BIOLOGY
LIBRARY

G

THE ANIMAL BEHAVIOR SERIES. VOLUME II

THE ANIMAL MIND'

A Text-book of Comparative
Psychology

BY

MARGARET FLOY WASHBURN, Pn.D.

i» * »

ASSOCIATE PROFESSOR OF PHILOSOPHY
IN VASSAR COLLEGE

THE MACMILLAN COMPANY
1908

All rights reserved

W<3

BIOLOGY

COPYRIGHT, 1908,
BY THE MACMILLAJSf COMPANY.

Set up amd «lectrotyped. Publish«d February, 1908.

Nortoooti

J. 8. Gushing Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.

PREFACE

THE title of this book might more appropriately, if not more
concisely, have been "The Animal Mind as Deduced from
Experimental Evidence." For the facts set forth in the fol-
lowing pages are very largely the results of the experimental
method in comparative psychology. Thus many aspects
of the animal mind, to the investigation of which experiment
either has not yet been applied or is perhaps not adapted, are
left wholly unconsidered. This limitation of the scope of
the book is a consequence of its aim to supply what I have
felt to be a chief need of comparative psychology at the
present time. Although the science is still in its formative
stage, the mass of experimental material that has been accu-
mulating from the researches of physiologists and psycholo-
gists is already great, and is also for the most part inaccessible
to the ordinary student, being widely scattered and to a con-
siderable extent published in journals which the average
college library does not contain. While we have books on
animal instincts and on the interpretation of animal behavior,
we have no book which adequately presents the simple facts.

Probably no bibliography seems to one who carefully
examines it entirely consistent in what it includes and what
it excludes. Certainly the one upon which this book is
based contains inconsistencies. The design has been to ex-
clude works bearing only upon general physiology, upon the
morphology of the nervous system and sense organs, or upon
the nature of animal instinct as such, and to include those
which bear upon the topics mentioned in the chapter headings.

v

• 177746

vi Preface

Within these limits, the collection of references upon no topic
is as full as would be necessary for the bibliography of a spe-
cial research upon that topic. Doubtless there are omissions
for which no excuse can be found. In one or two cases, where
the literature upon a single point is very large, as for example,
in the case of the function of the semicircular canals, only a
few of the more important references have been given.

One further comment may be made. The book through-
out deals with comparative rather than with genetic psy-
chology.

I gratefully acknowledge help from a number of sources.
To Professor Titchener I owe, not only my share of that
genuine psychological spirit which he so successfully imparts
to his pupils according to their ability, but various helpful
criticisms upon the present work, about half of which he
has read in manuscript. Dr. Yerkes has given me much
invaluable aid in securing access to material, and has very
kindly permitted me to see the proofs of his book on "The
Dancing Mouse." As editor of the series he has reviewed
my manuscript to its great advantage. Professors Georges
Bohn and George H. Parker have showed especial courtesy
in making their work accessible to me. Professor Jennings
has kindly allowed the use of a number of illustrations from
his book on "The Behavior of the Lower Organisms." My
colleague Professor Aaron L. Tread well has generously
helped me in ways too numerous to specify. But perhaps
my heaviest single obligation is to Professor I. Madison Bent-
ley, who has read the manuscript of the entire book, and
whose advice and criticism have been of the utmost benefit to

every part of it.

M. F. W.

VASSAR COLLEGE, POUGHKEEPSIE, N.Y.
October i, 1907.

TABLE OF CONTENTS

CHAPTER I

THE DIFFICULTIES AND METHODS OF COMPARATIVE PSY-
CHOLOGY

PAGES

§ i. Difficulties. § 2. Methods of obtaining Facts:
Anecdote. §3. Methods of obtaining Facts: Experi-
ment. § 4. Methods of Qbtaining Facts : the Ideal
Method. §5. Methods of Interpreting Facts . . 1-26

CHAPTER II

THE EVIDENCE OF MIND

§6. Inferring Mind from Behavior. §7. Inferring Mind

from Structure 27-36

CHAPTER III

THE MIND OF THE SIMPLEST ANIMALS

§ 8. The Structure and Behavior of Amreba. § 9. The
Mind of Amoeba. § 10. The Structure and Behavior
of Paramecium. § 11. The Mind of Paramecium.
§ 12. Definitions of Tropisms 37~S7

CHAPTER IV

SENSORY DISCRIMINATION: METHODS OF INVESTIGATION

§ 13. Preliminary Considerations. § 14. Structure as
Evidence of Discrimination. § 15. Behavior as Evi-
dence of Discrimination. § 16. Evidence from Structure
and Behavior Combined. § 17. Evidence for Discrimi-
nation of Certain " Lower " Sensation Classes . . 58-66

vii

viii Table of Contents

CHAPTER V
SENSORY DISCRIMINATION: THE CHEMICAL SENSE

PAGES

§ 18. The Chemical Sense in Ccelenterates. § 19. The
Chemical Sense in Flatworms. § 20. The Chemical
Sense in Annelids. § 21. The Chemical Sense in Mol-
lusks. § 22. The Chemical Sense in Echinoderms.
§ 23. The Chemical Sense in Crustacea. § 24. The
Chemical Sense in Arachnida. § 25. The Chemical
Sense in Insects. § 26. How Ants find Food. § 27. How
Ants find the Way Home. § 28. How Ants " recognize "
Nest Mates. § 29. How Bees are attracted to Flowers.
§ 30. How Bees find the Hive. § 31. How Bees "rec-
ognize" Nest Mates. § 32. The Chemical Sense in
Vertebrates 67-105

CHAPTER VI

SENSORY DISCRIMINATION: HEARING

§ 33. Hearing in Lower Invertebrates. § 34. Hearing
in Crustacea. § 35. Hearing in Spiders. § 36. Hear-
ing in Insects. § 37. Hearing in Fishes. § 38. Hear-
ing in Amphibia. §39. Hearing in Higher Vertebrates 106-119

CHAPTER VII

SENSORY DISCRIMINATION: VISION

§ 40. Some Problems connected with Vision. § 41. Vis-
ion in Protozoa. § 42. Vision in Coelenterates.
§ 43. Vision in Planarians. § 44. Vision in Annelids.
§ 45. Vision in Mollusks. § 46. Vision in Echino-
derms. § 47. Vision in Crustacea. § 48. Vision in
Spiders. § 49. Vision in Insects. § 50. Vision in
Amphioxus and in Fish. § 51. Vision in Amphibia.
§ 52. Vision in Other Vertebrates .... 120-147

Table of Contents

IX

CHAPTER VIII

SPATIALLY DETERMINED REACTIONS AND SPACE PERCEP-
TION

§ 53. Classes of Spatially Determined Reactions.
§ 54. Class I : Reactions to a Single Localized Stimu-
lus. § 55. Class II: Orienting Reactions: Possible
Modes of Producing Them. § 56. Orientation to Gravity :
Protozoa. § 57. Orientation to Gravity : Coelenterates.
§58. Orientation to Gravity : Planarians. §59. Orien-
tation to Gravity: Annelids. § 60. Orientation to
Gravity: Mollusks. § 61. Orientation to Gravity:
Echinoderms. §62. Orientation to Gravity : Crustacea.
§ 63. Orientation to Gravity: Spiders and Insects.
§64. Orientation to Gravity : Vertebrates. §65. The
Psychic Aspect of Orientation to Gravity. § 66. Orien-
tation to Light : Photopathy and Phototaxis. § 67. In-
stances of Photopathy and Phototaxis. § 68. Direction
and Intensity Theories of Phototaxis. § 69. The Eyes
in Phototaxis. § 70. Influences affecting the Sense of
Light Orientations. § 71. Mutual Influence of Light
and Gravity Orientations. § 72. The Psychic Aspect
of Orientation to Light. § 73. Orientation to Other
Forces .

148-189

CHAPTER IX

SPATIALLY DETERMINED REACTIONS AND SPACE PERCEP-
TION (continued)

§ 74. Class III : Reaction to a Moving Stimulus.
§75. Class IV: Reaction to an Image. §76. Methods
of Investigating the Visual Image: the Size Test.
§ 77. Methods of Investigating the Visual Image : the
Form Test. § 78. Class V : Reactions adapted to the
Distance of Objects. § 79. Some Theoretical Consid-
erations

190-204

Table of Contents

CHAPTER X

THE MODIFICATION OF CONSCIOUS PROCESSES BY INDI-
VIDUAL EXPERIENCE

PAGES

§80. Absence of Modification. § 81. Heightened Reac-
tion as the Result of Previous Stimulation. § 82. Ces-
sation of Reaction to a Repeated Stimulus. § 83. Varied
Negative Reaction to a Repeated Stimulus. § 84. Drop-
ping off Useless Movements : the Labyrinth Method.
§ 85. Dropping off Useless Movements : the Puzzle-box
Method. § 86. The Psychic Aspect of Dropping off
Useless Movements 205-246

CHAPTER XI

THE MODIFICATION OF CONSCIOUS PROCESSES BY INDI-
VIDUAL EXPERIENCE (continued}

§ 87. The Inhibition of Instinct. § 88. Inhibition in-
volving Discrimination of Successive Stimuli. § 89. In-
hibition involving Discrimination of Simultaneous
Stimuli. §90. Comparison of Methods. §91. "Visual
Memory " in Homing 247-269

CHAPTER XII
THE MEMORY IDEA

(m •

§ 92. Evidence for and against Ideas in Animals.
§ 93. The Primitive Function of Ideas. § 94. The ^

Significance of Stimuli from a Distance. § 95. Ideas of
Movement 270-284

*s

CHAPTER XIII "\\

SOME ASPECTS OF ATTENTION

§ 96. The Interference of Stimuli. § 97. Methods of
Securing Prepotency of Vitally Important Stimuli.
§ 98. The Peculiar Characteristics of Attention as a
Device to secure Prepotency 285-294

THE ANIMAL MIND

THE ANIMAL MIND

CHAPTER I

THE DIFFICULTIES AND METHODS OF COMPARATIVE
PSYCHOLOGY

§ i. Difficulties

[THAT the mind of each human being forms a region inac-
cessible to all save its possessor, is one of the commonplaces
of reflection. His neighbor's knowledge of each person's
mind must always be indirect, a matter of inference. \ How
wide of the truth this inference may be, even under the most
favorable circumstances, is also an affair of everyday ex-
perience: each of us can judge his fellow-men only on the
basis of his own thoughts and feelings in similar circum-
stances, and the individual peculiarities of different members
of the human species are of necessity very imperfectly com-
prehended by others. The science of human psychology
has to reckon with this unbridgable gap between minds as
its chief difficulty. The psychologist may look into his own
mind and study its workings with impartial insight, yet he
can never be sure that the laws which he derives from such
a study are not distorted by some personal twist or bias. For
example, it has been suggested that the philosopher Hume
was influenced by his tendency toward a visual type of
imagination in his discussion of the nature of ideas, which
to him were evidently visual images. As is well known, the
experimental method in psychology has aimed to minimize

2 The Animal Mind

the danger of confusing individual peculiarities with general
mental laws. In a psychological experiment, an unbiassed
observer is asked to study his own experience under certain
definite conditions, and to put it into words so that the ex-
perimenter may know what the contents of another mind are
like in the circumstances. Thus language is the essential
apparatus in experimental psychology; language with all
its defects, its ambiguity, its substitution of crystallized con-
cepts for the protean flux of actually lived experience, its
lack of terms to express those parts of experience which are
of small practical importance in everyday life, but which
may be of the highest importance to mental science. Out-
side of the psychological laboratory language is not always
the best guide to the contents of other minds, because it is
not always the expression of a "genuine wish to communicate
thought. *4^diPJi§jy2£ak louder thanwords," the proverb
says ; but when wordTar?l)acked'D^rgoocl faith they furnish
by far the safest indication of the thought of others. Whether,
however, our inferences are made on the basis of words or
of actions, they are all necessarily made on the hypothesis
that human minds are built on the same pattern, that what
a given word or action would mean for my mind, this it
means also for my neighbor's mind.

If this hypothesis be uncertain when applied to our fellow
human beings, it fails us utterly when we turn to the lower
animals. If my neighbor's mind is a mystery to me, how
great is the mystery which looks out of the 'eyes of a dog, and
how insoluble the problem presented by the mind of an
invertebrate animal, an ant or a spider ! We know that such
minds must differ from ours not only in certain individual
peculiarities, but in ways at whose nature we can only guess.
The nervous systems of many animals vary widely from our
Newn. We have, perhaps, too little knowledge about the

Difficulties and Methods 3

functions of our own to conjecture with any certainty what
difference this must make in the conscious life of such animals ;
but when we find sense organs, such as the compound eyes
of insects or crustaceans, constructed on a plan wholly diverse
from that of ours ; when we find organs apparently sensory in
function, but so unlike our own that we cannot tell what pur-
pose they serve, — we are baffled in our attempt to construct
the mental life of the animals possessing them, for lack of
power to supply the sensation elements of that life. "It is
not," said Locke, "in the power of the most exalted wit or
enlarged understanding, by any quickness or variety of
thought, to invent or frame one new simple idea in the mind"
(232, Bk. II, ch. 2); we cannot imagine a color or a sound
or a smell that we have never experienced; how much less
the sensations of a sense radically different from any that we
possess ! Again, a bodily structure entirely unlike our own
must create a background of organic sensation which renders
the whole mental life of an animal foreign and unfamiliar to
us. We speak, for example, of an "angry" wasp. Anger,
in our own experience, is largely composed of sensations of
quickened heart beat, of altered breathing, of muscular ten-
sion, of increased blood pressure in the head and face. The
circulation of a wasp is fundamentally different from that of
any vertebrate. The wasp does not breathe through lungs,
it wears its skeleton on the outside, and it has the muscles
attached to the inside of the skeleton*. What is anger like
in the wasp's consciousness? W« c$n ffrm n* atfeQuate
idea of it.

To this fundamental difficulty of the dissimilarity between
animal minds and ours is added, of course, the obstacle that
animals have no language in which to describe their expe-
rience to us. Where this unlikeness is greatest, as in the
case of invertebrate animals, language would be of little use,

4 The Animal Mind

since we could not interpret it from our experience ; but the
higher vertebrates could give us much insight into their
minds if they could only speak. We are, however, restricted
to the inferences we can draw from movements and sounds
that are made for the most part without the intention of
communicating anything to us. One happy consequence of
this fact, which to a slight extent balances its disadvantages,
is that we have not to contend with self-consciousness and
posing, which often invalidate human reports of introspec-
tion.

From these general considerations we can understand
something of the special difficulties that beset the path of the
comparative psychologist, who desires to know the contents
of minds below the human level. Knowledge regarding the
animal mind, like knowledge of human minds other than our
own, must come by way of inference from behavior. Two
fundamental questions then confront the comparative psy-
chologist. First, by what method shall he find out how an
animal behaves? Second, how shall he interpret the con-
scious aspect of that behavior?

§ 2. Methods of Obtaining Facts: The Method of Anecdote

The reading of such a book as Romanes's "Animal Intelli-
gence, " or of the letters about animal behavior in the London
Spectator, will reveal one method of gathering information
aboftfdiat anir^ls do. - This has been termed the Method
of Anecdote. It consists essentially in taking the report of
another person regarding the action of an animal, observed
most commonly by accident, and attracting attention because
of its unusual character. In certain cases the observer while
engaged in some other pursuit happens to notice the singular
behavior of an animal, and at his leisure writes out an account

Difficulties and Methods 5

of it. In others, the animal is a pet, in whose high intellectual
powers its master takes pride. It is safe to say that this
method of collecting information always labors under at
least one, and frequently under several, of the following dis-
advantages : —

1. The observer is not scientifically trained to distinguish
what he sees from what he infers.

2. He is not intimately acquainted with the habits of the
species to which the animal belongs.

3. He is not acquainted with the past experience of the
individual animal concerned.

4. He has a 'personal affection for the animal concerned,
and a desire to show its superior intelligence.

5. He has the desire, common to all humanity, to tell a
good story.

Some of these tendencies to error it is unnecessary to illus-
trate. A good example of the dangers of (2), lack of acquaint-
ance with the habits of the species, is given by Mr. and Mrs.
Peckham. They quote the following anecdote reported by
no less eminent and trained an observer than Wundt. "I
had made myself," says that psychologist, "as a boy, a fly-
trap like a pigeon cote. The flies were attracted by scatter-
ing sugar and caught as soon as they had entered the cage.
Behind the trap was a second box, separated from it by a
sliding door, which could be opened or shut at pleasure. In
this I had put a large garden spider. Cage and box were
provided with glass windows on the top, so that I could quite
well observe anything that was going on inside. . . . When
some flies had been caught, and the slide was drawn out, the
spider of course rushed upon her prey and devoured them.
. . . This went on for some time. The spider was some-
times let into the cage, sometimes confined to her own box.
But one day I made a notable discovery. During an absence

6 The Animal Mind

the slide had been accidentally left open for some little
while. When I came to shut it, I found that there was an
unusual resistance. As I looked more closely, I found that
the spider had drawn a large number of thick threads directly
under the lifted door, and that these were preventing my clos-
ing it. . . ."

"What was going on in the spider's mind?" Wundt asks,
and points out that it is unnecessary to assume that she
understood and reasoned out the mechanical requirements
of the situation. The whole matter can be explained, he
thinks, in a simpler way. "I imagine that as the days went
by there had been formed in the mind of the spider a deter-
minate association on the one hand between free entry into
the cage and the pleasurable feeling attending satisfaction of
the nutritive impulse, and on the other between the closed
slide and the unpleasant feeling of hunger and inhibited im-
pulse. Now in her free life the spider had always employed
her web in the service of the nutritive impulse. Associations
had therefore grown up between the definite positions of her
web and definite peculiarities of the objects to which it was
attached, as well as changes which it produced in the positions
of certain of these objects, — leaves, small twigs, etc. The
impression of the falling slide, that is, called up by association,
the idea of other objects similarly moved which had been held
in their places by threads properly spun; and finally there
were connected with this association the other two of pleasure
and raising, unpleasantness and closing, of the door" (446,

PP- SSJ-IS2)-

The Peckhams remark in criticism of this observation:

"Had Wundt been familiar with the habits of spiders, he
would have known that whenever they are confined they walk
around and around the cage, leaving behind them lines of
web. Of course many lines passed under his little sliding

Difficulties and Methods 7

door, and when he came to close it there was a slight resist-
ance. These are the facts. His inference that there was even
a remotest intention on the part of his prisoner to hinder the
movement of the door is entirely gratuitous. Even the
simpler mental states that are supposed to have passed
through the mind of the spider were the products of Wundt's
own imagination " (322, p. 230). The fact that the anecdote
was a recollection of childhood, so that it would probably be
impossible to bring any evidence from the character of the
web or other circumstance against the suggestion of Mr. and
Mrs. Peckham, is a further instance of the unscientific use
of anecdotal testimony.

An illustration of the third objection mentioned above,
the disadvantage of ignorance of the animal's individual his-
tory, is furnished by Lloyd Morgan. In describing his futile
efforts to teach a fox terrier the best way to pull a crooked
stick through a fence, he says that the dog showed no sign
"of perceiving that by pushing the stick and freeing the crook
he could pull the stick through. Each time the crook caught
he pulled with all his strength, seizing the stick now at the
end, now in the middle, and now near the crook. At length
he seized the crook itself and with a wrench broke it off. A
man who was passing . . . said, 'Clever dog that, sir; he
knows where the hitch do lie.' The remark was the charac-
teristic outcome of two minutes' chance observation " (282,
pp. 142-143). How many anecdotes of animals are based
on similar accidents?

It will be seen that in both the cases just criticised the error
lies in the interpretation of the animal's behavior. Indeed,
a root of evil in the method of anecdote consists in the fact
that observation in this form is imperfectly divorced from in-
terpretation. The maker of an anecdote is seldom content
with merely telling one what the animal did and leaving future

8 The Animal Mind

investigation and the comparative study of many facts to de-
cide what the animal's conscious experience in doing it was
like. The point of the anecdote usually consists in showing
that a human interpretation of the animal's behavior is pos-
sible. Here is shown the desire to tell a good story, which
we mentioned among the pitfalls of the anecdotal method;
the wish to report something unusual, not to get a just concep-
tion of the normal behavior of an animal. As Thorndike has
forcibly put it : "Dogs get lost hundreds of times and no one
ever notices it or sends an account of it to a scientific maga-
zine. But let one find his way from Brooklyn to Yonkers
and the fact immediately becomes a circulating anecdote.
Thousands of cats on thousands of occasions sit helplessly
yowling, and no one takes thought of it or writes to his friend,
the professor ; but let one cat claw at the knob of a door sup-
posedly as a signal to be let out, and straightway this cat be-
comes the representative of the cat-mind in all the books "

(393, P- 4).

All this is not to deny that much of the testimony to be
found in Romanes's " Animal Intelligence" and Darwin's
" Descent of Man" is the trustworthy report of trained ob-
servers ; but it is difficult to separate the grain from the chaff,
and one feels toward many of the anecdotes the attitude of
scepticism produced, for example, by this tale which an
Australian lady reported to the Linnaean Society. The burial
of some deceased comrades was accomplished, she says, by
a nest of " soldier ants" near Sydney, in the following fashion.
"All fell into rank walking regularly and slowly two by two,
until they arrived at the spot where lay the dead bodies. . . .
Two of the ants advanced and took up the dead body of one
of their comrades ; then two others, and so on until all were
ready to march. First walked two ants bearing a body,
then two without a burden; then two others with another

Difficulties and Methods 9

dead ant, and so on, until the line was extended to about forty
pairs, and the procession now moved slowly onward, followed
by an irregular body of about two hundred ants. Occa-
sionally the two laden ants stopped, and laying down the dead
ant, it was taken up by the two walking unburdened behind
them, and thus, by occasionally relieving each other, they
arrived at a sandy spot near the sea." A separate grave was
then dug for each dead ant. "Some six or seven of the ants
had attempted to run off without performing their share of
the task of digging; these were caught and brought back,
when they were at once attacked by the body of ants and
killed upon the spot. A single grave was quickly dug and
they were all dropped into it." No funeral procession for
them ! Of this story Romanes says, "The observation seems
to have been one about which there could scarcely have been
a mistake " (364, p. 91). One is inclined to think it just
possible that there was.

§3. Methods of Obtaining Facts : The Method of Experiment

Diametrically opposed to the Method of Anecdote and its
unscientific character is the Method of Experiment. An
experiment, properly conducted, always implies that the
conditions are controlled, or at least known; whereas igno-
rance of the conditions is, as we have seen, a common feature
of anecdote. The experimenter is impartial ; he has no de-
sire to bring about any particular result. The teller of an
anecdote wishes to prove animal intelligence. The experi-
menter is willing to report the facts precisely as he observes
them, and is in no haste to make them prove anything.
The conduct of an experiment upon an animal will, of course,
vary according to the problem to be solved. If the object is
to test some innate reaction on the animal's part, such as its
ordinary responses to stimulation or its instincts, one need

io The Animal Mind

merely place the animal under favorable conditions for ob-
servation, make sure that it is not frightened or in an abnormal
state, supply the appropriate stimulus unmixed with others,
and watch the result. If it is desired to study the process
by which an animal learns to adapt itself to a new situation,
one must, of course, make sure in addition that the situation
really is new to the animal, and yet that it makes sufficient
appeal to some instinctive tendency to supply a motive for
the learning process.

As one might expect, among the earliest experiments upon
animals were those made by physiologists with a view to
determining the functions of sense organs. The experimental
movement in psychology was slow in extending itself into the
field of the animal mind.

Romanes, whose adherence to the anecdotal method we
have noted, made in 1881, rather as a physiologist than as a
psychologist, a number of exact and highly valued experi-
ments on ccelenterates and echinoderms, which were sum-
marized in his book entitled " Jelly-fish, Star- fish, and Sea-
urchins," published in 1885. He has also recorded some
rather informal experiments on the keenness of smell in
dogs. Sir John Lubbock, in 1883, reported the results of
some experiments on the color sense of the small crustacean
Daphnia, and his book on "Ants, Bees, and Wasps," con-
taining an account of experimental tests of the senses and
"intelligence" of these insects, appeared in the same year.
A German entomologist, Vitus Graber, experimented very
extensively at about this period on the senses of sight and
smell in many animals. Preyer, the authority on child
psychology, published in 1886 an experimental study of the
behavior of the starfish. Loeb's work on the reactions of
animals to stimulation began to appear in 1888. Bethe's
experiments on ants and bees were published in 1898. Max

Difficulties and Methods n

Verworn, the physiologist, published in 1899 an exhaustive
experimental study of the behavior of single-celled animals.
With the exception of Preyer and Romanes, all these men
had but a secondary interest in comparative psychology:
Bethe, indeed, as we shall see, wholly rejects it. Lloyd
Morgan, who has written instructively on comparative
psychology, makes but a limited use of the experimental
method. Wesley Mills, professor of physiology in Me Gill
University, has studied very carefully the mental develop-
ment of young animals such as cats and dogs, but is inclined
to criticise the use of experiment in observing animals. The
work of E. L. Thorndike, whose " Animal Intelligence "
appeared in 1898, represents, perhaps, the first definite effect
of the modern experimental movement in psychology upon
the study of the animal mind. Thorndike's aim in this re-
search was to place his animals (chicks, cats, and dogs) under
the most rigidly controlled1 experimental conditions. The
cats and dogs, reduced by fasting to a state of "utter hunger,"
were placed in boxes, with food outside, and the process
whereby they learned to work the various mechanisms which
let them out was carefully observed. Since the appear-
ance of Thorndike's work the performance of experiments
upon animals has played much part in the work of psychologi-
cal laboratories, particularly those of Harvard, Clark, and
Chicago universities. The biologists and physiologists have
continued their researches by this method, so that a very large
amount of experimental work is now being done in compara-
tive psychology.

Despite the obvious advantages of experiment as a method
for the study of animal behavior, it is not without its dangers.
These were clearly stated by Wesley Mills in a criticism of
Thorndike's " Animal Intelligence" (273). They may be
summed up by saying that there is a risk of placing the

12 The Animal Mind

animal experimented upon under abnormal conditions in the
attempt to make them definite and controllable.1 Did not,
for example, the extreme hunger to which Thorndike's cats
and dogs were reduced, while it simplified the conditions
in one sense by making the strength of the motive to escape
as nearly as possible equal for all the animals, complicate
matters in another sense by diminishing their capacity to
learn? Were the animals perhaps frightened and dis-
tracted by the unusual character of their surroundings?
Thorndike thinks not (396); but whether or no he suc-
ceeded in averting these dangers, it is clear that they are
real. It is also obvious that they are the more threatening,
the higher the animal with which one has to deal. Fright,
bewilderment, loneliness, are conditions more apt to be met
with among the higher vertebrates than lower down in the
scale, and the utmost care should be taken to make sure that
animals likely to be affected by them are thoroughly trained
and at home in their surroundings before the experimenter
records results.

§ 4. Methods of Obtaining Facts: The Ideal Method.

The ideal method for the study of a higher animal involves
patient observation upon a specimen known from birth,
watched in its ordinary behavior and environment, and
occasionally experimented upon with proper control of the
conditions and without frightening it or otherwise ren-
dering it abnormal. The observer should acquaint him-
self with the individual peculiarities of each animal studied,
for there is no doubt that striking differences in mental capac-
ity occur among the individuals of a single species. At the
same time that he obtains the confidence of each individual

1 Cf. also Kline (222), and Vaschide and Rousseau (413).

Difficulties and Methods 13

animal, he should be able to hold in check the tendency to
humanize it and to take a personal pleasure in its achieve-
ments if it be unusually endowed. This is, to say the least,
not easy. Absolute indifference to the animals studied, if
not so dangerous as doting affection, is yet to be avoided.

§ 5. Methods of Interpreting Facts

We may now turn from the problem of discovering the facts
about animal behavior to the problem of interpreting them.
If an animal behaves in a certain manner, what may we con-
clude the consciousness accompanying its behavior to be
like? As we have seen, the interpretation is often confused
with the observation, especially in the making of anecdotes ;
but theoretically the two problems are distinct. And at the
outset of our discussion of the former, we are obliged to
acknowledge that all psychic interpretation of animal behavior
must be on the analogy of human experience. We do not
know the meaning of such terms as perception, pleasure,
fear, anger, visual sensation, etc., except as these processes
form a part of the contents of our own minds. Whether we
will or no, we must be anthropomorphic in the notions we
form of what takes place in the mind of an animal. Accept-
ing this fundamental proposition, the students of animals
have yet differed widely in the conclusions they have drawn
from it. Some have gone to the extreme of declaring that
comparative psychology is therefore impossible. Others
have joyfully hastened to make animals as human as they
could. Still others have occupied an intermediate position.

Descartes and Montaigne are the two writers antedating
the modern period who are most frequently quoted in this
connection. The latter had evidently a natural sympathy
with animals. In that most delightful twelfth chapter of the

14 The Animal Mind

second book of Essays, " An Apology of Raymond Sebonde,"
he gives free rein to the inclination to humanize them. I
quote Florio's translation: "The Swallowes which at the
approach of spring time we see to pry, to search and ferret
all the corners of our houses; is it without judgment they
seeke, or without discretion they chuse from out a thousand
places, that which is fittest for them, to build their nests and
lodging? . . . Would they (suppose you) first take water
and then clay, unlesse they guessed that the hardnesse of the
one is softned by the moistness of the other? . . . Why
doth the spider spin her artificiall web thicke in one place and
thin in another? And now useth one, and then another
knot, except she had an imaginary kind of deliberation, fore-
thought, and conclusion?" To ascribe such behavior to the
working of mere instinct, "with a kinde of unknowne,
naturall and servile inclination," is unreasonable. "The
Fox, which the inhabitants of Thrace use " to test the ice on a
river before crossing, which listens to the roaring of the water
underneath and so judges whether the ice is safe or not;
"might not we lawfully judge that the same discourse pos-
sesseth her head as in like case it would ours ? And that it is
a kinde of debating reason and consequence, drawne from
natural sense? 'Whatsoever maketh a noyse moveth,
whatsoever moveth, is not frozen, whatsoever is not frozen,
is liquid ; whatsoever is liquid, yeelds under any weight ? ' '

(277).

Descartes, on the other hand, writing some sixty years
later, takes, as is well known, the opposite ground. He
says in a letter to the Marquis of Newcastle, " As for the under-
standing or thought attributed by Montaigne and others to
brutes, I cannot hold their opinion." While animals surpass
us in certain actions, it is, he holds, only in those "which
are not directed by thought. . . . They act by force of

Difficulties and Methods 15

nature and by springs, like a clock, which tells better what the
hour is than our judgment can inform us. And doubtless
when swallows come in the spring, they act in that like clocks.
All that honey bees do is of the same nature " (99, pp. 281-
283). The statement of Descartes, contained in the letter
to Mersenne of July 30, 1640, that animals are automata, is
often misunderstood. Descartes does not assert that animals
are unconscious in the sense which that term would carry
to-day, but only that they are without thought. Sensations,
feelings, passions, he is willing to ascribe to them, in so far as
these do not involve thought. " It must however be observed
that I speak of thought, not of life, nor of sensation," he says
in the letter to Henry More, 1649; "I do not refuse to them
feeling ... in so far as it depends only on the bodily
organs " (99, p. 287). In this he does not go so far as some
modern writers, who decline to assert the presence of any
psychic process in the lower forms of animal life.

Turning to recent times, we find arguments very like those
of Montaigne used by the earlier evolutionary writers.
Darwin, for instance, says in "The Descent of Man," "As
dogs, cats, horses, and probably all the higher animals, even
birds, have vivid dreams, and this is shown by their move-
ments and the sounds uttered, we must admit that they pos-
sess some power of imagination" (89, p. 74). "Even brute
beasts," says Montaigne, "... are seen to be subject to the
power of imagination; witnesse some Dogs . . . whom we
ordinarily see to startle and barke in their sleep" (277, Bk. I,
ch. 20). " Only a few persons," Darwin continues, "now dis-
pute that animals possess some power of reasoning. Ani-
mals may constantly be seen to pause, deliberate, and resolve."
And he states that his object in the third chapter of the work
quoted is "to show that there is no fundamental difference
between man and the higher mammals in their mental facul-

1 6 The Animal Mind

ties" (89, p. 66). Romanes is evidently guided by the same
desire to humanize animals.

Now these writers were not led to take such an attitude
merely out of general sympathy with the brute creation, like
Montaigne ; they had an ulterior motive ; namely, to meet the
objection raised in their time against the doctrine of evolu-
tion, based on the supposed fact of a great mental and moral
gulf between man and the lower animals. They wished to
show, as Darwin clearly states, that this gulf is not absolute
but may conceivably have been bridged by intermediate stages
of mental and moral development. While this argument
against evolution was being pressed, the evolutionary writers
were very unsafe guides in the field of animal psychology,
for they distinctly "held a brief for animal intelligence,"
to use Thorndike's phrase. In more recent times interest in
both the positive and the negative sides of the objection drawn
from man's superiority has died out, and such special plead-
ing has become unnecessary.

•\ On the other hand, the fact that the greater part of the
experiments on animals were until the last ten or fifteen years
performed by physiologists has given rise to an opposite ten-
dency in interpreting the animal mind: the tendency to
make purely biological concepts suffice as far as possible
for the explanation of animal behavior and to assume the
presence even of consciousness in animals only when it is
absolutely necessary to do so. °Loeb in 1890 suggested the
theory which he has since elaborated, that the responses
of animals to stimulation, instead of being signs of " sensa-
tion," are in every way analogous to the reactions of plants
to such forces as light and gravity; hence unconscious "trop-
isms" (235). " Bethe in 1898 attempted to explain all the com-
plicated behavior of ants and bees, which the humanizing
writers had compared with our own civilization, as a result of

Difficulties and Methods 17

reflex responses, chiefly to chemical stimulation, unaccompa-
nied by any consciousness whatever (30). This revival, in an
altered form, of the Cartesian doctrine has met with energetic
opposition, especially from writers having philosophical
interests. At the present time the parties in the controversy
may be divided into three groups: those who believe that
consciousness should be ascribed to all animals; those who
hold that it should be ascribed only to those animals whose
behavior presents certain peculiarities regarded as evidence of
mind; and those who hold that we have no trustworthy
evidence of mind in any animal, and should therefore abandon
comparative psychology and use only physiological terms.

To the first group belong, among others, the French writer
Claparede, the Swiss naturalist Forel, and the Jesuit Was-
mann. The physiologist W. A. Nagel also takes a friendly
attitude toward the animal mind. In the second group may
be classed Loeb and H. Jordan. In the third belong the
physiologists Beer, Bethe, H. E. Ziegler, von Uexkiill, and
J. P. Nuel.

Claparede, Forel, and Wasmann maintain the existence of
consciousness in animals from widely different philosophical
points of view. The first-named is what is called a parallel-
ist; that is, he believes that mental processes and bodily
processes are not causally related, but form two parallel
and non-interfering series of events. In the study of animals,
both the physical and the psychical series should, he thinks,
be investigated. Biology should use two parallel methods:
the one ascending, attempting to explain animal behavior by
physical and chemical laws; the other descending, giving
an account of the mental processes of animals. Ultimately,
it may be hoped, according to Claparede, that both methods
will be applied throughout the whole range of animal life.
At present the ascending method is most successful with the

1 8 The Animal Mind

lowest forms, the descending method with the highest forms.
We cannot afford to abandon the psychological study of
animals, for our knowledge of the nervous processes under-
lying the higher mental activities is very slight; physiology
here fails us, and psychology must be left in command of
the field. The danger besetting the attempt at a purely
physical explanation of animal behavior is that the facts shall
be unduly simplified to fit the theory. Thus Bethe's effort
at explaining the way in which bees find their way back to
the hive as a reflex response, or tropism, produced by "an
unknown force," is highly questionable; the facts seem to
point toward the exercise of some sort of memory by the bees.
It is always possible, further, that the tropism is accompanied
by consciousness. A physiologist from Saturn might reduce
all human activities to tropisms, says Claparede in a striking
passage. "The youth who feels himself drawn to medical
studies, or he who is attracted to botany, can no more account
for his profoundest aspirations than the beetle which runs
to the odor of a dead animal or the butterfly invited by the
flowers ; and if the first shows a certain feeling corresponding
to these secret states of the organifm (a feeling of ' predilec-
tion' for such a career, etc.), how can we dare to deny to the
second analogous states of consciousness?" (75). If it is
argued that we have no direct, but only an inferential, knowl-
edge of the processes in an animal's mind, the argument is
equally valid against human psychology, for 'the psychologist
has only an inferential knowledge of his neighbor's mind (77).
Wasmann defends the animal mind from a different point of
view. For one thing, he believes that mental processes may
act causally upon bodily states. He accepts, in other words,
what is called iriteractionism, as opposed to parallelism.
Further, although he strongly opposes the doctrineTnatthe
reactions of animals are unconscious tropisms, and constantly

Difficulties and Methods 19

emphasizes their variability and modifiability through experi-
ence, he nevertheless believes that a gulf separates the human
from the animal mind. The term ''intelligence" which
most writers use to designate merely the power of learning
by individual experience, Wasmann would reserve for the
power of deducing and understanding relations, and would
assign only to human beings (425, 426). Although animals
have their instincts modified by sense experience, man
"stands through his reason and freedom immeasurably high
above the irrational animal that follows, and must follow,
its sensuous impulse without deliberation " (425).

Forel, in the third place, is what is .called a monist in meta-
physics. That is, he does not believe either that mind and
body are parallel, or that they interact causally, but that they
are two aspects of the same reality. "Every psychic phe-
nomenon is the same real thing as the molecular or neurocymic
activity of the brain-cortex coinciding with it " (132, p. 7).
The psychic and the physical, on this theory, should be
coextensive; not merely should consciousness in some form
belong to all living things, but every atom of matter should
have its psychic aspect, •n such a basis, Forel takes highly
optimistic views of the animal mind. In insects, of which he
has made a special study, it is, he thinks, "possible to demon-
strate the existence of memory, associations of sensory images,
perceptions, attention, habits, simple powers of inference
from analogy, the utilization of individual experience, and
hence distinct, though feeble, plastic individual deliberations
or adaptations " (132, p. 36).

The second of the three groups into which we divided
present-day writers on the interpretation of animal behavior
contains those who maintain not that all animals are conscious,
but that those whose behavior meets a certain standard may
be so considered. The nature of this test is a difficult prob-

2O The Animal Mind

lem. We shall therefore devote the next chapter to its con-
sideration; and as it necessarily plays an important part in
determining views regarding the animal mind, we shall '
postpone for the present the discussion of the second group.
The third group contains those biologists, conservative or
radical according to one's own position, who deny to cojipar-
ative_^s^chologyjhejightjo exist. The eminent neurologist
Bethe is a typical representative of the class. In his study of
the behavior of ants and bees he refuses to allow these animals
any "psychic qualities" whatever, and suggests the term
"chemo-reception" instead of "smell," to designate the in-
fluence which directs most of their reactions, — " smell" im-
plying a psychic quality (30). From his argument for the
probable absence of consciousness in ants and bees, as well
as in the crab (28), one might be inclined to put Bethe in the
second of the above-mentioned classes, for it is the lack of one
definite characteristic in the behavior of these animals, namely,
modification by individual experience, that makes him think
them unconscious. It becomes clear from other passages in
his writings, however, that he considers the presence of con-
sciousness even in animals that fein learn by experience, a
highly problematical and improper assumptidr!. In a foot-
note to a later article he says: " Psychic qualities cannot be
demonstrated. Even what we call sensation is known to
each man only in himself, since it is something subjective.
We possess the capacity of modifying our behavior [i.e. of
learning], and every one knows from his own experience that
psychic qualities play a part connected with this modifying
process. Every statement that another being possesses
psychic qualities is a conclusion from analogy, not a cer-
tainty; it is a matter of faith. If one wishes to draw this
analogical inference, it should be made where the capacity
for modification can be shown. When this is lacking, there

Difficulties and Methods 21

is not the slightest scientific justification for assuming psychic
qualities. They may exist, but there is no probability of it,
and hence science should deny them. Hence if one ventures
to speak of a Psyche in animals at all, one should give the
preference to those which can modify their behavior " (29).
But that Bethe himself prefers not to make the venture is
evident from statements in .the text of the same article. The
psychic or subjective, he says, is unknowable, and the only
thing we may hope to know anything about is the chemical
and physiological processes involved. "These chemo-physi-
cal processes and their consequences, that is, the objective
aspect of psychic phenomena, and these alone, should be the
object of scientific investigation " (29).

Together with Beer and von Uexkiill, Bethe shortly after-
ward published " Proposals for an Objectifying Nomen-
clature in the Physiology of the Nervous System." The
main purpose of this paper was to suggest that all terms
having a psychological implication, such as sight, smell,
sense-organ, memory, learning, and the like, be carefully
excluded from discussions of animal reactions to stimulation
and animal behavior generally. In their stead the authors
propose such expressions as the following: for responses to
stimulation where no nervous system exists, the term anti-
types; for those involving a nervous system, antikineses ;
the latter are divided into reflexes, where the response is uni-
form, and antiklises, where the response is modifiable. A
sense-organ becomes a reception-organ, sensory nerves are
receptory-nerves, and we have phono-reception, stibo-reception,
photo-reception, instead of hearing, smell, and sight. The
after-effect of a stimulus upon later ones is the resonance
of the stimulus .(20).

This attempt at an objective terminology meets the cor-
dial approval of H. E. Ziegler and J. P. Nuel. The former

22 The Animal Mind

declares that the concept of consciousness is worthless in the
study of animals, as no one knows whether an animal is
conscious or not. He suggests as additions to the new vocab-
ulary the term pleronomic to designate inherited reactions, and
enbiontic to signify acquired reactions (476). Nuel also
thinks that our ignorance of the mental states of animals
renders comparative psychology unscientific. He prefers the
"kinetic" to the psychic point of view; a sense-organ, in
man or beast, is an apparatus for reactions (297). In a book
on vision Nuel has suggested an objective terminology of his
own, where "ikonoreaction," for example, takes the place of
sight (2 96).*

It would seem that no serious objection could be raised
against the use of a purely objective nomenclature in physiol-
ogy, and that confusion might thereby be avoided, without
prejudicing the case of comparative psychology, which might
exist side by side with the other science, and reserve the terms
with psychic implications for itself. Wasmann, it is true,
objects to the new terminology on its own account, as cum-
brous and scholastic, and says that if Ziegler cannot use such
wprds as sensation, perception, seeing, and the like, without

* anthropomorphism, the fault is his own, and the fact should
not lead him to impose a new set of words on others (428).
These criticisms, however, are those of the conservative who
objects to anything new; all technical vocabularies are pe-
dantic, but it is impossible to take too many precautions
against confusion of ideas.

^^What attitude, now, shall we assume upon the broader
question raised by these writers, as to whether comparative
psychology is possible at all ? Must we accept the statement

1 Another instance of an attempt to use terminology without psychic im-
plications is to be found in R. Semon's "Die Mncme als erhaltendes
Princip im Wechsel des organischen Geschehens " (379).

Difficulties and Methods 23

that no knowledge whatever of the animal mind is obtain-
able? If so,^we must also admit that human psychology is
impossible. Our acquaintance with the 'mind of animals
rests upon the same basis as our acquaintance with the mind
of our fellow-man; both are derived by inference from
observed behavior. The actions of our fellow-men resemble
our own, and we therefore infer in them like subjective states
to ours : the actions of animals resemble ours less completely,
"but the difference is one of degree, not of kind. This argu-
ment in behalf of comparative psychology, which is brought
forward by Claparede (77), is opposed by Nuel with the
denial that human and animal psychology rest upon the same
basis (297); but no cogent proofs accompany Nuel's state-
ment. The physiologists would doubtless accept the other
horn of the dilemma, and reject human psychology along with
animal psychology ; but a scientific rigor which requires of us
to abandon the assertion of mind in our fellow-beings and
the study of that mind has pushed itself to absurdity. As
Jordan says, inferences upon a basis of probability form a
legitimate part of science (216). The mental processes in
other minds, animal or human, cannot indeed be objectively
ascertained facts ; the facts are those of human and animal
behavior; but the mental processes are as justifiable infer-
ences as any others with which science deals. The prime
necessity is merely that they shall be properly guarded. Cer-X
tain precautions are necessary when we infer the state of our j
neighbor's mind; certain added precautions are necessary /
when we infer states in the mind of an animal, and our asy
sertions should certainly diminish in dogmatism as we go
down the scale of animal life. But the psychologist, to whom,
as Titchener has put it, uthe facts and laws of mind are the
most real things that the world can show " (400), will never
consent to abandon the effort to probe the mysteries of other

24 The Animal Mind

minds, human or animal, until science consents to abandon
all hypotheses and inferences based on anything short of
perfect identity between instances.

Is it possible to state briefly the special precautions that
must be observed in interpreting animal behavior as accom-
panied by consciousness, granted that such interpretation is
admissible ? Jordan, while holding that the existence of the
animal mind may fairly be inferred under certain circum-
stances, holds that we are not justified in inferring the actual
quality of mental processes in animals. For this reason he
objects to the term " comparative psychology" (216). There
is no doubt that great caution should be used in regarding the
quality of a human conscious process as identical with the
quality of the corresponding process in the animal mind. For
example, we might say with a fair degree of assurance that
an animal consciously discriminates between light and
darkness ; that is, receives conscious impressions of different
quality from the two, yet the mental impression produced by
white light upon the animal may be very different frornjfee
sensation of white as we know it, and the impression procmced
by the absence of light very different from our sensation of
black. Black and white may, for all we know, depend for
their quality upon some substance existing only in the human
retina.

A second precaution concerns the simplicity or complexity
of the interpretation put upon animal behavior. Lloyd
Morgan, in his " Introduction to Comparative Psychology,"
formulated a conservative principle of interpretation which
has often been quoted as " Lloyd Morgan's Canon." The
principle is as follows : " 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 outcome of the exercise of one
which stands lower hi the psychological scale" (280, p. 53).

Difficulties and Methods 25

In other words, when in doubt take the simpler interpretation.
For example, a dog detected in a theft cowers and whines. One
possible mental accompaniment of this behavior is remorse;
the dog is conscious that he has fallen below a moral standard,
and grieved or offended his master. . A second is the antici-
pation of punishment ; the dog has a mental representation of
the consequences of his action upon former occasions, and
imagining himself likely to experience them anew, is terrified
at the prospect. $ A third possibility is that the dog's previous
experience of punishment, instead of being revived in the form
of definite images, makes itself effective merely in his feelings
and behavior; he is uncomfortable and frightened, he knows
not definitely why. It is evident that these three possibili-
ties represent three different grades of complexity of mental
process, the first being by far the highest. Lloyd Morgan's
canon enjoins upon us irj such a case to prefer the third
alternative, provided that it will really account for the dog's
behavior.

Now why should the simplest interpretation be pre-
ferred? We must not forget that the more complex ones
remain in the field of possibility. Positive assertions have no
place in comparative psychology. We cannot say that the
simplicity of an hypothesis is sufficient warrant of its truth,
for nature does not always proceed by the paths which seem
to us least complicated. The fact is that Lloyd Morgan's
principle serves to counterbalance our most important source
of error in interpreting animal behavior. It is like tipping
a boat in one direction to compensate .for the fact that some
one is pulling the opposite gunwale. /We must interpret the
animal mind humanly if we are to interpret it at all. Yet we
know that it differs from the human mind, and that the dif-
ference is partly a matter of complexity. Let us therefore
take the least complex interpretation that the facts of animal

26 The Animal Mind

behavior will admit, always remembering that we may be
wrong in so doing, but resting assured that we are, upon the
whole, on the safer side. The social consciousness of man is
very strong, and his tendency to think of other creatures,
even of inanimate nature, as sharing his own thoughts and
feelings, has shown itself in his past to be almost irresistible.
Lloyd Morgan's canon offers the best safeguard against this
natural inclination, short of abandoning all attempt to study
the mental life of the lower animals.

CHAPTER II
THE EVIDENCE OF MIND

§ 6. Inferring Mind from Behavior

IN the last chapter we saw that some recent writers uj
animal behavior and its interpretation, while refusing to ad-
mit the presence of consciousness in all forms of animal life,
yet hold that it can be proved to exist in certain forms. The
latter, it is maintained, display certain peculiarities of be-
havior that may be regarded as proofs of a psychic accompani-
ment. Into the nature of these proofs we may now inquire.

To begin with, can it be said that when an animal makes a
movement in response to a certain stimulus, there is an accom-
panying consciousness of the stimulus, and that when it fails ^
to move, there is no consciousness? Is response to stimu-
lation evidence of consciousness ? In the case of man, we /
know that absence of visible response does not prove that the
stimulus has not been sensed ; while it is probable that some
effect upon motor channels always occurs when consciousness
.accompanies stimulation, the effect may not be apparent to
an outside observer. On the other hand, if movement in
response to the impact of a physical force is evidence of con-
sciousness, then the ball which falls under the influence of
gravity and rebounds on striking the floor is conscious. Nor
is the case improved if we point out that the movements which
animals make in response to stimulation are not the equiva-
lent in energy of the stimulus applied, but involve the setting
free of energy stored in the animal as well. True, when a
microscopic animal meets an obstacle in its swimming, and

27

28 The Animal Mind

darts backward, the movement is not a mere rebound; it
implies energy contributed by the animal's own body. But
just so an explosion of gunpowder is not the equivalent in
energy of the heat of the match, the stimulus. Similarly it
is possible to think of the response made by animals to external
stimuli as involving nothing more than certain physical and
chemical processes identical with those existing in inanimate
nature.

If we find that the movements made by an animal as a re-
sult of external stimulation regularly involve withdrawal from
certain stimuli and acceptance of others, it is natural to use
the term " choice " in describing such behavior. But|if con-
sciousness is supposed to accompany the exercise of choice in
*J this sense, then consciousness must be assumed to accompany
/, '•''the behavior of atoms in chemical combinations.) When
hydrochloric acid is added to a solution of silver nitrate, the
atoms of chlorine and those of silver find each other by an
unerring " instinct" and combine into the white precipitate of
silver chloride, while the hydrogen and the nitric acid simi-
larly "choose" each other. Nor can the fact that behavior
in animals is adapted to an end be used as evidence of mind ;
for " purposive" reactions, which contribute to the welfare
of an organism, are themselves selective. The search for
food, the care for the young, and the complex activities which
further welfare, are made up of reactions involving "choice"
between stimuli; and if the simple "choice" reaction is on a
par with the behavior of chemical atoms, so far as proof of
consciousness goes, then adaptation to an end, apparent pur-
posiveness, is in a similar position.

Thus the mere fact that an animal reacts to stimulation,
even selectively and for its own best interests, offers no evi-
dence for the existence of mind that does not apply equally
well to particles of inanimate matter. Moreover, there is

The Evidence of Mind 29

some ground for holding that the reactions of the lowest
animals are unconscious. This ground consists in the ap-
parent lack of variability which characterizes such reactions.
In our own case, we know that certain bodily movements,
those of digestion and circulation, for example, are normally
carried on without accompanying consciousness, and that in
other cases where there is consciousness of the stimulus, as
in the reflex knee-jerk, it occurs after the movement is initiated,
so that the nervous process underlying the sensation would
seem to be immaterial to the performance of the movement.
These unconscious reactions in human beings are character-
ized by their relative uniformity, by the absence of variation
in their performance. Moreover, when an action originally
accompanied by consciousness is often repeated, it tends, by
what is apparently one and the same process, to become un-
conscious and to .become uniform. There is consequently
reason for believing that when the behavior of lower animals
displays perfect uniformity, consciousness is not present. On
the other hand, an important reservation must be made in the
use of this negative test. It is by no means easy to be sure
that an animal's reactions are uniform. The more carefully
the complexer ones are studied, the more are variability and
difference brought to light where superficial observation had
revealed a mechanical and automatic regularity. It is quite ^
possible that even in the simple, apparently fixed response of
microscopic animals to stimulation, better facilities for ob-
servation might show variations that do not now appear.

This matter of uniformity versus variability suggests a
further step in our search for a satisfactory test of the presence
of mind. Is mere variability in behavior, mere irregularity
in response, to be taken as such a test ? Not if we argue from
our own experience. While that portion of our own be-
havior which involves consciousness shows more irregularity

30 The Animal Mind

than the portion which does not, yet the causes of the irregu-
larity are often clearly to be found in physiological conditions
with which consciousness has nothing to do. There are days
when we can think clearly and recall easily, and days when
obscurities refuse to vanish and the right word refuses to come ;
days when we are irritable and days when we are sluggish.
Yet since we can find nothing in our mental processes to ac-
count for this variability, it would be absurd to take analogous
fluctuations in animal behavior as evidence of mind. So
complicated a machine as an animal organism, even if it be
nothing more than a machine, must show irregularities in its
working.

Behavior, then, must be variable, but not merely variable,
to give evidence of mind. The criterion most frequently ap-
plied to determine the presence or absence of the psychic is
a 'variation in behavior that shows definitely the result of
previous individual experience. "Does the organism," says
Romanes, "learn to make new adjustments, or to m6dify old
ones, in accordance with the results of its own individual ex-
perience ? " (364, p. 4). Loeb declares that "the fundamental
process which occurs in all psychic phenomena as the ele-
mental component " is "the activity of the associative memory,
or of association," and defines associative memory as "that
mechanism by which a stimulus brings about not only the
effects which its nature and the specific structure of the irritable
organ call for, but by which it brings about also the effects of
other stimuli which formerly acted upon the organism almost
or quite simultaneously with the stimulus in question."
"If an animal can be trained," he continues, "if it can learn,
it possesses associative memory," and therefore mind (243,
p. 1 2). The psychologist finds the term " associative memory "
hardly satisfactory, and objects to the confusion between
mental and physical concepts which renders it possible to

The Evidence of Mind 31

speak of a " mechanism" as forming an "elemental com-
ponent" in " psychic phenomena," but these points may be
passed over. The power to learn by individual experience
is the evidence which Romanes, Morgan, and Loeb will
accept as demonstrating the presence of mind in an animal.

Does the absence of proof that an animal learns by expe-
rience show that the animal is unconscious? Romanes is
careful to answer this question in the negative. "Because a
lowly organized animal," he says, "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 evi-
dence of the fact " (364, p. 3). Loeb, on the other hand, writes
as if absence of proof for consciousness amounted to disproof,
evidently relying on the principle of parsimony, that no
unnecessary assumptions should be admitted. "Our crite-
rion," he remarks, "puts an end to the metaphysical ideas
that all matter, and hence the whole animal world, possesses
consciousness" (243, p. 13). If learning by experience be
really a satisfactory proof of mind, then its absence in certain
animals would indeed prevent the positive assertion that all
animals are conscious ; but it could not abolish the possibility
that they might be. Such a possibility might, however, be
of no more scientific interest than any one of a million wild
possibilities that science cannot spare time to disprove. But
we shall find that learning by experience, taken by itself, is
too indefinite a concept to be of much service, and that when
defined, it is inadequate to bear the whole weight of proving
consciousness in animals. Such being the case, the possi-
bility that animals which have not been shown to learn may
yet be conscious acquires the right to be reckoned with.

The first point that strikes us in examining the proposed

32 The Animal Mind

test is that the learning by experience must not be too slow,
or we can find parallels for it in the inanimate world. An
animal may be said to have learned by experience if it be-
haves differently to a stimulus because of preceding stimuli.
But it is one thing to have behavior altered by a single pre-
ceding stimulus, and another to have it altered by two hun-
dred repetitions of a stimulus. The wood of a violin reacts
differently to the vibrations of the strings after it has "expe-
rienced" them for ten years; the molecules of the wood have
gradually taken on an altered arrangement. A steel rail re-
acts differently to the pounding of wheels after that process
has been long continued ; it may snap under the strain. Shall
we say that the violin and the rail have learned by individual
experience ? If the obvious retort be made that it is only in
living creatures that learning by experience should be taken
as evidence of mind, let us take an example from living crea-
tures. When a blacksmith has been practising his trade for
a year, the reactions of his muscles are different from what
they were at the outset. But this difference is not merely a
matter of more accurate sense-discrimination, a better " plac-
ing" of attention and the like ; there have been going on within
the structure of his muscles changes which have increased their
efficiency, and with which consciousness has had nothing to
do. These changes have been extremely slow compared to
the learning which does involve consciousness. In one or
two lessons the apprentice learned what he was to do; but
only very gradually have his muscles acquired the strength to
do it as it should be done. Now among the lower animal
forms we sometimes meet with learning by experience that is
very slow ; that requires a hundred or more repetitions of the
stimulus before the new reaction is acquired. In such a case
we can find analogical reasons for suspecting that a gradual
change in the tissues of the body has taken place, of the sort

The Evidence of Mind 33

which, like the attuning of the violin wood or the slow de-
velopment of a muscle, have no conscious accompaniment.
We must then ask the question : What kind of learning
by experience never, so far as we know, occurs unconsciously ?
Suppose a human being shut up in a room from which he can
escape only by working a combination lock. As we shall see
later, this is one of the methods by which the learning power
of animals has been tested. The man, after prolonged
investigation, hits upon the right combination and gets out.
Suppose that he later finds himself again in the same pre-
dicament, and that without hesitation or fumbling he opens
the lock at once, and performs the feat again and again, to
show that it was not a lucky accident. But one interpretation
of such behavior is possible. We know from our own ex-
perience that the man could not have worked the lock the
second time he saw it, unless he consciously remembered the
movements he made the first time ; that is, unless he had in
mind some kind of idea as a guide. Here, at least, there can
have been no change in the structure of the muscles, for such
changes are gradual; the change must have taken place in
the most easily alterable portion of the organism, the nervous
system; and further, it must have taken place in the most
unstable and variable part of the nervous system, the higher
cortical centres whose activity is accompanied by conscious-
ness. In other words, we may be practically assured that
consciousness accompanies learning only when the learning
is so rapid as to show that the effects of previous experience
are recalled in the guise of an idea or mental image of some
sort. But does even the most rapid learning possible assure
us of the presence of an idea in the mind of a lower animal ?
Where the motive, the beneficial or harmful consequence of
action, is very strong, may not a single experience suffice to
modify action without being revived in idea? Moreover,

34 The Animal Mind

animals as high in the scale as dogs and cats learn to solve
problems analogous to that of the combination lock so slowly
that we cannot infer the presence of ideas. Are we then to
conclude that these animals are unconscious, or that there is
absolutely no reason for supposing them possessed of con-
sciousness? Yerkes has criticised the "learning by expe-
rience" criterion by pointing out that " no organism . . . has
thus far been proved incapable of profiting by experience."
It is a question rather of the rapidity and of the kind of learn-
ing involved. "The fact that the crayfish needs a hundred
or more experiences for the learning of a type of reaction that
the frog would learn with twenty experiences, the dog with
five, say, and the human subject with perhaps a single ex-
perience, is indicative of the fundamental difficulty in the use
of this sign" (463). Nagel has pointed out that Loeb, in
asserting "associative memory" as the criterion of conscious-
ness, offers no evidence for his statement (294). The fact is
that while proof of the existence of mind can be derived from
animal learning by experience only if the learning is very
rapid, other evidence, equally valid on the principle of anal-
ogy, makes it highly improbable that all animals which learn
too slowly to evince the presence of ideas are therefore uncon-
scious. This evidence is of a morphological character.

§ 7. Inferring Mind from Structure

Both Yerkes and Lukas urge that the resemblance of an
animal's nervous system and sense-organs to those of human
beings ought to be taken into consideration in deciding whether
the animal is conscious or not. Lukas suggests that the cri-
teria of consciousness should be grouped under three heads :
morphological, including the structure of the brain and sense-
organs, physiological, and teleological. Under the second

The Evidence of Mind 35

rubric he maintains that "individual purposiveness " is char-
acteristic of the movements from which consciousness may
be inferred; that individual purposiveness pertains only to
voluntary acts, and that voluntary acts are acts "which are
preceded by the intention to perform a definite movement,
hence by the idea of this movement." We have reached the
same conclusion in the preceding paragraph. The third test
of the presence of consciousness, the teleological test, rests
on the consideration: "What significance for the organism
may be possessed by the production of a conscious effect by
certain stimuli ? " (252). This test, however, being of a purely
a priori character, would seem to be distinctly less valuable
than the others.

Yerkes proposes "the following six criteria in what seems
to me in general the order of increasing importance. The
functional signs are of greater value as a rule than the struc-
tural ; and within each of the categories the particular sign is
usually of more value than the general. In certain cases,
however, it might be maintained that neural specialization is
of greater importance than modifiability.
I. Structural Criteria.

1. General form of organism (Organization).

2. Nervous system (Neural organization).

3. Specialization in the nervous system (Neural spe-

cialization).
II. Functional Criteria.

1. General form of reaction (Discrimination).

2. Modifiability of reaction (Docility).

3. Variability of reaction (Initiative)" (463).

The terms "discrimination," "docility," and "initiative"
in this connection are borrowed from Royce's "Outlines of
Psychology " (372).

If resemblance of nervous and sense-organ structure to the

36 The Animal Mind

human type is to be taken along with rapid learning as co-
ordinate evidence of consciousness, it is clear that here also
we have to deal with a matter of degree. The structure of
the lower animals differs increasingly from our own as we go
down the scale. At what degree of difference shall we draw
the line and say that the animals above it may be conscious,
but that those below it cannot be? No one could possibly
establish such a line. The truth of the whole matter seems to
be this : We can say neither what amount of resemblance in
structure to human beings, nor what speed of learning, consti-
tutes a definite mark distinguishing animals with minds from
those without minds, unless we are prepared to assert that only
animals which learn so fast that they must have memory ideas
possess mind at all. And this would conflict with the argu-
ment from structure. For example, there is no good experi-
mental evidence that cats possess ideas, yet there is enough
analogy between their nervous systems and our own to make
it improbable that consciousness, so complex and highly
developed in us, is in them wholly lacking. We know not
where consciousness begins in the animal world. We know
where it surely resides — in ourselves; we know where it
exists beyond a reasonable doubt — in those animals of
structure resembling ours which rapidly adapt themselves to
the lessons of experience. Beyond this point, for all we
know, it may exist in simpler and simpler forms until we
reach the very lowest of living beings.

CHAPTER III
THE MIND OF THE SIMPLEST ANIMALS

§ 8. The Structure and Behavior of Amoeba

WE have seen in the last chapter that no one can prove
the absence of consciousness in even the simplest forms of
living beings. It is therefore perfectly allowable to speculate
as to what may be the nature of such consciousness, provided
that the primitive organisms concerned 'possess it. Per-
fectly allowable, yet also perfectly useless, many authorities
would argue ; the remoteness of the creatures from ourselves
in structure and behavior renders theorizing .about their
conscious experience, which is probably non-existent and
certainly unimaginable in any definite terms by us, the idlest
form of mental exercise.

Undeniably the formation of a positive notion regarding
the character arid content of psychic states in the mind, say
of an Amoeba, is next door to an impossibility. Yet it may
not be wholly a waste of time if we spend a few pages in the
attempt to discover wherein the simplest type, of mind, sup-
posing it to be that belonging to the simplest type of animal,
necessarily differs from our own. Some light, perhaps, may
be cast upon the growth of mental life -in complexity if we
try to make clear to ourselves what primitive consciousness is
not, though we may not be able to find in our own experience
any elements that shall properly represent what it is.

The first need is evidently information about the structure
and the behavior of a primitive animal. For this purpose the

37

38 The Animal Mind

Amoeba presents itself as a good subject. Structurally, it con-
sists of a single cell, as dp all the Protozoa, the lowest group
of animals ; it is so small that it can be studied only through
the microscope; its form, at least that of Amceba proteus, the
most typical species, is irregular and constantly changing in
locomotion or in response to stimulation. While the internal
substance of its body shows a certain amount of differentiation,
there is no trace whatever of special modifications that might
be supposed to serve for the conduction of stimuli to different
parts of the body, and thus represent the prototype of a ner-
vous system. Nor have any structures been found that could
conceivably be used for the special reception of stimuli ; that
is, there are no sense organs. So far as the anatomy of the
animal is concerned, then, it differs so widely from our own
that we could only conclude from it the absence of all those
features which our conscious experience involves.

Turning from structure to behavior, we find the external
activities of Amceba, that is, those not confined to the inner
processes of its cell body, to be superficially, at least, divisible
into two classes : movements of locomotion and responses to
stimulation. Amceba, though a water-dwelling animal, is
not a free-swimming one, but moves by crawling on a solid
body. This method of locomotion involves in A mceba proteus
changes of form on the animal's part, projections, called pseu-
dopodia, being sent out in advance of the movement of the
whole body. The protoplasm of the body shows in this pro-
cess certain flowing movements which are differently described
by different observers, and doubtless vary in different species :
thus Rhumbler finds that the protoplasmic currents move
backward along the sides of the animal and forward through
the middle in a way quite comparable to the behavior of cur-
rents in a drop of any fluid where the tension of the surface
is diminished in front, i.e., at the point toward which the drop,

The Mind of the Simplest Animals 39

in consequence of the diminished tension there, rolls. Such
movements, Rhumbler shows, can be reproduced by placing,
say, a drop of clove oil under the proper conditions of surface
tension (361, 362). Jennings, on the other hand, has ob-
served, at least in certain species of Amoeba, that the proto-
plasmic currents are all forward in direction, the movement
being really one of rolling, complicated by the attachment of
the lower part of the body to the solid object on which the
animal crawls. Mechanical conditions of surface tension
would not account for such currents (204, 206, 211). Del-
linger, finally, rejects both the surface tension and the "rolling"
theories, and from a study of side views of the moving Amoeba
concludes that progression occurs through the advancement
of the front end freely through the water and its subsequent
attachment, the rest of the body following through active con-
traction brought about by a contractile substance (98). The
problem is of great interest to the student of vital phenomena,
but its bearing on the question of mind in the Amceba is so
obscure that we need not consider it further, but may pass at
once to the study of the animal's reactions to special stimula-
tion.

These are, according to Jennings (206, 211), the foremost
authority on the behavior of the lowest organisms, three in
number; namely, the negative, the positive, and the food-
taking reactions. First, if an Amceba comes into strong con-
tact with a solid obstacle in its movements, or if a solution of
different composition from the water in which it lives strikes
against it, or if one side of it is heated, the animal responds
by contracting the part stimulated, releasing it from the sub-
stratum, and moving in another direction, usually one form-
ing only a small angle with the preceding one. If the whole
of one side or end receives a strong stimulus, if light falls on
one side, or an electric current is passed through the water, the

The Animal Mind

side stimulated — in the case of the electric current, the side
toward the positive pole — contracts as a whole, and the
movement takes place in the opposite direction. These phe-
nomena constitute the negative reaction (Fig. i).

