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ALBERT R. MANN
LIBRARY
AT
CORNELL UNIVERSITY

Cornell University Library
tudy

QL 767.N53
TT
002 995 847

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3 1924

Cornell University

Library

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

http://www.archive.org/details/cu31924002995847

COLOUR IN NATURE

COLOUR IN NATURE

A STUDY IN BIOLOGY

BY
|

MARION I NEWBIGIN
D.Sc. (LOND.)

LECTURER ON ZOOLOGY
IN THE ‘MEDICAL COLLEGE FOR WOMEN, EDINBURGH

LONDON
JOHN MURRAY, ALBEMARLE STREET
1898

PREFACE

THIS little book is an attempt to set forth in
systematic order the main facts at present known in
regard to the Pigments and Colours of Plants and
Animals. Although there are not a few books
which treat of the external aspects of the Colours of
organisms, it is a frequent complaint among those
engaged in actual research that, not only is the
physiological side of the subject inadequately treated
in the text-books, but the literature is also scattered
and difficult of access. It is hoped that the present
work may both help to supply this deficiency, and
also appeal to that wider public whose interest in
the problems of Colour has been aroused by the
existing general books on the subject. The popu-
larity of many of these is deservedly so wide, that I
have endeavoured throughout to avoid trenching on
ground already covered by them, and have rather
concentrated attention upon less familiar points.

vi COLOUR IN NATURE

The literature of the subject is so extensive that it
has been found impossible within the assigned limits
of space to attempt a complete bibliography. I
have therefore thought it most advisable to confine
myself as far as possible in the list of references
to papers actually consulted and discussed in the
text. Even thus abbreviated, the list will, I trust,
be of value to the workers in the subject.

I have to express my sincere obligations to Mr.
J. A. Thomson for much kind assistance, especially
with the literature.

COLLEGE OF MEDICINE FOR WOMEN,
EDINBURGH, September 1898.

CONTENTS

INTRODUCTION

Colour in Nature and in Organisms—Characters of the Colours of
Organisms and their General Importance—Method of Treatment
Adopted ‘ ; : é . Pager

CHAPTER I
THE COLOURS OF ORGANISMS

Colour Phenomena in General—Distinctions between Pigmental and
Structural Colours—Characters of Structural Colours and their
Classification— Production of Light by Organisms (Phosphor-
escence) % x ‘ 6

CHAPTER II
THE PIGMENTS OF ORGANISMS

Natural and Artificial Pigments— Classification of Pigments— Pig-
ments of Direct Physiological Importance—Derived Pigments
—Waste Products—Reserve Products—Introduced Pigments
—Distribution of Pigmental Colours—Spectroscopic Characters
of Pigments . ; ‘ 5 5 25

viii COLOUR IN NATURE

CHAPTER III

THE COLOURS AND PIGMENTS OF PLANTS

Pigments and Colours of Bacteria and Fungi—Chlorophyll and the
Associated Pigments—Colour and Pigments of Algee—Pigments
of Flowering Plants—Autumnal Coloration—Colours of Flowers
and Fruits—Meaning of Plant Pigments and Summary Page 51

CHAPTER IV
THE COLOURS OF PROTOZOA, SPONGES, AND CCELENTERA

Pigments of the Protozoa, their Characters, Variations, and probable
Origin—Coloration of Sponges and Ccelentera—Distribution of
Colours—The Colours of Corals and Sea-Anemones— Colour
Resemblances in the Ccelentera—The Effect of Light upon the
Development of Pigment—The Pigments of the Ccelentera—
Optical Colours 2 : : : : 73

CHAPTER V
COLOUR- PHENOMENA IN WORMS

Colours of Turbellaria and Nemertea—Pigments of Gephyrea—Colours
of the Chztopoda, Structural and Pigmental Colours — The
Pigments of the Capitellide—General Characters of Colora-
tion of Leeches and Origin of Markings—Pigments of Polyzoa
and the Origin of Pigmentation ‘. 5 3 96

CONTENTS ix

CHAPTER VI
THE COLOURS OF CRUSTACEA AND ECHINODERMA

Nature of Colours of Crustacea—Colour Variation— Pigments of
Crustacea: (1) Lipochromes, (2) Soluble Blues— Relations of
the Two—General Characters of Pigments—The Colours of
Echinoderma: (1) Star-fishes, (2) Brittle-stars, (3) Sea-urchins,
(4) Crinoids, (5) Holothurians—Pigments of Five Classes —
Relations to those of Crustacea . F Page 117

CHAPTER VII
THE COLOURS OF THE LEPIDOPTERA

Insect Coloration and its General Relation to Physiology— Pigments
of Caterpillars—Intrinsic and Derived Pigments—Mr. Poulton’s
Experiment—Meaning of Derived Pigments—Other Characters
of the Coloration of Caterpillars—Colours of Butterflies, Pig-
mental and Optical Colours—The Pigments of the Pieridae—
Pigments of other Butterflies—Origin of the Pigments of Butter-
flies—Contrast between the Pigments of Butterflies and those of
Caterpillars—Pigments and Mimicry . 138

CHAPTER VIII

THE COLOURS OF INSECTS IN GENERAL AND OF SPIDERS

Optical and Pigmental Colours in Insects—Colours of the different
Orders of Insects—Colours and Pigments of Orthoptera—Optical
Colours of Neuroptera—Coloration of Beetles—Pigments of
Hemiptera—Colour Resemblances between Diptera and Hymen-
optera—Contrast between Coloration of Lepidoptera and other

x COLOUR IN NATURE

Insects—General Aspects of Insect Coloration—Variation in
Colour: (1) Natural, (2) Artificially Produced—The Colours of
Spiders: (1) Optical, (2) Pigmental—Development of Colour
—Variation in Colour—Colours of the Sexes— Markings of
Spiders Page 163

CHAPTER IX

THE COLORATION OF MOLLUSCA AND OF INVERTEBRATES
IN GENERAL

Colours of the Mollusca: (1) of the Mantle, (2) of the Shell, (3) of
the Secretions—Pigments of the Mollusca— Green Oysters—
Characters of Coloration of Invertebrates—-Patterns and Mark-
ings of Organisms; Plants, Ccelentera, Leeches, Arthropoda,
Mollusca, Vertebrates—Meaning of Patterns . 4 184

CHAPTER X
THE COLOURS OF FISHES

The Colours of the Lower Vertebrates—General Characteristics of
Vertebrate Coloration—Colours of Fishes, their Characters and
Distribution in Tropical, Abyssal, and Temperate Forms—
Coloration of Flat-fishes—The Pigments and Structural Colours
of Fishes a ‘ . 207

CHAPTER XI

THE COLOURS OF AMPHIBIANS AND REPTILES

Colours and Pigments of Amphibia—The Colours of the Larvzee—
Development of Colour—The Relation of the Larval and Adult

CONTENTS xi

Coloration—Colour Variation in Larvae—Colours of Reptiles,
especially Lizards and Snakes—The Origin of Markings in
Snakes Page 225

CHAPTER XII
THE COLOURS OF BIRDS

General Characters of the Coloration of Birds—Sexual Coloration—
Distribution of Colour and Colour Variation—Food and Colour—
Pigments of Birds, their Characters and Distribution—Pigments
of Birds’ Eggs—Markings of Feathers . 242

CHAPTER XIII

THE COLOURS OF BIRDS (Continued)

The Structure of Feathers—Relation between Structure and Colour
—The Colours of Sun-birds, Humming-birds, and Birds of
Paradise, their Distribution and Characters—Markings of King-
fishers—General Characters of the Colours of Birds—Meaning
of Colour in Birds F s 3 : 5 262

CHAPTER XIV

THE COLOURS OF MAMMALS AND THE ORIGIN OF PIGMENTS

Coloration of Mammals—Pigments of Mammals—Colour of the Hair
and Skin in Man, and its Bearing on General Problems—Origin
of Melanin—Pigment and Waste Products-— Experimental
Evidence—Conclusions—Criticism of these Conclusions 286

xii COLOUR IN NATURE

CHAPTER XV
THE RELATION OF FACTS TO THEORIES
General Summary — Theories as to Origin of Colour: Poulton,
Wallace, Eimer, Cunningham, Simroth—Criticism of Natural

Selection—Criticism of Other Theories—Conclusion Page 300

REFERENCES z ; é : 327

INDEX i F . . 335

INTRODUCTION

Colour in Nature and in Organisms— Characters of the
Colours of Organisms and their General Importance —
Method of Treatment Adopted.

To those who have not followed closely recent
developments of Evolution Theory, the connection
between Biology and Colour may seem very remote.
The phenomena of colour, it may be said, are entirely
the province of the physicist; that the sky is blue
and the grass green are two facts of similar nature,
and the one is as inexplicable as the other. So
in general it may be said that it is simply a fact
of experience that most objects, whether animate
or inanimate, present themselves to our eyes as
coloured, and that it is therefore absurd to separate
the phenomena of colour as they appear in organisms
from the similar phenomena of inorganic nature. A
little reflection will, however, convince every one that
the biologist cannot afford to be indifferent to the
colours of the organisms with which he has to deal.
In the first place, they attract his attention because
of their frequently great intrinsic beauty and their
arrangement into patterns and markings which may

2 COLOUR IN NATURE

exhibit extraordinary constancy. Such markings
often tend to reappear in slightly modified forms in
a large series of nearly related organisms. Thus
Eimer has shown that the markings of the head,
often so conspicuous in the domestic cat, tend to recur
constantly throughout the whole of the Carnivora,
and in a large number of cases may be quite
definitely traced. This constancy of colour or
marking is not infrequently available for the purpose
of classification, or at least of ready identification,
and has therefore always attracted much attention.
But the point of interest about the colours of
organisms which has of late had most stress laid
upon it is one which, like so many recent develop-
ments of biological theory, has grown out of Darwin’s
work. As is well known, Darwin’s statement of the
Doctrine of Descent involved a clear formulation of
what has become widely known as the Struggle for
Existence, but what was in fact the first clear appreci-
ation of the intricacy of the relations existing between
organisms and their environment, the term including
both physical nature and other organisms. Darwin
endeavoured to prove that the balance of nature is
so finely adjusted that the slightest oscillation of one
part may affect parts apparently far removed from
it, and that the Struggle for Existence is so keen that
all specific characters are, as it were, maintained at
the point of the sword. Now we have just seen
that the colours and markings have always been
recognised as among the most constant of the
characteristics of organisms ; if, therefore, all specific
characters are preserved by virtue of their usefulness,
then surely the colours must be of supreme import-

INTRODUCTION 3
ance to the organisms——whence we have all the
recent developments of colour theories. For a
historical sketch of the way in which many theories
as to the meaning of colour in organisms have been
built up around the central doctrine of Natural
Selection, reference should be made to Mr. Wallace’s
Darwinism.

It is, however, of some importance to note that
the interest in the phenomena of colour thus mani-
fested has been very largely polemical in nature.
The supreme test of the value of a new scientific
hypothesis is that it explains series of phenomena
hitherto inexplicable, and the eagerness with which
the colours of organisms have been discussed and in-
vestigated has been chiefly stimulated by the hope
of adding fresh arguments to those already brought
forward in support of Darwin’s theory. We do not
propose at present to discuss the question of how
far this hope has been realised, but shall rather pass
on to consider a third aspect of the colours of
animals which has been as yet little dwelt upon.
This is the meaning of the colour in the functional
economy of the organism. The problems of colour
have of late years been so exclusively considered
from the outside—in connection with the relation
of the organism to its environment—that we are
apt to forget that the colours must also have a
meaning and a justification in the physiological
processes of the individual. The present method of
treating the subject is in large part the result of that
habit of regarding organisms as mere bundles of
qualities which has been so keenly criticised by a
recent writer. If we once realise that the colour of

4 COLOUR IN NATURE

an organism is not an isolated characteristic forced
upon it, as it were, from without, but may be merely
the outward expression of its constitution, we may
surely hope not only to be delivered from many
laborious hypotheses as to the use of colour in
particular cases, but also may perhaps learn some-
thing of the physiology of colour. The subject is
at least now sufficiently important to merit con-
sideration from a purely physiological standpoint.

We thus see that there are three reasons why it
is desirable that the Biologist should concern himself
with Colour in organisms. The first is the con-
spicuousness of colour phenomena in a merely
objective survey of animals and plants; the second
is the relation of these colours to current theories
of evolution; and the third is their importance in
comparative physiology.

The order of arrangement of these three is not
purely formal, but to some extent corresponds to
the historical order in which the subject has been
studied. Before Darwin, the colours of organisms
were chiefly studied as convenient marks by means
of which the organisms could be recognised. When
Darwin put forward his theory, which is based on
the supposition that all specific characters are of
supreme importance, it was a natural deduction
from the constancy of many colours that these must
be of great importance, and so we have all the
modern theories of colour (see Mr. Poulton’s Colours
of Animals). Again, now that the theory of Natural
Selection is no longer the centre of men’s thoughts,
and search is being made for a deeper analysis, it is
recognised that it is probable that the phenomena of

INTRODUCTION 5

colour have some connection with physiology, or
are at least not wholly accounted for when their
usefulness is proved (see Mr. Beddard’s Axzmal
Coloration, and, for the general question, Mr. Bate-
son’s great work on Variation).

Having now justified the intrusion of the bio-
logist into what appears to be the domain of the
physicist, we are at liberty to face the phenomena
of colour as they appear in organisms. As, however,
the relation of colour phenomena to the theory of
Natural Selection has had so much attention bestowed
upon it, we do not propose, at least in the first place,
to consider it here, but rather to direct attention to
the chemical, and where possible, to the physio-
logical aspects of the colours and pigments of
organisms. In a final summary it will be necessary
to consider the bearing of the facts upon theories.

CHAPTER I

THE COLOURS OF ORGANISMS

Colour Phenomena in General—Distinctions between Pig-
mental and Structural Colours—Characters of Structural
Colours and their Classification—Production of Light by
Organisms (Phosphorescence).

IT is not necessary here to consider in detail the
physical aspect of colour. Every one is more or less
familiar with the fact that, in general terms, objects
appear to us to be coloured because they absorb
certain of the elements of white light and reflect
the remainder. Thus the substance vermilion
appears to us to be red because it absorbs all the
components of the incident light except the red,
and this it reflects; it is a fundamental property of
vermilion that it possesses this power of selective
absorption. Practically all bodies do exercise
selective absorption to a greater or less extent,
but it is only when the absorption is marked in
the visible part of the spectrum that we definitely
recognise the bodies as coloured. When such
coloured bodies can be employed to impart their
own colour to animal or vegetable substances they

I THE COLOURS OF ORGANISMS 7

are ‘known in the arts as pigments, but to the
biologist the term includes only substances which
are produced by the activities of plants or animals.
The colour of the rose, for example, is due to a red
pigment present in the cells of the petals, which can
be extracted from those cells. This is the simplest
form of colour-production, and is the one which
commonly occurs in plants. Colours so produced
are called pigmental colours and can be recognised
by the following characters. Pigmental colours are
those produced by pigments, or substances of definite
chemical composition, which can by appropriate re-
agents be extracted from the coloured tissues, and
which react to light in the same way whether they
are within the tissues or outside of them. Tissues
or organisms showing only pigmental colours never
have a surface gloss, and the colour is not altered by
immersion in any medium which does not directly
attack the pigment.

Although the great majority of the colours of
plants are produced in this simple way, yet even
among them we have indications of that other kind
of colour-production which reaches its climax of
splendour in birds and butterflies. The gorgeous
tints of the humming-birds, which during life change
with every movement, are not produced by the
dyeing of the feathers with pigment, but are
phenomena of the same order as the colours of
the gems after which some of the birds are named.
We all know that the colours of the opal and of
many minerals are not due simply to a prime
property of the substance concerned, but are optical
effects dependent upon various external conditions

8 COLOUR IN NATURE CHAP

for their full manifestation. Many of the most
brilliant tints of animals are similarly optical colours,
but as they are produced by the special structure
of the coloured parts, they are more frequently
known as structural colours.

Structural or optical colours in organisms can
be recognised by the following tests. They usually
vary either with the angle of incidence of the light,
or with the change from reflected to transmitted
light ; tissues or organisms showing these colours
have a very marked surface gloss, and the colour
is usually destroyed by injury to this surface;
immersion in a neutral medium whose refractive
index is different from that of air, also usually
destroys the colour. Optical colours may further
be recognised by the negative character that the
coloured tissues do not yield to any reagent a
pigment of the same tint as that which they them-
selves possess. A peacock’s feather affords an
excellent example of a type of structural coloration
which is widely spread in birds. If the reader stand
in front of a window and hold a peacock’s feather in
his hand nearly on a level with the eye, and then,
still with the feather at arm’s length, slowly describe
a semicircle on his own axis, he will note that the
colours of the feather undergo a complete cycle of
changes. In this case, therefore, the colours change
with the change of the angle of incidence of reflected
light. If the feather be now held up to the light
it will be seen that the colours disappear, to be
replaced by a dull brown or black tint. Thus the
feather changes colour according as it is viewed by
reflected or transmitted light. The presence of a

I THE COLOURS OF ORGANISMS 9

marked surface gloss is, of course, very obvious, and,
though the size of the coloured particles is too small
to make it easy to note the effect produced by injury
to this surface, yet the disappearance of the colour
when the surface is thoroughly wetted with water or
oil can be observed very readily.

Having now distinguished generally between the
colours due to pigment and those which are the
result of a special structure, we must proceed to
consider these different methods of colour-production
in detail. As much less is known of the structural
colours than of the pigmental, we shall devote the
remainder of this chapter to the former, leaving the
latter for a further chapter.

CHARACTERS OF STRUCTURAL COLOURS AND
THEIR CLASSIFICATION

1. White is probably the simplest and most readily
understood of all structural colours, and, except in
rare cases, is always structural. The colours of the
lily, of white feathers, of Arctic mammals, are all due
to the same cause; namely, the total reflection of
light produced by the intercalation of numerous
bubbles of air, or some other gas, among colourless
solid particles. Thus in the lily the colourless cells
are separated by very numerous intercellular spaces
containing air. This is one of the simplest forms of
structural colour, not only because it is so readily
explicable physically, but also because there is no com-
plication arising from the presence of pigment in the
tissues, and because the modification of “ structure”
necessary to produce it is of the simplest nature.

10 COLOUR IN NATURE CHAP.

Most people are familiar with the analogous process
of producing a white colour by pounding up colour-
less glass, or crystals of blue sulphate of copper,
while the whiteness of snow, which has furnished so
many metaphors, is produced in a precisely similar
manner.

