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LIGHT AND THE BEHAVIOR
OF ORGANISMS

BY

S. O. MAST, Ph.D.

ASSOCIATE PROFESSOR OF BIOLOGY, GOUCHER COLLEGE,
JOHNSTON RESEARCH SCHOLAR, JOHNS
HOPKINS UNIVERSITY (1907-1908)

FIRST EDITION

FIRST THOUSAND

NEW YORK
JOHN WILEY & SONS

London : CHAPMAN & HALL, Limited

1911

Copyright, igio

BY

S. O. MAST

Stanbopc iPress

F. H. CrUSON COMPANY
BOSTON, U. S A

r

PREFACE

Parts I and II of this volume consist of an essay for
-hich the Cartwright Prize was awarded by the College
f Physicians and Surgeons of Columbia University in
909. The entire volume is the outgrowth of an intensive
and extensive study of the processes of orientation in plants
and animals, especially those without eyes, i.e., a study of
he perplexing and interesting question as to how these or-
canisms regulate their activities so as to bend or move
oward or from the source of stimulation. But while the
>ook deals primarily with the question of orientation, it
las a broader aspect and may be considered a treatise on
the behavior of organisms based on their reactions to light,
'he generality of the treatment of the subject of actions
1 organisms, including plants as well as animals, it is hoped
all make the work of value to all students of nature, espe-
cially to those interested in comparative psychology,
zoology, botany and physiology.

Throughout the work it has been my aim first of all to
tate precisely what organisms do under different condi-
tions of illumination, and then to consider the bearing of
the observed reactions on the various theories that have
been formulated regarding reactions in general. This aim
has made it necessary to present somewhat lengthy and
detailed descriptions of methods of stimulation and re-
sponses which, it is feared, may be rather tedious to those
who are interested only in the general aspect of the problem.
To such it will be of particular advantage to consult freely
the table of contents and the summaries.

The historical chapters which are found in Part I deal
with the origin and development of theories regarding
the activities of organisms, especially those associated with
light. No attempt has been made in these chapters to

y • • •

^ 111

iv PREFACE

review all the literature on behavior. Only such works
are here referred to as appear to have a theoretical bear-
ing, but many others are considered elsewhere.

Part II is devoted largely to the description and dis-
cussion of experimental observations on orientation made
by the author during the past five years. Only a few of
these have been previously published. The remaining
parts of the book are more general and contain relatively
much less original matter.

A large part of the experimental work connected with
this volume was done at Johns Hopkins University dur-
ing my residence as Johnston Research Scholar. To this
institution I am greatly indebted, not only for the scholar-
ship, but also for exceptional facilities placed at my com-
mand by the late Professor W. K. Brooks, Director of the
Zoological Laboratory during my residence, and for friendly
courtesies extended on every hand by other members of
the University. I am also under obligation to the Marine
Biological Laboratory of Woods Hole, Massachusetts, for
research facilities during the summer of 1907, and to the
United States Bureau of Fisheries for similar privileges
during the following two summers, and especially to the
Director of the Laboratory of the Bureau of Fisheries at
Woods Hole, Massachusetts, Doctor F. B. Sumner, for
generously supplying my needs. It is a pleasure to ac-
knowledge my further indebtedness to Professor H. S.
Jennings for his enthusiastic interest and support in the
work at all times and for critically reading the manuscript;
to Professors G. H. Parker and J. B. Watson for valuable
suggestions after reading much of the work in manuscript;
to Professor R. M. Yerkes for his thorough criticism re-
garding both composition and contents; and to my wife,
Grace Tennent Mast, for invaluable literary aid and criti-
cism. The author however must be held responsible for

all of the subject-matter. ^ ^ ,^

Samuel Ottmar Mast.

Baltimore, Maryland,
February 4, 1910.

TABLE OF CONTENTS

PART I

INTRODUCTION AND HISTORICAL REVIEW

CHAPTER I

General Introduction

CHAPTER n

Historical Review Concerning the Origin and Development of
Ideas and Theories Regarding Movements in Plants and Animals
WITH Special Reference to the Qu-estion of Tropisms

PAGE

1. Early Investigations and Ideas concerning Movement in Or-

ganisms S

2. First Attempts at Mechanical Explanation of Life Phenomena

— Galen, Harvey, Descartes, BoreUi, Ray 6

3. Period of VitaHsm 8

4. Return to Mechanical Explanations — Johannes Miiller, De

Candolle 8

5. Evolution and its Effect on the Study of Behavior in Plants

and Animals — Darwin, Bert, Graber, Romanes, Lubbock,
Preyer 9

6. Introduction of the Term " Tropism " and Development of its

Apphcation to Different Reactions — De Candolle, Knight,
Frank, Hofmeister, Darwin 11

7. Further Analysis of Reactions in Plants to Light — Sachs,

Strasburger, Engelmann, Darwin 13

CHAPTER III

Historical Review Concerning the Origin and Development of
Ideas and Theories Regarding Movements in Plants and Animals
with Special Reference to the Question of Tropisms (continued)

I, The Application of the Underlying Principle of Tropisms in
the Study of Animal Behavior as opposed to this Study from
the Point of View of Comparative Psychology — Loeb,

Verworn, Davenport, Radl and Others 23

V

Vi TABLE OF CONTENTS

2. More Thorough Experimental Analysis showing the Relative page

Importance of Internal and External Factors in Behavior —
Jennings, Holmes and Others 44

3. Summary of Historical Review 51

4. Various Definitions of Tropisms 53

5. Statement of Important Problems in the Study of Reactions

to Light 57

PART II

EXPERIMENTAL OBSERVATIONS AND DISCUSSIONS BEAR-
ING ON THE QUESTION AS TO HOW ORGANISMS
{ESPECIALLY THOSE WITHOUT EYES) BEND OR
TURN AND MOVE TOWARD OR FROM A
SOURCE OF STIMULATION

CHAPTER IV

Processes Involved in the Bending of Different Parts of Higher
Plants toward the Source of Light

I. Observations on Plumules of Indian Corn (Zea mays) and

Leaves of Tropaeolum 59

a. Introduction; b. Apparatus; c. Experiments; d. Re-
sults; e. Discussion.

CHAPTER V

Observations on Unicellular Forms in the Process of Attaining
and Retaining a Definite Axial Position with Reference to the
Source of Light

1. Myxomycetes and Rhizopods 74

2. Euglena 80

a. Description; b. Historical account; c. Orientation in
light from two sources; d. Material; e. Method of loco-
motion; /. Accuracy of orientation; g. Mechanics of
orientation in Euglena x in the crawling state; h. Dis-
cussion; i. Orientation of Euglena in the swimming
state; j. Threshold or sensitiveness when different sur-
faces are exposed to light; k. Function of the eye-spot.

3. Summary no

TABLE OF CONTENTS Vll

CHAPTER VI

Observations on Unicellular Forms' in the Process of Attaining
AND Retaining a Definite Axial Position with Reference to
THE Source of Light (continued)

PAGE

1. Stentor coeruleus 113

a. Introduction; b. Orienting reactions; c. Difference in
sensitiveness with different surfaces illuminated; d. Lo-
calized stimulation; e. Summary.

2. OeJjgonium Swarm-spores 123

a. Description; b. Material; c. Locomotion; d. Orienta-
tion in Kght.

3. Trachelomonas 128

4. Chlamydomonas alboviridis (Stein) 131

5. Chlorogonium 134

6. Paramecium i34

CHAPTER VII

The Factors Involved in the Process of Orientation in Colonial

Forms

1. Volvox globator and minor 136

2. Pandorina and Eudorina 146

a. Function of the eye-spots.

CHAPTER VIII

Observations on the Responses Involved in the Regulation of
Movement toward the Source of Light in Coelenterates

1. Hydra viridis 149

a. Historical review; b. Effect of light intensity on ac-
tivity; c. Orientation and locomotion; d. Reactions of
negative specimens; e. General conclusions.

2. Eudendrium Planulae 159

3. Eudendrium Hydranths 163

4. Reactions of Medusae 164

CHAPTER IX

Regulation in the Direction of Movement with Reference to the
Source of Light in Vermes, Fly Larvae, and Echinoderms

I. Arenicola cristata — Larvae 166

a. Description; b. Locomotion; c. Orientation; d. Me-
chanics of orientation; e. Discussion; /. Orienting
stimulation; g. Summary.

vm TABLE OF CONTENTS

PAGE

2. Blowfly Larvae — Musca sp. (?) 175

a. Introduction; b. Locomotion; c. Accuracy of orienta-
tion; d. Orientation in light from two sources; e. Orien-
tation and movement — (ij perpendicular to the direc-
tion of the rays — (2) toward a source of light; /. Sen-
sitive region; g. Effect of light intensity on rate of
locomotion; h. Method; i. Mechanics of orientation;
j. Discussion; k. Summary.

3. Earthworms : L98

Summary 205

4. Planaria 206

Summary 210

5. Echinoderms 211

CHAPTER X

Concerning the Question of Orientation in Mollusks, Arthropods
AND Vertebrates, with Special Reference to Circus Movements
• and their Bearing on this Question

1. General Account of Orientation 214

2. Circus Movements 215

3. Frogs and Toads 218

A. Bufo americanus.

a. Method; h. Orientation in light from two sources;
c. Orientation with one eye destroyed.

4. Caprella 223

a. Orientation; h. Discussion.

5. General Summary and Conclusions of Part II 228

PART III
GENERAL CONSIDERATION OF REACTIONS TO LIGHT

CHAPTER XI

Adaptation, Formation of Aggregations in Regions of a Given Light
Intensity and Different Methods of Response in Attaining
this Region and Remaining in it.

1. Introduction sho\\ing that Reactions in general are Adaptive. 236

2. Different Reactions observed in the Process of Collecting in

Regions having a given Condition of Illumination 239

TABLE OF CONTENTS ix

a. Random movements and avoiding reactions; b. Orienta- page
tion, change in sense of orientation, and avoiding re-
actions; c. Orientation and extent of movement limited
by environment; d. Orientation and movement directly
toward the place where the organism comes to rest;
e. Random movements and coming to rest in a given place.

CHAPTER XII

Reactions to Light which do not Result in Aggregation or

Orientation

1. Reactions to Shadows — Protective 247

2. Reactions to Shadows — Procuring Food 249

3. Reactions to Sudden Increase of Light Intensity 250

4. Reactions to Light caused by the Effect of Continued Illumi-

nation 252

5. Classification of Reactions to Light — Phototropism, Pho-

topathy . 253

6. Reclassification of Reactions to Light 256

(i) On the basis of the character of the stimulus.
a. Reactions to change of intensity; b. Reactions to con-
stant illumination; c. Reactions of questionable cause.
(2) On the basis of the fundamental causes of the response.
a. Reactions caused by the direct effect of light on the
reacting tissue; b. Reactions caused by an indirect effect
of light; c. Reactions due, not to any effect of light in
itself, but to what a given light condition or configura-
tion may represent.

7. Evolution of the Reactions to Light 262

CHAPTER XIII

Factors Involved in Regulating Reactions to Light — Variability

and Modifi ability in Behavior

I. Change in Sense of Reactions 265

a. Effect of intensity of light; b. Effect of change in tem-
perature — Original observations; c. Effect of chemicals
— Original observations; d. Effect of concentration of
the medium and mechanical stimuli; e. Effect of internal
changes.

X TABLE OF CONTENTS

CHAPTER XIV

Factors Involved in Regulating Reactions to Light — Variability
AND Modifiability IN Behavior (continued)

PAGE

1. Changes in Sensitiveness, in the Optimum, and in Various Other

Features regarding Reactions 288

2. General Summary of Part III 298

PART IV

REACTIONS IN LIGHT OF DIFFERENT WAVE-LENGTHS OR
; COLORS

CHAPTER XV
Energy, Photochemical Reactions, and Brightness

1. Energy Distribution in the Spectrum 304

2. Brightness Distribution in the Spectrum 305

3. Distribution of Actinic Effect in the Spectrum 308

CHAPTER XVI
Effect of Different Rays on the Reactions of Sessile Plants
I. Summary 319

CHAPTER XVII

The Relative Effect of Different Rays on the Reactions of

Unicellular Forms

1. Strasburger's Experiments 321

2. Engelmann's Experiments 322

a. Diatoms and Oscillaria with different species of Navicula
and Pinnularia as types; b. Cihates which have chlo-
rophyll with Paramecium bursaria as a type; c. Flagel-
lates with Euglena viridis as a type.

3. Verworn's Experiments 326

4. Experiments of Harrington and Leaming on Amoeba 327

5. Original Observations on Amoeba 328

a. Experiments with color filters; b. Experiments with the
solar spectrum.

TABLE OF CONTENTS XI

CHAPTER XVIII

Reactions of Multicellular Animals in Light Consisting of Waves

Differing in Length

PAGE

1. Experiments of Wilson on Hydra S33

2. Bert's Experiments on Daphnia 336

3. Lubbock's Experiments on Daphnia 337

4. Experiments of Yerkes on Simocephalus 341

5. Experiments of Graber on Various Animals 343

6. Loeb's Observations 346

CHAPTER XIX

Brief Consideration of the Reactions of Multicellular Animals
With Well-developed Eyes in Light Differing in Color — With
Special Reference to Color Vision.

1. Ants 348

2. Bees 352

3. Higher Crustacea — Experiments of Minkiewicz 355

4. Fishes 358

5. General Summary and Conclusions of Part IV 360

CHAPTER XX

Theoretic Considerations 366

Bibliography 379

Index 393

LIGHT AND THE BEHAVIOR
OF ORGANISMS

PART I

INTRODUCTION AND HISTORICAL REVIEW

CHAPTER I
GENERAL INTRODUCTION

That plants and animals respond to stimulation by
light Is a matter of common Information. It Is also well
known that many of the motile forms collect In regions
of a given Intensity of light; that many orient, some moving
or turning toward a source of light, others away from It;
and that many go toward a source of light under certain
conditions and away from It under others. The distribution
of the power to respond to stimulation by light Ii> the plant
and animal world has likewise been quite fully ascertained,^
and numerous accurate observations concerning the precise
methods of response have been recorded. There is how-
ever still much contention as to the explanation of these
phenomena, and It Is this that concerns us chiefly In this
work. In what manner and for what reasons do organisms
collect In regions of certain light Intensity? How do they

1 See Wiesner, 1879, 1881; Verworn, i88g, pp. 35-61; Nagel, 1896;
Davenport, 1897, pp. 182, 195; Radl, 1903, pp. 64-67; Washburn, 1908,
pp. 120-147; Congdon, 1908.

The works of these authors referred to by means of the dates following
each name, as well as those of all other authors similarly referred to in the
text, will be found in the bibliography.

I

2 LIGHT AND THE BEHAVIOR OF ORGANISMS

behave in light of different colors? What are the factors
involved in orientation, i.e., in attaining a definite axial
position with reference to the source of stimulation? How
do organisms regulate the direction of movement; how do
they remain oriented? What is the cause of reversal in
the sense of orientation? What controls variability and
modifiability in reactions to light? Are the reactions
adaptive? These are the principal problems before us,
problems which cannot fail to be of interest to all who are
in any way concerned with the activities of organisms.

Various solutions of these problems have been offered
by different investigators. Some say that motile plants
and animals orient and collect in light of a given intensity
because the particular intensity in which they congregate
pleases them more than any other, implying that there are
psychic phenamena involved in the process and indicating
that it is difference of light intensity in the field which
controls the direction of movement. Others say the re-
actions are not fundamentally adaptive and can be ex-
plained mechanically; that the movements of organisms
are, with few exceptions, regulated by the direction in
which the rays of light penetrate the tissue or by the angle
which the rays make with the sensitive surface or by the
relative intensity on symmetrical opposite sides. Light is
supposed by these investigators to act constantly as a
directive stimulus. The organisms are automatically con-
trolled by external factors. Still other authors claim that
the reactions to light are in general useful to the organism,
but that they can be accounted for mechanically and that
the essential controlling factor is a change of intensity on
the surface of the organism; that the other external factors
mentioned are of Importance only in so far as they make
such a change possible; that light does not act constantly
as an orienting stimulus, and that internal physiological
processes have much to do with the reactions. Some
maintain that only the more refrangible rays of the spec-
trum, those toward the violet end, are efficient in stimu-

GENERAL INTRODUCTION 3

lating the organisms, others appear to be equally positive
that all rays are active in this process, and still others say
that the stimulating efficiency of different rays varies in
different organisms and in the same organism under different
conditions.

Many investigators have apparently not thoroughly
analyzed the problems concerning reactions: To them the
question regarding orientation, e.g., has been merely: Is it
ray-direction or intensity difference that regulates this?
And with regard to this question they have failed to see
that there may be a vast difference in effect between
direction of rays in the field and direction through the
organism; between diversity in light intensity in \he field
and variation on different parts of the surface of the or-
ganism. Moreover they have failed to appreciate the
importance of difference in sensitiveness of different parts
of the reacting organism, and the consequent effect of
change in position on stirnulatlon.

An illustration will serve to emphasize the Importance
of distinguishing these characteristics. Suppose we have
an elongated opaque organism the anterior end of which
is more sensitive than the posterior, and suppose that
this organism is in a field of direct sunlight without any
other obstruction. Now It is evident that under such
conditions the Intensity In the field is uniform, but the
intensity on the Illuminated side of the organism may be
almost infinitely higher than that on its shaded side, since
no light can get through the organism, and If the organism
changes Its axial relation with reference to the ray direction,
the intensity on the surface may change just as much as it
would if the organism moved about in a field in which the
intensity was not uniform. Moreover if the organism takes
a position in which the sensitive anterior end is shaded by
the rest of the body it is of course in a lower effective inten-
sity of light than it would be If this end were illuminated.
Here again we see that a change in axial position In a field
uniformly illuminated may produce the same effect as

4 LIGHT AND THE BEHAVIOR OF ORGANISMS

movement from a region of one intensity to that of another.
And all this is dependent upon the direction of the rays in
the field whereas ray direction through the organism could
have no such effect. It is evident then that the question
" intensity difference or ray direction" may mean any one
of several things. This loose way of stating the problems
has led to much confusion.

Let us then, first of all, attempt to get a clear under-
standing of the questions involved in the reactions to light.
With this in view we shall consider the origin and develop-
ment of ideas concerning the movements in general of
plants and animals, and those induced through stimulation
by light in particular.

CHAPTER II

HISTORICAL REVIEW CONCERNING THE ORIGIN AND DE-
VELOPMENT OF IDEAS AND THEORIES REGARDING
MOVEMENTS IN PLANTS AND ANIMALS WITH
SPECIAL REFERENCE TO THE QUESTION
OF TROPISMSi

I. Early Investigations and Ideas concerning Movement in

Organisms

To primitive man motion was the criterion of life.
Everything that moved was aHve, not only plants and
animals but also various elements in nature, — water, wind,
fire, and the heavenly bodies. Motion was thought to
be under the control of higher beings, or the result of the
action of mind with which all living things were endowed.
The philosophers of early civilized races abandoned the
idea that all things which move are alive, but they still
considered that all physiological processes are due to vital
spirits. Aristotle (384-322 B.C.), thought that plants as
well as animals had souls. The pith was supposed to be
the seat of the soul in plants and all movements and other
phenomena characteristic of living things were regarded
as due to Its activity. During this period, all but a few
thinkers seemed to rest content that nothing more could
be learned about the cause or sequence of physiological
processes, and these few made only feeble attempts from a

^ The following works are the main sources of information regarding
the earlier views on plant and animal activity: History of Botany, by Julius
von Sachs (1875), translation revised by I. B. Balfour, Oxford (1890);
General Physiology, by Max Verworn (1894), translation second edition
by F. S. Lee, New York (1899); Contemporary Psychology, by Guido Villa,
translated by H. Manacorda, London, 1903.

5

6 LIGHT AND THE BEHAVIOR OF ORGANISMS

philosophical point of view at further analysis of causa-
tion. Not until the work of Galen (131-200 ± A.D.), four
hundred years later, was there anything approaching ex-
perimental analysis.

2. First Attempts at Mechanical Explanation of Life

Phenomena

Galen studied the structure of animals by direct obser-
vation, even practicing vivisection on pigs and monkeys,
and thus he sought to learn the functions of the various
organs. But others did not continue the experimental
work begun by him, and nearly thirteen centuries passed
without any progress. It was not until early in the six-
teenth century that interest in vital phenomena was again
aroused, and it was a century later before Harvey made his
important discovery on the circulatory system and pre-
sented mechanical explanations for many factors involved
in the process of circulation, all of them based on experi-
mental evidence.

A few years later, building on Descartes' idea " that the
bodies of animals and men act wholly like machines and
move in accordance with purely mechanical laws," Borelli
undertook to reduce the movements of the organic motor
apparatus to purely physical principles. The work of
Borelli formed the foundation of the iatromechanical
school, the members of which sought to explain all vital
phenomena in animals by the application of physical prin-
ciples. Other investigators of this period recognized the
importance of chemical reactions in animal activity, and,
under the leadership of Sylvius, founded the iatrochemical
school, a school which admitted the importance of physical
principles in explaining animal activity, but which strongly
emphasized the influence of chemical phenomena in vital
processes. The seventeenth century, and part of the
eighteenth, formed a period in which mechanical explana-

HISTORICAL REVIEW "J

tlons were offered for practically all reactions and other
physiological phenomena In animals, and the same may be
said with regard to plants, as will be shown in the following
pages.

Toward the close of the seventeenth century, the striking
movements of the sensitive plant, Mimosa, imported from
America, attracted considerable attention. Ray described
the movements of this plant In his " Historia Plantarum"
(1693), and although an apparent believer in the soul of
plants as defined by Aristotle, he tried to explain the move-
ments mechanically. He thought that they were due not
to sensations but to physical causes, — " Planta est corpus
vivens non sentlens." The leaves remain erect, he said,
because of the constant flow of sap Into them. When
touched, the tubes which carry the sap to them are par-
tially closed, and thus the supply of sap is diminished to
such an extent that the leaves are no longer held erect and
consequently droop. He was of the opinion, that plants
bend toward a window because of difference In rate of
growth on opposite sides due to difference In temperature.
It was known in a general way that an increase in tempera-
ture causes an Increase In the rate of growth In plants;
and Sharroc had found that the stem on which he was
experimenting grew toward that part of a window where
air entered through an opening. It was from these obser-
vations that Ray reached his conclusions.

At about the same time Dodart came to the conclusion
that physical contraction of the fibers on the moister side
of roots and their expansion on the moister side of stems
caused the former to turn down and the latter up.

Du Hamel, after studying the effect of light, temperature
and moisture on the direction of growth, concluded that
the " Richtung der Dampfe " in the vessels and around
the plant Is of prime Importance, and that if heat, light
and moisture have any influence on the direction, It Is
through their effect on the gases. Ridiculous explanations,
all of them, in the light of present knowledge! But, even

8 LIGHT AND THE BEHAVIOR OF ORGANISMS

SO, their importance cannot readily be overestimated, for
they formed the foundation of later work which led to
most fruitful results.

3. Period of Vitalism

It was fully realized before the close of the eighteenth
century that the mechanical explanations thus far pre-
sented were inadequate to account for many fundamental
phenomena at which they were directed. Especially was
this true with reference to movements of various kinds, in
both plants and animals. It led to the postulation of a
controlling principle in living beings, foreign to chemistry
and physics, a hypermechanical principle known as vital
force. Those who believed in this principle were called
vitalists. Some vitalists considered the postulated force
inscrutable, and consequently abandoned all hope of gain-
ing an insight into vital processes through experimental
means. Others, however, among the foremost of whom were
the botanist, De Candolle, and the famous physiologist,
Johannes Miiller, held the opinion that this force was
subject to further experimental analysis.

The prevalence of the former view was however un-
doubtedly the chief cause of stagnation in general physiology
in its broadest sense, during this period, for there was no
corresponding unproductive period in the development of
physical sciences. As a matter of fact many who had been
prominent investigators in both biological and physical
sciences, now abandoned the former, and devoted their
entire energies to the latter.

4. Return to Mechanical Explanations

Miiller realized the weakness of the iatromechanical
school as well as the inadequacy of pure philosophical
speculation. On the one hand he recognized the importance

HISTORICAL REVIEW 9

of speculation in guiding and unifying experimental work;
on the other he saw the necessity of founding philosophical
speculation on experimental facts. This broad view re-
sulted in much comparative work especially in physiology
and psychology, work which had a direct bearing on the
nature of psychic processes as well as on the nature of
physiological activity. Miiller worked on the higher
animals almost exclusively. His aim was to analyze the
phenomena of life as he found them in these organisms.
His followers, Wohler, Liebig, Helmholtz, du Bois-Rey-
mond, Lotze, Weber, Fechner and others, perpetuated this
aim, but they did not retain his breadth of spirit. Some
confined their investigations to the chemical side of physi-
ology, others to the physical side, and still others to pure
psychology. The question as to the origin and evolution
of vital phenomena, especially psychic phenomena, was not
yet prominent, if indeed it had been at all considered. The
behavior of lower animals had been studied to some extent,
but the Cartesian doctrine that there is no resemblance
between the mind of man and that of animals was still very
generally accepted.

5. Evolution and its Effect on the Study of Behavior of

Plants and Animals

With the establishment of the theory of evolution, there
appeared a new incentive in the study of animal behavior.
Darwin had demonstrated in a convincing manner the
structural interrelationship between various animals, in-
cluding man. It seemed clear that the complex anatomical
structures found in the higher animals had their origin
in the simpler structures found in the lower. Could the
same be said with reference to behavior? Did the mental
phenomena in man have their origin in the lower animals?
If so, then there must be some evidence of mental activity
in the lower animals, the psychic phenomena in these

10 LIGHT AND THE BEHAVIOR OF ORGANISMS

organisms must resemble those in man, and the Cartesian
doctrine must be wrong.

The importance of this problem was at once recognized
and a number of able investigators undertook its solution.
Prominent among these may be mentioned Darwin, Paul
Bert, Graber, Romanes, Lubbock and Preyer. It should
be emphasized that these investigators were not primarily
interested in explaining behavior either mechanically or
otherwise. Their principal aim was to throw light on the
origin of mental phenomena in man. Do the lower animals
have sensations? Do they have memory? Do they rea-
son? were questions which shaped their investigations.
These questions they sought to answer by studying the
behavior of animals under various conditions. Their
results seemed to indicate that the psychic phenomena in
animals differ from those in man in quantity rather than
in quality.

AVith reference to reactions to light they used what is
known as the preference method. Experimental condi-
tions were so arranged that the animals could get into
light of different intensities or different colors. The kind
of light in which they collected was supposed to be the kind
they preferred. The work was weak in that only end results
of the experiments were considered; it was never ascer-
tained precisely how the animals got into the region in
which they finally remained. Variation in the color or
in the intensity of light in the field was to these investi-
gators the controlling factor in the movement of animals.
They failed to consider the possible effects of the direction
of the rays, of variation in light intensity on the surface of
the animals, and of various internal factors. This led to
many erroneous conclusions. Still it must be said that
whatever one may think as to the point of view of these
investigators and the validity of their conclusions in general,
one cannot read with unprejudiced mind the account of
their work, especially that of Darwin, Lubbock, and
Romanes, without greatly admiring the keenness of their

HISTORICAL REVIEW II

observations and the Ingenuity of their experiments. The
point of view of these men dominated the field of animal
behavior from the middle of the nineteenth century until
the appearance of Verworn and Loeb well on toward 1890.
As has been stated, they studied the behavior of animals
with the express purpose of demonstrating the evolution
of psychic phenomena in man. These investigators w^ere
therefore not primarily interested in a physico-chemical
explanation of animal behavior.

The study of behavior in plants during this period was
however pursued with a very different aim. The question
as to the origin of mental phenomena influenced this study
but little, for it was generally conceded that plants were
devoid of all traces of psychic activity. There was con-
sequently nothing left but to attempt to account for their
behavior by means of physico-chemical analysis. Even
the vitalists realized that In the attempt of such analysis
lay the only hope of progress.

6. Introduction of the Term '' Tropism^' and Development of
its Application to Different Reactions

In 1806 De Candolle, a vltallst, succeeded in reversing
the daily periodic sleep movements of leaves by exposing
them to artificial light during the night and to darkness
during the day. The same year Knight showed by at-
taching developing seedlings to a rapidly revolving wheel
that the direction of growth of roots and stems Is regu-
lated by gravitation. He explained the directive action of
gravitation by assuming "that the root, being of a semi-
fluid consistence. Is bent downwards by its own weight,
while the nutrient sap in the stem moves to the underside
and causes stronger growth there, until by means of the
curvature so produced the stem assumes the upright posi-
tion." In 1828 Johnston found that roots In growing
downward can overcome considerable resistance and that
the direction of growth is therefore not due to their weight

12 LIGHT AND THE BEHAVIOR OF ORGANISMS

as Knight had assumed. About the same time Dutrochet
appHed endosmose and exosmose to explain the movement
of plants mechanically. In 1833 De Candolle proved that
it is light which causes plants to grow toward a window and
not difference in temperature on opposite sides as Ray had
thought one hundred forty years earlier. De Candolle
discovered that light retards growth in plants and con-
cluded that they turn toward a source of light because
growth is retarded on the more highly illuminated side.
The reaction according to De Candolle is due to difference
in intensity of light on opposite sides.

The turning toward the light was called heliotropism by
De Candolle (1835, Vol. 2, p. 609), who was, according to
Pfeffer (1906, pp. 154-155), the first to use this term. He
used it merely to indicate the exciting agency and not to
express the physiological response involved. Hofmeister
(1863, p. 86) added the terms positive and negative heli-
otropism; Frank (1870), invented the term geotropism; and
Darwin (1881), Rothert (1896) and Massart (1902) intro-
duced various special terms. While all these expressions
were at first very generally used to designate the relation
between the movement of the reacting organism and the
source of stimulation, they soon came to be used to desig-
nate also the processes underlying the reactions. De
Candolle's explanation of the reaction to light assumed a
direct effect of the external agent on the tissue involved in
the reaction; and the same was true with reference to
Knight's explanation of the reactions to gravity. The
cells in which the processes producing the curvatures took
place were supposed to be stimulated directly. The idea
of irritability, of transmission of stimuli, of a differentiation
between sensitive and reacting tissue, in plants had not
yet been promulgated. The term " tropism" then gradually
came to signify not merely turning, but turning due to the
direct effect of the stimulating agent on the tissue produc-
ing the movement, and this signification it has retained to
some extent to the present time.

HISTORICAL REVIEW 13

7. Further Analysis of Reactions in Plants to Light

Sachs was the first to point out the inadequacy of the
explanation brought forward by De Candolle. He and
others found negative as well as positive plant structures in
which the rate of growth was retarded by increase of inten-
sity of light. The bending from the source of light in these
structures could therefore not be due to difference in rate
of growth on opposite sides induced by difference in illu-
mination. Sachs was already of the opinion that gravita-
tion does not control the direction of growth in plants by
difference in the direct effect on the upper and lower sur-
faces of the reacting organ as Knight had assumed. He
says (1887, p. 696)/ " That in geotropic curvatures the
important point is only as to the direction in which gravita-
tion acts on the part of the plant, and that it is not in any
way a matter of a stronger effect on the lower side and a
feebler effect on the upper side, requires no proof." He was
profoundly impressed by the similarity between the re-
actions to light and those to gravity. This together with
the inadequacy of the explanations of De Candolle and
Knight led him to the conclusion clearly expressed in these
words (1887, p. 695): " It necessarily followed from this
that the standpoint assumed by De Candolle must be
abandoned, and that the whole subject of heliotropism
must be looked at in an entirely different way — a view
which impressed me the more, since according to all the
facts then known a striking agreement exists between
heliotropic and geotropic effects, and at the same time I
had even then come to see that geotropism and helio-
tropism are to be looked upon as phenomena of irrita-
bility. In addition to these reflections, also, I came to the
conclusion that in heliotropic curvatures the important
point is not at all that the one side of the part of the plant

* The original German edition appeared in 1882. Sachs first announced
his views on reactions to Hght in the preface of a paper by H. IMiiller in
1876.

14 LIGHT AND THE BEHAVIOR OF ORGANISMS

is illuminated more strongly than the other, but that it is
rather the direction in which the ray of light passes through
the substance of the plant;" (1882, p. 851) . . . ''dass es
sich bei den heliotropischen Kriimmungen gar nicht darum
handle, dass die eine Seite des Pflanzentheils starker als die
andere beleuchtet sei, dass es vielmehr nur auf die Richtung
ankomme, in welcher der Lichtstrahl die Pflanzensubstanz
durchsetzt."

It will thus clearly be seen that the term " ray direction,"
so frequently used to characterize Sachs' view in opposition
to intensity difference, is confusing. It expresses the truth,
but not the whole truth. Sachs did not refer to ray direc-
tion in general but to ray direction through the tissue, nor
did he oppose intensity difference in general. He had
nothing to do with the view of Bert and Graber that varia-
tion in illumination of the field regulates reaction to light.
He opposed the view of De CandoUe who states explicitly
that it is difference of intensity on opposite sides of the
reacting organ which causes heliotropic curvatures.

In the study of the reactions of sessile plants to light
there is but one phenomenon to consider — the turning of
the plant or some of its parts so as to assume a definite
position with reference to the source of light, i.e., orienta-
tion. In motile forms we have not only to deal with the
assumption of a definite axial position and movement but
we have also to deal with the phenomenon of aggregation.
How and why do certain unicellular organisms, for example,
collect in dense masses in certain regions of their environ-
ment? How is it that so many swarm spores, for instance,
collect on the side of the dish toward the source of light?
It was generally assumed that this phenomenon is due to
difference of intensity in the field, that these organisms in
some way select the illumination adapted to their needs
and remain there. But Nageli had observed as early as
i860 that flagellates and swarm spores collect at the side
of a porcelain dish nearest the window although the inten-
sity of light at this place is lower than elsewhere owing to

HISTORICAL REVIEW I 5

the shadow produced by the side of the dish. This fact led
some authors to conclude that these organisms avoid the
light, but this did not account for the fact that the swarm
spores collect also at the window side of a dish which pro-
duces no shadow and in which this part is most highly
illuminated. Cohn recognized this difficulty and con-
cluded in 1865, eleven years before Sachs announced his
ray-direction theory, that it is not difference of intensity
in the field but direction of the rays that regulates the
direction of movement in these organisms. He does not,
however, make it clear whether he means direction of the
rays through the tissue or direction in the field.

Sachs answered the question as to the cause of aggre-
gation in unicellular forms in a very simple way. He found
(1876, p. 241) that certain inanimate particles suspended
in water collect in definite regions when exposed to light
owing to currents caused by variation in temperature.
He was of the opinion that the movem.ent and aggregation
of unicellular forms under similar conditions were largely
if not entirely of the same nature.

For the express purpose of testing this opinion, Stras-
burger (1878, p. 552) studied the reactions of swarm spores
to light. He repeated the experiments of Sachs and ob-
tained confirmatory results, but concluded from detailed
microscopic observations on the movements of these organ-
isms that the aggregations formed in light under normal
conditions are almost entirely due to active swimming of
the swarm spores and not to currents in the water. Stras-
burger in this paper, however, incidentally supports the
general theory of Sachs on heliotropism. He found in
agreement with Nageli's observation (i860) that positive
swarm spores move toward a source of light even if in so
doing they pass from regions of higher light intensity into
regions of lower, and concluded just as Cohn (1865) had,
that this cannot be due to difference of intensity. He does
not however consider the fact that under the conditions of
his experiments the anterior ends of the spores were con-

1 6 LIGHT AND THE BEHAVIOR OF ORGANISMS

tinually more highly illuminated than the posterior, and
that this difference of light intensity might determine the
direction of movement; he merely states that this experi-
ment shows that the reactions are due to ray direction
without defining precisely what he means. Sachs, how-
ever, in referring to these experiments says (1887, p. 696),
" Even in the case of the influence of light on the move-
ment of swarm spores, the important point can only be as
to the direction of the rays of light, not as to whether the
given swarm spore is illuminated more strongly in front
or behind."

The excellent observations of Engelmann (1882-1883) on
the reactions of unicellular forms to light have a direct and
important bearing on the question of aggregation. Stras-
burger (1878) had observed that a sudden reduction of light
causes a definite reaction in swarm spores — " zitternde
Bewegung " — and others had seen similar responses to
sudden changes in the intensity of other stimulating agents.
But Engelmann seems to have been the first to point out
clearly the relation between such responses and aggre-
gation. He made detailed observations on the movements
of Paramecium bursaria, Euglena viridis. Bacterium photo-
metricum and other similar unicellular forms, in a field on
a slide containing a spot more highly illuminated than the
surrounding region. The illuminated spot, he says, acts
like a trap; the organisms in their random movements swim
into it without response, but when they reach the boundary
on the way out, they stop suddenly, turn back, and thus
remain in the illuminated area, which soon becomes crowded
with them. These observations are of such vital impor-
tance that it seems wise to emphasize them by quoting
directly from the author. Regarding Paramecium bur-
saria Engelmann says (1882, p. 393), " Ueberschreiten sie
z.B. zufallig die Granze von Licht und Dunkel, oder tauchen
sie auch nur mit der vorderen Halfte ihres Leibes eine
Strecke weit in das Dunkel ein, so kehren sie sofort um
nach dem Licht, wie wenn das Dunkel ihnen unangenehm

HISTORICAL REVIEW 17

ware." Referring to the reaction of Euglena in a drop
partially illuminated he says (1882, p. 395), " Dieses wirkt
wie eine Falle, denn einmal hineingekommen, gehen die
Euglenen in der Regel nicht wieder heraus. Sie kehren
an der Grenze des Dunkels immer so gleich wieder urn ins
Helle. Falls sie, was bei schnellem Vorwartsschwimmen
wohl einmal geschieht, gans ins Dunkel hineingekommen
sind, sistiren sie doch sofort die weitere Vorwartsbewegung,
drehen um eine ihres kurzen Axen, probiren — oft unter
bedeutenden Gestaltsanderungen — in verschiedenen Rich-
tungen fortzukommen bis sie endlich wieder ins Licht
gerathen." The effect of sudden reduction of light inten-
sity on Bacterium photometricum is described in the follow-
ing words (1883, p. no) : " Schwacht man nun plotzlich das
Licht ... so sieht man alle bis dahin im Gesichtsfeld
schwimmenden Bakterien fast im namlichen Moment eine
Strecke weit zuruck schiessen, einige, meist unter leb-
haftesten Rotation um ihre Langsaxe, stillstehen und
danach wieder die gewohnliche Bewegung aufnehmen.
Man erhalt vollstandig den Eindruck eines Erschreckens."

According to Engelmann none of the organisms men-
tioned above responds to an increase of intensity, nor do any
of them respond to a decrease, if it is sufficiently gradual.
The response is therefore dependent upon the time rate
of change.

Engelmann's account of aggregation in these organisms,
as far as it goes, has stood the test of time. He failed
however to grasp the importance of orientation and direct
movement toward the optimum. The reactions to sudden
changes of intensity described in this account are in all
essentials like those discovered by Jennings some fifteen
years later in his study of Paramecium. They have been
designated Schreckbewegungen by Pfeffer and motor reflex
and avoiding reaction by Jennings. They have much in
common with the reactions to shadows in many higher
forms, which Loeb (1893) claims are due to Unterschieds-
empfindlichkeit and Bohn (1908) says are due to "sensibi-

1 8 LIGHT AND THE BEHAVIOR OF ORGANISMS

lite differ entielle.'' The valuable experiments of Engelmann
on the behavior of unicellular organisms in microspectra
will be considered later (see Part IV).

Several very important contributions to the knowledge
of the reaction of plants, both in theory and in fact, were
made by Charles Darwin and his son Francis, in their
excellent work on " The Power of Movement in Plants "
(1880). (i) They made detailed observations on the move-
ment of different parts of plants in the absence of definite
external stimulations, and found that practically all parts
of plants — stems, leaves, roots, flowers, etc. — are constantly
performing circumnutation movements. From this they
concluded that tropic curvatures are brought about by
modification of movements already present, i.e., that tropic
stimuli are not the cause of movement but the cause of
modification of movement. (2) They studied the reaction
to light of plumules with the tips covered with small opaque
caps; of radicles with the tips cauterized by the application
of silver nitrate; and the reactions to gravity of radicles
with the tips removed, and found that these structures
responded normally after the tips were covered, removed
or injured, provided that they had been previously stimu-
lated, but that they did not respond if they were not stimu-
lated until after the operation. From these results they
concluded that plant-organs frequently have a sensitive
part separated by some distance from a reacting part which
is not sensitive, and that impulses originating in the former
are transmitted to the latter. (3) They studied the reac-
tions to light of certain plumules with one side covered
with an opaque substance, and of others not covered but
exposed at intervals, and concluded that the reactions are
due to difference in intensity on opposite sides but that the
principal factor in producing stimulation is a change of
intensity rather than absolute difference of intensity.

These conclusions are of such fundamental importance
that it seems advisable to insert the following quotations
from the authors' work cited above, (p. 485): "All ob-

HISTORICAL REVIEW 1 9

servers apparently believe that light acts directly on the
part which bends, but we have seen with the above described
seedlings^ that this is not the case. Their lower halves were
brightly illuminated for hours, and yet did not bend in the
least towards the light, though this is the part which under
ordinary circumstances bends the most." (p. 566), " We
believe that this case [referring to an experiment of Wies-
ner], as well as our own, may be explained by the excite-
ment from light being due not so much to its actual amount,
as to the difference in amount from that previously re-
ceived; and in our case there were repeated alternations
from complete darkness to light. In this, and in several
of the above specified respects, light seems to act on the
tissues of plants, almost in the same manner as it does on
the nervous system of animals." (p. 567), " It is an inter-
esting experiment to place caps over the tips of the cotyle-
dons of Phalaris, and to allow a very little light to enter
through minute orifices on one side of the caps, for the lower
part of the cotyledons will then bend to this side, and not
to the side which has been brightly illuminated during the
whole time." (pp. 568-569), " In the case of the radicles
of several, probably of all seedling plants, sensitiveness to
gravitation is confined to the tip, which transmits an influ-
ence to the adjoining upper part, causing it to bend towards
the center of the earth. That there is transmission of this
kind was proved in an interesting manner when horizon-
tally extended radicles of the bean were exposed to the
attraction of gravity for i or i\ h., and their tips were
then amputated. Within this time no trace of curvature
was exhibited, and the radicles were now placed pointing
vertically downwards, but an influence had already been
transmitted from the tip to the adjoining part, for it soon
became bent to one side, in the same manner as would have
occurred had the radicle remained horizontal and been
still acted on by geotropism. Radicles thus treated con-
tinued to grow out horizontally for two or three days, until
^ The tips of these were covered with opaque caps.

20 LIGHT AND THE BEHAVIOR OF ORGANISMS

a new tip was reformed; and this was then acted on by
geotropism, and the radicle became curved perpendicu-
larly downwards." ^ (pp. 572-573), " We believe that there
is no structure in plants more wonderful, as far as its
functions are concerned, than the tip of the radicle. If
the tip be lightly pressed or burnt or cut, it transmits an
influence to the upper adjoining part causing it to bend
away from the affected side; and, what is more surprising,
the tip can distinguish between a slightly harder and softer
object, by which it is simultaneously pressed on opposite
sides. If, however, the radicle is pressed by a similar
object a little above the tip, the pressed part does not
transmit any influence to the more distant parts, but bends
abruptly towards the object. If the tip perceives the air
to be moister on one side than on the other, it likewise
transmits an influence to the upper adjoining part, which
bends towards the source of moisture. When the tip is
excited by light (though in the case of radicles this was
ascertained in only a single instance), the adjoining part
bends from the light; but when excited by gravitation the
same part bends towards the center of gravity. In almost
every case we can clearly perceive the final purpose or
advantage of the several movements. Two, or perhaps
more, of the exciting causes often act simultaneously on
the tip, and one conquers the other, no doubt in accordance
with its importance for the life of the plant. The course
pursued by the radicle in penetrating the ground must be
determined by the tip; hence it has acquired such diverse
kinds of sensitiveness. It is hardly an exaggeration to say
that the tip of the radicle thus endowed, and having the
power of directing the movements of the adjoining parts,
acts like the brain of one of the lower animals; the brain
being seated within the anterior end of the body, receiving

^ This experiment was first performed by Ciesielski (1875). Darwin's
interpretation of the results has been questioned. See Francis Darwin's
interesting presentation of the controversy concerning this and related sub-
jects (1907, PP- 35-42; 69-76).

HISTORICAL REVIEW

21

Impressions from the sense-organs, and directing the several
movements."

This work of Darwin seems to have been set aside by
some of the most prominent Investigators of the day and
has even to this time not received recognition In accord
with Its importance. Loeb does not mention It at all.
Sachs refers to It In the following terms (1887, p. 689):
" In such experiments with roots not only Is great precau-
tion necessary, but also the experience of years and an
extensive knowledge of vegetable physiology, to avoid
falling Into errors, as did Charles Darwin and his son
Francis, who, on the basis of experiments which were
unskilfully made and Improperly explained, came to the
conclusion, as wonderful as it was sensational, that the
growing-point of the root, like the brain of an animal,
dominates the various movements in the root." The very
point which Sachs rejects has however been confirmed by
Pfeffer (1894), Czapek (1895, p. 244), Rothert (1894, p. 3),
and others. Czapek's experiment bearing on this point
is ingenious and convincing. He forced the apex of radicles

n

Fig. I. I. Seedlings of Lupinus albus (smaller size). The seedling (A) has
been removed from the klinostat after the apex is fixed in the glass cap k, and after
twenty-four hours has curved so as to place itself parallel with the perpendicular
line shown by the arrow. After Czapek, from Pfeffer (1906).

II. Seedlings of Setaria italica. The roots have been cut away down to the
rudiments w, the cotyledon [plumule] fixed in the glass tube a, and the seedling is
then placed horizontally. In A the hypocotyl has curved through 180°, and at
B has formed a complete coil. (Twice enlarged). After Darwin, from Pfeffer
(1906).

while being rotated on a clinostat to grow into small bent
tubes of glass closed at one end. When the seedlings were

2 2 LIGHT AND THE BEHAVIOR OF ORGANISMS

permanently fastened so that the base of the radicle was
horizontal and the tip vertical, there was no reaction,
but when so fastened that the base was vertical and the
tip horizontal, they responded by bending in the region
above the glass tube until the tip became vertical. (See
Fig. I.)

CHAPTER III

HISTORICAL REVIEW CONCERNING THE ORIGIN AND DEVEL-
OPMENT OF IDEAS AND THEORIES REGARDING MOVE-
MENTS IN PLANTS AND ANIMALS WITH
SPECIAL REFERENCE TO THE QUESTION
OF TROPISMS (continued)

I. The Application of tJie Underlying Principle of Tropisms in
the Study of Animal Behavior as opposed to this Study
from the Point of View of Comparative Psychology

Seven years after the appearance of " The Power of
Movement In Plants," by Darwin, Loeb began his work on
behavior of animals, at Wiirzburg, in an atmosphere per-
vaded by the spirit of Sachs. His first paper on the subject,
entitled " Die Orientierung der Thiere gegen das Licht
(thierischer Heliotropismus)," appeared in January, 1888.
A far more important and extensive paper bearing the
title " Der Heliotropismus der Thiere und seine Ueberein-
stimmung mit dem Heliotropismus der Pflanzen," was
brought out in pamphlet form the following year. Other
shorter papers followed from time to time. Most of these
papers, originally published in German, were translated
and published in English in Loeb's " Studies in General
Physiology," Chicago, 1905. These translations will be
referred to almost exclusively in the following pages.

Loeb took up the work in animal reactions with the idea
of explaining such reactions on chemical and physical
bases in opposition to the so-called anthropomorphic
explanations current at the time. His object was " to
find the agencies which determine unequivocally the direc-
tion of motion in animals." He writes (1905, Preface),
" I consider a complete knowledge and control of these
agencies the biological solution of the metaphysical problem

23

24 LIGHT AND THE BEHAVIOR OF ORGANISMS

of animal instinct and will." The author assumed that
these agencies had been fairly definitely ascertained with
reference to plants, and it was generally conceded that
their movements were not influenced by psychic phenomena.
He therefore began his work by attempting to show that
the reactions in plants and animals are controlled by the
same agencies, with the express purpose of proving that the
reactions of animals are not due to subjective (anthropo-
morphic) sensations as the work of Bert, Graber, Lubbock,
Romanes and others might lead one to infer. " I consider
it inadvisable," he says (1905, p. 16), ''to represent the
movements observed in animals as the expression of a
'color preference', or a 'color sensation', of a 'pleasurable'
or 'unpleasurable sensation', as do most animal physiolo-
gists and zoologists who have studied the effects of light in
the animal kingdom." (1906, p. 125), " It seemed to me
that we had no right to see in this tendency of animals to fly
into flame the expression of an emotion, but that this might
be a purely mechanical or compulsory effect of the light,
identical with the heliotropic curvature observed in plants.
I believed that the essential effect of the light upon these
animals might consist in a compulsory automatic turning
of the head toward the source of light, corresponding to the
turning of the head, or the tip, of a plant stem toward the
light; and that the process of moving toward the source of
light was only a secondary phenomenon. It seemed to me
also that if the stem of the plant could suddenly acquire
the power of locomotion, it would act exactly like the
animals which fly into the flame."

In his first paper Loeb deals with the reactions of certain
insect larvae. He found that positive larvae go toward
the light even when conditions are so arranged that in so
doing they must go into light of lower intensity. These
results lead to the following conclusions (1888, p. 2): " Die
Orientirung der Thiere gegen eine Lichtquelle wird bei den
Pflanzen (J. v. Sachs) bedingt durch die Richtung, in
welcher die Lichtstrahlen die thierischen Gewebe durchset-

HISTORICAL REVIEW 25

zen, und nicht durch die Unterschiede in der Lichtinten-
sitat auf den verschiedenen Seiten des Thieres." It is
evident from this quotation that Loeb at this time held
that the direction of the rays through the tissue is the con-
trolHng factor in orientation of animals; that is, that orienta-
tion in animals takes place just as Sachs had said it does
in plants; that it is not due to difference of intensity on
different parts of the organism, but to the direction in
which the directive rays pass through the tissue.

The results recorded in the second paper, dated 1889, are
in all essentials like those found in the first. The principal
points established are (i) that positive animals will pro-
ceed toward the window under conditions such that they
continually get into weaker light; (2) that only the more
refrangible rays are active in causing reactions. From
these results Loeb concludes as follows (1905, p. 3; first
edition, 1889): " The conditions which control the movements
of animals toward light are identical, point for point, with
those which have been shown to be of paramount influence in
plants.'' Five conditions are considered: (i) ray direction;
(2) wave length; (3) constancy of intensity; (4) limits of
intensity; (5) temperature. Two of them, the first and
the third, are of special interest to *is at present. I shall
therefore quote Loeb's words with reference to them* (1905,
p. 2), "So far as the light is concerned, the circumstance
which controls the orientation of the animal and the direc-
tion of its movements is the direction of the rays falling
upon the animal. The condition which is of importance
on the part of the animal is the symmetrical shape of the
body." It consequently appears that he, at this time,
no longer considered the direction in which the rays pass
through the tissue of the organism of special importance
but that he still regarded the direction in which they fall
upon it of importance. At the same time, however, he
accepted Sachs' theory as giving an adequate explanation
of orientation in plants and claimed that this theory also
holds for animals, for he says (1905, p. 89), "I showed

26 LIGHT AND THE BEHAVIOR OF ORGANISMS

that the law put forward by Sachs for the heliotropism
of plants, namely, that the direction of the rays of light
determines the orientation, holds good also for animals."
Elsewhere in the same paper he states this law explicitly
as follows (1905, p. 5): "Sachs came to the conclusion
that the direction in which the rays of light penetrate the
plant tissue determines the orientation of the plant toward
light." This statement of the law is correct, but it should
be emphasized that Sachs also said " that in heliotropic
curvatures the important point is not at all that the one
side of the part of the plant is illuminated more strongly
than the other." There is evidently much confusion here
in the application of Sachs' theory.

Do Loeb's conclusions in this paper show " that the law
put forward by Sachs for heliotropism of plants . . .
holds good also for animals"? He writes (1905, p. 28):
" From what has been said, no one, I believe, will doubt
that the direction of the progressive movements of the
caterpillars of Porthesia chrysorrhoea is determined by the
direction of the rays of light, and not by differences in
the intensity of the light in different parts of space. Posi-
tively heliotropic animals are compelled to turn their oral
pole toward the source*of light and to move in the direction
of the rays toward this source." And (1905, p. 53), " The
direction of the rays, and not the distribution of the intensity
of the light, in the test-tube, therefore, determines the direction
of the progressive movements.''' From these quotations it is
evident that Loeb means ray direction in general in opposi-
tion to difference in intensity in the field. He proved that
under the conditions of his experiments the direction of
motion is not governed by the difference of intensity in
the field. But this has nothing to do with Sachs' theory,
for this theory does not consider the effect of ray direction
in the field or " distribution of the intensity in the test-
tube." Sachs, as stated above, says very definitely that
it is the direction in which the rays pass through the tissue
and not difference of light intensity on opposite sides of the

HISTORICAL REVIEW 27

organism which regulates the movement. Consequently if
Loeb's explanation holds for animals and Sachs' for plants,
it is clear that the orientation in animals is not necessarily
regulated in the same way as in plants.

Sachs opposed the idea of De Candolle that difference
of intensity on opposite sides of the reacting organism con-
trols orienting reactions; while Loeb at this time opposed
the idea of Bert and Graber that difference of intensity in
the field determines the place of aggregation, and that
animals are " unterschiedsempfindlich." Sachs argued in
favor of ray direction through the tissue of the reacting
organ, Loeb in favor of ray direction in general. Failure
to recognize the difference between these views has led to
much confusion. It is on this account that the problem
has generally been so loosely stated in the terms ''Is it
ray direction or intensity difference? " — a question which
evidently cannot be answ^ered without an explicit state-
ment of the sense in which these terms are used.

Do Loeb's experimental results prove the absence of
sensations as factors in animal behavior as he assumes?
The experiments on which he bases his conclusions are
similar to those of Strasburger on swarm spores referred
to on p. 15. Loeb found that positive animals very gen-
erally move toward a source of light even if in so doing
they pass into regions of lower light intensity. He con-
cluded from this result correctly that this cannot be due
to variation in the intensity of light in the space, but
incorrectly that this disproves the existence of sensation,
for the animals with which he worked are more sensitive
to light at the anterior than at the posterior end. If they
enjoy light one would expect them to continue to face its
source even if the general illumination is decreased, be-
cause, if they should turn, the sensitive anterior end would
become shaded and this would cause a decrease in the
pleasant effect of light. The experimental results just
cited, therefore, do not prove the absence of sensation as a
controlling factor in the behavior of animals; neither do

2 8 LIGHT AND THE BEHAVIOR OF ORGANISMS

they show that It is not difference in light intensity on the
surface of the reacting organism which regulates orientation.

Let it be clearly understood that I am not arguing in
favor of psychic phenomena as factors in orientation.
Loeb's greatest service to the study of animal behavior was
his strenuous opposition to this idea, in spite of his failure
to demonstrate the absence of sensation as a factor in
reactions.

Let us now turn more directly to Loeb's later views on
orientation, or tropisms. These are clearly expressed and
explicitly stated in the following quotations. Referring
to the analogy between the effect of a constant electric
current and light, Loeb says (1897, p. 440): " Wir linden
hier erstens Wirkungen, die bei constanter Intensitat des
Lichtes unverandert andauern. Das sind die helio-
tropischen Wirkungen, die auf dem Einfluss des Lichtes
auf die Spannung assoziirter Muskelgruppen beruhen
('das Licht wirkt bei constanter Intensitat dauernd als
heliotropische Reizursache auf die Thiere') . . . Ich glaube
jetzt, dass hier eine vollkommene Analogie der Licht- und
Stromwirkungen zu Tage tritt, derart, dass auch, wie beim
Strom, die Licht-intensitdt dauernd die Spannung der
Muskeln beeinflusst, dass aber die Steilheit der Intensi-
tdtsschwankung die Fortleitung der Spannungsdndericng
bestimmt.

" Aber die Analogie zwischen der Stromwirkung und der
Lichtwirkung geht weiter. Als den fiir die heliotropische
Orientirung der Thiere wesentlichen ausseren Umstand
wies ich die Richttmg der Lichtstrahlen nach, wie das Sachs
bereits friiher fiir die Pflanzen gethan hatte. Das Wesen
der Orientirung fasste ich dahin auf, dass bei vollendeter
Orientirung Symmetriepunkte der Oberfldche des Thieres
unter gleichem Winkel von den Lichtstrahlen getroffen werden.''
An explanatory footnote (1905, p. 2), dated 1903, reads
as follows: ■" In these experiments it is presumed that the
animals move under the influence of only one source of
light. It is explicitly stated in this and the following papers

HISTORICAL REVIEW 29

that If there are several sources of Hght of unequal inten-
sity, the Hght with the strongest intensity determines the
orientation and direction of motion of the animal. Other
possible complications are covered by the unequivocal
statement, made and emphasized in this and the following
papers on the same subject, that the main feature in all
phenomena of heliotropism is the fact that symmetrical
points of the photosensitive surface of the animal must be
struck by the rays of light at the same angle. It is in full
harmony with this fact that if two sources of light of equal
intensity and distance act simultaneously upon a helio-
tropic animal, the animal puts its median plane at right
angles to the line connecting the two sources of light.
This fact was not only known to me, but had been demon-
strated by me on the larvae of flies as early as 1887, in
Wiirzburg, and often enough since. These facts seem to
have escaped several of my critics."

In these papers it is clear that the important factors in
orientation to light are considered to be: (i) symmetry of
the body; (2) the angle between the rays and the sensitive
surface on opposite sides; and (3) constant intensity,
functioning as it does in case of the electric current. Orien-
tation in light is supposed to be controlled by the direct
action of the external agent, on the locomotor tissue or
through a direct reflex arc. It is controlled unequivocally
by the external agent, which acts constantly as a directive
stimulus similar to the action of a constant electric current.

At this time Loeb evidently still placed much dependence
upon the assumed effect of the angle which the rays make
with the sensitive surface (ray direction), for if he con-
sidered merely intensity difference on opposite sides it
would be impossible for him to say as he does that when
organisms are exposed to light from " several sources . . .
of unequal intensity, the light with the strongest intensity
determines the orientation and direction of motion of the
animal." In a more recent discussion however he uses
the following expression (1906, p. 130)1 " We started w^th

30 LIGHT AND THE BEHAVIOR OF ORGANISMS

the assumption that the hehotropic reactions are caused by
a chemical effect of light; in all such reactions time plays
a role. We assume, furthermore, that if light strikes the
two sides of a symmetrical organism with unequal inten-
sity, the velocity or the character of the chemical reactions
in the photosensitive elements of both sides of the body is
different." This and the following quotation show that
he now considers orientation to be controlled by difference
of intensity on opposite sides, the very idea which Sachs
in his theory opposed.

In the following quotation he also brings out his idea as
to the direct effect of the agent on the reacting tissue with
reference to plants. Orientation in animals is supposed to
be just like this in principle; in animals, the agent is sup-
posed to affect the locomotor organs directly or through a
direct reflex arc (1906, p. 118) : '' How can light bring about
heliotropic curvatures? Let us suppose that light strikes
a plant on one side only, or more strongly on one side than
on the opposite side, and that it be absorbed in the super-
ficial layers of tissue of that side. In this case we assume
that on that side certain chemical reactions occur with
greater velocity than on the opposite side. What these
reactions are is unknown; we may think provisionally of
oxidations. This change in the velocity of chemical re-
actions either produces a tendency of the soft elements on
that side to contract a little more than on the opposite
side, or creates otherwise a greater resistance to those
forces which have a tendency to elongate or stretch the
plant, e.g., hydrostatic pressure inside the cells, or imbibi-
tion of certain tissue elements. The outcome will be that
one side of the stem will be stretched more than the oppo-
site side, and this will bring about a curvature of the stem.
Where the latter is soft at the tip, the bending will occur
only, or chiefly, in that region; and as the degree of softness
decreases rapidly from the tip downward, the result will
be that the tip will bend toward the source of light. This
result may possibly be aided by a greater photosensitive-

HISTORICAL REVIEW 3 1

ness of the extreme tip of the stem, although I am not aware
that this is an established fact."

It is strange that such a theory should have been sug-
gested to explain heliotropic curvatures in plants twenty-
six years after Darwin (see p. 18) proved that only the tips
of certain radicles and plumules are sensitive to light and
that the region where the curvature takes place is fre-
quently not at all sensitive, and several years after Pollock
(1900) had shown that traumatic stimuli are in many
instances transmitted from the tip of radicles to the motory
zone 5 to 8 mm. distant and produce curvatures toward
the uninjured side even if the cortex, the conducting tissue,
is cut on the side between the point of stimulation and the
motory zone. Moreover Loeb's theory fails utterly to
account for curvatures in structures having but a single cell
cavity as, for example, Vaucheria, the rhizoids of liver-
worts, and the hyphae of molds, all of which were known
to respond to light long before his theory was formulated.

Loeb's idea that the movements in plants and anim.als are
unequivocally controlled by external agents is emphasized
in the following quotations: (1905, p. 107), "By the help
of these causes it is possible to control the ' voluntary '
movements of a living animal just as securely and une-
quivocally as the engineer has been able to control the
movements in inanimate nature"; (1906, p. 128), "It
should be observed that the essential feature in these re-
actions is the compulsory turning of the head by the light,
which leaves the animal no choice, making all the cater-
pillars of Porthesia or all the plant lice of the same culture
behave exactly alike, just as in the case of a magnet all the
pieces of iron are compelled to behave ahke"; (1906,
p. 124), " The light would turn them automatically until
their axis of symmetry was in the direction of the rays of
light, and theanimal could then move only in this direction."

Thus we have seen that in 1906 Loeb asserts that orien-
tation in light is unequivocally controlled by the relative
intensity on symmetrically located sensitive parts of the

32 LIGHT AND THE BEHAVIOR OF ORGANISMS

organism; that light stimulates the locomotor organs con-
tinuously and directly or through a direct reflex arc. When
both sides are not equally illuminated one moves faster
than the other, causing the organism to turn until the light
intensity on the two sides is equal when they are both
equally stimulated and consequently move at the same rate.
This view he apparently still holds for he affirms it in
unquestionable terms in a recent address (1909, pp. 9-15):
" Zwei Faktoren bestimmen die Progressivbewegung der
Tiere unter diesen Bedingungen; der eine ist die symme-
trische Strukturdes Tieres und der zweite die photochemische
Wirkung des Lichtes (p. 9). . . . Wenn nun mehr Licht
auf eine Retina fallt als auf die andere, so werden auch die
chemischen Reaktionen, Beispielsweise die organischen
Oxydationen, in einer Retina mehr beschleunigt als in der
•andern; und dementsprechend werden in dem einen op-
tischen Nerven starkere chemische Anderungen auftreten als
in dem anderen (p. 11). . . . Diese Ungleichheit der che-
mischen Prozesse pflanzt sich von den sensiblen in die
motorischen Nerven und schliesslich in die mit denselben
verbundenen Muskeln fort. Wir schliessen daraus, dass bei
gleicher Beleuchtung der beiden Retinae die symmetrische
Muskelgruppe beider Korperhalften in gleicher Weiser
chemisch beeinflusst werden und somit in den gleichen
Kontractionszustand geraten ; wahrend wenn die Reaktions-
geschwindigkeit ungleich ist, die symmetrischen Muskeln
auf einer Seite des Korpers in starkere Tatigkeit geraten,
als auf der andern Seite. Das Resultat einer solchen un-
gleichen Tatigkeit der symmetrischen Muskeln beider
Korperhalften ist eine Anderung der Bewegungsrichtung
des Tieres " (p. 12).

In his earlier work Loeb appears to have held that all
reactions to light are due to constant intensity, but later
(1893, p. 265) he recognizes that some are due to change
in intensity. The former he calls heliotropic, the latter
photokinetic {unter s chieds empfindlich) . He characterizes the
difference between the two thus (1906, p. 135): " Helio-

HISTORICAL REVIEW 33

tropism covers only those cases where the turning to the
Hght is compulsory and irresistible, and is brought about
automatically or mechanically by the light itself. On the
other hand, there are compulsory and mechanical reactions
to light which are not cases of heliotropism; namely, the
reaction to sudden changes in the intensity of light." Orien-
tation is therefore, according to Loeb, never due to change
in light intensity. ''At a constant intensity light acts as
a continuous source of stimulation." When animals are
not oriented both sides are continuously stimulated but
one is stimulated more than the other. This causes one
side to move faster than the other " until symmetrically
situated points on the body of the animal are struck at the
same angle by equally strong rays of light."

In a recent paper (1907) Loeb again emphasizes this
difference between " heliotropism " and '' Vnterschieds-
empfindlichkeit.'' It is therefore evident that he was well
aware of the fact that certain animals respond to changes
in light intensity. This, however, is an old idea. As a
matter of fact it was the fundamental postulate of all who
thought that reactions are controlled by psychic phenomena.
And in his earlier work Loeb attempted to prove the ab-
sence of such phenomena, by showing that aggregation of
animals in a given light intensity is not due to difference of
intensity, i.e., that the animals are not "unterschiedsemp-
findlich.'' Later, however, he found that planarians collect
in regions of lowest intensity because they are " unter-
schiedsempfindlich "; (1907), '' Both forms of reaction may
occur in the same animal (e.g., Spirographis) , but this is
neither necessary nor the rule."

Loeb did not study the reactions of unicellular organisms
to light and it has been frequently stated that he did not
apply his theory to their reactions. Such statements, how-
ever, are erroneous as the following quotations will show:
(1905, p. 73), " Experiments on infusoria are already suffi-
ciently complete to show that Sachs's laws of heliotropism
also hold good for them. . . . Trembley's experiments on

34 LIGHT AND THE BEHAVIOR OF ORGANISMS

Hydra, however, show that in their case also the relation
is the same; at least it seems to me that Trembley's experi-
ments cannot be interpreted unless we assume that the
progressive movements of Hydra are determined by the
direction of the rays of light."

I have quoted Loeb rather freely in trying to present his
views, mainly because he and others have repeatedly main-
tained that critics have failed to understand his work,
particularly that referring to the cause of orientation and
aggregation in regions of certain intensity. These quota-
tions together with the discussion presented seem to warrant
the following summary statements concerning his work on
reactions to light.

(i) His object was to give a mechanical explanation of
behavior in opposition to so-called anthropomorphic ex-
planations of Bert, Graber and others.

(2) He proposed to do this by showing that the reactions
in animals, especially those due to stimulation by light, are
governed by the same law as those in plants.

(3) He accepted the explanation of orientation in plants
given by Sachs and states his theory correctly. Loeb's
conclusions however do not support this theory. He
confuses ray direction through the tissue with ray direction
in the field and difference of intensity on the surface of the
organism with diversity of intensity in the field.

(4) Loeb failed to consider the effect of difference in
sensitiveness to light between the posterior and anterior
ends of animals and the effect of change in axial position on
the relative illumination of these ends.

(5) His experimental evidence does not prove that the
direction of light rays functions in orientation except in so
far as it may produce difference of intensity on the surface
of the organism; nor does it prove the absence of sensation
in orientation.

(6) In 1888 Loeb held that orientation in animals is
controlled by the direction in which the rays of light pass
through the tissue. From 1889 to 1903 he advocated the

HISTORICAL REVIEW 35

idea that orientation is controlled by the direction in which
the rays strike the surface, or the angle they make with the
surface. His statements from 1906 to 1909 indicate that
he thinks that orientation is regulated by the relative inten-
sity of light on symmetrically located sensitive structures on
opposite sides of the organism, a view which Sachs strenuously
opposed.

(7) Loeb's theory of orientation with reference to plants
implies that the external agent acts on the motor apparatus
directly, and with reference to animals that it acts either on
the motor apparatus directly or through a direct reflex arc.

(8) He thinks that movements in plants and animals are
controlled unequivocally by external agents and that they
are not fundamentally adaptive. " Eine 'Auswahl' einer
passenden Beleuchtungsintensitat habe ich nie beobachtet"

(1909, p. 35).

(9) Reactions to light may be heliotropic or photokinetic.
The former are never due to change in light intensity, they
" are a function of the constant intensity; (the latter) a
function of the quotient of the change of intensity over
time," i.e., rate of change of intensity. There is a perfect
analogy between the effect of light and the effect of a
constant electric current.

(10) Aggregation in some forms is due to photokinetic
reactions.

(11) Loeb considers his theory applicable to the reactions
of the infusoria as well as to those of higher animals and
plants.

(12) He stands for an objective explanation of the be-
havior of animals in all his work, but he cannot be con-
sidered as the originator of this idea. Nor was he the first
to attempt to put it on an experimental basis.

Verworn was one of the first investigators in comparative
physiology in its broadest sense. He was of the opinion
that the fundamental physiological and psychological pro-
cesses are common to all animals and that they can be
solved in the simple forms more readily than in the more

36 LIGHT AXD THE BEHAVIOR OF ORGANISMS

complex. In this connection we are interested primarily
only in his investigations on the activities of the protozoa.
These were taken up in 1886, two years before Loeb's first
preliminary note on the reactions of animals appeared.
Verworn was probably the first to attempt an explanation
of the behavior of animals from a purely objective point of
view. In his papers many valuable observations are re-
corded on the collection of protozoa in given regions, and
on the orientation of these creatures when subjected to
stimuli of various sorts. Contrary to the idea of Loeb, he
concluded that the reactions to light are fundamentally
adaptive (1899, p. 60). His explanation of orientation is
of particular interest to us since it has frequently been
referred to in works on behavior. This he has presented
in his General Physiology^ (1899, P- 499) : "We will examine,
first, the forms that possess one fiagellum, such as many
Bacteria and flagellate I?ifusoria, and will select as repre-
sentative the delicate, green, flagellate-
infusorian Euglena, which, in summer,
^-j".^ ^.''5 by means of its countless numbers,

^^ changes the water of standing pools

into a deep green. The fiagellum of
the Flagellata is upon the anterior pole
of the body and moves through the
water in a screw-like path. For the
sake of simplicity its motion may be

considered as taking place in a single
Fig. 2. Scheme of the j^j^g_ j^ j^ ^^^^ ^^^^ ^^^^ -^ ^^^ij,
contraction of the ilagel- ^

lum of a flageilate-infus- lates about the Straight middle position

'^i^t^"s:nZ [Fig- 2I by means of alternate rhythmic

contractions toward the right (b) and
toward the left (bi) ; the swing out of the middle posi-
tion (a) into one of the two extreme positions (b or bi)
represents the phase of contraction; the return from one

1 The first edition of this volume appeared in 1894 at a time when Loeb
was emphasizing the importance of ray direction more strongly than he
did later.

HISTORICAL REVIEW 37

of the extreme positions into the middle position, the phase
of expansion. The fiagellum works, therefore, Hke an oar
that is moved alternately to the right and to the left at
the bow of a boat. It is evident that, while undisturbed and
having equal conditions upon all sides, the infusorian body
must move forward in a straight line, if the fiagellum beats
equally strongly toward the right and toward the left, i.e.,
if contraction and expansion occur with equal rapidity
toward the two sides. But if a contractile stimulus acts
upon the flagellate suddenly from one side, and if the long
axis of the body is not already turned in the direction of
the stimulus with the posterior pole toward its source, such
a position is assumed by means of a few strokes of the
fiagellum; for with every oblique or transverse position of
the long axis the fiagellum is stimulated to contract more
strongly upon the side upon which the stimulus falls than
upon the opposite side, it makes stronger strokes toward
the former than toward the latter side, and the result is
that the anterior part of the body is turned away from the
source of the stimulus. Exactly the same relations exist
here as in a boat moved by a single oar. The bow of the
boat also turns toward the opposite side when the boat is
propelled more strongly upon one side than the other.
The unequal strength of the flagellar stroke in the two
directions continues, and the anterior part of the body is
turned constantly more away from the source of the
stimulus, until the body has placed its long axis in the
direction of the incident stimulus. Then both sides of
the fiagellum become equally stimulated and the protist
swims in a straight line, so long as the stimulus continues.
Thus, negative chemotaxis, phototaxis, etc., appear in
uniflagellated Bacteria and Flagellata as a necessary result
of a unilateral excitation of contraction in the flagellum."

Orientation in forms possessing two fiagella and in forms
possessing numerous cilia is similarly explained. When an
organism of this sort is not oriented it is assumed that the
flagella or the cilia are more strongly stimulated on one side

38 LIGHT AND THE BEHAVIOR OF ORGANISMS

than on the other and that this causes them to beat more
or less effectively until the organism becomes directed
toward or from the source of stimulation, a direction it must
retain.

By careful reading of Verworn's theory, quoted above,
one is led to infer that he considered the flagella or cilia to
be stimulated directly. This, however, is not an essential
part of the theory. The essential point is that there is a
difference in the effect of the beat of the cilia on opposite
sides when these sides are differently illuminated. It does
not matter whether this is caused directly by the effect of
the stimulating agent on the cilia or indirectly through
impulses transmitted to the cilia from the body protoplasm.
An organism once oriented in accord with this theory must
remain oriented unless it is thrown out of orientation by
some other agent than that which has caused the orienta-
tion. Orientation according to this theory is direct. Light
acts constantly as a directive stimulus. Difference of in-
tensity on opposite sides of the organism causes unequal
action of the cilia on the two sides. Symmetrical location
of organs is essential in the organism.

It will thus be seen that Verworn's theory of tropisms
agrees with the theories of Loeb, especially the more recent,
in all essential points. These two authors, however, opposed
each other from the beginning. Loeb argued in favor of
ray direction, Verworn in favor of intensity difference;
neither seems to have known precisely what the other
meant. Verworn gives the following statement (1899,
p. 450) : " From the preceding consideration and by analogy
with the directive effects of other stimuli it is evident that
only the difference in the intensity of the light upon differ-
ent parts of the body can produce a directive effect; where
the stimulus acts upon the surface of the body from all
sides with equal intensity, the reason for a definite axial
position disappears, as is to be observed most clearly in the
action of chemical stimuli upon all sides. Although this is
obvious, some investigators, such as Sachs and Loeb, have

HISTORICAL REVIEW 39

believed that the direction of the rays is more responsible
for the manifestation of phototactic phenomena than are
differences in intensity. It is difficult to conceive this, for,
since the assumption of an axial direction is possible only
when differences exist at two different points of the surface
of the body, it is wholly mystical how the direction of the
rays, which is the same upon all sides of the body, can pro-
duce such an effect." Loeb is here classified with Sachs
where he claimed to belong. His experimental results and
conclusions are, however, from the beginning, more nearly
in harmony with the theory of Verworn than they are with
that of Sachs.

Verworn considers his theory applicable to orienting
reactions in unicellular forms induced by stimuli of various
kinds. He says (1899, P- 5^3) > " Thus the phenomena of
positive and negative chemotaxis, barotaxis, thermotaxis,
phototaxis and galvanotaxis which are so highly interesting
and important in all organic life, follow with mechanical
necessity as the simple results of differences in biotonus,
which are produced by the action of stimuli at two different
poles of the free-living cell."

In 1892 Oltmanns attempted to settle the dispute as to
the relative effect of ray direction and intensity difference
by studying the reactions of Volvox in an aquarium in
which the light became more intense gradually from one
end to the other. Such a distribution of light was pro-
duced by placing a hollow prism filled with a mixture of
India ink and glycerine-gelatine between the source of
light and the aquarium. The India-ink mixture of course
absorbed only a little light at the thin end of the prism, but
gradually more toward the thicker end. Under these con-
ditions the Volvox colonies collected in light of a given
intensity. Oltmanns says (1892, p. 195) that when the
prism was put between the source of light and a vessel
containing colonies which had a given direction of motion,
they changed their direction of motion almost instantly
and moved toward the region of optimum intensity. Olt-

40 LIGHT AND THE BEHAVIOR OF ORGANISMS

manns and others who used this method of producing light
of graded intensity assumed that the Hght rays in the
aquarium under such conditions were parallel with each
other and perpendicular to the side through which they
entered, and that the change in direction of motion when
the prism was put into place was due not to the direction
of the rays but to difference in light intensity. Oltmanns
does not make it clear in what sense he uses these terms.
He does not say whether he means difference of intensity in
the field or difference on the surface of the organism, ray
direction in the field or ray direction through the organism.
No matter, however, in which sense these terms were used,
the conclusion was not warranted, for it is clear from a
theoretical as well as from a practical standpoint, that the
rays of light, in the aquarium were neither parallel with each
other nor perpendicular to the side through which they
entered. The India-ink mixture contains numerous solid
particles of carbon in suspension, which, together with
particles in suspension in the water in the aquarium,
unquestionably diffuse the light in such a way that the rays
in the aquarium coming from the more highly illuminated
end are more numerous than those coming from the other
end, and so if the direction of the rays were the control-
ling factor one might expect the organisms to go toward
either end.

After reviewing the work of the preceding authors and
presenting some original experiments similar in method to
those of Strasburger, Davenport (1897) agrees with Loeb
in assuming two dissimilar sorts of locomotor responses
to light. These he designates phototaxis and photopathy.
Phototaxis he defines " as migration in the direction of the
light rays, and photopathy as migration toward a region
of greater or less intensity of light." He accepts the theory
of orientation as outlined by Sachs and formulates another
which is in all essentials like that of Loeb. He says (p. 209) :
" Let us first think of the way in which light acts on the
negatively phototactic (and photopathic?) earthworm.

HISTORICAL REVIEW 4 1

Represent the worm by an arrow whose head Indicates the
head end [Fig. 3, ^]. Let solar rays 55 fall upon it
horizontally and perpendicularly to its axis. Then the

S ii

Low Light Attunetnent

A <- .

Low Light Attunem.ent

Fig. 3. Diagram representing sunlight (SS) falling upon an elongated, bilateral
organism (represented by the arrow) whose head is at ^. After Davenport (1897,
p. 209).

impinging ray strikes it laterally, or, in other words, it is
illuminated on one side and not on the other. Since, now,
the protoplasm of both sides is attuned to an equal intensity
of light, that which is the less illuminated is nearer Its
optimum intensity. Its protoplasm is in a phototonic con-
dition. That which is strongly Illuminated has lost its
phototonic condition. Only the darkened muscles, then,
are capable of normal contraction; the brightly illuminated
ones are relaxed. Under these conditions the organism
curves towards the darker side; and since its head region
is the most sensitive, response begins there. Owing to a
continuance of the causes, the organism will continue to
turn from the light until both sides are equally illumi-
nated ; i.e. until it is in the light ray. Subsequent
locomotion will carry the organism in a straight line, since
the muscles of the two sides now act similarly. Thus
orientation of the organism is effected. The same ex-
planation . . . will account, mutatis mutandis, for positive
phototaxis."

It Is evident that this theory assumes a direct effect of
the stimulating agent on the locomotor organs. Daven-
port thus claims that orientation may be brought about in
two ways: " Light acts directly either through difference
in intensity on the two sides of the organism, or by the

42 LIGHT AND THE BEHAVIOR OF ORGANISMS

course the rays take through the organism " (p. 210). He
assumes that changes of Hght intensity do not result in
orientation but that stimulation caused by such changes
may determine the position of organisms in the field in
some such way as described by Engelmann. He says
(p. 211), " Two kinds of effects are produced by light:
one by the direction of its ray — phototactic; the other by
the difference in illumination of parts of the organism —
photopathic."

Holt and Lee (1901) studied the behavior of Stentor
coeruleus in an aquarium receiving light through a prism
similar to the one used by Oltmanns, and found that the
animals collected at the darker end. In conclusion they
support Verworn's theory; but from the preceding dis-
cussion of the effect of the prism on the direction of
rays it is evident that the validity of this conclusion is
questionable.

Radl's work on reactions to light was almost entirely
confined to the Crustacea and insects. In 1903 after a
rather extensive review and criticism of the results and
theories of others, and an exposition of his own work, he
arrived at two conclusions which are of interest in this
connection. One has reference to the mechanics of orien-
tation, the other to the explanation of negative reactions.

His theory of orientation is based on the conception that
change in the direction of motion is brought about by
unequal stimulation of symmetrical points on the surface
of the organism, a conception which lies at the foundation
of all the theories thus far presented, excepting that of
Sachs and the first one of Loeb. While all of these differ
in some respects, they are alike in that they assume the
external agent to act through the effect of chemical changes
in the organism. Radl proposes to explain orientation as
the direct effect of light on the organism. He says (1903,
p. 151) : " Alle Autoren, welche bisher dieses Thema beriihrt
haben, haben an indirekte Wirkungen des Lichtes gedacht,
dass namlich durch dasselbe chemische Veranderungen

HISTORICAL REVIEW 43

hervorgerufen werden, welche erst die Reaktionen des
Organismus direkt beelnflussen. . . . Gegeniiber diesen
Anschauungen mochte ich das Problem des Phototropismus
als direkte Wirkung des Lichtstrahls auf den Organismus
auflfassen. Wenn wir namlich konsequent unsere x\uffas-
sung der Orientierungserscheinungen durchfiihren woUen,
so miissen wir auch den Phototropismus als Folgeerschein-
ung aus dem Spiel zweier Krafte, einer ausseren und einer
inneren auffassen — ich bemiihe mich wenigstens umsonst
mir vorzustellen, dass die Sache anders sein konnte. Die
aussere Kraft ist in diesem Falle der Lichtstrahl; derselbe
muss eine Druckkraft auf den Organismus ausiiben, ich
glaube eine ahnliche Druckkraft, wie auf uns etwa der
Luftstrom driickt. Diese Vorstellung scheint recht phan-
tastisch zu sein, ich sehe jedoch keinen anderen Ausweg.
Es ist nicht notig, dass dieser Druck gross sei, er kann sehr
fein sein, aber ein Druck, welcher eine Richtung hat, muss
es sein, wenn iiberhaupt eine Orientierung, eine Drehung
entstehen kann."