Secondly, the reaction to solid bodies sometimes takes a
positive form. In this case a pseudopodium is pushed for-
ward in the direction of the stimulus, and the animal moves

toward the solid. As the nega-
tive reaction serves the purpose
of avoiding obstacles, so the
positive reaction is useful in
securing contact with a support
on which to creep, and with
food. It seems to be given in
response to weak mechanical
stimuli, stronger ones producing
the negative reaction. No
chemicals have been found to
occasion it, but weak chemical
stimulation very likely cooper-
ates with mechanical stimula-
tion when the positive reaction
is given to food.
Thirdly, there is the food-taking reaction. This consists,
for Amceba proteus, in the pushing forward of a pseudopodium
on either side of the particle of food that has come into con-
tact with the animal ; the bending over of the ends of the pseu-
dopodia so as to grasp the food, while "a thin sheet of pro-
toplasm" spreads from the upper surface of the animal over
it ; and the final fusion of the ends of the pseudopodia and the
ends of this sheet, so as to take the food directly into the ani-
mal's body. The reaction may occur anywhere on the body
surface, there being no specialized mouth. It appears to be

FIG. i. — Negative reaction of
Amoeba to stimulation by a glass
rod. a. Application of the stim-
ulus, b. Change of direction
of movement. After Jennings

(211).

The Mind of the Simplest Animals 41

made only in response to edible substances, hence there is
doubtless some chemical peculiarity about the stimulus which
makes it effective (Fig. 2).

These three reactions make up, together with the ordinary
crawling locomotion, the variety of the Amoeba's experience
as displayed in behavior, with the addition of a peculiar set
of movements occurring in the absence of all mechanical stimu-
lation. When an Amoeba is floating in the water, through some
chance, unattached to any solid, "such a condition," says

FIG. 2. — Food-taking reaction of Amoeba, i, 2, 3, 4, successive stages.
After Jennings (211).

Jennings, "is most unfavorable for its normal activities ; it can-
not move from place to place, and has no opportunity to obtain
food." Its mode of getting out of the difficulty is to send out
"long, slender pseudopodia in all directions," until "the body
may become reduced to little more than a meeting point for
these pseudopodia" (211, p. 8). As soon as one of these
"feelers" comes in contact with a solid, it attaches itself, and
the whole animal following soon takes up its normal crawling
locomotion.

§ 9. The Mind of Am&ba

Now what light does the behavior of Amoeba, thus described
in its various forms by Jennings, throw upon the nature of the

42 The Animal Mind

animal's possible consciousness? The first thought which
strikes us in this connection is that the number of different
sensations occurring in an Amoeba's mind, if it has one, is
very much smaller than the number forming the constituent ele-
ments of our own experience. We human beings have the
power to discriminate several thousand different qualities
of color, brightness, tone, noise, temperature, pressure, pain,
smell, taste, and other sensation classes. Thus the content
of our consciousness is capable of a great deal of variety. It
is hard to see how more than three or four qualitatively dif-
ferent processes can enter into the conscious experience of an
Amoeba. The negative reaction is given to all forms of strong
stimulation alike, with the single exception of food. We shall
in the following chapter discuss more fully the nature of the
evidence that helps us to conjecture the existence of different
sensation qualities in an animal's mind ; but it is clear that
where an animal so simple in its structure as the Amoeba
makes no difference in its reactions to various stimuli, there
can be no reason for supposing that if it is conscious, it is
aware of them as different. The reaction to edible sub-
stances is, however, unlike that to other stimulations. The
peculiarity of edible substances which occasions this differ-
ence must be a chemical one. In our own case, the classes
of sensation which result from the chemical peculiarities of
food substances are smell and taste ; evidently to a water-
dwelling animal smell and taste would be practically indis-
tinguishable. We may say, then, that supposing conscious-
ness to exist in so primitive an animal as the Amceba, we
have evidence for the appearance in it of a specific sensation
quality representing the chemical or food sense, and standing
for the whole class of sensations resulting from our own
organs of smell and taste. The significance of the positive
reaction is harder to determine. It seems to be given in re-

The Mind of the Simplest Animals 43

sponse not to a special kind of stimulus, but to a mechanical
or food stimulus of slight intensity. In our own experience,
we do not have stimuli of different intensity producing sen-
sations of different quality, except in the cases of temperature
and visual sensations. We do, however, find that varying the
strength of the stimulus will produce different affective quali-
ties ; it is a familiar fact that moderate intensities of stimula-
tion in the human organism are accompanied by pleasant-
ness, and stronger intensifies by unpleasantness. The motor
effects of pleasantness and unpleasantness in ourselves are
opposite to each other in character. Pleasantness produces
a tonic and expansive effect on the body, unpleasantness a
depressive and contractive effect. In the Amoeba, the posi-
tive and negative reactions seem to be opposed. The essen-
tial feature of the negative reaction is the checking of move-
ment at the point stimulated ; that of the positive reaction is
the reaching out of the point stimulated in the direction of the «
stimulus. This much evidence there is for saying that besideTV
a possible food sensation, the Amoeba may have some dim K
awareness of affective qualities corresponding to pleasantness J I/
and unpleasantness in ourselves. It should, however, be~~~
borne in mind that wide differences must go along with the
correspondence. In us, pleasantness brings a thrill, a "bodily
resonance," due to its tonic effect upon the circulation, breath-
ing, and muscles ; unpleasantness has also its accompaniment
of vague organic sensation, without which we can hardly con-
ceive what it would be like. In an Amoeba, it is clear that
this aspect, as found in human consciousness, must be wholly
lacking. Again, in the human mind pleasantness "and un-
pleasantness are connected with various sensation qualities
or complexes; we are pleased or displeased usually "at"
something definite. The vagueness of the affective qualities
in an Amoeba's consciousness can only be remotely suggested

44 The Animal Mind

by our own vague, diffused sense of bodily well-being or ill-
being ; and this is undoubtedly given its coloring in our case
by the structure and functioning of our internal organs.

As for the peculiar behavior of an Amoeba suspended in
the water and deprived of solid support, the stimulus for this
must lie within the cell body itself. If any consciousness
accompanies it, then the nearest human analogy to such
consciousness is to be found in organic sensations, and these,
as has just been said, must necessarily be in the human mind
wholly different in quality from anything to be found in an
animal whose structure is as simple as the Amoeba's.

A consequence of this lack of qualitative variety in the sense
experiences of an Amoeba is a lack of what we may call com-
plexity of structure in that experience. The number of
stimulus differences which are in the human mind represented
by differences in the quality of sensations is so great that at
any given moment our consciousness of the external world is
analyzable into a large number of qualitatively different sen-
sations. At the present instant the reader's consciousness
"contains," apart from the revived effects of previous stimu-
lation, many distinguishable sensation elements, visual, audi-
tactile, organic, and so on. The Amoeba's conscious-
.ess, if it possesses one, must have a structure inconceivably
impler than that of any moment of our own experience.

A second point in which the mind of an Amceba must, if it
exists, differ from that of a human being, consists in its entire
lack of mental imagery of any sort. Not only has the Amceba

t three or four qualitatively different elements in its expe-
rience, but none of these qualities can be remembered or
revived in the absence of external stimulation. How may
we be sure of this ? If our primitive animal could revive its
experiences in the form of memory images, it would give some
evidence of the influence of memory in its behavior. Indeed,

The Mind of the Simplest Animals 45

as we shall learn, it is possible, in all probability, for an ani-
mal's conduct to be influenced by its past experience even
though the animal be incapable of reviving that experience in
the form of a memory image. Therefore, if we find no evi-
dence that the Amoeba learns, or modifies its behavior as the
result of past stimulation, we may conclude a fortiori that it
does not have memory images.

Now it would be stating the case too strongly to say that
past stimulation does not affect the behavior of Amoeba at all.
In the first place, this animal shows, in common with all other
animals, the power of " getting used" to certain forms of
stimulation, so that on long' continuance they cease to provoke
reaction. "Thus," Jennings says, "Amoebae react negatively
to tap water or to water from a foreign culture, but after trans-
ference to such water they behave normally" (211, p. 20).
Such cessation of reaction occurs when the continued stimulus
is not harmful. In a sense, it may be called an effect of ex-
perience; but there is clearly no reason for supposing that
it involves the revival of experience in the form of an idea or
image. We have parallel phenomena in our own mental
life. A continued stimulus ceases to be "noticed," but the
process involves rather the disappearance of consciousness
than the appearance of a memory image. Jennings, how-
ever, is inclined to think that preceding stimulation may
modify the Amoeba's behavior in a way more nearly suggest-
ing memory in a higher type of mind. He describes an
interesting observation to illustrate this. A large Amoeba,
c, had swallowed a smaller one, b, but had left a small canal
open, through which the swallowed one made efforts to escape,
which were several times foiled by movements on the part of
the large Amoeba toward surrounding it again. Finally it
succeeded in getting completely out, whereupon the large
Amoeba "reversed its course, overtook b, engulfed it com-

46 The Animal Mind

pletely again, and started away." The small Amoeba con-
tracted into a ball and remained quiet until through the move-
ments of the large one there chanced to be but a thin layer
of protoplasm covering it. This it rapidly pushed through,
escaped completely, and was not pursued by the large Amoeba
(211, pp. 17-18), (Fig. 3).

Of this performance Jennings says, "It is difficult to con-
ceive each phase of action of the pursuer to be completely
determined by a simple present stimulus. For example . . .
after Amoeba b has escaped completely and is quite separate
from Amoeba c, the latter reverses its course and recaptures
b. What determines the behavior of c at this point ? If we
can imagine all the external physical and chemical conditions
to remain the same, with the two Amoebae in the same relative
positions, but suppose at the same time that Amoeba c has
never had the experience of possessing b} — would its action
be the same? Would it reverse its movement, take in b,
then return on its former course ? One who sees the behavior
as it occurs can hardly resist the conviction that the action at
this point is partly determined by the change in c due to the
former possession of b, so that the behavior is not purely
reflex" (211, p. 24).

If it is true that an Amoeba which had not just uhad the
experience of possessing 6" would not have reversed its move-
ment and gone after b when the latter escaped, still we cannot
think it possible that c's movements in so doing were guided
by a memory image of b. It may be supposed that the recent
stimulation of contact with b had left a part of c's protoplasm
in a condition of heightened excitability, so that the weak
stimulus offered perhaps by slight water disturbances due
to b's movements after escaping produced a positive reaction,
although under other circumstances no reaction would have
been possible. In any case, there is no evidence that Amoeba's

48 The Animal Mind

behavior is influenced by stimulation occurring earlier than
the moments just preceding action ; no proof of the revival
of a process whose original effects have had time to die out ;
and it is upon such revival that the memory images which
play so much part in our own conscious life depend.

Let us consider for a moment some of the results of the
absence of this kind of material in the possible mental pro-
cesses of Amoeba. In the first place, such a lack profoundly
affects the character of the experiences which the animal
might be supposed to receive through external stimulation.
If we call the possible conscious effect of a mechanical stimu-
lus upon the Amoeba a touch sensation, the term suggests,
naturally, such sensations as we ourselves experience them.
In normal human beings touch sensations are accompanied
by visual suggestions, more or less clear, of course, according
to the visualizing powers of the individual, but always present
in some degree. Fancy, for example, one of us entering a
room in the dark and groping about among the furniture.
How constantly visual associations are brought into play !
Not once is a mere touch impression apprehended without
being translated into visual terms ; the forms and positions of
the articles encountered are thought of immediately as they
would appear if the room were lighted. The difficulty we
have in thinking of a touch sensation with no visual associa-
tions illustrates the difference between our sense experience
and that of an animal incapable of recalling images of past
sensations.

It is equally obvious that in the absence of memory ideas,
not only must the Amoeba lack processes of imagination and
reasoning, but there can be nothing like the continuous self-
consciousness of a human being, the " sense " of personal iden-
tity, which depends upon the power to revive past experiences.
It is even possible that the "stream of consciousness" for an

The Mind of the Simplest Animals 49

Amoeba may not be a continuous stream at all. Since its
sensitiveness to changes in its environment is less developed
than that of a human being, and there are no trains of ideas
to fill up possible intervals between the occurrences of out-
side stimulation, the Amoeba's conscious experience may be^v
rather a series of " flashes" than a steady stream. And for 7
the Amoeba, again, we must remember that even such a series
would not exist as such ; the perception of a series would involve
the revival of its past members. Each moment of conscious-
ness is as if there were no world beyond, before, and after it.
Another consequence of that simplicity of structure which
results both from the rudimentary powers of sensory discrimi-
nation and from the absence of memory ideas in the Amoeba's
mind is that there can be no distinction, within a given mental
process, between that which is attended to and that which is
not attended to, between the focus and the margin of conscious-
ness. Given a consciousness which at a certain moment is
composed of the qualitatively different elements A, B, C, and
D, we can understand what is meant by saying that A is
attended to, is in the foreground of attention, while B, C,
and D remain in the background. But given, on the other
hand, a creature whose conscious content at a certain time
consists wholly of the qualitatively simple experience A, it
is evident that attention and inattention are meaningless
terms. Different moments of its consciousness may differ
in intensity; but attention, involving, as it does, clearness
rather than intensity, arises only when mental states have
become complex and possess detail and variety within their
structure.

§ 10. The Structure and Behavior of Paramecium

Although Amoeba represents in structure the simplest form
of animal life, its behavior in response to stimulation is rather

50 The Animal Mind

more complex than that of some other members of the type
Protozoa. There is a large group of single-celled animals
called Ciliata, from the fact that their bodies are covered with
little hairlike protoplasmic filaments or cilia which serve as
organs of locomotion by acting like tiny oars. A common
representative of the group is Paramecium. The structure of
this animal is distinctly more specialized than that of Amoeba.
Not only are the cilia modified locomotory structures, but
there is a definite region for food -taking. A groove extends
obliquely down one side of the body, terminating at its
lower end in a mouth. The cilia along this oral groove
beat with especial vigor and create currents which sweep
food particles to the mouth. Paramecium swims rapidly
through the water with a spiral motion of its body, due
to the facts that the aboral cilia beat more strongly than
the rest, and that the animal compensates for the turning
thus occasioned by turning on its long axis. Its reactions
to stimulation Jennings has shown to be only two in
number. First, there is a very definite avoiding or negative
reaction. This is given in response to decided mechanical
stimulation at the anterior end, as when the animal
swims rapidly against an obstacle, and also in response
to chemical stimulation, to strong ultra-violet rays (167), and
to temperatures above or below a certain middle region called
in this case, as in analogous cases with other animals, the
optimum. For Paramecium it lies between 24° and 28° C.
The negative reaction consists, according to Jennings, of the
following process : the animal darts backward, reversing the
beat of its cilia, turns toward the aboral side (that opposite to
the oral groove) by increasing the beat of the oral cilia and
lessening the compensating rotation, and continues on a
forward course that is now at an angle with its former line of
motion. If this new course carries it clear of the stimulus,

The Mind of the Simplest Animals 51

it continues on its way ; if not, repeated contact with the stim-
ulus causes a second reaction, the Paramecium always turning
in the same direction, so that ultimately it avoids the source
of stimulation (194, 211) (Fig. 4). Differing strengths of
stimulus produce the reaction with different degrees of vio-
lence. When a very strong stimulus is encountered, the
animals "respond first by swimming a long way backward,
thus removing themselves as far as possible from the source
of stimulation. Then they turn directly toward the aboral

FIG. 4. — Negative reaction of Paramecium. A is the source of stimulation.
1-6 are the successive positions of the animal. After Jennings (211).

side, — the rotation on the long axis completely ceasing.
In this way the animal may turn directly away from the drop
[the stimulus] and retrace its course" (211, p. 50). On the
other hand, when the stimulus is very weak the reaction may
be reduced to the following form: the Paramecium " merely
stops, or progresses more slowly, and begins to swing its
anterior end about in a circle." As long as it does not thus
get out of range of the stimulus, the movement is continued.
"When the anterior end is finally pointed in a direction from
which no more of the stimulating agent comes, the Paramecium
swims forward " (211, p. 51). Evidently, however, these are

52 The Animal Mind

but differing degrees of a reaction whose essential features are
the same.

While Paramecium definitely avoids by means of this neg-
ative reaction certain chemicals introduced into the water, it
shows a tendency to collect in the neighborhood of others.
Such is the case with weak acids, with a bubble of oxygen if
air has been long excluded from the slide, and with carbon
dioxide, which in water of course produces acid (214). Jen-
nings pointed out that the inclination of Paramecium to
gather in groups is very likely due to the attraction for them
of the carbon dioxide which they excrete. But he has also
shown that this "attraction" to certain chemicals does not
mean the presence of a special positive reaction. The fact
is that when the animals collect in a drop of weak acid, for
example, they are not drawn .toward the acid. They simply
happen, in their ordinary movements, to swim into it, and on
entering it show no disturbance whatever. But when they
come to the edge of the drop on their way out, they give the
negative reaction to the surrounding water. In this way they
are, as it were, trapped within the drop.

The nearest analogue to a positive reaction in Paramecium
consists in the fact that sometimes, when they come into
contact with a solid, instead of darting backward, the animals
merely cease moving, and extending stiffly the cilia which
touch the object, remain at rest (Fig. 5). The utility of
this behavior is that around decaying vegetable matter, the
kind of solid oftenest found in the animal's ordinary environ-
ment, there is apt to be a supply of food in the way of bacteria ;
it is a good anchorage. What characteristics of the stimulus
determine that this "contact reaction," rather than the
negative reaction, shall be given? Does weak mechanical
stimulation occasion it, as happens with Amoeba's positive
reaction ? Evidence in favor of this is offered by the fact that

The Mind of the Simplest Animals 53

the contact reaction is more likely to occur if the animal
comes against the solid when swimming rather slowly. Jen-
nings reports also that individuals vary. " Often all the indi-
viduals in a culture are thus inclined to come to rest, while
in another culture all remain free-swimming,
and give the avoiding reaction whenever they
come in contact with a solid " (211, p. 60). This
would suggest that some individuals are in a state
of greater excitability than others, so that a given
stimulus acts more strongly upon them. On the
other hand, there is a possibility that qualitative
as well as intensive differences in the stimulus
are responsible for the contrasting reactions.
"In general," says Jennings, Paramecium
"shows a tendency to come to rest against loose
or fibrous material; in other words, it reacts
thus to material with which it can come in
contact at two or more parts of the body at once.
To smooth, hard materials, such as glass, it is much less
likely to react in this manner " (211, p. *6i). Perhaps, then,
the spatial distribution of the stimulus over several points
of the body surface increases the probability of a contact
rather than an avoiding reaction.

Certain other forms of behavior in Paramecium involve
the taking up of a definite position with reference to some
constant stimulus, and are therefore termed by Jennings
"orienting reactions." In the first place, if there is a current
in the water, the animals will head up-stream. Jennings
explains this as due to the giving of avoiding reactions in
response to the disturbing effects of the current on the cilia
until, with the Paramecium's head up-stream, the current no
longer tends to reverse the cilia. Analogous reaction is given
to gravity ; the animals direct their heads upward, and swim

FIG. 5.—
Positive
thigmotaxis
in Parame-
cium. After
Jennings
(211).

54 The Animal Mind

in that direction. The cause of this has been the subject of
some dispute, which we shall discuss in a later chapter; but
the response to gravity seems in any case not to involve a new
form of reaction. Further, Paramecium reacts to the cen-
trifugal force produced by whirling a horizontal tube around
a vertical axis just as it does to gravity ; that is, it orients itself
in such a way as to swim toward the axis, in the opposite
direction to the pull of the force (211, p. 78).

To an electric current the response of Paramecium is
more complicated. When the current is weak the animals
move toward the cathode. This appears to be caused simply
by the giving of the negative reaction so long as the front end
of the animal is turned toward the anode, and is thus being
stimulated. But if the current is made gradually stronger,
the movement toward the cathode grows slower and finally
stops. Further increase in the intensity of the current causes
the animal to swim backward toward the anode, and finally
to burst into pieces. This reversal of movement Jennings
has found to be due to the fact that the cilia nearest the cathode
have their direction reversed ; as the current is made stronger,
this effect is increased, until finally it balances and prevails
over the beat of the forward cilia (21 1, pp. 82 ff.). The animal's
movements are thus really discoordinated by the action of
the strong current. The effect seems a pathological one, and
probably need not be taken into account in considering the
normal life of the infusorian; as Jennings says, "The re-
action to electricity is purely a laboratory product" (211,

p. 168).

§ ii. The Mind of Paramecium

If we now compare the behavior of Paramecium with that
of Amoeba in order to draw conclusions with regard to the
possible consciousness of the former, we find that although the
mechanism of reaction is decidedly more complicated in Para-

The Mind of the Simplest Animals 55

mecium than in Amoeba, there is rather less possibility of
variety in the conscious experience of the ciliated protozoon.
The reversal of cilia, the rolling toward the aboral side,
form a more elaborated and specialized mode of withdrawal
than does the simple checking of protoplasmic flow at one
region of the body. But, supposing Paramecium to be con-
scious, the significance for its consciousness of the negative
or avoiding reaction is even less clear than that of the corre-
sponding behavior in Amoeba. The negative reaction in
Amoeba is contrasted on the one hand with a positive reaction,
opposite in character, given to stimuli of less intensity, so that
the opposition of unpleasantness and pleasantness in the
human mind is clearly suggested ; and on the other hand with
a food-taking reaction, given to edible substances, and hinting
at a differentiation corresponding to that between touch and
taste in our own experience. The only approach to a positive
reaction in Paramecium is the coming to rest in contact with
solids, and this does not present any very striking analogy
with such expressions of pleasantness as we are acquainted
with. Further, Paramecium has no special food-taking
reaction at all. The fact seems to be that its greater speed of
motion has developed its negative reaction at the expense of
the others. It does not need to reach out in a typical positive
reaction, for its rapid dashing through the water greatly
increases its natural chance of getting food. The whirling
of the oral cilia brings it edible as well as inedible substances.
It does, however, much need in its headlong career a means
of avoiding the dangers into which it may rush, and so we
find the very definite and well-adapted negative reaction
dominating the field. If, then, the mind of an Amoeba is
thought of as capable of three or four qualitatively different
experiences, that of a Paramecium must be even less
favored.

56 The Animal Mind

> Is there any evidence of the presence in Paramecium of
JL&ie revival of past experiences in any form ? The answer to
\this question must be negative, as in the case of Amoeba.
Immediately preceding stimulation does have some effect
upon the response to present stimulation, but these effects
are all of such a character as to suggest rather the disap-
pearance of possible consciousness than the recall of a memory
image. Jennings mentions several instances. "If a Para-
mecium is subjected to a strong induction shock, it fails for
some time thereafter to react to weak shocks, though at the
beginning it reacted to these" (211, p. 100). Such a result
is due probably to fatigue. "Paramecia which have been
living at the usual temperatures show a temperature opti-
mum of about 24 to 28 degrees ; if they are kept for some
hours at a temperature from 36 to 38 degrees, the optimum
rises to 30 or 32 degrees. A change in the individuals in-
duced in this way is commonly spoken of as acclimatiza-
tion " (211, p. 101). Further, "if a bit of filter paper is placed
in a preparation of Paramecia, the following behavior may
often be observed. An individual swims against it, gives the
avoiding reaction in a slightly marked way, swimming back-
ward a little; then it swims forward again, jerks back a
shorter 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 "(211, p. 101). Be-
havior of this type, where a stimulus at first occasions the
negative reaction, but on immediate repetition ceases to do
so, we shall find very common among the lower forms of
animals; it suggests simply that the stimulus acts less and
less strongly on repetition, not that the effects of its earlier
application are consciously recalled.

Jennings's work on other ciliate Protozoa, as well as on the

The Mind of the Simplest Animals 57

group known as Flagellata, the members of which have in
place of cilia a long whiplike protoplasmic filament, and
move by lashing it to and fro in the water, indicates that in all
of them the negative reaction is the principal feature of be-
havior (199, 21 i), and that if any of them possess minds, those
minds are of quite as rudimentary a type as that of Amoeba,
and very likely rendered even more so, as far as qualitative
variety of experience goes, by the predominance of the neg-
ative reaction due to greater speed of motion.

§ 12. Definitions of Tropisms

Before passing to the study of higher forms of animal life,
we may note the meaning of a few technical terms used in
describing the behavior of simple animals especially. The
direct motor response of an animal to an external stimulus is
known as a tropism, from the Greek word meaning "to
turn." Various prefixes are attached to this term to indicate
the nature of the stimulus concerned; thus phototropism
means the reaction of an animal to light; chromotropism,
reaction to color ; thigmotropism, reaction to contact ; chemo-
tropism, reaction to chemical stimulation; rheofropism,
reaction to currents ; geotropism, reaction to gravity ; electro-
tropism, reaction to the electric current; anemotropism,
reaction (e.g., in winged insects) to wind. Some writers
have used instead of tr'opism the word taxis, from the Greek
word meaning "to arrange," speaking of chemotaxis, thigmo-
taxis, and so on. Phototaxis has, as we shall see, a rather
special significance distinct from phototropism. When an
animal gives a positive reaction in response to a stimulus,
it is said to be positively chemotropic, or phototropic, as the
case may be ; when its reaction is negative, it is called nega-
tively chemotropic, phototropic, and so on.

CHAPTER IV
SENSORY DISCRIMINATION: METHODS OF INVESTIGATION

§ 13. Preliminary Considerations

ONE of the most important points in which the human
mind differs from the mind of the lowest animal forms con-
sists, we have seen, in the enormously greater number of
different sensations which enter into human experience, as
compared with the small number of sensory discriminations
possible to the simpler animals. Much of the experimental
work that has been done on animals has been directed
toward discovering what discriminations they make among
the stimuli acting upon them, and to the results of this work
we shall give our attention in the next chapters. But first
we ought to get a clearer idea of just what kind of evidence
is needed to indicate the existence of a variety of sensations
in an animal's mind.

At the outset, we must remind ourselves that, in the ab-
sence of any satisfactory proof that the lower animal forms
have minds at all, and the equal absence of any proof that they
have not, all our conclusions about the number and kind of
their possible sensations must remain subject to the proviso
that they possess consciousness. Further, a point that was
mentioned on page 24 must again be emphasized. No evi-
dence of discrimination between two stimuli on an animal's
part can do more than show us that for the animal they are
different; just what the quality of the sensation resulting
from each may be, whether it is identical with any sensation

58

Sensory Discrimination 59

quality entering into our own experience, we cannot say.
The light rays which to us are red and blue may for an
animal's consciousness also differ from each other, and yet
if our experience could be exchanged for the animal's, we
might find in the latter nothing like red and blue as we know
them.

Thus much being premised, what sort of evidence can
be obtained that an animal does discriminate between two
stimuli? Again, as in considering the evidence for the
existence of consciousness in general, there is an argument
from structure and an argument from behavior.

§ 14. Structure as Evidence of Discrimination

The argument from structure consists primarily in the fact
that an animal possesses sense organs recognizably like
our own. If a creature has an organ suggesting strongly the
construction of the human cochlea, or an organ with a lens
and a membrane composed of rods and cones, it is highly
probable that auditory stimuli in the one case and light in the
other produce specific sensations. This argument from the
morphology of sense organs. is, however, limited in two ways.
First, it is only a small part of the animal world whose sense
organs resemble ours closely enough to make the analogy
safe. And secondly, we do not after all know very much
about the relation of our own sense-organ structure to function.
We know, for example, that our own organ with a lens and
retina gives us visual sensations, but we cannot say with
certainty which structures in the retina furnish brightness
sensations and which color sensations, nor do we know any-
thing about the retinal structures that underlie different
qualities of color sensations. We can say that sensations
of hearing come from the ear, but no one can tell us how
to judge from the structure of the ear what range and

60 The Animal Mind

fineness of pitch discriminations exist in its possessor's
mind. No investigator has yet succeeded in relating the
different qualities of smell and taste to differences in the
end organs.

4 § 15. Behavior as Evidence of Discrimination

The argument from behavior is as follows : If an animal
reacts in a different way to two qualitatively unlike stimuli,
then, providing that it is conscious at all, it may be supposed
to receive qualitatively unlike sensations from them. If it
always reacts in the same way to both, then both may be sup-
posed to be accompanied by the same sensation quality.
Obviously these statements need further discussion. For
one thing, it may be urged that in our own case the same
external reaction is often made to stimuli that are nevertheless
consciously discriminated. A man may eat with relish and
without observable difference in behavior, for example, foods
that yet give him perfectly distinguishable smell and taste
sensations. Precisely this objection holds against a method
of experimentation, formerly a good deal used, which may
be called the Preference Method of testing discrimination.
Vitus Graber, for instance, attempted to find whether ani-
mals belonging to a variety of species could discriminate
colors, by offering them the choice of two compartments
illuminated each with a different color. Clearly, if the ani-
mals chose one compartment as often as the other, it would
be rash to conclude that the two lights produced for them
indistinguishable sensation qualities. There might simply
be the absence of any preference, along with perfect dis-
crimination. The fact is that in all experiments upon
animals, whether to determine their power of distinguishing
stimuli or their power of learning by experience, the first
requisite is to give the animal what we commonly call a

Sensory Discrimination 61

motive. That is, the conditions of the experiment must be
so arranged that some already present tendency to act,
whether inborn in the animal or acquired by previous ex-
perience, shall be appealed to.

This is increasingly the case, the higher the animal worked
with stands in the scale. The higher animals have what
might be called a large reserve fund of discriminations. That
is, they are capable of making many more selective reactions
to stimuli than they need at a given moment actually to use.
Hence in their case the experimenter must make a careful ad-
justment of conditions to bring out exactly the discrimination
wanted. He must either make the performance of the re-
action pleasant or its non-performance unpleasant to the ani-
mal. A monkey, for example, confronted by a set of glass
tumblers covered each with a differently colored paper, may
behave toward them all in precisely the same way; yet if
food be put regularly in the blue -tumbler, whose position in
the row is varied, it becomes worth the monkey's while to
make use of his discriminative powers, and he may show by
his different behavior toward the blue tumbler that it pro-
duces on him a different impression from the others.

With simpler animals the problem is less difficult. If
an animal is capable only of a half dozen different ways of
responding to stimulation, we may with comparative safety
assume that it has less opportunity to hold them in reserve ;
and if such an animal invariably reacts in the same way to
two different forms of stimulus, or if the variations in its re-
sponse are not correlated with differences in the stimulation,
it becomes probable that the two stimuli produce in its as-
sumed consciousness identical sensation qualities. Thus it is.
not the number of stimuli to which an animal reacts that!
can be taken as evidence of the qualitative variety of itsf
sensations, but the number of stimuli to which it gives dif-

62 The Animal Mind

ferent reactions. When Jennings, for instance, says that
Amoeba " reacts to all classes of stimuli to which higher
animals react" (211, p. 19), we cannot conclude that it pos-
sesses all classes of sensations that higher animals possess,
for its reactions to these different stimuli are but little varied
according to the kind of stimulus.1

§ 1 6. Evidence from Structure and Behavior Combined

As a matter of fact, the argument from structure needs
confirmatory evidence from behavior. For clearly the mere
presence of a sense organ bearing sufficient likeness to our
own to admit of conjecturing its function would be of no
value as proof unless it were shown that the sense organ actu-
ally functioned. In order to do this, it would be necessary to
show that the animal reacted to the stimulus conjectured as
appropriate to the sense organ, and that removal of the organ
profoundly modified the reaction. Thus we shall find that
many experiments to test sensory discrimination have been
made by the method of extirpating a sense organ and studying
the effect on behavior. Thelnethod has many disadvantages,
the chief of which lies in the fact that it is hard to say which
disturbances in behavior are due actually to the loss of the
organ and which to the more widespread effects of the opera-
tion. Yet this much may be said for the combination of
proof from structure and behavior involved in the Method
of Extirpation, if we may so call it : where an animal reacts
to a certain stimulus, for instance light, when a sense organ is
intact, and fails to react to light, though otherwise normal,
when the organ is removed, there arises a possibility that light

1 One of many reasons for the unsatisfactoriness of a recent article by A.
Olzelt-Newin, entitled " Beobachtungen iiber das Leben der Protozoen"
(299), lies in the author's uncritical acceptance of the hypothesis that reac-
tion to a special kind of stimulus means a special kind of sensation.

Sensory Discrimination 63

may produce in the animal's consciousness a specific sensation
quality, even although the animal ordinarily reacts to light
in a manner indistinguishable from that of its responses to
other stimuli. Though light and mechanical stimulation,
for example, both ordinarily produce a negative reaction,
yet if light brings about its effect only through the medium
of a specialized structure with which mechanical stimuli
are not concerned, then along with the probable un-
pleasantness accompanying the negative reaction there may
go a quality peculiar to the functioning of that special
structure.

Another mode of combining evidence from structure with
evidence from behavior is by the use of localized stimuli. If
an animal gives a response, which in itself may have nothing
to mark it off from responses to other stimuli, when a special
kind of stimulation is applied to certain regions of the body,
and only then; while the other stimuli produce better re-
actions when applied elsewhere, then the suggestion is
given that different sense organs are involved, and the same
possibility arises of different sensation qualities.

Two other forms of evidence whereby from behavior a
differentiation of sensory structures can be argued, and from
differentiation of sensory structures possible differences of
sensation quality, may be mentioned. The first of these con-
sists in showing that reactions to different stimuli may be
independently fatigued. The natural inference is that a
specific nervous apparatus belongs to each stimulus. The
second lies in demonstrating that the reactions to different
stimuli occur with different degrees of rapidity. If there is a
marked difference in the reaction times of an animal to different
forms of stimulation, each, again, may be supposed to affect
its own nervous pathway. A modification of this method
consists in noting the influence of a stimulus upon the time of

64 The Animal Mind

reaction to another nearly simultaneous stimulus. If such an
influence can be shown, it is evident that the force producing
it has some effect on the nervous system. By combining
this method with that of extirpating a sensory structure,
indications may be obtained that the nervous effect of the
auxiliary stimulus is dependent on a definite receptive
apparatus, and hence is probably accompanied by a special
sensation. This method was used by Yerkes to demonstrate
hearing in frogs (456, 462, 464).

One further consideration offers itself to the student of
animal responses to stimulation. It has been the special en-
deavor of Jennings to point out the fact that these responses,
instead of being wholly accounted for by the characteristics
of the stimulus, are determined in part by the internal,
physiological condition of the animal (211). We shall
therefore note often in the course of the following pages cases
where difference of reaction is due to internal rather than to
external causes.

§ 17. Evidence for Discrimination of Certain "Lower"
Sensation Classes

Bearing all these points in mind, let us proceed to survey
the evidence for variety in the sensations of animals. In
the lowest forms, such evidence must be derived entirely from
behavior. That from the presence of a sense organ is almost
wholly lacking. And although various stimuli, as we have
seen, produce reactions in Amoeba, yet there is only one case
where these reactions are strikingly different according to the
quality of the stimulus applied. This instance consists in the
distinction between food-taking reactions, given to edible sub-
stances, and the responses to mechanical stimulation. The
sense of touch, undoubtedly, must play a part in the mental
life of the lowest animals that have consciousness at all.

Sensory Discrimination 65

But the earliest distinction between a touch quality and a
quality that is other than touch seems to occur when food
sensation and contact sensation are differentiated. It is
possible that warmth and cold also appear as distinct sensation
qualities in the experience of low forms of animals, but we
have little real evidence of the fact. No organs of tempera-
ture sensation are definitely known even in human beings.
And the responses of low animals to thermal stimulation are
not specialized. They consist usually of negative reactions,
given when the animal is subjected to a temperature either
above or below, but especially above, the " optimum"; and
these reactions are not different from the ordinary negative
type, suggesting unpleasantness rather than a specific sensa-
tion quality. In some cases the sensibility to thermal stimu-
lation has been found to be differently distributed from that
to other classes of stimuli. But in any case, sensations of
warmth and cold are probably in no member of the animal
kingdom differentiated into any greater number of qualita-
tively distinct sensations.

The sense of touch, also, shows but little internal differen-
tiation. Its importance, so far as we can judge, is rather on
the spatial than on the qualitative side. The sense quality
of pain we naturally think of as the accompaniment of the
negative reaction in its more violent forms, given to a stimulus
that is injuring the organism. Organic and kinaesthetic
sensations are hard to trace in the lower animals ; for animals
whose structure differs widely from our own, the qualities
of these two classes must remain beyond the power of our
imagination. That differences in physiological condition
such as are produced by hunger, satiety, or fatigue involve
differences of accompanying organic sensation in the con-
sciousness of the animal manifesting them is possible.
Kinsesthetic sensations, as we shall see, are apparently con-

F

66 The Animal Mind

cerned in the processes whereby many animals have learned
to traverse a labyrinth path.

The three classes of sensation whose existence in the ani-
mal mind can be most satisfactorily traced are the chemical
sense, under which smell and taste belong, the sense of hear-
ing, and the sense of sight. To the study of these the follow-
ing chapters will be devoted. Since the manifestations of the
chemical sense in the lowest forms of animals consist chiefly
in a differentiation of response to food and to mechanical
stimulation, the contact sense or sense of touch will, in dis-
cussing these forms, be considered along with the chemical
sense.

CHAPTER V

SENSORY DISCRIMINATION: THE CHEMICAL SENSE

§ 1 8. The Chemical Sense in Ccdenterates

WE have already discussed the responses to mechanical,
chemical, and food stimulation in those members of the Pro-
tozoa whose behavior has been most carefully studied, and
may begin the present chapter with an account of the corre-
sponding reactions in the lowest of the Metazoa, or many-
celled animals, the coelenterates. Although externally the
forms of different families of coelenterates differ widely, yet
the general plan of structure is the same in all : the body of
the typical ccelenterate is a hollow sac, whose walls consist
of two layers of cells, food being taken into a mouth at one
end of the sac, and the arrangement of cells being on the plan
of circular symmetry. In the phylum of the coelenterates are
included sea-anemones, jellyfish, the little green or yellow
Hydra, sponges, corals, and ctenophores.

Hydra (Fig. 6), one of the simplest coelenterates, shows
a food reaction distinct from the contact reaction. Me-
chanical stimulation is followed by withdrawal of the ten-
tacles, and by contraction of the stem. This behavior may ,
be called a negative or avoiding reaction, and no positive^
reaction to a mechanical stimulus has been observed. The
food-taking reaction, on the other hand, consists in the seiz-
ing of the food by the tentacles. It seems to be given in
response to a combination of chemical wjth mechanical
stimulation, such as is offered by contact with a solicTedible

67

68

The Animal Mind

object (418). Shall we say that Hydra possesses, then, a
food sensation and a contact sensation that are distinguish-
able in its consciousness, provided such consciousness exists ?
It may be that the contrast between the two is more nearly
analogous to that between pleasantness and unpleasantness
in our own experience, for the food-taking reaction in Hydra

is the only form of the
positive reaction, and the
response to mere contact
is distinctly negative in
character. The influence
of physiological condition
in Hydra's reactions is
shown by the fact that
although ordinarily the
food response is brought
about only by contact
with food, if the animal
is very hungry any
chemical stimulation,
even quinine, will pro-
duce it (418). This
blunting of discrimina-
tion has, of course, the
adaptive aspect that the
starved animal can afford to lose no chances, and suggests the
analogy from our own experience of the loss of intellectual
discrimination in moments of intense emotion. For the
emotion too represents a situation where the organism can-
not afford to lose chances by hesitating in reaction long
enough for nice discrimination.

In Tubularia crocea, a ccelenterate belonging to the family
of hydroids which form colonies of many individuals on a

FIG. 6. — Hydra, mth, mouth; *, tentacle.
After Parker.

Sensory Discrimination: the Chemical Sense 69

common stem, food and contact stimuli do not produce dif-
ferent reactions, but have different degrees of efficiency in V
bringing about response. When a grain of sand was placed in
contact with the tentacles on one side and a bit of meat in a
corresponding position on the other side, the reaction was
almost invariably in the direction of the meat. Filtered meat
juice allowed to flow upon the distal tentacles produced a reac-
tion 82 per cent of the time, while carmine water was effective
only 15 per cent of the time. Further, if the distal tentacles
were touched several times with a needle, they remained
closed ; but if the second stimulus used was a piece of meat,
the tentacles opened out and waved about (319). Whether
in such a case as this the possible conscious accompaniments
of the responses are to be regarded as qualitatively different /f
sensations, or only as different degrees of intensity of the same
sensation, it is difficult to say. Another hydroid, Corymorpha
palma, gives no response whatever to meat juice; only irri-
tating chemicals produce reactions, whose character appears
to be tactile (402).

In the sea- anemones or actinians we find behavior in re-
sponse to food stimulation as distinguished from contact
stimulation varying in different representatives of the group.
Generally speaking, the food reaction seems to be more marked
than the contact reaction. W. H. Pollock a number of years
ago reported his observation that certain unnamed sea- anem-
ones opened out if food were suspended near them in the
water, and referred the phenomenon to "a sense of smell"
(343). Adamsia rondeleti winds its tentacles around bits
of sardine meat and passes them from tentacle to tentacle
toward the mouth. When balls of filter paper softened with
sea water are substituted, the feeding reaction is wholly lack-
ing. Either the tentacles fail to react at all, or the ball is
"felt of " slowly with no attempt to seize it, or it is momentarily

7<D The Animal Mind

seized and then dropped. If the paper ball be soaked in fish
juice, on the other hand, it is seized as eagerly as the fish
meat. A negative reaction, consisting in the withdrawal of
the tentacles affected, may be produced by applying a bit
of paper soaked in quinine solution or by the discharge of qui-
nine solution from a pipette near the tentacles (237, 288). A
peculiar form of negative reaction has been observed in Adam-
sia, and more strikingly in Cerianthus, when a paper ball soaked
in fish juice has been passed from tentacle to tentacle till it
has nearly reached the mouth. The process is suddenly re-
versed, and the ball is passed back from one tentacle to another
till it reaches the outside edge and is dropped off. Nagel,
the observer, thinks the stimulus for this change of reaction
is the gradual wearing off of the "sapid parts" of the ball
during its passage toward the mouth — it might be the
squeezing out of the meat juice — and calls special atten-
tion to the fact that the reaction whereby the paper is got rid
of is wholly different from the ordinary reaction of a tentacle
to mechanical stimulation, which, as we have seen, does not
involve seizing the object at all. A tentacle touched by a bit
of moistened filter paper ordinarily responds, if at all, by a
mere contraction without the winding seizure of the object.
Touched by the same object "handed on" to it by a tentacle
nearer the mouth than itself, it seizes the paper and passes it
on to the tentacle beyond it. The cause of this difference in
behavior seems to lie in the processes that have been taking
place just previously. Nagel does not hesitate to say that a
psychic process must be involved, but its details are not easy
to construct (291).

Another sea-anemone, Aiptasia, has but one ring of ten-
tacles, and like Tubularia crocea, instead of showing different
responses to contact stimulation alone and to contact plus food
stimulation, it merely reacts with greater emphasis to the

Sensory Discrimination : the Chemical Sense 7 *

latter. In both cases the tentacles wind around the object,
contract, and direct themselves toward the mouth (291).
Again the question arises whether the possible accompanying
sensations differ in quality or only in intensity. One species
of Aiptasia, A. annulata, however, does react differently
to filter paper soaked in crab juice and to plain filter paper
(207), showing that even within a genus the capacity for stim-
ulus discrimination may differ. In like manner one sea-
anemone, Actinia, will take filter paper soaked in acetic acid,
while another, Tealia, rejects it (127).

Metridium, a common sea-anem-
one of our coasts, has its tentacles
covered with cilia which have a con-
tinual waving motion toward the tip
of the tentacle (Fig. 7). If particles
of an inedible substance are dropped
on a tentacle, no definite reaction
occurs, but the particles are carried
by the ordinary motion of the cilia
out to the tentacle tip, where they drop off. When a bit of
crab meat, or some meat juice, is dropped on a tentacle, the
latter contracts and curls over with the tip directed toward
the mouth. The ciliary movement continuing in its usual
direction now of course carries the food toward the mouth.
Applying food to the lips on either side of the mouth causes
a different response. The cilia on these lips ordinarily wave
outwards; when food is brought in contact with them their
motion is reversed, and the food is thus passed into the
mouth. In Metridium, then, there is no specific rejecting
reaction for inedible substances (303).

Various instances of the effect of physiological condition
upon response to food stimulation in sea-anemones have been
noted. Adamsia loses the power to discriminate between edi-

FIG. 7. — Metridium. After
Parker.

72 The Animal Mind

ble and inedible substances when very hungry (291). Sagar-
tia davisi will also swallow inedible substances if hungry
enough (403). Stoiachactis helianthus will give either a
positive or a negative reaction to food according to its condi-
tion of hunger or satiety (207). The reaction of Metridium
to food may vary decidedly with the degree of hunger (3) ;
although it will continue taking food as long as the process is
mechanically possible (211). Fatigue has also been shown
to affect the food responses of Metridium and other sea-
anemones ; specimens that have been fed meat and filter paper
alternately will after a time refuse to take filter paper (207,
291, 303). This behavior was thought by Nagel to indicate
that the animal had discovered the deception practiced upon
it ; but apparently the real cause is fatigue ; showing itself first
T» with reference to the weaker of the two stimuli (3).

As regards the localization of the sensitive elements, au-
thorities, and probably species, differ. Nagel finds the ten-
tacles most sensitive (291) ; Loeb observed that the stump of
the animal has discriminative reactions (237), while Fleure
and Walton state that in the species tested by them the mouth-
region is most responsive to chemical stimulation (127).

A certain amount of discrimination between mechanical
stimuli is asserted of these animals by Romanes. "I have
observed," he says, " that if a sea- anemone is placed in an
aquarium tank and allowed to fasten upon one side of the
tank near the surface of the water, and if a jet of sea water
is made to play continuously and forcibly upon the anemone
from above, the result of course is that the animal becomes
surrounded with a turmoil of water and air bubbles. Yet
after a short time it becomes so accustomed to this turmoil
that it will expand its tentacles in search of food, just as it does
when placed in calm water. If now one of the expanded ten-
tacles is gently touched with a solid body, all the others close

Sensory Discrimination: the Chemical Sense 73

around that body in just the same way as they would were they
expanded in calm water " (366, p. 48), although the solid
stimulus is decidedly less intense than that offered by the
bubbles. Similarly, Fleure and Walton find that certain
species show little reaction to accidental contact with a pebble
that is moved, but react quickly to a finger (127).

The body of a typical medusa or jellyfish consists of a bell-
shaped " umbrella " from the edge of which tentacles depend.
Hanging from the middle like the clapper of the bell or the
handle of the umbrella is the manubrium, at the end of which
is the mouth. In the medusa Carmarina hastata no differen-
tiation in reaction to contact and food stimulation appears,
merely a readier response of the tentacles to the latter ; but
we do find whatever evidence for the existence of a specific
sensation quality is furnished by localized sensitiveness, for
the skin of the under side of the umbrella, and of the manu-
brium, is very sensitive to mechanical stimulation, and wholly
insensitive to chemical stimulation, while the tentacles, as
has just been stated, react, by shortening and twisting them-
selves about the object, more readily to chemical than to
mechanical stimulation. A mechanical stimulus applied to
any part of the under edge of the umbrella produces after
from one to three seconds a movement of the manubrium tip
toward the point stimulated (289, 291).

The little medusa Gonionemus murbachii (Fig. 8) shows, on
the other hand, two well-defined different responses to special
stimulation : motor reactions and food-taking reactions. The
motor or swimming reactions are given in response to me-
chanical stimulation and to the presence of food near the
animal in the water; but the food-taking reaction occurs
only in response to food (solution of fish meat) ; very rarely
a weak inorganic chemical stimulus will produce the begin-
ning of the response. An important exception to the usual

74

The Animal Mind

inefficacy of mechanical stimuli in bringing about the feeding
reaction occurs when a moving mechanical stimulus is used ;
this very quickly produces the early stages of the food-taking
response. Special reactions to stimuli in motion are wide-
spread throughout the animal kingdom; their significance
will be discussed in the chapter on Space Perception. The
food-taking response in Gonionemus shows a marked coor-
dination of movements; if the food touches one or more
tentacles, these contract and twist about it ; they then bend
toward the manubrium, and the margin of the bell also

bends in ; the manubrium
swings over toward the
bell and envelops the
food with its lips (451).
Another ccelenterate
whose reactions to chemi-
cal stimulation have been
observed is the cteno-

FIG. 8. -Gonionemus. After Hargitt. ph()re

body is an elongated oval, with longitudinal ciliated ridges,
having the mouth slit at the end which is normally uppermost
when the animal is at the surface of the water, and at the
opposite end an otolith or statolith organ lying between two
flattened " polar plates." The significance of this organ will
be considered later. The aboral region is far more sensitive
than any other to mechanical stimulation ; the slightest touch
on one of the polar plates causes the animal to shorten itself
and fold in the plates. The aboral end, being the hind end of
the creature, is not usually brought into contact with objects.
Nagel, who studied the animal, suggests that this region, being
sensitive to changes in pressure, may enable the animal to
right itself when it rises to the surface with the aboral end
up, as the change from water to air pressure could not fail

Sensory Discrimination : the Chemical Sense 75

to stimulate the polar plates. Nagel apparently made no
experiments on the behavior of Beroe with reference to food
stimuli; for chemical stimulation he used picric acid, dilute
hydrochloric acid, quinine, strychnine, saccharine, coumarin,
vanillin, and naphthalin. To all these unwonted stimuli the
animal responded by some form of negative reaction, indicat-
ing possible unpleasant feeling. The edges of the mouth,
where the nerves end in bulblike structures, reacted to
quinine, vanillin, and coumarin by stretching the mouth into
a circular form instead of its usual slitlike shape ; suggesting
an effort to get rid of the stimulus. Precisely similar re-
actions were produced by stimulation with lukewarm water.
Nagel concludes that the organs for chemical and ther-y^
mal stimulation are identical; whether the sensation quali-'
ties are different is, he thinks, an open question. There is
at least no evidence that they are different (289, 291).

§ 19. The Chemical Sense in Flatworms
Next to the coelenterates zoologists place the phylum of the
Platyhelminthes or flatworms, which possess a bilaterally
instead of a radially symmetrical structure. Many repre-
sentatives of the group are parasitic, and so far as the writer
is aware, no extended study of the reactions of these forms to
stimulation has been made. Most of our knowledge in re-
gard to the sensory life of the flatworms is confined to the class
Turbellaria, including the common freshwater and marine
planarians. These are small slow-moving creatures which
crawl about on solid objects under water or on films covering
the surface. The mouth is situated on the ventral side of
the body, sometimes quite far removed from the head end
(Fig. 9). One chief interest of planarians to physiologists
has lain in their remarkable power to regenerate parts lost
by mutilation.

76

The Animal Mind

Planaria maculata, a common freshwater planarian, re-
v, sponds to stimulation by two forms of negative reaction, a posi-
'• tive reaction, and a feeding reaction. The negative and posi-
tive responses are given either to mechanical or to chemical
stimuli, the former being produced by strong,
the latter by weak stimulation. Hence they
.do not suggest correlation with qualitatively
7 different sensation contents, but rather with
unpleasantness and pleasantness. The two
forms of negative reaction correspond to dif-
ferences in the location of the stimulus. If the
head end of the body is stimulated strongly on
one side, the head is turned awray from that
side. If the posterior part of the body is
strongly stimulated, the animal makes power-
ful forward crawling movements. The signifi-
cance of local differences in stimulation for
response and for possible consciousness, again,
will more properly be discussed in a later
chapter. As has just been said, both weak
chemical and weak mechanical stimulation
cause Planaria maculata to give a positive
reaction by turning its head in the direction of
the stimulus, which need not be in actual con-
tact with the body (316). A planarian will
follow an object such as the point of a pin
moved in front of it, and one planarian will
follow the trail of another that happens to come within the
proper distance. Similarly, the neighborhood of food will
cause the animal to turn toward it. Bardeen has suggested
that the so-called "auricular appendages," two small movable
prominences on the animal's back near the head end, which
are specially sensitive to touch, may be " delicate organs

FIG. 9.— Plana-
rian, dorsal
view. After
Woodworth.

Sensory Discrimination : the Chemical Sense 77

capable of stimulation by slight currents in the water set up by
the minute organisms that prey" upon the animal's food; so
that the positive reaction when given to food may be really a
response to mechanical stimulation (10). As Pearl, however,
found that chemicals, diffused in the water, would produce
positive responses (316), it is probable that Planaria maculata
is directly sensitive to chemical stimulation, though it re-
sponds thereto in the same way as to mechanical stimulation.
A land planarian, Geodesimus bilineatus, is reported by
Lehnert to perceive food at distances from four to five times the
length of its body, and he does not describe the positive reaction
as given in response to any other than food stimulation (231).

The food-taking reaction in Planaria maculata is made
under the influence of combined mechanical and chemical
stimuli, in contact with the pharynx or the ventral side of the
animal. When an object which has occasioned the positive
reaction is reached, the head folds over it and grips it, contract-
ing so as to squeeze it. The substance being thus brought into
contact with the pharynx, swallowing movements are pro-
duced if the proper stimulus is given. Bardeen was inclined
to think that contact with a soft substance constituted the
proper stimulus, as he found that hard particles placed on the
pharynx were not swallowed (9). Pearl, however, believes
that mechanical and chemical stimulation must combine.
The former alone does not suffice, for swallowing movements
are not evoked when one planarian crawls over another; the
latter alone is insufficient, for placing the animal in a sugar
solution has no effect. If chemical and mechanical stimula-
tion are united, the reaction is given whether the chemical is
edible or not ; Pearl found it occurring in response to sodium
carbonate (316).

Evidence of the influence of physiological condition upon
the reactions of planarians is furnished by the fact that the

78 The Animal Mind

resting planarian shows a decidedly lowered susceptibility to
stimulation. Bardeen found that if the animal was not
already in motion, it gave no positive response to food in
its neighborhood (10).

§ 20. The Chemical Sense in Annelids

In our own experience, as has been said, the "food sense"
is represented by the two senses taste and smell, the stimulus
for the one being fluid, and that for the other gaseous, so that
the latter enables us to perceive objects at a distance. For
water-dwelling animals, such as most of those whose behavior
, we have been describing, the distinction evidently cannot
' well be drawn. If such an animal perceives food at a dis-
tance, the stimulus is necessarily diffused through the water,
and Lloyd Morgan has proposed the term " telaesthetic taste"
for the sense which makes such perception possible (279,
p. 256). The term indicates that this sense corresponds to
taste in an air-dwelling animal because the stimulus is fluid,
but differs in that it allows perception of a distant object, as
taste in the ordinary sense does not. In the most familiar
representative of the Annelida or segmented worms, the com-
mon earthworm, as in the land planarian, a distinction analo-
gous to that between smell and taste in our own sensory
experience may be made ; in the leeches and marine annelids
it cannot.

Gentle and continuous mechanical stimulation produces in
the earthworm "positive thigmotaxis " ; that is, the animals
have a tendency to crawl and lie along the surface of solids
(387). That there is some discrimination of edible from
inedible substances when in contact with the body Darwin
thought probable from the apparent preference of the worm
for certain kinds of food (91). In the earthworm Allolobo-
phora fatida we find a differentiated response to contact

Sensory Discrimination: the Chemical Sense 79

and chemical stimulation. These worms live in barnyard
manure. When placed on scraps of shredded filter paper
moistened with water they refuse to burrow; when the filter
paper is wet with a decoction of the manure they burrow as
soon as they come into contact with it. The adequate stimu-
lus for burrowing is thus a combined mechanical and chemical
one; the chemical stimulus alone is insufficient, for filter
paper thus prepared has no effect on the worms unless they
are actually in contact with it (387). Using the human terms,
the case is one of taste rather than smell. Nagel suggests
that the earthworm's chief use for a chemical sense is to help
it find the moisture which is necessary to its life (292) ; but
curiously enough Allolobophora fatida seems to have no power
of doing this from a distance. Smith found that a worm
would crawl around a wet spot on paper until its skin dried,
without crawling into it. If by accident it happened to touch
the moist place, it would enter and remain there (387). There
seems no satisfactory evidence that worms respond to chemicaL
stimulation from a distance by positive reactions, although/
Darwin believed that they found buried food by "the sense
of smell" (91). Chemical stimuli not in contact with the
body do produce negative reactions (292), but these reactions X
do not differ from the responses to strong mechanical stimu-
lation. They are of various forms — turning aside, with-
drawing into the burrow if the tail is already inserted, squirm-
ing, and so on, the differences being correlated with differences
in the intensity and location of the stimulus and in the excita-
bility (physiological condition) of the animal. But nothing
in the character of the response suggests that negative reaction
to a chemical stimulus has a different conscious accompani-
ment from that of negative response to a mechanical stimulus.
The most natural interpretation of them all on the psychic
side is that of unpleasantness, increasing in intensity as the X

8o The Animal Mind

reaction takes a more violent form.1 The time occupied in
reacting has, however, recently been made a basis for dif-
ferentiating the response to different chemicals. It was found
that if the worms were suspended by threads, and their an-
terior ends dipped into solutions of sodium, ammonium,
lithium, and potassium chlorides, the animals reacted to these
substances with diminishing promptness in the order just
given. The differences in reaction time were marked. Now
all four of these substances produce in man nearly the same
taste quality, salt, for which the common constituent chlorine
is therefore held responsible. The sodium, lithium, ammo-
nium, and potassium ions have apparently but little effect on
the human taste organs. Since the earthworm reacts with
decided time differences to the four, it may be that its taste
organs are specifically affected by each, and that different taste
Y qualities may be occasioned in its consciousness, supposing it
/ to be conscious (314). In other members of the annelid family,
such as the leeches and marine worms, we know little of the
differentiation between food and contact senses. That some of
them respond to odorous substances is stated by Nagel (292).

§ 21. The Chemical Sense in Mollusks

In the case of the Mollusca, also, there is little satisfactory

r evidence on the subject of the chemical sense. The Acephala,

to which the clam, oyster, and scallop belong, do not take

1 W. W. Norman argued that the squirming reactions of worms, and
the corresponding reactions of other animals to injurious stimulation, can-
not be taken as evidence of an accompaniment of disagreeable conscious-
ness, because of the fact that when the worm, for instance, is cut in two,
the squirming movements are confined to the posterior piece, while the head
end crawls away undisturbed. The head end, he urges, containing the
cerebral ganglia, ought to be the part capable of suffering, but it gives no
reaction (295). We cannot, however, conclude from the absence of a reac-
tion under abnormal conditions that its possible conscious accompaniment
in the normal state is also done away with.

Sensory Discrimination: the Chemical Sense 81

food by active movements ; hence, of course, they can have no
specific feeding reactions. Chemical sensibility, distributed
over the surface of the body, has been observed in lamelli-
branchs, a branch of the Acephala (292). Gasteropods, in-
cluding snails and slugs, have, owing to their active food
taking, more use for a chemical sense; in marine snails it
seems rather definitely localized in the feelers (292). Yung
found in the snail Helix pomatia that smell was most acute
at the end of the feelers, but that the animal even when
deprived of its feelers could distinguish perfume. Taste
he found best developed near the lips, and touch sensibility
distributed over the body, but especially toward the end of
the feelers (472, 474).

§ 22. The Chemical Sense in Echinoderms

In the phylum of the echinoderms, under which are classed
starfish and sea-urchins, the " circular symmetry" of body
structure characteristic of the ccelenterates reappears. Star-
fish were found by Romanes many years ago to show, besides
pronounced negative reactions to strong or injurious me- V
chanical stimulation, what he called a sense of smell. Its
manifestations depended on the physiological condition of
the animal ; that is, upon its degree of hunger. If kept several
days without food a starfish would immediately perceive its
presence and crawl toward it. " Moreover, if a small piece of
the food were held in a pair of forceps and gently withdrawn
as the starfish approached it, the animal could be led about the
floor of the tank in any direction." By cutting off various
parts of the rays, Romanes found that "the olfactory sense
was equally distributed throughout their length;" and he
also showed that the ventral and not the dorsal surface of the
body was concerned, by varnishing the latter, which left the
reactions unaffected, and by observing that when a bit of food

82 The Animal Mind

was placed on the back it remained unnoticed (365, pp. 321-
322). Preyer reported great individual differences in the
responses of starfish to food stimulation ; while certain speci-
mens were unmoved by the neighborhood of food, an indi-
vidual of another species came from more than six inches
away and fell upon it (350). Whether the unlikeness of
behavior was due to the species difference or to a difference
in the degree of hunger, does not appear.

§ 23. The Chemical Sense in Crustacea

The highest invertebrate animals belong to the phylum of
the Arthropoda, like the annelid worms in their segmented
structure, but more highly organized in many respects. The
body of a typical arthropod consists of a series of segments,
one behind another, each segment with a pair of appendages.
The higher an arthropod stands in the scale, the more modi-
fication and differentiation of function there is in the seg-
ments and appendages; the former often become consoli-
dated, and the latter become modified for swimming,
walking, or sensory purposes. The lowest grand division of
the Arthropoda is that of the Crustacea.

As the animals of this group are covered with a hard out-
side shell, sensitiveness to touch and chemical stimulation is
ordinarily referred to certain hairs scattered over the body,
and to the modified appendages of the anterior segments which
we commonly know as "feelers," the large and small antenna?.
That mechanical contact stimuli in certain Crustacea give rise
-JLto specialized reactions is evidenced by observations on the
hermit crab. This animal, as is well known, has acquired
the instinct of taking up its abode in empty shells, most com-
monly those of some gasteropod mollusk. When wandering
about in search of a dwelling, the crab's reactions to the ob-
jects it meets show adaptation to the character of the stimulus,

Sensory Discrimination : the Chemical Sense 83

for it will not investigate a glass tube or ball; the smooth
surface seems not to be the adequate stimulus for beginning
the movements involved in exploring and entering a shell (41).
The responses of Crustacea to food stimulation vary, as
might be expected, with different genera and species. Nagel
finds the role of the food sense in aquatic Crustacea very in-
significant; they occasionally show antennal movements in
the presence of food, he says, but are not guided to it (292).
That general restlessness is shown by various Crustacea in the
neighborhood of food, but not in contact with it, has been ob-
served by Bell in the crayfish (22), by Holmes in the amphipod
Amphithoe longimana (180), by Bateson in shrimps and
prawns (n), and by Bethe in the green crab (28). Bethe
arranged a series of aquaria one above the other, with a con-
nection between them, and found that when food was placed
in the uppermost compartment the crabs in the lower ones
were successively excited as the food juices diffused themselves
from each compartment to the one below. In the amphipod,
the small antennae and the mouth parts appeared to be the
regions especially sensitive to food stimulation; if the food
touched one of the former, the animal instantly made a dart
for it. Touching the antennule with a needle very rarely
caused such a reaction (180). Bateson 's shrimps and prawns
had their food sensibility located chiefly in the antennules,
though if food was placed very near them they would show
disturbance even though deprived of antennules (n). This
was the case also with Holmes's amphipod. Bell, on the other
hand, found the whole body of the crayfish sensitive to chemi-
cal stimulation, and no evidence that the small antennae were
especially concerned. The crayfish's reactions to contact with
food were such as to direct the stimulus toward the mouth ;
negative reactions of rubbing, scratching, and pulling at the
affected part were obtained by stimulation with acids, salts,

84 The Animal Mind

and other irritants (22). Evidences of irritation by the
neighborhood of asafoetida were observed also by Graber in
Pagurus (152).

In some Crustacea the sense of smell is possibly concerned
in guiding the male to the female. Certain copepods which
daily migrate from near the surface of the water to greater
depths and back again have had this behavior explained as a
result of the reactions of the females to light, plus the tendency
of the males to follow the females. That the latter is an
affair of chemical stimulation is indicated by the fact that the
females were sought even when concealed hi tubes (304).
In the case of some other Crustacea, however, the sexes do not
seem to be aware of each other's neighborhood until they come
into actual contact (182, 184).

§ 24. The Chemical Sense in Arachnida

The two most important divisions of the phylum Arthro-
poda, besides the Crustacea, are those of the Arachnida and
Insecta. Spiders, as is well known, have highly developed
^ responses to mechanical stimulation ; the web-making species
in particular are sensitive to very slight web vibrations. The
food reactions of spiders have never, so far as the writer knows,
been tested, but various observers report sensitiveness to
chemical stimulations, such as those produced by odorous
oils, not in contact with the body. Spiders of the family
Attidae would react to glass rods dipped in such oils and
brought close behind them, but would not react to clean glass
rods when similarly placed (320). The reactions seem to be
of a negative character (351), and, of course, in all such cases
it remains uncertain whether the possible conscious accom-
paniment is a specifically olfactory unpleasantness or an un-
pleasant irritation of the body surface. Pritchett found that
irritating and non-irritating oils gave negative reactions (351) ;

Sensory Discrimination: the Chemical Sense 85

but an oil that belongs, for us, to the latter class might belong
to the former in the case of a spider. If the sensibility were
sharply localized, that fact would point in the direction of a
specific olfactory sensation ; but while some authorities think
the spider's feelers or palpi are smell organs (25), others
believe that sensibility to chemical stimulation is distributed
over the body (258, 351). Nagel finds no specific organ of
smell and little smell sensibility in spiders (292).

A member of the Arachnida which presents but slight super-
ficial resemblance to the spiders is Limulus, the horseshoe
crab. Limulus shows taste reactions, but no response to
smell stimuli. If the mandibles at the base of the legs be
rubbed with inedible objects, there is no reaction. Similar
negative results are obtained by holding strong-smelling food
close to the mouth or jaws. But if an edible substance be
rubbed on the mandibles, strong chewing movements take
place. Ammonia or acid vapor will produce these same
chewing reflexes, but the claws make snapping movements
"as though to pick away some disagreeable object." If a wad
of blotting paper wet with ammonia or acid be laid on the
mandibles, the chewing movements are reversed and the object
is sometimes picked up by the claws and removed. Patten
found organs which he believed to be gustatory on both the
mandibles and the claws (315). Pearl observed no gustatory
reactions in the free-swimming embryo of Limulus (317).