The fact that the whiteness of all these sub-
stances is due to an optical effect and not to a
pigment should be thoroughly grasped, otherwise
those not accustomed to dealing with colour pheno-
mena will find much difficulty in comprehending
structural colours in general. White sunlight is
produced by the combination of all the tints of the
rainbow. When objects permit light to pass com-
pletely through them, we call them transparent; when
they reflect all the rays of the light uniformly, we call
them white. This whiteness may be produced in
one of two ways. A substance such as “Chinese
white” is white because it is a property of the
particles of which it is composed to reflect equally
all the rays of incident light ; it is further a familiar
fact that Chinese white can be employed to impart
its own colour to other objects, that is, it can be
employed as a pigment. Snow, on the other hand,
is white, not because its individual particles reflect
the light—on the contrary they are transparent—but
because these transparent particles are separated by
bubbles of air. The incident light in passing from
the one medium to the other is bent or refracted, and
the result is the appearance of whiteness. A white
colour in organisms, except in very few cases, is
similarly produced, and is not due to pigment.

Other structural colours are to be accounted for

I THE COLOURS OF ORGANISMS II

in a closely similar fashion. Glass is a colourless or
transparent substance, when powdered it is white,
when cut with prismatic edges it displays all the
colours of the rainbow, and yet the qualities of the
glass remain unaltered. These are of course very
familiar facts, but it is important to realise also that
many of the most brilliant colours of organisms are
produced in a similar fashion—are adventitious and
not due to the essential properties of the coloured
substance. It is only in rare cases, however, that
bright colours are produced in a way capable of
simple physical explanation. There are usually
complications arising from the presence of some
amount of pigment, from the superposition of tissues,
or from the complex nature of the individual tissues.
Such brilliant optical colours occur apparently only
in cuticular structures. It is a curious fact that,
although such structures are of course not cellular,
nor living, yet their colour frequently fades very
rapidly after death; that dragon-flies, for example,
lose in a very short time all their gorgeous tints is a
fatt only too well known to collectors. This may be
due to loss of water, or to changes in the underlying
tissues.

Structural colours are most brilliant and con-
spicuous in birds and insects, but it is chiefly in the
former that they have been studied. Dr. Gadow has
especially studied the structural colours’ of birds,
and he divides them into two classes, according to
their behaviour as regards incident light. Thus
certain structural colours, such as green and blue,
are unchanging in reflected light, and are then

1 For a purely physical treatment see a pamphlet by B. Walter.

12 COLOUR IN NATURE CHAP.

not readily distinguishable from pigmental colours.
These Gadow classifies as Oljective Structural colours.
Again many colours change in tint according to the
angle at which they are viewed. Such metallic
colours may be classified as Subjective Structural
colours.

2. Of Objective Structural colours, green and blue
afford the best examples. Green seems to be usually
produced by a combination of a yellow pigment and
a structural modification, wherefore green feathers
usually appear yellowish in transmitted light. The
display of a blue colour again seems, at least in birds
and probably in insects, to be always associated with
the presence in the tissues of a dark-coloured pig-
ment. A blue colour in the feathers of birds is always
confined to certain parts of the feather, and its
presence is associated with considerable modifications
of feather structure. Blue is not however confined
to the feathers, but may occur also on bare patches
of skin, as in some of the Paradise birds, in the
cassowary, etc. Curiously enough, a blue colour in
the skin seems to be particularly unstable, fading as
rapidly after death as does the blue colour in the
abdomen of some dragon-flies.

The exact physical cause of green and blue colours
is still doubtful, but the fact that the colours are
structural is readily perceived both by the strong
surface gloss and by the disappearance of the colour
in transmitted light.

3. While unvarying blue and green tints in birds
rather heighten the general effect than display in
themselves great beauty, the Subjective Structural
colours, on the other hand, display the most ex-

I THE COLOURS OF ORGANISMS 13

quisitely varying tints, and it is to them that many
birds and insects owe their wonderful flashing beauty.
These colours glow with all the tints of the rainbow,
and change with every changing ray of light. Such
metallic colours are of course not uncommon pheno-
mena in the inorganic world, and are displayed, for
example, with extraordinary brilliancy on a polished
slab of the mineral Labradorite ; among organisms,
although suggested elsewhere, they attain their
maximum brilliancy in birds and insects. In birds
their degree of development varies greatly, for we
find them ranging from the dull greenish gloss of
some of the female humming-birds to the gorgeous
colouring of many of the males in humming-birds
and birds of Paradise. Among birds metallic colour
is apparently always associated with the presence in
the feathers of dull brown or black pigments, which
are necessary for the production of the colours. It
is also associated with a modification of the feather
structure which in many cases renders the feathers
unfitted for the purpose of flight. Here, as in the
case of objective structural colours, the exact physical
causation of the colours is unknown.

In insects the colours are equally bright, but have
had little attention bestowed upon them; it is still
doubtful whether the colours in them are or are not
associated with the presence of dark pigment.

4. The presence of a pigment is not, however,
essential to the production of structural colours ; the
common earthworm, for example, exhibits a faint
iridescence which is due to the presence of numerous
fine lines on its colourless cuticle, these fine lines
producing interference of light. Although the colour-

14 COLOUR IN NATURE CHAP.

ing is very slight in the earthworm, it is well known
that some of the marine worms, eg. the sea-mouse
(Aphrodite), are covered with numerous bristles which
exhibit brilliant iridescent colours. Again, the colours
of mother-of-pearl are of course produced by structure
only, without any assistance from pigment. For con-
venience of reference, Structural Colours may perhaps
be arranged as follows, retaining Gadow’s distinction
of objective and subjective colours.
Structural Colours :—
1. Those not dependent upon the presence of a
pigment.
(a) Due to total reflection; white colour of
some flowers, of some feathers, of hair, etc.
(8) Due to striation of the surface, occurrence
of thin plates, etc.; iridescence of bristles
and cuticle of worms, of mother-of-pearl, etc.
2. Those dependent on the presence of a pigment.
(a) Objective structural colours; blue and
green feathers.
(8) Subjective structural colours; metallic
colours of many birds and insects.
Structural colours are of extreme interest, not
only on account of their wonderful beauty, but also
on account of the difficult questions connected with
their origin. It is to some extent possible to corre-
late pigment-production with the physiology of the
organism, but this seems extremely difficult in the
case of structural coloration. We may note, how-
ever, that structural colouring attains its greatest
perfection among birds and butterflies, and both
groups are noted for the extraordinary development
of their cuticular structures. The delicate beauty of

I THE COLOURS OF ORGANISMS 15

the sculpturing of butterflies’ scales has been extolled
by most possessors of a microscope, while savage
and civilised races are alike in their admiration for
the feathers of birds. The fact that organisms so
widely separated as are birds and butterflies are
alike in exhibiting both exquisite structural colora-
tion and a wonderful development of structures
arising from the cuticle, suggests that the structural
colours are in origin merely a result of extreme
differentiation of the cuticle, and therefore produced
by the same cause which gave rise to this differentia-
tion. The presence of brilliant iridescence in some
of the mud-inhabiting worms is therefore not quite
inexplicable, fer here also we find that the cuticle
shows a considerable amount of differentiation. We
can also further understand how it is that the highest
pitch of perfection is attained in birds and butterflies,
when we consider that in both cases the colouring’
occurs in connection with structures which are of
supreme importance to the species, that is, with the
feathers of the bird and the scales, which are but
outgrowths of the wings, in butterflies.

PRODUCTION OF LIGHT BY ORGANISMS (PHOS-
PHORESCENCE), ITS DISTRIBUTION AND MEANING

The production of light, the phenomenon commonly
though incorrectly spoken of as phosphorescence, is
so striking a characteristic of many organisms that
it deserves notice here in our consideration of colours.
It is unnecessary to enter into a detailed account of
the theories of Dr. Carpenter and others as to the
probable use of this characteristic, because these

16 COLOUR IN NATURE CHAP.

are already familiar to most people. It is more to
our purpose to note the organisms among which
it occurs, and the special parts with which it is
associated.

To begin with the simplest organisms—the fact
that many decaying substances shine in the dark
has long been known, and modern bacteriologists
have been able to isolate the specific micro-organisms
which possess, under certain conditions, this property
of evolving light. Just as in the case of pigment
production, so also the phosphorescence depends upon
the nature of the organism and the character of its
surroundings. Thus Photo-bacterium phosphorescens
evolves light during the fermentation of sugar if free
oxygen be present and the temperature be favour-
able (3°-35° C.).

Among the Protozoa, phosphorescent forms are
‘most noticeably represented by the genus /WVocézluca,
but some Radiolarians, such as Thalassicola, are also
luminous. In MVoctiluca the phosphorescence is said
by Allman to be produced in the cortical layer of
protoplasm.

Among multicellular plants phosphorescence is
apparently confined to the fungi. It is an old
controversy whether it does or does not occur in
flowering plants, but most authorities answer the
question in the negative. In the fungi the
luminosity is marked in several species of Agaricus.
The phenomenon is only manifest in the presence of
free oxygen, is associated with oxidative processes,
and is dependent upon the temperature. The light
is said to be usually of blue or greenish colour, but
in some cases is quite white.

I THE COLOURS OF ORGANISMS 17

In the multicellular animals it is curious to note
that phosphorescence is most marked in pelagic and
in abyssal animals, though also of course occurring
elsewhere, e.g. in the glowworm, some centipedes, etc.

In the Ccelentera, many of the Meduse are
especially remarkable for their luminosity; their
colouring is thus described by A. Agassiz, “The jelly-
fishes, sparkling and brilliant in the sunshine, have
a still lovelier light of their own at night. They send
out a greenish-golden light, as lustrous as that of
the brightest glowworm, and in a calm summer
night the water, if you but dip your hand into it,
breaks into shining drops beneath your touch.”

As to the remaining groups of the Ccelentera, we
find well-marked luminosity in the Siphonophora, in
both divisions of the Anthozoa, and in the Ctenophora.
In the first group we may specially note PAysalza, the
Portuguese Man-of-War, which Agassiz describes
as appearing like a fire-balloon at night. Of the
Anthozoa, the Alcyonaria include the most brilliantly
phosphorescent forms; the Gorgonide being often
described as forming luminous forests at the bottom
of the sea, where those of the deep-sea forms which
retain their eyes are supposed to congregate. Lumin-
ous forms occur also, however, among the Zoantharia,
where the deep-sea Sagartia abyssicola, for example,
is said to secrete abundant phosphorescent mucus.

In the Ctenophora phosphorescence is so common
as to be practically a class character, and the members
of the group are frequently very important in the
production of surface phosphorescence.

In Worms certain of the marine Annelids, such
as Chetopterus and the Syllidz, are luminous.

Cc

18 COLOUR IN NATURE CHAP.

Among Echinoderms phosphorescence seems to
be known only among the Ophiuroidea, where it has
been described both in Mediterranean and in Atlantic
species.

Among the Crustacea phosphorescence is exceed-
ingly common, especially among deep-sea forms. In
some cases the luminosity is confined to the eyes, in
others, as in many of the deep-sea Schizopoda, there
are special luminous organs placed behind the eyes
or above the legs, in other cases again the phosphor-
escence is diffused.

In the terrestrial Anthropods phosphorescence is
known among various insects, glowworms, fireflies,
etc, and among centipedes, but it is not so common
as among marine forms.

Among the Mollusca phosphorescence occurs
among pelagic forms such as Phyllivhoé, and among
burrowing forms such as Pholas, whose luminosity
has long been known.

Next to the Ccelentera the Tunicatd are perhaps the
forms which display the most universal and brilliant
phosphorescence. Every one knows Moseley’s de-
scription of the brilliancy of the light emitted by
Pyrosoma, the phosphorescent fire-flame. Many
other pelagic Tunicates display the same pheno-
menon to a lesser degree. The Tunicates display so
many striking analogies to the Ccelentera that this
physiological one is perhaps not remarkable.

Finally, we have the phenomena of phosphores-
cence admirably displayed in fishes, especially the
pelagic and abyssal forms. In the pelagic forms
the sense-organs and the lateral line are often phos-
phorescent, while in some cases there are special

I THE COLOURS OF ORGANISMS 19

luminous organs. Many of the deep-sea fishes are
very markedly phosphorescent ; the phosphorescence
may be limited to spots on various parts of the body
surface, or there may be special luminous organs on
the head, while in many cases there are delicate
tactile processes phosphorescent at the tip; in some
cases the fins are themselves luminous.

As to the details of the mechanism of phosphor-
escence, Prof. Panceri of Naples made some interest-
ing observations more than twenty years ago. He
studied many marine animals, especially Pyrosoma,
Phyllirhoé, Pholas, and others. His papers display
a combination of careful observation and acute
deduction which make them models of scientific in-
vestigation. By the employment of very ingenious
methods he demonstrated the fact that in Pyrosoma
the luminosity is due to two cell-clusters in each
ascidiozooid. As each colony contains thousands of
individuals, the number of luminous spots is enormous
even in a relatively small colony. The cell-clusters
lie “on each side of the anterior end of the branchial
sac,” not far from the nerve ganglion, and consist of
spherical glandular cells. They contain a substance
soluble in ether, probably of fatty nature, which
apparently becomes luminous when oxidised. In
ordinary circumstances the light is only excited by
a mechanical stimulus, and then is first aroused at
the stimulated point and spreads more slowly
throughout the colony. These “luminous currents ”
are not nearly so rapid as in some of the Ccelentera,
Pennatula for example, and to this is probably due
the fact that when Moseley wrote his name on a
large Pyrosoma it stood out in “ letters of fire,” which

20 COLOUR IN NATURE CHAP.

of course could not occur if the propagation of the
stimulus were rapid. Under ordinary conditions the
phosphorescence is stimulated by the shock of the
waves against the sensitive organism, but it may be
also produced by various chemical stimuli, such as
fresh water, alcohol, and ether. The action of the
two latter agents is very interesting. If they come
into direct contact with the luminous organs they
extinguish the light instantly, while if they do not
reach the organs they act as powerful stimulants of
the light. The luminosity ceases at death, but the
fluid obtained from the living organism by crushing
retains its phosphorescent power for some time.

In the Medusze the luminosity is due to the
epithelial cells, and in consequence of the delicacy of
these the light is communicated to surrounding objects.
In Phyllirhoé the light is produced by the ganglion
cells of the nervous system, and by certain peculiar
nerve cells (Miiller’s cells), and is again apparently due
to a special substance occurring in the luminous cells.

In the Copepoda the phosphorescent substance
seems to be produced in special glands situated at
various parts of the body, and the light, according to
Giesbrecht, is only produced when the substance
secreted comes into contact with sea-water. In
Metridia longa, according to Vanhoeffen, the light
appears especially immediately behind the head, and
at a point close to the posterior end of the abdomen.
The living organism examined in light is colourless,
except at these same points which appear as “ moss-
green” spots ; these spots are apparently the phos-
phorescent glands. Other phosphorescent Copepods
display similar phenomena.

I THE COLOURS OF ORGANISMS 21

We cannot here enter into a description of the
phenomena of phosphorescence as they appear in all
luminous organisms, but may conveniently add to
the descriptions already given, an account of the
process in terrestrial forms. For this purpose we
may take Prof. C. Emery’s account of the phosphor-
escence of Lucitola italica, the Italian glow-insect.
Here in the male the luminous organs are placed on
the two last abdominal segments, each one appearing
as a continuous surface although originally formed in
two halves. In the female there are two luminous
spots at the sides of the ventral surface of the fifth
abdominal segment, the following two (the last) ab-
dominal segments appear in dried specimens of the
same pale colour as the fifth but are not luminous.
The luminous organs consist each of a double layer,
a ventral transparent one and a more dorsal chalk-
white one containing masses of urates. Some of
the large tracheal tubes run along the inner surface
of the dorsal plate, and give off perpendicular
branches which pass downwards and penetrate into
the substance of the transparent layer, where they
branch very freely, and come to lie close beneath
the skin.

A surface view of the luminous organ shows a
series of round or oval spots corresponding to little
masses of transparent cells surrounding the termina-
tion of these tracheal branches; it is these spots
which are luminous during life. They turn brown
when treated with osmic acid, and are separated by
broad darker intervals. More careful examination
shows that these spots correspond to little cylinders
formed by clusters of cells—the tracheal end-cells—

22 COLOUR IN NATURE CHAP.

which are placed near the ends of the fine tracheal
capillaries. The substance of the luminous organ
is made up of parenchyma cells, most easily perceived
in the transparent layer. The result of his examina-
tion convinced Emery that the luminous organ was
a specialised portion of the fatty body, the arrange-
ment of the trachez being one of the most marked
indications of specialisation.

His explanation of the luminosity is as follows:
the parenchyma cells (fatty cells) secrete the luminous
substance, from them it is taken up by the tracheal
end-cells, and, being here exposed to the action of
the oxygen of the tracheal system, undergoes a pro-
cess of oxidation resulting in the formation of light.
This process can only occur in regions where the
chitinous lining of the trachee is very thin, as in the
capillaries ; the free branching of the slender tracheal
capillaries in the luminous organs especially provides
for this. Ina later paper Emery is inclined to lay
more stress upon the parenchyma cells than upon the
tracheal end-cells as the seat of luminosity. The
luminosity is then due to the oxidation (or combus-
tion) of a probably useless body stored in the cells of
the organ (the fatty body). To this description we
may perhaps add that recent investigations tend to
emphasise the importance of the fatty body of insects
as an organ connected with excretion.

As to the nature of the light emitted by the
different phosphorescent organisms, the observations
are not sufficiently numerous to draw any conclusions.
Moseley found that the blue and violet rays were
absent in the light of three deep-sea Alcyonarians,
while surface forms displayed light of various colours ;

I THE COLOURS OF ORGANISMS 23

but the observations are too limited for any con-
clusion to be drawn.

