taking, 151

Amoeba, food taking, 59, 68;
locomotion, 66; role of
surface tension in move-
ments, 67-69

anemones, habits, 153; varied
reactions, 146, 147

Chilomonas, chemotaxis, 27

Didinium, capture of food,
78

earthworms, varied reactions
of, 148, 149

Euglena, phototaxis, 44, 45

Paramoecium, chemotaxis, 23-
26; motor reflex in, 73, 74;
thigmotaxis, 35; varia-
tions in behavior, 76, 77

Spirillum, chemotaxis, 27

starfish habits, 154-155, 158-
159, 181, 182

Stentor, modifiabiHty of be-
havior, 83

tropisms, 21
Jesse, E., intelligence in alligator,
230

Keeble, F., 163, tidal rhythms in
Convoluta, 156

Kinnaman, A. J., 279, imitation in

Rhesus monkeys, 272
Kirby, W. and Spence, W., 216,

definition of instinct, 91
intelligence of beetle, 206
Knauer, F. K., intelligence of frogs,

228
Kollmann, J., 189, intelligence of

octopus, 189
Kreidl, A., 32, fishes coming to

sound of bell, 224
functions of statocysts in

Palaemon, 31, 32

Labidocera, geotaxis, 31

phototaxis changed by con-
tact, 50

Lacordaire, J. T., intelligence of
instincts, 191

Lady-beetles, geotaxis, 32

Lamarck, J. B., evolution of
instinct, 115, 118 ff

Lamar kian theory of instinct, 115-
131, 136-138

Lambs, undefined instinct of young,
96

Landois, H., 231

Lasius, intelligence, 212, 213

variability of cocooning in-
stinct, 132

Lecaillon, A., 231

Lee, F. S., 61

Leibnitz, intelligence of animals,
232

Leucocytes, chemotaxis, 26

Lewes, G. H., 138, origin of
instinct, 116

LiUie, R. S., phototaxis of Areni-
cola, 52, 53

Limpets, homing, 187, 188

Limulus, segmental reflexes, 16

Littorina, tidal rhythms, 156,
157

Lizards, intelligence, 229

Lobster, phototaxis, 55, 56
rheotaxis, 38, 57

290

INDEX

Loeb, J., 10, 17, 31, 61, conscious-
ness in animals, 4, 6

Amphipyra, tropisms of, 36

geotaxis in Antennularia, 30;
in Cerianthus, 30; in
insects, 32

Gonionemus, rhythmic con-
tractions of, 16

heliotropism, 19, 41, 46

photokinesis and phototaxis,
41

phototaxis, change of in cope-
pods, Gammarus and Pol-
ygordius, 49

stereotropism, 33

theory of tropisms, 18-21, 60
Loxocephalus, forming groups, 24
Loxophyllum, behavior, 16, 25, 35,
79-81, 141

chemotaxis, 25

reaction of pieces, 80

reactions to repeated stimula-
tion, 141

rhythms, 16

thigmotaxis, 35
Lubbock, Sir J., 216, communica-
tion in ants, 214; in bees,
215

homing of bees, 193

intelligence of ants, 211-214

intelligence of wasp, 192
Lukas, F., 17

Lyon, E. P., compensatory mo-
tions, 40

phototaxis in Palaemonetes,
49, 56

rheotaxis in fishes, 37, 38, 57

Mactra, reaction to shadows, 41
Magpie, imitation, 257
Mammals, intelligence, 232 flf

compensatory motions, 39
Man, instincts of, 100
Marshall, H. R., 10, 179
Massart, J., chemotaxis absent in
Polytoma, 25

Massart, J., effect of temperature

on phototaxis of Chrom-

ulina, 49
geotaxis in lower organisms, 29
Mast, S. O., 61, 89; Arenicola

larvoe, phototaxis of, 53

Didinium, behavior of, 77-79;

Eudendrium, phototaxis, 52;

Stentor, phototaxis, 44

Maxwell, S., thigmotaxis of Nereis,

36
May-fly. See Heptagenia.
McCook, H. C, 216
Mendelssohn, M., 61
Metalnikow, S., food taking in

Paramcecium, 72
Metridium, food taking, 151

varied reactions of, 160, 161
Michener, E. J., effect of chemicals

on phototaxis, 50
Migration, in birds, 108
Mills, T. W., 258
Milne-Edwards, H., intelligence of

animals, 260
Minkiewicz, C., 61, disguisement of

spider crabs, 104
reactions to colored lights, 47
Mobius, K., 231, 279, intelligence