The maximum pressure of direct sunlight having an
intensity of 5000 ± candle meters is only 0.4 mg. on one
square meter of black surface, and only twice as great on
an equal area of reflecting surface. According to this
theory then, an organism responding to o.i candle meter
would have to be stimulated by light not to exceed 0.000016
mg. In view of this fact it is not likely that this theory
will ever be seriously considered. It has been presented
here merely as a matter of historical interest.

Radl's view as to the difference between positive and
negative reactions is equally untenable. He concludes his
discussion on this subject with the following paragraph
(1903, p. 103): '*Ich glaube nun, dass der Unterschied
zwischen positivem und negativem Phototropismus ahnlich
wie beim Menschen nicht ein Unterschied in der Orien-
tierung, sondern nur in der Lokomotion ist; dass das Tier
in beiden Fallen gegen die Lichtquelle gleich orientiert ist,
jedoch nicht gleiche Muskeln spannt." It is of course

44 LIGHT AND THE BEHAVIOR OF ORGANISMS

well known that contrary to Radl's conclusion, most of
the organisms which face the source of stimulation when
positive, turn and face in the opposite direction when
negative.

2. Afore thorough Experimental Analysis Showing the Rela-
tive Importance of Internal and External Factors in Behavior

None of the investigators thus far mentioned studied the
behavior of lower organisms in sufficient detail to be able
to tell from direct observation precisely what takes place
in the reactions. It was well known from direct observa-
tion that many of these organisms form dense aggregations
under certain conditions and that they frequently orient
when subjected to certain stimuli; but just what takes
place during the process of aggregation and orientation was
with a few exceptions known only theoretically.

Jennings was the first to supply this deficiency in obser-
vation. He began his investigations on this subject in
1897 by working out in minutest detail precisely what
movements are involved in the formation of the dense
aggregations so frequently seen in cultures containing
paramecia. His work differs from that of his predecessors
in this line largely in that, while they, with the possible
exception of Engelmann, studied mass movements and end
results, he studied the individuals. He was interested
not so much in the aggregations as in the process of their
formation. How does each individual get there? and
why does it stay there? were prominent questions in his
mind.

The observations on the formation of aggregations of
paramecia were followed by similar observations on the
reactions of representative species of the various groups of
protozoa and lower metazoa to various sorts of stimuli.
All of this work is characterized by unity of purpose, keen-
ness of observation and simplicity of method.

The results of all of Jennings' work, published in nu-

HISTORICAL REVIEW 45

merous papers, were brought together and systematized in
the well known book on the " Behavior of Lower Organ-
isms " (1906). I shall refer to this book almost exclusively
in trying to present his views on the factors involved in the
phenomena in which we are especially interested — aggre-
gation in regions of given light intensity, orientation and
change in sense of reaction.

Aggregation in a region having a given light intensity
may be formed, according to Jennings, in either of two ways.
(i) The organisms get into the region just as they would
into any other region, merely by swimming about in an
aimless manner, without orientation and without direct
movement toward the region. When they get to the limit of
the region and are about to pass out into light of a different
intensity the sudden change to which they are subjected
produces a stimulation which causes a definite reaction.
This reaction consists chiefly In a sudden turn toward
a given side, frequently after backing some distance, and
procedure on a new course. They respond with this
reaction every time they come to the edge of the region and
therefore remain In this region. Other individuals behave
in the same way and this results In an aggregation. " Motor
reflex " was the first term applied to this method of reaction
with Its various modifications; later it was designated
" motor reaction," and finally " avoiding reaction." The
essential feature In the avoiding reaction is the fact that
the organism always turns toward the same side regardless
of the place of application of the stimulus. The side toward
which it turns is determined by Internal factors. Thus It
is that the direction of turning bears no definite relation to
the position of the source of stimulation. The organism
may turn directly toward it or away from It or at any
angle to It. The method of aggregation thus described by
Jennings for Paramecium is in all essentials like that de-
scribed by Engelmann in 1882 and 1883 for Paramecium
bursaria, Euglena, Bacterium photometrlcum and other
organisms.

46 LIGHT AND THE BEHAVIOR OF ORGANISMS

(2) In place of getting into regions of a given light in-
tensity by mere wandering movements, organisms may
orient and move directly toward such regions, and the
avoiding reaction may keep them in this region just as
described above, or they may remain because it is illumi-
nated by light of optimum intensity. If they get into
light of lower intensity they become positive and return
to the optimum directly after becoming oriented. If they
get into light of higher intensity they become negative
and orient in the opposite direction, which again causes
them to return to the optimum intensity. The organ-
ism usually tries numerous positions before it becomes
oriented. Many errors are made before the successful posi-
tion is attained; many directions of motion are tried; one
is selected. Jennings has designated this method of orien-
tation as orientation by " trial and error," or more recently
merely by ''trial." Some seem to be of the opinion that the
trial movements are haphazard movements, that they are
not definitely determined. In answer to this Jennings says
(1906a, p. 452): ''The behavior may perhaps be most
accurately characterized as ' selection from among the
conditions produced by varied movements.' In general we
find that many organisms are so constituted that internal
conditions (permanent or temporary) will produce under
stimulation movements that are varied in precisely such
a way as to subject the creature to as varied environmental
conditions as possible, and thus give it an opportunity to
select what is nearest the optimum. Every one of these
movements is, of course, as absolutely determined as the
most orthodox tropism, only the determining factor is not
the localization of the stimulus (or other external factor)
alone.

" Certain recent writers have seemed to imply that there
is a contrast between the 'trial and error' method, and
behavior that is definitely determined by structural and
other internal conditions. It needs to be emphasized,
perhaps, that the behavior which I and others have char-

HISTORICAL REVIEW 47

acterized by this phrase is very precisely determined by
structural and other internal conditions; indeed, its dis-
tinguishing feature is the fact that it is thus determined by
such conditions, rather than exclusively by the external
conditions."

Jennings places particular emphasis on the idea that
" activity does not require present external stimulation."
This is an idea of which Darwin made much in his work on
movement in plants. To explain orientation, Darwin said,
we do not need to account for movement; it is only neces-
sary to account for change in the direction of movement.
Jennings applies this idea to the orientation of animals.
The animals are in motion; the question is, how is the
direction of motion regulated so as to result in orientation?
He says that in many of the infusoria it is regulated by
means of the avoiding reaction. "This reaction" (1906,
p. 79) "consists in successively 'trying' not only different
directions of locomotion, but also different positions of the
body axis. As soon therefore as a position is reached in
which the disturbance causing the reaction no longer exists,
the reaction of course stops; the animal therefore retains
this axial position."

It will thus be seen that orientation in these forms is,
according to Jennings, not brought about by a direct turn-
ing of the anterior end of the body toward or away from
the source of stimulation. It is not due to unequal stimu-
lation of points symmetrically situated on the body; the
external agent does not act constantly as a directive stimu-
lus. " The position of orientation is not one in which a
median plane of symmetry takes up a definite position
with reference to the external agent." Not all reactions
resulting in orientation are however of this sort. Many
organisms have the power of turning directly toward or
aw.ay from the side stimulated; in these orientation may
take place directly, as Jennings clearly states in the follow-
ing words (1906, p. 271), " In the symmetrical Metazoa
we of course find many cases in which the animal turns

48 LIGHT AND TH'E BEHAVIOR OF ORGANISMS

directly toward or away from the source of stimulation,
without anything in the nature of preliminary trial move-
ments." Reactions which show a definite relation to the
localization of the stimulus '' include perhaps the greater
number of the directed movements of the organisms."

It is evident, judging from these quotations, that Jen-
nings does not hold that all organisms orient by means of
avoiding reactions. He does not oppose the idea of direct
orientation by means of differential response to localized
stimulation. He opposes the view that this is the only
method of orientation and the view that orientation is
caused by the direct effect of the external agent on the
locomotor organs. He holds that the power of differential
response to localized stimulation is derived from other
methods of reaction, as described in the following quota-
tions and abstracts (1906, pp. 306-308): "First we have
the simple phenomenon that when a portion of an organism
is stimulated this portion may respond by contraction,
extension, or other change of movement." Such local
reponses to local stimulation we find in Amoeba, Hydra,
Sagartia, flatworms and many other soft-bodied animals,
and even in man when the electrode of a batter}^ is applied
directly over a muscle. " In many cases we find that the
relation of the movement to the source of stimulation is
brought about indirectly through selection from among
varied movements. The organism tries moving in many
directions, till it finds one in which there is no stimulus to
further change. ... In still other cases the reaction shows
a definite relation to the localization of the stimulus, yet
it is not due to local reaction of the part stimulated,
nor is it brought about by trial. If an infusorian is stimu-
lated at the anterior end it swims backward; stimulated
at the posterior end it swims forward. Both these move-
ments are reactions of the entire organisms, all the motor
organs of the body concurring to produce them; they are
not produced by local reactions of the organs at one end
or the other. . . . Such behavior apparently represents

HISTORICAL REVIEW 49

not a primitive condition, but a product of development."
" To a change leading away from the optimum (in either
plus or minus direction)" the organism responds in such a
way as to tend to return to the optimum. " Thus are pro-
duced the so-called positive and negative reactions."

The essential characteristics in behavior, as analyzed by
Jennings, are clearly set forth in the following quotations
(1906, pp. 283-292). Internal factors: ** Activity does
not require present external stimulation. . . . Activity
may change without external cause. . . . Changes in
activity depend on changes in physiological states. . . .
Reactions to external agents depend on physiological
states. . . . The physiological state may be changed by
progressive internal processes, particularly those of metabo-
lism. . . . The physiological state may be changed by the
action of external agents. . . . The physiological state
may be changed by the activity of the organism. . . .
External agents cause reaction by changing the physio-
logical state of the organism. . . . The behavior of the
organism at any moment depends upon its physiological
state at that moment. . . . Physiological states change in
accordance with certain laws. . . . The resolution of one
physiological state into another becomes easier and more
rapid after it has taken place a number of times."

Different factors on which behavior depends : "We have
seen that the behavior of the organism at a given moment
depends on its physiological state, and that it therefore
secondarily depends upon all the factors upon which the
physiological state depends. Hence we cannot expect the
behavior to be determined alone by the present external
stimulus, as is sometimes maintained, for this is only one
factor in determining the physiological state. The be-
havior at a given moment may depend on the following
factors, since these all affect the physiological state of the
organism :

" I. The present external stimulus.

''2. Former stimuli.

50 LIGHT AND THE BEHAVIOR OF ORGANISMS

3. Former reactions of the organism.

4. Progressive internal changes (due to metabolic pro-
cesses, etc.).

''5. The laws of the resolution of physiological states one
into another.

" All these factors have been strictly demonstrated by
observation and experiment, even in unicellular organisms.
Any one of these alone, or any combination of these, may
determine the activity at a given moment."

External factors (p. 299): "We may sum up the external
factors that produce or determine reactions as follows:
(i) The organism may react to a change, even though neither
beneficial nor injurious. (2) Anything that tends to inter-
fere with the normal current of life activities produces
reactions of a certain sort ('negative'). (3) Any change
that tends to restore or favor the normal life processes may
produce reactions of a different sort ('positive'). (4)
Changes that in themselves neither interfere with nor assist
the normal stream of life processes may produce negative
or positive reactions, according as they are usually followed
by changes that are injurious or beneficial. (5) Whether a
given change shall produce reaction or not, often depends
on the completeness or incompleteness of the performance
of the metabolic processes of the organism under the exist-
ing conditions. This makes the behavior fundamentally
regulatory."

Reactions and change in the sense of reactions are,
therefore, according to Jennings, adaptive ; and if this be
true, an explanation of them must be looked for along the
same lines as an explanation of any other adaptive charac-
teristic in organisms, functional as well as structural.

Finally we may refer to the "selection of random move-
ments" as a factor in orientation, as put forward by
Holmes (1905). He studied the reactions to light of earth-
worms and blow-fly larvae and found that when these
animals are stimulated they turn in many directions,
apparently feeling about until they become directed away

HISTORICAL REVIEW 51

from the source of stimulation. From these observations
he concluded that "orientation is produced indirectly by
following up these chance movements which bring respite
from the stimulation."

This conclusion is in perfect harmony with that of Jen-
nings regarding the orientation of protozoa. The only
difference between the orienting reactions in the two classes
of animals mentioned is that the unicellular forms studied
by Jennings turn in different directions by means of the
avoiding reaction, i.e., they always turn toward a struc-
turally defined side, while the metazoa investigated by
Holmes are not thus limited in their direction of turning.
Not all protozoa however are limited in the direction of
turning. Lacrymaria olar, for example, swings its long
anterior proboscis-like appendage about in all directions
and there appears to be no limitation set to the direction
in which it may turn.

Holmes contrasts the random movements w^ith forced
reflexes, and characterizes the former as " elements of
spontaneous, undirected activity." This statement natu-
rally leads to the conclusion that the direction of motion
in random movements is not definitely determined. It is
however hardly probable that Holmes intends to convey
such an idea, for it is undoubtedly true that the direction
in random movements is as definitely and absolutely deter-
mined as it is in the avoiding reaction or in forced reflexes.
The difference is merely that the factors involved are
different in the different methods of reaction.

3. Summary of Historical Review

(i) During the early periods of civilized man all living
things were held to be endowed with a soul which was
responsible for all activity.

(2) Mechanical explanations of activity received but
little attention until early in the seventeenth century, the
period of Harvey, Descartes and Borelli.

52 LIGHT AND THE BEHAVIOR OF ORGANISMS

(3) This period resulted in the origin of the iatromechani-
cal and iatrochemical schools. The object of these schools
was to explain all vital phenomena on purely physical and
chemical principles.

(4) The failure to accomplish this purpose led to the
origin of the doctrine of vital force, during the first years
of the eighteenth century. This resulted in a period of
stagnation in research in this line which continued until
the appearance of Johannes Miiller, De CandoUe and many
others, early in the nineteenth century.

(5) The establishment of the doctrine of evolution by
Darwin and the consequent interest in the origin of mental
phenomena in man led to special activity in the study of
behavior of animals from the psychological point of view,
and numerous anthropomorphic explanations of their
activity.

(6) In plants activity was studied from the physico-
chemical point of view during this period. This study
resulted in the development of the idea that the actions
are definitely controlled by external agents, e.g., the direc-
tion of growth in roots and stems by gravity, moisture,
light, etc. The reactions thus definitely controlled were
called tropisms. At first the term tropism was used merely
to indicate the relation between the direction of bending
and the position of the source of stimulation (De Candolle,
1832). Tropisms were however in general regarded as
reactions unequivocally controlled by external agents.

(7) The study of animal behavior from the physico-
chemical point of view was first taken up by Verworn and
Loeb in 1886 and 1887. The activity of the different
organs in animals had been studied from this point of view
for nearly three centuries, but not the reactions of the
animal as a whole. Loeb attempted to show that the
behavior In plants and animals is essentially the same, and
concluded that the behavior of animals is very largely un-
equivocally controlled by external agents. He and his
followers therefore described reactions in animals in terms

^ HISTORICAL REVIEW 53

of tropisms in opposition to the anthropomorphic descrip-
tions current at that time. Animals go toward a source of
light neither because it is useful for them to do so nor
because they enjoy light or can see, but because they are
positively heliotropic. But what is the underlying cause
of tropisms? What are the mechanics involved in the
processes described by this term? Loeb applied the theo-
ries developed by botanists to answer these questions and
developed others (see p. 25). Verworn and other in-
vestigators added new ones or suggested modifications.
Thus it came about that the term tropism came to have
a multiplicity of meanings.

(8) Some of the explanations of behavior offered under
the name tropism were founded on the idea that the external
agent acts directly or through a direct reflex mechanism on
the locomotor organs. This idea together with others
assuming unequivocal control of behavior by external
factors, Jennings and his followers found to be untenable
in their studies on the behavior of the lower organisms.
The new features introduced by this school have been
clearly set forth above ; it will therefore not be necessary to
emphasize them here.

4. Various Definitions of Tropisms

The term tropism was first used by De Candolle in 1832.
He called the bending of plants toward the light helio-
tropism, indicating merely the relation between the direc-
tion of bending and the source of stimulation. Later the
term tropism came to signify not only the bending or orient-
ing but also the explanation of the process. Thus for every
new explanation the term received a new signification, and
this has naturally led to much confusion. Let us point
out some of the different meanings which have been applied
to the term heliotropism.

(i) Sachs in 1876 concluded, as stated above, that
orientation of plants is due not to difference in light inten-

54 LIGHT AND THE BEHAVIOR OF ORGANISMS

sity on the surface as De Candolle held, but to the direction
in which the rays pass through the tissue. HeHotropism,
to some of those who agreed with Sachs, meant orientation
due to direction of rays through the tissue, to others merely
orientation due to ray direction in general.

(2) Darwin in 1880 said orientation in plants is due to
modification of circumnutation. It is regulated by differ-
ence of intensity on opposite surfaces, probably changes of
intensity, and he used the term heliotropism to indicate
this.

(3) In 1888 Loeb maintained that orientation in animals
is controlled by the direction in which the rays pass through
the tissue, that is, in the same way in which Sachs had said
it was controlled in plants. In 1889 he still held that light
reactions in plants and animals are governed by the same
lawa. But now he says symmetrically located points on
the photosensitive surface must be struck by light at the
same angle. " Light automatically puts the plant or the
animal into such a position that the axis of symmetry of
the body, or organ, falls into the direction of the rays of
light." Heliotropism is however used not only to express
this explanation of orientation, which differs materially
from that of Sachs, but also to indicate movement toward
or from the source of light. In his later work, he abandons
the idea of the importance of the angle between the sen-
sitive surface and the light rays and substitutes the idea
that it is relative intensity on opposite sides which governs
orientation. Thus heliotropism received a new significa-
tion. His most recent views are expressed in the following
quotations (1906, pp. 135, 138): " Heliotropism covers only
those cases where the turning to light is compulsory and
irresistible, and is brought about automatically or mechani-
cally by the light itself. ... If the current curves of
radiating energy, e.g., light rays, strike an animal on one
side only, or on one side more strongly than on the sym-
metrical side, the velocity or the kind of chemical reactions
in the symmetrical photosensitive points of both sides of

HISTORICAL REVIEW 55

the body will be different. The consequence will be in a
positively heliotropic animal a stronger tension or tendency
to contract in the muscles connected with the photosensitive
points of the one side of the body than in those connected
with the opposite side." This view is affirmed in a recent
address (1909).

(4) It is ordinarily assumed that Verworn"- considers
orientation in the lower forms to be due to the direct effect '
of the external agent on the locomotor appendages. If,
e.g.t one side is more highly illuminated than the other the
cilia beat more or less effectively on that side and thus
produce orientation. This process is termed heliotropism
or phototaxis.

(5) " Two kinds of effects are produced by light " accord-
ing to Davenport (1907, pp. 210, 211), " one by the direc-
tion of the rays . . . either through difference of intensity
on the two sides of the organism, or by the course the rays
take through the organism — phototactic; the other by
the difference in illumination of parts of the organism —
photopathic."

(6) Yerkes says (1903, p. 361), "All those reactions in
which the direction of movement is determined by an
orientation of the organism which is brought about by the
light are phototactic; and all those reactions in which the
movement, although due to the stimulation of light, is not
definitely directed through the orientation of the organism
are photopathic."

(7) To Radl (1903) heliotropism means orientation due
to difference in light pressure on unequally illuminated
symipetrically located surfaces.

(8) Holmes (1905) calls orientation by selection of ran-
dom movements phototaxis (heliotropism).

(9) Barrows (1907, p. 530) and Walter (1907, p. 149)
suggest "asymmetrical response to asymmetrical stimula-
tion" as a criterion of tropisms; and because the organisms
worked on respond thus they conclude that their reactions
are tropic. According to this criterion it is of course evident

56 LIGHT AND THE BEHAVIOR OF ORGANISMS

that every diflferentlal response to a localized stimulation
even in a human being may be a tropic response.

(10) To Bohn forced orientation constitutes a tropism;
(1908, p. 78), " L' orientation est directe; I'animal est attire
sans qu'il pitisse resister: il y a la un ' tropisme' au sens de
Loeb "; (p. 80), " On n'a pas besoin de nier la 'volonte' de
Tanimal; on peut dire que ces impulsions sont plus fortes
qu'elle. On ne peut nier les tropismes."

(11) Parker apparently considers any reaction which
carries an animal toward or away from the source of stimu-
lation as tropic; he says (1908, p. 426), " Since amphioxus
swims away from a source of light, it is negatively photo-
tropic." Minkiewicz (1907, p. 47), uses the term tropism
in much the same sense, as does also Hadley, who defines
it and photopathy as follows (1908, p. 201) : " A phototactic
reaction [is] one in w^hich the organism tends to place the
longitudinal axis of the body parallel to the direction of
the rays and to approach or recede from the source of those
rays. ... A photopathic reaction is one in w^hich an or-
ganism, without previous assumption of a body-orientation,
^selects' regions of optimal light-intensity."

(12) Washburn (1908, p. 57) refers to tropisms as "the
direct motor response of an animal to an external stimulus,"
and Torrey defines the term similarly but somewhat more
definitely. He says (1907, p. 319): *' In heliotropism as
well as in galvanotropism, the oriented organism is in a
condition of physiological stimulation, and . . . the re-
sponse to stimulation is local." This definition is in all
essentials like those of Verworn and Loeb.

(13) Driesch (1908, p. 11) says, "A tropism ... is a
directed movement of a growing part of a plant or hydroid
determined by the direction of a directed agent."

(14) Wheeler (1910, p. 515) considers reactions which
*' involve an adaptive orientation" as tropic.

(15) Jennings (1909, p. i) suggests the following defini-
tion: " The tropism includes those reactions in which the
organism takes and maintains a definite orientation — places

HISTORICAL REVIEW 57

the axis of its body in a definite position — with relation
to some external source of stimulation."

It is evident from these statements that nearly every
reaction in living organisms comes under one or another
of the various definitions given to the term tropism. To
say that an organism is tropic or not tropic means but little
until the sense in which this term is used is defined. Failure
to do this has led to serious misunderstanding. I have no
objection whatever to the term tropism if used in its
original sense, or in any other definite sense. At present,
however, it conveys so many different meanings that it
inevitably leads to confusion. I shall therefore avoid using
it in the following analysis of reactions to light.

5. Statement of Important Problems in the Study of

Reactions to Light

In this analysis we shall ai n to keep in mind the various
factors suggested as important in the different tropism
theories and other explanations of behavior. We shall ask
ourselves: is orientation direct, does the organism turn
directly toward or away from the source of stimulation, or
does it become oriented after a series of preliminary move-
ments? How is the stimulus causing orientation pro-
duced: by direction of rays through the organism in accord
with the theory of Sachs; by absolute difference of intensity
on symmetrically located points on the sensitive surface in
accord with the theories of Loeb and Verworn; or by changes
of intensity on the surface in accord with the ideas of Engel-
mann, Darwin, and Jennings? Does light act constantly
as a directive stimulation similar to the action of a constant
current of electricity in accord with Loeb's theory of trop-
ism, or does it act only when the organism turns out of its
course so as to produce changes of intensity, as suggested
by Jennings? Is orientation due to the direct effect of
light on the locomotor appendages in accord with the
theory of Verworn and the analysis of Torrey, to the indi-

58 LIGHT AND THE BEHAVIOR OF ORGANISMS

rect effect through a direct reflex arc as suggested by Loeb,
or is the whole organism more or less involved in the re-
action in accord with the ideas of Jennings and Holmes?
If orientation is direct, precisely what movements are
involved in the process? Are the avoiding reactions due
to differential response to localized stimulation, as held by
some, or is the direction of turning in such reactions abso-
lutely determined by the structure and physiological state
of the organism? Are the reactions to light in general
adaptive and modifiable in accord with Jennings' analysis,
or are they fixed and forced and unequivocally controlled
by the external agent in accord with Loeb's ideas? Are
the more refrangible rays most active in stimulating all
organisms as claimed by Loeb and Davenport, or are some
organisms stimulated more by waves of a certain length,
and others by waves of a different length as claimed by
Verworn and Nagel? These questions and others we shall
attempt to answer in the following pages.

PART II

EXPERIMENTAL OBSERVATIONS AND DISCUS-
SIONS BEARING ON THE QUESTION AS TO HOW
ORGANISMS (ESPECIALLY THOSE WITHOUT
EYES) BEND OR TURN AND MOVE TOWARD
OR FROM A SOURCE OF STIMULATION

CHAPTER IV

PROCESSES INVOLVED IN THE BENDING OF DIFFERENT

PARTS OF HIGHER PLANTS TOWARD THE

SOURCE OF LIGHT

I . Observations on Plumules of Indian Corn {Zea mays) and
Leaves of Nasturtium ( Tropaeolum)

a. Introduction. — It is well known that many plant
structures have a sensitive zone which may be separated
by some distance from the motory zone and that impulses
are transmitted from the one to the other. Darwin (1880),
Pfeffer (1894), Czapek (1900), Pollock (1900), Haberlandt
(1904) and others demonstrated this for leaves and plumules
stimulated by light and for radicles stimulated by gravita-
tion and injury (cauterization). Newcombe (1902, p. 346)
also proved that impulses due to stimulation by water
currents are transmitted in radicles. In radicles the dis-
tance of transmission of impulses is frequently over 10 mm.,
while in leaves it is often several centimeters.

Just how the external agent produces the stimulus is not
known, although it is generally supposed that it is by caus-
ing chemical changes. With regard to light it has been a
question as to whether the orienting stimulation is depend-

59

6o LIGHT AND THE BEHAVIOR OF ORGANISMS

ent upon the direction in which the rays pass through the
tissue or upon difference of intensity on opposite sides of
the reacting organ. Sachs (see p. 13) originated the former
view and Miiller and others supported it, while Darwin,
Wiesner and Oltmanns were prominent champions of the
latter. Darwin also emphasized in particular the impor-
tance of change in intensity. Pfeffer (1906, p. 228) says
that the experimental results and the arguments offered in
support of either view are not conclusive.

Darwin exposed monocot plumules (stems of young
seedlings) with one side covered with India ink in front of
a window and found that they did not bend straight toward
the window, but deflected toward the uncovered side. This
result seems to indicate that the curvature is due to differ-
ence in light intensity on the surfaces. Pfeffer (1906, pp. 3,
229), however, considers it inconclusive, largely on account
of the possible effect of the India ink on transpiration
(evaporation). Oltmanns studied the curvature of plants
grown behind a hollow prism containing India ink and
glycerine gelatine so arranged that the light intensity
decreased from right to left, and found that they deflected
toward the brighter end of the field. He therefore con-
cluded in favor of difference of intensity as the controlling
factor in orientation. His results, however, are not con-
clusive, owing to the diffusion of light by the particles of
India ink in suspension (see p. 40).

b. Apparatus. — In the following work the objections
to the experiments of Darwin and Oltmanns were elimi-
nated by the use of an apparatus known as the light grader
modified to suit the conditions of the experiments. The
important features in the construction of this apparatus
will be understood readily by referring to Fig. 4. The
walls of the apparatus are all light-proof and dead black
inside, so as to prevent reflection. The outline of a cross
section at any point is square. The upper portion of the
front wall of the vertical part of the apparatus is hung on
hinges forming a door. From the bottom of this door is

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 6l-

Fig. 4. I. A vertical section of the light grader. The lens (a), which is a seg-
ment of a cylinder, has its longitudinal axis lying in the plane of the section; b,
stage; c, Nernst glower; d, non-reflecting background; e, mirror;/, light rays; g,
opaque screens. Distance from glower of lamp to stage, one meter.

II. Stereographic view of light, lens, and image; a, lens; b, field of light pro-
duced by the image of the glower (c); d, opaque screen, which lies flat on lens and
contains a triangular opening which causes a gradation in the light intensity of
the field (b).

hung a loose vertical curtain, which can be so opened that
observations can be made without admitting Hght. The
source of hght Is a Nernst glower, which Is parallel with the
minor axis of the lens. It is mounted In front of a small
opening In a light-proof box painted dead black Inside,
which thus forms a non-reflecting background. The glower

62 LIGHT AND THE BEHAVIOR OF ORGANISMS

and stage are at the conjugate focal points of the lens, and
therefore at equal distances (50 cm.) from it. The plano-
convex cylindrical lens used is 25 cm. long, 10 cm. wide and
has a radius of curvature of 12.5 cm.

A cylindrical lens will not form a single definite
image of an object, but rather a series of images, since by
means of it light is focused only in reference to one plane.
If, then, the object, e.g., a Nernst glower, is placed at one
of the conjugate focal points so that the distance from the
lens to the glower is equal to that from the lens to the
image, and the glower is so arranged that it is perpendicular
to the axis of the lens, the image will not consist of a narrow
band of light as large as a glower, which would be true if
the segment of a sphere were used as the lens, but it will
consist of a comparatively large field of light, the length of
which is proportional to the functional length of the lens,
while the width is equal to the length of the glower, regard-
less of the functional width of the lens (see Fig. 4). But
since the amount of light which passes through the lens is
directly proportional to the functional width of the lens
and the width of the field is constant, it is clear that the
intensity of light in the field, if we disregard the amount of
light absorbed by the lens, must also be theoretically pro-
portional to its functional width. Direct measurements of
the light intensity with different functional widths of the
lens proved this to be true within the limits of error. If,
then, the lens be covered with an opaque screen containing
a triangular opening, the base of which is parallel with the
minor axis of the lens as represented in Fig. 4, there will
result a rectangular field of light in which the intensity
gradually diminishes from the end produced by light which
passes through the base of the triangular opening to the
opposite end, where theoretically it fades into darkness.
Practically, however, it was found to be impossible to cut
the apex of the triangular opening so as to prevent an
apparent line at the end of least intensity. Since the light
intensity of the field is proportional to the functional width

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 63

of the lens, it Is evident that the rate of diminution in
intensity depends upon the ratio of the altitude of the
triangular opening to the length of its base; i.e., decreasing
the altitude or increasing the base causes an increase in
the rate of diminution, and vice versa. Yerkes (1903) was
the first to make use of a cylindrical lens in studying re-
actions to light.

c. Experiments. — In these experiments the light grader
was placed in a horizontal position in such a way that the
glower was vertical. The lens was covered with an opaque
screen containing two triangular openings with the apexes
facing each other and only a millimeter apart. In this
way two parallel horizontal beams of light were produced,
the intensity of which gradually diminished from side to
side (see Fig. 5). The object of having two beams was to
neutralize any possible effect from diffusion of light by the
lens.

A single plumule at a time was exposed In one of these
beams of light. In some cases it was allowed to grow up
into it from a small pot of sphagnum in which It was ger-
minated; In others the seedlings were transferred to the
light grader after the plumules were about one centimeter
long.

In former experiments with this apparatus aquatic or-
ganisms were used; it was therefore necessary to expose
them in an aquarium containing water. Under such con-
ditions it is Impossible to eliminate light reflected from the
glass walls of the aquarium and from particles in suspension
in the water. With the plumule growing in air, however,
and with only one exposed in the beam of light at a time,
it is evident that all such reflections are done away with.
Thus the objections to Oltmanns' experiments with the
hollow prism have been obviated, and likewise those brought
forward against Darwin's work.

All the following experiments were performed in a large
dark room. During the first part of the work the apparatus
was situated several meters from a dead black wall upon

64 LIGHT AND THE BEHAVIOR OF ORGANISMS

which the beams of light fell and were absorbed. The
altitude of the triangular openings in the screen over the
lens was 7 mm. and the base 50 mm. The beams of light
thus produced were 14 mm. wide and 20 mm. high at the
focal point in the light grader, the place where the plumules
were exposed. At this point the light intensity in each
beam decreased from side to side at the rate of 2 ca. m.^
per mm., it being 100 ca. m. at one side and zero at the
other. From these data the intensity at any part could
readily be calculated. In order to ascertain the intensity
to which the plumules w^ere exposed it was therefore neces-
sary only to learn their position in the field ; and to calculate
the difference of intensity on opposite sides it was sufficient
to know their diameter, the difference in all parts of the
field being 2 ca. m. per mm. width.

During the first part of the work the movements of each
plumule were recorded by tracing its shadow cast upon a
sheet of paper held in a vertical position a few centimeters
back of it. The shadow was thus traced at the beginning
of the experiment and again at definite intervals. At first
only a few tracings were made in twenty-four hours. It
was however soon found that owing to marked circum-
nutating movements and to surprisingly indefinite lateral
deflections it was necessary to locate the position of the
plumules at 30 to 60 minute intervals (see Fig. 5).

By this method only the lateral and vertical movements
of the plumule were recorded. There was no record of the
movement toward the source of light; in some of the later
experiments however this movement also w^as recorded.
A fine pointer was fastened so that the sharp end was
10 cm. above the tip of the plumule. A glass plate was
then fastened in a horizontal position one meter above the
pointer. By proper illumination the sharp end of the
pointer and the tip of the radicle could clearly be seen
throu^^h the glass plate, and it was not difficult to fix a

^ The a bbreviation ca. m. will be used for the term candle meters through-
out this vc>lume.

\

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 65

dot of ink in line with these on the plate by sighting through
a small circular hole in a piece of opaque paper. The
horizontal movements of the tip of the radicle could thus
be quite accurately recorded by making dots on the plate
in line with the pointer and the tip of the radicle, at any

->

n

->

■>

13 12

Fig. 5. Tracings of shadow of a plumule of corn showing its reaction in light of
graded intensity, three-fourths natural size. I. Cross section of two beams of
light as used in the experiment; intensity at a and a', zero; at b and b', 100 ca. m.;
I, 2, 3, 4, 5, 6, 7, successive positions of plumule at intervals of 60 minutes, right
side more highly illuminated than left; 8, 9, 10, 11, 12, 13, same with left side more
highly illuminated than right. It will be seen that the plumules deflect slightly
toward the more highly illuminated side under both conditions.

II. Side view of plumule showing amount of curvature toward source of light at
close of experiment, n, direction of light.

desired intervals, and connecting them with a line. The
records thus made represent the movement of the radicle
magnified ten times. The direction of the rays was recorded
by tracing the edge of a ruler placed on the glass plate in
such a position that the edge was in line with the shadow
of the plumule cast on a white surface temporarily arranged
for the purpose (Fig. 6).

The intensity of light to which the plumules were ex-
posed varied from about 2 to 14 ca. m. In most of the
experiments they were exposed to the lowest intensity, the
edge of the plumule at the beginning of the experiment
jDeing in close contact with that side of the beam of light
which had the lowest intensity (see Fig. 5).

66

LIGHT AND THE BEHAVIOR OF ORGANISMS

B

<^o4^- '"^-^ 9:40 A.M.
11:00
.11:30

12:30 P.M.
1:00

1:55

2:30

d

9:15 A.M:.

10:00

Fig. 6. A-E. Courses taken by tips of plumules in bending toward the glower
in a graded beam of light; magnified five times. The dots represent the position
at time indicated. The large arrows indicate direction of rays; the small ones the
direction of movement of plumules; d, side of the beam having the lowest light
intensity; /, side having highest intensity. In E the beam was reversed between
i.ooand 2.00 p.m. It will be seen that in every case except A the plumules de-
flected slightly toward the more highly illuminated side. See text.

d. Results. — Under these conditions the reactions of
36 plumules, 14 of wheat (Triticum vulgare) and 22 of corn
(Zea mays), were studied and recorded with the following
results: of the 14 wheat plumules studied 6 deflected toward
the more highly illuminated side, 3 toward the less highly
illuminated side, and 5 did not appreciably deflect in either
direction. Of the 22 corn plumules 13 deflected toward th^
more highly illuminated side, 2 toward the less highly

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 67

illuminated side, and 7 did not definitely deflect toward
either side.

These results seem to indicate that it is difi"erence in light
intensity on the organism which regulates the direction of
movement. The lateral deflections are, however, as indi-
cated in Fig. 6, relatively small. The maximum is scarcely
more than 2 mm. in a movement of 10 mm. tow^ard the
source of light. Considering the conditions of the experi-
ments superficially one would expect a much greater deflec-
tion if the direction is regulated by the relation in light
intensity on different parts of the surface. A corn plumule
frequently has a diameter of over one millimeter at a point
not more than one millimeter from the tip, well within the
sensitive zone. In such a plumule placed in contact with
the edge of the beam of light having the lowest intensity,
the difference of intensity between the surface facing the
glower and that facing in the opposite direction is appar-
ently not as great as the difference of intensity between
the two sides. Consequently one might conclude that if
the movement is regulated by difference of intensity, the
plumule should bend at least as far toward the highly
illuminated edge of the beam as toward the glower.

There are however serious objections to such a conclusion.
In the first place it is not known whether or not the sensitive
tissue extends to the surface. It may be that it is restricted
to the central portion of the plumule and that it is very
narrow, so that the intensity difference on opposite sides
of this tissue is relatively slight under the conditions of the
experiment. In the second place It is evident that light
can affect the tissue only by penetrating it, and since the
rays strike the surface facing the glower nearly at right
angles, and the more highly Illuminated side at a very small
angle, much more light will penetrate the former than the
latter. And in the third place, under the conditions of
the experiment, the Illumination of the two sides will be
equalized by the movement of the plumules much sooner
than will that of the two surfaces.

6S> LIGHT AND THE BEHAVIOR OF ORGANISMS

However this may be, It must be conceded that while the
results of these experiments indicate that orientation is due
to diversity of light intensity on the reacting organ, they
do not definitely settle the question.

Much more convincing results were obtained toward the
close of the work when it occurred to me that it would be
possible to prevent the bending toward the glower entirely,
without vitiating the results, by reflecting the beam of
light and illuminating the surface directed away from the
glower as well as that facing it. A small mirror of finest
quality 5 mm. X 2 cm. was therefore supported in the beam
of light in a vertical position 3 cm. from the plumule. By
careful manipulation and frequent adjustment it was pos-
sible to keep the intensity on the surface directed toward
the glower and the one opposite nearly the same, while the
difference of Intensity on the right and left sides was nearly
twice as great as it was when the beam was not reflected.
The reactions of 4 plumules of Zea mays were studied under
these conditions. All deflected definitely toward the more
highly Illuminated side, as represented In Fig. 7. These
results seem to prove conclusively that orientation In plu-
mules of the gramineae (grasses) is in some way regulated
by difference in light Intensity on opposite sides, and that the
direction in which the rays enter the tissue Influences the
direction of motion only in so far as this may produce
unequal illumination of different parts of the sensitive
tissue.

A number of experiments were made with young nas-
turtium (Tropaeolum) leaves In graded light. Different
parts of the leaf blades were thus subjected to different
intensities. In some experiments one-half of the blade was
entirely in the shadow. I was unable to detect any influ-
ence of the unequal illumination of the blade on orientation.
The leaves turned toward the source of light just as they
did when the blades were entirely Illuminated by light of
equal Intensity throughout. The circumnutatlon move-
ments in these leaves were so great, however, that it would

a

M ,, + -I y -'

10:io A.M

11;30

5:10

m

d

8:05 A.M. %-j

l:i5 P.M.

11:25 P.M. He

7.40

5:00
4:30

3:30

a

t t "

d

12:30 P.M.

1:00'

m

1:45

5:15

:05
9-00-11:80 A.M

m

n

o

n

'1

;.Vi

E F

Fig. 7. ^-Z). Courses taken by tips of plumules of Indian corn (Zea mays) as
viewed from above in a beam of graded light (a) which was reflected from the
mirror m so as to illuminate the two surfaces equally. Magnified five times. The
arrows (a) indicate the direction of the rays from the glower, and the other arrows
the direction of movement of plumules; d, side of beam of light having lowest
intensity; /, side having highest intensity (see F below). Movement in the direc-
tion of the rays of light was caused by imperfect adjustment of mirror producing
unequal illumination from the glower and mirror.

E, Cross section of beams of Hght. i, outline of shadow of plumule at the be-
ginning of Course C above, i.io p.m.; 2, shadow at 2.10 p.m.; 3, shadow at 11.25 P-M.

F, I, shadow of plumule at beginning of Course D above, 9.00 a.m.; 2, same at
5.15 P.M. The light intensity at 0 was zero; at n, about 200 ca. m. The increase
of intensity in the field from side to side was about 14 ca. m. per mm. It will be
seen that the plumules deflected strongly toward the side most highly illuminated.

69

70 LIGHT AND THE BEHAVIOR OF ORGANISMS

have been impossible to detect anything but rather de-
cided effects. It is hoped that these experiments may be
extended.

e. Discussion. — The conclusion arrived at above that
orientation is regulated by the difference in light intensity
on opposite sides of the plumules is in direct opposition to
Sachs' theory (see p. 13) of orientation. It opposes that
of Loeb in so far as he attaches importance to the idea
that symmetrically situated points on the surface must be
struck by light at the same angle when the organism is
oriented (see p. 28). It neither confirms nor contradicts
Loeb's and Verworn's idea (see pp. 29, 38) as to the direct
effect of the external agent on the motory tissue. Nor does
it bear on the question proposed by Darwin (p. 18) that
orientation is due exclusively to modification of circum-
nutations. It is entirely possible that the lateral illumi-
nation causes an increase as well as a change in the direction
of the movement.

Superficially the evidence seems to indicate clearly that
orientation is direct, that there is nothing corresponding
to selection of random movements (see p. 50). However,
it is impossible to say in how far even very slight circum-
nutating changes in position may affect diversity of light
intensity within the individual cells in the sensitive zone,
and in how far such changes in position may be interpreted
as trial movements. Owing to the possibility of such
variations in illumination within the cells, due to very
slight changes in the position of the plumule, it is also
impossible to decide whether the stimuli which cause
orientation are due to constant intensity or to change of
Intensity.

These experiments have no bearing on the question as
to how curvature resulting in orientation in the plumules
is produced. The experiments of Darwin and others, how-
ever, showing that there Is a distinct sensory and motory
zone in these structures, demonstrate clearly that it is not
due to the direct effect of the illumination on the tissues

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 7 1

which produces the curvature, as Loeb's theory quo ted above
demands. The mechanism involved is undoubtedly far
more complex than this theory indicates. It may be similar
to that offered by Pollock to explain the curvatures in roots.
He says (1900, p. 59): " The stimulus is transmitted from
the sensitive root tip to the curving parts, in the cortical
parenchyma. The effect of the stimulus is to increase the
normal tension between cortical parenchyma and axial cylin-
der on the side that becomes convex, and to decrease or re-
verse the normal tension between the cortical parenchyma
and the axial cylinder on the side that becomes concave.
The change in tension also extends to the different layers of
the cortical parenchyma on the concave side, the outer
layers becoming negative with respect to the inner ones.
So much has been demonstrated. The evidence is in
favor of the view that the tensions on the concave side
are changed by the protoplasm becoming more permeable
to water, some of which passes out into intercellular spaces,
possibly to be taken up by the convex cells, which later
contain more water than the concave cells. The shorten-
ing of the concave side may be masked sometimes by a
certain amount of growth." This theory does not account
for curvature in structures having but a single cell cavity,
like the hyphae of molds, rhizoids of liverw^orts, and some
algae, all of which are known to respond to light by bending
toward or from its source. That these reactions cannot be
accounted for on the basis of osmotic changes was pointed
out by Hofmeister as early as 1867.

Very little is known concerning the fundamental factors
involved in orientation in other plant structures than those
mentioned, although much work has been done on them,
especially on the leaves. Darwin (1881) was the first to
attempt to locate the sensitive structure in the leaf. He
found that neither quality nor intensity of reaction is
affected by shading the blade, and concluded that the
petiole perceives the light. V^oechting (1888) came to
quite the opposite conclusion in experiments on malva

72 LIGHT AND THE BEHAVIOR OF ORGANISMS

and other plants. Krabbe (1889) supported Darwin in his
conclusion, as did also Rothert (1894) and Czapek. Haber-
landt (1904), on the other hand, maintains not only that
the blade is functional in light perception, but also that the
curved and thickened outer walls of the epidermal cells act
as lenses and focus the light on the protoplasm within, and
that orientation is regulated by responses due to the dis-
tribution of the intensity of light within the cells of the
epidermis. Kneip (1907) covered the upper surface of the
blades of Tropaeolum with a thin layer of paraffin oil whose
index of refraction is about 0.143 greater than the index of
cell sap. The oil consequently inverted the lens effect of
the curved walls of the epidermal cells and thus caused a
dispersal of the rays within the cell. Kneip found however
that the leaves treated thus responded to light much like
those not treated, and concluded (p. 136), '' that the lens
action is of no importance in the leaves studied." Haber-
landt (1909) however does not agree with this conclusion.
He claims that the fact that leaves still respond to light
after the epidermis is covered in such a way as to neutralize
the focusing effect of the curvature of the outer cell walls,
merely shows that the effect of these walls can be dispensed
with and not that it is useless, and holds that after the lens
effect of these walls is neutralized the light intensity is still
unequal on the inner surface of the cells, when the light
strikes the epidermis obliquely, and that this may cause
orienting responses, but that the focusing effect of the
curved outer walls of the cells enhances the promptness
and precision of the orienting responses.

Various other experiments aside from those mentioned
above have been carried out, but the results obtained lead
to no definite conclusions concerning the function of the
lens action of the epidermal cells, nor do they give any
clear notion as to the mechanism of orientation in plants.
About all that can be said is that leaves generally take a
position such as to facilitate photosynthesis, and that the
chloroplasts within the cells likewise assume what may be

BENDING OF HIGHER PLANTS TOWARD THE LIGHT 73

termed an optimum position. The reactions are adaptive.
In some instances if the Hght is too intense the chloroplasts
are found along the side walls which are more or less nearly
parallel with the incident rays. In others the leaves turn
so that the edge of the blade faces the light. In all proba-
bility both the petiole and the blade are sensitive to light,
at least in some leaves, but the method of regulating the
movements is still a mystery.

CHAPTER V

OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS
OF ATTAINING AND RETAINING A DEFINITE AXLA.L
POSITION WITH REFERENCE TO THE
SOURCE OF LIGHT

I. Myxomycetes and Rhizopods

All the Rhizopods and the plasmodia of Myxomycetes
that are known to react to Hght are negative, as was shown
by Baranetzsky (1876, pp. 328, 340), Stahl (1884, p. 167),
Engelmann (1879, p. 3), Davenport (1897) and others.
The contention of Hofmeister (1867, p. 20) that plasmodia
are positive in Hght of very low intensity has not been
confirmed.

Davenport (1897, p. 186) exposed specimens of Amoeba
proteus under a compound microscope in a small horizontal
beam of direct sunlight with all other light intercepted by
means of opaque screens and found that they orient directly.
They make no preliminary trial movements in this process.
If the direction of the rays is changed they always turn from
the source of light at once, never toward it. There is no
evidence of selection of random movements in these animals.
The same is probably true in case of other Rhizopods and
Myxomycetes, although there are no investigations which
bear directly on this point. Baranetzsky (1876) found that
even a slight increase in illumination causes a distinct
retardation in streaming movements of Myxomycetes.
Engelmann (1879) observed that light thrown upon a
pseudopod of Pelomyxa palustris causes it to be withdrawn
suddenly. Harrington and Leaming (1900) found that a
sudden increase in light intensity causes a retardation in
the movement of Amoeba. Ewart (1903, p. 69) says that
protoplasmic streaming in cells in general is retarded by

74

OBSERVATIONS ON UNICELLULAR FORMS 75

increase in light intensity, and Pringsheim (1879, pp. 334,
367) maintains that local retardations in streaming move-
ment can be produced by local stimulation. Jennings
(1904) has shown the same to be true for Amoeba when
stimulated mechanically and chemically.

After completing this part of the manuscript I had the
opportunity of observing the orienting reactions in Amoeba
proteus in detail, and also the effect of different rays on the
reactions. I shall insert a description of the former here;
the latter will be discussed in Part IV.

In studyingorientation numerous specimens were mounted
under a large cover glass supported by a ring of vaseline so
as to give them ample room for moving about and to pre-
vent the solution from drying up. The specimens thus
enclosed could be kept in excellent condition for several
days. The observations were made under a compound
microscope situated in diffuse daylight without any screen
around it. Mirrors were so arranged that two horizontal
beams of direct sunlight were reflected upon the stage at
right angles to each other after passing through 8 cm. of
water to eliminate the heat. Specimens exposed in one
of these beams without any light from the substage were
found to direct their course in a general way from the source
of light. In one instance, after a slide had been exposed
for fifteen minutes, there were eleven specimens in one field
of the low power, all but two of which were moving from
the source of light. In another field there w^ere twelve
specimens; all but four of these were directed from the
source of light. Of these four, two were proceeding at
right angles to the rays and two were going toward the light.
In still another field containing nine specimens, seven were
negatively oriented, one positively and one at right angles
to the rays. Orientation, however, was not very precise in
any of the specimens. The amoebae usually took a sort
of zigzag course. Pseudopods were frequently seen to
extend toward one side for some distance, then stop as
though they had been checked, after which new ones were

76 LIGHT AND THE BEHAVIOR OF ORGANISMS

ordinarily seen to extend on the opposite side for some dis-
tance, and stop, etc.

The details in the process of orientation were observed
as follows: a specimen which had oriented in one beam of
light was selected, after which the light in this beam was
intercepted and that in the other simultaneously turned
on. The reaction of numerous specimens to a change in
the direction of the rays was thus observed and the move-
ments in several were recorded by means of camera sketches
made at short intervals. A typical record is presented in
Fig. 8, although a majority of the specimens observed
did not orient as precisely and definitely as did the one
represented in this record. By referring to Fig. 8 it will be
seen that the amoeba under observation gradually turned
from the side most highly illuminated, sending out pseudo-
pods only on the shaded side. What is the cause of this?

If direct sunlight is thrown upon an amoeba which is
active in diffuse daylight, all movement stops instantly,
but there is ordinarily no immediate contraction of any of
the pseudopods. After a few moments of exposure new
pseudopods usually appear at the posterior end, and not
until these begin to form do the old ones begin to retract.
In changing the direction of the rays so that the amoebae
become strongly illuminated from the side, as described
above, the distribution of the light intensity on the different
pseudopods is changed since different surfaces become
exposed. Judging from our preceding statement it might
be expected that this change of light intensity would
inhibit the protoplasmic streaming in the pseudopods on
the illuminated side. I could, however, never be quite
certain that it did, although it often appeared so. The
difficulty in observation here lies in the fact that without
any change of illumination the pseudopods form, extend a
varying distance, then stop and retract while others form
elsewhere. When a pseudopod stops after the direction
of the rays is changed it is consequently impossible to be
certain that it would not have stopped had the light not

OBSERVATIONS ON UNICELLULAR FORMS

77

3:48 P.M.

3:54

8:57.5

0.5 mm:

FiG. 8. Camera drawing representing different stages in the process of orien-
tation of Amoeba proteus. i, Amoeba oriented in light nn before light //is turned
on; 2-9 successive positions at the time indicated on each after light II is turned on.
Arrows represent the direction of streaming of protoplasm in pseudopods. In
those which do not contain arrows there was no noticeable streaming at the time
the sketch was made. // and nn, direction of light.

78 LIGHT AND THE BEHAVIOR OF ORGANISMS

been changed. If light acts directly on the protoplasm
it might also be expected that in a pseudopod laterally
illuminated, the flow on one side would be retarded, thus
causing it to curve. But no evidence of this could be seen.
How then does orientation take place if the pseudopods
which are present continue and do not turn from the source
of light? There is but one way that I can see, and that is
by the inhibition of the formation of new ones on the more
highly illuminated side of the organism.

Since we know that an increase of intensity inhibits
streaming in the pseudopods of Amoeba it seems strange
that no one has thus far been able to see any reaction in an
amoeba in passing from a region of one intensity to that of
another. Davenport (1897, p. 186) studied their move-
ments in a field " separated by a sharp line into a light and
dark half," but could detect '* no effect resulting from the
change from light to dark or the reverse." I made ob-
servations much like those of Davenport, and found that
when the amoebae came in contact with the light area
they usually stopped and proceeded in a different direction,
as represented in Fig. 9. The light area used in these
experiments was about 0.5 mm. square and had very
definite edges and a high intensity. It was produced by
focusing a limited area of a luminous Welsbach mantle on
the slide by means of the mirror and an Abbe condenser.
These observations were made in a dark room and no light
except the small beam from the Welsbach rrantle reached
the microscope.

By referring to Fig. 9 it will be seen that after one
pseudopod came in contact with the illumination and was
stopped, the amoeba did not at once proceed in the opposite
direction so as to avoid the light, but sent out other pseu-
dopods at only a slight angle with the first, apparently
trying to get around the obstacle in this way. The char-
acter of the response did not change after the first pseudopod
came in contact with the light, or after the second and the
third came in contact with it. But after the fourth became

OBSERVATIONS ON UNICELLULAR FORMS

79

exposed the direction of motion was nearly reversed. This
indicates that the reaction was modified, that the response
to a given stimulus depends upon the preceding experience.

Fig. g. Sketches representing the reactions of an amoeba proceeding toward
an intense area of light the rays of which were perpendicular to the slide. L, field
of light formed by focusing a section of a Welsbach mantle on the slide, i-io,
successive positions of the amoeba a little less than one-half minute apart. Arrows
indicate direction of streaming in pseudopods.

In view of these facts it is probably true that the orienta-
tion of all of the rhizopods in light is due to a local response
to a local stimulation, a direct inhibition of the movement
of the part most highly illuminated. This would of course
result in the prevention of the formation of pseudopods on
the more highly illuminated side, and the organism would

8o LIGHT AND THE BEHAVIOR OF ORGANISMS

turn until both sides are equally illuminated, and symmet-
rically located points on the body equally stimulated.

Such a method of orientation is in harmony with much
in Verworn's theory and also with the essentials in Loeb's.
It does not, however, support the idea connected with
these theories, that a constant intensity produces a constant
directive stimulation.

Jennings (1904) has shown that certain amoebae roll over
and over in their movement. The protoplasm on the
underside in relatively low light intensity is constantly
coming to the surface into a greater intensity, and moreover
the beginning of every laterally directed pseudopod in those
forms which do not roll necessarily causes a change in the
light intensity of the protoplasm in it. Thus it is clear that
the protoplasm is being continuously subjected to changes
of intensity. And while the rate of movement in the animal
as a whole is no doubt influenced by constant light inten-
sity, much as it is by temperature, it may be that orienting
reactions are responses solely to changes in light intensity,
— in negative organisms to a rather sudden increase of
intensity.

This method of orientation is opposed to the idea of
Sachs (see p. 14), that the direction in which the rays
penetrate the tissue is of importance in orientation, and
also to that of Loeb (see p. 28) with reference to the im-
portance of the angle between the rays and the surface.

2. Euglena

a. Description. — Euglena is a minute elongated or-
ganism. The posterior extremity ends in a spinelike
process; the anterior end is rounded off rather bluntly.
The different species vary greatly in size; some are not over
o.oi mm. long and o.ooi mm. in diameter, while others are
nearly fifty times as large. The forms most commonly met
with average about o.i mm. in length and 0.015 mm. in
diameter. Nearly all are green, having numerous chloro-

OBSERVATIONS ON UNICELLULAR FORMS

8l

plasts of various forms. They have a contractile vacuole
which opens to the exterior at the anterior end, and a brown
pigment spot known as the eye-spot, in close connection with
the vacuole. They exist m three states, — free-swimming,
crawling and encysted (Fig. lo). In the free-swimming

C 'p

F

-0.01 mm.

Fig. io. Sketches of Euglena, showing general structure of dififerent forms.
A and C, Euglena :*; sp. (?) in crawling state; B, probably a form of E. viridis; D,
E, E. deses; e, eye-spot; v, contractile vacuole; ch, chloroplasts; space in B limited
by dotted lines well filled with small chloroplasts; n, nucleus; c, caudal spine; p,
pigment granules which appear to be composed of same substances as eye-spot,
— these were found in only a few specimens. E, shows typical curvature toward
dorsal surface while swimming in direction indicated by arrow. F, eye-spot highly
magnified; s, surface view; a, view from anterior end. The convex surface is
directed outward, mm., projected scale. All outlines were made with camera
from specimens killed in iodine. Contractile vacuoles and nuclei were sketched
free-hand from Uving specimens.

state they have a flagellum frequently nearly as long as
the body. Wager (1900) found that in E. viridis it passes
down through the opening of the contractile vacuole and
divides into two branches, each of which is attached to the
wall of the vacuole. One of these branches contains an
enlargement which lies directly opposite the eye-spot, as

^2 LIGHT AND TEE BEHAVIOR OF ORGANISMS

represented in Fig. ii. Under certain conditions some
forms cast off the flagellum, sink to the bottom and crawl

about in a manner to be described in
detail later. In the encysted state, as
is well known, they are inactive.

h. Historical account. — It has long
been known that these organisms in
their free-swimming state orient and
-c.v. swim toward a source of light, and
Stahl (1880, p. 410) found that if the
light intensity is high they become
negative, i.e., they swim away from
the source of light. Engelmann (1882,
Fig. II. Side view of p. 396) observed that if Euglenae are

anterior end of Euglena ,1 1 • j ^ • ' ±.

viridis, after Wager;., eye- Hiountcd on a slide contammg a spot
spot; /, flagellum; e./., en- of relatively Strong light they collect

largement in flagellum; c.r., • j • . i • . • ,

contractile vacuole. ^"^ dense masses m^ this spot just as

Paramecia collect in regions contain-
ing a little CO2. They swim into it without any ap-
parent reaction, but when they reach the boundary on
the way out they stop suddenly, turn around, and thus
remain in the illuminated area. Engelmann called this
reaction Schreckhewegung, shock-movement, and Jennings,
avoiding reaction. Engelmann also proved that the ante-
rior end of E. viridis is more sensitive than the posterior.
Jennings (1904) however was the first to demonstrate
the connection between the shock-movement, the sudden
turning when subjected to a decrease in illumination,
and orientation, although the idea expressed in the fol-
lowing words shows that Engelmann (1882, p. 395) was
also very near the truth in this matter: " Falls sie, was
bei schnellem Vorwartsschwimmen wohl einmal geschieht,
gans ins Dunkel hineingekommen sind, sistiren sie doch
so fort die weitere Vorwartsbewegung, drehen um eine
ihrer kurzen Axen, probiren — oft unter bedeutenden
Gestaltsanderungen — in verschiedenen Richtungen fortzu-
kommen bis sie endlich wieder ins Licht gerathen."

OBSERVATIONS ON UNICELLULAR FORMS 83

Jennings found that as Euglena swims on its spiral course
it rotates on its long axis so as to keep the side containing
the eye-spot constantly facing out, and that when it is
stimulated it always turns toward this side, which is desig-
nated the dorsal side. The process of orientation is de-
scribed as follows (1906, p. 138): "The Euglenae are
swimming about at random in a diffuse light, when a
stronger light is allowed to fall upon them from one side.
Thereupon the forward movement becomes slower and
the Euglenae begin to swerve farther than usual toward the
dorsal side. Thus the spiral path becomes wider and the
anterior end swings about in a larger circle and is pointed
successively in many different directions. In some part
of its swinging in a circle the anterior end of course be-
comes directed more nearly toward the light ; thereupon the
amount of swinging decreases, so that the Euglena tends
to retain a certain position so reached. In other parts
of the swinging in a circle the anterior end becomes less
exposed to the light; thereupon the swaying increases, so
that the organism does not retain this position, but swings
to another. The result is that in its spiral course it suc-
cessively swerves strongly toward the source of light, then
slightly away from it, until by a continuation of this process
the anterior end is directed toward the light. In this
position it swims forward. The course of Euglena in
becoming oriented is shown in " Fig. 12.

Orientation in Euglena is, therefore, according to Jen-
nings, indirect. The stimulus resulting in orientation is
due to changes in light intensity on the organism. The
direction of the rays functions in orientation only in so far
as it makes such changes possible. Changes of intensity
on the organism may be due to movement from a region
of one intensity to that of another, or to a change in the
axial position of the organism with reference to the source
of light. There Is no evidence that orientation is due to a
constantly acting directive stimulus In accord with Loeb's
theory of troplsms. Jennings does not deny that the

84

LIGHT AND THE BEHAVIOR OF ORGANISMS

Euglenae are affected by light after they are oriented. He
thinks, however, that whatever such effects may be, they
are relatively unimportant in the process of orientation.

^— f

^\-~.e_,^^"~^\

b{

}a

4^

L 4

^^_

Fig. 12. Illustration of the devious path followed by Euglena in becoming
oriented when the direction of the light is reversed. From i to 2 the light comes
from above; at 2 it is reversed. The amount of wandering {a-h) varies in different
cases. After Jennings (1906, p. 137).

Torrey (1907, pp. 317, 319) criticizes the analysis pre-
sented by Jennings in the following terms: " My analysis of
their responses, based upon the figure which Jennings him-

OBSERVATIONS ON UNICELLULAR FORMS 85

self has drawn, with text description, leads to quite a
different conclusion from his. The figure indicates that
Euglena is both unterschiedsempfindlich and heliotropic.
At a (Fig. 12) the reversal in the direction of the light,
which has been coming from the direction in which the
creature has been swimming, produces a sudden change in
intensity of stimulation, a shock which results in the swerv-
ing from the previous course, as indicated between a and c.
The organism recovers rapidly, only to be subjected to the
constant stimulus of a steady light from one direction to
the end of the experiment. The result of the action of the
constant stimulus is a path, from c to 5, so perfectly in
harmony with the tropic schema, that, in spite of Jennings'
descriptions and elucidations, I can only wonder at his
running so boldly and so far into the enemy's camp. . . .
In heliotropism . . . the oriented organism is in a condi-
tion of physiological stimulation, and . . . the response to
stimulation is local ; finally, . . . the interpretation of the
behavior of heliotropic organisms on the basis of general
changes concerning the whole organism, not only does not
accord with the main facts, but is rather psychical than
physiological in character."

It is thus evident that while Torrey recognizes that
Euglena responds to change of light intensity, he considers
that orientation is due to the local effect of unequal stimu-
lation of symmetrically situated points on the body, and
that after the organism is oriented it is held upon its course
by constantly acting directive stimulation. He does not,
however, explain where the symmetrically located points
which are subject to local stimulation are situated in Euglena.
They might be conceived to be in the flagellum or in the
body. In the former case It would imply direct action of
the point stimulated, In the latter a reaction in harmony
with the location of the stimulus, i.e., if the stimulus is
applied to the left side of the body the flagellum would
strike toward the left; if applied to the right side. It would
strike toward the right, etc.

86 LIGHT AND THE BEHAVIOR OF ORGANISMS

If Euglenae actually orient by local response to local
stimulation, as Torrey assumes, or if light acts constantly
as a directive stimulus in accord with Loeb's theory, one
should be able to find evidence of it in these organisms in
the crawling state. With this in mind, therefore, I took
up the study of specimens in this state.

Before entering on the description of the reactions in
Euglena bearing directly on the problem just stated, I shall
however refer briefly to the question of orientation in
light from several sources, since the experimental results
obtained under these conditions throw some light on the
idea of Sachs, that the direction of the rays through the
organism regulates orientation, and on Loeb's idea that
symmetrically located points on the sensitive surface must
be struck by rays at the same angle when an organism is
oriented.

c. Orientation in light from two sources. — In studying
the movement of Euglenae in light from two sources,
Nernst glowers in a dark room were so arranged and
screened as to produce two small horizontal beams of light
which crossed each other at right angles in the aquarium.
One glower was stationary. The other was mounted on a
horizontal track so that it could easily be pushed nearer
to or farther away from the aquarium. Thus the relative
intensity from the two glowers could be changed without
any change in the direction of the rays. Several species of
Euglena in the free swimming state, and two, Euglena deses
and Euglena x in the crawling state, were used in these
experiments. The results were the same in all.

When the light from the two glowers was equal and the
Euglenae positive they moved in a general way toward a
point very nearly halfway between the glowers. But when
it was unequal, they moved toward a point nearer the
source from which the more intense light came. Negative
specimens take the same general course but in the opposite
direction. This experiment is particularly striking if the
glower on the track is gradually moved from a position in

OBSERVATIONS ON UNICELLULAR FORMS 87

which the Hght intensity from it is much lower than that
from the stationary glower to a position in which it is much
higher. Under such conditions one can clearly see these
organisms, especially the free-swimming forms, gradually
change their direction of motion through an angle of nearly
90°. (Just how this change is brought about will be
demonstrated later.) By regulating the relative intensity
of the light from the two sources, it is thus possible to
cause Euglenae to move toward any point between the two
sources of light without changing the direction of the rays.
It is evident then that the direction of the rays does not
absolutely control the direction of motion. These results are
in harmony with those which I obtained in experiments on
Volvox (1907, p. 134). Identical results were also obtained
in light from two sources with Stentor coeruleus, Trachelo-
monas, Chlamydomonas, Oedogonium swarm-spores, Eu-
dorina, Pandorina, Planulae of Eudendrium, Limulus
polyphemus larvae, Musca larvae, Allolobophora foetida,
medusae of Bougainvillea superciliaris, trochophores of
Hydroides dianthus, Arenicola larvae, zoeae, several forms,
and Leptoplana tremellaris. Judging from these results
it is highly probable that all individuals without image-
forming eyes orient in the same way under like conditions.
All of these forms can be induced to change their direc-
tion of motion by varying the relative light intensity on
opposite sides of the body, or by changing the intensity
on the same side, without changing the direction of the rays.
It may therefore be concluded that difference in the inten-
sity of light on opposite sides, or a change of intensity on
the same side of the body of all these creatures, may deter-
mine orientation independently of the direction of the
rays. The orientation of organisms without image-form-
ing eyes can therefore not be explained by the application
of Sachs' ray direction theory, nor are the orienting reac-
tions in harmony with the statements of Loeb expressed
in the following quotations: (1905, p. 2), " It is explicitly
stated in this and the following papers that if there are

88 LIGHT AND THE BEHAVIOR OF ORGANISMS

several sources of light of unequal intensity, the light with
the strongest intensity determines the orientation and
direction of motion of the animal. Other possible compli-
cations are covered by the unequivocal statement, made
and emphasized in this and the following papers on the
same subject, that the main feature in all phenomena of
heliotropism is the fact that symmetrical points of the
photosensitive surface of the animal must be struck by
the rays of light at the same angle. It is in full harmony
with this fact that if two sources of light of equal intensity
and distance act simultaneously upon a heliotropic animal,
the animal puts its median plane at right angles to the line
connecting the two sources of light. This fact was not
only known to me but had been demonstrated by me on
the larvae of flies as early as 1887, in Wiirzburg, and often
enough since. These facts seem to have escaped several
of my critics; " (p. 61), " When the diffuse daylight which
struck the [Musca] larvae came from two window^s, the
planes of which were at an angle of 90° with each other, the
paths taken by the larvae lay diagonally between the two
planes. . . . This experiment was recently published by an
American physiologist as a new discovery to prove that I
had overlooked the importance of the intensity of light!"
(p. 82), '' The direction of the median plane or the direction
of the progressive movements of an animal coincides with
the direction of the rays of light ... if there is only a
single source of light. If there are two sources of light of
different intensities, the animal is oriented by the stronger
of the two lights. If their intensities be equal, the animal
is oriented in such a w^ay as to have symmetrical points of
its body struck by the rays at the same angle;" (p. 268),
" Attention need scarcely be called to the fact that if rays
of light strike the animal [larvae of Limulus polyphemus]
simultaneously from various directions, and the animal is
able to move freely in all directions, the more intense rays
will determine the direction of the progressive movements."
Note that this animal is in the list mentioned above (p. 87).

OBSERVATIONS ON UNICELLULAR FORMS 89

Under the conditions of the experiment described above,
the organisms mentioned do not move in a direction parallel
with the rays, neither do they necessarily orient so " that
symmetrical points of the photosensitive surface [are]
struck by the rays of light at the same angle," nor does
" the light with the strongest intensity determine the
orientation and direction of motion."

Toads (Bufo americanus) were the only animals w^ith
image -forming eyes that were tested with reference to
orientation in light from two sources (see p. 87). If the
intensity from the two sources is unequal they usually hop
directly toward the stronger light and pay no attention to
the weaker. This is in accord with Loeb's explanation
given above. But if the intensity from the two sources is
equal, they go toward either one and not toward a point
between the two, as Loeb's explanation demands. In none
of the organisms studied are the orienting reactions such
as are demanded by Loeb's explanation. These results
will be referred to in connection with the discussion of the
importance of equal stimulation of symmetrical points on
the animal.

Let us now return to our study of the reactions of Euglena
in the crawling state and to the problem suggested by
Torrey's criticism of Jennings referred to above. Is orien-
tation in Euglena due to light acting constantly as a direc-
tive stimulation similar to the effect of a constant electric
current, or to an intermittent effect, a response to change
of intensity only, in accord with Jennings' explanation?

d. Material. — During the months of November and
December excellent material for this study was discovered
in a puddle of water fed by a drain from a dwelling house
at Windsor Hills, Baltimore. The bottom of the puddle
was covered with a dense green layer composed almost
entirely of two species of Euglena, — E. deses and another
species which was somewhat like viridis but could not be
positively identified. It will be referred to as Euglena x.
Most of the E. deses had fiagella, but the E. x with very

QO LIGHT AND THE BEHAVIOR OF ORGANISMS

few exceptions had none. The latter were considerably
smaller than the former. They averaged nearly 0.08 mm.
in length and somewhat more than 0.015 mm. in diameter.
A fairly good idea of the form and structure may be obtained
by referring to Fig. 10. It will be seen in this figure that
the caudal end terminates in a spinelike process, and that
the eye-spot, in close contact with the canal leading from
the contractile vacuole, forms an angle of about 45° with
the long axis of the body. The eye-spot has the form of a
flattened disk somewhat curved, so as to fit around the
canal.

e. Method of locomotion. — It is frequently stated that
Euglenae in this state progress by amoeboid movements,
i.e., by streaming movements. I was, however, unable to
detect anything resembling streaming movements in any
of the several different species studied in the crawling state.
Many do change their form very much by contracting in
various ways, and some may move slightly by thrusting
the anterior end forward and then drawing up the posterior
end, but progression in this way is relatively unimportant.

The process of locomotion without flagella appears to be
much the same in all forms observed. It was however
studied in detail only in Euglena x. While in motion these
organisms usually are considerably curved, being convex
on the ventral surface, the side opposite the eye-spot.
They rotate on the long axis either entirely over to the left,
as seen from the posterior end, or only halfway, then back
again, lying on the dorsal surface during this apparent
rocking movement. During either of these rotating move-
ments both ends appear to move back and forth. The
posterior end however moves laterally much less than the
anterior. In many instances it continues forward in nearly
a straight path, while the anterior end progresses on a
spiral course of considerable relative width.

While thus rotating the organisms appear to slide along,
moving forward a little with each turn of the body. They
progress at the rate of about 0.3 mm. per minute. Pre-

OBSERVATIONS ON UNICELLULAR FORMS 91

cisely what factors are involved in causing the forward
movement I was not able to ascertain. Only very slight
contractions can be seen at any time and no streaming
movements at all.

The posterior end is in much closer contact with the sub-
stratum than the anterior. If currents of water are passed
back and forth over the Euglenae it can be seen that the
anterior end is free, for it moves with the current. Fre-
quently specimens are found attached to the slide with
only the tip of the caudal spine in contact with the sur-
face. In such specimens the whole body sw^ings about with
the current. They are held fast by an adhesive substance
which they secrete. The presence of such a substance can
be detected by passing a small glass rod across the path of
a crawling individual near its posterior end, or by pushing
the rod about on a slide containing numerous Euglenae
which have been crawling about for a short time. If this
is done the end of the rod soon becomes covered with a
substance to which cling numerous Euglenae attached
usually only at the posterior end. It is however not likely
that the extrusion of the secretion forces the Euglenae along,
as is supposed to be true in the case of diatoms. The body
appears to become alternately more and less curved as they
rotate in such a way as to force them forward. The caudal
spine appears to be used as a sort of lever in this movement.
They can however move without the use of the spine, for
moving specimens were repeatedly seen in which the point
of the spine was not in contact with the slide at all. This
was evident especially in specimens which rotated only
partially over and then back again.

As these creatures crawl along, rotating on the long axis
with the anterior end progressing on a spiral course, the
dorsal surface, the surface containing the eye-spot, always
faces the axis of the spiral. This is just the opposite of
Jennings' observations on Euglena viridis in the free-
swimming state. I found however that E. acus and a
few other species swim with the dorsal side facing the axis

92 LIGHT AND THE BEHAVIOR OF ORGANISMS

of the spiral and that E. deses swims with one side facing
the axis.

Euglenae in the crawHng state, just as in the free-swim-
ming state, may be either negative or positive in their
light reactions. The crawhng specimens worked on were
however negative to Hght of surprisingly low intensity
throughout the entire work. But very few were found
which were positive even in diffuse sunlight during the
middle of the day, unless the sky was covered with very
dense clouds. The cause of reversal in the sense of orien-
tation will be discussed elsewhere.

/. Accuracy of orientation. — In the study of their
reactions to light, the Euglenae w^ere exposed either to
sunlight direct and diffused, or to light from a Nernst
glower, a Welsbach burner or a carbon filament. When
exposed to light from a single source, e.g., a Nernst glower
so arranged that there is as little reflection as possible,
Euglenae orient and move nearly straight toward or away
from the light with little deviation, if they are strongly
positive or negative; but if they are not, as is frequently the
case, they deviate much. Even under the most favorable
conditions there is however little similarity between Eugle-
nae moving toward a source of light and iron filings moving
toward a magnet, a comparison sometimes met with in the
literature on reactions to light. In studying Euglenae one
always finds specimens which do the unexpected thing.
Their reactions are very much less dependent upon external
conditions than are the reactions of iron filings. To come
to a full realization of this, one need only consider the fact
that these organisms may be negative or positive in almost
any light intensity or they may not react at all. To predict
with any degree of accuracy what these organisms are going
to do under given conditions, it is necessary to know much
about the history of their past reactions.

g. Mechanics of orientation in Euglena x in the crawling
state. — Nernst glowers mounted in front of a non-reflect-
ing background (see Fig. 4) and properly screened in a large

OBSERVATIONS ON UNICELLULAR FORMS 93

dark room were used in all quantitative work, and in all
work in which it was desirable to regulate the direction
of the rays. I have elsewhere pointed out the advan-
tageous features of these glowers for such work (1906,

P- 363)-

The general movements of Euglenae could readily be
followed under a Braus-Driiner binocular, but it was found
necessary to use a compound microscope in working out
the details in the reactions owing to the small size of the
organisms. They progress so slowly however that every
movement can easily be followed even under the highest
magnification. They are consequently very favorable for
the work in hand notwithstanding their minute size.

In studying the process of orientation the microscope was
placed either in front of two windows in the laboratory
so situated that the general direction of the light entering
them was at right angles on the stage, or in the dark room
in a similar relative position in front of two Nernst glowers
(see Fig. 13). The two glowers were mounted so that the
rays were practically parallel with the plane of the stage.
One was stationary; the other was mounted on a track so
that the distance between it and the aquarium could readily
be varied, and thus the intensity of the light from it on the
stage changed without any change in the direction of the
rays. Both glowers were of the same kind and both were
in the same circuit, so that any fluctuation in the current
affected both alike. The relation in light intensity from
the two sources could thus be regulated as desired. The
glowers were so screened that only a small beam from each
reached the stage, and this could readily be cut off from
either or both.

The Euglenae were either mounted on a slide under a
cover-glass or exposed in a rectangular glass aquarium made
for the purpose by cementing slides together with balsam
and linseed oil. After they had oriented in light from one
of the two sources, the light from that source was cut off
and that from the other turned on simultaneously. In

94 LIGHT AND THE BEHAVIOR OP ORGANISMS

this way their reactions during the process of reorientation
could be studied in detail. The following description of
this process refers to E. x in the crawling state.

If the light in which positive organisms are oriented is
decreased in intensity without a change in the direction of
the rays, e.g., by pushing back the Nernst glower on the
track, they respond in a characteristically definite way.
If the decrease is relatively slight the anterior end is merely
turned toward the ventral surface, the whole body becomes
more curved and the spiral course of the anterior end
becomes wider. If however the decrease is considerable,
they frequently stop in their forward motion and turn the
anterior end toward the ventral surface to such an extent
that the two halves of the organism form a right angle.
In this condition they continue to rotate, turning over and
over in the same spot, and appear to be squirming and
twisting about aimlessly. They soon however straighten
again and continue on their way toward the source of light,
having apparently become acclimatized to the change in
light intensity. If the intensity is increased there is no
response in positive Euglenae. Negative individuals, on
the contrary, respond precisely as described above if the
light intensity is increased, but not at all if it is decreased.
If the specimens however are only slightly positive or nega-
tive they may be caused to respond with this twisting reac-
tion either by increasing the intensity or by decreasing it.
In order to induce this reaction it is necessary to change
the intensity at a certain rate. If the glower is moved back
very slowly and steadily, no reaction whatever is seen.
A sudden decrease of intensity then without any change in
the direction of the rays produces a definite reaction in
positive individuals, and a sudden increase of intensity
produces the same reaction in negative individuals. These
reactions are in accord with the shock effects of Engelmann
and Pfeffer and Unlerschiedsempfindlichkeit of Loeb. They
are not due to an absolute change of intensity but to the
time rate of change of intensity. The amount of change

OBSERVATIONS ON UNICELLULAR FORMS 95

necessary to induce a reaction will be discussed later
(p. 105).

If the intensity from the two sources of light arranged as
described above is equal and the beams which reach the
stage of the microscope are alternately cut off with an
opaque screen so as to change the direction of the rays
suddenly without changing the intensity, it appears as
though the Euglenae if positive always turn directly toward
the source of light, never away from it no matter in what
position they are or which surface becomes illuminated
when the ray direction is changed. These results would
seem to indicate that there is here a local response to a
local stimulation, or at least differential response to local-
ized stimulation. I was firmly convinced of the truth of
this for several days, as were also other members of the
laboratory who observed these reactions. Further work
however demonstrated the fallacy of this conclusion.

By very careful observations under the high power it
was found that if the ventral surface, the surface opposite
the eye-spot, faces the source of light, after the direction
of the rays is changed, there is no immediate reaction.
The Euglenae continue on their course as though no change
had taken place until the rotation on the long axis carries
the dorsal surface over into a position in which it faces the
light. As soon as this surface, the surface containing the
eye-spot, faces the light there is a definite reaction. The
Euglenae turn the anterior end toward the ventral surface
more or less sharply, i.e., away from the source of light, but
they continue to rotate so that the ventral surface soon
faces the light again; but it is evident, owing to the curva-
ture in the body, that the anterior end is now directed more
nearly toward its source than it was when this surface faced
the light during the preceding rotation. While in this
position, the body is somewhat straightened, so that the
anterior end is not carried back as far during the following
rotation, and when the dorsal surface comes to face the
light it is directed more nearly toward its source than it

96

LIGHT AND THE BEHAVIOR OF ORGANISMS

was when the organism was in this position before, as
represented in Fig. 13. This reaction is repeated during
each complete rotation. Every time the eye-spot becomes

$ ^ I * t ».

'■' i'/

i/ <'

o

•<= ^«

-«^

-^<

-^0<

-^i^^

-^S

-«a

-<^

■<r-

-<«s

^

n

-«*.

Fig. 13. Euglena sp. (?) in crawling state, showing details in process of orien-
tation; V, contractile vacuole; es, eye-spot; n, o, direction of light; a-c, positions
of Euglena with light from n intercepted; c-m, positions after light from n is turned
on and that from o cut off so as to change the direction of the rays. If the ray
direction is changed when the Euglena is in position c there is no ^-eaction until it
reaches d. Then it suddenly reacts by bending away from the source of light to e,
after which it continues to rotate and reaches position/, where it gradually straight-
ens to g, and rotates to h, when the eye-spot again faces the light and the organism
is again stimulated and bends to i, from which it proceeds to j, etc., to m, where
it is practically oriented. If the ray direction is changed when the Euglena is at d,
it responds at once and orients as described above. If the intensity from n is
lower than that from o the organism may respond at once when the ray direction
is changed no matter in which position it is. (Compare with orientation in Stentor,
Fig. 14.)

OBSERVATIONS ON UNICELLULAR FORMS 97

more strongly illuminated the organism responds by bend-
ing, and when it becomes shaded the creature gradually
straightens out and resumes its normal form again; thus
the anterior end becomes directed more and more nearly
toward the source of light until the organism reaches an
axial position in which the eye-spot is no longer exposed to
sufficient changes in illumination during the process of ro-
tation to cause a bending reaction. The organism therefore
continues in this direction, i.e., more or less nearly toward
the source of light. Orientation is frequently brought about
in two or three rotations. It is clear that during this pro-
cess light does not act continuously as an orienting stimulus.
Euglena responds with reactions leading to orientation only
when the dorsal side is turned toward the source of illu-
mination, not when the ventral side is exposed. And it
should be emphasized that the first movement in the re-
sponse is a bending away from the source of light, toward
which it later becomes oriented.

It is evident from the above description that turning into
such a position that the eye-spot faces the source of light
produces a stimulation which results in a definite reaction.
In this reaction the organism always bends the anterior
end toward the ventral surface. It appears at first thought
as though this reaction were due to the illumination of the
eye-spot. It will however be demonstrated that this is
not true.