§ 25. The Chemical Sense in Insects

Throughout all the branches of the animal kingdom thus v
far mentioned, the chemical sense has functioned chiefly as
a food sense. There has been but little evidence jotJhe
development of qualitative discrimination within the sense
itself. Thatis, while in many cases an animal can apparently
distinguish the edible from the inedible, and gives negative

s/

86 The Animal Mind

reactions to irritating chemicals, one would hardly be jus-
tified in saying that it possesses more than one food sensation
quality; while in our own case, of course, though we make
comparatively little use of the sense of smell, the qualitative
discriminations possible by its means are many. But we
come now to a group of animals where there appears a remark-
able development of qualitative variety in the sensations
resulting from chemical stimulation; namely, the Insecta.
As the reactions of animals to mechanical stimulation, on the
other hand, offer evidence of little qualitative difference in
the accompanying sensations, we shall give but slight atten-
tion to them in what follows.

To begin with, there is evidence that taste and smell are
distinct in many insects. The water beetle Dytiscus margina-
lis, found apparently unresponsive to food at a distance, will
bite with especial eagerness at filter paper soaked in what
Nagel calls "a pleasant solution" (292). Ants fed honey
mixed with strychnine will taste it and then stop, and will do
this even when the antennae and mouth palpi are removed,
indicating that the taste organs are in the mouth itself (130).
Similar results have been obtained from similar tests on wasps,
and it has been observed that wasps so treated will hesitate
when offered pure honey afterward (439).

Vitus Graber tested the reactions of various insects to odors
by the method which we called on page 60 the Method of Pref-
erence. This was Graber 's favorite mode of studying the
effect of stimuli upon animals. Applied to olfactory stimuli
it consisted in offering a choice between different compart-
ments, containing each a different odor. The animal's power
of discrimination was argued from the tendency to choose cer-
tain odors rather than others. Such preferences were shown
by the insects (152). The method, however, as was noted
above, is unsatisfactory, because discrimination might exist

Sensory Discrimination : the Chemical Sense 87

where preference did not. Another criticism urged against
Graber's experiments is that the odors used were too strong
and irritating. The insects displayed choice between odors
even when their antennae were removed ; but there is much
evidence to show that the antennae are the true organs of smell
in insects. Various flies and beetles which are in the habit of
laying their eggs in putrefying flesh will not react to it when
their antennae are removed, and it has been shown that insects
which seem to find their mates by response to olfactory stimu-
lation, fail to do so when deprived of antennae (130). Inter-
esting "compensatory movements" have been seen in silk-
worm moths with one antenna removed ; they turned, that is,
in the direction of the remaining antenna (219). We shall
note movements of this class later in insects with one eye
blackened, and in fish with one auditory nerve cut. The
exploring movements of the antennae which certain insects
make in seeking a proper place to lay their eggs in have been
taken as evidence of the smell function of these organs (324).
It may be, then, that the reactions of insects without antennae
observed by Graber were due rather to the irritating than
to the properly olfactory character of the stimuli.

The function of the chemical sense in the mating processes
of insects is one of the most remarkable phenomena connected
with the sensory reactions of animals. Forel says he had a
female Saturnia moth shut up in his city room, and that
within a short time a number of males came and beat against
the window (130). Riley hatched in Chicago some moths
from the Ailanthus silkworm, which were carefully confined.
No other specimens were known to exist within hundreds of
miles. A virgin female was put in a wicker cage on an ailan-
thus tree, and a male, with a silk thread tied around the abdo-
men for identification, was liberated a mile and a half away.
The next morning the two were together (363).

88 The Animal Mind

The most interesting observations on the sense of smell
as used in the mating of insects, however, are those of Fabre.
A cocoon of the "Bombyx du chene" a species of which Fabre
had not seen a specimen in the locality for twenty years, was
brought to him, and from it a female hatched. Sixty males
sought her within a few hours after she reached maturity.
Fabre noticed in this and other cases that shutting the female
in an air-tight box prevented the males from being guided to
her, but that the smallest opening was enough to allow the
odor to escape ; that the males were not in the least confused
or led astray by placing dishes of odorous substances about,
and that they would seek anything on which the female had
rested for a time, a fact which suggests that the stimulus is
a secretion of the body, as it is known to be in silkworm moths.
Fabre offers the suggestion that smell stimuli as they are op-
erative in the animal kingdom generally may be of two classes :

(1) substances which give off particles in vapor or gas, and

(2) substances which give off a form of vibration. Our own
olfactory sense is limited to the first class of stimuli, but some
animals, notably insects, may be sensitive to both (115).
Certainly the marvellous sensitiveness involved in these mating
reactions suggests a kind of response to stimulation unknown
in human experience.

§ 26. How Ants find Food

In many ways the Hymenoptera are the most interesting
of insects, particularly those members of the order which
have developed community life. Their reactions to chemical
stimulation have been the subject of a large mass of literature,
some of the more important results of which we may now
undertake to survey, considering ants, bees, and wasps suc-
cessively. Sir John Lubbock was among the earliest observ-
ers to indicate the great importance of chemical stimuli

Sensory Discrimination : the Chemical Sense 89

in the life of ants. In the first place, he demonstrated that it
is by chemical stimulation that ants are able to follow each tf
other to supplies of food; or to larvae, for an ant's behavior to
an ant larva found in the course of its wandering is like its
behavior to food; the larva is picked up and carried to the
nest. Lubbock put some larvae on a glass plate at a little
distance from one of his artificial ant nests, and set a similar
empty plate beside it; he then made a bridge of a strip of
paper leading from the nest toward the plates, and connected
each of them with this bridge by a separate short paper strip.
He placed a marked ant at the larvae ; she picked up one and
returned to the nest. She soon appeared followed by several
others ; when she had reached the larvae, and before the others
had arrived at the dividing of the ways, Lubbock exchanged
the short strips, so that the one over which the marked ant
had passed now led to the empty plate. The following ants
all took this path, indicating that they were guided by some
trace which her footsteps had left. Lubbock was inclined
to think, however, that some kind of communication must
have passed between the marked ant and her fellows in the
nest to induce them to follow her, and also that this communi-
cation might on occasion convey some notion of the quantity
of food or larvae to be had. He placed three glass plates near
an ant nest, connecting each of them with the nest by means
of a paper strip. On one plate he put a heap of several hun-
dred larvae, on the second two or three only; the third was
empty. He put marked ants on each of the plates, and cap-
tured all the ants which they led back with them. Many
more ants came to the plate with the larger heap of larvae
than to the others. Lubbock explained this by supposing
that the ant from that dish had in some way communicated
to the nest the greater numbers at her disposal (248, pp.
172 ff.). Obviously it would be enough to suppose that the

QO The Animal Mind

smell of food or larvae about an ant returning laden to the
nest is a stimulus to her nestmates to follow her; that this
smell is stronger, the larger the stock she has found, and
hence acts as a more powerful stimulus.

§ 27. How Ants find the Way Home

Lubbock's experiments indicated also that in finding their
way back to the nest ants make more use of smell than of sight.
One only of these observations need be described. Lub-
bock placed larvae in a dish on a table connected by a bridge
with an ant nest. He accustomed the ants to go back and
forth from the dish to the nest along a path which he diversi-
fied by artificial scenery, such as rows of bricks along either
side, and a paper tunnel. When the- path was thoroughly
learned he moved the bricks and the tunnel so that they led
in a different direction; the ants, however, were not at all
disconcerted by this cataclysm of nature, but followed the
same track as before, evidently guided by their own footprints
(248, p. 259). The direction of the light is not without some
influence, however. When two candles that had stood near
the nest were moved to the opposite side, some of the ants
were confused (248, p. 268). Bethe, whose object is to show
that all ant behavior is a series of unconscious reflexes to
chemical stimuli, made the following attempt to study the
formation of a new ant path. He placed near the entrance
to a nest a large sheet of paper covered with lampblack, on
which the footsteps of the ants could be traced. On this paper
he placed a supply of food. When an ant had found the food,
in going back to the nest she always followed the path by
which she had come, except that when the original path had
crossed itself in loops, the ant omitted the loops in returning.
Other ants followed the same path, though they all had a ten-
dency to cut off curves (30). Wasmann, the ardent opponent

Sensory Discrimination : the Chemical Sense 91

of Bethe's reflex theory, looks with suspicion on this tendency
gradually to straighten the path, and thinks an animal re-
flexly drawn along by a chemical stimulus on the ground
would make no improvements in the route (426). The evi-
dence that it is the sense of smell, if not a smell reflex, that
guides the ants remains, however, strong. Bethe further
points out that the chemical stimulus deposited by the feet
of the ants is volatile. If a strip of paper be placed across
an ant path, the ants on coming to it stop, quest about, and
are delayed until one accidentally runs across the strip and
others follow. (Why, asks Wasmann, if they are being re-
flexly drawn along, do they not merely stop short when the
stimulus fails, instead of hunting for it ?) The piece of paper
is thus gradually adopted into the ant road; if it is subse-
quently removed, the ants stop and are bewildered at the
place where it was, showing that the earlier traces of their
footsteps, under the paper, have evaporated. Again, Bethe
thinks he has evidence that the chemical stimulus left by the
feet of ants going from the nest is different from that deposited
by those going to the nest, and that ants on the way. home
will not follow a track made by the feet of other ants on the
outward journey, and vice versa (30). That they will follow
their own individual track in either direction is shown by
the smoked paper experiment just described, and also by an
experiment of Fielde's, where ants finding their way through
a labyrinth from a heap of pupae to the nest and back
again followed each one its own trail without regard to the
others (218). Bethe found that when the usual road to an
ant nest had been interrupted by the removal of a heap of
sand, and the road across the breach had been established
solely by incoming ants, the outgoing ants refused to follow
it, and made a new road for themselves (30). Wasmann
thinks this may have been done merely on account of the

92 The Animal Mind

faintness of the recently established path as compared with
the old one (426). Bethe observed also that if a strip of paper
had been adopted into an ant road, and was then while an
ant was on it rotated through 180 degrees, the ant stopped
and was disturbed on coming to the end of it (30). Experi-
ments on rotating ants were made also by Lubbock (248),
and seem to give puzzling and conflicting results; it is not
clear why even on the assumption that there is a difference
in odor between the road to the nest and that from the nest,
an ant on a road which led both ways should have found her
course interrupted by rotation. One fact, Bethe thinks,
shows that even assuming two road smells is not enough.
Ants of certain families (Lasius) which habitually make regu-
lar and frequented roads, can if they come upon one of these
roads in wandering at once take the proper direction, either
to or from the nest. Evidently the mere presence of two
smells would not enable them to do this. Bethe suggests
that the particles of the two chemical substances are also
differently polarized, so that one of them can be followed only
in one direction, the other in the opposite direction (30).
Wasmann objects to this that an ant returning on its own traces
would destroy them, as the opposite polarizations would can-
cel; and that similar confusion would occur on a narrow
and much frequented road (426). He and Forel (132) both
think that, granting the distinction between the outward
and inward paths, which is made by only a few families of
ants, the direction is most probably given by a perception of
the "smell form" of the footsteps obtained through the
antennae.

Recently C. H. Turner has come to the conclusion that ants
are not guided " slavishly" or reflexly by the odor of their
tracks in finding the way to and from the nest. He made
a small cardboard stage from which an inclined cardboard

Sensory Discrimination: the Chemical Sense 93

bridge led down to the artificial ant nest. Ants and pupae
were placed on the stage. After the ants had through
random movements learned the way down the incline
a second incline was placed so as to lead from the opposite
side of the stage to the nest. No ants went down this way.
The inclines were then exchanged so that the one bearing the
scent of the ants' footprints was on the opposite side and the
unscented incline in the old place ; the ants continued to go
down in the old place. It is unsafe to criticise an experiment
without having actually seen it, but Turner's account does not
exclude the possibility that the ants were guided in setting
out on their homeward course by the scent of their footprints
on the cardboard stage, which seems to have remained un-
changed. He confirms Bethe's observation that the path-
ways to and from the nest are different, but does not find
that even a single ant follows her own footsteps in both di-
rections. The direction of the light, not smell, is the ruling
factor in pathfinding, according to Turner, who offers the
following experimental evidence. When the stage with the
first incline was arranged as before and a 16 c.p. lamp
placed near the side to which the incline was attached, the ants
learned to go down the pathway to the nest. When the in-
cline on the opposite side was added, and, after waiting a time
to make sure that no ants went down that way, the lamp
was moved to the other side, marked disturbance was shown
by the ants. "In most cases, some would finally go down
the new incline; in a few cases after the lapse of several
minutes all went down the new incline." Altering the in-
tensity of the light had no effect ; the disturbance was caused
by any decided change in its direction (408).

In all probability, different species of ants vary in the
degree to which they make use of smell as a guide. Pieron,
for example, finds that Formica cinerea depends more upon

94 The Animal Mind

vision than Lasius fuliginosus, which is guided largely by
smell (325), and the same conclusion is reached by Was-
mann (426). Pieron calls attention to still another factor
which he calls muscular memory, influential in all the species
he tested, but especially so in Aph&nogaster barbara nigra.
If an ant of this species is returning to the nest and steps on
a bit of paper covered with earth, she may be moved bodily
to some distance without seeming to notice the fact. In such
a case, on being set down, she continues her march "in the
new region, following a direction absolutely identical with that
which she was following in her return to the nest, and she does
not stop or seem disturbed until after a more or less prolonged
march, often about equal to the distance that separated her,
at the moment when she was displaced, from the entrance to
the nest." Even if her displacement occurs near the entrance
to the nest, she will go on past it and stop at a distance about
equal to that which she would ordinarily have had to traverse.
Evidently smell is not here concerned (325). The ant would
seem to be like a little machine wound up to go just so far
and to take just such turnings. We shall mention this
observation again in a later chapter.

§ 28. How Ants "recognize" Nest Mates

Another problem of ant life to which smell appears to
furnish the key is that of the recognition of nest mates. It has
long been known that an ant entering a strange nest, though
of the same species, is likely to meet with rough treatment,
and even be put to death. Now Forel found in 1886 that
ants of the genus Myrmica whose antennae were removed
would attack their own nest mates (130). It seems probable
that each nest of ants has a peculiar odor which is the basis
of the distinction between friends and foes. Bethe tested the
smell theory by dipping an ant first in weak alcohol, then

Sensory Discrimination : the Chemical Sense 95

in water, and then in the juices obtained by crushing the
bodies of a number of ants of another species. He found
that an ant thus treated would be attacked and killed by
its own nest mates, but could be introduced, though not
so easily, into the nest whose odor it now presumably bore,
even though its appearance was quite different from that
of the ants therein (30). Wasmann repeated these experi-
ments with much less success than Bethe ; bathing Myrmica
ants with essence of Tetramorium ant did not preserve
them from final destruction at the jaws of the latter,
though it delayed their fate; nor did much bathing with
foreign nest odors induce their nest mates to attack ants
of the species Lomechusa strumosa, though they seemed
disturbed at first. Wasmann apparently thinks other fac-
tors besides smell, vision perhaps, enter into the recogni-
tion process (426). Bethe, in a later paper, suggests that
Wasmann's negative results may have been due to the fact
that the nest smell very quickly returns to the ants after
it has been removed ; he himself took account only of the
first reaction of other ants toward the one subjected to
treatment (31).

Fielde, as the result of a study of the genus Stenamma,
concludes that each ant is the bearer of three distinct odors :
the individual odor, which enables her to follow her own
trail in a labyrinth, and the reception of which depends upon
the tenth segment of the antennae ; the race odor, depend-
ent on the eleventh segment; and the nest odor, depend-
ent on the twelfth (118). In a later article she concludes
that the nest odor of the worker ants is derived from their
queen mother; that the odor of the queen is unchanging,
and is imparted to her eggs. The worker, however, gradually
changes its odor. Queens of diverse odors may be produced
by the influence of males that are the offspring of worker

96 The Animal Mind

mothers and have the differentiated worker odor. A young
ant isolated from the pupa stage until many days old will
single out its queen mother from queens of other species,
but will show decided suspicion of older sister worker ants.
A mixed nest formed of newly hatched ants of different
species was separated for seven months. On rejoining each
other, the ants showed hostility ; their odor, Fielde argues,
had changed. But young ants of one species were received
by those of the other species. Fielde does not hesitate to
introduce the psychic factor and say that the latter remem-
bered the odor of the young ones, having been associated with
it in their own youth. The suggestion might be made that
the young ants had not as yet developed any specific odor,
but this is opposed by the observation that newly hatched
Lasius ants from a strange colony were not received by a nest
of Stenammas, while young Lasius ants from a colony with
which the Stenammas had been acquainted in youth were
accepted eleven months after the latter had been segregated.
It is an affair of the memory, Fielde is assured ; and she says,
"If an ant's experience be narrow, it will quarrel with many,
while acquaintance with a great number of ant odors will
cause it to live peaceably with ants of diverse lineage, pro-
vided the odors characterizing such lineage and age environ
it at its hatching" (123). Bethe held that an ant's own nest
odor offered no stimulus to it at all, but that fighting reflexes
were occasioned by any foreign nest odor (30). Many facts,
however, seem to tell against this view; among others, the
early observation of Forel that a Myrmica ant deprived of
its antennae attacks everything in sight (130). It should,
according to Bethe's theory, live peaceably with all.

Thus we see that in spite of some divergence of testimony,
there is evidence that ants have a variety of qualitatively
different smell experiences : the smell of food and of larvae,

Sensory Discrimination : the Chemical Sense 97

probably distinct, though there is no experimental proof of
the fact; the individual smell of an ant's own footsteps;
a possible distinction, in some species, between the smell of
the outgoing and that of the incoming paths ; and the different
odors which seem to be responsible for the discrimination be-
tween nest mates and foreigners. If it is true, as Fielde
maintains, that loss of the eighth and ninth segments of the
antennae renders an ant incapable of caring for the young,
then the recognition of larvae and pupae does depend upon a
specific odor (118). Forel makes an interesting distinction
between the sense of smell in insects with immovable anten-
nae and the same sense where the antennae, as in ants, can
be moved about over objects. In the former case it is as
with us a qualitative sense pure and simple, giving informa- *.,
tion of objects at a distance ; in the latter case it is a contact
sense, and may give rise to spatial as well as qualitative per-
ceptions. He compares the antennae to a pair of olfactory
hands, and points out how such organs may allow of the per-
ception of the " smell form" of objects (132).

§ 29. How Bees are attracted to Flowers

In bees the sense of smell is equally well developed. But
no topic in comparative psychology has been more hotly .
disputed than the use which bees make of this sense, and the *
extent to which they depend, rather, upon sight. Darwin
(90) and H. Muller (284, 285) thought both color and
fragrance influential in attracting insects. Plateau main-
tains that the chief influence attracting bees to flowers is
smell, and that color has little effect. He made a number of
experiments in which the brightly colored corollas of flowers
were cut off without disturbing the nectaries, and claims to
have found that the visits of bees to the mutilated flowers
were as frequent as before (336-338, 339, 341). On the other

98 The Animal Mind

hand, Giltay obtained opposite results; the flowers whose
corollas were removed were neglected by bees, while those
which were covered so as to be invisible, but not so as to
prevent the odor from escaping, were also unnoticed (144).
Josephine We*ry found that the proportion of bees visiting
flowers with intact corollas to those visiting flowers with the
corollas removed was 66 : 18 (434). Kienitz-Gerloff criticises
Plateau's figures and the accuracy of his experiments (220).
Forel found that a bee with the antennas and all the mouth
parts removed, hence probably incapable of smell, returned
to flowers for honey, though of course without success (130).
Andreae thinks that among diurnal insects those which live
on the ground, and take but short flights, are more influenced
by smell ; while the freely flying insects are attracted by the
sight of flowers (5).

§ 30. How Bees find the Hive

Most complicated of all is the problem as to how bees find
their way back to the hive. It is obvious that the simple ant
method of following a chemical trail is ruled out for in-
sects that fly. Bethe abandons the puzzle as insoluble (30).

Von Buttel-Reepen attempts at length, and with a vast
amount of apic lore, to refute his position. It would be im-
possible to give more than the briefest statement of the argu-
ments of both sides. Bethe maintains that the smell of the
hive does not guide the bees back to it, because he found
that if the hive were rotated slowly enough to allow the cloud
of nest smell at the opening to move with the opening, the bees
returning would not follow it for more than 45°, but would
go to the place where the opening had been. He thinks
they are not guided by sight, because when he completely
changed the appearance of the hive, masking it with branches,
and other coverings, the bees were not disconcerted, but flew

Sensory Discrimination : the Chemical Sense 99

straight to the mouth of the hive. He brings other evidence
against the vision hypothesis which we shall discuss in
Chapter XI. An unknown force, he concludes, guides
the bee in its homing flight (30). Von Buttel-Reepen
believes that visual memory will explain all the facts;
that the bees were not disturbed by the altered appearance
of their hive because they knew their way so thoroughly
that nothing could disturb them by the time they had
come so nearly home. The visual memory required is, he
admits, of a peculiar sort, which we shall consider in a later
chapter. The odor of the hive does cooperate with vision in
certain cases ; when a stock of bees has been moved without
their knowledge, they fly out without making any " orienting
flight," as they commonly do on leaving a new place, a fact
that is one of the evidences for the visual memory theory.
Nevertheless, many of them succeed in finding their way back,
and then, if their hive is placed among a number of others,
von Buttel-Reepen thinks they " smell" their way back to
the right one. He mocks at Bethe's unknown force, on the
ground that it must sometimes lead the bee to the hive and
sometimes back to the place where food has been found (72).
Bethe attempts to answer this by saying that the force acts
in cooperation with the physiological condition of the animal ;
the laden bee follows it to the hive, the bee with the empty
crop is led back to the food supply (32). Of course one may
say what one pleases about the modus operandi of an unknown
force without fear of disproof, but also without carrying much
conviction.

§ .31. How Bees "recognize" Nest Mates

The nest smell, which characterizes each hive and prevents
the reception of strangers, who are treated precisely after the
fashion of ants in similar circumstances, is composed accord-

ioo The Animal Mind

ing to von Buttel-Reeperi of the following odors : the indi-
vidual odor of different workers ; the family odor, common
to all the offspring of the same queen; the larval smell and
food smell; the drone smell, the wax smell, and the honey
smell. There are various ways in which the mode of reaction to
a foreign nest smell is modified. If two bee stocks are placed
side by side, and one has the queen and entire brood removed,
it will go over to the other stock and be kindly received. One
can understand that the attraction of the queen and brood odor
may overcome the tendency of the foreign nest smell to repel
the invaders, but it is harder to see why the more fortunate
stock should allow itself to be invaded. Further, a bee laden
with honey can get itself received by a foreign stock that has
exchanged hives with it, where an unladen bee is attacked;
here the smell of the honey may overcome the foreign smell.
As is well known, two alien stocks may be united by sprin-
kling them with some odorous substance. The queen odor
is the strongest factor in the nest smell ; in swarming it over-
comes the tendency to return to the old nest, and queen-
less swarms will join themselves to foreign swarms having a
queen. The apparent attention paid to the queen while
laying eggs, the gathering of workers around her trilling their
antennae toward her, suggest strongly that her odor is
pleasant to them. The queen herself, however, is perfectly
indifferent to any foreign nest smell, and will beg food of any
bee, even those which are angrily crowded around her cage
in a foreign hive. Drones also will go from stock to stock,
and are always peacefully received until drone-killing time
begins. It has usually been supposed that the unrest dis-
played by a bee stock when deprived of its queen is due to the
absence of the queen odor, and it seems almost certain that
this must be a powerful influence, though von Buttel-Reepen
thinks it is not the only influence, for he has observed that if

Sensory Discrimination : the Chemical Sense 101

the queen be replaced in the honey space, removed from the
rest of the hive, the bees will quiet instantly, before the smell
has had time to diffuse itself. Also, bees sometimes behave
as if they had lost their queen when she is only put in a cage,
and her odor is perfectly accessible (72).

It is clear that bees as well as ants are capable of dis- >*/
tinguishing a considerable number of smell qualities. Prob-
ably the same thing is true of the social wasps. In the
solitary wasps, however, we find less evidence of a highly
developed sense of smell, or rather of a great variety of smell
reactions, and the solitary bees are very likely less influenced
by smell than the social bees. In the interesting study of the
solitary wasps by Mr. and Mrs. Peckham, it appears that
sight plays a far more important role than smell for these
insects and the return to the nest in particular seems to be
almost entirely an affair of sight (322, 323). In general
the greatest development of qualitative variety in the sense of
smell is found in the social Hymenoptera, and is probably
a product of the social state. Perris, however, noted that
the solitary wasp Dinetus was much disturbed in finding its
nest hole if he had placed his hand over the hole during
the wasp's absence, and thought the odor of his hand was
distracting to the insect (324).

§ 32. The Chemical Sense in Vertebrates

Although the vertebrates stand at the head of the animal
kingdom, yet in point of complexity of structure and behav-
ior the lowest vertebrate is far below the highest members p
of the invertebrate division. When we undertake to study
the responses to special stimulation displayed by this
same lowest vertebrate, the little Amphioxus or lancelet,
it is like going back to the earthworm. The only kind of
evidence that contact, chemical, and temperature stimuli

IO2 The Animal Mind

produce specific sensation qualities is found in the fact that
^sensibility to them is differently localized, and may be in-
dependently fatigued. To weak acid, the head end of the
animal is most sensitive, the posterior end less, the middle
least ; to contact with a camel's-hair brush, the two ends are
equally sensitive and more so than the middle; to a current
of warm water the order of sensitiveness is head end, mid-
dle, posterior end (311).

For fishes, as for all aquatic animals, the distinction be-
V- tween smell and taste becomes obscure. The neighborhood
of food not in actual contact with the body seems to stir fish
to activity, but not to direct their movements. Bateson
(12) and Herrick (165) both obtained evidence of this; Nagel,
on the other hand, declares that fish do not perceive food at a
distance except by sight, and that the function of the first pair
of cranial nerves in these animals must remain uncertain (292).
The well-developed character of these " olfactory" nerves
and lobes, whose function in higher vertebrates is certainly
connected with smell, would argue against the supposition that
smell can be wholly lacking in fishes. It is generally agreed
that a contact food sense exists in fish; Nagel, however,
holds that its organs are situated only about the mouth (292),
while Herrick has good experimental proof that fishes which
have "terminal buds," structures resembling taste buds,
distributed over the skin, are also sensitive to food stimulation
applied to different regions of the skin. He thinks that Nagel's
negative results were due to the fact that instead of food stimuli
in his experiments he used chemicals with which the fish
would not normally be acquainted (165). Nagel thinks the
role of the chemical sense in Amphibia also is negligible (292),
and there is no experimental evidence, to the writer's knowl-
edge, indicating specific taste or smell sensations either in Am-
phibia or in reptiles. In birds the high development of both

Sensory Discrimination: the Chemical Sense 103

sight and hearing, and the fact that almost all reactions are
made in response to the stimuli for these senses, masks the ^
presence of olfactory sensitiveness if it exists. Taste, birds
seem to have ; the chicks experimented on by Lloyd Morgan,
for example, displayed disgust at picking up bits of orange
peel instead of yolk of egg (281, pp. 40-41). Xavier Raspail
is, so far as the writer knows, the only observer who has ex-
pressed a definite opinion in favor of the sense of smell in
birds. He thinks they abandon eggs that have been handled
because they detect the fact by smell ; that they find buried
grubs by smell, and are guided by this sense to concealed food
and water. The last statement he supports by the observa-
tion that their tracks lead straight to hidden food on their
first visit to it, showing that it was not found by accident

(359)-

When we come to the Mammalia, however, we find in the
great majority of types a very high development of qualitative V"
discrimination in the sense of smell. Hunters know it to
be the chief defensive weapon of wild animals, and it has
retained great keenness in many domesticated ones, — the
cat, for instance, which will be awakened from slumber in
the garret by the odor, quite unsuspected of human nostrils,
of some favorite food being prepared in the kitchen, and is
thrown into ecstasy at a faint whiff of catnip. The dog, how-
ever, is the hero of this field of mental prowess. The ex-
periments of Romanes on the power of a favorite setter to
track his scent are well known. In one of them he collected
a number of men, and told them to walk in Indian file,
"each man taking care to place his feet in the footprints of his
predecessor. In this procession, numbering twelve in all,"
Romanes says, "I took the lead, while the gamekeeper
brought up the rear. When we had walked two hundred
yards, I turned to the right, followed by five of the men ; and

IO4 The Animal Mind

at the point where I had turned to the right, the seventh man
turned to the left, followed by all the remainder. The two
parties . . . having walked in opposite directions for a con-
siderable distance, concealed themselves, and the bitch was
put upon the common track of the whole party before the
point of divergence. Following this common track with
rapidity, she at first overshot the point of divergence, but
quickly recovering it, without any hesitation chose the
track which turned to the right." It had previously been
ascertained that she would not follow the scent of any
other man in the party save her master, and failing him,
the gamekeeper. "Yet ... my footprints," continues Ro-
manes, "in the common track were overlaid by eleven
others, and in the track to the right by five others. More-
over, as it was the gamekeeper who brought up the rear,
and as in the absence of my trail she would always follow
his, the fact of his scent being, so to speak, uppermost in
the series, was shown in no way to disconcert the animal
following another familiar scent lowermost in the series"
(367). Such behavior indicates not only that the dog can
experience a variety of smell qualities, which is also the case
with us human beings, but that it has the power to analyze
a fusion of different odors and attend exclusively to one
component, a power that we lack almost entirely. When
we experience two smell stimuli at the same time, it is but
rarely that we can detect both of the two qualities in the
mixture; usually one of them swamps the other, or else a
new odor unlike both results. But the dog, and probably
many other animals, can analyze a smell fusion as a trained
musician analyzes a chord. In this respect, if not in the
variety of smell qualities, the olfactory sense has undergone
ti degeneration in us, and so far as we can judge, the fact
' is due to the habit of relying rather upon the sense of sight.

Sensory Discrimination: the Chemical Sense 105

Even in the case of the monkey, Kinnaman reports that the
animals he was testing with regard to their power of dis-
criminating the size, shape, and color of vessels in one of
which food was placed, always looked, never smelled, for
the food (221).

CHAPTER VI

»
SENSORY DISCRIMINATION: HEARING

§ 33. Hearing in Lower Invertebrates

THE sense of hearing, in all air-dwelling animals, is that
sense whose adequate stimulus consists in air vibrations;
for human beings these vibrations may reach a frequency of
50,000 (single vibrations) in one second and still produce an
auditory sensation. But the meaning of the term " hearing "
for water-dwelling animals, and hence for most of the lowest
forms of animal life, is more difficult to determine. Tn the
Protozoa it seems to have no meaning at all ; the reactions of
these animals to water vibrations are indistinguishable from
their reactions to mechanical stimulation. But in some of
the ccelenterates the possibility of a specific auditory sensa-
tion quality has been suggested by the discovery of a pecul-
iar sense organ. While varying in its structure in different
genera and orders of ccelenterate animals, this organ con-
sists typically of a small sac, filled with fluid and containing
one or more mineral bodies. Apparently these latter could
operate in connection with a stimulus only when the stimulus
was constituted by shaking the animal, or in some way dis-
turbing its equilibrium. They might then serve as means for
the reception of water vibrations, as the ear serves for the
reception of air vibrations ; they might, in short, be primitive
organs of hearing. Accordingly the term " otocysts " was
given to organs of this type wherever they were found in the
animal kingdom, and the mineral bodies in the otocysts were
called otoliths.

1 06

Sensory Discrimination : Hearing 107

But experiments upon coelenterates have entirely failed to
show that animals of this class react to sounds (in, 415,291).
And in some coelenterates, as well as in higher animals having
the same type of organ, the removal of the so-called otocysts
has been found to involve disturbance of the animal's power
to keep its balance and maintain a normal position. Hence
Verworn has suggested that for "otocyst" and "otolith"
the terms "statocyst" and "statolith" might appropriately
be substituted (415). In jellyfish, indeed, even the balancing
function of the statocyst organs appears doubtful; and it is
possible that they function in response to shaking and jar-
ring (286, 291). In any case, there is no evidence whatever
of a specific auditory sensation in the consciousness, if such
exists, of ccelenterate animals.

Nor has any reaction to sound been demonstrated in either
the flatworms or the annelid worms; their sensitiveness to
vibrations seems to be an affair of mechanical stimulation.
Darwin's experiments on this point are well known. The
earthworms which he observed were quite insensitive to
musical tones, but when the flower pots containing their
burrows were placed on a piano, the worms retreated hastily
as soon as a note was struck (91). Most observers agree that
mollusks also react only to mechanical jars (e.g., 101), and
that the statocyst organs found in some mollusks have
no auditory function. Bateson, however, records that a cer-
tain lamellibranch, suspended by a thread in a tank, re-
sponded by shutting its shell when a sound was produced by
rubbing a finger along the glass side of the tank (12), and
Bethe demands to know of what possible use as static organs
the statocysts in fixed mollusks can be (27). The echino-
derms are apparently insensitive to auditory stimuli (350,

365).

io8 The Animal Mind

§ 34. Hearing in Crustacea

In the Crustacea the function of the statocyst organs has
been the subject of much dispute. They are in this group of
animals sometimes closed sacs with statoliths, sometimes open
sacs containing grains of sand. Most commonly the organs
are situated in the basal segment of the small antennae.
There is usually inside the sac a projection bearing several
ridges of hairs, graded in size, which tempt to the hypothesis
that they respond to vibrations of different wave lengths, as
the fibres of the basilar membrane of the human cochlea are
supposed by the Helmholz theory to do. Hensen, indeed,
placing under the microscope the tail of a small shrimp,
Mysis, whose statocyst is situated in that region, observed
that the long hairs of the tail vibrated in response to musical
tones, from which he infers that the statocyst hairs may do so *
(163). In 1899 he was still inclined to believe that the latter
can serve no other than an auditory function (164). Never-
theless the weight of authority is in favor of regarding the
"sac" in Crustacea as a static rather than an auditory organ.
The only evidence of sound reaction in two shrimp-like
forms, Palaemon and Palaemonetes, was a " flight reflex"
given by some individuals when sounds were produced very
near them in the water; and although this response ceased
when the statocysts were destroyed, the fact is of little sig-
nificance, as other reflexes also were abolished by the opera-
tion (19). To sounds made by tapping the wall of the aqua-
rium Palasmonetes reacted by leaping away from the wall
nearest to it, even though the leap was made toward the sound.
When both statocysts were removed, the reactions were still
made, but not so markedly nor at so great a distance from the

*

1 This observation is sometimes incorrectly quoted as if the hairs concerned
were actually the statocyst hairs. Cf., for example, Morgan, 279, p. 266.

Sensory Discrimination : Hearing 109

sound. A similar response to the striking of a partially
submerged glass jar was seen in a decapod, Virbius zos ten-
cola, which has no statoliths (349). Mysis has been found
to react to sounds when the statocysts are destroyed (27).
The fiddler crab, which is amphibious, responds in water to
vibrations by retreating slowly from the vibrating walls, and
does the same when blinded and deprived of its statocysts,
but gives no reaction when the antennae and antennules are
removed. On land these animals do not respond to sounds,
only to vibrations produced in the earth, for instance by
stamping (349). No sound reactions have been found in
the crayfish (21). In short, such responses to vibrations as
occur among the Crustacea seem affairs rather of mechanical
than of true auditory stimulation; nevertheless Bethe (27)
and Hensen (164) are both inclined to believe, as did Delage,
who first called attention to the static function of the statocysts
(97), that they may be auditory organs also. The "static
sense" of Crustacea will be discussed later.

§ 35. Hearing in Spiders

In spiders the same difficulty arises, of deciding whether
the reactions to sound are tactile or auditory. There are no
statocysts, but the delicate hairs on the body and legs of the
animal have been held to be auditory organs. Dahl, a
number of years ago, found them responding to the tones of a
violin (86, 87), but this test, which Hensen applied to Mysis,
is of very doubtful significance ; as Prentiss suggests, the hairs
on the back of the human hand do the same (349). When
various species of spiders were tested by holding tuning forks
near them or their webs, only the web-making species gave any
response. These latter would not react to ordinary noises,
nor to the sound of a small fork, but to the humming of a
large fork they responded always by raising the front legs,

1 10 The Animal Mind

and sometimes by dropping from their webs (320). Two
Texan species that were experimented upon by placing them
in a cage free from vibration gave no response whatever to
tuning forks of various pitches or to other sounds (351).
It seems, then, highly probable that spiders are sensitive
only to vibrations communicated to their webs, and very
likely these furnish tactile rather than specific auditory stimu-
lation. The observation of Boys may be quoted: "On
sounding an A fork, and lightly touching with it any leaf or
other support of the web or any portion of the web itself,
I found that the spider, if at the centre of the web, rapidly
slews around so as to face the direction of the fork, feeling
with its fore feet along which radial thread the vibration
travels. Having become satisfied on this point, it next darts
along that thread till it reaches either the fork itself or a junc-
tion of two or more threads, the right one of which it instantly
determines as before. If the fork is not removed when the
spider has arrived it seems to have the same charm as any fly,
for the spider seizes it, embraces it, and runs about on the
legs of the fork as often as it is made to sound, never seeming
to learn by experience that other things may buzz besides its
natural food. If the spider is not at the centre of the web
at the time that the fork is applied, it cannot tell which way
to go until it has been to the centre to ascertain which radial
thread is vibrating." If, however, it has followed the fork
to the edge of the web, and the fork is then withdrawn and
brought near again, the spider reaches out in its direction.
If the spider is at the centre of the web and a sounding fork is
brought near without touching the web, the spider does not
reach for it, but drops down at the end of a thread. If the
fork touches the web again, the spider climbs the thread and
finds the spot very quickly (69).

Sensory Discrimination : Hearing 1 1 1

§ 36. Hearing in Insects

The sense of hearing in insects also is problematical.
When the insect makes a sound itself, which, as in the case of
crickets, is connected with the mating process, it would seem
a priori highly probable that it can hear. Various structures
have been designated as auditory organs, the finely branched
antennae of mosquitos and gnats, on the same doubtful
evidence that they have been found to vibrate in response to
musical tones (264) ; and in the Orthoptera certain very
peculiar structures situated on the front legs of grasshoppers
and crickets, and in the first segment of the abdomen in
locusts. These structures Graber called chordotonal organs,
and he felt convinced from experimental tests that they
were auditory. The cockroach, Blatta, while running about
the room will stop, he says, for an instant when the strings of
a violin are struck. A blinded specimen, hung by a thread,
became violently agitated at a sudden tone from a violin.
A water insect, Corixa, although undisturbed by the water
vibrations produced by pushing a bone disk toward it in the
water, gave decided reactions when the disk was connected
with an electric bell. Other water beetles were still more
sensitive. That they distinguished pitch differences Graber
thought probable from the fact that he observed reactions
of different degrees of violence to sounds of different pitch;
and their discrimination of intensity changes he thought
demonstrated by the fact that if a continuous tone, sounding
while a water beetle is swimming about, be made suddenly
louder, the speed of the insect's movements visibly increases.
It is going rather far, however, to pass from the evidence that
insects discriminate sounds made by their own species from
other sounds to the conclusion that "they like us have the
capacity to analyze, at least to a certain degree, these

H2 The Animal Mind

peculiar clangs or noises, and to distinguish clearly from one
another the partial tones that compose them" (149).
iTower thought that he had observed the potato foppfle,
reacting to the sound of^a^U£ingjork^f404). Will noted
responses from a male beetle to the strioTulation of a female
of its species enclosed in a box 15 cm. away (439). Radl
has recently made the suggestion that the organs which
Graber called chordotonal organs, and which contain a fibre
stretched between two points of the integument, represent
a kind of transition between " Gemeingefuhl" and hearing.
In support he offers the following evidence: the fibres re-
semble the tendons in which some muscles end, and are very
likely developed from tendons; the organs exist in insects
that have no use for hearing, such as grubs shut up in fruits;
insects have not been shown to respond to pure tones, but only
to noises such as the cricket's chirping, which for us affect
Gemeingefuhl. Further, there is no evidence that hearing
ever guides insects to each other; in short, it is but a rudi-
mentary sense, and its organs are those which serve also to
register muscular activity. It is, in insects, a " refined
muscular sense" (357).

The auditory sense, if it exists in insects, is very likely
confined to those which produce sounds, and its qualities
limited within the range of such sounds. Most species of
ants, for instance, produce no sound that the human ear,
even with the aid of a microphone 248), can detect, although
certain East Indian species are reported to make a loud hiss-
ing noise when disturbed (424), and some American species
are said to chirp (108, 437). Ch. Janet maintains that ants of
the Myrmicidae make a stridulating noise (190, 191). The
weight of evidence is also against the existence of sound
reactions in ants ; careful experiments by Fielde and Parker
on a number of species led to the conclusion that the only

Sensory Discrimination : Hearing 113

vibrations responded to were those which were communicated
through the solid on which the ants stood, and received
through the legs (125). It is probable that the observers
who have come to opposite conclusions have not in every
case been careful to exclude the possibility of such vibration
of the substratum. Wasmann, for instance, thinks he has
seen reactions to sound; he noted that ants in an artificial
nest raised their antennae and lifted the fore part of their
bodies when he scratched with a needle on some sealing wax
with which the nest had been mended (423). He also quotes
Forel's account (129) of a species which makes an " alarm
signal" by striking the ground with its abdomen: this,
remarks Wasmann naively, must be perceived by the ants,
" otherwise it would not be an alarm signal"! (424). If
perceived, it may of course be as a tactile rather than an
auditory sensation. Weld has observed reactions to the
sound of whistles and tuning forks in several species of ants,
and even concludes that they perceive the direction from
which sounds come ; but since, of the four observations upon
which this latter opinion is based, two were cases where the
ants hurried toward the sound and the others cases where
they backed away from it, the possibility of mere coinci-
dence seems not to be excluded (433).

As regards the auditory sense in bees, there is again a
difference of opinion. They do, of course, make sounds,
and sounds of different quality, under different conditions.
Yet Lubbock entirely failed to get bees to respond to any kind
of sounds artificially produced (248), while Bethe urges that
the sounds produced by bees are involuntary, like the sounds
of our own breathing and heart-beats, and that there is no
more evidence that bees can hear them than that we can hear
these sounds in our own case (32). Forel is positive that
insects in general cannot hear (130). Von Buttel-Reepen,

H4 The Animal Mind

on the other hand, who knows bees thoroughly, thinks that
the sense of hearing plays a considerable part in their life.
He believes that the disturbance produced by the loss of a
queen is communicated to the whole hive by the peculiar
wailing noise made by some members and instinctively
imitated by the others, and that this disturbance is calmed
by a similar dissemination of the " happy humming" pro-
duced on her restoration — hearing playing a more important
part than smell. The starting of a swarm, he thinks, is also
largely a matter of sound communication. The process be-
gins by the coming out of certain bees which push in among
the bees hanging at the entrance of the hive and stir them up
to swarming by making sounds. The "swarm-tone" is
peculiar and often disturbs the inhabitants of neighboring
hives that are not ready to swarm. Also, a swarm can be
guided to a new dwelling if a few bees are taken there ; they
call the others by loud humming. If during this process the
new hive is moved, the bees will go on for a few moments in
the direction in which they started, then slowly turn, guided
by the tone. A few may keep on in the original direction.
We may look with suspicion, however, upon von Buttel-
Reepen's suggestion that these latter, having passed beyond
hearing of the call, are guided by the recollection of the tone
they heard at first ! He refers also to the shrill noise made by
the young queens ready to swarm, and to the peculiar uneas-
iness produced when a strange queen is being attacked, and
resulting, he thinks, from her " cries of pain" (72).

§ 37. Hearing in Fishes

Throughout the vertebrate animals there exist structures
bearing analogy to our own ears, whose function might there-
fore be supposed to be auditory. But in the lowest verte-
brates the only structures of the human ear represented are

Sensory Discrimination : Hearing 115

the semicircular canals, and these suggest a static rather than
an auditory organ. The cyclostomes, eel-like and semi-
parasitic forms classed below the true fishes, have a pair of
sacs one on either side of the head, containing mineral bodies,
and each leading into one or two semicircular canals. In
the true fishes the sac has two chambers, marked off from
each other by a constriction. Three semicircular canals open
from the foremost chamber, two lying in the vertical plane,
and one in the horizontal plane. The chambers contain
"statoliths" and fluid.

That the semicircular canals in fishes have a static l func-
tion has been shown by experiments to be described later. Is
the fish ear also an organ of hearing? Again authorities
differ, and it is probable that species differ. Kreidl got no
response from goldfish when vibrating rods were placed either
in the water or in the air near the water. Only when the fish
were made more sensitive by strychnine did they react, and
only to noise, not to tone. They reacted quite as well, more-
over, when the ears were removed ; whence it was concluded
that their sensitiveness to noise resided in the skin (227, 228).
A similar negative conclusion regarding auditory sensation
has been reached by F. S. Lee (230) and by O. Korner as a
result of experiments on twenty- five species (223). On the
other hand, Bigelow found that the goldfish on which he ex-
perimented were sensitive in their normal condition, but in-
sensitive when the auditory nerves were cut, and thinks that
Kreidl's operation did not remove the whole of the fish's ear
(33). Triplett thought both perch and goldfish were excited
by the sound of whistling, which usually preceded their being
fed (407). Parker tested the killifish, a species of minnow,
using the sustained slow vibrations (40 complete swings per

1 The word "static" is here used to mean "relating to equilibrium" in
general, not to static equilibrium as distinguished from dynamic equilibrium.

n6 The Animal Mind

second) of a bass viol string placed on one side of the aquarium
as a sounding board. The fish cage was suspended in the
aquarium from an independent support. Normal fish re-
sponded to the vibrations, usually by movements of the fin,
96 per cent of the time. Fish in which the nerves to the ears
had been cut responded in 18 per cent of the tests; those in
which the skin had been made insensitive, but the ears left,
94 per cent. Since causing the string to vibrate jarred the
whole aquarium somewhat, these experiments were checked
by others where the stimulus was produced by placing the
stem of a vibrating tuning fork against the sounding board.
The results were the same as in the first set of tests. Parker
concludes that the ears of the minnow are certainly organs
for the reception of sound ; but as he obtained no such reac-
tions from dogfish, he is inclined to think that different species
vary (305, 306). Tests by Zenneck on Leuciscus rutilus, L.
dobula, and Alburnus lucidus also led to the conviction that
these fish, at least, could hear. A bell was struck by elec-
tricity under water, and occasionally a piece of leather was
placed upon it at the point where the clapper struck. In the
latter case the mechanical vibrations produced were, it was
held, the same as those occasioned by the actual ringing of the
bell, but the sound vibrations were destroyed. The fish
reacted by swimming instantly away from the neighborhood
of the bell when it was rung, but not when the leather was
used; hence, apparently, they reacted to sound (475).

Widely distributed among fishes is a curious set of structures
known as the lateral-line canals. Along each side of the fish,
extending from head to tail, there is a row of pores opening
into a long canal, which at the head divides into three branches,
one going upward above the eye, a second below the eye, and
a third down toward the lower jaw. The functions of these
canals have given rise to much discussion among zoologists, an

Sensory Discrimination : Hearing 117

exhaustive history of which will be found in Parker's mono-
graph entitled "The Function of the Lateral-line Organs
in Fishes." The problem seems to have been solved by
Parker's own experiments. He first proved experimentally
that the canals played no part in responses to the following
stimuli : light, heat, salinity of the water, food, oxygen dis-
solved in the water, carbon dioxide, foulness of the water,
hydrostatic pressure, steady currents flowing through the
water, and sound. When, however, the water in the aqua-
rium was made to vibrate slowly, about six times per second,
the fish made certain characteristic reactions, differing
somewhat for the four or five species observed, but always
failing to appear when the lateral-line nerve was cut. Parker
concludes that "the stimulus for the lateral-line organs (a
water vibration of low frequency) is a physical stimulus inter-
mediate in character between that effective for the skin (de-
forming pressure of solids, currents, etc.) and that for the ear
(vibrations of high frequency), and indicates that these organs
hold an intermediate place between the two sets of sense organs
named" (309). The ear is thus regarded as actually derived
from the lateral-line canal, as this in turn was derived from
the skin. We may suppose that at least three different sensa-
tion qualities result from stimulation of the skin, the canals,
and the ear, where hearing can be shown to exist.

§ 38. Hearing in Amphibia

Emergence from the water, on the part of adult Amphibia,
is accompanied by disappearance of the lateral-line canals,
and consequently of whatever sensations these mediate. In
the frog, the ear has a tympanic membrane lying at the sur-
face of the head. A single bone, the columella, with one
end against this membrane, lies across the middle ear. The
internal ear is not essentially different in structure from that

n8 The Animal Mind

of the fish; there is no cochlea. Yerkes has made an
interesting study of the reaction of frogs to sound. He
found that they occasionally ''straightened up and raised the
head as if listening" when other frogs croaked or made a
splash by jumping into the water. To no other sound did he
get any apparent response, nor was it possible to make frogs
in their native habitat jump or show any uneasiness by pro-
ducing any sort of noise, so long as the experimenter remained
invisible. "Apparently," Yerkes says, "they depend almost
entirely upon vision for the avoidance of dangers." It is of
course highly improbable that an organ should be adapted
only to the reception of the croaking of other frogs and the
splash of water, and not to noises made in imitation of these ;
and Yerkes suggests that the frogs may hear many sounds to
which they respond by inhibiting movement as a measure of
safety. This view is confirmed by the results of experiments
where the breathing movements of the frog's throat were
registered by means of a lever resting against it and recording
on smoked paper. Evidence from change of the breath-
ing rate was obtained of the hearing of sounds ranging from
fifty to one thousand single vibrations a second (456). Later,
it was shown that sounds, although they did not, when given
alone, cause the frogs to react, modified the responses to other
stimuli, reinforcing or inhibiting them according to the inter-
val between the sound and the other stimulus. This effect
was noticed both when the frogs were in the air and when they
were under water. It was more marked in the spring (the
mating season) than in the winter. That it concerned the
special auditory sense- apparatus, and hence may have been
accompanied by true auditory sensations, was shown by the
fact that it disappeared when the auditory nerves were cut.
Sounds ranging from fifty to ten thousand single vibrations
a second were effective (462, 464). This, of course, does not

Sensory Discrimination : Hearing 119

mean that the frog perceives such sounds as differing in pitch.
The absence of a cochlea throws doubt on such a supposition ;
the sensation differences are probably much cruder than
would be the case for a human being.

§ 39. Hearing in Higher Vertebrates

The reptilian ear does not differ markedly from that of
amphibians. The writer knows of no experiments upon the
sense of hearing in reptiles.

The cochlea is supposed to be the portion of the human
ear upon which the power to distinguish pitch differences
rests. Yet birds have no cochlea, though if we grant that
animals which produce sounds are those which are able to
hear them, some birds at least must be capable of pitch
discriminations of wide range and great acuteness. The
powers of imitation so often evidenced in bird song are
proof that this is the case.1 The sense of hearing, so long
absent or problematical in the ascending scale of animal
forms, reaches great importance in the life of birds and
mammals. How far various mammals have the same
range of auditory qualities that a human being has, what
their capacity for pitch discrimination may be, has been
but little investigated. Raccoons have been taught to dis-
criminate between the note Aj on a harmonica and the
note A'", climbing on a box to be fed when the high
note was sounded and staying down when they heard the
low one (82). It is probable that the variety of auditory
qualities entering into the experience of the highest verte-
brates is large.

1 Interesting evidence of this power in a bird which might not have been
supposed to possess it was obtained by Conradi, who found that English
sparrows reared by canaries acquired recognizable bits of the canary song
(83).

CHAPTER VII
SENSORY DISCRIMINATION: VISION

§ 40. Some Problems connected with Vision

IN this chapter we shall consider one aspect only of the
reactions of animals to light stimulation ; namely, the question
whether such stimulation produces in the possible conscious-
ness of a given animal any sensations qualitatively unlike those
accompanying other forms of stimulation, and if so, how
many such specifically visual sensations, qualitatively differ-
ent from each other, the animal may be supposed to be capable
of receiving. The spatial aspect of vision will for the present
be neglected.

Even with this restriction, the photic reactions of animals
present a series of problems of enormous complexity. One
especially difficult question is, it is true, postponed : the ques-
tion as to just what happens when an animal seeks or avoids
light. The so-called orientation of animals, that is, their
assumption of a definite position with reference to a force
acting upon them at a certain point, is a subject more closely
connected with spatial than with qualitative discrimination;
and though, as we shall see, the seeking or avoiding of
light by an animal by no means always involves orientation
of the body, yet the complex distinctions that have to be drawn
in connection with this subject will be more fully discussed
under the head of orienting reactions. But puzzles enough are
left for the present chapter. What, for instance, is the mean-
ing of the fact that the rays beyond the violet end of the spec-
trum, invisible to us, produce effects upon certain animals?

120

Sensory Discrimination : Vision 121

Are they seen, or do the sensations accompanying them rather
resemble those produced by an irritating chemical? What
kind of sensation quality may we suppose exists in the con-
sciousness of an animal whose responses to light are mediated
by the skin, not by the eyes ? When an animal discriminates
in its reactions between rays that to our eye differ in color, is
the discrimination one of color qualities, or of differences in
brightness, such as the spectrum offers to a totally color-blind
person ? And if a colored ray does not produce a color sen-
sation in the consciousness of a given animal, that is, if the
animal is color-blind, does it produce the same brightness
sensation that it would produce in a color-blind human being ?
These questions will constantly suggest themselves, but in
most cases the evidence will be insufficient to settle them.

§ 41. Vision in Protozoa

Many of the Protozoa, as we know, react to light. Amoeba
gives a negative response when light falls upon it from the
side ; that is, it moves away from the light, and Jennings con-
jectures that this probably occurs by the contraction of the
part of the body nearest the light, which is what would happen
if the light were a mechanical stimulus (211, p. n). Blue
light has the same effect as white light, and red light has no
effect at all ; but the reactions of Amceba to light of different
colors differ only in degree, and do not indicate any qualita-
tive difference of accompanying conscious processes (162).
Nor, if the reaction to light is really identical with the negative
reaction in general, can we conclude that any specific visual
sensation accompanies it. The same holds true of the re-
sponses of various ciliate and flagellate Protozoa to light.
These all, so far as observed, take place by the ordinary nega-
tive or avoiding reaction ; some of the animals give it on pass-
ing from a region of less to one of greater illumination, and

122 The Animal Mind

thus "seek" the darker regions, while others give it when
undergoing a change in the reverse direction, and hence are
" positively phototropic." But if nothing distinguishes the
negative reaction to photic stimuli from the negative reaction
to any other stimulus, then nothing shows the existence of a
sensation quality peculiar to the effect of light — unless a
special receptive apparatus can be demonstrated. In a flag-
ellate Protozoon called Euglena, a pigment spot exists near
the anterior end. Now although pigment apparently is not,
as Hesse (176) has emphasized, a necessary constituent of
visual organs, yet its occurrence always suggests some rela-
tion to light, as it is essentially a kind of matter having the
property of absorbing light. Euglena gives the negative re-
action on entering a shadow. Is its pigment spot really an
"eye spot" and concerned in this response? Apparently
the reaction occurs before the pigment spot has entered the
shadow, and as soon as the transparent tip lying in front of
the pigment spot has been pushed into the shaded region
(no). It is uncertain, then, what the function of the pigment
spot is. But in another organism, which is structurally
intermediate between the single-celled and the many-celled
forms, pigment spots do play a r61e in light reactions. This
organism is called Volvox, and it is really a colony of globular
flagellates, each with its flagellum turned outward, and each
with an "eye spot." Very weak light has no effect on the
movements of Volvox; moderate light causes movement
toward the source of light, and very strong light causes move-
ment away from the source (183). Accurate observation of
these movements indicates that the eye spots are essential to
them; each individual responds to a change of illumination
of its eye spot (262). This much evidence, then, we have that
if Volvox possesses consciousness, changes of light intensity
produce in it a specific sensation.

Sensory Discrimination : Vision 123

§ 42. Vision in Coslenterates

Turning to the coelenterates, we find that Hydra shows
no response to light other than a tendency to come to rest
in the more illuminated parts of the vessel containing it
(406, 444). Very strong light, however, makes it wander
about until it happens to reach a more shaded region.
Thus if the animal is subjected to light either above or
below a certain "optimum," it is restless. A "vague uneasi-
ness" is the kind of psychic accompaniment to this behavior
most naturally suggested ; repeated strong mechanical stimu-
lation will also make the animal wander about. Nothing
points to the existence of a visual quality. Blue and green
light are more frequented by Hydra than red and yellow
light; this parallels the effect of colored rays on Amoeba
(444). Widely distributed through the animal kingdom is a
kind of equivalence, for reaction purposes, between blue or
violet and white light on the one hand, red light and dark-
ness on the other.
i

On the hydroid colonies of Tubularia no change of light
intensity operated as a stimulus (319). In Actinians the
only evidence that the reactions due to light differ from those
otherwise produced lies in the greater slowness of the former.
Many sea-anemones are wholly unaffected by photic stimu-
lation, Sagartia lucia and Metridium, for example (160).
Many others have been found to contract when illuminated
(150, 207, 291). Eloactis producta expands its tentacles only
in light of low intensity, taking about fifteen minutes to do so
when covered with a hood, and retracting in five minutes
when the light is restored. This retraction is decidedly
slower than that produced by mechanical stimulation (160).
That the responses to light are more marked in animals which
have been living in comparative darkness than in those

124 The Animal Mind

taken from illuminated spots, has been shown both for sea-
anemones and for Hydra (127).

Many Medusae or jellyfish also react to light more slowly
than to other forms of stimulation. It is true that on Sarsia,
a form tested by Romanes many years ago, light seemed to
act as quickly as any other stimulus. If a flash of light were
allowed to fall on the animal while it was moving about,
"prolonged swimming movements" ensued; if it was at rest,
it gave only a single contraction — another instance of the
effect of physiological condition upon reaction. Sudden
darkening produced no reaction, whence Romanes concluded
that "it is the light per se and not the sudden nature of the
transition from darkness to light which in the former experi-
ment acted as the stimulus." There are, however, as we
shall see, other animals in which an increase of illumination
brings about response where a decrease fails, and vice versa.
When a beam of light was thrown into a bell- jar containing
many Sarsiae and placed in a dark room, "they crowded into
the path of the beam and were most numerous at that side
of the jar which was nearest the light." "There can thus,"
concludes Romanes, "be no doubt about Sarsia possessing
a visual sense" (365, p. 41). But as these reactions are not
differentiated in any way, they cannot be taken as evidence of
a specific sense, unless indeed they depend on a specialized
sensory structure. This latter Romanes found to be the
case ; Sarsia has pigment spots on the margin of its bell, and
its response to light ceased when these were destroyed.
Tiaropsis, another jellyfish studied by the same observer,
gave further evidence of "a visual sense" in the fact that it
responded to light more slowly than to mechanical stimulation.
In Gonionemus, both difference in reaction time and depend-
ence of response on a special organ indicate that light may
produce a specific sensation, always granting the presence of

Sensory Discrimination : Vision 125

consciousness. Yerkes found that this jellyfish, unlike Sarsia,
reacts in the same manner in passing either from sunlight to
shadow or the reverse. In both cases it stops swimming and
sinks to the bottom. A sudden change of illumination, there-
fore, checks its activity. On the other hand, if when the
light falls upon it the animal is at rest, it becomes active
again ; but sudden decrease of illumination has no effect upon
the resting animal. The inhibitory effect of strong light fall-
ing upon the jellyfish while in motion Yerkes explains as a
special adaptation. For one case of such increase of illumi-
nation occurs when the animal swims, bell upward, to the
surface on being disturbed; the light of the surface is of
course normally stronger than that in the lower regions. The
inhibition of activity resulting causes the animal, after turning
over, to sink slowly, bell downward, with expanded tentacles.
This is a position that gives it a better chance of catching food
and carrying it to the lips than is offered by the right-side-up
posture, where food would have to be carried downward
against the upward current occasioned by the sinking of the
animal. Light is not the only factor in producing the inver-
sion at the surface, however, for it will occur in darkness.
When swimming, Gonionemus moves toward the light if the
latter is fairly intense, but comes to rest in the shaded portions
of the vessel containing it. The reaction time to light is
much slower than that to other stimuli, but the animal re-
sponds most promptly when certain pigmented bodies at the
base of the tentacles are exposed to the stimulus. If the mar-
gin of the bell containing these bodies is cut off, no reaction
to light can be obtained (451, 458, 470). A great variety of
structures apparently sensory in function is found on the bell
margin of different genera and species of Medusae. Some
of them are statocysts. Others suggest a visual function,
and in the Cubomedusae there are fairly well developed eyes.

126 The Animal Mind

§ 43. Vision in Planarians

In planarians, unmistakable eyes are present, yet appar-
ently the reactions to light are not wholly dependent upon
them. The general effect of photic stimulation on the plana-
rian is to stimulate it to movement; it comes to rest in the
shaded portions of a vessel (9, 239, 243, 169). To a certain
extent, light directs the movement of the animal away from it
(313). But Hesse found that one species of planarian with
much more highly organized eyes than another reacted to
light decidedly less; the strength of the light reaction does
not, he concludes, correspond to the development of the light
perception. The latter depends on the number of sensitive
elements in the eye, the former on the habits of the animal
and the feeling tone aroused by the light (169). This " feeling
tone" may apparently be connected with a skin sensation.
Decapitated and hence eyeless planarians respond to light,
but more slowly (243), and with less definite reference to the
direction of the light (313).

§ 44. Vision in Annelids

The earthworm's sensitiveness to light is also dermal,
although Hesse believes that he has found visual organs in
certain structures in the skin, especially at the upper lip and
the tail end (168). However this may be, the effectiveness of
light as a stimulus is not confined absolutely to any one region
of the body. When the worms are in a normal condition,
attached to their burrows, the combined effect of light and the
contact stimulus at the tail produces the ordinary negative
reaction of withdrawal into the burrow (91, 179). The only
evidence that light is accompanied by a specific conscious-
ness is to be derived again from the fact that the reaction time
to light is much longer than that to mechanical stimulation.

Sensory Discrimination: Vision 127

If the worm is detached from the burrow, it will take a course
leading it more or less obliquely away from the light ; if it is
crawling in passages between glass plates, which allow it the
choice between only two paths, one straight toward the light
and the other straight away, it takes the latter about 95 per
cent of the time (38 7 ). Graber used his ' ' Preference Method ' '
on earthworms, employing a box with two compartments, one
illuminated with diffused daylight, the other dark. At the
end of every hour the number of worms in each compartment
was counted. That in the darkness was on the average 5.2
times as great as that in the light. When ground glass was
substituted for the dark screen, making the compartment
under it about half as light as the other, the number in the
lighter compartment was about .6 the number in the darker,
though still moderately light, portion of the box, thus show-
ing that the worms were sensitive to comparatively small
differences in intensity. Graber also placed colored glasses
over the compartments, with the following results : the worms
preferred red to blue even when the former was much lighter
than the latter, indicating that the preference was determined
by the wave length and not by the brightness of the light;
they preferred green to blue under similar conditions, and red
to green. They emphatically preferred white light from
which the ultra-violet rays had been subtracted to ordinary
white light, 6.7 times as many being found in a compartment
covered by a screen impervious only to ultra-violet rays (150).
The effect of ultra-violet rays on many animals is very dele-
terious (167). The avoidance of the ultra-violet rays and
the seeking of red by negatively phototropic animals is almost
universal.

The part of the body of the earthworm affected by the light
influences the reaction. Darwin indeed reported that the
worms withdrew into their burrows only when light fell on

128 The Animal Mind

the head end (91), but decapitated worms were found by
Graber to show the same light and color "preferences" as
normal ones, though in a less marked degree (150), and Yung
obtained evidence that sensitiveness to light is distributed
over the body (473). According to Hesse the anterior end
of the worm is most sensitive, the tail next, and the middle
region least (168). Not only the region, but the amount of
body surface affected, also makes a difference. When the
whole length of the worm was illuminated, the percentage of
reactions was to that obtained where the front third only was
involved as 26 to 10.2, while the relative occurrence of re-
sponses when the middle third and the posterior third alone
were stimulated is represented by the figures 2.4 and i re-
spectively (312). The effect of colored rays has been found
to be proportionate to their intensity ; that is, the green and
yellow regions of the spectrum are most effective (473).

Although the ordinary response of the earthworm to light
is negative, it has been possible to determine experimentally
a positive phototropism in Allolobophora fatida for very
low intensities, and the emergence of worms from their
burrows at nightfall has been referred to this tendency to
seek very faint light (i).

No parallel in our own experience can be found for the sen-
sation received by an eyeless animal from light. In many
of the marine worms, however, well- developed eyes exist, but
not such as are capable of giving clear images. The function
of the eyes of marine worms seems to be chiefly that of receiv-
ing stimuli from shadows. Many tube-dwelling worms will
withdraw into their tubes if a shadow is cast upon them, and
the term "skioptic" has been suggested for this class of reac-
tions (158, 173, 373). The leech Clepsine shows the same
kind of behavior ; the slightest shadow cast on the surface of
the water in a dish where these animals are resting quietly

Sensory Discrimination : Vision 129

will cause them to reach up and sway from side to side in
apparent search for prey (438).

A curious effect of colors on tube-dwelling worms has been
observed. When placed under either blue or red glass, the
sensory activities of the worms seemed to be inhibited for
a time, the effect being more striking in the case of the red
glass. When brought suddenly from under blue glass into
ordinary white light, the worms showed intensified reactions ;
while bringing them from under red glass to white light in-
hibited their reactions for from two to five minutes (158).