of pike, 219, 220
intelligence of a chimpanzee,

266
Mocking-bird, imitation, 257
Moggridge, J. T., on young trap

door spiders, 98
Molluscs, change of responsiveness

to light and shadows, 143
intelligence, 187-189
Molothrus, imperfect egg-laying

instinct, 96
Monkeys, 260 ff
Montgomery, T. H., on young

Epeiras, 98
Moore, A., geotaxis in Paramcecium,

30
Moore, A. R., habit formation in
starfish, 154, 155

INDEX

291

Morgan, C. Lloyd, 10, 114, 138, 190,
animal intelligence, 248
behavior of protozoa, 63
consciousness in animals, 4
deferred instincts, 96
evolution of instinct, 121
fear of hawks not innate, 123
homing of limpets, 187-189
imitation in chicks, 254
intelligence in chicks, 165; in
dog, 234, 235; in stickle-
back, 226
instinct and internal impul-
sion, 108, 109
intelligence in solitary wasp,

203, 204
Lamarckism rejected, 124
organic selection, 136
principle of, 159, 234
varied behavior of Clepsine,
111, 112
Morgan's law, 159, 234
Morse, M., tidal rhjrthms of

Littorina, 157
Mosquitoes, geotaxis, 32

reaction of larvae to shadows,
142
Moth, tropisms, 18
Motor reflex, in Euglena, 28, 44, 45
in Loxophyllum, 79, 80
in Paramoecium, 15, 73, 74, 76
in Stentor, 44, 81, 82
Mound-building bird, instinct of

flight, 97
Murbach, L., geotaxis in Gon-

ionemus, 30
Mysis, geotaxis, 31

Nagel, W. A., 61, food taking in
anemones, 150, 151
light reactions of molluscs, 41

Natural selection, origin of instinct
by, 134 ff

Necturus, intelligence, 227

Nephelis, phototaxis, 44

Nereis, thigmotaxis, 36

Nervous system, of crayfish, 101
Nightingale, variation in singing

instinct, 133
Norman, W. W., pain sensations in

lower animals, 8
Notonecta, phototaxis, 55
Nuel, J. P., 61

Octopus, intelligence, 188, 189
Ocypoda, intelligence, 184
Odynerus, instinct, 118
Oelzelt-Newin, A., 90, food taking

in Loxophyllum, 80
Orang, intelligence, 267
Orbitolites, thigmotaxis, 34
Orchestia, phototaxis, 46, 48, 49,

51, 55
Organic selection, 136-138
Osborn, H. F., organic selection,

136
Osmia, parental instincts, 120
Ostrich, imitation in yoimg, 254
Oxytricha, forming of groups, 24
thigmotaxis, 35

Pachygrapsus, intelligence, 187
Pagurus, intelligence, 186
Pain, 6, 164 ff
Palsemon, 32

statocysts, 31 ff
Palaemonetes, change of photo-
taxis, 49
Paley, W., conception of instinct,
108

definition of instinct, 91
Paramoecium, behavior, 15, 35, 71

chemotaxis, 21, 23-25, 27,
75, 76

choice of food, 72

effect of hunger on, 150

electrotaxis, 59, 60

forming groups, 24

geotaxis, 29, 30

interference of reactions, 140

modifications of behavior, 76,
77, 140

292

INDEX

Paramoecium, motor reflex, 15, 73,
74,76
reaction to ultra violet rays,
75

swimming, 72
thermotaxis, 58, 75
thigmotaxis, 35, 73, 76
Parker, G. H., food taking in
anemones, 150, 151
geotaxis of Labidocera, 31
phototaxis of Labidocera, 31,
50; of Vanessa, 55
Parrot imitation, 257
Patellas, homing, 188
Pawlow, J. P., formation of habita

by stomach, 159
Peal, imitation in jungle pheasant,

254
Pearl, R., 163, goniotaxis in
Planaria, 36
reactions of Planaria, 53, 147
variability in behavior of
Planaria, 147
Pechuel-Losche, E., intelligence of

baboons, 267, 269
Peckham, G. W. and E. G., 216,
homing of wasps, 193-196
instincts of solitary wasp, 119,

120, 131, 132
intelligence in solitary wasp,

203, 204
variability of instinct in soli-
tary wasp, 131, 132
Perch, inteUigence, 220, 221
Pfeffer, W., chemotaxis of bacteria,
22
chemotaxis of spermatozoida
of ferns and mosses, 26
Pfungst, O., 258
Phototaxis, 19-21, 36, 38, 40 ff, 82,

186, 192
Photokinesis, 41, 42
Pieron, H., intelligence of Cras-
sius, 221
intelligence of Triton, 227
kinaesthetic reflex, 199