If the light from the two sources arranged as described
above is not equal, and the tw^o beams which reach the
aquarium are alternately intercepted, it is evident that the
organism will be subjected simultaneously to a change in
the direction of the light rays and a change of light intensity.
If the stronger light is thrown upon the Euglenae after
they are oriented in the weaker, they orient just as de-
scribed above, but if the weaker is turned on after they are
oriented in the stronger there is an immediate reaction,
no matter which surface, the ventral or the dorsal, happens
to be exposed at the time. If it is the dorsal and the

98 LIGHT AND THE BEHAVIOR OF ORGANISMS

difference of Intensity between the light in the two beams
is not too great, orientation takes place just as described
above; but if it is the ventral surface which is exposed, it
frequently happens that the organism first becomes directed
away from the source of light and then toward it only after
repeated reactions. The first step in all these reactions,
regardless of how they are induced, is the same. It consists
of a bending of the anterior end toward the ventral surface.
It has been demonstrated (i) that this reaction can be
induced in positive Euglenae by reducing the light intensity
of the field without changing the direction of the rays, no
matter which surface is illuminated, and (2) that it can be
induced without any variation in the light intensity of the
field by changing the direction of the rays from one in
which the anterior end is illuminated to one in which the
dorsal surface is illuminated, or (3) it can be induced by the
rotation of the organism on the long axis from a position
in which the ventral surface is exposed to one in w^hich the
surface containing the eye-spot is exposed. Since the reac-
tion under the first condition can be due only to a change of
intensity on the whole or some part of the organism it is
evident that the reaction under the second and third con-
ditions is likewise due to a change of intensity. But since
the light intensity of the field is constant under these
conditions it is evident that the decrease of intensity must
be restricted to a portion of the body and that it must be
due to the shading of one part by another owing to the
movement of the organisms. Our observations show that
a change in the position of the organisms, from one in
which the ventral surface is illuminated to one in which
the dorsal surface is exposed, causes a reaction. Such a
change in position must therefore produce a change of
intensity on the sensitive parts of the organism. This may
be conceived to be due to the eye-spot's acting as an opaque
screen and casting a shadow when it faces the light on some
highly sensitive protoplasmic structure located near it
(see Fig. 11), or to the location of the more highly sensitive

OBSERVATIONS ON UNICELLULAR EORMS 99

material in such a position that it is more strongly affected
when the ventral surface is illuminated than it is when the
dorsal surface is illuminated. The function of the eye-spot
will be referred to again later.

Orientation in negative specimens takes place precisely
as it does in positive specimens. The reactions resulting
in orientation however are induced by an increase of inten-
sity in place of a decrease, as in the case of positive speci-
mens; and a change from a position in which the eye-spot
faces the light to one in which the ventral surface is exposed
induces the avoiding reaction, while in positive specimens
it is a change from the latter to the former which causes this
reaction.

After having thus worked out the details in the orienting
reactions in Euglenae in the crawling state, I made obser-
vations on specimens in the free-swimming state and found
the reactions to be essentially the same. A brief account
of these observations will be found below (p. 102).

h. Discussion. — The orientation of Euglena in the
crawling state confirms in general the description of the
orientation in the free-swimming state given by Jennings
(see p. 83). When the organism is not oriented every
change from a position in which the light strikes the ventral
surface to one in which it strikes the dorsal, and vice versa,
due to rotation on the long axis, may be considered a
" trial movement." If such a trial movement results in a
decrease of light intensity on the sensitive protoplasm in
the organism it responds with a definite reaction, after which
it repeats the trial movements. Thus it continues until it
becomes so directed in its course that rotation on the long
axis no longer produces sufficient change of intensity on
the sensitive part to induce a reaction. It is evident that
this condition is fulfilled when the organism moves toward
or away from the general source of light. Orientation can
take place in an absolutely constant intensity of light in
the field. It is however always induced by reactions which
are due to changes 0} intensity on some part of the organism.

lOO LIGHT AND THE BEHAVIOR OF ORGANISMS

This is of course due to the successive illuminating and shad-
ing of different parts of the organism owing to its move-
ments. There is no evidence indicating that light, acting
constantly as a directive stimulus similar to the action of
the electric current, has any influence on orientation of
Euglena in accordance with the idea of Loeb supported by
Torrey. This however does not mean that light does not
act constantly on the organism, for it is probable that It
does, much in the manner of temperature. The evidence
bearing on the point in question is however not conclusive.
Euglena does become more active when the Intensity is
increased, but it Is Impossible to say whether this increase
in activity Is due to absolute intensity or to the change
of intensity on certain structures caused by the rotation
of the organism. The fact that the movement of the
organism does cause changes of Intensity on different
structures in It makes the problem as to the effect of con-
stant light intensity an exceedingly difficult one to reach
experimentally.

Nageli (i860, p. 102) concludes that In swarm spores the
rate of movement Is Independent of the light intensity, and
Strasburger (1878, p. 624) comes to the same conclusion.
" Die Schnelligkeit der Bewegung wird durch das Licht
nicht beeinflusst, doch bewegen sich die Schwarmer je
grosser die LIchtintensItat ist, in um so geraderen Bahnen."
Pfeffer (1884, p. 375) also is of the opinion that chemical
stimulation of fern spermatozolds causes no acceleration of
movement. Holmes (1903, p. 323) however says: " It was
found that, as the Volvox travelled towards the light, their
movement was at first slow, their orientation not precise,
and their course crooked. Gradually their path became
straighter, the orientation to the light rays more exact and
their speed more rapid. After travelling over a few spaces
(centimeters), however, their speed became remarkably
uniform until the end of the trough was reached." I came
to the same conclusion in my study of Volvox (1907, p. 150),
but was of the opinion that the Increase In rate of movement

OBSERVATIONS ON UNICELLULAR FORMS lOi

is dependent more upon the time of exposure to light than
upon the increase of intensity.

The experimental difficulty of course lies in the fact
pointed out above, that the movement of the organism
itself causes change of intensity on different structures
in it. The fact however that Volvox, e.g., in swimming
toward a source of light into regions of higher intensity
without changing its orientation, swims more slowly as it ap-
proaches the region of optimum intensity and finally stops
altogether, seems to show very clearly that the intensity
affects the rate of movement. But the results of Holmes
also show that the rate bears no definite relation to the
absolute intensity. There is much need of more experi-
mental data on this subject.

Does Euglena always turn toward or from the side
stimulated? Is orientation due to differential response to
localized stimulation? Or is the organism stimulated as
a whole with a reaction dependent more or less upon the
structure of the organism? If we are correct in our assump-
tion that there is a highly sensitive protoplasmic structure
located in the anterior end of Euglena, it is likely that it
is always stimulated by light in the same place regardless
of the portion of the surface exposed. Judging from this
alone one might conclude that the stimulus acts as a local
sign. But the fact that Euglena in the crawling state
always bends toward the ventral surface when stimulated
by light, while specimens in the free-swimming state always
turn toward the dorsal surface when stimulated, contra-
dicts this conclusion and supports the idea that the stimulus
acts upon the organism as a whole. This is again in direct
opposition to Torrey's statement (1907, p. 319), " The
interpretation of the behavior of heliotropic organisms on
the basis of general changes concerning the whole organism,
not only does not accord with the known facts, but is
rather psychical than physiological in character." It is
difficult to see how the fact that an organism always turns
in the same direction when it is stimulated as a whole can

I02 LIGHT AND THE BEHAVIOR OF ORGANISMS

be considered a criterion of psychic activity. For all that
Is known to the contrary Euglena may be conscious. It
may Indeed have anthropomorphic sensations accompany-
ing each reaction. But surely no one would consider the
fact that It always turns toward the same side and does
not respond In accordance with the theory of localized
stimulation as Indicating that it has such sensations.

i. Orientation of Euglena in the swimming state. —
Early in December two species, Euglena deses and a form
much like viridls but somewhat larger, were found in the
free-swimming state. They were however not very active;
the specimens of E. deses studied swam only at an average
rate of approximately 0.25 mm. per minute and rotated
only about eleven times per minute; the other species moved
somewhat faster. The reactions In both could readily be
followed under a magnification of 150 diameters. Their
orienting reactions were studied In the same manner as
were those of Euglena In the crawling state, and they were
found to be practically the same.

A decrease of the light Intensity in the field without a
change In the direction of the rays produces definite reac-
tions, (i) There is a slight bending of the anterior end
toward the dorsal surface. (2) The whole organism turns
toward the dorsal surface by the action of the flagellum.
(3) Their spiral course becomes wider. If the decrease is
considerable they may be thrown out of orientation entirely
and turn about several times before they become oriented
again.

If the direction of the rays Is changed without the light
intensity's being changed, there Is usually no reaction,
just as m crawling specimens, until in the process of rota-
tion the surface containing the eye-spot comes to face the
light; then there is a sudden turning toward this surface,
i.e., toward the source of light. In many instances the
turning is so sharp immediately after the dorsal surface
becomes Illuminated that it may appropriately be desig-
nated as a jerk or a twitch. This reaction is repeated

OBSERVATIONS ON UNICELLULAR FORMS 103

every time this surface is turned toward the light, each
reaction resulting in directing the anterior end more nearly
toward the source of light, until both surfaces are so nearly
equally illuminated throughout the entire rotation that
the change of intensity is no longer sufficient to cause a
reaction.

If the light intensity is decreased at the same time that
the ray direction is changed, the reaction described above
always occurs immediately after the change is made, regard-
less of the surface illuminated. All however turn toward
the dorsal surface. This results in movement in all direc-
tions and apparent confusion. When Euglenae are taken
from the culture jars and first exposed in the aquarium they
are more sensitive and respond to slighter changes than
they do after they have been exposed for some little time.
Among these one finds many specimens which always re-
spond immediately after the ray direction is changed, even
if the intensity remains the same, no matter which surface
faces the light after the change is made. This indicates
that a change in the direction of the rays from that in
which the anterior end is illuminated to one in which the
ventral surface is illuminated causes the same response as
a decrease of intensity. It may therefore be concluded
from this and preceding observations that the organism is
most stable^ when the anterior end faces the source of light,
less stable when the ventral surface faces it, still less stable
when the dorsal surface is exposed, and least stable when
the posterior end is directed toward the source of light. I
was able to ascertain roughly the amount of reduction in
light intensity required to induce the avoiding reaction
with each of these different surfaces illuminated excepting
that on the posterior end. The results of this work to-
gether with a description of methods will be found below.

The orienting reaction of free-swimming specimens is
in all essentials like that of the crawling specimens. It

* It is meant by this that it requires a greater change of light intensity
in the field to induce a reaction.

I04 LIGHT AND THE BEHAVIOR OF ORGANISMS

takes place just as Jennings represents (Fig. 12), with the
exception that if the direction of the rays is changed with-
out any change of the intensity, orientation may take place
without an increase in the diameter of the spiral course
represented in Fig. 12, a-c. The organisms may orient by
increasing the swerving only toward the source of light
after its position is changed, not in the opposite direction.

The fact that these free-swimming specimens of Euglena
in certain physiological states do not respond at all after
the position of the source of light is changed from one in
which the anterior end is illuminated to one in which the
ventral surface is exposed, until the organism rotates so as
to expose the dorsal surface; that as soon as this surface
faces the light there is a sudden twitching turn toward the
source of light ; and that this reaction is repeated every time
the surface containing the eye-spot comes to be illuminated
in the course of the rotation on the axis, shows very clearly
that orientation in the free-swimming state as well as in
the crawling state is not due to a constantly acting stimulus,
as Torrey assumes.

Unequal stimulation of symmetrically located points, as
an important factor in causing orientation in accord with
the theories of Verworn and Loeb, is of course out of the
question in this form. If in heliotropism the results are
a function of the constant intensity, as Loeb maintains
(1906, p. 135), it must be admitted that there is no evidence
indicating that Euglena is heliotropic.

j. Threshold or sensitiveness when different surfaces are
exposed to light. — The difference in sensitiveness of the
organism with different parts of the surface illuminated
was measiared in the following way: positive specimens
were exposed in the small slide aquarium^ to light from the
glower on the track. After they had oriented, the intensity
was suddenly decreased without any change in the direction
of the rays, by sliding the glower away until it could be

^ An aquarium made of glass slides glued together with balsam boiled
in linseed oil.

OBSERVATIONS ON UNICELLULAR FORMS 105

clearly seen that a majority responded by definitely in-
creasing the width of the spiral The point at which such
response was given varied much with different individuals
under different conditions and could therefore not be accu-
rately ascertained. It was however discovered that if the
organisms were oriented in 61 ca. m. the intensity had to
be decreased to 17 ca. m. before unquestionable response
resulted. This shows that under the conditions of the
experiment it requires a decrease of 44 ca. m., or over 66
per cent of the total intensity, to induce the avoiding
reaction when the light strikes the anterior end.

By changing the position of the movable glower, the re-
lation between the intensities of light from the two glowers,
here arranged as in many of the preceding experiments,
was so adjusted that when the Euglenae were suddenly
exposed in the stronger light after they had oriented in the
weaker, nearly all responded at once, regardless of the sur-
face illuminated. Those in which the ventral surface was
exposed turned away from the source of light; those with
the dorsal surface illuminated turned toward it, and the
rest turned in various other directions. The reaction is
very striking under these conditions, although of course it
was possible to ascertain only approximately the change
of intensity necessary to produce it. It was found after
many trials that the least reduction of light intensity with
a simultaneous change in the direction of the rays, which
caused this reaction in a majority of the specimens, was
32 ca. m., the intensity of the light from the stationary
glower being 61 ca. m. and that from the movable glower
29 ca. m. It will of course be understood that individuals
frequently respond to much smaller changes of intensity,
depending upon their physiological state. With the an-
terior end exposed then, a reduction of 44 ca. m. without a
change in the direction of the rays is sufficient to cause the
avoiding reaction. With the ventral surface exposed a
reduction of 32 ca. m. together with a simultaneous change
in the direction of the rays causes the avoiding reaction,

lo6 LIGHT AND THE BEHAVIOR OF ORGANISMS

and with the dorsal surface exposed the same reaction is
induced by a change in the direction of the rays without a
decrease of intensity. These results lead to the conclusion
that a change of the organism from a position in which the
anterior end faces the so'urce of light to one in which the
dorsal surface faces it, results in a reduction of effective
Itght intensity of approximately 44 ca. m. in a total inten-
sity of 61 ca. m. Since the anterior end of Euglena is
nearly transparent, such a relatively large reduction seems
possible only if there is a highly sensitive bit of protoplasm
so situated that the eye-spot casts a shadow on it when the
light strikes the dorsal surface.

k. Function of the eye-spot. — Wager (1900, PI. 32,
Fig. 2) observed an enlargement in the flagellum, situated
very near the concave surface of the eye-spot (see Fig. 11).
It may be that this is highly sensitive to changes in light
intensity and that the eye-spot functions as an opaque
screen casting a shadow upon the enlargement whenever
the dorsal surface is exposed. It may however also function
in absorbing the rays when the ventral surface or the
anterior end is exposed, much as the retinal pigment func-
tions in the eye of higher forms, or it may function some-
what like the pigment cups in planarians, amphioxus, etc.

The only evidence we have with reference to the function
of the eye-spot aside from that presented above is given by
Engelmann (1882, p. 396). He says, in substance, refer-
ring to this structure in Euglena viridis, that if a sharp
shadow is gradually brought from the posterior end of a
swimming Euglena toward the anterior, there is no reac-
tion until the shadow reaches the colorless anterior portion
of the organism which contains the eye-spot. In the case
of large individuals moving into a shadow, the reaction
could be seen to be given before the eye-spot was in dark-
ness. The colorless anterior end is therefore the primary
light recipient region, but the eye-spot may still function
secondarily, as do the pigment cells in the retina of higher
animals. These observations of Engelmann have been

OBSERVATIONS ON UNICELLULAR FORMS 1 07

widely quoted, and it is generally assumed that they prove
that the sensitive portion of Euglena is located anteriorly
from the eye-spot, and some hold that they even prove
that the eye-spot does not function in light reactions at all.
My observations on Euglena, however, seem to indicate
that the portion most sensitive to light lies in close proximity
with the inner surface of the eye-spot, not in front of it.

I repeated the experiment of Engelmann as follows: An
opaque screen containing a rectangular opening 2X3 cm.
was placed between the microscope and a Welsbach burner
as near the globe of the burner as possible. A piece of tin
was then hung inside the globe of the burner a few milli-
meters from the Welsbach mantle and so arranged that
one of the straight edges could be seen through the open-
ing in the screen. By means of the Abbe condenser that
portion of the mantle exposed was focused on a slide under
the microscope, containing Euglena deses and E. viridis (?),
also E. triqueter and other species. The edge of the tin
focused on the slide gave a strikingly sharp edge between
the light area and the shadow. The reactions of the
Euglenae were studied as they approached this edge. Both
low and high power were used in the observations. The
relation of intensity between light and shadow could be
regulated by manipulating the iris diaphragm connected
with the Abbe condenser, and the light area could easily
be shifted by turning the mirror. In this way it was pos-
sible to move the shadow of the tin over any portion of the
specimens while they were in motion. The Euglenae under
observation swam about very slowly, E. deses at the rate
of approximately 0.3 mm. per minute and viridis (?) not
much faster. Every movement and reaction could be
distinctly seen. I was however able to confirm Engel-
mann's conclusions only in part. The Euglenae generally
reacted before the entire body entered the shadow, and no
response was observed when the posterior end was shaded
until the shadow reached the anterior end, proving in
accord with Engelmann's conclusion that the anterior end

lo8 LIGHT AND THE BEHAVIOR OF ORGANISMS

is undoubtedly more sensitive than the posterior. Speci-
mens were also repeatedly seen to react as soon as the
anterior end in front of the eye-spot came into the shadow,
but many were seen to turn about before the anterior end
reached the shadow at all, presumably owing to causes
other than changes in light intensity. I was therefore
at no time certain that those which reacted when only
the tip of the anterior end touched the shadow would not
have reacted had they not come in contact with the shadow.

But suppose that those which did react before the eye-
spot was shaded were stimulated by the shadow on the
anterior end in front of the eye-spot, would this prove that
the eye-spot is not a light recipient organ or that there is
no highly sensitive structure back of the part stimulated?
It evidently would not, for as soon as the anterior end
touches the shadow, the light which is reflected from it
onto the structures in the interior of the body before it
reaches the shadow is cut off. The light intensity on
structures which are not in the shadow at all is therefore
reduced as well as that on those which are in the shadow,
and it may be that the decrease of intensity on the former
causes the reaction.

The possible effect on structures in Euglena near the eye-
spot, due to shading merely the tip of the anterior end, can
readily be illustrated by noting the effect if one looks into
the mouth of a test tube full of translucent jelly and throws
a shadow on the closed end. The reduction of light will
of course affect the eye at once, although it may be a con-
siderable distance from the shadow.

If there were a differentiated bit of protoplasm highly
sensitive to variation in light intensity, located in close
proximity to the eye-spot on the side facing the interior
of the body, one might even expect the organism to react
before the anterior end reaches the shadow at all, for, since
there is no light reflected from the shaded area, it is evident
that merely turning the anterior end toward it would result
in a decrease of light intensity on the postulated sensitive

OBSERVATIONS ON UNICELLULAR FORMS 1 09

structure, and moving toward the shadow would decrease
it still more.

There is consequently nothing in connection with the
observations of Engelmann which contradicts the idea that
the eye-spot in Euglena functions as an opaque screen, and
that there is a bit of protoplasm which is highly sensitive
to changes in light intensity in close contact with it. The
hyaline protoplasm at the anterior end condenses the light
so that it is most intense in the neighborhood of the eye-
spot. This can be seen in Euglena in direct sunlight. It is
however much more marked in Chlamydomonas and the
zooids of Eudorina and Pandorina (Figs. 17 and 21). If
the light is thus actually focused on the most sensitive
structure of the organism it is easy to see how changes in
the general direction of the rays could produce marked
changes of intensity on this structure. Aside from acting
as an opaque screen the eye-spot may, of course, as already
stated, also function as an absorptive background.

In Trachelomonas hispida the eye-spot is situated very
near the middle of the anterior end (see Fig. 16). If it
functions by shading the sensitive portion or by absorbing
the rays in this form it is highly probable that the sensitive
portion consists of a minute structure situated very near
it, perhaps in the hollow of the concave surface. In some
of the forms however the location of this structure seems
to show that it does not function as a screen. In Vol vox,
Pandorina and Eudorina the eye-spots are situated on the
outer posterior surface of the zooids. It is difficult to see
how they could function as screens in these forms (see Fig.
21). The same difficulty is encountered in some forms of
Chlamydomonas in which this structure is situated near
the posterior end (see Fig. 17). It does however not seem
necessary to assume that the eye-spot functions precisely
the same in all forms. While it may function both as an
opaque screen and as an absorptive background in Euglena,
it may possibly function only by absorbing light rays in
Volvox and Chlamydomonas,

no LIGHT AND THE BEHAVIOR OF ORGANISMS

3. Summary

(i) Some species of Euglena exist in three states, — • free-
swimming, crawling and encysted.

(2) While in the crawHng state they push themselves
along at the rate of about 0.3 mm. per minute by alter-
nately curving and straightening the body very slightly as
they rotate on the long axis. There is no evidence of loco-
motion by means of amoeboid movement.

(3) In this state they orient fairly accurately in light.
They may be either positive or negative.

(4) When exposed to light from two sources they may
move toward or from a point located anywhere between the
two sources. The location of this point depends upon
the relation in amount of light from the two sources. If
the light from one source is stronger than that from the
other, this point will lie nearer the source from which the
stronger light comes.

The orientation of sixteen other species in light from two
sources was ascertained. All of those without image-form-
ing eyes, fifteen in number, oriented just like Euglena.
The one with image-forming eyes always moved directly
toward one or the other of the sources of light, never
toward a point between them. It is therefore evident that
Loeb's statement regarding this point will not hold for
any of these organisms.

(5) If the intensity is decreased without any change in
the direction of the rays, positive Euglenae in the crawling
state always respond by bending the anterior end toward
the ventral surface. This may be termed a shock-move-
ment, or avoiding reaction, or a bending reaction.

(6) A change from a position in which the ventral sur-
face faces the source of light to one in which the dorsal, i.e.,
the surface containing the eye-spot, faces it, induces the
bending reaction. Such a change in position therefore pro-
duces the same result as does a reduction in the light inten-
sity. These reactions can consequently be induced either

OBSERVATIONS ON UNICELLULAR FORMS III

by changing the direction of the rays or by changing the
light intensity of the field.

(7) The bending reactions are induced wherever the light
strikes the dorsal side of Euglena owing to its rotation on
the long axis. This reaction is repeated until the organism
is oriented and rotation no longer causes a change of illumi-
nation on its dorsal and ventral surfaces. It remains ori-
ented because, while it proceeds in this direction, there are
no stimulations which induce the bending reaction.

(8) The intensity can be so gradually changed that there
is no response. The bending reaction is therefore depend-
ent upon a time rate of change, and orientation is conse-
quently also due to a time rate of change in the light
intensity.

(9) The results of these experiments support Jennings'
conclusion that orientation in Euglena is brought about by
selection from trial positions. This of course does not
mean conscious selection.

(10) It is probable that light has a constant effect on the
activity of Euglena much as temperature does, but there is
no evidence that such activity has anything to do with the
process of orientation, as the explanations of Loeb, Ver-
worn, and Torrey demand.

(11) Orientation is not dependent upon the direction in
which light rays pass through the tissue, in accordance
with Sachs; nor is it dependent upon the angle between
the rays and the sensitive surface, or the unequal stimula-
tion of symmetrically located points on the surface, as Loeb
assumes; nor upon the effect of the stimulating agent upon
the locomotor organs directly, or through a direct reflex
arc, as the theory of Verworn demands; nor upon light
acting constantly as a directive stimulus, in accord with
Loeb's idea supported by Torrey.

(12) The most highly sensitive portion of Euglena is
probably situated near the concave surface of the eye-spot,
and the eye-spot probably functions in casting a shadow
on the highly sensitive substance when the light strikes the

112 LIGHT AND THE BEHAVIOR OF ORGANISMS

dorsal surface. The eye-spot may also function in absorb-
ing the rays.

(13) There is no evidence in these experiments bearing
on the question of anthropomorphic sensation. The results
do not exclude its presence. The reactions are due to
changes in light intensity, and every change, for all that is
known to the contrary, may cause a sensation.

CHAPTER VI

OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS
OF ATTAINING AND RETAINING A DEFINITE AXIAL
POSITION WITH REFERENCE TO THE
SOURCE OF LIGHT (continued)

I. Stentor coeruleus

a. Introduction. — Davenport (1897) and Holt and Lee
(1901) worked out the general features in the light reactions
of Stentor coeruleus. They found that these animals are
negative and that they orient rather accurately. Holt and
Lee concluded that orientation takes place in accord with
Verworn's theory, that light acts constantly as a directive
stimulation. If one side is more highly illuminated than
^e other the cilia beat more effectively on the illuminated
side. This causes the animal to turn directly from the
source of light until it is oriented and both sides are equally
illuminated. Jennings (1904) found that an increase in the
light intensity of the field causes a definite reaction in Sten-
tor regardless of the direction of the rays or the surface
exposed at the time the change is made. This reaction was
designated the avoiding reaction. It consists of turning
toward the right aboral side. The organisms may stop
and turn very sharply or they may simply swerve farther
towards this side as they proceed on their spiral course.
By means of this turning the anterior end is directed toward
various points in space. Sooner or later it becomes directed
away from the source of light and the organism is oriented.
This direction is retained because when the anterior end is
turned from the light it is not subjected to changes in light
intensity as the animal rotates on its axis and continues
on its spiral course.

Orientation, therefore, according to Jennings, is brought

113

114 LIGHT AND THE BEHAVIOR OF ORGANISMS

about by reactions which are induced by a change in the
effective intensity. This may be due to actual change of
the intensity of the field, to a movement from a region of
one intensity to that of another, or to a change of intensity
on the surface of the organism caused by changing the sur-
face turned toward the source of light. Direction of rays
and difference of intensity in the field are functional in the
process of orientation only in so far as they may influence
change of intensity on the organism. Orientation is not
induced by a constantly acting directive stimulus; it is the
result of a response to a time rate of change of intensity, a
shock-effect, U titer schieds em pfindlichkeit.

Working independently of Jennings I obtained (1906)
results which were in all essentials in harmony with his.
Jennings assumed that the anterior end of Stentor is more
sensitive than the posterior. I proved that Stentors are
more sensitive to light when the anterior end is exposed
than they are when any other surface is exposed. The
minimum threshold in animals stimulated by rays perpen-
dicular to the long axis was found to be 1.2 ca. m., and
that in those stimulated by light striking the anterior end
only 0.25 ca. m.

h. Orienting reactions. — I was of the opinion that while
the avoiding reactions no doubt play a large part in orien-
tation of Stentor, a direct effect of light as a constantly
acting stimulus in orientation might be discovered by a
careful investigation with this in mind.

Three methods were used in this investigation: (i) Water
was removed from under the cover glass until the space
between it and the slide became so narrow that the Stentors
could no longer rotate on their axes. They were then illu-
minated so that various surfaces were successively exposed.
If light acts constantly as a directive stimulus one might
expect the cilia on the illuminated side to strike back more
vigorousl^^ than those on the shaded side regardless of the
surface exposed. I was however unable to observe any
relation between the rate of movement of the cilia and the

OBSERVATIONS ON UNICELLULAR FORMS 1 15

surface illuminated as indicated by the currents in the
water.

(2) A number of attached Stentors in a small rectangular
glass aquarium were repeatedly suddenly exposed in light
intensity of 8000 ca. m. produced by a Nernst lamp. When
they were thus exposed they were directed toward various
points of the compass, so that various parts of the surface
faced the source of light in different individuals. A few
always contracted immediately after each exposure, others
began to swing about the point of attachment, some clock-
wise and others counter-clockwise, but all turned toward
the ventral surface. The cilia must consequently beat the
same in all individuals no matter which surface is exposed
to the light. There is therefore no evidence in these results
that light acts constantly as a directive stimulus.

(3) After Stentors had oriented in light from a single
Nernst glower, the glower was slightly moved to one side
so as to change the direction of the rays slightly, and the
method of reorienting was observed. It was found that
under such conditions the Stentors merely swerve farther
away from the source of light each time after the oral side
is directed toward it in the process of rotation. Thus they
soon become oriented again. There is no definite avoiding
reaction in this process of orientation. The organisms
never increase the swerving toward the source of light ; they
always increase it in a direction which tends to turn the
anterior end from the light. Does light act as a constantly
directing stimulation in this process of orientation or does
it act by causing repeated successive stimulations due to
changes of intensity on some part of the surface of the
organism as in Euglena? Is Stentor heliotropic according
to Loeb's definition or is it unterschiedsempfindlich? The
following experimental observations will furnish answers to
these questions.

Two Nernst glowers were arranged and screened so as
to produce two small beams of light which crossed at right
angles in a small aquarium containing numerous Stentors.

Il6 LIGHT AND THE BEHAVIOR OF ORGANISMS

The light intensity from each of the two glowers was equal.
The direction of the rays could therefore, without any alter-
ing of the intensity, be changed by alternately intercepting
the light in each of the two beams. If the ray direction is
thus changed after the Stentors are oriented in one of the
beams of light, one side will of course be directed toward
the light. If it chances to be the aboral side and the
Stentors are not very strongly negative, they continue on
their course just as though the direction of the rays had
not been changed, until in the process of rotation the oral
side comes to face the light; then the organism responds in
one of two ways: it may stop suddenly and sometimes back
a little and turn sharply toward the aboral side; that is, it
may respond with the avoiding reaction, or it may merely
swerve farther from the source of light on its spiral course
as represented in Fig. 14. When the oral side again comes
to face the light the organism is again stimulated and it
again swerves farther from the source of light. This reac-
tion is repeated once during each rotation until the oral
side is nearly equally exposed to the light throughout the
entire rotation. This is evidently true when the anterior
end is directed away from the source of light. If the or-
ganism responds with the avoiding reaction it turns more
directly from the source of light and thus becomes more
rapidly oriented, as represented in Fig. 14.

Why does Stentor respond when the oral side faces the
light and not when the aboral side faces it in the same
intensity? If Stentors are oriented in light of a given
intensity and the intensity is decreased without any change
of ray direction there is no response; but if it is increased
they respond in one of two different ways, depending upon
the amount of increase. If the increase is relatively slight
they merely swerve more strongly toward the oral side; and
since this side always faces out when the organism swims
on its spiral course, the result is that the course is made
wider. If the intensity increase is greater the creature
stops suddenly, turns toward the aboral side, sometimes

^^

Fig. 14. Stentor coeruleus in the process of orientation. The curved line
represents the spiral course; the arrows m and n the direction of light from two
sources; a-f, dififerent positions of Stentor on its course; 0, the oral surface; ab, the
aboral surface. At a the Stentor is oriented in Hght from m, n being shaded. If
n is exposed and tn shaded simultaneously when the Stentor is in position b, there
is usually no reaction, if the intensity has not been changed, until it reaches c and
the oral side faces the light; then the organism may respond bj^ suddenly stopping,
backing and turning sharply toward the aboral side, as indicated by the dotted
outline, and become oriented at once; or it may merely swerve more or less toward
the aboral side without stopping. At e the oral side is again exposed and the
organism is again stimulated and it again swerves from the source of light. This
process is continued until the oral side is approximately equally exposed to the
light in all positions on the spiral course. If the Stentor is at c when n is exposed
it responds at once and orients as described above. If the light from « is more
intense than that from m, or if the organism is very sensitive when n is exposed and
m shaded, it responds at once no matter in which position it is. If it is at b it turns
toward the source of light, but now repeats the reaction, successively turning in
various directions until it becomes oriented, 117

Il8 LIGHT AND THE BEHAVIOR OF ORGANISMS

backing slightly at the same time, and goes ahead swerving
sharply toward the ventral surface and the oral side. This
throws the animal out of orientation; the first method of
response does not. This shows that the reaction is due to
an increase of intensity on some part or on the whole of
the body. It is evident that the intensity of light on the
sides of Stentor changes, if it rotates while it is illumi-
nated from the side, owing to its own shadow; and since it
reacts only when the oral side is carried from a position in
which it is shaded to one in which it is illuminated, it is
clear that this side must be more sensitive than the aboral,
or perhaps better, that the animal is more sensitive when
the oral side is exposed than it is when the aboral side is
exposed.

If the light intensity is increased at the same time that
the direction of the rays is changed as described above, all
the organisms respond with the avoiding reaction at once no
matter which side faces the light, just as in the case of Eu-
glena. In this response some may be seen to turn upward,
some downward, and others to the right or left. They all
turn toward the aboral side. The direction in which they
turn therefore depends upon the position of this side when
the change is made. Stentors frequently respond thus w^hen
the direction of the rays is changed without varying the
intensity. This takes place when the organisms are highly
sensitive.

It is clear from this description that the orienting reac-
tions in Stentor and Euglena are the same in princible.
Both organisms can orient in a field of uniform light of
constant intensity. The stimuli causing orientation are
however due to changes of intensity on the sensitive struc-
tures in the body. Such changes of intensity in a field of
light of uniform and constant intensity are caused by the
shadows produced by one part passing over other parts as
the organisms rotate. There is no evidence that the direc-
tion of the rays functions in orientation excepting in so
far as it may influence changes of intensity; nor is there

OBSERVATIONS ON UNICELLULAR FORMS 119

any evidence that light acting constantly somewhat like a
constant electric current, has any effect on orientation as
Loeb's explanation of orientation demands.

c. Difference in sensitiveness with different surfaces
illuminated. — The threshold of reaction in Stentors varies
so much in different individuals and In the same individual
in different conditions that quantitative results are of little
value unless they can be correlated with causes of variation.
A few measurements made may however be of interest
in showing the relative stability of these organisms with
different parts of the surface exposed.

On February 12, specimens fresh from the culture jar
were put into the aquarium with the two Nernst glowers
arranged as described above. The intensity from the two
glowers was equal; it was 321 ca. m. When the ray direc-
tion was changed by intercepting alternately the beams of
light after the Stentors had become oriented, practically
all of them responded immediately with the avoiding reac-
tion regardless of their position when the change was made.
Some turned toward the light, others away from it, and the
remainder turned in various other directions. After these
specimens had been experimented upon for about fifteen
minutes only those responded immediately in which the
oral side faced the light when the direction of the rays
was changed. The rest did not respond until after they had
rotated sufficiently to expose the oral side. In responding
they gave either the avoiding reaction or merely swerved
farther from the source of light as they continued on their
spiral course. In both methods of reaction they always
turned directly from the source of light, never toward it.
In casually studying Stentors under these conditions only,
one might readily conclude that orientation is always direct
and that it is due to local response to a local stimulation.
This however is not the case.

As soon as the observations described above were com-
pleted, I put the Stentors into darkness, left them for a
short time and then exposed them to light from the

I20 LIGHT AND THE BEHAVIOR OF ORGANISMS

movable glower in an intensity of 150 ca. m. After they
had oriented the glower was suddenly pulled toward the
aquarium until it could be clearly seen that many of the
specimens responded with the avoiding reaction. By re-
peating this many times it was found that it required an
increase from 150 ca. m. to 444 ca. m. (or 294 ca. m.) to
throw them out of orientation. This may then be called
the threshold with the posterior end illuminated.

Frequently during the progress of the preceding experi-
ments the ray direction was changed and the light intensity
increased simultaneously. It was found that when the
intensity was thus increased from 150 ca. m. to 321 ca. m.
(or 171 ca. m.), nearly all responded at once regardless of
the surface turned toward the source of light after the
change was made; and under these conditions they could
be seen to turn toward the light as w^ell as from it. But
when the intensity was increased from 226 ca. m. to 321
ca. m. (or 95 ca. m.), only those responded in which the
oral side faced the light after the direction of the rays was
changed, and these also responded when the ray direction
was changed without an increase in light intensity, as repre-
sented in Fig. 14.

Judging from these results a change in the position of a
Stentor from one in which the posterior end faces the source
of light to one in which the oral side faces it, is equivalent to
increasing the intensity nearly threefold ; and a change from
a position in which the aboral side is illuminated to one in
which the oral side faces the light is equivalent to doubling
the intensity. These considerations show clearly how a
stimulation in a field of uniform and constant light intensity
can be produced by change of intensity. The fact that
Stentors are so much more sensitive when the oral surface
is illuminated than when the aboral surface or the posterior
end is exposed, points toward the presence of a highly sensi-
tive region in the neighborhood of the oral opening in Sten-
tors. The precise location of this region is a subject for
future investigation. The following experiments however

OBSERVATIONS ON UNICELLULAR FORMS 121

indicate that it is not in the membranellae or in the ridge
from which they project.

On February 27, 3.30 p.m., a number. of Stentors were
put into a one-half-normal glycerine solution. In the course
of about two minutes the entire ridge with the membranellae
in nearly all the specimens was thrown off, after which the
anterior end was rounded and the oral opening was tightly
closed. They were then transferred to normal culture fluid,
in which they swam about much like normal specimens.
They rotated on the long axis, proceeded on a spiral course
and responded with the avoiding reaction when they were
mechanically stimulated or when suddenly exposed to strong
light. The threshold for light reactions was however much
greater than normal. At 8.30 p.m. there was no indication
that regeneration had begun, but on the following morning
nearly all the specimens were normal again.

d. Localized stimulation. — Are the reactions in Stentor
differential responses to localized stimulation ? In Euglena,
it will be remembered, we were obliged to answer this ques-
tion in the negative. The results described above seem to
show that there is a highly sensitive structure in Stentor
at' the anterior end near the oral side. It is therefore
probable that whenever these animals are stimulated by
light they are stimulated in this region regardless of which
surface is exposed; and if this is true it is evident that the
fact that these organisms may turn toward the illuminated
side or toward the shaded side does not prove that they
do not give a differential response to localized stimulation,
nor does it prove that they do. We have therefore no
conclusive evidence bearing on this question with reference
to Stentor.

Summary

(i) Stentor coeruleus collects in shaded regions either by
orienting and swimming directly toward such regions or by
wandering into them aimlessly. They remain in the shaded
region because whenever they come to the edge of it, the

122 LIGHT AND THE BEHAVIOR OF ORGANISMS

increase of intensity causes them to respond with the avoid-
ing reaction.

(2) An increase in the intensity of Ught in which Stentors
are oriented, without any variation in the direction of the
rays, causes them to respond either with the avoiding reac-
tion or by simply swerving farther toward the oral side,
making the spiral course wider.

(3) Orientation in Stentor takes place essentially as it
does in Euglena. It is caused by changes in light intensity
on the sensitive tissue in the organisms. If, without a
change in the intensity, the direction of the rays is changed
so that the side of the organism instead of the end is exposed,
there is no reaction, provided the Stentors are not highly
sensitive, except when the oral side comes to face the light
in the process of rotation on the long axis. Then the reaction
may consist either of the avoiding reaction or merely of a
greater swerving from the source of light. Both result in
orientation. This shows that orientation is brought about
by reactions due to changes of intensity and not to light
acting constantly as a directive stimulus in accord with
the theories of Loeb and Verworn.

(4) If the animals are highly sensitive or If the light
intensity is Increased when the direction of the rays is
changed, they respond no matter which side is exposed
after the change is made. In this response they turn in all
directions, toward the light as well as away from it.

(5) Stentors may orient in a field of light which is uni-
form and constant In Intensity; but the orientation even
under such conditions Is due to a change of Intensity. This
change is caused by the movement of the animal which
results In alternately Illuminating and shading different
parts of the organism.

(6) Orientation may be said to be duo to selection from
trial movements, just as In Euglena, ' en In those cases
where Stentor never errs by turning definitely toward the
light, for during every rotation the relatively highly sensi-
tive oral side is alternately shaded and Illuminated until

OBSERVATIONS ON UNICELLULAR FORMS 1 23

the organism becomes directed from the source of Hght.
Thus it is that the rotation itself constitutes a trial move-
ment.

(7) There is no evidence that light acting continuously
has any influence on orientation. These organisms are not
heliotropic in accord with Loeb's definition of this term.

(8) Stentors probably are more active in higher than in
lower light intensity. But even here it is impossible to say
whether the greater activity is due to stimulations pro-
duced by constant intensity or by changes of intensity,
since even in a field of absolutely constant light intensity
the movements of the organism cause the more sensitive
parts to become alternately shaded and illuminated.

2. CEdog07iium Swarm-spores

The reactions of swarm-spores to light have been studied
but little. Most observers have merely recorded the fact
that they do respond to stimulation by light and that they
may be negative or positive. Strasburger (1878, p. 591)
found that if exposed in glass jars they collect near the
surface of the water at the side facing the window, but he
says that they orient very indefinitely and that he therefore
did not attempt to analyze the reactions.

These organisms are very nearly radially symmetrical.
It is in such forms, rather than in asymmetrical forms like
Euglena and Stentor, that one might expect to find a defi-
nite relation between the direction of turning and the side
illuminated. In such forms one might also expect orienta-
tion to be the result of light acting constantly as a directive
stimulation. I was therefore much interested in working
out the details in the reactions of these creatures.

a. Description. — (Edogonium swarm-spores are in gen-
eral very much Uke an egg in form (Fig. 15). At the smaller
end, the anterior, there is a colorless mound-shaped eleva-
tion. The rest of the body is green. At the base of this
mound-shaped elevation there is a band of cilia. I was

124 LIGHT AND THE BEHAVIOR OF ORGANISMS

unable to find any indication of an eye-spot in living speci-
mens although I spent much time in looking for it. Stras-
burger however claims to have observed a red pigment
spot after treatment with acetic acid. There is great varia-
tion in the size of the spores; some of those used in these
experiments were several times as large as others. I am
not certain, however, that they were all of the same species.
In general they are considerably smaller than Paramecia.

Fig. 15. Oedogonium swarm-spores. In responding with the avoiding reac-
tion they always turn toward the same side. This side bears no definite relation
to any visible asymmetry in their structure. A turned toward the more concave
side, i.e., from the side near which the nucleus was located. B turned toward
the more convex side, near which the nucleus was found. C was very nearly sym-
metrical in form. It turned toward the side containing the nucleus. D, diagram
representing the direction of the stroke of the cilia during the process of turning.
This can be very distinctly seen under high magnification.

b. Material. — Swarm-spores were obtained in great
numbers in midwinter by adding fresh water to a jar which
contained numerous Qildogonium filaments, and letting it
become slightly stale. In those jars which were in diffuse
sunlight the swarm-spores collected near the surface of
the water on the side facing the window. In those in
direct sunlight they usually collected on the opposite side.
They are quite strongly negative in their reactions to
gravity; this accounts for the collection near the surface
of the water.

c. Locomotion. — ■ They swim on a spiral course of vary-
ing width and rotate on the long axis just as do Euglena
and Stentor. The smaller end is usually ahead but they
reverse freely, and under some conditions they swim with
the larger end ahead almost constantly. They rotate clock-
wise when the smaller end is ahead, but in the opposite

OBSERVATIONS ON UNICELLULAR FORMS 1 25

direction when the larger end is ahead. Early in the fore-
noon after the jars have been in darkness all night the
spores usually all swim with the larger end ahead, but later
in the day they proceed with the other end foremost. I
was however unable to induce this change by keeping them
in darkness during the day. The following quotation from
my notebook serves to emphasize this peculiar reversal :
On the morning of January 10 all swam with the larger
nonciliated end forward rotating counter-clockwise as seen
from the posterior end. Most of them swam rather actively
in closed curves circling toward the left. In the afternoon

I was surprised to find all the spores swimming about with
the smaller end, the end containing the cilia, ahead. They
were very abundant in the jar and quite active. The jar
was in strong light all day, part of the time in direct sun-
light, and although they were most numerous on the wall
of the jar facing the window, they gathered on the side
under the cover-glass farthest from thewindowwhen exposed
to the direct rays of the sun. On the morning of January

II there were but very few motile specimens, but all that
were observed excepting one swam as those found on the
preceding morning did, i.e., with the larger end ahead. At
2.30 P.M. they were slightly more numerous and nearly all
swam with the smaller end ahead. I am, at present, un-
able to account for this reversal in locomotion.

These organisms are so small, move so rapidly, and are so
nearly S3^mmetrical that it was impossible to ascertain under
normal conditions whether or not they always turn toward
the same side in their orienting reactions. They were
therefore mounted in a solution of quince-seed jelly. In
this solution they swim about very slowly; they stop fre-
quently, back some distance, turn toward one side and
then proceed on a new course; that is, they respond with
the avoiding reaction. If the solution contains consid-
erable jelly they frequently swim with the posterior end
ahead.

By focusing attention upon specimens in which the

126 LIGHT AND THE BEHAVIOR OF ORGANISMS

nucleus, ordinarily located near the surface, was visible,
and upon others in which opposite sides had a slightly dif-
ferent curvature, it could be seen (i) that the same surface
continually faces out as they proceed on the spiral course,
precisely as in asymmetrical forms; and (2) that a given
individual always turns toward the same side in giving the
avoiding reaction. The side toward which they turn bears
no definite relation to the location of the nucleus or to the
curvature of the side. Some turn toward the more convex,
others toward the more concave surface. These organisms
then, although symmetrical, respond with the avoiding
reaction, when mechanically stimulated, just like asym-
metrical forms. They stop, usually back quite a distance,
turn toward a given side, and then proceed on a new course
(Fig. 15). Since there is no known asymmetric structure
which bears any definite relation to the direction of turning,
and since the organism always turns toward the same side
no matter which point on the surface comes in contact with
a solid, it is evident that as far as the facts are known they
indicate that there is no differential response to a localized
stimulation. The same is true in case of the symmetrical
form Didinium nasutum. During the turning process it
was clearly seen in both Didinium and the swarm-spores
that the cilia strike forward on one side and backward on
the opposite side, showing a remarkable differentiation in
function.

d. Orientation in light. — The process of orientation was
studied in negative forms in direct sunlight. In the quince-
seed jelly solution the spores do not orient definitely enough
to make it possible to work out their orientation reactions,
and positive specimens under normal conditions orient very
indefinitely. The method of procedure in this study was in
general like that followed in the observations on Euglena
and Stentor.

A small beam of light direct from the sun was allowed to
fall on the stage of the microscope, and another beam was
reflected at right angles to it with a mirror. The light in

OBSERVATIONS ON UNICELLULAR FORMS 1 27

each of the two beams was then alternately intercepted, and
thus the direction of the rays was changed. It was found
that in the process of orientation under such conditions,
the swarm-spores always turn away from the source of
light, never toward it, but in this turning they merely
swerve farther in their spiral path every time that the course
in the spiral is directed away from the source of light, and
not so far when it is directed toward it. Since the same
side is always directed out in the spiral it is evident that
they do not turn directly from the source of light. They
turn from the source of light only when a given side faces
the light, not when the opposite side faces it.

The orientation therefore takes place in these swarm-
spores just as it usually does in Euglena and Stentor when
the ray direction is changed without a change of intensity
of light. This shows that the organisms are more sensitive
when one side is illuminated than they are when the op-
posite side is exposed, just as was shown to be true in
asymmetrical forms. They are however not very readily
stimulated by changes of intensity; it was impossible to
induce the avoiding reaction in this way. If mounted on a
slide containing a bright area in a dark field, the collection
in the brightarea is not definite, as it is in the case of Euglena
under similar conditions; the swarm-spores usually pass out
and in without any apparent response. If the light Inten-
sity Is only moderately changed after they are oriented
there Is but a slight increase in the width of the spiral path.
If it Is changed much the spores immediately turn and
swim up, since they are strongly negative In their reactions
to gravity.

Orientation in these symmetrical forms Is then governed
by the same factors as it is in the asymmetrical forms. It
Is due to changes of light intensity on the organism. These
changes are produced in a field of constant intensity by the
rotation on the axis. There is no evidence that light acting
constantly as a stimulus has any effect on orientation. The
swarm-spores are more sensitive when one side is exposed

128 LIGHT AND THE BEHAVIOR OF ORGANISMS

than they are when the opposite side is exposed. They
always turn toward a given side, which, as far as can be
seen, is not structurally defined. There is no evidence indi-
cating differential response to localized stimulation.

3. Trachelomonas

No detailed observations on the light reactions of Trache-
lomonas have been recorded. The reactions of this organ-
ism are of interest to us here chiefly because it is very
nearly radially symmetrical and because it has a very promi-
nent eye-spot located very near the middle of the anterior
end, a location quite different from that in any other form
of which I know.

Trachelomonas hispida, the species studied most care-
fully, is ellipsoidal in form, about 0.02 mm. long and 0.015
mm. wide. It is surrounded by a dark brown rough brittle
test of considerable relative thickness. A single large flagel-
lum, frequently three times as long as the body, projects
through a hole in this test at the anterior end. A relatively
large contractile vacuole appears to communicate with the
exterior through this hole. This suggests that the flagellum
may possibly extend into the contractile vacuole as it does
in Euglena viridis. The eye-spot is reddish brown in color
and has the general form of a thin curved disk (Fig. 16).
It is located between the contractile vacuole and the anterior
end and appears partially to surround a canal leading from
the former to the exterior. The eye-spot is very irregular
in outline and appears under an oil immersion lens to con-
sist of a number of small granules embedded in a homo-
geneous matrix. The granules project in the form of
marked knob-like elevations on the convex surface, making
it appear very rough. In most specimens similar granules
were found lying about loose in the neighborhood of the
eye-spot. The test is so nearly opaque in many specimens
of hispida that little can be seen through it, while in some
other species studied it is actually black, so that nothing

OBSERVATIONS ON UNICELLULAR FORMS

129

can be seen of the structure Inside. The tests can however
be readily removed. If the cover-glass Is allowed to press
lightly on the organism It splits open and the cell within
with its prominent eye-spot and bright green color escapes.
All these forms react definitely to light; they are negative
in strong light and positive in weak. They orient quite

Fig. 16. I. Trachelomonas hispida showing structure; cv, contractile vacuole;
t, dark brown test nearly opaque; s, spines on surface; n, nucleus; ch, chloroplast;
e, eye-spot.

II. Different view of same specimen showing that the eye-spot is nearly cen-
trally located. Note rough edges of eye-spot and loose granules composed of same
material.

III. End view of eye-spot.

mm., projected scale. I and II drawn with camera as seen under oil immersion.

IV. Sketches of specimens seen in the process of leaving the test preparatory
to fission. They frequently swim about in the naked elongated state for some
time.

accurately and proceed on a spiral course much like Eu-
glena. It is remarkable how sufficient light to cause
a stimulation can get through the dense black tests of
some specimens.

In the study of the reactions of these organisms they were
mounted in water under a large cover-glass which was sup-
ported by a thin ring of vaseline. In this way evapora-
tion was prevented. The specimens thus mounted lived

130 LIGHT AND THE BEHAVIOR OF ORGANISMS

for over a month and Increased in numbers. When they
are about to divide they crawl out through the opening in
the test at the base of the flagellum and then they may
swim about in the naked elongated state (Fig. 16) for some
time before they divide and form new tests. When the
test is new it is nearly transparent. It was in specimens in
this condition and in those in the naked state that the reac-
tions were observed.

The orienting reactions were studied just as they were
in the forms already described. Trachelomonas was found
to orient just like Euglena in the free-swimming state. If
the light intensity is moderately decreased without changing
the direction of the rays they swim in a wider spiral; if
much decreased, they turn sharply in all directions. They
always turn toward the convex surface of the eye-spot. If
the ray direction is changed without a change of inten-
sity only those with the convex surface of the eye-spot
directed toward the light react immediately, the rest not
until this surface becomes exposed in the process of rotation.
The reaction consists in simply swerving farther toward
the light each time that the eye-spot faces the source of
light; thus they soon become oriented. If the ray direction
is changed with a simultaneous decrease in the light inten-
sity, all react at once. Under such conditions they turn
from the light as well as toward it.

It is clearly evident that turning the convex surface of
the eye-spot tow^ard the source of light produces the same
effect as a decrease in the light intensity of the field. There
is practically the same amount of protoplasm on all sides
around this structure, and as far as can be seen under the
best oil immersion lens this protoplasm is the same on all
sides. If this is true the shadow of the eye-spot should
have the same effect whether illuminated from the concave
or the convex surface. The fact that it does not indicates
that there is a highly sensitive bit of protoplasm close to
the eye-spot on the concave surface.

OBSERVATIONS ON UNICELLULAR FORMS

131

4. Chlamydomonas alhoviridis (Stein)

Chlamydomonas Is a small green egg-shaped organism
usually less than o.oi mm. In length. It has two or four
flagella, a contractile vacuole which appears to open to the
exterior at the base of the flagella, and a distinct eye-spot
located near the surface In various positions on the side of
the body, sometimes nearer the posterior than the anterior
end (Fig. 17). In Euglena and Trachelomonas the eye-
spot is situated near the contractile vacuole and the base

-0.03 miii7

Fig. 17. I. Chlamydomonas alboviridis. II. Chlorogonium, showing struc-
ture and form, v, contractile vacuole; 7i, nucleus; ch, chloroplasts; e, eye-spot;
mm., projected scale.

of the flagellum; and It has been suggested that the eye-
spot Is nothing more than a collection of w^aste material
deposited by this vacuole. In Chlamydomonas It is, how-
ever, so far from the contractile vacuole that there does not
appear to be any relation between the two structures.
These organisms usually swim with the end containing the
flagella ahead, but sometimes they swim for short distances
with the opposite end foremost.

I was interested In this form chiefly because it appeared
as though its reactions might have some bearing on the

132 LIGHT AND THE BEHAVIOR OF ORGANISMS

function of the eye-spot, since in this species it is located
well toward the posterior end of the body.

These creatures react very definitely to light. They are
positive in weak and negative in strong light, and swim on
a spiral course. The intensity however in which they are
positive or negative varies greatly in different individuals
and in the same individual under different conditions. It
is very difficult to follow their movements since they are
so small and swim so rapidly. Jennings (1904, p. 64) found
that they "react to a decrease in illumination by a sudden
turn to one side, by an increase in the width of the
spiral, and by a change in the course just as happens
in Euglena and Cryptomonas." But he was '' unable to
determine the relation of its structure to the spiral path
and to the direction of turning in the reaction." My
observations confirm the conclusions of Jennings as stated
above.

The orienting reactions in these organisms were studied
just as they were in Euglena. They were alternately
exposed in each of two beams of light which crossed on the
stage of the microscope at right angles. If the light from
the two beams is equal and the organisms are not very
sensitive, all turn toward the source of light, none in the
opposite direction, when the direction of the rays is sud-
denly changed. This they do by swerving farther in one
direction than in the opposite as they proceed on their
spiral course, just as do Euglenae and Stentors under similar
conditions. But if the light from one source is more
intense^ than that from the other, and the intensity is
increased at the same time that the direction of the rays is
changed, many of them stop and turn sharply, some toward
the light, others away from it. This reaction is very strik-
ing; there is apparent confusion for some time after the ray
direction and the intensity are simultaneously changed,
whereas if the ray direction is changed without a change

^ The light from one source was 100 ca. m.; that from the other 160 ca. m.
in these experiments.

OBSERVATIONS ON UNICELLULAR FORMS 133

of intensity there appears to be perfect order, all the speci-
mens turning gradually toward the light.

The fact that these organisms turn in various directions
when the intensity is decreased regardless of the side illu-
minated indicates that they always turn toward a struc-
turally defined side. I was however unable to follow the
reactions under these conditions so as to see if there was
a definite relation between the location of the eye-spot
and the direction of turning. But by carefully studying
specimens swimming about slowly in an optimum light
intensity, I saw that with very few exceptions they turn
toward the side on which the eye-spot is situated. A
few specimens which were loosely entangled in debris
were seen to turn in the opposite direction. This was
probably due to some interference with the movement
of the flagella. It may therefore be concluded that
Chlamydomonas always turns toward the side contain-
ing the eye-spot.

The fact that positive specimens turn toward the source
of light when the ray direction is changed without a change
of intensity, and that they turn toward the side containing
the eye-spot, indicates that they are most sensitive when
the side without the eye-spot is illuminated, for it is a de-
crease of intensity which causes a reaction in these organ-
isms when they are positive. It therefore follows that a
change from a position in which the eye-spot is on the
shaded side to one in which it is on the illuminated side has
the same effect as a decrease of intensity. Is this decrease
due to the shadow cast by the eye-spot? In Euglena it
seems likely that it is. In some specimens of Chlamydo-
monas, however, this structure is situated so near the
posterior end that it is difficult to see how it could
function in this way. The long axis of one of the speci-
mens represented in Fig. 17 would have to be at an
angle of nearly 45° with the direction of the light before
the eye-spot would cast any shadow on structures in the
body.

134 LIGHT AND THE BEHAVIOR OF ORGANISMS

5. Chlorogonium

Chlorogonlum is a green spindle-shaped organism with
two flagella and a very prominent bright reddish eye-spot
located very near the surface, only a short distance from
the anterior end (Fig. 17). This organism is frequently
found in cultures containing Chlamydomonas.

The reactions in Chlorogonium are essentially like those
in Chlamydomonas. They were studied in much the same
way in both forms, but it was much easier to follow these
reactions in the former than in the latter form.

The eye-spot in Chlorogonium is favorably situated to
function by shading the interior.

6. Paramecium

The assumption held by some investigators that the
power to react to light is common to all protoplasm is
probably wrong. It is well known that Paramecia and
many other protozoa do not respond to light of ordinary
intensity. If all protoplasm can be stimulated by light,
one would certainly expect these forms to show some evi-
dence of response when suddenly subjected to powerful
illumination.

At noon on a perfectly clear day in July I arranged a
double convex lens 10 cm. in diameter so as to focus the
direct rays from the sun on a slide under the mic oscope.
The light was passed through distilled water in order to cut
out the heat rays. The light at the focal point was at least
500,000 ca. m. in intensity. This extremely intense light
was repeatedly flashed upon the Paramecia as they swam
about under the microscope, but there was no evidence of
any response whatever. It is altogether probable then
that the power to respond to light is not common to all
protoplasm.

The fact that Paramecia do respond to ultra-violet rays
as shown by Hertel (1904) has no bearing on this question.

OBSERVATIONS ON UNICELLULAR FORMS 135

The wave length of the rays used in Hertel's experiment
was only 280 fxij,, while the length of the shortest wave of
the visible spectrum is a little over 400 ^t//. Paramecia
continuously exposed to the ultra-violet rays are injured
almost immediately; their movements become uncoordi-
nated and they die in from 10 to 50 seconds. It seems
probable, then, that these rays stimulate Paramecia because
of their injurious effect.

CHAPTER VII

THE FACTORS INVOLVED IN THE PROCESS OF ORIEN-
TATION IN COLONIAL FORMS

I. Volvox glohator and minor

Many interesting observations have been made on the
light reactions of Volvox since Leeuwenhoek discovered
this organism over two hundred years ago. The details in
the reactions have however only recently been worked
out. In 1907 I published an extensive paper on this sub-
ject, and the following account is based largely on this
paper.

Volvox is an organism somewhat like a hollow sphere
slightly elongated. The largest colonies are nearly i mm.
in diameter and the smallest can readily be seen with the
naked eye. Each colony is composed of from 200 to 22,000
individuals and each individual consists of a single cell
known as a zooid. The zooids, interconnected with proto-
plasmic strands, are arranged side by side so as to form a
wall inclosing a cavity. They are very much like Chlamy-
domonas in structure and color. Each one contains tw^o
fiagella and an eye-spot which is situated on the outer pos-
terior surface. The eye-spots at the anterior end of the
colonies are from eight to ten times as large as those at
the posterior end much as in Pandorina represented in
Fig. 21.

The colonies usually rotate counter-clockwise on the long
axis, like Euglena, but they seldom swim on a spiral course.
They orient and swim toward a source of light or away
from it in a general way; they do not however orient very
accurately. Colonies swimming horizontally toward a com-
pact source of light usually deflect either to the right or to
the left or up or down. The more strongly positive the

136

ORIENTATION IN COLONIAL FORMS 137

colonies are the more nearly parallel with the rays they
swim. If the position of the source of light is changed
without a change of intensity, they change their direction
of motion until the course bears to the light rays the same
relation that it had before. In thus changing their direction
of motion they always turn directly toward the source
of light without any preliminary movement. No matter
which surface is illuminated there is an apparent differential
response to localized stimulation. There is no evidence of
trial movements in the colonies taken as a whole. They
never turn in the wrong direction as Euglena and Stentor
frequently do, even if the intensity and the ray direction
are changed simultaneously. There is no evidence that the
sensitiveness of the colonies depends upon the surface
exposed as was found to be true in many unicellular forms,
nor is there any indication of an avoiding reaction when
the intensity is changed. If the intensity is much decreased
they merely stop forward progress and, because of the effect
of gravity, the anterior end turns up. They do not aggre-
gate extensively in highly illuminated areas in a dark field
as Euglena and various other forms do. They pass from
darkness into light and vice versa without any apparent
reaction.

There is no evidence that the direction of rays through
the organism, in accordance with Sachs' theory, or that the
angle between the rays and the sensitive surface as suggested
by Loeb controls orientation. This process is regulated by
the relative intensity of light on opposite sides of the colony.
The following facts prove this to be true: i. If exposed to
light from two sources they swim toward any point between
them. The location of this point depends upon the relative
amount of light from the two sources as indicated in Fig. 18.
2. In the light grader (see Fig. 4) so arranged as to produce
a field of light which consists of parallel rays, but in which
the intensity gradually diminishes from side to side so that
one side of a colony, swimming toward the source of light,
is more strongly illuminated than the other, it deflects

138 LIGHT AND THE BEHAVIOR OF ORGANISMS

toward the more strongly illuminated side as represented in
Fig. 19.

Difference in light intensity on opposite sides of the

c»-

d

\f

mmmsm?m§^;

a

f

Fig. 18. Representation of the movement of Volvox when subjected to light
from two sources, a, plate glass aquarium 8 cm. long and 8 cm. wide; b, 222-volt
Nernst glower, 66 cm. from aquarium (distance from aquarium constant); c, iio
volt glower (distance from aquarium variable); d, screen; e, point of introduction
of Volvox; /, direction of light rays; i, 2, 3, 4, courses of Volvox exposed to light
from both glowers: i, with no-volt glower igg cm. from aquarium; 2, with iio-
volt glower 99 cm. from aquarium; 3, with no-volt glower 49 cm. from aquarium;
4, with I ID- volt glower 24 cm. from aquarium; x-y, course of Volvox when exposed
to light from glower b only; y-z, course when exposed to light from glower c only.

colonies, then, causes them to turn until the two sides are
equally illuminated. "The turning^ may be conceived to
be due to an increase in the backward phase of the stroke

1 Mast, 1907, pp. 151-154.

ORIENTATION IN COLONIAL FORMS

139

on the shaded side, or a decrease in the same phase on the
illuminated side or a decrease in the forward phase on the
shaded side, or an increase in this phase on the illuminated
side. Can it be ascertained which of these is the cause of
the difference between the effect of the stroke of the flagella
on the shaded sides and that of those on the illuminated
side of the colonies?

Fig. 19. Graphic representation of the total average difference in deflection
due to difference in light intensity on opposite sides of Volvox colonies, compiled
from numerous experimental records, a, plate-glass aquarium 8 cm. wide and
15 cm. long; b, light rays; c, c', points where the colonies were introduced; d, aver-
age course with the region of highest light intensity to left; e, average course with
strongest illumination to the right. Light intensity at (/) the middle of field 57.12
candle meters. From the rniddle the intensity gradually increased toward either
end where it was 442.68 candle meters. Intensity at c, 327 candle meters, at c',
263 candle meters.

"If the light intensity of the field is suddenly decreased
while colonies of Volvox are swimming horizontally toward
it, they stop forward motion, the longitudinal axes take a
vertical position due to the effect of gravity, and then the
colonies swim slowly upward. It is not at all difficult to
find specimens in which this upward swimming is just suffi-

140 LIGHT AND THE BEHAVIOR OF ORGANISMS

cient to overcome the effect of gravity, and under such con-
ditions they appear to be hanging in the water motionless.
They are, however, rotating on their longitudinal axes. If
now the light intensity, to which these apparently motion-
less organisms are exposed, is increased they soon begin to
turn toward its source; but in so doing they swim upward,
as represented in the accompanying diagram (Fig. 20).

Fig. 20. Diagram representing the reaction of a Volvox colony when the light
intensity is suddenly changed, a, outline of colony; b, longitudinal axis; c, light
rays; d, point in the course where the light is suddenly decreased; e, point where
it is suddenly increased; /, course taken by colony. In continuing from e, the
side of the colony facing the source of light travels over a shorter distance than
the shaded side. Consequently the backward stroke of the fiagella on the latter
side must be more effective than that of those on the former.

"In thus swimming upward and horizontally toward the
source of light, it is clear that the effect of the backward
stroke of the fiagella increases both on the shaded side and
on the illuminated side, for both sides move forward. But
the shaded side moves farther than the illuminated side,
consequently the increase in the effect of the backward
stroke must be greater on the former than on the latter.
The difference in the effect of the stroke of the flagella on op-
posite sides, which results in orientation of positive Volvox
colonies, is, therefore, due to a greater increase in the back-
ward stroke of the flagella on the shaded side than of those
on the illuminated side.

"If the light thrown upon apparently motionless colonies
is quite intense, they frequently may be seen to sink 4 or

ORIENTATION IN COLONIAL FORMS 141

5 mm. immediately after the light is turned on, but while
they are sinking this short distance, they apparently become
acclimated and soon turn toward the light, and at the same
time swim upward, just as described above. During the
time in which these colonies sink they continue to rotate
in the same direction as before. The sinking must then
be due to a decrease in the effect of the backward stroke
of the flagella on all sides, and this decrease is due to an
increase in light intensity. But when the colonies turn
toward the source of light, and at the same time swim
upward, it is evident that the increase in light intensity
must cause an increase in the backward phase of the stroke
of the flagella on all sides, for if this were not true there
could be no upward motion. The side nearest the source
of light, however, passes over a shorter distance than the
opposite side, as will readily be seen by referring to the dia-
gram, and therefore the increase in the effect of the back-
ward phase must be greater on the latter than on the
former. But the light intensity is greater on the former
than on the latter (a paradox). When the light intensity
in the field is increased the effect of the backward phase of
the stroke of the flagella may be increased or decreased on
all sides. If it is increased the effect is most marked on the
side in lowest light intensity. Furthermore, if the light is
strong the colonies turn toward its source more rapidly and
do not swim upward so far and thus make a sharper curve
than when it is weak; but the stronger the light the greater
the difference between the intensity on the shaded and that
on the illuminated side. It therefore follows that the
greater the difference in intensity on these sides, the greater
the difference in effect of the backward phase of the stroke
of the flagella, the effect being greatest on the side least
illuminated. These considerations support the conclusion
arrived at above, i.e., that the factors which regulate the
activity of the colonies, as a whole, are different from those
which regulate the direction of motion.

"We have thus demonstrated that while orientation is

142 LIGHT AND THE BEHAVIOR OF ORGANISMS

due to difference in light intensity on opposite sides of the
colonies, it is brought about in positive specimens by the
flagella striking backward with greater effect on the side
in lowest light intensity than elsewhere. I suggest the fol-
lowing explanation of this:

" First, it must be remembered that the organism con-
stantly rotates on its longitudinal axis. If then a colony
is so situated that one side is more highly illuminated than
the opposite, it is clear that the zooids will constantly be
carried from a region of higher to a region of lower light
intensity, and vice versa. They are thus subjected to con-
stant changes in strength of illumination. As stated above,
the flagella strike backward with greater vigor on the shaded
side than on the opposite one and, therefore, it is evident
that as the zooids reach the region of lower light intensity,
in other words when the light intensity to which they are
subjected decreases, they increase the effect of the back-
ward stroke of the flagella, i.e., they attempt to turn toward
a structurally defined side (the side facing the anterior end
of the colony). This is precisely what Euglena does when
it passes from a region of higher to one of lower light inten-
sity, i.e., it turns toward a structurally defined side, the
larger lip. The individuals in a colony then respond with a
motor reaction induced by change in light intensity; they
react on the same basis as do Euglena, Paramecium, Stentor
and other unicellular forms, in their trial and error reactions,
? but owing to the way in which they are interrelated, and
to the rotation of the colony on the longitudinal axis, this
reaction of the zooids causes orientation in the colony as a
whole, without error.

*' This explanation of orientation in entire colonies holds
also for orientation in segments. As previously stated, only
those segments orient which have such a form that they
can rotate. As they rotate the cut surface constantly faces
the center of the spiral, so that if the axis of the spiral is
not directed toward the source of light, the outer surface
where the zooids are situated is alternately turned toward

ORIENTATION IN COLONIAL FORMS 143

the light and away from it. Thus the zooids are carried
from regions of higher to regions of lower light intensity
and vice versa, and the motor reaction is induced just as it
is in entire colonies.

" Orientation in negative colonies can be explained in
precisely the same way as that in positive ones, assuming
merely that in this condition the zooids respond with the
motor reaction when they pass from lower to higher light
intensity instead of when they pass from higher to lower
(as is true when the organisms are positive) . The backward
stroke then becomes most effective on the side most highly
illuminated."

It is altogether likely that the orientation In Volvox Is
not entirely due to the change of light intensity on the
zooids caused by difference of intensity on the colonies as a
whole and rotation on the long axis as described above ; but
that as in Euglena and other forms discussed above, the
sensitiveness of the zooids depends upon the surface exposed.
When the zooids are carried from the Illuminated to the
shaded side It is clear that they are not only transferred
from a region of higher to a region of lower light intensity,
but the surface turned toward the light Is also changed.
This change in Itself may, as in various unicellular forms
studied, cause a reduction of Intensity on certain structures
within the zooids by the movement of shadows of other
structures, and consequently an orienting stimulus. The
orientation of segments Indicates that this factor plays a
very important part in the orientation of the colonies.

Take for example a segment formed by cutting a colony
in half lengthwise. The zooids in this segment lie side by
side and are some distance apart, being connected by thin
strands of protoplasm. Most of the substance In the cavity
runs out when the colony Is cut, and what remains is trans-
parent. It is therefore evident that there is no more sub-
stance to shade the zooids when the Inner surface of the
segment faces the source of light than there is when the
outer faces it. If the segments are in a field of uniform

144 LIGHT AND THE BEHAVIOR OF ORGANISMS

intensity the zooids are not carried from regions of higher
to regions of lower Ught intensity and vice versa as described
above. Under such conditions then the orienting reactions
must be due entirely to the shading of certain structures
within the zooids by other structures also within them, as
the surfaces turned toward the source of light change.

Orientation in Volvox is, according to this account, due
to changes in light intensity on the zooids as a whole
together with changes of intensity on certain structures in
the zooids, made possible by difference of intensity on the
surface of the colony and rotation on the long axis. The
change of intensity is caused by the movement of shadows
of certain structures in the organism cast upon other struc-
tures. These shadows are present in a field of uniform
intensity. There is no evidence that the direction of the
rays in the field or through the body, or that the angle the
rays make with the surface of the body, is of importance
in orientation excepting in so far as it may affect difference
of intensity in the individual zooids or the colony as a
whole. Nor is the symmetry of the organism of prime
importance, for, as was stated above, asymmetrical seg-
ments of various forms orient nearly as accurately as entire
colonies. Orientation may take place in constant light
intensity quite as well as in a field having various intensities.
Difference of intensity in the field does however determine
the distribution of the colonies. They are negative in light
of high intensities, positive in that of low and neutral in
that of optimum intensity. Orientation then, whether posi-
tive or negative, tends to direct the colonies to the area of
optimum illumination. Light acts as a constant directive
stimulus on the colonies as a whole, but there is no evidence
that there is a directive stimulus without change of inten-
sity, for the reacting elements, the zooids, and especially
the structures within the zooids, are not subjected to con-
stant intensity.

It is probable however that constant intensity affects the
activity of these organisms somewhat as temperature does.

ORIENTATION IN COLONIAL FORMS 145

They become negative in high intensity and appear to be
more active in some intensities than in others, but since it
is practically impossible to subject the different parts of the
colonies to constant intensity owing to the shadow of one
part on another and the movement of the organism, it is
impossible to say whether or not they would become nega-
tive or more active if there were no such change of light
intensity. This subject was dealt with more in detail under
Euglena.

Bancroft (1907, p. 163) intimates that orientation of
Volvox in light takes place in the same way as it does in a
constant electric current; that it is not due to changes of
intensity but to constant intensity; and that it is therefore
a tropic reaction in accord with Loeb's definition. He says
(p. 162): ** It has been shown that the galvanotropic orien-
tation of Volvox is brought about by a cessation or great
diminution in the stroke of the flagella at one pole of the
organism. This diminution in activity of the flagella
appears to be the only way in which Volvox is capable of
responding to stimuli. Nothing in the nature of a motor
reflex has ever been observed in this organism so far as I
know. The flagella always strike most strongly backward.
We have then the simplest possible kind of a mechanism
for bringing about galvanotropic orientation. The current
diminishes the activity of the flagella at one pole of the
colony and consequently the activity of the flagella at the
other pole causes the organism to turn in that direction.
We have here a tropism reduced to its lowest terms. There
is nothing of the nature of trial and error present at all."

We have clearly demonstrated above that orientation of
Volvox in light is not due to a " cessation or great diminu-
tion of the stroke of the flagella at one pole of the organ-
ism " as it is in a constant electric current. It is due to
an acceleration in the backward stroke of the flagella on
the shaded side. This is caused by a response to a decrease
in the light intensity on the zooids as a whole together with
a decrease in intensity on certain structures within the

146

LIGHT AND THE BEHAVIOR OF ORGANISMS

zooids, owing to the rotation of the colonies on the long
axis and the consequent transfer of the zooids from the
illuminated to the shaded side. This response is similar to
the avoiding reaction in Euglena, Trachelomonas, Chlamy-
domonas and other unicellular forms.

If Bancroft is correct in his description it is evident that
the factors involved in orientation in a galvanic current
and in light are not the same, and that the orienting reac-
tions in light are not tropic according to Loeb's definition.

2. Pandorina and Eiidorina

These organisms are much like V^olvox in structure.
They are however very much smaller and contain only

-0.1 mniT

Fig. 21. I. Eudorina; II, Pandorina, showing structure and form; a, anterior
end; z, zooids; ch., chloroplasts, all the zooids are well filled with them; e, eye-
spots — note difference in size at opposite poles, and location on outer posterior
surface of zooids. Each zooid has two flagella, — only a few of them are repre-
sented. Eudorina is surrounded by a hyaline layer the outline of which is repre-
sented by a dotted line. Outlines made with camera; mm., projected scale.

III. Ej'e-spot greatly magnified; n, surface view; m, side view. The flat surface
is directed outward and slightly posteriorly.

from 32 to 64 zooids (Fig. 21). By means of methods
similar to those used in studying Volvox it was found that

ORIENTATION IN COLONIAL FORMS 147

the process of locomotion and the reactions in both Pan-
dorina and Eudorina are in all essentials like those in Vol-
vox. They are negative in strong and positive in weak
illumination, but the degree of intensity in which they are
positive or negative varies greatly. They always swim
with the end containing the larger eye-spots ahead. They
usually rotate counter-clockwise on the long axis and pro-
ceed on a straight path. Only a very few colonies were
found to swim in a spiral course. If the light intensity is
decreased or increased with or without a change in the
direction of the rays there is no shock effect, nothing
resembling an avoiding reaction. If the general direction
of the rays is changed, positive specimens turn directly
toward the source of light without any preliminary move-
ments whatever, and negative specimens always turn in the
opposite direction. If exposed to light from two sources
so situated that the rays cross at right angles they swim
toward or from a point situated between the two sources.
The location of this point depends upon the relative inten-
sity of light from these sources. Orientation in these forms
takes place just as it does in Vol vox.

a. Function of the eye-spots. — The eye-spots in both
Eudorina and Pandorina are located on the outer posterior
surface of the zooids just as in Vol vox. They have the
form of a segment of a sphere. The flat surface which is
slightly concave faces out. Those at the anterior end of
the colonies are much larger than those at the posterior, as
represented in Fig. 21. The former are nearly 2.5 ^u in sur-
face diameter, and 0.9 m thick; the latter are only approxi-
mately 0.6 fjL in diameter, but relatively thicker than the
former. They are reddish brown in color and stand out
boldly in strong illumination from below, showing that they
are comparatively opaque. In direct sunlight they become
luminous, giving off a greenish blue light, and as the colo-
nies rotate they sparkle and glitter, presenting a wonderfully
beautiful spectacle. After having been in direct .sunlight
for some time they are also luminous in diffused sunlight,

148 LIGHT AND THE BEHAVIOR OF ORGANISMS

but not so in darkness. No evidence of this property was
seen in the eye-spot of Euglena. It therefore appears that
the eye-spots in these two forms differ in composition, and
it may be that they function differently. At any rate,
judging from their location I am unable to see how they
could function in Volvox, Pandorina or Eudorina by shading
structures in the interior, as they appear to in Euglena,
unless they function only when the colonies are negative.
The fact however that the eye-spots at the anterior are
much larger than those at the posterior end is strong evi-
dence in opposition to such a view. If these structures func-
tion in light reactions in these forms at all, they must
function either as an absorptive background somewhat like
the retinal pigment in the eye or as direct light recipient
organs as I suggested (1907, p. 112).

In our work on Euglena it was pointed out that the
hyaline protoplasm at the anterior end condenses the light
and brings it to a focus in the neighborhood of the eye-spot
when the organism is oriented, possibly on the structure
most sensitive to light. In Pandorina and Eudorina each
zooid acts as a condensing lens. In direct sunlight a highly
illuminated spot can be clearly seen at the surface directed
away from the source of light in each zooid, even in those
well filled with chloroplasts. The focusing of the light is
much more definite in these forms than it is in Euglena.
It is evident that every lateral movement of the organism
causes a change in the location of the point on which the
light is focused in the zooids, and this of course produces
definite and marked changes in light intensity. It may be
that the eye-spots, located near the posterior end of the
zooids as they are, function in some way in connection with
such changes of the focal point.

CHAPTER VIII

OBSERVATIONS ON THE RESPONSES INVOLVED IN THE

REGULATION OF MOVEMENT TOWARD THE

SOURCE OF LIGHT IN COELENTERATES

Only a few of the animals belonging to this group orient
in light. Many do not respond to light at all. In others
the general activity depends upon the light intensity. Some
get into regions of optimum light intensity by means of
orientation, others by random wandering movements. In
both cases they come to rest in this region. The former
method is much more effective than the latter. If animals
have the power to orient they can move directly toward
the region of optimum intensity and consequently get there
much more quickly than do those which reach such regions
by random movements.

In this paper we shall consider only a few species, all of
which show some evidence of orientation.

I. Hydra viridis

a. Historical review. — Trembley (1744) seems to have
been the first to record experimental observations on the
effect of light on the movements of Hydra. He exposed
the animals in a glass jar covered with an opaque case con-
taining an opening on one side, and found that they migrated
toward the opening. He did not however record the details
in the method of migration. Loeb (1905, p. 73) referring
to these experiments says, " Trembley 's experiments on
Hydra, however, show that in their case also the relation
is the same," i.e., "that Sachs's ^ laws of heliotropism . . .

^ Sachs, it will be remembered, claimed that orientation is controlled by
the direction in which the rays pass through the organism.

149

150 LIGHT AND THE BEHAVIOR OF ORGANISMS

hold good. ... It seems to me that Trembley's experi-
ments cannot be interpreted unless we assume that the
progressive movements of Hydra are determined by the
direction of the rays of light."

Wilson (1891) found that while Hydra viridis usually
collects on the bright side of the dish, it collects in shaded
regions if the light is very intense ; that it aggregates more
freely in a blue field than in a yellow or white field ; and that
it collects more abundantly in the blue field even if it con-
tains no more blue rays than the white field and is therefore
of a much lower intensity. He found that a change from
light to darkness or from blue or white light to light of
other colors causes the animals to wander about more.
When they pass into blue or white light they tend to come
to rest. This explains their collection in blue and white
light. Wilson thinks that the Hydras may also go toward
the source of light directly; that the collection at the more
highly illuminated side of the aquarium is not entirely due
to random wandering. Washburn (1908, p. 123) however
says, "Hydra shows no response to light other than a tend-
ency to come to rest in the more illuminated parts of the
vessel containing it."

The following experiments were undertaken with these
questions in mind: i. Do Hydras wander about more in
darkness than in light? 2. Do they move directly toward
a source of light? 3. Do they orient? 4. What factors
are involved in orientation ?

b. Effect of light intensity on activity. — Experimental
results recorded in literature show that Hydra is in general
more active in sub and supra optimum intensities than in
optimum intensity. The following observation shows, how-
ever, that total darkness seems to inhibit movement. On
April II at 10 a.m. six green Hydras were taken from the
culture which was in strong diffuse light, put into some
water in a small rectangular aquai;ium and placed in total
darkness without the temperature's being changed. They
soon became attached to the bottom of the vessel in posi-

MOVEMENT TOWARD LIGHT IN COELENTERATES 1 51

tions represented by dots (a) in the accompanying diagram
(Fig. 22). At 12 M. all were still in the positions where

10:-20

10:05

•

11:00 A.M.

^V

^3/f

/ 8:00 A.M.
8:00 [ /

/^

/ 11:00

\ Ky^

11:00

11:00

x^. v^

Fig. 22

o,y

Fig. 23

4:55 P.M.

■<r-
<-
<-

n

Fig. 24

Fig. 22. Movement of Hydra viridis in total darkness, a, position at begin-
ning of course, 8 a.m., jy; x, position at 8 a.m., j\; o, 9 a.m., I'V; y, 8 a.m., y\.
Fig. 23. Movement of the same specimens after exposure to light. 8 to

11 a.m., j\. Note that they have become much more active.

Fig. 24. Movement of H. viridis toward source of light, n; intensity

12 ± ca. m. All the looping movements of each specimen are represented from
three o'clock until the close of the experiment. The dots represent points of
attachment. The animals extended in various directions from each point but
usually traveled only toward source of light.

they first became attached. They were well expanded and
the anterior ends were variously directed. At 2.15 p.m.
one had moved about 3 mm., the rest not at all. At 8 a.m.
the following morning four had moved to positions desig-

152 LIGHT AND THE BEHAVIOR OF ORGANISMS

nated x. At 9 a.m. the next morning two had taken posi-
tions represented by 0. During the next 24 hours three
moved to positions marked y; all were well contracted and
motionless. They were now exposed to diffuse sunlight
without being moved or jarred. They began to expand
almost immediately and soon began to move about. The
paths taken are indicated in Fig. 23. After 11 a.m. they
did not again change their positions until the close of the
experiment 24 hours later.