With regard to phosphorescence in general it
appears to be most common in the relatively simple
organisms of the pelagic fauna, and next to these in
the abyssal fauna. If, as there seems much reason
to believe, it depends upon processes of oxidation,
its presence in the deep-sea forms is very remarkable.
The phosphorescent organs of the deep-sea forms,
especially the fishes, occur most frequently in con-
nection with nervous organs, especially with those
delicate tactile processes so characteristic of these
animals. A good example of this is found in the
luminous “ lures” of the deep-sea fishing frog. The
statement that this form has adopted the salmon
poacher’s method and does its fishing by means of a
luminous lure, seems at first sight to demand a teleo-
logical explanation, but when we remember that the
lures are greatly developed tactile organs the fact
that they are luminous is less inexplicable, in origin
at least. The whole subject of the phosphorescence
of abyssal forms is exceedingly puzzling. We are all
familiar with the pictures of the abysses of the ocean,
of the chill darkness illuminated by fitful gleams of
phosphorescent light, but it is difficult to decide how
far these representations are justifiable. It is a
somewhat obvious remark that no one has seen this
lower world, but is one which is perhaps not altogether
unnecessary. Many deep-sea organisms phosphoresce
when brought to the surface, but there is no proof
that they do so under the ordinary conditions of their
life. Has the increased amount of oxygen in the
surface waters no intensifying effect? Then also the

24 COLOUR IN NATURE CHAP. I

Alcyonarians have been described as forming luminous
forests in the ocean depths, but Agassiz, Panceri,
and others insist that in the cases studied by them,
the luminosity is dependent upon mechanical and
chemical stimuli, and we are always being told of
the calm stillness of the ocean abysses. The swim-
ming of fishes among these forests may set up
currents which produce luminous flashes, but surely
those elaborate hypotheses which assume that the
Alcyonarians give sufficient light for the other organ-
isms to see each other’s colours by, are a “ baseless
fabric” of the imagination.

With regard to the phosphorescence of the deep-
sea fishes, it is interesting to note that Agassiz
considers it to be a characteristic of these abyssal
fishes which belong to groups, most of whose
members are pelagic, rather than of abyssal fishes
in general.

As to the meaning of phosphorescence little can
be said. It appears not improbable that like pig-
ments it may arise in different ways in different
organisms. It is at least difficult to believe that
a process like that described by Emery for Luczola,
which seems to be purely excretory, can be entirely
homologous with the luminosity of nervous structures.
The luminous substance in all cases is apparently of
the nature of a fat, and it is unnecessary to emphasise
the fact that complex fatty substances tend to occur
in association with nerve tissues.

The subject has an important bearing upon the
general one, for phosphorescence, no less than
colour, has been frequently dismissed with a few
magic words upon Natural Selection.

CHAPTER II

THE PIGMENTS OF ORGANISMS

Natural and Artificial Pigments—Classification of Pigments—
Pigments of Direct Physiological Importance— Derived
Pigments—Waste Products—Reserve Products—Intro-
duced Pigments—Distribution of Pigmental Colours—
Spectroscopic Characters of Pigments.

ALTHOUGH, as we have seen, some of the most
beautiful colours of organisms are optical effects, or
are produced by a combination of a pigment and a
certain structure, yet the vast majority of the colours
of plants, and the colours of many animals, owe their
origin to pigments only. Before proceeding to con-
sider the colours of the separate groups of animals
and plants, we shall look for a little at pigments in
general.

The pigments or colouring-matters which are
most familiar to artists and those engaged in the
industries, are very frequently either artificial organic
substances, such as the now familiar aniline dyes,
or are inorganic compounds of the metals, such as
ultramarine and Scheele’s green. Some, however,
of the natural organic pigments are used in the arts.
As is well known, the ancients obtained their Tyrian

26 COLOUR IN NATURE CHAP.

purple from various small shell-fish, such as Murex
and Purpura, while the different preparations of
carmine and cochineal, saffron, and hematoxylin or
logwood are other familiar examples of pigments
produced by animals or plants which are habitually
employed as colouring-agents. The greater number
of the pigments of plants and animals are, however,
too fugitive to be so utilised, and in consequence are
in their pure state unfamiliar to most people. Such
are chlorophyll itself, the blue, red, orange and
yellow pigments of flowers, and the green, red,
yellow and orange pigments of animals, most of
which are destroyed by light or other agents with
great rapidity. It is indeed worth noting that most
of the natural dyes are of vegetable origin, and that
they are usually inconspicuous and apparently unim-
portant during the life of the plant, cf. hama-
toxylin, brasilin, etc. So, too, when they are of
animal origin, they are not usually important in
producing the coloration of the living creature, cf.
Murex, the coccus insect, etc. It is partly in con-
sequence of the fact that the pigments important
in producing coloration are so fugitive and so
difficult to obtain in a pure state, that so little is
really known of their relations and constitution.
Many of them, indeed, have hardly been investigated
at all.

We may note here a point which will be dwelt
on in detail in its proper place, that a pigment
found in an animal need not necessarily be formed
by that animal. There are some instances of
pigments which are transferred from one animal
or plant to the tissues of another by the food with

II THE PIGMENTS OF ORGANISMS 27

apparently little or no change in composition. A
more remarkable case still is found in the bones of
certain fishes (Belone, Protopterus, Lepidosiren), of an
amphibian, and of a lizard. In these the bones are
coloured a bright green and have been found to
contain vivianite, a mineral consisting of a phosphate
of iron. This mineral has also been found in con-
nection with fossil bones, but is there probably the
result of a chemical change during the process of
fossilisation.

In the vast majority of cases, however, the pigments
of plants and animals are definite organic compounds,
produced by the organism in which they occur.
Such pigments are of very varied characters and
composition, some being substances of great com-
plexity, while others are relatively simple. Just as
in the laboratory of the chemist, a coloured substance
capable of being employed as a pigment may be
produced at many stages in a series of chemical
transformations, so in the laboratory of nature
pigments may occur at many stages in metabolism.
This at once suggests an interesting question—is
there any relation between the colour and the
chemical composition of pigments? That is, are the
pigments of simple chemical composition usually
certain particular colours, and those of complex
structure other colours? Such statements have
under various forms been made repeatedly, and later
we must consider them in detail, but there is as yet
no evidence to show that the colour of pigments
can be employed as a direct means of classification.

The classification of the pigments of plants and
animals is indeed a matter of profound difficulty.

28 COLOUR IN NATURE CHAY?

The chemistry of most is so imperfectly known that
the composition cannot be employed as a basis, while
the suggestions as to physiological function, with
which the literature of the subject is full, are for the
most part merely guesses. But while we admit that
it is at present impossible to draw up a logical
classification, it may be well to mention certain
categories into which some of the better known
pigments are said to fall. These are as follows :—

1. Pigments of direct physiological importance, as
in respiration, etc.
Native }2. Derivatives of such pigments.
Pigments. Waste products or modifications of such.
Reserve products or pigments associated with
reserves.
5. Introduced pigments.

ale

The pigments falling under these heads we shall
consider in some detail.

1. PIGMENTS. OF DIRECT PHYSIOLOGICAL
IMPORTANCE

The pigments in this group have always attracted
much attention, and have been attentively studied in
many cases. As types we may take hemoglobin
and chlorophyll, the one so characteristic of higher
animals, the other of plants.

Hemoglobin.—As is well known, hemoglobin, the
red colouring-matter of the blood of Vertebrates, is
a compound of a pigment containing iron (hematin)
and a proteid, which is one of the few as yet
obtained in crystalline condition. Its great im-
portance is due to the fact that, owing probably to

Wi THE PIGMENTS OF ORGANISMS 29

the contained iron, it possesses the power of forming
a loose combination with oxygen, and so of acting as
arespiratory pigment. Of its origin little is known,
but there seems to be some reason to believe that in
development it arises from the chromatin of the
nucleus. In a recent paper on iron-compounds in
animal and vegetable cells by Dr. Macallum, the
author puts forward the theory that the chromatin,
that part of the nucleus which readily takes up stains,
is an iron-holding nucleo-albumen. This chromatin
has the power of fixing free oxygen, and the forma-
tion of hemoglobin results in the conversion of the
nuclein into the pigment hzmatin ; the substance re-
taining its primitive power of forming a combination
with oxygen. Further, Dr. Macallum regards the
diminished amount of hemoglobin in the blood in
anzemia not as the prime cause of the disease, but as
one of the results of the deficiency of iron-containing
compounds in the nuclear chromatin ; this deficiency
being the prime cause of the various symptoms of the
disease. The distinction may seem unimportant,
but it is not so in reality ; tor if Macallum’s view be
correct, it carries us one point further. Anzemia is
recognised in our own species by the pallor of the
lips and skin produced by the diminution in the
number of red blood corpuscles; it is therefore
natural to conclude that the disease is caused by
the insufficient amount of hemoglobin present. If,
however, it is proved that this absence of hemoglobin
is itself merely a consequence of something deeper
which affects the whole organism, we are at least a
little nearer the problem of the primary meaning of
hzemoglobin.

30 COLOUR IN NATURE CHAP.

So far we have spoken of hemoglobin as the
supremely important pigment of Vertebrates, as a
pigment perpetually justified by its usefulness, but
we must notice.that it has a wide distribution in
the animal kingdom and occurs under circumstances
where usefulness is difficult to prove, although this
has not prevented the assertion being widely made.

In the striped muscles of Vertebrates, for example,
it is widely but irregularly distributed, often in
a single species being invariably absent in some
muscles, and invariably present in others. Of a
well-marked distinction between red and white flesh
the rabbit is a familiar example; the common fowl
is another as well known to the physiologist as to the
epicure. Other notable cases are those of the fish
Hippocampus, where only the muscles of the dorsal
fin are red, and of the rare fish Luvarus, where the
difference between red and pale muscles is very well
marked ; but it would be easy to multiply examples
almost indefinitely. Among the unstriped muscles
of Vertebrates, hemoglobin is said to be found only
in the wall of the rectum.

Among Invertebrates, hemoglobin shows the
same peculiarities of distribution as in the muscles
of the Vertebrates. Thus it is present in the peri-
visceral fluid of some Turbellarians, of Glycera, and
of Phoronis ; in the hemolymph of Lumbricus, Tubz-
fex, and other Annelids; in the muscles of the
pharynx in Buccinum undatum, Littorina,and other
Gasteropods ; in the sheath of the nerve-cord in
Aphrodite aculeata; in the cephalic slits of Nemer-
teans ; and so on.

It is well known that in the case of the red

Il THE PIGMENTS OF ORGANISMS 31

and the pale muscles of Vertebrates the difference
of colour is usually associated with differences in
chemical composition and histological character, and
in certain cases at least with physiological differences
in relation to the reaction to stimuli. It is there-
fore sometimes supposed that the contained hemo-
globin is of respiratory importance, and is the cause
of the physiological characters. On the other hand,
in certain cases as in the rabbit, the association
between the colour and the other characters is not
very close, and it is to be noted that insects also
possess two kinds of muscle distinguished znter alia
by their colour; in their case the extensive develop-
ment of the tracheal system forbids the idea that the
difference is caused by the presence or absence of a
special respiratory pigment.

In Invertebrates the difficulties connected with the
supposition that hemoglobin is always of supreme
importance as a respiratory pigment are even greater.

It is said that haemoglobin is especially necessary
to Lumbricus on account of its peculiar habitat ; that
its presence in the head-slits of Nemerteans is
essential for the oxidation of the brain; that it is
present in the muscles of the buccal mass of Littorina
because these muscles are especially active; and so
on. On the other hand, many large marine worms
have no hemoglobin, whatever their habitat, and
many Gasteropods have none in their buccal muscles.
Can we suppose that these muscles are less active in
the limpet, the snail, and many others than in
Littorina ?

The more these questions are considered, the
more difficult does it become to suppose that hamo-

32 COLOUR IN NATURE CHAP.

globin in these cases is functional in a degree at
all comparable to that seen in Vertebrates. If it
were, we should expect that when once acquired.
by the members of a group it would be retained by
all their descendants, however widely they might
diverge in other respects ; and the irregular distribu-
tion of hemoglobin among the Invertebrate groups is
contrary to this supposition.

If, however, we accept Macallum’s theory that
hemoglobin is a modification of nuclear chromatin,
then it may quite well be that it has a primary
meaning in metabolism which accounts for its
presence in Invertebrates and in the muscles of
Vertebrates, while under certain conditions, as in the
case of the hemoglobin of the blood of almost all
Vertebrates, it may acquire supreme importance as
a respiratory pigment. Such a suggestion would
explain many peculiarities of distribution which are
at present exceedingly puzzling.

Hemoglobin is of course a chemical compound
of great complexity. Just as it is itself probably
formed by the modification of a still more complex
substance, so it in its turn undergoes a process of
breaking down, during which it gives origin to a
number of simpler substances. Some of these form
pigments which will have to be considered under
Group 2.

Chlorophyll—Chlorophyll is, like hemoglobin, a
compound of considerable complexity, but owing to
its great instability its exact composition is unknown.
We have, further, no exact knowledge as to the part
it plays in the metabolism of the plant. It is of
course a familiar fact of experience that green plants

re THE PIGMENTS OF ORGANISMS 33

when exposed to sunshine can decompose carbonic
acid and set free oxygen, and that it is by virtue of
their chlorophyll that they can do this, but beyond
this we have nothing but a host of rival theories.
The most interesting. of these is perhaps that of
Schunck, who considers that chlorophyll carries
carbonic acid from the air to the assimilating proto-
plasm, just as hemoglobin carries oxygen. Macallum
holds that chlorosis in plants and animals is due
to the same cause, the absence or deficiency of
iron in the nuclear chromatin ; and this in spite of
the fact that it is improbable that chlorophyll con-
tains iron. Thus the absence of chlorophyll in a
plant grown without iron is only one of the con-
sequences of the general unhealthiness. In reference
to this view of Macallum’s, it is interesting to note
that according to Nencki there is a close relation
between hzmatoporphyrin, a derivative of hzmo-
globin, and one of the derivatives of chlorophyll
green.

As is well known, chlorophyll is, at any rate in
solutions, an extremely unstable pigment, fading in
light with great rapidity. In natural conditions
it is associated with one or more pigments belonging
to a widely spread group of pigments—the lipo-
chromes, characterised by their colour, which varies
from red to yellow, as well as by other properties.
It is uncertain whether these subsidiary pigments
exercise any influence in the process of assimilation,
or whether it is not the true chlorophyll or chloro-
phyll green which is alone active. Lipochromes are
not the only pigments which occur in association
with chlorophyll, but the subject will be treated

D

34 COLOUR IN NATURE CHAP.

again in further detail when we come to consider
the colours of plants.

So far as is yet certainly known, chlorophyll is
the only pigment by means of which organisms can
avail themselves of the carbon of the atmosphere,
though it is quite possible that some other pigments
may yet be shown to possess this property. The
suggestion has been made in the case of the bright
pigments of coral polypes, as well as of various
other pigments, notably of the purple pigment of
Beggiatoa roscopersicina (see Chap. II1.).

Pigments fulfilling the function of hemoglobin
are perhaps fairly numerous, but there is some doubt
connected with many of these so-called respiratory
pigments. Of those certainly respiratory, hemo-
cyanin is by far the most important.

The question as to whether any other pigments
besides those of the nature of chlorophyll and the
respiratory pigments are of direct physiological
importance, is a somewhat difficult one. According
to many, some pigments, such as the colouring
matter of the Red Alge and the dark pigments of
many animals, are of great importance in protecting
delicate tissues from the injurious effects of certain
of the rays of white light.

Again, the pigment described by Dr. MacMunn as
enterochlorophyll, which is so common in connection
with the alimentary tract in many Invertebrates, is
supposed by some to be of importance in digestion
or assimilation. There are other suggestions of the
same kind, but there is as yet little certainty, the
functions of the large majority of pigments being
quite unknown.

II THE PIGMENTS OF ORGANISMS 35

2. DERIVED PIGMENTS

The second group of pigments includes those which
are formed by the decomposition or breaking down
of the pigments of the first group. These are prob-
ably always useless so far as any direct function is
concerned, and it might be thought that they should
be simply classed as waste products. It seems, how-
ever, better at present to reserve the term waste
products, at least in animals, for those results of
nitrogenous metabolism to which the term is usually
applied by physiologists. We have already mentioned
that hemoglobin undergoes a series of retrogressive
changes, resulting in the production of various
pigments. Most of these, in Vertebrates at any rate,
are of little importance in producing coloration, are
speedily eliminated from the body, and in our own
species are chiefly of interest to the practical physician.
There are, however, some exceptions. Melanin, the
dark pigment which colours the hair or skin of most
mammals, and which, as we have seen, is indirectly
of importance in producing many of the gorgeous
tints of birds, is thought by some authorities to be
a derivative of hemoglobin. Again, the frequently
exquisite colours of birds’ eggs are almost certainly
due to pigments derived from the blood. It is quite
possible that this group of pigments may ultimately
turn out to be a large one, but as yet hemoglobin
is one of the few pigments whose metamorphoses
have been fully worked out.

36 COLOUR IN NATURE CHAP.

3. WASTE PRODUCTS

The next group, the pigments which are definitely
waste products, or are produced by the modification
of waste products, has only been seriously studied
very recently. The researches of Mr. F. Gowland
Hopkins in this country, and of Dr. Urech abroad,
as well as of others, have demonstrated within the
last few years the extremely interesting and im-
portant fact that the colours of the butterflies
belonging to the family of the Pieridae are due to
pigments which are modifications of the ordinary
waste products of the organism. Hopkins’s discovery
that the yellow pigment which he calls lepidotic acid,
found in the wings of the Pieridz, occurs also as one
of the normal waste products of the organism, is one
of extreme interest to the comparative physiologist.
It is well known that in higher Vertebrates death
follows with extreme rapidity upon the removal of
the kidneys, and it is usually stated by physiologists
that it is the nitrogenous waste products themselves,
or their precursors, which poison the animal; in the
case of butterflies, however, we have a modification
of uric acid stored up in the wings, functioning as a
colour-producing substance, while the same substance
is eliminated from the body as a waste product at
the time of the emergence from the pupa state. We
are thus forced to suppose that the wings of butter-
flies, being relatively non-vital parts, can have
poisonous substances stored up in them without
injury to the organism, and that therefore the
utilisation of waste products as colouring agents can

II THE PIGMENTS OF ORGANISMS 37

only occur in cases where the coloured structures are
not intimately connected with the blood system.
Another substance of this group which among
fishes is widely utilised as a colouring agent is
guanin, This substance, used in the manufacture
of artificial or Roman pearls, is colourless or chalk-
like, but it occurs in the skins of fishes as small
crystals which frequently display a beautiful iri-
descence and a pearly lustre. It is to these crystals,
mingled with pigments, that the soles, the cod family,
and numerous others owe their frequently beautiful
colour. The crystals occur in the scales, and also
in the deep layers of the skin, in the peritoneum
and in the air-bladder, as well as elsewhere. It is
interesting to note that in Elasmobranchs, which are
ancient fishes, although guanin is present in the skin
it has no metallic lustre, such as it exhibits in many
of the modern bony fishes. Guanin is not of course
a pigment in the strict sense of the word, but it is
of much importance in producing coloration in fishes,
and its composition makes its occurrence in the skin
of great interest. It may seem that undue stress
is laid upon these’ peculiar colouring matters, but
they are of much interest, because while of most
pigments the chemical relationships are unknown,
the substances of the uric acid and related groups
have been tolerably well worked out. This is
partly on account of their importance in practical
medicine, for we all know that the production of
an excess of uric acid or its imperfect elimination
from the body is in man associated with painful
diseases. Now in systematic zoology we dis-
tinguish higher from lower animals by the fact that the

38 COLOUR IN NATURE CHAP.

former, in the current phrase, display more differ-
entiation and more integration. Differentiation
displays itself most obviously in an increasing
complexity of parts, but integration is shown in
the increasingly perfect physiological relation of
parts. In the evolution of organisms an increasingly
rapid elimination of waste must have been a factor
which made for progress ; no machine can work well
if it is choked with its own waste. Waste products
can therefore be employed as colouring agents only
under unusual conditions or in organisms at a rela-
tively low level in evolution. Thus we see that the
comparative study of pigments must yield important
contributions to comparative physiology in general.