The fact that animals which display positive phototropism
show also an "aversion" to red and a tendency to seek colors
that contain the ultra-violet rays, while negatively phototropic
animals avoid light that has ultra-violet rays, and seek red,
which lacks these rays, has pointed to the probability that
apparent color discriminations in the lower forms of animals
are really reactions to the intensity of the light, and espe-
cially to the intensity of the ultra-violet rays. This position,
however, has recently been questioned by Minkiewicz. He
has succeeded in changing the reactions of a Nemertean
worm, Linens ruber, to colored light, while its response to
white light remained unaltered. When placed in diluted sea
water the animal would, after a day, direct itself toward violet
rays, although still negative in response to white light. On
the fourth day the ordinary " chromotropism " was restored;
that is, the worm sought red rays. After two or three weeks
of life in the diluted sea water, on being restored to ordinary
sea water the worm's chromotropism was again inverted,
becoming positive to the violet rays, while negative phototrop-
ism persisted. Moreover, intermediate stages in the passage
from the red- to the violet-seeking phase were observed; a
stage where, still positive to red, the animal ceases to distin-
guish red from yellow, and others where it seeks violet, but has

130 The Animal Mind

become indifferent to green and yellow. These stages last for
several hours, but corresponding ones have not been observed
in the passage from the violet phase back to the red phase ;
perhaps they occurred too rapidly to be noted (274).

§ 45. Vision in Mollusks

In the phylum Mollusca we find eyes of all grades of de-
velopment, from mere pigment spots in certain Acephala to
the elaborate eye of the squid, with its lens, iris, and contrac-
tile pupil. Such an eye is fully capable of forming an image.
Among the Acephala there are many instances of reaction to
light in the absence of all visual organs. The sensitive parts
are commonly the siphons, which are projected from the
shell to take in currents of water containing nourishment, and
withdrawn in response to sudden darkening in some cases, to
sudden illumination in others, and in still other instances to
either (102, 290, 373). In Pecten varius, which has eyes
on the border of its " mantle," Rawitz found that a shadow
would cause reaction provided that it fell simultaneously upon
a considerable number of the eyes, from which he concludes
that they may cooperate in a kind of mosaic vision (360).

In snails, although the eyes are undoubtedly concerned in
light reactions, a certain amount of skin sensitiveness has
been shown. Helix aspersa, a negatively phototropic ani-
mal, when blinded, reacted one-half as many times to light
as when normal; H. nemoralis, positively phototropic, only
one-eighth as many times; from which the suggestion was
derived that the " dermal light-sense" may be more effective
in negative than in positive animals (441). Very interesting
observations on periodic changes in the responses of marine
gasteropods to light have been made by Bohn (55), but these
will be more fully considered in a later chapter. The
cephalopods, with their highly developed eyes, offer an inter-

Sensory Discrimination : Vision 131

esting field for the study of visual reactions, which is as
yet almost untouched.

§ 46. Vision in Echinoderms

The starfish and sea urchin, among the echinoderms,
depend for their responses to light upon pigment or eye spots
on the arms. They are positively phototropic, but lose this
tendency if the eye spots are removed ; a fact which furnishes
some evidence for the existence of a specific visual quality
(365, 398). Romanes found the sensitiveness to light so great
in the individuals examined by him that they discriminated
between ordinary pine boards used to cover the face of the
tank containing them, and the same boards painted black,
light being in both cases admitted through a narrow slit (365).

Various sea urchins have been found responsive to shadows.
One, Centrostephanus longispinus, has not even the rudi-
ment of an eye. This animal in diffuse daylight seeks the
darkest corner and turns its aboral pole to the light. A
sudden shadow falling on it causes it to direct its spines
toward the shaded side. The reaction time involved is de-
cidedly longer than that to mechanical stimulation, and more-
over, although pieces of the animal will react to the latter,
responses to shadows depend on keeping the system of radial
nerves intact. Hence von Uexkiill, who made the above
observations, concluded that a special set of nerve fibres is
concerned in photic reactions (410). Dubois had suggested,
from studies on the mollusk Pholas dactylus, that in such
cases the pigment changes which occur, under the influence of
light, over the surface of the body, furnish the stimulus1 (102),

1 The pigment changes, Dubois thinks, cause contraction of a muscular
layer lying underneath the pigment, which contraction excites the nerve
endings. This arrangement, which he terms a "sysfeme avertisseur," he be-
lieves to be involved in the reactions of low forms of animals to various
stimuli.

132

The Animal Mind

but von Uexkull thinks this impossible, as the light reactions
occur before the pigment changes do. This migratory pig-
ment, he believes, acts merely as a screen; the source of
excitation for the optic fibres may lie in another pigment
which he has extracted and found very sensitive to light (410).

§ 47. Vision in Crustacea

The spatial aspect of vision assumes great importance in
the arthropods, both because of the precision of their orien-
tation to light in
many cases, and
because of the
peculiar func-
tions of the com-
pound eye so
common in this
group. This or-
gan appears to be
specially adapted
to the vision of
moving objects
(Fig. 10). It con-

FiG. 10. — Diagrammatic representation of the compound sists essentially

eye of a dragon-fly. C, cornea; K, crystalline cone; Q£ & number of
P, pigment; R, nerve rods of retina; Fb, layer of

fibres; G, layer of ganglion cells; Rf, retinal fibres; simple CVCS SO

Fk, crossing of fibres. After Glaus. Crowded together

that the common cornea is, as it were, faceted, each facet
belonging to an eye. These facets are lens shaped, and back
of each lies a refractile crystalline cone. Behind these, in
turn, are nervous structures, the rods or retinulae, each sepa-
rated from its neighbors by a pigment sheath. Light rays
passing through each corneal facet probably produce a single
spot of light on the retinula, and the total image is thus a

Sensory Discrimination : Vision 133

mosaic formed of these spots. Into its characteristics, how-
ever, we need not enter. In the present chapter we are
concerned only with the evidence that light stimulation in
general, and light of different wave lengths in particular,
produces specific sensations.

That the visual reactions of Crustacea are accompanied by
a special visual sensation, if we suppose these animals to be
conscious, is sufficiently evidenced by their dependence on
the eyes. To movements and shadows the responses are for
the most part given. Bateson, watching shrimps and prawns,
noted that they apparently could not see their food when it
had been taken from them and lay near at hand, but quickly
raised their antennae when an object was passed between
them and the light (i i). The little fairy shrimp, Branchipus,
will stop swimming as soon as the edge of a shadow falls
upon it. "Skioptic" reactions in the family of Cirripedia,
to which the barnacles belong, were noted by Pouchet and
Joubert in 1875, as well as the fact that those individuals
which were attached to rocks, where a sudden shadow
might mean danger, reacted, while those attached to floating
objects, and therefore exposed normally to light fluctua-
tions, did not (348). The problem as to whether light
of different colors produces different sensations in the
crustacean consciousness was the subject of experiments
a number of years ago, in which the Preference Method
was used. Sir John Lubbock arranged to have a sunlight
spectrum thrown on a long trough containing Daphnias,
tiny crustaceans belonging to the lowest sub-class, that
of the Entomostraca (Fig. n). Daphnia is decidedly
positive in its phototropism. At the end of ten minutes
glass partitions were slipped across the trough at the approxi-
mate dividing lines of the spectral colors. The number of
animals in each compartment was then counted. The ex-

134 The Animal Mind

periment was repeatedly performed, and the greatest num-
ber was always found in the yellow-green region (249, 250).
Bert obtained similar results with the use of an electric light
spectrum; but besides throwing all the colors at once upon
the vessel, he allowed each color to act separately through a
narrow opening, and noted the speed of the positive response
produced. That the "preference" shown for yellow- green
light is not a matter of color vision, but of response to the
greater intensity of the light in this region of the spectrum,
was suggested by Bert (24), and Merejkowsky showed that

the larvae of Balanus
and Dias longiremis
manifested no color
preference when the
colors were made of
equal intensity (269).
Lubbock attempted

FIG. ii. — Daphnia. at, antenna; atl, antennule; to prove the exist-
OC,eye. After Yerkes. ence Qf qualitative

as distinguished from intensive discrimination by various
modifications of the experiment, but without entirely con-
clusive results (251, pp. 221 fL). Finally, Yerkes, working
on Simocephalus, a form closely related to Daphnia, found
that when a gaslight spectrum was used, the animals col-
lected in the red-yellow region, that of greatest intensity for
such light ; and that if this region had its intensity diminished
by a screen of India ink or paraffin paper, the crustacean
moved out of it (448). In all probability, then, the reactions
of these forms are not accompanied by qualitatively different
color sensations corresponding to light of different wave
lengths.

That Daphnia seeks a region affected by the ultra-violet
rays of the spectrum in preference to darkness, although the

Sensory Discrimination : Vision 1 35

two look alike to our eyes, was shown by Lubbock (251, pp.
215 ff.). An effect of physiological condition suggesting the
law of general adaptation in human vision is evidenced by the
fact that individual Daphnias which have been a long time
in darkness will respond to a lower intensity than those which
have been long exposed to illumination (94). Many curious
results of physiological condition upon orientation to light
in Crustacea will be discussed later.

Experiments on the reactions of the crayfish, which is
moderately negative in its phototropism, to light coming
through colored glasses indicate that the animal seeks red
when the light falls vertically, but shows no marked prefer-
ence when light is passed horizontally through the glass.
The tendency to seek red is characteristic of negatively photo-
tropic animals, but in this case it seemed to be stronger
even than the tendency to seek black. No definite proof
of a specific color reaction is, however, offered (21). The
positive reactions to light of Pycnogonids, or sea spiders,
a curious group of animals whose classification is uncertain,
have been found to depend on the presence of a visual organ

(79).

§ 48. Vision in Spiders

Spiders do not have the compound eye, but a number of
ocelli, or simple eyes; the typical fully developed inverte-
brate eye with cornea, lens, vitreous humor, rod layer, and
pigmented layer in the retina^the latter lying in front of the
nerve fibres supplying the retina, instead of behind them as
in the vertebrate eye. Experiments have been made on color
discrimination in spiders; some by the Preference Method,
where the spiders showed an inclination for red when offered
a choice of compartments illuminated through red, green,
blue, and yellow glass (320) ; others by attempting to form an
association between paper of a certain color and the spider's

136 The Animal Mind

nest. This latter, containing eggs, was surrounded by col-
ored paper, and when a spider had become accustomed to
going in and out over the paper, another color was substituted,
and a false nest made in another place, surrounded by the
original strips of paper. The spider under these circum-
stances showed some confusion and tendency to go to the
false nest (321). The experiments with Daphnia have,
however, suggested a fundamental source of error in experi-
ments on the color vision of animals. A human being who
is totally color-blind is nevertheless able to discriminate
among objects that to a normal eye have different colors,
because such objects take on to the color-blind eye different
shades of gray. It is always possible, then, unless special
precautions are taken, that an animal's apparent discrimi-
nations of color may be really brightness discriminations,
in some way analogous to those made by the color-blind
person. No such precautions were taken in the experiments
just described, and the color sense of spiders remains unproved.
In blind and blinded myriapods, the family to which the
centipede belongs, skin sensitiveness to light is shown (329,

335)-

§ 49. Vision in Insects

The compound eye again occurs in insects, together with
ocelli or simple eyes, the latter usually placed in the middle
of the head. The respective functions of the two kinds of
eyes are not definitely known, though there is a possibility
that the ocelli may serve for near vision and for vision in faint
light. Plateau, however, finds that insects with the com-
pound eyes blinded and the simple eyes intact are unable to
see even in faint light, and has but a poor opinion of the
usefulness of the latter. Caterpillars, which have only sim-
ple eyes, depend, he thinks, chiefly on their long hairs or on
their feelers to warn them of the approach of obstacles (332).

Sensory Discrimination : Vision 137

On the color sense of insects there are first the old ex-
periments of Graber by the Preference Method, whose most
definite result was to show that positively phototropic insects
prefer colors containing the ultra-violet rays, while the nega-
tively phototropic ones prefer red, from which these rays are
absent. No proof that the discriminations were made on the
basis of color proper rather than brightness was forthcoming
(151). Similar observations were made by Lubbock on ants,
which in their underground life are negatively phototropic,
the eggs and larvas apparently needing darkness in order to
develop, but on their foraging expeditions are comparatively
indifferent to light. They showed a preference for red when
tested, and a tendency to avoid the ultra-violet rays, so marked
that they preferred bright daylight from which these rays
had been extracted by chemical screens, to darkness that con-
tained the ultra-violet (248, pp. 207 fL). Graber suggested
that the ultra-violet rays produce a skin sensation in the ants ;
but Forel agrees with Lubbock that the effect is visual, be-
cause he found that varnishing the eyes made the ants in-
different to ultra-violet (130). Ants of the family Lasius seem
to be normally insensitive to these rays (134). It is just pos-
sible, then, that a visual sensation of quality wholly foreign
to our experience may accompany the action of ultra-violet
rays on insects. Loeb has noted that the relative effect of
violet and ultra-violet vibrations, as compared with that of the
rest of the spectrum, is greater, the less developed the visual
organ (233).

Lubbock's experiments on the color sense of bees are more
to the point than those on ants, for they were made not by the
Preference Method, but by associating a color with food. No
precaution, however, was taken against the brightness error.
He found that bees which had eaten honey from blue paper
would pick out the blue pieces from a number of differently

138 The Animal Mind

colored papers, whose positions were altered during the ex-
periments (248). Forel got similar results, and reports that
a bumble bee thus trained would select all the blue objects
in the room for special examination (130). Lubbock's
tests with wasps gave negative results.

We have already noted the dispute as to how far visual
sensations in general are involved in the reactions of bees to
flowers, and have seen that Plateau maintains their relative
unimportance in this connection, as compared to smell.
Besides the experiments which we have quoted on pp. 96-
97, he adduces the facts that he could never persuade insects
to alight upon artificial flowers, though these were not dis-
tinguishable by human eyes from real ones (336-338) ; that
bees show no preference for flowers of any particular color
(339) ; and that they often make errors, in alighting on closed
buds, seed pods, and wilted flowers, which indicate defective
vision (341). But Josephine W£ry and others have noted
that bees do seek artificial flowers (434). Even Plateau
does not deny that an insect may perceive flowers from a
distance, "whether because it sees the color in the same way
that we do, or because it perceives some kind of contrast
between the flowers and their surroundings" (339).

Von Buttel-Reepen gives one or two instances to show
that the color perception of bees is sometimes influential in
helping them to recognize their own hives. He reports a case
where a stock of bees had been driven from their hive and
scattered. The front of the hive was blue. Some of the
bees tried to find their way into other hives, and selected for
their efforts those which had blue doors. This authority
believes, moreover, that the sense of sight has occasionally
something to do with the reception of bees into a foreign hive.
" Robber bees," which steal honey from strange hives, when
they begin their downward career, approach the strange

Sensory Discrimination : Vision 1 39

dwelling with a peculiar hesitating flight; afterwards, says
von Buttel-Reepen, they become "frecher." He declares
that when attempting to alight before a foreign hive they are
often driven off by the rightful occupants before their odor
can have been noticed, and ascribes this reaction to the sight
of their hesitating method of approach. On the other hand,
when a broodless stock joins itself to one that has a brood, the
latter is induced to receive them peacefully because of their
assured manner (72).

The majority of bee students incline to the belief that bees
are guided back to their hives from long flights by visual
memory, though the phenomena are not easy to explain.
Solitary wasps, it seems highly probable from experiments,
find their nests by sight ; but this subject will be more fully
discussed in Chapter XI.

§ 50. Vision in Amphioxus and in Fish

The vertebrate eye differs in origin and in structure from
any form of invertebrate eye, the most striking difference in
structure being perhaps the situation of the pigmented layer
of the retina behind the nerve-fibre layer, which is respon-
sible for the existence of the blind spot where the trunk of
the optic nerve breaks through the retinal layer. Another
point of unlikeness consists in the fact that the invertebrate
optic nerves do not cross on their way to the brain, while in
the vertebrates there is either total or partial crossing of the
fibres. In both the vertebrate and the simple invertebrate eye
the image is formed by means of a lens, although Nagel has
suggested that the function of the lens in the lower forms of eye
is rather to collect the light than to produce an image (293).

The reactions of Amphioxus to light offer as chief evidence
that they are accompanied by a specific sensation quality
the fact that they may be fatigued independently of other

140 The Animal Mind

reactions. The only structures suggesting a visual function
are pigment spots on the back near the head, and other pig-
ment cells distributed down the back. Amphioxus makes
negative reactions to light, especially when the light, from
which heat rays have been excluded by passing it through
water, is directed at any point on the back, the most sensitive
region lying just back of the eye spot (225, 311). The eye
spot itself, and the front end of the animal, are insensitive.
Fatiguing the light reactions had no effect on response to
other forms of stimulation (311). Attempts to test the
4 'color preferences" of Amphioxus by illuminating different
parts of a trough with differently colored lights gave negative
results (225). A skin sensibility to light has been observed
also in larval lampreys, which will give negative reactions
even when the optic nerves are cut (310). Blind fish have
been found to react to light, apparently through the skin (107).
Among the many animals whose supposed color prefer-
ences Graber tested were two species of fish, but no con-
vincing proof of their powers of color discrimination was
obtained (151). Bateson placed food on differently colored
tiles, and observed that the fish picked it off most readily
from white and pale blue, and least readily off dark red and
dark blue; which establishes little save that the bait was
probably more conspicuous on the former (12). Professor
Bentley and the writer succeeded in getting fairly conclusive
evidence that one fish, of the common variety of chub,
Semotilus atromaculatus, could associate a given pigment with
food. Two dissecting forceps were used, alike except that
to the legs of one were fastened, with rubber bands, small
sticks painted red, while to those of the other similar green
sticks were attached. The forceps were fastened to a wooden
bar projecting from a wooden screen, which divided the
circular tank into two compartments, and hung down into

Sensory Discrimination : Vision 141

the water. Food was always placed in the red pair of forceps,
which were made frequently to change places with the green
ones ; and the fish was caused to enter the compartment half
of the time on one side, and half of the time on the other.
This was to prevent identification of the food fork by its posi-
tion or the direction in which the fish had to turn. The animal
quickly learned to single out the red fork as the one important
to its welfare, and in forty experiments, mingled with others so
that the association might not be weakened, where there was
no food in either fork, and where the forceps and rubber bands
were changed so that no odor of food could linger, it never
failed to bite first at the red. Moreover, the probability that
its discrimination was based upon brightness was greatly
lessened by using, when we experimented without food, a
different red much lighter than that in the food tests. The
fish successfully discriminated red from blue paints in the
same way, and it was afterwards trained, by putting food
in the green fork, to break the earlier association and bite
first at the green (421),

§ 51. Vision in Amphibia

The fact that the commonest form of color blindness in
human beings affects the qualities red and green, and that
these colors have the most restricted area of visibility, might
tempt one to the belief that ability to distinguish red and green
is a late acquisition in the animal kingdom. So far, com-
parative psychology offers no support for this view. The
fish whose behavior has just been described certainly made
some sort of distinction between the colors red and green.
And the only evidence of color vision in the Amphibia is
evidence that frogs discriminate, in some fashion, between
red and white, although the difference to the frog may be one
of brightness merely. Yerkes, in studying the frog's power

142 The Animal Mind

to learn by experience, caused it to go through a simple
labyrinth leading to a tank of water. At the point where the
first choice between two paths occurred, a red card was
placed on one side and a white card on the other. When the
frog had learned to take the correct path, toward the white,
the cards were exchanged, without any other alteration in the
conditions; and the decided confusion of the animals in-
dicated that they had discriminated between the red and
white cards and had learned to react with reference to this
discrimination (454).

Two species of frogs tested by Ellen Torelle showed posi-
tive phototropism, associated, as usual, with a tendency to
prefer blue to red light (401). The frog's phototropism,
moreover, persists even when the animal is blinded, although
in the normal animal the eyes are involved in the reaction,
since it occurs when the skin is covered and the eyes left
intact (224, 308). Skin sensitiveness to light has been
noted also in salamanders (103). The nature of the "dermal
light sensation" remains a mystery. It can hardly, in frogs,
be a painful irritation, since it produces a positive response ;
and it is not due to heat rays, for it occurs when these are
intercepted by passing the light through water. As Parker
says, radiant heat and light, " distinct as they seem to our
senses, are members of one physical series in that they are
both ether vibrations, varying only in wave length" (308).
While, then, the nerve endings in human skin are sensitive
only to the slowest of these vibrations, the heat rays, those in
the skin of the frog may respond to the whole series, with
what accompanying sensation qualities we cannot say.

§ 52. Vision in Other Vertebrates

In some reptilian eyes, and in those of all birds, a few
fishes, and Ornithorhyncus, there are attached to the ends of

Sensory Discrimination : Vision 143

the cones in the retina transparent, colored globules like little
drops of oil. The significance of these colored drops is
wholly unknown. If they really transmit to the cones only
those colors which they seem to, then the color sensations of the
animals possessing them must be wholly different from ours.

No experimental tests on color vision in reptiles have
been made, so far as the writer is aware. As for birds, in
the palmy days of the doctrine of sexual selection we should
have felt quite sure that the bright colored plumage of many
species indicated ability to distinguish colors. Some experi-
mental evidence of this power has been obtained. A chick
that had learned to pick out bits of the yolk of a hard-boiled
egg from the white was given bits of orange peel, which he
seized, but seemed to find exceedingly distasteful. After-
wards he was for some time suspicious of the bits of yolk.
On the other hand, after having learned to avoid bad-tasting
black and yellow caterpillars, he did not transfer his aversion
to black and yellow wasps ; probably their points of difference
from caterpillars were so numerous that the resemblance of
color was not attended to (281, pp. 40-41).

Color vision in the English sparrow and the cowbird has
been tested by a method previously used on monkeys. A
number of glasses of like size and shape were covered inside
and out with differently colored papers, including red, yellow,
blue, green, dark and light gray. These glasses were placed
in a row on a board, and food was put always in the same
glass, the position of which, however, was changed in the
different experiments. The sparrow and cowbird learned
to pick out the right vessel under these conditions (345).
Somewhat similar tests were fairly successful with pigeons,
which were also experimented on by Graber's method of
allowing a choice between compartments illuminated through
differently colored glass. Although the pigeons showed no

144 The Animal Mind

tendency to avoid any particular color, they indicated a pref-
erence for green and blue. This result it was attempted to
verify by pneumographic tests, and a greater quickening of
breathing was recorded under green and blue lights than
under other light stimuli (371).

Raccoons have been trained to discriminate cards of dif-
ferent colors and brightnesses in the following pairs : black-
white, black-yellow, black-red, black-blue, black-green,
blue-yellow, red-green. The two last-named discriminations
proved decidedly more difficult, and one of the four raccoons
tested never learned to distinguish red from green or blue
from yellow (82).

In none of the above described experiments, however, is
the brightness error eliminated. Kinnaman's color tests on
monkeys did make an attempt in this direction. The monkeys
were tested with glass tumblers covered with papers of differ-
ent colors, and when it had been shown that they were able
to identify a vessel of a particular color as associated with
food, the possibility that their discriminations might have
been based on brightness rather than color was investigated in
the following way. First, the animals' power of distinguishing
different shades of gray was tested, and it was found that
they could barely detect a difference considerably greater
than that between the ''brightness values" of the colors
used ; that is, the grays that a color-blind human being would
have seen in place of the colors. Secondly, this result was
confirmed by covering the glasses with gray papers varying
in brightness somewhat more than did the colors used, and
finding that the monkeys distinguished these grays decidedly
less well than the colors. Thirdly, it was proved that a
colored glass could be picked out correctly many times from
among three others covered with gray paper of the same
brightness as the color (221).

Sensory Discrimination : Vision 145

The most elaborate and careful experiments that have yet
been made on vision in the lower animals are those of
Yerkes on the Japanese dancing mouse. The method con-
sisted in teaching the animals to associate one of two differ-
ently illuminated compartments with a disagreeable electric
shock. In the perfected experiments on brightness dis-
crimination, the illumination of the compartments was
varied in intensity by arranging a light above each. One
light could be kept at a constant height and the other raised
or lowered. Weber's Law was proved to hold for the one
individual tested ; the ratio of the difference in brightness to
the absolute brightness being about one-tenth, between the
limits of 5 and 80 hefners of absolute brightness. For test-
ing color discrimination, after a series of experiments with
colored papers, a somewhat similar apparatus was used,
the light being filtered through colored screens (Fig. 12).
No ability to discriminate green and blue was displayed
unless the two were made very different in brightness. Light
blue and orange, green and red, violet and red, were dis-
criminated even when their brightnesses were considerably
varied. Yet the possibility that these discriminations were
made on the basis of brightness rather than color differences
is suggested by an interesting kind of evidence. After a
mouse had learned to choose, for example, green rather than
red, it was offered a choice between light and darkness,
and showed a uniform preference for the former, although
untrained mice do not. This looks as though the green had
been previously chosen as the lighter of the two colors. If
such were the case, the brightness values of the colors for the
mouse must be quite different from those which they have for
a human being. In fact, there are reasons for thinking that
the red end of the spectrum is much darker to the mouse's
than to the human eye. Even allowing for the possibility

FlG. 13. — Color discrimination apparatus used by Yerkes on the dancing mouse.
A, nest-box; B, entrance chamber; R, R, red niters ; G, green filter; Z,,L, incan-
descent lamps in light box ; 5, millimeter scale on light box ; /, door between
A and B; O, O, doors between alleys and A.

Sensory Discrimination : Vision 147

of discriminations based on brightness differences other than
those appreciable to human beings, Yerkes concludes that
the mice have a certain degree of ability to distinguish red,
green, and violet as colors. The mouse retina seems to be
lacking in cones (469).

CHAPTER VIII

SPATIALLY DETERMINED REACTIONS AND SPACE PER-
CEPTION

§ 53* Classes of Spatially Determined Reactions

MODIFICATION of the behavior of animals with reference
to the spatial characteristics of the forces acting upon them
appears at the very beginning of the scale of animal life, and
throughout is quite as important as modification with refer-
ence to the kind or quality of such forces. It assumes a num-
ber of distinct forms. Some of these suggest to us, interpret-
ing them as we must on the basis of our own experience, no
conscious aspect at all; they seem rather mechanical effects
upon a passive organism. In other cases, it appears possible
that the mental process which we know as space perception,
involving the simultaneous awareness of a number of sensa-
tions consciously referred to different points in space, may
accompany the reaction of an animal with reference to the
spatial relations of its environment. And sometimes we can
only say that differences in the space characteristics of a
stimulus may modify the accompanying sensation in some
manner which yet apparently does not involve space per-
ception as we know it.

Our task in the following pages will then be to examine
the different ways in which animal behavior is adapted to
the spatial characteristics of stimuli, and to ask which of these
suggest as their conscious accompaniment some form of space
perception. A classification of spatially determined responses

148

Spatially Determined Reactions 149

that is not, indeed, ideally satisfactory, but may serve our
purpose, divides them into five groups : —

1. Reactions adapted to the position of a single stimulus
acting at a definite point on the body.

2. Reactions to a continuous stimulus, which involve the
assumption of a certain position of the whole body with refer-
ence to the stimulus : orienting reactions.

3. Reactions to a stimulus that moves, i.e. that affects
several neighboring points on the body successively.

4. Reactions adapted to the relative position of several
stimuli acting simultaneously.

5. Reactions adapted to the distance of an object from the
body.

These forms of behavior will be successively discussed.

§ 54. Class I: Reactions to a Single Localized Stimulus
Responses to stimulation that are adapted to the point
of application of the stirmqliis are to be found among very
simpleanimals. They may be subdivided into three groups :
first, cases where the part of the animal that reacts is the part
directly affected by the stimulus; second, cases where the
whole animal reacts by a movement in the appropriate direc-
tion; and third, cases where a part of the body not directly
affected by the stimulus moves toward the point stimulated.

i. Amoeba furnishes an example of the first class. Its
negative reaction occurs by the checking of protoplasmic
flow at the point where a strong mechanical stimulus affects
the body; its positive reaction by a flowing forward of the
protoplasm at the point where a weak stimulus acts, and its
food-taking reaction by an enveloping flow on both sides of
the point stimulated. This would seem to be the most
primitive way of adapting response to the location of a stimu-
lus: the effect is produced just where the force acts, as it

150 The Animal Mind

might be upon a piece of inanimate matter. In no animal
with a nervous system, probably, is the process quite so simple.
The bell of the jellyfish contracts at the point where a stim-
ulus, mechanical or photic, is applied ; yet although these
responses are made when the nervous system is thrown out of
function, they occur more slowly, and in the normal animal
the nervous tissue is probably involved, while, of course, a
long conduction pathway is traversed when, to use a familiar
illustration, the baby pulls back its hand from the candle
flame.

2. Paramecium and other infusoria, planarians, the
earthworm, and various other animals give us illustrations
of movements of the entire body differing according to the
point affected by a single stimulus. If the front half of
Paramecium is touched, the animal gives the typical avoiding
reaction of darting backward and turning to one side; if
the hinder end be touched, it moves forward (211, p. 50).
On the other hand, it makes no difference in its reactions
to stimuli affecting either side of the body; the turning is
always to the aboral side even when the stimulus comes from
that direction (211, p. 52). If strong mechanical stimula-
tion be applied to the head end of a planarian, there is
a response which seems to belong under type (i) : the head
is turned away from the stimulus. If the hinder region is
touched, strong forward crawling movements of the body
are produced. The positive reaction in the planarian,
turning the head toward the stimulus, also suggests type (i),
but in reality it has been shown by Pearl to be a far more
complex affair than the mere flow of protoplasm at the
stimulated point, and to involve the contraction of several
sets of muscles (316). The earthworm creeps backward
if the front half of the body is affected, turns away from
a stimulus applied to the side of the anterior end, and creeps

Spatially Determined Reactions 151

forward if the stimulus affects the posterior half of the body
(210). In general, a reaction of type (2) rather than type (i)
will occur in proportion to the degree in which an organism's
movements are coordinated and it tends to act as a whole.

3. One of the prettiest examples of the most highly co-
ordinated form of response to a single localized stimulus,
namely, movement of some other part of the body toward
the point affected, is to be found in the swinging over of the
jellyfish's manubrium toward the spot on the bell touched
by food. "In the typical feeding reaction," says Yerkes,
"the manubrium bends toward the food. If during such
a movement the piece of food be moved to the opposite
side of the bell, the manubrium, too, in a few seconds will
bend in the opposite direction, that is, again toward the
food " (451). The sea urchin responds to mechanical
stimulation by moving the spines toward the place stimu-
lated (410). In the higher animals this form of reaction has
largely superseded other methods of adapting behavior to
a stimulus acting at a definite point. Where grasping
appendages exist, the obvious device is to move them toward
the point of stimulation in order either to seize or to remove
the object. This involves not merely that the effects of the
stimulus shall diffuse so as to involve general locomotor
movements, but that the effect shall be exerted very defi-
nitely upon a particular set of muscles in a particular way.
The "scratch-reflex" of mammals, and the reaction whereby
a frog rubs its hind leg on the spot of skin affected by a drop
of acid, are further examples.

What can we say regarding the conscious accompani-
ment of the reactions described under these three heads?
When a stimulus applied at point a brings about a reaction
different from that produced by precisely the same stimulus
acting on point b, are the accompanying sensations different,

152 The Animal Mind

supposing the animal concerned to be conscious? If they
are, the difference must be what has been called a difference
in local sign. There is certainly no evidence that space
perception is concerned. Space perception in our own ex-
perience always involves the simultaneous awareness of sev-
eral stimuli. But where a single stimulus only is operative,
the fact that reaction to it is modified by its location cannot
mean that the relations of that location to the location of
other stimuli are perceived. The truth is that space per-
ception is so constant a factor in our own experience that
we cannot imagine how a single sensation can be modified
in connection with change of place of the stimulus, where
space perception does not exist. A touch at any point
on the skin of a human .being is referred to a definite
point in a constructed space, tactile and visual; it is
given its proper place in a complex of sensations. What
modification of it would correspond to its location if it stood
alone in consciousness we cannot now conceive.

§ 55. Class II : Orienting Reactions; Possible Modes of
Producing Them

Various forces, such as gravity, light, electricity, centrif-
ugal force, currents of water and air, are all influences
causing certain organisms to bring their bodies into a defi-
nite position. Such reactions, involving the direction of
the whole body with reference to a continuous force acting
upon it, are known as reactions of orientation. There are
various ways in which they might conceivably take place.

(a) They might be due to the "pull" of a force upon
the passive body of an animal. In the case of gravity or
of a current of -wind or water, if one part of the body were
heavier or offered more surface to the force, the position
assumed could be explained without supposing any activity

Spatially Determined Reactions 153

on the animal's part. In such a case there would be no
reason for thinking of the reaction as conscious.

(b) The response might be due to the effect of a force
acting unevenly upon the two sides of the body, and thereby
unevenly affecting the motor apparatus on the two sides,
thus causing the animal to turn until the forces acting upon
symmetrical points were balanced. This, although involv-
ing activity on the animal's part, would not, if the force
acted directly on the muscles, suggest any conscious ac-
companiment.

(c) The orientation might take place by a negative
reaction on the animal's part to a definite stimulus given
when the animal was in any other than the final, oriented
position. If gravity were the force in question, the stimu-
lus might be the pressure exerted within the body by particles
of different density or by the fluid or mineral bodies in a
statocyst organ. If the stimulus were light, the organism
might be oriented by giving the negative reaction when its
head entered a region either brighter or darker than the
optimum illumination. In such cases, where the ordinary
negative reaction is the only one involved, there is no reason
to suppose the occurrence of any conscious accompaniment,
other than the possible unpleasantness connected with that
reaction.

(d) •rientation to gravity might occur through a special-
ized "righting" reaction, given in response either to a stimu-
lus within, say, a statolith organ, or, as in the planarian, to
the absence of accustomed contact stimulation on one sur-
face of the body. The reaction in these cases being a special-
ized one, it is possible that a peculiar sensation quality might
be involved.

(e) Orientation might take place through a movement
occurring when the position of several stimuli perceived

154 The Animal Mind

simultaneously was disturbed, and tending to restore them
to their original position. This is the principle involved,
as we shall see, in explaining the rheotropism or current
orientation of fishes, and the anemotropism, or orientation
to air currents, of insects, as due to an instinct to keep the
visual surroundings the same. And this form of orienta-
tion alone suggests a true space perception as its conscious
accompaniment.

Such being the conceivable ways in which orientation may
be brought about, what are the observed facts? They may
be considered under the heads of orientation to gravity, to
light, and to other forces.

§ 56. Orientation to Gravity: Protozoa

To this form of reaction the term "geotropism" or "geo-
taxis " has been applied. In various Protozoa negative geo-
tropism, or a tendency to rise against the pull of gravity, has
been observed : first by Schwartz in two single-celled organisms
frequently classified as plants, Euglena and Chlamydomonas
(378); and eight years later by Aderholcl, who suggested,
without accepting it, the theory that the orientation may be
due simply to the greater weight of one end of the organ-
ism's body (2). This view was maintained by Verworn : the
action of gravity, he urged, must be purely passive. It can-
not operate as a stimulus to active response on the animal's
part, for a stimulus is always a change in environment, and
gravity is a constant force (416). This ignores the fact that
the animal's relations to gravity may change though
gravity does not. According to Verworn's theory, the
geotropic orientation of a single- celled organism takes place
through a series of "little falls" whereby the heavier end is
directed downward. Massart opposed this view on the
basis of observations which showed that the actual move-

Spatially Determined Reactions 155

ments of Jthe organisms did not correspond to it, but were the
result of active orientation. If response to gravity is passive,
then dead animals should fall through the water in the same
position as that assumed by living animals when oriented
to gravity. Massart experimented with various Protozoa by
killing them and studying their methods of sinking, which
he found not always the same as the attitudes assumed in
response to gravity (259). There is always the possibility,
however, that the methods employed to kill may change the
specific gravity of some part of the body. Jensen offered
the theory that reaction to gravity may be due to the difference
in the water pressure on the two ends of the animal. He
asserted that when the air pressure on the water was reduced
by exhausting the air above, there was an increase in the
geotropism, indicating a relative rather than an absolute
sensibility to pressure (215), but Lyon points out that this
process may affect the animals in various other ways besides
altering the air pressure. Increasing the air pressure, or
protecting the surface with oil, has no effect upon geotropism,
Lyon finds, and he urges that Jensen's theory requires
enormous sensibility to pressure differences on the organism's
part, as great as that needed by a human being to note the
difference between the air pressure on the head and that on
the feet (255). Another suggestion was offered by Daven-
port, namely, that negatively geotropic organisms swim in
the direction where the greatest resistance to their progress
is offered. This is like one theory put forward to explain
rheotropism, or the tendency of animals to swim against
currents, and anemotropism, or the "head against wind"
movement of insects; and as R£dl (355) first and Lyon (254)
afterward pointed out, it assumes the fact to be explained,
for only if an animal actively opposes a force, will that force
exert more pressure at one point of its body than at another.

156 The Animal Mind

The theory cannot explain why an animal at rest should be
oriented. Another argument that tells against it is offered
by experiments showing that animals placed in solutions
of the same density as their own bodies, in which, therefore,
they have no weight, still display negative geotropism, and
that the direction of the response is not reversed when the
fluid is made heavier than the animals (255). Lyon's own
theory, accepted by Jennings, is that the stimulus for geo-
tropism is furnished by the action of gravity within the body
of the organism, upon substances of different weight which
exert varying pressures and take up different positions ac-
cording to the position of the body (255).

It has been shown that the reactions of Paramecium to
gravity are modified by a variety of conditions. Negative
geotropism, in a sense their normal condition, is favored by
plentiful food supply and by an increase in temperature
within certain limits; positive geotropism, movement down-
ward, may be brought about temporarily by mechanical
shock, by salts and alkalies, by temperature changes (278,
388) to which, however, the animals may adapt themselves;
with less constancy by increase in the density of the fluid
containing them, and with lasting effect by lack of food.
It has been suggested that the downward movement under
these circumstances is protective, since it shields the animals
from surface agitation of the water, from surface ice, and
from failure of the surface food supply (278). We shall
see that similar conditions often change the direction of an
animal's response to light.

§57. Orientation to Gravity: Ccdenterates
Among the ccelenterates, geotropism is shown by certain
hydroids, whose stems have a tendency to curve upward
and their " roots " a tendency to grow vertically downward

Spatially Determined Reactions 157

when the animals are placed in a horizontal position (402).
The sea-anemone Cerianthus, whose normal position is head
upward, will right itself if placed in any other position,
though the righting reaction may be inhibited by contact
stimulation on the side of the animal. It ordinarily lives
with the body enclosed in a tube, and when taken from its
proper habitat it seems to " prefer" a position, even hori-
zontal, where the sides of the body are in contact with a
solid, to a vertical position with its sides uncovered (237).
The righting reaction of Hydra is not determined by gravity
at all; the animal will take any position, vertical or hori-
zontal, but "seeks" always to have its foot in contact with
a solid (418). Some actinians have shown an interesting
modification of gravity reaction through what we may call
habit. Six specimens of Actinia equina were selected that
had been fixed to the rocks in an " upside-down " position,
that is, with the mouth end downward ; and six others that
had been right side up. In the first experiment all were
placed upside down; the tendency to right themselves was
decidedly stronger in those which had been previously erect.
Similarly, when twelve selected in the same way were all
placed right side up, the ones that had previously been in
the reversed position showed a certain inclination to reassume
it (143). On the other hand, the orientation of the polyp
Corymorpha palma to gravity was entirely unaffected by
keeping the animal for a long time in a position where it
could not right itself; it assumed the upright position as
soon as it was set free (402).

It was noted in the chapter on hearing that the peculiar
organs occurring in certain Ccelenterata and in many other
animals, which were originally called otocysts because of
their supposed auditory function, have had their name
changed to that of statocyst since it has appeared that they

158 The Animal Mind

subserve chiefly orientation to gravity. In jellyfish, removal
of these organs does not seem to affect the animal's power of
keeping its balance; apparently equilibrium is maintained
here by the simple action of gravity, for dead jellyfish float
in the right-side-up position (286, 291). It has been sug-
gested that the statocyst organs are for the reception of
stimuli produced by shaking, to which medusae are ap-
parently sensitive (291). Negative geotropism exists in
Gqnionemus, which swims to the_ surface of the water when
disturbed (470). fnTctenophors, the statocyst organ, which
is usually at one pole of the body, has been found to function
as an organ for the maintenance of equilibrium (415).

§ 58. Orientation to Gravity: Planarians
A good example of a specially developed reaction having
for its result the " righting" of an animal in an abnormal
position is offered by the behavior of a planarian that has
been turned over so that its back rests on the surface of
support. The reaction consists of a turning of the body,
beginning with the head end, about the long axis, so that
a spiral form is assumed. The dorsal surface of the animal
is convex, the greatest thickness of the body being in the
middle line. When the planarian lies on its back, it thus
naturally tips to one side, like a keeled boat out of water.
This side, being brought into contact with a solid, gives
a reaction analogous to the negative one, that is, it extends
or stretches. Such a stretching of one side when the planarian
is right side up would of course produce a turning in the
opposite direction, a negative reaction. In this case, however,
the opposite side does not contract to allow of turning, but
maintains the same length. The necessary result is that
the body is thrown into a spiral: as soon as the ventral
surface of the head comes into contact with the solid, in

Spatially Determined Reactions 159

consequence of the turning, the negative reaction of that
end ceases. Thus the righting is progressively accomplished
(316). The whole response can hardly be classed under
the head of geotropism. Like that of Hydra, it is not made
as the result of the pull of gravity, but is a reaction to contact
stimulation; the animal will crawl in an upside-down posi-
tion as readily as any other provided that the ventral sur-
face and not the dorsal is in contact with a support.

§ 59. Orientation to Gravity: Annelids
Geotropism, in the marine worm Convoluta roscoffensis,
has been found to fluctuate with the rise and fall of the tides,
even when the animal is removed to an aquarium. In
normal life the worms burrow in the sands at rising water,
and come to the surface when the tide retreats. Prolonged
exposure to air, or increase in the intensity of the light,
causes them to move down the slope of the shore to moist
places. These movements in the normal environment are
represented by upward and downward movements of the
animal when confined in a glass tube. Keeble and Gamble
thought these oscillations in geotropism did not occur in
darkness, and that the stimulus bringing them about was
photic. When the summation of light stimuli passes a cer-
tain amount, they maintained, positive geotropism appears ;
when the after effect of light stimulation is dissipated, the
negative phase recurs (140). Bohn, however, finds that the
oscillations do persist in darkness, and that their primary
cause is the mechanical shock of the waves, as is further
indicated by the observation that shaking the tube will
cause the worms to descend (35). The geotropism of Con-
voluta is dependent on the statocyst (140).

160 The Animal Mind

§ 60. Orientation to Gravity: Mollusks

Among Mollusks, the slug has had its reactions to gravity
carefully observed. When placed in a horizontal position
on an inclined glass plate, these animals tend to turn either
upward or downward, moving either with or against the
force of gravity. Davenport and Perkins found that the
same individuals differed at different times in this respect,
and concluded that the sense of the geotaxis was determined
by obscure conditions. They also found that an inclination
of only 7.5° on the part of the glass plate, representing only
13° of the full force of gravity, is sufficient to make the slugs
orient themselves with reference to the pull of the earth,
though the precision of such orientation increases as the
angle increases (95). Frandsen thought it was the weight
of the posterior part of the body that determined whether
the movement should be up or down: that the natural
tendency of all was to go downward, but that in some in-
dividuals the posterior part, which is poorly controlled,
was heavier than the anterior, and pulled the animal around
head upward (135). The statocyst organs in a cephalopod,
Eledone, have been shown to function in maintaining
equilibrium (137).

§ 61. Orientation to Gravity: Echinoderms

Very interesting righting reactions, in the starfish and sea
urchin, are described by Romanes. The starfish rights
itself by twisting around the tips of two or three of its rays
until the suckers in the ventral side have a firm hold of the
supporting surface, and then continuing the twisting, always
in the same direction on the different rays, until the whole
body is turned. The sea urchin, "a rigid, non-muscular and
globular mass," with relatively feeble suckers, has a much

Spatially Determined Reactions 161

harder time of it, and does not succeed in pulling itself over
unless it is perfectly fresh and vigorous. It occasionally
rests for some time when it has reached a position of stability
halfway over, before continuing the process (365).

Lyon has observed marked negative geotropism in the
larvae of the sea urchin. He was unable to test Davenport's
theory of the nature of the geotropic response by putting the
animals in a solution of the same density as their own bodies,
for the reason that such a fluid was too dense and sticky
(being made of gum arabic and sea water) for them to swim
in. That the response was merely a passive one he
thinks improbable, because the larvae from eggs that have
been rapidly rotated, or "centrifuged," as it is called, have
all the pigment on one side of their bodies and may therefore
be supposed to have their ordinary balance disturbed; yet
they rise to the surface just like the rest (256).

§ 62. Orientation to Gravity: Crustacea
That the statocyst organs in Crustacea are probably con-
nected with equilibrium rather than with hearing we have
already seen. Delage in 1887 found that Mysis, Palaemon,
and other forms displayed serious disturbance of equilibrium
when both eyes and statocysts were destroyed, showing that
the e^es_also play a part in the maintaining of balance (97).
The eyes have been found to cooperate with the statocysts
in the fiddler crab, Gelasimus, and also in another decapod,
Platyonichus (78). Neither of these has statoliths. Penaus
membraneus, on the other hand, was found to be permanently
disoriented by destruction of the statocysts or even removal
of the statoliths, while blinding produced no great dis-
turbance, probably because of the animal's nocturnal habits
(19, 138). Young crayfish with the statocysts destroyed
will swirn upside down as readily as right side up (71).

1 62 The Animal Mind

But the prettiest evidence for the static function of the
statocysts was obtained when powdered iron was substituted
for the mineral bodies in the open statocysts of Palaemon.
It was found that when a magnet was brought near, the animal
would respond by taking up a position corresponding to the
resultant of the pull of the magnet and that of gravity (226).

Specific righting reactions occur in many Crustacea,
though in some cases these seem to be merely the incidental
effects of their ordinary locomotion. Branchipus, the fairy
shrimp, normally swims upside down; if turned right side
up when moving along the bottom of the vessel, it continues
to move in this position without showing any disturbance
until it happens to rise a little from the bottom, when ap-^
parently the weight of the body pulls it around into the
usual upside-down position. The crayfish has two methods
of righting itself: a quick "flop" executed with the tail,
and a slow and laborious raising of itself on one side and
tipping over (96).

Many Crustacea show marked responses to gravity:
for example, Parker found decided negative geotropism in
the females of the marine copepods whose depth migrations
he studied. It seems to be needed to counteract the tendency
of the animals to fall to the bottom by their own weight (304).
In certain copepods, light was observed to change the sense
of the response to gravity, not by taking its place as a directive
stimulus, but apparently by producing some physiological
change in the animals. Their normal geotropism was
positive, that is, they had a tendency to move downwards.
In darkness, however, their geotropism became negative.
They were also negatively phototropic to strong light. If,
when in the negatively geotropic phase, they were illuminated
from below by intense light, from which they would ordinarily
have moved away, the change from negative to positive

Spatially Determined Reactions 163

geotropism induced by the light was of sufficient influence
to make them move downward toward it (113). Other
facts regarding the relation of geotropism and phototropism
are mentioned on pp. 182 ff.

§ 63. Orientation to Gravity: Spiders and Insects
| Spiders and insects have no statolith organs. Bethe
^thinks that equilibrium is maintained in their case as a
natural result of the position of the centre of gravity and the
distribution of air in the body. He supports this view by
experiments in which dead insects, allowed to fall through
the air, assume the normal position, and is inclined to think
that all animals without special static organs maintain their
balance in this way (27). Negative geotropism in certain
insects, as evidenced by a tendency to creep from horizontal
planes up vertical ones, was observed by Loeb (234). In
light the eyes of insects have probably much to do with
maintaining equilibrium. Certain aquatic insects, in ex-
periments where the light was made to strike them only
from below, as soon as they left the support on which they
were resting, turned themselves upside down (355).

§ 64. Orientation to Gravity: Vertebrates
It has long been known that in vertebrates the static
function resides in the ear, and especially in the semicir-
cular canals (e.g., 70, 85, 128, 147). Various experimenters
have noted that operations on the ears of fishes disturb the
equilibrium of these animals. Sewall, indeed, found that
section of the semicircular canals in the shark had no effect
on its balancing powers, although operations on the vesti-
bule and ampullae did disturb movement (380) ; and
Steiner got no effect on equilibrium from removing the
contents of the labyrinth (391). Errors in method and

164 The Animal Mind

observation probably influenced these results. Loeb found
that severing the auditory nerve or removing the statoliths
from the dogfish caused the fish to incline toward the operated
side and to roll the eyes in that direction (238). Total
extirpation of one labyrinth in the perch was observed by
Bethe to make the fish curve toward the affected side. The
fish Scardinius showed a tendency to curve toward the
opposite side (27). Lee's experiments on the dogfish showed
a very definite relation between the position of the canal
operated upon and rolling movements of the fish. Cutting
the front canals caused the fish to dive forward, cutting the
rear canals made it dive backward, and cutting the canal
on either side made it roll over toward that side. A natural
explanation of this behavior is to suppose that the absence
of stimulus from the cut canal produces the same effect that
rolling the fish in the opposite direction, and thus diminish-
ing the pressure of the fluid in the canal, would produce.
The fish " feels as if" it were being rolled over, and makes
movements to regain its equilibrium. When the nerves
supplying the ears on both sides were cut, the fish became
perfectly indifferent to its position and would float upside
down without any effort to right itself. The vestibule and
otoliths of the fish ear are thought by Lee to be concerned
with statical equilibrium; that is, with the maintenance of
position while the fish is at rest, while the canals are concerned
with balance during motion (dynamic equilibrium) (230).
It may be added that experiments on the sea horse indicate
that destruction of the labyrinths in this animal has no effect
on equilibrium: the upright attitude is due to the position
of the air bladder and is assumed even by dead animals (139).
That vision may materially aid in maintaining equilibrium
in vertebrates is indicated by evidence from various sources,
among others, the observation of Bigelow that goldfish in

Spatially Determined Reactions 165

which the nerves supplying both ears had been cut recovered
after two or three weeks and could swim quite normally
except when they were placed in a large body of water and
made to swim rapidly, when they showed no power of pre-
serving their balance (33). Their successful performance
of slower movements was very likely due to the use of
sight.

Sensory impulses from the body muscles themselves un-
doubtedly cooperate with those from the semicircular canals
in the maintenance of balance. They are evidently involved
in the peculiar withdrawing movements by which land ani-
mals, even puppies, kittens, and young rats whose eyes have
not opened, save themselves from falling when they reach
the edge of the object on which they have been crawling
(271, 384). Water-dwelling animals, accustomed to plunge
off solid supports, lack this protective instinct; Yerkes
showed that among several species of tortoises, some land-
dwelling, some amphibious, and some aquatic, the first men-
tioned were much more reluctant than the second to crawl
off the edge of a board, and the second more reluctant than
the third (459).

§ 65. The Psychic Aspect of Orientation to Gravity
Glancing back over these examples of the responses made
by animals to gravity, we note that while in some cases the
earth's attraction appears to act mechanically upon the
animal, causing the body passively to assume a certain
position, the common method of bringing about orientation
seems to be that some structure in the body, placed in an
abnormal position, presents a stimulus which brings about
a compensatory movement. This structure may be heavier
particles of the body substance, as probably is the case in
Paramecium ; it may be a statolith, or the fluid in the laby-

1 66 The Animal Mind

rinth; it may be the eyes. In any case, what shall we say
about the sensation quality involved ? Perhaps the reactions
produced are wholly reflex. Perhaps the statolith or the
canal fluid produces a specific sensation quality. Or perhaps,
as Verworn thinks, the sensation quality is merely that of
pressure (415). Whatever its nature, spatial perception,
the perception of the spatial relations between several stimuli
simultaneously apprehended, plays no part in the orientation
of animals to gravity.

§ 66. Orientation to Light: Photopathy and Phototaxis
One of the first facts that confronts the student of the
ways and means by which animals become oriented to visual
stimuli is the distinction which Loeb drew between what he
called heliotropism and sensibility to difference (Unterschieds-
empfindlichkeit) (239) ; and to indicate which the terms
" phototaxis " and " photopathy " have also been applied.
The phenomena are as follows. Strasburger, working on
the swarm spores of certain plants, thought he had evidence
that their reactions to light evinced not so much a tendency
to seek a certain intensity of illumination, as a susceptibility
to the direction from which the light came. He placed over
the vessel containing them an India ink screen, thicker at
one end so as to cause gradations in the intensity of the light
reaching the vessel. When the light fell perpendicularly
through this screen, the distribution of the swarm spores
through the vessel was nearly uniform ; that is, the differences
of intensity had no effect. When the screen was removed,
and the light fell at an angle, the spores immediately oriented
themselves to its direction, and preserved this orientation
even when the screen was replaced (392). The word
phototaxis, instead of being used to designate any reaction
to light, has been narrowed to designate the tendency to

Spatially Determined Reactions 167

orient with reference to the direction rather than to the
intensity of the light.

Now on the other hand there are some cases where animals
apparently seek or avoid light without being oriented at all ;
that is, without having their bodies placed in a definite position
during the movement. Planar ians are an example of this.
Increased intensity of light stimulates them to activity ; they
crawl about until they reach a shaded portion, where they come
to rest. Their movements are not directed straight away from
the light ; in other words, it does not negatively orient them,
but it excites them and the shadow brings them to rest (239).
To cases such as this, where a certain intensity of light
stimulates activity while a different intensity may inhibit it, but
where no orientation of the body with reference to the direction
of the rays occurs, the term "photopathy" may conveniently
be applied. Bohn suggests that the tendency of certain ani-
mals to come to rest in shaded portions may really be an
expression of fatigue produced by the action of the light (55).

A second problem arises in connection with the mechanism
of phototaxis. How does light orient an animal? Does
it exert an effect upon the muscles or locomotor organs of the
body that is equivalent to pulling the animal around into the
required position? Or does the organism become oriented
because in a series of movements those which would bring it
out of the oriented position are corrected by negative reactions ?
Again, if the effect of light upon the body is direct, producing
orientation by bringing the animal at once into line with the
light rays, is this effect produced by the direction of the light
rays as they pass through the body, or by the fact that in any
other than the oriented position two symmetrical points on
opposite sides of the body are illuminated with unequal
intensity, a theory of phototaxis which would bring it into
nearer relation with photopathy?

1 68 The Animal Mind

§ 67. Instances of Photopathy and Phototaxis

The phenomena of orientation to light in different groups
of animals suggest now one, now another of these questions.
In Protozoa, although attempts have been made to show that
orientation is produced by the direct effects of light on sym-
metrical points, according to the observations of Jennings
(206) and Mast (261), it seems to be due to negative reactions
given when the organism, in its ordinary swimming move-
ments, either passes into a region of greater or less illumina-
tion, or swings its anterior end " to ward or away from the
source of light, so that it is shaded at one moment and strongly
lighted at the next." That is, the reactions are caused, not
by the direction of the light rays as such, but by differences
in the intensity of illumination. Strasburger's results, in
which the swarm spores moved toward the light into a region
of less intense illumination, Jennings holds were due to the
fact that "turning the sensitive anterior end away from the
source of the light" would diminish the effective illumination
of the animal more than passing into the slightly less illu-
minated region. That is, the two ways of changing the inten-
sity of the stimulus, moving forward into a darker region,
and turning the head end away from the light, are here
opposed : the latter effect is stronger than the former, hence
the organisms do not turn the head end from the light,
or rather they make the negative reaction when it is so
turned, and do move toward the shaded region. "If the
difference in intensity of light in different parts of the drop
were increased till the change in illumination due to pro-
gression is greater than the change due to swinging the
anterior end away from the source of light, then the positive
organisms would gather in the more illuminated regions"
(211, p. 148).

Spatially Determined Reactions 169

In Volvox, also, orientation is held by Oltmanns (298) and
Mast (262) to be an affair of intensity differences rather than
of light direction. The reaction of a Volvox colony, which in
moderate light is positively phototropic, occurs in consequence
of a response by each individual in the colony given when,
as the colony rotates, that individual passes from a higher
to a lower intensity of light.

In Hydra, the effect of light is photopathic rather than
phototactic. We have seen that these animals, when sub-
jected to light either above or below a certain " optimum"
intensity, wander about until they reach a region of the right
degree of illumination ; their movements manifest no definite
orientation (444). One sea-anemone, Actinia cereus, ob-
served by Bohn, does show an oriented response to light.
Weak light causes expansion of its tentacles perpendicularly
to the light rays. If the light is increased, the tentacles "tend
to orient themselves in the direction of the rays, and finally
converge in a bundle parallel to that direction," a response
which has the effect of protecting them from the intense
light (62).

The medusa Gonionemus offers an instance of opposition
between photopathy and phototaxis, the former being nega-
tive, the latter positive, in daylight. That is, it moves toward
the light when swimming, but being less active in darkness
than in light, it comes to rest, and hence tends to collect,
in darkened regions. Intense light gives a negative photo-
taxis. Sudden decrease and sudden increase of light intensity
have alike the effect of temporarily inhibiting activity. On
swimming either from shadow into sunlight, or from sunlight
into shadow, the medusa stops, turns over, and sinks to the
bottom. But when this effect has been produced by entering
shadow, the animal, on again becoming active, may move
in any direction ; when it has been produced by entering sun-

170 The Animal Mind

light, the medusa on beginning to move again "usually turns
in such a way as to move back into the shaded region." This
effect Yerkes, who first observed it, thinks due to the contrac-
tion of the bell on the more illuminated side ; that is, it is a
definite reflex to a localized stimulus. Orientation results
from the fact that the greater intensity of stimulus on one side
of the bell produces contraction at that point (451, 470, 468).

We have already seen that planarians offer an illustration
of photopathy. Light is not, however, wholly without effect
on the direction of the animal's movements. It has been
found that when planarians are placed with the head toward
the source of light, they have a distinct tendency to turn out
of the path, while if their heads are directed straight away
from the light, their tendency is to keep in the path (313).
It is probable that when the animal turns its head toward the
light, its movement is checked by a negative reaction. An
attempt has been made to show that photopathy, rather than
response to the direction of the light rays as such, governs the
responses of the land planarian Bipalium kewense to photic
stimuli. The apparatus was arranged so that a shadow was
thrown from above upon an area of light coming horizontally
from one side. Although the animal is negatively phototropic
to a marked degree, it would crawl toward the source of light
in order to get into the shadow (81). The explanation for
this may very likely correspond to that offered by Jennings for
the reverse behavior of Strasburger's swarm spores. That
is, the planarian might have obtained a greater diminution
of light intensity by moving into the shadow than by turning
aside from the path of the rays. The one possibility excluded
is that the negative reaction of planarians is a response to the
direction of the light rays as such.

The same evidence for photopathy, as distinct from photo-
taxis produced by direction of light, has been obtained for the

Spatially Determined Reactions 171

earthworm Allolobophora fatida (81). Yet the movements
of earthworms are oriented by light ; as we have seen, they
tend to move away from a source of light. This orientation
Holmes believes to take place by the checking of random
movements of the head in the direction of the light. In the
crawling movements stimulated when light is thrown upon
the worm, the head is turned from side to side. If it happens
to be turned toward the light, it is withdrawn. Holmes ex-
plains the observation of Parker and Arkin that the head of
the worm is much more apt to turn from the light than toward
it (312), by saying that account was probably taken here only
of the first decided turn made. He himself experimented by
lowering a worm, crawling on a wet board, while its body was
in a straight line and contracted, into a beam of light at right
angles to the body, and noting the first movement of the
head. This was found to be twenty-seven times away from
the light and twenty-three times toward the light. A similar
method of orientation by "trial and error'' was observed in
the leech and in fly larvae by Holmes (185).

E. H. Harper, on the other hand, working on the earth-
worm PericluBta bermudensis, declares that if the light is
strong enough there are no random movements of the head at
all, but that the first movement is a direct reflex away from
the light. When the light is only moderate, the appearance
of random movements is due to the fact that the worm is less
sensitive in a contracted than in an expanded state. Loco-
motion consists in a series of contractions and expansions,
and "as each extension begins in a state of lower sensibility,
the anterior end may be projected toward the light, only to
be checked when its increase of sensibility with extension
makes the stimulus appreciated" (161). A similar sugges-
tion that orientation may occur either by a definite reflex or
as the outcome of random movements, according to the ani-

172 The Animal Mind

mal's physiological condition, is to be found as early as the
work of Pouchet on fly larvae. He noted that the courses
taken by the larvae away from the light were either straight,
"or they present to right and left indentations due to the
wavering movements which the animal makes ... in a
certain number of cases, as if to take at each instant a new
direction." These individual differences might have been
accounted for, says Pouchet, by differing degrees of hunger
in the larvae (347).

Phototaxis in certain tube-dwelling marine worms was ob-
served by Loeb. Spirographis spallanzanii gradually curves
its tube until its oral end faces the direction from which the
rays of light come ; and another marine worm, whose tube is
absolutely stiff, adapts itself to a change in the direction of
the rays by curving the newly -formed portions of the tube as
it constructs them (236).

Attempts to show the independence of photopathy and pho-
totaxis by causing a positively phototactic animal to move
toward the source of light even when, by an arrangement of
screens overhead, such movement brings it into a region of
dimmer illumination, have been made with apparent success
on the crustacean Daphnia (93). That no increase in the
intensity of the light will reverse Daphnia's positive phototaxis
is also evidence that photopathy, the seeking of an optimum
intensity, is absent in these Crustacea (457). Simocephalus,
being made to collect in the brighter regions of a trough and
showing no orientation to light rays entering the trough at
right angles, seemed to display photopathy independent of
phototaxis (448). It is very difficult, however, to be sure in
such experiments that the direction of the light rays and the
intensity of the illumination are really independently varied,
for the diffusion of light by floating particles and its reflection
from the sides of the trough offer disturbing factors. The

Spatially Determined Reactions 173

amphipod Talorchestia longicornis, which moves toward the
light but comes to rest in shaded portions, seems to combine
positive phototaxis with negative photopathy (181). Loeb's
observations on the larvae of the arachnid Limulus, the horse-
shoe crab, and upon insect larvae, may also be mentioned
here. When strongly negative, the former moved away in
the line of rays of sunlight falling obliquely from a window
upon the vessel containing them ; the shadow of the window
bar lay across the vessel, and the animals continued to move
through it in the same direction, although, on passing out from
it, they went into a more brightly lighted region (239). A
similar illustration of phototaxis without photopathy was
found in the caterpillars of the Porthesia moth, which give a
positive response, and in fly larvae, which are negative (235).

§ 68. Direction and Intensity Theories of Phototaxis
The problem as to whether orientation to light is brought
about by the influence of the direction of light rays as such, or
by the fact that light falling upon an oriented organism from
a given direction affects symmetrical points with different
degrees of intensity, is one requiring much nicety of discrimi-
nation between concepts. Loeb, in his earliest discussion of
the subject, expresses himself positively in favor of the former
hypothesis. "The orientation of animals to a source of light
is, like that of plants, conditioned by the direction in which the
light rays traverse the animal tissue, and not by the difference
in the light intensity on the different sides of the animal"
(233). To this Bohn urges as a "fundamental objection"
that "the 'luminous rays' which strike a living body have,
save in wholly exceptional cases, various directions, being
reflected, diffused and refracted by neighboring bodies" (55).
Certainly if definite orientation to light occurred only when an
animal's body was traversed by rays in one predominant direc-

174 The Animal Mind

tion, it would be of little practical service. The other view,
that the important factor is the difference in the intensity of
stimulation of opposite points on the unoriented animal's
body, is that held by Verworn (417). Holmes points out that
no crucial test experiment of the two hypotheses has ever
been made. Such an experiment would require that a semi-
transparent animal should have two symmetrical points on its
body, a and b, stimulated with exactly equal intensity, each
by a ray of light coming from a different direction. Under
such circumstances, according to the theory of Verworn, the
animal ought to move straight forward (181). An attempt
to get evidence was made by Davenport and Cannon in a
study of Daphnia. They proposed the following question:
Do positively phototactic animals move more rapidly toward
their optimum intensity than toward an intensity below the
optimum? If orientation is determined, as the Verworn
theory supposes, by the relative intensity of light on different
points of the organism, then the absolute intensity of the
light ought not to affect it. If, on the other hand, the direc-
tion of the rays orients the animal, then precision of orienta-
tion should increase as the absolute intensity approaches the
optimum. Daphnia was found to move somewhat less
rapidly toward the light when the intensity of the latter was
reduced ; this fact was held to be due to diminished precision
of orientation and hence to tell for the theory of Loeb1 (93).

§ 69. The Eyes in Phototaxis

The directive theory of phototaxis is of little significance
in connection with the reactions to light of organisms whose
bodies are opaque and which have eyes. For the eyes seem
to be fundamentally concerned in orientation to light. That

1 A discussion of the intensity, and direc^on theories will be found in Holt
and Lee's article on "The Theory of Phototactic Response" (187).

Spatially Determined Reactions 175

this is the case in Daphnia was shown by Radl, who placed
the animal under a microscope in such a way that only the
eyes could be moved. When the light coming from below
was diminished, the eyes rolled upward; when the light
coming from above was diminished, they rolled downward.
The precise positive phototaxis of Daphnia, Ra"dl thinks, is
primarily an eye movement, the body being turned to follow
the eyes (354). Indeed, Radl is of the opinion that in all
animals having eyes, the essential feature of phototropism is
eye-orientation, wholly analogous to fixation in the case of
human vision (356). In amphipods, blackening of one eye
of a positively phototropic animal caused a turning toward the
blackened side, as if the animal were trying to restore the
missing illumination; similar experiments upon negative
animals produced turning toward the other side (181). Like
phenomena have been observed in other Crustacea, in
mollusks, annelids, and insects. Bonn, like Radl, is in-
clined to explain the light tropisms of animals with eyes as
entirely due to an effect, either tonic or inhibitory, according
as the animal is positive or negative, of light acting through
the eyes upon the muscles of the same side of the body. If
one eye received more light than the other, a positive animal
would turn toward the darker side because the muscles on
the side toward the light would be more strongly stimulated.
A negative animal would turn toward the light because of
inhibition of muscular activity on that side. Orientation
may then be effected in a normal animal when the eyes
equally illuminated (55).