This shows that darkness inhibits movement in Hydra
viridis ; that they become exceptionally active when exposed
to light after having been in darkness for some time, and
that they apparently become acclimatized readily, since
they come to rest in the same intensity after having been
exposed a few hours. The inhibition of movement by
darkness may however not be due to absence of direct
stimulation by light, but to the effect of darkness on photo-
synthesis.

c. Orientation and locomotion. — In the study of the
movements of Hydras, they were put into water from the
culture jar, i cm. deep, in a rectangular aquarium 2X5X8
cm. made by cementing slides together. The aquarium
was exposed in the dark room to light from a 50-candle-
power Nernst glower situated 2 meters from the end of it.
The glower w^as so arranged that the rays w^ere parallel with
the bottom and sides of the aquarium. The end of the
aquarium was covered with an opaque screen containing
an opening such that the sides and surface of the water
were in darkness, so that reflection from various surfaces
might be reduced as much as possible. All the light except
that in the beam w^hich fell on the end of the aquarium was
absorbed by screens.

At 10 A.M., April II, six green Hydras were put into the
aquarium near the end opposite the glower. They became
attached very soon and stretched out in various directions,
some toward the source of light others away from it.
Twenty-four hours later they were near the middle of the

MOVEMENT TOWARD LIGHT IN COELENTERATES 153

aquarium, and the following morning (April 13) they were
all at or near the end facing the light. It therefore took
them nearly 48 hours to move a distance of 8 cm. There
were now nine specimens; three buds had been set free.
The aquarium was turned end for end so that the Hydras
were again at the end farthest from the glower. During
the following 24 hours several of the specimens traveled
the entire length of the aquarium, 8 cm. They therefore
moved as far during the 24 hours as they had during the
preceding 48 hours. This shows that they became much
more active after they had been exposed in a given light
intensity 48 hours than they were at first. There was at
no time any indication of orientation. The specimens
appeared to face in all directions equally, but they appeared
to move quite directly toward the source of light.

In the following experiment more detailed observations
on movement were made. Several specimens were put into
the aquarium near the end farthest from the light at 9 a.m.,
April 16. They became attached almost at once. At 12.30
all were removed but the five which had been most active
during the preceding hours. The courses taken by these five
specimens are recorded in Fig. 24. Two of the specimens,
A and B, were attached to the surface film. One remained
there during the entire experiment, but the other, B, went
to the bottom after moving a short distance. The surface
of the water was in darkness, but the Hydras hanging from
it extended into the light. The animals moved from place
to place by stretching out the body, attaching the tentacles
to the substratum, and then pulling up the foot and fasten-
ing it again near the end bearing the tentacles, sometimes
on the same side it had been and sometimes on the oppo-
site side. They progress by what may be called the loop-
ing method. From 3 o'clock to the close of the experiment
every progressive change in position of the specimens A
and B, and most of those of the other three specimens, were
recorded. During the periods between the looping move-
ments the specimens contracted and expanded from time

154 LIGHT AND THE BEHAVIOR OF ORGANISMS

to time and bent in various directions, remaining in any
given position only a few minutes, so that during the periods
in which the foot was fixed, the oral end was directed toward
various points of the compass. But it will be seen by refer-
ring to the figure that the looping movements were in
general directed toward the source of light. The only
movements in the opposite direction occurred in specimens
Cand E. The former moved in this direction but once, the
last looping move it made during the experiment; the latter
was not seen in the act of moving. It did however reach
the end of the aquarium farthest from the source of light
where it came to the surface and remained to the end of
the experiment.

d. Reactions of negative specimens. — At 11.35 a.m.,
April 17, two specimens were exposed in the small rectangu-
lar aquarium to direct sunlight. They became very active
at once and bent from side to side, expanding and con-
tracting frequently. One changed its position by looping
five times in a little more than 15 minutes, the other by
looping three times. Both proceeded from the source of
light nearly as directly as the positive specimens studied
moved toward it. There was no apparent relation between
the direction of bending and the source of light, just as
was found to be true in positive specimens. The anterior
end appeared to be directed toward the source of light as
much of the time as away from it, but locomotion occurred
only when this end was directed from the light. After
having been exposed a little over 15 minutes they lost their
attachment to the bottom and became perfectly quiet,
apparently having been injured by the intense light.

As already stated the anterior end of Hydra is successively
directed toward various points of the compass. After re-
maining in a given position a few minutes the animals usually
contract, turn toward one side and expand again. Some-
times however they bend and turn so as to change their
position without contracting. There is no definite relation
between the direction of bending and the source of light.

MOVEMENT TOWARD LIGHT IN COELENTERATES 155

There is however in positive specimens a tendency to retain
the position in which the oral end is most highly illumi-
nated. This was demonstrated as follows:

Ten specimens were exposed in the rectangular aquarium
in the dark room. As they traveled toward the light the
direction in which they faced was recorded at intervals.
These records appear in the table given below.

TABLE I.

Anterior end directed ^

Time of
observation

Toward

From source

Perpendicular

to direction

of light

soiirce of light

of light

3-45

7

3

0

4. 10

5

2

3

4.20

6

2

2

4-35

4

2

4

5.00

6

2

2

5-3°

3

2

5

6. 10

5

3

2

6.30

8

0

2

7-15

3

3

4

8.00

4

2

4

9.00

7

I

2

9-30

5

3

2

10.00

6

I

3

10.30

5

I

4

Total

74

27

39

' In making the table all those specimens in which the oral end was near a plane passing
through the foot and perpendicular to the direction of the rays were recorded in column 4;
all in which this end was definitely to the right, i.e., toward the light, in column 2; and those
to the left, in column 3.

The table shows very clearly that the oral end of the ten
specimens studied was directed approximately toward the
source of light nearly three times as much as from it. If
the direction of locomotion depended merely upon the
direction in which the oral end points, one would expect
these organisms in the positive state to loop from the source
of light more than one-third as often as toward it. This

156 LIGHT AND THE BEHAVIOR OF ORGANISMS

however is not true; movement from the source of Hght is
ordinarily relatively rare. There must therefore be a
greater tendency to travel when the oral end faces the
light than when it faces in any other direction.

It may then be stated that Hydra tends to orient with
the anterior end directed either toward or away from the
source of light depending upon whether the specimens are
positive or negative, and that it tends to travel in the direc-
tion in which it orients. There are evidently two appar-
ently independent phenomena involved here, orientation
and locomotion. How can these phenomena be explained ?
Let us first consider orientation.

Since Hydras tend to expose the anterior end to light
when they are positive and to shade it when they are nega-
tive, it is probable that the oral end in this organism, as in
Euglena and Stentor, is more sensitive to light than other
parts of the body. If this is true it may be that orientation
is, as Jennings suggests (1906, p. 213), '' due to the fact
that when it turns this end away, the change to relative
obscurity at the anterior end causes further movement, till
the light again falls on the anterior end." This explana-
tion fits the facts, as far as they are known, fairly well. It
is however difificult to see, since Hydra frequently retains a
position in which the anterior end is shaded for more than
two minutes, how " the change to relative obscurity at the
anterior end could cause further movement." There is
certainly no reaction in these animals comparable to the
avoiding reaction or shock movements in the lower forms,
for sudden changes of intensity even if extremely great
produce no immediate reactions. If it is assumed that the
organism tends to become oriented by random movements
and tends to remain oriented because of inhibition due to the
illumination of the anterior end, this dif^culty is obviated.

As already pointed out, locomotion ordinarily occurs only
when Hydra is approximately oriented. Positive speci-
mens travel only when the oral end Is illuminated, not when
it is shaded. It Is therefore evident that the light itself

MOVEMENT TOWARD LIGHT IN COELENTERATES 157

has something to do with this movement. It is not due
entirely to internal changes. The organism must be af-
fected differently when the anterior end is illuminated than
it is when this end is shaded. A decrease in illumination
ordinarily causes increase in activity, but this fact cannot
be the cause of locomotion after orientation, for if it were,
we should expect the greatest tendency to move when the
anterior end is directed from the source of light in place of
toward it.

One thing is clear from our results stated above, that is,
that the light condition which tends to inhibit turning in
various directions also tends to cause locomotion. From
this it may be concluded that orientation and locomotion
are phenomena w^hich are regulated by different processes.
It may be that the former is dependent largely on difference
of intensity on opposite sides of the organism and the latter
upon an action of light similar to that of heat.

e. General conclusions. — It can be definitely stated
then that Hydra in the positive state reaches a position in
which the anterior end faces the light by random move-
ments; that it remains in this position longer than in any
other, and that it ordinarily starts to travel only when it is
in this position. The last two statements prove that light
affects it differently when the oral end is exposed than when
it is shaded.

It is impossible to say whether Hydra tends to retain
the position in which the light strikes the oral end because
of an inhibition due to an increase in effective light inten-
sity, when the body Is turned from a position in which the
oral end is shaded to one in which It Is Illuminated; or
whether it tends to remain oriented because of Increase in
motion due to a reduction in effective intensity when the
anterior end is turned away from the light; or whether the
tendency to retain the oriented position is due not to a
change of Intensity, but to the fact that In this position
the anterior end is most highly illuminated and that the
inhibition is due to the effect of constant intensity.

158

LIGHT AND THE BEHAVIOR OF ORGANISMS

While the tendency in Hydra to remain oriented may
then be the result of stimulation by constant intensity,
there is no evidence whatever that light acts constantly as
a directive stimulus. Light may however affect locomotion
by acting constantly, much as temperature does; but even
in this case it is impossible to be certain that such effects
are not due to changes of intensity, for the shadows of
some parts of the body move almost constantly over other
parts, owing to the fact that this animal is quiet only for
short periods.

?o-

2

Fig. 25. I. Reaction of an attached Hydra to a constant electric current of
moderate intensity. 1-5, successive stages in the reaction.

II. Successive stages in the reaction of a Hydra to the electric current when
the foot is unattached. The foot becomes directed toward the anode. After
Pearl (1901).

The fact that there is no definite relation between the
direction of turning and the side illuminated shows that
neither the symmetry of the body, nor the angle the rays
make with the surface, nor the direction of the rays through
the body, nor local response to local stimulation, nor dif-
ferential response to localized stimulation, can be of special
importance in orientation in light.

The difference between the orienting reaction of Hydra
in light and in a constant electric current is striking. As

MOVEMENT TOWARD LIGHT IN COELENTERATES 159

stated above, in light there is no evidence whatever of direct
bending toward or from the source of stimulation. In a
constant electric current however it bends directly until
the long axis is in line with the direction of the current, as
represented in Fig. 25. The electric current in the process
of orientation acts constantly as a directive stimulation;
the reaction is tropic, according to Loeb's definition. Light
does not act constantly as a directive stimulation; orienta-
tion is the result of " selection of random movements; " the
reactions are not in accord with Loeb's definition of
tropism.

2. Eudendrium Planulae

The planulae of Eudendrium are set free during the latter
part of July and the first part of August. In the labora-
tory they are usually liberated early in the forenoon, after
which they immediately begin to travel toward the light.
These organisms are cone-shaped, about i mm. in length
and about 0.2 mm. in diameter at the larger end when
expanded; when contracted they are shorter and consider-
ably wider at the larger end. They are light reddish in
color and consist of numerous similar cells so arranged as
to inclose a cavity. All the cells are well covered with
comparatively short cilia on the outer surface.

Eudendrium planulae are always in contact with the
substratum. They move along something like planaria.
Locomotion seems to be due entirely to the action of the
cilia, but the planulae are never found swimming freely
through the water like infusoria. There is no evidence of
constant rotation on the long axis. It may be, however,
since all sides appear the same, that they move with dif-
ferent sides in contact with the substratum at different
times. Locomotion is very slow, the average rate being
only about i cm. in 15 minutes. Every movement of this
creature can therefore be easily followed even under high
magnification. The larger end is always ahead, and this
end is constantly turned from side to side very slowly, and

l6o LIGHT AND THE BEHAVIOR OF ORGANISMS

raised slightly from time to time during the process of
locomotion. As the organism proceeds on its course it
secretes a mucous substance in the form of a delicate fiber
which can be readily detected by pushing a needle across
the path a short distance from the posterior end. The
planula and the needle usually adhere so firmly to the
mucous fiber that the former can be lifted to the surface of
the water.

Hargitt (1904, p. 272) referring to the reactions of Euden-
drium planulae says, " At the height of the breeding season
their numbers were large and they promptly swam directly
toward the strongest light with great uniformity. By inter-
posing a dark screen between this source of light and allow-
ing another from the opposite side to operate upon the
aquarium, there w^as an almost instantaneous response, the
entire number almost without exception facing directly
about, like a body of soldiers at command, and moving
without deviation in the opposite direction, that is, toward
the second source of light." When studied en masse in
light from a window^ they do appear to orient very accu-
rately; but if attention is focused on individuals it soon
becomes evident that there is considerable variation in the
direction of motion. This becomes still more evident if the
reactions of individuals are studied in light from a single
compact source. Thus several active specimens which ap-
peared to be moving directly toward the window were
selected and exposed one at a time in light of about the
same intensity from a Nernst glower so arranged as to
eliminate practically all refraction and reflection from the
different substances in the aquarium, th€ glass walls,
the surface of the water, and particles in suspension in the
water. The angle between their direction of motion and the
direction of the rays was frequently measured, and it was
found that it varied all the way from o to 10° and even
more in a few individuals. If a number of specimens are
put into the aquarium at the same time and at the same
point, it is found that as they proceed toward the source

MOVEMENT TOWARD LIGHT IN COELENTERATES i6l

of light, some deflect to the right, others to the left, so that
the group gradually becomes wider and wider. When
exposed to light from two sources they may travel toward
any point between, as stated under Euglena.

They are positive in strong as well as in weak light, but
if the intensity is very high, as, e.g., direct sunlight, or
quite low, they do not orient so accurately as they do in
light of moderate intensity, and the lateral movements of
the anterior end are more pronounced. In general the more
strongly positive the planulae are, the more accurately they
orient and the less they swing the anterior end from side
to side. From time to time the anterior end also becomes
flatter and broader. The lateral movements of the anterior
end, as well as the process of becoming broader, are due to
internal contractions. If the ray direction is but slightly
changed after the planulae are oriented, they do not turn
directly toward the source of light in its new position, but
merely swing the anterior end a little farther toward it
each time. In the meantime the body gradually turns so
as to become oriented again. If however the direction of
the rays is changed to such an extent that the sides of the
organism become fully exposed, they with very few excep-
tions appear to turn toward the light at once. In this
process they swing the anterior end laterally until it nearly
if not quite faces the source of light. It is thus frequently
bent at right angles to the posterior end. The anterior
end often swings back after turning but never so far as it
was before. The lateral turning is a slow steady movement
due, no doubt, to contraction of the tissue in the planulae
and not to the action of the cilia, for it is much more rapid
than the forward movement, which is entirely due to the
action of the cilia, and moreover no currents indicating
unequal ciliary action on opposite sides could be detected.

There Is no definite reaction if the intensity is suddenly
decreased or increased, nothing similar to an avoiding reac-
tion or a shock movement. I have seen the planulae pass
from darkness into strong light, consisting of rays perpen-

l62 LIGHT AND THE BEHAVIOR OF ORGANISMS

dicular to the bottom of the aquarium, without any reaction
excepting perhaps a very sHght increase in the lateral move-
ment of the anterior end.

The fact that these organisms turn directly toward the
light when the side is illuminated apparently shows that
they have the power of differential response to localized
stimulation, that they can orient without preliminary trial
movements. The fact that the anterior end is constantly
being turned from side to side, and that orientation may
take place if the ray direction is only slightly changed by
merely swinging this end a little farther toward the source
of light each time that it turns in that direction in the
regular process of lateral movement, shows that under cer-
tain conditions orientation takes place by the trial method.
The fact that the planulae can move toward any point
between two sources of light shows that neither the direc-
tion of rays through the body nor the angle the rays make
with the surface is of importance in orientation. If this
be true, the orienting stimulus must be due to difference of
intensity or a change of intensity on opposite sides of the
body, especially on the anterior end. The lateral swinging
movements serve to magnify the difference or change of
intensity on this end, and thus make it possible for the
animal to orient more accurately than it otherwise could.

The turning of the anterior end appears to serve in direct-
ing the course much as a cane serves a blind man in keeping
him on the path. The man may go directly toward his
goal without deviation and still it is evident that he orients
and keeps on his course by the trial method. Every move-
ment of the cane is a trial movement. Likewise every
lateral movement of the anterior end in the planulae and
many other organisms, as well as separate movements of
the antennae, eyes, and other special organs in various
forms may be trial movements. It is therefore clear that
the mere fact that an organism moves directly toward a
source of stimulation is not sufficient evidence to show that
its orientation and direction of movement are not regulated

MOVEMENT TOWARD LIGHT IN COELENTERATES 163

by the trial method, as has been assumed by some investi-
gators.

While the planulae of Eudendrium may undoubtedly
orient by differential response to localized stimulation it
is at present impossible to say whether such stimulation is
due to changes of light intensity or to constant intensity.
The question as to the importance of lateral movements of
the anterior end in orientation, and the cause of stimulation
will be referred to more in detail under the reactions of fly
larvae and earthworms.

3. Eudendrium Hydranths

After the planulae are a few days old the anterior end
becomes attached to the substratum and they soon develop
into hydranths, which bend toward the source of light as
they grow. In order to study this process of bending
toward the light, I placed on the stage of the compound
microscope, a small aquarium containing hydranths which
had bent so that the distal end was nearly horizontal, and
turned it so that one side of the organisms faced the light,
and then projected a selected specimen with a camera
lucida. This same individual was projected later from
time to time, and in this way its movements were definitely
recorded. It was found that the hydranths turn directly
toward the source of light. There was no indication of cir-
cumnutation movements. The bending takes place only
in the region of growth. All sides elongate but the shaded
side elongates more than the illuminated side. The organ-
isms bend toward the source of light very slowly. In all
the individuals studied it required 48 hours or more to turn
through 90°.

In the orientation of this organism It seems probable that
light acts as a constant directive stimulation. But the
knowledge we have concerning the process hardly warrants
even a suggestion as to the probable mechanism involved.
The bending may possibly be due to contraction as sug-

1 64 LIGHT AND THE BEHAVIOR OF ORGANISMS

gested by Loeb (1906, p. 121): " The heliotropic curvature
consists here in the stem undergoing a stronger contraction
or shortening on the more strongly illuminated side of the
polyp than on the opposite side." The fact however that
the stem elongates on all sides does not favor this view,
although growth might possibly mask shortening due to
contraction. Unequal rate in growth may have something
to do with the bending, since it takes place only in the region
of elongation and is an exceedingly slow process.

Loeb thinks the orienting reactions in Eudendrium are
the same as those in plants. He says (1906, p. 120), '' The
same phenomena of heliotropism w^hich we find in plants
we find also in sessile animals; and the identity of the
heliotropic reactions in these two groups of organisms is so
complete that it would be at any time possible to demon-
strate the phenomena and laws of plant heliotropism in such
animals, and vice versa.'' The identity Loeb maintains
exists here is in all probability extremely superficial.

4. Reactions of Medusae

Many medusae do not react to light at all ; others respond
to changes of intensity by contracting; and still others
become more active with change in the illumination. Only
a few are know^n to orient. Both Yerkes and Morse have
shown that Gonionemus murbachii orients under certain
conditions, although very indefinitely. It apparently turns
directly toward or away from the source of light without
any preliminary movements. These organisms appear to
have the power of differential response to localized stimula-
tion. As to how light produces the orienting stimulation
nothing is known. Many of the light reactions of this form
are clearly due directly to change of intensity, while others
appear to be due to the effect of constant light intensity.

The medusae of Bougainvillea superciliaris orient far more
accurately than does Gonionemus or any other medusa of
which I know, but even in these orientation is not accurate

MOVEMENT TOWARD LIGHT IN COELENTERATES 165

enough to warrant definite conclusions regarding the me-
chanics involved in this process.

These creatures are only about i mm. in diameter. They
have four prominent reddish brown spots symmetrically
situated on the margin of the bell. From the tissue sur-
rounding each of these spots there project three short
tentacles which are much contracted when the medusae
swim. The medusae are negative in their reactions to
gravity, and positive to light of intensities ranging from
weak diffused sunlight to intense direct sunlight. This
causes them to collect at the surface of the water in the sea
and to swim toward regions of highest light intensity.

In swimming toward the source of light they frequently
turn to the right or left rather sharply so as to produce a
zigzag course. The turning from side to side indicates that
light does not act as a constant directive stimulation. If
however the ray direction is changed through 90° the
medusae turn directly toward the source of light without
any preliminary trial movements. It may be then that
they are stimulated only when they turn a certain amount
and that, owing to the power of differential response to
localized stimulation, they always turn toward the light
after such stimulation and consequently remain oriented,
in a general way. If orientation is due to differential
response to localized stimulation the stimulation may be
caused by an increase of intensity on the illuminated side
or a decrease on the shaded side. We have however no
evidence bearing on this question.

If exposed to light from two sources they swim toward a
point between them. The location of this point depends
upon the relative intensity of light from the two sources.

CHAPTER IX

REGULATION IN THE DIRECTION OF MOVEMENT WITH

REFERENCE TO THE SOURCE OF LIGHT IN VERMES,

FLY LARVAE, AND ECHINODERMS

Many of the organisms belonging to these groups respond
very definitely to stimulation by light. In some, the
response results in orientation, in others it consists merely
of an increase or decrease in rate of movement, and in
still others it consists chiefly of a sudden contraction. We
shall concern ourselves here primarily with forms which
orient, emphasizing particularly the orienting reactions.
The reactions of the blowfly larvae will be discussed in
this section owing to their worm-like structure and
method of locomotion.

I . A renicola cristata — Larvae

a. Description. — Arenicola deposits its eggs in great
numbers in masses of jelly-like substance. In the course
of a few days the eggs develop into finger-shaped free-swim-
ming larvae about 0.3 mm. long. These organisms are
strongly positive in their reactions to light and negative in
their reactions to gravity. They contain two ciliary rings,
one near the anterior and the other near the posterior end ;
these are connected by a median ventral band of cilia. On
either side near the anterior end is an eye-spot. Lillie
(1903, p. 345) says, " Each [eye-spot] consists of a compact
clump of pigment or excretory granules on the surface of
the brain." I studied the eye-spots in living specimens
slightly flattened with the cover-glass, under an oil immer-
sion lens and found that they consist of a brownish granular
cup-shaped portion which partially surrounds an ellipsoidal

166

VERMES, FLY LARVAE, AND ECHINODERMS 167

hyaline portion and that this is directed dorso-anterio-later-
ally. The hyaHne structure in the eye-spot is probably highly
sensitive to light, while the pigmented portion appears to
serve in shading the inner surface so as to admit light only
from in front and from the side on which the eye-spot is
located.

b. Locomotion. — The larvae have two methods of loco-
motion. They swim by means of the cilia for a few days
then settle to the bottom and crawl. The crawling move-
ment is brought about by means of muscular contraction
much as in many other annelids. During this method of
progression they are slightly negative and there is scarcely
any indication of orientation. In the free-swimming posi-
tive state however they orient quite accurately. Both
methods of reaction are adaptive. The positive reaction
in the free-swimming state serves to keep the larvae at the
surface of the water and causes them to scatter widely.
The negative reaction serves to keep them at the bottom
and to guide them into the mud where most of their future
days are to be spent.

In swimming they proceed much like the dilates. They
rotate counter-clockwise on the long axis and travel on a
spiral course. The ventral surface, the surface containing
the median band of cilia, constantly faces out in the spiral,
contrary to what might be expected if these cilia are
functional. The organism Is slightly curved, the ventral
surface being concave. It may be that this causes the
constant swerving toward this surface, which together with
rotation on the long axis results in the spiral course.

c. Orientation. — Orientation is not so accurate as is
generally assumed. If casually observed It is true that the
larvae do appear to move directly toward the source of
light, but if the course of a given individual exposed to light
from a single compact source is carefully followed it is found
that there are frequent deviations. As the organisms pro-
ceed they frequently turn the anterior end slightly but
suddenly toward either side by means of muscular contrac-

l68 LIGHT AND THE BEHAVIOR OF ORGANISMS

tlon. This causes the spiral course to become very irregu-
lar and makes it appear as though they were constantly
being thrown out of orientation and reorienting. Their
general course is toward the source of light but it is a very
irregular course.

d. Mechanics of orientation. — If the direction of the
rays of light is changed after the larvae are oriented they
all appear to turn directly toward the source of light in its
new position without preliminary trial movements. What
is the mechanism involved in this apparent direct orienta-
tion? If one edge of a cover-glass is supported so that it is
a little higher than the other, and if the larvae mounted in
water on the slide are forced to travel toward the lower
edge, they soon reach a place where the cover- glass is so
near the slide that they no longer rotate. Under such con-
ditions the animals lie usually on the ventral surface, but
some specimens are found on either side or on the dorsal
surface. No definite movement is seen in those on either
side, excepting occasionally a slight forward motion. But
in those on either surface, the anterior end is constantly
seen to move from side to side with a slight jerky motion.
This lateral movement of the anterior end is undoubtedly
due to muscular contraction. If one of the specimens with
the dorsal surface up is selected and light thrown upon it
from such a direction that the rays strike its side at right
angles, the lateral movement toward the side illuminated
is at once much increased and the organism turns in that
direction. By using two sources of light so situated that
the rays cross at right angles in the region where the speci-
men is located, and then alternately intercepting the light
from each of the two sources, it can be seen clearly that the
larva, by muscular movement, turns the anterior end toward
the source of light directly. There is no trial reaction in
this process. It is an asymmetrical response to an asym-
metrical stimulation. This does not however mean that
both sides of the organism are stimulated in accord with
the theories of orientation of Loeb and Verworn. The reac-

VERMES, FLY LARVAE, AND ECHINODERMS 169

tion may be due to an Increase in illumination on one side
or a decrease on the other. The stimulation may be local
and the reaction a differential response. We shall refer to
this problem again later.

The larvae are so small and move so rapidly in the free-
swimming state that it is exceedingly difficult to follow
their movements in detail during the process of orientation.
By carefully observing this process, however, in a low tem-
perature by means of which the rate of movement is much
reduced, it was found that it takes place somewhat as fol-
lows: If the source of light is changed in its position after
a free-swimming specimen is oriented, reactions occur im-
mediately only if either eye-spot is fully exposed after the
change is made. If the ventral or the dorsal surface is
directed toward the source of light after the ray direction
is changed, there is no reaction until the organism has
rotated through 90° as it proceeds on its spiral course, and
one of the sides comes to be illuminated. Then the anterior
end is turned sharply toward the source of light, frequently
to such an extent as to form a right angle with the posterior
end. This causes rapid swerving on the spiral toward the
light and speedy orientation (see Fig. 26). I was unable
to detect any change in the course due to ciliary action.

e. Discussion. — The method of orientation in Arenicola
larvae has some features in common with that of Euglena
in the free-swimming state. In both forms there is a defi-
nite reaction whenever an eye-spot comes to face the source
of light as they proceed on their spiral courses. This reac-
tion consists of a turning toward the side containing the
eye-spot and a swerving on the spiral course in the same
direction, and this results in orientation. The larvae how-
ever, having two eye-spots, can turn toward the source of
light in two positions in the spiral, whereas the Euglenae
can turn toward It in only one. If one were to imagine two
Euglenae united so as to form an organism with two eye-
spots facing in opposite directions, it would not be difhcult
to conceive the organism thus formed capable of turning

lyo

LIGHT AND THE BEHAVIOR OF ORGANISMS

^ .■y ^, ^/
f f f f

717

\f ^'

I n

Fig, 26. I. Arenicola larva in the free-swimming state, proceeding on a spiral
course, m, n, directions of light; a-h, different positions on the spiral; b, dorsal
surface up, right eye-spot toward n; d, ventral surface up, left eye-spot toward n.
If the ray direction is changed by simultaneously exposing n and shading w when
the larva is in position a or c, no reaction takes place until it reaches b or d, then
it bends the head sharply toward the source of light and turns in its course. In
the former position it turns toward the right side of the body, in the latter toward
the left. This indicates that the larvae have the power of differential response
to locaUzed stimulation, and that the orienting stimulus may be due to a change
of light intensity.

II. Much enlarged sketch showing the general structure and position of the
eye-spots as seen under an oil immersion objective. The eye-spots are com-
posed of a dark brownish caplike portion, y, which partially surrounds a colorless
portion, x, directed slightly dorso-laterally; z, band of cilia.

VERMES, FLY LARVAE, AND ECHINODERMS 1 71

toward the source of light in two positions on the spiral
course just as Arenicola larvae do. If in place of a union
of two individuals we should have a union of three, it would
result in an organism that could turn toward the source of
light in three positions on the spiral. And if a sufficient
number were united it is clear that the organism could turn
toward the source of light in all positions on its course.
Such organisms we have in the colonial forms Volvox,
Eudorina, and Pandorina. All of these consist of numerous
individuals united, and all can turn toward the source of
light directly no matter which side is illuminated. It is
however probable that this analogy is, with reference to
Arenicola larvae, merely superficial.

/. Orienting stimulation. — In positive Euglenae it was
found that orientation is due to a reaction caused by a
reduction in effective light intensity due either to a change
in the intensity of the field or to rotation of the organism
owing to the fact that it is more sensitive when the ventral
surface is illuminated than when the dorsal surface is. In
Volvox the orienting stimulus is likewise due to a reduction
of effective intensity. To what is it due in Arenicola lar-
vae? Is it due to a decrease of intensity caused by the
shadow of the pigment on the hyaline portion of the eye-
spot on the side turned from the light, or to an increase of
intensity on this structure in the eye-spot turned toward
the light, or to an absolute difference of intensity on the
two sides in accord with the theories of Verworn and Loeb?

Two methods were used in attempting to answer these
questions. In both the larvae were mounted under a large
cover-glass supported by means of a ring of vaseline. The
cover was then pressed down until the space became so
narrow that the larvae could not rotate, (i) A piece of
sheet metal containing an opening i cm. square was hung
about 3 mm. from a Welsbach mantle. From the middle
of one side of the opening there projected nearly to the
center a spinelike process. The incandescent mantle was
focused on the slide by means of the plane mirror and Abbe

172 LIGHT AND THE BEHAVIOR OF ORGANISMS

condenser where it produced a sharply defined rectangular
area of intense Ught with a narrow triangular shadow pro-
jecting from one side. By manipulating the mirror I was
able to change the position of this area of light so as to
illuminate or shade any part of a larva fairly accurately in
spite of its microscopic size. All light except that from
the opening in front of the mantle was thoroughly elimi-
nated by means of suitable screens. Without going into
detail regarding the numerous observations made at dif-
ferent times, the redactions may be summarized as follows:
(a) If the anterior end is suddenly illuminated the larva
bends from side to side vigorously, (b) If the light inten-
sity on one eye-spot is increased without changing that on
the other, it bends both ends rather sharply toward the
illuminated side, (c) If the intensity on either eye-spot is
decreased it also bends toward the illu ninated side, (d) If
any portion back of the eye-spots is shaded or illuminated
there are no definite reactions. This shows that the ante-
rior end (probably the eye-spots) is the most sensitive part
of the larvae if it is not the only sensitive part, and that if
the light intensity is increased or decreased on either side
regardless of the direction of the rays, the larvae turn
toward the more highly illu ninated side.

(2) Two sources of light were so arranged and screened
as to produce small horizontal beams which crossed at right
angles on the stage. The larvae exposed in the light from
these two beams oriented toward a point approximately
halfway between the two sources. If the light from one
source was now intercepted they turned directly toward the
other, and when it was again exposed they returned to their
former position. These reactions do not occur in all in-
stances nor is the orientation always precise and definite,
especially if the larvae are not in prime condition. They
were however seen in so many cases that there can be no
doubt concerning the conclusions stated above.

What bearing have these conclusions on the problem in
hand? It is evident that when the light in one beam is

VERMES, FLY LARVAE, AND ECUINODERMS 173

intercepted after the larvae are directed toward a point
between the two, the intensity on the side facing this beam
is decreased more than that on the opposite side, and when
this Hght is turned on again after the larvae are directed
toward the other, the intensity on the same side is increased
more than it is on the opposite side. Under both condi-
tions however we find that the larvae turn toward the side
most highly illuminated. Under one therefore they turn
toward the side on which the intensity is increased, under
the other from the side on which it is decreased. It is
evident then that if the orienting stimulus is due to change
of intensity, it may be due to an increase as well as to a
decrease of intensity. And if this is true the organism must
in some way perceive the difference between a stimulus due
to an increase and one due to a decrease of illumination,
for in response to the former it turns toward the point of
stimulation whereas in response to the latter it turns from
this point.

It can be definitely stated then that orientation in Areni-
cola larvae is due to difference of intensity on opposite sides.
Whether it is the result of light acting constantly as a
directive stimulation like a constant electric current, or
whether it is the result of reactions due to changes of inten-
sity on the sensitive structures in the organisms brought
about largely by its movements, is a question concerning
which our evidence does not warrant a definite conclusion.
The facts that the organisms are frequently thrown out of
orientation as they proceed toward the source of light, and
that the anterior end is almost constantly turned from side
to side speak in favor of the latter. The organism is not
held definitely on its course as one would expect in case of
light acting constantly as a directive stimulation in accord
with the definitions of tropisms of Loeb and Verworn. It
must however be remembered that even if the organism is
under the influence of a constantly acting directive stimu-
lus it might turn from side to side frequently owing to the
effect of internal processes or other external stimuli.

174 LIGHT AND THE BEHAVIOR OF ORGANISMS

Summary

(i) Arenicola larvae are positive to light in their free-
swimming state. They rotate on the long axis and swim
on an irregular spiral course owing to frequent sharp lateral
movements of the head.

(2) If held under a cover-glass so that they cannot
rotate, it is found that the head is suddenly turned directly
toward the source of light when either of the two sides
is illuminated, frequently to such an extent that the
anterior end of the body is at right angles to the posterior
end.

(3) The larvae have two prominent eye-spots, which con-
sist of a hyaline portion partly surrounded by an opaque
caplike structure. The hyaline portion, which is probably
sensitive to light, is directed dorso-anterio-laterally. In
the free-swimming state orientation takes place by a greater
swerving toward the source of light on the spiral course
every time an eye-spot faces the light just as in Euglena.
In Arenicola, since it has two eye-spots, the increase in
swerving takes place in two different positions on the spiral,
i.e., twice during a complete rotation on the long axis. In
Euglena, since there is but one eye-spot, it takes place only
in one position on the spiral, or once during a complete
rotation. Euglena however responds when the eye-spot
faces the source of light because when it is in this position
the eye-spot shades the tissue. It responds only to a
decrease of intensity while it is positive. Arenicola larvae,
on the contrary, respond to either an increase or a de-
crease of intensity on the sensitive tissue on either side,
but they always turn toward the more highly illuminated
side.

(4) The stimulus causing this reaction, a reaction by
means of which the organism orients, is probably due either
to a decrease or to an increase of intensity on either side.
The orienting reaction is probably a differential response
to a localized stimulus. Our evidence however does not

VERMES, FLY LARVAE, AND ECHINODERMS 175

warrant a definite conclusion on this point. Orientation
may be the result of a response regulated by the absolute
difference of intensity on opposite sides, that is, it may be
due to the action of light owing to continued intensity
rather than to its action owing to change of intensity.

2. Blowfly Larvae — Musca sp.{?)

a. Introduction. — The reactions of the blowfly larvae
to light were described by Loeb in 1890. He exposed the
larvae in front of a window in diffused and direct sunlight
and found them to be negative and to orient very accurately.
He says (1905, p. 57), " They crept with mathematical pre-
cision in the direction of the rays. When a shadow was
thrown on the board by a penholder, it could be noticed
that the animals moved away from the light in a direction
exactly parallel to the edge of the shadow. . . . They
acted as though they were impaled on the ray of light which
passed through their median plane. When I turned the
board around, the animals immediately turned about also,^
and again placed their median planes in the direction of
the rays." Loeb, assuming that the method of orientation
in this form is the same as it is in others, concluded that it
is controlled by the same factors. In this form as in others
light acts constantly as a directive stimulus and " the main
feature ... is the fact that symmetrical points of the
photosensitive surface of the animal must be struck by the
rays of light at the same angle." (1897, p. 440), " Ich
glaiihe jetzt, dass hier eine vollkommene Analogie der Licht-
und Stromwirkungen zu Tage tritt, derart, dass audi, wie beim
Strom, die Lichtintensitdt dauer?id die Spannung der Muskeln
heeinflusst, dass aber die Steilheit der Intensitdtsschwankung

die Fortleitung der Spanmmgsdnderiing bestimmt

Das Wesen der Orientirung fasste ich dahin auf, dass bei
vollendeter Orientirung Symmetriepimkte der Oberfldche des
Thieres unter gleichem Winkel von den Lichtstrahlen getroffen
werden," According to Loeb then, if the position of the

176 LIGHT AND THE BEHAVIOR OF ORGANISMS

source of light is changed, the larvae turn immediately and
directly from the light in its new position until both sides
are again struck by the rays at the same angle.

Holmes' observations of the orienting reactions of fly
larvae do not support^ Loeb's theory. He found among
other things (1905, p. 105), " If a strong light is thrown
upon a larva from one side it may swing the head either
towards or away from the light ... In the animals here
described there is, so far as I can discover, no forced orien-
tation brought about by the unequal stimulation of the
two sides of the body, but an orientation is produced
indirectly by following up those chance movements which
bring respite from the stimulus. I do not deny that there
may be an orienting tendency of the usual kind, but if
there is it plays only a subordinate role in directing the
movements of the animal. The orientation of these forms
is essentially a selection of favorable chance variations of
action and following them up."

b. Locomotion. — The blowfly larvae move from place
to place entirely by means of muscular contraction. They
proceed somewhat as follows: the anterior end is raised,
thrust forward toward one side, fastened to the substratum,
and then the posterior end is pulled forward, after which
the anterior end is again raised and thrust forward, now
toward the opposite side, fastened, and the posterior end
again drawn up. The anterior end is thus turned alter-
nately toward the right and left quite regularly during the
process of locomotion. The extent of this lateral move-
ment varies much, but it is usually great enough so that
the extremity of the anterior end is nearly at right angles
to the direction of locomotion (see Fig. 31).

In drawing forward the posterior end the whole body
contracts, but the contraction is greater on the ventral than
on the dorsal surface, forming an arch, in which the extreme
anterior end is nearly vertical and the sensitive tip (Fig. 30)
well drawn under so as to be hidden from view. Thus the
tip of the anterior end becomes alternately thrust out and

VERMES, FLY LARVAE, AND ECHINODERMS 177

exposed, retracted and concealed. This is of importance
in the orienting reactions as will be seen later (Fig. 31).

c. Accuracy of orientation. — The orientation of organ-
isms is generally supposed to be far more accurate than
it really is. This is no doubt due to the fact that many of
the observations on light reactions have been made in light
which is more or less diffused and the direction of which
is not thoroughly under control. In testing the accuracy
of orientation in the blowfly larvae they were exposed on a
piece of smooth moist black paper on a glass plate in a
small horizontal beam of light from a Nernst glower. The
course taken by the larvae was traced on the paper in white
ink with a small pen held about i cm. in front of the end
of the larvae. In this way the movements could be quite
accurately traced. Neither the presence of the pen nor
the ink on the paper made any appreciable difference in the
course taken. The courses of four different individuals are
given in Fig. 27. All of the specimens tested deviated con-
siderably; those used in Fig. 27, B and C, deviated toward
the left in all the trials; the one used in Fig. 27, A, to the
right, and that in Fig. 2^], D, to the right in some trials
and to the left in others. In Fig. 27, A, the head move-
ments are represented more in detail than they are in the
others. It will be seen that the lateral movements to the
right and the left alternate quite regularly. The posterior
end takes a much more regular course than the anterior.
In direct sunlight orientation is somewhat more accurate
and the lateral movements are not so pronounced. But
I failed to find any specimens which " crept with mathe-
matical precision in the direction of the rays, . . . exactly
parallel to the edge of a shadow," or which " acted as
though they were impaled on the ray of light which passed
through their median plane," as Loeb states.

d. Orientation in light from two sources. — In the study
of orientation in light from two sources the larvae were
exposed on moist black paper just as in the preceding experi-
ment, in a field of light composed of two small horizontal

lyS

LIGHT AND THE BEHAVIOR OF ORGANISMS

beams, one from each of two Nernst glowers so situated
that the beams crossed at right angles. One of the glowers
was stationary and the light from it constant. The other
was mounted on a track so that the light from it could be

Fig. 27. The lines i, 2, etc., in ^, B, C, D, represent courses taken by four
different blow-fly lar\'ae in light of 78 ca. m. intensity, n, direction of horizontal
rays from single Xernst glower. Orientation is not as accurate as one would ex-
pect if light acts constantly as an orienting stimulus in accord with the theories of
Sachs, Loeb, and Verworn. See text.

varied. The paths taken by the larvae under the different
conditions are represented in Fig. 28.

It will be seen by referring to the figure that the larvae
can move in a direction leading from any point between the
two sources of light just like all the other lower organisms
tested under these conditions. Loeb says (1905, p. 61),

VERMES, FLY LARVAE, AND ECHINODERMS 179

" When the diffuse daylight which struck the [fly] larvae
came from two windows the planes of which were at an
angle of 90° with each other, the paths taken by the larvae

Fig. 28. Direction of movement of fly larvae in light from two sources; n, m,
direction of rays; i, course in light from n and m 50 and 5.5 ca. m. respectively;
2, course in light from n and m, 50 and 15 ca. m. respectively; 3, course in light
from n and m, 50 and 60 ca. m. respectively. Lines i, 2, and 3 represent the aver-
age direction of several courses taken by each of three larvae,

lay diagonally between the two planes; " and (p. 2), " It is
explicitly stated in this and the following papers that if
there are several sources of light of unequal intensity, the
light with the strongest intensity determines the orientation

l8o LIGHT AND THE BEHAVIOR OF ORGANISMS

and direction of motion of the animal. Other possible com-
plications are covered by the unequivocal statement, made
and emphasized in this and the following papers on the
same subject, that the main feature in all phenomena of
heliotropism is the fact that symmetrical points of the
photosensitive surface of the animal must be struck by the
rays of light at the same angle. It is in full harmony with
this fact that if two sources of light of equal intensity and
distance act simultaneously upon a heliotropic animal, the
animal puts its median plane at right angles to the line
connecting the two sources of light. This fact was not
only known to me, but had been demonstrated by me on
the larvae of flies as early as 1887 in Wiirzburg, and often
enough since. These facts seem to have escaped several of
my critics."

It is evident without further discussion that the reac-
tions of fly larvae in light from two sources are not in accord
with Loeb's conclusions. When exposed in light from two
sources of different intensity the stronger does not deter-
mine the orientation and direction of motion, nor are sym-
metrical points on the photosensitive surface struck by the
rays of light at the same angle.

e. Orientation and movement — (i) perpendicular to the
direction of the rays — (2) toward a source of light. — The
following experiments bring out clearly the importance of
intensity in the orientation of fly larvae. A small horizontal
beam of light from a single Nernst glower was thrown on the
black paper used in the preceding experiments. In this
beam a small vertical post was erected so as to produce a
well-defined narrow shadow. By means of a mirror this
shadow was illuminated with rays of light either perpendic-
ular to or parallel with its edges, as represented in Fig. 29.
The intensity of light in the shadow could be regulated by
changing the position of the mirror. It was always con-
siderably lower than that in the field on either side.

If a specimen taken from darkness is placed on the plate
in the shadow with its anterior end directed toward the

VERMES, FLY LARVAE, AND ECHINOhERMS i8l

mirror, i.e., toward the source of light, it soon begins to
crawl and turn so as to direct the anterior end away from
the mirror, but In attempting this the anterior end extends
into the direct light owing to the narrowness of the shadow.
This produces a stimulus which causes it to withdraw and
swing in the opposite direction, where it soon comes Into
the intense direct sunlight again. It continues this swing-
ing and crawling from one side of the shadow to the other

Fig. 29, Representation of arrangement of apparatus used to produce light
conditions in which negative fly larvae crawl toward the source of light or perpen-
dicular to the rays, a, glass plate; b, b', beams of Hght; c, shadow cast by the
opaque standard d; e, fly larva; g, Nernst glower; s, opaque screen; m, m', mirrors,
relatively much farther from the glass plate than represented. The larva is placed
in the shadow, c, which is illuminated by light reflected from either the mirror m'
or m. The intense illumination on the anterior end whenever it projects beyond
the shadow prevents the larva from turning around and shows that under the
conditions of the experiment it is the change of light intensity on the anterior end,
and not the direction of the rays or the relation of intensity on symmetrically
located sensitive points, which regulates the direction of movement.

for a short time, but soon comes to travel more nearly
parallel with the edges of the shadow and consequently
extends into the light much less frequently. If the rays
from the mirror are perpendicular to the edges the larva
also remains in the shadow, but usually It crawls along near
the edge farthest from the mirror. The negative larvae can

l82 LIGHT AND THE BEHAVIOR OF ORGANISMS

thus be forced to move toward a source of light or at any
angle with the rays.

If exposed in a narrow shadow in a field of light con-
sisting of rays perpendicular to the substratum they crawl
along in the shadow. If the anterior end chances to pro-
ject out into the light it is stimulated and turned in the
opposite direction. The direction of the rays here is how-
ever perpendicular to the substratum; under the preceding
conditions it was parallel with the substratum, yet the reac-
tion is the same under both conditions. It is of course due
to a change of intensity and is not primarily dependent
upon the ray direction. In a narrow shadow in the field
consisting of vertical rays the larvae can also be made to
move toward or perpendicular to light rays in the shadow
just as under the conditions described above. Cole (1907)
obtained similar results in experiments on Bipaliumkewense.

These results indicate that in their movements the larvae
attempt to keep the sensitive anterior end in the lowest
possible light intensity regardless of the direction of the
rays or the angle between them and the sensitive surface.
Loeb claims that the larvae follow the direction of the rays
even if in so doing they go from regions of lower into regions
of higher light intensity. He says (1905, p. 58), " I put
the almost fully grown larvae into a test-tube and placed it
horizontally on the table, with its longitudinal axis perpen-
dicular to the plane of the window. The sun's rays made
a small angle with the window. By means of a screen I
arranged the test-tube so that only diffuse light fell through
the window upon the half turned toward the window, while
direct sunlight fell on the half turned toward the room.
At the beginning of the experiment the animals were all
on the window side of the test-tube. They immediately
moved from the shaded part into the direct sunlight on the room
side, and remained there.'' Do these results prove Loeb's
conclusions? Can the reactions described in the quota-
tion be explained on the assumption that orientation reac-
tions are due to difference of intensity?

VERMES, FLY LARVAE, AND ECHINODERMS 183

Loeb says (p. 58), " When the animals crossed the bound-
ary from diffuse Hght into direct sunlight, the reaction
caused by the increase in the intensity of the light did
not take place until a half or a third of the body was in the
sunlight (because in all phenomena of stimulation some time
elapses between the application of the stimulus and the re-
action to it). The animal checked its movement and turned
its head through an angle of 90°- 130° from side to side. If
in so doing the head again came into the shade the animal
returned into the shade; but if this did not happen, as was
more usually the case, the animal continued its movement
into the sunlight." Under the conditions of the experiment
quoted above there was therefore no cause for turning until
one-half or one-third of the body was in direct sunlight.
It is evident that after the anterior end is in the sunlight,
it is in lowest light intensity when it is directed from the
source of light. Consequently if the larvae did start to
turn around so as to get back into the shaded region, the
effective intensity would be increased and this would at
once cause them to turn the head back again to the position
in which it is shaded. Moreover Loeb says, as quoted
above, that if the head came into the shadow in turning,
the animal returned into the shade. It is therefore evident
that there is nothing in the observations of Loeb inconsist-
ent with the idea that the orienting reactions are due to
difference or change of intensity on the surface of the organ-
ism. Nor do these observations show that these reactions
are not accompanied by anthropomorphic sensations as
Loeb intimates, since every reaction shows that the larvae
assume positions such that there is a minimum exposure of
the sensitive anterior end.

/. Sensitive region. — Both Loeb and Holmes assume
the anterior end to be the most sensitive part of the fly
larvae with reference to stimulation by light. The follow-
ing experiments on the effect of intensity on the rate of
locomotion indicate that this is not only the most sensitive
region, but that it is the only region sensitive to light. At

i84

LIGHT AND THE BEHAVIOR OF ORGANISMS

the anterior end there are two minute cone-shaped pro-
tuberances not over 0.5 mm. apart (Fig. 30). These pro-
tuberances can barely be seen with the naked eye when
the anterior end is extended, and not at all when it is con-
tracted. Judging from their connection with the nervous
system, I am inclined to believe that they are light recipient
organs.

a.sp

D

I I I I
5 mm^ >

Fig. 30. Musca larva. A, side view anterior end expanded; a.sp., anterior
spiracular process showing seven spiracular papillae; o.t, optic tubercle. After
Hewitt (1908, PL 30, Fig. g). B, camera outline, dorsal view showing anterior
end expanded. C, same showing anterior end contracted, and optic tubercle
withdrawn and turned under as it is during the process of looping. D, dorsal
view of entire animal.

g. Effect of light intensity on rate of locomotion. — The
effect of light intensity on the rate of locomotion in fly
larvae was tested under three conditions: (i) with the entire
larva exposed; (2) with the posterior third exposed; and
(3) with the posterior three-fourths exposed.

h. Method. — A glass plate 25 cm. square was covered
with two sheets of filter paper over which was placed a
sheet of smooth black paper. This was then thoroughly

VERMES, FLY LARVAE, AND ECHINODERMS 185

soaked In water and the plate so arranged that the edges
of the filter paper which projected over the edge of the
glass plate extended into water kept in a vessel below. In
this way a smooth surface containing a constant amount
of moisture was produced. It was found that such condi-
tions are very essential in quantitative work with the larvae,
especially that of constant moisture, since their rate of loco-
motion depends much upon the amount of moisture in the
surface upon which they crawl; either too much or too little
causes marked retardation. Two fine white threads were
placed parallel with each other on the paper 15 cm.^ apart,
so as to form a definitely limited course upon which to try
the speed of the larvae under different light conditions. A
horizontal beam of light 4 cm. wide was projected from a
Nernst glower upon the glass plate perpendicular to the
threads. The light in this beam halfway between the
threads was 7 ca. m. in intensity. The time it required a
given larva taken from the culture jar kept in total dark-
ness to travel the distance between the threads was accu-
rately ascertained by means of a stop watch. At the end
of the course the larva was allowed to crawl onto a piece
of black paper supported on a section lifter and then trans-
ferred to the starting point without changing its orientation,
and allowed to continue on its course w^ith the least disturb-
ance possible. After having ascertained the time required
to crawl 15 cm. in the beam of 7 ca. m. intensity, the plate
was turned through an angle of 90° so as to expose the
larva in a similar beam of light but one of a much higher
intensity. The intensity of this second beam of light in
the middle of the course was 3888 ca. m. It was produced
by a group of three Nernst glowers so arranged that a cross
section formed a small triangle. When the current was on,
the three glowers appeared much like a highly illuminated

^ Through some oversight I failed to record the distance between the
threads. I am not quite positive whether it was 15 cm. or 10 cm. This
however does not invalidate the results recorded in the following table since
they are comparative in every case.

i86

LIGHT AND THE BEHAVIOR OF ORGANISMS

solid rod several times as large as a single glower. It there-
fore cast a sharp shadow, a point of importance in the
following experiments. The time required to travel the dis-
tance was recorded just as under the preceding conditions.
The rate of movement was now obtained alternately under
the two conditions. The results appear in Table II. This
table shows that it required on an average 46.4 seconds to

TABLE II

Distance

Time in seconds

Light intensity,
3888 ca. m.

Light intensity,
7 ca. m.

15 cm.

(( u

(I u
a ii

44.4
43-2
42.4
44.8
42.

47-2

46.8

44.

46.6

47-4

Total Average

43-36

46.4

travel 15 cm. in an Intensity of 7 ca. m., and 43.36 seconds
to travel the same distance in an intensity of 3888 ca. m.
Under the former conditions the larvae therefore crawled
at the rate of 0.321 cm. per second, and under the latter
at the rate of 0.345 cm. per second, a difference of only
0.024 cm. per second, due to a difference of 3881 ca. m. of
light.

In studying the effect on the rate of locomotion of expos-
ing the posterior third and three-fourths of the larvae, the
apparatus was arranged just as described above. The time
required to travel 15 cm. in 7 ca. m. intensity was first
ascertained with a given larva, then the larva was trans-
ferred to the starting point, and after it had crossed the
thread a small beam of light 3888 ca. m. in intensity from
the three glowers was thrown on the posterior end and held
there by means of moving along by the side of the larva a
screen containing a small rectangular opening. The time

VERMES, FLY LARVAE, AND ECHINODERMS 1 87

required to complete the course was thus alternately
obtained under each of the two conditions. The results
obtained with one-third exposed are recorded in Table III;

TABLE III

Time in

seconds

Date

Distance

Posterior \ in

Entire larva

3888 ca. m.

in 7 ca. m.

Jan. 29

15 cm.

36.95

37-35

" 30

50.64

51.40

(( ii

36.77

36.67

11 u

35-38

35-18

41.10

41.75

(( a

43-66

43-58

" 31

69.45

71-5

ii ii

42.64

42.64

ii a

42.01

41.96

Feb. I

40.3

40.4

Total average

43-89

44.24

those with three-fourths exposed in Table IV. Each figure
in columns three and four in the tables represents the aver-

TABLE IV

Distance

Time in

seconds

Date

Posterior f in

Entire larva

3888 ca. m.

in 7 ca. m.

Jan. 29

15 cm.

38.94

40.73

ii li

36.56

38.12

" 30

47-84

48.18

39-05

40.01

li ((

53-23

54.30

" 31

42.18

42.26

u u

46.60

46.80

(( n

41.78

43.01

Feb. I

42.9

42.8

Total average

43-23

44.02

1 88 LIGHT AND THE BEHAVIOR OF ORGANISMS

age of ten trips across the course made by different indi-
viduals. The total average as seen in Table III, with the
entire larva exposed in 7 ca. m. light intensity, is 44.24
seconds, and that in 7 ca. m. intensity with the posterior
third of the body in 3888 ca. m. intensity, is 43.89 seconds,
a difference of only 0.35 seconds in traveling 15 cm. In
Table IV the total average in 7 ca. m. intensity is 44.02
seconds; and in 7 ca. m. with f of the body exposed in 3888
ca. m. intensity, it is 43.23 seconds, a difference of 0.79
seconds in traveling 15 cm. By comparing these results
with those recorded in Table II it will be seen that there
is very little difference in rate of locomotion under the
different conditions of illumination, i.e. larvae entirely ex-
posed in 3888 or 7 ca. m. or the posterior one- third or
three-fourths exposed in 3888 ca. m. This seems to indi-
cate that the tissue sensitive to light is restricted to the
anterior tip of the body. The difference in rate of loco-
motion under the different conditions can be accounted
for by assuming it to be caused by the light reflected from
the highly illuminated posterior end of the body upon the
sensitive anterior end.

The exposure of the side of the body to the very intense
light from the three glowers has apparently no effect what-
ever on orientation. The larvae continue as directly on
their course as though they were exposed only to light of
7 ca. m. intensity from the single glower. As a matter of
fact all but the very tip of the anterior end can be illumi-
nated by this intense lateral light without causing any
noticeable deviation in the direction of motion. If how-
ever the tip is exposed there is a sudden sharp turning
of the anterior end either toward or from the source of
light.

The results recorded in the last two tables indicate either
that the tissue sensitive to light in fly larvae is confined to
the extreme anterior end, or that light of constant intensity
has no effect on the rate of locomotion, the increase in rate
due to increase in light intensity when the entire organism

VERMES, FLY LARVAE, AND ECHINODERMS 189

is exposed being due to change of intensity caused by the
extension and contraction of the anterior end.

i. Mechanics of orientation. — Holmes (1905, p. 105)
says, " If a strong light is thrown upon a larva from one
side it may swing the head either towards or away from
the light," intimating that it is turned in one direction as
often as in the other. I exposed various individuals to sud-
den lateral illumination by direct sunlight, or light of nearly
equal intensity from the three glowers, at different times
and recorded the direction in which the anterior end turned.
The results appear in Table V. It will be seen by referring

TABLE V
Number of times anterior end is turned

From source of

Toward source of

light

light

6

6

13

8

9
8

13
26

7
26

26

24

19
28

31

22

3
8

3
8

27

21

Total 177

165

to this table that in all there were 177 turns from the light
to 165 toward it, i.e., nearly the same number in both
directions. Later however I obtained results very different
from these. They are recorded in Table VI. The results
recorded in Table VI show that when a larva is first exposed
to intense unilateral illumination, it turns toward the source
of light practically as frequently as from it, and orientation
is indirect, but that after being exposed for some time it
turns considerably more often from the source of light than

IQO

LIGHT AND THE BEHAVIOR OF ORGANISMS

toward it, and if then exposed to lateral illumination of a
lower intensity it seldom turns toward the source of light
at all, and orientation is direct.^

TABLE VI

-

Number of times anterior end is turned

Date

From source of
light

Toward source
of light

Feb. 6 Direct sunlight

u fi ^ Same individual 1

^5
34

3S

45

25
16

( Direct sunlight (
u (T { Same individual /

( Direct sunlight )

<< ^ ( Same individual /

12

5

1 Diffused light \

Feb. 7 New individual

12.30 P.M. Direct sunlight

( Same individual {

I.OO P.M. ^ ^.^^^^ g^^ijgj^^ f

, ( Same individual )
^•3° ^•^- i Diffused light \

29
4S

21
17

2

How are these results to be explained? It is ordinarily
supposed that the higher the illumination the more direct
the orientation in such organisms as fly larvae. The results
above indicate the opposite to be true. Have these crea-
tures the power of differential response to localized stimu-
lation, as the final results recorded in the table seem to
indicate ?

During the normal process of locomotion, as already
stated, the larvae alternately swing the anterior end slightly
to the right and left at the same time that they thrust this

^ The variable results recorded in Table VI show very clearly the im-
portance of studying reactions under different conditions, and also that
statistical results in the study of reactions may be very misleading. This is
particularly true in case of organisms which readily become acclimatized,
as the blowfly larvae do. If the larvae are exposed to direct sunlight half
an hour or so they frequently lose all power of response to lower intensities
and sometimes respond no longer even in direct sunlight.

VERMES, FLY LARVAE, AND ECHINODERMS 191

end forward. When the anterior end is thus extended the
two cone-shaped elevations at the very tip (Fig. 30) become
fully exposed; and these, owing to the lateral movements
of the anterior end, face alternately to the right and the
left. When the animal fastens the anterior end to the sub-
stratum and pulls up the posterior end, the cone-shaped
structures cannot be seen. They appear to be drawn in,
and the whole anterior end is turned under somewhat as
the arch is formed in the looping process. This causes the
tip to be thoroughly concealed and shaded.

When the larvae are first exposed to sudden lateral illu-
mination in direct sunlight, they respond immediately by
throwing the anterior end toward one side violently, no
matter in what position this end chances to be. If it hap-
pens to be directed from the source of light when the sun-
light is flashed upon the organism, it turns toward the source
of light, and if the sunlight is immediately intercepted after
the larva turns, it will continue in the direction toward
which the anterior end points; if it is not intercepted, the
anterior end is thrown in the opposite direction, and then
the larva may follow this turn and become oriented imme-
diately, or it may swing the end back and forth a few times
before becoming oriented. If the anterior end faces the
light when it is exposed to the sun it is first thrown in the
opposite direction and orientation takes place just as de-
scribed above. The anterior end is thus turned in the direc-
tion opposite to that in which it is when the exposure is
made. It is therefore evident that under these conditions
the larvae will turn toward a strong unilateral illumination
about as often as from it.

According to the tables, however, turning toward the
source of light becomes less frequent after the organism is
exposed for a time and much less frequent if the intensity is
decreased. What is the cause of this? If the larvae are
carefully observed when they are suddenly exposed to lat-
eral illumination by diffuse light, it is found that they
respond immediately only if the anterior end is turned

192 LIGHT AND THE BEHAVIOR OF ORGANISMS

toward the source of light when the exposure is made (Fig.
31). If this end is in any other position, there is no reac-
tion whatever until the organism, in its normal process of

•

locomotion, extends it toward the source of light. Then
it is at once turned from the light to such an extent that it
frequently makes a right angle w4th the posterior end.
Later it is swung back, but only part way. The tip is
however exposed and so the animal may be stimulated
again, after which it again turns sharply from the source of
light. This process is repeated until the organism has
turned to such an extent that the anterior end is practically
as much exposed when it turns in one direction as it is when
it turns in the other. The great preponderance of lateral
movements from the source of light and direct orientation in
diffuse light therefore do not indicate that fly larvae have
the power of differential response to localized stimulation.

But why does the organism turn toward the light if the
lateral illumination is very intense? Whenever the larva
is stimulated, it turns the anterior end in a direction oppo-
site to that in which this end is when it receives the stimu-
lus. The tip of the anterior end is relatively very sensitive;
in diffuse light the larvae are stimulated only when this end
is extended and fully exposed, but in very intense light,
owing to the transluc-ency of the surrounding tissue, it is
stimulated no matter in what position the anterior end is;
consequently if this end is turned from the source of light
when the organism is exposed it is at once turned sharply
in the opposite direction, i.e., toward the light.

j. Discussion. — It has already been demonstrated that
neither the direction of the rays through the organism, in
accord with Sachs' theory, nor the angle between the rays
and the sensitive surface, in accord with Loeb's explana-
tion, is of importance in explaining the orienting reactions
of the fly larvae. Nor is the direction of the rays in the
field of importance except in so far as it may produce
difference of intensity on the body. How then are the
orienting stimulations produced ? Are they due to light

n

n

m

Fig. 31. The process of locomotion and orientation in blow-fly larvae, a-j,
diflferent positions taken during the process; m, n, direction of light rays. The
anterior end of the larvae is quite regularly turned from right to left during the
process of locomotion. If n is exposed and m shaded simultaneously when a larva
is at d, it turns sharply to e, then loops to /, turns and expands to g, where the
sensitive anterior end becomes fully exposed and consequently stimulated. This
causes the larva to turn sharply at once to h, where it becomes attached and loops
to i, expands and turns toj and is again stimulated, after which it repeats its former
response, etc., until it is oriented and the oral end is no longer subjected to marked
changes of intensity, as it swings back and forth in the process of locomotion. If
n is exposed when the larva is in position h, no reaction takes place until it expands
and turns to c, then it responds as described above. If the light from n is much
more intense than that from m, or if the larva is in a very sensitive state it responds
at once when n is exposed no matter in which position it is. If it is at a or 6 it throws
the anterior end sharply toward n, then in the opposite direction, after which it
orients as described above. It may however wave the anterior end back and
forth several times before it orients. 193

194 LIGHT AND THE BEHAVIOR OF ORGANISMS

acting constantly as a directive stimulus similar to stimula-
tion by a constant electric current, or to absolute difference
of intensity on symmetrically located points in the sensitive
surface, or to changes of intensity?

According to the idea of Loeb that light acts constantly
as a directive stimulation, the organism is continuously
stimulated by light on both sides. When one side is more
highly illuminated than the other, that side becomes stimu-
lated more than the other and causes a more rapid move-
ment of the locomotor organs connected with the sense
organs of that side. This of course causes the organism to
turn until both sides are equally stimulated. We have
demonstrated that in a light intensity of 3888 ca. m. the
rate of locomotion is only 0.024 mm. per second greater than
in an intensity of 7 ca. m. (Table II). If then the rate of
motion of the two sides of the organism under discussion is
due to the absolute intensity on the two sides in accord
with Loeb's theory, and if one side were exposed to an
intensity of 3888 ca. m., while the other is exposed to an
intensity of 7 ca. m., the former would move only 0.024 nim.
per second faster than the latter. It would therefore require
several seconds for a fly larva to become oriented even with
a difference of intensity on opposite sides amounting to
nearly 4000 ca. m. ; whereas it actually requires only a frac-
tion of a second for the larvae to orient under conditions in
which the greatest difference of intensity could not possibly
be more than 5-10 ca. m. The theory that orientation is
due to light acting constantly as a directive stimulation is
therefore inadequate to account for the orientation of fly
larvae. Moreover the fact that the larvae, when exposed
to moderate light intensity, respond only when the anterior
end comes to be fully exposed to the light in the process of
locomotion, shows clearly that the orienting stimulation is
not acting constantly.

The symmetry of the body with reference to the location
of the sensitive surface seems to be of no special importance
as far as orientation is concerned in this organism. I was

VERMES, FLY LARVAE, AND ECHINODERMS 195

unable to obtain any evidence of the power of differential
response to localized stimulation. The orienting reactions
could readily be explained by assuming the area sensitive
to light to be restricted to a small mass of substance located
in the middle of the very tip of the anterior end.

The idea that the stimulations leading to orientation are
due to changes of intensity (in fly larvae an increase only)
on the sensitive surface seems to fit the facts as far as known.
Stimulations thus produced cause an increase in the lateral
head movements somewhat similar to the avoiding reac-
tions and shock movements in the lower forms. Owing to
the difference in exposure of the anterior end, the move-
ments from the source of light are increased more than
those toward the light. This continues until the organism
is directed away from the source of light and the change of
intensity on the anterior end is no longer sufficient to cause
a response.

The process of orientation in the fly larva is strikingly
similar in principle to that in Euglena and Stentor. Sten-
tor, e.g., is most sensitive when the oral side is exposed; the
fly larvae when the anterior end is exposed. When Stentor is
not oriented the highly sensitive oral side is alternately fully
illuminated and shaded by means of rotation on the long
axis. In the fly larvae the alternate illuminating and shad-
ing of the sensitive anterior end is brought about by the
swinging of the head from side to side. If the intensity is
not high, Stentor never turns toward the light; it responds
only after the oral side is turned toward the light. This
response consists in a rapid swerving from the source of
light and eventually results in orientation. Likewise the
fly larva under similar conditions responds only after the
anterior end is exposed, and the response consists in sharp
turning from the source of light, which on repetition results
in orientation. Stentor makes no mistakes in the pro-
cess of orientation under these conditions. It never turns
toward the source of light, but still there are constant trial
movements during the process of orientation. The same

196 LIGBT AND THE BEHAVIOR OF ORGANISMS

is true with regard to the fly larvae. In Stentor every rota-
tion on the spiral course may be considered a trial move-
ment. If the organism is not oriented it swerves a little
farther from the source of light in each rotation after the
oral side is turned toward the light, until this side is equally
exposed throughout the entire rotation. Just so every lat-
eral movement of the fly larvae may be considered a trial
movement. If the organism is not oriented the anterior
end becomes much more fully exposed when it is turned
toward the light than when it faces in the opposite direc-
tion. This produces a stimulation and causes it to be
turned farther than usual in the opposite direction, but it
is swung back again, receives another stimulation, and is
turned still farther from the light. Thus the organism may
be considered to try different positions by swinging the
anterior end back and forth. This trial process does not
cease after the organism is oriented; the anterior end con-
tinues to swing from side to side. If it is subjected to but
little difference of light intensity as it swings from side to
side, there is no response and the organism continues as it
is directed, but if it is subjected to considerable difference
of intensity it responds and turns as described above.

The fly larva presents an excellent example of an organ-
ism guided fairly directly on its course by successive trial
movements, and shows again that the mere fact of accurate
orientation is not a satisfactory criterion of direct orienta-
tion. Of course it is not necessary to assume that this
organism consciously tries different positions in the process
of orientation.

In how far do the reactions of the fly larvae agree with
the explanation Holmes presented with reference to them ?
Is "orientation in these forms . . . essentially a selection of
favorable chance variations of action and following them
up" ? The answer to this question depends entirely upon
what is meant by chance variations. It is therefore evi-
dent that a statement with reference to it would add little
or nothing to our analysis.

VERMES, FLY LARVAE, AND ECHINODERMS 197

Summary

(i) Fly larvae are negative in their light reactions in all
intensities to which they respond. They become acclima-
tized very readily so that after they have been exposed in a
given intensity for about half an hour they fail to respond
unless the intensity is increased.

(2) The tip of the anterior end is the only sensitive region
on the larvae. On this tip there are two cone-shaped struc-
tures which probably are light recipient organs.

(3) In locomotion the larvae turn the anterior end slightly
from side to side with considerable regularity; but if sud-
denly exposed to high intensity they throw the anterior end
from side to side violently.

(4) Absolute difference in light intensity has but little
effect on the rate of movement. In 7 ca. m. it was found
to be 0.321 cm. per second; in 3888 ca. m., 0.345 cm. per
second.

(5) Unilateral illumination of the posterior third or three-
fourths of the body has practically no effect on the rate of
locomotion.

(6) The process of orientation in the fly larva is similar
in principle to that in Euglena and Stentor. It is brought
about by reactions which are similar to the avoiding reac-
tions or shock movements of the lower organisms. These
reactions are due to changes of light intensity on the sensi-
tive anterior end; and the changes of intensity are due
largely to the lateral movements of this end.

(7) Orientation is the result of trial movements, but it is
doubtful whether it could be considered as the result of selec-
tion of random movements as defined by Holmes (1905).

(8) In light from two sources they may take a path
extending from any point betw^een them. The location of
this point depends upon the relation in intensity of light
from the two sources.

(9) There is no evidence indicating differential response
to localized stimulation. If the fly larva has the power of

igS LIGHT AND THE BEHAVIOR OF ORGANISMS

such response at all, it is but little developed and is of very
little importance in the general reactions to light.

(lo) Neither the direction of the rays through the organ-
ism, nor the angle with the surface, nor the symmetry of the
sensitive surface, nor absolute difference of intensity on the
body, is of importance in orientation excepting in so far as
they may influence change of intensity on the anterior end.

(ii) There is no evidence indicating that the orienting
reactions in fly larvae are tropic in accord with Loeb's
definition of this term.

3. Earthworms

The light reactions of various earthworms have been
studied by a number of investigators, several of whom
directed special attention to the process of orientation.
Parker and Arkin (1901), Miss Smith (1902), and Adams
(1903) made observations on the direction of movement of
the anterior end when illuminated from one side, and found
that it turned from the light more often than toward it,
indicating, since these organisms are ordinarily negative,
that orientation is direct. Holmes, however (1905), is of
the opinion that the animals actually start to turn toward
the light just as often as from it, but that the movements
toward the light are inhibited owing to the greater exposure
as the end expands. This causes marked movements only
in the direction from the source of light. He believes that
Parker and Arkin and others may have failed to take into
consideration the slight preliminary movement which occurs
before the actual extension takes place, and that this may
account for the preponderance of negative turning recorded
by these investigators. Holmes, taking account of all the
minute preliminary movements, says (p. lOi): "In the two
specimens employed the first detectable turn was away from
the light 27 times and towards the light 23 times. After a
few extensions the worm in nearly all cases soon turned
and crawled away from the light. The first detectable

VERMES, FLY LARVAE, AND ECHINODERMS 199

movement of the earthworm seems, therefore, to be nearly
as Hkely to be towards the Hght as away from it. The
shght preponderance of negative turns may be due to the
fact that some of the smaller trial movements were over-
looked, to a slight direct orienting effect of the rays, or
merely to chance."

Harper (1905), working on Perichaeta bermudensis and a
species of Lumbricus in various light intensities, concluded
that in light of comparatively low intensity orientation is
indirect and that there are numerous random movements,
but in direct sunlight, especially if the worms have previ-
ously been kept in darkness, orientation is direct and ran-
dom movements are almost entirely eliminated.

I undertook the study of the reactions of Allolobophora
foetida with the express purpose of ascertaining the effect
of constant light intensity on the rate of movement with
different portions of the animal highly illuminated, thinking
that it might be possible thus to demonstrate the difference
between the action of light as an orienting stimulus, and
a stimulus affecting the general activity of the organism.
The rate of movement in this form however is so irregular
that I found it impossible to obtain results worthy of con-
sideration. I therefore turned my attention to direct obser-
vation of the process of orientation.

In locomotion the earthworm usually swings its anterior
end from side to side, but not nearly so regularly as do blow-
fly larvae. If after a specimen is oriented in a beam of
light, the ray direction is suddenly changed so as to illumi-
nate the side, one of four different kinds of movements may
result: (i) a contraction of the anterior end; (2) an exten-
sion of the anterior end ; (3) sudden raising of the anterior
end frequently accompanied by swinging from side to side,
or (4) direct turning either toward or from the source of
light in the plane of the substratum. If the animal is
active, the lateral movements of the anterior end are more
pronounced and regular during its locomotion than if it is
sluggish. When exposed to unilateral illumination in such

200 LIGHT AND THE BEHAVIOR OF ORGANISMS

a condition the anterior end is simply turned sharply in the
direction opposite to that in which it is when it receives
the stimulus, just as in the case of blowfly larvae. Thus
it is turned toward the source of light about as often as
from it, regardless of the light intensity. I found this to be
true in direct sunlight, contrary to Harper's conclusion, as
well as in light of lower intensities.

If the animal however is rather sluggish so that there is
little lateral movement of the anterior end it turns from the
source of light with very few exceptions. The direction in
which the anterior end started to move after exposure to
lateral illumination in six such specimens is recorded in
Table VII. These specimens were allowed to orient in light
of 15 ca. m. intensity, after which they were suddenly ex-
posed to a horizontal beam of light, ordinarily of higher
intensity, from one side. There was but little lateral move-
ment of the anterior end after the specimens were oriented
in the lower intensity, and they moved so slowly that the
direction in which the anterior end started to turn after
one side was illuminated could be clearly seen.

TABLE VII

Number of times anterior end

Intensity of

turned

Condition of
specimens used

Lateral Illumi-
nation

From source

Toward source

Time

of light

of light

200 ca. m.

21

4

Fresh specimen
taken from
darkness

?

200 ca. m.

22

3

Same specimen

?

50 ca. m.

23

2

Fresh

12 .00

200 ca. m.

25

0

Same

12.15

12 ca. m.

24

I

(( <(

12-55

200 ca. m.

25

0

(( <<

3-5°

In a few other sluggish specimens the exposure to unilat-
eral illumination was not made until after they had come
to rest in the light of 15 ca. m. Under such conditions the
animals did not react at all until a few moments after

VERMES, FLY LARVAE, AND ECHINODERMS 201

the exposure, then they very slowly extended and turned
the anterior end from the source of light every time. The
movements were so slow that they could be readily fol-
lowed in detail under a hand lens. There was no evidence
of even the slightest preliminary turning toward the source
of light. It must therefore be concluded that these animals
have the power of differential response to localized stimula-
tion by light. This conclusion is in harmony with the
results of Parker and Arkin, Miss Smith, Adams, and
Harper.

Parker and Arkin also found that if only the middle or
the posterior third of the body is exposed there are more
negative head movements than positive. I was unable to
confirm these results. Specimens were repeatedly allowed
to orient in a horizontal beam of light of 15 ca. m. intensity
on smooth moist black paper supported by a glass plate as
described above; and after they had started to crawl away
from this source of light a portion of the body was exposed
in an intensity of 200 ca. m. to lateral illumination from a
Nernst glower. The glower was mounted vertically so that
it cast a well-defined sharp shadow. By means of an opaque
screen containing a rectangular opening a beam of light
could be thrown upon any portion of the body and held
there by moving the screen in harmony with the movement
of the animal. The orientation of specimens was thus fre-
quently studied and the direction of movement carefully
noted while theycrawled across the field, a distanceof 20 cm.,
alternately with and without some portion of the body
exposed to unilateral illumination. The exposure of any
portion back of the sixth segment had no appreciable effect
on the direction of motion. If however the screen was at
any time brought forward so that the relatively intense
lateral rays fell on the anterior end there was always an
immediate response if the specimens were active. If the
anterior end chanced to be directed from the source of light
in its swinging movements when the exposure was made, it
was at once thrown sharply toward the light; if it chanced

202 LIGHT AND THE BEHAVIOR OF ORGANISMS

to be directed toward the source of light, it was thrown
equally strongly in the opposite direction. These results,
together with some statistical tabulations of direction of
head movements with different portions of the body back
of the sixth segment exposed to unilateral illumination, indi-
cate that, while these portions are no doubt sensitive to
light, only the anterior end is immediately functional in
regulating orientation. Jennings (1906a, p. 442) arrived
at practically the same conclusion with reference to other
stimuli.

Parker and Arkin in their experiments used a Welsbach
gas burner as a source of light. Owing to the width of the
luminous part of the burner it is evident that a shadow of an
object in light from such a source w411 not have sharp edges;
and likewise a beam produced by means of a screen con-
taining an opening will not have well-defined edges. In
such a beam there is a region of uniform highest inten-
sity in the middle and a region of graded intensity on
either side. By calculations based on the data furnished
by Parker and Arkin it was found that in the beam of light
at the place where they exposed the earthworms the region
of uniform highest intensity was 10 mm. wide and the
region of graded intensity on either side was 16 mm. wide.
Outside of this on either side there was another region 15
mm. wide faintly illuminated by light reflected from the
water screen. It is evident that when the middle or pos-
terior end of a specimen of Allolobophora foetida, usually
only about 4 cm. long, is exposed in the region of greatest
intensity in such a field, the anterior end will be exposed to
the weaker light in the adjoining region. It may be then
that the preponderance of negative head movements found
by Parker and Arkin with the posterior portion of Allolo-
bophora exposed to relatively strong lateral illumination,
was due to the effect of the weak illumination on the an-
terior end. Since the light in the beam becomes gradually
weaker as one proceeds outward, it is clear that the anterior
end of the worm will be in higher light intensity when the

VERMES, FLY LARVAE, AND ECHINODERMS 203

middle is in the region of strongest illumination, than when
the posterior end is there. One would therefore expect a
greater proportion of negative head movements under the
former conditions than under the latter, which is just what
Parker and Arkin found.

The tabulated conclusions of these authors are formu-
lated with the supposition that all positive head movements
in Allolobophora exposed to lateral illumination are " due
to other stimuli than light " (I.e., p. 153). My observa-
tions do not confirm this conclusion. As already stated, it
was found that if the anterior end is bent toward either
side when the exposure is made, it simply turns toward the
opposite side regardless of the direction of the rays. Orien-
tation in these forms is by no means entirely due to differen-
tial response to localized stimulation. Selection of random
movements or trial movements, as Holmes, Harper and
Jennings pointed out, undoubtedly plays a very large part
in the process of orientation in the earthworm under ordi-
nary conditions.

The swinging movements of the anterior end are in the na-
ture of trial movements. They may be induced by external
conditions, but their character and direction are determined
by the structure of the organism and various physiologi-
cal processes. They make it possible for the organism
to orient much more accurately than it otherwise could.
When the anterior end is directed straight ahead and the
organisms are oriented, this end is more or less shaded and
not in a position to be readily stimulated by changes in the
direction of the greatest illumination. If it were immov-
ably fixed to the rest of the body in this position the entire
organism might turn toward either side considerably with-
out receiving an orienting stimulation. In place of turning
the entire body it raises the anterior end so as to magnify
the difference of intensity on opposite surfaces, extends it
so that it becomes more sensitive, and swings it from side
to side so that the different surfaces become alternately
shaded and illuminated, thus producing changes of inten-

204 LIGHT AND THE BEHAVIOR OF ORGANISMS

sity. If the change of intensity is greater when the oral
end is turned toward the right than when it is turned
toward the left, it is stimulated and bends farther toward
the left. The direction of bending is generally independent
of the direction of the rays, but the extent of bending
usually is not.

Holmes and Harper both pointed out that swinging
movements toward the source of light are checked because
the animal becomes more and more sensitive as the anterior
end extends toward it. This end is however not only
checked under such conditions, it is also stimulated and
swings farther in the opposite direction. This is an impor-
tant factor in the process of orientation, that can hardly be
said to be included in the explanation of orientation by
selection of random movements, as described by Holmes.

It may now be asked: Is the orienting stimulus in this
form due to a change of light intensity or to the effect of
light acting constantly as a directive stimulus ?

There is no doubt that a change of intensity causes defi-
nite reactions in the earthworm, and that reactions thus
produced may result in orientation either by the selection
of random or trial movements, or by inducing more defi-
nitely prescribed movements, as in the blowfly larvae. But
this does not indicate that constant light cannot also pro-
duce orienting stimulations. There is however no evidence
showing that it does. Even in case a worm lies perfectly
quiet and very gradually starts to turn from the light w^hen
laterally illuminated, as can be readily demonstrated, it is
impossible to say whether the stimulus causing the reaction
is due to the effect of constant intensity or to change of
intensity. And when the worm is in motion, ever extending
and contracting the anterior end and changing its position
so that the effective intensity is continually changing, it is
of course impossible to predict what would take place if
the sensitive elements could be exposed to light having a
constant intensity. If light is thrown upon a specimen
which is perfectly quiet it begins to move, but in this case

VERMES, FLY LARVAE, AND ECHINODERMS 205

also it Is Impossible to say whether the activity Is caused
by a change of Intensity or by constant Intensity.

The Idea of Verworn, Holt and Lee, Loeb, and Torrey
that when an organism Is oriented both sides are equally
stimulated and consequently move at equal rates, and that
when it is not oriented the two sides are unequally stimu-
lated and therefore move at unequal rates, thus causing
orientation, has no experimental support in the reactions
of the earthworms.

Summary

(i) All the earthworms that react to light are ordinarily
negative. There is, however, some evidence that some at
least are positive in very low light Intensity.

(2) They orient fairly accurately under some conditions
and move away from the source of Illumination.

(3) The entire surface of the earthworms is probably
sensitive to light, but the anterior end Is much more sensi-
tive to light than any other part of the body. Our evidence
Indicates that in AUolobophora orientation is entirely con-
trolled by the sensory elements In the first five or six seg-
ments. The anterior end is most sensitive when extended,
as shown by Harper.