In this connection we may notice the interest-
ing fact that, in discussing the evolution of the fcetal
membranes in Vertebrates, Dr. Richard Semon con-
nects the appearance of the allantois (the “hyper-
trophied urinary bladder”) with the acquisition of the
metanephros, a more efficient excretory organ than
the simpler mesonephros. It is certain that the
blood in the lower Vertebrates contains a far larger
amount of nitrogenous waste than that of the higher.
We shall see later that as we ascend in the scale in
Vertebrates we have a corresponding diminution in
the amount of nitrogenous waste products deposited
in the skin. Now our acquaintance with the
physiology of the Invertebrates is exceedingly
limited, and it may well be that a knowledge of
their pigments may be of much help in deciding
general questions as to the phylogenetic position
and the relative importance of structures connected
with the excretion of waste.

1 THE PIGMENTS OF ORGANISMS 39

4. RESERVE PRODUCTS

The next group of pigments includes those which
are actually reserve products or are associated with
such. Although the literature of the subject contains
numerous mention of pigments described as reserve
products, it is still doubtful whether the description
is accurate in any one of the cases, Carminic acid,
the pigment of the coccus insect, is very frequently
described as a reserve, but this is doubtful; it is a
glucoside, and therefore a carbohydrate is produced
by boiling it with dilute acid, but nothing is certainly
known as to its function.

The term reserve product is applied with more
plausibility to various members of the large series of
lipochromes or fat-pigments. The lipochromes are
pigments, varying from yellow through orange to red
in colour, which in the dry state give a blue colour
with concentrated sulphuric or nitric acid, and which
are soluble to a greater or less degree in all the
solvents of fats, as well as in fats themselves. They
are divisible into two series, according as they do or
do not form compounds with the caustic alkalies.
Both series occur in animals, but the second only in
plants. In plants the best known lipochrome is
carotin, which is widely distributed, and is, according
to Carl Ehring, a cholesterin fat. The chemical
nature of the other series is still unknown.

The lipochromes occur frequently, though not
invariably, in association with fat in organisms.
From the red flesh of the salmon, for example, it
is possible to extract an oil containing the pink

40 COLOUR IN NATURE CHAP.

colouring-matter in solution. Now during the breed-
ing season, when the salmon is fasting, the fat which
is so abundant in the muscles is transferred from
these to the ovaries, and with the fat the pigment
is also carried, so that the muscles become pale in
colour. A very similar process has been described
by Zopf in the case of a fungus (P2lobolus). Here
in the endospores, the gemmz, and the zygospores,
drops of oil occur intensely coloured with a lipochrome
pigment. When the gemmz or spores germinate,
the oil-drops disappear, and with them the pigment
also disappears. Zopf in consequence describes this
pigment as a reserve product. It seems, however,
safer as yet merely to admit that lipochrome pigments
frequently occur in association with reserves, leaving
the question as to whether the pigments themselves
are capable of being employed in metabolism to be
determined in the future. It is extremely unlikely
that the lipochromes are always or even usually of
the nature of reserves, as they occur in a variety of
structures where such a significance seems impossible.
They are extremely common pigments both in plants
and animals, and we shall have to recur to them very
frequently in the course of the following pages.

5. INTRODUCED PIGMENTS

The last group of pigments that we shall consider
here includes those which are not produced by the
organism in which they occur, but are obtained from
other organisms used as food and are transferred
apparently almost unaltered to the tissues. The
colouring-matter of green oysters, which was formerly

II THE PIGMENTS OF ORGANISMS 41

said to be obtained from the diatoms of their food,
used to be given as an example; another is the case
worked out recently by Mr. Poulton (1893), showing
that the green pigment of some caterpillars is derived
from the green leaves upon which they live. Another
example, which in some degree resembles Mr.
Poulton’s case, is one mentioned by Zopf (1892). In
studying the colouring-matter of the fungus P2lobolus
already mentioned, he found that a parasite growing
on the fungus took up not only the drops of oil but
also the pigment associated with the oil, the result
being that parasite and host were similarly coloured.
This is interesting, because it probably has some
bearing on certain of the cases of colour-resemblance
which are now so numerous in the literature of colour.

The above five sets of pigments have been con-
sidered in some detail, because it is quite possible
that ultimately the greater number of the pigments
at present known may be found to fall into one or
other of these categories. In the meantime, however,
there is no proof of this, and there are very many
pigments which it is quite impossible to classify. On
this account, we shall. simply consider them system-
atically under the organisms in which they occur.
Before doing this it may be well, in the absence
of a complete classification of pigments, to consider
for a little the distribution of pzgmental colours in
organisms.

THE DISTRIBUTION OF PIGMENTAL COLOURS

Blueas a pigmental colourisexceedingly rare among
the more complex animals, but is not uncommonamong

42 COLOUR IN NATURE CHAP,

plants and the simpler forms of animal life. Thus
the blue colours of many butterflies, of the feathers
and skin of birds, of the mandrill among mammals,
are all due to structural coloration, while the blue
of hyacinths, of some jelly-fish, and of the lobster
are pigmental colours. In flowering plants, as will be
seen in the chapter on plant pigments, a blue colour
is in some cases associated with an alkaline condition
of the cell-sap, but beyond this there is still much
uncertainty. In animals a blue colour occurs especi-
ally in surface forms, whatever their relations, eg.
jelly-fish, molluscs as in Janthina, and Tunicates. It
is also remarkable as one of the conspicuous colours
of coral-reefs where it occurs in corals, sea-anemones,
Turbellarian worms and starfishes; the blue colours
of the cuttles and fishes of the same situation are no
doubt optical. In general, therefore, in animals blue
pigmental colours occur in organisms of simple struc-
ture exposed to strong light. A comparative in-
vestigation of the blue pigments of surface animals
would be of great interest, but has yet to be made.
The two main hypotheses as to origin are on the one
side that of the protective value of the colour, and
on the other of the direct action of light as the
important factor; it is not improbable that it may
turn out that it is abundance of free oxygen which
is the really important point. In general the blue
pigments of animals are characterised by their usual
solubility in water, their instability, and the fact that
they frequently give either the litmus reaction or some
modification of it on acidification. To what extent
this indicates affinity among them is at present quite
unknown ; apparently most are readily reduced.

II THE PIGMENTS OF ORGANISMS 43

Looking at the distribution of blue pigments
more in detail, we find that while blue and purple
colours are common in the Flowering Plants, there
is much uniformity in the pigments, which seem all
to belong to the axthocyan series. In the lower
plants blue pigments are more numerous. Thus
various blue and purple pigments have been described
among Bacteria—the blue or purple colour which
develops on decaying meat is a good example.
These pigments have been compared by some to
natural aniline dyes. With these, and as yet of little
interest except to the chemist, we may put the
bluish-green colours of certain Fungi.

A more interesting pigment is phycocyan, which
occurs in the bluish-green Algz (Cyanophycez). It
is a beautiful pigment, blue in transmitted and blood-
red in reflected light, and is said by Molisch to be of
albuminoid nature. It is soluble in water, and has
been obtained in crystalline condition.

In animals blue pigments seem to be more
numerous than in plants. They are found in various
Protozoa, as in Massula and Stentor; in the former
it seems possible that the pigment is taken up from
the blue-green Algze of the food. In the Ccelentera
blue pigments are exceedingly common, especially
in shallow-water forms in warm seas, and in the
pelagic jelly-fish. In worms and in Echinoderms
we find that, although blue colours occur, the pig-
ments producing them do not appear to have been
isolated. Similarly in insects, where blue is very
common, blue pigments have not yet been described,
although it is improbable that the blue in all cases
is structural.

44 COLOUR IN NATURE CHAP.

In Crustacea there is probably only one series of
blue colouring-matters, the so-called soluble blues,
which will be more fully described in the chapter on
the group.

Among Mollusca we have notably the blue pig-
ment of Janthina and the purple one of Aplysza, the
undescribed colouring matters of the shells of mussels,
of the mantle of 77dacna, and of others.

Among Vertebrates blue pigments do not appear
to have been as yet described, except in the case of
the blue pigments of the eggs of many birds. These
pigments are apparently derivatives of haemoglobin.

A green colour is of course almost universally
distributed among plants, where it is due to the
important pigment chlorophyll. Among animals
green like blue is rare as a pigmental colour, except
in simple organisms. The questions connected with
the green colours of animals have been greatly com-
plicated by the common habit of hastily assuming
that any pure green colour in animals is likely to be
due to chlorophyll. Chlorophyll occurs apparently
in many Protozoa, in Hydra viridis, in the fresh-water
sponge Spongilla, and in some Turbellarian worms.
Its occurrence in numerous other animals, especially
worms and insects, has been often asserted, probably
in some cases because the gut contained unaltered
chlorophyll which gave the characteristic spectrum.
“Modified chlorophyll” is said by Mr. Poulton to
colour the tissues of many caterpillars. A group of
pigments which resemble chlorophyll, and which
have been often mistaken for it, are apparently
widely spread among Invertebrates. They usually
occur in connection with internal organs, and are

II THE PIGMENTS OF ORGANISMS 45

therefore only in rare cases of direct importance in
coloration. Of this group, ‘“ enterochlorophy]ll,”
bonellin, and chetopterin are good examples ;. the
two latter give rise to external coloration in the
forms in which they occur. We shall make frequent
reference to these pigments in the course of the
following chapters.

The occurrence of green colour in organisms from
which yellow lipochromes only can be extracted. is
an exceedingly common phenomenon, which has been
observed in many different animals. The green
colour is doubtless due in many cases to the structure
of the coloured parts ; in others it may be due to a
combination of the lipochrome with a base ; there is
little doubt that the green colour of many Crustacea
is in part due to this cause. Apart from chlorophyll
and lipochrome combinations, green pigments occur
freely in many Ccelentera, in many worms, and in
some Mollusca; in these cases they are usually soluble
in alcohol and often give banded spectra, but most
have been very imperfectly investigated. In Verte-
brates green pigments are almost absent; as in the
case of blue pigments, the most marked exception is
probably the green pigment which colours birds’ eggs,
though according to Sorby the green colour is here
produced by a mixture of blue and yellow. As a
colour, green in animals is most abundant among
those inhabiting trees or herbage, but is here very
frequently structural. In marine animals it seems to
occur in large masses chiefly in coral-reefs, where the
corals are often largely green. In these situations
green plants are relatively rare, and accordingly the
green colouring-matter has been supposed by Dr.

46 COLOUR IN NATURE CHAP.

Hickson to perform the same functions as chlorophyll.
It is apparently not chlorophyll (see Chap. IV.).

Yellow is an exceedingly common and widely
distributed colour, occurring most frequently, however,
in animals in which other pigments are also present.
It not infrequently occurs in the form of spots or
stripes upon a dark ground, although it is common
in flowers as a ground colour. In most cases yellow
is due to the presence of yellow lipochromes, which
are perhaps the most universally distributed of all
pigments. In view of their exceedingly wide dis-
tribution, it is perhaps remarkable that yellow animals
should not be more common than they are. Apart
from the yellow lipochromes we have as important
yellow pigments lepidotic acid, the waste product
which occurs in the wings of.some butterflies, and
the yellow pigments of the eggs of birds. The
yellow or tawny colour of the hair in many mammals
is not due to a special yellow pigment, but to the
uniform distribution .of a small amount of dark
pigment.

A ved colour is perhaps always due to pigment,
and red pigments are fairly numerous. Large
numbers of red animals are coloured with lipochrome
pigment ; the red lipochromes indeed begin at the
Protozoa and extend upwards to birds. Their
colour varies from deep orange to pure rose-red, but
all have an indescribable fatty appearance which
makes them readily recognised by any one accus-
tomed to working with lipochromes. Red is very
common among marine animals, especially among
Crustacea, Echinoderma, some Mollusca, Protozoa,
and Ccelentera. In Crustacea it occurs among some

II THE PIGMENTS OF ORGANISMS 47

surface forms, but attains its greatest brilliancy
among those inhabiting deep water, which often
exhibit a wonderfully pure and bright scarlet tint.
All these red tints are due to lipochromes. The red
blood-pigment hemoglobin is frequently important in
coloration in simple forms, especially worms, but the
effect is produced by the shining through of the
red blood, and not by the deposition of hemoglobin
as a superficial pigment. In butterflies red pigments
important in coloration occur which are probably
modified waste products. Among plants red pig-
ments are probably usually either lipochromes or
anthocyans; red is an exceedingly common colour
among plants.

Black and deep brown pigments are very widely
distributed in the more complex animals, and are
virtually absent from plants and simple animals.
As exceptions we may note the presence of a brown
pigment in Hydra fusca and the “brown body” of
Polyzoa. The former, according to Miss Greenwood,
is a waste substance, the latter will be discussed
under Polyzoa. That their distribution is not wholly
dependent upon structure is, however, shown by the
fact that they are rare in Crustacea and exceedingly
common in Insecta, slightly developed in most
Chztopoda and often marked in Hirudinea. There
is some evidence to show that they do not commonly
occur’ in connection with structures containing car-
bonate of lime, which may serve to explain their
rarity in Crustacea. At least it is noticeable that in
Mollusca very dark brown or black pigments do not
usually occur in the shells, while they may be common
in the mantle, internal organs, or: secretions, as in

48 COLOUR IN NATURE CHAP.

cuttles. The question whether there is any genetic
connection between the dark pigments of animals
belonging to different orders is one of great interest,
but one which it is at present impossible to answer.
Dark pigments are mostly very stable and insoluble;
where they have been analysed they have been
usually found to contain nitrogen, and in some cases
sulphur as well. They are probably of no further
use in metabolism, and seem often to tend to increase
in the course of development, and to be more abundant
in dominant than in weak species. Many have
regarded them as directly waste products increasing
with increase of metabolism, a question which we
shall discuss later.

Pale brown pigments may arise in so many
different ways that nothing of a general nature can
be profitably said of them. In plants and perhaps
in some insects tannin may play the part of a brown
pigment.

White as a pigmental colour is rare. Often
purely structural, it is sometimes due to the depo-
sition of fat in the subcutaneous tissues. In the
Pieride among butterflies white is due to uric acid
which here plays the part of a white pigment.

THE SPECTROSCOPIC CHARACTERS OF PIGMENTS

The pigments with which we have been concerned
in this chapter are recognised by their colour, their
reaction to various chemical reagents, and finally by
their spectroscopic characters. Most people are
familiar, at least by hearsay, with the important part
which has been played by the spectroscope in

11 THE PIGMENTS OF ORGANISMS 49

modern chemistry, and have heard of the wonderful
discoveries as to the composition of the sun and
some of the stars which have followed from the
spectroscopic examination of the light emitted by
these bodies. Now we have already seen that
many of the pigments of plants and animals are
extremely fugitive and unstable, many of them also
are exceedingly difficult to extract from the tissues.
By means of the spectroscope, however, the coloured
tissues may often themselves be directly examined,
and on account of the readiness with which the
operation may be performed, a large number of
pigments are known chiefly by their spectra. Now
even in the case of the metals, we find an authority
like Mr, Crookes saying that “inferences drawn from
spectrum analysis per se are liable to grave doubt,
unless at every step the spectroscopist goes hand in
hand with the chemist.” If this be true of radiant
matter spectroscopy, where the spectra are capable
of extraordinarily accurate study, how much more
likely is it to be applicable to the spectrum analysis
of pigments, where the methods are as yet clumsy
and inadequate! Thus we find as a matter of
practical experience, that it has been found impos-
sible in the aniline industry to employ the spectra
as a test for affinity. Again, in spite of the
common assertion that pigments yielding identical
spectra are themselves identical, we find that three
distinct pigments—hemoglobin, carmine, and turacin
—are described by competent authorities as giving
spectra which are virtually identical. Turacin, a
pigment discovered by Professor Church, is of
purplish-violet colour, and occurs in the feathers of
E

50 COLOUR IN NATURE CHAP. II

certain birds. It is incapable of existing in states
of oxidation and reduction, and contains copper and
not iron, so that it is unlikely that it is closely
related to haemoglobin. Carmine again is not
nearly related either to turacin or haemoglobin.
The spectrum test in this case seems to break down
utterly. In spite of numerous difficulties of this
kind, the literature contains numerous identifications
of pigments based only upon the use of the spectro-
scope ; thus a recent observer describes chlorophyll
in the skin of the horse! In the following pages
identifications based only upon spectra have been
almost entirely omitted, for the evidence is con-
sidered insufficient.

CHAPTER III

THE COLOURS AND PIGMENTS OF PLANTS

Pigments and Colours of Bacteria and Fungi—Chlorophyll
and the Associated Pigments—Colour and Pigments of
Algze—Pigments of Flowering Plants—Autumnal Colora-
tion—Colours of Flowers and Fruits—Meaning of Plant
Pigments and Summary.

IN considering the colours and pigments of plants
we may conveniently begin with the colours of
Bacteria and of Fungi, and then pass to the con-
sideration of the colouring of chlorophyll-containing
plants.

The colours of Bacteria are often surprisingly
brilliant. The red spots which Micrococcus prodigiosus
forms on moist bread, the violet colour which
sometimes appears on decaying meat, are familiar
cases in point, but less familiar forms are often
equally bright. There are some facts of interest
in regard to the position in which the pigments
occur, and the conditions necessary for their formation.