: 01

ion \

are ]

§ 70. Influences Affecting the Sense of Light Orientations
In no class of animal responses to stimulation is the effect
more dependent upon the cooperation of a number of condi-
tions than in those involving orientation to light. Many in-

176 The Animal Mind

fluences have been found to reverse the sense of light reactions,
transforming negatively phototropic into positively photo-
tropic animals, and vice versa. That such reversal should
occur in response to increase or decrease of the intensity of
the light is what one would naturally expect ; if a certain in-
tensity of illumination is favorable to the life processes of an
animal, it would seem appropriate for it to seek light of that
intensity but avoid light of greater intensity. Many animals,
like Gonionemus, are positive to light of moderate intensity
and negative to strong light (451). The females of the crusta-
cean Labidocera migrate to the surface of the water at night-
fall because, like the earthworm, they react positively to faint
light ; and move downward at sunrise because they are nega-
tive in their response to intenser light (304). On the other
hand, Holmes observed that Orchestia agilis, an amphipod
crustacean, would, if brought from strong to weaker light, be-
come negative for a short time ; the meaning of such a change
it is difficult to conjecture (181). Sudden reduction of light
causes a temporary negative phase also in Convoluta roscof-
fensis (140).

Prolonged action of^Jight__ma,y alter phototropism : the
"depth migrations," that is, the periodical movements toward
and away from the surface of the water, in the free- swimming
larvae of the barnacle, Balanus, are due apparently to the fact
that an exposure of several hours to light will make positive
animals negative, even though the light at the end of the
period of exposure is decidedly fainter than it was at the be-
ginning (153). The positive reactions of the water insect
Ranatra increase in violence the longer the light acts ; on the
other hand, after being kept in darkness for several hours,
Ranatra is negative on first being taken out (186). Daphnias
kept in darkness for a time become decidedly negative to dif-
fused daylight, whereas if kept in light they would have been

Spatially Determined Reactions 177

positive. A sudden change in light intensity, either brighten-
ing or darkening, has the effect of making positive Daphnias
temporarily negative (302).

Temperature changes influence response to light. The ob-
vious suggestion here would be that since increased tempera-
ture often accompanies increased intensity of light, animals
that are positively phototropic only up to a certain degree of
illumination ought to become negative when the temperature
is decidedly raised. This, however, is by no means always
the effect produced by increased temperature. Strasburger's
swarm spores became positive in higher temperatures, nega-
tive in lowered ones (392). Orchestia agilis, which we have
just seen becomes temporarily negative on being brought from
strong into weak light, may be made positive again if the water
is slightly warmed. When the same animal is dropped into
water, it becomes strongly negative, but it will show a positive
response if the water is heated almost to a fatal point (181).
On the other hand, the copepods and annelid larvae studied by
Loeb were made negative by increased, positive by lowered,
temperature. Other crustaceans, e.g. Daphnia (457), had
their responses to light unaffected by a fairly wide range
of temperature changes.

Increasing or decreasing the-devmty ^Jhe_juiaier will also
affect phototropism. In some copepods diluting the water
produced negative responses to light, while increasing its den-
sity brought about those of the opposite sign (239). Diluting
the water produced negative phototaxis in the larvae of
Palaemonetes (257). Parker failed to find any such effect in
the case of the copepods studied by him (304). W. Ostwald
has called attention to the possibility that "internal friction''
between the organism and the medium may affect various
tropisms. Freshly caught Daphnias, which are negative
or indifferent, quickly become positive if gelatine or quince

178 The Animal Mind

emulsion is added to the water. Since they would become
so in time anyway, Ostwald thinks the mechanical friction
of the sticky liquid simply acts as a " sensibilator " and brings
on this positive phase sooner (302).

Change in_the purity of the water, also sometimes produces
change of sign in the response to light. The amphipod
Jassa, negative in ordinary sea water, becomes positive in
foul sea water (181). The presence of chemicals is an influ-
ence probably identical with the one just mentioned. Vari-
ous Crustacea have had the direction of their reactions
changed by carbonic or other acids, ammonium salts, ether,
chloroform, paraldehyd, and alcohol (244). The ultra-violet
rays will make positive Balanus larvae temporarily negative

(245).

The^ statejof hunger_or satiety in an animal must be reck-
oned with: the caterpillars of Porthesia, for example, are
decidedly positive when hungry, much less so when fed (236).
The slug Limax maximus, ordinarily negative to strong light,
is positive to light of any intensity when hungry (135).

Mechanical stimulation is most striking in its effect on light
reactions. Pouchet in 1872 noted that fly larvae after having
been shaken fail to display their usual orientation to light
(347). The copepod Temora longicornisy usually negative,
can be made positive by shaking it (239). Very curious
phenomena of a similar nature have been observed in the
case of some Entomostraca. Certain individual specimens
of the ostracod Cypridopsis appeared to be decidedly positive,
others negative. Careful experimental analysis of the condi-
tions revealed the following as the true state of affairs. The
animals are predominantly negative. But contact with a
mechanical stimulus has the effect of making them positive;
thus a negative animal that is picked up in a pipette, or merely
comes in contact with the end of the trough in swimming away

Spatially Determined Reactions 179

from the light, may become positive. In course of time such
a positive animal will become negative of its own accord, so
to speak, without further mechanical stimulation, but such
stimulation, if applied, makes it negative at once (405).

Similar experiments upon Daphnia and Cypris gave results
of the same general character. The strong positive tendency
of the former may, by several times taking the animal up in a
pipette, be made very temporarily negative; the opposite
effect could not be well tested because of the difficulty of pre-
serving the negative state long enough to experiment on it.
In the case of Cypris, an animal temporarily negative could be
made positive by picking it up, but the positive phase could
not be similarly reversed. No other sudden stimulus pro-
duces the effect which is thus induced by mechanical contact
(449). And no possible analogy from our own experience
suggests itself ; the phenomenon remains a mystery.

The effect of contact was observed by Holmes in the ter-
restrial amphipod Orchestia agilis. The most permanent
phase of these animals is positive, although they are at rest
under seaweed on the beach by day. But when they are
thrown into the water, they become strongly negative, no
matter what the intensity of the light ; and to a considerable
extent this effect is independent of the temperature (181). In
the case of the copepod Labidocera astiva, being picked up in
a pipette will make the females, ordinarily positive, negative
for a time. The males are normally slightly negative, but
picking them up, instead of reversing this tendency, increases
it (304). The strong positive phototropism of the "water
scorpion" Ranatra, an hemipterous insect, may be made nega-
tive by handling, and especially by dipping in water (186).

Periodical changes in the sense of response to light have
been observed in animals subjected to periodical changes in
environment. The gasteropod mollusk Littorina lives on

180 The Animal Mind

the rocks of the seacoast in regions where it is covered
with water at high tide and exposed to the air at low tide.
According to the height at which they are found, some of these
animals undergo the alternations of wetness and dryness at
the ordinary tidal periods, twice a day, while others are
reached by the water only at the special high tides occurring
every fourteen days. Mitsukuri showed that when the waves
of a rising tide cover these mollusks, they display negative
phototropism and seek shelter in rock cavities ; while as soon
as they are again exposed to the air, their phototropism be-
comes positive and they emerge in search of food. Further,
he found that a Littorina whose phototaxis was negative
could be made positive by being subjected to the action of a
stream of water for a time (275). Bohn later studied the
effects of placing black or white screens near the animals at
various angles to their crawling movements, and found that
the black screens exerted an attractive influence at certain
times, the white screens at others. These changes in the
"sense" of the phototropism correspond in time to the oscil-
lations of the tide, even though the animals are studied in the
laboratory; they tend gradually to grow less pronounced,
however, under such circumstances. Further, the level from
which the Littorinas are taken influences the nature of their
response to light. Those from high levels, "which undergo
prolonged and intense desiccation, habitually move following
the direction of the luminous field in the negative sense ; the
Littorinas from low levels, which undergo only short and
slight desiccation, move, habitually, following the direction
of the luminous field in the positive sense." The former
become positively phototropic at the time of highest water,
the latter negatively phototropic at the time of low water.
In all cases, the tendency is for the animals to become nega-
tive at low-water time. The attraction of the dark screens

Spatially Determined Reactions 181

represents that of the dark surface of the rocks (55). Similar
oscillations corresponding to the periodicity of the tides were
observed in the annelid Hedista diver sicolor (55) and in the
sea-anemone Actinia equina (65).

A further influence upon light reactions which is doubtless
involved in the formation of the rhythms just described, has
been emphasized by Bohn; namely, the " hydratation " or
desiccation — the wetness or dryness — of the tissues. The
oscillations of Hedista just mentioned may be explained by
supposing that when the annelid is dry, light has the power
of exciting muscular movements. This means that when the
worms have accidentally crept into the shade, they stop, giv-
ing the effect of negative photopathy. If one eye has its
illumination diminished, there is an inhibition of muscular
activity on that side, and consequently a turning in that direc-
tion. At the period of high tide, when the muscles are wet,
the action of light on the animal is inhibitory and the above
phenomena are reversed. When the Littorinas observed by
Bohn are decidedly moist or decidedly dry, black and white
screens exert an influence that is proportional to their area;
the attractions and repulsions seem irresistible, "the mollusk
in the neighborhood of shelter or food continues on its way
toward the screen as if drawn by a fatal force, as if it saw and
felt nothing." But when the tissues of its body are in an
intermediate stage between "hydratation" and desiccation,
large screens have no effect upon it ; it reacts to small objects
in its neighborhood. "The animal seems, as it were, to dis-
engage itself from the influence of external forces, seems no
longer to behave like a pure machine : it goes to the stones
and seaweed where it may find shelter and nourishment, as
if it saw and was conscious of them" (55).

The state_ofjrest or movement^ still another factor. The
"mourning cloak" butterfly, ^Vanessa antiopa, on coming to

1 82 The Animal Mind

rest in bright sunlight, orients itself with the head away from
the light. When it moves, on the other hand, it flies toward
light of any intensity (307). Bohn also has noted that cer-
tain butterflies orient themselves when alighted in such a
way that the posterior part of the eyes is toward the light.
When in this position there is a tendency for the wings to be
spread apart, while when the insect is facing the light the
wings are closely folded (55). The effect on the wings was
noted in Vanessa also, and, it is suggested, may have some
function in bringing the sexes together (307). The pomace
fly when at rest is not oriented at all. Light exerts upon it
merely the effect of stimulating it to movement, a "kinetic,"
not a directive, effect. When the movement has been started,
however, it is directed toward the light : positive phototaxis
appears. But owing to the kinetic influence of the light,
when the insects have been long exposed to sunlight they tend
to come to rest in the more shaded portions, with their heads
away from the light, for this is the position in which they
are least stimulated to movement. The kinetic effect in-
creases with the intensity of the light, but its " directive
effect," through which orientation is secured after the move-
ment is started, was at least in one case lost under intense
light (74).

The background, finally, sometimes determines the sense
of the reaction. Keeble and Gamble found that while the
crustacean Hippolyte varians would move toward the light
whether it was on a white or black background, Macromysis
inermis was negative on a white ground and positive on a
black ground (218).

§ 71. Mutual Influence of Light and Gravity Orientations

Orientation to light and orientation to gravity are not with-
\ put mutual influence in determining the behavior of an

-

Spatially Determined Reactions 183

animal. Supposed instances of this have been noted in the
case of the periodically changing geotropism of Convoluta
roscoffensis (140) and in the copepods observed by Esterly
(113). The relations of gravity and light responses in the
larvae of the squid, a cephalopod mollusk, seem to be as
follows. The larvae have a tendency to rise to the surface of
the water both in darkness and in light, suggesting negative
geotropism. Two test tubes were arranged by Loeb, one
lying horizontally and at right angles to a window, the other
inclined at an angle of 45 degrees from the upright position,
and with the upper end directed away from the window.
Larvae were placed in both tubes ; those in the former showed
positive phototaxis by collecting at the end nearest the win-
dow, but those in the latter gave evidence that their negative
geotropism was stronger than their positive phototaxis by
rising to the upper end, although it was farthest from the
source of light (239). It is not usual for geotropism thus to
come off victorious in a contest with other tendencies. Jen-
nings says, " As a general rule the reaction to gravity is easily
masked by reactions to other stimuli " (211, p. 150). In the
mollusks observed by Bohn, the tendency in ascending or
descending the rocks is to orient the body in the line of the
greatest slope. When light and gravity are acting together
upon the animal, its movement seems to be a resultant of the
two, but if the mollusk is made to move on a vertical plane,
gravity thus exerting its maximal force, the influence of the
light disappears altogether; and if the animal is put in an
upside-down position by further tipping of the surface, the
sense of its phototropism is reversed; that is, it may be
repelled instead of attracted by a dark screen (55).

A curious tendency has been noted by many observers in
insects with both eyes blinded ; namely, to fly straight up into
the air. Forel thought they did so because in no other

184 The Animal Mind

direction could they escape obstacles (130) ; but this fact they
would have to learn by experience, for which, in some cases
at least, they do not take time. Plateau believed the rising
into the air was due to sensations produced by the action of
the light on the surface of the body, leading the insects in
the direction of the strongest light, which usually comes from
above. He supported this view by showing experimentally
that a blinded insect would not rise if set free at night, while
on the other hand, if liberated in a lighted room, it would, in
spite of the blinding, fly toward the light or the lightest part
of the ceiling (332, 334). In the butterfly Vanessa, Parker
thinks the rising due to negative geotropism, as the insect
flew upward in a darkened room (307). Axenfeld suggested
that it might be caused by light penetrating the integument
of the head (7).

§ 72. The Psychic Aspect of Orientation to Light

What shall be said of the psychic aspect of all this complex
mass of facts regarding the orientation of animals to light?
If such orientation occurs in some animals by the immediate
action of light on the body tissues, either by virtue of the direc-
tion of its course through them, or by the relative effects on
the motor apparatus, at symmetrical points, of stimulations
differing in intensity, there is no analogy for this in our own
experience. We are not pulled about into line by the direct
action of light on our bodies, and we cannot imagine what the
conscious accompaniment of such a process would be. If
orientation occurs through the giving of a negative reaction
whenever the body chances to move out of the oriented posi-
tion, we may conjecture that the negative reaction is, here as
elsewhere, accompanied by unpleasant consciousness ; whether
also by a specific visual sensation will be evidenced by the
existence of a sense organ or by any other of the arguments

Spatially Determined Reactions 185

mentioned in Chapter IV. If the effect of light is merely
" kinetic," causing no orientation, but movement about until
the animal chances to come into the shadow, vague restless-
ness or uneasiness is the human experience most closely
resembling its possible conscious accompaniment. In none
of these cases does spatial perception appear to be concerned.
Where, however, the response to light depends upon the eyes,
the accompanying psychic process may have a spatial char-
acter. Even though the eyes do not give clear images, if the
reaction is determined by the greater intensity of illumination
on one eye than on the other, it is possible that the visual field
present to the animal's consciousness may contain gradations
of intensity arranged side by side in a spatial pattern. An
important advance from mere phototropism to visual space
perception is made, according to Ra"dl, when an animal's eyes
are oriented by "a dark point in light space" rather than by
" a bright point in dark space," but the conditions that render
such orientation possible he does not attempt to define, other
than by suggesting that they are connected with the structure
of the eye itself (358).

§ 73. Orientation to Other Forces

One force, which, as was noted in Chapter III, produces
orientation, namely, the electric current, we shall leave out
of account. It is not a stimulus to which animals are nor-
mally subject, and though its action on living matter is of
great interest to the physiologist, the comparative psycholo-
gist's difficulty in finding a psychic interpretation for the facts
may justify setting them aside. Similar considerations apply
to orientation to centrifugal force. There remain the orien-
tations that have been termed respectively " rheotropism "
and " anemotropism," responses to currents of water and to
currents of air.

1 86 The Animal Mind

The tendency shown by many aquatic animals to orient
themselves with head up-stream, and to swim against the cur-
rent, was formerly thought to be a response to the pressure
exerted by the current — a reaction leading the animal to re-
sist pressure. Lyon, however, pointed out that this explana-
tion assumes rheotropism on the animal's part. It is because
the animal opposes the current that the current exerts any
pressure. If it merely allowed itself to be carried passively
along, and if the current surrounding the animal flowed with
uniform velocity in all its parts, no stimulus whatever could
be exerted by the water pressure (254). It seems probable
that eyeless animals do not, as a matter of fact, orient them-
selves against a current of this sort, and that rheotropism in
their case occurs when a current of unequal velocity disar-
ranges their movements, or when they are in contact with a
solid body. Thus Jennings has suggested that in Parame-
cium the reaction is due to the fact that unless the animal has
its head to the current, the flow of the latter will interfere
with the normal backward stroke of the cilia, causing negative
reactions until the disturbance is removed by proper orienta-
tion (211, p. 74). In animals with eyes, however, there is
reason to think that apparent rheotropism is largely an affair
of vision. Lyon's theory of rheotropism in fishes is that the
fish orients itself and swims in such a way that its surround-
ings, the bottom of the stream, for example, shall appear to the
sense of sight to be at rest, an hypothesis which, as we shall
see, was adopted by R£dl to explain the "hovering" of insects
in one place (355). Lyon supports it by experiments where
the bottom or sides of the aquarium were caused to move in
the absence of any current in the water, and the fish was
found to follow them. When the fish was placed in a re-
volving glass cylinder, it followed the revolutions, although
there was a slow current, of course, in the same direction,

Spatially Determined Reactions 187

against which, on the pressure theory, the fish should have
moved. Still more decisive was the experiment where young
fish were placed in a corked bottle full of water which was
submerged and put near a wall covered with algae. When the
bottle was moved in one direction, all the fish went to the
opposite end, although no current could have been produced.
Again, a wooden box with ends of wire netting, the bottom
covered with gravel and the sides with seaweed, was used;
fish (Fundulus) were placed in it, and the box was held
lengthwise in a strong current. The fish oriented them-
selves, but as soon as the box was released and allowed to
float away, they lost their orientation, though their relation
to the current was in no way altered. Blind fish, Lyon found,
oriented themselves by touch, sinking to the bottom. There
does, however, appear to be, in some cases, a genuine pres-
sure reaction to current, for when water is rushing through a
small hole into a tank containing blind fish, they keep their
heads to the current without touching anything. Here the
different parts of the stream have different velocity, and pres-
sure stimuli are actually applied to the skin. There must
be pressure reaction, also, when fish actually swim up-stream
instead of merely maintaining their places against a current
(155). Such a reaction was displayed, probably, by some
shrimps which, being in the water with the fish in the revolv-
ing tank experiment, did swim against the current instead of
with it (254).

Some very interesting behavior touching on this same point
was observed by Garrey in a school of the little fish called
sticklebacks. He noted that if any object was moved along
the side of the aquarium containing them, the whole school
would move along a parallel line in the opposite direction.
If an individual fish happened to be heading directly toward
the object, it would turn in the opposite direction from the

1 88 The Animal Mind

one in which the object was moved ; if it was heading some-
what in the opposite direction already, it would turn farther
in that direction until parallel with the object's line of motion ;
if it was heading somewhat in the same direction as the object,
it would "back off hesitatingly," and reverse itself by a turn
in either direction, usually taking the way around toward
which it was already partially headed, if the object was rapidly
moved, but the other way around if the object's motion was
slow. At first sight this behavior seems to display an instinct
precisely opposite to that of keeping the visual field constant.
Yet the sticklebacks, when placed in a cylindrical glass tank
inside of a black and white striped vessel, moved with the
latter when it moved, proving that they possessed the usual
tendency shown by Lyon to be involved in rheotropism.
Garrey points out that movement in the opposite direction is
produced not when the whole visual field moves, but when it
is at rest, and one object in it moves. Can it be, he asks, that
the moving object "fixes the attention" of the fish and pro-
duces an apparent motion of the background in the opposite
direction, which motion the fish follows? (141.)

Rheotropism in water arthropods may be similarly ac-
counted for, and in the opinion of Ra"dl, this same tendency
explains the habit swarms of insects have of hovering over the
same place, a phenomenon which Wheeler thought might be
due to odors emanating from the soil (435). Insects will often
be found to follow an object over or under which they are
grouped in the air, if it be moved (355). Swarms of insects
may be noted in the air over a country road, following its
windings and apparently oriented by the contrast between
the road and the dark banks on either side. When, however,
resting insects turn so as to keep their heads to the wind, the
reaction is evidently really due to the wind and not to their
visual surroundings (370). Probably the disturbance to their

Spatially Determined Reactions 189

wings produced by any other position causes them to rest only
in the "head-on" orientation.

The responses of animals to different intensities of heat
seem not to involve a definite orientation of the body. A tem-
perature above the optimum produces wandering movements,
which cease when the animal happens to reach the proper
temperature (265, 268, 457). If we were to adopt the ter-
minology applied to light reactions, we should say that ther-
mopathy rather than thermotaxis is the rule.

CHAPTER IX

SPATIALLY DETERMINED REACTIONS AND SPACE PERCEPTION

(continued)

§ 74. Class III: Reactions to a Moving Stimulus

SPECIALIZED response to a stimulus in motion, that is,
one which successively affects several neighboring points on
a sensitive surface, is also frequently met with in animal be-
havior. Its usefulness is obvious : a stimulus in motion is
very commonly a living creature, hence either an enemy or
food. In any case it must be reacted to with extreme prompt-
ness. Reactions of this class may be distinguished as tactile
or visual according as the moving stimulus is mechanical or
photic.

We find good examples of specialized reactions to motile
touch in the ccelenterates. The sea-anemone, Aiptasia, gives
its most violent reaction, involving all the tentacles at once,
when touched by a moving object (291). The medusa
Gonionemus makes, in the case of a moving mechanical
stimulus, its single exception to the rule of responding by
the feeding reaction to edible substances only. The tentacles
will be wound corkscrew fashion about a glass rod drawn
across them, they bend in toward the mouth, and the bell
margin bearing them contracts ; the feeding reaction goes no
further, however. But the response is differentiated from that
to any other form of stimulation by its greater speed : the
reaction time is from .3 to .35 of a second, compared with .4 to
. 5 of a second for other stimuli (451). Special vigor and speed

190

Spatially Determined Reactions 191

generally characterize reactions to contact with moving ob-
jects. In eliciting the scratch-reflex of dogs, an object drawn
along the skin is decidedly more effective than one pressed
against the skin for the same length of time (382, p. 184).
The physiological effect is probably, Sherrington says, the
same as that involved in the "summation" of successive slight
stimuli applied at the same point. As is well known, the latter
will bring about a response of considerable violence, though
each one acting alone would apparently be without effect.

Is it likely that these responses to moving stimuli in contact
with the skin involve the perception of movement as a form
of space perception; that is, a perception of the successive
positions occupied by the stimulus and their relative direc-
tion ? I think we may say that they probably do not, in the
lower animal forms at least. And a chief reason for saying
so lies in the fact that the reactions are so rapid. To perceive
the spatial relations of stimuli, or any other relations, is a
process not favored by great speed of response. The quicker
the reaction, the less clear the perception of its cause : such
seems to be the general law. The sensation accompanying
contact with a moving object may differ in intensity from that
accompanying a resting stimulus ; it may, in the lower forms,
differ qualitatively in some way not represented in our own
experience, but it can hardly be connected with the more
complex psychic processes involved in any form of space per-
ception.

In vision, also, there are special arrangements for reacting
to moving stimulation. The sensitiveness of many animals
to changes of light intensity, although not a direct adaptation
to the spatial characteristics of a stimulus, serves the same
purpose, for changes in light intensity are oftenest brought
about by objects in motion. In the mollusk Pecten varius,
a transition from shadow vision to movement vision is illus-

192 The Animal Mind

trated : the animal closes its shell when a shadow is moved so
as to fall on its eye spots in rapid succession (360). Generally
speaking, the simple invertebrate eye, however, is adapted to
respond to changes in light intensity rather than to moving
objects. Plateau found that caterpillars, which have only
simple eyes, could see moving objects no better than those at
rest (333), and Willem was inclined to think snails saw rest-
ing objects better than moving ones (441). On the other
hand, the compound eye is specially formed to be affected by
moving stimuli. The crayfish will react to anything of fairly
good size in motion, but is apparently unable to avoid sta-
tionary objects in its path (21). The poor vision of the com-
pound eye for resting objects is shown by the ease with which
insects may be captured if the movements of the captor are
very slow. They may be readily approached, also, if the
movements are all in the line of sight, that is, directly toward
the insect, so that successive facets of the compound eye are
not affected, as would be the case in lateral movements. Let
the reader try bringing the hand slowly straight down over
a fly, and see how much closer he can come before the fly is
disturbed than he can if the hand is moved from side to side.
Plateau, from experiments on different orders of insects, con-
cludes that "visual perception of movement" is best devel-
oped in the Lepidoptera (moths and butterflies), Hymen-
optera (ants, bees, and wasps), Diptera (flies), and Odonata
(dragon-flies); that the distance at which movements can
be seen does not exceed two metres, and averages 1.5
metres for diurnal Lepidoptera, 58 cm. for Hymenoptera,
and 68 cm. for Diptera (335).

It is possible that response to a moving stimulus received
through the eye may be accompanied by spatial perception
of movement, although if the eye is compound, the experience
must differ from our own visual movement perception.

Spatially Determined Reactions 193

§ 75. Class IV: Reaction to an Image

By an image is meant the perception of simultaneously oc-
curring but differently located stimuli as having certain spa-
tial relations to each other. Through its means, or that of
the nervous processes underlying it, there arises the possi-
bility of adapting reaction not merely to the location of a
single stimulus, but to the relative location of several stimuli.
Responses may thus be adjusted not only to the direction of
an object but to its form. On the basis of such adjustments
a whole new field of possible discriminations is opened up.

The commonest arrangement for the production of a visual
image is the double convex lens, which collects the rays of
light diverging in their reflection from an object and brings
them together again upon the sensitive retina. The lenses
found in many simple invertebrate eyes seem, however, very
ill adapted to the image-producing function. It is probable
that they serve rather to intensify the effect of the light rays
by bringing them together, than to give a clear-cut image
(293). In the eye of certain invertebrates, such as the Nauti-
lus, a gasteropod mollusk, while there is no lens, the opening
admitting the light rays is so small that an inverted image
might be formed through it, such as may be obtained through
a pinhole. It is unlikely, however, that this eye is really an
image-producing organ. Hesse includes under image-form-
ing eyes only the camera or convex-lens eye, the mosaic eye,
and the superposition eye. The last is a peculiar form of com-
pound eye where light can pass from one section to another,
and where the image is formed by the cooperation of various
refracting bodies (176).

The simplest and vaguest conceivable visual image would
be that of a visual field whose different parts should differ
in brightness. An eye capable of furnishing indications

194 The Animal Mind

merely of the direction from which the greatest illumination
comes might produce this kind of an image, which would of
course not allow the perception of objects, only that of
brightness distribution. ( We have already seen that the orien-
tations of certain animals to light seem to be produced through
a tendency to take such a position that the two eyes shall be
equally illuminated. If the two visual fields are combined in
the case of such animals, as they are in our own binocular
vision, under ordinary conditions the oriented position would
give a visual field whose brightness is equal throughout,
while any other position would give greater brightness
at one side of the field. If they are not combined, if there
is no binocular vision, we cannot imagine what the resulting
perception is. That the direction from which the light comes
influences ants in finding their way is, we have seen, the opin-
ion of Lubbock (248 ) and of Turner (408). It was found not
to be important to white rats in learning a labyrinth path (43i).1

§ 76. Methods of investigating the Visual Image: the
Size Test

The presence of a visual image that is something more than
a visual field of graded brightnesses has been tested by meth-
ods which may be divided into two groups : those which
investigate the effect of stimuli differing in area but of the
same intensity, and those which test discrimination of the
form of objects.

Bohn's observations on the mollusk Littorina show that its
reactions are influenced by the size of the illuminated or dark-
ened surface, as well as by the intensity of the light. When
neither very wet nor very dry, Littorina will react to small
objects in its neighborhood, whereas in an extreme state of
" hydratation " or desiccation it responds to the attraction or
repulsion of the larger screens with fatal uniformity (55).

Spatially Determined Reactions 195

Plateau attempted to test the responses of certain Dip-
tera to the size of an opening admitting light, by placing them
in a dark room, into which light entered from two sources.
One was a single orifice large enough to let the insects out ;
the other was covered with a net whose meshes were too fine
to allow them to pass. The amount of light from the two
sources could be made equal. When this was done, the
insects, which were positively phototropic, sought the two
equally often ; if the light from either was made more intense,
they went to that one. Plateau concluded both that the flies
could not see the netting and that the area of the light source
did not affect them (328). On the other hand, Parker
found that the mourning-cloak butterfly did discriminate
areas, flying to the larger of two sources of equally intense
light (307).

This method of testing the image-forming power of an
animal's eyes has recently been elaborated by L. J. Cole.
He subjected animals with decided positive or negative
phototropism to the influence of two lights made equally
intense but differing in area, one coming through a piece of
ground glass 41 cm. square, the other a mere point. Eye-
less animals, the earthworm, for example, reacted equally
often to each light. Animals whose eyes from their structure
have been judged capable of perceiving merely the direction
of light rays, such as the planarian Bipalium, confirmed the
argument from structure by showing little more discrimination
than the eyeless ones. On the other hand, animals with
well-developed compound or camera eyes, for example certain
insects and frogs, did distinguish between the lights, going,
if positively phototropic, toward the one of larger area;
if negatively phototropic, away from it (80).

Discrimination of boxes differing in size, but alike in form,
placed in a row along a board, food having been put in one,

196 The Animal Mind

was imperfectly learned by two Macacus monkeys ; the errors
leaned in the direction of taking the larger vessel (221).
Raccoons were taught to distinguish perfectly between two
cards, one 6jx6| inches square and the other 4^X4^,
shown successively. The animals were to climb on a box
for food when the larger card was shown and to stay down
when the smaller one appeared. As we shall see later, L. W.
Cole, the experimenter, thinks the learning gave evidence not
only of a spatial image, but of a memory image (82).

One apparent effect of size upon visual perception relates
to the distance at which an object produces a reaction.
Caterpillars, for example, are described as giving evidence of
seeing a slender rod extended toward them at a distance of
about a centimeter; large masses they reacted to at some-
what greater distance (333). It is highly doubtful whether
this means that the simple eye of the caterpillar could give
a perception of two objects as differing in size if they were
equally distant. Myriapods, which make very little use of
sight and do not perceive their prey until they touch it, give
evidence of seeing an obstacle having a rather broad surface,
the size of a visiting card, at a distance of about 10 cm.,
if it is white and reflects much light, or if it is blue;
but not if it is red, — another indication of the relation be-
tween white and blue light, red light and darkness, noted on
p. 123 (329).

§ 77. Methods of investigating the Visual Image: the

Form Test

The second method of studying visual images, that of
testing an animal's power to discriminate forms, has been
applied chiefly to the higher vertebrates. Bumblebees, to be
sure, were thought by Forel to evince a capacity to distin-
guish a blue circle from a blue strip of paper when they had

Spatially Determined Reactions 197

previously found honey on a blue circle, even though the two
had been made to exchange places. They flew first to the
place where the blue circle had been, but did not alight upon
the strip. Wasps, also, according to Forel, distinguished
among a disk, a cross, and a band of white paper, going first
to the form on which they had last found honey (130).
Various species of birds were experimented on by the method
of placing cards carrying simple designs over glasses covered
with gray paper, food being placed always under the same
card. The English sparrow and the cowbird both learned to
distinguish a card bearing three horizontal bars and one
bearing a black diamond from each other and from plain
gray cards. On the other hand, the sparrow, curiously
enough, did not succeed in discriminating vessels of different
form ; the cowbird was not fully tested with these, but gave
some evidence that it was learning (344, 345) . Pigeons were
only moderately successful in a similar test (371).

Many dogs have been taught to distinguish printed letters
on cards; Sir John Lubbock's poodle "Van" is a familiar
example. Van learned to pick out cards marked "Food,"
"Bone," "Out," "Water," and the like, and to present each
on its appropriate occasion. It took him ten days to begin to
make the first step of distinguishing between a printed card
and a plain one ; in a month this was perfected and in twelve
more days, when he wanted food or tea, he brought the right
card one hundred and eleven times and the wrong one twice.
The second mistake consisted in bringing the word "door"
instead of "food," indicating that it was really the look of the
words that he distinguished (251, p. 277 f.).

The dancing mouse could not learn to distinguish two equal
illuminated areas of different forms (469). Raccoons learned
to discriminate a round card from a square one (82). Thorn-
dike taught the two Cebus monkeys under his observation to

198 The Animal Mind

come down to the bottom of the cage for food when a card
bearing the word "Yes" printed on it was exposed, and to
stay up when one bearing the letter "N" was shown. The
conditions seem to have been complicated, however, by the
fact that the two cards were not placed in quite the same
position. Further tests with cards carrying various designs
showed varying degrees of capacity to distinguish them on
the part of the monkeys (397). Kinnaman got negative
results with his two Macacus monkeys in attempting to
train them to distinguish cards such as those used in the later
experiments of Porter on birds. His monkeys, however,
proved able to distinguish vessels of different forms, "a wide-
mouthed bottle, a small cylindrical glass, an elliptical tin
box, a triangular paper box, a rectangular paper box, and a
tall cylindrical can.'7 These vessels differed in size as well
as in form (221).

Special evidence of the comparative development of the
visual image in different genera of ants is suggested by
Wasmann to be furnished by the facts of mimicry. Certain
insects belonging to orders other than the Hymenoptera
inhabit ants' nests, and have in many cases become more or
less modified to resemble their hosts. Wasmann thinks
that these resemblances, which have been established on
account of their protective value, are in insects living among
ants of well-developed visual powers, such as would deceive
especially the sense of sight, while in the "guests" of ants
whose vision is poor, the mimicry is adapted to produce
tactile illusions (426).

§ 78. Class V : Reactions adapted to the Distance of Objects
The factors that make possible the perception of the third
dimension, depth, or distance outward from the body, in in-
vertebrate animals are little known. Certain invertebrates do

Spatially Determined Reactions 199

give evidence of the power to judge distance. The hunting
spiders, for example, which do not make webs, but pursue
their prey in the open, leap on it from a distance of several
inches. Dahl thinks distinct vision is limited to two centi-
meters (88), and Plateau says capture is not attempted until
the prey is within this distance (332). The Peckhams,
however, tested a hunting spider by putting it at one end of a
narrow glass case sixteen inches long, at the other end of
which a grasshopper was placed. When eight inches from
its victim, the spider's movements changed, and at four
inches the leap was made1 (321).

Reactions of this character, where the animal makes a
single movement adapted to the distance of an object from it,
are almost the sole evidence we can get of accurate perception
of the third dimension. The alleged performance of the
jaculator fish, which, as described by Romanes, "shoots its
prey by means of a drop of water projected from the mouth
with considerable force and unerring aim," the prey being
"some small object, such as a fly, at rest above the surface
of the water, so that when suddenly hit it falls into the water,"
would involve distance perception (364, p. 248). The catch-
ing of insects on the wing by various amphibians, reptiles,
and birds has the same significance. A salamander cau-
tiously stalking a small fly will not strike until it gets within
a certain distance. In Necturus and in other animals the
pause just before snapping at food has been suggested to be
for the purpose of proper fixation (438).

Yerkes's tests of the so-called "sense of support" in tor-
toises indicate some power of estimating distance by vision

1 Porter observed that the distance at which spiders of the genera Argiope
and Epeira could apparently see objects was increased six or eight times if
the spider was previously disturbed by shaking her web (346). This, of
course, does not refer to the power to judge distance.

2OO The Animal Mind

in these animals. He experimented, it will be remem-
bered, with individuals belonging to three classes: land-
dwelling, water-dwelling, and amphibious. The first men-
tioned would crawl off the edge of a board 30 centimeters
above a net of black cloth only with much reluctance when
their eyes were uncovered ; when blindfolded they would not
move at all. The water tortoises plunged off without hesita-
tion from a height of 30 centimeters, but hesitated slightly
at 90 centimeters, although some individuals would take the
plunge at once even from a height of 180 centimeters. When
blindfolded, all of the water tortoises rushed off at any height.
The land-and-water-dwelling tortoises hesitated at 30 centi-
meters and at 90 centimeters showed a conflict of impulses,
trying to catch themselves, before launching off. When
blindfolded they would not leave the board at all, though they
moved about upon it freely (459).

Some of the most important conditions of distance per-
ception in our own experience are lacking in the lower
vertebrates and in invertebrates. Stereoscopic vision, the
appearance of solidity given to objects by the fact that the
visual fields of the two eyes combine, thus producing blending
of two slightly different views of the object looked at, has
been held to be dependent on the partial crossing of the op-
tic nerves on their way to the brain, whereby each retina sends
nerve fibres to both hemispheres of the brain. This arrange-
ment does not appear in the animal kingdom below the birds ;
whatever function it plays in space perception is, then,
absent from reptiles, amphibians, fish, and invertebrates.
Certainly stereoscopic vision cannot exist in animals whose
eyes are so placed that the same object cannot be seen by
both, as is the case with most fishes. -In birds, whose eyes
are situated too far toward the sides of the head for the
same object to cast its images on the foveas or centres of the

Spatially Determined Reactions 201

two retinas, there appears to be a secondary fovea in each
eye, so placed as to suggest that it serves binocular vision,
while the primary fovea is used for monocular vision. Con-
vergence, the turning of the eyes toward each other to bring
the two images of an object on the central part of the retinas,
which is an important aid to human estimations of distance,
is also necessarily lacking in animals without binocular
vision. A third factor in our own perceptions of distance,
the accommodation of the crystalline lens, that is, the altera-
tion of its convexity through the pull of the accommodation
muscle to enable it to focus objects at different distances,
has been carefully studied in connection with the lower
animals by Beer. \ Through experiments on the refractive
powers of eyes dissected from the dead animal, he reached the
conclusion that no invertebrates but cephalopods have the
power of accommodation.^ It is rudimentary or lacking also in
some members of the fish, lizard, crocodile, snake, and mam-
mal families. In cephalopods, fishes, amphibians, and most
reptiles, the process of accommodation does not involve a
change in the form of the lens, but an alteration in the dis-
tance between the lens and the retina. The device of in-
creasing the curvature of the lens for vision of near objects
appears first in certain snakes, and is found throughout the
higher vertebrates (18).

Where accommodation does not exist, as in most inverte-
brates, it is possible to trace other arrangements for adapting
vision to the distance of the object seen. Thus in com-
pound eyes, part of the eye may be adapted to near vision and
part to far vision. This is suggested by the fact that some
of the little tubes, or ommatidea, of which the compound eye
is composed, diverge from each other by a less angle than
others, indicating that they are suited to the reception of more
nearly parallel rays. In insects with both simple and com-

2O2 The Animal Mind

pound eyes one form may be used for near and one for far
vision. Spiders appear to have the principal eyes adapted
for far vision and the auxiliary eyes for near vision, while one
spider, Epeira, has part of the hinder median eye adapted to
each (176).

§ 79. Some Theoretical Considerations
The temptation is strong to speculate upon the essential
nature of the conditions which make possible true space
perception, the simultaneous experiencing of sensations that
are referred to different points in space. Such speculation
must be of the most tentative description, yet the following
suggestions seem not wholly unwarranted by the facts. For
one thing, it looks probable that the ability to suspend im-
mediate reaction is essential to space perception. Can a
spatial complex of sensations occur in the experience of an
organism unless that organism is capable of receiving a number
of stimuli on a sensitive surface and of suspending, for a
brief period at least, all reaction ? Let us take as an example
of such a complex a visual field, within which different color
and brightness qualities are arranged in definite order, some
above, some below, some to the right, others to the left.
Could such a balance of tendencies to move the eye as is
involved in the simultaneous perception of a number of
elements preserving regular space relations to each other have
been brought about unless no single one of the tendencies were
irresistible? One can readily imagine an eye functioning
in such a way that every stimulation of it, though occasioned
by rays from several different directions acting simultaneously,
should issue at once in a resultant movement. Would not the
accompanying consciousness be a single resultant sensation,
rather than a complex of spatially ordered elements? It is
a good deal easier, of course, to ask than to answer such
questions.

Spatially Determined Reactions 203

/' Again, the power of getting true spatial images seems to
be bound up closely with the power of moving the sensitive
surface. We get our best tactile space perceptions through
active touch, involving movement of the hands and fingers;
our visual space perceptions are profoundly influenced by eye
movements. Where the movements of an animal's body as a
whole are very rapid, as in the case of winged insects, this
fact may compensate for the immovability of its eyes.
Forel, as we have seen, thinks that insects which can explore
objects by moving the antennae, bearing the organs of smell,
over them, may have smell space perceptions, such as are un-
known to our experience ; they may perceive the shape and
size of odorous patches as we could do if our organs of smell
were on our hands (132). Now, movement of a sense organ
brings about the same result that movement of a stimulus
across a resting sense organ does ; that is, the stimulus affects
different points of the sensitive surface in succession. But
the vital significance of the two is quite different ; movement
of an object across a resting sense organ means very likely
that the object is alive ; it must be instantly reacted to, and the
speed of the reaction is unfavorable to the formation of a true
space perception. Movement of the sense organ, however,
gives a series of impressions on successive points of the sensi-
tive surface, from a resting object. While the sense organ is
being moved, it is probable that other reactions of the animal
will be suspended. Whether any part in the formation of
that complex conscious content which we call a spatial
image, consisting of different sensations simultaneously
apprehended, is played by the "lasting over" of the impres-
sions on one sensitive point after the stimulus has passed on to
the next, a phenomenon which we find both in touch and in
sight sensations, it is impossible to say. (^We are, however,
apparently justified in the statements that the essence of

2O4 The Animal Mind

space perception, as distinct from other conscious processes
that may accompany spatially determined reactions, is the
presence of an image in the sense above defined, and that a
movable sense organ is an important condition for the pro-
duction of such an image.)

CHAPTER X

THE MODIFICATION OF CONSCIOUS PROCESSES BY
INDIVIDUAL EXPERIENCE

(THE reactions of animals to stimulation show, as we re-
view the various animal forms from the lowest to the highest,
increasing adaptation to the qualitative differences and to the
spatial characteristics of the stimuli acting upon them. It is
therefore possible to suppose that the animal mind shows
increasing variety in its sensation contents, and increasing
complexity in its spatial perceptions. But besides this
advance in the methods of responding to present stimu-
lation, the higher animals show in a growing degree the
influence of past stimulation. While a low animal may
apparently react to each stimulus as if no other had affected
it in the past, one somewhat higher may have its reaction
modified by the stimulation which it has just received. An
animal still more highly developed may give evidence of being
affected by stimuli whose action occurred some time before ;
and finally, in certain of the vertebrates, perhaps, as in man,
conduct may be determined by the presence in consciousness
of a memory idea representing a past stimulus. " Learning
by experience," or "associative memory," as we saw in Chap-
ter II, has been regarded as the evidence par excellence of the
existence of mind in an animal. That it does not serve this
purpose to entire satisfaction was also pointed out in that
earlier chapter, and will be more clearly apparent as we survey
in the following pages the various ways in which an organ-

205

206 The Animal Mind

ism's past experience may modify its behavior, asking each
time what the possible conscious aspect of the modification
may be.

§ 80. Absence of Modification

In the first place there presents itself for consideration the
case of animals that meet a situation by repeating the same
reaction over and over again. For example, Paramecium
encounters an obstacle in its path. It performs the only
reaction in its power, the avoiding reaction; it darts back-
ward, rolls to one side, and proceeds forward at an acute
angle to its former course. Suppose that the obstacle is so
large that the animal strikes it again. The negative reaction
is repeated and again repeated if need be until the course
is sufficiently altered to carry the Paramecium clear of the
obstacle. To behavior of this sort Jennings has extended
the term "trial and error" (206, p. 237). The expression
was first used by Lloyd Morgan to distinguish between the
human method of solving a problem and the dog's method,
the latter being called "trial and error" (282, p. 139). Mor-
gan meant that the dog does not attempt to reason the matter
out beforehand, making use of his previously acquired knowl-
edge before beginning to act ; but that he attacks it at once
in some manner derived from individual experience or racial
inheritance. If this method fails, he tries another similarly
derived, and so on until one method proves successful.
Paramecium also tries over and over again, although what
it tries is always the same thing. Whether Paramecium's
behavior is really shown to be akin to the dog's, by calling
both "trial and error," is questionable, however ; the resem-
blances between the performances of an animal that invariably^
responds with the same reaction until it chances to be carried
beyond the reach of the stimulus, and those of a human being
who successively "thinks of," that is, recalls the ideas of

Modification by Experience 207

various devices until the right one is obtained, is but super-
ficial. Certainly the behavior of the Paramecium, "trial
and error" though it may be, is not learning, and gives no
evidence either for or against consciousness as its accompani-
ment. If it has a subjective side, the unpleasantness that
is most naturally regarded as the accompaniment of a negative
reaction would seem to be modified in no way by the repeated
performance of the reaction.

§ 81. Heightened, Reaction as the Result of Previous

Stimulation

/

But even in the lowest animals the effect of a stimulus is
often, as we have seen, altered by the "physiological con-
dition" of the animal, and this condition is commonly the
result of the stimulation previously received. Sometimes the
influence is in the direction of increasing the violence of the
response. Thus in the earthworm Jennings points out that
various stages of excitability may exist, due to the action of
previous stimulation and varying all the way from a state of
rest, where a slight stimulus produces no effect, to a condi-
tion of violent excitement, where moderate stimulation will
cause the animal to "whip around" into a reversed position
or wave its head frantically in the air (210). This increased
excitability suggests the "nervous irritation" produced in a
human being by an accumulation of disagreeable stimuli;
but an increased unpleasantness is the only obvious interpre-
tation of its psychic aspect.

§ 82. Cessation of Reaction to a Repeated Stimulus

While response to a given stimulus may thus be altered by
reason of the fact that other stimuli have been acting upon
the animal just previously, certain interesting modifications of
reaction occur when the same stimulus is repeatedly given.

208 The Animal Mind

One form of such modification is found where the stimulus
is of moderate intensity and not harmful to the animal.
The Ciliata Vorticella and Stentor, which spend a part of their
time attached to solids by a contractile stem, contract at the
first application of a moderately intense mechanical stimulus,
but fail to react at all when the stimulus is several times
repeated (203). Hydra responds to mechanical stimulation
by contraction, but gets used to the process when repeated
and gives no further reaction (418). The sea-anemone
Aiptasia reacts by a sharp contraction to a drop of water
falling on it ; later it ceases its response to this stimulus. If
exposed to light, it contracts and remains in this state for some
hours, but afterwards expands again (207). The annelid
Bispira voluticornis was found by Hesse to give no further
response to sudden shadows when the stimulus was frequently
repeated (173). Von Uexkiill reports that the sea urchin
Centrostephanus longispinus ceased to respond to shadows
after three successive stimulations (410). Nagel observed
that certain eyeless mollusks which react to sudden darkening
very quickly get used to the stimulus and cease to respond;
often after one reaction they decline to react for several hours.1
The mollusks that responded to sudden brightening rather
than to shadows, that were in NageFs phrase photoptic rather
than skioptic, took longer to become accustomed to repeated
stimulation, but did so by gradually weakening their reaction
(290). A web-making spider that was found by the Peck-
hams to drop from its web at the sound of a large tuning
fork declined to disturb itself after the stimulus had been

1 The opposite phenomenon is reported by Rawitz of the mollusk Pecten,
whose response to a shadow was the shutting of its shell. Repeated or long-
continued shadowing, instead of doing away with the reaction, caused the
animal to remain with closed shell for a long time ; an intensification of the
reaction which suggests the effect of summation of stimuli (360). We may
infer that the stimulus in such a case is injurious.

Modification by Experience 209

repeated from five to seven times (320). Ants "become
used" to the ultra-violet rays which they ordinarily avoid
(ng).

If learning by experience be extended to cover every case
where an animal reacts to a stimulus differently because of
earlier stimulation, then this is learning by experience. An
interesting point suggests itself in regard to the permanency
of such learning. In case the animal the next day responds
with less vigor to the excitant which it got used to the day
before, there would seem some plausibility about the inter-
pretation of Nagel, who says with that inclination in favor
of the psychic which always characterizes him, that the be-
havior of his mollusks "makes the assumption of a certain
power of judgment in these animals unavoidable. The
animal recognizes that the repeated shadow is not due to the
presence of an enemy or other danger" (290). On the other
hand, of course, it is perfectly conceivable that an animal
might go through such a process of judgment and still be
unable to remember it the next day. However, if we find
that only very recent stimulation has any effect, the suggestion
that this effect is due to some purely physiological alteration
in the organism lies near at hand.

As a matter of fact, the higher the animal the more lasting
appears to be the result of "getting used" to a stimulus.
For instance Hydra, if it is allowed to reach full expansion
after having contracted at a touch, will respond to the
seco'nd touch just as it did to the first ; the stimuli, to influence
each other, must come in quick succession. The relation
of loss of reactive power to the interval between the stimuli
was prettily shown by Hargitt for a tube-dwelling marine
worm, Hydroides dianthus. Shadows were thrown from a
pendulum whose rate could be varied, and it was found that
if a full second intervened between the stimuli, the reaction
p

2io The Animal Mind

would always be given ; if the interval was half a second, after
the first few stimuli many of the worms failed to react, while
if the interval was only a quarter of a second, almost all of
them became indifferent (158). Mrs. Yerkes observed that
the same annelid would often fail to respond to shadows
repeated at intervals of from 5 to 10 seconds, and that 95
out of 200 responded when the interval was from one to two
minutes (447). On the other hand, the spider experimented
on by the Peckhams for some time reacted each day to the
sound of the fork by dropping from its web until the sound
had been repeated some half dozen times; but after the
fifteenth day it would not drop at all (320). There is an
adaptive aspect to this difference between Hydroides and the
spider. An animal that has little power to discriminate
among stimuli could not afford to suspend its negative re-
action for any length of time, for another stimulus, indis-
tinguishable from the one to which it had become accustomed,
might happen along and end its career. But a creature with
greater capacity for qualitative discrimination can safely
suspend reaction for a considerable period to one out of the
many stimuli which it is capable of discriminating.

Where the effect is temporary, the most obvious suggestion
as to its cause is fatigue. In our own experience this word is
used chiefly with reference to motor processes; we perceive
a certain signal, but are too fatigued to respond. On the
sensory side, when a repeated stimulus is no longer perceived,
we call the phenomenon one of adaptation. That the failure
of Stentor to respond to successive stimuli is not due to motor
fatigue appears quite certain to Jennings, since under favor-
able conditions he has obtained reactions from the animal
for a far longer period than that occupied by the process of
getting used to slight mechanical stimulation (203). And in
most of the cases cited, the acclimatizing process seems to

Modification by Experience

211

occur too rapidly to make fatigue of the motor apparatus
probable. The most natural analogy to the phenomenon in
our own experience is sensory adaptation, such as we find, for
instance, in the fact that a moderate weight laid on the skin
ceases after a time to be felt. The psychic accompaniment
of such modification of behavior is probably, if it exists,
merely the gradual disappearance of all sensation.

Another case of the cessation of reaction to a repeated stim-
ulus is reported by Wasmann of ants in an artificial nest,
which assumed the fighting attitude in response to the move-
ment of a finger outside the nest, but after two or three repeti-
tions of the motion were no longer disturbed (426). Where
animals as high in the scale as the ant and spider are con-
cerned, it is possible that this process of getting used to a
stimulus may involve rather a dulling of emotion than a dis-
appearance of sensation.

That adaptation is itself adaptive hardly needs to be empha-
sized. As Jennings suggests, if the sea-anemone that con-
tracts at the first ray of light were to remain contracted in
steady illumination, it would lose all chance of getting food
under the new conditions (207). The negative reactions
ordinarily involve interruption of the food-taking process,
and it is important that they should not be continued in re-
sponse to stimulation that is relatively permanent. Hargitt
thinks that the loss of reaction to repeated shadows which he
observed in marine worms may be an adaptation to the vary-
ing illumination caused by ripples at the surface of the water

(158).

Does such loss of reactive power ever occur in connection
with a positive or food-taking reaction? One would expect
that a single condition would bring it about under such cir-
cumstances ; namely, loss of hunger. And, as a matter of
fact, observers of the feeding processes in many lower forms

212 The Animal Mind

have found that these cease or turn into negative responses
when the animal is satiated ; although Pieron indeed reports
that while the responses of Actinia equina and A. rubra to
mechanical stimulation cease on repetition of the stimulus,
those to food stimulation continue indefinitely (327). If the
change from food-taking to negative reaction has a conscious
accompaniment, this might naturally be thought of as a
change from pleasant to unpleasant affective tone. One
very interesting case of such a change in the feeding reaction
occurs in the sea-anemone. Nagel observed that if a ball of
filter paper soaked in fish juice were placed upon one of the
tentacles of Adamsia, it was seized as eagerly as a ball of fish
meat, but that when this deception had been several times
repeated, the ball was held for a shorter period each time, and
was finally rejected as soon as offered. Nagel is inclined to
think that this is learning by experience, and points out that
the psychic life of Adamsia must possess little unity, for the
"experience" of one tentacle does not lead other tentacles to
reject the paper balls at once (291). Parker finds similar
behavior in Metridium, and explains it by saying that the
filter paper offers but a weak food stimulus, and that " the suc-
cessive application of a very weak stimulus is accompanied by
... a gradual decline in the effects, till finally the response
fails entirely" ; in other words, that we have adaptation to a
food stimulus (303). Jennings fed Aiptasia alternately with
pieces of crab meat and with filter paper soaked with meat
juice, the result being that the fifth piece of filter paper was
rejected — but so was the crab meat thereafter. Jennings
came to the conclusion that the phenomenon is due simply
to loss of hunger on the animal's part, and that where Parker
found that the crab meat would be taken after the filter paper
was refused, it was because the latter was a weaker stimulus
and naturally was the first to call forth the effects of

Modification by Experience 213

satiety. The objection to the hunger hypothesis is that
other tentacles of the same animal will react after one
tentacle has stopped; satiety ought surely to affect the
entire organism (207). Allabach, in the light of these
researches, made a careful study of Metridium. She dis-
poses of the psychic learning by experience theory of Nagel
by saying that the only experience upon which the animal
could reject the filter paper must be experience that it is not
good for food. This could be learned only by swallowing it ;
but the failure of the reaction occurs just as well when the
animal is prevented from swallowing the filter paper. That
the phenomenon is not one of adaptation to weak stimuli
is shown by the fact that it may be brought about by succes-
sive feedings with meat which is not allowed to be swallowed.
It cannot be due to loss of hunger, for this is experimentally
shown to affect all the tentacles at once. Allabach concludes
that it is simply a case of local fatigue of the tentacles. The
taking of food by a tentacle involves the production of a con-
siderable quantity of mucus, the immediate supply of which
is probably exhausted after a few reactions, and a short period
of rest is required (3).

This explanation seems, however, not precisely adapted to
the most recently published experimental results bearing upon
the point ; those of Fleure and Walton. They tested Actinia
with a scrap of filter paper once every twenty-four hours,
placing it on the same tentacles, which usually carried it to
the mouth, where it was swallowed and later ejected. After
from two to five days the mouth would no longer swallow the
fragment, and in two more days the tentacles refused to take
hold of it. Other tentacles could be "deceived" at least
once or twice after this, but very soon manifested the inhibi-
tion, indicating that a nervous connection and not merely
local fatigue was involved. All traces of the "learning"

214 The Animal Mind

were lost after from six to ten days' interval. Another anem-
/ one, Tealia, "learned" more quickly than Actinia (127).

Modification of behavior closely analogous to this was
observed in fishes by Herrick. Catfish, when the barbels
were touched with a bit of meat, immediately seized it. If
a piece of cotton wool were used instead of the meat, they
made the same reaction, but after this experience had been
repeated a certain number of times they ceased to respond
to the cotton, although they still took meat eagerly, showing
that neither hunger nor fatigue was involved. Moreover,
the "learning" would persist for a day or two. "I rarely,"
says Herrick, " after the first trials, got a prompt ' gustatory '
reflex with the cotton" (165). In these cases it looks very
much as though we had to deal with a real discrimination
between stimuli, a type of behavior which will be considered
under a later heading.

§ 83. Varied Negative Reactions to a Repeated Stimulus

Another way in which reactlfc^ a ^petition of the same
stimulus becomes modified is as^^^vs : the animal under a '\
strong stimulus tries , one after c^^^g) different forms of t
negative reaction until one of them is successful in getting rid *
of the stimulus. Here is a genuine case of trial and error,
where, however, different reactions are tried. The Stentor
furnishes us with a typical example of this : when attached by
its stem and stimulated strongly a number of times in suc-
cession, it first tries the ordinary negative reaction, bending
over and to one side. Next, it reverses momentarily the
direction in which its cilia are whirling. If this, several
times repeated, does not succeed in getting rid of the stimulus,
the animal contracts strongly upon its stem. This also is
continued for some time, but if the stimulus is kept up, too,
the Stentor finally breaks away and swims off (203).

Modification by Experience 215

There are many examples of similar behavior in other
animals. Hydra, which sometimes displays the phenome-
non of adaptation by refusing to react at all to repeated
stimulation, in other cases tries first the ordinary negative
response of contraction, and later moves away from the region
it has been occupying (418). Frandsen found that if the slug
Limax maximus has a tentacle touched several times in suc-
cession, it at first withdraws the tentacle and turns away from
the stimulus. Later, it may move toward and push against
the stimulus, and do the same if the touch is on the side of
its body, resisting and curving around the obstacle — another
way, of course, of getting rid of it (135). Preyer, again,
observed a very pretty instance of this sort of behavior in the
starfish. He slipped a piece of rubber tubing over the middle
part of one of the arms of a starfish belonging to a species in
which those members are very slender, and found that the
animal tried successively various devices to get rid of the
foreign body, to wit, the following: rubbing it off against
the ground, shaking it off by holding the arm aloft and wav-
ing it pendulum-wise in the air, holding the tube against the
ground with a neighboring arm and pulling the afflicted arm
out, pressing other arms against the tube and pushing it off,
and, finally, as a last resort, amputating the arm. This, says
Preyer, is intelligence, for the emergency is not one normal to
the animal, and it is adapting itself to new conditions (350).
It would, however, be demanding too much even from intel-
ligence to suppose that the starfish's behavior is entirely
new. A human being, capable of ideas, could only, in a
similar predicament, "think of," that is, call up, ideas of the
behavior which on former occasions somewhat resembling
the present had proved effective. Do such cases of the trial
of different devices indicate that the animal concerned calls
up any kind of idea or image of each device, before putting

216 The Animal Mind

it into practice? Decided evidence in favor of such a sup-
position might be furnished if the " trial and error" needed
to be gone through with only once. A human being brought
into such conditions and guiding his conduct by ideas would,
if placed in a similar emergency soon afterwards, immediately
recall the idea of the successful action and waste no time over
the unsuccessful ones. But we have no reason to think that
such is the fact with our primitive animals. Preyer's star-
fish, when confined by large flat-headed pins driven into the
board on which it lay, close up in the angles between its
arms, managed to escape by trying a large variety of move-
ments, and gradually diminished, Preyer says, the number of
useless movements made in successive experiments (350).
O. C. Glaser, on the other hand, has recently found that the
echinoderm Ophiura brevispina does not improve at all with
practice in removing obstructions from its arms. The very
versatility of the starfish, this writer thinks, tells against
its perfecting any one movement through experience (145).
Stentor and Hydra go through the same series of reactions
each time, without apparently being influenced by their
previous behavior. And again we must remind ourselves
that there is no reason why their conduct, adaptively regarded,
should be otherwise. An animal with so little power of dis-
tinguishing qualitative differences among stimuli cannot be
in any way aware that the stimulus which affects it a second
time is going, as in the previous case, to be so persistent that
the ordinary negative reaction will not get rid of it. Further,
each reaction of the series performed by the animal is more
disturbing to its ordinary course of life than the preceding one.
The Stentor can bend to one side and still continue the food-
taking process ; if it reverses its ciliary action, feeding must be
momentarily interrupted ; while contraction on the stem and
breaking loose from its moorings are still more serious in-

Modification by Experience 217

fractions of the normal routine. It would be decidedly
disadvantageous to take the last step while there was any
chance that milder measures might prevail.

In all probability, since the behavior just described has no
permanent effect upon the animal, it is physiologically due, as
Jennings suggests (208), to the overflow of the nervous energy
set free by the stimulus into first one channel and then an-
other. In most cases the movements resulting are all adapted
to getting rid of the stimulus, though only one of them is
successful in so doing ; but we have on record one case where,
in a supreme emergency, the stimulus being not only repeated
but increased in intensity, every possible outlet is tried,
whether it has any fitness to the situation or not. This was
observed by Mast, testing the effect of increased temperature
on the reactions of planarians. The first influence of such
increase from 23 degrees to 26 degrees C., is to produce
heightened activity and positive reactions. Then, from 26
degrees to 38 degrees, the reactions are negative. From 38
degrees to 39 degrees, violent crawling movements set in, and
then, curiously enough, the righting reaction is given, per-
fectly irrelevant, of course, to the conditions. Finally, the
anterior and posterior ends are turned under, the central part
is arched upward, and the animal falls over forward on its
back (260).

In all these cases where repetition of the same stimulus
produces successively different forms of the negative reaction
increasing in violence, it is most natural to think of the psychic
accompaniment as an increasing degree of unpleasantness.
In our own experience, repeating a stimulus does not alter
the quality of the resulting sensation, except where the struc-
ture of a special sense organ is a modifying factor, as in the
case of visual after-images. Repetition of the stimulus does
with us human beings diminish the intensity of the accompany-

218 The Animal Mind

ing sensation; but this process is the natural accompani-
ment, as we have seen, of diminishing reaction, not of varied
and increasingly violent reaction. A decidedly disagreeable
stimulus acting repeatedly on a human being may produce
unpleasantness that grows more and more intense until it
is unbearable ; the behavior of a human being under such cir-
cumstances is much like the animal behavior we have just
been describing. Various movements calculated to get rid
of the stimulus are tried, each more energetic than the last.
Hence, if the lower animals behaving thus are conscious, we
may plausibly assert that their consciousness under these
circumstances is increasingly unpleasant. But the human
experience in such a case would be, or might be, further char-
acterized by the presence of ideas. That is, the human being
would think of the different ways to get rid of the stimulus
one after another. This many, at least, of the animals that
try different negative reactions are apparently incapable of
doing. We judge that they are so by the simple fact that on
being subjected after an interval to the same presumably dis-
agreeable stimulus, they do not at once make the reaction that
was previously successful in getting rid of it. A human being,
recalling that reaction in idea, would be able to do so. We
shall see in the next chapter that many animals, while they
do not learn the successful reaction from a single experience,
do gradually diminish the number of unsuccessful ones made
in a series of experiences. It may be that as more experimen-
tal evidence is accumulated, this will be found to be the case
throughout the whole animal kingdom, but at present it
looks as though the lowest forms may, when an injurious
stimulus is repeatedly given, pass through their whole reper-
toire of negative reactions in one experience after another,
without any shortening of the process. Trial and error this
may be called : learning it is not.

Modification by Experience 219

§ 84. Dropping off Useless Movements: the Labyrinth
Method

The next form of modification of behavior by individual
experience which we shall consider occurs when an animal,
under the influence of some stimulus which it strives either
to get rid of or to get more of, goes through a series of reactions
until one proves successful ; on being after an interval of time
placed in the same situation, the unsuccessful movements are
fewer, and further repetition causes them to be dropped off en-
tirely. This is the mode of behavior which was first brought
into clear relief by the experiments of Thorndike on chicks,
dogs, and cats. Since then an increasing number of inves-
tigators have shown its existence in a large number of forms.
One of the simplest methods for the testing of this sort of learn-
ing is the labyrinth method. In its developed form it was first
used, I believe, by Small in his work on white rats, and was
suggested by the natural habits of the animal, which is, of
course, accustomed to run about through narrow passages.
The plan consists in placing food, or something else attractive
to the animal, at the end of a series of passages containing
a number of false turns. The labyrinth used for the rats
was very complicated, being in fact a replica in wire netting
of the Hampton Court maze, but much simpler ones have
since been employed for other animals. One advantage of
the labyrinth method is that it requires nothing of the animal
except what is perfectly natural to it, namely, locomotion.

The lowest forms which have been thus far tested by this
means are certain Crustacea. The crab Carcinus granulatus
was placed in a very simple labyrinth with only two points
where a choice between the right and wrong paths was pos-
sible. At the end of the labyrinth was the aquarium, and the
crab's discomfort out of the water served as the occasion stir-

220

The Animal Mind

G

ring it to activity. In fifty experiments the path had not been
perfectly learned, although the time was greatly reduced. A
still simpler path was then offered the animal by placing a
wire screen partition in the middle of the aquarium, with an
opening in the centre and food on the other side ; and in ten
trials the time occupied by the crab in finding the food was
much lessened. Still neither of the two animals tested had
learned to go straight to the opening, but each followed a
habit of its own, one moving directly toward the food, hunt-
ing for an opening
near it, and then
going to the mid-
dle where the
opening was; the
other always fol-
lowing the edge of
the screen all the
way around until
it came upon the

FIG. 13. — Labynnth used by Yerkes and Huggms in ex- . r

periments on the crayfish. T, compartment from Opening ^453/*
which animal was started; P, partition at exit; A labyrinth off 6 r-

G, glass plate closing one exit. . .

ing only a single

choice of passages was used in testing the crayfish ; again one
end of the box communicated with the aquarium (Fig. 13).
About halfway down the length of the box a partition put in
longitudinally divided it into two passages, one of which was
closed at the end by a glass plate. In sixty trials the animals,
which had originally chosen the correct passage 50 per cent
of the time, came to choose it 90 per cent of the time. A
second series, with a single animal upon which more tests a
day were made, resulted in the formation of a perfect habit in
two hundred and fifty experiments. The glass plate was then
shifted to the other passage, and the crayfish was naturally

Modification by Experience

221

completely baffled for a time, but succeeded in learning the
new habit (47 1 ). Ants of the species Stenammafulvum piceum
have been tested in a labyrinth by Fielde. The observations
indicated that this ant's tendency to be guided by the chemical
traces of its own footsteps militated to a certain extent against
shortening the path by dropping off useless turnings. Each
ant followed her own previous trail through the labyrinth to
the nest. Yet some tendency for the movements to become

FIG. 14. — Labyrinth used by Yerkes in experiments on the frog. A, box open-
ing into maze ; E, entrance ; T, tank ; G, glass plate ; P, partition ; 7C,
electric circuit whereby animal could be given shock on entering wrong
passage; C, K, cells and key; R, R, red cardboard; W, W, white card-
board.

automatic and independent of the smell clew was shown by
the fact that when an ant had gone over the path many times,
a portion of the track might be obliterated without inter-
rupting her course 1 (118).