(4) These animals have the power of differential response
to localized stimulation. Under certain conditions, if one
side Is Illuminated, they always turn toward the shaded
side without preliminary movements and therefore orient
directly.

(5) They frequently swing the anterior end frorn side
to side continuously during the process of locomotion. If
light is thrown on one side under such conditions they turn
the oral end sharply in the direction opposite that In which
It chances to be when it receives the stimulation. They
may therefore turn toward the source of light first and
become oriented only after several more preliminary move-
ments. Or they may be In such a state that they are only
stimulated when the anterior end Is extended toward the

2o6 LIGHT AND THE BEHAVIOR OF ORGANISMS

source of light, not when it is turned in the opposite direc-
tion. Under such conditions they will of course never turn
* sharply toward the source of light. The swinging of the
anterior end may however be considered as a trial move-
ment and orientation consequently indirect.

(6) The swinging movements of the anterior end increase
the possible accuracy of orientation in case of direct as well
as indirect orientation. By means of them the animal takes
its bearing, if this anthropomorphic term may be permitted.

(7) The stimulations which lead to orientation are ordi-
narily unquestionably due to change of intensity on the
sensitive surface.

(8) Light may possibly have some effect on orientation
by acting constantly as a directive stimulus, but there is
no evidence indicating that it has. There is no evidence
showing that these animals are tropic in accord with Loeb's
definition.

4. Planaria

It is well known that nearly all the planarians respond
to stimulation by light, but it is frequently assumed that
they orient only very indefinitely and that the effect of
light consists largely in making them more or less active.
Loeb (1906, p. 136) says, " If fresh-water Planarians are
put into such a circular glass dish, they show very little or
no tendency to move in the direction of the rays of light,
creeping along in an irregular manner and gathering not at
the negative or positive side of the jar, but on both sides
. . . where, on account of the refraction of light, the
intensity is a relative minimum."

More recent investigations have however shown that
many of these forms orient fairly accurately, and it is pri-
marily these investigations that concern us at present.

In the planarians the power of differential response to
localized stimulation seems to be more highly developed
than in the earthworms, and orientation in light seems to be
brought about largely by such responses. There are how-

VERMES, FLY LARVAE, AND ECHINODERIIS 207

ever also numerous head movements which bear no definite
relation to the location of the stimulus.

Cole, studying the reactions of Bipalium kewense to
sources of light of different size, refers to the process of
orientation as follows (1907, p. 365): "Like most plana-
rians, it creeps with an even, gliding motion, the head being
slightly raised and waved to right and left apparently in
searching movements, as the worm crawls forward. . . .
Bipalium kewense is exceedingly sensitive to light, of even
a very low intensity, falling upon It from the side, and
responds Immediately by turning away from the light."

Walter (1907) made an extensive study of the light reac-
tions of the following species: Planaria maculata; Planaria
gonocephala; Phagocata gracilis; Dendrocoelum lacteum;
and Bdelloura Candida. He found that all of these species
orient more or less accurately under certain conditions and
concluded that orientation '' Is primarily due to asymmetri-
cal response resulting from asymmetrical stimulation." He
also found that these animals frequently respond by raising
the anterior end and throwing It from side to side, and
noticed that such movements are caused either by sudden
increase or by sudden decrease of intensity. These move-
ments, however, he thinks are functional in orientation only
" by assisting an organism to secure asymmetrical stimula-
tion " (1. c, p. 153).

I made some observations on the orienting reactions of
Leptoplana tremellarls and several other polyclads, all of
which were positive in their light reactions, and found that
all of these forms orient directly. There Is no evidence of
preliminary trial movements in the process. The lateral
head movements are always slight and frequently appar-
ently absent. When exposed to light from two sources all
of these forms crawl toward a point between the lights.
The location of the point toward which they move, however,
depends upon the relative Intensity of light from the two
sources. It is always nearer the source from which the
more intense light comes. This indicates that orientation

2o8 LIGHT AND THE BEHAVIOR OF ORGANISMS

is regulated by the relative intensity of light on opposite
sides.

It may now be asked: What is the cause of the orienting
stimulus ? Is it a change of intensity or constant intensity ?
There is direct evidence showing that the organism responds
both to changes of Intensity and to constant intensity. If
a planarian passes suddenly from a region of a given inten-
sity into a region of a higher or lower intensity, it responds
by suddenly turning the head from side to side; and when
subjected to constant light of different intensities it may
become more or less active. In working with fresh-water
planarlans I frequently observed that if they were exposed
to constant illumination after they had been at rest for
some time, they did not respond for several minutes; when
they did respond they first moved very slowly and very
gradually became more active. Walter (1907, p. 63) records
similar observations. It may of course be argued that even
in this case it is the change of intensity due to turning on
the light that arouses the animals. It Is however hardly
probable that It Is, since the response Is frequently not
apparent until after the animals have been exposed for
several minutes. It appears that a difference In constant
light Intensity not only causes the planaria to become active
or to come to rest, but that it also affects the rate of move-
ment after they are active.

In experiments on Planaria gonocephala in constant illu-
mination of different intensities from directly above, Walter
(1907, p. 57) found an average rateof locomotion of 0.57 mm.
per second in darkness, 0.63 mm. per second in 431 ca. m.
The highest rate, 0.75 mm. per second, was in 39 ca. m.
It v/ill thus be seen that the greatest increase due to a
difference In constant Intensity Is 0.18 mm. per second.

It is therefore clear that a change of Intensity causes a
comparatively rapid response, and difference in absolute or
constant Intensity a comparatively slowresponse. This leads
to a conclusion arrived at previously several times in these
chapters, that while an organism may be stimulated both

VERMES, FLY LARVAE, AND ECHINODERMS 209

by a sudden change of light Intensity and by constant inten-
sity, the processes involved are different. A sudden change
of intensity acts much like mechanical contact or a change
in an electric current. Constant intensity on the other
hand acts like constant temperature.

Our original questions still remain: Is orientation due to
light acting through change of intensity? Or is it due to
constant intensity, both sides of the organism being stimu-
lated constantly, but unequally when one side is more
highly illuminated than the other, thus causing difference
in rate of movement of the two sides ? The maximum dif-
ference in rate due to absolute difference of intensity, as we
have seen, is only 0.18 mm. per second. It is evident then
that the greatest difference in rate of locomotion on the
two sides due to absolute difference of intensity could not
be more than 0.18 mm. per second; and if orientation is
due to this it is clear that the orienting process would be
exceedingly slow, very much more so than it actually is.

Our evidence then indicates that orientation in these
forms is due not to light acting constantly as a directive
stimulus similar to the action of a constant electric current
in accord with Loeb's theory, but to reactions caused by
intermittent action of light through changes of intensity
on some part of the sensitive surface. These changes may
of course be due to the movements of the organism or to
changes in the direction of illumination.

It is evident that if the anterior end turns from side to
side, or if the ray direction is changed, the intensity on one
side becomes higher while that on the other becomes lower.
Is the orientation due to the former or to the latter? In
Euglena it was demonstrated that if the specimens are posi-
tive the orienting stimulus is due to a decrease of effective
intensity; if negative, to an increase of effective intensity.
In Planaria there appear to have been no observations bear-
ing directly on this point. I frequently observed, however,
that when positive polyclads crawling toward a source of
light come into contact with a sharp shadow at either edge

210 LIGHT AND THE BEHAVIOR OF ORGANISMS

of the beam in which they are exposed, so as to shade one
side, they turn directly from the shaded side, indicating
that the orienting stimulus in positive Planaria, as in Eu-
glena, is due to a decrease in effective intensity on some part
of the sensitive surface and that orientation is brought
about directly by differential response to localized stimuli.

Summary

(i) Planaria may collect in regions of optimum light
intensity either by wandering into such regions and coming
to rest or by orienting and crawling directly toward such
reg ons and coming to rest. The latter method is of course
more effective than the former.

(2) In some forms orientation is very indefinite; in others
it is fairly accurate.

(3) In locomotion there are frequent lateral head move-
ments. These may be accelerated either by sudden increase
or by sudden decrease in light intensity. They appear to
be independent of the location of the stimulus.

(4) Orientation at least in some forms is due largely to
differential response to localized stimulation. The lateral
head movements no doubt function by increasing localized
stimulations, and thus make it possible for the organism to
direct its cour-e more efficiently than it otherwise could.

(5) Light acts on Planaria by virtue of both changes of
intensity and constant intensity. Responses to changes
of intensity are comparatively rapid ; responses to constant
intensity comparatively slow. The effect of constant light
is similar to the effect of constant temperature.

(6) The orienting stimulus appears to be due to changes
of effective intensity on some part of the sensitive surface.
In positive specimens it is, as in Euglena, probably due to a
decrease, in negative specimens to an increase of intensity
on one side.

(7) There is no evidence indicating that light owing to
constant intensity is functional in the process of orientation.

VERMES, FLY LARVAE, AND ECHINODERMS 211

5. Echinoderms

The Echinoderms are peculiar in that they can move
with any side ahead. In reversing the direction of move-
ment they ordinarily do not turn around but merely move
with the opposite side ahead. Some appear to be perma-
nently positive and others permanently negative, while still
others may be either positive or negative, depending upon
circumstances.

Washburn (1908, p. 131) says that the starfish and sea
urchins depend for their response to light upon pigment or
eye-spots on the arms. This conclusion is based largely
upon the observations of Romanes, who obtained no reac-
tions after the tips of the arms bearing these structures
were amputated. In some species however the response
appears to be independent of the eye-spots, for Cowles
found that in Echinaster crassispina the reactions to Hght
were normal three hours after one centimeter had been cut
from the tip of each arm.

Jennings (1907), working on Asterias forreri, a negative
starfish, found that it moves toward the shaded side no
matter whether the side is shaded by the substance in the
starfish itself or by some other object, showing that the
direction of motion is regulated by difference of intensity
on the surface regardless of the direction of the rays. If
illuminated from one side it therefore moves from the source
of light because the side opposite the light is shaded. If
the position of the source of light is changed it alters its
direction of motion at once, ordinarily by simply proceeding
with the side ahead which has become shaded. Bohn (1908),
however, working on several different species, found that
there, is some tendency to turn after the direction of the
light is changed so that a given ray will be ahead.

The lack of orientation in moving from a source of light
is much more striking in the holothurians, which are superfi-
cially at least much more definitely bilaterally symmetrical.
Pearse (1908, p. 278) describes the process in the holothu-

212 LIGHT AND THE BEHAVIOR OF ORGANISMS

rian Thyone briareus as follows: " In a series of twenty-four
reactions the locomotion in every case carried the ani-
mal away from the light to the end of the dish, but there
was no definite orientation of the body in relation to the
light. In ten of these negative responses the anterior end
was ahead as the individual moved; in nine instances the
posterior end preceded the anterior; and in five the loco-
motion was straight toward the right or left. Not one of
the eight individuals [used in this experiment] moved in
every case with the anterior or posterior end in front."

Very little is known about the actual mechanism involved
in these reactions. It seems clear however that it is the
shading of part of the body that produces the stimuli which
regulate the direction of motion, for if a shadow is thrown
on one side of a specimen of Asterias forreri, e.g., in such a
way as not to produce any change of intensity on the
exposed side, it moves toward the shaded side; and the
continued movement toward the shaded side seems to be
due to a constant difference in absolute light intensity on
opposite sides. If this is true the direction of movement is
regulated by light acting constantly as a directive stimulus.
It must however be borne in mind that there is no orienta-
tion in these organisms. They move with any side ahead
much like an amoeba. In Amoeba, it will be remembered
that the formation of pseudopods on the illuminated side
is probably checked by the action of the light, and that this
results in movement toward the shaded side of the organism.
It may be that in the echinoderms light has a similar effect
on the extension of the tube feet. If it has, direction of
movement is of course regulated by changes of intensity on
these structures. This idea is supported by the fact dis-
covered by von Uexkiill (1897) that a single spine or pedi-
cellaria connected with a piece of shell responds to stimuli
practically as it does in the entire animal, showing that the
parts of this organism are capable of independent action,
and the same is probably true of other organs in this form
and also in the other echinoderms.

VERMES, FLY LARVAE, AND ECHINODERMS 213

After I had completed this part of the work, Cowles
reported to me personally that he found that positive star-
fish, when placed on the dorsal surface, always extend the
tube feet toward the shaded side of the body and turn from
the source of light, whereas in the normal position they
extend the tube feet toward the light. This seems to show
clearly that the direction of locomotion is not regulated by
the direct effect of light on these structures, as suggested
above.

CHAPTER X

CONCERNING THE QUESTION OF ORIENTATION IN MOL-

LUSKS, ARTHROPODS AND VERTEBRATES WITH

SPECIAL REFERENCE TO CIRCUS MOVEMENTS

AND THEIR BEARING ON THIS QUESTION

I. General Account of Orientation

We have found in our work on the lower metazoa that
as the organisms become more complex and the structures
more highly differentiated, the power of differential response
to localized stimulation by light becomes more highly
developed and plays an increasingly more important part
in orientation; and trial and random movements become
of relatively less importance. In the mollusks, arthropods
and vertebrates, there is little evidence of preliminary trial
movements in the process of orientation in light. If the
ray direction is changed these forms turn directly until
their direction of motion bears the same relation to the
source of light it previously had. I found this method of
orientation to hold in the following forms: Limnea columella,
Cyprls sp. (?), Daphnia sp.(?), Scapholeberisarmata, zoeae,
several species belonging to the Anomura and Brachyura,
Caprella sp. (?) and Bufo americanus. The work of Cole,
Bohn, Parker, Holmes, Yerkes, Harper, Hadley, Torelle
Radl and others shows the same to be true for numerous
other species belonging to these groups.

The fact that these forms orient in light without prelimi-
nary movements does not however indicate the absence of
trial movements when subjected to other stimuli and is no
argument against the " trial and error " hypothesis in gene-
ral as Bohn (1908, pp. 77, 82) seems to Imply; nor does it
show how light acts as a stimulus. It probably means that
with reference to stimulation by light the power of differen-

214

MOLLUSKS, ARTHROPODS AND VERTEBRATES 215

tial response to localized stimulation has become so highly
developed that trial movements are largely eliminated.
Jennings (1906a, p. 453) makes the following characteris-
tically clear statement regarding this question: " It is, of
course, very true, as Harper ('05) remarks, that definitely
localized reaction methjods are developed as we rise higher
in the scale, yet it appears to be equally true that if we
mean by ' trial and error ' the performance of varied move-
ments, subjecting the organism to varied conditions, certain
of which are selected, then this also becomes more highly
developed and more used by organisms as we ascend the
scale. We must not forget that this expression ' trial and
error ' was originally based on the behavior of such highly
developed organisms as the cat, dog and monkey ; and doubt-
less there is no organism which uses this method to any such
extent as does man. Whenever the external conditions do
not furnish a precise determining factor for the movements
yet some sort of reaction is required, any organism is forced
to have recourse to this style of behavior, performing varied
movements till a condition is reached that relieves the organ-
ism of the necessity of continuing these movements. In its
highest form we call this experimentation."

2. Circus Movements

In Euglena, Stentor and some of the other lower forms
it was demonstrated that the orienting stimulus is due to
a change of light intensity. Is there any evidence as to how
the local stimulus which leads to orientation in the higher
forms is produced ?

It has been found by a number of investigators working
on different forms that if one of two symmetrically located
sense organs is in any way prevented from functioning, the
organism no longer orients but continually turns toward
one side when stimulated. Loeb and others found this to
be the case in several different animals with one-half of
the brain destroyed. Holmes (1901, p. 220) found that the

2l6 LIGHT AND THE BEHAVIOR OF ORGANISMS

positive terrestrial amphipods and several different flies
with one eye blackened turn continuously toward the func-
tional eye. Parker (1903, p. 463) working on Vanessa
antiopa, and Radl (1903, p. 61) on the flies Dexia carini-
frons and Musca domestica, obtained similar results. The
fact that these organisms, all of which are positive, thus turn
continuously toward the functional eye seems to show that
the orienting stimulus is not necessarily and exclusively due
to a decrease of intensity in these forms, as it is in positive
Euglena and many other organisms. It may be due to the
continued action of light on the eye. A change of light
intensity does however undoubtedly produce a stimulus
which may result in orientation.

The performance of circus movements has frequently
been brought forward in support of Loeb's theory of orien-
tation stated in the following quotation (1906, p. 139):
" It seems that in animals the region at the oral pole is, as
a rule, more sensitive than the rest of the body. Conse-
quently the tension of the muscles determining the position
of the head or oral pole is more intensely affected by dif-
ferences in the intensity of light than that of the muscles
of the rest of the body. The head is consequently bent
until its symmetrical photosensitive points are again struck
at the same angle by the rays of light. The tension of the
symmetrical muscles of the head then again becomes equal,
and the head must remain in this position unless other forces
disturb its orientation. The rest of the body follows the
orientation of the head."

The precision with which some organisms with but one
functional eye perform circus movements does indeed
appear to add support to this explanation of orientation.
Recent investigations have however thrown considerable
doubt on the earlier interpretation of these movements.
It has been found that they are not so regular and constant
as was formerly supposed. Carpenter (1908, p. 486), experi-
menting on Drosophila with one eye blackened, observed
that they " crept in a fairly direct path toward the light,

MOLLUSKS, ARTHROPODS AND VERTEBRATES 217

although a tendency to deviate toward the side of the nor-
mal eye regularly occurred." Radl (1903, p. 62) says,
" Die Calliphora vomitoria bewegt sich fast ebenso gerade
mit einem geschwarzten Auge, wie wenn sie auf beiden
sieht, und es ist mir nicht leicht, diese Erscheinung zu
erklaren." Holmes (1905) discovered that Ranatra with
one eye blackened at first deviates strongly toward the
functional eye in going toward a source of light, but that
this deviation decreases and that the path becomes much
more nearly direct after repeated trials, indicating that the
animal learns to adjust itself to the new conditions and that
its reaction mechanism is not so simple as Loeb's theory
demands. This is still more clearly demonstrated by the
interesting observations of Holmes on the fiddler crab, Uca
pugnax. I cannot do better than to quote his conclusions
based on these observations (1908, p. 496): " The point of
principal interest in the phototaxis of the fiddler crabs is the
relation of their lateral orientation to the theories of tro-
pisms. Can we regard orientation as a direct response in
which the animal is involuntarily forced into line, or is it
rather to be considered as coming under the pleasure-pain
type of behavior, and as therefore related to the voluntary
seeking of a certain end which is exhibited in the behavior
of higher forms ? In order to explain the orientation of a
highly organized form like an insect or crustacean in which,
in most cases, response to light takes place through the
eyes, we may assume that light falling more strongly on one
eye sets up impulses which are transmitted more or less
directly to the leg musculature. We may assume that the
extensors of the opposite side are stimulated, or the flexors
on the same side, or both, and that in consequence of this
distribution of impulses the animal moves until its body
is in line with the rays. In such a case the movements
involved in orientation are the same as those employed in
ordinary locomotion, only the activity of the legs on one or
the other side is accentuated according to the position of
the body in relation to the direction of the rays.

2l8 LIGHT AND THE BEHAVIOR OF ORGANISMS

'* In the fiddler crab, however, the case Is different, and
we cannot explain the phenomenon In this way. The legs
of the fiddler move In a plane approximately at right angles
to the sagittal plane of the body, but they are capable of a
certain amount of forward and backward motion which
may be employed to change the direction of locomotion.
The movements Involved In orientation are different from
those employed In ordinary running. They are special
movements employed to check deviations from a certain
course, a circumstance w^hlch would greatly complicate any
attempt to explain orientation as a comparatively direct
response. The results of observations on fiddler crabs tend
to confirm the conclusion reached In studies made on the
phototaxis of Ranatra, namely, that light Is followed much
as an animal pursues any other object of Interest, such as
prey, or Its mate, and until we can give a physiological
explanation of these phenomena we are not, I believe. In a
position to give a satisfactory explanation of orientation to
the direction of the rays of light."

A similar Idea regarding reactions to light was expressed
by Graber much earlier. He says (1884, p. 248), *' Um ein
analoges Beispiel aus einer andern Sinnessphare anzufiihren,
so benimmt sich hier [exposed to light differing In color or
intensity] die Raupe offenbar ganz ahnllch wie In dem Fall,
wenn ihr als Futter einerselts Nesselkraut und andererselts
Irgend eine andere Pflanze vorgesetzt wird, Indem sle con-
stant das letztere verschmaht und das erstere ergrelft, und
in dem Sinne konnen wir also auch ganz gut von einem
Farbengeschmacke reden."

3. Frogs and Toads

The conclusions of Holmes as far as they refer to orienta-
tion In animals with image-forming eyes are strongly sup-
ported by the observations of Miss Torelle on the response
of the frog to light and by the orienting reactions of the
American toad Bufo americanus described below. Miss

MOLLUSKS, ARTHROPODS AND VERTEBRATES 219

Torelle says (1903, p. 471): " Tests were made at midday
on a level tract of ground about two acres in extent which
contained neither trees nor any object that could cast a
shadow. Six frogs were tried. When freed, each moved
indifferently toward any point of the compass, but usually
kept on moving in the direction in which it began to move.
In several trials no movement resulted; the frog crouched
low between short bunches of grass, its head held close to
the ground. When dark black or dark brown screens were
placed in the middle of this area and the frogs placed
within five yards of them, the movement was toward and
into the shadow of the screen, where they usually remained
indefinitely." In various other experiments it was found
that frogs exposed in direct sunlight hopped toward shadows
in the neighborhood, no matter if this required movement
perpendicular to the rays of light. When first exposed the
frogs turned toward the light, but after being in this position
a few moments they turned and hopped toward the shadows.
After they are in the shade they usually turn so as to face
the light. These reactions seem to show that the frogs go
toward the shadow because they see and perceive it. The
following reactions of toads lead to the same conclusion.

A. Bufo americanus

a. Method. — Two horizontal Nernst glowers were so
arranged in a large dark room that the rays crossed at right
angles above a black table one meter square. The two
beams of light, 20 cm. wide at the place of intersection,
were parallel with the plane of the table, and the lower edge
of the beams was just high enough to clear the table, which
was therefore not illuminated. Both beams were absorbed
by the dead black walls of the room, which were several
meters from the table. The light intensity in the middle of
the field from one glower was 12.5 ca. m., and that from
the other was 25 ca. m.

h. Orientation in light from two sources. — On July 7,
at 7 P.M., seven toads, two large ones and five small ones.

2 20 LIGHT AND THE BEHAVIOR OF ORGANISMS

were collected and brought to the laboratory. These speci-
mens were exposed one at a time in light from a single
source; they all oriented directly and fairly accurately. If
placed on the table in the beam of light so that one side
faced the glower they turned slowly but directly until they
faced the light and then hopped or walked toward its source,
stopping frequently for a few moments at intervals on the
way. The light from both glowers was now turned on and
the specimens were exposed, one at a time, in the beam
having the lower intensity, in such a position that after
they oriented and moved toward the source of light they
soon reached the more intense lateral light from the second
glower, which of course illuminated one side. I was some-
what surprised to find that ordinarily there was no apparent
reaction whatever when the specimens reached the beam
of lateral light, although the intensity of this was twice as
high as that in which they were oriented. The toads con-
tinued on their way just as though there' had been no
lateral illumination. All the other organisms studied in
light from two sources proceeded toward or from a point
between the sources of light. The toads always went
directly toward one or the other of the two sources.

Each of the seven specimens under observation was
studied while it crossed the field six times, and all but one
continued toward the glower which produced the beam in
which they were first oriented, without any perceptible
deviation on account of the lateral light from the second
glower. Only one specimen turned when it reached the
lateral illumination. It did not however go toward a point
between the two glowers as might have been expected, judg-
ing from results of previous experiments on other forms.
It proceeded directly toward the glower which produced
the lateral illumination. In no instance was there any evi-
dence of movement toward a point between the two
sources.

These results do not support Loeb's recent statement
regarding orientation in light from two sources. He says

MOLLUSKS, ARTHROPODS AND VERTEBRATES 2 21

(1909, p. 13), " Sind zwei gleich starke Lichtquellen in
gleichem Abstand vom Tier vorhanden, so bewegt sich das-
selbe senkrecht zur Verbindungslinie der beiden Licht-
quellen weil dann beide Augen in gleicher Weise vom Licht
beeinflusst werden Darin untersche det sich, wie Bohn
richtig bemerkt, die maschinenmassige heliotropische Reak-
tion der Tiere von der nicht durch Heliotropismus bedingten
Bewegung eines Menschen zu einer von zwei Lichtquellen."
Toads, exposed to light from two sources, as stated above,
do not proceed toward a point between them. Judged by
the criterion of Loeb and Bohn, their reactions under these
conditions are therefore like those of a human being under
similar conditions and not like those of other animals
( Tiere) .

c. Orientation with one eye destroyed. — Toads with
the lenses ^ removed from the eyes so that the power of
forming images is destroyed, frequently orient fairly accu-
rately and hop or walk toward a source of light. When
exposed to light from two sources they move toward a
point between them, contrary to what occurs in specimens
that can see.

In specimens with one eye destroyed there is a slight
tendency to deflect toward the injured eye. The head in
such specimens is inclined toward the blind side as though

^ No special precaution was taken to destroy the retina in the eyes from
which the lens was removed. This seemed unnecessary since I was not
primarily interested in the question of distribution of sensitive elements.
Parker (1903 and 1905) demonstrated very clearly that the skin of some
frogs and fishes is sensitive to light, and Pearse (19 10) showed the same for
various amphibia. Many of these creatures can orient fairly accurately
with the retina of both eyes entirely destroyed. My aim was to test the
effect on orientation of unequal stimulation on opposite sides. In the toad
with the lens removed, the eye fills with a substance which, while it may not
be absolutely opaque, certainly intercepts nearly all the light, so that even
if the retina is functional in both eyes, one receives much more light than
the other. The important point here is that when the toads with the lens
of one eye removed as described in the text are oriented and move toward
the light they are not equally stimulated on symmetrically located sensitive
points.

222 LIGHT AND THE BEHAVIOR OF ORGANISMS

the animals were trying fully to expose the intact eye and
still travel toward the light. Hadley (1908, p. 187) ob-
served somewhat similar reactions in positive lobster larvae
with one eye removed, as did also Miss Torelle (1903) in
frogs with one eye covered. Thus it will be seen that the
toads and frogs and lobster larvae tend to deflect toward
the blind eye, while butterflies, amphipods and flies tend
to deflect toward the functional eye.

On June 27 several toads were exposed to light from one
glower, and from these the three which oriented most accu-
rately w^ere selected. The lens was then removed from one
eye in each toad. The following day they w^ere exposed
250 cm. from a 50 candle power Nernst glower in a beam of
light 45 cm. wide. The beam of course became narrower
as the source of light was approached. If set down so that
one side was illuminated all of these specimens turned
directly so as to face the source of light, but in all the head
was turned toward the blind side so that there was a distinct
curve in the spine, and after turning thus they hopped or
walked toward the glow^er, deflecting slightly toward the
injured side. They almost always reached the edge of the
beam of light before getting to its source and one passed
out into the shadow a few times.

On July 14 the lens was removed from one of the eyes
of a large active toad which had oriented rather accurately.
The toad was then immediately exposed in the beam of
light. It hopped toward the glower apparently as accu-
rately as it did before the operation, but there w^as a tend-
ency to turn the intact eye toward the light whenever it
came to rest after each leap. The light intensity was much
reduced but still the specimen went directly toward the
glower. The following day this toad was again exposed;
it now went toward the source of light even more nearly
directly than on the preceding day. There was no longer
any appreciable tendency whatever to turn toward the
blind side.

These results show that in this form and in all the other

MOLLUSKS, ARTHROPODS AND VERTEBRATES 223

forms which orient after one eye is destroyed difference of
effective intensity on opposite sides does not regulate ori-
entation. " The head is . . . [not necessarily] bent until
its symmetrical photosensitive points are . . . struck at
the same angle by the rays of light," nor is it necessary
that both eyes be influenced alike, as Loeb's theory de-
mands. It appears that these organisms, as Holmes says,
referring to the fiddler crab, follow light " much as an
animal pursues any other object of interest, such as prey,
or its mate."

Graber (1884, p. 226) found that Rana esculenta tends to
collect in light of relatively low intensity and we have seen
that Miss Torelle found that the frogs she studied tended
to collect in shaded places, but Parker (1903) and Cole
(1907) found that when frogs are exposed to light in a dark
room they are positive, apparently regardless of the inten-
sity of the light. I found the same true with reference
to Bufo. How can these contradictory phenomena be
accounted for? In a dark room containing but a single
compact source of light such as was used by Parker, Cole
and myself in these experiments, it is not likely that an
animal can see anything but the source of light. If then
the frogs and*toads are guided by sight in their movements,
as their reactions indicate, it is evident that they must go
toward the source of light if they go at all. In these reac-
tions light no doubt acts continuously as an orienting stimu-
lus, and the direction of the rays must necessarily, if they
go toward an object because they see it, guide them on
their way. This of course does not imply that the reac-
tions are controlled by psychic phenomena. The process
of orientation is however, without doubt, far more compli-
cated than the theory of Loeb demands.

4. Caprella

What can be said with regard to orientation in the
higher forms with eyes incapable of forming images ? Such
animals are of course not able to see. They cannot follow

224

LIGHT AND THE BEHAVIOR OF ORGANISMS

an object because of its size or orm. The eyes function
merely in distinguishing different degrees of intensity or
movement perhaps. Caprella seems to possess eyes of this
kind. At any rate when exposed to Hght from two sources
it swims toward any point between them, depending upon

•3 mm

Fig. 32. Camera outline of Caprella sp. (?). A, dorsal view; B, side view,
showing the extent of bending during locomotion. See text.

their relative intensity, indicating that orientation is not
regulated by the same factors as in the toad.

An idea of the general form and structure of the form on
which the following observations were made can be gained
by referring to Fig. 32. This creature is found in abun-
dance attached to Eudendrium colonies. I did not ascer-
tain the species.

a. Orientation. — While attached this form shows no
indication of orientation, but when free it usually swims
toward a source of light fairly accurately. When it swims
slowly the anterior end, with its appendages spread out, is
thrown toward the ventral surface until it is nearly at right

MOLLUSKS, ARTHROPODS AND VERTEBRATES 225

angles with the rest of the body, after which it returns more
slowly with the appendages partially folded. When it
swims rapidly the posterior end strikes backward at the
same time that the anterior end straightens, and moves
forward when the anterior end strikes backward. Thus the
creature alternately bends and straightens in rapid succes-
sion and forces itself through the water. Ordinarily it
swims on either the right or the left side. When it changes
its direction of motion it always turns toward the dorsal
surface. If the ray direction is changed it turns directly
toward the source of light if the dorsal surface is illumi-
nated, but if the ventral surface faces the light it first
rotates on its long axis through 180° and then turns.

Orientation is not brought about by unequal action of
symmetrically located sensory and locomotor organs. W^hen
the organism turns, either the entire anterior end alone or
both ends must strike differently than when it swims
straight ahead. The appendages on opposite sides of the
body act the same. The organism as a whole must be
involved in the process of orientation. This cannot be due
to a " compulsory automatic turning of the head " toward
the source of light, in accord with Loeb's explanation of
orientation. In some way or another these creatures do
keep the anterior end directed toward the general source
of illumination. Just how this is done I am unable to say.
It may be that the organism receives a localized stimulation
when it turns so that the anterior end is no longer directed
toward the light, and orients by means of a differential
response to such a stimulation. There certainly is no evi-
dence indicating that light acts constantly as a directive
stimulation, nor is there any indicating that " the head
is . . . bent until its symmetrical photosensitive points are
. . . struck at the same angle by the rays of light." The
process of orientation is by no means as simple as these
explanatory quotations from Loeb demand.

h. Discussion. — Orientation in Corethra larvae, which,
according to the work of Harper (1907) bend sharply alter-

2 26 LIGHT AND THE BEHAVIOR OF ORGANISMS

nately to the right and left in the process of locomotion,
supports the conclusion reached in the study of Caprella,
as do also the orienting reactions of zoeae and lobster larvae.
Lobster larvae in the earlier stages always orient with the
posterior end toward the source of light. When they are
positive they swim toward the light with this end ahead.
When they are negative they swim in the opposite direction
with the anterior end ahead. Hadley (1908, pp. 264-276)
showed that if the direction of the rays is changed in any
way after the larvae are oriented, they immediately and
directly turn until the anterior end is shaded again. If the
position of the source of light is so changed that the dorsal
or the ventral surface is exposed, it is evident that the sides
may still be equally illuminated. If the dorsal surface, for
example, becomes illuminated when the ray direction is
changed they turn toward the ventral surface. This shows
very clearly that orientation in these forms is not regulated
solely by relative intensity of light on symmetrically located
structures, for the illumination on opposite sides throughout
this whole reaction may have been equal. I observed simi-
lar reactions in the zoeae of several different species of the
Brachyura and Caridea. In some of these animals the eyes
extend laterally beyond the surface of the body so far that
both are illuminated no matter whether the anterior or
posterior end is directed toward its source.

In all these forms it is evident that the turning toward the
dorsal or the ventral surface, when the source of light is
lowered or raised, must be due to the fact that different
portions of the eyes become illuminated, unless it is regu-
lated by vision. If it is, the former orientation appears to
be a differential response to a localized stimulation. The ob-
servations of Holmes and others indicate that orientation in
a number of other arthropods also depends upon the surface
of the eyes exposed. Holmes (1905) covered different parts
of the eyes of Ranatra with an opaque substance and found
that it responds just as though the environment in the
direction of the blackened portion of the eyes were dark.

MOLLUSKS, ARTHROPODS AND VERTEBRATES 227

The fact that many animals with image-forming eyes
respond to size of the luminous area rather than to intensity
difference as shown by Parker (1903) and Cole (1907), is
not at all opposed to the idea of Holmes stated above. In
this fact we have an answer to the much-discussed and
perplexing question as to why moths fly toward a candle at
night and not toward the moon. The reason clearly is that
in moonlight there are large illuminated areas all about,
while in candle light the objects about are so faintly illumi-
nated that the moths do not react to light reflected from
them. But this answer is clearly superficial. The question
is: Why do the Mourning-cloak butterflies, e.g., fly toward
large illuminated patches rather than toward the sun, which
is much brighter ? Parker (1903, p. 465) says it is because
the larger area makes a larger " spot on the retina." " I
therefore believe that Vanessa antiopa stays near the ground
on bright sunny days because its flight is directed by large
bright retinal spots rather than by small ones, even though
the latter are of vastly greater intensity." These answers
are no doubt correct, but they are not fundamental. The
reactions referred to above are in general adaptive, and an
explanation of them must be sought along the same general
lines as an explanation for any other question involving
adaptation. It is frequently said that organisms in water
are limited in their movements toward the sun by the sur-
face of the water, but that insects flying in air are not thus
limited. Referring to Labidocera Parker says (p. 462):
"Their positive phototropism is held in check by their inabil-
ity to pass above the surface of the water. No such barrier
holds the butterfly to the earth." It is evident that this is
true only in a restricted sense. Insects flying up in the air
soon find their limit, and of course no one knows how many
have tried this very thing during the process of fixing the
adaptive habit of reacting positively to large luminous
areas rather than to small ones of much higher intensity.
Thus it may be that they have learned to go toward the
larger luminous areas in preference to smaller ones much

2 28 LIGHT AND THE BEHAVIOR OF ORGANISMS

as they have learned to follow their mates or other objects
of interest.

The fact that an insect flies into a candle flame at night
or a bird against a lighthouse tower and loses its life does
not indicate that its light reactions are not in general adap-
tive, as many assume. One might as well say, if a creature
in a room flies toward a window and is injured by striking
the glass, that its behavior is not adaptive. The environ-
ment in both cases contains artificial factors which animals
rarely experience. It remains for future investigators to
demonstrate whether or not insects and birds can learn
to avoid such pitfalls as candles and lighthouses.

Whether or not the flight of birds against a lighthouse is
tropic, as Cole suggests in the following quotation, depends
upon the sense in which the term tropism is used: (1907,
p. 410) " The way in which migrating birds often, on stormy
nights, gather about lighthouses and dash into the glass only
to be killed, recalls strongly the flying of moths into a flame,
and it seems possible that this is an expression of photo-
tropism in birds which is ordinarily inhibited by other
responses." It seems probable that all creatures which fly
are guided on their course by sight at least with reference
to their immediate environment. It is well known that
birds seldom fly against the lighthouse windows unless the
night is dark and stormy. Under such conditions the light
intensity all about is very low, so that when the birds get
near the lighthouse they can see nothing but the light in it,
and if they are to fly toward something they can see, they
must evidently fly toward this light. If this is true the
factors involved in this phenomenon are similar to those
involved in regulating their flight toward any other object.

5. General Summary and Conclusions

The several summaries at the close of the different sec-
tions in this part and the table of contents should be re-
ferred to for a general idea of the subjects treated and the

MOLLUSKS, ARTHROPODS AND VERTEBRATES 229

results and conclusions reached. Here we aim only to
bring together the more important factors involved in the
process of orientation in different organisms.

(i) The plumules of Zea mays and probably of all the
other gramineae bend toward the more highly illuminated
side of the sensitive region regardless of the direction of the
rays. Our experimental results do not bear on the question
as to whether or not orientation is due to a modification of
circumnutating movements as maintained by Darwin. Nor
do they warrant a conclusion as to whether the stimulation
causing bending is due to a change of intensity on some
part of the sensitive region in accord with Darwin's sug-
gestion, or to constant intensity, all sides being continuously
stimulated in proportion to the intensity to which they are
exposed, in accord with the theories of De Candolle, Loeb,
Verworn and others. The bending may be due to a dif-
ferential response to a localized stimulation.

(2) In the myxomycetes and rhizopods orientation is in
all probability due to a local response to a local stimula-
tion. Light retards the activity of the protoplasm and
thus prevents the formation of pseudopods on the more
highly illuminated side. It is impossible to say whether
light acts constantly as a directive stimulation, all parts of
the organism being continuously stimulated in proportion
to the absolute intensity to which they are exposed (Loeb,
Verworn, etc.), or whether it acts only through changes of
intensity, the prevention of formation of pseudopods on the
illuminated side being due to the increase in light intensity
on the protoplasm as the pseudopods are thrust out.

(3) In Euglena, Stentor, Trachelomonas, Chlamydomo-
nas, Chlorogonium, Oedogonium swarm spores and prob-
ably in all other ciliates and flagellates which orient in
light, orientation is due to definite responses to changes of
light Intensity on the sensitive part of the organism. The
changes of intensity are ordinarily due to the movement of
shadows of one part of the body over another part. This
is caused largely by the rotation on the long axis. In

230 LIGHT AND THE BEHAVIOR OF ORGANISMS

Euglena and Trachelomonas the most highly sensitive part
is probably restricted to a relatively small mass of proto-
plasm in close proximity with the concave surface of the
eye-spot. The eye-spot probably functions in shading this
structure when its convex surface faces the light. The
eye-spot may also function by absorbing light in a manner
similar to that in the choroid coat in the eye. In Chlamy-
domonas and the volvocineae, judging from its location, the
eye-spot cannot function by shading the sensitive part of
the organism, as it appears to in Euglena. In these forms
it can only function by absorbing light, if it functions in
light reactions at all.

The difference in sensitiveness with different surfaces
exposed is surprisingly great in Euglena and Stentor, and
probably in all the other ciliates and flagellates. In Stentor,
under the conditions of the experiment it requires an in-
crease in intensity from 150 ca. m. to 444 ca. m. to induce
a reaction when the posterior end faces the light, while a
change from a position in which the aboral to one in which
the oral surface is exposed causes a reaction without any
change of intensity in the field, showing the marked effect
of the shadow of the former surface on the latter. There is
no evidence that the direction of the rays functions except
in so far as it may affect change of intensity. Nor is there
any evidence that light acts constantly as a directive stimu-
lus similar to the effect of the constant current in accord
with Loeb's theory of tropisms.

In Euglena the avoiding reaction is not a differential re-
sponse to a local stimulus, for in the crawling state it bends
toward the ventral surface, while in the free-swimming
state it bends toward the dorsal surface. The direction
of bending is governed by internal factors. The reac-
tions caused by changes of intensity result in directing the
organisms toward various points of the compass. As soon
as they reach a position in which the rotation on the long
axis no longer causes a change of intensity on the sensitive
region there is no longer any cause for turning; they there-

MOLLUSKS, ARTHROPODS AND VERTEBRATES 23 1

fore continue in this direction. Orientation in these forms
takes place in principle precisely as Jennings' description
demands.

(4) The same factors are involved in the process of
orientation in the colonial forms, Volvox, Eudorina and
Pandorina, as are involved in the orientation of the ciliates
and flagellates. The orienting stimulus is due to a change
of intensity on some part of the zooids. This is due (i) to
the change in the surface of the zooids exposed as the
colonies rotate, and (2) to the transfer of the zooids from
the illuminated side of the colony to the shaded side
and vice versa. The change of intensity causes a definite
response in the zooids, a shock movement or avoiding reac-
tion, which consists in an effort to turn toward the side fac-
ing the anterior end of the colony. Owing to the way in
which the zooids are united there are no errors in the process
of orientation in the colonies as a whole. They never turn
in the wrong direction as many of the protozoa frequently do.
The reaction of Volvox in light is not like the reaction of
this form in a constant electric current, as Bancroft assumes.

(5) Hydra viridis moves fairly accurately toward or from
a source of light. When attached it does however not take
a definite axial position with reference to the direction of
the rays. It may bend toward any point of the compass.
There is no definite relation between the direction of bend-
ing and the side illuminated. But the anterior end is
directed toward the source of light a greater part of the
time, and the animal usually travels only in this direction.
When the anterior end is shaded the animal tends to turn,
but when this end is illuminated it tends to travel. This
indicates that the tendency to become oriented and the
tendency to travel are not the result of the same factors.
The former may be due to the stimulation by light owing
to a change of intensity, the latter to the action of light
owing to its constant intensity. There is no evidence indi-
cating that the reaction in light is the same as the reaction
in a constant electric current.

232 LIGHT AND THE BEHAVIOR OF ORGANISMS

All the other coelenterates studied orient directly. We
are however unable to say how light produces the orient-
ing stimulus. Orientation probably is due to a differential
response to a localized stimulation.

(6) In Arenicola larvae orientation takes place much as
it does in the flagellates and ciliates; but the larvae can turn
directly toward the right or the left side. They appear to
have the power of differential response to localized stimula-
tion; the stimulation is probably due either to a reduction
of light intensity on the shaded side, or to an increase of
intensity on the illuminated side.

In the blowfly larvae only the very tip of the anterior
end is sensitive to light. They turn this tip to the right
and left alternately during the process of locomotion. If it
becomes more highly illuminated as it turns toward either
side it is turned farther in the opposite direction. The
larvae apparently continually test their position with refer-
ence to the light. The lateral head movements make it
possible for these creatures to orient much more accurately
than they otherwise could. The orienting stimulus is un-
doubtedly due to an increase of intensity.

The earthworms can turn directly from the light. They
probably have the power of differential response to localized
stimulation by light. The random movements of the ante-
rior end may serve to localize the stimulation. The direc-
tion of movement is, however, not entirely determined by
localized stimulation. It is largely influenced by internal
factors. The worms frequently turn toward the source of
light when stimulated. The " selection of random move-
ments," essentially as described by Holmes, is an important
factor in orientation.

The anterior end of the earthworm is much more sensitive
to light than the rest of the body. Lateral illumination of
any portion of Allolobophora back of the sixth segment does
not appreciably affect orientation, although this part of the
animal is undoubtedly sensitive to light.

The planarians orient directly. A sudden change of

MOLLUSKS, ARTHROPODS AND VERTEBRATES 233

intensity causes definite lateral head movements. In posi-
tive specimens, where one side is shaded, they turn toward
the side which is not shaded, indicating that it is a decrease
of intensity on the shaded side that causes orientation.

There is no evidence indicating that the direction of the
rays or the angle between the rays and the sensitive surface
functions in orientation in any of the forms referred to
under vermes, except in so far as it may cause change of
intensity; nor is there any evidence that light acts con-
stantly as an orienting stimulus.

The echinoderms do not orient; they can move with any
part of the body ahead. When the direction of the light
is changed they change the direction of motion by moving
with another part of the body ahead. Precisely how the
direction of motion is regulated is unknown.

(7) The moUusks, arthropods and vertebrates all orient
directly. There is little evidence of preliminary trial move-
ments in this process in these forms. Orientation is prob-
ably due to differential response to localized stimulation.
In some of the positive mollusks the orienting stimulus
appears to be due to a decrease of intensity on the shaded
side. With reference to the way in which light produces
the stimulation in the other forms in these groups our
evidence does not warrant a conclusion.

The animals with image-forming eyes no doubt orient
and go toward a source of light much as they go toward
any other object of interest to them.

(8) Orientation may then be due (a) to local response
to local stimulation, as in the rhizopods and myxomycetes;
{h) to shock movements, avoiding reactions, as in the dil-
ates, flagellates, colonial forms, vermes and larvae of various
kinds; {c) to differential response to localized stimulation,
as in some of the coelenterates, the vermes and all the
higher forms; {d) to sight, as in many animals with image-
forming eyes.

(9) No evidence was found indicating that the direction
of light in the field or through the tissue (Sachs) functions

234 LIGHT AND THE BEHAVIOR OF ORGANISMS

in orientation of any organisms without image-forming eyes
except in so far as it may produce difference of intensity on
different parts of the organism. Those organisms which
orient by sight are of course guided by the direction of the
rays reflected from the object toward which they go.

(10) There is no evidence indicating that symmetrically
located points on the surface must be struck by light at the
same angle when organisms are oriented, as Loeb maintains
(1905). The only organisms which do not travel toward or
from a point located anywhere between two sources of light
are those with highly developed image-forming eyes. Such
forms, as far as experimental evidence indicates, always go
toward one or the other of the two sources of light. With
but one functional eye these animals still orient fairly
accurately, although under such conditions symmetrically
located points are unequally stimulated.

(11) There is no conclusive evidence, except perhaps in
animals with image-forming eyes, showing that light acts
continuously as a directive stimulus, that symmetrically
located sides are continuously stimulated, — equally when
the light intensity on them is equal, unequally when it is
not, and that this regulates orientation by regulating the
rate of motion of the locomotor apparatus on the two sides
as is demanded by the theories of De Candolle, Loeb, Ver-
worn, Davenport and Radl.

(12) There is no conclusive evidence showing that orien-
tation in light is ever due to tropic reactions in any organ-
isms, if the definitions of tropisms given by Loeb, Verworn,
or Radl are used as criteria.

(13) All organisms that respond to light at all respond
to changes of intensity. In some the response to such
changes results in orientation, in others it does not. They
are all tinterschiedsempfindlich, in accord with Loeb's defi-
nition of this term. They all respond to time rate of
change in light intensity. The idea of reactions to change
of intensity is however not original with Loeb, as is some-
times assumed. The explanations of reactions to light

MOLLUSKS, ARTHROPODS AND VERTEBRATES 235

given by Engelmann, Bert, Graber, Lubbock, Romanes,
Darwin and others, all of whom preceded Loeb, were
largely founded on this idea.

(14) Light no doubt acts on organisms without a change
of intensity much as constant temperature does, making
them more or less active and inducing changes in the sense
of orientation; but there Is no conclusive evidence showing
that light acting thus ever functions In the process of
orientation.

PART III

GENERAL CONSIDERATION OF REACTIONS

TO LIGHT

CHAPTER XI

ADAPTATION, FORMATION OF AGGREGATION IN REGIONS

OF A GIVEN LIGHT INTENSITY AND DIFFERENT

METHODS OF RESPONSE IN ATTAINING THIS

REGION AND REMAINING IN IT

I. Introduction Showing that Reactions in General

are Adaptive

Thus far we have dealt almost exclusively with the mech-
anism of orientation. In this and the following chapters
we shall deal with reactions to light from a much broader
point of view. We shall consider not only the various differ-
ent methods of response leading to a general classification,
but also the various factors which control the different
responses, leading to a discussion of the nature of stimula-
tion and the cause of reactions.

Under natural environmenta conditions organisms are
usually found in places well suited for the continuance of
their life processes. Sometimes they crowd together and
form dense aggregations in such regions. This is especially
true in case of unicellular organisms. Euglena, Chlamydo-
monas, Volvox and other similar forms collect in the more
highly illuminated regions of their environment during dark,
cloudy days, early in the morning and late in the evening,
and in shaded places when the sunlight is very intense. A
certain amount of light is necessary for the well-being of
these organisms since they depend upon photosynthesis In

236

ADAPTATION AND AGGREGATION 237

the process of feeding, but too much is fatal. Stentor
coeruleus, Amoeba and myxomycetes thrive in total dark-
ness. They are always found in regions of comparatively
low light intensity. The same is true of fly larvae. Nega-
tive response to light tends to keep these creatures buried in
cadavers where they find food. It is ordinarily only under
artificial conditions that the reactions of organisms to light
prove fatal. Positive reactions to candle, lamp and light-
house destroy untold numbers of moths and flies and bees
and beetles and birds, but who has seen such fatalities under
natural conditions ! Under such conditions the responses
to light direct these animals to the advantage of their well-
being. When an insect or a bird in a room, a bee in a flower
or a pomace fly in a wormhole of a decaying apple is excited
it flies directly toward the light and ordinarily escapes. It
could not be expected to react differently in the presence of
a candle surrounded by darkness, since it receives the same
general stimulation and has had no experience with the
consequences. Many water-inhabiting larvae are strongly
positive even in light so intense that it is injurious. They
do not become negative and escape danger. Under natural
conditions the strong positive response serves to scatter
them far and wide, and under such conditions there is no
need for a negative response. The surface of the water
limits the distance they can proceed toward the light and in
their natural environment they experience none of sufficient
intensity to be injurious. As they grow older many of them
lose their positiveness and become negative, and now their
reactions guide them to the bottom into dark places, where
they spend most of their adult life.

Sand fleas are usually found in dark places under masses
of seaweed on the beach. If they are disturbed they
become strongly positive. This response directs them
toward the water, from which the stronger light usually
comes. After they are in the water they become negative.
This response directs them to the bottom and into dark
crevices. Under natural conditions their reactions to light

238 LIGHT AND THE BEHAVIOR OF ORGANISMS

are adaptive, but if they are confined in a glass jar with a
bunch of seaweeds at one end and an intense light at the
other they collect at the illuminated side and remain there
and die, whereas if they had become negative they would
have been saved by the shelter of the seaweeds. But could
they, even from a rational point of view, be expected to
react otherwise under conditions which neither they nor
their ancestors have experienced?

If the positive reaction which guides a moth into a candle
is a non-adaptive reaction, then the positive reaction which
guides the wolf searching for food into a baited pitfall must
also be considered non-adaptive. But what would happen
to the wolf if he did not react positively to food; does not
the trapper expect him to get into the pitfall ? Is not the
flight of the moth into the flame, after all, precisely what
one would expect if its reactions to light in general are
adaptive?

Even in case of positive reactions of animals which live
in darkness, as e.g. the caterpillar of the willow borer and
the mud-inhabiting crustacean Cuma rathkii, referred to by
Loeb (1906, p. 159), or the cave-dwelling fishes mentioned
by Eigenmann (1899), it is probable that the reactions were
inherited from ancestors in which they were adaptive.
Reactions to light which are non-adaptive, except under
artificial conditions, are certainly rare. And whatever the
fundamental cause may be, it is evident that organisms in
general in their natural habitats tend to aggregate in regions
which have conditions adapted to the needs of their life
processes. We shall have occasion to emphasize this in
the following paragraphs. These conditions may differ for
different species and different individuals and for the same
individual from t'me to time.

Of course the fact that reactions are adaptive does not
explain their origin. Natural selection may tell us why
organisms are as they are. It shows why adaptive charac-
teristics continue to exist while the non-adaptive ones do
not, but it does not tell us how they originated or why they

ADAPTATION AND AGGREGATION 239

are at all. It may be, for all that is known to the contrary,
that Loeb is correct in his assumption that the reactions of
organisms have their origin in fortuitous chemical combina-
tions (see Chapter XX), and that those organisms which
live in high light intensity do so because they are positive in
their reactions to light, while those which live in low inten-
sity do so because they are negative. It must however be
conceded that we have as yet but very few, if any,- acts
which bear directly on this question.

2. Different Reactions Observed in the Process of Collecting
in Regions having a given Condition of Illumination

Let us now proceed to ascertain precisely how the dif-
ferent organisms respond so as to get into and remain in
favorable light conditions. This has been thoroughly inves-
tigated for a number of different forms. It is by no means
the same in all. We shall consequently consider the dif-
ferent methods under several different headings: — a. Ran-
dom movements and avoiding reactions; h. Orientation,
change in sense of orientation, and avoiding reactions; c.
Orientation and extent of movement limited by environ-
mental conditions; d. Orientation and movement directly
toward the place where the organism comes to rest; e. Ran-
dom movements and coming to rest in a given place It
should be emphasized here that our aim in this section is
not merely to discuss methods of aggregation but also to
set forth all different methods of reactions with the view of
classifying them.

a. Random movements and avoiding reactions. — Quite
a number of the unicellular organisms get into the region of
optimum intensity by random movements. They swim
about aimlessly hither and thither, testing conditions in
many different places. Thus they sooner or later get into
an optimum intensity, where they remain, not by coming
to rest, but because whenever they reach the boundary of
the optimum region they are stimulated and consequently

240 LIGHT AND THE BEHAVIOR OF ORGANISMS

return. Engelmann (1882) was the first to observe and
record thus method of aggregation in detail. He says (1882,
P- 393)' referring to the aggregation in light of Paramecium
bursaria, Stentor viridis and other chlorophyll-bearing cili-
ates: *' Ueberschreiten sie z. B. zufallig die. Granze von Licht
und Dunkel, oder tauchen sie auch nur mit der vorderen
Halfte ihres Leibes eine Strecke weit in das Dunkel ein, so
kehren sie sofort um nach dem Licht, wie wenn das Dunkel
ihnen unangenehm ware." The collection of Euglena in
an illuminated area is described as follows (p. 395) : " Dieses
wirkt wie eine Falle, denn einmal hineingekommen, gehen
die Euglenen in der Kegel nicht wieder heraus. Sie kehren
an der Grenze des Dunkels immer so gleich wieder um ins
Helle. Falls sie, was bei schnellem Vorwartsschwimmen
wohl einmal geschieht, ganz ins Dunkel hineingekommen
sind, sistiren sie doch so fort die weitere Vorwartsbewegung,
drehen um eine ihres kurzen Axen, probiren — oft unter
bedeutenden Gestaltsanderung — in verschiedenen Richt-
ungen fortzukommen bis sie endlich wieder ins Licht
gerathen." In the last sentence we have an intimation of
orientation by trial, a method so thoroughly worked out
and clearly stated by Jennings later. The clearest state-
ment however which Engelmann made with reference to
aggregation by the method under consideration is found in
his article on Bacterium photometricum (1883, p. no):
" Schwacht mann nun plotzlich das Licht ... so sleht
mann alle bis dahin im Gesichtsfeld schwimmenden Bak-
terien fast im namlichen Moment eine Strecke weit zuriick-
schiessen, einige, meist unter lebhaftester Rotation um ihr
Langsaxe stillstehen und danach wieder die gewohnllche
Bewegung aufnehmen. Man erhalt Vollstandig den Ein-
druch eines Erschreckens." The reactions to sudden
change of intensity in Bacterium photometricum has since
been designated '' Schreckbewegung." It is in all essentials
like the reaction of Paramecium to changes in chemical
concentration which Jennings has designated " motor re-
flex " or " avoiding reaction," and somewhat similar to the

ADAPTATION AND AGGREGATION 241

reaction of serpullds to sudden changes of light intensity.
Loeb designated the power in serpuUds to respond thus
" Unterschiedsempfindhchkeit."

Besides those already mentioned various other organisms
have been found to aggregate by this method, notably Sten-
tor coeruleus, Trachelomonas, Chlamydomonas, Chlorogo-
nium, Phacus and some swarm-spores, but so far as known
Bacterium photometricum is the only form which is entirely
dependent upon random movements to get into the region
of optimum illumination. All the others are only partly
dependent upon random movements in this. Under certain
conditions they orient and proceed directly toward the
region of most favorable illumination. Many forms how-
ever which make use of orientation at times in reaching the
optimum light are entirely dependent upon random move-
ments and avoiding reactions in case of other sources of
stimulation, notably chemicals.

Reactions of the sort described above have usually been
referred to as photopathic. They are supposed by some
to be fundamentally different from orienting reactions,
which are often called tropic. The former are supposed
to be due to the action of light owing to difference or change
of intensity, the latter to the action of light owing to con-
stant intensity or direction of rays. This distinction how-
ever will not hold, for we have clearly demonstrated that
orientation in many organisms is due to changes of light
intensity on the sensitive tissue.

b. Orientation, change in sense of orientation, and avoid-
ing reactions. — Many organisms, as stated above, have
the power of orienting and moving directly toward the
region of favorable illumination. In relatively low light
intensities they are positive; in relatively high they are
negative; in favorable intensity they do not react to light;
consequently they tend to remain, but they do not come
to rest.^ The rate of movement is apparently not decreased.

^ Loeb has recently impugned these statements. We shall refer to this
matter in detail later (see footnote, p. 266).

242 LIGHT AND THE BEHAVIOR OF ORGANISMS

The organisms simply do not orient but swim about in an
apparently aimless way. If in their wandering they get
out of the optimum they orient and return. Under certain
conditions the avoiding reaction serves to prevent them
from leaving the optimum, just as described under (i) above,
but this reaction functions mainly in the process of orienta-
tion, as set forth in detail for many different forms in the
part of this volume devoted to that subject.

Practically all the lower motile forms which react to light
at all make use of this method of getting into the optimum
and remaining there. In this group we may put a few
rhizopods, numerous flagellates, some ciliates, several colo-
nial forms, a few rotifers, at least one coelenterate, a num-
ber of vermes, and some insect larvae.

The advantage of orientation over random movement in
getting to the optimum is evident, for it usually guides the
organisms directly there. Conditions can however be so
arranged that the greatest amount of light does not come
from the portion of the field most highly illuminated. On
the floor in front of a window for instance the region of
highest illumination is some distance from the window.
From this region toward the window the intensity decreases,
but the window is still the source of strongest illumination.
Under such conditions the organisms may be led astray.
If they are positive they proceed in the direction from which
the strongest light comes and may thus be carried directly
from the optimum. This was the case in a number of
Loeb's experiments. It led him and others to the erro-
neous conclusion that difference of intensity is of no impor-
tance in regulating reactions to light in many forms. They
failed to realize that the process of orientation is in itself
an attempt on the part of the organism to attain optimum
environmental conditions. Its purpose is essentially the
same in all organisms without eyes, plants as well as ani-
mals, sessile organisms as well as motile. When an organ-
ism is oriented it is in its most favorable light conditions
as far as the immediate surrounding is concerned, for in

ADAPTATION AND AGGREGATION 243

positive individuals the most sensitive part is then fully
exposed to the light, whereas in negative individuals it is
shaded. In sessile forms nothing more can be accomplished,
but in motile forms still more favorable conditions are ordi-
narily attained by locomotion after orientation, that is,
movement toward or from the source of greatest illumina-
tion, depending upon whether the organism is positive or
negative.

The fundamental principle involved in orientation, as
shown in Part II, is the same in all organisms without
image-forming eyes, although the process differs much. Ori-
entation is the result directly or indirectly of the effect of
illumination on life processes. Movement toward unfavor-
able conditions generally induces orienting reactions. The
reaction leading to orientation is however frequently not a
response to an immediate unfavorable condition. It may
be a response to a sign quite as much as the sudden con-
traction which closes the valves of a mussel when a shadow
passes over it. The shadow in itself is of no particular con-
sequence to the mollusk, but what follows may be. Just so
the change of intensity has no particular influence on the
life processes of Euglena, e.g., but what follows may have.
Thus it is evident that orientation in the lower forms is
dependent upon the power of discrimination between dif-
ferent degrees of intensity — " Unterschiedsempfindlich-
keit " (Loeb), '* sensibilite differentielle " (Bohn). In many
of these forms orientation is undoubtedly, and in all it is
probably, a response to change of light intensity on some
part of the organism. At any rate it has in no instance
been demonstrated that it is, as Loeb states, ** a function
of the constant intensity," that orientation in light is like
orientation in an electric current.

c. Orientation and extent of movement limited by
environment. — Many organisms orient and proceed toward
or from the source of light as far as the physical conditions
of the environment will permit and collect there. Areni-
cola and Eudendrium larvae, zoeae and many other aquatic

244 LIGHT AND THE BEHAVIOR OF ORGANISMS

forms, particularly in the early stages of development, aggre-
gate at the surface of the water in this way. They do not
have a response similar to the avoiding reaction in many
unicellular forms, nor do they become negative even if the
light intensity is increased to such an extent that it is
undoubtedly injurious. Conditions can readily be so ar-
ranged that they will go toward the light into chemical
solutions, temperatures and concentrations where they are
killed. Still their reactions under natural conditions are
highly adaptive. Owing to their strong positive reaction
they are brought to the surface of the water and scattered
far and wide. Under natural conditions they apparently
never or at least seldom experience light of such an intensity
that it is injurious, or a combination of other environmental
conditions such that their positive reactions to light prove
fatal. The power to become negative in excessively high
light intensity would consequently be of no special benefit
to these organisms.

d. Orientation and movement directly toward the place
where the organism comes to rest. — Most of the organisms
with well-developed eyes, together with some few without,
may be placed under this head. They go directly to a
given place and remain because they come to rest there.
Thus we find according to Bohn (1908) that Littorina and
some starfishes under certain conditions go directly toward
rocks and other objects and come to rest in their shadows.
Butterflies and various other forms, according to Parker
(1903) and Cole (1907), go directly toward large luminous
areas and come to rest there. And frogs in direct sunlight
were found by Torelle (1903) to go directly toward shadows,
even if in so doing they had to move perpendicular to the
direction of the light rays. But after they were in the
shadow they turned so as to face the light, an adaptive reac-
tion throughout; for under the conditions of the experiment
It was probably advantageous for the frogs to be pro-
tected by the shade and to face the light when in it, since
thus they were most likely to see both food and enemies.

ADAPTATION AND AGGREGATION 245

e. Random movements and coming to rest in a given
place. — Probably all organisms that react to light at all
are affected in their movements by the action of light owing
to the absolute intensity. Such effects are not dependent
upon the time rate of change of intensity but upon the
actual amount of light energy present and the time of
exposure. Light tends to inhibit the movements of Plas-
modia, and some bacteria, while total darkness has the
same effect on purple bacteria, Volvox, Euglena, Chlamy-
domonas, Hydra, some planaria, earthworms and a number
of other organisms. In most of these organisms this action
of light is either insufficient to have any apparent effect on
aggregation or it is masked by other stronger reactions.
In planaria however the influence of relatively low light
intensity has a marked effect on the process of aggregation.
Some of these animals, as Loeb points out (1906, p. 136),
do not orient. When exposed in a dish in front of a window
they wander about aimlessly until they get into the darker
regions, where they come to rest and consequently remain.
Thus it is that they collect in the region of lower light
intensity. Loeb considers this reaction of planaria '' a
function of the quotient of the change of intensity over
time," i.e. time rate of change. I am however of the
opinion that it is a function of the absolute intensity, that
the action of light in these reactions is similar to the action
of heat, for active planarians respond by raising the head
and throwing it from side to side, both when the light is
suddenly decreased and when it is suddenly increased.
There is no indication of an immediate decrease in locomo-
tion under such conditions (Walter, 1907, p. 71), as would
be expected if the aggregation were due to a change of
intensity. And moreover, if the intensity is suddenly in-
creased after the planarians have come to rest, they do not
become active at once. Walter says (1907, p. 63) that the
interval between sudden increase in illumination and re-
sponse under such conditions "was often several minutes."

CHAPTER XII

REACTIONS TO LIGHT WHICH DO NOT RESULT IN
AGGREGATION OR ORIENTATION

We have demonstrated in the preceding pages that reac-
tions to sudden changes of Ught intensity may result in
orientation followed by aggregation, or in aggregation with-
out orientation. In either case the immediate response
to the change of intensity is an abrupt change in direction
of motion called Schreckbewegung, or avoiding reaction.
Most of the organisms considered respond thus to an
increase of intensity under some conditions and to a
decrease under others. The immediate response under the
two conditions is precisely the same; but in case of orien-
tation the former leads to locomotion away from the source
of light, the latter to locomotion toward it; and in case of
aggregation, the former results in collections in regions of
relatively low light intensity, the latter in regions of rela-
tively high intensity.

There are many organisms which respond to sudden
changes of light intensity much like those referred to above.
They contract suddenly or change their direction of locomo-
tion abruptly, but these reactions ordinarily result neither
in orientation nor in aggregation. Most of these reac-
tions are responses to shadows, i.e. to a sudden decrease
in light intensity. These organisms do not all however
react in the same way, and may consequently be divided
into several groups as follows : — i . Reactions to shad-
ows— protective; 2. Reactions to shadows — procuring
food; 3. Reactions to sudden increase of light intensity;
4. Reactions to light caused by the effect of continued
illumination.

246

REACTIONS TO LIGHT 247

I . Reactions to Shadows — Protective

Many animals respond only to shadows, i.e., to a sudden
decrease in light intensity, not to a gradual decrease or to
an increase, no matter how sudden or how great. Such
reactions to shadows are widely distributed. They are
highly protective in all instances, and consequently vary
somewhat in accord with the different habits of the different
animals. The holothurian Thyone briareus contracts when
the light intensity is suddenly reduced, frequently to such
an extent that any portion protruding above the sand and
mud in which the animal lives is entirely withdrawn (Pearse,
1908, p. 277). Several different sea urchins, according to
von Uexkiill (1897), turn the spines toward the shaded part,
apparently to ward off an approaching enemy. Various
tubicolous annelids have been observed to contract violently
and jerk back into their tubes when an object passes be-
tween them and the source of light, e.g., Amphitrite bombyx
(Dalyell, 1853), Branchiomma koUikeri (Claparede, 1868),
Serpula (Ryder, 1883), Hydroides dianthus (Andrews,
1891, pp. 285, 296), Serpula uncinata (Loeb, 1893, p. 103),
Spirographis spallanzani (Nagel, 1896, p. 76), Potamilla
oculifera, Sabella microphthalmia and Protula intestimum
(Hargitt, 1906, p. 310), and Bispira voluticornis (Hesse,
I899). Shadows cause Pecten (Rawitz, 1888), Avicula,
Area, and Cardium (Patten, 1886) to close their valves rap-
idly. Nagel (1896, pp. 18-77) observed similar reactions in
23 species of lamellibranchs without eyes, 4 species of gastro-
pods with the eyes removed, and several blind arthropods.
I have also frequently seen the short-neck clam Maya ara-
naria close its siphon and contract, and Littorina littorea
retract rapidly into its shell. The hermit crab Pagurus
also darts back into its stolen home when a shadow is cast
on it. Mosquito larvae, ordinarily found at the surface of
the water, scurry to the bottom at the approachof a shadow,
and the killifish Fundulus responds much in the same way.
Barnacles, according to the observation of Pouchet and

248 LIGHT AND THE BEHAVIOR OF ORGANISMS

Joubert (1875), stop their respiratory movements and con-
tract if they are attached to objects some distance beneath
the surface where a shadow might indicate the approach of
an enemy, but they do not respond to decrease in Ught inten-
sity if attached to objects floating on the surface, where
shadows can have no such signification. All of these reac-
tions are very much like the involuntary closing of the
eyelids caused by shadows in higher forms.

All of the organisms in this class are readily acclimated
to changes of intensity, and in all, the reduction can be
made so gradual that they do not respond, showing that the
response is dependent upon the time rate of change. Those *
organisms which react by contracting expand again very
soon, even if the intensity remains constant at the lowest
point reached during the application of the stimulus. This
shows that the response is due to the process of changing
the intensity and not to the absolute difference in the
amount of light (at different times) before and after the
change takes place. Von Uexkiill says that the response
in sea urchins usually fails after three or four trials. Nagel
found the same with regard to moUusks. He says (1896,
p. 29), '* Das auffallendste an den Beschattungsreaktionen
ist nun aber die Art, wie sich die Tiere an ofter wiederholte
Reize gewohnen.

" Ich lasse beispielsweise den Schatten eines Bleistiftes
liber die ausgestreckten Siphonen einer Herzmuschel hin-
streifen. — Sie schliesst blitzschnell die Siphonen.

" Ich warte einige Minuten und wiederhole den Versuch.
— Jetzt schliesst die Muschel ihre Siphonen nicht mehr,
sondern es zucken nur die Rander derselben ein wenig
zusammen.

" Ich warte wieder einige Minuten, und wiederhole den
Versuch zum drittenmale, und jetzt bleibt jede Reaktion
aus. Nun kann ich aber auch den Grad der Verdunklung
um das hundertfache steigern, indem ich als schatten-
werfenden Korper statt des Bleistiftes ein Buch oder
ein grosses Stiick Karton verwende: Trotzdem bleibt die

REACTIONS TO LIGHT- 249

Muschel regungslos, der Schatten geniert sle in keiner
Weise.

'' Noch auffallender ist die Erscheinung bei der Auster
und unserer Malermuschel. Wenn diese einmal durch einen
Schatten erschreckt worden sind, dann bleibt jede weitere
Beschattung ohne Erfolg. Erst wenn eine Stunde oder
mehr seit dem ersten Versuche ohne Storung verflossen ist,
sind die Muscheln wieder fiir den Schattenreiz empfang-
lich." I have frequently noticed that the hermit crab
(Pagurus), mosquito larvae and tubicolous worms, especially
Hydroides, soon fail to respond to ordinary shadows if they
are kept in a place where the shadows frequently occur,
but under such conditions they still respond to reduction
of intensity greater than they ordinarily experience. The
experimental results of Mrs. Yerkes (1906) and Hargitt
(1906, 1909) on Hydroides dianthus lead to the same con-
clusion. Hargitt's observations (1909, p. 158) are of especial
interest in showing the relation between the reaction and
the habitat. He found that specimens taken from a depth
of about twenty fathoms did not respond to shadows which
caused very definite reactions in specimens taken in shallow
water. Whatever the immediate physiological cause of all
these reactions may be, it is evident that they are admirably
adapted to protect the organism against the attack of
enemies. There are however animals in which similar re-
actions are found which serve quite a different purpose.
These are included in the following group.

2 . Reactions to Shadoivs — Procuring Food

Whitman observed that even a very faint shadow causes
the leech, Clepsine, to become restless and stretch up and
sway from side to side, apparently in search of something
to seize. Bateson records similar reactions in shrimps and
prawns. When a shadow passes near these animals they
raise their antennae and swing them about. The primary
cause of reaction in these animals is probably the same as

250 LIGHT AND THE BEHAVIOR OF ORGANISMS

it is in the preceding group. In both, the reaction has to
do with the well-being of the individuals responding. But
in the one it has to do with protection against mechanical
injury, while in the other it has to do with the process of
procuring food. The important point is that the shadow
in itself is of no particular importance in either case, but
what follows may be. It is a response to a sign just as
truly as is the reaction of a dog to which Brooks refers in
the following characteristically convincing words (1907,
P- 53) » " The kick is a sign of something which may follow,
and the actions which do follow are not the effect of the
kick, for they are directed or adjusted, either consciously
or unconsciously, to an event of which it is only the fore-
runner."

3. Reactions to Sudden Increase of Light Intensity

There are reactions to changes of intensity which appear
to be more directly concerned with the effect of light than
are those just referred to. They are mostly responses to
an increase of intensity and may be due to the effect of the
change of intensity or to the effect of the absolute illumina-
tion. An earthworm, e.g., jerks back into its burrow when
light is flashed upon it, as definitely as Hydroides does when
it is suddenly shaded. If however the intensity is gradually
increased it may not react at all. This is therefore evi-
dently a reaction which is primarily dependent upon the
time rate of change of light intensity. A sudden decrease
of intensity produces no such reaction. I have frequently
observed similar reactions in Stentor coeruleus, as did also
Bronn in the actinia, Edwardsia and Cerianthus; Nagel
(1896) in the sea squirt, Ciona, and in several different
mollusks, and Parker (1908, p. 419) in Amphioxus. Parker
says: " In all the tests I carried out, I never observed a
reaction to a rapid diminution of light, and the reactions
to light that did occur were always the result of a rapid
increase of intensity. When an animal was resting quietly

REACTIONS TO LIGHT 251

on its side in a shaded aquarium and a beam of sunlight
was suddenly thrown upon it, it would usually respond by
one or two vigorous locomotor leaps, after which it might
come to rest even in the sunlight. If now the sunlight was
suddenly cut off, no response followed. That this failure
to respond was not due to exhaustion from over-exposure
to light was easily shown by quickly throwing on the sun-
light a second time, whereupon a reaction much like the
first one usually followed immediately."

The jellyfish Sarsia contracts, according to Romanes
(1885, p. 41), if suddenly illuminated while it is at rest,
and Yerkes (1902) observed similar reactions in Gonione-
mus. A decrease in illumination produces no response in
either of these forms if they are at rest, but in case of
Gonionemus in the active state the movements are imme-
diately checked either by an increase or by a decrease of
intensity, after which the medusae turn over and sink to
the bottom. While a sudden decrease of intensity does not
call forth a response in Gonionemus when at rest, prolonged
exposure to light of low intensity causes it to become active.
Thus its light reactions are such as to keep it in moderately
illuminated regions. In Sarsia in the active state, on the
other hand, increase in illumination tends to cause increase
in activity. It does not inhibit movement as it does in
Gonionemus. A number of animals respond to both a
decrease and an increase of light intensity. Most of these
respond more definitely to the former than to the latter,
but there are some which appear to respond to both in the
same way and equally definitely. Nagel says (1896, p. 74),
e.g., that Helix hortensis draws into its shell when suddenly
illuminated, much as it does when suddenly shaded. Such
reactions are of considerable theoretic importance : they will
be referred to again later.

252 LIGHT AND THE BEHAVIOR OF ORGANISMS

4. Reactions to Light Caused hy the Effect of Con-
tinued Illumination

In all of the animals referred to above, the reaction is
clearly a response to change of intensity, except in the case
where Gonionemus becomes active after having been sub-
jected to low light intensity for some time. In the actin-
ians Aiptasia, Cerianthus and Eloactis the response appears
to be due primarily to the action of light owing to its abso-
lute intensity.

" Aiptasia annulata," Jennings says (1905, p. 459), " is
very sensitive to light, expanding in darkness, but con-
tracting after a few seconds when exposed to strong light.
In ordinary daylight the animal remains contracted for
some hours, but after such a period most specimens extend
in spite of the light. In comparative darkness the animals
direct the disk toward the source of light, through a con-
traction on the side of the column exposed to the light.
After remaining undisturbed for a long time in an aquarium
that is fairly well lighted, the animals give up their orienta-
tion with respect to the strongest source of light; with less
light they retain it."

Regarding Eloactis, Hargitt says (1907, p. 277) that they
begin to retract almost immediatelyafter exposure to diffuse
daylight. " This reaction is not sudden or general at once,
as in such creatures as the earthworm, but begins in a some-
what indefinite movement of the body, accompanied by
similar movements of the tentacles, followed very soon by
a slow but definite retraction of the entire body within the
tube, often including likewise the tentacles as well." In
direct sunlight the reaction is more striking. " In some
cases the reaction was so definite and prompt as to leave the
impression on the observer that the creature was possessed
of something akin to visual sensation."

While, as stated above, these reactions appear to be pro-
duced by the action of light owing to constant intensity, it
may even here be due to the effect of the changes of inten-

REACTIONS TO LIGHT 253

sity. The principal reason for thinking it is due to the
effect of constant intensity is the fact that there is no
immediate response when the intensity is changed. But
this retardation and slowness in reaction may be due to the
general character of the animals. As a matter of fact we
have as yet presented no conclusive experimental evidence
showing that reactions are dependent upon any action of
light other than that due to changes of intensity, although
we have several times intimated that in all probability the
activity of many organisms is affected by constant inten-
sity. The strongest evidence we have in support of this
is found in connection with observations on the change in
sense of reaction.

However this may be, there is in this group no evidence
of a reaction to a sign. The reaction is undoubtedly a
direct response to the light itself.

In organisms with image-forming eyes the reactions are
preeminently responses to signs, at least in so far as the eyes
function in the responses. These animals are not primarily
interested in light as light, but in what may follow a given
light condition, e.g., an image on the retina. At first
thought it seems as though here were a clear case of stimu-
lation due to the action of light through constant intensity,
for objects can be seen for some time without changing the
light configuration on the retina. No new image is how-
ever formed on the retina without change of intensity, and
it is consequently evident that here, too, the stimulation
may be due to changes of intensity rather than to constant
intensity.

5. Classification of Reactions to Light — Phototropism,

Photopathy

Reactions to light have ordinarily been classified as
phototropic (phototropism) or phototactic (phototaxis) and
photopathic (photopathy). In some instances '' helio " has
been substituted for " photo." Organisms which orient and

2 54 LIGHT AND THE BEHAVIOR OF ORGANISMS

move toward or from a source of light are usually termed
phototactic, those which orient but do not move as photo-
tropic, and those which do not orient but still react have
been termed photopathic. The adjectives positive and
negative are ordinarily used in connection with these terms
to signify whether the organisms go or bend toward the
source of light or in the opposite direction ; or whether they
collect in regions of higher intensity or in those of lower
intensity. The terms mentioned above have however been
used not only to signify direction of movement, but also
to designate the nature of the stimulation and the response
as set forth in Part I, under definitions of tropisms.

Loeb says (1906, p. 135): '' Heliotropism covers only
those cases where the turning to the light is compulsory
and irresistible, and is brought about automatically or
mechanically by the light itself. On the other hand, there
are compulsory and mechanical reactions to light which
are not cases of heliotropism; namely, the reaction to sud-
den changes in the intensity of light. ... In the former
case the results are a function of the constant intensity,
in the latter a function of the quotient of the change of
intensity over time." All cases of orientation are con-
sidered by him to be due to heliotropism, i.e., to the effect
of light by virtue of its " constant intensity." All other
reactions to light are due to changes of intensity. In this
class Loeb puts the contraction of the tubicolous annelids
Serpula and Spirographis and the collection of Planaria in
regions of low light intensity. These reactions, he says,
are due to Unterschiedsempfindlichkeit — sensibility to dif-
ference of Intensity; those resulting In orientation are not.

Davenport (1897, pp. 210, 211) maintains that "Two
kinds of effects are produced by light: one by the direction
of its ray — phototactic; the other by the difference In illu-
mination of parts of the organism — photopathic. . . .
Light acts directly either through difference In Intensity on
the two sides of the organism, or by the course the rays
take through the organism." Here again we have two

REACTIONS TO LIGHT 255

classes, — phototropism and photopathy. All cases of ori-
entation belong to the former, and all cases where organisms
aggregate without orientation, to the latter. Davenport
mentions Planaria torva, Daphnia and Volvox as examples
of organisms which are photopathic. Both he and Loeb
affirm that there are some organisms which are both photo-
tropic and photopathic. The one mentions Daphnia as an
example, the other Spirographis.

Yerkes (1903, p. 361) refers to this problem as follows:
" The motor reactions of organisms to light, so far as known
at present, are of two kinds: phototactic and photopathic.
In both intensity of the light, not the direction of the rays,
is the determining factor. All those reactions in which the
direction of movement is determined by an orientation of
the organism which is brought about by the light are
phototactic; and all those reactions in which the movement,
although due to the stimulation of light, is not definitely
directed through the orientation of the organism, are photo-
pathic. . . . /\n organism which selects a particular in-
tensity of light and confines its movements to the region
illuminated with that intensity is photopathic." Thus it is
seen that Yerkes, like Loeb and Davenport, divides reac-
tions into two classes and ascribes both kinds of reactions
to a given individual in certain cases. All three authors
agree in designating orienting reactions as photopathic or
phototropic, but they do not agree in their explanation of
the process of orientation. Loeb claims it is due to the
effect of constant intensity; Davenport thinks in some cases
it is due to the direction in which the rays pass through
the organism, and in others to the effect of difference of
intensity on opposite sides; and Yerkes maintains that it is
due to difference of intensity in all cases. But if the light
intensity on the surface of an organism is not uniform
almost every movement of the organism produces changes
of intensity on some part of it. It is therefore evident that
orientation (phototropism), according to Loeb, Davenport
and Yerkes, may be due respectively to the effect of con-

256 LIGHT AND THE BEHAVIOR OF ORGANISMS

stant intensity, direction through the tissue, or change of in-
tensity. We have however clearly demonstrated in Part II
of this volume that there is no experimental evidence
proving that direction of the rays through the tissue or
difference of light intensity on different parts of the body
is functional in the orientation of any organism, except-
ing in so far as it may cause changes of intensity on the
organism.

Photopathy, or Unterschiedsempfindlichkeit, as Loeb
calls it, is due to change of intensity according to Loeb, to
difference of intensity on the organism according to Daven-
port, and to difference of intensity in the field according to
Yerkes. It is at once evident however that there may be
agreement in these apparently different statements. If an
organism is so illuminated that the light intensity on dif-
ferent parts of the body differs, every moment is almost
certain to cause changes of intensity on some part of the
organism, and if the intensity is not uniform in the field an
organism, of course, cannot move about without causing
changes of intensity on its surface. It may be, then, that
the fundamental factor involved in photopathy, according
to all of these authors, is change of intensity.

It is thus evident that the reactions grouped under photo-
tropism and photopathy by the authors mentioned may all
depend upon changes of intensity, and that the two phe-
nomena may be fundamentally the same. If this be true,
then the classification of reactions to light as photopathic
and phototropic is without a foundation. Are there, then,
no differences in these reactions which will serve as a basis
for a classification?