As to the first point, the pigment may occur
within the cells of the colony. In 1873 Professor
E. Ray Lankester described an interesting peach-

52 COLOUR IN NATURE CHAP.

coloured Bacterium found in stagnant river- water,
in which the peculiar pigment was confined to the
protoplasm of the individuals and did not extend
into the jelly surrounding the colonies. Mr. Slater
has similarly described a form producing a bright
red pigment which is confined to the cells. On the
other hand, in the case of Micrococcus prodigziosus,
the red pigment is confined to the masses of
mucilage surrounding the colonies and does not
occur within the cells at all, while in yet other cases
the pigment may be found only in the substance in
which the colonies are living.

As to the conditions under which the pigments
are produced, it is well known that MWzcrococcus pro-
digiosus loses its power of producing pigment at high
temperatures, while light is necessary for the produc-
tion of the purple pigment of Beggiatoa roscopersicina.
This pigment is of especial interest because, according
to Engelmann (1888), its presence in the organism is
associated with the power of breaking up carbonic
acid and setting free oxygen, therefore it has a
function equivalent to that of the chlorophyll of
green plants. In the case of Bacillus pyocyaneus,
according to Gerrard, the power of pigment produc-
tion is dependent upon the nature of the medium, as
well as upon the particular race of the microbe.

As a point of interest with regard to the chemical
nature of the pigments, we may notice that in 1889
Zopf described a yellow pigment formed by Bacterium
egregium as a lipochrome, and stated that this was the
first time that lipochromes had’ been described as
products of bacterial action; he has since described
other cases. The bright red Aicrococcus pigment is

1II THE COLOURS AND PIGMENTS OF PLANTS 53

soluble in alcohol but not in water, and is said to be
not improbably related to the aniline dyes—a state-
ment which has also been made for the blue pigment
found on decaying meat. The pigment of the peach-
coloured Bacterium described by Professor Lankester
was found to be very insoluble, but gave a distinct
three-banded spectrum.

The pigments of Bacteria have been chiefly studied
as a means of identifying the organisms producing
them, and a further account of them here is un-
necessary. Their main interest lies in their frequent
brilliancy and variety. Bacteria are remarkable for
the enormous number of chemical substances which
are the result of their activity ; it would be strange if
among these there were not some capable of pro-
ducing colour, but the remarkable brightness of many
of the tints is of considerable theoretical importance.
The fact that the production of the pigment is a
factor both of external conditions and of the con-
stitution of the organism suggests that bacteriologists
may one day have something to say on the general
problem of the conditions necessary for pigment
formation.

With regard to the higher Fungi, it is a familiar
fact that they frequently display great brilliancy of
colour. This is especially true of their fructifications
which on barren ground are often a not inconsider-
able factor in the production of the bright tints of
autumn. Although these “toadstools” are regarded
with disfavour by many people on account of their
uselessness for culinary purposes, their beauty,
especially when seen in natural conditions among
yellow bracken and glowing brambles, can hardly be

54 COLOUR IN NATURE CHAP.

denied. It is not, however, the conspicuous forms
only which are brightly coloured. A species of Pezzza
produces the brilliant green colouring matter which
so often stains decaying wood, and the yellow spots
which £cédium causes on the leaves of the barberry
are common enough objects in spring. Zopf has
devoted considerable attention to the pigments of
Fungi, and has examined many of them. He has
shown that many Fungi contain one or several
different lipochromes, and, as has been already
mentioned, these lipochromes may be taken up un-
altered by parasites. We have also already noticed
Zopf’s view that the lipochromes function as reserves.
Besides the lipochromes, a very large number of pig-
ments in Fungi have been described and named.
The chemical characters of some are known, of others
unknown, but in few cases, if any, has any definite
function been assigned to them. Thus we find in
the Fungi a great number of pigments of very diverse
colours and chemical composition and of unknown
function. Notwithstanding this last fact, the colora-
tion exhibits some at least of the characters displayed
by organisms whose colour is thought to be of supreme
importance to them. In the large toadstools the
bright colour is usually confined to the upper and
conspicuous surface of the pzleus, and this surface
is not infrequently marked or spotted with great
regularity. These facts are of considerable import-
ance with regard to theories as to the (proximate)
origin and meaning of colour. They show us that
not only pigments, but also regularity of coloration
may occur in organisms in cases where the relations
to other organisms are too simple for us to suppose

MI THE COLOURS AND PIGMENTS OF PLANTS 55

that either pigment or coloration can be of direct
use. When we come to consider the phenomena of
colour in organisms whose relations to other organisms
are extremely complex, we shall find that there is
a constant tendency to look for the cause or the
justification of the colour phenomena in these complex
relations. It is therefore important to realise that it
is illogical to seek for a complete explanation of
colour phenomena in complex organisms while those
of relatively simple ones remain unexplained.

CHLOROPHYLL AND THE ASSOCIATED PIGMENTS

This brief account of the colours of the plants not
containing chlorophyll must suffice, for it is impossible
here to go into further details. We shall treat the
colours and colouring-matters of the chlorophyll-
containing plants in greater detail, for in them the
colours are as a whole more striking and beautiful,
and the pigments are much more fully known.

In these plants chlorophyll is of course by far the
most important pigment, and sometimes indeed the
only one. For details as to its chemical characters
reference must be made to the text-books, but we
may here recall one or two points as to its occurrence.
Chlorophyll occurs, generally speaking, in all plants
except the Fungi; it is especially abundant in the
leaves or vegetative organs, and occurs in associa-
tion with definite parts of the cell protoplasm—the
chlorophyll corpuscles. It is frequently associated
with other pigments which may mask it, or may
replace it in special regions of the plant. This
partial replacement is especially noticeable in the

&

56 COLOUR IN NATURE CHAP.

Flowering Plants, and it is only here that it is
seriously regarded as the result of a process of
selection. The unequal distribution of chlorophyll
is, however, well marked in some of the lower plants.

As to the associated pigments, we find that in
Flowering Plants the chlorophyll of green leaves is
always mingled with a greater or less amount of a
yellow lipochrome usually known as xanthophyll.
The different tints of leaves are caused in great part
by the varying amounts of the two pigments present,
a large amount of xanthophyll giving the leaf a
yellowish tint. The exact relation of this yellow
pigment to the colouring-matter of leaves blanched
by growing in darkness is uncertain, but the two are
probably at least nearly related. We know from
investigations on the lipochromes of animals that
these pigments are not only peculiarly unstable, but
that a number of them frequently occur simultane-
ously in an organism, and are then very difficult to
distinguish from one another. This fact enables us
to understand readily how it is that the questions
whether there is more than one lipochrome associated
with chlorophyll, and whether etiolin (the colouring-
matter of blanched leaves) is or is not identical with
xanthophyll, remain undecided. In considering the
colours of flowers and fruits we shall find that
xanthophyll, or pigments closely related to it, may
play an important part in the economy of the plant,
but the function of the xanthophyll connected with
chlorophyll is unknown, if indeed it possess any.
The only suggestion that has been made in this
matter is that the xanthophyll may have some
function in protecting the protoplasm of the cells

III THE COLOURS AND PIGMENTS OF PLANTS 57

from the injurious effect of certain of the elements of
white light by absorbing them, or that it in some
way assists the assimilating action of chlorophyll.
As yet, however, there is little evidence for either
of these suggestions, and the majority of authors are
quite silent as to the function of xanthophyll.

COLOUR IN ALG

A similar association of chlorophyll with other
pigments, especially lipochromes, is often well seen
in the Algz, where the pigments seem more varied
than in flowering plants. In many cases they com-
pletely mask the chlorophyll, while in others the
unequal distribution of the chlorophyll produces
colour effects which show a striking resemblance to
those seen in the flowering plants.

The masking is especially well seen in the case
of the colouring-matter of the Floridez or Red Sea-
weeds, which has further had an important function
ascribed to it. These Alge for the most part live in
deep water, and are chiefly known to those who are
not botanists by the beautiful reddish-pink fronds of
Delesseria so often cast up on the seashore during
the summer months. Like De/esseria the Floridee
are nearly all red or violet when living, but if placed
in cold, fresh water the red pigment dissolves out
and leaves the seaweed green. The solution in
water of the red pigment, known as phycoerythrin,
fluoresces strongly from red to yellowish-green. The
chlorophyll with which it is associated in the Floridex
is said to differ in some respects from the chlorophyll
of other plants. Now, as we have said, the Red

58 COLOUR IN NATURE CHAP.

Algz are typically deep-water forms, and it is well
known that water absorbs the various rays of light
unequally, so that the yellow and red rays do not
penetrate to great depths, but are stopped long before
the blue and violet. Engelmann (1883) experimented
upon the effect of the different rays of the spectrum
upon the process of assimilation in various plants,
and found that while green plants, generally speaking,
assimilate best in red light, the Red Algz assimilate
best in blue light, and can therefore live at depths
impossible to other plants. This does not of course
assist us as to the meaning of the phycoerythrin,
but Kerner has supplemented Engelmann’s observa-
tions by the suggestion that the red pigment absorbs
the blue and violet rays and converts them into red
-rays, so that according to him the presence of the
red pigment is absolutely necessary to these Alge.
The suggestion is perhaps chiefly of interest because
it is one of a great number which have been made
lately as to the effect of pigments on the incident
rays of light, and which assign a physiological im-
portance to pigments hitherto regarded as useless.
Nothing is known as to the chemical affinities of
phycoerythrin. It is possibly of proteid nature,
and like numerous other pigments has been, on ex-
tremely doubtful evidence, accredited with a respira-
tory function.

We do not propose to go into further detail on
the subject of pigments associated with chlorophyll,
but may mention one or two cases in the Alge of
that replacement of chlorophyll by other pigments
in special regions which is so common in the higher
plants. In many of the unicellular Alge, as in many

III THE COLOURS AND PIGMENTS OF PLANTS 59

of the Protozoa, there is a constant oscillation between
red and green as a ground colour. It seems most
probable that this, as in higher plants, is due to
a destruction of the chlorophyll green and a conse-
quent predominance of an associated lipochrome. In
Algze it occurs nowhere in such an instructive way
as in the brittle-worts Chara and Nitella. Here
virtually the whole plant is coloured green by chloro-
phyll, but the reproductive organs, and especially the
antheridia, as they ripen become a bright red, the
colour being due to the red chromatophores which
replace the chlorophyll corpuscles. The whole
process is exactly analogous to that which occurs
during the ripening of red fruits like the rose-hip.
Quite similar is the process which occurs during
the maturation of the antheridia of brown seaweeds
like Fucus. Here the plant owes its colour to a
combination of chlorophyll green, a brown pigment,
and a lipochrome. In the oospheres all these three
are retained, but in the antherozooids the brown
and the green disappear, and the orange-coloured
lipochrome remains in the chromatophores and gives
rise to the orange coloration. The retention of the
chlorophyll in the female elements and its disappear-
ance from the male many would regard as an
illustration of the greater vegetativeness of the
female. These two examples may suffice to show
that the processes which give rise to the colours of
Angiosperms have very well-marked analogues among
Cryptogams.

60 COLOUR IN NATURE CHAP.

PIGMENTS OF FLOWERING PLANTS

Among the flowering plants chlorophyll is of
course the supremely important pigment. With it
as already seen are associated lipochrome pigments
whose nature and amount determine the exact shade
of green displayed by the vegetative organs. In
flowering plants not only are the organs connected
with reproduction often brightly coloured, but the
vegetative organs themselves may also display
brilliant pigments. Among the pigments two series
are of special importance—these are first the lipo-
chromes, and second the anthocyans. Of the lipo-
chromes it is not necessary to say anything further
at present, but the anthocyans merit more detailed
consideration.

Anthocyan, or the series of pigments included
under this name, occurs dissolved in the cell-sap,
and varies in colour from blue to red. It is an
exceedingly common pigment in the higher plants,
occurring alike in vegetative and reproductive organs,
and is readily soluble in water. By steeping the
rind of an apple or slices of beetroot in water, a red
solution of the pigment is readily obtained. If an
alkali such as caustic soda or ammonia be cautiously
added, the colour changes from red to blue, green,
and yellow successively. Finally, on adding excess
of alkali the solution becomes colourless. If the
alkali added be ammonia, this may be removed by
boiling, when the blue colour will once more reappear.
This change from red to blue is of course the litmus
reaction, so familiar to all who have worked in a

I THE COLOURS AND PIGMENTS OF PLANTS 61

chemical laboratory. It is exceedingly characteristic
of certain of the anthocyans, and is of much im-
portance in the production of the colours of flowers
and fruits.

The chemistry of the anthocyans has not yet
been fully worked out, but it seems probable that
they are derivatives of tannin, probably oxidation
products. The colouring-matters of the different
varieties of grapes, as well as of the leaves of the
vines bearing them, are said to be due to oxidation
products of the tannic acids which are so abundant
in these plants, and these colouring-matters are
undoubtedly of anthocyan nature. There is also
other evidence pointing in the same direction.
Tannin is probably a substance of no further use to
the plant, and therefore it is probable that the
anthocyan pigments come into the group of waste
products. We say probable only because, while
plant physiology is so imperfectly known as at
present, it seems hardly justifiable to apply to plants
terms which in relation to animals have a perfectly
definite meaning.

As to the proximate importance of anthocyan to
the plant, there are endless suggestions and hypo-
theses. Into all these we cannot enter here, but as
a type may take a recent interesting paper by
Professor Stahl on the meaning of anthocyan in
brightly coloured foliage leaves.

Stahl studied the bright colours of the leaves of
certain tropical plants, and decided that these could
not be regarded as warning colours—a conclusion
which is probably well founded! He therefore set
himself to discover some physiological function. He

62 COLOUR IN NATURE CHAP.

found by experiment that the red parts of plants
absorbed 1.82 per cent more heat than green parts,
and he considers that this fact explains the peculiar
distribution of this red colouring-matter, its presence
in young shoots, Alpine plants, etc. He believes
that it also explains its occurrence in wind-fertilised
flowers, in many Dicotyledons, in Gymnosperms, in
Cryptogams, and so on, for the increased absorption
of heat in these cases would aid the growth of the
pollen tube, etc. It does not, however, explain the
occurrence of red pigment in the leaves of tropical
plants, where it is often associated with the simul-
taneous occurrence of white spots, which have, of
course, exactly the contrary effect as regards heat
absorption. Stahl finds that such plants inhabit
shady places, and his opinion is that the simul-
taneous existence of red and white spots aids trans-
piration by producing an unequal absorption of heat.
The white spots are produced by the presence of
large air-spaces beneath the epidermis, the result being
that the spots cool more slowly at nights and so
receive less dew, and therefore render transpiration
possible even at a low temperature. The paper con-
tains some other observations which we need hardly
note here. The reviewer of Stahl’s paper in the
Botanische Zeitung calls it a model of biological
investigation, and in its lavish employment of in-
genious hypotheses it may at least be regarded as a
type of many biological investigations,

Another investigation which seems to throw more
light upon the peculiarities of the distribution of
anthocyan is one by Molisch. In working at antho-
cyan, Molisch (1889) attempted to obtain a solu-

I THE COLOURS AND PIGMENTS OF PLANTS 63

tion from the brightly coloured leaves of some species
of Coleus. He found, however, that on boiling these
leaves with water, he did not obtain a coloured
solution, although the leaves themselves lost their
colour. If, however, the colourless solution be
evaporated to dryness, anthocyan is left behind in
its blue or violet form. Similarly if the decolorised
leaves be dried at a gentle heat they regain their
bright colours. The reason for this remarkable
phenomenon Molisch found to be as follows. Dur-
ing life the cell-sap in these coloured leaves is faintly
acid or neutral in reaction, but the protoplasm is as
usual distinctly alkaline. At the moment of death
the alkaline protoplasm mingles with the cell-sap,
and the alkalinity of the protoplasm is strong enough
to completely destroy the anthocyan colour (cf. action
of alkalies as explained above). Molisch also found
that leaves which are not decolorised on boiling are
remarkable for the large amount of acid contained
in the cell-sap, this being apparently large enough to
neutralise the alkalinity of the protoplasm. By
further experimentation Molisch convinced himself
that the decolorisation only occurred when the cells
containing anthocyan were in contact with cells very
rich in chlorophyll. This he illustrated by a very
pretty experiment. He took the leaves of a species
of Saxifrage, Saxzfraga sarmentosa, in which the epi-
dermal cells of the leaf are very rich in anthocyan,
and removed a small part of the epidermis from
a leaf. Both the piece of epidermis and the re-
mainder of the leaf were then kept at a temperature
sufficient to kill the cells, and so bring the cell-sap
into contact with the protoplasm. In a quarter of

64 COLOUR IN NATURE CHAP.

an hour the part of the epidermis still in contact.
with the leaf became decolorised, while the severed
portion completely retained its colour. As the
chlorophyll can hardly be supposed to have a direct
effect on the anthocyan, Molisch concludes that the
“conditions for the formation of alkaline substances
must be especially favourable in chlorophyll-con-
taining cells.’ This is a conclusion of great interest
and importance. If cells which contain much chloro-
phyll also contain strongly alkaline substances, and
therefore, as we may reasonably suppose, tend to
destroy any anthocyan pigment which may be
formed, then this is equivalent to saying that the
conditions which are favourable to the development
of chlorophyll are unfavourable to the development
of anthocyan, and wzce versa. This is a very tempt-
ing conclusion, for it seems to explain many points
in the distribution of anthocyan which have hitherto
been very puzzling. Broadly speaking, anthocyan
tends to appear conspicuously in many leaves and
shoots in early spring, in many leaves in autumn, in
flowers, and in fruits. Its function is often said in
the case of vegetative organs to be the protection of
chlorophyll from the injurious effects of excessive
light, but this does not account for its appearance in
autumnal leaves. If, however, we may assume that
chlorophyll and anthocyan are in antithesis to one
another, we can readily understand why the latter
should appear in spring, before the power of assimi-
lation is completely established, in autumn when it
is beginning to disappear, in ripening flowers and
fruits where it is more or less completely lost, and
also in the leaves examined by Stahl which grew

III THE COLOURS AND PIGMENTS OF PLANTS 65

in shady places, in conditions unfavourable to the
development of much chlorophyll.

AUTUMNAL COLORATION

From this consideration of the common pigments
of the higher plants, we may pass to an account of
the peculiarities of the colours, selecting as examples
the colours of autumnal leaves and of flowers and
fruits.