A simple form of the labyrinth method has been used on
fish (Fundulus), which were kept by a screen in the sunny end
of an aquarium, the darkened end being also the place where
they were fed. One upper corner of the screen was cut out.
In a couple of days, allowing six or eight trials a day, the fish
learned to swim straight up to this corner (394). A more
complicated labyrinth was also used, but the time required

1 Cf. the observations of Pieron referred to on p. 94.

222

The Animal Mind

to learn it is not stated. The greater speed and ease of
locomotion in the fish as compared with the crustaceans
may have been one factor concerned in the former's greater
rapidity of learning.

With the green frog, the labyrinth pictured in Figure 14
was used. After one hundred trials, practically no errors were
made (454). Another animal whose learning powers have

been tested by this

method is the turtle.

The labyrinth was
distinctly more com-
plex than that used
for the frog. It in-
volved four blind
passages, and led to
the turtle's comfort-
able, darkened nest.

During the first
four trips the time
was reduced from

FIG. 15. — Labyrinth used by Yerkes with turtles. ,, . J r

A, starting point; F, blind alley; 3, 4, 6, thirty-five minutes to

inclined planes. three minutes and

thirty seconds; in the fourth trip the animal took two
wrong turns. The time of the fiftieth trip was thirty-five
seconds. In a second labyrinth (Fig. 15), two inclined
planes were introduced, up and down which the turtles had
to crawl. This labyrinth took them longer to traverse, and
the time curve shows greater irregularity, rising, for instance,
to seven minutes on the forty-fifth trial, after having been as
low as two minutes and forty-five seconds at the thirty-fifth.
The process of shortening the path was observed very prettily
in connection with the inclined planes. The turtles had to
turn about as soon as they had reached the bottom of the

Modification by Experience 223

descending plane. They soon began to make the turn before
they got to the bottom, and finally to throw themselves over
the edge as soon as they reached the top (450).

Some of Thorndike's experiments on chicks involve the
labyrinth method, others what we shall call the puzzle-box
method. The chicks were confined in small pens, with food
outside. In some cases they could get out by running to a
particular spot, or up an inclined plane; in other cases by
pecking or pulling at something. Both sorts of action were
learned; obviously the former, involving simple locomotion
on the animal's part, are the ones which concern us at pres-
ent (393). Porter found that the English sparrow quickly
learned the Hampton Court maze (344), and that the vesper
sparrow and cowbird learned a simpler form in twenty or
thirty trials (345). Pigeons tested by Rouse acquired the
ability to traverse four different labyrinths, and it was noted
that their experience with the earlier ones seemed to help
them in the later ones (371).

White rats observed by Small learned the Hampton Court
maze, in nine experiments made at intervals of two days,
so well that they committed only two errors in the ninth
test, but the significance of this time is obscured by the
fact that the rats were allowed to run freely about the laby-
rinth every night (385). Watson's earlier work with the
white rat was designed to compare the learning processes of
the young with those of the adult animal. The rat is born
unable to care for itself, and before those observed by Wat-
son had reached the age of twelve days, they were unable to
find their way by a simple labyrinth path back to the
mother. At twenty-three days of age they learned a
labyrinth more quickly than adults, probably because of
their greater activity, although for the same reason they
made more useless movements. The object of the research

224 The Animal Mind

was to test Flechsig's theory that learning depends upon the
presence of medullated fibres in the central nervous system;
this was found to be unconfirmed, since at twenty-four days,
when the rat is psychically mature, the medullation of its
fibres is highly imperfect (430).

In his later experiments on white rats Watson's aim was
to investigate the nature of the sensations which guide them
through a labyrinth. The results will be discussed a few
pages farther on (431).

Allen's work on the guinea pig was intended for comparison
with Watson's study of the white rat, because the young
guinea pig comes into the world, not helpless like the baby rat,
but well equipped on both the sensory and motor sides. In the
labyrinth, here, the mother was put at the end of the maze,
and the sight and smell of her were supposed to serve as the
stimulus to activity. Before the young animals had reached
the age of two days, they did not succeed in learning a com-
paratively simple path, but at that age they did learn it, and
proved the fact when the wire netting box in which they were
placed was turned about, by pushing at the place where the
opening had formerly been. At three days, they learned a
more complex labyrinth, and appeared to possess the learning
capacity of adults (4).

In Yerkes's study of the Japanese dancing mouse, the
reactions to irregular and to regular labyrinths were com-
pared, and it was found that a maze of the latter type, that is,
one where left and right turns alternated, was more quickly
learned and more perfectly mastered than an irregular one.
Yerkes urges the importance of keeping account of the errors
made by an animal as well as the times occupied in traversing
a maze (469). Watson's later work on the white rat gives
only the turns (431). In many cases, especially with animals
not naturally active, the time values have little significance;

Modification by Experience 225

an animal in a sluggish mood may traverse a path very
slowly and yet make no errors.

Kinnaman taught two Macacus rhesus monkeys the Hamp-
ton Court maze. That they had an anticipatory idea of the
pleasure in store for them at the centre he thinks evidenced
by the fact that they would begin to smack their lips audibly
on reaching the latter part of their course. Yet for them, as
for the rats, one of the most persistent errors lay in taking the
wrong turning at the outset (221).

What is the mental aspect of the process of learning a laby-
rinth ? Does it involve that form of memory which consists
in the revival of images of past experience ? Or is it simply
the gradual formation of a habit of movement, at no stage of
which a memory image functions ? In the first place, we may
note that no method less calculated to involve images could
well be devised. A human being in such a labyrinth as that at
Hampton Court, with all his wealth of image- forming and
controlling power, is at a loss to make use of it for his guid-
ance. Secondly, there are various phenomena displayed in
the experiments which tell against the image theory. For
one thing, the slowness of the learning process in the simple
labyrinths indicates that memory in this sense is not concerned.
When an animal has the choice between two passages only,
if it possessed the power of recalling, in any terms whatever,
a memory image of its previous experience, surely thirty or
forty trials would not be required before the right path was
taken at once. Again, the nature of the errors made in some
cases suggests that memory images are not present. For in-
stance, when Small's two rats had learned the complicated
labyrinth almost perfectly, the one error in which they both
persisted lay in taking the wrong turn at the entrance. Now
this, it is safe to say, would be the very first error that a being
which guided itself by images would eliminate. It might be
Q

226 The Animal Mind

difficult to remember, in the image- forming sense, the later
turns, but surely "turn to the right" or "turn to the left"
would present itself in some sort of terms at the entrance, if
the animal could have memory ideas at all. Furthermore, it
is very difficult to interpret the learning process here as a case
of association of ideas. In Small's labyrinth, two kinds of
errors could be made: the one would land the animal in a
cul-de-sac, the other simply meant taking a longer passage
when a shorter one would suffice. If the former came to be
avoided as the result of the calling up of a memory idea, this
idea might be that of being brought up short and compelled
to retrace one's steps, but how are we to imagine the idea
of a shorter path as balanced against that of a longer path ?
Small says they must be "distance or temporal ideas in tactual-
motor terms," and urges that our own lack of experience of
such ideas should not make us doubt their existence in the
rat mind; but Thorndike's position, that no ideas are in-
volved at all, that the rat merely comes gradually to "feel
like" taking one turn rather than the other, seems more
probable.1 In other words, we have the formation of a habit
of movement rather than an association of ideas.

But though ideas may not be involved, the further question
remains as to what kind of peripherally excited sensations are
influential in the learning process. This question really
resolves itself into two. First, by what "clews" does the
animal guide itself in learning the labyrinth path? Second,
do these clews continue necessary to its guidance when the
habit is formed? The two parts of the problem have not
always been kept distinct by those who have used the method.

As regards the first part, Small obtained evidence that his
white rats were not guided merely by the smell of their own
tracks in finding their way to the centre of the maze, from

1 See his review of Small's work, Psych. Rev., vol. 8, p. 643.

Modification by Experience 227

various observations, among others the fact that they ran all
over the passages in their earlier trials, so that smell might
have guided them wrong as well as right (385). This conclu-
sion was confirmed by the experiments of Watson, who found
that rats with the olfactory lobes removed learned the laby-
rinth as readily as normal rats (431). Yerkes, in his labyrinth
tests upon the crawfish and the frog, excluded smell as a means
of guidance by washing the labyrinth out between trials (454,
471). For certain species of ants, as we know, smell is the
dominant factor. Visual clews seem to be used by different
animals to different degrees. The frog displayed a disturb-
ance of its habit when red and white cards placed on either
side of the passage were interchanged (454). The crawfish
seemed to recognize and draw back from the screens in the
blind passage before running against them (471). The pigeons
tested by Rouse, when required to go through the labyrinth in
darkness, were obliged to relearn it, although they made the
first turn correctly. Perhaps, Rouse suggests, the stimulus to
the first turn was the sound of the door lifted to admit them,
or the touch of the narrow entrance (371). On the other
hand, Small found that altering the direction of the light had
little effect on the performances of his white rats. He also
placed wooden pegs painted red, at each division of the paths,
in the middle of the correct path, and caused the labyrinth
thus arranged to be learned by hitherto untrained rats. They
did not learn it any faster through the presence of these visual
hints, nor, when it had been learned, were they at all discom-
posed by the removal of the pegs (385). Allen's guinea pigs
did not alter their behavior with alteration of the position of
colored cards placed as guiding marks (4). And Watson's
blinded rats learned the labyrinth as readily as normal
ones (431).

Rouse found that the pigeon could make use of auditory

228 The Animal Mind

stimuli as clews. He arranged to have an ordinary electric
bell rung whenever the birds entered a wrong alley, and a
wooden bell sounded when they emerged and took the right
course. After they had learned the path under these condi-
tions, the two kinds of sound stimuli were interchanged, and
the result was a certain amount of confusion on the part of
the birds. Another device consisted of a board with electric
wires, laid on the floor of the labyrinth. A bell was rung
whenever a pigeon stepped on the board, and the bird was
given an electric shock; when the experience had been
repeated a number of times, the pigeons would show uneasi-
ness at the sound of the bell, wherever they happened to be in
the labyrinth (371). In white rats, Watson found that par-
tial deafness, produced by throwing the middle ear out of
function, had no effect on the ability of the rats to learn the
path (431).

Various stimuli may, then, serve as clews in the process of
learning the labyrinth. But in certain cases, neither visual,
olfactory, nor auditory stimuli seem to be at all concerned.
This was true of Watson's white rats, and probably of Allen's
guinea pigs. Special tests were made to investigate the role
of tactile sensations in the labyrinth performances of these
animals. With the guinea pigs, a cardboard labyrinth was
substituted for that of wire netting, and a black cloth placed
on the floor (4). Watson's white rats, when the vibrissae
(long whiskers) were removed, learned the maze as well as
normal rats. Those which had already learned it manifested
some disturbance on having the vibrissae removed, showing
a tendency to bump into the partitions and hug the walls.
Watson does not report whether this same disturbance failed
to appear in the rats without vibrissae that were learning the
labyrinth for the first time ; he merely gives the times occu-
pied in traversing the course to show that the learning process

Modification by Experience 229

was normal. It would be very strange and quite out of accord
with the general behavior of animals in labyrinths if a prac-
tised rat should be more disturbed by the removal of an accus-
tomed stimulus than an unpractised one. No effect on the
learning power of the rats was produced by making their
paws anaesthetic (431). Yerkes has shown, although with-
out operating on his subjects, that the Japanese dancing
mouse does not necessarily depend on sight, smell, or touch
for guidance in the labyrinth (469). It should be noted that
proof of an animal's ability to learn a maze when deprived
of a certain class of sensations does not show that it normally
makes no use of those sensations in the learning process.

As a matter of fact, the stimuli which originally give the
"clews" in the case of the white rats must be the rats' own
movements. "Muscular sensations dependent on the direc-
tion of turning," " kinsesthetic sensations," are the only
elements in our own experience that suggest themselves as
possibilities where an animal learns the maze equally well
when blinded, anosmic, deaf, and partially deprived of touch
(431). But this is not the same as saying that when an ani-
mal has learned the labyrinth, it is "guided by kinaesthetic
sensations." Nor can we show that an animal was not guided
by some other stimuli, say visual ones, in learning the laby-
rinth, when we prove that having once learned it, the animal
is not disturbed by the removal of these stimuli. For when
the labyrinth path has been learned, the habit may be in a
sense quite independent of the very stimuli that served to form
it, precisely as the pianist becomes independent of the notes
in playing a familiar piece. The fact that Yerkes's frogs
were disturbed, after the habit had been formed, by the inter-
change of the cards, indicates that visual stimuli were still
important to them; but if they had not been disturbed by
such interchange, when they were fully practised, it would

230 The Animal Mind

not have proved that the cards had played no part in forming
the habit.

In the same way, it is not likely that a thoroughly prac-
tised animal needs to have in consciousness even kinaesthetic
sensations. Watson attempts to describe the processes in
the mind of a practised rat as follows : " What leads up to
the act of turning ? The ' feeling ' (probably only vaguely
' sensed ') which may be expressed anthropomorphically in
these terms: *I have gone so far, I ought to be turning
about now ! ' " " If the turn is made at the proper stage
. . . the animal may be supposed thereby to get a * reassur-
ing feeling,' which is exactly comparable from the stand-
point of control to the experience which we get when we
touch a familiar object in the dark" (431, pp. 95-6). I do
not think these before and after ' feelings ' are necessarily
present at all in the consciousness of an animal whose
labyrinth habit is fully formed. Such an animal has be-
come a little machine which takes so many steps along a
straight path, turns to the right, takes so many more steps,
and so on until the performance is complete. If, indeed,
it makes an error in this process, then the kinaesthetic sen-
sations may come into play, but otherwise there would seem
to be no reason for assuming in the fully practised animal
consciousness of any stimulus except the initial one which
starts it on its path.

Very curious are the results obtained by Watson when the
entire labyrinth was turned through an angle of 90 degrees.
Although no turn which the animals had to take was in any
way altered by this proceeding, the rats showed decided con-
fusion, the blind rats as much as the others. This latter
fact would indicate that alteration in the direction of the
light was not the source of the confusion ; but when the maze
was rotated through 180 degrees, the blind rats were not

Modification by Experience 231

disturbed, while the others were. More investigation, de-
cidedly, is needed before we can decide, as Watson does,
" either that static sensations have a r61e, or . . that the rat
has some non-human modality of sensation which, whatever
it may be, is thrown out of gear temporarily by altering the
customary relations to the cardinal points of the compass."

One or two incidental observations regarding the behavior
of animals in labyrinths are strongly suggestive of the auto-
matic character of the movements involved. An animal that
has gone astray on the path will often find the way back to
the starting-point, and from there traverse the whole road
rapidly and unerringly (e.g. 450, 431), apparently in the same
way that a piano player who has a piece " at his fingers' ends,"
but has stumbled in a passage, can go through with entire
success if he starts over again. As piano players know, in
such a case it is much better not to attend to stimuli at all,
but to think of something else ; the movements will take care
of themselves better if consciousness intervenes as little as
possible.

Again, in the process of learning a labyrinth, habits of
movement are often formed that are of no use whatever ; that
do not lead to success, and hence cannot be guided in any
sense by the animal's experience of their pleasant consequences.
Rouse and Small both report this tendency to form useless
habits, and in the case of some salamanders observed by the
writer, which never finally mastered the labyrinth they were
placed in, habits of going elaborately wrong would make their
appearance and persist for several days, each animal re-
maining true to its individually acquired tendency. The
mere fact that the movements were accidentally performed
two or three times in succession created a persistence in doing
them, although they led to no pleasurable consequences what-
ever.

232

The Animal Mind

§ 85. Dropping off Useless Movements: the Puzzle-box

Method

The dropping off of useless movements is further illustrated
in those experiments where animals are required to work some
kind of mechanism. This may be called briefly the puzzle-
box method. It is obviously an advance in difficulty over the
labyrinth method in that it requires the formation of a new

FIG. 1 6. — Puzzle box used in Porter's work on birds. A B, one method of attach-
ing string to latch; C, a second method. In the first, the loop at B had to
be pulled; in the second, the string had to be pushed in.

impulse rather than the mere guidance of an old one; it does
not merely direct the animal in the performance of something
that he would do anyway, but causes him to do something
that he otherwise would not do. Yet the distinction is not
so fundamental as it seems.

The puzzle-box method has been tried with birds, rats, cats,
dogs, raccoons, and monkeys. Thorndike, its originator,
made some experiments of this type on chicks ; the animals
were confined in pens, from which they could be released by

Modification by Experience 233

pecking at a string or some such object. In other cases, as
we have seen, these tests should be classed rather with the laby-
rinth method, as requiring merely that the chick should run
out at a given definite place (393). Porter tested English
sparrows with boxes containing food, which could be entered
by pulling a string fastened to a latch, or by pushing the string
into the wire netting with which one side of the box was cov-
ered (Fig. 1 6). The sparrows learned very quickly; one of
them by the tenth test had left out all unnecessary move-
ments (344). In later experiments a cowbird and a pigeon
also learned to open a similar box. Before beginning the
test the birds were accustomed to being fed in the box with the
door open. Their first success in opening the door lay in
accidentally clawing or pecking at the proper point, and in
later trials the action was simplified ; thus the birds learned
not to attack other parts of the box, to use the bill instead of
the claws, and to stand on the floor beside the box instead of
hopping upon it. A point of some interest arises in connection
with the fact that one or two of the birds, for instance the male
pigeon, opened the door in the simplest possible way, although
not very quickly, the first time they tried it, and that these
birds showed very little improvement in speed through sub-
sequent trials ; whereas the ones that had the most difficulty
about the first execution of the act ultimately reduced their
speed much below that of the others. It is possible, as Porter
suggests, that "greater difficulty and therefore more vigorous
activity on the part of the animal in the initial trials of any
series may naturally be expected to lead to more rapid prog-
ress in the later ones" (345). In Rouse's test of the pigeon
by the puzzle-box method, it showed less aptitude than that
displayed by the English sparrow (371).

Small tested his white rats with two boxes containing food.
One could be entered by digging away the sawdust which was

234 The Animal Mind

banked around the lower end of the box, if the digging was
done in a particular place; the other, by tearing off strips
of paper which held shut a spring door. The result of the
earlier series of experiments with the first-mentioned box
was that after an hour and a half on the first day one rat
happened to dig in the right place and entered. The second
day this rat took only eight minutes, and the thirteenth day
only thirty seconds, to enter. With the second box there
was always a tendency to begin by digging, and even in the
thirteenth experiment, where the rat got in by biting off the
papers in fifteen seconds, she began by two strokes of dig-
ging. In a later test with this box the rat chanced to be ex-
tremely hungry, and dug violently for several seconds, indi-
cating a blunting of the discriminative powers by hunger,
analogous to that which we have found in very low animals.
Like Porter subsequently, Small found that "if a rat happens
to succeed by several methods, as, e.g., biting, clawing, butting,
there is a strongly marked tendency to select the most ex-
peditious and effective method. This apparent selection,
however, is rather a matter of inertia than of prevision."
The rats were later trained to discriminate between the two
boxes, being sometimes presented with one and sometimes
with the other. Such experiments, however, may properly
be classed under the head of another method which we shall
presently discuss. Great individual differences were found
an^ng the rats : two of the four tested never learned to get
into the boxes so long as they were with their more energetic
companions, but merely profited by the activity of the latter
(386). Watson's puzzle-box experiments on the white rat
were designed, like his labyrinth tests, to compare the powers
of the adult with those of the young. The results were
practically the same as those of the labyrinth tests, except
that in the box experiments, where mere activity counts for

Modification by Experience

235

less than in a labyrinth, the adult rats solved the problems
in less time than that occupied by the young ones (430).

In Thorndike's work on cats and dogs, the investigator
placed the animals themselves in the boxes, and food on the
outside, so that the problem was not how to get in but how
to get out. The getting out could be accomplished in various

FIG. 17. — Puzzle box used in Thorndike's experiments on cats.

ways, such as pulling a wire loop, clawing a button around,
pulling a string at the top of the box, poking a paw out and
clawing a string outside, raising a thumb latch and pushing
against the door, and so on (Fig. 17). The animals, on
being first put into the box, made all sorts of movements
in their struggles to get out; the right movement was hit
upon by accident. Only very gradually, as the experiment
was repeated again and again, were the useless movements
omitted, until finally the right one was performed at once
(393)- Wesley Mills has criticised these pioneer experi-
ments of Thorndike's on the ground that the animals were
under such unnatural conditions and in such an extreme
state of hunger that they profited by experience more slowly

236

The Animal Mind

than might otherwise have been the case (273); and this
may have been to a certain extent true. In testing monkeys
with puzzle boxes Thorndike placed the food on the
inside and the monkeys on the outside. He found a
marked difference between the speed of their learning and

that shown by the cats and
dogs. " Whereas the latter
were practically unanimous, i
save in the cases of the very i
easiest performances, in show-
ing a process of gradual learn-
ing by a gradual elimination
of unsuccessful movements
and a gradual reinforcement
of the successful one, these are

FIG. 1 8. — Combination fastening used 'unanimous, Save in the very
in Kinnaman's work on monkeys. , , .

The figures indicate the order in hardest, in Showing a prOCCSS
which the parts of the combination of Sudden acquisition by a

rapid, often apparently instan-
taneous abandonment of the unsuccessful movements and
selection of the appropriate one, which rivals in suddenness
the selections made by human beings in similar perform-
ances " (397). Kinnaman further complicated the box tests
with his Macacus monkeys by constructing "combination"
fastenings, which required the performance of a set of actions
in a certain order, and found that these were mastered by
the animals (221) (Fig. 18).

Cole's work on the raccoon, finally, indicates that in speed
of learning this animal stands " almost midway between the
monkey and the cat," while "in the complexity of the associa-
tions it is able to form it stands nearer the monkey." The
raccoons, like the monkeys, learned combination locks,
although they did not learn to perform the various move-

Modification by Experience 237

ments involved in a definite order. They showed an inter-
esting tendency to skip at once to the movement that im-
mediately preceded the opening of the door (82).

The question arises, as in the case of the labyrinth ex-
periments, whether, when the animal has learned the proper
movements to open a box, he opens it by "remembering"
the movement ; that is, by having some kind of an idea or
image of it in his consciousness, or whether we have to do
with the formation of a habit by a process in which ideas
are at no time involved. Here, again, the gradual character
of the learning process, where it is gradual, points to the
absence of ideas; a human being who had once hit by
accident upon the right way to open a lock could hardly fail
on being confronted with it a second time, at not too great
an interval, to recall an idea of the successful movement
and perform it at once, without any unnecessary accom-
panying movements. We have seen an approach to this
state of things in the monkeys; accordingly it is possible
that they may learn by means of ideas. On the other hand,
rapid learning, where the action is very simple and closely
connected with the animal's instincts, does not necessarily
mean the presence of ideas; in certain cases there may
exist arrangements for the rapid modification of an instinctive
mechanism which do not involve the production of images
at all. The most we can say is that slow learning, by gradual
elimination of the useless movements, indicates, so far as
we can judge, the absence of any guiding idea of the action.
Other evidence against the idea hypothesis was derived by
Thorndike from various facts. In the first place he found
in the animals observed by him an entire lack of what has
been termed inferential imitation.

Imitation in animals has by some writers, notably Was-
mann, been classed as a special method of learning by

238 The Animal Mind

experience (426), and from one point of view it is. But
imitation may be, as various authors have pointed out, of
at least two different types. The first may be called in-
stinctive imitation, and is widespread throughout the animal
kingdom. It occurs when the sight or sound of one animal's
performing a certain act operates as a direct stimulus, ap-
parently through an inborn nervous connection, to the per-
formance of a similar act by another animal. "If," says
Lloyd Morgan, "one of a group of chicks learns by casual
experience to drink from a tin of water, others will run up
and peck at the water and will themselves drink. A hen
teaches her little ones to pick up grain or other food by peck-
ing on the ground and dropping suitable materials before
them, the chicks seeming to imitate her actions. . . . In-
stinctive actions, such as scratching the ground, are performed
earlier if imitation be not excluded " (281, pp. 166-167).
Imitation in this sense is hardly so much a method of learn-
ing by experience as a method of supplying experience.
An animal may perform an act the first time because, through
inherited nervous connections, the sight of another animal's
performing it acts as a stimulus. But it will continue to
perform the act, in the absence of any copy to imitate, only
if the act is itself an instinctive one, like drinking in birds,
or becomes permanent by reason of its consequences, just as
would be the case if its first performance had been accidental
rather than imitative. As a matter of fact, instinctive imita-
tion seems usually to be concerned with actions themselves
instinctive.

Inferential imitation, or what Morgan calls reflective
imitation, is a different affair. It is the case where an animal,
watching another one go through an action and observing
the consequences, is led to perform a similar act from a
desire to bring about the same result. The most natural

Modification by Experience 239

description of the subjective side of this process in a human
being would be to say that the sight of the other individual's
behavior "suggests the idea" of similar behavior on one's
own part. Inferential imitation would then not differ
fundamentally from any other case of learning by ideas.
Now Thorndike, in his experiments on cats and dogs, found,
as we have said, no evidence of this type of imitation. A
cat put in a puzzle box did not learn the way out any sooner
for watching, even repeatedly, the performances of a cat
that knew how to get out. With monkeys, Thorndike's
most extensive tests were made to find whether the animal
would learn to open a box from seeing the experimenter
himself do it, and his results were again, on the whole,
negative (393). Small's white rats also showed no ability
to profit by each other's experience in this way. One of
each of the pairs first experimented on solved the problems
presented; the other, instead of either attacking them for
itself or learning by watching the successful one, contented
itself with stealing the food secured by the latter (386).
Imitation, according to Yerkes, plays no considerable role
in the learning processes of the dancing mouse (469).

On the other hand, Kinnaman's monkeys did give some
indications of learning by inferential imitation. In one
case, the box had to be opened by pulling out a plug. One
monkey failed to work the mechanism, and gave up in despair.
Another one then came out of the cage, the first one follow-
ing. Number two went to the box, seized the end of the plug
with its teeth and pulled it out. The box was set again,
and monkey number one rushed to it, seized the plug as
number two had done, and got the food. She immediately
repeated the act eight times. A second and similar observa-
tion was made where the mechanism was a lever (221).
Hobhouse found that cats, dogs, elephants, and monkeys

240 The Animal Mind

were aided in their learning processes if he "showed" them
how to do the thing (177). Whether this was inferential
imitation in the sense that they got the idea of the action
and of its result by watching him, or whether they were
merely aided in focussing their attention on the important
object, the string, hook, or lever, it is difficult to be sure.

Berry found that the white rats he experimented on
manifested a type of imitative behavior which he is inclined
to regard as intermediate between instinctive and fully in-
ferential imitation. "When two rats were put into the box
together," he says, "one rat being trained to get out of the
box and the other untrained, at first they were indifferent
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's movements 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 to get out by working at the spot where he had
seen the trained rat try " (26).

Now, so far as the light cast by this evidence for and against
inferential imitation or the presence of ideas in the animal

ind is concerned, the matter seems to stand as follows.

e cannot be sure that Kinnaman's monkeys really had
an idea of the proper action suggested to them by seeing
their companions perform it ; the case might have been one
of instinctive imitation, taking here a form more elaborate
than was seen in cats and dogs because more complicated
movements are natural to the monkey than to the lower
mammals. If it is certain that Berry's uneducated rat
began to watch the actions of the educated one more closely

Modification by Experience 241

as a result of its observation that the latter succeeded where
it failed, then, although the imitation was confined merely
to investigating the same general locality as that attacked
by the trained rat, and not extended to an actual performalSce
of the movement, it would seem to be inferential in type.
But precisely this certainty is apparently rather hard to
attain. Again, when an animal is assisted in learning by
watching a human being who undertakes to show him how,
is he given an idea of the act and its results, or does he merely
have his attention called to the important part of the mechan-
ism to be worked? True inferential imitation is hard to
prove. On the other hand, the failure of an animal to show
inferential imitation does not mean that the animal cannot
have ideas, f We cannot conclude that an animal is incapable
of ideas because it does not have them suggested to it under
circumstances that would suggest them to our minds.)

Again, Thorndike noticed that while after a time the cats
that had been caused to go into the box and let themselves
out before being fed would go into the box of their own
accord, cats that had been from the first dropped into the
box at the top did not learn to go into it of themselves. He
argues that if the cat had been able to have the idea of being
in the box, as a necessary prelude to food, it would have
been able to pass from the idea of being dropped in to that
of going in itself. Further, he found that he could not train
the animals to do a certain act by forcibly putting them
through it, and concludes that if they had been capable of
having the idea of the act, it would have been suggested to
them by this process (393). But in each of these cases,
the experience which is supposed to give the animal the idea
of the act is one that has some points of likeness with the
act and other points of decided difference. The experience
of seeing another animal perform a movement is very different

242 The Animal Mind

from that of performing it oneself ; the experience of being
picked up and dropped into a box is very different from that
of walking in through the door, and the experience of being
forcibly held and put through a movement differs from do-
ing it without restraint. To the human mind, accustomed to
more analysis of its own experiences, one of these would
suggest the other, but we cannot argue that because such an
association is not made in the animal's mind, therefore the
latter is incapable of ideas, any more than we could conclude
a total absence of ideas in the consciousness of a man to
whom a primrose by the river's brim does not suggest thoughts
of the moral government of the Universe.

Moreover, the raccoon, according to Cole, presents some
of these very indications of ideas in its learning processes.
In the first place, the raccoons, unlike Thorndike's cats,
did "run back into boxes into which they had hitherto been
lifted." They were picked up by the nape of the neck
and dropped into the boxes. On the thirty-third trial in
the case of one raccoon, "she turned . . . and went quickly
back into the box. She opened the door in six seconds,
came out, was fed for a moment from the bottle, and then
immediately re-entered the box." It is, then, at least possible
that the idea of being in the box was suggested to them from
the experience of being dropped in. In the second place,
unlike Thorndike's subjects, the raccoon learned to work a
fastening by being put through it. For example, raccoon
number two had failed to learn to raise a horizontal hook.
"To make it a certain failure, I waited thirty-two minutes
while he worked steadily. I put him through five times
by raising the hook with his nose. He then succeeded in
three and four-tenths seconds, then in seven and two-tenths,
and so on." Other examples are not quite so clear-cut as
this, but there is ample evidence that putting the raccoon

Modification by Experience 243

through the proper movement greatly facilitated the learning
process. Further evidence of the presence of ideas in the
raccoon's mind will be considered later (82). Yerkes has
found that the dancing mouse is aided in learning by being
put through the act (469). Hobhouse thinks that his cats,
dogs, elephants, and monkeys showed that their actions were
guided by an idea of the result, instead of being merely ac-
quired reactions to stimuli, because they varied the means
to the end. "In opening the sideboard drawer, Jack
[a dog] not merely pulls, but learns for himself how to get
his head into the drawer without shutting it again, altering
the method when he once hurts himself, and finding another.
So again, I have seen him, when standing up to pull open the
door of his box by means of a wire, accidentally pushing it
with his paws again as he let go. At a second trial he was
careful to avoid this, dropping the wire and pushing his
nose in as soon as there was room. Similarly, I have seen
the elephant shift the box that she was opening when she
had found that in a certain position the door would slam to
again before she could get her trunk in." These bits of
behavior, in Hobhouse's opinion, indicate that the animals
have ideas of the changes they wish to bring about (177).
While it may be rash to assert of any particular higher
animal that its consciousness never 'contains ideas, yet the
slow acquisition observed in many of these experimental
tests certainly gives evidence of a process of learning whose
essence consists simply in the gradual dropping off of un-
necessary movements. Upon the nature of this process,
psychology can throw little light. Thorndike declares that
the successful movement is "stamped in" because pleasure
results from it, while the unsuccessful movements are
"stamped out" because no pleasure results. The terms
" stamping in " and " out " must refer to some effect upon

244 The Animal Mind

the nervous system, and of this effect we know no more than
that it exists. Jennings says that the disturbance set up in
the organism by the stimulus, by hunger or confinement,
as the case may be, not finding an outlet by one path of
discharge, seeks others in succession until one is found which
relieves the disturbed condition. This, we have seen, he
and others have found to be the case in very low forms of
animal life. But the crucial part of the phenomena we are
now considering is described in the following sentence :
" After repetition of this course of events, the change which
leads to relief is reached more directly, as a result of the
law of the readier resolution of physiological states after
repetition " (208). And that is all we know of the matter.
But we may well note the probability that a habit, in the
sense of a fixed way of action not innate in the individual,
may originate in two ways : first, by the loss of conscious
control in the case of a set of actions originally voluntary
and guided by ideas; and second, by the gradual increase
of speed and accuracy in the performance of a series of
actions, never at any time guided by anything but external
stimuli. In our own experience, the first kind of habit
formation has been of so much interest that it has diverted
attention from the second. Yet the latter is shown con-
stantly in the growth of skill that comes through the mere
repetition of a series of movements, apart from the " knowing
how," which means conscious control.

§ 86. The Psychic Aspect of dropping off Useless Move-
ments

The conscious aspect of learning by dropping off useless
movements must consist largely in the mere shortening of
a period of unpleasantness and unrest. The useless move-
ments are unpleasant, the successful one brings pleasure;

Modification by Experience 245

when the latter comes to be performed at once, the con-
sciousness accompanying must be wholly pleasant. Further, k
the puzzle-box experiments differ from the labyrinth ex- \
periments in that the successful movement is not merely
turning in one direction rather than another, a habit which
might ultimately become perfectly independent of external
stimuli; but a reaction upon a particular object. It is
evident that in the course of learning, this object, at the
outset unnoticed, must come to stand in the centre of the
animal's consciousness, the focus of its attention, as soon
as it is perceived at all. Such a change would constitute
another feature of the consciousness accompanying this
form of learning, in addition to the diminishing unpleasant-
ness which goes along with the dropping off of unnecessary
movements. A special aspect of experiments on imitation
is connected with this process of learning to focus attention
on ike proper object. Can an animal have his attention
" called" to the object by watching some one else operating
on it, or must the object gain its power to determine reaction
solely by the animal's own experience of the consequences
connected with it? Hobhouse's experiments on dogs, cats,
and other animal subjects led him to the conclusion that \
watching his performance did materially assist the animal \
in this respect. His method was to perform the action of
pulling a lever or string repeatedly while the animal was
watching; and that this process facilitated learning he
thinks evident from the fact that after it had been several
times repeated, the behavior of the animal changed in a
marked degree; instead of being random, it was definitely
directed at the proper object. The suddenness of the transi-
tion from random to definite behavior indicates, in Hobhouse's
opinion, the effect of being shown. Thus, for example,
his dog Jack "after being once shown, . . . learnt to pull

246 The Animal Mind

a stopper out of a jar with his teeth. The stopper fitted
into a large round glass jar, and could be lifted with the
teeth by a projecting peg. I lifted it out for him once, and
left him to deal with the jar, which he did by knocking it
over and rolling it all about the room until the meat was
jerked out. At the second trial he pulled at the stopper him-
self with his teeth; and he repeated this many times" (177).
A cat with which the writer is acquainted stands on his hind
legs and touches a door handle with his paw when he wishes
to be let out. He has never succeeded in letting himself
out by any such method. It is possible that the habit may
have been acquired from the fact that the door is sometimes
opened for him after he has done so ; but this is by no means
always the case. He is often left to mew for some time
after he has pawed the handle. There is, then, the pos-
sibility that observing human beings open doors may have
caused the handle to occupy the focus of his attention;
but one's attitude toward this hypothesis should be ex-
tremely cautious.

CHAPTER XI

THE MODIFICATION OF CONSCIOUS PROCESSES BY INDI-
VIDUAL EXPERIENCE (continued)

§ 87. The Inhibition of Instinct

IN still another form of experiment that has been devised
to study the ways in which animals learn by experience, the
object has been to secure the complete inhibition of an in-
stinctive action. Obviously two factors will come into play
here, — the strength of the instinct and the force of the modi-
fying experience. The latter factor we might suppose to be
strongest when the performance of an instinctive action
could be made attended with pain, and less strong when
the performance of some action opposed to instinct has been
found to be accompanied by pleasure.

The. first case we find apparently illustrated by Morgan's
chick in his dealings with a bee ; he needed but one experi-
ence with that insect to inhibit entirely, the next day, his
instinct to peck at it (281, p. 53). On the other hand,
Bethe denied consciousness to the crab because, although
every time it went into the darkest corner of the aquarium
it was seized by a cephalopod lurking there, it did not in six
experiences learn to inhibit its negative phototropism ; nor
did the crabs learn not to snap at meat, though several times
when they did so they were seized by the experimenter (28).
The case of the crabs is not, however, fairly comparable
with that of the chick, for the latter was not really obliged
to inhibit his pecking instinct altogether, but only to direct

247

248 The Animal Mind

it away from a certain object, while the crabs had no outlet
at all for their photic and nutritive instincts. Very likely
a longer course of training than that employed by Bethe
might have succeeded in suppressing the instincts. The
chick's case really belongs to a form of learning which we
shall consider later on; that where the inhibition depends
on the discrimination of different stimuli. The purest
instance of the kind of modification of behavior by ex-
perience at present concerning us is furnished by Mobius's
experiments with the pike (276), afterward repeated by
Triplett with perch (407). The pike was kept in one half of
an aquarium, separated by a glass screen from the other
half, in which minnows were swimming about. The pike
naturally dashed at them, and received a bump on its nose
whenever it did so. After a considerable period of this
sort of experience, the glass screen was removed, and the
minnows were allowed to swim freely around the pike,
when it was found that the latter's instinct to seize them
had been Wholly inhibited by the disagreeable consequences of
such action. Triplett's description of the occasional struggles
of the instinct to assert itself is extremely interesting. An
analogous case is offered by Goldsmith's account of the
shell-inhabiting fish, Gobius, which, when a glass partition
was placed between it and its shell domicile, dashed against
the glass for a time, but after three and a half hours went
around it, and the next day did so after only a quarter of an
hour's unsuccessful attempting to get through the parti-
tion (146).

The second kind of training in the inhibition of an in-
stinct, where the performance of an action opposed to instinct
is made to produce pleasure, is illustrated by Spaulding's
work on "Association in Hermit Crabs." He found that these
animals, which are positively phototropic, could be trained

Modification by Experience 249

to go into the darkened part of the aquarium to get food,
and finally to do so even if no food was there (389). Es-
pecially striking as an example of this kind of learning is
the behavior of the insect called the water scorpion in the
experiments of Holmes mentioned on page 179. With its
head directly away from the light, and the right eye blackened,
the natural tendency of this positively phototropic insect was
to turn to the left. Yet after a sufficient amount of training
in a position where the natural tendency was to turn toward
the right, the animal, on being replaced with its back to the
light, turned toward the right, an action directly contrary
to instinct having been thus brought about by experience,
as Holmes thinks, and as we may certainly conjecture, of its
pleasurable consequences (186). When the " flight-reflex"
comes gradually to be inhibited in animals that are being
tamed, we have another instance, of this type of learning
(e.g., 106).

The chief psychological question involved in the con-
sideration of that form of learning by experience which
involves the inhibition or reversal of an instinct is whether
there is in the animal's mind an actual representation of
the effects of the actions which constitute the animal's train-
ing. Does the pike, confronted with the minnow, recall
the bump on its nose? Where the learning is very rapid,
this always remains possible. Where the process is slower,
however, the simpler hypothesis would be that the pleasure
and pain of the results operate directly on the animal's
tendencies to move, without the intervention of images.
In the experiments where the results are painful, the stimulus
at first produces, through the animal's inherited nervous
connections, a movement toward it. This movement, un-
der the peculiar circumstances of the case, occasions pain,
and pain brings about a negative reaction of withdrawal.

250 The Animal Mind

It is possible that, as a consequence of the general tendency
of the nervous system to establish short-cuts, after repetition
of this experience, the appearance of the stimulus may stir
up the negative reaction soon enough to inhibit the positive
one altogether; through the operation of the same law
whereby in learning a foreign language we first pass to the
meaning of a word by way of the sound of the English word,
but later make the association without any intervening link.
On the psychic side, the object which was at first agreeable
has simply become disagreeable. As for the cases where the
instinct has been reversed by means of pleasurable conse-
quences, the training of the hermit crab was accomplished by
pitting a stronger against a weaker instinct. Nothing but the
natural victory of the stronger innate tendency to move was
required to make the crab go into the dark part of the aqua-
rium to be fed, when the food was actually there ; but what
made it continue to do so when the food was removed?
The representation of the resulting pleasure, Spaulding says ;
shall we admit this, or confine ourselves to physiological
terms, and say that the nervous energy involved in the sight
of the dark corner has come to find its natural outlet in
movements toward that corner, through the repetition of
these movements as a result of the operation of the stronger
food instinct? The case of the water scorpion certainly
suggests rather the operation of a blind habit than the effect
of any representation of pleasure. Here the light-seeking
instinct is, as it were, pitted against itself ; shall we say that
the animal, guided by a representation of the pleasure it
has previously derived from turning to the left, does so now,
when the slightest turn to the right would actually give it
that pleasure? Possibly, but the behavior looks more like
the working of a mechanical habit of turning.

Modification by Experience 251

§ 88. Inhibition involving Discrimination of Successive
Stimuli

In other experiments requiring the inhibition of an
instinct, the animal is caused to discriminate between two
nearly similar stimuli, to execute the action when one is
presented and to inhibit it when the other one appears.
Such tests, as we have seen, are very commonly adopted
to investigate sensory discrimination. The principle is the
same whether the action to be inhibited is an instinct or an
acquired habit. The experiments may be divided into two
classes: in the first only one stimulus is given at a time,
and the animal in consequence is sometimes required to
inhibit its response entirely; in the second, two or more
stimuli are given simultaneously , and the animal simply
has to choose among them on the basis of its past experiences.
Experiments belonging in the former class were successfully
performed by Thorndike with Cebus monkeys. Both of his
subjects learned to come down to the bottom of the cage
to be fed when the experimenter took a piece of food in his
left hand, and to stay up when he took it with his right hand,
by being fed in the first case and not in the second. One
of the monkeys learned to discriminate in like manner be-
tween cards carrying different figures; the other one failed
with the cards and learned to react or inhibit reaction only
in connection with different movements of Thorndike's
hands (397). Professor Bentley and the writer made some
experiments on the chub by this method, which gave wholly
negative results. The red and green forceps, each con-
taining food, were plunged one at a time into the water;
the fish was allowed to get the food from the red forceps,
but the green ones were withdrawn before it had a chance
to bite. The time which the fish took to rise and snap at

252 The Animal Mind

the forceps was measured by a stop-watch, and in the course
of 131 experiments the animal was not found to rise to the
green any less promptly than to the red. In other words,
no tendency to inhibit reaction to the green was shown,
although our later experiments proved that the fish 'could
distinguish the two (421). Apart from the difference in
.intellectual level between the fish and the monkey, it is
probable that the food- taking instinct was stronger in the
former, which came directly from the wild state, where it
could afford to lose no chances of nourishment. Dahl's
observation that the spider Attus arcuatus refused to take
house flies after having been presented with one smeared
with oil of turpentine, although it seized a gnat, is also a case
of inhibition involving discrimination of successively offered
stimuli (88). Cole, in his very interesting experiments on
the raccoon, raises the question whether discriminations of
this type do not involve memory images, and answers it in
the affirmative. He used the method to test discrimination
of colors, tones, forms, and sizes; the results have been
noted in earlier chapters. The cards used were placed on
levers so that by a touch they could be pushed up and
down. The animals learned to climb up for food
when one of two differently colored cards was shown, and
to stay down when the other one appeared; to distinguish
in a similar way between a high and a low tone, between
a round and a square card, and between a card 6| x 6J
inches and one 4^ x 4^ inches square. Of course the action
of climbing up was not itself purely instinctive, but had
become associated with the food instinct. The raccoons
also hit upon the trick of clawing up the cards themselves,
and if the one that appeared was the " no-food" card, they
would either claw it down again and pull up the other, or
proceed at once to pull up the other, leaving the "no-food"

Modification by Experience 253

one also up. Since the cards were shown successively, Cole
concludes that "remembrance of the card just shown was
required for a successful response." "Why," he asks,
"should the animal put the red card down if it did not fail
to correspond with some image he had in mind, and why
when he put the green up should he leave it up and go up on
the high box for food if the green did not correspond with
some image he had in mind?" (82). It seems to the writer
that the supposition of an image is unnecessary, except
possibly in the experiments requiring discrimination of
sizes. It is perfectly possible, as we know from our own
experience, to react to one stimulus and not to another with-
out going through a comparison of the two, unless the differ-
ence between them is merely one of degree. It might have
been possible for a human being to discriminate between
the larger and the smaller cards only by calling up a memory
image of the card not shown and comparing it with the one
before him ; it surely would not have been necessary for him
\ to use images in the reactions to colors, forms, and tones.
I And if a human being, accustomed to much dependence on
\ memory ideas, could get on without them here, surely a
\ raccoon could. Even in judgments of degree, all laboratory
\psychologists know that human beings have a strong tendency
\to make absolute rather than comparative judgments, and
use memory ideas but little. Better evidence of the use of
images is furnished by the following method: "Three
levers were placed on the displayer. 'One, on being raised,
displayed white, another orange, another blue. The plan
was to display white, orange, and blue consecutively, then
to display the same blue three times. I fed the animal if
he climbed upon the high box on being shown the series
white, orange, blue, and did not feed him after the series
blue, blue, blue." That is, the stimulus immediately pre-

254 The Animal Mind

ceding the reaction was the same in both cases. The differ-
ence lay in the foregoing stimuli. The series " white, blue,
red, food, and red, red, red, no food" was also used. The
raccoons learned to respond properly, ''though," Cole con-
tinues, "I never completely inhibited the animals' tendency
to start up on seeing white or blue, which were precursors of
the red which meant food. Thus the animals all anticipated
red on seeing its precursors, which in itself seems good
evidence of ideation. Many times, however, they turned
back after starting at blue or white and looked for the red,
then climbed up once more, thus showing that the red was
not a neglected element of the situation, but an expected
color which they generally waited to see, but sometimes
were too eager to wait for." Two of the raccoons had been
previously trained in two-color series, while one had ex-
perienced only the three-color series. The former showed
a decided tendency to go up at the second color when there
were three. The latter had been trained first on the series
"white, orange, blue, food; blue, blue, blue, no food;"
then on the series "white, blue, red, food; red, red, red, no
food." "Although blue, his former food signal, was," in
the second series, "placed second as a no-food color, he made
the mistake of reacting to it only ten times in the first fifty,
because it was not third, while he did go up to the final 'no-
food ' red twenty-seven times because it was third. It seems
certain, therefore, that raccoons are able to learn to dis-
tinguish one object or movement from two and two from
three, a species of counting not different from that which
anthropologists ascribe to primitive man." Certain details
of the behavior of the raccoons in these tests are significant.
"Each one, on seeing the first red, would drop down from
a position with both front paws on the front board to stand
on all fours in front of it and merely glance up at the sue-

Modification by Experience 255

ceeding reds. As soon as the white appeared, however,
the animal would lean up against the front board, claw
down the white and blue, but never the final red."

Now Cole thinks that the learning of this trick by the rac-
coons proved that "the animal retains an image of the cards
which just preceded red." The only alternate supposition
seems to him to be that they always reacted to the number
of the card in the series, which, if the series were irregularly
given, would not have been the same in successive trials. To
suggest one's own interpretation of animal behavior that one
has not seen, in the place of the experimenter's interpretation,
requires some temerity, but to the present writer the most
natural way of accounting for the raccoon's performances
would be the supposition that in the series white, blue, red,
for instance, at the end of which they were fed, the occurrence
of white threw them into a state of expectancy, of readiness
to climb up on the box ; this was heightened by the blue, and
finally "discharged" into action by the red. During this
process they may have had an anticipatory image of the
blue and of the red. But when the red came they did not
stop to call up memory images of the preceding colors,
and decline to act until they had assured themselves that
those were blue and white instead of red. Preparedness
to act was probably already secured by the actual occur-
rence of the white card at the beginning of the series. In
other words, while images may have been present, they
were images with a future, not a past reference. A human
being reacting to a series of stimuli in this fashion would
but rarely, in case his attention had wandered during the
giving of the first two stimuli, have to recall them as
memory images before reaction, but he might very likely
have anticipatory images of the stimuli to come while
waiting for them. For reasons that will be later men-

256 The Animal Mind

tioned, it seems probable that anticipation rather than ret-
rospection is the primitive function of ideas.

"It may still be objected," continues Cole, "that retaining
an image while you raise three, or even six, colors is hardly
retention at all, so short is the time. Of course the fact that
the animals made steady and rather uniform progress for six
days would show that the impression was not effaced in twenty -
four hours. Number one, however, was given a review of his
first three-color work after an interval of eighteen days. He
did not respond to the three blue cards at all, and made but
one mistake in twenty trials to the series white, orange, blue,
though he did start up at orange six times. The visual images
of the colors must therefore have been retained for eighteen
days with sufficient clearness to permit successful responses."
A certain confusion of thought is evident in this paragraph.
The visual images of course were not retained for eighteen
days; what was retained was, possibly, the capacity to have
the visual image of the third color in the series suggested by
the actual occurrence of the second. The length of time
this capacity persisted is quite irrelevant to the question as to
whether visual ideas were really present. An animal incapable
of having ideas might retain the effect of previous stimulation
for a long period, and an animal that had ideas might lose the
power of having a particular one suggested to it by a given
stimulation after a few hours. What we should really like to
know is whether the raccoons could think of color number
three if color number two were not actually shown them a
few seconds earlier; whether they could "think over" the
whole performance when the apparatus was not there; in
short, how free and unhampered by the control of present
sense stimulation their use of ideas can be. Cole concludes,
"We are . . . forced to believe that the raccoon retains visual
images." We are, at least, shown some reason for thinking

Modification by Experience 257

that memory ideas connected with immediately preceding
peripheral stimulation may occur in the raccoon's mind.

§ 89. Inhibition involving Discrimination of Simultaneous

Stimuli

Experiments of the second class, where the different stimuli
are simultaneously presented, have been made by Kinnaman
on monkeys, by Cole on raccoons, by Porter and Rouse on
birds, and by Yerkes on the dancing mouse. Kinnaman's
Macacus monkeys entirely failed to discriminate cards with
different figures on them when one card was placed on a box
with food and the other on an empty box (221). The English
sparrow and the cowbird, on the other hand, both learned to
do this. Monkeys and birds alike learned to discriminate
glasses covered with differently colored papers, and the posi-
tion or number of a vessel in a series (221, 345, 371). Cole's
raccoons learned to discriminate a black from a white glass,
and, with more difficulty, a red from a green one (82). The
monkeys were able to distinguish fairly well differences in
size, and in the form of the vessel. The birds were not
tested with size differences, and Porter's birds failed to
discriminate the vessel forms; Porter suggests that the
monkeys may have been helped by the fact that the
vessels Kinnaman used differed in size as well as in form.
Rouse found that his pigeons did tolerably well in learning
form differences (371). Our own experiments with the chub,
where the red and green forks were presented together and the
fish learned rather quickly to bite at the red rather than the
green even when both were empty (421), also illustrate this
method, as does the case of the chick stung by the bee, who,
on the basis of this experience, pecks at other insects but
V avoids bees (281). Similarly, Forel's bees and wasps, which
were trained to pick out pieces of paper of particular colors

258 The Animal Mind

and forms because they have been previously fed from them,
exhibit behavior which belongs to this class (130). The
method was used also by Yerkes in the experiments to test
brightness, color, and form discrimination in the dancing
mouse, described on pp. 145 and 197. Yerkes prefers to
establish discrimination by associating disagreeable rather
than agreeable experiences with one of the alternatives,
finding that the motive thus constituted works with greater
uniformity. Hence his mice were given slight electric shocks
when they made the "wrong" choice, instead of being fed
when they made the " right" one. He describes three dif-
ferent types of behavior on the part of the mice in making
the choices, which he calls choice by affirmation, choice by
negation, and choice by comparison. The first is illustrated
when the mouse enters the right compartment at once, the
second when it goes to the wrong compartment and turns
away from it, the third when it vacillates for some time be-
tween the two (469).

§ 90. Comparison of Methods

The methods just described have something in common
with, and something different from, the puzzle-box method.
In both cases a particular object, offering certain peculiarities
to the senses, and distinguished from other objects, ultimately
comes to occupy the focus of consciousness ; but in this method
of choice, the other objects are themselves connected, either
by instinct or acquired impulses, with a particular reaction
which has to be checked. No definite tendency has to be in-
hibited in the puzzle-box method; it is only necessary for
random movements to be dropped off. On the other hand,
these experiments where inhibition becomes dependent on
the presentation of a particular stimulus, differ from tests
like those on the hermit crabs or the water scorpion, in that the

Modification by Experience 259

latter require the inhibition of an entire instinct. The case
of the hermit crab, that of the chub presented with one pair
of forceps at a time, and that of the chub required to choose
between two differently colored forceps, may represent three
lessening degrees of the amount of inhibitory influence exerted
by experience. The hermit crab entirely abandoned its
ordinary method of reacting to light. The whole instinct
vanished. The chub, if it had refused as a result of expe-
rience to rise to the green fork when it was presented alone,
would have suspended an instinctive action so far as that
particular stimulus was concerned, and would have been
condemned to inactivity simply because no other stimulus
appealing to the nutritive instinct presented itself. The chub
offered a choice between two forks is not required to suspend
action at all, save for the brief interval necessary to discrimi-
nate between them. We should naturally expect that this
third state of affairs would be the easiest to bring about, and
such seems to be the case. It would probably be effected most
quickly when one of the stimuli was associated with positive
pain, instead of with mere absence of pleasure ; hence, very
likely, the extremely rapid learning of Morgan's chick.
Obviously the difference between the first and second of the
two cases just cited is at bottom one of degree, not of kind.
"A whole instinct" means, of course, a reaction to a whole
class of stimuli ; but the process by which light, for instance,
is discriminated from other forms of stimulation cannot be
ultimately different from the process by which one kind of
light stimulation is distinguished from another kind.

As we survey the processes of learning involved in all these
methods, the labyrinth method, the puzzle-box method, the
method of inhibition, and the method of inhibition with choice,
we find that they are all cases of the checking of movements
which do not involve positively pleasurable results. Their

260 The Animal Mind

psychological aspect in every case means, while the learning
is going on, the diminution of unpleasantness and the increase
of pleasantness ; apart from this, when the learning is com-
pleted, it differs in the different cases as regards the part played
by the consciousness of certain more or less accurately dis-
criminated objects. As the learning process proceeds, such
objects, as we have seen, come to stand in the focus of atten-
tion, so that to the cat put in the puzzle-box the string that
opens the door is instantly attended to ; the chick, half auto-
matically pecking at various objects on the ground, becomes
vividly conscious of the appearance of a bee among them ;
the monkey becomes aware of the difference in color between
two vessels otherwise quite similar.

§ 91. Visual Memory in Homing

Doubtless all the phenomena which animals exhibit in these
various experimental tests are displayed also in their ordinary
and normal life. There is one mode of behavior, however,
the existence of which has been established by careful ob-
servation of an animal in its proper environment, that does
not easily find an analogue among the facts we have been
describing. I refer to the exercise of visual memory by bees
and wasps. The case of the bee, indeed, finding its way from
repeated excursions back to a hive which remains in the same
place, may ultimately involve the formation of a habit of
movement like that displayed in experiments by the labyrinth
method. We have already noted on pp. 138, 139 some of the
evidence that bees are, at least in their earlier flights from the
hive, guided back by visual memory. Lubbock found that
bees from a hive near the seashore, when taken out on the
water and liberated, were unable to find their way home, al-
though the distance was less than their usual range of flight
on land; and he ascribes their failure to the lack of visual

Modification by Experience 261

landmarks to guide them (248). Bethe, who thinks bees are
guided home neither by vision nor by smell, but by an un-
known force to which they respond reflexly, also liberated
some bees at sea about 1700-2000 metres from their hive,
which was near the foot of Vesuvius and beside some very tall
and conspicuous trees. The bees failed to return, yet Bethe
thinks, if they were guided by vision, the mountain and the
trees should have aided them to do so (32). It may well be,
of course, that bees cannot see objects at such a distance.
Besides his observation that changing the appearance of a hive
did not disturb the bees in their homing flight, Bethe urges
against the visual memory hypothesis an observation on a
hive which had on one side of it a garden, and on the other
side a town, which he thinks the bees never visited, as food
was to be had in abundance in the garden. Yet when lib-
erated in the town they flew back to the hive with an accuracy
certainly not born of their acquaintance with the locality (30).
Von Buttel-Reepen, however, doubts whether the bees really
never visited the town. Bethe's most striking illustration of
his unknown force, however, is derived from his "box-ex-
periments." If a number of bees are carried in a box some
distance from the hive, on being liberated they fly straight
up in the air. Some of them will return to the hive, but if
the distance is great enough, many will drop back upon the
box. Now if the box has moved only a few centimeters
away during the flight of the bees, they will drop back to the
precise spot where it was, and take no notice of its new loca-
tion. If they were guided by vision, Bethe urges, they could
easily see the box (30, 32). This, says von Buttel-Reepen, is
arguing that their visual memory must be like ours if it exists
at all ; it may be a memory, not of the appearance of the box,
but of its locality. He himself, repeating Bethe's experiments,
observed the bees on dropping back after their upward flight,

262 The Animal Mind

hunting not at the place where the box had been, but at a
height which was about that of their home-hive entrance. He
thinks that an important feature of the bee's visual memor}
consists in a power of accurately estimating height above the
ground. If the entrance to the hive be raised or lowered 30
cm., all the returning bees will go to the old place, and
it will be hours and sometimes days before they find the
new one. Moreover, the same bees tend to return to the same
corner of the opening each time. When a row of hives had
been arranged, some with openings in front and others with
openings at the side, bees which had been driven home ir
haste by a storm would sometimes try to enter the wrong hive
but if their home hive opened on the side, they would attempl
to enter the foreign hive on the corresponding side (72).

It may be granted that there is much evidence in favoi
of the use of visual memory by bees, although the differences
which must exist between the visual perceptions gained by
the compound eye and those of our own experience neces-
sarily complicate the phenomena and make them hard tc
interpret. In the solitary wasps, although Fabre is inclinec
to assume a " special faculty " of homing, independent o:
visual memory, basing his assumption on experiments when
the wasps returned to their nests, from which they had beer
transported in a box to a distance of three kilometers (115
Series I) ; yet the evidence obtained by the Peckhams seem;
fairly conclusive in favor of memory for visual landmarks
The solitary wasps have been shown by the observations o
the Peckhams to depend upon sight for the return to th(
nest (322, 323), and the same conclusion is indicated for th<
social wasps by Enteman (112). The Peckhams' belief ii
the visual memory of solitary wasps rests first upon the fac
that the wasp, upon completing her nest, always spend
some time in circling about the locality, in and out amon]

Modification by Experience 263

the plants, as if she were making a careful study of the
region. On leaving the nest a second time she omits this
process and flies straight away. A similar "locality sur-
vey" is made by hive bees and by social wasps. Secondly,
the Peckhams argue that if the wasp does not remember
her nest by landmarks, it ought to make no difference to her
when the surroundings are altered in any way. They found,
however, that a wasp of one species could not discover her
nest when a leaf that covered it was broken off, but found
it again without trouble when the leaf was replaced. Another
wasp abandoned the nest she had made for herself with much
labor, because the Peckhams, to identify the spot themselves,
drew radiating lines from it in the dust. A third argument
against the existence of a special sense of direction is the fact
that wasps sometimes are unable to find their nests. In one
case the Peckhams dug up the nest of a wasp and she made
another five inches away. After an absence of three hours the
wasp returned, and seemed to be puzzled as to whether the old
spot or the new one were the place of her nest. " At first she
alighted upon the first site and scratched away a little earth,
and then explored several other places, working about for
twelve minutes, when she at last found the right spot." Simi-
larly, when a wasp that was carrying her prey left it for a few
moments to go to the nest, as many of them do, apparently to
see that all is right there, if any of the surrounding objects
were altered she often had great difficulty in finding the prey
again. On one occasion a wasp of another species dug its
nest in the midst of a group of nests of the Bembex wasp.
These latter are usually dug in a wide bare space of earth
which has no vegetable growth to serve as a landmark. When
the intruder had finished her nest, it looked just like the Bem-
bex holes. She went away, secured a spider, and when she
returned she could not find her nest. " She flew, she ran, she

264 The Animal Mind

scurried here and there, but she had utterly lost track of it.
She approached it several times, but there are no landmarks
on the B. field. After five minutes our wasp flew back to look
at her spider," which she had dropped about three feet away,
" and then returned to her search. She now began to run into
the B. holes, but soon came out again, even when not chased
out by the proprietor. Suddenly it seemed to strike her that
this was going to be a prolonged affair, and that her treasure
was exposed to danger, and hurrying back she dragged it
into the grass at the edge of the field, where it was hidden.
Again she resumed the hunt, flying wildly now all over the
field, running into wrong holes and even kicking out earth
as though she thought of appropriating them, but soon
passing on. Once more she became anxious about the
spider, and, carrying it up on to a plant, suspended it there.
Now she seemed determined to take possession of every
hole that she went into, digging quite persistently in each,
but then giving it up. One in particular that was close by
the spider seemed to attract her, and she worked at it so
long that we thought she had adopted it, for it seemed to
be unoccupied. At last, however, she made up her mind
that all further search was hopeless, and that she had
better begin de novo; and forty minutes from the time that
we saw her first she started a new nest close to the spider, as
though she would run no more risks" (322). An occurrence
of this kind certainly lends color to the ' recognition of land-
marks ' theory. On the other hand, the Bembex wasps them-
selves find their nests with unerring accuracy, though there
is no landmark in the field. Fabre noted that Bembex wasps
could not be led astray by any modification of either the look
or the smell of their nests, and thought a peculiar form of space
memory, unparalleled in our own experience, must be in-
volved in the nest-finding of this species (115, Series I). A

Modification by Experience 265

similar kind of memory for pure locality, if one may so term
it, is maintained by Goldsmith to exist in a fish, Gobius, which
lives in a shell. If its shell habitation is moved during its
absence, the fish seeks it in the place where it previously was
(146). Bouvier, repeating Fabre's experiments on Bembex,
obtained a different result. When a stone, for example,
that had been at the mouth of a Bembex nest was moved
a distance of 2 dm., the wasp, returning, went to the
stone. Bouvier accordingly maintains the visual landmark
hypothesis (68). Ferton holds the same view with regard
to a species of wasp that makes its nest in shells. If during
successive absences on the wasp's part the shell is moved from
position A to position B, and later from B to C and from C to
D, the wasp, returning, goes in turn to each of the positions
that the shell has occupied. "In time, she omits to go to A,
then to B. Little by little, the image of the previous locations
of her nest is effaced in the insect's memory." When she
has found it, after each displacement, she makes a new
"locality survey," before starting off again (116).

The performances of carrier pigeons in finding their way
home have been the subject of a considerable literature,
and many theories which it would take a disproportionate
amount of time to discuss.1 While the facts are not easy to
explain, careful observations on young pigeons indicate that
their powers are acquired, not innate, and that they are in-
fluenced by visual landmarks (375). What guides the flight
of migrating birds over vast stretches of water, or the young
migrants in those species where young and old fly by different
routes, remains a mystery.

If we take the case of the solitary wasp as typical of guid-
ance by visual landmarks, it is to be noted that no gradual

1 For an account of them see Claparede, " La faculte d'orientation loin-
taine (Sens de direction, sens de retour) " (76).

266 The Animal Mind

elimination of useless movements occurs, as in many species
the nest is revisited but once. Nor is there any opportunity
for the elimination of errors through their unpleasant conse-
quences. Doubtless the wasp does not choose the best possi-
ble route for her return to the nest, and doubtless she would
improve upon it if she made repeated journeys ; but at least
she performs at the first trial definite, not random, responses
to stimuli that are new to her ; responses that are not wholly
due to inherited nervous mechanism. We have here a kind
of behavior that is not in any sense "trial and error."

Without undertaking the difficult task of explaining it
fully, one or two aspects of this form of profiting by experience
may be noted. The wasp, when she has finished digging her
nest, makes a "locality survey" ; that is, she circles about the
neighborhood in flight for a few minutes. This conduct on
her part is doubtless instinctive. During the process she
receives a number of specific visual stimuli. On her flight
in search of prey, several visual landmarks probably impress
her. When she has secured her spider or caterpillar, she
begins the return flight. We cannot attempt to explain all
the mysteries connected with this, but at least we can say that
the flight back to the nest, and the alighting and burying the
prey, are instinctive actions which are carried out only under
the influences of the same visual stimuli that the animal re-
ceived on its locality survey and its outward flight. It is
essential to their performance that the wasp's nervous system
should receive stimulation like that which it has received a
short time previously. The case differs from the formation
of a habit, such as we saw illustrated in the labyrinth experi-
ments ; for while in a habit the action becomes dependent on
a certain kind of stimulus repeatedly received, here it is not
the frequent previous experience of the stimulus that renders
it and it alone effective, — for the features of the locality

Modification by Experience 267

may have been quite new to the wasp when she dug her nest
there, — but its recent previous experience.

The great thing to be desired with regard to the effect of
individual experience upon behavior is that it shall be rapid.
One might at first thought say also, "that it shall be per-
manent." But it is probable that permanence of impression,
though valuable, is less so than speed of modification; for
too great permanence of one impression would interfere with
the formation of new ones. We have been often told of the
value that attaches to the capacity for forgetting. Judged
by /the standard of rapidity, the guidance of action by a
stimulus that has been experienced only once before, though
recently, is a form of modification through experience dis-
tinctly superior to the guidance of action by repeated stimu-
lation. It has been noted already that if the effect of a stimu-
lus is very painful, the stimulus does not need to be repeated ;
but obviously it is better for an animal to modify its behavior
rapidly without undergoing pain, which must tell upon its
vital energies.