6. Reclassification of Reactions to Light

Reactions to light may conveniently be classified (i) on
the basis of the character of the stimulus, and (2) on the
basis of the fundamental causes of the response.

(i) On the basis of the character of the stimulus we

REACTIONS TO LIGHT 257

obtain the following groups: a. Reaction to change of Inten-
sity; b. Reactions to constant Illumination; c. Reactions of
questionable cause.

a. Reaction to change of intensity. — Response to change
of Intensity on the surface of the organism may result In
orientation or merely In a change In position or direction
of motion, {a) Orientation: Examples — Euglena, Chlam-
ydomonas, Trachelomonas, CVilorogonlum, ^swarm-spores,
Volvox, Stentor, Planaria, earthworms and fly larvae, {b)
Changes In direction of motion: Examples — Shock reac-
tions, or avoiding reactions which do not result In orienta-
tion, In all the forms mentioned above, {c) Changes In
position : Examples — Sudden contraction In the tublcolous
worms, Gonlonemus, a few actlnlans, various moUusks and
arthropods, and Amphloxus. {d) To these a fourth divi-
sion may be added consisting of reactions to shadows In
Clepslne, shrimps, prawns, mosquito larvae and Fundulus.

b. Reactions to constant illumination. — Constant or
continuous Illumination affects the sense of the reaction of
organisms and their general activity, and it may possibly
produce orientation In some forms. Whenever a positive
organism becomes negative In light or vice versa, It Is In all
probability due to the action of light owing to Its continued
intensity, the absolute amount of light energy received, the
product of the Intensity and time of exposure. Reversal
In the sense of reaction Is not common to all organisms which
respond to light, but the general activity of all probably
depends upon the absolute amount of light energy received,
much as the activity depends upon the temperature or heat
energy received. The aggregation of Planaria In regions
of low light Intensity Is no doubt In part due to this effect
of light, since they come to rest even In a field uniformly
illuminated from above In such a way that there Is no per-
ceptible change of Intensity on any part of the organism.
The time of exposure is an important element In this
response. This Is contrary to Loeb's conclusions regarding
the cause of aggregation of Planaria. He claims the aggre-

258 LIGHT AND THE BEHAVIOR OF ORGANISMS

gation of Planaria to be due to Unterschiedsempfindlichkeit,
sensibility to changes of intensity, and classifies the reac-
tions with those of Serpula and Spirographis to shadows.
I can however see no similarity between the responses of
these organisms.

c. Reactions of questionable cause. — There are many
responses in which it is as yet impossible to be certain as
to what characteristic of light causes them. The orienta-
tion of Amoeba for example is probably due to the direct
effect of the increase in light intensity on the protoplasm,
but for all that is known to the contrary it may be due to
continued illumination rather than to change in illumina-
tion. With regard to the orientation of the sessile plants
and animals, as well as all animals with well-developed
eyes, and many of the lower forms (entomostraca. Hydras
sea anemones, and the larvae of Arenicola, Limulus and
various crabs), experimental results do not as yet warrant
a definite conclusion as to whether the reaction is due to
the effect of change of intensity or continued illumination.
It should however be emphasized again that in no case has
it been demonstrated that orientation is " a function of the
constant intensity " as maintained by Loeb. Nor is there
any evidence indicating that the direction of the rays
through the tissue has any direct effect on this process.

(2) On the basis of the fundamental cause of response,
the reactions to light may be classified as follows: a. Reac-
tions caused by the direct effect of light on the reacting
tissue; b. Reactions caused by an indirect effect of light;
c. Reactions due, not to any effect of light in itself, but to
what a given light condition or configuration may represent.

a. Reactions caused by the direct effect of light on the
reacting tissue. — Examples: Inhibition of protoplasmic
streaming In the rhizopods and plasmodia and in numerous
different plant cells, probably reversal in the sense of reac-
tion and the change in sensitiveness, and the general activity
of some organisms at least.

b. Reactions caused by an indirect effect of light. —

REACTIONS TO LIGHT 259

Examples: Shock-movements and orientation in Euglena,
Chlamydomonas, Trachelomonas, Chlorogonium, swarm-
spores, Volvox and Stentor; orientation in all higher plants,
especially those in which the sensory zone is separated from
the motory zone, as in the plumules of grasses; and the
shock-movements in Edwardsia, Cerianthus, Gonionemus,
various moUusks, fly larvae, earthworms, and Amphioxus.
Light may have a marked effect on the life processes of all
these organisms, but there is no evidence indicating that
the slight changes of intensity required to induce reactions
affect these processes. The organisms mentioned above
are interested, not in the light condition which causes the
reaction, but in that which ordinarily follows such a con-
dition if the position or the direction of movement is not
changed. Euglena for example may respond when the
intensity on the colorless anterior end is reduced by a
small fraction of a candle-meter. It cannot be of any
special importance to the organism to keep this colorless
end illuminated, but it is of the greatest importance to
keep the green portion back of it illuminated, for light is
necessary in the process of photosynthesis. Likewise the
slight increase of intensity necessary to cause an earthworm
to withdraw into its burrow, or to cause a negative Euglena
to give the avoiding reaction, cannot be injurious to either
of these organisms, for both thrive in light much stronger
than that required to produce these reactions. I have kept
earthworms continuously exposed to strong diffuse daylight
(150 ca. m.) in excellent condition for weeks, whereas at
night a candle-meter of light flashed on them Is often suffi-
cient to cause violent contraction. The light condition
which causes this response in earthworms is not Injurious,
but the Illumination that usually follows if they do not re-
spond may be. Then, too, there is another factor involved
here. Exposure ordinarily puts the worms at the mercy
of the birds which prey upon them. Thus the light may
be a sign of an enemy to the worms and in this regard they
belong in the following group, for this phase of the response

26o LIGHT AND THE BEHAVIOR OF ORGANISMS

is not a reaction to light at all, but a reaction to what light
represents.

The distinguishing characteristic which differentiates the
responses in the organisms in this and the following group
is however superficial. The fundamental principle involved
in the reactions of the organisms in both is the same, for
the reactions in the former as well as those in the latter are
responses to signs. To those in this group the stimulating
light condition is a sign of another condition of light either
more or less intense than the one to which they respond;
to those in the next group it is a sign of an object. And it
is this more or less intense light, or the object represented,
that is of vital importance to these organisms, not the con-
dition of light to which they respond.

c. Reactions due, not to any effect of light in itself, but
to what a given light condition or configuration may repre-
sent. — (a) Reactions caused by shadows or a sudden
decrease -in light intensity, representing either enemies or
food : Examples — the sudden contraction or movement of
tubicolous annelids, numerous echinoderms, mollusks and
arthropods, and the response of Clepsine, shrimps, prawns,
mosquito larvae and Fundulus. In case of Clepsine the
shadows undoubtedly represent food; in the rest, with the
probable exception of shrimps and prawns, it represents
enemies, (b) Reactions to sudden exposure to light or
increase of Intensity probably representing enemies, espe-
cially in case of earthworms : Examples — Arenicola and fly
larvae, earthworms and a few mollusks. (c) Reactions
caused by the size of the luminous area in connection with
intensity: Examples — Butterflies (Vanessa), Water scor-
pion (Ranatra) and frogs (Rana and Acris). In some of
these organisms the positive reaction to a large area in
preference to a small one of the same intensity prevents
flight toward the sun, and it probably has something to do
with mating, (d) Reactions caused by size, form, varia-
tion In shadow and color of luminous area: Example —
The higher animals, especially man. In these organisms

REACTIONS TO LIGHT 261

the reactions are associated with objects which have some
vital relation to their existence as food or a source of dan-
ger. In the higher animals, man in particular, pleasure
and other emotions enter in as factors in the response, and
the objects represented by the condition of light which
causes the response may consequently have additional signi-
fications. I do not however wish to be understood as advo-
cating the exclusion of such factors in the reactions of lower
organisms, for, while they have not been demonstrated in
these organisms, they may exist for all that is known to the
contrary.

In the organisms in Group a all the tissue appears to be
equally sensitive to light. In many of those in Group b,
Euglena, Stentor and Planaria, for example, some parts of
the body are undoubtedly more sensitive than others, and
it may be that the sensitive tissue is confined to a small
area definitely located, as for instance in Euglena near the
eye-spot. This tissue serves to distinguish changes of inten-
sity, but owing to its positional relation to non-sensitive
tissue (see Fig. 11) which intercepts the light from one side,
and the movement of the organism, especially the rotation
on the long axis, it serves also to locate the direction from
which the strongest light comes. Thus we have in these
organisms structures which may be termed direction eyes.
In Planaria and Arenicola larvae the highly sensitive tissue
is nearly surrounded by opaque tissue which admits light
only from one side (see Fig. 26), and consequently serves to
locate more accurately the direction from which the light
comes. In these organisms the lateral head movements
are also of importance in locating the direction of the light.
While the photosensitive tissue may be confined to limited
regions in some of these forms, we are certain that it is not
in others. Planaria and earthworms e.g. are known to
respond after the more highly sensitive anterior end has
been removed. Histological investigations in the latter
seem to indicate that the photosensitive elements are fairly
well distributed over the entire body surface.

262 LIGHT AND THE BEHAVIOR OF ORGANISMS

Most of the organisms in Group c have image-forming
eyes. The simplest of these appear to serve merely to distin-
guish between difference in size and location of illuminated
areas, while the more complex serve to distinguish form,
distance, and color as well. In many of these organisms
the tissue which is sensitive to light is not confined to the
eyes. Many of the fishes and amphibia respond to light
alter the eyes, including the retina, have been removed, and
there are also a number of blind species which respond to
light.

Nearly all of the reactions classified above are probably
responses to changes of intensity. They are no doubt asso-
ciated with chemical changes caused by changes of light
intensity, and affected by other chemical changes dependent
upon the effect of continued illumination, the absolute
intensity, and the time of exposure. We shall refer to this
matter again in the final chapter.

7. Evolution of Reactions to Light

It is not my purpose to discuss the problem of the evolu-
tion of the reactions to light. Nothing of importance could
at present be added, in such a discussion, to the ideas of
Jennings on this question expressed in his treatment of the
development of behavior (1906, pp. 314-327). I shall there-
fore merely suggest, without argument, the order in which
the reactions to light seem to have appeared.

The most primitive responses to light are probably due
to the effect of continued illumination on synthetic and
growth processes in green plants. Responses of this nature
we may assume to have been the basis for the origin of all
others, which probably appeared somewhat in the order
following:

(i) Change in the rate of locomotion dependent upon
the absolute amount of light energy received. No orienta-
tion, but probably aggregation at the optimum intensity —
Bacteria.

REACTIONS TO LIGHT 263

(2) Contraction of naked protoplasm due to sudden
changes In the Hght Intensity, i.e., changes In the amount
of hght energy received, resulting In orientation in some
instances — Amoeba.

(3) Fixed responses (avoiding reactions) caused by sud-
den changes of intensity, the nature of the response depend-
ent upon the structure of the organism. No orientation,
but aggregation at the optimum — Bacteria.

(4) Reactions similar to those under (3), but more defi-
nitely circumscribed by the structure of the body, especially
the localization of tissue sensitive to light. Definite orien-
tation and movement directly toward the optimum — Sten-
tor, etc.

(5) Reactions to a sign. The change in illumination
which causes the response is of no consequence to the organ-
nism, but the illumination which would follow If it did not
respond may be — Euglena, Volvox, etc.

(6) Reactions to a sign. The change of intensity
(shadow) which causes the response represents objects
which may be beneficial or injurious, food or enemies —
Clepsine Hydroides, etc.

(7) Reactions to a sign. The light condition or con-
figuration which causes the response represents objects, not
by means of shadows cast by them, but by means of the
light reflected from them expressing size, form or color —
Animals with image-forming eyes.

CHAPTER XIII

FACTORS INVOLVED IN REGULATING REACTIONS TO LIGHT
— VARIABILITY AND MODIFIABILITY IN BEHAVIOR

Everyone who has ever attempted observations on the
behavior of organisms with precise methods, knows that
variability even in the lower forms under constant external
conditions is one of the striking characteristics in reactions.
There are internal as well as external factors involved in
determining what the organism is to do. Just what these
are and how they influence reactions is a question of pri-
mary importance.

Many organisms turn or move toward a source of light
under certain conditions and away from it under other
conditions. They may be either positive or negative; that
is, the sense of orientation and response, in general, may be
reversed. Nearly all organisms turn through i8o° when
the sense of orientation changes so that they always move
with the same end ahead. There are however some excep-
tions. Radl (1903, p. 91) claims that Daphnia may swim
about in various directions with the back constantly facing
the source of light. Bohn (1905, p. 8) found that young
European lobster larvae always swim with the posterior end
directed toward the source of light, so that when they are
positive this end is ahead, and when negative it is behind.
Hadley (1908, p. 260) observed the same in the larvae of
the American lobster, as did also Lyon (1906) in several
larval stages of Palaemon. I observed similar methods of
locomotion in the larvae of several other decapod Crustacea.
In many of the lower forms orientation results from re-
sponses to changes in light Intensity. When these forms are
negative they respond only to an Increase of. intensity, and
when they are positive only to a decrease. It will be our

264

REGULATION OF REACTIONS 265

primary aim in this chapter to consider the factors involved
in producing these changes.

I. Change in Sense of Reactions

a. Effect of intensity of light. — Famintzin (1867, p. 20)
appears to have been the first to observe and record,
although not very definitely, that the sense of reaction in
organisms depends upon the intensity of the light. He
placed a shallow dish containing Chlamydomonas and Eu-
glena in diffuse daylight and a similar one in direct sunlight,
and covered about three-fourths of each at the room side
with an opaque screen. In the diffuse light the organisms
collected at the window side of the dish; in the direct sun-
light they collected in the shadow of the screen at the
opposite side. In 1872 Miiller recorded the observation
that seedlings of Lapidium, w^hich bend toward the source
of light if it is moderately strong, turn and bend in the oppo-
site direction if it is very intense, e.g. direct sunlight.
Strasburger (1878, p. 572) was perhaps the first to actually
see motile organisms turn about when the light intensity
was changed and swim in the opposite direction. He found
that various swarm-spores, which were strongly negative in
a given light intensity, became positive when the microscope
was moved farther from the window, but that they turned
about and swam in the opposite direction when the micro-
scope was again brought to the window. " Falls die
Schwarmer nicht zu grosse Neigung haben sich niederzu-
setzen, kann dies Spiel beliebig wiederholt werden."

Reactions similar to those mentioned above were seen
by Stahl (1880, p. 412) in Vaucheria, by Berthold (1882)
in marine algae, by Verworn (1889, p. 50) in diatoms, by
Wiesner (1880, p. 38) in tendrils of Ampelopsis and Vitis,
by Oltmanns (1897, p. i) in Volvox, Phycomyces and vari-
ous seedlings, by Lubbock (1884) and Ostwald (1907) in
Daphnia, by Wilson (1891) in Hydra, by Frandsen (1901)
in Limax, by Radl (1901, p. 83) in Simocephalus sima, by

2 66 LIGHT AND THE BEHAVIOR OF ORGANISMS

Adams (1903) In Allolobophora foetida, by Parker (1902,
p. 119) in Labidocera, by Yerkes (1902) in Gonionemus, by
Groom and Loeb (1890, p. 169) in nauplii of Balanus, by
Loeb (1905, p. 276) in Polygordius larvae,^ by Hadley (1908)

1 In an address published after this part of the manuscript was finished
Loeb says (1909, p. 34) that if organisms are positive in a given Kght inten-
sity they are positive in every intensity to which they respond at all, and
that forced suggestions in connection with the theory of natural selection
are responsible for the idea that they aggregate in the intensity of light
best suited for their general welfare. ''Man hat nun auch versucht, zu
zeigen, dass die Organismen eine 'Lichtstimmung' besitzen und ihren
Hehotropismus so regulieren, dass sie stets in diejenige Lichtintensitat
kommen, welche fiir ihr Gedeihen am besten geeignet ist. Ich glaube, dass
es sich hier ebenfalls um eine den Forschern durch die extreme Zuchtwahl-
theorie aufgezwungene Suggestion handelt. Ich habe an einer grossen Zahl
von Organismen Versuche angestellt, aber ich habe bei klarer Anordnung
der physikalischen Versuchsbedingungen auch niemals eine einzige Erschei-
nung gefunden, welche fiir eine derartige Anpassung spricht. Es hat sich
stets herausgestellt, dass positiv heliotropische Tiere gegen Licht jeder
Intensitat, sobald nur die Reizschwelle iiberstiegen wird, positiv helio-
tropisch sind. . . , Eine 'Auswahl ' einer passenden Beleuchtungsintensitat
habe ich nie beobachtet."

I am unable to understand how anyone can accept the statements quoted
above in the face of the numerous records to the contrary; nor can I reconcile
these statements with those of Loeb in earlier publications. He says (1905,
p. 272), "Groom and I described some observations at Naples on the be-
havior of the naupHi of Balanus perforatus, and certain other marine animals,
which were at times negatively heliotropic, and at other times positively
hehotropic. We found that the intensity of the light determines the sense
of hehotropism in these animals. Above a certain intensity light makes
these animals negatively heliotropic, and this the more quickly the greater
the intensity of the Hght. By lamphght the animals were always positively
heliotropic." (p. 276), "The heliotropism of Polygordius larvae can also
be influenced by Hght. This influence consists chiefly in the fact that direct
sunlight makes positively heliotropic animals negative. I did not succeed
in making negatively heliotropic larvae positive by exposing them to weak
light." The idea that these larvae do not become positive in weak Hght
is however not supported by Loeb's observations as recorded in the same
paper a few paragraphs farther on. Referring to Polygordius larvae which
had become negative in direct sunlight he says (p. 277), "When later I
carried the animals back into the north room and kept the temperature
constant at i5°-i6° C, they again became positively hehotropic in the course

REGULATION OF REACTIONS 267

in lobster larvae, and by various investigators in a number
of other organisms.

It will thus be seen that reversal in the sense of orienta-
tion caused by the effect of light is widely distributed among
living organisms. Is this effect of light due to stimulation
caused by the process of changing the intensity, as in case
of orientation in Euglena, for example, or the contraction
of Hydroides ? Or is it due to continuous illumination, as
in case of the activity of many organisms ? In other words,
is it due to the time rate of change, or to constant intensity ?
I have frequently observed that Chlamydomonas, Euglena,
Volvox, and other similar forms do not become negative at
once if the light intensity is suddenly increased above the
optimum. These organisms must be exposed to the higher
intensity for some little time before the sense of reaction is

of twenty minutes "; and on the following page he says, "I again took some
animals which had become positively hehotropic in the north room, and
convinced myself first of all that at a constant temperature of 20° C. they
would become negatively heliotropic in direct sunlight in a few minutes.
I then returned them to the north room, and here the animals again be-
came positively heliotropic at the same temperature in the course of fifteen
minutes."

It would be difficult to state in more exphcit terms that the nauplii of
Balanus and the larvae of Polygordius are negative in strong fight and
positive in weak than Loeb has done in the passages quoted from his pub-
lication of 1905, and it would be equally difficult to state more explicitly
that they are not negative in strong and positive in weak light than he has
stated in his address of 1909.

If these organisms are negative in strong and positive in weak light, as
Loeb's experiments indicate, it is evident that their reactions tend to keep
them in light of moderate intensity, an idea quite in harmony with those
Loeb rejects in his recent address. As a matter of fact it is not at all diflicult
to find Chlamydomonas, Euglena, Volvox, or any other similar organisms
in such a state that they are neutral in a given light intensity, positive in a
lower intensity, and negative in a higher. I have repeatedly observed this in
all of these forms as well as in several others; and in case of Volvox I have
many times observed, as stated elsewhere, that the colonies collect in great
numbers in the open spaces between pond-lily leaves and other water plants
on dark, cloudy days, but that they collect in shaded places when the sun
is bright.

268

LIGHT AND THE BEHAVIOR OF ORGANISMS

reversed. This is clearly shown in the following observa-
tions on Volvox, graphically represented in Fig. 33. By
referring to path A it will be seen that the colony introduced
at n was positive to light from the three glowers as well as

a

0*0(7

Fig. 33- The lines A and B represent the courses taken by single Volvox colo-
nies as seen in water 2 cm. deep in a plate-glass aquarium e. (The paths are rep-
resented in approximately accurate proportions); g, a group of three 222-volt
Nernst glowers in a vertical position; a, carbon arc; /, direction of light rays; d,
opaque screens; fin', path with glowers exposed and arc shaded; cc', path with arc
exposed and glower shaded; c'n, path with both glowers and arc exposed.

to that from the arc, but that it became negative after
swimming toward the arc for a short distance from c, turned
about and moved across the aquarium to c'. That is, at
the end of the experiment the colony was negative to a
much lower light intensity than at the beginning. The
arc was approximately 250 candle power. It was 15 cm.

REGULATION OF REACTIONS 269

from the point where the organism became negative. The
light intensity at this point was therefore 1 1,1 11 ± ca. m.
But the colony was still negative after having crossed the
aquarium, a distance of nearly 8 cm., or nearly 23 cm.
from the arc, i.e., in an intensity of 4726 ± ca. m., \vhich
is 6385 ± ca. m. less than the intensity in which it first be-
came negative. Similar results are represented in path 5,
but unfortunately the distances between the sources of light
and the aquarium, in this exposure, were not recorded.

The colony which produced path B was positive to the
light from the arc when first put into the aquarium at c,
but after moving toward the source of light a few centi-
meters it became negative, turned about and moved in the
opposite direction. When it reached c' the glowers were
exposed and the colony promptly changed its direction of
motion and proceeded on a course directed from a point
between the two sources of light. This point, however, was
much nearer the arc than the glowers, the light from the
former being much more intense than that from the latter.
When the light from the arc was cut off at w, the colony
was found to be negative to the comparatively weak light
from the glowers. It consequently changed its course and
moved from this source; but after continuing about 3 cm.
it became positive, turned about and moved toward the
glowers to n' , and probably would have continued farther
had it not been prevented from doing so by the wall of the
aquarium. It will be noticed that the point 11' , where the
colony was still positive at the end of its course, was about
3 cm. nearer the glowers than n, where it proved to be nega-
tive, and nearly 7 cm. nearer than the point where it
changed its course from negative to positive. That is, the
organism was positive at n' in a much higher light intensity
than that in which it was negative at n and at the point
where it changed from negative to positive.

This shows that there were very striking changes in the
optimum in these colonies. It also shows that the reversal
in sense of reaction was not due to an effect produced by

270 LIGHT AND THE BEHAVIOR OF ORGANISMS

the processes of changing the intensity; for if it had been
the colony on path A would have turned from the source
of light at c in place of toward it, and then from it after
having been exposed to the high intensity for some little
time. The fact that the colony moved toward the arc
light some little distance after turning at c, and that it was
negative in a much lower light intensity a little later, shows
clearly that there is some time required to bring about the
changes in the organism w^hich determine whether it shall
be negative or positive. Reversal in the sense of reaction
is not merely dependent upon the intensity but also upon
the time of exposure. It is probably a function of the
product of intensity and time. It is therefore evident that
the change in sense of orientation in these lower forms is due
to continued illumination, while orientation is due to change
in the intensity of illumination. If all reactions are regulated
by chemical changes, there must be at least two different
sets of chemicals involved, one which is influenced by
changes of intensity, another by constant intensity. I have
discussed the possible nature of the chemical changes asso-
ciated with reversal in reactions in a former paper (1907,
pp. 1 57-161), and shall refer to it in this volume under
theoretic considerations. Chapter XX.

Reversal in the sense of reaction is of the greatest impor-
tance to the well-being of organisms, for, as showm in the
preceding chapters, it tends to keep them in the optimum
illumination. This is true whether the change of intensity
causes reversal in orientation or merely a change in the
avoiding reactions, shock-movements, for both of these
methods tend to produce aggregations at the optimum. A
change in light intensity does not however induce reversal
in all organisms which respond to light. I was unable to
obtain positive reactions in Stentor coeruleus, Amoeba,
and fly larvae; and the same is true for many of the pla-
narians, including the land planarian Bipalium kewense,
and some other worms, especially in certain stages of their
development.

REGULATION OF REACTIONS 271

There are also many organisms which never become nega-
tive in their responses. This is true of the great majority
of higher plants and various animals. I exposed the ento-
mostracan Scapholeberis armata, and Caprella, Leptoplana
tremellaris and the early stages of Eudendrium, Arenicola,
Limulus, and many other forms in light of 15,000 ± ca. m.
intensity, and found that they remained positive although
many were soon injured by the intense light. Carpenter
(1908) was unable to make Drosophila negative to light,
although he exposed specimens in over 480,000 ca. m., an
intensity which produced convulsive reflexes and was un-
doubtedly injurious. Those organisms mentioned above
which do not become positive thrive in darkness. There
is, as stated in the preceding chapters, consequently no need
for a positive reaction to light. Those which do not become
negative thrive in strong light. Under natural environ-
mental conditions they rarely meet with intensities so high
as to be injurious. In these animals there is, then, no need
for negative reactions.

Holmes says (1901, p. 233), " Talorchestia longicornis is
strongly and permanently positive both in weak and strong
light." These animals however come to rest in shaded
spots and are usually found under drifts of seaweeds.
Orchestia agilis, which is found in similar places, is negative
when first exposed, but it soon becomes positive, '* the more
quickly the stronger the light." After it is positive it
" remains so even in the strongest light, but it may be
rendered temporarily negative to exposure to light of lower
intensity." Similarly Holmes (1905) found that Ranatras
are negative when first taken from darkness and later posi-
tive, after which they remain positive as long as they are
in the light, no matter how intense the light may be or how
long they may be exposed. In general, exposure to light
tends to make them positive, whereas darkness tends to
quiet them and make them negative. It should be empha-
sized here that not only the intensity but also the time of
exposure has to do with these reactions. Holmes says that

272 LIGHT AND THE BEHAVIOR OF ORGANISMS

after seventeen Ranatras, negative in a given light inten-
sity, had been exposed for an hour and forty minutes, all
became positive.

The positive reactions in all of the animals just referred
to prove fatal under certain conditions, and Loeb (1905, pp.
42, 74) claims that the caterpillar of the willow borer and
the mud-inhabiting crustacean Cuma Rathkii are positive,
although in their natural environment they are never ex-
posed to light. Here, then, we have a number of reactions
which do not lead the organisms to their optimum, reactions
which under certain conditions are clearly not adaptive.
But, as already shown, these reactions are non-adaptive only
under artificial conditions. It was however reactions of this
character that led Loeb (1906, p. 159) to conclude "that
the tropisms could not have been acquired by the way of
natural selection," and to formulate an explanation of their
origin which we shall consider later, Chapter XX.

b. Effect of change in temperature. — It is well known
that temperature affects the activity of organisms. If it is
increased above normal, organisms ordinarily become more
active and more sensitive to other stimuli until the optimum
is reached, when their activity and their response to other
stimuli decrease, as they usually do when the temperature
is decreased below normal. Changes in temperature may
however have quite a different effect on some organisms.

Strasburger (1878, p. 606) found that haematococcus
swarm-spores, which were positive in a given light intensity
at 16° to 18° C, became negative when the temperature was
decreased to 4° C.,and more strongly positive when increased
to 35° C. He obtained similar though somewhat less striking
results with other swarm-spores. The degree of change of
temperature required to cause a reversal in reaction to light
was found to vary with the different organisms and with
the same organism under different conditions. Strasburger
says (p. 610), " SInd die photometrischen Schwarmer, mit
denen experimentirt werden soil, auf sehr hohe Lichtinten-
sitat gestimmt, so wird es, um sie auf den negativen Rand

REGULATION OF REACTIONS 273

des Tropfens heriiberzubringen, niederer Temperatur bediir-
fen, als wenn sie auf geringere Helligkeitsgrade gestimmt
waren. Im ersten Falle wirken Licht und Temperatur
sich so zu sagen entgegen, im letzteren so zu sagen
gleichsinnig."

The experimental results of Massart (1891, p. 164) on the
flagellate Chromulina confirm those of Strasburger in that
a decrease in temperature causes these organisms to become
negative to light. Loeb (1905, p. 274) however observed
that an increase in place of a decrease in temperature causes
a change in the sense of reaction. He says that Polygordius
larvae which were strongly negative in a given light inten-
sity at 11° became strongly positive when the temperature
was lowered to 6°, and negative again when it was raised;
that others which were positive at 24° became negative at
29°; and that still others positive at 17° became negative
at 24°. Loeb claims to have found similar reactions in
marine copepods. And Miss Torelle discovered a reversal
in reactions in the frog Rana clamata, but here it is again a
decrease of intensity that causes negative reactions. She
says (1903, p. 487), " A rise in the temperature to 30° C.
accelerates the rate of the positive response. A lowering
of the temperature to 10° C. produces movements away
from the light." Holmes (1905, p. 323) observed similar
reactions in Ranatra. He says, '' Raising the temperature
tends to accentuate the positive phototaxis in Ranatra and
lowering it tends to produce the negative reaction. In
several experiments two dishes containing Ranatras were
set before a window so as to receive the same amount of
light. As the specimens had been previously kept in the
dark, they showed a negative reaction. Into one dish warm
water was poured raising the temperature from about 20° C.
to nearly 30° C. In a few minutes the specimens in the
warmer dish became positive, the ones in the cool water
still showing a negative phototaxis. Ranatras transferred
to the cooler dish soon became negative, while those which
were picked up in the same way and dropped back into the

274 LIGHT AND THE BEHAVIOR OF ORGANISMS

warm water from which they were taken soon resumed their
positive reaction."

Change in temperature does not however cause reversal
in reactions to Hght in all organisms. Strasburger (p. 608)
discovered that while the swarm-spores of Ulothrix, Ulva
lactua, Chaetomorpha aerea, and Chytridium vorax respond
much like those of haematococcus, those of Scystosiphon
lomentarium, Chilomonas curvata, Botrydium and Bryop-
sis could not be induced to reverse by changing the tem-
perature; and the same is true for the copepod Labidocera,
and for Daphnia pulex and Cypris, according to the work
of Parker (1902, p. 117) and Yerkes (1900, p. 417). I have
observed the same in a number of organisms referred to
below. It must be admitted that the above statement with
reference to the copepods is somewhat too broad, since
Parker tested the reactions only in 10°, 30°, and 35°, and
Yerkes studied only the effect of increase in temperature
above the normal.

Original observations. — Observations on the effect of
changes in temperature on reactions to light in microscopic
forms were made by mounting them on a Pfeffer warming-
stage under a large cover-glass sealed and supported by
means of a ring of vaseline. The Pfeffer warming-stage
consists of a glass cell 1X6X8 cm. with three holes in the
ends, one for a thermometer, the other two for water inlet
and outlet. It is fastened on the stage of a compound
microscope and admits of observation under either low or
high power. The temperature is regulated by passing hot
or cold water through the openings, and recorded by means
of the thermometer. It was thus possible to subject the
organisms to gradual or sudden changes in temperature
ranging from a little above zero to nearly 100°.

May II, 1908, at 9 a.m. a few drops of solution were
taken from some collected the preceding day and mounted
on the warming-stage. The solution contained numerous
specimens of Euglena viridis, Euglena deses, Phacus trique-
ter, and a few specimens of Euglena spiragyra and Phacus

REGULATION OF REACTIONS 275

longlcaudus. A large majority of all of these species were
strongly negative at 22°, In light of 250 ca. m., when first
mounted, but after they had been exposed from two to
three minutes they became strongly positive without any
change In temperature or light Intensity. The temperature
was now gradually lowered, and as this proceeded the organ-
isms became less and less active. At about 12° nearly all of
them came to rest and the Euglenae contracted and became
nearly spherical as If about to encyst. Thus the organisms
lay motionless as the temperature decreased to 8° and
finally to 5°. But after having been In this low temperature
for nearly five minutes, they gradually became active again
and swam about, first In an apparently aimless fashion, but
later as definitely and rapidly from the source of light as
they had been swimming toward It at 22°. They thus
became negative In the low temperature without any change
In the Intensity of the light. Is this reversal In the sense
of reaction due to the effect of changing the temperature, or
is it due to the absolute difference in temperature? The
following has reference only to Euglena viridls, although
the reaction of the other species mentioned above is similar
to that in this form. After the Euglenae used in the ob-
servations referred to above had been subjected to 5° for
some minutes, the temperature was gradually raised and it
was found that they were still negative at 8°, but positive
at 12°. After having been at 12° for six minutes, the tem-
perature was again decreased, and now It was found that the
organisms remained active and positive at a temperature
even below 5°. They did not come to rest at 8° as they
had when first exposed to decrease in temperature. The
temperature was now allowed to rise gradually to about
22° in 250 ca. m. About half of the Euglenae collected on
the side toward the light and the rest on the opposite side.
When the slide was turned end for end, the two groups
immediately began to swim in opposite directions in two
columns which met and passed near the middle of the field,
the positive column above, near the cover-slip, the negative

276 LIGHT AND THE BEHAVIOR OF ORGANISMS

below, near the slide, owing no doubt to the fact that the
source of Hght was somewhat above the level of the stage.
About half of the Euglenae were now evidently negative and
the rest positive, but half an hour later nearly all were nega-
tive, although there had been no change in light intensity
or temperature. When the temperature was reduced they
became still more strongly negative. After keeping the
temperature between 5° and 8° for three minutes, it was
rapidly raised to 22°; the Euglenae were now very strongly
positive. They fairly streamed toward the source of light.
The temperature was now again reduced and held at 5° for
a few minutes, during which the organisms were negative.
It was then slowly raised, and many became positive at
12°, after which it was once more reduced and held at 5°
for five minutes, during which the organisms were negative,
and then again slowly increased. Many of the Euglenae
now became positive at 8°.

We have thus seen the same individuals within the
course of a few minutes in constant light intensity reverse
in the sense of reaction several times. We have seen them
come to rest as the temperature decreased and become active
again as it decreased still farther. W^e have seen them
change from a condition in which they were negative at
22° and positive at higher temperature to one in which
they were positive at 8° and negative at lower tempera-
ture. These observations were repeated many times under
different conditions with the same general results. Similar
changes in reactions to light were also repeatedly produced
by changes in temperature in different species of Chlamy-
domonas, Trachelomonas, Chlorogonium and Volvox.

These results show: (i) That a decrease in heat energy
tends to cause a change in the sense of reaction to light
from positive to negative and an increase tends to cause a
change from negative to positive. A decrease in heat
energy, therefore, produces the same changes in the re-
actions to light as an increase in light energy. (2) That
the reactions to light of a given intensity depend not only

REGULATION OF REACTIONS 277

Upon the absolute temperature at the time of the observa-
tion, but also upon the preceding temperature, the time rate
of change in temperature, and the time of exposure at a
given temperature. (3) That reversal in the sense of reaction
may take place without any change in temperature or light
intensity. Reversal in the sense of reaction, therefore,
seems to be due to the effect of constant temperature and
time of exposure rather than to the effect of change in
temperature.

In some of the forms mentioned above the changes are
very indefinite, but in Chlamydomonas alboviridis they
are even more pronounced and striking than in Euglena.
In studying Chlamydomonas it was also found that only
under certain conditions will changes in temperature
cause reversal in the sense of reaction to light. Thus, for
example, it could not be reversed in specimens which had
been in total darkness for twenty-four hours. These
specimens were negative at 22° in light having an inten-
sity as low as 150 ca. m., and they remained negative
when the temperature was raised until they died at a little
over 40°.

No change in the sense of reaction to light could be
induced by varying the temperature in either direction
between zero and the maximum in the following forms:
Stentor, various species of zoeae, Scapholeberis armata,
Daphnia, Cyclops, Cypris and a small water spider.
Change in temperature, however, has certain effects on
the reactions in Daphnia, Cyclops, Cypris and the water
spider which are similar to those observed in Euglena.
On May 16 several specimens of each of these species were
exposed in light having an intensity of 160 ca. m. They
were neutral at room temperature (22°), and swam about
slowly without orienting or aggregating. When the tem-
perature was decreased they gradually became more and
more quiet, and finally sank to the bottom motionless, but
when the temperature reached 8° they became active
again, and soon collected at the side of the dish nearest the

278 LIGHT AND THE BEHAVIOR OF ORGANISMS

source of light. When the temperature was decreased
still more they became strongly positive. This experi-
ment was repeated several times with similar results.
Increase in temperature above normal ordinarily causes
these organisms to become more strongly positive until a
maximum is reached, when the movements become irregu-
lar and reaction to light ceases. In no instance was it
found that any of these organisms became negative owing
to changes in temperature. The interesting point in these
observations is the fact that they become quiet as the
temperature decreases and then active again when it
decreases still further, just as in case of Euglena, but the
former' become only more strongly positive, whereas the
latter change from positive to negative.

Not all entomostraca can be made positive by decreasing
the temperature. On June i, Alona gracilis was found in
great abundance in a Paramecium culture jar. A few speci-
mens of Cypris were also found in the same jar. They were
strongly negative at room temperature (25°) in light of
250 ca. m. The temperature was lowered to freezing, but
the organisms were continuously negative whenever they
responded at all.

It is thus evident that in some organisms a decrease in
temperature causes negative responses to light, whereas
in others it causes positive responses. How this is brought
about is very difficult to see from a physico-chemical point
of view, although there are chemical compounds in which
decrease in temperature facilitates reactions caused by
light, as shown in Part IV of this volume. The fact that
the organisms become quiet as the temperature decreases,
and then active again as it decreases still more, is particu-
larly puzzling. In some organisms the change in the sense
of reaction caused by change in temperature is clearly
adaptive, and from this point of view we get some light
on the causes of the change in reaction, but of course only
a superficial explanation for adaptation is itself a problem.
In Euglena and frogs, for instance, the negative reaction

REGULATION OF REACTIONS 279

to light In low temperature takes them from the surface
and prevents their freezing, while in Polygordius larvae
and those entomostraca which become negative when the
temperature increases the reactions take them out of the
warm surface water. I am however not positive that
the surface water becomes warm enough to injure these
creatures. If it does not, then those reactions are appa-
rently not adaptive.

c. Effect of chemicals. — That the reactions to light in «
some organisms are closely associated with the chemical
constituents of the environment was clearly shown by the
experiments of Englemann referred to elsewhere. In these
experiments, Englemann found that the green ciliates,
Paramecium bursaria and Stentor viridis, respond to light
only when the oxygen pressure is below normal, but he did
not note any actual reversal in reaction due to changes in
the chemical condition of the medium. Loeb (1904, p. 2)
however states that Gammarus pulex, which is " naturally
negatively heliotropic " can be made positive by adding
small quantities of any of the following substances to the
water: carbon dioxid, hydrochloric, oxalic or acetic acid,
various narcotics, " such as ether, chloroform, paralde-
hyde, alcohol or esters" and ''all the ammonium salts,
ammonium hydrate included." The alkalis, excepting
NH4OH, urea, oxygen and hydrogen, on the other hand,
only excite these creatures; they do not cause reversal in
the reaction. Similar results were obtained in experiments
on Cyclops and Daphnia. The former however can also
be made negative, if it is in the positive state, by the addi-
tion of NaOH. "Attempts to make sea-water Gammarus
positively heliotropic by CO2 have failed." Holmes (1901)
observed that the amphipod Jassa becomes positive when
placed into foul sea water.

The fact that chemicals so very different in their general
properties as acids, alkalis and narcotics may have the same
effect on the sense of the reactions of these organisms seems
to show that the effect of the different chemicals is not

28o LIGHT AND THE BEHAVIOR OF ORGANISMS

specific. The chemicals appear to produce changes in the
general state of the organism as a whole or a unit. This
idea is strongly supported by the observations on Arenicola
larvae to be presented later, and by the work of Holmes
(1905, p. 317) on Ranatra. He found that any condition
which causes an increase in activity accentuates the posi-
tive reactions to light, while any condition which quiets
the organisms tends to make them negative. " The causes
that produce the negative reaction are, as a rule, those
which lead to diminished activity and excitement. Cold,
exposure to darkness, and the quieting effect of contact
stimuli lead to a condition of lessened excitability and,
perhaps as a result of this, to a negative reaction to light."
The same is probably true of many other insects. When
a moth becomes quiet it is likely to crawl into dark crevices,
but when it is disturbed it flies toward the light, and the
more it is stimulated the more energetically positive it
becomes. The pomace fly Drosophila is often found in
dark cavities in decaying fruit. If it is disturbed it im-
mediately flies out and escapes. Carpenter showed that
the stronger it is stimulated the more strongly positive it
becomes. Many similar instances could be cited. The
strong positive reactions to light in these forms may lead
them into fatal surroundings, but ordinarily they are of
the greatest importance, for they guide them to places of
safety.

Original observations. — On May 29, 1908, a solution
containing numerous specimens of Daphnia, Cypris, Cy-
clops, a small w^ater spider about 0.5 mm. in diameter, and
various insect larvae, all taken from a shallow pond the
preceding day, were exposed in light of 200 ± ca. m. Some
of the individuals of the different species were negative
but most of them were neutral. In these there was no
apparent response to light. They remained equally scat-
tered throughout the aquarium and swam slowly about.
Pure CO2 was now allowed to bubble through the water
very slowly. Nearly all of the organisms except the water

REGULATION OF REACTIONS 281

spiders soon became more active and began to swim
toward the light side of the aquarium, where in the course
of a very few moments they formed a dense aggregation.
Those which had been negative as w^ell as those which were
neutral had become positive. They remained at the more
highly illuminated side of the aquarium only four to five
minutes, then gradually scattered about again. A little
more CO2 was then added to the water and the organisms
became positive again. This process was repeated several
times. When air was forced through the water they
scattered almost immediately, and became indifferent to
light, or sometimes slightly negative. It is consequently
not the agitation which makes them positive when CO2 is
allowed to bubble through the water. These results seem
to indicate that the change in reaction to light is dependent
upon the change in amount of CO2 as well as upon the
absolute amount.

In these experiments the water spiders became quiet
when the CO2 was added and sank to the bottom, but in
some later experiments they also became strongly positive.
In Stentor, Chlamydomonas, Vol vox and Scapholeberis no
change in sense of reaction could be induced by means of
adding CO2. They become quiet and sink to the bottom
after the CO2 reaches a certain concentration. I was
unable to change the sense of reaction to light in several
different zoeae and in the larvae of Hydroides dianthus
by means of hydrochloric-acid solutions. The hydroides
larvae were exposed in sea-water solutions of HCl varying
in strength from n/250, in which they were immediately
killed, to w/7250, in which their response was normal in
every respect, both in light intensities so low that they were
positive and so high that they were negative.

In Arenicola larvae however the sense of reaction to
light can be reversed by means of various chemical solu-
tions. These larvae are strongly positive during the first
few days, even in very intense light. They swim freely
through the water near the surface. Later they settle to

282 LIGHT AND THE BEHAVIOR OF ORGANISMS

the bottom and become slightly negative to light. On
August 6, 1909, a considerable number of larvae, a few
hours after they had emerged from the egg-strings, were
put into each of several small glass aquaria containing sea
water. The larvae were strongly positive in the light
' intensity in which they were exposed. To one of the
aquaria distilled water was added drop by drop, until the
larvae no longer responded to light ; to another concentrated
sea water was added, and to each of the others a weak
solution of one of the following chemical compounds:
chloroform, adrenalin, atropin, carbonic acid, hydro-
chloric acid, acetic acid, magnesium sulfate, magnesium
chlorid, ammonia and sodium hydrate.

After the larvae became neutral in each solution, they
were left undisturbed. If they became positive in the
course of a few minutes, as frequently happened, the solu-
tion was made stronger until they became neutral again.
In the solutions containing carbon dioxid, caffeine, dis-
tilled water or concentrated sea water, the larvae became
negative in the course of a few minutes and collected at the
side of the aquaria farthest from the source of light, but in
no instance was the negative reaction as marked and
precise as the positive had been. There was no very
definite orientation in the negative specimens, no such
streaming from the source of light as there is toward it
under normal conditions. Many of the larvae in each
solution settled to the bottom of the aquaria and remained
there, having apparently lost all power to respond to light.
The larvae in the solution containing magnesium sulfate,
magnesium chlorid, hydrochloric acid, acetic acid, ammonia,
sodmm hydrate or atropin, also became negative, but it
required a much longer time. In some of these solutions
the larvae did not become negative until several hours
after the compounds had been added.

After becoming negative the larvae usually remain so
permanently, or at least for several hours. For example,
those which became negative in the afternoon of August 6

REGULATION OF REACTIONS 283

were still negative the following morning. Many of those
in diluted sea water and in sea water containing CO2 were,
however, positive after being in over night. In these
aquaria there were two collections, one at the end toward
the light and one at the opposite end. In those solutions
containing ammonia, sodium hydrate or magnesium sul-
fate the aggregation at the negative side of the aquarium
was much more pronounced the following morning than it
had been the preceding evening. Larvae taken from any
of these solutions and put into normal sea water became
positive in the same light intensity almost immediately in
every instance. I did not succeed in producing reversal
in reactions with chloroform or adrenalin, nor did I suc-
ceed by changing the temperature. The experiments under
these conditions were, however, not very extensive.

It is at once evident that there is a striking difference
between the reversal in reaction in such forms as Chlamydo-
monas and iVrenicola larvae. In the former the change is
comparatively sudden, sharp and definite, and the nega-
tive orientation is as accurate and precise as the positive
orientation. In the latter the change is comparatively
slow and indefinite, and negative orientation is much less
precise than positive orientation. In Arenicola larvae it
appears that any condition which acts as a depressant
tends to cause the young positive larvae to become nega-
tive. These larvae become negative under normal condi-
tions as they grow older. Depressants apparently hasten
the appearance of this state, and under their influence
larvae become negative earlier than they otherwise would.

d. Effect of concentration of the medium and mechani-
cal stimuli. — We have already stated the fact that Areni-
cola larvae become negative both in concentrated and in
diluted sea water. Loeb (1893, pp. 94, 96) was able to
make negative Polygordius larvae positive by adding i to
1 .3 per cent. NaCl to the sea water and positive individuals
negative by diluting the sea water with 40 to 60 per cent,
fresh water. Similar results were obtained with cope-

284 LIGHT AND THE BEHAVIOR OF ORGANISMS

pods. Minklewicz (1907, p. 50) says that Lineus ruber,
which is ordinarily negative in the regions of the spectrum
toward the violet end and positive in those toward the red
end, becomes positive in the former and negative in the
latter if subjected to a solution consisting of 25 to 80 c.c.
of distilled water and 100 c.c, of sea water, but that the
reactions to white light remain negative. There may then
be, according to Minkiewicz, a reversal in the sense of reac-
tion to light of given wave lengths without a reversal in the
sense of reaction to white light.

In working on the light reactions of Temora longicor-
nis, a copepod, Loeb (1905, p. 282) noticed that the animals,
ordinarily negative, were frequently positive immediately
after being caught. This change in the sense of reaction
was due probably to mechanical agitation. Miss Towle
(1906, p. 345) obtained similar results. She found that
the light reaction of Cypridopsis could be temporarily
changed from negative to positive by taking the animals
up in a pipette or by making them pass through a maze
constructed with needles, but that they could not be
changed in the opposite direction. In certain organisms
however precisely the opposite change takes place. Holmes
(1905, p. 319) observed that Ranatra becomes negative if
it is handled under water or taken from the water and
dropped in again. He also (1901) thinks that the fact
that Orchestia gracilis is positive in air and negative in
water may be due to the contact stimulus of the water.
It is of interest to note that while these animals are per-
manently negative in sea water they become positive in
fresh water shortly before they die. The copepod Labi-
docera, which is ordinarily positive to light, can, according
to Parker (1902, p. 117), be made temporarily negative by
vigorously ejecting it from a pipette into sea water several
times.

e. Effect of internal changes. — There are many organ-
isms which respond to light in one way during part of their
existence and in a different way, or perhaps not at all,

REGULATION OF REACTIONS 285

during another. Thus we find the plumules of many of the
grasses (gramineae) very sensitive to light during the early
stages of development and not at all later. Fly larvae are
strongly negative, but the imagos are positive. Loeb
(1906, p. 134) found that the nauplii of Balanus are positive
when they leave the egg, but that they become negative
soon afterward. I have observed similar changes in reac-
tions in the larvae of Arenicola, Limulus, and Hydroides
dianthus, in various larvae of crabs and in the medusae of
Bougainvillea. There is a striking peculiarity connected
with the change in the sense or reaction of Limulus larvae.
When these animals proceed toward the source of light
they always swim, but when they proceed from it they
always crawl. They usually swim most of the time
when they are young and are positive, but when they get
older they nearly always crawl on the bottom and are
negative. If specimens which are crawling from the
source of light are agitated until they swim they proceed
toward the light, but as soon as they touch the bottom
and begin to crawl they go away from the light. I have
repeatedly seen specimens in a glass dish swim toward the
source of light against the side of the dish, sink to the bot-
tom and crawl from the light several centimeters, then
start up again and swim toward the light, and so on,
repeating the process many times. Contact seems to
have something to do with the sense of reaction here, but
the fundamental causes of the changes are no doubt rooted
in the developmental changes in the organism associated
with its habits. In nearly all of the species mentioned
above the changes in the sense of the reactions are un-
doubtedly adaptive. When the larvae first leave the egg
they are strongly positive, and swim out in various direc-
tions from the site of their birth so as to become widely
scattered. Later, when the developmental processes pre-
pare them for sedentary life, they become negative and
consequently go to the bottom, where they become at-
tached or burrow in the mud. It is not probable that the

286 LIGHT AND THE BEHAVIOR OF ORGANISMS

different conditions of light are of vital importance to these
creatures. The reactions are in reality responses not to
light, but to what light represents. They appear to have
learned to use light as a guide in directing their course in
accord with the demands of their state of development
and general habits.

Hadley (1908) made a very thorough study of the
changes in the photic reactions of lobster larvae. He
found that they are positive for about two days after
hatching, after which they become negative and remain
so until shortly before molting, when they again become
positive. Both the early second-stage, and the third-
stage larvae are negative but as in the first stage they
become positive before molting. The fifth and later stages
are persistently negative.

It has long been known that changes in light cause
daily periodic movements in plants, the so-called sleep
movements of leaves and flowers, and that these move-
ments continue for some time if the plant is kept in con-
tinuous illumination. Pfeffer (1906, p. 108) says, ** The
periodic movements are at first pronounced, both in con-
stant light and in darkness, in the case of the leaves
of Acacia lophantha, Mimosa pudica, Impatiens noli-me-
tangere, and Sigesbeckia orientalis, and they continue
to be perceptible until after the lapse of four to eight
days."

Similar after effects have been noted in certain animals.
Mitsukuri (1901) observed that the mollusk Littorina is
negative when under water during high tide and positive
when it is exposed to the air at low tide. Bohn (1905 and
1907) made similar observations on Littorina, Hedista
diversicolor, and Actinia equina, and claims for them
that these periodic changes in the sense of reactions to
light continue in harmony with the tide for some days in
specimens confined in aquaria where they are not directly
affected by the tides. I was unable to confirm the results
recorded by Bohn in observations on Littorina littorea at

REGULATION OF REACTIONS 287

Woods Hole. Nor was I able to confirm them in obser-
vations on several related species at the Tortugas.

We have thus presented various instances in which an
organism is positive under given external conditions at
one time and negative under precisely the same condi-
tions at another time. In some cases this change in
reaction requires a long time, in others only a few moments,
as e.g., in the reaction of Vol vox represented in Fig. 33. It
is evident that such changes must be regulated by inter-
nal factors, that they must be due to alterations within
the organism itself. As a matter of fact, all reactions are
directly controlled by internal factors which are in turn
influenced by external factors. The interesting point here
is however the fact that we may have movements and
change in movements without any immediate changes in
the environment. Many instances of this have been cited
by Jennings (1906), especially in Chapter XVI.

The facts (i) that the reactions may be affected in the
same way in a given organism by so many contrasting
conditions, including concentration and dilution of medium,
high and low temperature, acids, alkalis, narcotics and
salts; (2) that the same change in external conditions may
cause opposite reactions in different organisms, e.g., a rise
in temperature causes some to become negative and others
positive; and (3) that the sense of reaction may change
without any immediate external change, — indicate that
these responses are due not to a direct and specific effect
of the environment on some definite chemical compound
within the organism, but rather to the effect on the organ-
ism as a whole.

CHAPTER XIV

FACTORS INVOLVED IN REGULATING REACTIONS TO

LIGHT — VARIABILITY AND MODIFIABILITY IN

BEHAVIOR (continued)

I. Changes in Sensitiveness, in the Optimum, and in Vari-
ous Other Features Regarding Reactions

The sensitiveness and the optimum vary greatly in
different organisms and in the same organism under differ-
ent conditions. In some the optimum is nearly total
darkness, in others it is direct sunlight, 5000 ± ca. m.
Some are negative in extremely low intensities, others are
positive in equally low intensities. The flatworm Bipa-
lium kewense, e.g., avoids light so weak that it barely
affects the human eye, and responds to the slightest
changes in illumination; and the plants Lepidium sativum,
Amaranthus melancholicus ruber, Papaver paeoniflorum,
and Lunularia biennis bend toward the source of light
in an intensity as low as 0.00033 ca. m. (Figdor, 1893).
Some organisms are usually negative in direct sunlight,
and the intensity may be changed thousands of candle
meters without a response. Other organisms are positive
in equally high light intensity. What interests us here
chiefly is not the difference in response in different species,
but variability in response, and the changes in sensitive-
ness and in the optimum in given individuals and the
regulation of such changes.

Strasburger (1878) found that if swarm-spores are kept
in light of relatively high intensity their optimum is much
higher than if they are kept in weak illumination. These
organisms, then, adapt themselves in some way to the

288

REGULATION OF REACTIONS 289

environmental conditions. They tend to become attuned,
as Strasburger puts it, to the Hght intensity of their en-
vironment; they become accUmated. The sensitiveness and
the optimum, as well as the reactions in general, at any
given time, depend upon the preceding exposure of the or-
ganism. This is well illustrated by the behavior of Volvox
as observed by the writer.

On July 30, 1904, at 5 p.m., it was found that Volvox,
which had been collected at 6 a.m. and kept in the dark all
day, responded definitely to light of 0.16 ca. m. intensity,
and quite definitely to light of 0.14 ca. m. This is the
lowest intensity to which any response was obtained at
any time. Specimens collected shortly after 12 M., July 14
and 15 respectively, and tested as soon as brought into
the laboratory, responded to light of 0.50 to 0.83 ca. m.
The sky was clear on both of these days, but the organ-
isms were found among the water plants in more or less
shaded places.

It was found at different times that after being exposed
to direct sunlight a few moments the colonies did not
respond even to an intensity as high as 500 ca. m. We
have thus observed the threshold to vary from 0.14 to
500 ca. m., and this variation seems to have been due
largely to preceding exposure to light. The threshold is
higher in colonies pre\dously exposed to strong light than
in those exposed to weak light.

The optimum light intensity for practically all Volvox
colonies is somewhat lower than that of direct sunlight,
5000 ± ca. m., but sometimes it is very much lower; it
varies greatly. This variation is clearly shown in the fol-
lowing observation :

After a few very cloudy days the sun came out at il a.m.,
July 24, 1904, and the sky became exceptionally clear and
remained so the remainder of the day. At 2 p.m. Volvox
colonies were found in abundance freely exposed to the
sunlight. Some of the colonies were collected and taken
to the laboratory, where it was accidentally discovered that

290 LIGHT AND THE BEHAVIOR OF ORGANISMS

they were negative in light intensities in which this organ-
ism had formerly always been found to be strongly posi-
tive. I then tested the colonies for the optimum and was
greatly surprised to find that they were negative to all
light intensities above 0.57 ca. m. In light from 0.57 to
0.29 ca. m., the lowest intensity to which they were ex-
posed, their reactions were indefinite. There was no indi-
cation of any positive reaction whatever.

At different times a number of colonies were taken from
a given jar and half of them put into each of two similar
vessels containing equal amounts of water. One of the
vessels was then exposed to direct sunlight and the other
covered so as to exclude all light. After having been in
this condition a short time the reactions of the colonies in
the two vessels were compared by exposing both to the
same light intensity. In such cases it was always found
that the specimens which had been in direct sunlight were
negative to light of lower intensity than those which had
been in darkness. These results indicate that the colo-
nies had not become acclimated to the high light intensity.
But they do become acclimated under certain conditions,
judging from the observations of Oltmanns, who says
(1892, p. 190), that he covered two lots of Vol vox with
the same kind of prisms, July 31, in the evening. One of
these lots with its prism was kept in darkness until 9 A.M.,
August I, the other was exposed to light. During the
following three days it was found that those which were
in darkness until 9 a.m. collected in regions of lower light
intensity than the others. Strasburger found the same to
be true with reference to the reactions of swarm spores.
It seems strange that the effect upon the optimum in
colonies exposed for so short a time could, as Oltmanns
states, be still observed after three days.

There are some indications that when Volvox is negative
to light of low intensity it becomes positive when exposed
to a much higher intensity. This is shown by the follow-
ing observations :

REGULATION OF REACTIONS 29 1

August 23, 1904, was a bright, clear day. At 4 p.m.
specimens were collected in a place which had been well
exposed to the sun much of the afternoon. Soon after
reaching the laboratory, these specimens were found to be
positive in light intensities varying from 230 to 1400 ca. m.
The colonies not used in these tests were put into a liter
jar and placed in strong diffuse sunlight in a west window.
Here many of the colonies soon aggregated on the side of
the jar farthest from the source of light. At 5.45 p.m.,
after having been in the window about an hour, they were
found to be negative to an intensity of 230 ca. m. and at
6.45 P.M. to an intensity as low as 3 ca. m. They seemed
to become more strongly negative the longer they were left
in the window, although the light from 6.30 P.M. on was
quite dim. At the close of the experiment, 7 p.m., certain
colonies which had been strongly negative to an intensity
of 230 ca. m. were found to be positive to an intensity
of 400 ca. m. The following day these organisms were
exposed again to light of 1400 ca. m. and to various
lower intensities, but there were no indications of negative
reactions.

In certain cultures of attached specimens of . Stentor
coeruleus kept in low light intensity I have seen some
specimens respond definitely by violent contraction to a
sudden increase of illumination of even less than 120 ca. m.,
while other specimens in the same culture did not re-
spond at all, even to a much greater increase. In other
cultures under the same environmental conditions none
of the specimens could be made to respond even by flash-
ing the most intense direct sunlight (5000 ± ca. m.) upon
them.

Free-swimming individuals at times avoid even the
faintest illumination, while at other times they are found
in strong, diffuse daylight. These creatures apparently
become accustomed to light very readily. They were
often observed to give very definite responses In diffuse
light when first taken from a culture jar, and none at all

292 LIGHT AND THE BEHAVIOR OF ORGANISMS

after they had been exposed five minutes. Many similar
instances have been cited in the preceding pages, notably
those with reference to reactions to shadows.

It will thus be seen that there is a tendency in organisms
toward adaptation to environmental conditions. Exposure
to low intensity tends to lower the optimum and increase
the sensitiveness, w^hile exposure to high intensity tends
to produce the opposite effect. But momentary exposure
to high intensity, as we have seen in Volvox, may actually
lower the optimum. The reaction of an organism depends
not only upon the rate of change in illumination and the
intensity, but also upon the time it is exposed.

Among the most interesting and conclusive observations
on variation and modification in reactions to light are
those of Mrs. Yerkes (1906) and Professor Hargitt (1906)
on the annelid, Hydroides dianthus. Hydroides, as pre-
viously stated, ordinarily jerks rapidly back into its tube
when the light intensity is suddenly decreased, but it does
not respond when the intensity is increased. This is
clearly a reaction to a sign. The decrease of intensity,
the shadow, is of no direct consequence to these creatures,
but what ordinarily follows the shadow, an attack of an
enemy, may be.

Mrs. Yerkes was primarily interested in modification of
behavior. She selected two specimens, one of which did
not respond at all to a given reduction of light intensity
and the other responded only once. Both however re-
acted definitely when lightly touched. For ten days
these two specimens were subjected to a series of stimu-
lations consisting of shadows followed by light tactile
stimuli. The first day one responded to the shadow,
alone, three times in forty trials, and the other only once.
In the former there was a great increase in the number of
responses to shadow from the fourth to the eighth day,
then a slight falling off. In the latter the increase was not
so great, but still it was definite, especially from the second
to the fifth day. It thus appears that these creatures

REGULATION OF REACTIONS 293

learned to react to the shadow, the sign of the tactile
stimulus that regularly followed it.

Hydroides becomes acclimated to a given stimulus with
surprising rapidity. Mrs. Yerkes (1906, p. 442) found that
in sixteen tests out of twenty-seven with different spec-
imens the animals responded to shadows passed over
them at regular intervals only from one to three times,
after which the decrease of intensity did not appear to
affect them at all.

Hargitt records similar results in a paper published a
few months earlier than that of Mrs. Yerkes, and again in
a later paper. He observed (1909, p. 179) that specimens
taken at a depth of from eight to fifteen fathoms react to
shadows only in an indefinite way, and that many do not
respond at all, indicating clearly that the response depends
upon past experience as well as upon present conditions.
Jennings (1906) and others have observed similar effects
of other stimuli on numerous different species.

One of the most interesting features in the behavior of
Hydroides, and one that has been most accurately recorded,
is the variation in the time that these animals remain in
the tubes after responding to a given reduction in light
intensity. In a series of ten trials Mrs. Yerkes (1906)
found the time to vary from 15 to 240 seconds, and in
another series of sixty trials from 10 to 710 seconds.
There is no apparent regularity in this variation. The
author says, referring to the last series mentioned above
(p. 447) : " The period of retraction is short the first three
times — 19 to 34'' — but the fourth time it is nearly four
minutes. For the next thirteen times it ranges from
eighteen to ninety-three seconds; then comes another
period of nearly four minutes followed by nineteen con-
tractions which last from twelve to eighty-five seconds
each and then a contraction of nearly twelve minutes'
duration. Thus after the fourth, eighteenth, thirty-
eighth and sixtieth trials the animal remained contracted
for a relatively long period, varying from four to twelve

294 LIGHT AND THE BEHAVIOR OF ORGANISMS

minutes, whereas the intervening contractions seldom
lasted more than one and a half minutes and are usually
less than thirty seconds."

Hargitt (1909) extended these observations on the vari-
ability in reactions of the tubicolous annelids. Of especial
interest are his results with experiments on specimens
taken in deep water where shadows are very faint as com-
pared with specimens taken from shallow water where
changes of light intensity are striking. The following
tables illustrate the character of these reactions (pp. 170,

TABLE VIII
Showing reactions of specimens from deep waters ^

August 9, II A.M. August 9, 2 P.M.

Temperature, 22° C.

Temperature, 22,5° C.

I
2

3
4

5
6

7
8

9
10

II

12

13
14

15
16,

!;■
18.

19'

20.

A

4-12

B

+ 10
o
o

+10

+

o

+

o
o
o
o
o
o
o
o

D

E

+

o

o

A

B

+ 18

+35

+

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

D

+30

E

+ 20

^ The numbers preceded by + represent time in seconds animals re-
mained in tubes after stimulation. Minus sign indicates failure to respond;
zero indicates that animal was in tube when stimulus was applied. The
stimulus consisted in turning off a i6-candle-power electric lamp. The
intensity is not recorded. Interval between successive stimuli, usually
5 minutes. (Hargitt, 1909, pp. 159 and 170.)

REGULATION OF REACTIONS

295

TABLE IX

Showing reactions of two specimens F and G, from shallow water.

Legend same as for Table VIII. See footnote.

I

o

3
4

5
6

7
8

9

lO

II

12

13

14

15
i6

17

i8

19

20

lO A.M.
22°

40

30
30

60

45
100

300
40

30
180

43
180

75

75
470

50
60

150

G

360
60

180
60

50

50
90

60

35
120

300

60

90

105
90

130

150

150
225

90

2 P.M.
23°

15
10

12
10
18

15
12

13

15

12

45

75
20

20

12

15
II
18

20

30

45

30

45

50

35
80

100
40

no
90

15

90
90

105
80

85

45

The fact that there is no " definite law in relational
sequence " in the reactions of Hydroides, especially the
fact that the time that a given specimen remains in the
tube varies so much without any observable regularity or
relation with environmental changes, has led Hargitt to
conclude that behavior of organisms cannot be explained
by the application of purely physical principles and to
sympathize " with a tendency to postulate the presence of
certain psychic factors."

However one may regard Hargitt's conclusion, his results
seem to show clearly that the immediate environment
at any given time will not account for the reactions of
Hydroides at that time, that they are dependent upon

296 LIGHT AND THE BEHAVIOR OF ORGANISMS

internal as well as external factors, and that if the inter-
nal processes, physiological changes, do account for the
variability in the reactions these processes cannot be run-
ning their course with any degree of regularity.

Among the crustaceans and the higher forms varia-
bility and modifiability in reactions to light are com-
mon. Holmes (1905) found that Ranatra with the left
eye blackened tends to turn to the right in going toward
a source of light, but after several trials it goes nearly
directly toward it. " In the first trial the insect veered
over constantly to the left, passed by the lamp and went
off from the table before it turned around. In the fol-
lowing trials a marked tendency to turn to the left is also
shown; frequently the insect makes one or more complete
circus movements to the left before reaching the light.
At the eleventh trial its course is corrected for the first
time by a turn to the right side, but, instead of going
straight up to the light, it performed a complete circus
movement to the left before reaching it. The next time
the course was corrected by a sharp turn to the right and
the circus movement was dispensed with. At the next
trial the course was corrected in the same way, and at the
fourteenth attempt the insect deviated only slightly to
the left side and then turned to the right to reach the
lamp. In the following ten trials it reached the light by
a nearly straight path. Whenever it began to turn away
from the light to the left it corrected its course by a direct
turn in the opposite direction instead of going around in
a complete circle as at first."

Spaulding (1904) observed that hermit crabs are ordi-
narily positive. They usually collect in the more highly
illuminated regions of an aquarium. But he found that
after shading the part of the aquarium in which the crabs
were fed every time that food was introduced, they soon
came to the part shaded even before food was put in,
quite contrary to their ordinary reaction to light.

In even casually studying the behavior of bees, wasps,

REGULATION OF REACTIONS 297

ants and various insects in their natural environment, one
can hardly fail to see that their reactions to light are any-
thing but fixed. Ants, for example, ordinarily avoid the
light. They are said to be negative. But they are not
always found in the dark recesses of their nests. Does
this mean that the sense of reaction changes? Light
undoubtedly guides them at times, and the sense of
reaction changes frequently, but sight no doubt plays a
part in the reactions. If a nest containing pupae or
larvae is opened, a given ant may often be seen, in caring
for the young, to travel back and forth repeatedly from
the brightest sunlight to the dark cavities of the nest.
Here it is evident that the ordinary negative response to
light has been modified. Again, the flight of bees from the
extreme darkness of the hive out into the brightest sun-
light, through shadow and sunshine, into and out of the
cavities of flowers and back into the darkness of the hive
again, offers another striking example of variability in
response to light, for it is no doubt light that guides these
organisms in many of their movements, although that in
which they are primarily interested is not light, but the
objects represented by different conditions or configurations
of light.

We have thus seen that the reactions to light depend
upon various agents, and that they are modifiable and
variable to a certain degree in all organisms from the
lowest to the highest, i.e., that they are in general adap-
tive and regulatory. But in none of the lower forms
have such striking adaptive changes in reaction to light
been observed as Jennings records with reference to other
stimuli in his interesting description of the behavior of
Stentor and some other organisms (1906, pp. 170-179).
There is at present no greater need in the study of the
behavior of lower organisms than a comprehensive and
thorough quantitative study of the relative activity of
the different factors involved in regulation.

298 LIGHT AND THE BEHAVIOR OF ORGANISMS

General Summary of Part III

(i) Reactions to light are in general adaptive. There
are, however, certain reactions which are clearly injurious
and often fatal; .as, for example, the flying of insects into a
flame and the positive reactions of organisms which live in
darkness. But the positive reactions of insects are ordi-
narily advantageous. It is only under artificial condi-
tions that they prove fatal, and the ancestors of many
animals which now live in darkness lived in the light.
Positive reactions were probably advantageous to them,
and the power to respond thus was probably inherited by
the offspring, in which it is useless.

(2) Organisms ordinarily collect in light conditions
which facilitate life processes. They get into an optimum
light condition either by orienting and moving directly
toward it or by random movements; and they remain
either because they come to rest there or because they
respond with the avoiding reaction when they reach the
limit of the region of optimum light. All of the reactions
involved in aggregation are responses to changes of inten-
sity, with the probable exception of those in which the
organisms come to rest at the optimum. In these, aggre-
gation is no doubt due to the effect of continued illumina-
tion or constant intensity.

(3) Many organisms react to light without orienting
or aggregating. In nearly all cases these reactions are
sudden contractions or changes in direction of movement
caused by sudden changes in light intensity, as, for ex-
ample, the jerking into its tube of Hydroides when a
shadow passes over it. Most of these reactions are highly
protective against the attack of enemies, but some serve to
indicate the presence of food, as in the case of Clepsine.
These are clearly reactions to signs. It is not the shadow
in which these organisms are interested, but what ordi-
narily follows. There are, however, organisms which re-
spond to the effect of light more directly, as, for example,

REGULATION OF REACTIONS 299

the contraction of the sea anemone Edwardsia when light
is flashed on it. It is the change of intensity that causes the
response, but there is no evidence of a reaction to a sign
here. These creatures are directly interested in the effect
of the light which produces the response. Continued illu-
mination probably affects the activity of all organisms that
respond to light, and change in the sense of reaction
when due to the action of light is in all probability due to
the effect of continued intensity rather than to change of
intensity.

(4) The actions of organisms may change without any
change whatever in external conditions. They may, for
example, be positive to light of a given intensity under
given conditions at one time and negative to the same
intensity under precisely the same external conditions at
another. This change in reaction must be due to internal
factors. It may take place in course of a few moments, as
in Volvox, or it may require weeks, as in the case of the de-
velopment of the fly, the larva of which is negative while
the imago is positive. Then, again, it may be periodic,
as in the sleep movements of many of the plants.

(5) High light intensity ordinarily causes organisms in
the positive state to become negative. There are, how-
ever, organisms which do not become negative no matter
how high the intensity is, e.g., many plant structures and
numerous aquatic larvae. Under natural conditions they
do not experience illumination so strong that it is injuri-
ous; there is, therefore, no need for a negative response.
The positive reactions of insects and other forms which
frequently prove fatal under artificial conditions are adap-
tive under natural conditions.

Reversal in the sense of reaction is not a response to a
change of intensity. It does not take place until some
time after the change is made. There is a time element
involved here. It is due to changes occurring within the
organisms, caused by continued light conditions, not by
changes in such conditions.

300 LIGHT AND THE BEHAVIOR OF ORGANISMS

(6) Decrease in temperature causes swarm-spores, Eu-
glena, Chlamydomonas, Vol vox and other similar organ-
isms in the positive state to become negative. Increase
of intensity causes the opposite change. Decrease in heat
energy, therefore, causes the same change in reaction in
these forms as increase in light energy. In other forms
however this is not true. Polygordius larvae, for example,
become negative when the temperature is increased. In
many organisms changes in temperature do not cause re-
versal in the sense of reaction.

(7) In Gammarus pulex, Cyclops, Daphnia, Cypris, a
small water spider, and various insect larvae, addition of
CO2 causes the reactions to light to become strongly
positive. In Gammarus various acids and narcotics and
all the ammonium salts also cause strong positive reactions.
In Cyclops sodium hydrate causes positive specimens to
become negative. In Stentor, Chlamydomonas, Volvox,
and Scapholeberis carbon dioxid does not cause a change
in the reaction. In Arenicola larvae various narcotics,
acids, alkalis and neutral salts — in general, apparently
any substance which acts as a depressant — cause a change
from positive to negative reactions. In Ranatra any con-
dition which tends to make the animal quiet produces
negative reactions, while any condition which excites it
tends to produce positive reactions.

(8) The sense of reaction in most organisms is only
temporarily affected by concentration of the medium or
mechanical stimulation. Arenicola larvae become nega-
tive in both concentrated and diluted sea water, while Poly-
gordius larvae become positive in the former and negative in
the latter.

Mechanical stimulation appears to cause Temora and
Cypris to become positive, while it causes Ranatra, Or-
chestia and Labidocera to become negative.

(9) The fact that change in sense of reaction can be
produced by such a variety of different means seems to
show very clearly that this change is not due to a specific

REGULATION OF REACTIONS 301

interaction between external and internal chemical con-
stituents, as, for example, a relation between acids and
alkalis, but to an effect on the general state of the organ-
ism as a whole.

(10) Variability and modifiability in response to a given
external condition are striking characteristics in the be-
havior of all living beings. The optimum and sensitive-
ness in swarm-spores, diatoms, Euglena, Stentor, Volvox
and other similar organisms have been found to change in
accordance with the environment. If they are exposed to
strong illumination for some time the optimum intensity
increases and the sensitiveness decreases. If exposed to
low intensity the opposite change takes place. Not only
the light intensity, but also the time of exposure, is active
in these changes. Momentary exposure may produce
results just the opposite from those due to continued
exposure. Changes in response frequently occur with-
out any immediate changes in the environment. These
changes are regulated by internal factors, physiological
processes. Many of the responses to a given light con-
dition are extremely variable. This is due to the fact that
numerous factors, both internal and external, are involved
in these responses. Concerning the internal factors little
is as yet known.

PART IV

REACTIONS IN LIGHT OF DIFFERENT WAVE-
LENGTHS OR COLORS

CHAPTER XV

ENERGY, PHOTOCHEMICAL REACTIONS AND BRIGHTNESS

It is assumed by some investigators that the reactions
to light in lower forms, plants as well as animals, are all
induced by waves of approximately the same length, and
that these waves are the more refrangible in the spectrum,
the so-called actinic rays, the rays which are generally sup-
posed to have the greatest effect on chemical reactions.

Davenport (1897, p. 202) closes a brief review of the
literature on this subject with the following words: '* Thus,
without multiplying cases, the results of experiments may
be summed up as follows: positively phototactic or posi-
tively photopathic organisms are such only in the presence
of the blue rays." Referring to experiments with numer-
ous different animals, Loeb says (1905, p. 294),^ " [I] found
a universal confirmation of the fact . . . that the more
strongly refrangible rays of the visible spectrum are the
most active heliotropically, as in the case of plants."

Other investigators have, however, arrived at different
conclusions. They claim that all the rays in the visible
spectrum and some in the ultra-violet may be active in
stimulating organisms and that not all organisms are
equally stimulated by the different rays. After disagree-
ing with Loeb's statement that the shorter waves are the
more efficient in all plants and animals, Nagel adds (1901,
p. 294), " Es ist sehr wahrscheinlich dass fiir sehr viele

^ Original in Pfiiiger's Arch., Vol. 54, 1893.

302

ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 303

lichtempfindliche Thiere das Maximum der Reizwirkung
im Gelbgriin liegt, dass sie mit anderen Worten lichtemp-
findliche Substanzen besitzen die dem Sehpurpur der
Wirbelthieraugen ahnUch sind." Pfeffer (1906, p. 175)
maintains that " the relative efficiency of the different rays
is not the same in all plants," and Verworn says (1889,
p. 60), " Es hat sich herausgestellt, dass die meisten Protis-
ten nur auf bestimmte Farben, d. h. Strahlen von bestimm-
ten Wellenlangen reagiren, welche durchaus nicht fur alle
die gleichen sind." It is therefore evident that with ref-
erence to the reactions of the lower forms there are con-
tradictory opinions as to the efficiency of the different rays
of light. The same may be said in regard to animals with
image-forming eyes.

Let us review the more important of the experiments
which have led to these contradictory opinions and try to
formulate the conclusions to which they lead.

In this review we shall first attempt to ascertain the
efficiency of different parts of the spectrum in producing
reactions in various plants and animals, then we shall
compare this with the distribution of energy in the spec-
trum, with the distribution of brightness as judged by the
human eye, and with the distribution of actinic or photo-
chemical effect.

Sunlight, as is well known, consists of ethereal vibrations
composed of waves varying in length from approximately
390'''' to 760''''. Aside from varying in length, light waves
may also vary in amplitude, and then there may be innu-
merable combinations of waves of different lengths. These
three different physical characteristics of light are said to
produce different specific subjective sensations in man,
known respectively as color-tone or hue, brightness or
shade, tint or saturation. The subjective sensations are
in all probability in some way associated with the effects
of light on chemical changes in the retina.
__ Monochromatic light, or light having a fixed color-tone,
consists of waves which are equal in length. In accord

304 LIGHT AND THE BEHAVIOR OF ORGANISMS

with the above supposition there are, therefore, theoreti-
cally as many different color-tones in the visible solar
spectrum as there are different wave lengths. Practically,
however, these numerous theoretical hues are divided, usu-
ally into six classes, — • violet, blue, green, yellow, orange and
red. Authorities differ but little as to the point of division
in the spectrum between the different colors. Selecting
the classification most commonly found we shall refer to
wave lengths 390 to 430^" as violet, 430 to 490'"" as blue,
490 to 560''^ as green, 560 to 590^'' as yellow, 590 to 630^^
as orange, and 630 to 760"" as red.

I, Energy Distrihittion in the Spectrum

The distribution of energy In the solar spectrum as well
as that in various artificial spectra has been thoroughly
investigated, as has also that of the effect of different
rays on a number of different chemical reactions and the
distribution of brightness as judged by the human eye.
In a recent paper Nichols (1905) has summarized much of
the work on the energy in the visible spectrum. He gives
the curves of distribution for the normal or grating spec-
trum of the following sources of light: Hefner lamp,
acetylene lamp, petroleum lamp, illuminating gas with
different burners, Welsbach mantle, various electric incan-
descent carbon filaments, magnesium flame, direct and
diffuse sunlight and a few others. While the distribution
of energy in the spectrum differs considerably with the
different sources of light, it corresponds in that the energy
toward the red end is much greater than that toward the
violet (Fig. 34). There is only one exception to this: in
case of magnesium oxide the energy is " greater in the
violet thaji in the yellow and green " {I.e., p. 159).

In the normal gas spectrum the energy increases very
gradually from the beginning of the violet to the begin-
ning of the green at about 500^^, then it Increases very
rapidly and reaches its maximum at the end of the red.

ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 305

At 450^^^ (with a bat's wing burner) the energy is repre-
sented by o.ii; at 500^'' by 0.52; at 700'''' by 12.70. In
the red, then, the energy is 100 times as great as in the
violet and about 20 times as great as in the green. In
case of direct sunUght the maximum energy in the normal
spectrum is between the yellow and the orange. From
this point there is a gradual decrease toward the red end
and a somewhat more rapid decrease toward the violet. The
difference in energy in different parts of the normal sun-
light spectrum is not nearly so great as in the normal gas
spectrum. In the prismatic spectrum of both gas and
sunlight it is, however, much greater than in the normal
spectrum, owing to the relatively greater condensation of
the rays toward the red end. Whereas the maximum in
the normal sunlight spectrum is between the yellow and the
orange at about 600''", in the prismatic spectrum it is,
according to Langley (1884), at the end of the red or per-
haps even in the infra-red. (Fig. 34.)

2. Brightness Distribution in the Spectrum

The determination of relative brightness of different
parts of the spectrum is a matter of considerable com-
plexity, owing largely to the difficulty of comparing differ-
ent colors with reference to brightness and to the fact that
the relative brightness of the different colors varies with
the absolute intensity, a characteristic known as Pur-
kinje's phenomenon. In spite of these difficulties the
results obtained by eight or more different methods, all
refined in every detail, are in close agreement, indicating
that the conclusions stated below are in all probability
reliable.

The method most widely employed Is the direct com-
parison of the different colors with white light of known
intensity. By means of this method Fraunhofer in 18 17
located the maximum brightness in the prismatic solar spec-
trum in the neighborhood of the line D. In 187 1 Vierordt

3o6 LIGHT AND THE BEHAVIOR OF ORGANISMS

obtained results which agree fairly well with those of
Fraunhofer, by means of finding the amount of white light
required to make a given color imperceptible when added
to it. The results of Vierordt are graphically given in
Fig. 34.

0

■^

E

D

(

7 B

J

T^

V

18

/\

\

/\

\

IG

/

'\

\

V

/

>

y\

\

\

14

/

/

1

\

\,

\

/

/f}

1
1

1

i

\

\

\

A

12

\

1

V

\

/

/i

V

1

^ 1

1

\

V

f

^

10
8

/ 1

' 1 /

v\

X

\

/>

^^^^

/

1 /

1 /
1/

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400 460 520 580 G40 700 760

Fig. 34. Curves representing the relative distribution of brightness and energy
in the spectrum, i, brightness curve for color-bHnd persons showing that the
red end is considerably shortened (after Konig, i8gi); 2, brightness curve for
normal eye in prismatic solar spectrum (from Davenport, 1897, p. 159, after Vier-
ordt, 1873, p. 17); 3, 4, and 5, brightness curves for normal eye in normal gas-
light spectrum of low, medium, and high intensities respectively (after Haycraft,
1897, p. 141); 6, energy curve for solar prismatic spectrum (after Langley, 1884,
P- 233); 7, energy curve for normal gas-light spectrum constructed from data
given by Nichols, 1905, p. 151. The value of the ordinates in most of these curves
is arbitrary. A-G, approximate positions of Fraunhofer lines in spectrum.

These curves show clearly that the distribution of brightness in the spectrum is
not proportional to the energy of the diflferent parts and that consequently bright-
ness must depend upon the length of the waves as well as upon their amplitude.

Haycraft (1897, p. 140) ascertained the brightness dis-
tribution in a normal gas spectrum produced with Hilgar's
large spectroscope and diffraction grating, by means of
the so-called flicker method. The results obtained by this
method, graphically recorded in Fig. 34, agree very well

ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 307

with those Haycraft obtained by measuring the size of
the pupil in different parts of the spectrum and by deter-
mining the distance at which small areas differing in color
become invisible. It will be seen by referring to Fig. 34
that the maximum brightness obtained in these experiments
in light of high intensity is very near the Fraunhofer line D,
that is, between the yellow and the orange, which agrees
with the maximum in the solar spectrum obtained by
Fraunhofer and Vierordt. It is interesting to note that
in low light intensity the maximum is in the green, and
that this corresponds fairly well with the maximum for
color-blind individuals.