We are all in this climate familiar with the fact
that the chlorophyll of stems and leaves is short-
lived: we all know how the delicate yellow-green of
spring leaves deepens into the dark green of summer,
and then disappears in the yellows and reds of
autumn, while these in their turn lose their glory
before the chill blasts of winter. Chlorophyll is
bound up with the assimilating power of the plant,
and as this power diminishes, the chlorophyll which
is its outward expression disappears also. This is,
however, true only of the green colouring-matter, the
chlorophyll-green, and not of the associated pigments.
The chlorophyll-green is probably reabsorbed along
with starch and any other useful substances which
may be in the leaf, while the yellow xanthophyll
remains behind in the form of oily drops, set free by
the disintegration of the chlorophyll corpuscles. In
the simplest case, eg. that of straw, there is thus
produced a uniform yellow coloration, the chlorophyll
being completely removed and the xanthophyll only
left. To produce the splendour of our October woods,
however, other factors have to be introduced. In
the first place the removal of the chlorophyll is often

F

66 COLOUR IN NATURE CHAP.

slow and partial, giving rise to the appearance of a
variegated leaf. Further, owing either to the for-
mation of several lipochromes, or to the unequal
distribution of a single lipochrome, parts of the
leaf become a deeper shade of yellow or orange,
sometimes becoming intensified to a dull red, while
in some cases there is produced a special red
anthocyan pigment. There are therefore three
main factors in the production of the tints of
autumn: (1) the disappearance of the chlorophyll-
green, (2) the increasing prominence of the lipo-
chromes, and (3) the development of anthocyan.
Other changes of minor importance also occur.
Thus the general effect is often heightened by the
dull brown colours assumed by the leaves of such
trees as the oak and the beech. These colours are
produced by the oxidation of the tannins of which
these trees contain such an abundant supply. These
substances are probably useless, and are got rid of
in the falling leaves and the bark. Although these
changes tend to occur with great regularity every
autumn, it is a matter of common experience that
they are to a large degree dependent upon the
weather, a fine dry autumn with a touch of frost
being specially favourable to the development of
brilliant colouring. Autumn colouring is of great
interest in a comparative study of coloration. There
is no reason to suppose that the colouring is of the
slightest use to the trees, and yet it often displays
to an extraordinary degree that beauty and perfectness
which we are accustomed to regard as the result of
the action of Natural Selection. It is further of
fundamental importance in the investigation of the

III THE COLOURS AND PIGMENTS OF PLANTS 67

causes of the colours of flowers, for the three
processes already mentioned as the factors in the
production of autumnal coloration are precisely the
same as the processes which produce the colours of
flowers. If the tints of autumn arise naturally from
the check to vegetation produced by the first breath
of frost, then we may reasonably suppose also that
the colours of flowers are also in origin the natural
result of diminished vegetative power.

Although in our climate the majority of our
plants shed their leaves in autumn in a cloud of
glory, yet we have of course some which retain their
leaves for several years, or do not shed them all
at once, being, as we say, evergreen. Even here,
however, the diminished vegetative power is frequently
seen in the partial modification of the chlorophyll.
The leaves of many evergreens assume a reddish
colour in winter, and this is due to the partial dis-
appearance of chlorophyll from the corpuscles, and
its replacement by red oily drops, probably of
lipochrome nature. These red drops disappear again
in spring when the leaves assume their normal green
colour. This red pigment differs from that of most
autumnal leaves in being confined to the chlorophyll
corpuscles, while in most cases a red colour is due to
anthocyan dissolved in the cell-sap.

COLOURS OF FLOWERS AND FRUITS

The subject of autumnal coloration leads up to
the colours of flowers and fruits. These in the
general case are due either to anthocyan pigments
dissolved in the cell-sap, or to lipochrome pigments

68 COLOUR IN NATURE CHAP.

contained in solid bodies, known as chromoplasts or
chromoleucites.

Anthocyan pigment colours the petals of hyacinths,
bluebells, roses, etc.; and such fruits as grapes, blae-
berries, cranberries, and so on. The property of
colour-change which we have already seen it to
possess is of considerable importance in the pro-
duction of the colours of flowers and fruits, for when
anthocyan is present in, for example, the cells of
petals, its tint depends in part on the degree of
acidity of the cell-sap.

Thus we are all familiar with the change in the
colour of the flowers as they develop, which is
frequently so conspicuous a feature in various
members of the natural order Boraginacee. The
forget-me-not is pink in bud and blue when full-
grown, the pink colour occasionally persisting as a
variation. The colour-change is associated with a
diminished acidity of the cell-sap of the cells of the
petals. The sap is at first strongly acid, but as the
flower develops the acid disappears. Most flowers
which in natural conditions are blue show as a varia-
tion, or under cultivation, a tendency to become pink,
eg. pink hyacinths, pink Campanulas, etc.; a fact
which seems to indicate that the amount of acid
present in plants tends to vary, or is in an unstable
condition. Such a variation though most common
in cultivation probably also occurs in natural con-
ditions. Thus the common milkwort, Polygala
vulgaris, may be found in the same locality under at
least three different varieties, with blue, pink, or
white flowers respectively. It is reasonable to
suppose that this variation is the result of variation

III THE COLOURS AND PIGMENTS OF PLANTS 69

in the amount of acid present in the cell-sap. Other
plants again show an extraordinary constancy in the
tint of the anthocyan of the petals, the fruitlessness
of florists’ efforts to produce blue roses is said to be
the result of some peculiarity of constitution of the
pigment which makes the tint impossible.

The colours of the lipochromes vary from yellow
through orange to red, and they colour such flowers
as daffodils, jonquils, yellow and red lilies, such fruits
as those of the tomato, the melon, the honeysuckle,
the asparagus, the lily-of-the-valley, and so on. It
is interesting to note, however, that the pure red
lipochromes, as distinct from the orange, are rare if
not entirely absent in plants (see Chap. II. under
Lipochromes). As to origin, the chromoplasts arise
in-much the same way as chlorophyll corpuscles do,
and indeed in many cases the unripe fruit or un-
developed floral leaves contain ordinary chlorophyll
corpuscles. As development proceeds the chlorophyll
disappears just as in the case of autumnal leaves,
and the highly coloured lipochrome is left. Various
names have been given to the different lipochromes
of flowers and fruits, but there is some reason to
suppose that they all arise from the pigments normally
associated with chlorophyll. Sometimes the lipo-
chromes occur in ripe fruits and flowers as crystals,
the original envelope of protoplasm having entirely
disappeared. The colours of most flowers and fruits
are caused either by anthocyan or lipochrome pig-
ments, or by a combination of the two, but there are
a few which are coloured by a yellow pigment of
unknown affinities dissolved in the cell-sap. Such are
the colouring-matters of the orange and of the dahlia.

70 COLOUR IN NATURE CHAP,

We have thus considered the common colouring-
matters of flowers and fruits, and seen that in general
terms they fall into two groups, the fixed or lipo-
chrome pigments, occurring in the form of solid
particles in the cell, and the free or anthocyan pig-
ments occurring in solution in the cell-sap, and
varying in tint according to its reaction. In flowers,
further, we frequently find a pure white colour, which
we have already seen to be an optical colour caused
by the presence of air-spaces between colourless cells.
Colours which are not primary colours are caused by
a superposition of differently coloured elements, or of
colourless elements on coloured ones.

Besides the actual bright colour, flowers are often
conspicuous by the beauty and regularity of their
markings. Sometimes this is of exceedingly simple
nature, and bears an obvious relation to the nature
of the parts. Thus in the snowdrop the petals are
delicately veined with green, and in the wood-sorrel
with violet. A veining with green recalls the appear-
ance of some autumnal leaves when the chlorophyll
seems to linger longest at the sides of the veins of
the leaf. In the same way the purple veins of wood-
sorrel (Oxalis) are quite similar to the reddened veins
of many leaves in spring, and just as in leaves the
red colour may occasionally spread over the whole
leaf, so it is not very uncommon to find in the wood-
sorrel as a variation that the petals have become
completely purple. This occurs especially in shady
places. The markings of many petals are, however,
of far more complex nature, and bear no obvious
relation to the structure of the parts. Such are, for
example, the markings on the labellum in many

1 THE COLOURS AND PIGMENTS OF PLANTS 71

species of orchids, and on the corolla of the foxglove.
There seems little doubt that these markings are in
many cases employed by insects as landmarks in
their search for honey, and they have been in con-
sequence termed honey-guides. In many cases, how-
ever, they are much more complex than seems
necessary for this function, and are by no means
limited to flowers containing honey ; their meaning
and origin are still very doubtful.

MEANING OF PLANT PIGMENTS AND SUMMARY

Looking at the colours of flowers and fruits as a
whole, we may say that all the processes which give
rise to their brilliant colours have a parallel in the
vegetative shoot. The prominence of lipochromes
and the development of anthocyan are paralleled in
the autumnal coloration of leaves. The development
of a white colour is paralleled by the occasional
partial albinism of leaves, which occurs either as a
result of injury by other organisms, or in some
instances as a natural condition, eg. in the lung-
wort (Pulmonaria). Even the complex markings of
petals are dimly foreshadowed in the veining of
leaves.

All this is fairly obvious, but when we attempt
to discuss further the prime meaning of colour in
plants, the difficulties are very great. Of the five
groups of pigments described in the last chapter, we
have in plants the first represented by chlorophyll ;
the third possibly represented by the anthocyan
pigments, which are apparently derived from tannins,
and are probably useless substances; the fourth,

72 COLOUR IN NATURE CHAP, III

according to Zopf, represented by the ubiquitous
lipochromes ; and even the fifth group is mentioned
by Zopf as being represented among the Fungi. It
is curious that in spite of the fact that chlorophyll is
such a complex pigment, and can be made to yield
numerous coloured derivatives in the laboratory,
there is no evidence that in natural conditions any
of the plant pigments are produced by its decom-
position. The assertion that the xanthophyll of
leaves is derived from it has been repeatedly made,
but not on any good grounds.

As to the primary meaning of chlorophyll in
metabolism we know nothing. Macallum seems to
believe that it is in some way directly connected with
the nuclear chromatin.

As to the lipochromes, we know neither their
primary meaning nor their proximate use. They
are perhaps universally distributed in plants, and
occur in association with reserves in the shape of oil
and fat. They are probably always either yellow or
orange, the pure red lipochromes not occurring in
plants, and these yellow lipochromes are perhaps
invariably found in association with fat. We have
already seen that Ehring regards the carotin of the
tomato as a cholesterin fat.

The remaining pigments are probably all of the
nature of waste or useless substances. Among them
are included the anthocyans, the colouring-matters
of bark, and of some woods like brasilin, hematoxylin,
etc. The pigments of Fungi are more numerous
than those of flowering plants and are little known.

CHAPTER IV

THE COLOURS OF PROTOZOA, SPONGES, AND
CQELENTERA

Pigments of the Protozoa, their Characters, Variations, and
probable Origin—Coloration of Sponges and Ccelentera
—Distribution of Colours—The Colours of Corals and
Sea-Anemones—Colour-resemblances in the Ccelentera
—The Effect of Light upon the Development of Pigment
—The Pigments of the Ccelentera—Optical Colours.

THE pigments of the Protozoa are mostly very plant-
like, chlorophyll and lipochromes being exceedingly
common. There is often an interesting alternation
between chlorophyll and red or yellow lipochrome
pigments, very similar to that seen in the vegetative
organs of the higher plants. Thus Hematococcus
is sometimes red and sometimes green: it is always
red in the resting stage, and gradually acquires the
power of movement and a green colour simultaneously.
Sometimes in this form and in Huglena there is
merely a red ring. round the nucleus; when the
green colour develops it begins at the periphery
and gradually spreads inwards. Rostafinski found
that the red colour is produced by a combination of
two lipochromes, distinguished by their solubilities,

74 COLOUR IN NATURE CHAP.

—an interesting fact, because this association or lipo-
chromes in pairs is widely spread throughout both
the animal and vegetable kingdoms. In Euglena
sanguinea the colouring-matter may be absent with-
out apparently any resulting specific difference. The
red colour occurs chiefly in spring, in autumn, in the
dry condition and in bright sunshine, a state of
affairs quite comparable to that which obtains in the
higher plants. According to Engelmann (1882), the
red colouring-matter is capable of evolving oxygen
as well as the green.

Among other Protozoa which do not contain
chlorophyll, red pigments- sometimes occur, often
being found only in the so-called eye-spots. In
mass the red colouring-matter may sometimes render
the tiny organisms very conspicuous ; thus Agassiz
speaks of Globigerina occurring in floating masses of
scarlet colour, and forming an appreciable factor in
the coloration of the ocean-surface.

A brown colouring-matter, perhaps identical with
the pigment of Diatoms (diatomin), seems to have
a wide distribution among the Protozoa, but the
question whether it is an intrinsic or a derived pig-
ment is as yet undetermined. It occurs occasionally
in the cortical layer of Vortzcella for example.

Among the Protozoa containing chlorophyll or
lipochromes we must also mention the Radiolaria.
Many of these contain the so-called yellow cells,
which are little masses of protoplasm apparently
coloured by chlorophyll, plus some other pigment.
There is strong evidence in support of the conclusion
that these are unicellular alge living in symbiosis
with the Radiolarians (Geddes, Brandt). Others

Iv. THE COLOURS AND PIGMENTS OF PROTOZOA 75

contain phzodia or phzodellz, which are similar
masses impregnated with fine granules of brown
pigment of unknown characters. Several sug-
gestions as to function have been made, but none
seems well established. Karawaiew considers that
they play an important part in the assimilation of
food, but in what respect is not quite apparent.
Among the Foraminifera, pigments perhaps of
lipochrome nature are very common. We have
already spoken of Globigerina as being bright red
in mass; a similar pigment is described in some
detail by Fritz Schaudinn in Myxotheca arenilega.
This is a very large form, and in it the whole of the
protoplasm is coloured a bright Pompeian red by
means of a finely granular pigment. The pigment
is soluble in alcohol, and was found to be absent
in only two cases out of a large number examined.
The organism was observed to feed on Copepoda,
which are often very brightly coloured organisms,
and we must allow the possibility that the pigment
was derived from the food. It may be thought that
this suggestion is too freely made for the colours of
the Protozoa, but it should be remembered that in
organisms of such great simplicity it is difficult to
clearly distinguish between pigment directly intro-
duced with the food and intrinsic pigment; in their
case derived pigment has not quite the same mean-
ing as in the case of ccelomate animals. Even in
the Ccelomata, indeed, colouring-matter introduced
into the gut with food may have a direct importance
in coloration ; many of the transparent herbivorous
worms such as Nemerteans or Annelids are coloured
green by the contents of the gut. It is likely that

76 COLOUR IN NATURE CHAP.

many of the colouring-matters of the Protozoa are
as adventitious and unimportant as these, but of the
physiology of the Protozoa relatively little is known.

Apart from the lipochrome pigments and chloro-
phyll we find that, as already seen, blue and violet
pigments are not uncommon in the Protozoa. In
Stentor ceruleus notably, the presence of a blue pig-
ment in the alveolar layer has long been known.
According to Biitschli the pigment may be blue,
red, rusty-yellow, or coffee-brown. It was examined
spectroscopically in 1873 by Prof. E. Ray Lankester,
who found that it gave a three-banded spectrum.
More recently it has been again investigated by
Mr. Herbert Johnson, who has made a special study
of the American species of Stentor.

In several of these chlorophyll is present in the
form of the so-called zoo-chlorella, but these are
entirely absent from S. cwrudeus, which is the largest
form and the one which is hardiest in aquaria. In
it there is abundant pigment present in the ectoplasm,
the pigment being arranged in stripes which corre-
spond to ribs or stripes in the ectoplasm. The stripes
are very inconspicuous in species in which pigment
is absent, but can be demonstrated in these by
differential staining. The occurrence of pigment in
these simple organisms showing a differential dis-
tribution associated with morphological differences
in structure and physiological differences in reaction
to chemical agents, is a fact of much theoretic interest.
It will be noted that the result of the differential
distribution is to produce a simple form of marking
—a true coloration in the common use of the
term.

tv THE COLOURS AND PIGMENTS OF PROTOZOA 77

As to the pigment itself, according to Johnson
it normally varies in tone from bright sky-blue to
pale sea-green or even dull bluish-gray, but if the
organisms are kept under unfavourable conditions
it becomes reduced in quantity and changes to
a yellowish-brown colour. This change always
occurred when the Stentors were artificially divided,
and Johnson never found the blue colour to be
regained when once lost—a curious fact (the
statement, of course, refers to forms kept in con-
finement). Individuals are sometimes found which
are almost devoid of pigment. Schuberg observed
a fact confirmed by Johnson that the pigment is
thrown out of the living Stentors apparently in the
same way as that in which fecal matter is got
rid of; the pigment tends especially to accumulate
near the point of attachment in forms which have
remained long in one place.

The blue pigment is extremely stable, not being
dissolved by alcohol, ether, etc, nor attacked by
acids or alkalies.

Another Protozoan (WVassula) also contains a
blue pigment which is probably derived from the
Oscillatoria of the food.

Another beautiful violet pigment, apparently of
unknown characters, is described by Dr. O. Niisslin
in a Protozoan (Zoonomyxa violacea) found in the
Herrenwieser Lake. This organism has its proto-
plasm filled with numerous small violet vacuoles,
sufficiently abundant to colour the whole organism
violet. The pigment has a superficial resemblance
to one described by Greeff in Amphzzonella violacea,
but in that form the pigment is granular, while here

78 COLOUR IN NATURE CHAP.

it is in solution. The colouring-matter is very
susceptible to the action of reagents, being destroyed
by very dilute alkalies and acids, iodine or alcohol.
It also disappears after death or encystment. This
extreme instability is interesting, for, as we have seen,
it is characteristic of so many blue or violet pigments.

In general the pigments of Protozoa seem to be
usually chlorophyll, lipochromes, or blue or violet
pigments of unknown relations. The notes given
above are obviously incomplete, but the pigments
have been little investigated. The notes may, how-
ever, be sufficient to emphasise the points of import-
ance about the pigments of the group. These are
briefly as follows: in spite of the fact that the colours
show great variability, we find every now and again
in the group complex and unstable pigments of vivid
and beautiful tint; these may in some cases, as in
that of chlorophyll, have an important function, but
such functions are entirely unknown—it would seem
that in some instances the pigments are merely
introduced with the food ; there is also considerable
evidence to show that the pigments vary in tint in
harmony with the varying physiological conditions of
the organism. In all these respects the pigments of
the Protozoa afford an interesting commentary on
those of higher animals.

THE COLOURS OF SPONGES

About the colours of sponges it is not at present
possible to say much. It is a familar fact of observa-
tion that they are exceedingly variable and often
bright ; red, orange, yellow, green, and dull colours

1v THE COLOURS AND PIGMENTS OF SPONGES 79

are all common. The green colour of the fresh-
water Sjongilia is usually asserted to be due to
chlorophyll, but green tints occur also in marine
forms. Sponges whose colours vary from greenish-
yellow to red almost all contain lipochromes. The
pure red lipochrome known as tetronerythrin is
widely spread, and is often associated in sponges with
a peculiarly penetrating odour like that of ozone.
Krukenberg was of opinion that in these sponges the
pigment played some part analogous to that of
chlorophyll in plants, being of importance in a pro-
cess of assimilation. It does not appear that his
observations have been repeated or confirmed.