Furthermore, we may notice that while recency of experi-
ence is more valuable than frequency of experience as regards
the rapidity with which behavior is modified, nothing can
take the place of frequency where permanence is concerned.
It is not to be desired that the wasp should remain perma-
nently subject to the influence of these particular landmarks.
On the contrary, their influence must be effaced, in order that
she may dig and stock other nests in other localities. The
lasting character of the effect produced on the animals in the
labyrinth and box experiments has, on the other hand, been
proved by nearly all experimenters. One of Small's rats
opened a puzzle box with speed and accuracy after an inter-
val of forty days, during which time she had had no tests
whatever. The birds tested by Porter remembered the maze

268 The Animal Mind

well after an interval of thirty days, and one of Thorndike's
monkeys opened a puzzle box at once, eight months after his
last previous experience with it.

Most rapid of all the ways in which conduct may be modi-
fied by experience is the method of the memory idea. The
wasp, finding her way back to her nest, is guided by the actual
recurrence of certain stimuli which she has experienced on her
outward flight. As she has needed no frequent experience
of those stimuli to make them effective, her behavior is far
superior to that of the frog in the labyrinth. But if she pos-
sessed the power to direct her course by the memory image
of a stimulus rather than by its actual recurrence, still more
time would be saved. Suppose, for instance, that on the
devious flight in search of prey three landmarks, A, B, and
C, had impressed themselves upon the wasp's consciousness.
Suppose that the distance in a straight line from A to C is
less than the distance via B. Now if, on the return trip,
the wasp can have in mind not only C, which is actually
before her, but the memory of B and A and of their relative
position, she can greatly shorten her course by flying straight
from C to A. Not only is it unnecessary for an animal capable
of memory images to wait for the repetition of stimuli many
times, before its behavior is modified; it does not need to
wait for the complete repetition of a series of stimuli even once,
for the earlier members of the series being given, the later
ones are suggested in idea.

Yet here, too, frequency of repetition is the condition of
permanence. I think it may be said that if an animal is
capable of having a memory image at all, a single recent ex-
perience of two stimuli, fully attended to, is enough to make
one of them call up the image of the other. But if the asso-
ciation is to be permanent, nothing but frequent repetition
will serve. It is customary to say that a single very vivid ex-

Modification by Experience 269

perience may suffice as well as a repeated experience to pro-
duce long retention. But the fact is that a single vivid ex-
perience amounts to a repeated experience, because it is
usually recalled so often in idea. If we could imagine a
person's having a very forcible impression, which he did not
once recall for a long period of years, it is doubtful whether
he could recall it at the end of that time with anything like
the success which attends his recollection of the familiar sur-
roundings of his childhood.

CHAPTER XII
THE MEMORY IDEA

§ 92. Evidence for and against Ideas in Animals

IN the last chapter we have seen that the behavior of the
lower forms of animal life, at least, can be fully explained
without supposing that the animals concerned ever consciously
recall the effects of a previously experienced stimulus in the
entire absence of the stimulus itself. We must admit that it
is not easy to prove the possession by any animal of memory
in the sense of having ideas of absent objects, rather than in the
sense of behaving differently to present objects because of past
experience with them. The dog shows clearly that he remem-
bers his master in the latter sense by displaying joy at the
sight of him. Can we be sure that he has remembered him
in the former sense during his absence ; that is, that he has
had a memory image of him? Certain pieces of negative
evidence have been noted. Where an animal learns to work
a mechanism by gradually dropping off unnecessary move-
ments, it looks as if its conduct were not guided by an idea of
the right movements, for the association of ideas as we know
it is so rapid a process that a single experience of two stimuli
together is enough to enable one to revive the other in the form
of a memory idea, provided that the experience was recent.
When an animal hag learned to run through a complicated
labyrinth almost without error, but still persists in taking the
wrong turning at the outset, we are surely justified in saying
that if it has ideas, it does not use them as a human being
would, for some kind of idea of the right way to start the laby-

270

The Memory Idea 271

rinth course would certainly be formed in a human mind after
a very few experiences. Thorndike's attempts to make cats
and dogs learn by inferential imitation, and by putting them
through the movements required, while they do not show
absence of ideas in the animals' minds, indicate that ideas
were not suggested to his subjects under circumstances which
would have suggested them to human beings. Cole's op-
posite results, however, weaken Thorndike's conclusions.
Further, the way in which instinctive actions are often per-
formed by animals indicates that ideas are not present as they
would be to a human being's consciousness. Human beings
do some things from instinct, but the doing of them may be
accompanied by ideas ; a mother's care for her child involves
ideas of the child's happiness or suffering, and of its future.
;Enteman's account of the worker wasp which, lacking other
'food to present to a larva, bit off a portion of one end of the
larva's body and offered it to the other end to be eaten, jsug-
gests a peculiar limitation of ideas in the wasp's mi*ra, at
least while this particular function was being performed (112).
rrhe cow, which had lamenteol at being deprived of her calf,
and on having the stuffed skin of her offspring given to her,
licked it with maternal devotion until the hay stuffing pro-
truded, when she calmly devoured the hay (^79, p. 334), had
perhaps experienced some dim ideas connected with her loss,
but certainly her consciousness was more absorbed by the
effects of present stimulation and less occupied with ideas
than a human mother's would have been.

On the other hand, certain features of animal behavior are
held by most people to be indications that the creatures thus
acting have ideas of absent objects. Dogs and cats are sup-
posed to dream because they snarl and twitch their muscles
in sleep ; but, as Thorndike has pointed out, such movements
may be purely reflex and unaccompanied by any conscious-

272 The Animal Mind

ness whatever. A dog shows depression during his master's
absence, but his state of mind may be merely vague discom-
fort at the lack of an accustomed set of stimuli, not an idea
of what he wants. A cat, indeed, once observed by the
writer, did behave as a human being would do to whom an
idea had occurred, when, on coming into the house for the
first time after she had moved her kittens from an upper
story to the ground floor, she started upstairs to the old nest,
stopped half way up, turned and ran down to the new one.
But errors of interpretation are possible at every turn of such
observations. An attempt was made by Thorndike to test
experimentally the presence of ideas in the minds of the cats
he was studying by the puzzle-box method. He sat near the
cage where the cats were kept, and having made sure that the
cats were looking at him, he would clap his hands and say,
" I must feed those cats." Aften ten seconds he would take
a piece of fish, go to the cage and hold it through the wire
netting; the cat, of course, would climb up and get the food.
After from thirty to sixty trials the cat learned to climb up
when it heard him clap his hands and speak, without waiting
for him to get the fish. But it is not certain that the hand-
clapping came thus to suggest to the cat an idea of the ex-
perimenter's taking the food and coming to the cage ; rather,
in the course of so many repetitions, the clapping of the hands
may have become a direct stimulus to the act of climbing up,
although Thorndike thinks that the ten seconds7 interval ren-
dered this improbable (393). Cole, as we have seen, has
observed behavior in the raccoon that might well be regarded
as involving ideas (82).

Despite the difficulty of proving that animals have memory
ideas, it is not likely that any such gulf separates the human
mind from that of the higher animals as would be involved
in the absence from the latter of all images of past experiences.

The Memory Idea 273

That ideas occur in far less profusion and with far less free-
dom of play in the animal mind that possesses them at all
than in the human mind ; that even the highest animal below
man lives far more completely absorbed in present stimula-
tions than does the average man, seems also practically cer-
tain. In the lack of more definite knowledge on the subject,
we may discuss a few related questions that suggest them-
selves with regard to, first, the primitive function of ideas;
secondly, the relation of ideas to qualitative differences in
sensation; and thirdly, the nature and possible origin of
"movement ideas."

§ 93. The Primitive Function of Ideas

(i) What would be the most obvious and fundamental
use of ideas to an animal ? In our own experience, ideas of
absent objects have, among the various functions they sub-
serve, two that are rather definitely contrasted, which may be
termed the backward and the forward reference of ideas.
On the one hand, we recall past experiences purely as such;
we indulge in "the pleasures of memory," letting the at-
tention wander over trains of ideas recognized as belonging
to the past. On the other hand, we form ideas of experiences
we expect to have in the future, ideas which are derived, it
is true, from what has happened in the past, but which in-
volve a very different attitude on our part from that required
by mere retrospection, — the attitude, namely, of anticipation,
of preparing to act appropriately to the situation present in
idea. Now if we ask which of these two functions of the idea
is practically the more important, we cannot hesitate to say
that the second is. To recall the past, except for the pur-
pose of anticipating the future, is an intellectual luxury. As
Bentley remarks, "The primary use of the image, we surmise,
was to carry the organism beyond j;he limits of the immediate

274 The Animal Mind

environment, and to assist it in foreseeing and providing
for 'the future/ Its function seems, then, to have been a
prophetic one; it was a means to what we may term remote
adaptation. . . . The past, being less important than the
future, must have been known as such later " (23).

It is in making possible the anticipation of a coming
stimulus, thus preparing the way for reaction, that the mem-
i ory image is most fundamentally useful. Can we form any
conception of the conditions under which it would most
naturally make its appearance, in its simplest, most rudi-
mentary form ? Let us suppose that an animal's behavior in
a certain case requires a definite series of stimulations for its
guidance. The acts concerned have been performed several
times, so that when the reaction to number one in the series
has occurred, the motor apparatus concerned in the reaction
to number two is slightly innervated, although the actual
giving of the second stimulus is necessary to produce the
movement. Now if the stimuli follow each other in quick
succession, this tendency for one movement to help, as it
were, in starting the next, would result finally in the perform-
ance of the whole set of reactions " automatically," with
lapsing consciousness. But suppose the sequence is slow,
or that one stimulus in the series is delayed. It is important,
perhaps, that the series of movements shall not go on until
the delayed stimulus acts. During this time of waiting, it
may weH be that the nervous energy prepared for the next
reaction, besides innervating to some degree the motor
mechanism that will be needed, overflows into the sensory
centres which the anticipated stimulus is to stir to full activity.
The result for consciousness is an idea, an image, though
perhaps rather vague, of the stimulus waited for. Why, it
may be asked, have we made the process by which motor
centres become " associated," so that habits are formed, and

The Memory Idea 275

the innervation of one centre in a series involved in successive
reactions produces innervation of the next, fundamental,
and suggested that the process of " association " whereby
ideas are brought into consciousness is secondary and de-
rived? Simply because we find the formation of motor
habits far down in the animal kingdom, long before there is
any evidence of the existence of ideas. It is interesting to
note that Judd has recently advanced the theory that the phys-
iological process underlying the ''association of ideas" may
involve the motor pathways (217). In any case, we may be
pretty sure a priori that the primary function of the memory
idea or image is to anticipate and prepare the way for re-
action to a coming stimulus.

§ 94. The Significance of Stimuli from a Distance

(2) Another question that arises in connection with the
origin of the memory idea bears on the possible significance
of that increase in ability to react to stimuli from a distance
which we find characterizing the higher animals. An
important difference must exist between the stimuli from
objects directly in contact with an organism's body, such as
give rise to touch, temperature, taste, and pain sensations in
our own experience, and those which proceed from objects at
a distance, such as forms of vibratory energy and odors. This
difference consists in the fact that the former have a more
direct and instant effect upon the organism's welfare, and in
consequence demand more rapid reaction, than the latter.
A stimulus in immediate contact with an animal's body may
have a harmful or a beneficial influence at the moment of its
impact ; it may be food to be seized or an enemy to be es-
caped, and the seizing or escaping must be done on the instant.
On the other hand, if an animal possesses the power, belong-
ing in increasing degree to the higher animals, of reacting to

276 The Animal Mind

an influence proceeding from an object still at a distance, it
becomes safe for it to delay the reaction after the stimulus is
given. The danger is not so imminent, the food is not yet
within reach ; the full motor response to stimulation may be
suspended for a short interval without imperilling the life
interests of the animal., Now what is the import of this
delay between stimulus and reaction for the memory idea?
It seems probable that the reproduction of a sensory image
by central excitation demands that its original stimulus shall
have left upon the nervous system a relatively permanent
effect. We may distinguish three grades of animal behavior
in response to stimulation. First, there is the condition where,
so far as we can see, the animal does not learn by jndividual
experience. A stimulus entering such an organism, and
sending its energy out again through whatever motor paths
are available, leaves so little effect upon the substance through
which it passes that the animal behaves toward a second
stimulus of the same kind precisely as it did toward the first.
In the next place, we have the grade where the animal learnsj
by experience, without having the power to recall an image of
its experience. The chick stung by a bee very likely cannot
have later the image of a bee suggested to him, but he can and
does refrain from picking up the next bee he sees. Here the
stimulus has modified the behavior of the animal, and has
left a relatively permanent effect of some sort upon the ner-
vous substance; but renewed stimulation from without is
necessary before this modification makes itself apparent.
Finally, when we have the possibility of an image, purely
centrally excited, and not leading immediately to movement ;
when a process similar to the original may be set up, not by
an influx of energy from without, but by the weaker nervous
current coming from some other central sensory region, it is
evident that the nervous substance must have been far more

The Memory Idea 277

profoundly affected by the original stimulus than it was in
either of the before-mentioned cases. What characteristics
of a stimulus would determine how strongly and deeply it
would affect the nervous substance through which its energy
passed ? Its intensity, the quantity of that energy, of course ;
but still more emphatically the length of time the energy
remained in the centres concerned, without being drained
off into motor paths and transformed into bodily movement.
Not merely the strength, but the duration of the current de-
termines how deep a path it shall dig out for itself.

Now, as we have seen, stimuli that are in a position to help
or harm an organism at the instant of their contact with its
body are stimuli demanding immediate motor reaction. In
such cases, the energy of the stimulus is deflected at once
into the appropriate motor path ; it is not delayed long enough
in the sensory regions to produce any. permanent change there.
But where the animal possesses a capacity to be affected by
light and sound, which cannot help or harm at the moment
of their action upon its body, then reaction may be postponed ;
then the current of energy sent by the stimulus into the ner-
vous substance is not at once drained off, but may linger
sufficiently long to produce whatever alteration, whatever
impress upon sensory centres, is needful to insure their
subsequent functioning as the basis of a memory image.
The delay between stimulus and reaction, made possible by
sensitiveness of the organism to stimuli only indirectly affect-
ing its welfare, may then supply time for the nervous modi-
fication to be produced that is later to underlie the memory
image, as the delay occupied in waiting for an expected
stimulus offers a chance to bring this modification into play
and call the image to consciousness. The same principle
also helps to explain why the human mind gets its clearest
and most controllable memory images from the senses whose

278 The Animal Mind

stimuli do not indicate direct contact of a beneficial or harm-
ful object with the body; while the closer and more direct
the stimulation, as for instance in touch and organic sensa-
tions, the obscurer the image.1

Many of the foregoing sentences are taken from an article
by the writer which appeared in 1904 (420). A very interest-
ing discussion of the significance from the neurological stand-
point of reaction to stimulation from a distance is to be found
in Sherrington's recently published book on "The Integrative
Action of the Nervous System" (382, pp. 324 ff.). Sher-
rington proposes the term "distance receptors" for those
receptive organs "which react to objects at a distance," and
declares that "the distance receptors contribute most\to the
uprearing of the cerebrum." The -most important signifi-
cance of the power to act in response to distant objects
Sherrington finds to be that it allows an interval for pre-
paratory adjustment, "for preparatory reactive steps which
can go far to influence the success of attempt either to obtain
actual contact or to avoid actual contact with the objec^."
That these preparatory steps may also involve the germ of
the memory image is clearly suggested by Sherrington.
"We may suppose," he says, "that in the time run through by
a course of action focussed upon a final consummatory event,
opportunity is given for instinct, with its germ of memory,
however rudimentary, and its germ of anticipation, however
slight, to evolve under selection that mental extension of the
present backward into the past and forward into the future
which in the highest animals forms the prerogative of more
developed mind. Nothing, it would seem, could better

1 An exception may be taken to this statement so far as smells are con-
cerned. Some people seem to have difficulty in getting memory images of
odors. For the writer, such images are among the most vivid and most
readily controlled in her experience.

The Memory Idea 279

insure the course of action taken in that interval being the
right one than memory and anticipatory forecast." The
present writer's views regarding the significance of the delay
made possible by reaction through "distance receptors,"
while independently formed, find thus most valuable support.

§ 95. Ideas of Movement

(3) A very striking difference between man and most of
the lower animals lies in the immensely greater number of
different movements, each adapted to some feature of the
environment, that man is able to perform. When we think
of the enormous variety of muscular adjustments of which the
human race as a whole is capable, and compare it with the
limited power of an earthworm to react upon its surround-
ings, the small extent of its motor repertoire, the gulf that
separates them is highly impressive. And the conscious
experience of an animal must be profoundly modified by the
number and variety of the motor coordinations it has under its
control ; not only because sensory discriminations in general
involve differentiation of motor reaction, but because that
breaking up of the crude mass of sense impressions into
smaller masses which we call the perception of external
objects depends so largely on what the animal is able to do
with objects. Think, for example, of a creature able to
move in response to its environment, but not able to alter
the relative position of different features of that environment ;
not able, in plain words, to pick up a single object and move
it about. " Objects" for such an animal simply would not
exist. There would be a vague background of sensation
qualities, but no sharply defined groups of such qualities.
An object to the human mind is essentially a bit of expe-
rience with which things can be done ; which can be moved
about independently of its surroundings, "handled," used

280 The Animal Mind

for one purpose or another. The perception of objects
as distinct entities increases with the power of making defi-
nitely coordinated and adjusted motor responses to them.
That one important condition to the production of such re-
sponses lies in the possession of a grasping organ, a highly
movable member that can seize objects firmly and thus move
them about, is self-evident. The elephant and the monkey,
which. possess such organs, must have far more definite per-
ceptions of objects, as individual entities to be separated
from their backgrounds and used, than any other lower
animals. But to the acquisition of the most complicated and
perfect systems of motor reactions another factor contributes.
This factor is the movement idea. A movement idea is
the revival, through central excitation, of the sensations,
visual, tactile, kinaesthetic, originally produced by the per-
formance of the movement itself. And when such an idea
is attended to, when, in popular language, we think hard
enough of how the movement would "feel" and look if it
were performed, then, so close is the connection between
sensory and motor processes, the movement is instituted
afresh. The movement is willed by attending to the idea of
it. This is the familiar doctrine expounded by James in
Chapter XXVI of his " Psychology " (189). Recently it has
been pointed out that the "willing" of a movement by no
means always or even usually involves preliminary attention
to a movement idea (e.g. 445). This is undoubtedly true.
Nearly or quite all the movements executed by a man in the
ordinary course of a day are movements that he has made
many, many times before. And movements that have been
repeatedly made come to be made in response to stimuli
that through association have been substituted for the origi-
nal processes inducing movement. When I see my handker-
chief on the floor I do not need to think beforehand of what

The Memory Idea 281

stooping and picking it up will feel like; the sight of that
object in that position sets off the appropriate movement
directly. When the soldier hears the command "Halt!"
he does not first think of stopping; the sound stops him.
But the important consideration is not what conditions de-
termine old movements, movements that have been many
times performed by the individual. The superiority of an
animal consists largely in its power to learn new movements
rapidly. And whenever we ourselves learn really new
movements, we find that an essential part of the process is
the presence of a movement idea in the focus of attention.

Such processes as those involved in learning the type-
writer, in learning to play golf, in acquiring any new set of
muscular adjustments, certainly involve calling up in the
form of ideas the sensory experiences obtained from actually
moving. We have to " think" where the fingers must go,
how the arms must swing ; the trainer who instructs us puts
forth every effort to suggest to us the proper look and feel of
the movements themselves. He must, of course, in so doing
recall to us the ideas of the movements already familiar to
us which are most nearly similar to the required new ones.
Where nothing similar can be found, the training is likely to
fail. The difficulty experienced by an average human being
in learning to move his ears consists essentially in the fact that,
never having done anything remotely similar to moving his
ears, he has no movement idea to call up. He cannot move
them because he cannot "imagine" how it would feel to
move them.

Thus the power to attend to a memory idea of the sensa-
tions formerly involved in the performance of a movement is
a very important factor in the rapid acquisition of new move-
ments. And one reason why the lower animals in general
learn new movements but slowly may be connected with a

282 The Animal Mind

lack of development of the power to attend to movement
ideas. For the slight development of this power in most of
the lower animals there is at least one obvious reason. The
life of an animal in natural conditions demands that its
attention shall be constantly directed outward. It is en-
gaged in continual watchfulness for food and enemies. The
stimuli which come to it from external objects demand all
its mental energies ; the successful animal is the wide-awake,
alert animal. How can it, with every available avenue of
sense wide open to the external world, with every unit of
mental capital invested in watching and listening and smell-
ing, spare any mental energy to attend to the sensations from
its own movements ? It sees the prey, it makes an elaborate
series of movements in response to the sight ; but if it were to
attend for one instant to the sensations from the movements
themselves, there would be a relaxation of its watchfulness
of external things that might mean the escape of the prey.
But unless it attends to the sensations resulting from move-
ment, it will not reproduce them in idea. That which is
unattended to when originally experienced is ordinarily not
recalled.

It would thus seem as though one condition which must be
fulfilled if movement ideas are to play an important part in a
creature's experience were that the animal should, for a time
at least, be set free from the pressure of the practical hand-to-
hand struggle for the means of existence, and thus enabled
in safety to attend to its own movement sensations. Animal
play, at first thought, offers an instance of such liberation
from practical necessities. But as Groos has shown, animal
play is not so unpractical as it looks (154). It is simply the
exercise of the same instincts upon which in other circum-
stances the animal's welfare depends. The attention is
absorbed in external objects quite as much in play as in

The Memory Idea 283

the actual chase or warfare. The kitten watches the string,
for which she has no practical use, as intently as she watches
the bird for which she does have a practical use; the dogs
rolling over and over each other are nearly as absorbed in each
other's movements as if they were in deadly combat.

That relief from practical necessity which will serve the
purpose we are considering is to be found not in play, but
in infancy. If a creature spends the period during which
its nervous system is undergoing most rapid development
in a state of complete shelter and protection from external
danger, with all its vital needs supplied, then the nervous
energy which under other conditions would be expended in
the processes underlying attention to external stimuli is
free to be so devoted that attention will be directed toward
the creature's inner experiences. The human baby, while
he may be interested in lights and sounds, in external impres-
sions, does not need to be alert and watchful lest he miss his
dinner or be dined on himself; his attention is free to be
expended on his own movement experiences as well as on
anything else. That young children do go through a stage
of intense interest in the sensations resulting from their own
movements is a fact made clear from many observations.
The curious period of "self-imitation" in the child when it
repeats for an indefinite period the same movement or sound,,
over and over again (8), is very likely a period of vivid
attention to movement sensations ; and just as the movement
will take place if we attend exclusively to the idea of it, so
here the child's developed attention to the sensations result-
ing from the movement reinstates the movement itself.

That the prolonged period of human infancy is of advan-
tage to the intellectual life of man because it means plasticity,
the absence of fixed instincts that would take the place of
acquisition by individual experience, was first pointed out by

284 The Animal Mind

Fiske (126). But quite as important is the fact that in pro-
longed infancy we have the opportunity for acquiring the
habit of that attention to our own movements which is the
prerequisite for the movement idea. There are, as we have
seen, various ways of learning by experience — slow ways
that do not involve ideas, and the rapid way that does. The
great advantage of man over most of the lower animals is not
so much in the fact as in the method of his learning. One
of the most vital meanings of the long period of helplessness
and dependence constituting human infancy lies in the fact
that by relieving from the necessity of attending exclusively
to external objects, it renders possible attention to the sensa-
tions resulting from movement; and thus, by supplying an
essential condition for the revival of such sensations in idea,
it opens the way for the control of movement through the
movement idea.

CHAPTER XIII
SOME ASPECTS OF ATTENTION

THE student absorbed in reading "does not hear" an ap-
proaching footstep. That is, a stimulus which would under
other circumstances produce an effect loses a great part of
its influence because of the fact that another stimulus is al-
ready upon the field. This other stimulus need not be more
intense, that is, need not involve more physical energy,
than the one which is gnored. It does not win the victory
by a mere swamping of its rival through its superior quantity.
A man may walk along city streets, his eyes and ears bom-
barded with brilliant lights and loud sounds, and yet the
centre of his consciousness may be a train of ideas, repre-
senting in their physical accompaniment in his cortex a
quantity of energy insignificant compared with that of the
external stimuli pouring in upon him. Psychologists com-
monly express this fact by saying that while the strength of
a stimulus conditions the intensity of the mental process ac-
companying it, the clearness of that process depends upon
attention.

§ 96. The Interference of Stimuli

Attention, then, is the name given to a device, whatever
its nature, whereby one stimulus has its effectiveness in-
creased over that of another whose physical energy
may be greater. What happens in the simpler forms of
animal life when two stimuli, requiring different reactions,
operate simultaneously? We may quote from Jennings the

286 The Animal Mind

facts about Paramecium. "If the animal is at rest against
a mass of vegetable matter or a bit of paper, . . . and it
is then struck with the tip of a glass rod, we find that at first
it may not react to the latter stimulus at all." "A strong
blow on the anterior end causes the animal to leave the solid
and give the typical avoiding reaction." "If specimens
showing the contact reaction are heated, it is found that they
do not react to the heat until a higher temperature is reached
than that necessary to cause a definite reaction in free-swim-
ming specimens." "On the other hand, both heat and cold
interfere with the contact reaction. Paramecia much above
or much below the usual temperature do not settle against
solids with which they come in contact, but respond in-
stead by a pronounced avoiding reaction." "Specimens in
contact with a solid react less readily to chemicals than do free
specimens. . . . On the other hand, immersion in strong
chemicals prevents the positive contact reaction." "The
contact reaction may completely prevent the reaction to
gravity," and to water currents. It also modifies the reaction
to the electric current. While a part of the influence exerted
by the contact reaction on other responses may be purely
physical, due to the fact that an actual secretion of mucus
may occur whereby the animal "sticks fast" to the solid,
yet this alone does not explain the facts, for the cilia that are
not attached do not behave normally. The reaction to gravity
regularly yields whenever opposed to the action of any other
stimulus (211, pp. 92 ff.).

Sometimes the action of one form of stimulation merely
affects the form of the response to another, as in the case where
abnormal temperature causes the avoiding instead of the
positive reaction to be given to solids. In other cases, re-
action to one of the stimuli is suppressed or weakened. The
facts suggest that the influential stimulus is either the ont

Some Aspects of Attention 287

that is on the field first (the contact reaction may prevent re-
sponse to temperature, or abnormal temperature may modify
the contact reaction), or the one that is the more important
(gravity yields always to other stimuli).

In some higher animals the effects of interference of stimuli
have been noted. The earthworm will not respond to light
if feeding (91) or mating (179). In the turbellarian Con-
voluta roscoffensis light is victorious over heat in determining
reaction. The animals in their positively phototropic phase
will remain in the heated light end of a vessel until they perish.
Light and gravity are more nearly balanced in their effects.
Convoluta is negatively geotropic, yet if the brightest region
is below the surface, the animals will go there. But if this
region is only a little brighter than the surface, they will
stay at the surface, gravity dominating (140). The sea
urchin shows in its behavior a somewhat similar relation
between mechanical and chemical stimulation. If weak
acid is dropped into the water containing specimens of Arba-
cia, their spines begin to interlace. A slight shaking will
restore them to the normal position, but if more acid be
added, no mechanical stimulation will overcome the effect
of the chemical (409). Various facts concerning the inter-
relations- of gravity and light as stimuli have been noted in
Chapter IX. A very interesting case of the suppression of
one reaction by another is reported by Holmes in his obser-
vations on the water insect Ranatra. The positive response
of this insect to light, very precise and striking, may be
wholly suspended when the animal is feeding, when a num-
ber of individuals are collected, when the insect stops to
clean itself, or even "by the sudden appearance of a large
object in the field of vision," behavior which is strongly
suggestive of the "distraction of attention" in a human
being (186). Roubaud, in a study of the behavior of some

288 The Animal Mind

species of flies that live on the seashore, feeding on dead
fish and the like, says that they will abandon the " head-on"
position which they regularly assume toward the wind, if
attracted by the odor of food (370).

Wherever we find that one class of stimuli regularly
yields to another if the two act together, it is safe to assume
that the prepotent stimulus is more important to the organ-
ism's welfare than the vanquished one. And while we can-
not without more ado call such cases of the interference of
stimuli as are found in very simple animals cases of attention,
and ascribe to their psychic accompaniments all the character-
istics of attention as a feature of our own experience, yet we
may assert that they have in common with attention the sig-
nificance of being a device to secure reaction to the most vitally
important of several stimuli acting at once upon the organism.

§ 97. Methods of securing Prepotency of vitally Important

Stimuli

An inanimate object acted upon by several forces at once
is determined in its motion by their relative intensity. Con-
ceivably, an extremely simple form of animal life, when
subjected to two stimulations acting together, would also
respond in a way answering precisely to the relative strength
of the two. It is easy to see what would be the disadvantage
of such a state of affairs for the animal. The weaker of
the two stimuli might be of far greater significance for organic
welfare than the stronger. For example, it would often be im-
portant that an animal should be able to respond to a very
faint food stimulus rather than to any of the stronger forces
acting upon it. Evidently a prime need of animal life is
some arrangement whereby weak but important stimuli
shall be given the preference in determining reaction over
stronger but less vitally necessary ones. Sense organs are

Some Aspects of Attention 289

one such device. The comparatively slight amount of
chemical energy coming from a bit of food may have its
effectiveness for the nervous system greatly increased through
its reception by a structure adapted to use the whole of it
to advantage. Light stimulation involves a quantity of
energy that is insignificant in comparison with tljgLjgrosser
forces acting on an organism; yet falling on the retmaT^the-
energy is economized and magnified through the stored--up
chemical forces it sets free. Thus a weak stimulus may by
a sense organ be made powerful to determine reaction.
Another arrangement to the same effect is the peculiarity
of the nervous system whereby, through an arrangement
akin to the summation of faint stimuli, a moving stimulus,
one acting successively upon neighbor ng points of a sensitive
surface, produces an effect disproportionate to its intensity.
A moving stimulus is a vitally important stimulus ; it means
life, and hence may mean food or danger. The response to
it is in most cases adapted rather to its importance than to
its physical strength A third arrangement for the securing
of reaction to vitally important stimulation lies in the existence
of preformed connections in the nervous system, which bring
it about that the path of the excitation produced by one stimu-
lus is clear to the motor apparatus, while that of another is
closed. Reactions of this sort we call instinctive. The
nesting bird responds to the sight of building material rather
than to that of objects offering equally strong stimulation
to the optic nerve ; the cat sits at the mouse hole, the parent
animal responds to the faintest cry of the offspring, because
these stimuli have the right of way by virtue of inherited
nervous connections.

Finally, a weak stimulus may determine reaction and be
victorious over a stronger one because of nervous pathways
formed throught he individual's own experience. The conse-

290 The Animal Mind

quences of reaction to it in the individual's past may operate
to secure reaction to it in the future. To the cat in a puz-
zle box, the string that must be pulled to let it out offered
originally no stronger stimulus to action than any other ob-
ject in sight ; but after sufficient experience the string comes
to dominate the situation and determine the cat's behavior.
If the experience of consequences is slowly acquired, by
many repetitions, the process of reacting to an object originally
indifferent may be unaccompanied by any ideas of the con-
sequences of such reaction. If it is rapidly acquired, we
know that we human beings at least accompany our reactions
by calling up the results of our past reactions in the form of
memory ideas.

§ 98. The Peculiar Characteristics of Attention as a Device
to secure Prepotency

We have suggested that attention is a means of securing
reaction to the vitally important stimuli acting upon an
organism. Does reaction to a stimulus always mean atten-
tion to the sensation accompanying that stimulus ?

This question may best be answered by examining the
characteristics of the attention process as we know it. In
attention, the details of the object attended to become clear
and distinct. That is, attention is a state where discrimina-
tion is improved. Further, attention involves varying de-
grees of effort, and these are marked by varying intensity of
certain bodily processes. Attention under difficulties is
accompanied by a rigid position of the body, by holding the
breath, and by various muscular effects, aside from the pro-
cesses which, like frowning, are concerned with the adaptation
of the sense organ to receive an impression. These general
bodily effects of attention are all such as to suggest that the
body is to be kept as quiet as possible during the attentive

Some Aspects of Attention 291

state. In other words, no reaction is to be made to the
object attended to except such as may be necessary to allow
its being carefully discriminated from other objects. At-
tention, in its intenser degrees, at least, seems to involve a
state of suspended reaction.

Not every case, then, of response adapted to the vital im-
portance of a stimulus is a case that suggests as its psychic
aspect attention to the accompanying sensation. When, for
example, a reaction of especial speed is made to contact with
a moving stimulus, the speed of the reaction would itself
indicate that the sensations produced are not attended to.
The proper situation for attention would be the situation in
which the reaction needs to be suspended until the stimu-
lus is fully discriminated. Now such careful discrimination
does not appear to be characteristic of reactions that are
largely based on inherited nervous structures. Many facts
concerning the instincts of animals, that is, their inherited
reactions, indicate that these are extremely rough adjust-
ments of behavior to environment until refined by individual
experience. Hudson observed, for example, that newly born
lambs on the South American plains had a tendency to run
away from any object that approached them, and to follow
any object that receded from them. They would follow
his horse for miles as he rode along, and would run away
from their own mothers when the latter moved toward them.
He explained this as adapted to the fact that ordinarily
their first duty, on making their appearance in the world,
is to keep up with the receding herd, while an approaching
object is more likely to be an enemy (188). Later, this
rough adjustment is modified; they learn by experience
not to run away from their mothers, and not to follow indis-
criminately any leader.

If it is true that instinct unmodified by experience is

292 The Animal Mind

adapted to general rather than to special features of environ-
ment, it seems likely that the phenomena of attention as we
know them are found chiefly in connection with those re-
sponses to vitally important stimulation which are determined,
in part, at least, by the individual experience of the reacting
animal, for these are the responses requiring most careful
discrimination among stimuli, and the delay of reaction until
such discrimination has been made.1 Putting the matter
in a slightly different way, we may say that purely inherited
responses can be adapted only to certain broad, roughly
distinguished classes of stimuli, for these alone are common
to the experience of all members of the species. Nothing
but individual experience can bring to light the importance
for welfare of certain particular stimuli, for the significance
of these would vary with the experience of each individual
animal. Among the lower animals, attention probably
reaches its highest pitch where the response most needs to be
suspended in order that the stimulus may be fully discrim-
inated. The rabbit or wild bird crouching motionless close
to the ground, watching each movement of a possible enemy,
suggests strongly to our minds a condition of breathless
attention. Whether such an interpretation is the true one
depends very much, I should say, on the extent to which
past individual experience has refined the animal's powers of
discrimination. Mere "freezing to the spot" may be an
inherited reaction, useful in time of danger, but more anal-

1 In this connection Franz's recent experimental demonstration that the
frontal lobes, long regarded as the seat of the neural processes underlying
attention, are concerned in the functioning of recently learned reactions, is
of especial interest. Franz found that cats and monkeys which had been
trained to work mechanisms lost the power to do so when the frontal lobes
were extirpated, although habits of older date, such as responding to a call,
were preserved (136, 136 a).

Some Aspects of Attention 293

ogous in its psychic aspect to the blank emptiness of the
hypnotic trance than to alert, watchful attention.

Yet although, in so far as attention is a state favoring
discrimination of stimuli, it is involved in that part of an
animal's behavior which is derived from individual expe-
rience, since pure instinct discriminates but roughly; in so
far as it is still one of the devices for securing reaction to
stimuli of vital importance, its root must lie in instinct. No
object wholly unrelated to some fundamental instinct can
hope to secure attention, for the great classes of vitally
important stimuli have all of them preformed paths in the
nervous system by which their reactions are secured. What
individual experience does is to refine upon the adaptations
which instinct makes possible ; to bring about the connection
of certain stimuli, originally indifferent, with the performance
of an instinctive response, or to produce a checking of the
instinctive response when certain individual peculiarities
of a stimulus that would otherwise call it forth become
evident. For instance, an animal learns by experience to
come at the call of a human being who feeds it ; the sound,
originally without effect on its reactions, has come to be
connected with the nervous mechanism of an instinct. The
chick pecking at small objects on the ground learns by ex-
perience to inhibit this instinctive response with reference
to objects having certain peculiarities originally undiscrimi-
nated, but now in some way emphasized through painful
circumstances accompanying his previous encounter with
them.

The most fundamental characteristic of attention, then,
is perhaps that aspect of it which has been called abstrac-
tion, the diminished effectiveness of stimuli not attended to.
By virtue of this aspect we recognize that attention belongs
with instinct as being concerned in securing the prepotency

294 The Animal Mind

of vitally important stimulation. On the other hand, the
further characteristic of attention, namely, that it is a state
of suspended reaction involving careful discrimination of
stimuli, suggests that its functioning is connected rather with
the refining and modifying influence of individual experience
acting on instinct, since here alone do we find delayed re-
action and accurate stimulus discrimination.

The highest grade of attention, the final triumph of vital
importance over mere intensity of stimulation, is to be found
where the focus of attention is occupied by an idea or train
of ideas. When a process purely centrally excited holds the
field and makes the individual deaf and blind to powerful
external stimuli pouring in upon his sense organs, then he
is superior to the immediate environment at least. This
form of attention occurs, probably, only when the vital im-
portance of the idea attended to has been learned through
that most rapid form of individual acquisition of experience
which involves the revival of the past in idea. It has been
called derived attention. The ideas attended to are held
in the focus of consciousness and analyzed through the power
of associated ideas. The inventor holds to his problem, the
student to his task, in spite of distractions, because of the
consequences which he thinks of as likely to result. It
seems unlikely that attention in this final form occurs among
the lower animals. While ideas are probably present to
some extent in the minds of the higher mammals, they are
hardly so far freed from connection with external stimuli that
the animal can shut out the world of sense from its conscious-
ness and dwell in a world of ideas.

BIBLIOGRAPHY

The following is a list of the books and articles consulted hi the
preparation of this work. Not all of them are cited in the text.

1. ADAMS, G. P., 1903. On the negative and positive photo-

tropism of the earthworm, Allolobophora foetida, as de-
termined by light of different intensities. Am. Jour.
Physiol., vol. 9, p. 26.

2. ADERHOLD, R., 1888. Beitrag zur Kenntniss richtender

Krafte bei der Bewegungen niederer Organismen. Jena.
Zeitschr. f. Naturwiss., Bd. 22, S. 310.

3. ALLABACH, L. F., 1905. Some points regarding the behavior

of Metridium. Biol. Bull., vol. 10, p. 35.

4. ALLEN, J., 1904. The associative processes of the guinea-

pig. Jour. Comp. Neur. and Psych., vol. 14, p. 293.

5. ANDREW, E., 1903. Inwiefern werden Insekten durch

Farbe u. Duft der Blumen angezogen? Beihefte z.
botan. Zentralblatt, Bd. 15, S. 427.

6. AXENFELD, D., 1896. Die Rontgenschen Strahlen dem

Insektenauge sichtbar. Cent. f. Physiol., Bd. 10, S. 436.

7. 1899. Quelques observations sur la vue des arthropodes.

Arch. ital. biol., t. 31, p. 370.

8. BALDWIN, J. M., 1894. Mental development: methods and

processes. New York.

9. BARDEEN, C. R., 1901. Physiology of Planaria maculata.

Am. Jour. Physiol., vol. 5, p. 1.

10. 1901. Function of the brain in Planaria maculata.

Ibid., vol. 5, p. 175.

11. BATESON, W., 1887. Notes on the senses and habits of

some Crustacea. Jour. Mar. Biol. Assoc. United Kingdom,
vol. 1, p. 211.

12. 1887. On the sense-organs and perceptions of fishes.

Ibid., vol. 1, p. 225.

13. BEER, TH., 1892. Die Accommodation des Vogelauges.

Pfliigers Arch., Bd. 53, S. 175.
295

296 The Animal Mind

14. BEER, TH., 1894. Die Accommodation des Fischauges.

Ibid., Bd. 58, S. 523.

15. 1897. Die Accommodation des Kephalopodenauges.

Ibid., Bd. 67, S. 541.

16. 1898. Die Accommodation des Auges bei den Rep-

tilien. Ibid., Bd. 69, S. 507.

17. 1898. Vergleichend-physiologische Studien zur Sta-

tocysten-Function, I. Ueber den angeblichen Gehorsinn
u. das angebliche Gehororgan der Crustaceen. Ibid.,
Bd. 73, S. 1.

18. 1898. Die Accommodation des Auges bei den Amphi-

bien. Ibid., Bd. 73, S. 501.

19. 1899. Vergleichend-physiologische Studien u. s. w.,

II. Versuche an Crustaceen. Ibid., Bd. 74, S. 364.

20. BEER, TH., BETHE, A., u. VON UEXKULL, J., 1899. Vor-

schlage z. einer objektivirender Nomenclatur in der Phy-
siologic des Nervensystems. Biol. Cent., Bd. 19, S.
517.

21. BELL, J. C., 1906. The reactions of the crayfish. Harvard

Psych. Studies, vol. 2, p. 615.

22. 1906. The reactions of the crayfish to chemical stimuli.

Jour. Comp. Neur. and Psych., vol. 16, p. 299.

23. BENTLEY, I. M., 1899. The memory image and its quali-

tative fidelity. Am. Jour. Psych., vol. 11, p. 1.

24. BERT, P., 1869. Sur la question de savoir si tous les animaux

voient les memes rayons que nous. Arch, de physiol.,
t. 2, p. 547.

25. BERTKAU, P., 1885. Ueber die Augen u. ein als Gehororgan

gedeutetes Organ der Spinnen. Sitzungsber. d. nieder-
rhein. Gesellsch., Bd. 42, S. 218, 282.

26. BERRY, C. S., 1906. The imitative tendencies of white

rats. Jour. Comp. Neur. and Psych., vol. 16, p. 333.

27. BETHE, A., 1894. Ueber die Erhaltung des Gleichgewichtes.

Biol. Cent., Bd. 14, S. 95.

28. 1898. Das Centralnervensystem von Carcinus moenas,

II. Arch. f. mikr. Anat., Bd. 51, S. 447.

29. 1898. Die anatomische Elemente des Nervensystems

u. ihre physiologische Bedeutung. Biol. Cent., Bd. 18,
S. 843.

Bibliography 297

30. BETHE, A., 1898. Durfen wir den Ameisen u. Bienen psych-

ische Qualitaten zuschreiben ? Pfliigers Arch., Bd. 70, S.
15.

31. 1900. Noch einmal uber d. psychischen Qualitaten der

Ameisen. Ibid., Bd. 79, S. 39.

32. 1902. Die Heimkehrfahigkeit der Ameisen u. Bienen.

Biol. Cent., Bd. 22, S. 193, 234.

33. BIGELOW, H. B., 1904. The sense of hearing in the goldfish,

Carassius auratus. Am. Nat., vol. 38, p. 275.

34. BINET, A., 1894. The psychic life of micro-organisms.

Authorized translation. Chicago.

35. BOHN, G., 1902. Contributions a la psychologic des anne*-

lides. Bull. Inst. gen. psych., Paris, t. 2, p. 317.

36. 1903. Observations biologiques sur les are"nicoles.

Bull. Mus. d'hist. nat., t. 9, p. 62.

37. 1903. Sur les mouvements oscillatoires des Convoluta

roscoffensis. C. r. Acad. Sci., Paris, t. 137, p. 576.

38. 1903. Les Convoluta roscoffensis et la theorie des

causes actuelles. Bull. Mus. d'hist. nat., t. 9, p. 352.

39. 1903. Actions tropiques de la lumiere. C. r. Soc.

Biol., Paris, t. 55, p. 1440.

40. 1903. Sur la phototropisme des artiozoaires supe*rieurs.

C. r. Acad. Sci., Paris, t. 137, p. 1292.

41. 1903. De revolution des connaissances chez les ani-

maux inarms littoraux, I. Les crustace*s. Bull. Inst.
ge"n. psych., Paris, t. 3, p. 590.

42. 1904. Cooperation, hie"rarchisation, integration des

sensations chez les artiozoaires. C. r. Acad. Sci., Paris,
t. 138, p. 112.

43. 1904. Intervention des influences passees dans les

mouvements actuels d'un animal. C. r. Soc. Biol., Paris,
t. 56, p. 789.

44. 1904. Les premieres lueurs de 1'intelligence. Bull.

Inst. ge*n. psych., Paris, t. 4, p. 419.

45. 1904. Periodicity vitale des animaux soumis aux

oscillations du niveau des hautes mers. C. r. Acad. Sci.,
Paris, t. 139, p. 610.

46. 1904. Oscillations des animaux littoraux synchrones

des mouvements de la marge. Ibid., t. 139, p. 646.

298 The Animal Mind

47. BOHN, G., 1904. Mouvements de manege en rapport avec les

mouvements de la mare*e. C. r. Soc. Biol., Paris, t. 57,
p. 297.

48. 1904. Attractions et repulsions dans un champ lumi-

neux. Ibid., t. 57, p. 315.

49. 1904. Influence de la position de Fanimal dans 1'espace

sur les tropismes. Ibid., t. 57, p. 351.

50. 1904. L'anhydrobiose et les tropismes. C. r. Acad.

Sci., Paris, t. 139, p. 809.

51. 1904. Theorie nouvelle du phototropisme. Ibid.,

t. 139, p. 890.

52. 1905. Les receptions oculaires. Bull. Inst. gen.

psych., Paris, t. 5, p. 171.

53. 1905. Les causes actuelles et les causes passe*es.

Rev. scient., t. 3, pp. 353, 389.

54. 1905. Mouvements rotatoires d'origine oculaire.

C. r. Soc. Biol., Paris, t. 58, p. 714.

55. 1905. Attractions et oscillations des animaux marins

sous Tinfluence de la lumiere. Memoires Inst. gen.
psych., Paris, vol. 1, p. 110.

56. 1905. Des tropismes et des 6tats physiologiques.

C. r. Soc. Biol., Paris, t. 59, p. 515.

57. 1905. L'eclairement des yeux et les mouvements

rotatoires. Essais et erreurs dans les tropismes. Ibid.,
t. 59, p. 564.

58. 1905. Impulsions motrices d'origine oculaire chez les

crustace*s. Bull. Inst. ge"n. psych., Paris, t. 5, p. 42.

59. 1906. Les tropismes, les reflexes, et 1'intelligence.

L'anne*e psych., t. 12, p. 137.

60. 1906. Sur les courbures dues a la lumi&re. C. r. Soc.

Biol., Paris, t. 61, p. 420.

61. 1906. Sur les mouvements de roulement influence's

par la lumiere. Ibid., t. 61, p. 468.

62. 1906. Mouvements en relation avec 1'assimilation

pigmentaire chez les animaux. Ibid., t. 61, p. 527.

63. 1907. L'influence de l'e*clairement passe sur la matiere

vivante. Ibid., t. 62, p. 292.

64. 1906. Observations sur les papillons du rivage de la

mer. Bull. Inst. ge*n. psych., Paris, t. 6, p. 285.

Bibliography 299

65. BOHN G., 1907. Le rhythme nycthem^ral chez les actinies.

C. r. Soc. Biol., Paris, t. 62, p. 473.

66. 1907. Les etat physiologiques des actinies. ' Bull.

Inst. gen. psych., Paris, t. 7, p. 81.

67. BONNIER, G., 1905. L'accoutumance des abeilles et la

couleur des fleurs. C. r. Acad. Sci., Paris, t. 141, p. 988.

68. BOUVIER, E. L., 1901. Les habitudes de Bembex. L'anne*e

psych., t. 7, p. 1.

69. BOYS, C. V., 1880. The influence of a tuning fork on the

garden spider. Nature, vol. 23, p. 149.

70. BREUER, J., 1891. Ueber die Function der Otolithen-

Apparate. Pfliigers Arch., Bd. 48, S. 195.

71. BUNTING, M., 1893. Ueber die Bedeutung der Otolithenor-

gane fur d. geotropischen Functionen von Astacus fluwa-
tilis. Ibid., Bd. 54, S. 531.

72. BUTTEL-REEPEN, H. VON, 1900. Sind die Bienen Reflex-

maschine? Biol. Cent., Bd. 20, S. 97, 177, 209.

73. CALKINS, M. W., 1905. The limits of genetic and compara-

tive psychology. British Jour. Psych., vol. 1, p. 261.

74. CARPENTER, F. W., 1905. Reactions of the pomace fly to

light, heat, and mechanical stimulation. Am. Nat.,
vol. 39, p. 157.

75. CLAPAREDE, E., 1901. Les animaux sont-ils conscients?

Rev. phil., t. 51, p. 24. Trans, in Internat. Quart.,
vol. 8, p. 296.

76. 1903. La faculte d 'orientation lointaine (Sens de

direction, sens de retour) . Arch, de psych., t. 2, p. 133.

77. 1905. La psychologic comparee est-elle legitime?

Ibid., t. 5, p. 13.

78. CLARK, G. P., 1896. On the relation of the otocysts to

equilibrium phenomena in Gelasimus pugilator and Platy-
onichus ocellatus. Jour. Physiol., vol. 19, p. 327.

79. COLE, L. J., 1901. Notes on the habits of pycnogonids.

Biol. Bull., vol. 2, p. 195.

80. 1907. An experimental study of the image-forming

powers of various types of eyes. Proc. Amer. Acad. Arts
and Sciences, vol. 42, p. 335.

81. 1907. Influence of direction vs. intensity of light in

determining the phototropic responses of organisms.

30O The Animal Mind

Abstract in Jour. Comp. Neur. and Psych., vol. 17, p.
193.

82. COLE, L. W., 1907. Concerning the intelligence of raccoons.

Jour. Comp. Neur. and Psych., vol. 17, p. 211.

83. CONRADI, E., 1905. Song and call-notes of English sparrows

when reared by canaries. Am. Jour. Psych., vol. 16, p.
190.

84. CUENOT, L., 1891. Etudes morphologiques sur les e*chino-

dermes. Arch, de biol., t. 11, p. 521.

85. CYON, E., 1878. Experimentelle Untersuchungen iiber

die Function der halbzirkelformigen Canale. Bibl. de
I'e'cole des hautes Etudes, section des sciences naturelles,
t. 18.

86. DAHL, F., 1883. Ueber die Horhaare bei den Arachnoiden.

Zool. Anz., Bd. 6, S. 267.

87. 1885. Das Gehor- und Geruchsorgan der Spinnen.

Arch. f. mikr. Anat., Bd. 24, S. 1.

88. 1885. Versuch einer Darstellung der psychischen

Vorgange in der Spinnen. Vierteljahr. f. wiss. Phil.,
Bd. 9, S. 84, 162.

89. DARWIN, C. R., 1874. Descent of man and selection in

relation to sex. New York.

90. 1877. Effects of cross and self-fertilization in the vege-
table kingdom. New York.

91. 1883. The formation of vegetable mould through the

action of worms, with observations on their habits. New
York.

92. DAVENPORT, C. B., 1897-1899. Experimental morphology.

2 vols., New York.

93. DAVENPORT, C. B., and CANNON, W. B., 1897. On the de-

termination of the direction and rate of movement of
organisms by light. Jour. Physiol., vol. 21, p. 22.

94. DAVENPORT, C. B., and LEWIS, F. T., 1899. Phototaxis of

Daphnia. Science, N.S., vol. 9, p. 368.

95. DAVENPORT, C. B., and PERKINS, H., 1897. A contribution

to the study of geotaxis in the higher animals. Jour.
Physiol., vol. 22, p. 99.

96. DEARBORN, G. v. N., 1900. The individual psycho-physiol-

ogy of the crayfish. Am. Jour. Physiol., vol. 3, p. 404.

Bibliography 301

97. DELAGE, Y., 1887. Sur une fonction nouvelle des otocystes

comme organes d 'orientation locomotrice. Arch, de zool.
expe"r., lie serie, t. 5, p. 1.

98. DELLINGER, 0. P., 1906. Locomotion of Amcebse and allied

forms. Jour. Ex. Zool., vol. 3, p. 337.

99. DESCARTES, R., 1640, 1649. The page references in the text

are to the translation in Torrey's The philosophy of
Descartes in extracts from his writings. N.Y., 1892.

100. DREW, G. A., 1906. The habits, anatomy, and embryology

of the giant scallop, Pecten tenuicostatus. Univ. of Maine
Studies, no. 6.

101. 1907. The habits and movements of the razor-shell

clam. Biol. Bull., vol. 12, p. 127.

102. DUBOIS, R., 1889. Sur le mecanisme des fonctions photo-

dermatiques et photogeniques dans le siphon du Pholas
dactylus. C. r. Acad. Sci., Paris, t. 109, p. 233.

103. 1890. Sur la perception des radiations lumineuses

par la peau chez les protees aveugles des grottes de la
Carniole. Ibid., t. 110, p. 358.

104. 1890. Sur la physiologic compare*e des sensations

gustatives et tactiles. Ibid., t. 110, p. 473.

105. 1890. Sur la physiologic compared de 1'olfaction.

Ibid., t. Ill, p. 66.

106. EDINGER, L., 1899. Haben die Fische ein Gedachtniss?

Allgemeine Zeitung, Beilage, Oct. 21 and 23. (Transla-
tion in Smithsonian Report, 1899, p. 375.)

107. EIGENMANN, C. H., 1899. The blind fishes. Biol. Lectures,

Marine Biol. Lab., Wood's Hole, 1899, p. 113.

108. EMERY, C., 1893. Zirpende und springende Ameisen.

Biol. Cent., Bd. 13, S. 189.

109. ENGELMANN, T. W., 1879. Ueber Reizung des contraktilen

Protoplasmas durch plotzliche Beleuchtung. Pflugers
Arch., Bd. 19, S. 1.

110. 1882. Ueber Licht- und Farbenperception niedersten

Organismen. Ibid., Bd. 29, S. 387.

111. 1887. Ueber die Functionen der Otolithen. Zool.

Anz., Bd. 10, S. 439.

112. ENTEMAN, M. M., 1902. On the behavior of social wasps.

Pop. Sci. Mo., vol. 61, p. 339.

302 The Animal Mind

113. ESTERLY, C. 0., 1907. Reactions of copepods to light and

to gravity. Am. Jour. Physiol., vol. 18, p. 47.

114. EWALD, K., 1892. Physiologische Untersuchungen iiber

das Endorgan des Nervus octavus. Wiesbaden.

115. FABRE, J. H., 1879-1904. Souvenirs entomologiques.

9 vols. Paris.

116. FERTON, C., 1905. Notes detaillees sur Pinstinct des

hymenopteres melliferes et ravisseurs. Ann. Soc. entom.
France, t. 74, p. 56.

117. FIELDE, A. M., 1901. A study of an ant. Proc. Philadel-

phia Acad. Nat. Sci., vol. 53, p. 425.

118. 1901. Further study of an ant. Ibid., vol. 53, p.

521.

119. 1903. Supplementary notes on an ant. Ibid.,

vol. 55, p. 491.

120. 1903. Artificial mixed nests of ants. Biol. Bull.,

vol. 5, p. 320.

121. 1903. A cause of feud between ants of the same species

living in different communities. Ibid., vol. 5, p. 326.

122. 1904. Observations on ants in relation to tempera-
ture and to submergence. Ibid., vol. 7, p. 170.

123. 1904. The power of recognition among ants. Ibid.,

vol. 7, p. 227.

124. 1905. The progressive odor of ants. Ibid., vol. 10,

p. 1.

125. FIELDE, A. M., and PARKER, G. H., 1904. The reactions

of ants to material vibrations. Proc. Philadelphia Acad.
Nat. Sci., vol. 56, p. 642.

126. FISKE, J., 1874. Outlines of cosmic philosophy. 2 vols.,

Boston.

127. FLEURE, H. J., and WALTON, C. L., 1907. Notes on the

habits of some sea-anemones. Zool. Anz., Bd. 31, S. 212.

128. FLOURENS, P., 1842. Recherches expe"rimentales sur les

propriet^s et les fonctions du systeme nerveux dans les
animaux vertebres. Paris.

129. FOREL, A., 1874. Les fourmis de la Suisse. Zurich.

130. 1888. Sur les sensations des insectes. Recueil zool.

suisse, t. 4, no. 2.

131. 1900-1901. Sensations des insectes. Rivista d;

Bibliography 303

biologia generate, vol. 2, pp. 561, 641; vol. 3, pp. 7,
241, 401.

132. FOREL, A., 1904. Ants and some other insects. Trans, by

W.M. Wheeler. Chicago.

133. 1906. La me*moire du temps chez les abeilles. Bull.

Inst. gen. psych., Paris, t. 6, p. 258.

134. FOREL, A., and DUFOUR, H., 1902. Ueber die Empfind-

lichkeit der Ameisen fur ultraviolett und Rontgensche
Strahlen. Zool. Jahrbuch, Abth. f. Systematik, Bd. 17,
S. 335.

135. FRANDSEN, P., 1901. Studies on the reactions of Limax

maximus to directive stimuli. Proc. Amer. Acad. Arts
and Sciences, vol. 37, p. 185.

136. FRANZ, S. I., 1906. Observations on the functions of the

association areas (cerebrum) in monkeys. Jour. Am.
Med. Assoc., vol. 47, p. 1464.

136a. 1907. On the functions of the cerebrum : the frontal

lobes. Arch, of Psych., no. 2.

137. FROHLICH, A., 1904. Studien iiber die Statozysten

wirbelloser Tiere, I. Versuche an Cephalopoden, und
Einschlagiges aus der menschlichen Pathologic. Pflugers
Arch., Bd. 102, S. 415.

138. 1904. Studien u. s. w., II. Versuche an Krebsen.

Ibid., Bd. 103, S. 149.

139. 1905. Ueber den Einfluss der Zerstorung des Laby-

rinthes beim Seepferdchen, nebst einigen Bemerkungen
tiber das Schwimmen dieser Tiere. Ibid., Bd. 106, S. 84.

140. GAMBLE, F. W., and KEEBLE, F., 1903. The bionomics of

Convoluta roscoffensis, with special reference to its green
cells. Quar. Jour. Micr. Sci., vol. 47, p. 363.

141. GARRET, W. E., 1905. A sight reflex shown by stickle-

backs. Biol. Bull., vol. 8, p. 79.

142. GAUBERT, P., 1892. Recherches sur les organes des sens et

sur les systemes integumentaires, glandulaires, et mus-
culaires des appendices des Arachnides. Ann. des Sci.
nat., 7e serie, Zool., t. 13, p. 31.

143. GHINST, VAN DER, 1906. Quelques observations sur les

actinies. Bull. Inst. gen. psych., Paris, t. 6, p. 267.

144. GILTAY, E., 1904. Ueber die Bedeutung der Krone bei den

304 The Animal Mind

Bluten u. iiber das Farbenunterschiedungsvermogen der
Insekten, I. Jahrb. f. wiss. Bot., Bd. 40, S. 368.

145. GLASER, O. C., 1907. Movement and problem solving in

Ophiura brevispina. Jour. Exp. Zool., vol. 4, p. 203.

146. GOLDSMITH, M., 1905. Recherches sur la psychologic de

quelques poissons littoraux. Bull. Inst. gen. psych.,
Paris, t. 5, p. 51.

147. GOLTZ, F., 1870. Ueber die physiologische Bedeutung der

Bogengange des Ohrlabyrinthes. Pfliigers Arch., Bd.
3, S. 172.

148. GRABER, V., 1882. Die chordotonalen Sinnesorgane u.

das Gehor der Insekten, I. Arch. f. mikr. Anat., Bd. 20,
S. 506.

149. 1883. Die chordotonalen u. s. w., II. Ibid., Bd. 21,

S. 65.

150. 1883. Fwndamentalversuche iiber d. Helligkeits- u.

Farbenempfindlichkeit augenloser u. geblendeter Thiere.
Sitzungsber. d. kais. Akad. d. Wiss., Wien, math.-natur-
wiss. Klasse, Bd. 87, Abth. 1, S. 201.

151. 1884. Grundlinien zur Erforschung des Helligkeits-

und Farbensinns der Thiere. Prag und Leipzig.

152. 1889. Ueber die Empfindlichkeit einiger Meerthiere

gegen Riechstoffe. Biol. Cent., Bd. 8, S. 743.

153. GROOM, T. T., and LOEB, J., 1890. Der Heliotropismus der

Nauplius von Balanus perforatus und die periodischen

Tiefenwanderungen pelagischer Tiere. Ibid., Bd. 10,
S. 160.

154. GROOS, K., 1898. The play of animals. Trans, by E. L.

Baldwin. New York.

155. GURLEY, 1902. The habits of fishes. Am. Jour. Psych.,

vol. 13, p. 408.

156. HACHET-SOUPLET, P., 1900. Examen psychologique des

animaux. Paris.

157. HANDL, A., 1887. Ueber d. Farbensinns der Thiere u. die

Vertheilung der Energie im Spektrum. Sitzungsber. d.
kais. Akad. d. Wiss., Wien, math.-naturwiss. Klasse,
Bd. 94, S. 935.

158. HARGITT, C. W., 1906. Experiments on the behavior of

tubicolous annelids. Jour. Exp. Zool., vol. 3, p. 295.

Bibliography 305

159. HARGITT, C. W., 1907. Further observations on the behav-

ior of tubicolous annelids. Abstract in Jour. Comp.
Neur. and Psych., vol. 17, p. 199.

160. 1907. Notes on the behavior of sea-anemones. Biol.

Bull., vol. 12, p. 274.

161. HARPER, E. H., 1905. Reactions to light and mechanical

stimulation in the earthworm, Perichaeta bermudensis.
Ibid., vol. 10, p. 17.

162. HARRINGTON, N. R., and LEAMING, E., 1899. The reaction

of Amoeba to lights of different colours. Am. Jour.
Physiol., vol. 3, p. 9.

163. HENSEN, V., 1863. Studien iiber das Gehororgan der

Decapoden. Zeit. f. wiss. Zool., Bd. 13, S. 319.

164. 1899. Wie steht es mit der Statozysten-Hypothese ?

Pflugers Arch., Bd. 74, S. 22.

165. HERRICK, C. J., 1903. The organ and sense of taste in fishes.

Bull. U.S. Fish Comm., vol. 22, p. 237.

166. HERRICK, F. H., 1895. The American lobster. Ibid.,

vol. 15, p. 1.

167. HERTEL, E., 1904. Ueber Beeinflussung des Organismus

durch Licht, speziell durch die chemisch wirksamer
Strahlen. Zeit. f. allg. Physiol., Bd. 4, S. 1.

168. HESSE, R., 1896. Untersuchungen iiber die Organe der

Lichtempfindungen bei niederen Thieren, I. Die Organe
der Lichtempfindungen bei den Lumbriciden. Zeit. f.
wiss. Zool., Bd. 61, S. 393.

169. 1897. Untersuchungen u. s. w., II. Die Augen der

Platyhelminthen, insonderheit der tricladen Turbellarien.
Ibid., Bd. 62, S. 549.

170. 1897. Untersuchungen u. s. w., III. Die Sehorgane

der Hirudineen. Ibid., Bd. 62, S. 671.

171. 1898. Die Lichtempfindung des Amphioxus. Anat.

Anz., Bd. 14, S. 556.

172. 1898. Untersuchungen u. s. w., IV. Die Sehorgane

des Amphioxus. Zeit. f. wiss. Zool., Bd. 63, S. 456.

173. 1899. Untersuchungen u. s. w., V. Die Augen der

polychaten Anneliden. Ibid., Bd. 65, S. 506.

174. 1900. Untersuchungen u. s. w., VI. Die Augen eini-

ger Mollusken. Ibid., Bd. 68, S. 379.

The Animal Mind

>. HESSE, R., 1901. Untersuchungen u. s. w., VII. Von den
. Arthropodenaugen. Ibid., Bd. 70, S. 347.

176. — — 1902. Untersuchungen u. s. w., VIII. Weitere

Thatsachen. Allgemeines. Ibid., Bd. 72, S. 565.

177. HOBHOUSE, L. T., 1901. Mind in evolution. London.

178. HODGE, C. F., and AIKINS, H. A., 1895. The daily life of a

protozoan. Am. Jour. Psych., vol. 6, p. 524.

179. HOFFMEISTER, W., 1845. Die bis jetzt bekannten Arten

aus der Familie der Regenwtirmer. Braunschweig.

180. HOLMES, S. J., 1900. Habits of Amphithoe longimana

Smith. Biol. Bull., vol. 2, p. 165.

181. 1901. Phototaxis in Amphipoda. Am. Jour. Physiol.,

vol. 5, p. 211.

182. 1902. Observations on the habits of Hyallella den-

tata. Science, N.S., vol. 15, p. 529.

183. 1903. Phototaxis in Volvox. Biol. Bull., vol. 4,

p. 319.

184. 1903. Sex recognition among amphipods. Ibid.,

vol. 5, p. 288.

185. 1905. The selection of random movements as a factor

in phototaxis. Jour. Comp. Neur. Psych., vol. 15, p. 98.

186. 1905. The reactions of Ranatra to light. Ibid.,

vol. 15, p. 305.

187. HOLT, E. B., and LEE, F. S., 1901. The theory of photo-

tactic response. Am. Jour. Physiol., vol. 4, p. 460.

188. HUDSON, W. H., 1895. The naturalist in La Plata. Lon-

don.

189. JAMES, W., 1890. The principles of psychology. 2 vols.

New York.

190. JANET, C., 1893. Note sur la production des sons chez les

fourmis et sur les organes qui les produisent. Ann. Soc.
ent. France, t. 62, p. 159.

191. 1894. Sur les nerfs de Pantenne et les organes chordo-

tonaux chez les fourmis. C. r. Acad. ScL, Paris, t. 118,
p. 814.

192. 1893-1905. Les fourmis, les guepes et les abeilles.

Paris.

193. JENNINGS, H. S., 1897. Studies on reactions to stimuli in

unicellular organisms, I. Reactions to chemical, os-

Bibliography 307

motic, and mechanical stimuli in the ciliate protozoa.
Jour. Physiol., vol. 21, p. 258.

194. JENNINGS, H. S., 1899. Studies, etc., II. The

mechanism of the motor reactions of Paramecium. Am.
Jour. Physiol., vol. 2, p. 311.

195. 1899. Studies, etc., III. Reactions to localized stimuli

in Spirostomum and Stentor. Am. Nat., vol. 33, p. 373.

196. 1899. The behavior of unicellular organisms. Biol.

lectures, Marine Biol. Lab., Wood's Hole, 1899, p. 93.

197. 1899. The psychology of a protozoan. Am. Jour.

Psych., vol. 10, p. 503.