In case of color-blind individuals the difficulty of com-
paring intensity of different colors is of course obviated.
The brightness distribution in the spectrum has been
ascertained in several cases, all of which are in approxi-
mate agreement, the maximum being in the green near
the Fraunhofer line E (Fig 34), in fairly close agreement
with the maximum for the normal eye in the spectrum of
low light intensity.

By comparing the curves in Fig. 34 it is at once evident
that the brightness distribution in the spectrum is not
proportional to the energy. The distribution of brightness
in the normal solar spectrum, the prismatic solar spectrum
and the normal gas spectrum agrees fairly well in certain
respects, while the distribution of energy in these spectra
is very different. Brightness sensation is therefore asso-
ciated with some specific effect of the length of light waves,
as well as with the amplitude of the wave. Color sensa-
tion, on the other hand, is associated with the specific effect
of the length of the waves and with the effect of combina-
tion of waves of different lengths. The specific effect of
waves of a given length and amplitude is no doubt due to
chemical changes in the retina. Visual purple is most
rapidly bleached by the rays in the spectrum between the
lines D and E, the region containing the rays which are
absorbed most readily, and the region which contains the

3o8 LIGHT AND THE BEHAVIOR OF ORGANISMS

maximum brightness. There is probably some interrela-
tion between these phenomena. It is however not our
purpose to discuss theories of vision. We wish merely to
emphasize that experimental results appear to show that
brightness is not proportional to the energy in light, that
it is a function of wave length as well as of amplitude.

3. Distribution of Actinic Effect in the Spectrum

That light causes profound changes in chemical com-
pounds is a matter of common information to all familiar
with the process of photography. The fact that the
shorter waves of the spectrum, the ultra-violet, violet and
blue are chiefly active in causing changes in the halogen
salts of silver and various other metals used in this pro-
cess, is at least in part responsible for the idea that photo-
chemical changes in general are largely if not entirely
brought about by the action of the shorter waves, which
are usually referred to as the actinic rays.

Photochemical reactions are far more numerous in both
the inorganic and the organic realms than is generally
supposed. Davenport (1897, pp. 1 61-165) brought to-
gether many instances under the following heads: syn-
thetic, analytic, substitutional, isomeric, polymerismic,
fermentative effects of light. Recent investigations have
made known others which are of especial interest to us.
Most important among these are numerous reversible
reactions, reactions which take place in one direction in
daylight or in light of a given wave length, and in the
opposite direction in darkness or in light of a different
wave length.

The following reversible equations are referred to in a
recent paper by Stobbe (1908) on photochemical reactions.
The first five are quoted by Stobbe, the rest were dis-
covered by him. In these equations the arrows indicate
the direction in which the reaction takes place in the
different conditions of light with which they are labeled.

ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 309

In the first equation, for example, the reaction proceeds
toward the right in the Hght and toward the left in
darkness.

light

1. 2 AgCl ^=f AgaCl + C1.

dark

light

2. 2 C14H10 ( / C28H20.

dark

light

3. 5 Agl ;=f Agia + 2 Ag2l.

dark

4. Tetraphenyldihydrotriazin.

light
White c ' rose red.
dark

5. Dimethyloxalessigesterphenylhydrazon.

light
White < ^ citron yellow,
dark

Cells — C — Cells CeHsCH

6. Triphenylfulgid, OC-C C-CO

I O 1

light .
Orange yellow < ^ light brown.

dark

blue or violet (440 to 550"")
Orange yellow < ^ dark brown.

red or yellow (550 to 700"")

H CI2

C C

^ \ / \

HC C CCl

7. jS-Tetrachlor-a-ketonaphtalion, I li II

HC C CCl

% / \ /

C C

HO

ultra-violet

White ( ' violet.

yellow green

310 LIGHT AND THE BEHAVIOR OF ORGANISMS

In the last two equations it is clearly shown that the
longer waves as well as the shorter are actinic. Stobbe
investigated the reactions of numerous so-called fulgides in
the different rays of the spectrum C'Steinheilschen Spectro-
graphen") and found seventeen which behave much like
triphenylfulgid. There is however considerable varia-
tion as to the specific effect of the rays in the different
forms. In general the shorter waves cause the fulgides to
become darker in color, while the longer ones cause them
to become lighter. But in some it is the violet which pro-
duces the dark shades, while in others it is the ultra-violet
or the blue. " Je mehr sich die Farbe eines Fulgides ver-
tieft, je w^eiter sich die Absorption eines Fulgides nach
dem rothen Ende des Spectrums erstreckt, um so weiter
riickt auch die Erregungszone nach derselben Richtung
vor" (1908, p. 31).

In white light the fulgides turn dark, just as in mono-
chromatic light, but strange as it may appear the reac-
tion is much less pronounced, even if the white light has
more of the effective rays than the monochromatic light of
any given region in the spectum. The relatively feeble
effect of white light must be due to the presence of the
longer waves, which, as represented in the equation above,
tend to produce the lighter shades and consequently retard
the production of the darker. It may be well to call
attention to the fact in passing that the investigations of
Lubbock on Daphnia, of Wilson on Hydra and of Wiesner
on some of the higher plants show that as in the ful-
gides, monochromatic light consisting of certain rays is
more effective in causing reactions than the same light in
combination with other rays.

It is evident from the last two equations that the longer
rays as well as the shorter may have a specific photo-
chemical effect. Triphenylfulgid, e.g., is changed from dark
brown to orange yellow by the longer waves and not by
the shorter. There are many other reactions which are
induced only by the longer waves. Among the most

ENERGY, PHOTOCIIEMICALS AND BRIGHTNESS 31 1

important of these the process of photosynthesis in plants
furnishes an excellent example. The maximum for this
process lies in the red very near the Fraunhofer line C.
This is not solely due to the fact that the rays in this
region are more readily absorbed than those in the adjoin-
ing regions, for the violet rays are also absorbed, and here
there is no appreciable effect on photosynthesis. In
solutions which contain ferrocyanide or certain other
coloring matter the longer waves are also more effective
than the shorter, and pure ozone, which is changed to
oxygen only in the ultra-violet, is similarly acted upon by
the visible rays if chlorine be added. These various
examples inevitably lead to the conclusions that while the
shorter rays may induce chemical changes in more sub-
stances than the longer, they cannot be considered as the
only actinic rays. The relative efficiency of the different
rays depends first of all upon one or more of the com-
pounds between which the photochemical reaction is tak-
ing place, but it also, at least in certain cases, depends upon
the presence of substance in which no apparent change is
taking place.

Many of the photochemical reactions are exothermal.
For example, the light conditions which induce the ful-
gides to become dark are much more effective in lower
temperature than in higher. According to Stobbe it
requires nearly ten times as much light energy to produce
a given change at 100° as it does to produce the same
change at 87°. A decrease in heat energy , therefore, pro-
duces the same effect as an increase in light energy, a
statement which at first thought appears self-contradictory.
As a matter of fact, however, it merely demonstrates the
independence of these two forms of energy in producing
chemical reactions.

It is evident from what has thus far been presented
that the actinic distribution in the spectrum is not
proportional to distribution of energy. There are many
well-known photochemical reactions which occur only in

312 LIGHT AND THE BEHAVIOR OF ORGANISMS

ultra-violet, the region of the spectrum which contains
least energy.

Precisely how light produces chemical changes is un-
known, but it is clear that only those rays which are
absorbed can be effective. The efficiency is however not
proportional to the absorption. According to the excel-
lent researches of Luther and Forbes (1909), the reaction
between quinine and chromic acid is only affected by the
rays absorbed by the quinine, and not at all by those
absorbed by the chromic acid. Only about 4 per cent of
the light absorbed by the quinine is changed to chemical
energy. The ultra-violet and violet are most readily
absorbed, but the green is most efficient, i.e., a greater
amount of chemical action is caused by a given amount
of light energy absorbed in the green than by the same
amount absorbed in the violet and ultra-violet, showing
clearly that the efficiency is not proportional to the ab-
sorption. The same is true in case of photosynthesis,
which is supposed to be due to the action of light ab-
sorbed by the chlorophyll. Chlorophyll dissolved in alcohol
has, according to Reinke (1884), a prominent absorption
band in the red, a weak band in the orange, the yellow
and the green, while from 500/^^ on, i.e., in the blue and
violet, practically all light is absorbed. The maximum rate
of photosynthesis however takes place in the red, from
which it decreases rapidly in either direction, so that be-
yond the green in the region of maximum absorption there
is scarcely any photosynthesis.

The specific effect of the different rays on chemical
reactions as well as on brightness sensation is evidently a
function of the length of the waves and the rate of vibra-
tion. The effect of the different rays is not proportional
to the energy in these rays, but the effect of light of a given
wave length is of course dependent upon the amplitude of
the waves, their intensity, as well as upon their length.

CHAPTER XVI

EFFECT OF DIFFERENT RAYS ON THE REACTIONS OF

SESSILE PLANTS

It Is evident that as in inorganic and organic compounds
and in man, so in the lower organisms, the reactions to
light may be due to or at least associated with a specific
action of the length of the light waves, or with the ampli-
tude of the waves, or with a combination of waves of
different lengths. In experiments on the effect of colored
light on organisms it is therefore essential to know what
sort of light is being used as a stimulating agent ; many
results are unreliable because this was not known, or at
least is not recorded. The colors used were frequently
produced by means of solutions or colored glass which
transmit waves varying much in length. In case of red
glass, e.g., there is usually some orange and yellow and fre-
quently a little blue, violet or ultra-violet transmitted as
well as the red. A reaction in such light, apparently due
to the longer waves may actually be due to the shorter, or
to the specific effect of the combination. Then, too, the rela-
tive intensity of the different colors was often not consid-
ered. There was then the possibility that the reactions
were due to intensity rather than to color.

The bearing of this discussion becomes evident when
we consider the fact that some organisms are sensitive to
light of extraordinarily low intensity; e.g., Figdor (1893)
found the plants Lepidium sativum, Amaranthus melan-
cholicus ruber, Papaver paeoniflorum, and Lunularia biennis
to respond to light as weak as 0.00033 ca. m. In organ-
isms so extremely sensitive it is evidently impossible to
be certain as to what causes a reaction if they are not
subjected to monochromatic light of known intensity. In

313

314 LIGHT AND THE BEHAVIOR OF ORGANISMS

organisms which are not very sensitive it is, however, un-
hkely that a few stray foreign rays mixed with a given
color will alter the reactions. In reviewing the work on
reactions in colored light and formulating conclusions it
will consequently be necessary to consider carefully the
methods used in such work.

Poggioli (1817) was the first to study the relative effect
of light waves of different lengths on the reactions of
plants. He exposed seedlings of Brassica and Raphanus in
different parts of the spectrum and found that they turn
toward the source of light in the red as well as in the
violet, but that the reaction in the latter occurs much
more rapidly than in the former. He does not mention
the reaction in other parts of the spectrum. These
results seem to have remained unchallenged for twenty-
five years, when Payer (1842), after studying the reac-
tions of different seedlings in a solar prismatic spectrum
and behind different color media, came to the conclusion
that red, orange, yellow and green act like darkness, and
that blue is more active than violet. This conclusion,
however, although supported by Sachs, is not in harmony
with the experimental results of Gardner, Dutrochet and
Pouillet, Guillemin, Wiesner and others. Gardner (1844)
found that all the seedlings in a trough which extended
beyond the solar prismatic spectrum into the ultra-violet
and the infra-red bent toward the sources of light, but that
they deflected slightly toward the indigo. The deflection
toward the indigo was no doubt due to light reflected by
the seedlings in this region of the spectrum. Dutrochet
and Pouillet (1844) obtained similar results in their experi-
ments with the roots of white and black mustard in a
strong solar prismatic spectrum. They concluded that
all the rays, including ultra-violet and infra-red, cause the
roots to bend from the source of the light, but that the
blue is most active.

Guillemin (1858) made a more detailed study of this
subject than had previously been made. His methods

REACTION OF PLANTS IN COLORS 315

were excellent. He studied the reactions of white mus-
tard and cress seedlings in three different solar prismatic
spectra produced respectively by means of flint glass,
rock salt, and quartz. All diffuse light was eliminated by
means of suitable screens, and twenty-five different tests
were made. In all three spectra there were two regions
of maximum effect. The primary maximum lay in the
violet or ultra-violet in all cases, and the secondary maxi-
mum between the infra-red and the green. The minimum
effect in all cases was in the blue near the Fraunhofer
line F.

These conclusions were in the main confirmed by the
thorough work of Wiesner (1879). Wiesner studied the
reactions of stems and roots of several different seedlings
behind thoroughly tested absorbing media and in differ-
ent parts of a direct sunlight spectrum produced by means
of the " Soleil'sche apparat mit Flintglasprisma." The
spectrum " showed the Fraunhofer lines clearly " and was
consequently relatively pure. I shall mention but one of
the numerous experiments the results of all of which agree
in general. In this experiment numerous pots of Vicia
sativa were distributed in the spectrum from infra-red to
ultra-violet. The seedlings between the violet and the ultra-
violet began to bend first, then those to the right and left,
and later those in the red. In the yellow and orange there
was no reaction. Wiesner states the results in the fol-
lowing words (1879, p. 190): " Schon nach ij Stunde
waren die an der Grenze zwischen Violett und Ultra-
violett (H-J) befindlichen Pflanzchen nach vorn genelgt.
Nach Ablauf von etwa f Stunde folgten die im mittleren
Violett und Ultraviolett aufgestellten; eine Viertelstunde
spater neigten sich die im Indigo stehenden, 10 Minuten
hierauf die im Blau, nach weiteren 20 Minuten die im
Griin und Ultraroth stehenden, sodann, nach einer Viertel-
stunde die im aussersten Roth, und nach einer weiteren
Viertelstunde die im Roth von B-C. Die Keimlinge in
Gelb und Orange standen jetzt, d. i. nach vollen 3 Stunden,

3i6

UGHT ANV THE BEHAVIOR OF ORGANISMS

noch vollig aufrecht. Eine Stunde spater hatten die vom
Indigo bis ins Ultraviolett reichenden Keimlinge sich
stark hankenformig gegen die Lichtquelle hinge wendet,
gleichzeitig neigte sich das im Orange stehende Pflanzchen
schwach vor. Der im Gelb befindUche Keimling bheb
aber bis ans Ende des Versuches vollkommen aufrecht."
The roots of Sinapis alba (white mustard) were found to
respond in the spectrum in all essentials like the stems of
Vicia sativa, except that they turned from the light in place
of toward it.

Fig. 35. Graphic representation of the reaction of several plants in light diflfer-
ing in wave-length. A-H represent Fraunhofer lines in the spectrum. The
curves i, 2, and 3 were constructed by tabulating as ordinates the reciprocals of
the time required to induce a response under the different light conditions: i, stem
of Vicia seedling; 2, stem of cress seedling; 3, etiolated willow shoot. After Wiesner
(1879, p. 191).

The results of numerous observations on the reactions
of different seedlings behind different absorbing media all
of which were spectroscopically tested are graphically rep-
resented in Fig. 35. It is interesting to note that these
results agree fairly well with those obtained in the spec-
trum, although the different colors were in no instance
monochromatic. Wiesner claims that yellow is not merely

REACTION OF PLANTS IN COLORS 317

neutral, but that it actually causes a retardation in reac-
tion when mixed with other active rays.

Miiller (1872) obtained varying results in his study of
the reactions of seedlings in the solar prismatic spectrum.
He found the region of maximum effect for cress to be in
the blue at line F, and that for Sinapis alba in the yellow
and upper part of the green between lines D and E. He
claims that this difference is due to the difference of absorp-
tion of light under different conditions and in different
plants. The farther a plant is from the prism the farther
the maximum extends toward the red end of the spectrum.
It may even extend into the infra-red. In case of cress
seedlings, e.g., set in a row extending from the prism, it
was found that those farthest away were neutral in violet
and blue but still reacted in green and yellow, whereas
those nearer the prism responded most strongly in the
violet and blue. Miiller thinks this is due to difference in
absorption of different rays under different conditions.

The work of Sachs in 1864 and later, as already stated,
led to conclusions similar to those of Payer. Sachs
studied the reactions of various seedlings under double-
walled bell jars, some of which were filled with ammoniacal
solution of copper hydrate and others with potassium
bichromate. The former transmitted violet to green in-
clusive, the latter yellow to red. The seedlings under
the copper solution curved strongly, while those under
the chromate remained straight. Similar results were ob-
tained behind cobalt and ruby glass. These results are
not in accord with those of Guillemin, Wiesner and others,
who found that seedlings responded in the longer wave
lengths. The fact that Sachs could get no reaction in red
produced by sunlight passed through a solution of potas-
sium bichromate was probably, as Pfeffer points out
(p. 176), " the result of feeble intensity of the light used,
or of the special properties of the experimental material."
The experiments of Kraus (1876) with colored screens
show that the stalks of the perithecial heads of the fungus

3l8 LIGHT AND THE BEHAVIOR OF ORGANISMS

Claviceps microcephala turn toward the source of light
nearly as rapidly in the red as in the blue, and Brefeld
obtained similar results for Pilobolus microsporus and
Pilobolus crystallinus.

Some of the contradictory results mentioned above are
evidently due to the fact that the authors did not take into
account the effect of the time of exposure and the inten-
sity of the light. If the blue, for instance, is more active
than the red, the maximum curvature will take place in
the blue in weak illumination, while a minimum curvature
will take place in this region in very strong illumination;
for plants either become negative in high light intensity or
fail to respond. Recently Blaauw (1909) made a thorough
investigation of the reactions of plants in different regions
of the spectrum with these facts in mind. He calculated
the relative efficiency of different rays in terms of energy
contents and time of exposure of the reacting organ, and
found for oats seedlings, Avena sativa, that in medium light
intensity and time of exposure there is a slight reaction from
the red end of the spectrum to the green, 500^^, then a
rapid increase to a maximum in the indigo 465^'', and a
decrease to zero well in the ultra-violet. For equal energy
and time of exposure the reaction is 2600 times greater in
the region of maximum efficiency than in the red, yellow
and green, twice as great as between the violet and the
ultra-violet, 390^^, and only four times as great as in the
ultra-violet at 365''^.

In case of the fungus Phycomyces, Blaauw found the
effect of red and yellow relatively much greater than in
Avena. In other respects the distribution of efficiency in
the spectrum was found to be similar in the two forms.

In spectra of very high intensity produced by means of
a grating it was found that the minimum rate of curvature
for Phycomyces is in the indigo, and that there are two
regions of maximum rate of curvature, one in the red, the
other in the violet. This, the author maintains, is due to
the fact that Phycomyces is positive in weak light (100-

REACTION OF PLANTS IN COLORS 319

150 ca. m. seconds), neutral in strong light (100,000-
200,000 ca. m. sec.)/ and negative in very strong light
(2,000,000-12,000,000 ca. m. sec). However, the red and
yellow rays are, under all conditions, relatively more effi-
cient in the molds than in green plants. The difference
in the distribution in the spectrum of the stimulating
efficiency in different intensities of light may possibly
account for the discrepancies in the results of former
investigation in this field.

Summary

(i) According to Gardner all visible rays are active in
producing curvature in plants; according to Dutrochet
and Pouillet, Guillemin and Miiller, plants respond to all
visible rays and some ultra-violet and infra-red as well;
according to Wiesner they respond to all rays in the visi-
ble spectrum except some yellow and orange, and possibly
some beyond at either end; according to Kraus and Bre-
feld the red is nearly as active as the blue in causing
reactions in the molds; according to Blaauw all the visible
rays of the spectrum and some ultra-violet produce cur-
vature in oats seedlings and molds, but the longer waves
are relatively more active in the latter than in the former;
according to Sachs and Payer only the shorter waves are
active. Thus it is seen that all but Sachs and Payer
obtained reactions to the longer as well as to the shorter
waves of the spectrum, and since nearly all of these inves-
tigators used relatively pure prismatic colors and efficient
methods in other respects, it is evident that the great bulk
of evidence goes against Sachs' conclusion that only the
shorter waves are active in light reactions in plants.

(2) The experimental results of all the investigators

^ " Candle-meter seconds " indicates the product of the time of exposure
and the intensity of the light. This, Blaauw maintains, is a constant for
the threshold of a given plant, that is, the higher the intensity, the shorter
the time of exposure required to produce a reaction.

320 LIGHT AND THE BEHAVIOR OF ORGANISMS

mentioned above agree in that they indicate that the
region of maximum effect for all the plants tested is lo-
cated somewhere toward the violet end of the spectrum.
It may therefore be definitely concluded that the distri-
bution in the spectrum of stimulating efficiency for plants
is not primarily dependent upon energy or brightness as
judged by the human eye, since the maximum for these is
located in the yellow.

(3) The reactions to light in plants are in all proba-
bility associated with photochemical changes induced by
the light. We have seen that photochemical changes are
specific. If, e.g., one chemical reaction takes place only in
blue and another only in green, other conditions being the
same, it may be concluded that the chemical constituents
taking part in the two reactions are not the same. The
experimental results presented above show that the effect
of the different rays on reactions is not the same for all
plants. It is therefore probable that the photochemical
changes associated with the reactions to light are not the
same in all plants. I do not consider this point definitely
established, owing to the possible effect of difference in
selective absorption of light in the different plants. It is
however more strongly supported by the observed reac-
tions in unicellular forms than by those in sessile plants.

CHAPTER XVII

THE RELATIVE EFFECT OF DIFFERENT RAYS ON THE
REACTIONS OF UNICELLULAR FORMS

The first observations on the effect of different colors
on the movement of unicellular forms were made by
Cohn in 1865, nearly fifty years after similar observations
had been made on sessile plants by Poggioli (18 17).
Cohn's account of his observations is very brief. He
studied the movements of swarm spores in colors differ-
entiated by means of colored glass and concluded that the
blue rays are the most effective and that the red act like
total darkness. He says (1865, p. 222), '' Die Organismen
werden von den blauen Strahlen am starksten angezogen,
wahrend sich die rothen wie totale Finsterniss verhalten."

I. Strashiirger's Experiments

Much more extensive and conclusive results were
obtained by Strasburger (1878), w^ho also studied swarm-
spores of various kinds, but principally Botrydium.
Strasburger exercised the utmost precaution in his experi-
ments. Many of the observations were made in a dark
room in light of different colors produced by a quartz
prism in a horizontal beam of direct sunlight. The slit
in the opaque screen over the prism was only 0.4 mm.
wide and the spectrum at the point of exposure 55 mm.
long. The Fraunhofer lines could be clearly seen. It is
therefore evident that there was but little intermingling of
rays of different lengths in the spectrum. In addition to
the spectrum, colored glass and various solutions were
used. The results led Strasburger to conclude that the
blue, indigo and violet light alone cause orienting reactions,
but that yellow, red and green cause a quivering movement

321

322 LIGHT AND THE BEHAVIOR OF ORGANISMS

(" zitternde Bewegung ") in some swarm-spores. ''Die
blauen indigofarbigen und violetten Strahlen sind allein
auf die phototaktischen Schwarmer von Einfluss und liegt
das Maximum der Wirkung im Indigo " (p. 623), These
conclusions in general support those of Cohn, but it appears
quite probable that in higher light intensity the swarm-
spores would have been found positive in the longer waves
as well as the shorter, since, as Strasburger states, the red
and yellow produced indefinite reactions in the low inten-
sity in which they were exposed. It is worthy of note
that Strasburger and Wiesner both obtained practically
the same effect on reactions with slightly impure colored
light produced by means of filter screens as they did with
monochromatic light produced by means of prisms. And
the same appears to be true in all other experiments on
organisms without eyes, in which such a comparison has
been made, as, e.g., in the observations of Harrington and
Learning on Amoeba and in my own work on the same
organism referred to in detail later. This indicates that
the reactions of these organisms in waves of a given length
are not appreciably affected by the presence of waves of a
different length and that extreme precautions to eliminate
all foreign rays are not necessary in studying their reac-
tions to different colors. In more sensitive forms, however,
especially in those with well-developed eyes, monochro-
matic light is indispensable.

2. Engelmann s Experiments

Engelmann's experiments on unicellular forms in spec-
tral colors are of the highest character and the greatest
interest. He exposed the organisms in monochromatic
light of solar, gas, and electric microspectra thrown on the
slide by means of a prism attached to a microscope, and
noted not only the regions in which they aggregated, but
also the reactions during the process of aggregating. He
divides these creatures into three classes based upon their
different reactions, as follows:

UNICELLULAR FORMS AND COLOR 323

a. Diatoms and oscillaria with different species of Na-
vicula and Pinnularia as types (1882, p. 390). — These
organisms are active in dayhght or in the yellow, orange
and red of the spectrum, but become quiet if the Hght
intensity is decreased or if they are transferred to the green,
blue or violet regions. In these colors they may however
become active again if the intensity is sufficiently in-
creased. It is evident that owing to these reactions they
would tend to collect in the spectrum toward the violet
end, or at least leave the opposite end, since they become
quiet whenever they chance to get out of the yellow,
orange or red. This region however has the greatest
stimulating efficiency. Engelmann thinks that the reac-
tions of these organisms are dependent upon the liberation
of oxygen, and that they react primarily to the longer
light waves because of the liberation of oxygen when they
are exposed to them. This conclusion is based upon the
fact that a reduction of oxygen pressure without a change
of light intensity or color causes cessation of movement.
It appears then that these organisms do not respond to
sudden changes in light intensity and that the reaction to
light is dependent upon the absolute intensity rather than
upon change of Intensity.

b. Ciliates which have chlorophyll with Paramecium
bursaria as a type (1882, p. 393). — Engelmann found that
these organisms do not react to light at all, unless the
oxygen pressure is reduced below the normal. If the air
surrounding the preparation Is replaced with hydrogen
they become very active (" sehr unruhig ") and aggregate
in the region of highest light intensity. Under such con-
ditions they also collect in the red of the gas or solar micro-
spectrum between the lines B and C at 650-700''''.
Engelmann describes the process of aggregation as fol-
lows: " Ueberschreiten sie z. B. zufalllg die Granze von
Licht und Dunkel, oder tauchen sie auch nur mit der vor-
deren Halfte Ihres Leibes eine Strecke welt in das Dunkel
ein, so kehren sie sofort um nach dem Licht, wie wenn das

324 LIGHT AND THE BEHAVIOR OF ORGANISMS

Dunkel ihnen unangenehm ware." The process of col-
lecting in regions of given light conditions in these forms
is therefore precisely the same as that which Jennings
found some fifteen years later with reference to the col-
lection of Paramecium caudatum and aurelia in regions
containing carbon dioxide.

It is evident that in Paramecium bursaria a reduction of
light intensity or a change from regions illuminated by
the longer waves to one illuminated by the shorter causes
a definite response. Whether or not this response would
under certain conditions result in orientation in these
organisms as it does in Euglena was unfortunately not
ascertained. Nor was it ascertained whether or not they
ever become negative, i.e., respond to an increase of inten-
sity or to a change from the shorter waves to the longer.

Engelmann thinks that the reactions of these organisms
to light is regulated by the oxygen liberated, that it is a
change in the oxygen pressure that produces a stimula-
tion. The response w^hich results in aggregation in these
organisms is however, undoubtedly, at least indirectly
dependent upon the time rate of change of light intensity
rather than upon the absolute intensity, and their reaction
system and method of collection in a given region are
evidently quite different from those in diatoms.

c. Flagellates with Euglena viridis as a type (1882,
P- 395)-^ — The light reactions of the organisms in this
class are not primarily dependent upon . oxygen pressure.
They respond to light in the same way whether the oxygen
pressure is normal or above or below normal, unless it is
carried to such extremes that all activity ceases.

These organisms were found to form dense aggregations
in the more highly illuminated regions, just as Paramecium
bursaria does when the oxygen pressure is below normal,
but in the microspectrum they collect in the blue near
the Fraunhofer line F, 470-490^^, not in the red where

^ Bacterium photometricum (1883, pp. 95-124), a form on which Engel-
mann worked later, might also be included here.

UNICELLULAR FORMS AND COLOR 325

P. bursarla collects. The method of collection of Euglena
is described as being the same as that of P. bursaria. The
region in which they aggregate " wirkt wie eine Falle,
denn einmal hineingekommen, gehen die Euglenen in der
Regel nicht wieder aus. Sie kehren an der Grenze des
Dunkels immer sogleich wieder nur ins Helle." Engelmann
does not mention the fact that these forms orient and that
they may become negative, and of course did not realize
that owing to these reactions they can move directly
toward the optimum, so that the aggregation is not
entirely due to random movements. According to the
work of Engelmann, then, a reduction of light intensity or
a change from blue in the region of the Fraunhofer line F
to any other color of the spectrum stimulates Euglena and
causes it to turn and proceed in a different direction.

Like Euglena, so specimens of Bacterium photometricum
collect in the more highly illuminated regions of their
environment, but in the microspectrum, unlike Euglena,
most of them collect in the infra-red between 800 and
900"^, some in the orange between 580 and 610"" and a
few elsewhere. In general they collect in those regions
where the absorption bands for the coloring matter they
contain are found. Their reactions to light are independ-
ent of oxygen pressure. The method of collection is
described as being similar to that of Euglena. A reduc-
tion of intensity or a change In color from that in which
they collect to any other causes the bacteria to swim sud-
denly backward (" zuriick schiessen ") 10 to 20 times
their length, after which they proceed in the ordinary
way again. This reaction has been designated " Schreck-
bewegung." An increase of intensity or movement into
the regions of the spectrum where they collect does not
produce the " Schreckbewegung," neither does a gradual
decrease of intensity. Engelmann found but little evi-
dence indicating that these organisms orient, and did not
ascertain whether or not they become negative in very
high intensity.

326 LIGHT AND THE BEHAVIOR OF ORGANISMS

3. Verworn's Experiments

The experiments of Verworn (1889) yielded results
which led him to conclude that the diatom Navicula bre-
vis reacts only to the shorter waves, while Oscillaria reacts
to all the waves in the visible spectrum. " Als die allein
wirksamen Lichtstrahlen erwiesen sich auch bei den Dia-
tomeen die kurzwelligen " (p. 49); " Die Versuche, welche
sich auf die Ermittelung der wirksamen Strahlen bezogen,
hatten das ganz unvermuthete Ergebniss, dass Strahlen
von alien Wellenlangen ungefahr von der Linie a bis
iiber G hinaus die Bewegungen der Oscillarien beeinflussen.
. . . Die Ansammlungen der Oscillarien war nach Ein-
schaltung von Rubinglas oder Kalibichromatlosung, auch
im Halbdunkel, ebenso vollkommen, wie bei Anwendung
von griinem Glas, Kobaltglas, Kupferoxydammoniaklosung
oder reinem Sonnenlicht " (p. 51). In these experiments
Verworn mounted the organisms on a slide and studied the
movements under a microscope, surrounded by a tight case
which was black inside. The case contained an opening
15 X 20 mm. in one side to admit light. Colored glass
plates or flat flasks containing different solutions could be
so adjusted as to admit only light which passed through
them. The colored media used were thoroughly tested
spectroscopically (" genau spektroskopisch untersucht ").
Five different media were used. The red glass transmitted
red and orange (600-740"") ; the cobalt glass, blue and violet
(410-510"") and a little infra-red; the green glass, yellow
and green, and a little orange and blue (480-600^''') ; the
potassium bichromate solution, red, orange, yellow, and a
little green (550-740"") ; and the ammoniacal solution of
copper hydrate, blue and considerable green (430-520^0-

The Oscillaria were positive; they collected at the side
nearest the light, even if it was of very low intensity
(" Halbdunkel "), no matter which one of the media was
in the opening. The diatoms, on the other hand, moved
toward the light only in the shorter waves; they did not

UNICELLULAR FORMS AND COLOR

327

respond behind the ruby glass or the potassium bichro-
mate solution, and only indefinitely behind the green
glass even in direct sunlight. It is therefore evident that
the distribution in the spectrum of stimulating efficiency
is not the same in these two forms, for the diatoms react
only to the shorter waves, while Oscillaria reacts to the
longer as well as to the shorter. Whether or not the
reaction of Oscillaria to the different rays is proportional
to the energy can however not be definitely ascertained
from the data at hand.

4. Experiments of Harrington and Learning on Amoeba

One of the most interesting of the investigations on the
effect of different colors on the reactions of protozoa is
that of Harrington and Leaming (1900). These authors
projected amoebae on a screen with a Zeiss photomicro-
graphic apparatus, and studied the effect of sudden changes
in color and intensity on the movements. The light rays
were differentiated by means of Bierstadt's colored celloi-
din plates. The results of numerous observations, all
recorded in detail, are summarized in the following table,
which has been slightly modified (p. 16). The table shows
clearly that the violet is more effective than any other
color tested, except white, in causing retardation in move-
ment, and since orientation is the result of such retarda-
tions in movement the violet must also be more effective
in regulating this phenomenon.

TABLE X

White

Violet

Green

Yellow

Red

White following

0

-5?

— I

— I

— I sec.

Violet

+ 5

0

— 20

-24

-9 "

Green

+ 5

+ 12

0

?+l2

0 "

Yellow "

+ 1

+ 3

0

0

0 "

Red

+ 1

+ 2

0

0

0 "

Zero indicates that there was no change in movement when the light
conditions were changed as indicated; the numbers preceded by — indicate
the average time in seconds before streaming stopped or decreased; and the
numbers preceded by + indicate the average time before streaming started
or increased.

328 LIGHT AND THE BEHAVIOR OF ORGANISMS

It is claimed that a sudden increase of intensity causes
an immediate cessation of movement either in white Hght
or in light containing only the shorter waves, but that
" after a few minutes [exposure] streaming will commence
under any light if the amoeba be a fairly active individual"
(p. 1 6). The authors appear to think that some colors
actually cause an increase in the rate of movement in the
amoebae: " That red is the most powerful excitant to flow
is indicated by the shorter latent period after quiescence
in white light." There is however little evidence sup-
porting this conclusion.

Harrington and Leaming did not ascertain the intensity
of light transmitted by the different filters, and the light
was not spectroscopically examined, so that the purity of
the colors used remains unknown. Similar results were
however obtained in spectral colors, but the authors un-
fortunately have not described how the spectrum was
produced or what kind of light was used as a source.
Their results have occasionally been seriously questioned.

5. Original Observations on Amoeba

Owing to the questionable character of some of the
results of Harrington and Leaming and to the fact that
the region in the spectrum of maximum effect on the
movement could not be definitely located from their data,
it seemed desirable to have the experiments repeated.

a. Experiments with color filters. — Early in June,
1909, Dr. H. S. Jennings put at my disposal an excellent
culture of Amoeba proteus, which had come up in hay
infusion used in rearing Paramecia. Specimens of this
culture were studied both in a solar prismatic spectrum
and in different colors produced by means of filters which
were kindly furnished by Dr. R. P. Cowles. The filters
were prepared and spectroscopically tested in the physical
laboratory of Johns Hopkins University. The red was
transparent from 620^^ out, opaque from 450 to 590^^" and

UNICELLULAR FORMS AND COLOR 329

faintly transparent from 380 to 450''''. The blue was trans-
parent from 430 to 490^^ and from 690^'' out, and opaque
from 590 to 670''''. The green was transparent from 380 to
400"^, from 450 to 550'''' and from 680"'' out. It was
opaque between 580 and 660 ''^ and faintly transparent
between 400 and 450''''.

Several amoebae were mounted under a large cover-
glass surrounded by a thin ring of vaseline so as to pre-
vent evaporation, and give ample space for free movement.
In this inclosure they were found to remain active and
in excellent condition for several days. The observations
were made on the stage of a compound microscope under
a magnification of about 150 diameters with very faint
illumination from the mirror. A beam of direct sunlight
which passed through 8 cm. of water was thrown on the
slide at an angle of about 45° with the stage.

The organisms were exposed to light of different colors
by intercepting the beam with the colored filters. It was
found that amoebae which moved actively in weak diffuse
light ceased moving shortly after being suddenly exposed
to strong red light, but soon began again. If they were
now exposed to green the movement again ceased; the
same was true for blue after green and for direct sunlight
after blue. A change from direct sunlight to blue, blue
to green, or green to red, produced no apparent effect.
After being exposed to any color or any combination of
colors for a short time the movement was resumed. In
direct sunlight or in blue light it required longer than in
green or red. As a matter of fact, in these two colors, in
the red in particular, there was no cessation of movement
in some specimens, and only a slight decrease in others,
while in still others the movement stopped entirely. In
case of direct sunlight or blue, on the other hand, the
movement stopped abruptly in nearly every specimen
almost as soon as exposed. Similar but somewhat more
detailed results were obtained in the spectrum.

b. Experiments with solar spectrum. — In these experi-

330 LIGHT AND THE BEHAVIOR OF ORGANISMS

ments a horizontal beam of direct sunlight was passed
through a vertical prism and thrown on the mirror below
the stage of the microscope, from which it was reflected
to the slide. By manipulating the mirror the amoebae on
the slide could be suddenly subjected to light in any part
of the spectrum, and the color to which they were exposed
could be instantaneously changed.

The vertical slit in the opaque screen over the face of
the lens was 2 mm. wide and the spectrum on the slide
nearly 3 cm. long. There was consequently some over-
lapping of rays in adjacent parts of the spectrum, but there
was no intermingling of rays in distant parts. For example,
in the red there was some orange, but no rays of shorter
wave lengths.

The amoebae were examined in daylight so faint that
they could scarcely be seen. After a specimen active in
this light had been selected, it was suddenly exposed to
any desired part of the spectrum and the reaction noted.

Many observations were made on numerous individuals
between 10 a.m. and i p.m., June 16 and 18. The sky
was clear, and the intensity of light consequently at a
maximum, approximately 5000 ca. m. Without going into
details with reference to reactions of individual specimens
it may be stated that the effect of sudden exposure td
red,^ yellow or violet after very faint diffuse sunlight was
essentially the same. There was in many specimens a
slight decrease in rate of movement, in some a momentary
cessation, and in others no apparent reaction whatever.
In the green the effect was similar to that in red, yellow
and violet, only somewhat more marked. To obtain the

* The wave lengths are designated in terms of color.

Red = 630-760'^''

Orange = 590-630^^

Yellow = 560-590'^''

Green — 490-560'^''

Blue = 430-490'^''

Violet = 395-430''''

Ultra-violet = 340-395''^

UNICELLULAR FORMS AND COLOR 331

effect described above it is necessary (i) to have amoebae
in a certain condition, (2) to keep them in as low light
intensity as possible before exposing, and (3) to use very
intense light. When exposed in blue after having become
active in any other color or in diffuse sunlight, all move-
ment stopped instantly in nearly all specimens observed.
But there was no apparent contraction; the animals re-
tained almost the exact form they had before the exposure.
After remaining quiet a few seconds, the streaming of the
protoplasm in the anterior pseudopods slowly began again,
but now it nearly always proceeded in the same direction.
Gradually new pseudopods were formed, usually at the
posterior end, and as these developed the old ones were
slowly withdrawn. The rate of movement ordinarily in-
creased at such a rate that after 30 to 60 seconds it was
again normal. If any other part of the spectrum was
flashed on an amoeba which had become active in the blue
there was no apparent reaction, but when such a specimen
was exposed to direct sunlight it was clearly seen, in some
instances, that the streaming ceased again.

The results obtained in these experiments lend support
to the general conclusions of Harrington and Leaming.
They differ from their results only in a few details. I
found red and yellow to have a slight effect on the move-
ment of Amoeba. Harrington and Leaming did not, prob-
ably owing to deficiency in light intensity or to exposure
in too great an intensity preceding the exposure to red or
yellow. I found only a very slight stimulation in the
violet, whereas they recorded no difference between the
effect of blue and violet.

It may then be definitely concluded that the blue rays,
430 to 490"", have a very marked effect on the rate of
movement of Amoeba, while the violet, green, yellow,
orange and red rays have only a slight effect; and since
the direction of movement is in all probability regulated
by changes in the rate, it is evident that the blue rays are
also of primary importance in this process.

332 LIGHT AND THE BEHAVIOR OF ORGANISMS

It will be seen at once that the effect of different parts
of the prismatic solar spectrum on the movement of
Amoeba is not proportional to the energy contents, for
the energy gradually increases as one proceeds from the
violet toward the red end, whereas the region of maximum
stimulation for this animal is in the blue, from which it de-
creases toward both ends. Nor is it proportional to the
brightness as judged by the human eye, for the yellow is
much brighter than any other part of the spectrum. In
fact, under the conditions of the experiment, one could
hardly bear to look through the microscope when the
yellow was reflected, while in the case of blue, the region of
maximum stimulation for Amoeba, there was no unpleasant
stimulation whatever to the eye.

One of the most interesting characteristics in the re-
actions of unicellular forms is the variation in the loca-
tion of the region of maximum stimulating efficiency in
the spectrum. The experimental results presented above
indicate that for the swarm-spores it is in the indigo, for
Amoeba it is in the blue, while Oscillaria appears to be
stimulated equally by all the visible rays. This indicates
that, as in plants so in unicellular forms, the chemical
changes associated with the reactions to light are not the
same in all of the different species (see p. 320).

CHAPTER XVIII

REACTIONS OF MULTICELLULAR ANIMALS IN LIGHT
CONSISTING OF WAVES DIFFERING IN LENGTH

I. Experiments of Wilson on Hydra

Among the most thorough and rehable experiments on
the reactions to Hght of different wave lengths are those
of Wilson (1891) on Hydra. Wilson had "a fraternity of
Hydras five hundred to a thousand strong all of which
had arisen in [a large] aquarium [in a north room] from a
group of three or four progenitors" (Footnote, p. 415).
The window side of the aquarium was divided into equal
areas which were covered with s.trips of red, yellow, green,
blue or colorless glass. In some instances there were two
areas of each color, one with a single thickness of glass,
the other with two, producing two fields of the same color
but o*r different intensity.

The activities of the animals and the changes in posi-
tion were studied and recorded for a week, during which
the glass strips were frequently rearranged. It was
found that the Hydras tend to collect in the colorless
region and in the more intensely illuminated regions of
each color, i.e., back of the areas covered with only one
thickness of glass, but that with reference to the different
colors they tend to aggregate in the blue even if the inten-
sity in it is much lower than that in any other region.
The tabulated results of two series of observations will
serve to emphasize this. These results were obtained
after rearranging the plates so as to change the color of
the different regions.

333

334 LIGHT AND THE BEHAVIOR OF ORGANISMS

TABLE XI. (After Wilson, i8gi, p. 424-)

Yellow decrease [in number of Hydras] .... 56 per cent.

Red " " " " " 55 " "

Green " " " " " 70 " "

Blue increase " " " " 92 " "

TABLE XII. (After Wilson, p. 427.)

Total increase [in number of individuals during period of
observation] 421 to 674, i.e., 60 per cent.

Blue, increase 327 per cent.

Yellow, decrease 30

Dark screen, decrease 37

Daylight, increase 30

a (I

Wilson says that hydras are positive in blue, that they
go fairly directly toward the source of light, and that the
other colors are inactive, but he does not show definitely
how the aggregations in the blue are formed. We shall
discuss the movement in different colors more in detail
later.

The colors produced by the plates of glass used were
not monochromatic. Thorough spectroscopic examina-
tion showed that the red transmitted a little orange, the
yellow some green, orange and red, the green some yellow
and a trace of red, and the blue some indigo and violet
and a trace of green and red. It is not at all likely that
such slight transmission of foreign colors as represented
above modifies the reactions of organisms that have not
well-developed eyes and are no more sensitive to light
than Hydra, although much has been said regarding this
and many results have been branded worthless owing to
the use of slightly impure colors. Wilson fortunately con-
firmed the results obtained with colored glasses by critical
tests in a spectrum produced by focusing light from an
Argand gas burner on a narrow slit in an opaque screen
in front of a large carbon bisulphide prism. '' The appa-
ratus was placed in a perfectly dark underground room and
every pains was taken, by the use of suitable screens, etc..

MULTICELLULAR ANIMALS AND COLOR 335

to exclude from the aquarium all light excepting that
proceeding from the prism" (Footnote, p. 430). In the
spectrum, which was about three inches long, the hydras
showed a very marked tendency to collect in the lower
blue, from line G to line F, and for a slight distance in the
green. They were wholly indifferent to the lower rays,
the violet and ultra-violet, as well as to all those above the
lower green, including the infra-red. It should be empha-
sized that they are not negative in these colors. This was
shown both by their reactions in the spectrum and by
those under colored glass.

The observations of Wilson seem to prove conclusively
that the blue is most active in stimulating both Hydra
viridis and Hydra fusca. This stimulating activity of the
blue is specific; it bears no definite relation to the distri-
bution of energy or of brightness, both of which are fairly
definitely located for the gas-light spectrum, the region of
maximum energy and brightness being well toward the
red end.

One of the striking peculiarities in the results obtained
by Wilson is the fact that whereas hydras collected most
abundantly in the regions of highest intensity under given
color conditions, more were regularly found in the blue than
in daylight, when they were given a choice between these
two conditions of light (see Table XII), although the latter
contained at least as much blue as the former and was of
course more intense owing to the presence of other rays.
This result is similar to that obtained by Lubbock (1888)
on daphnias, which were found to collect more freely in
yellow and green light than in daylight.

Loeb appears to doubt the accuracy of these results.
Referring to those of Lubbock he says (1905, p. 10):
" One half of a dish was covered by a yellow screen; the
other half was left uncovered. In the uncovered half, 1,904
animals collected, while 3,096 gathered under the yellow
screen. From this Lubbock concludes that Daphnia has
a * preference ' for ' yellow.' But one would suppose that

336 LIGHT AND THE BEHAVIOR OF ORGANISMS

in the uncovered part of the dish there was at least as
much yellow light as under the yellow screen; or did the
majority 'hate' the blue light?" Referring to observa-
tions on Porthesia chrysorrhoea he says (1905, p. 29),
" This experiment shows that the more refrangible rays have
the same effect as mixed rays.''

The methods of both Lubbock and Wilson were how-
ever such as to leave little room for doubt regarding their
results. It is interesting to note that the recent work
of Stobbe (1908) on the photochemical changes in the
organic compounds known as fulgides demonstrates reac-
tions which proceed more rapidly in monochromatic light
of a given intensity than in the same light in combination
with other rays. These experiments have been referred to
under the section on photochemical reactions, p. 310. It is
likely that the chemical changes in Hydra and Daphnia
associated with their light reactions are of the nature of
certain fulgides.

2. Bert's Experiments on Daphnia

The first experiments dealing with the effect of differ-
ent colors on the reactions of multicellular animals were
made by Paul Bert in 1868. Bert was interested in
color vision. He attempted to answer the question as to
whether the specific effect on the different rays in the
spectrum is the same in the inferior animals as it is in
man. He attacked the problem from a purely psychologi-
cal point of view, as the title of his paper published in
1869 indicates: " Sur la question de savoir si tous les ani-
maux voient les memes rayons que nous."

An electric-light spectrum was thrown on the flat side
of an aquarium covered with an opaque screen containing
a narrow vertical slit. Daphnias were exposed to the
different colors of the spectrum, and it was found that
they collected at the side of the aquarium nearest the
light, no matter which part of the spectrum was allowed

■MULTICELLULAR ANIMALS AND COLOR 337

to enter the slit. The organisms were therefore positive
in all colors; the reactions however were more rapid in
the yellow and green than in other parts of the spectrum.
Bert says (1869), " II fut facile de remarquer, qu'elles
accouraient beaucoup plus rapidement au jaune ou au
vert qu'a toute autre couleur."^

When the opaque screen was removed so as to expose
the daphnias to the entire spectrum at once, most of them
collected in the yellow and green. Bert was of the opinion
that the distribution of effect in the spectrum is the same
in Daphnia as it is in man, and he concluded that the col-
lection of Daphnia in the yellow and green part of the
spectrum and the great activity in these colors is not
due to color vision, but to the fact that the light intensity
in this part of the spectrum is higher than elsewhere.
He was of the opinion that light affects these animals
much as it does the human being, with reference to
brightness, that the yellow for them as for man is the
brightest and consequently the most effective part of the
spectrum.

Results similar to those recorded by Bert were obtained
by Merejkowsky (1881) on Dias longiremis and larvae of
Balanus, by Lubbock (1881) on Daphnia, and by Yerkes
(1900) on Simocephalus.

Merejkowsky states that he exposed Dias longiremis and
Balanus larvae in light of different colors but of equal
brightness and found no evidence of preference. The valid-
ity of these results is, however, questionable, since it is by
no means certain that the brightness of the different colors
used in these experiments was actually equal.

3. Lubbock's Experiments on Daphnia

Lubbock's experimental methods and results are far
more convincing than those of Merejkowsky. He pub-
lished his interesting observations on Daphnia in the

^ Taken from Loeb (1905, p. 9). Page in original not given by Loeb.

338 LIGHT AND THE BEHAVIOR OF ORGANISMS

Journal of the Linnean Society in 1881. The following
account is however taken from a later publication (1882,
pp. 212-231).

In these experiments Lubbock projected a prismatic
solar spectrum arranged by Professor Dewar at the Royal
Institute, vertically downward on a wooden trough 14
inches long and 4 inches wide. In this trough he put
50 specimens of Daphnia pulex, scattered them equally
through the water, and after ten minutes inserted glass
partitions so as to divide the trough into compartments
corresponding in size with the five principal colors of the
spectrum. He then recorded the number of individuals in
each, after which he repeated the process. The total for
ten trials follows: 5 in the violet, 32 in the blue, 298 in
the green, 74 in the yellow, 90 in the red and one in the
dark part of the spectrum.

In comparing these results it is necessary to consider
the fact that in a prismatic spectrum the red and green
are each much more than twice as wide as the yellow; and
the blue and violet are each wider than the green. Lub-
bock allowed three-fourths of an inch for the yellow and
two inches for the green. Correcting for this difference
in width the calculated number in the yellow would have
been 196. Lubbock concludes (p. 214), "It will be
observed . . . [that] there were more Daphnias in pro-
portion, as well as absolutely, in the green, although the
yellow is the brightest portion of the spectrum."

It was also found that when daphnias were exposed in
the green, yellow and red of a normal spectrum, they col-
lected in the green rather than in the red. In these experi-
ments the region of highest intensity in the middle of the
field was shaded. After ten minutes' exposure 410 speci-
mens were found in the green end, 14 in the shaded area
and 76 in the red. These results indicate clearly that if
brightness alone controls the reactions of Daphnia, it
must be different for them than it is for the human eye.
Numerous convincing experimental results showing that

MULTICELLULAR ANIMALS AND COLOR 339

this animal is positive in ultra-violet indicate the same
thing.

Lubbock's primary object in this work, however, was to
test the color vision of Daphnia. I can do no better than
to quote at length the experiments which bear on this
question:

'' I placed (March 26) fifty Daphnias in a trough (i),
covering over one half of it w^th a pale green, and another
fifty in a trough (2) half of w^hich w^as covered with yellow
(aurine). On one side was a similar trough (3), one end
of which was shaded by a porcelain plate; and on the
other side a fourth trough (4), one end of which had a
little, though but little, extra light thrown on it by means
of a mirror. As before, I counted the Daphnias from
time to time, and turned the troughs round. All four
were in a light room, but not actually in direct sunshine.
Thus, then, in one trough I had half the water in some-
what green light; in the second trough, half the water in
yellow light; in the third, one half was exposed and the
other somewhat darkened; while the fourth, on the con-
trary, gave me a contrast with somewhat more vivid
light. If, then, the Daphnias w^ent under the green and
yellow glass, not on account of the color, but for the sake
of shade, then in trough 3 a majority of them would have
gone under the porcelain plate. On the other hand, if the
porcelain plate darkened the water too much, and yet the
open water was rather too light for the Daphnias, then in
the fourth trough they would, of course, have avoided the
illuminated half. The results show that the third trough
was unnecessary, still I may as well give the figures; the
fourth proves that the Daphnias preferred a light some-
what brighter than the ordinary diffused light of the room.
Of course it does not follow that the effect of color is the
same as with us " (p. 226).

The results of twenty trials are recorded, but since all
are essentially the same I shall quote only the following
five (p. 227) :

340

UGHT AND THE BEHAVIOR OF ORGANISMS

TABLE XIII.

(After

Lubbock, p. 227.)

Trough I

Trough 2

Trough 3

Tro

ugh 4

Green

White

Yellow

White

Ex-
posed

Dark-
ened

Illumi-
nated

Unillumi-
nated

light

light

light

light

half

half

half

half

March 28

7-30

2>2>

17

34

16

35

15

30

20

7-5°

32

18

37

13

27

23

32

18

8.10

34

16

2>3>

17

29

21

30

20

8.35

36

14

35

15

26

24

^2>

17

9.05

26

24

27

23

Z?>

17

35

15

•

161

89

166

84

150

100

160

90

In another series of experiments in which one half of
one dish was covered with a ruby glass and that of another
dish with blue glass, the majority of the animals collected
in the uncovered portion of both dishes. It is evident
from the results in troughs 3 and 4 above that the daph-
nias were positive to the highest light intensity used in
these experiments. It may then be assumed that they
were positive to the light conditions in which the majority
collected in troughs i and 2, and in the experiments with
red and blue. In the former however they collected in
the part of the trough having the lower intensity, i.e., in
the green and yellow respectively, in preference to white,
whereas under all the other conditions they collected in
that portion of the trough having the higher light intensity.

While these results do not demonstrate subjective color
sensation, I am unable to see how they can be explained
without assuming a specific effect depending upon the
length of the waves or the color iadependent of intensity
or brightness.

Lubbock's conclusions are very cautiously summed up
in the following paragraphs (p. 231):

" My experiments, I think, show that while the Daph-
nias prefer light to darkness, there is a certain maximum
of brilliancy beyond which the light becomes inconven-

MULTICELLULAR ANIMALS AND COLOR

341

iently bright to them, and that they can distinguish be-
tween hght of different wave-lengths. I suppose it would
be impossible to prove that they actually perceive colors;
but to suggest that the rays of various wave-lengths pro-
duce on their eyes a different impression from that of
color, is to propose an entirely novel hypothesis.

" At any rate, I think I have shown that they do dis-
tinguish between rays of different wave-lengths, and prefer
those which to our eyes appear green and yellow."

The striking positive reaction to yellow and green in
preference to white light of a higher intensity seems to
indicate that Daphnia is negative to the other rays of the
spectrum. This question has been discussed elsewhere.

4. Experiments of Yerkes on Simocephalus

Yerkes (1899) made a very thorough study of the re-
actions of Simocephalus vetulus, a form similar to Daphnia,
both in gas and sunlight spectra. Every reasonable pre-
caution was taken in the manipulation of the apparatus.
The method employed was like that used by Lubbock on
Daphnia. The following table shows the relative numbers
which collected in the different regions of the two spectra:

TABLE XIV. (After Yerkes, 1899.)

Gas spectrum

Sunlight spectrum

Red

24.7 per cent

23-3 " "
6.2 " "

5-3 " ''
0.8 " "

9.7 per cent

35.2 " "
14.6 " "

25-5 " "
08" "

Yellow

Green

Blue

Violet

Since Simocephalus is positive even In direct sunlight, it
is safe to say that It Is positive to the light conditions of
that part of the spectrum In which it aggregates. The
table above shows clearly that in the gas spectrum most of

342 LIGHT AND THE BEHAVIOR OF ORGANISMS

the specimens collected in the red and yellow, whereas in
the sunlight spectrum most of them collected in the yel-
low, green and blue. In the former, then, they collected
nearer the red end than in the latter, and since the bright-
est part of the gas spectrum is somewhat nearer the red
end than it is in the sunlight spectrum, Yerkes concluded
that the reactions of Simocephalus are dependent upon
intensity rather than upon color.

The results appear to me to demonstrate that intensity
is undoubtedly a factor in the reactions of Simocephalus,
but they do not appear to demonstrate that the reactions
are independent of the length of the waves of light. The
distribution in the spectrum of the power to stimulate
Simocephalus agrees roughly with that of brightness for
the human eye. If corrections are made for the difference
in width between yellow and blue in the sun spectrum,
then the 25 per cent for the blue, which is fully twice as
wide as the yellow, will be reduced to about twelve per
cent, and the percentage in the green will also be reduced
somewhat, showing that the yellow is the most active by
far. This however is not in opposition to the conclusions
reached from Lubbock's results, that the reactions depend
upon the length of the waves as well as upon the ampli-
tude. On the contrary, the fact that the distribution in
the spectrum of stimulating efficiency in these forms
does not correspond with the distribution of energy, indi-
cates that the reactions are dependent upon the wave
length, possibly in some such way as brightness is depend-
ent upon the wave length. This does not mean that the
chemical changes and the mechanism in general is the
same in these forms as that associated with brightness
sensations in man; and of course it does not demonstrate
the presence of brightness sensations in these Crustacea,
as the conclusions of Bert and Lubbock might lead one
to suspect.

Considering the results of all the experiments on the
daphnias referred to above, it may be concluded: (i) that

MULTICELLULAR ANIMALS AND COLOR 343

these organisms, contrary to the hypothesis of Loeb and
Davenport, are most strongly affected by the green and
yellow rays; (2) that this effect is dependent primarily
upon the length of the waves and secondarily upon the
amplitude or energy; (3) that these organisms can be
stimulated by the ultra-violet and by all the rays in the
visible spectrum, except perhaps those near the infra-red;
(4) that the stimulating efficiency is not proportional to
the energy contents; and (5) that the distribution in the
spectrum of stimulating efficiency in these organisms dif-
fers from that in a majority of the lower forms, in which
it has been clearly demonstrated that the blue or violet
rays are the most active. This indicates that the chemical
changes associated with the reactions are not the same in
all organisms.

5. Experiments of Graher on Various Animals

The experiments of Graber on the reactions of animals
to colored light are the most extensive of any yet made in
this line. He tested 54 different species, — 5 mammals,
7 birds, 2 reptiles, 3 amphibia, 2 fishes, 3 moUusks, 27
insects, 2 spiders, and 3 worms. Nearly all of these
species have well-developed eyes. Besides these in the
normal state a blinded amphibia and a blinded insect were
tested.

Following the work of Bert and Lubbock, Graber at-
tacked the problem from the psychological point of view.
The foremost question with him appears to have been:
Do the animals perceive color? In nearly all the experi-
ments he studied the change of distribution of the animals
in suitable boxes or troughs, the two halves of which were
illuminated with light of different colors or different inten-
sities, and recorded the number which collected in each
half of the inclosure. He does not state how the animals
reacted so as to aggregate in one or the other of the light
conditions. The aggregations may have been due to

344 LIGHT AND THE BEHAVIOR OF ORGANISMS

positive orientation to the light conditions in which the
animals collected, or to negative orientation to the other
condition; or the organisms may have wandered into the
light in which they collected by random movements and
have remained because of a definite reaction when the
border of this area was reached, somewhat similar to the
reactions of Paramecium on reaching the limit of an area
containing carbon dioxide; or they may have remained
because they came to rest somewhat as planarians come
to rest in a given light condition under some circum-
stances. It is therefore impossible to be certain as to the
interpretation of many of the results. Moreover only
a limited number of different colors was used, so that
the maximum effect in the spectrum cannot be located.
We shall consequently consider only a few of Graber's
observations.

Colored glass and solutions were used almost exclusively
to differentiate the rays. These were all, however, thor-
oughly examined spectroscopically, and the relative inten-
sity of the light transmitted was also fairly accurately
ascertained. In this work Graber had the assistance of
the physicist Professor Mach. The lower forms only are of
interest to us here.

Lumbricus is well known to be negative in its light reac-
tions. Graber's results are in harmony with this. In a
trough one half of which was shaded he found over five
times as many in the shaded region as in the illuminated
region. When one half of the trough was red and the other
blue there were nearly five times as many in the former as
in the latter, and in the case of red and green, and green
and blue, the majority collected in the light having the
longer wave lengths. In all these experiments the worms
were undoubtedly negative to the light conditions they
avoided.

The red in these experiments contained rays between
6io and 710"'^; the blue rays between 550 and 570'"'",
those between 700 and 720'^''', and those below 540'^'^; the

MULTICELLULAR ANIMALS AND COLOR 345

green rays between 450 and 600'^'^. In the red-blue and
green-blue tests the intensity of the red and green was
twice as high as that of the blue. In the red-green test it
was a little more than twice as high in the red as in the
green. In these tests, however, the worms consistently
collected in the light having the higher intensity, whereas
in white hght the opposite was true. However, when
white light containing ultra-violet was contrasted with
white light without, but of decidedly higher intensity, the
great majority collected in the light having the higher
intensity.

Considering the conditions of these experiments I do
not hesitate to conclude that blue, violet and ultra-violet
are more efficient in causing reactions in Lumbricus than
green or red. Similar conclusions are strongly supported
by the reactions of various other species, notably the
snail Limnaeus stagnalis and several insect larvae as well
as imagos. In numerous other instances, however, as
already intimated, the results are not conclusive; in these
it is questionable whether the quality of light has any
specific functions and it is impossible to say to what the
change in distribution is due.

The following conclusion of Graber is undoubtedly not
warranted (1884, p. 245): " Als eines der allerwichtigsten
und interessantesten Ergebnisse meiner vergleichenden
Lichtgefiihl-Studien betrachte ich die Tatsache, dass die
leukophilen oder weissholden Tiere mit geringen Ausnahmen
alle blatdiehend, die leukophohen oder dunkelholden hingegen
rotliehend sind^

The results of these experiments appear to indicate that
in general animals which are positive in white light are
also positive in blue, whereas those which are negative in
white are negative in blue, not positive to red, as Graber's
conclusions would suggest. This is however not univer-
sally true, as the experiments on Daphnia clearly show.

The ideas in Graber's conclusions expressed in the quo-
tations above and elsewhere, that animals actually perceive

346 LIGHT AND THE BEHAVIOR OF ORGANISMS

color and that the reactions are controlled by subjective
sensation, are evidently without experimental foundation.
They are reached purely through human analogies.

6. LoeVs Observations

The accounts of Loeb's experiments on the effect of
different colors on reactions are found in two papers,
published in 1890 and 1893 respectively. Of the two
colors, red and blue, used in these experiments, the former
was produced by means of a solution of potassium bichro-
mate or ruby glass, and the latter by means of " cobalt
glass or an ammoniacal solution of copper." In each of
these two colors the reactions of the following animals
were studied: Musca larvae, plant lice, caterpillars of
Porthesia chrysorrhoea, moths of Sphinx euphorbia and
Geometra piniaria, various copepods, the meal worm Tene-
brlo molitor, and larvae of the June bug Melolontha vul-
garis, Limulus polyphemus, and Polygordius.

Loeb maintains that these forms react in blue light
essentially as they do in white; that the negative forms are
negative in the blue light and the positive forms positive;
and that the red rays have a slight effect on the move-
ment of some of the animals but none on that of others.
He concludes that in all of these animals " the more
strongly refrangible rays of the visible spectrum are the
most active heliotropically, as in the case of plants "
(1905, p. 294). He also says (1905, p. 73), " I have con-
firmed the identity of animal with plant heliotropism on
crabs (Gammarus locusta, Cuma Rathkii), naked snails
and worms (leeches, planarians, earth-worms and others),"
but he does not state definitely that he studied the reac-
tions of these forms in colored light.

In harmony with the results of Graber's experiments, as
well as with those of various other investigators, Loeb's
results show fairly clearly that blue is more efficient than
red in stimulating the organisms he tested, but they do

MULTICELLULAR ANIMALS AND COLOR 347

not show that it is more efficient than yellow or green or
any other rays in the spectrum. It is difficult to under-
stand how he could conclude that in animals '' the more
strongly refrangible rays of the visible spectrum are the
most active heliotropically, as in the case of plants," and
'' that the more refrangible rays have the same effect as
mixed rays " (1905, p. 29), after studying reactions in but
two different colors.

CHAPTER XIX

BRIEF CONSIDERATION OF THE REACTIONS OF MULTI-
CELLULAR ANIMALS WITH WELL-DEVELOPED EYES
IN LIGHT DIFFERING IN COLOR — WITH SPECIAL
REFERENCE TO COLOR VISION

In the lower animals with image-forming eyes the reac-
tions to colors are very much more complicated than in
those without them. In these there is but little evidence
that any one color is much more efficient than another.
They may be positive to or may select a given color at
one time and a very different one at another. Investiga-
tions in this line are still few and methods inadequate. We
shall present only a few of the more conclusive. Foremost
among these may be mentioned those of Lubbock on ants,
bees and wasps. Of these we shall devote special atten-
tion to the work on ants and bees.

I. Ants

In his earlier experiments with ants Lubbock (1895,
p. 186) placed strips of glass differing in color, or glass jars
containing colored solutions, side by side over an artificial
nest.^ After leaving them for a few minutes he recorded
the number in each of the different colors, then rearranged
the color media and repeated the process. In twelve
different observations there was a total of 890 ants under
the red, 544 under the green, 495 under the yellow and
only 5 under the violet. The results of numerous other
observations under similar conditions were in all essentials
like them.

^ These color media were spectroscopically tested by Professor Dewar.

348

COLOR VISION 349

In later experiments Lubbock used more refined methods.
He had a prismatic electric-hght spectrum thrown upon a
nest especially prepared for the purpose. Ten different
experiments were made with this, the results of which were
similar. I shall quote one in full:

" Professor Dewar kindly prepared for me a condensed
pure spectrum (showing the metallic lines) w^ith a Siemens'
machine, using glass lenses and a mirror to give a perpen-
dicular incidence when thrown on the nest. ... I arranged
the light and the ants as before, placing the pupae in the
ultra-violet, some being distinctly beyond the bright
thalline band. The ants began at once to remove them.
At first many were deposited in the violet, some, however,
being at once carried into the dark beyond the red. When
all had been removed from the ultra-violet, they directed
their attention to those in the violet, some being carried,
as before, into the dark, some into the red and yellow.
Again, w^hen those in the violet had all been removed,
they began on the pupae in the red and yellow, and car-
ried them also into the dark. This took nearly half an
hour. As I had arranged the pupae so that it might be
said that they were awkwardly placed, we then turned the
nest round, leaving the pupae otherwise as they had been
arranged by the ants; but the result of moving the nest
was to bring some of them into the violet, though most
were in the ultra-violet. They were, as before, all carried
into the dark space beyond the red in about half an hour.

'* We then turned the glass round again, this time
arranging the end about the length of the spectrum be-
yond the end of the violet visible to our eyes. They began
clearing the thalline band, carrying some into the violet, but
the majority away further from the spectrum. In a quarter
of an hour the thalline band had been quite cleared; and
in half an hour a band beyond, and equal to the thalline
band, those in the violet being left untouched. After
the pupae in the ultra-violet portion had all been moved,
those in the violet were also carried away and deposited

3 so LIGHT AND THE BEHAVIOR OF ORGANISMS

about twice as far from the edge of the violet as the further
edge of the bright thaUine band " (1895, pp. 203-205).
Considering the results of all these different experi-
ments, Lubbock concluded (p. 199) " that: (i) ants have the
power of distinguishing colours; (2) that they are very
sensitive to violet; and it would also seem (3) that their
sensations of colour must be very different from those
produced upon us." We shall discuss these conclusions
later.

Many experiments with various color and intensity con-
ditions were performed, in which the light in one part of
the nest was passed through carbon bisulphide so as to
eliminate the ultra-violet. It was found in general that,
other conditions being equal, the ants avoid the light con-
taining ultra-violet rays. These rays, although invisible
to man, appear therefore to stimulate the ants somewhat
like the rays which are visible. These results agree with
those of Lubbock on Daphnia and those of Graber on the
earthworm, and ten different species of insects as well
as a few other forms. It is also well known that para-
mecia and various bacteria can be stimulated by ultra-
violet. Stimulation by these rays therefore appears to be
fairly common among animals.

Lubbock assumes that ultra-violet as well as the visible
rays in the spectrum produces color sensation in ants.
He says (1895, p. 220): " These experiments seem to me
very interesting. They appear to prove that ants per-
ceive the ultra-violet rays. Now, as every ray of homo-
geneous light which we can perceive at all appears to us
as a distinct colour, it becomes probable that these
ultra-violet rays must make themselves apparent to the
ants as a distinct and separate colour (of which we can
form no idea), but as unlike the rest as red is from yellow,
or green from violet."

Very few will agree with Lubbock in assuming that he
has demonstrated color vision — subjective sensation in
ants. His results however are reliable. There can be no

COLOR VISION 351

question but that these creatures while in their nests
avoid white light, and particularly rays of the shorter
wave lengths, and that red is much less efficient in stimu-
lating them than any other color. The effect of the dif-
ferent rays is at least to some degree specific. The dis-
tribution of efficiency in stimulating ants in the prismatic
solar spectrum is certainly not proportional to the dis-
tribution of energy or brightness as judged by the human
eye. But why this is true and what mechanism is involved
in the process of avoiding the shorter waves are questions
upon which Lubbock's results have no definite bearing.

Can the reactions of ants to colors be explained by
assuming that they are negative to rays of the shorter
wave lengths, and that they are oriented by the light in
the sense of Loeb's definition of heliotropism, or in any
other definite way? In the absence of larvae or pupae there
is some evidence indicating that ants orient and move
from light containing the shorter waves, and that their
movements are fairly definitely controlled by external con-
ditions, but in the presence of these organisms there is no
evidence showing that their reactions are thus definitely
controlled. Under such conditions internal factors must
have much to do with the reactions. In transferring
larvae and pupae, as in the experiment of Lubbock quoted
above, a given individual may pass back and forth many
times from the red end of the spectrum to the violet and
ultra-violet before all the young are deposited in the red
or beyond. It cannot be maintained that they become nega-
tive to violet light when they are carrying their young and
positive when they are not; for this opposes the fact that
in the absence of larvae and pupae they avoid the violet.
During the process of transferring their young the ants
cannot therefore be considered either negative or positive
to the violet or to any other color or condition of illumi-
nation. These reactions must be regulated primarily by
internal factors. What these factors are is a question con-
cerning which there is yet very little knowledge. That the

352 LIGHT AND THE BEHAVIOR OF ORGANISMS

reactions are adaptive, that it is to the advantage of the
larvae and pupae, as well as to the adults, to be in darkness
or in rays of longer wave lengths while in their nests,
rather than in those of shorter, can scarcely be questioned.
And Lubbock's suggestion that " ants do not like light in
their nests, probably because they do not deem it safe,"
if liberally interpreted, may not be so far from the truth
as some investigators assume (see Loeb, 1905, p. 13).

2. Bees

1

In the study of the effect of different colors on the
behavior of bees Lubbock showed the same characteristic
thoroughness and ingenuity manifested in his work on
ants. Several pieces of paper which differed in color were
pasted to glass slips, upon each of which a drop of honey
was placed. A bee was then taken from a hive, marked
and placed near the honey on one of the glass slips. After
the bee had taken honey to the hive and returned several
times the glass slip was removed to a distance of from one
to three feet, and one of a different color was put in its
place. When the bee now returned it seldom went to the
honey over the new color in the old position; it usually
returned to that over which it had been accustomed to
collect honey, although it was now in a new position.
The reactions to various different colors were tested in this
way. In case of blue being the original color visited by
the bee, it returned 31 times to the blue, two times to the
green and not at all to the yellow, orange, red and white,
one of which was substituted for the blue between each
visit. In case of green being the original color the bee
returned to the green 20 times, to the blue twice, to the
yellow once and not at all to the other colors. In case of
orange it returned 20 times to the orange, and but twice to
other colors which were not recorded. These experiments
extended over several days and a number of different bees
were used.

COLOR VISION 353

In a second set of experiments Lubbock trained a bee
to come to a lawn for honey placed on a piece of colorless
glass. He then procured several similar pieces of glass of
different colors, arranged them on the lawn so that they
were all about one foot apart, and put a drop of honey on
each. After the bee returned it was frequently disturbed,
so that it was compelled to sip honey several times, either
over the same color or over different ones, before it left for
the hive. The order in which it visited the different colors
was recorded, and every time the bee left for the hive the
relative position of all the glass plates was changed.

Many different series of observations were made in this
way under various conditions; the results in all are however
essentially the same. I shall therefore present only a por-
tion of one series. In this series (1895, p. 307) the bee came
first to the blue 31 times, to the green 10, to the orange 11,
colorless 5, red 14, white 19, and to the yellow 9 times.
This shows that the blue is visited much oftener than any
other color, although the bee was trained to get honey
from the colorless piece of glass.

Graber (1884, pp. 167-174) obtained similar results in
comparing the effect of red with that of blue by means of
a different method. He inclosed the bees in a box one
half of which was illuminated with red, the other half with
blue light. In some experiments the blue was much
brighter than the red, in others the red was brighter than
the blue. In every test a majority of the individuals
inclosed collected in the blue.

The first set of experiments led Lubbock to conclude
" that bees possess the power to distinguish colours "
(p. 302) ; and the second, that they prefer blue.

Perhaps the most interesting of Lubbock's results is the
demonstration that honey bees can be trained to select
any given color. This shows that they can in some way
distinguish color and that the different rays and combina-
tions of rays must have a specific effect on them; but it
does not prove that they have color vision, for color-blind

354 LIGHT AND THE BEHAVIOR OF ORGANISMS

persons can also distinguish different colors if they differ
in brightness as did those in Lubbock's experiments.
It does, however, demonstrate that internal factors are of
primary importance in these reactions. The selection of
a given color now and a different one some other time,
flight into the pitch darkness of their home at one moment
and out into the brightest sunlight the next, is surely not
the result of orientation unequivocally controlled by the
immediate environment. These reactions can be explained
only upon the assumption that some internal condition
regulates the change in reaction.

Concerning the second conclusion of Lubbock, that bees
" prefer blue," it must be said that if this is anthropo-
morphically interpreted there is no solid foundation for
the conclusion, but if it is merely intended to indicate that
shorter waves having a given amount of energy stimulate
bees more strongly than longer waves having the same
amount of energy, there can be no doubt as to its validity,
unless the bees used in the experiments of Lubbock and in
those of Graber had been accustomed to collect honey from
blue flowers before the tests were made.

Aside from those already mentioned there are numerous
other references to the reactions to color in ants, bees
and other arthropods in the literature on these subjects.
Among these may be mentioned those of Minkiewicz (1907),
Keeble and Gamble (1900), and Bell (1906) on decapod
Crustacea, those of the Peckhams and McCook on spiders,
and those of Graber, Forel, Plateau, Buttel-Reepen, Bethe,
Bulman, Miss Fielde, Darwin, Muller and Bonnier on ants,
bees, wasps and other insects. Much of the work of the in-
vestigators in the last group was directed toward the ques-
tion as to the influence of the color of flowers on the visits
of insects with its bearing on their evolution. Most of
the results of this work favor the negative of this question,
but nearly all of these investigators agree that insects have
color vision, although their evidence is far from conclusive.
With reference to reaction to color, none of the work of any

COLOR VISION 355

of the authors mentioned above is as thorough as that
of Lubbock. And since it leads to no essentially new or
contradictory conclusions, with the exception of that of
Bethe, it would be of but little value to review it here.
That of Minkiewicz is nevertheless somewhat out of the
ordinary, and it may consequently not be out of place to
devote a few paragraphs to it.

/

3. Higher Crustacea — Experiments of Minkiewicz

Before presenting the work of Minkiewicz on the Crus-
tacea, it will be necessary to refer briefly to his earlier obser-
vations on the nemertean Lineus ruber, since these form
the basis of his later work.

Minkiewicz exposed these worms in horizontal beams
of light of different colors produced by means of a prism,
colored glass or tissue paper. Under normal conditions
they were found to be negative in blue or green and positive
in red or yellow. But if left for several hours in 100 c.c.
of sea water diluted with 25 to 80 c.c. of distilled water,
they became positive to the more refrangible rays of the
spectrum. The striking peculiarity of these reactions is
that in colorless light the organisms wxre negative under
all conditions. He says (1907, p. 48): '' I have not as yet
found, in spite of long continued researches, a single means
of transforming the negative phototropism of Lineus into
positive phototropism by agents either chemical, osmotic
or thermic. Thus, for example, the animal remains nega-
tive until its death in the presence of white light whatever
the concentration of the sea water."

The author concludes that all the chromatic rays have a
specific action independent of each other and of white light.

Among the Crustacea, Minkiewicz experimented with
spider crabs and hermit crabs. His studies on the former
were devoted primarily to Maja verrucosa and Maja squi-
nado, but he claims to have made analogous observations
on different species of Pisa, Inachus and Stenorynchus.

356 LIGHT AND THE BEHAVIOR OF ORGANISMS

It is well known that many of the spider crabs fasten
seaweeds and other objects to the carapace. Minkiewicz
placed the crabs into aquaria with the sides and bottom
uniformly colored and added bits of colored paper, some
corresponding to the walls of the aquarium and others not.
He claims that the animals selected those pieces which
harmonized with their environment in color, and fastened
them to the surface of the body and legs so that they became
inconspicuous. " Les resultats sont ... vraiment f rap-
pants, la couleur du costume correspondant toujours
precisement a celle du milieu " (1907, p. 41). In a black
aquarium however there was no evidence of selection, and
it appears that the animals were unable to distinguish
between green and yellow.

The author also says that if the crabs are left on a
given color for some time and then transferred to an
aquarium which is variegated in color, they come to rest
in that part which corresponds in color with that from
which they were taken.

In the experiments on the hermit crabs Minkiewicz
illuminated the two halves of an aquarium with light
of different colors, placed the animals so that the two
eyes were exposed to light of different colors, and found
that the creatures turned toward the color indicated by the
arrows below; i.e., in case of black and red in the aquarium
they went toward the red, in case of red and yellow, toward
the yellow, etc., as indicated:

black -^ red — > yellow -^ blue -^ violet-^ green — > white.

If the crabs are kept in a jar and exposed to their own
excreta for some time their reactions to colors gradually
change as follows:

Normal : red — > blue ^^ green.
red -^ green — ^ blue,
green -^ red — > blue,
green -^ blue — > red.

COLOR VISION 357

Minkiewicz maintains that these reactions cannot be due
to intensity difference, since the light in the yellow and green
under the conditions of the experiments was more intense
than that in any other color, and the organisms were posi-
tive to blue in the presence of yellow, but positive to green
in the presence of violet.

The most interesting of the results obtained by Minkie-
wicz refer to the change in reaction to different colors.
The spider crabs apparently become positive to the color
which is dominant in the environment. Lineus and
Pagurus, positive to a given color under certain conditions,
become negative to that color under different conditions,
or positive to some other color, while they remain con-
tinuously positive to white light. These reactions have
much in common with those of the honey bee to different
colors. They show that the creatures, especially the spider
crabs, can distinguish colors. This of course does not
demonstrate the power of subjective color sensations.
It does however indicate that the different rays cause
different changes in the organisms; in other words, that
they have specific effects w^hich are in some way related
to the wave lengths. There is, however, no evidence bear-
ing on the question as to whether or not these effects are
analogous to those associated with brightness sensation or
with color sensation in man or with neither.

The results of Minkiewicz must unfortunately be accepted
with reserve, since he does not describe his methods in
sufficient detail to warrant definite conclusions as to their
validity, and they have as yet not been confirmed, although
similar experiments have been made on other forms.

Pearse has recently (1909) repeated the experiments
of Minkiewicz at Woods Hole, Mass., using Libinia emar-
ginata in place of Maja. He obtained no evidence what-
ever of decoration in harmony with the environment. I
have observed many of Pearse's experiments and repeated
some myself, and feel justified in saying without going
into details, that there was no evidence of color selec-

358 LIGHT AND THE BEHAVIOR OF ORGANISMS

tlon. Bateson (1887) records similar results in work on
Stenorynchus. Recently I have again tested crabs for
selection of color. At the Tortugas Islands numerous
specimens of each of three species, not yet definitely
identified, were used in these tests. A large proportion
of all of the animals observed decorated profusely in the
colored aquaria used; but I found no evidence whatever
of harmony between the color of the substance selected
and that predominating in the environment, although the
methods used by Minkiewicz were closely imitated.

4. Fishes

I shall refer to but two other experiments on the subject
of reactions to colors, one by Washburn and Bentley on
the creek chub Semotilus atromaculatus, the other by
Reighard on the marine gray snapper, Lutianus griseus.

Washburn (1908, p. 140) gives the following description
of their experiment: '' 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 compart-
ments, and hung down into the water. Food was always
placed in the red pair of forceps, which were made fre-
quently 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 position or
the direction in which the fish had to turn. The animal
quickly learned to single out the red fork as the one impor-
tant 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 for-
ceps and rubber bands were changed so that no odor of
food could linger, it never failed to bite first at the red.

COLOR VISION 359

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 discrimi-
nated 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."

Reighard made his experiments on a school of gray
snappers, a form which usually inhabits the water under
a dock at one of the Dry Tortugas Islands. The gray
snappers feed on atherina, a small fish found in abundance
near the shore. They take these fish, even if they have
been killed in formalin and stained any color, but if Cassi-
opea tentacles are fastened to the atherinas they soon learn
to avoid them. After the gray snappers had learned to
reject red atherinas with tentacles it was found that they
also rejected red ones without, but that they still took
those stained any other color. For example, when blue
and red atherinas were thrown in together they took only
the blue, and this was true even if some of the red ones
were of a much brighter shade and others of a much darker
shade than the blue ones.

This seems to prove that the selection could not have
been due to difference in brightness, such as a color-blind
person can perceive in the different colors, and it led the
author to conclude that ^ray snappers have color vision.