In Hircinta variabtls and some other sponges
there is a red pigment which, according to Kruken-
berg, is very similar to the pigment of the Red
Alge, and which is readily decolorised by reducing
agents. Some of the Aplysinidze contain pigments
of the type described by Krukenberg as uranidines,
which are of yellow colour, but tend to rapidly undergo
oxidative changes which turn them black. In other
forms, 2g. Chondrosia, dark pigments occur—a fact
of some interest, because these are rare in simple
organisms.

THE COLOURS OF SEA-ANEMONES, CORALS,
JELLY-FISH AND THEIR ALLIES (CCELENTERA)

The group of animals whose colours we have next
to consider includes some of the most beautiful of
existing organisms. Beautiful as are the changing tints
of birds and butterflies, they lack for many people
the subtle fascination possessed by the delicately

80 COLOUR IN NATURE CHAP.

translucent colours of the Ccelentera. That some part
at least of the charm is due to childish reminiscences
of tales of coral islands and tropical seas, few would
deny, but apart from this, many of the polypes and
sea-anemones of our own shores are adorned with
tints which afford intensest pleasure to a colour-
loving eye.

The Ccelentera include a large number of forms,
which are almost all marine, and are found in greatest
abundance in warm seas. Many of them are of
sedentary habit, and frequently of peculiarly plant-
like appearance ; many in their method of growth
or in their peculiar shape present a strong superficial
resemblance to seaweed. All are characterised by
relatively great simplicity of structure, and therefore,
in accordance with the principles which we have
already considered, their colours are due to the
presence of pigment in the coloured tissues, and not
to effects of structure. As a group, therefore, the
Ccelentera are plant-like not only in general habit,
but also in the development of a large series of
brilliant pigments. Owing to their simplicity of
structure and the absence of true internal skeleton, all
are during life more or less transparent or translucent,
and this translucency adds an extraordinary delicacy
to their colouring. Unfortunately this colouring
cannot be seen in preserved specimens, both on
account of the fugitive nature of the pigments, and
on account of the loss of the transparency during the
process of preservation. During life the transparent
appearance is largely increased by the habit which
most possess of distending the tentacles and the
body by means of sea-water, while death is usually

1v THE COLOURS AND PIGMENTS OF CCQELENTERA 81

accompanied by a process of contraction which not
infrequently destroys all beauty of form as well as of
colour. All these causes combine to render it practi-
cally impossible, at least with present methods of
technique, to adequately preserve the large majority
of the Ccelentera. Frequenters of museums will
recall the glass models or crudely tinted chromo-
lithographs which usually fill the cases devoted to
the group. Although, therefore, the beauties of the
coral-reefs must to most of us be merely a matter of
hearsay, yet it should be emphasised that Ccelentera
are inhabitants of temperate as well as of tropical seas,
and that many a pool on the British coast will dis-
play organisms whose colouring differs in degree
only, and not in kind, from that of the denizens of
the most marvellous coral-reef.

It is not necessary here to discuss the structure
of the Ccelentera ; we need only recall the fact that
most of the organisms included in this class are either
polypes of the nature of a sea-anemone, or jelly-fish
of the familiar type, and that both these forms may
occur in the course of a single life-history. For our
purpose it is sufficient to think of a polype as con-
sisting of a hollow column fixed at one end to some
solid body, and bearing a mouth surrounded by
waving contractile tentacles at the free end. Such
polypes may grow singly as do the sea-anemones, or
they may grow together in “colonies.” The polypes
in such colonies are connected together by a fleshy
substance traversed by numerous tubes. If lime be
laid down in considerable amount in this fleshy
substance, the colony forms a “coral.” This “ coral”
or limy skeleton is really outside the individual

G

82 COLOUR IN NATURE CHAP.

polypes, but owing to the way in which these are
spread over it, they have a superficial appearance of
enclosing the coral within their own soft parts.

DISTRIBUTION OF COLOURS

The coral-reefs of warm seas are largely formed
of such colonial sea-anemones, the simpler colonial
forms which do not produce a limy skeleton being
notably absent from these regions. In their living
condition on the reefs the polypes themselves, as
well as the skeleton with its organic covering, are all
coloured, often with the brightest of tints. Further,
it seems to be relatively rare for the whole colony to
be of one tint. Sometimes the polypes are sharply
contrasted in colour with the coral, sometimes the
youngest portions of the colony differ entirely in
colour from the older; while in those forms in which
the individual polypes attain a considerable size, the
tentacles may be banded or tipped with a colour
quite different from the ordinary ground - colour.
According to Dr. Hickson (4 Naturalist in N.
Celebes, London, 1889) the commonest tint is a deep
greenish-brown, and next to that, and especially in
the younger parts, a bright green. Again, a study
of the descriptions and plates in Mr. Saville
Kent’s great book on the Barrier Reef of Australia
shows that after these come shades of red, pink, and
yellow, and more rarely electric blue. According to
Saville Kent there is great and striking variation
in tint both within the limits of a species and even
during the course of the life-history of a single
colony.

Iv THE COLOURS AND PIGMENTS OF CCELENTERA 83

Although descriptions of coloured organisms do
not as a rule convey much idea of their beauty, it may
illustrate the problem before us if we give details of
some of the corals studied by Mr. Saville Kent during
his sojourn at the Barrier Reef. It is perhaps not
unnecessary to repeat that one must read into the
list of tints something of that purity of colour, that
varying light and shade, which have made the glories
of spring and autumn a never- failing source of
artistic inspiration.

THE COLOURS OF CORALS AND SEA-ANEMONES

Among the masses of coral which go to form the
reef, the different species of Madrepora, or stag’s
horn coral, are usually very conspicuous. In this
country specimens of Madrepora are quite familiar
as slender branching stems studded with tiny
openings, but they reach us always in the white or
bleached condition ; in their natural condition almost
all are brightly coloured. Thus in one species
(M. prostrata) the whole colony is usually bronze-
green with yellow tips, but it may be bright green
with yellow tips, or more rarely shrimp-pink with
yellow tips. Again, another species is remarkable
in having the ends of its branches crowned by
larger cells than those which constitute the branches
themselves. In this case the branches are pale
yellow to white, the polypes being light brown to
greenish-yellow; the large terminal cells are a
delicate china or bright turquoise blue, their con-
tained polypes being of an emerald-green tint.
Another species, 1/. hebes, shows a large amount of

84 COLOUR IN NATURE CHAP.

variation. Most frequently it is a dark brown colour
with white extremities ; sometimes the whole surface,
including the polypes, is a vivid grass-green. Mr.
Saville Kent describes a large colony which when
first examined was pinkish-brown with greenish-
white growing apices, the polypes throughout being
a light emerald- green. On revisiting the colony
after two years, it was found that the surface of the
colony was of a clear seal-brown with white or pale
lilac-blue tips, while the polypes were a clear red-
brown, the tips of the tentacles only being a greenish-
white. This variation is exceedingly marked, but
the author seems to intimate that it was accompanied
by a retardation of the growth of the colony. It is
unnecessary here to give further details as to the
colouring of the Madrepores ; the above descriptions,
which are taken almost verbatim from The Great
Barrier Reef, are sufficient to prove the statement
that these corals are remarkable for their contrast of
colour, the contrast being produced by the respective
colours of polypes and ground-substance, or by the
different tints of mature and growing regions. The
colours mentioned are exceedingly common through-
out the genus.

The same brilliancy of tint is observable among
the other corals; the banding of the surface of
individuals is especially remarkable among the large
sea-anemones or the solitary corals. The occurrence
of bands of colour, especially round the tentacles, is
admirably shown in many of our native sea~anemones,
as are also the essential peculiarities of colouring.
Note, for example, the green and magenta colouring
of Anthea cereus, the bright turquoise beads at the

1v THE COLOURS AND PIGMENTS OF CCELENTERA 85

base of the tentacles of the common Acéznia, and so
on. These beads are little clusters, batteries, of sting-
ing-cells, and are sometimes called nematospheres.
In some of the large tropical anemones the tentacles
are greatly branched after the fashion called pinnate,
and some of the pinnz are modified to form nemato-
spheres, which are thus borne like little fruits on the
surface of the tentacles. These are especially well
developed in Aeterodactyla, and there display the
most wonderful beauty of colour. Often they are
a brilliant translucent violet, with an apical spot of
vivid emerald-green ; in another species they are a
bright amethyst with a terminal spot of a darker
tint. These nematospheres occur in clusters, and
when viewed under the low power of the microscope,
may, according to Mr. Saville Kent, “be appropriately
compared to currant-like fruit, carved out of amethyst,
with a crystal of amethyst inserted, to represent the
cicatrix of the antecedent flower.” The occurrence
of such detailed beauties of form and colour in these
simple organisms is of great theoretic interest.
Among other interesting organisms included in the
Ccelentera we must notice the blue coral (He/zopora).
This obtains its name from the fact that its colour is
more permanent than that of most corals, and the
coloration is therefore quite marked in dried speci-
mens. Curiously enough, the blue colour is more
intense in the middle of the coral than on its outer
surface, which is simply of a blue-gray colour. The
polypes are small and pale-coloured. This form is
of great interest because the special group to which
it belongs, the Alcyonaria, do not now form as a rule
continuous living skeletons, although many fossil

86 COLOUR IN NATURE CHAP.

corals belonged to it. The only other existing reef-
building Alcyonarian coral is Zubipora, the organ-
pipe coral; here the coral itself is of a bright red
colour, and the polypes either brownish-red with
green-tipped tentacles, or entirely of a delicate green
colour.

COLOUR - RESEMBLANCES IN THE CQ@ELENTERA

We have already noted the plant-like habit of
growth which causes so many of the simpler Ccelentera
to be popularly mistaken for seaweed, but there are
also a few detailed resemblances which have been
described. Thus the species of the sea-anemone
genus Actinodendron have much-branched tentacles
which resemble the fronds of seaweeds or ferns very
closely. In the case of A. alcyonotdeum the habitat
is in sandy pools in the corners of coral rocks; the
long tentacles float freely in the water and sting with
extreme severity when touched; the colour is dull and
seaweed-like. It is suggested by Saville Kent that
the resemblance to seaweed entices organisms within
the reach of the deadly tentacles, and that this is
therefore an example of “alluring coloration.” <A
species of a related genus (Megalactes griffithst) has
tentacles of similar nature, and inhabits similar
situations, but is distinguished by a relatively elabo-
rate system of coloration, being marked by radiating
white lines on a ground of lilac, pale sea-green,
gray and buff. Again, in the species of Heterodactyla,
already mentioned for their elaborately beautiful fruit-
like nematospheres, the branched tentacles are grass-
green in colour and “ present the aspect of aggregated

tv THE COLOURS AND PIGMENTS OF CCGELENTERA 87

tufts of fine, brightly-coloured moss, or, yet more
appropriately, certain varieties of the very finely
divided leaves of cultivated parsley.” So also one
of the species of mushroom coral, Fungia crassiten-
taculata, when in the fully extended condition, “ bears
a considerable resemblance to a crowded growth of
the common green seaweed, Exteromorpha.’ There
is, however, great colour-variation in the species, and
it is only the green variety which presents this
appearance. Finally, we may mention the case of
the “lettuce-corals ” (Zrvidacophyllia), which, especially
in the green species, are said to closely resemble in
their peculiar method of growth leaves of lettuce or
endive. This is an interesting case of resemblance,
for it would require an exceedingly enterprising
biologist to construe it as a case of protective or
alluring coloration.

THE EFFECT OF LIGHT UPON THE DEVELOP-
MENT OF PIGMENT

On this subject there are a few interesting observa-
tions. Mr. Saville Kent observed that in Euphyllia
glabrescens the tentacles were frequently green or brown
with paler tips, but where they were completely shaded
from the light they were quite transparent and
colourless with faintly tinted tips. Similarly, forms
like Symphyllia which only unfold their tentacles at
night, have these transparent or colourless, while the
exposed parts are coloured with the usual brown or
green pigments. As a curious exception to this rule
he found that a species of Dendrophyllia, D. coccinea,
was always coloured a bright red, even when shaded.

88 COLOUR IN NATURE CHAP.

Similarly, D. rvamea also displays the same deep tint
whether it grows in shallow water (7-8 fathoms) or
at great depths (600 fathoms).

In connection with the effect of light we may
mention the prevalence of blue colours among the
pelagic jelly-fish, which many would regard as directly
due to the action of light.

THE PIGMENTS OF THE CC:LENTERA

The number, beauty, and great variability of the
tints of the Ccelentera make the question of the
nature of their pigments one of great interest, but, as
is so often the case, the bright pigments are very
unstable and their examination is a matter of great
difficulty. In consequence, the observations which
have been made are, in most cases, very incomplete.
We shall not attempt to give a detailed account of
the pigments already described, but shall merely
describe the characters of the better known of them.

We have already emphasised the predominance
of a green tint in the sessile Ccelentera, and the
frequent tendency for this tint to be in whole or in
part replaced by another colour, such as blue, pink,
or brown. The brightness of the tint has suggested
to many the possibility that the pigment might be
chlorophyll or some related colouring-matter, impart-
ing to the organism the power of taking carbon from
the air and evolving free oxygen. Among the more
recent supporters of this view we have Prof. Hickson,
who during his stay in N. Celebes remarked on the
absence of Algz in the neighbourhood of coral-reefs,
and suggested that their green pigment might enable

1v THE COLOURS AND PIGMENTS OF CCELENTERA 89

the coral polypes to physiologically replace the miss-
ing plants. The suggestion has the more force in
view of the fact that many anemones have in their
inner layer the so-called “ yellow cells,” which many
regard as symbiotic Algze. Further, Prof. Geddes
some years ago succeeded in showing that some
green anemones, ¢g. Anthea cereus, possess in sun-
light the power of evolving free oxygen. Kruken-
berg was unable to confirm Geddes’s results as to
the evolution of oxygen, but subjected the pigments
of Anthea to a careful examination. He -chose
specimens of Anthea which were bright green with
purple tips to the tentacles; and though he found
that an alcoholic extract contained a mixture of
pigments, he could not succeed in persuading himself
that it contained chlorophyll. He speaks with con-
siderable reserve on the question of the affinities of
the pigments, and carefully guards himself against
an absolute denial of the existence of symbiotic
Algz ; his point simply being that there is consider-
able evidence against the hypothesis that Axthea
contains chlorophyll. Observations on other pig-
ments which have been made since Krukenberg’s
work, seem to justify us in speaking a little more
decidedly on the subject of this pigment.
Krukenberg’s observations may be summarised as
follows: he obtained a pigment which dissolved in
alcohol to form a solution which varied in colour
from brown to green, showed distinct red fluorescence,
and gave a banded spectrum ; the addition of acid
turned the green solution blue, and added a new
band at the junction of the yellow and the green, as
well as altering the’ position of the other bands.

90 COLOUR IN NATURE CHAP.

These observations taken together seem to show that
the pigment belongs to an interesting group of
colouring - matters, which, as already mentioned,
includes the pigments of the worms Boneliza and
Chetopterus, of the “liver” of Mollusca and of some
other animals, all of which present some superficial
resemblances to chlorophyll. In view of the fact
that the cells of the “liver” in Mollusca contain
yellow-brown granules which certainly contain the
characteristic pigment (often called enterochloro-
phyll), I am of opinion that it will probably be
found that the so-called yellow cells of Azthea are
merely granules of the characteristic pigment, prob-
ably mingled with some other, Dr. M‘Munn (1885)
on spectroscopic grounds certainly denies that the
Anthea pigment corresponds to his “enterochloro-
phyll,” but as the animals contain a mixture of
pigments, it seems not improbable that the anomalous
appearances noticed by him were due to the simul-
taneous occurrence of several pigments in the same
solution. The resemblances to the chztopterin
group are certainly very striking. Now these
“yellow cells” are widely distributed among sea-
anemones, and if we may assume a similar wide
distribution of a pigment of the chetopterin group,
it is possible to account for some of the remarkable
colour-phenomena of the Ccelentera; these may be
merely the result of the properties of the dominant
pigment.

The better known pigments of the chztopterin
group, viz. chetopterin, bonellin, and “ entero-
chlorophyll,” show two marked peculiarities. In the
first place, associated apparently with their complex

v THE COLOURS AND PIGMENTS OF CCELENTERA 91

spectrum, they exhibit a lack of definiteness and purity
of tone; in dilute solutions chetopterin is a delicate
pure green, but when concentrated it becomes muddy
and shows strong red fluorescence. It seems not un-
reasonable to suppose that this peculiarity will affect
the nature of the tint in the living organism. Thus
if the same pigment occurs in very delicate trans-
lucent tissues, and in denser and more opaque ones,
it seems probable that the characteristic differential
absorption will result in striking difference of tint in
the two cases. May not this account for some of
those marked differences in colour between old and
young parts of a colony, between polypes and “coral,”
between tentacles and body, and so on, upon which
we have already dwelt?

In the second place, the pigments of the cheto-
pterin group are remarkable for their extreme sus-
ceptibility to the action of reagents, especially to
acids and alkalies. Acids produce remarkable varia-
tions in tint corresponding in part to new pigments ;
it is probable that a considerable number of colouring-
matters can be produced by the action of different
reagents. Now Krukenberg noticed that the colour
of sea-anemones varied according as they secreted a
tryptic ferment (ze. one which acts in an alkaline
medium) or a peptic one (acting in an acid medium) ;
it seems therefore probable that the colour in any
given case will depend upon the reaction of the
surrounding tissues. This may agai account for
some of the local variations in tint, while it is not im-
probable that the vivid green colour so often seen may
be due to a pigment derived from the pigment of the
“ yellow cells.” Chzetopterin and enterochlorophyll

92 COLOUR IN NATURE CHAP.

at least can be made to yield very bright green
derivatives.

Dr. M‘Munn (1885) has described by means of
the spectroscope a number of other pigments in sea-
anemones, but too little is known of these to make it
profitable to detail here their names and properties.
There is, however, one interesting pigment, called by
Moseley polyperythrin, which deserves further notice.
Moseley found this pigment in a number of simple
stony corals and in a few anemones and jelly-fish,
almost all from deep water. It is of a deep madder-
brown colour, and in the case of the corals sometimes
coloured uniformly both the soft parts and the coral,
sometimes the soft parts only. Further, in some
specimens it was uniformly distributed, while in
others it occurred in streaks or was totally absent.
The pigment dissolves in acidulated alcohol or in
dilute acid to form a pink solution with green
fluorescence, and gives a spectrum which in some
respects resembles that of the pigments of the
chetopterin group. It seems not improbable that
this pigment is the result of the modification of a
pigment allied to the “chlorophyll” of Azthea.