198. 1899. Studies, etc., IV. Laws of chemotaxis in

Paramecium. Am. Jour. Physiol., vol. 2, p. 355.

199. 1900. Studies, etc., V. On the movements and motor

reflexes of the flagellata and ciliata. Ibid., vol. 3, p. 229.

200. 1900. Reactions of infusoria to chemicals: a criti-
cism. Am. Nat., vol. 34, p. 259.

201. 1900. Studies, etc., VI. On the reactions of Chilo-

monas to organic acids. Am. Jour. Physiol., vol. 3, p. 397.

202. 1902. Artificial imitations of protoplasmic activities

and methods of demonstrating them. Jour. Applied
Micros., vol. 5, p. 1597.

203. 1902. Studies, etc., IX. On the behavior of fixed

infusoria (Stentor and Vorticella) with special reference
to the modifiability of protozoan reactions. Am. Jour.
Physiol., vol. 8, p. 23.

204. 1904. Physical imitations of the activities of Amreba.

Am. Nat., vol. 38, p. 625.

205. 1904. The behavior of Paramecium. Additional

features and general relations. Jour. Comp. Neur. and
Psych., vol. 14, p. 441.

206. 1904. Contributions to the study of the behavior of

lower organisms. Carnegie Institution Publications.
Washington.

207. 1905. Modifiability in behavior, I. Behavior of sea-
anemones. Jour. Exp. Zool., vol. 2, p. 447.

208. 1905. The method of regulation in behavior and in,

other fields. Ibid., vol. 2, p. 473.

209. 1905. The basis for taxis and certain other terms in the

308 The Animal Mind

behavior of infusoria. Jour. Comp. Neur. and Psych.,
vol. 15, p. 138.

210. JENNINGS, H. S., 1906. Modifiability in behavior, II.

Factors determining direction and character of move-
ment in the earthworm. Jour. Exp. Zool., vol. 3, p. 435.

211. 1906. Behavior of the lower organisms. New York.

212. 1907. Habit formation in the starfish. Abstract in

Jour. Comp. Neur. and Psych., vol. 17, p. 190.

213. JENNINGS, H. S., and JAMIESON, C., 1902. The movements

and reactions of pieces of ciliate infusoria. Biol. Bull.,
vol. 3, p. 225.

214. JENNINGS, H. S., and MOORE, E. M., 1902. On the reac-

tions of infusoria to carbonic and other acids, with special
reference to the causes of the gatherings spontaneously
formed. Am. Jour. Physiol., vol. 6, p. 233.

215. JENSEN, P., 1893. Ueber den Geotropismus niederer

Organismen. Pflugers Arch., Bd. 53, S. 428.

216. JORDAN, H., 1905. Einige neuere Arbeiten auf dem Gebiete

der "Psychologic" wirbelloser Thiere. Biol. Cent.,
Bd. 25, S. 451, 473.

217. JUDD, C. H., 1905. Movement and consciousness. Yale

Psych. Studies, N.S., vol. 1, p. 199.

218. KEEBLE, F., and GAMBLE, F. W., 1902. The colour physiol-

ogy of the higher Crustacea. Phil. Trans. Roy. Soc.,
London, vol. 196B, p. 295.

219. KELLOGG, V. L., 1907. Some silk-worm moth reflexes.

Biol. Bull., vol. 12, p. 152.

220. KIENITZ-GERLOFF, 1898. Prof. Plateau und die Blumen-

theorie. Biol. Cent., Bd. 18, S. 417.

221. KINNAMAN, A. J., 1902. Mental life of two Macacus rhesus

monkeys in captivity. Am. Jour. Psych., vol. 13, pp. 98,
173.

222. KLINE, L. W., 1899. Suggestions toward a laboratory

course in comparative psychology. Ibid., vol. 10, p. 399.

223. KORNER, 0., 1905. Konnen die Fische horen? Beitrage

zur Ohrenheilkunde, S. 93.

224. KORANYI, A. VON, 1892. Ueber die Reizbarkeit der Frosch-

haut gegen Licht und Warme. Cent. f. Physiol., Bd. 6,
S. 6.

Bibliography 309

225. KRAUSE, W., 1897. Die Farbenempfindungen der Amphi-

oxus. Zool. Anz., Bd. 20, S. 513.

226. KREIDL, A., 1893. Weitere Beitrage zur Physiologie des

Ohrlabyrinthes. Sitzungsber. d. kais. Akad. d. Wiss.,
Wien, math.-naturwiss. Klasse, Abth. 3, Bd. 102, S. 149.

227. 1895. Ueber die Schallperceptiou der Fische. Pflii-

gers Arch., Bd. 61, S. 450.

228. 1896. Ein weiterer Versuch iiber das angebliche

Horen ernes Glockenzeichens durch die Fische. Ibid.,
Bd. 63, S. 581.

229. LECAILLON, A., 1904. Sur la biologic et la physiologic

d'une araignee (Chiracanthum carnifex). L'annee psych.,
1903, pp. 10, 63.

230. LEE, F. S., 1894. A study of the sense of equilibrium in

fishes. Jour. Physiol., vol. 15, p. 311.

231. LEHNERT, G. H., 1891. Beobachtungen an Landplanarien.

Arch. f. Naturgeschichte, Bd. 57, S. 306.

232. LOCKE, J., 1689. Essay on the human understanding.

233. LOEB, J., 1888. Die Orientirung der Tiere gegen das

Licht. Sitzungsber. d. phys.-med. Ges., Wiirzburg,
1888, S. 1.

234. 1888. Die Orientirung der Tiere gegen die Schwer-

kraft der Erde. Ibid., S. 5.

235. 1890. Der Heliotropismus der Tiere und sein Ueber-

einstimmung mit dem Heliotropismus der Pflanzen.
Wiirzburg.

236. 1890. Weitere Untersuchungen iiber den Heliotro-
pismus der Tiere. Pfliigers Arch., Bd. 47, S. 391.

237. 1891. Untersuchungen zur physiologischen Morpholo-
gic der Tiere, I. Ueber Heteromorphose. Wiirzburg.

238. 1891. Ueber Geotropismus bei Tieren. Pfliigers

Arch., Bd. 49, S. 177.

239. 1893. Ueber kiinstliche Unwandlung positiver helio-

tropischer Tiere in negativ heliotropische und umge-
kehrt. Ibid., Bd. 54, S. 81.

240. 1894. Beitrage zur Gehirnphysiologie der Wiirmer.

Ibid., Bd. 56, S. 247.

241. — — 1894. Zur Physiologie und Psychologic der Actinien.

Ibid., Bd. 59, S. 415.

3io The Animal Mind

242. LOEB, J., 1897. Zur Theorie der physiologischen Licht- und

Schwerkraftwirkungen. Ibid., Bd. 66, S. 439.

243. 1900. Comparative physiology of the brain and

comparative psychology. New York.

244. 1904. The control of heliotropic reactions in fresh

water crustaceans by chemicals. Univ. of Cal. Pub.,
vol. 2, p. 1.

245. 1906. Ueber die Erregung von positivem Helio-

tropismus durch Saure, insbesondere Kohlensaure, und
von negativem Heliotropismus durch ultraviolette Strah-
len. Pfliigers Arch., Bd. 115, S. 564.

246. 1906. Ueber die Summation heliotropischer und

geotropischer Wirkungen bei den auf der Drehschiebe
ausgeloster compensatorische Kopfbewegungen. Ibid.,
Bd. 116, S. 368.

247. 1907. Concerning the theory of tropisms. Jour.

Exp. Zool., vol. 4, p. 151.

248. LUBBOCK, J., 1883. Ants, bees, and wasps. New York.

249. 1883. On the sense of colour among some of the

lower animals, I. Jour. Linn. Soc., London, Zool.,
vol. 16, p. 121.

250. 1884. On the sense of colour, etc., II. Ibid., vol. 17,

p. 205.

251. 1888. On the senses, instincts, and intelligence of

animals, with special reference to insects. New York.

252. LUKA.S, F., 1905. Psychologic der niedersten Thiere. Wien

und Leipzig.

253. LYON, E. P., 1898. The functions of the otocyst. Jour.

Comp. Neur. and Psych., vol. 8, p. 238.

254. 1904. On rheotropism, I. Rheotropisrn in fishes.

Am. Jour. Physiol., vol. 12, p. 149.

255. 1905. On the theory of geotropism in Paramecium.

Ibid., vol. 14, p. 421.

256. 1906. Note on the geotropism of Arbacia larvae.

Biol. Bull., vol. 12, p. 21.

257. 1906. Note on the heliotropism of Palsemonetes larvae.

Ibid., vol. 12, p. 23.

258. McCooK, H. C., 1889-1893. American spiders and their

spinning work. 3 vols.

Bibliography 311

259. MASSART, J., 1891. R6cherches sur les organismes infe*ri-

eurs, III. La sensibilite" a la gravitation. Bull. Acad.
roy. Belgique, t. 22, p. 158.

260. MAST, S. O., 1903. Reactions to temperature changes in

Spirillum, Hydra, and fresh-water planarians. Am.
Jour. Physiol., vol. 10, p. 165.

261. 1906. Light reactions in Stentor caeruleus. Jour.

Exp. Zool., vol. 3, p. 359.

262. 1907. Light reactions in lower organisms, II. Volvox.

Jour. Comp. Neur. and Psych., vol. 17, p. 99.

263. MAYER, A. G., and SOULE, C. G., 1906. Some reactions of

caterpillars and moths. Jour. Exp. Zool., vol. 3, p. 415.

264. MAYER, A. M., 1874. Researches in acoustics. Am. Jour.

Science and Arts, III Series, vol. 8, p. 89.

265. MENDELSSOHN, M., 1895. Ueber den Thermotropismus

einzelliger Organ ismen. Pfliigers Arch., Bd. 60, S. 1.

266. 1902. Re"cherches sur la thermotaxie des organismes

unicellulaires. Jour, de physiol. et de pathol. ge*n., t. 4,
p. 393.

267. 1902. R4cherches sur Pinterfe'rence de la thermo-
taxie avec d'autres tactismes et sur la me'canisme du
mouvement thermotactiqUe. Ibid., t. 4, p. 475.

268. 1902. Quelques considerations sur la nature et la

role biologique de la thermotaxie. Ibid., t. 4, p. 489.

269. MEREJKOWSKY, C., 1881. Les crustace"s infeVieurs distin-

guent-ils les couleurs? C. r. Acad. Sci., Paris, t. 93,
p. 1160.

270. METCALF, M. M., 1900. Hearing in ants. Science, N.S.,

vol. 11, p. 194.

271. MILLS, T. W., 1894-1896. The psychic development of

young animals and its physical correlations. Montreal.

272. 1898. The nature and development of animal intelli-
gence. New York.

273. 1899. The nature of animal intelligence and the

methods of investigating it. Psych. Rev., vol. 6, p.
262.

274. MINKIEWICZ, C., 1907. Chromotropism and phototropism.

Trans, in Jour. Comp. Neur. and Psych., vol. 17, p. 89.

275. MITSUKURI, K, 1901. Negative phototaxis and other

312 The Animal Mind

properties of Littorina as factors in determining its
habitat. Annot. zool. japonenses, 4.

276. MOBIUS, K., 1873. Die Bewegungen der Thiere und ihr

psychischer Horizont. Schrift. d. naturwiss. Ver. f.
Schleswig-Holstein, Bd. 1, S. 113.

277. MONTAIGNE, M. DE, 1580. Essays. Florio's translation.

278. MOORE, A., 1903. Some facts concerning geotropic gather-

ings of Paramecium. Am. Jour. Physiol., vol. 9, p.
238.

279. MORGAN, C. L., 1891. Animal life and intelligence. Boston.

280. 1894. Introduction to comparative psychology.

London.

281. 1896. Habit and instinct. London.

282. 1900. Animal behaviour. London.

283. MORSE, M., 1906. Notes on the behavior of Gonionemus.

Jour. Comp. Neur. and Psych., vol. 16, p. 450.

284. MULLER, H., 1873. Die Befruchtung der Blumen durch

Insekten und die gegenseitigen Anpassungen beider.
Leipzig.

285. 1882. Versuche ttber d. Farbenliebhaberei der Ho-

nigbiene. Kosmos, Bd. 6, S. 273.

286. MURBACH, L., 1903. The static function in Gonionemus.

Am. Jour. Physiol., vol. 10, p. 201.

287. NAGEL, W. A., 1892. Die niederen Sinne der Insekten.

Jena.

288. 1892. Der Geschmacksinn der Actinien. Zool. Anz.,

Bd. 15, S. 334.

289. 1893. Versuche zur Sinnesphysiologie von Beroe

ovata und Carmarina hastata. Pfliigers Arch., Bd. 54,
S. 165.

290. 1894. Beobachtungen iiber den Lichtsinn augenloser

Muscheln. Biol. Cent., Bd. 14, S. 385.

291. 1894. Experimentelle sinnesphysiologische Unter-

suchungen an Coelenteraten. Pfliigers Arch., Bd. 57,
S. 495.

292. 1894. Vergleichend physiologische und anatomische

Untersuchungen iiber d. Geruchs- und Geschmacksinn
und ihre Organe. Zoologica, Heft 18.

293. 1896. Der Lichtsinn augenloser Thiere. Jena.

Bibliography 313

294. NAGEL, W. A., 1899. Review of Loeb: Vergleichende

Gehirnphysiologie u. s. w. Zool. Cent., Bd. 6, S. 611.

295. NORMAN, W. W., 1900. Do the reactions of the lower ani-

mals against injury indicate pain sensation? Am. Jour.
Physiol., vol. 3, p. 271.

296. NUEL, J. P., 1904. La vision. Paris.

297. 1905. La psychologic compared est-elle l£gitime?

Reponse a M. Claparede. Arch, de psych., t. 5, p. 326.

298. OLTMANNS, F., 1892. Ueber die photometrischen Bewe-

gungen der Pflanzen. Flora, Bd. 75, S. 183.

299. OELZELT-NEWIN, A., 1906. Beobachtungen iiber das

Leben der Protozoen. Zeit. f. Psych, und Physiol. der
Sinnesorgane, Bd. 41, S. 349.

300. OSTWALD, W., 1903. Zur Theorie der Richtungsbewegun-

gen niederer schwimmender Organismen, I. Pfliigers
Arch., Bd. 95, S. 23.

301. 1906. Zur Theorie u. s. w., II. Ibid., Bd. Ill, S. 452.

302. 1907. Zur Theorie u. s. w., III. Ueber die Abhangig-

keit gewisser heliotropischer Reaktionen von der inneren
Reibung des Mediums, sowie u. d. Wirkung "mechan-
ischer Sensibilatoren." Ibid., Bd. 117, S. 384.

303. PARKER, G. H., 1896. The reactions of Metridium to food

and other substances. Bull. Mus. Comp. Zool., Harvard,
vol. 29, p. 105.

304. 1901. Reactions of copepods to various stimuli and

the bearing of this on daily depth migrations. Bull. U. S.
Fish Com., vol. 21, p. 103.

305. 1902. Hearing and allied senses in fishes. Ibid.,

vol. 22, p. 45.

306. 1903. The sense of hearing in fishes. Am. Nat., vol.

37, p. 185.

307. 1903. The phototropism of the mourning cloak

butterfly, Vanessa antiopa Linn. Mark Anniversary
Volume, p. 453.

308. 1903. The skin and the eyes as receptive organs in

the reactions of frogs to light. Am. Jour. Physiol.,
vol. 10, p. 28.

309. 1904. The function of the lateral-line organs in fishes.

Bull. Bureau of Fisheries, vol. 24, p. 183.

314 The Animal Mind

310. PARKER, G. H., 1905. On the stimulation of the integu-

mentary nerves of fishes by light. Am. Jour. Physiol.,
vol. 14, p. 413.

311. 1907. The interrelation of sensory stimuli in Am-

phioxus. Abstract in Jour. Comp. Neur. and Psych.,
vol. 17, p. 197.

312. PARKER, G. H., and ARKIN, L., 1901. The directive influ-

ence of light on the earthworm, Allolobophora fcetida.
Am. Jour. Physiol., vol. 5, p. 151.

313. PARKER, G. H., and BURNETT, F. L., 1901. The reactions

of planarians with and without eyes to light. Ibid.,
vol. 4, p. 373.

314. PARKER, G. H., and METCALF, C. R., 1906. The reactions

of earthworms to salts. Ibid., vol. 17, p. 55.

315. PATTEN, W., 1893. On the morphology of the brain and

sense organs of Limulus. Quar. Jour. Micr. Sci., vol.
35, p. 1.

316. PEARL, R. J., 1903. The movements and reactions of

fresh-water planarians. Ibid., vol. 46, p. 509.

317. 1904. On the behavior and reactions of Limulus in

early stages of its development. Jour. Comp. Neur. and
Psych., vol. 14, p. 138.

318. PEARL, R. J., and COLE, L. J., 1901. The effect of very

intense light on organisms. Report Mich. Acad. Sci.,
1901, p. 77.

319. PEARSE, A. S., 1906. Reactions of Tubularia crocea. Am.

Nat., vol. 40, p. 401.

320. PECKHAM, G. W. and E. G., 1887. Some observations on

the mental powers of spiders. Jour. Morph., vol. 1,
p. 383.

321. 1894. The sense of sight in spiders, with some observa-
tions on the color sense. Trans. Wis. Acad. Sciences,
Arts, and Letters, vol. 10, p. 231.

322. 1898. On the instincts and habits of the solitary

wasps. Wis. Geol. and Nat. Hist. Survey, Bull. 2.

323. 1905. Wasps, social and solitary. Boston.

324. PERRIS, E., 1850. Me*moire sur le siege de Todorat dans

les article's. Ann. sci. nat., Zool., Se"rie 3, t. 14, p. 149.

325. PIERON, C.; 1904. Du role du sens musculaire dans Torien-

Bibliography 315

tation de quelques especes de fourmis. Bull. Inst. ge"n.
psych., Paris, t. 4, p. 168.

326. PIERON C., 1904. Contribution a 1 'etude du probleme de la

reconnaissance chez les fourmis. C. r. 6e Congres inter-
nat. de Zool., p. 482.

327. 1906. Contribution a la psychologic des actinies.

Bull. Inst. ge"n. psych., Paris, t. 6, p. 40.

328. PLATEAU, F., 1885. Recherches expe*rimentelles sur la

vision chez les arthropodes. Les insectes distinguent-ils
la forme des objets? Bull. Acad. roy. Belgique, Hie
serie, t. 10, p. 231.

329. 1886. Recherches sur la perception de la lumiere par

les myriapodes aveugles. Jour, de Tanat. et de la physiol.,
t. 22, p. 431.

330. 1887. Observations sur les moeurs du Blaniulus

guttulatus. 'C. r. Soc. ent. Belgique, t. 31, p. 81.

331. 1887. Recherches experimentelles, etc., I. BuU.

Acad. roy. Belgique, t. 14, p. 407.

332. 1887. Recherches experimentelles, etc., II. Ibid.,

t. 14, p. 545.

333. 1888. Recherches expe"rimentelles, etc., III. Ibid.,

t. 15, p. 28.

334. 1888. Recherches expe*rimentelles, etc., IV. Ibid.,

t. 16, p. 159.

335. 1888. Recherches expe*rimentelles, etc., V. Ibid.,

t. 16, p. 395.

336. 1895. Comment les fleurs attirent les insectes, I.

Ibid., t. 30, p. 466.

337. 1896. Comment, etc., II. Ibid., t. 32, p. 505.

338. 1897. Comment, etc., III. Ibid., t. 33, p. 301.

339. 1899. La choix des couleurs par les insectes. Me*-

moires Soc. zool. France, t. 12, p. 336.

340. 1899. La vision chez I'Anthidium manicatum. Ann.

Soc. ent. Belgique, t. 43, p. 452.

341. 1902. Observations sur les erreurs commises par les

hymenopteres visitant les fleurs. Ibid., t. 46, p. 113.

342. PLATT, J. B., 1899. On the sp. gr. of Spirostomum, Para-

mecium, and the tadpole in relation to the problem of
geotaxis. Am. Nat., vol. 33, p. 31.

3i 6 The Animal Mind

343. POLLOCK, W. H. (with addendum by Romanes), 1883. On

indications of the sense of smell in Actiniae. Jour. Linn.
Soc. London, Zool., vol. 16, p. 474.

344. PORTER, J. P., 1904. A preliminary study of the psychology

of the English sparrow. Am. Jour. Psych., vol. 15, p.
313.

345. 1906. Further study of the English sparrow and other

birds. Ibid., vol. 17, p. 248.

346. 1906. The habits, instincts, and mental powers of

spiders, genera Argiope and Epeira. Ibid., vol. 17, p. 306.

347. POUCHET, 1872. De Tinfluence de la lumiere sur les larves

de dipteres privies d'organes exterieurs de la vision.
Rev. et mag. de zool., He serie, t. 23, pp. 110, 129, 183,
225, 261, 312.

348. POUCHET and JOUBERT, 1875. La vision chez les Cirrhipedes.

C. r. et Me"moires Soc. Biol., Vie serie, t. 2, p. 245.

349. PRENTISS, C. W., 1901. The otocyst of decapod Crustacea.

Bull. Mus. Comp. Zool., Harvard, vol. 36, p. 165.

350. PREYER, W., 1886. Ueber die Bewegungen der Seesterne.

Mitth. a. d. zool. Stat. zu Neapel, Bd. 7, S. 27, 191.

351. PRITCHETT, A. H., 1904. Hearing and smell in spiders.

Am. Nat., vol. 38, p. 859.

352. PUTTER, A., 1900. Die Reizbeantwortungen der ciliaten

Infusorien. Zeit. f. allg. Physiol., Bd. 3, S. 406.

353. 1900. Studien iiber Thigmotaxis bei Protisten.

Arch. f. Anat. u. Physiol., physiol. Abth., Supplement-
band, S. 243.

354. RADL, E., 1901. Ueber d. Phototropismus einiger Arthro-

poden. Biol. Cent., Bd. 21, S. 75.

355. 1901. Untersuchungen iiber d. Lichtreactionen der

Arthropoden. Pflugers Arch., Bd. 87, S. 418.

356. 1903. Untersuchungen iiber die Phototropismus der

Tiere. Leipzig.

357. 1905. Ueber das Gehor der Insekten. Biol. Cent.,

Bd. 25, S. 1.

358. 1906. Einige Bemerkungen und Beobachtungen iiber

die Phototropismus der Tiere. Ibid., Bd. 26, S. 677.

359. RASPAIL, X., 1899. On the sense of smell in birds. Trans.

in Smithsonian Report, 1899, p. 367.

Bibliography 317

360. RAWITZ, B., 1888. Der Mantelrand der Acephalen. Jena.

Zeit., Bd. 22, S. 415.

361. RHUMBLER, L., 1898. Physikalische Analyse von Lebens-

erscheinungen der Zelle, I. Bewegung, Nahrungsauf-
nahme, Defakation, Vacuolen-Pulsation und Gehausebau
bei lobosen Rhizopoden. Arch. f. Entwicklungsmech.,
Bd. 7, S. 103.

362. 1905. Zur Theorie der Oberflachenkrafte der Amoben.

Zeit. f. wiss. Zool., Bd. 83, S. 1.

363. RILEY, C. V., 1895. The senses of insects. Nature, vol. 52,

p. 209.

364. ROMANES, G. J., 1883. Animal intelligence. New York.

365. 1885. Jellyfish, starfish and sea-urchins. New York.w

366. 1885. Mental evolution in animals. London.

367. 1887. Experiments on the sense of smell in dogs.

Nature, vol. 36, p. 273.

368. ROMANES, G. J., and EWART, J. C., 1881. Observations

on the locomotor system of Echinodermata. Phil.
Trans. Roy. Soc., London, vol. 172, pt. 3, p. 855.

369. ROSENTHAL, J., 1905. Physiologic und Psychologic. Biol.

Cent., Bd. 25, S. 713, 741.

370. ROUBAUD, 1907. Instincts, adaptation, resistance au

milieu chez les mouches des rivages maritimes. Bull.
Inst. gen. psych., Paris, t. 7, p. 61.

371. ROUSE, J. E., 1906. The mental life of the domestic pigeon.

Harvard Psych. Studies, vol. 2, p. 580.

372. ROYCE, J., 1903. Outlines of psychology. New York.

373. RYDER, J. A., 1883. Primitive visual organs. Science,

N.S., vol. 2, p. 739.

374. SCHNADER, M., 1887. Zur Physiologic des Froschgehirns.

Pflugers Arch., Bd. 41, S. 75.

375. SCHNEIDER, G. H., 1905. Die Orientirung der Brieftauben.

Zeit. f. Psych, u. Physiol. d. Sinnesorgane, Bd. 40, S.
252.

376. SCHNEIDER, K. C., 1905. Grundzuge der vergleichender

Tierpsychologie. Biol. Cent., Bd. 25, S. 666, 701.

377. SCHRODER, C., 1901. Experimentelle Studien liber Bliiten-

besuch, besonders der Syritta pipiens L. Allg. Zeit. f.
Ent., Bd. 6, S. 181.

318 The Animal Mind

378. SCHWARZ, F., 1884. Der Einfluss der Schwerkraft auf die

Bewegungsrichtung von Chlamydomonas und Euglena.
Sitzungsber. d. deutsch. hot. Gesell., Bd. 2, S. 51.

379. SEMON, R., 1904. Die Mneme als erhaltendes Princip im

Wechsel des organischen Geschehens. Leipzig.

380. SEWALL, H., 1884. Experiments on the ears of fishes with

reference to the function of equilibrium. Jour. Physiol.,
vol. 4, p. 339.

381. SHARP, B., 1884. On the visual organs in Lamellibranchiata.

Mitth. a. d. zool. Station zu Neapel, Bd. 5, S. 447.

382. SHERRINGTON, C. S., 1906. The integrative action of the

nervous system. New York.

383. SHUFELDT, R. W., 1900. Notes on the psychology of fishes.

Am. Nat., vol. 34, p. 275.

384. SMALL, W. S., 1899. Notes on the psychic development

of the young white rat. Am. Jour. Psych., vol. 11, p.
80.

385. 1899. An experimental study of the mental processes

of the rat, I. Ibid., vol. 11, p. 133.

386. 1900. An experimental study, etc., II. Ibid., vol. 12,

p. 206.

387. SMITH, A. C., 1902. The influence of temperature, odors,

light, and contact on the movements of the earthworm.
Am. Jour. Physiol., vol. 6, p. 459.

388. SOSNOWSKI, J., 1899. Untersuchungen iiber die Verander-

ungen der Geotropismus bei Paramecium aurelia. Bull.
Internat. Acad. Sci., Cracovie, 1899, p. 130.

389. SPAULDING, E. G., 1904. An establishment of association

in hermit crabs, Eupagurus longicarpus. Jour. Comp.
Neur. and Psych., vol. 14, p. 49.

390. STAHL, E., 1878. Ueber d. Einfluss des Lichtes auf die

Bewegungserscheinungen der Schwarmsporen. Bot. Zeit.,
Bd. 36, S. 715.

391. STEINER, J., 1888. Die Functionen des Centralnerven-

systems und ihre Phylogenese, II Abth., Fische. Braun-
schweig.

392. STRASBURGER, E., 1878. Die Wirkung des Lichtes und der

Warme auf Schwarmsporen. Jena. Also in Jena. Zeit.,
n. F., Bd. 5, S. 572.

Bibliography 319

393. THORNDIKE, E. L., 1898. Animal intelligence. Psych.

Rev. Monograph Supp., vol. 2, no. 4.

394. 1899. A note on the psychology of fishes. Am. Nat.,

vol. 33, p. 923.

395. 1899. The instinctive reactions of young chicks.

Psych. Rev., vol. 6, p. 282.

396. 1899. A reply to "The nature of animal intelligence

and the methods of investigating it." Ibid., vol. 6, p. 412.

397. 1901. The mental life of the monkeys. Psych. Rev.

Monograph Supp., no. 15.

398. TIEDEMANN, F., 1815. Beobachtungen iiber d. Nerven-

system und d. sensiblen Erscheinungen der Seesterne.
Deutsches Arch. f. d. PhysioL, Bd. 1, S. 161.

399. TITCHENER, E. B., 1902. Were the earliest organic move-

ments conscious or unconscious ? Pop. Sci. Mo., vol. 60,
p. 458.

400. 1905. The problems of experimental psychology.

Am. Jour. Psych., vol. 16, p. 208.

401. TORELLE, E., 1903. The response of the frog to light.

Am. Jour. PhysioL, vol. 9, p. 466.

402. TORREY, H. B., 1904. Biological studies on Corymorpha,

I. C. palma and its environment. Jour. Exp. Zool.,
vol. 1, p. 395.

403. 1904. Habits and reactions of Sagartia davisi. Biol.

Bull., vol. 6, p. 203.

404. TOWER, W. L., 1906. Evolution in chrysomelid beetles of

the genus Leptinotarsa. Carnegie Pub.

405. TOWLE, E., 1900. A study in the heliotropism of Cypridop-

sis. Am. Jour. PhysioL, vol. 3, p. 345.

406. TREMBLEY, A., 1744. M6moires pour servir a I'histoire

d'un genre de polypes d'eau douce. Paris.

407. TRIPLETT, N. B., 1901. The educability of the perch.

Am. Jour. Psych., vol. 12, p. 354.

408. TURNER, C. H., 1906. A preliminary note on ant be-

havior. Biol. Bull., vol. 12, p. 31.

409. UEXKULL, J. VON, 1897. Ueber Reflexe bei den Seeeigeln.

Zeit. f. Biol., Bd. 34, S. 298.

410. 1897. Der Schatten als Reiz fur Centrostephanus

longispinus. Ibid., Bd. 34, S. 319.

320 The Animal Mind

411. UEXKULL, J., VON, 1900. Die Wirkung von Licht und

Schatten auf die Seeeigeln. Ibid., Bd. 40, S. 447.

412. 1900. Ueber die Stellung der vergleichender Physi-
ologic zur Hypothese der Tierseele. Biol. Cent., Bd. 20,
S. 497.

413. VASCHIDE, N., and ROUSSEAU, P., 1902. Etudes exp6ri-

mentales sur la vie mentale des animaux. Rev. scient.,
t. 19, pp. 737, 777.

414. 1903. Etudes, etc. Ibid., t. 20, p. 321.

415. VERWORN, M., 1891. Gleichgewicht und Otolithenorgan.

Pflugers Arch., Bd. 50, S. 423.

416. 1899. Psycho-physiologische Protistenstudien. Jena.

417. 1899. General physiology. Trans, by F. S. Lee.

London.

418. WAGNER, G., 1904. On some movements and reactions of

Hydra. Quar. Jour. Micr. Sci., vol. 48, p. 585.

419. WARREN, E., 1900. On the reaction of Daphnia magna

to certain changes in its environment. Ibid., vol. 43, p.
199.

420. WASHBURN, M. F., 1904. A factor in mental development.

Phil. Rev., vol. 13, p. 622.

421. WASHBURN, M. F., and BENTLEY, I. M., 1906. The estab-

lishment of an association involving color discrimination
in the creek chub, Semotilus atromaculatus. Jour. Comp.
Neur. and Psych., vol. 16, p. 113.

422. WASMANN, E., 1891. Die zusammengesetzten Nester und

gemischter Kolonien der Ameisen. Miinster.

423. 1891. Zur Frage nach den Gehorsvermogen der

Ameisen. Biol. Cent., Bd. 11, S. 26.

424. 1893. Lautausserungen der Ameisen. Ibid., Bd. 13,

S. 39.

425. 1897. Vergleichende Studien iiber d. Seelenleben der

Ameisen und der hoheren Tiere. Freiburg i. B. Trans.,
St. Louis, 1905.

426. 1899. Die psychischen Fahigkeiten der Ameisen.

Zoologica, Heft 26.

427. 1899. Instinkt und Intelligenz im Thierreich. Frei-
burg i. B. Trans., St. Louis, 1903.

428. 1900. Einige Bemerkungen zur vergleichenden

Bibliography 321

Psychologie und Sinnesphysiologie. Biol. Cent., Bd. 20,
S. 342.

429. WATKINS, G. P., 1900. Psychical life in protozoa. Am.

Jour. Psych., vol. 11, p. 166.

430. WATSON, J. B., 1903. Animal education. Univ. of Chicago

Contributions to Philosophy, vol. 4, no. 2.

431. 1907. Kinsesthetic and organic sensations: their role

in the reactions of the white rat to the maze. Psych.
Rev. Monograph Supp., vol. 8, no. 2.

432. WEIR, J., 1899. The dawn of reason. New York.

433. WELD, L. D., 1899. The sense of hearing hi ants. Science,

N.S., vol. 10, p. 766.

434. WERY, J., 1904. Quelques experiences sur Tattraction

des abeilles par les fleurs. Bull. Acad. roy. de Belgique,
pt. 1, p. 1211.

435. WHEELER, W. M., 1899. Anemotropism and other tro-

pisms in insects. Arch. f. Entwicklungsmech., Bd. 8,
S. 373.

436. 1900. A study of some Texan PonerinaB. Biol. Bull.,

vol. 2, pp. 1, 43.

437. 1903. Ethological observations on an American ant.

Jour, fur Psych, und Neur., Bd. 2, S. 31, 64.

438. WHITMAN, C. O., 1898. Animal behavior. Biol. Lectures,

Marine Biol. Lab., Wood's Hole, 1898, p. 285.

439. WILL, F., 1885. Das Geschmacksorgan der Insekten.

Leipzig.

440. WILLEM, V., 1891. Sur les perceptions dermatologiques.

Bull, scient. de la France et de la Belgique, t. 23, p.
329.

441. 1892. De la vision chez les mollusques gasteropodes

pulmones. Arch, de biol., t. 12, p. 57.

442. 1892. Les gasteropodes percoivent-ils les rayons

ultra- violets ? Ibid., t. 12, p. 99.

443. 1892. Observations sur la vision et les organ es visuels

de quelques mollusques prosobranches et opisthobranches.
Ibid., t. 12, p. 123.

444. WILSON, E. B., 1891. The heliotropism of Hydra. Am.

Nat. vol. 25, p. 413.

445. WOODWORTH, R. S., 1906. The cause of a voluntary move-

T

322 The Animal Mind

ment. No. 12 in Studies in philosophy and psychology by
former students of Charles Edward Garman. Boston.

446. WUNDT, W., 1894. Lectures on human and animal psy-

chology. Trans, by J. E. Creighton and E. B. Titchener.
London.

447. YERKES, A. W., 1906. Modifiability of behavior in Hy-

droides dianthus. Jour. Comp. Neur. and Psych., vol. 16,
p. 441.

448. YERKES, R. M., 1900. Reactions of Entomostraca to

stimulation by light, I. Am. Jour. Physiol., vol. 3, p. 157.

449. 1901. Reactions, etc., II. Reactions of Daphnia and

Cypris. Ibid., vol. 4, p. 405.

450. 1901. The formation of habits in the turtle. Pop.

Sci. Mo., vol. 58, p. 519.

451. 1902. A contribution to the physiology of the nervous

system in the medusa Gonionemus murbachii, I. The
sensory reactions of Gonionemus. Am. Jour. Physiol.,
vol. 6, p. 434.

452. 1902. A contribution, etc., II. The physiology of the

nervous system. Ibid., vol. 7, p. 181.

453. 1902. Habit formation in the green crab, Carcinus

granulatus. Biol. Bull., vol. 3, p. 241.

454. 1903. The instincts, habits, and reactions of the

frog, I. Associative processes of the green frog. Har-
vard Psych. Studies, vol. 1, p. 579.

455. 1903. The instincts, etc., II. Reaction time of the

green frog to electrical and tactual stimuli. Ibid.,
vol. 1, p. 598.

456. 1903. The instincts, etc., III. Auditory reactions of

frogs. Ibid., vol. 1, p. 627.

457. 1903. Reactions of Daphnia pulex to light and heat.

Mark Anniversary Volume, p. 361.

458. 1904. The reaction time of Gonionemus murbachii

to electric and photic stimuli. Biol. Bull., vol. 6, p.
84.

459. 1904. Space perceptions of tortoises. Jour. Comp.

Neur. and Psych., vol. 14, p. 17.

460. 1904. Inhibition and reinforcement of reactions in

the frog. Ibid., vol. 14, p. 124.

Bibliography 323

461. YERKES, R. M., 1905. Concerning the genetic relations of

types of action. Ibid., vol. 15, p. 132.

462. 1905. The sense of hearing in frogs. Ibid., vol. 15,

p. 279.

463. 1905. Animal psychology and criteria of the psychic.

Jour. Phil., Psych., and Sci. Methods, vol. 2, p. 141.

464. 1905. Bahnung und Hemmung der Reactionen auf

tactile Reize durch akustische Reize beim Frosche.
Pflugers Arch., Bd. 107, S. 207.

465. 1906. Objective nomenclature, comparative psychol-
ogy, and animal behavior. Jour. Comp. Neur. and Psych.,
vol. 16, p. 380.

466. 1906. Mutual relations of stimuli in the frog, Rana

clamata Daudin. Harvard Psych. Studies, vol. 2, p. 545.

467. 1906. Bonn's studies in animal behavior. Jour. Comp.

Neur. and Psych., vol. 16, p. 231.

468. 1906. Concerning the behavior of Gonionemus.

Ibid., vol. 16, p. 457.

469. 1907. The dancing mouse. New York.

470. YERKES, R. M., and AYER, J. B., 1903. A study of the

reactions and reaction time of the medusa Gonionemus
murbachii to photic stimuli. Am. Jour. Physiol., vol. 9,
p. 279.

471. YERKES, R. M., and HUGGINS, G. E., 1903. Habit forma-

tion in the crawfish, Cambarus affinis. Harvard Psych.
Studies, vol. 1, p. 565.

472. YUNG, E., 1892. La fonction dermatoptique chez le ver de

terre. C. r. et Trav. Soc. Helv. Sci. nat., 1892, p. 127.

473. 1893. La psychologic de 1'escargot. Ibid., 1893,

p. 127.

474. 1903. Recherches sur le sens olfactif de 1'escargot

(Helix pomatid). Arch, de Psych., t. 3, p. 1.

475. ZENNECK, J., 1903. Reagiren die Fische auf Tone? Pflu-

gers Arch., Bd. 95, S. 346.

476. ZIEGLER, H. E., 1900. Theoretisches zur Tierpsychologie

und vergleichenden Neuropathologie. Biol. Cent., Bd. 20,
S.I.

INDEX

Accommodation of lens, 201.

Acephala, chemical sense, 80 f.; hear-
ing, 107; vision, 130.

Actinia, 71, 157, 169, 181, 212 f.

Actinians, chemical sense, 69 ff.; reac-
tions to light, 123; geotropism, 157;
learning, 211, 212 f.

Adamsia, 69 ff., 212.

Adaptation, 45, 56, 210 f.

Affective qualities, in mind of Amceba,
43. See Pleasure and Unpleasant-
ness.

Aiptasia, 70 f., 190, 212.

Alburnus, 116.

Allolobophora, 78 f., 128, 171.

Amoeba, 38 ff., 121, 149.

Amphibia, chemical sense, 102; hear-
ing, 117 f.; vision, 141 f.; learning,
222, 231.

Amphioxus, 101 f., 139 f.

Amphipods, 83, 175.

Amphithoe, 83.

Anecdote, method of, 4 ff.

Anemotropism, 57, 154, 185, 188 f.,
288.

Annelids, chemical sense, 78 ff.; hear-
ing, 107; vision, 126 f., 172; geo-
tropism, 159 ff.; learning, 209 f.

Anticipatory function of ideas, 256,

273-

Ants, taste, 86; food-finding, 88 f.;
nest-finding, 90 f.; nestmates, 94 ff.;
hearing, 112 f.; vision, 90, 93, 198;
mimicry, 198; learning, 209, 211,
221, 227.

Aphaenogaster, 94.

Arachnids, chemical sense, 84 ff . ; hear-
ing, 109 f.; vision, 135 f.; learning,

2O8, 2IO.

Arbacia, 287.
Argiope, 199.
Association, 274 f.
Associative memory, 30, 205.
Astacus, 83, 109.

Attention, 49, 241, 245, 260, 282,

285 ff.
Attidae, 84.
Attus, 252.

Background, influence of, in light
reactions, 182.

Balanus, 133, 134, 176, 178-

Bees, chemical sense, 97 ff.; hearing,
113 f.; vision, 137 ff., 196 f.; learn-
ing, 257, 260 ff.

Beetles, 86, in.

Behavior, as evidence of discrimina-
tion, 60 ff.

Bembex, 263 ff.

Beroe, 74 f.

Binocular vision, 194.

Bipalium, 170, 195.

Birds, chemical sense, 102 f.; hearing,
119; vision, 142, 143 f.; learning,
119, 143, 197, 223, 227, 233, 257, 268.

Bispira, 208.

Blatta, in.

Bombyx, 88.

Branchipus, 133, 162.

Brightness, in relation to color vision,
i2i, 129, 133 f., 141, 144. 145-

Canary, 119.

Carcinus, 83, 219 f.

Carmarina, 73.

Cat, 103, 165, 235 f., 239, 271, 272,

289, 290, 292.
Caterpillars, 192, 196.
Catfish, 214.
Cebus, 197, 251.
Centrifugal force, orientation to, 54,

152, 185.

Centrostephanus, 131, 208.
Cephalopods, 130, 160.
Cerianthus, 70, 157.
Chemical sense, Amceba, 40, 42;

cceTenterates, 67 ff.; flatworms, 75 ff.;

annelids, 78 ff.; mollusks, 80 f.;

325

326

Index

echinoderms, 81; Crustacea, 82 S. ;
insects, 84 ff.; vertebrates, 101 flf.

Chemicals, effect on phototropism, 178.

Chemotropism, 40, 51, 52, 57.

Chick, 103, 143, 223, 232 f., 238, 247,
257, 293.

Chlamydomonas, 154.

Choice, as evidence of mind, 28.

Chordotonal organs, in f.

Chromotropism, 57, 129.

Cirripedia, 133.

Clepsine, 128 f.

Cockroach, in.

Coelenterates, structure, 67; chemical
sense, 67 ff.; hearing, 106 f.;
reactions to light, 123 f., 169 ff.;
geotropism, 156 ff.; localized re-
actions, 190; learning, 208, 209,
211 ff., 215.

Color, 121, 127, 128, 129, 133, 134,
135, 136, 137, 138, 140, 141, i43»
144, 145-

Convergence, 201.

Convoluta, 159, 176, 183, 287.

Copepods, 84, 162 f., 177, 183.

Corixa, in.

Corymorpha, 69, 157.

Cow, 271.

Cowbird, 143, 197, 223, 233, 257.

Crab, 83, 219 f., 247.

Crayfish, 83, 109, 135, 161, 162, 192,
220 f., 227.

Crustacea, chemical sense, 82 ff. ; hear-
ing, 108; vision, 132 ff. ; geotropism,
161; learning, 219 f.

Ctenophors, 74.

Cyclostomes, 115.

Cypridopsis, 178.

Cypris, 179.

Daphnia, 10, 133 ff., 172, 174, i75>

176 f., 178.

Density of water, effect on phototaxis,

177 f.; on geotaxis, 156, 161.
Dermatoptic sensations, 120 f., 126 ff.,

130, 136, 137, 139 f., 142, 184.
Dias, 134.
Difficulties of comparative psychology,

i ff.

Dinetus, 101.

Diptera, 172, 173, 178, 195.
Direction theory of phototaxis, 167,

173 ff-

Discrimination, evidence of, 58 ff.;

method of, 251 ff.; in attention,

291 ff.
Distance, perception of, 196, 198 ff.;

effect of stimulation from a, 275 ff.
Dog, 25, 103 ff., 165, 197, 235 f., 239,

243, 245 f., 270.
Dogfish, 1 1 6, 164.
Dreaming in animals, 15, 271.
Duration of light stimulation, effect on

phototaxis, 176.
Dytiscus, 86.

Ear, in fish, 114 f., 163 ff. ; in amphibia,
117; in reptiles, 119; in birds, 119.

Earthworm, 78 f., 107, 126 ff., 150 f.,
195, 207, 287.

Echinoderms, chemical sense, 81;
hearing, 107; vision, 131; geotro-
pism, 160 f.; learning, 208, 215 f.

Electrotaxis, 54, 57, 152, 185.

Eledone, 160.

Elephant, 239, 243.

Eloactis, 123.

Epeira, 199, 202.

Euglena, 122, 154.

Evolutionary writers, attitude toward
animal mind, 15 f.

Experiment, method of, 9 ff.

Extirpation of sense organs, 62.

Eyes, in protozoa, 122; in coelenterates,
125; in planarians, .1^6; in annelids,
126, 128; in mollusks, 130; in
echinoderms, 131; in Crustacea, 132;
in spiders, 135; in insects, 136; in
amphioxus, 140; in vertebrates, 139,
142 f.; in phototaxis, 174 f.; in
balancing, 161, 165; in spatially de-
termined reactions, 192, 193 ff.

Fatigue, 63, 65, 72, 167, 210 f., 213.

Fish, chemical sense, 102; hearing,
115 f. ; vision, 140 f.; space per-
ception, 186 ff. ; equilibrium, 163 f.;
learning, 214, 221, 248, 251 f., 257, 259.

Flagellata, 57, 122.

Flowers, attraction of insects to, 97 f.,
138- ^

Form discrimination, 196 ff.

Formica, 93.

Fovea, 200 f.

Frequency, effect of, 266 ff.

Frog, 118 f., 141 f., 195, 222, 227, 229.

Index

327

Frontal lobes, function of, 292.
Fundulus, 115 f., 187, 221.

Gasteropods, chemical sense, 81; vision,

130; learning, 215.
Gelasimus, 109, 161.
Geotropism, 57; in protozoa, 53, 154 ff.,

286; in ccelenterates, 156 ff.; in

planarians, 158 f.; in annelids, 159;

in mollusks, 160; in echinoderms, 160

f . ; in Crustacea, 161 ff . ; in spiders and

insects, 163; in vertebrates, 163 ff. ;

influence on phototropism, 162, 182 ff.,

287.

Gobius, 248, 265.
Goldfish, 115, 164 f.
Gonionemus, 73 f., 124 f., 150, 151,

158, 169, 190.
Grasping organ, significance of, 151,

280.

Grasshoppers, in.
Guinea pig, 224, 227, 228.

Habit, 244, 274 f.

Hearing, in protozoa, 106; in ccelen-
terates, 107; in planarians, 107; in
earthworm, 107; in mollusks, 107;
in echinoderms, 107; in Crustacea,
108 f. ; in spiders, 109 f. ; in insects,
no; in fish, 115 f.; in amphibia,
118 f. ; in birds, 119; in mammals,
119; as clew in maze, 227 f.

Hedista, 181.

Heliotropism, 166. See Phototropism.

Helix, 81.

Hermit crab, 82, 248 f., 259.

Hippolyte, 182.

Hovering of insects, 186, 188.

Homing, in ants, 90 ff., in bees, 98 ff .,
260 ff.; in wasps, 101; in birds, 265.

Hunger, effect on phototaxis, 178; on
food-taking, 72, 81, 211 f.; on dis-
crimination, 234.

Hydra, 67, 123, 157, 169, 208, 209, 210,

215, 216.

Hydratation, 181, 194.
Hydroides, 209 f.

Ideas, absence in Amoeba, 44 ff.; learn-
ing by, 268 f . ; evidence for, in animals,

216, 225 f., 237 ff., 249 f., 252 ff., 270
ff.; primitive function of, 256, 273 f.;
relation to stimulation from a dis-

tance, 275 ff.; of movement, 279 ff.;
attention to, 294.

Identity, sense of, 48.

Image, spatial, 193 ff.

Imitation, instinctive, 238; inferential,
23 8 ff., 271; in bird song, 119; self, 283.

Infancy, 283 f.

Inhibition, method of, 247 ff.

Insects, chemical sense, 85 ff.; hearing,
in ff.; vision, 136 ff.; anemotrop-
ism, 1 88; geotropism, 163; learning,
221, 249, 260 ff.

Instinct, inhibition of, 247 ff.; unac-
companied by ideas, 271; relation
to attention, 289, 291 ff.

Intensity of light, influence on photo-
taxis, 176.

Intensity theory of phototaxis, 167,
173 ff.

Interactionism, 18.

Interference of stimuli, 285 ff.

Interpretation, methods of, 13 ff. ;
schools of, 17 ff.; precautions in,
24 ff.

Jaculator fish, 199.
Jassa, 178.

Killifish, 115 f.

Kinaesthetic sensations, 65, 229 f.

Kinetic effect of light, 182.

Labidocera, 176, 179.

Labyrinth method, 219 ff. ; crab, 219 f. ;
crayfish, 220 f. ; ant, 221; fish, 221
f.; tortoise, 222; frog, 222; chicks,
223; birds, 223; rat, 219, 223 f.;
guinea pig, 224, 227, 228; monkey,
225; dancing mouse, 224, 229.

Lamb, 291.

Landmarks, use of, in homing, 90, 99,
139, 260 ff.

Language, animal, 3 f., 113, 118.

Lasius, 92, 94, 96, 137.

Lateral-line organs, 116 f.

Learning by experience, as criterion
of mind, 30 ff ., 205 ; forms of, 205 ff .

Leech, 78.

Leuciscus, 116.

Light, reactions to. See Phototropism,
and Vision.

Limax, 160, 178, 215.

Limulus, 85, 173.

328

Index

Lineus, 129.

Littorina, 179 f., 183, 194.

Local sign, 152.

Locality survey, 99, 262 f., 266.

Localized stimulus, reaction to, 149 £f.

Loligo, 183 f.

Lomechusa, 95.

Macacus, 196, 198, 225, 236.

Macromysis, 182.

Mammals, chemical sense, 103 ff.;
hearing, 119; vision, 144 ff.; learn-
ing, 223 ff., 233 ff., 251 ff., 257 f.

Marginal bodies, 107 f.

Mechanical stimulation, reaction to,
39 ff., 50, 53, 65, 82 ; effect on photo-
taxis, 178 ff. ; on reaction to temper-
ature, 286; on reaction to chemical
stimuli, 287.

Medusae, structure, 73; reactions,
73 ff.; static function in, 107; re-
sponse to light, 124 f.; localized
reactions, 150, 151.

Metridium, 71, 72, 123, 212 f.

Mimicry, 198.

Mind, evidence of, 27 ff.

Mollusks, chemical sense, 80 ff.; hear-
ing, 107; vision, 130; geotropism,
160; learning by experience, 208 f.,
215.

Monism, 19.

Monkey, 105, 144, 196, 197, 198, 225,
236, 237, 239, 240, 251, 257, 268,
292.

Mouse, dancing, 145, 197, 224, 229,
239, 243, 258.

Movement ideas, 279 ff.

Movement of sense organ, 203 f.

Moving stimulus, reaction to, 74, 149,
190 ff., 203, 289.

Myriapods, 136, 196.

Myrmica, 95, 96.

Mysis, 108, 161.

Nautilus, 193.
Necturus, 199.
Nest-finding. See Homing.
Nest-smell, in ants, 94 ff.; in bees,
99 ff.

Objective nomenclature, 21 ff.
Objects, perception of, 279 f.
Ophiura, 216.

Orchestia, 176, 177, 179.
Organic sensation, 3, 43, 44, 65.
Orientation, to centrifugal force, 54, 152,

185; to current, see Rheotropism;

to electric current, see Electrotaxis ;

to gravity, see Geotropism; to light,

see Phototaxis.

Orienting reactions, 53, 149, 152 ff.
Orthoptera, in.
Otocyst, 106, 107, 157.
Otolith, 106, 107.

Pagurus, 84.

Pain sensation, 65, 80, 249 f., 267.

Palaemon, 108, 161, 162.

Palaemonetes, 108, 177.

Parallelism, 17.

Paramecium, 49 ff., 150, 156, 186,

206 f., 286.

Past stimulation, effect of, 45 ff., 56 ff.,

207 ff.

Pecten, 130, 208.
Penseus, 161.
Perch, 248.
Perichaeta, 171.

Periodical fluctuations, in geotropism,

159; in phototropism, 179 ff.
Permanence of learning, 267 f.
Photopathy, 166 ff.
Phototaxis, 57, 166 ff.
Phototropism, 57, 122 ff.
Physiological condition, effect of, on

reactions, 64, 68, 71, 77, 81, 135,

175 ff., 207.
Physiologists, attitude of, toward animal

mind, 16 ff.
Pigeon, 143 f-, 197, 223, 227 f., 233,

257; carrier, 265.
Pike, 248.
Pitch discrimination, in insects, in;

in birds and mammals, 119.
Planarians, structure, 75; contact and

food reactions, 76 ff.; hearing, 107,

vision, 126; localized reactions,

150 f.; righting reaction, 158 f.;

photopathy, 167, 170; learning, 217.
Platyonichus, 161.
Play of animals, 282 f.
Pleasure, as accompaniment of positive

reaction, 43; as motive, 243, 247 ff.,

258, 260.
Pomace fly, 182.
Porthesia, 178.

Index

329

Preference method, 60, 133, 135, 137,

140.
Prepotency of vitally important stimuli,

288 ff.
Protozoa, structure and behavior,

37 ff., 49 ff.; mind, 41 ff., 54 ff.;

reactions to light, 121 f., 168 f.;

geotropism, 154 ff.; learning, 208,

214.
Putting through a movement, effect on

learning, 241, 271.
Puzzle-box method, 232 ff.; birds,

232 f.; rats, 233 f.; cats and dogs,

235; monkeys, 236; raccoons, 236 f. ;

general character, 232, 245, 260.
Pycnogonids, 135.

Raccoon, 119, 144, 196, 197, 236 f.,

242 f., 252 ff., 257, 272.
Ranatra, 179, 249, 287.
Rat, 219, 223 f., 226 ff., 233 f., 239,

240 f., 267.

Reaction time, as test of sensory dis-
crimination, 63 f., 80, 118.
Recency, effect of, 266 ff.
Repeated stimulation, effect of, 207 ff.,

266 ff.
Reptiles, chemical sense, 102; vision, 119,

142, 199 f.; " sense of support," 165,

199 ;> learning, 222 f.
Rheotropism, 53, 57, 154, 183 ff., 185.
Righting reactions, 153, 157, 158,

162.

Sagartia, 72, 123.

Salamander, 142, 199.

Sarsia, 124.

Saturnia, 87.

Scardinius, 164.

Sea-horse, 164. ,

Sea-urchin, Si, 131, 160 f., 2o8,_28j.

Self -imitation, 283.

Semicircular canals, 115.

Semotilus, 140 f., 251 f., 257, 259.

Sense organs, significance of, 59 ff.,

288 f.

Sex reactions, 84, 87 f.
Shadows, reaction to, 125, 128, 130,

Shark, 163.
Silkworm, 87.
Simocephalus, 134, 172.
Size discriminations, 194 ff.

Skioptic reactions. See Shadows.
Smell, in annelids, 78 f.; in mollusks,

81; in echinoderms, 81; in crus-

tacea, 84; in spiders, 84; in insects,

86 ff.; in vertebrates, 101 ff.; as

guide in labyrinth, 226 f.
Snail, 81, 130.
Space perception, 148, 152, 154, 166,

185, 186 ff., 191, 192, 193 ff., 202 ff.
Sparrow, English, 119, 143, 197, 223,

233» 257'» vesper, 223.
Spatially determined reactions, 148 ff.
Speed of reaction, influence on con-
sciousness, 202.
Spiders, chemical sense, 84; hearing,

109 f. ; vision, 135 f., 199, 201 f. ;

learning, 6 f., 208, 210.
Spirographis, 172.
Starfish, 81, 131, 160, 215.
Statocyst, 107, 108, 109, 153, 157 f.,

159, 160, 161 f., 163, 165 f., 216.
Statolith, 107, 108, 109, 153.
Stenamma, 95, 96, 221.
Stentor, 208, 214, 216.
Stereoscopic vision, 200 f.
Stickleback, 187 f.
Stoiachactis, 72.
Structure, as evidence of mind, 34 ff.;

as evidence of discrimination, 59 ff.
"Support, sense of," 165, 199 f.
Suspended reaction, relation to space

perception, 202; to ideas, 375 ff.;

to attention, 291 ff.
Swarm spores, 166, 170.

Talorchestia, 173.

Tealia, 71, 214.

Temora, 178.

Temperature, reaction to, 189 ; Amoeba,
39; Paramecium, 50, 56, 286; Beroe,
75; Amphioxus, 101 f.; planarians,
217; sensation, 65; influence on pho-
totaxis, 177, 179, 287.

Tetramorium, 95.

Thigmotaxis, 52 ff., 57, 78.

Tiaropsis, 124.

Tortoise, 165, 199, 222.

Touch, 48, 55, 64 ff., 67, 69, 71, 72 ff.,
76 f., 78, 82, 84, 86.

Trial and error, 167, 168, 171, 206 f.,
216 ff., 266.

Tropism, 16, 18, 57.

Tubularia, 68 f., 132.

330

Index

Ultra-violet rays, reaction to, ito, 127,
134 f., 137, 209; effect on photo-
taxis, 178.

Uniformity of reaction as evidence of
absence of consciousness, 29.

Unpleasantness, 43, 55, 207, 317, 244 f.,
249 f., 258, 260.

Useless movements, dropping off, 219
ff.; persistence of, 231.

Vanessa, 181, 184, 195.

Vertebrates, chemical sense, 101 ff.;
hearing, 114 ff.; vision, 139 ff.;
reactions to gravity, 163 ff.; learn-
ing, 214, 221 ff., 232 ff., 248 f.,
251 ff.

Virbius, 109.

Vision, in protozoa, 121 ff.; in ccelen-
terates, 123 ff.; in planarians, 126;
in annelids, 126 ff.; in mollusks, 130;
in echinoderms, 131 f.; in Crustacea,
132 ff.; in spiders, 135 f.; in myria-
pods, 136; in insects, 136 ff.; in
Amphioxus, 139 f.; in fish, 140 f.
in amphibia, 141 f.; in reptiles, 142
in birds, 143 f.; in mammals, 144 ff.
use in homing, 90, 93, 99, 139, 260 ff.
in rheotropism, 183 ff.; in labyrinth,
227 f.; binocular, 200 ff.

Volvox, 169.

Vorticella, 208.

Wasps, social, 101, 257, 262,
solitary, 101, 139, 197, 262 ff.

271;

INDEX OF NAMES

The numbers refer to the pages on which the work of the writers is cited, whether their
names appear in the text or not.

Adams, 128.
Aderhold, 154.
Allabach, 72, 213.
Allen, 224, 227, 228.
Andreae, 98.
Arkin, 128, 171.
Axenfeld, 184.
Ayer, 158.

Baldwin, 283.

Bardeen, 76, 126.

Bateson, 83, 107, 133, 140.

Beer, 17, 21, 108, 161, 201.

Bell, 83, 109, 135, 192.

Bentley, 140, 251 f., 257, 273.

Berry, 240 f.

Bert, 134.

Bertkau, 85.

Bcthe, 10, n, 17, 18, 20 f., 83, 90 ff.,

95 f., 98 f., 107, 109, 161, 163, 164,

247 f., 261.
Bigelow, 115,^164 f.
Bohn, 82, 130, 157, 159, 167, 169, 173,

175, 180 f., 182, 194.
Bouvier, 265.
Boys, 1 10.
Breuer, 163.
Bunting, 161.
Burnett, 126, 170.
Buttel-Reepen, von, 98 ff., 138!., 261 f.

Cannon, 135, 172, 174.

Carpenter, 182.

Claparede, 17, 18, 23, 265.

Cole, L. J., 170, 171, 195.

Cole, L. W., 119, 144, 196, 197, 236 f.,

242 f., 252 ff., 257, 271, 272.
Conradi, 119.
Cyon, 163.

Dahl, 109, 199, 252.

Darwin, 8, 15, 16^79, 107, 126, 127,

287.
Davenport, 135, 155, 160, 172, 174.

Dearborn, 162.
Delage, 109, 161.
Deflinger, 39.
Descartes, 13, 14-15.
Drew, 107.
Dubois, 130, 131.
Dufour, 137.

Edinger, 249.
Eigenmann, 140.
Emery, 112.
Engelmann, 106, 122.
Enteman, 262, 271.
Esterly, 162 f., 183.

Fabre, 88, 262, 264 f.

Ferton, 265.

Fielde, 91 f., 209, 221.

Fiske, 284.

Flechsig, 224.

Fleure, 72, 213.

Flourens, 163.

Forel, 17, 19, 86, 87, 92, 94, 96, 97, 113,

137, 183 f., 196 f., 203, 257.
Frandsen, 160, 178, 215.
Franz, 292.
Frohlich, 160, 161, 164.

Gamble, 159, 176, 182, 183, 287.

Garrey, 187 f.

Ghinst, von der, 157.

Giltay, 98.

Glaser, 216.

Goldsmith, 248, 265.

Goltz, 163.

Graber, 10, 60, 86 f., in, 112, 127, 140.

Groom, 176.

Groos, 282.

Gurley, 187.

Hargitt, 123, 129, 209 f., an.
Harper, 171.

Harrington,
Hensen, 108, 109.

331

332

Index of Names

Herrick, 102, 214.

Hertel, 127.

Hesse, 122, 126, 128, 193, 202, 208.

Hobhouse, 239 f., 243, 245 f.

Hoffmeister, 126, 287.

Holmes, 83, 122, 171, 173, 174, 175,

176, 177, 178, 179, 249, 287.
Holt, 174.
Hudson, 291.
Huggins, 220.
Hume, i.

James, 280.

Janet, 112.

Jennings, 39 ff., 45 ff., 50 ff., 62, 64, 121,
123, 150, 156, 168, 170, 183, 186, 206,
207, 208, 210, 211, 212, 214, 217,
244, 285 ff.

Jensen, 155.

Jordan, 17, 23, 24.

Joubert, 133.

Judd, 275.

Keeble, 159, 176, 182, 183, 287.

Kellogg, 87.

Kienitz-Gerloff, 98.

Kinnaman, 105, 144, 196, 198, 225,

236» 239, 240, 257.
Kline, 12.
Koranyi, 142.
Korner, 115.
Krause, 140.
Kreidl, 115, 162.

Learning, 121.

Lee, 115, 164, 174.

Locke, 3.

Loeb, 10, 16, 17, 30, 31, 34, 70, 72,

137, 157, 163, 164, 166, 167, 172,
173, 176, 177, 178, 183.

Lubbock, 10, 88, 89, 90, 92, 133, 137,

138, 194, 197, 260.
Lukas, 34 f.

Lyon, 155, 156, 161, 177, i86f.

McCook, 85.
Massart, 1 54 f .
Mast, 122, 168, 169, 217.
Mayer, in.
Mendelssohn, 189.
Merejkowsky, 134.
Metcalf, 80.
Mills, ii, 165,235.

Minkiewicz, 129.

Mitsukuri, 180.

Mobius, 248.

Montaigne, 13 f., 15.

Moore, 156.

Morgan, 7, n, 24 ff., 31, 108, 143, 206,

238, 247, 259, 271.
Murbach, 107, 158.

Nagel, 17, 34, 69, 72, 73, 74, 75, 79,
80, 85, 102, 106, 107, 123, 128, 130,

139, 158, 190, 193, 208, 209, 212.

Norman, 80.
Nuel, 17, 21, 22.

Oelzelt-Newin, 62.
Oltmanns, 169.
Ostwald, 176 ff.

Parker, 71, 80, 84, 102, 112 f., 115 ff.,
117, 126, 128, 139 f., 142, 162, 170,
171, 176, 177, 179, 181 f., 184, 195,

212.

Patten, 85.

Pearl, 76 f., 85, 150, 158 f.

Pearse, 69, 123.

Peckham, 5, 6, 7, 84, 101, no, 135 f.,

199, 208, 262 ff.
Perkins, 160.
Perris, 101.
Pieron, 94, 212, 221.
Plateau, 97 f., 136, 138, 184, 192, 195,

196, 199.
Pollock, 69.
Porter, 143, 197, 199, 223, 233, 257,

267.

Pouchet, 133, 172, 178.
Prentiss, 109.
Preyer, 10, 107, 215, 216.
Pritchett, 84, no.

Radl, 155, 163,. 175, 185, 186, 188.

Raspail, 103.

Rawitz, 130, 192, 208.

Rhumbler, 38 f.

Riley, 87.

Romanes, 4, 8, 9, 10, n, 16, 30, 31, 72,

81, 103 f., 107, 160 f., 199.
Roubaud, 188, 287.
Rouse, 143 *-, 197, 223, 227 f., 231, 233,

257-

Royce, 35.
Ryder, 130.

Index of Names

333

Schneider, 265.

Schwartz, 154.

Sewall, 163.

Sherrington, 191, 278.

Small, 165, 219, 225 ff., 231, 233 f.,

239, 267.

Smith, 78, 79, 127.
Sosnowski, 156.
Spaulding, 248 ff.
Steiner, 163.
Strasburger, 166, 168, 177.

Thorndike, 8, n, 12, 16, 197 f., 219,
221, 223, 232 f., 235 f., 237 ff., 251,
268, 271 f.

Tiedemann, 131.

Titchener, 23.

Torelle, 142.

Torrey, 69, 72, 157.

Tower, 112.

Towle, 178 f.

Triplett, 115, 248.

Turner, 92 f., 194.

Uexkiill, von, 17, 21, 131, 208, 287.

Vaschide, 12.

Verworn, n, 107, 154, 158, 166, 174.

Wagner, 68, 157, 208, 215.

Walton, 73, 213.

Wasmann, 17, 18 f., 22, 90 ff., 95,

113, 198, 211.
Watson, 194, 223 f., 227, 228 ff.,

234 f.
Weld, 113.
We"ry, 138.
Wheeler, 188.
Whitman, 128 f., 199.
Will, 86, 112.
Willem, 130, 192.
Wilson, 123, 169.
Woodworth, 280.
Wundt, 5 ff.

Yerkes, A. W., 210.

Yerkes, R. M., 34, 35, 64, 73 f., 118 f.,
134, 141 f., 145 ff-, 151. 158, 165,
169 f., 172, 176, 177, 179, 189, 190,

197, 199 f., 220 f., 222 f., 224, 227,

229, 239, 257 f. )

Yung, 8 1, 130.

Zenneck, 116.
Ziegler, 17, 21, 22.

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