It will be seen that the conclusion that fishes have color
vision, both in the work of Washburn and Bentley and in
that of Reighard, rests primarily upon the fact that the
animals discriminated between red of different shades and
blue or green, and upon the assumption that the brightness
of the different parts of the spectrum is practically the same
for fishes as it is for man — that their eyes are stimulated by
all the rays from the infra-red to the ultra-violet somewhat
as ours are. While this may be true, it has not been posi-
tively demonstrate-d. As a matter of fact, there are reasons
for believing that the red end of the spectrum for fishes

360 LIGHT AND THE BEHAVIOR OF ORGANISMS

and some other vertebrates, e.g., the dancing mouse and
color-blind persons, is much darker than it is normally
for man, and that the visible spectrum for these forms is
somewhat shortened at this end. It is evident that the
red, which appeared brighter than the blue to the human
eye, may have actually appeared darker to the fishes, and
if this be true the discrimination may have been made
on the basis of brightness. There consequently remains
some doubt as to the validity of the conclusion stated
above.

Even in the birds and mammals the question of color
vision is not settled, although these animals can undoubt-
edly distinguish different regions in the spectrum. But
since it is not our object to discuss this subject we shall
refer the reader to the excellent researches of Porter (1904,
1906) on the birds, Yerkes (1907) on the dancing mouse,
Kinnaman (1902) and Watson (1909) on the monkey, and
Cole (1907) on the raccoon.

5. General Summary and Conclnsio?ts of Part IV

(i) The energy curve in both normal and prismatic
spectra is much the same for nearly all sources of light.
Beginning with the violet end it rises more or less gradu-
ally to a maximum at the red end, 760''^. In the normal
or grating spectrum for sunlight, however, the maximum
is in the orange at 610"". From this point the energy
decreases slightly toward the red end.

(2) The location in the spectrum of the maximum effect
on photochemical reactions depends primarily upon the wave
length and the chemical substances which take part in the
reaction; and secondarily upon the absorption of light, the
distribution of energy and the presence of substances which
apparently do not take part directly In the reaction. The
reaction between quinine and chromic acid, e.g., takes place
most rapidly in the ultra-violet, whereas Triphenylfulgid is
changed from the black form to the yellow most rapidly in

COLOR VISION 361

the presence of red, orange and yellow rays, and photosyn-
thesis in plants proceeds most rapidly in the red and orange.
The photochemical reaction in a given substance is therefore
primarily dependent upon the length of the light waves.
The reaction between quinine and chromic acid is affected
only by the light absorbed by the quinine, but the effect
is not proportional to the absorption. The maximum ab-
sorption takes place in the ultra-violet, but the maximum
efficiency is in the green. This shows that the location
of the maximum effect is dependent upon the power of
absorption and upon the distribution of energy as well as
upon wave length. Ozone in the presence of chlorine is
changed to oxygen by the visible light rays, while pure
ozone is not, showing that photochemical reaction in a
given substance may depend upon the presence of other
substances.

(3) The distribution of brightness as judged by the human
eye is approximately the same for both normal and pris-
matic spectra of sunlight and gaslight. But the distri-
bution of energy in these spectra differs considerably. It
is therefore evident that brightness must be primarily a
function of the length of the waves and secondarily a func-
tion of the amplitude or energy contents. In lower in-
tensity the maximum is near the green, in higher it is in
the orange. In color-blind persons it is usually in the
green.

(4) In the higher plants the maximum rate of curvature
in the spectrum takes place at the lower limit of the violet,
but there is a secondary maximum in the red. In some
fungi, however, the rate of curvature takes place under
potassium bichromate as rapidly as it does under an
ammoniacal solution of copper hydrate. That is, it takes
place as rapidly in the longer waves of the spectrum as it
does in the shorter. In the swarm-spore the region of
maximum stimulation in the solar prismatic spectrum is in
the indigo near the Fraunhofer line G. In Amoeba, Euglena
and Hydra it is in the blue, in Paramecium in the ultra-

362 LIGHT AND TEE BEHAVIOR OF ORGANISMS

violet, in Daphnia and Simocephalus in the yellow or green,
in Bacterium photometricum probably in the infra-red
with a secondary maximum in the orange, while in Oscillaria
all rays appear to be equally efficient. In plasmodia,
protoplasmic streaming in cells, earthworms, some moUusks,
and a number of insects and spiders, the region of maximum
stimulation is probably somewhere toward the violet end
of the spectrum, although it has not been definitely lo-
cated. In nearly all organisms without image-forming
eyes^ the relative stimulating efficiency of the different rays
is apparently constant under different conditions, but in
the forms with eyes there is evidence that it varies. Some
of the spider and hermit crabs, a number of insects and
spiders and many higher forms may be positive to certain
rays under certain conditions and to others under other
conditions. Bees and fishes can undoubtedly distinguish dif-
ferent regions in the spectrum. They can be trained to select
any of the primary colors of the spectrum by associating
these colors with food. That is, they are positive to (or
select) one color at one time and another at a different time.
Just what mechanism is involved in this power of selection
is unknown. Whether it is on the basis of brightness or on
the basis of color vision or neither is a matter concerning
which experimental evidence does not warrant a definite
conclusion. Many organisms react to ultra-violet much
as they do to visible rays. This is in harmony with the
following quotation from Schafer referring to man (1898,
p. 1055) : '' The invisibility of the infra-red rays is prob-
ably due to insensitiveness of the retina, while the ultra-
violet rays fail to be seen, partly, at any rate, owing to
absorption by the ocular media."

(5) The presence of certain rays retards the reaction to
others in a number of organisms. According to Wiesner,
some plants react to red more strongly than to red mixed
with yellow. And according to Lubbock and Wilson

^ Euglena appears to be an exception to this. According to the researches
of Engelmann it becomes positive to red in low oxygen pressure.

COLOR VISION 363

Daphnia reacts more strongly to green or yellow and Hydra
to blue than to white light. These reactions have much
in common with the reversible photochemical reactions of
certain chemical compounds, particularly the fulgides, in
which the reaction in one direction proceeds most rapidly
in the shorter wave lengths and in the opposite direction
in the longer, while in a mixture of rays the reaction pro-
ceeds more slowly in one or the other direction, depending
upon the relative amount of the different rays.

(6) Considering the results set forth above it is evident
(a) that, contrary to the hypothesis of Sachs, Loeb and
Davenport, the shorter waves are not the more active in
all plants and animals; the different rays do not have
the same relative stimulating efficiency in all organisms;
and (b) that the stimulating efficiency of the different
rays, probably in all organisms, is not proportional to the
energy they contain, but that for a given ray or color there
is a definite relation between the energy and the stimula-
tion which is probably in accord with Weber's law.

(7) Light, as we have seen, causes reactions between
many different chemical compounds. In these reactions
the different rays have a specific effect. That is, certain
reactions are produced only by waves of a given length and
others only by waves of a different length. If reactions
in a given chemical solution take place in light waves of
a given length, and those in another solution in waves of a
different length, we may be fairly certain that the reacting
compounds in the two solutions differ. The reactions of
organisms are caused by, or at least associated with, chem-
ical changes in the organisms. The organisms probably do
not react to the external agents directly, but to the chem-
ical changes within produced by these agents. Since the
reaction to the different rays is not the same in different
organisms, it is clear that the chemical changes associated
with the reactions in the different organisms are not the
same. For example, in Amoeba the maximum power of
stimulation is in the blue; this must be associated with

364 LIGHT AND THE BEHAVIOR OF ORGANISMS

certain chemical changes which are produced by the blue.
In Daphnia the maximum is in the yellow and green, and
in many of the plants it is in the violet. These reactions,
too, are associated with chemical changes, but since these
chemical changes are caused by rays differing in wave
length from those which are most efficient in Amoeba, the
chemical compounds must also be different unless the dif-
ference in the effect of the different rays can be accounted
for by assuming the presence of certain inactive substances
which influence the chemical reaction, or by assuming
selective absorption on the part of the organism. It is
however not likely that the difference in reaction to differ-
ent rays can be explained thus. We may then conclude
that the chemical changes associated with reactions are
not the same in all organisms. While we do not at present
know what these changes are, there are prospects that
future investigations along this line may demonstrate the
nature of some of them at least, especially after the photo-
chemical reactions in organic and inorganic substances have
been more thoroughly investigated.

(8) In plants and the lower organisms on which the
relative stimulating efficiency of the different rays is fairly
constant the chemical changes accompanying the reactions
may be relatively simple, but in the higher forms in which
the relative stimulating efficiency of the different rays
varies it seems evident that the chemical changes must be
very complicated. If this is true, it contradicts Loeb's
general conclusion that- the reaction mechanism associated
with photic responses in plants is the same as that in
animals, that " the dependence of animal movements on
light is in every point the same as the dependence of plant
movements on the same source of stimulation " (1905,
p. 81).

(9) Honey bees, some fishes, birds and mammals, and
probably some of the decapod Crustacea and spiders, can
unquestionably, and many of the lower forms with well-
developed eyes can probably, distinguish the different

COLOR VISION 365

regions of a spectrum. Whether the mechanical processes
associated with this discrimination are analogous to those
associated, with brightness or color vision in the human
being or neither is not known, but the processes in these
forms are undoubtedly very different from those associ-
ated with the reactions in simpler forms, e.g., in Amoeba
or plants.

CHAPTER XX

THEORETIC CONSmERATIONS

The following points have been established in the pre-
ceding pages:

(i) Movement and change in movement, both in rate
and direction, may take place without any immediate exter-
nal change.

(2) Sudden changes in light intensity on any sensitive
structure in an organism may cause reactions ; ^ for exam-
ple, the orientation of Euglena and the retraction of the
tubicolous annelids.

(3) Continued illumination without any variation of
intensity probably affects the rate of locomotion in all
organisms which respond to light, and it may cause changes
in direction of movement by inducing a reversal in the
sense of reaction. The time of exposure, as well as the
absolute intensity, is functional in this. In fact the prod-
uct of the time of exposure and the intensity is probably,
within certain limits, constant in producing a given stimu-
lus, no matter what the relative value of the two factors is.

(4) A sudden increase and a sudden decrease of light
intensity may under certain conditions produce the same
reaction, e.g., the contraction of Helix hortensis, the avoid-
ing reaction in Euglena and the raising and throwing of
the anterior end from side to side in planarians. Such
responses are more striking in some cases of stimulation
by temperature than in case of stimulation by light. Para-
mecium, for instance, gives the avoiding reaction to de-

^ In this discussion we shall consider anything which causes a change
of movement a stimulus, and any response to a stimulus a reaction. A
reaction, then, is either a change in rate of movement or in direction of
movement.

366

THEORETIC CONSIDERATIONS 367

crease as well as to increase of temperature, even if the
change is only slight.

(5) A given condition of illumination may inhibit one
kind of movement in an organism and cause movement
of another kind. When the oral end of Hydra viridis
is fully illuminated the swinging about the point of at-
tachment is inhibited and locomotion is produced. (See
Chapter VIII).

(6) An increase in the general illumination of an organ-
ism may cause an increase in activity, while a sudden de-
crease of intensity causes a still greater increase in activity
in the same organism at the same time. If the light inten-
sity on a Volvox colony under certain conditions is increased,
all of the zooids in the colony become more active, but those
on the shaded side of the colony become most active. The
rotation of the colony on the longitudinal axis causes a sud-
den decrease of intensity on the sensitive part of the zooids
as they are carried to the shaded side of the colony (see
Chapter VII), and the greater increase in activity of the
zooids on the shaded side is no doubt due to this sudden
decrease of intensity, while the activity of all the zooids
is probably augmented by the effect of the continued
illumination.

(7) An increase in light energy may produce the same
effect on reactions as a decrease in heat energy. Chla-
mydomonas, for example, becomes negative in constant
temperature if the light intensity is increased or in con-
stant illumination if the temperature is decreased. (See
Chapter XIII).

(8) Acids, certain narcotics and salts, and at least one
alkali, may cause a change in the sense of reaction from
negative to positive in Gammarus pulex. Any condition
which acts as a depressant may cause Ranatra or Arenicola
larvae to become negative.

(9) The stimulating effect of the different rays in the
spectrum is specific. But it is not the same in all organisms.
With a given amount of energy some are most strongly

368 LIGHT AND THE BEHAVIOR OF ORGANISMS

stimulated by blue, others by violet or ultra-violet, others
by green and yellow, and still others by red and infra-red
(see Summary to Part IV, Chapter XIX).

(10) Reactions to light are variable, modifiable, and in
general adaptive, (a) An attached specimen of S ten tor
coeruleus, for example, may contract suddenly when light
of a given intensity is flashed upon it, or It may merely
swing about its point of attachment or it may not respond
at all. Hydroides may remain in its tube after stimulation
by a given decrease of intensity only a few seconds, or It
may remain for several minutes. This difference In re-
sponse to the same external conditions must be due to
internal factors, {h) Wherever there is a reaction to a
sign, it Is probable that the response to a given external
condition has been modified. For example, Euglena under
certain conditions responds to a very slight decrease In
light intensity on the colorless anterior end, which Is In
itself of no consequence to the organism, but this slight
decrease In illumination Is usually followed by a greater
decrease on the entire body if there Is no change In the
direction of locomotion, and It Is of course for the welfare
of the organism to prevent this. It Is probable that
originally no response was given until the injurious condi-
tion was realized. Many similar illustrations are found
in organisms that respond to shadows which announce
the approach of an enemy, {c) Adaptation and regulation
are striking characteristics in nearly all reactions to light.
The reactions are adaptive not only under constant con-
ditions, but also under varying conditions, for if the environ-
ment Is changed the reactions change to meet the demands
of the new circumstances. Jennings has well said (1906,
P- 338): "Regulation constitutes perhaps the greatest
problem of life. How can the organism thus provide for
its own needs? To put the question In the popular form,
How does it know what to do when a difficulty arises?
It seems to work toward a definite purpose. In other
words, the final result of its action seems to be present in

THEORETIC CONSIDERATIONS 369

some way at the beginning, determining what the action
shall be. In this the action of living things appears to
contrast with that of things inorganic."

Let us now see in how far the various theories concern-
ing behavior account for the phenomena set forth above.
The more prominent of these are those of Loeb, Jennings
and Driesch. We have already referred to some theories
elaborated to account for the reactions of plants to light
(see Chapter IV) . These we shall not consider again here.

Loeb's theories refer to two features in behavior: (i) the
direct cause and regulation of any given reaction and (2)
the origin of adaptive reactions, (i) He says (1906, p. 130)
that the reactions " are caused by a chemical effect of
light " and then continues as follows, showing how the
reactions are regulated: "We assume . . . that if light
strikes the two sides of a symmetrical organism with
unequal intensity, the velocity or the character of the chem-
ical reactions in the photosensitive elements of both sides
of the body is different; that in consequence of this differ-
ence the muscles, or contractile elements, on one side of the
organism are in a higher state of tension than their antago-
nists." He claims (p. 131) that " it [is] possible by the use
of chemicals to control the precision and sense of the
heliotropic reactions " and that this and other facts prove
that reactions to light are caused by the chemical changes
produced by the light. Very few will agree that Loeb
has proved his point here. But practically every one
assumes that light does cause chemical changes in organisms
and that these changes affect the reactions. Many, how-
ever, do not agree with Loeb in the idea that they are the
direct and immediate cause of the reactions to light, as
his elucidation indicates. The fact that acids, narcotics,
salts, alkalis or any condition which acts as a depressant
may produce the same effect on the reactions of certain
organisms to light seems to Indicate that the reactions
are, at least In some Instances, due to a general effect on the
organism as a whole.

370 LIGHT AND THE BEHAVIOR OF ORGANISMS

There are however photochemical reactions which are
suggestively similar to some of the photic reactions in
organisms. We have shown in Part IV that in some com-
pounds the reactions proceed in one direction in one light
condition and in the opposite in another; that the action
of the different rays of light is specific, and that in some
of these chemical reactions heat and light tend to produce
opposite effects. The first two of these reactions are
somewhat similar to the change in the sense of reactions
produced by changes in light intensity in many organisms
and to the specific reactions to different rays. The last
is similar to the effect of heat and light on the sense of
reaction in Chlamydomonas and various other organisms
referred to under (7) above. If then these organisms
contain chemical compounds which are affected by light
like those referred to above, we can account for the reac-
tions mentioned by assuming that they are due to the
effect of the light on the chemical changes in the organism.
In case of Volvox I was also able to account for a number
of peculiarities in the process of reversal in the sense of
response by the assumption of reversible photochemical
reactions within the organism (Mast, 1907, pp. 157-161).
And we might account for the fact that an increase in
illumination produces the same effect as a sudden decrease,
as in the case of the zooids of Volvox (see (6) above),
by assuming two photochemical reactions, one dependent
upon the time rate of change of intensity, the other upon
the absolute intensity and the time of exposure. In this
same way the inhibition of one sort of movement and the
augmentation of another in the same organism (Hydra)
might be accounted for. So we might continue and account
for modifiability, variability, adaptation, etc., by various
other assumptions. But all of these assumptions regard-
ing chemical changes are so extremely hypothetical that
speculation based on them has at present but little
value. And it is important to realize that the common
belief that light in some way influences the activity

THEORETIC CONSIDERATIONS 371

of organisms by chemical changes which it causes within
them, is as yet founded almost entirely on such hypo-
thetical assumptions — assumptions which are problems,
not solutions.

(2) Loeb's explanations of the origin of adaptive reac-
tions to light is found in the following quotation (1906,
p. 160): " The fact that cases of tropism occur even where
they are of no use, shows how the play of the blind forces
of nature can result in purposeful mechanisms. There is
only one way by which such purposeful mechanisms can
originate in nature; namely, by the existence in excess of
the elements that must meet in order to bring them about.
In green plants and in some animals the positive heliotrop-
ism is useful; yet there exists probably an endless number
of heliotropic animals for which their heliotropism is about
as useless as is galvanotropism. The prerequisites for
heliotropism are a symmetrical body form, which seems
to be present in almost all organisms — although some
asymmetries exist — and the presence of photosensitive
substances, which is not quite so common, but certainly
not infrequent. Some of the regular substances found
in protoplasm seem to turn readily into a photosensitive
form. As the two conditions mentioned above are quite
common, the laws of probability make it necessary that
in a certain number of cases both conditions will be fulfilled,
and then we may expect heliotropic actions. If it now
occurs that in an organism the turning to the light helps
it to find its food, as is the case with certain caterpillars,
e.g., Porthesia chrysorrhoea, or the stems of green plants
whose starch is manufactured by light, we have a ' purpose-
ful mechanism.' Again, according to the laws of probabil-
ity, the number of animals in which the three groups of
conditions meet is much smaller than where only two meet.
The tropisms thus furnish an insight into the origin of
purposeful reactions by the blind forces of nature." The
difficulty with this hypothesis is that it does not fit the
facts. It rests primarily upon the assumption that there

372 LIGHT AND THE BEHAVIOR OF ORGANISMS

exists an endless number of organisms whose reactions to
light are useless. Indeed, according to this theory, there
must be more organisms in which the reactions to light
are or have been useless than there are in which they
are purposeful, for the author states, as quoted above,
that *' the number of animals in which the three groups
of conditions [purposeful mechanism] meet is much smaller
than where only two meet [useless reactions]." We have
demonstrated that, while there are isolated instances,
mostly under artificial conditions, in which orientation and
subsequent locomotion (heliotropism) lead to fatal results,
the orienting reactions are in general useful to the organism
in its life processes, and the same may be said regarding
all other reactions to light. Thus, it is evident that Loeb's
theory of the origin of purposeful reactions is not in har-
mony with the observed facts.

Jennings' theory of behavior is founded upon the idea
that the reactions are fundamentally '* purposeful." He
admits that light and other external agents cause chemical
changes in the organism, and that all reactions are deter-
mined by chemical changes or states ; but that the chemical
change or state which causes a given reaction is not directly
and entirely the result of the external condition which
precedes the reaction; that what an organism does under
a given condition depends upon what it and its ancestors
have done and experienced in the past as well as upon the
present external conditions. The reactions are above all
things regulatory. External conditions are not the direct
cause of reactions.

Reactions are defined as changes in the activity of or-
ganisms. Such changes may occur under constant exter-
nal conditions. They are therefore due primarily to
internal changes. External factors cause reactions not
directly, but indirectly, by altering internal processes
(physiological states). Variability in reaction to given
external conditions is due to changes in physiological
states. If an organism responds to light of a given inten-

THEORETIC CONSIDERATIONS 373

sity in a given way now, and to the same intensity in another
way later, it is because the physiological state of the organ-
ism has changed. When external changes persistently
follow each other, as, for example, shadow and contact in
case of the attack of an enemy on Hydroides, the shadow
produces a certain physiological state. This state is
resolved into another by contact, and this results in a
reaction. Repetition tends to cause the resolution of the
first physiological state into the second, without contact,
and consequently a reaction to the shadow which was
formerly given only to a contact stimulus. Thus we have
the origin of a reaction to a sign, response to a representative
stimulus, as Jennings terms it. The shadow in the case
mentioned above represents the contact; it is a sign of the
approach of danger. All of this the author has elaborated
in a most masterful way in his book " Behavior of the
Lower Organisms " (1906). Every step in the develop-
ment of the theory is supported by numerous experimental
facts and all seems to fit what is known concerning the
reactions of organisms. Reactions, according to this
theory, are, as stated above, primarily due to physiologi-
cal states. External agents ordinarily produce reactions
through the effect they have on these states. By the
application of this idea all the different phenomena con-
nected with reactions to light as summarized at the begin-
ning of this chapter can be accounted for.

But what are these physiological states and of what do
they consist? That there are such states in organisms
cannot reasonably be doubted, and that the reactions are
dependent upon them much as Jennings assumes, seems to
me to have been well established in his work. But what
regulates the physiological states is a question concern-
ing which we have as yet but little knowledge. Jennings
assumes that they are regulated entirely objectively, i.e.,
by the interaction of external and internal physico-
chemical processes. This is of course a legitimate assump-
tion, an assumption which indeed has some experimental

374 LIGHT AND THE BEHAVIOR OF ORGANISMS

support, especially in the fact that changes in metabolism
cause changes in reaction. " Hungry animals react
positively to possible food while satiated ones react nega-
tively to the same stimuli." Paramecium bursaria is
positive to light in solutions deficient in oxygen, whereas
it does not react under normal conditions. After Volvox
has been resting in darkness for some time it responds
to light in a manner very different from the response given
when it is active. Jennings (1906, pp. 251-253 and else-
where) cites several other similar instances indicating that
reactions depend upon physiological states, but he frankly
admits that ** it is rarely possible to observe them [physio-
logical states] directly," especially in the lower organisms,
in which "the real data of observation are the actions;
if we considered these alone, we could only state that a
given organism reacts under the same external conditions
sometimes in one way, sometimes in another. This would
give us nothing definite on which to base a formulation
and analysis of behavior, so that we are compelled to assume
the existence of changing internal states. This assump-
tion, besides being logically necessary, is, of course, sup-
ported by much positive evidence drawn from diverse
fields, and there is reason to believe that in time we shall
be able to study these states directly. Before we can
come to a full understanding of behavior, we shall have to
subject the physiological states of organisms to a detailed
study and analysis, as to their objective nature, causes,
and effects " (p. 251).

And again, after giving a most excellent description of
the reactions of Stentor, in which he shows that these
creatures sometimes respond in at least five different ways
to the same stimulus, Jennings says (p. 177): "Since in
each of these cases the external conditions remain through-
out the same, the change in reaction must he due to a change
in the organism. The organism w^hich reacts to the carmine
grains by contracting or by leaving its tube must be differ-
ent in some way from the organism which reacted to the

THEORETIC CONSIDERATIONS 375

same stimulus by bending to one side. No structural
change is evident, so that all we can say is that the physio-
logical state of the organism has changed. The same organ-
ism in different physiological states reacts differently
to the same stimuli. It is evident that the anatomical
structure of the organism and the different physical or
chemical action of the stimulating agents are not sufficient
to account for the reactions. The varying physiological
states of the animal are equally important factors. In
Stentor we are compelled to assume at least five differ-
ent physiological states to account for the five different
reactions given under the same conditions." It is thus
evident, without further argument, that while there is
some evidence bearing on physiological states, we know
but little about their nature and regulation. Even in
those cases where it appears evident that they are depend-
ent upon metabolic processes, it must be said that we
know practically nothing about their regulation, since we
know almost nothing concerning the fundamentals in
metabolism. It is evident, then, that for all that is known
to the contrary, subjective factors, entelechies, or psy-
choids, factors foreign to inorganics, may have a hand in
controlling physiological changes and consequently the
reactions. Such factors have been postulated by the vital-
ists and neovitalists, notably by Hans Driesch.

Driesch postulated a non-energetic factor to account for
form regulation and regulation in behavior. He claims
that if certain organisms, starfish eggs, for instance, are
divided into halves in any direction, each half will produce
a new individual. Such organisms, he says, form harmo-
nious equipotential systems. Every part has the same
potency (future possibilities) as every other part, no
matter how the whole is divided. No machine^ (using
the term in its broadest sense), he holds, could account for

^ "A machine is a typical configuration of physical and of chemical
constituents, by the acting of which a typical effect is attained." (Driesch,
Vol. I, pp. 138, 139.)

376 LIGHT AND TEE BEHAVIOR OF ORGANISMS

this. Referring to genesis, Driesch asserts that an egg
must be considered as a whole, a unit, an entity, an individ-
ual, but during every step In the process of development It
Is still a whole, an Individual, although It may have been
divided many thousands of times. Now he asks (Vol. I,
p. 225), " Can you Imagine a very complicated machine,
differing In the three dimensions of space, to be divided
hundreds and hundreds of times and In spite of that to
remain always the same whole ? " and adds (p. 226), '' We
say It Is a mere absurdity to assume that a complicated
machine, typically different In the three dimensions of
space, could be divided many many times, and in spite
of that always be the whole: therefore there cannot exist
any sort of machine as the starting-point and basis of
development." Acting, too, he affirms, cannot be ex-
plained by the application of physico-chemical principles
alone; and It is this part of his analysis which concerns us
in particular. " In acting," he says (Vol. II, p. 69), " there
may be no change in the specificity of the reaction when the
stimulus is altered fundamentally, and again, there may
be the most fundamental difference In the reaction when
there is almost no change in the stimulus." In other
words, each constituent of the effect does not depend upon
each constituent of the stimulus, '' but one whole depends
on the other whole, both ' wholes ' being conceivable In
a logical sense exclusively " (p. 81). The author supports
his contention still further by referring to the historical
basis of acting. He says (p. 81), " Firstly, the effects that
are given off In acting occur In a field of natural events very
different from that of the stimuli received historically:
sensations belong to one, movements to another field.
Secondly, the historical basis serves only as a general
reservoir of faculties, the specific combinations of the
stimuli received historically being preserved by no means
in their specificity, but being resolvable into elements; these
elements then — transferred, however, to another sphere
of happening — are rearranged into other specificities

THEORETIC CONSIDERATIONS 2)77

according to the Individuality of the actual stimulus in
question."

Thus it is maintained that acting or behavior cannot
be accounted for on the basis of physics and chemistry.
There must be a factor involved here which is not active in
the inorganic realm. This factor is postulated as a non-
energetic regulatory factor. It is supposed to prevent
reactions — activity or becoming — by compensating po-
tentials, i.e., by transferring kinetic into potential energy,
and to regulate reactions and becoming by setting " free
into actuality what it has itself prevented from actuality,
what it has suspended hitherto " (p. i8o). Thus it is
that this non-energetic factor, entelechy, psychoid, is sup-
posed to regulate development, becoming and action in
organisms. It requires the same amount of energy under
certain conditions to fire a gun to-morrow at lO A.M. as it
does at II a.m., and just as much to fire it east as it does
to fire it west. In some such way, I understand the author
to assume that psychoid can regulate behavior without
energy. It does jiot create reactions, but it regulates
them with regard to time and direction.

Admitting the operation of a factor of this sort it is a
simple matter to explain the regulation of physiological
states and all of the puzzling phenomena in behavior previ-
ously referred to, but in such an explanation we still have an
u'nknown factor to account for, the psychoid ; and concern-
ing this some maintain nothing can be learned, for it is evi-
dent that if there is such a factor at work in behavior dif-
ferent things can happen under precisely the same physico-
chemical conditions. And if this be true, how can we hope
to progress experimentally? According to Driesch's theory
every act is definitely and absolutely determined, although
not mechanically. With a given physico-chemical con-
stellation and a psychoid in a given state, precisely the same
things will always occur. If there is a psychoid of this
sort, a factor which has different states but which always
acts tie same in any given state, it seems to me that by

378 LIGHT AND THE BEHAVIOR OF ORGANISMS

experimental investigation it will be possible to learn some-
thing concerning its nature, just as we have learned, at
least in part, the character and manifestation of electricity,
gravity and other similar concepts.

It is however highly essential in all investigation and
discussion bearing on such concepts to differentiate clearly
the two points of view from which they may be considered,
the scientific and the metaphysical. From a scientific
point of view entelechy and psychoid, like gravity, electri-
city and chemical affinity, can be used only to indicate the
facts observed, not the cause of the phenomena. From this
point of view gravity indicates merely the fact that bodies
tend to approach each other, not the cause of this tendency.
It is only in the realm of metaphysics that all of these con-
cepts, psychoid and entelechy, as well as gravity, electricity
and chemical affinity, are looked upon as causal agents.

Entelechy, then, from a scientific point of view, merely
indicates certain facts concerning regulation which ap-
parently do not fit into any of our physical or chemical
concepts. It has no more to do with the cause of these
acts than chemical affinity has with the cause of chemical
reactions. It is a name for certain phenomena just as is
electricity. Whether or not there are any such phenomena
is the question at issue, and our only hope of agreement
in an answer lies in further investigations. But until this
question is settled it must be said that those who maintain
that there are no factors functional, no phenomena, in liv-
ing matter that are not also found in irorganic matter, that
there are no entelechies, are certainly no more scientific
than those who maintain the opposite, for the fundamental
phenomena, the distinguishing characteristics of living
matter, have not as yet been accounted for mechanically.
To say that they can be is prejudging the future quite as
much as to say that they cannot be. Convictions are
valuable, but dogmatic statements as to what can or cannot
be done in the future have no place in science, a.s has
been repeatedly demonstrated.

BIBLIOGRAPHY

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379

380 LIGHT AND THE BEHAVIOR OF ORGANISMS

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388 LIGHT AND THE BEHAVIOR OF ORGANISMS

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INDEX

Acacia, 286.

Acclimatization, Euglena, 103; Vol-
vox, 141; Hydra, 152; Stentor,
119; Musca larvae, 189, 190, 197;
to change of intensity, 248, 249;
288-297.

Acids, effect of, on reversal of reac-
tions, 279-283, 300.

Acris, 260.

Actinia, reaction to sudden increase
of intensity, 250; reaction to con-
tinued illumination, 252; 257.

Actinia equina, 286.

Adams, orientation in earthworms,
198; 201; 266.

Adaptation, Verworn on, 36; in re-
actions in plants, 72; in Arenicola
larvae, 167; in reactions of but-
terflies, 227; in Euglena, Chlamy-
domonas, Volvox, Stentor,
Amoeba, etc., 236-239; natural
selection, 238, 239; 285; 292;
297; 298; in ants, 351; 368;
chemical regulation of, 370.

Aggregation, method of in : Euglena
(Engelraann), 16, Paramecium
Qennings), 45, Volvox, 144, coe-
lenterates, 149, Planaria, 206,
frogs, 219, general, 239-245; cause
of, 242, 243.

Aiptasia annulata, reactions of
(Jennings), 252.

Algae, 265.

Alkalis, effect of, on reversal of re-
actions, 279-283, 300.

AUolobophora foetida, 199; 266. See
Earthworm.

Alona, 278.

Amaranthus, 288; 313.

Amoeba, 48; reactions to light, 74-
80; process of orientation, 76-79;
effect of change of light intensity
on movement of, 76-79; adapta-
tion, 237; 257; 263; 270; 322;
reactions of, in spectrum, 327-332,
361; effect of change of intensity
and color on movement of, 328,

2>2>o, 331; 2>(^yy 365-

Ampelopsis, 265.

Amphibia, function of skin in re-
sponse, 262; 343. See Bufo.

Amphioxus, reaction to sudden in-
crease of intensity, 250; 257; 259.

Amphipods, 216.

Amphitrite bomb>'x, 247.

Andrews, 247.

Animal behavior, effect of theory of
evolution on, 9, 10; relation to
psychic phenomena, see Psychic
phenomena; summary of Loeb's
ideas on, 34, 35; analysis of (Jen-
nings),49, 50; Drieschon,374-378.

Annelids, tubicolous, reactions to
shadows, 247. See Hydroides.

Anomura, 214.

Ants, modification in reactions of,
296, 297; reactions to colors,
348-352; effect of ultra-violet
rays on, 349; sensation in, 350;
change in reactions of, 351.

Area, 247.

Arenicola larv^ae, orientation in light
from two sources, 87; description
of, 166, 167; locomotion, 167;

393

394

INDEX

accuracy of orientation, 167; me-
chanics of orientation, 168-171,
174, 232; orientation compared
with that in Euglena and Volvox,
169, 171; distribution of sensitive
tissue, 172; aggregation of, 243;
258; 260; reversal in reactions,
271, 280-283, 285; 367.
Aristotle, regulation of behavior,

5; 7-
Arkin, orientation in earthworms,

198; 201; 202.

Arthropods, 233; 257; 260.

Asterias forreri, 211; 212.

Atherina, 359.

Avena, reactions in spectrum, 318.

Avicula, 247; 2)2y, 326.

Avoiding reaction, 17; defined, 45;
compared with "Schreckbe-
wegung, " 82, no; in Euglena,
82-86, 92-106; in Stentor, 113-
121; in swarm-spores, 125, 126;
137; in Volvox, 142; in Pandorina
and Eudorina, 147; in Euden-
drium, 161; 233; effect of, on
aggregation, 239-243; 246; 257.

Bacteria, 36; 37; 350; 362.
Bacterium photometricum, 16; 17;

45; aggregation of, 240, 241; 263;

324; reactions of, in spectrum,

325-
Balanus, nauplii of, 266; change in

reactions, 285; 337.
Bancroft, reaction to electricity in

Volvox, 145, 146; 231.
Baranetzsky, movements of myxo-

mycetes, 74.
Barnacles, reactions to shadows,

247.
Barrows, definition of tropism, 55.
Bateson, 249; decoration in crabs,

357-
Bdelloura Candida, 207.

Bees, adaptation, 237; modification
in reactions of , 296, 297; reactions
to colors, 352-355; change in
reactions, 352-354; 364.

Bell, 354.

Bentley, 358.

Bert, 10; 14; 24; 27; 34; 235; reac-
tions of Daphnia in spectrum,

336, 337; 342; 343-
Berthold, 265.
Bethe, 354.
Bierstadt, 327.
Bipalium kewense, 182; reactions

to light, 207; 270; sensitiveness of,

288.
Birds, flight of, into lighthouse, 228;

adaptation, 237; color vision in,

343, 360, 364.

Bispira voluticornis, 247.

Blauuw, reactions of plants in spec-
trum, 318, 319.

Blowfly larvae. See Musca larvae.

Bohn, 17; definition of tropism, 56;
211; 214; 221; 243; 244; 264;
periodic movements, 286. See
Criticism.

Bonnier, 354.

Borelli, founder of iatromechanical
school, 6; 51.

Botrydium, 274; 321.

Bougainvillea superciliaris, medusae
of, orientation in light from two
sources, 87, 165; orientation, 164,
165; description of, 165; 285.

Brachyura, 214; orientation in zoeae
of, 226.

Branchiomma kollikeri, 247.

Brassica, 314.

Brefeld, 318; 319.

Brightness, distribution in spectrum:
normal, 305-308, 361, in color-
bhnd individuals, 307; cause of,
307; distribution of, in spectrum
compared with that of energy, 361.

INDEX

395

Bronn, 250.

Brooks, response to a sign, 250.

Bryopsis, 274.

Bufo americanus, orientation in
light from two sources, 89, 219-
221; 214; orientation with one eye
destroyed, 221, 222.

Bulman, 354.

Buttel-Reepen, 354.

Butterfly, Mourning-clock. See Va-
nessa antiopa.

Calliphora vomitoria, 217.

Caprella, 214; structure and loco-
motion, 224; 271.

Carbon dioxid, effect of, on rever-
sal of reactions, 279-283, 300.

Cardium, 247.

Caridea, orientation in zoeae of, 226.

Carpenter, on circus movements,
216; reactions of Drosophila, 271,
280.

Cartesian doctrine, 9.

Cassiopea, 359.

Cerianthus, 250; 252; 254.

Change of intensity, 17; orientation
by, in Euglena, 99. See Light.

Chaetomorpha aerea, 274.

Chemicals, changes in, related to
reactions, 270, 278, 308-312, 320,
363, 364, 370; effect of, on re-
versal in reactions, 279-283, 300,
367; extent of effect of, on reversal
in reactions, 280; reactions of,
reversible in Hght (Stobbe), 308-
312; effect of different rays on re-
actions of, and cause of, 312, 360,
363; effect of mixed rays of light
on reactions of, 310, 363; same
compared with effect of mixed
rays on organisms, 335, 336; effect
of, on reactions of organisms in
spectrum, 323-325, 2,2>2, 343-

Chilomonas curvata, 274.

Chlamydomonas, orientation in
light from two sources, 87; func-
tion of eye-spot, 109, 133; struc-
ture of, 131; mechanics of orien-
tation, 132, 133; 134; 136; 146;
229; 230; adaptation, 236; aggre-
gation of, 241; 257; 259; reversal
in reactions and effect of tempera-
ture on, 265, 267, 277, 280, 300;
reversal in reactions compared
with same in Arenicola, 283; 367;

370.

Chlorogonium, structure of, 134;
function of eye-spot in, 134; ag-
gregation of, 241; 257; 259.

Chromulina, 273.

Chytridium vorax, 274.

Ciesielski, 20.

Ciliates, 229; 230; 232; 233; 240;
aggregation of, 242; reactions in
spectrum, 323, 324.

Ciona, 250.

Circus movements, in various species,
215-218.

Claparede, 247.

Classification, of reactions to light,
253-262.

Claviceps, 318.

Clepsine, reactions to shadows, 249;
257; 260; 263; 298.

Cohn, ray-direction and movement
of organisms, 15; effect of different
rays (unicellular forms), 321, 322.

Cole, L. J., 182; reactions of Bi-
pahum to light, 207; 214; 223;
effect of size of light area on re-
actions, 227; 228; 244.

Cole, L. W., 360.

Color, wave-length of, 304; energy
in, 304, 305; brightness of, 305-
308; effect of, on reactions:
chemical, 308-312, Daphnia, 310,
336-341, Hydra, 310, 333-33^,
plants, 310, 313-320, unicellular

396

INDEX

forms, 321-332, Simocephalus,
341-343, higher animals, 343-346,
ants, 348-352, bees, 352-355,
higher Crustacea, 355-358, fishes,
358-360, birds and mammals, 360;
effect of impurity of, on reactions,
310, 313, 322, 335, 340, 362;
selection of, in crabs, 356-358.
See Spectrum.

Concentration of medium (mechani-
cal stimuli). See Reactions.

Copepods, 273; 274.

Corethra larvae, orientation in (Har-
per), 225.

Cowles, 211; direction of righting
reactions in starfishes, 213; 328.

Crab, fiddler: reactions to light, 217,
218; Hermit, see Pagnrus; 258;
effect of color on reaction (decora-
tion), 355-358.

Criticism, of Darwin by Sachs, 21;
of Loeb's theories of orientation,
26, 27, 31, 70, 80, 83, 87-89, 104,
no. III, 119, 122, 137, 144, 164,
168, 171, 173, 177-180, 182, 183,
188, 192-195, 198, 205, 206, 209,
220, 221, 223, 225, 229, 230, 233-
235, 242, 258, 351, 363, 364; of
Loeb and Sachs by Verworn, 38;
of Sachs' ray-direction theory, 70,
80, 87, III, 137, 144, 158, 182,
192, 198, 233; of Pollock's theory
of curvature in roots, 71; of Jen-
nings by Torrey, 84, 85 ; of Torrey
on orientation in Euglena, 85, 86,
loi, 104, III, 205; of Radl's the-
ory of orientation, 43, 234; of
Bancroft on orientation, 145; of
Verworn's theory of orientation,
104, 122, 168, 171, 173, 205, 229,
234; of Parker and Arkin on ori-
entation in earthworms, 202, 203;
of Holt and Lee on orientation,
205; of Bohn on orientation, 220,

221; of Davenport on orientation,
234; of Loeb on cause of aggre-
gation in Planaria, 245; of classi-
fication, 255, 256; of Loeb on
regulation and adaptation, 266,
267, 272, 369, 371, 372; of Loeb
on cause of change in reactions,
287, 301; by Loeb on Lubbock's
experiments on Daphnia, 335, 336;
of Loeb on reactions in spectrum,
2,2>^, 346, 347, 363, 364; of Sachs'
hypothesis on effect of different
rays, 363; of Davenport on reac-
tion in spectrum, 363; of Jennings
on regulation, 377; of Driesch on
regulations, 378.
Crustacea, 42; 238; reactions to
shadows, 249; decapod, 264; 272;
354; modifiability in reactions,

355-358; 364.

Cryptomonas, 132.

Cuma rathkii, adaptation, 238; 272;
346.

Cyclops, 277; 279; 280-283; 300-

C>T)ridopsis, 284.

C>T)ris, 214; 274; 277; 278; 280-283;
300.

Czapek, curvature of roots con-
trolled by root-tip, 21; 59; 72.

Daly ell, 247.

Daphnia, 214; 255; orientation of,
264; 265; 274; 277; 279; 280-283;
300; effect of different wave-
lengths on reactions of, 310; reac-
tions in spectrum, 335-341, 362;

345; 350; 364-
Darkness, effect of, on movement:
in Hydra, 152; in Hydra, etc.,

245-
Darwin, 9; 10; 12; theory of orienta-
tion in plants, 18-21; transmis-
sion of stimuli in plants, 19; 23;
31; 47; 52; definition of tropism.

INDEX

397

54; 57; 59; 60; 63; 70; location of
sensitive structure in leaves, 71;
229; 235; 354.

Davenport, theory of orientation,
40-42; definition of tropism, 55;
58; reactions of Amoeba to light,
74; movement in Amoeba affected
by change in light intensity, 78;
reactions of Stentor, 113; 234;
phototaxis and photopathy com-
pared, 254-256; effect of different
rays on reactions: 302, criticism
of, 343, 363; photochemical reac-
tions, 308.

De Candolle, vitalism, 8; sleep
movements of leaves, 11; theory
of orientation in plants (heliotrop-
ism), 12; 13; 14; 27; 52; defini-
tion of tropism, 53; 54; 229; 234.

Decoration, in crabs, 356-358.

Dendrocoelum lacteum, 207.

Descartes, philosophy of movement,

6; 51-

Dewar, 338; 349.

Dexia carinifrons, 216.

Dias longiremis, 337.

Diatoms, 265; reactions of, in spec-
trum, 323, 326.

Didinium nasutum, 126.

Dodart, curvature of roots, 7.

Driesch, definition of tropism, 56;
369; theory of vitalism, 374-378.

Drosophila ampelophila, 216; effect
of intense light on, 271; change in
reaction of, 280.

Du Bois-Reymond, 9.

Du Hamel, cause of curvature in
plants, 7.

Dutrochet, osmosis and movement
of plants, 12.

Dutrochet and Pouillet, 314; 319.

Earthworms, orientation of (Daven-
port), 40, 41; 50; locomotion, 199;

trial in orientation, 200-206, 232;
distribution of sensitive tissue in,
201-205; accuracy of orientation,
205; reaction to sudden increase
of light intensity, 250; 257; 259;

260; 343; 346.

Echinaster crassispina, 211.

Echinoderms, method of locomo-
tion, 211; orientation of, 21 1-2 13;
233; aggregation of starfishes, 244;
reactions of sea urchins to shad-
ows, 247; 260.

Edwardsia, 250; 259; 299.

Eigenmann, 238.

Eloactis, reactions of (Hargitt),

252.

Energy, distribution in spectrum,
304-308, 360; distribution in spec-
trum compared with stimulating
efficiency, 332, 343.

Engelmann, reactions of unicellular
forms to light, 16, 17; 42; 44; 57;
stimulation of pseudopods, 74;
method of aggregation in Euglena,
etc., 82; 94; function of eye-spot,
106-109; 235; methods of aggre-
gation compared with ideas of
Jennings, 240, 2,23, 324; 279; reac-
tions of unicellular forms in spec-
trum, 322-325.

Entelechy, as a factor in regulation,

377-

Entomostraca, 258; 271; 278.

Eudendrium, hydranths, mechanics
of orientation, 163, 164.

Eudendrium, planulae of, orienta-
tion in light from two sources, 87;
description and locomotion, 159;
accuracy of orientation, 160; me-
chanics of orientation, 161-163;
aggregation of, 243; 271.

Eudorina, orientation in light from
two sources, 87, 147; function of
eye-spot, 109, 147; structure of,

398

INDEX

146; locomotion of, 147; orienta-
tion and change in sense of, 147,
231; 171; aggregation of , 242.

Euglena, viridis, 16; method of ag-
gregation (Engelmann), 17; 36;
45; description of, 80-82; aggrega-
tion of (Engelmann), 82; orienta-
tion of (Jennings) , 83, 84, (Torrey),
84, 85; orientation in light from
two sources, 86, 87, no; different
species and collection of, 89, 90;
locomotion of, 90, no; accuracy
of orientation, 92; mechanics of
orientation, 92-99, 102-104, no;
discussion of orientation, 99-102;
distribution of sensitive tissue,
104-106, in; function of eye-
spot, 98, 99, 102, 106-109, in;
sensation in, 112; 115; 118; 122-
137; 142-148; 156; 161; 174; orien-
tation compared with that in Are-
nicola larvae and Musca larvae,
169, 171, 195; 209; 210; 215; 216;
229; 230; adaptation, 236; 243;
257; 259; 261; 263; reversal in
reaction and effect of tempera-
ture, 265, 267, 274-279; deses,
viridis, spiragyra, 274; 301; reac-
tions in spectrum, 324, 325, 361;
366; 368.

Evolution, effect of theory of, on
behavior, 9, 10; 52; natural selec-
tion, 238, 239; of reactions to
light, 262, 263.

Ewart, protoplasmic streaming, 74.

Eyes, function of, in reactions: flies,
216, 217, fiddler crab, 217, Rana-
tra, 218, toads and frogs, 219-
223, arthropods, 226, 227, birds,
228, 233; function of, in aggrega-
tion, 244.

Eye-spot, in Euglena: structure of,
81, 82, function of, 98, 99, 102,
106-109, in; in Trachelomonas :

structure of, 128, 129, function
of, 109, 130; in Chlamydomonas:
function of, 109, 133; in Chloro-
gonium, function of, 134; in Eu-
dorina and Pandorina, structure
and function of, 147, 148; lumi-
nosity of, in direct sunlight, 147;
in Arenicola lar\^ae: 166, function
of, 171, 172, 174; function of, in
starfish, 211; summary of function
of, 230.

Famintzin, change in sense of re-
action, 265.
Fechner, 9.
Fielde, 354.
Figdor, sensitiveness of plants, 288,

313-
Fishes, adaptation in blind, 238;
function of skin in response, 262;
343; color vision in, 358-360,

364-
Flagellata,36;37; 229; 230; 232; 233;

aggregation of, 242; reactions in

spectrum, 324, 325.
Flies, circus movements in, 216,

217; function of eyes in reactions,

216, 217; adaptation, 237. See

Musca.
Forbes, 312.
Forel, 354.
Frandsen, 265.
Frank, geotropism, 12.
Fraunhofer, brightness in spectrum,

305-307.
Frogs, aggregation of, 244; 260; re-
versal in reactions, 273; 278; See

Bufo.
Fulgides, reversible in light, 308-

310.
Fundulus, reactions to shadows, 247;

257; 260.
Fungi, reactions of, in spectrum,

317-319-

INDEX

399

Galen, experiments on animals, 6.
Galvanotropism, compared with

heliotropism, 28, 29; 56; 119; in

Vol vox, 145, 146,
Gamble, 354.

Gammarus, 279; 300; 346; 367.
Gardner, reactions of seedlings in

spectrum, 314, 319.
Geometra, 346.
Geotropism, 13.
Gonionemus murbachii, orientation

in, 164; 251; 252; 257; 259; 266.
Graber, preference method, 10; 14;

24; 27; 34; theory of reactions,

218; 223; 235; effect of color on

reactions of higher animals, 343-

346; 350; 353; 354-
Gramineae, 68; 229; 259; change in

reactions, 285.
Groom, 266.
Guillemin, reactions of seedlings in

spectrum, 314, 317, 319.

Haberlandt, 59; function of epider-
mal cells in reactions to light, 72.

Hadley, definition of tropism, 56;
214; 222; orientation in lobster
larvae, 226; reversal in reactions
of lobster larvae, 264, 266, 286.

Haematococcus, 272; 274.

Hargitt, 160; 247; reactions of Hy-
droides, 249; reactions of Eloactis,
252; variation and modification
of reactions of Hydroides, 292-

295-

Harper, orientation in earthworms,
199; 200-205; 214; 215; orienta-
tion in Corethra larvae, 225.

Harrington and Leaming, move-
ment of Amoeba, 74; 322; reac-
tions of Amoeba in spectrum, 327,
328, 331.

Harvey, experiments of, on the cir-
culatory system, 6; 51.

Haycraft, brightness in spectrum,
306, 307.

Hedista, 286.

Hehotropism, origin of term, 12;
Sachs' theor>' of , 13-16; compared
with galvanotropism, 25-3 1 ; Loeb's
theory of, 28-33; compared with
Unterschiedsempfindlichkeit, 32,
2,2), 254-256; Verworn's theory of,
36-38; defined, 53-56, 253-256;
of Euglena, 85, 104; compared
with galvanotropism in Volvox,
145, 146; 164; 227; in birds (Cole),
228; 266; in plants and animals
compared, 346. See Orientation.

Helix hortensis, 251; 366.

Helmholtz, 9.

Hertel, response to ultra-violet in
Paramecium, 134.

Hesse, 247.

Hewitt, 184.

Hofmeister, heliotropism, 12; curva-
ture in single-celled structures,
71; Plasmodia, 74.

Holmes, orientation: selection of
random movements as a factor in,
50, 51, 196, 197; definition of trop-
ism, 55; 58; effect of light inten-
sity on rate of movement, 100,
loi ; orientation in Musca larvae,
176; 183; 189; trial movements in
earthworms, 198; 203; 204; 214;
on circus movements, 215, 217;
on orientation, 218; 223; 226; 227;
232; reversal in reactions, 271,
273, 279, 280; effect of contact on
reactions, 284; modification in re-
actions of Ranatra, 296.

Holothurian, orientation of, 211.

Holt and Lee, 42; reactions of Sten-
tor, 113; 205.

Hydra, 34; 48; effect of light inten-
sity on activity, 150-152; orienta-
tion and locomotion, 1 51-15 7,

400

INDEX

231; acclimatization, 152; reaction
of negative specimens, 154; dis-
tribution of sensitive tissue in,
156; reaction to electric current
compared with reaction to light,
158; 258; 265; effect of different
wave-lengths on reactions of, 310;
reactions in spectrum, 333-335;

361, 363; 367; 370.
Hydroides dianthus, orientation in
light from two sources, 87; reac-
tions to shadows and acclimatiza-
tion, 247, 249; 250; 263; 280; 285;
variation and modification in reac-
tionsof, 292-295,368; 298;366;373.

Impatiens, 286.

Inachus, 355.

Indian corn. See Zea 7nays.

Infra-red, effect of : on seedlings, 31 4-

319, on bacteria, 325, on Daphnia,

343; cause of invisibihty, 362; 368.
Infusoria; 36.
Insects, 42, 343.
Intensity-difference, 3, 4; effect of,

on movement of organisms, 15.

See Light.

Jassa, 279.

Jellyfishes. See Sarsia, Gonionemus
and BougainvUlea.

Jennings, 17; work compared with
that of predecessors, 44; method
of aggregation of lower organisms,
45, 46; motor reaction, motor
reflex, av'oiding reaction defined,
45; theory of orientation (trial and
error), 46-49; external stimulus
not necessary for activity in or-
ganisms, 47; direct orientation,
48; analysis of behavior, 49, 50,
372-375; on regulation in be-
havior, 50, 368, 372; 51; 53; defi-
nition of tropism, 56; 57; 58;

stimulation of Amoeba, 75 ; move-
ment of Amoeba, 80; 82; orienta-
tion in Euglena, 83, 84; 89; 91; 99;
104; 112; orientation of Stentor,
113; 114; reactions in Chlamydo-
monas, 132; orientation in Hydra,
156; 202; 203; orientation in
Asterias, 211; ''trial and error"
defined, 215; 231; theory of orien-
tation compared with that of
Engelmann, 240, 323, 324; reac-
tions of Aiptasia, 252; 262; 287;
293; 297; 328; 369.

Johnston, 11.

Joubert, 248.

Keeble and Gamble, 354.

Kinnaman, 360.

Kneip, function of epidermal cells in
orientation of leaves, 72.

Knight, effect of gravitation on
curvature of roots, 11, 12; 13.

Krabbe, 71.

Kraus, reactions of fungi in spec-
trum, 317, 319.

Labidocera, 227; 266; reversal in
reactions, 274, 284, 300.

Lacrymaria olar, 51.

Langley, 305.

Lapidium, 265, 288, 313.

Larvae, Limulus, 87; Musca, 87;
Arenicola, 87; lobster, 226; adap-
tation, 237; aggregation of, 242;
mosquito, 247;Palaemonetes, 264;
Polygordius, 266.

Leaves, orientation of, 71-73.

Leeches, 346. Sec CJepsine.

Leeuwenhoek, 136.

Leptoplana tremellaris, orientation,
in light from two sources, 87; 207;
orientation of, 207; 271.

Liebig, 9.

Life, primitive ideas of, 5, 51.

INDEX

401

Light, change of intensity on plants
(Darwin), 19; graded by means of
prism, 39, 60; pressure of, and
supposed effect on orientation, 43;
graded by means of lens, 60, 61;
effect of constant, and change of
intensity of, on reactions of:
Euglena, 16, 83, 85, 98, 99, 100-
112, Stentor, 113-115, 118-123,
Swarm-spores, 127, Trachelomo-
nas, 130, Chlamydomonas, 132,
133, Volvox, 139, 143-145, Eudo-
rina and Pandorina, 147, Hydra,
157, Eudendrium, 161, 163, Me-
dusae, 165, Arenicola larvae, 172-
175, Fly larvae, 191-197, earth-
worms, 204, Planaria, 208, 210,
general, 229-235, 241, 245, 253-
258, 299, 366, 367; effect of, on
movement, 245; reactions to sud-
den decrease of, 247-250; reac-
tions to sudden increase of, 250,
251; reactions to continued illumi-
nation, 252, 253; effect of, on
reversal in reactions, 265-272,
299; characteristics of, 303; effect
of, on chemical reactions, 308-312.

Light grader, 60-62. ,

Lillie, 166.

Limax, 265.

Limnaeus stagnalis, 345.

Limnea columella, 214.

Limuluspolyphemus,larvaeof, orien-
tation in light from two sources,
87; 88; 258; 271; reversal in reac-
tions, 285; 346.

Lineus ruber, 284; effect of color on
reactions, 355.

Littorina, aggregation of, 244; 247;
periodic movements caused by
tides, 286.

Lobster larvae, orientation and
change in sense of orientation,
226, 264; 267.

Loeb, 11; 17; 21; object of observa-
tions on reactions of animals, 23,
24, 34; control of movement in
animals and plants identical, 25,

26, 164, 346; on relation between
sensations and animal behavior,

27, 28; first theory of orientation
(ray direction), 24, 25, 34; second
theory of orientation (angle of
rays), 28, 29, 35; third theory of
orientation (intensity difference),
29-31, 35, 221; effect of constant
intensity compared with change
of intensity, 32, 33; extent of
appHcation of theories, 33, 34;
ideas on animal behavior sum-
marized, 34-36; theory of orien-
tation (tropism) compared with
Verworn's, 39; 40; 42; 52; 53)
definition of tropism, 54-58; 70;
80; 83; 86; theories criticized. See
Criticism; theories of orientation
applied to: Volvox, 137, 144, 146,
Hydra, 149, 150, 159, Arenicola
larvae, 173, earthworms, 205, Pla-
naria, 209, Caprella, 225, ants,
351; 363; 364; on orientation in
Eudendrium, 164; 168; 171; orien-
tation in Musca larvae, 175, 176,
182, 183; 177; 178; 180; 192; 194;
198; reactions of planarians to
light, 206; on circus movements,
215; 216; 220; 223; 229; 230;
234; 238; on origin of reactions,
239-243; cause of aggregation in
Planaria, 245; 247; heliotrppism
compared with Unterschiedsemp-
findlichkeit, 254-257; 258; on
adaptation and change in sense
of reaction, 266, 267, 272, 273,
285; effect of chemicals on reac-
tions, 279; effect of concentration
of medium on reactions, 283; 284;
effect of different colors on reac-

402

INDEX

tions: 302, 346, 347, criticism of,
335, 336, 343, 346, 347; on cause
and regulation of reactions, 369; on
origin of adaptive reactions, 371.

Lotze, 9.

Lubbock, 10; 24; 235; 265; on effect
of different colors on reactions of :
Daphnia, 310, 335-343, 362, ants,
348-352, bees, 352-355-

Lumbricus, 199; reactions in colors,
344. See Earthworms.

Lunularia, 288; 313.

Lupinus albus, 21.

Luther and Forbes, photochemical
reactions, 312.

Lutianus griseus, 358.

Lyon, 264.

Mach, 344.

Machine, defined (Driesch), 375.

Maja, verrucosa and squinado, 355;

357-
Mammals, 343; 360; color vision in,

364.

Massart, 12; reversal in reactions 273.

Maya aranaria, 247.

McCook, 354.

Medusae, reactions to light, 164, 165.

Melolontha, 346.

Merejkowsky, 337.

Metazoa, 47.

Mimosa, 7; 286.

Minkiewicz, definition of tropism,
56; effect of concentration of me-
dium on reactions, 284; 354; reac-
tions of Crustacea to color, 355-
358.

Mitsukuri, reversal in reactions, 286.

Modifiability, in behavior, 264-301;
368; chemical regulation of, 370.
See Acclimatization.

Mollusks, 233; 243; reactions to
shadows, 247; acchmatization,
248; 257; 259; 260; 343,

Monkey, color vision, 360.

Morse, reactions of Gonionemus, 164.

Mosquito larvae, reactions to
shadows, 247; 249; 257; 260,

Moths, flight of, into flame, 227, 228;
adaptation, 237; change in reac-
tions, 280; 346.

Motor reaction, defined, 45,

Motor reflex, 17; defined, 45; 240.

Mouse, dancing, color vision, 360.

Movement, random, as a factor in
orientation, 50, 51, 157; rate of,
in: swarm-spores and Vol vox, 100,
loi, Euglena, 102, no. Hydra,
151, Musca larvae, 184-189,
earthworms, 199, Planaria, 208;
random, in Musca larvae, 189-192,
196; random, as a factor in orien-
tation of earthworms, 203, 232;
effect of random, on aggregation,
239-241, 245; periodic, in plants,
286; periodic, in Littorina, etc.,
286; cause of change in, 366.

Muller, H., 13; 354.

Miiller, J., vitalism, general physi-
ology, 8, 9; 52.

Muller, N. J. C, 60; change in sense
of reactions (seedlings), 265; re-
actions of seedlings in spectrum,

317, 319-
Musca larvae, 50; orientation in

light from two sources, 87, 177,
197; 88; orientation according to
Loeb, 175; locomotion, 176; ac-
curacy of orientation, 177; orien-
tation perpendicular to the rays,
180-183; distribution of sensitive
tissue, 183, 184, 188; effect of light
intensity on rate of locomotion,
184-189, 197; random movements,
189-192, 196; acclimatization of,
189, 190, 197; mechanics of orien-
tation, 189-197, 232; orientation
compared with that in Euglena

INDEX

403

and Stentor, 195, 197; trial and
error, 196, 197; circus movements,
216; 257-260; 270; change in re-
actions, 285; 346.

Mustard. See Sinapis.

Myxomycetes, 74; 229; 233; adapta-
tion, 237.

Nageli, early observations on move-
ment of flagellates and swarm-
spores, 14; 15; effect of light in-
tensity on rate of movement, 100,

Nagel, 58; 247; on reactions td
shadows and acclimatization, 248;
250; 251; effect of different rays
on reactions, 302.

Narcotics, effect of, on reversal of
reactions, 279-283, 300.

Natural selection and adaptation,
238, 239, 272.

Nemertean, 355.

Nernst glower, 61; 86; 92.

Newcombe, transmission of stimuli,

59-
Nichols, distribution of energy in

spectrum, 304, 305.

Oats. See Avena.

Oedogonium swarm-spores, orienta-
tion in light from two sources, 87;
description of, 123, 124; collection
of material, 124; locomotion of,
124; reversal in sense of orienta-
tion, 125; mechanics of orienta-
tion, 126-128; aggregation of, 127,
241; 229.

Oltmanns, experiments on ray direc-
tion and intensity difference, 39,
40; 42; 60; 63; 265; 290.

Optimum intensity, effect of, on
aggregation, 242; variation in,
288-297, 301.

Orchestia, reversal in reactions, 271,
284, 300.

Orientation in light, in plants and
animals compared (Loeb), 24, 25;
plumules of Zea mays: 63-69, dis-
cussion of, 70-71; in leaves, 71-73;
in Amoeba: 76-79, discussion of,
80; in Euglena (Jennings), 83, 84;
from two sources (Euglena, etc.),
86-89, 219-221, 224; accuracy of,
in Euglena, 92; mechanics of, in
Euglena crawling, 92-99; discus-
sion of, in Euglena, 99-102; in
negative Euglena, 99; mechanics
of, in Euglena swimming, 102-104;
mechanics of, in Oedogonium
swarm-spores, 126-128; in Trache-
lomonas: accuracy of, 129, me-
chanics of , 129, 130; mechanics of,
in Chlamydomonas, 132, 133; me-
chanics of, in Volvox, 137-144,
231; in Volvox compared with
that in Euglena and Stentor, 142;
in Eudorina and Pandorina, 147,
231; in Hydra, 157, 231; in Euden-
drium planulae, 161-163; in Eu-
dendrium hydranths, 163, 164;
in Arenicola larvae, 167-171, 232;
in Musca larvae, 189-197, 232;
in earthworms, 198-205, 232; in
Echinoderms, 21 1-2 13; in Bufo,
219-223; in Caprella, 224, 225;
in Corethra larvae, 225; in lobster
larvae, 226; in zoeae of Brachyura
and Caridea, 226; mechanics of
(general), 233-235; effect of, on
aggregation, 241-244; fundamen-
tal cause of, 243; in Daphnia,
264.

Oscillaria, reactions of, in spectrum,
323, 326, 327, 362; distribution of
stimulating efficiency in spectrum,

Ostwald, 265.

Oxygen, effect of, on reactions, 279,
323-325, 362, 374-

404

INDEX

Pagurus, reactions to shadows, 247;
249; modification in reactions of,
296.

Palaemon, 264.

Pandorina, orientation in light from
two sources, 87, 147; function of
eye-spot, 109, 147; 136; structure
of, 146; locomotion of, 147; orien-
tation and change in sense of,
147, 231; 171; aggregation of, 242.

Papaver, 288; 313.

Paramecium, 17; 45; 82; reactions
to light, 134, 135, 361; 142; 324;
344; 350; 366.

Paramecium bursaria, method of
aggregation (Engelmann), 16; 45;
aggregation of, 240; effect of oxy-
gen on reactions of, 279; reactions
in spectrum, 323, 324; 374.

Parker, definition of tropism, 56;
orientation in earthworms, 198;
201; 202; 214; on circus move-
ments, 216; 221; 223; effect of
size of illuminated area on reac-
tions, 227; 244; 250; 265; 274;
effect of contact on reactions,
284.

Patten, 247.

Payer, reactions of seedlings in spec-
trum, 314, 317, 319.

Pearse, orientation in holothurians,
211; 221; 247; decoration in crabs,

357.

Peckham, 354.

Pecten, 247.

Pelomyxa palustris, 74.

Perichaeta bermudensis, 199.

Pfeffer, 12; 17; 21; 59; 60; 94; 100;
warming-stage, 274; sleep move-
ments in plants, 286; effect of dif-

«

ferent rays on reactions, 303; 317.
Phacus, aggregation of, 241; 274.
Phagocata gracihs, 207.
Phalaris, 19.

Photokinesis, defined, 35. See TJnter-

schiedsempfindlichkeit.
Photopathy, defined by Davenport,

40, 41; 55; 56; 241; defined, 253-

256; compared with phototaxis,

254; 302.
Photosynthesis, 311, 312, 361.
Phototaxis, defined by Davenport,

40; 55; defined by Hadley, 56;

217; defined, 253-256; compared

with photopathy, 254; 302. See

Heliotropism.
Phototropism. See Heliotropism.
Phycomyces, 265; 318.
Physiological states, dependence of

reactions upon, 49; effect of, on

reversal in reactions, 284-287;

Jennings on, 372-375-
Pilobolus, 318.
Pinnularia, 323.

Pisa, 355-

Planaria, accuracy of orientation,
206; locomotion and rate of loco-
motion, 207-210; method of aggre-
gation, 210; 254; 257; 261; 270;
346; 366; maculata, 207; gono-
cephala, 207; torva, 255.

Plants, theory of orientation in:
(Darwin), 18-21, (Loeb), 30; re-
actions of, in light, 59-73; sensi-
tiveness of, 288, 313, 319; effect
of different wave-lengths on re-
actions of, 310, 313-320, 361;
effect of mixed rays on reactions
of, 362. See Table of Contents.

Planulae of Eudendrium, 87. See
Eudendrium.

Plateau, 354.

Plumules, orientation of, 63-69.

Poggioli, effect of different rays on
reactions of plants, 314, 321.

Pollock, transmission of stimuU in
roots, 31, 59; theory of root curva-
ture, 71.

INDEX

405

Polygordius larvae, 266; reversal in
sense of reaction, 273, 279, 283,
300; 346.

Porter, 360.

Porthesia chrysorrhoea, 31; 336; 346;

371-
Potamilla oculifera, 247.

Pouchet, 247.

Pouillet. Sec Dutrochet.

Prawns, 249; 257; 260,

Preference method, 10; 343.

Preyer, 10.

Pringsheim, protoplasmic streaming,

75-
Problems, in reactions to light,

statement of, i, 2, 3, 57, 58.
Protozoa, 51. See Table oj Contents.
Protula intestimum, 247.
Psychic phenomena, in organisms,

9, 10, 27; in Euglena, 102; in frogs

and toads, 223, in ants, 350, in

bees, 353.
Psychoid, defined, 377.
Purkinje's phenomenon, 305.

Raccoon, color vision, 360.

Radl, theory of orientation (pres-
sure of hght), 42, 43; cause of
change in sense of orientation,
43; definition of tropism, 55; 214;
on circus movements, 216, 217;
234; 264; 265.

Rana, 260; 273.

Ranatra, 217; 218; 226; 260; reversal
in reactions, 271, 273, 280, 284,
300; modification in reactions of,
296; 367.

Raphanus, 314.

Rawitz, 247.

Ray, explanation of movement in
plants, 7; orientation in plants, 12.

Ray direction, as used by Sachs, 14;
effect of, on orientation in animals
(Loeb), 24-26; compared with dif-

ference of intensity, 27; Oltmanns
on, 39, 40; function of, in orienta-
tion: plumules, 68, 69, Euglena,
83, 87, no. III, Stentor, 114, 118,
Hydra, 150, Musca larvae, 180-
182; 137; 144; 241; Davenport on,
254-256.

Reactions to light, distribution of, i;
preference method, 10; problems in,
i~3> 57? 585 ill various organisms.
See Table of Contents; classification
of, 253-256; reclassification of, 256-
262; evolution of, 262, 263.

Reactions, to shadows, 247-250; to
sudden increase of light intensity,
250, 251; to continued illumina-
tion, 252, 253, 257; to change of
intensity, 257; of questionable
cause, 258; caused by direct effect
of light, 258; caused by indirect
effect, 258-260; caused by what
light represents, 260; fundamental
cause of, 258-260, 278, 279, 298,
300, 320, 363, 366; reversal in,
264-267, 355; cause of reversal in,
267-287, 299; extent of reversal
in, 271; effect of light on reversal
in, 265-272, 299; effect of tem-
perature on reversal in, 272-279,
300; effect of chemicals on re-
versal in, 279-283, 300; effect of
concentration of medium and me-
chanical stimuh on reversal in,
283, 284, 300; effect of internal
changes on reversal in, 284-287;
variability and modifiability in:
general, 288-297, 361, 362, in ants,
351, in bees, 352-354, in higher
Crustacea, 355-358, in fishes, 358-
360; effect of mixed colors on, 310,

316, 335, 340, 362, 363-
Regulation, in behavior, 50, 377;
Chapters 13, 14, and 20. See
Jennings.

4o6

INDEX

Reighard, color vision in fishes, 359,
360.

Reinke, chlorophyll-absorption band,
312.

Reptiles, 343-

Rhizopods, 74; 79; 229; 233; aggre-
gation of, 242; 258.

Romanes, 10; 24; 211; 235; 251.

Roots. See Plants.

Root-tip, function of, in reactions,
19, 20.

Rothert, 12; 21; 72.

Ryder, 247.

Sabella microphthalmia, 247.

Sachs, J. von, ray-direction theory
of orientation in plants (heho-
tropism, geotropism), 13-16; ag-
gregation of organisms due to
currents, 15; criticism of Darwin's
idea of function of root-tip in
movement of roots, 21; 24; 25-30;
3,y, 34; 38; 39; 40; 42; definition
of tropism, 53, 54; 57; 60; 70; 80;
86; ray-direction theory criticized,
87; 112; ray-direction theory ap-
plied to Vol vox, 137, 144; 149)
233; 314; effect of different colors
on reactions of seedlings, 317; 319;
See Ray Direction ajid Criticism.

Sagartia, 48.

Sand fleas, adaptation, 237, 238,

Sarsia, 251.

Scapholeberis armata, 214; 271; 277;
280; 300.

Schafer, cause of invisibility of infra-
red and ultra-violet, 362.

Schreckbewegung, origin and mean-
, ing of term, 17; in Euglena, 82;
no; 161; 233; 240; 246; 257; 259;
in bacteria, 325.

Scystosiphonlomentarium, 274.

Sea anemone, 258; 299.

Sea squirt, 250.

Sea urchin. See Echinoderms.

Seedlings, orientation in light, 59-
73 ; change in sense of orientation,
265; reactions in spectrum, 314,
See Plants.

Semotilus atromaculatus, 358.

Sensation. See Psychic phenomena.

Sensibilite differentielle, compared
with motor reflex, 17; 243.

Sensitiveness, with different surfaces
exposed: in Euglena, 104-106, in
Stentor, 114, 115, 119; variation
in, 288-297, 301; in plants, 288,

3^3, 319-
Serpula, 247; 254; 258.

Serpula uncinata, 247.

Setaria italica, 21.

Shadows, reactions to, 247-250.

Sharroc, 7.

Shock-movement. See Schreckbewe-
gung.

Shrimps, 249; 257; 260.

Sigesbeckia, 286.

Sign, reaction to, 243; 250; 259-263;
286; 292-295; 298.

Simocephalus sima, 265; reactions
in spectrum, 337, 341-343, 362.

Sinapis alba, reactions in spectrum,
316, 317.

Smith, orientation in earthworms,
198; 201.

Spaulding, modification in reactions
of hermit crabs, 296.

Spectrum, energy distribution in,
304, 305; brightness distribution
in, 305-308; actinic effect, dis-
tribution in, 308-312, 360; reac-
tions: of plants in, 313-320, of
unicellular forms in, 321-332, of
Amoeba in, 327-332; distribution
of stimulating efficiency in: plants,
314-320; swarm-spores, 321, 322;
diatoms and Oscillaria, 323, 326,
327, ciliates, 323, flagellates, 324,

INDEX

407

bacteria, 325, Amoeba, 327, 330-
332, Hydra, 334, 335, Daphnia,
337-341, Simocephalus, 341-343,
worms, spiders, insects, moUusks,
fishes, amphibia, reptiles, birds,
mammals, 343-346, ants, 348-352,
bees, 352-355, higher Crustacea,
355-358, fishes, 358-360, birds and
mammals, 360, general, 361, 362,
367; for fishes and dancing mouse,

359, 360.

Sphinx, 346.

Spider, water, reversal in reactions,
277, 300; 343; 354; color vision
in, 364.

Spirographs spallanzani, 2)y') 247;
254; 255; 258.

Stahl, 74; light reactions in Euglena,
82; 265.

Starfish. See Echinoderms.

Stenorynchus, 355; 358.

Stentor coeruleus, 42; orientation in
light from two sources, 87, 115,
116; distribution of sensitive tissue
in, 114, 119-121; mechanics of
orientation, 114-119, 122; reac-
tions when attached, 115; aggre-
gation of, 121, 241; activity in
different intensities of light, 123;
124; 126; 127; 132; 137; 156;
orientation compared with that
in Musca larvae; 195; 215; 229;
230; adaptation, 237; 250; 257;
259; 261; 263; 270; 277; 280;
variation in sensitiveness, 291,
301, 368; 297; 300; variation in
reactions, 374.

Stentor viridis, aggregation of, 240;
effect of oxygen on reaction, 279.

Stimulation, differential response to
localized, in: Euglena, 83, 10 1,
Stentor, 121, Oedogonium swarm-
spores, 128, Volvox, 137, Hydra,
158, Eudendrium, 162, medusae,

164, 165, Arenicola larvae, 174,
fly larvae, 195, earthworms, 200,
201, 205, Planaria, 206, 210, vari-
ous species, 214, Caprella, 225,
arthropods, 226; general, 48;
Summary of, 230-235.
Stimulus, transmission of, in plants,
12, 21; fundamental cause of
(Jennings), 50; 59; orienting,
cause of in: Euglena, 94, 98,
Stentor, 118, Oedogonium swarm-
spores, 127, Paramecium, 134,
135, Volvox, 144, Hydra, 157,
Eudendrium, 163, Arenicola lar-
vae, 1 71-174, Musca larvae, 194,
195, earthworms, 204, 206, Plan-
aria, 208, 210, Echinoderms, 212,
Ranatra, 218, frogs and toads,
219, 223, gramineae, myxomy-
cetes, rhizopods, and various other
organisms, 229-235; a sign as a
cause of, 243; cause of, in reac-
tions to shadows, 250; cause of
(constant intensity), 252, 253;
cause of, in reactions to light
(Davenport), 254-256; character
of, 256-258.
Stobbe, on reversible photochemical

reactions, 308-312; 336.
Strasburger, reactions of swarm-
spores to light, 15, 123; 16; 27;
40; effect of light intensity on
rate of movement, loc; 124;
reversal in reactions of swarm-
spores, etc., 265, 272-274; change
in optimum, ^88; 290; reactions of
swarm-spores in spectrum, 221,
222.
Swarm-spores, 87; rate of movement
in different intensities of light,
100; aggregation of, 241; 257; 259;
change in sense of reaction, 265;
272; 274; reactions of, in spec-
trum, 321, 322, 361.

4o8

INDEX

Sylvius, founder of iatrochemical

school, 6.
System, harmonious equipotential,

375-

Talorchestia, 271.

Temora longicornis, 284; 300.

Temperature, effect of, on reversal
in reactions, 272-279, 300; effect
of, compared with that of light,
276, 367; extent of effect of, on
reversal in reactions, 277.

Tenebrio, 346.

Theory, of orientation in plants:
De Candolle, 12, Sachs, 13-16,
Darwin, 18-21; Loeb: first theory
of orientation (ray direction), 24,
25, 34, second theory (angle of
rays), 28, 29, 35, third theory
(intensity difference), 29-31, 35;
of orientation in animals (Daven-
port), 40-42; Radl's, 42, 43; Jen-
nings' (trial and error), 46-49;
Torrey's, 84, 85; Holmes', 218;
Graber's, 218; of local response
to local stimulation, 86. See
Criticism.

Threshold, in Euglena, 104-106; in
Stentor, 114, 115, 119; in plants,
288; in Volvox, 289-291.

Thyone briareus, 212; reactions to
shadows, 247.

Tissue, sensitive, distribution of: as
a factor in orientation, 3, in
plants, 21, 59, 71, in Hydra, 156,
157, in Arenicola larvae, 172, in
]\Iusca larvae, 183, 184, 188, 197,
in earthworm, 201, 205; effect of,
in reactions, 261, 262.

Toads. See Bufo and Frogs.

Torelle, 214; reactions of frogs to
light and shadow, 218, 219; 222;
223; 244; reversal in reactions
(frog), 273.

Torrey, definition of tropism, 56; 57;
criticism of Jennings on Euglena,
84, 85; 89; 100; loi; 104; 112; 205.

Towle, effect of mechanical stimu-
lation on reaction, 284.

Trachelomonas, orientation in light
from two sources, 87; function of
eye-spot, 109, 130; description of,
128; structure of eye-spot, 128,
129; accuracy of orientation, 129;
mechanics of orientation, 129, 130;
146; 229; 230; aggregation of, 241;

257; 259-
Trembley, 33, 34; observations on

movements of Hydra, 148, 149.

Trial and error, orientation by, 46;
in Euglena, 99; in Stentor, 123;
in Volvox, 142, 145; in Euden-
drium, 162; in JMusca larvae, 196;
in earthworms, 198, 199, 203, 206;
214; defined by Jennings, 215;
Engelmann on, 240.

Titicum vulgare, 66.

Trochophores, Hydroides, 87.

Tropaeolum, 59; 68; 72.

Tropism, introduction of term and
original meaning, 11, 23, 52; de-
fined, 53-57; 83.

Uca pugnax. See Fiddler Crab.

Uexkiill, von, 212; 247; 248.

Ulothrix, 274.

Ultra-violet, effect of, on: Paramecia,
134, 135, Daphnia, 339, 343, chemi-
cal reactions, 310, 311, 360, ants,
349, seedlings, 314, 315, 319,
Lumbricus, 345; cause of invisi-
bility of, 362; 368.

Ulva lactua, 274.

Unterschiedsempfindlichkeit, com-
pared with motor reflex, 17; 27;

2>y, 85; 94; 114; lis; 241; 243;
compared with heliotropism, 254-
256; 258.

INDEX

409

Vanessa antiopa, 216; reactions to
light, 227; aggregation of, 244;
260.

Variability, in reactions, 264-301;
367; chemical regulation of, 370.
See Modifiahility.

Vaucheria, 31; 265.

Vermes, aggregation, 242. See
Earthworm.

Vertebrates, 233.

Verworn, 11; 35; theory of orienta-
tion (tropism), 36-38; same com-
pared with Loeb's theory, 38, 39;
general application of theory, 39;
42; 52; 53; definition of tropism,
55; 56; 57; 70; 80; 104; 112; 113;
122; 168; 171; theory of orienta-
tion applied to: Arenicola larvae,
173, earthworms, 205; 229; 234;
265; on effect of different colors
on reactions, 303; reactions of
unicellular forms in spectrum, 326,
327. See Criticism.

Vicia sativa, 315, 316.

Vierordt, brightness in spectrum,

305-307-
Vision, as a factor in orientation,

218, 219, 223, 224, 233; color:
in Daphnia, 339,'' in ants, 350, in
bees, 354, in fishes, 358-360, in
birds, 360, in monkeys, 360, in rac-
coon, 360, in dancing mouse, 360,
in general, 364, 365.

Visual purple, 307.

Vitalism, origin and early ideas on,
8; 52; theory of (Driesch), 374-

378.

Vitis, 265.

Voechting, 71.

Volvox, Oltmann's experiments on,
39; orientation in light from two
sources, 87; rate of movement in
dififerent intensities of light, 100,
loi; function of eye-spots, 109;

structure of, 136; locomotion, 136;
distributionof sensitive tissue, 137;
mechanics of orientation, 137-144,
231; orientation of segments, 142,
143; orientation in negative col-
onies, 143; change in sense of ori-
entation, 145, 267-270, 280, 287,
299, 300; orientation in light com-
pared with orientation in a gal-
vanic current, 145, 146; 148;
orientation compared with that in
Arenicola larvae, 169, 171; adap-
tation, 236; aggregation of, 242;
255; 257; 259; 263; 265; change
in sensitiveness and optimum,
289-292, 301; 367; 370; 374.

Wager, eye-spot in Euglena, 81, 82,
106.

Walter, definition of tropism, 55;
reactions of Planaria, 207, 245.

Washburn, definition of tropism, 56;
on movement in Hydra, 150;
211.

Washburn and Bentley, color vision
in fishes, 358, 359.

Wasps, modification in reactions of,
296, 297.

Watson, 360.

Weber, 9; law of, 363,

Wheeler, definition of tropism, 56.

Whitman, reactions of Clepsine to
shadows, 249.

Wiesner, 19; 60; 265; effect of differ-
ent rays on reactions of plants,
310, 314-3171 322, 362.

Willow borer, adaptation in cater-
pillar of, 238.

Wilson, reactions of Hydra to light,
150; 265; effect of different rays
on reactions of Hydra, 310, 362;
reactions of Hydra in spectrum,

333-335-
Wohler, 9.

4IO

INDEX

Yerkes, Ada W., reactions of Hy-
droides and modification in, 249,
292, 293.

Yerkes, R. M., definition of tropism
(phototaxis, photopathy), 55; 63;
reactions of Gonionemus, 164; 214;
251; classification of reactions to
light, 255, 256; 266; 274; reactions

of Simocephalus in spectrum, 341-
343; 360.

Zea mays, orientation of plumules,

59-73, 229.
Zoeae, orientation in light from

two sources, 87; 226; 243; 277;

280.