Pigeon, compensatory motions, 39
Pike, intelligence, 219
Planaria, reactions to chemicals,
28,53
reactions to light, 41
reactions to mechanical stimu-
lation, 53, 141
reactions to repeated stimula-
tion, 141
thigmotaxis, 36
varied response to stimuli,
144, 147
Plants, reactions, 15
Play instinct, 109
Pleasure, 6, 55, 164 ff
Pleurosigma, tidal rhythms, 156
PoUstes, intelligence, 195, 196
Polyergus, stupidity, 210, 211

variation in cocoon spinning,
132, 133
Polytoma, lack of chemotaxis, 25
Pouchet, G., phototaxis of Temora,

50
Preyer, W., 163, behavior of star-
fish, 181
human instincts, 100

Promethus moth, instincts of, 124,

125
Protozoa, internally initiated activ-
ity, 109
intelligence, 63-65, 86 ff, 181
reactions, 15, 21 ff, 44-47, 49,
68-60, 63 ff, 140, 141, 150
rhythms, 16

tropisms, 18, 21 ff, 44-47, 49,
5S-60, 70 ff
Prowazek, S. von, 90
Psammobia, reaction to light, 41
Putter, A., 61, 90
Pycnogonids, phototaxis, 56

Raccoon, intelligence, 242-245, 255
Rddl, E., 62, hovering in flies, 38
movement of eye of Daphnia,
67

INDEX

293

R^I, E., phototaxis of Arthro-
poda, 55
relation of phototaxis to vis-
ion, 56
Rae, Dr., fear in young ducks, 123
Ranatra, behavior after decapita-
tion, 105
Interference of reactions, 5,

140
phototaxis, 48, 51, 52, 55, 56,
144
Random movements, in chemo-
taxis, 24-26
in phototaxis, 21, 22, 41-45
r61e of in learning, 168
Rat, imitation, 255, 256
Raven, imitation, 257
Reason, in apes and monkeys,
274 ff
in insects, 203 flf
in mammals, 232 ff
Reflex action, 11 ff
Reflexes, 2, 11 ff

of pecking, swallowing, 176-
177. See also Motor
Reflex,
Reflex action and instinct, 99 ff,

116, 117
Reighard, J., 231, intelligence of

fishes, 223, 224
Reimarus, H. S., 114
Reptiles, intelligence, 229, 230
Reuter, O. M., intelligence of ants,

193
Rhea, no innate fear of man, 122
Rheotaxis, 34, 37 ff, 57
Rhesus monkey, intelligence, 262- Shepherd, W., 279, imitation in

Romanes, G. J., 10, 114, 138, 279,
animal inteUigence, 2
definition of instinct, 91, 92
evolution of instinct, 120,

121
homing of bees, 196
intelligence of dog, 250
moth flying to candle, 20
Romanes, Miss, intelligence of
Cebus, 265, 269, 278, 279

Sachs, H., tropisms, 19
Sagartia, effect of hunger on food
taking, 150
geotaxis, 30
Schaeffer, A. A., 90, 231, food tak-
ing in Paramoecium, 72
selection of food in Stentor, 85,

86
intelligence of frog, 228, 229
Schneider, G. H* 114, 190, in-
stinct, 1
intelligence of octopus, 188,
189
Schneider, K. C, 114
Schrader, M., 17, segmental re-
flexes in frog, 107
Sense of direction, in ants, 193
in bees, 193, 196-202
in horses, 247
in wasps, 193-196, 200,
201
Serpula, reactions to light and
shadows, 41
varied reaction to shadows,
145

265
imitation, 271-274
Rhumbler, L., 90, behavior of

Amoeba, 68
Rhythms, in behavior, 16, 79, 80,

155-158
Righting movements, in Loxo-
phyllum, 81
in starfish, 154, 155, 158

Rhesus monkeys, 272, 273
Sherrington, C. S., 17, reflex action,

11
Small, W. S., 258, imitation in

white rat, 255
Smith, S., 90, educabiUty of

Paramoecium, 86-89
Sokolowsky, A., 279
Solen, reaction to shadows, 41

294

INDEX

Sondheim, M., 216, intelligence of

damsel-fly, larvae, 191,

192

Spallanzani, Abbe, clasping instinct

of brainless male frog, 106

Sparrows, fear in, 122

Spaulding, D. A., 114, instincts of

chicks, 99
Spaulding, E. G., 190, learning in

hermit crabs, 186, 187
Spence, W. See Kirby.
Spencer, H., 10, 114, 138, 179,
definition of instinct, 91,
107
Lamarckian theory of instinct,
127

mental evolution, 1, 9
origin of instinct, 116, 117
pleasure and pain, 167, 172-

175
relation of intelligence to other
activities, 139
Spider crabs, instincts for disguise-

ment, 104
Spiders, devouring their eggs, 96
geotaxis, 32
instincts of young, 98
Spirillum, chemotaxis, 27

geotaxis, 29
Spirostomum, geotaxis, 30
Stahl, E., chemotaxis in myxomy-

cetes, 23
Staphylococcus, attracting leu-
cocytes, 26
Starfish, habit formation, 154-155,

158
Statkewitsch, P., reaction of Para-
moecium to induction
shocks, 144
Statocysts, functions of in Crusta-
cea, 31
function in Ctenophores, 30
Stentor, effect of hunger on, 150
modifiability of behavior, 49,

82-86, 141, 145, 150
phototaxis, 44, 49, 82

Stentor, reactions, 15, 81-86

rhythms, 16
Stereotropism, 33 ff, 42
Stickleback, intelligence, 226
Stoichactis, varied reactions, 146
Stone, intelHgence of dog, 250, 251
Strasburger, E., 62, effect of tem-
perature on phototaxis of
swarm spores, 49
Sunfish, intelligence, 223
Surface tension theory of move-
ments of Amoeba, 67
Swallows, instinct of flight, 97
Swarm spores, change of photo-
taxis, 47

Talorchestia, photokinesis, 42

phototaxis, 42, 54
Temora, phototaxis changed by

contact, 50
Temperature, effect on phototaxis,
49
reactions of organisms to, 58,
59
Terns, fear in, 96, 97

feigning death, 96, 97
undefined instinct of young, 96
Thermotaxis, 58, 59

in ParamcEcium, 58, 75
in Planaria, 59
Thigmotaxis, 33-36, 42
Thompson-Seton, E., 258
Thomdike, E. L., 231, 258, 279,
animal intelligence, nature
of, 235-242, 251, 252, 255
imitation in monkeys, 270
intelligence in cats, 166, 167
intelligence of monkeys, 261,
262
Titchner, E. B., 10
Toads, intelUgence, 227, 228
Toenjies, H., 258
Torelle, E., 62

Torrey, H. B., effect of hunger on
food reactions of Sagartia,
160

INDEX

295

Torrey, H. B., geoUxis in Cory-

morpha, 31
geotaxis in Sagartia, 30
Tortoise, intelligence, 229, 230
Towle, E., phototaxis of Cypri-

dopsis, 50
Triplett, N., 231, intelligence of

perch, 220, 221
Triton, intelligence, 227
Tropisms, 2, 18 ff
Turner, C. H., 216, homing of

burrowing bees, 198, 199

homing of mud dauber, 200

Twain, Mark, imbecility of ants, 95

Uexkull, J. von, 17, 163, 190, intelU-
gence in Eledone, 189
tonus, 149

Verwom, M., 62, 90, function of
statocyst in Ctenophores,
30

geotaxis, 34

thigmotaxis, 34

thigmotaxis in Oxytricha, 35
Voltaire, 91
Volvox, phototaxis, 47
Vorticella, change of behavior, 89

reactions, 15

rhythms, 16

Wagner, G., reactions of Hydra,

141, 145
Wagner, M., 217, bumblebees, 208,

209, 215
communication in bees, 215
homing of bees, 193
WaUace, A. R., 280
Walter, H. E., 62, reactions of

planarians to changes of

light intensity, 142-143;

to repeated jarring, 141
Walton, C. L. See Fleure, 159.
Washburn, M. F., 10
Washburn, M. F., and Bentley, I.

M., 231

Washburn, M. F., intelligence of

chub, 222
Wasmann, E., 10, 114, 217, com-
munication in ants, 214
courage in bees, 207
imitation in ants, 210, 254
inteUigence of animals, nature

of, 181
intelligence of ants, 193, 205,

211
intelligence of apes, 268, 276,

277
plasticity of behavior, 139
Wasps, homing, 193-196, 200, 201
intelligence, 192-196, 200, 201,

203, 204
instincts, 119, 120, 131, 132
Watkins, G. P., 90
Watson, J. B., 258, 280, imitation

in monkeys, 270, 272
Weismann, A., 138, contra La-

marckism, 127
Wheeler, W. M., 217, imitation in

ants, 209, 210
White, G., intelligence in tortoise,

229, 230

Whitman, C. O., 138, evolution of

instinct, 115, 116, 121, 135

Lamarckism, 124

intelUgence of Necturus, 227

origin of instincts in pigeons,

135
varied behavior of Clepsine, 111
Willcox, M. A., 190
Witmer, L., 280, intelligence of
chimpanzee, 266, 267
imitation in chimpanzee, 272,
273
Wodsedalek, J., intelligence of

May-fly nymph, 192
Wundt, W., 138, origin of instinct,
118

Yerkes, A. W., 163, reactions of
Hydroides to shadows,

296 INDEX

Yerkes, R. M., 190, 231, 258, Yoakum, G. S., 259

intelligence in Carcinus, Yung, E., 190
184; in crayfish, 184; in

frog, 228; in turtles, 230; Zell, T., 259

variability of behavior in Zeigler, H. E., 114

dancing mouse, 133, 134 Zurn, F. A., 259

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