Another interesting series of pigments, apparently
not allied to the preceding, are those producing the
blues and browns of the surface jelly-fish. There is
reason to believe that the blue colour of Cyanea, of
the common Aurelia, of Rhizostoma, and of Vellela
are all due to a pigment called by Krukenberg
Cyanein, which is recognised by the following
characters. It is soluble in water, especially in
water containing neutral salts (¢,¢. sea-water), It is
unaltered by weak acids, but strong acid, alcohol,

1v THE COLOURS AND PIGMENTS OF C@LENTERA 93

benzol, chloroform, phenol, heat, etc., destroy the blue
colour and give a precipitate of dull red or brown
colour. The blue solution in water gives three (or
two?) bands in its spectrum, but Professor E. R.
Lankester (1870) did not succeed in getting these
bands with Véellela; the reddish-brown substance
apparently gives no bands. Further, in R/zzostoma
Cuviert the blue colour is somewhat variable; it
chiefly occurs in young specimens; the older, especially
when carrying eggs, are of dirty red or reddish-brown
tint. This must surely be due to the modification of
the unstable blue pigment under the influence of
some specific change in the chemical characters of the
protoplasm of the organism. Then again, Professor
M'‘Kendrick found that the brown colouring-matter of
the jelly-fish Chrysaora was soluble in hot sea-water,
yielding a dark brown solution of acid reaction, which
gave a reddish-brown precipitate with strong alkali.
It seems probable that this colouring-matter is also
the result of a modification of cyanein.

These cases show that in all probability the
number and variety of the tints in the Ccelentera
are due in large part to physical or chemical changes
in a few complex pigments. As to the cause of these
changes, it must be noted that the pigments are here
deposited in internal structures where active meta-
bolism is going on, and not, as in many higher
animals, in superficial inert tissues. The physiology
of the higher Vertebrates shows that in them there is
a metabolism of pigment as active as any that can
occur in coral polypes; but while in the latter every
change is apparent on the surface, in the former the
changes are concealed by the intervening passive

94 COLOUR IN NATURE CHAP.

tissues, and the colour has a characteristic permanence,
The varying translucency and delicacy of the tissues
in the Ccelentera further determines the apparent
colour of the pigments, so that we may almost
describe these as simple forms of optical colours. It
is likely that the frequent variation of colour in simple
organisms is to be accounted for in a similar fashion.

Among the more special colour-phenomena of
the group, we may notice the comparative rarity of
the lipochromes. They are said to occur in the red
coral of commerce, in the skeleton and soft parts of
some of the Gorgonide, probably in the Dendrophyllia
mentioned above, and in a few others.

The colouring-matter of the blue coral Helzopora
cerulea also deserves mention. It is very different
from any other blue pigment known, and is very
insoluble. If, however, the coral be decalcified with
hydrochloric acid, the pigment is set free, and may
be dissolved in alcohol. The solution gives no bands
and turns green with alkali. It seems not improbable
that the colouring-matter is metallic in origin, perhaps
a salt of some metal (cf the green colour of the bones
of Belone, etc.).

From the simplicity of the structure of the
Ceelentera it is naturally to be expected that true
optical colours will be absent. These are not,
however, quite unknown, for the calcareous axes of
some of the Gorgonians display brilliantly iridescent
colours. Agassiz (Voyage of the Blake, vol. ii. pp.
144,145) describes the species of /7idogorgia as having
axes of a bright emerald-green or of burnished gold,
while others have a lustre like mother-of-pearl. All
the iridescent forms inhabit deep water, and the

1v THE COLOURS AND PIGMENTS OF CCELENTERA 95

colours are probably due to the same cause as the
colours of shells, which also tend to be especially
iridescent when found in deep water.

We have already spoken of the prevalence of
phosphorescence in the group.

CHAPTER V

COLOUR-PHENOMENA IN WORMS

Colours of Turbellaria and Nemertea—Pigments of Gephyrea—
Colours of the Chzetopoda, Structural and Pigmental
Colours—The Pigments of the Capitellidae— General
Characters of Coloration of Leeches and Origin of Mark-
ings—Pigments of Polyzoa and the Origin of Pigmentation.

AS central and unspecialised forms, the worms are
of considerable interest in a comparative study of
colour, and there are several interesting facts in
regard to them which make it desirable that we
should consider the more important groups suc-
cessively.

The somewhat grim associations which cluster
about the Platyhelminths do not lead us to expect
bright colours among them, and yet in point of fact
the free living forms often exhibit great brilliancy.

Among the Turbellaria pigment frequently occurs
in the cells of the epidermis, in the interstices between
these cells, or in the parenchyma of the body.
Many, such as Convoluta, contain in the cells of this
parenchyma the so-called symbiotic Alge of green
or brown colour. These chlorophyll cells have no
cellulose envelope, and often contain pyrenoids.

CHAP. V COLOUR-PHENOMENA IN WORMS 97

The only detailed investigations on the pigments
of Turbellaria appear to be those of Moseley, who
made some observations on two species of Rhyncho-
demus found in New South Wales. Of these one
was blue and the other red, the two living together
under somewhat similar conditions. The blue
pigment was insoluble in alcohol, turned red with
acids, and then dissolved in alcohol; alkali restored
the blue colour. This at once suggested that the
red pigment was due to an acid reaction in the tissues
of the red species, and was simply a modification of
the blue. Moseley could not, however, succeed in
turning the red pigment blue with alkali, and found
that it was insoluble in acidified alcohol. He was
forced therefore to the conclusion that the two
pigments are entirely different ; no further investiga-
tion seems to have been made on the subject.

It is interesting to note that in spite of the
simplicity of the Turbellarians, the coloration is not
necessarily completely uniform, but may show an
arrangement into bands and spots. Thus the species
of Geoflana often show a dorsal stripe of green,
orange, or purple, while the rest of the body may
show spots of blue, brown, or yellow on a dull
ground.

The pigments of the Nemertea have been even
more neglected than those of the Turbellaria, though
with less excuse. We quote the following eloquent
description of their colours from Professor M‘Intosh’s
monograph :—“ The colours of many species of the
group are of such beauty as to attract even the
casual observer, while in this respect also they widely
deviate from their supposed allies the parasitic

H

98 COLOUR IN NATURE CHAP.

worms. The richest purples appear on velvety skins
of deep brown or black, each of the soft and mobile
folds giving shades that vary in intensity and lustre.
Bright yellow contrasts with dark brown, white with
vermilion brown and dull pink, while individual
uniformity is characterised by such hues as rose-
pink, white, green, yellow and olive, the gradation of
colour in the various parts of a single specimen
being so subtle that enthusiasm as well as skill is
necessary in the artist who sets himself to the task
of faithful delineation” (Ray Soc. vol. xxxiv. p. 2).
This description is based on the British forms only,
and the tropical are said even to surpass these in
splendour. In addition to the beauty of colour,
we find that simple: forms of marking, such as
longitudinal and transverse stripes, are common.
Professor M‘Intosh speaks also of the “silver sheen,”
and the “ever-changing iridescence of the active
cilia,’ and considers that in point of beauty and
variety the Nemerteans even surpass the Annelids.
Light seems to have some influence on colour, but
the exact extent of the influence is apparently not
determined. Thus we are told that some of the
most brightly coloured live in crevices and dark
corners, but that under natural conditions the colour
varies according to the amount of exposure to light.
In captivity Lzxeus marinus turned pale, especially
in the anterior region, but others, eg. Amphiporus
lactifloreus, developed more pigment so that the skin
became opaque and of a deeper colour. Some are
transparent, and the food contained in the gut
(Alge, etc.) shines through and produces a marked
and peculiar coloration. There is virtually no sexual

Vv COLOUR-PHENOMENA IN WORMS 99

difference in colour, but in some cases, as in Amphi-
porus pulcher, the eggs are bright red, and by shining
through the thin body wall, produce a striking effect
on the coloration of the female. In simple animals
this primitive form of colour-difference in the sexes
is not uncommon. :

Biirger’s recent monograph with its beautiful plates
confirms M‘Intosh’s descriptions without adding very
much new information as to colour. Biirger describes
the epithelium as consisting of three kinds of cells—
slender thread-like cells, interstitial cells, and gland-
cells.) The abundant pigment may occur in any of
the three. In Zzmweus and the related forms it occurs
in the gland-cells, whose secretion is often grass-
green. In Memertopsis peronea, on the other hand,
the pigment is confined to the interstitial cells, which
form two long dorsal stripes of red-brown colour.
Biirger describes numerous instances of protective
coloration and of marked colour - resemblances
between armed and unarmed forms found living
together. His figures show again the frequency of
bands both longitudinal and transverse, but I have
not been able to find any suggestions as to their
meaning or origin. The pigments also have not yet
been investigated.

The remaining flat-worms, being parasitic, display
no brilliancy of colour.

THE PIGMENTS OF THE GEPHYREA

In the small order of the Gephyrea known as the
Echiuroidea a very interesting pigment occurs. The
curious form known as Bonellia viridis is bright

100 COLOUR IN NATURE CHAP,

green in colour, and was for long supposed to
contain chlorophyll. The pigment is said to occur
in the skin and sub-epidermic cells of the female,
and in the wandering cells which partially fill the
reduced body cavity in the degenerate male. It is
readily soluble in alcohol, the solution being green
to brown according to the degree of concentration,
and displaying a blood-red fluorescence. The
solution gives a beautiful and complex spectrum,
which changes when acid is added. The addition
of acid also changes the colour from violet to blue
according to the amount added. The pigment was
given the name of bonellin by Sorby, and has been
studied by numerous investigators. Both Sorby
and Krukenberg showed conclusively that it is not
chlorophyll, but the superstition dies hard, and may
be still found in many text-books. Professor E.
Ray Lankester (1897) has recently re-examined the
pigment, and shown that it occurs in the organism
in the alkaline condition—a point of some interest.
There can be little doubt that bonellin is a member
of the group of pigments spoken of in the last
chapter, which may conveniently be called the
chetopterin group. These pigments contain nitrogen
but no copper, and their function is quite unknown.
The curious point in connection with bonellin itself
is that it is only known in Bonelha viridis, although
a green colour is common among the Echiuroids. A
new British species of Thalassema, described by
Professor Herdman, is of an extremely vivid green
colour, the tint being somewhat similar to that of
Bonellia, Nevertheless the pigment was found to
be soluble in water, to give a one-banded spectrum,

v COLOUR-PHENOMENA IN WORMS 101

to form in alcohol a pure green solution without
fluorescence, and so to offer a marked contrast to
bonellin. It seems most probable that this pigment
is related to some of the derivatives of bonellin.
The same pigment appears to occur in smaller
amount in some other species of Zhalassema and
in Hamingza. Both these genera are further stated
by Lankester to contain hemoglobin in the peri-
visceral fluid and muscles.

In the Sipunculoidea, another order of Gephyreans,
integumental pigments appear to be absent, but in
Sipunculus and Phascolosoma the oxidised blood
contains a deep red pigment called by Krukenberg
hemerythrin. This resembles hemocyanin in being
colourless when reduced, and probably in having
a respiratory function. It belongs to a group of
pigments called by Krukenberg the Floridines,
characterised by their solubility in water and
glycerine, and their insolubility in the usual organic
solvents, such as alcohol, ether, chloroform, etc., as
well as by the readiness with which they undergo
oxidation. This pigment is similar to the one
already mentioned as occurring in the sponge
Hircinia ; it is, however, difficult to believe that it
can be respiratory there.

THE COLOURS OF THE CHATOPODA

The colours of the Chzetopoda often display great
beauty, and may be structural or pigmental, or due to
a combination of the two. One of the simplest forms
of structural colour is that displayed by the earth-
worm, where the cuticle exhibits a faint iridescence

102 COLOUR IN NATURE CHAP.

due to a system of fine lines on the surface. With
the greater development of the cuticle in the marine
worms, there is also a greater elaboratign of struc-
tural colour.

As a type of coloration in Polychzetes we may
ake Nerezs diversicolor, a very common worm on
our shores. This species shows considerable colour-
variation, the upper surface being a pure bronze,
greenish-brown, green, or pinkish, while the lower
surface is bright pink or flesh-coloured, the whole
body showing a very distinct metallic sheen in
addition. These bright colours are most distinct in
the large adult specimens found in the Laminarian
zone, and fade very rapidly after death, whether the
specimens are preserved in alcohol or formalin.
Preserved specimens are dead-white in colour, but
still show a faint sheen.

Taking such a form as a starting-point, we have
on the one hand worms in which the colour is
predominantly structural, and on the other those in
which it is predominantly pigmental. Structural
colours are especially marked in cases where the
characteristic bristles attain great development. In
the sea-mouse (Afhrodite), for example, the bristles
form a dense felt-like mass exhibiting beautifully
iridescent tints, which are in life much obscured
by the mud with which the animals are usually
covered. The beautiful golden crown of bristles
which Pectinaria belgica protrudes from its tube,
is another example of structural colour occurring in
connection with specialised cuticular structures.

Colours due to pigments are also often exceed-
ingly beautiful in these marine worms. The vivid

v COLOUR-PHENOMENA IN WORMS 103

green of Eulalia viridis, of Sabella, and of many
others, the pink of Terebella and Euchone rosea, and
the bright red filaments of C7irratulus, are examples
of the bright tints which are so widely spread in the
group.

Before passing on to consider what is known as
to the nature of the pigments, we may note the
different ways in which the external coloration of
the Annelids is produced. In the smallest and
simplest forms there is no pigment either in the
blood or tissues, and the animals are therefore
transparent and colourless. But when they are
herbivorous, as is often the case, the contents of the
food-canal may give them a green colour. The next
stage may be described as the condition when the
tissues retain their transparency and lack of pigment,
but the coloured blood shines through the thin
body wall, and gives the animals a distinct and often
bright colour. This is well seen in the bright red
Tubifex of ditches, the green Sadel/a in its sand-
tubes on the seashore, and others. Again, the
tissues may be devoid of pigment, but may be too
opaque to allow the blood-pigment to shine through
except in certain regions. Thus, for example, in
Gordiodrilus tenuis the genefal colour is a cream
white produced by the coelomic corpuscles, but the
body is marked by longitudinal red stripes produced
by the shining through of the larger blood-vessels.
Generally, however, we find that forms in which the
tissues are too opaque to allow the blood to be
directly visible not infrequently develop pigment
in the tissues. Thus the common earthworm con-
tains a certain amount of pigment scattered among

104 COLOUR IN NATURE CHAP.

the muscles ; and some earthworms owe their very
dark colour to pigment occurring in this situation,
or present in considerable quantity in the cells
of the peritoneal epithelium. Accompanying the
differentiation which gives size and opacity to the
body, there is frequently in Annelids, as already
noticed, a differentiation of the cuticle which gives
rise to structural colour. It is curious to note that,
contrary to the usual rule that bright colours in
worms when not due to structure are the result of
the shining through of coloured internal structures,
we find that the bright colours of the little Oligo-
chetes belonging to the genus #olosoma are. said
to be due to coloured oil-globules contained in the
skin (see Beddard, Ann. and Mag. Nat. Hist. vol. ix.
pp. 12-19). The oil-globules are in the different
species blue-green, yellow-greeen, or orange-red, and
may be of lipochrome nature.

As to the pigments themselves, if we bear in mind
the statement just made that, when bright, the colours
of worms are due in the general case to coloured in-
ternal tissues, it must be obvious that the pigments
are usually those which by hypothesis are of direct
physiological importance. Hzmoglobin, which is
widely and irregularly distributed in the group, is
one of these. It is often exceedingly conspicuous
in the more delicate forms; but even in the larger
and more opaque worms it may shine through the
thin-walled gills or tentacles. The green pigment
which occurs in the blood or ccelomic fluid of
Sabella, Branchiomma, Spirographis, and Siphono-
stoma, is another example. This pigment has been
accredited with important respiratory functions. It

v COLOUR-PHENOMENA IN WORMS 105

was called chlorocruorin by Professor E. Ray Lan-
kester, who stated that it is capable of existing in
an oxidised (green) and a reduced (red) condition.
Krukenberg denies this and regards the appearances
observed by Professor Lankester as due to the
marked dichroism of the green solution. The
solution exhibits a two-banded spectrum, and the
pigment is destroyed by strong acid or alkali.

In connection with the supposed respiratory
function it is perhaps worth notice that, according to
Paul Langerhans, the colour of the blood in Sadel/a
variabtlis may vary from a clear yellow-green to a
dark brown ; while it is an old observation that in
worms in general the colour of the blood varies
much, being very frequently different in the various
species of a genus.

Another interesting green pigment is that which,
as we have already noticed, colours the aberrant
worm Chetopterus. This pigment is of greenish
colour, and is confined to the walls of the anterior
part of the alimentary canal. Like bonellin and
“enterochlorophyll,” it gives a complex, exceedingly
beautiful spectrum which changes on the addition of
alkali or acid, and it in other respects shows a relation
to these two pigments.

Another green pigment of brighter tint but ap-
parently of simpler nature occurs in Eulalia viridis,
and probably in other forms. In Zvw/alza itself it is
apparently intermixed with a yellow pigment, but
in the eggs occurs pure. It is soluble in water
and alcohol, turns slightly blue with acids, and is
destroyed by strong alkali. It closely resembles
the green pigment of Zhalassema already mentioned.

106 COLOUR IN NATURE CHAP.

The other pigments of the bristle-bearing worms
are not well known; it is possible that some of the
pink pigments are lipochromes.

As to the colours of Polychetes in general, the
striking features are the presence of optical colours,
the number of green forms, and the absence of those
beautiful and elaborate markings which are so char-
acteristic of the leeches. The plan of the colora-
tion is throughout much simpler; when brilliant,
it seems to be usually dependent upon coloured in-
ternal tissues shining through the skin. A great
number of forms, moreover, especially those inhabit-
ing tubes, are colourless and transparent.

The colours of the Oligochztes, such as the earth-
worm, hardly merit separate notice: they are mostly
dull or inconspicuous, but in some cases show a
tendency to develop simple patterns. Of this tend-
ency our own brandling (Lumbricus fetidus) affords
a convenient example.

THE PIGMENTS OF THE CAPITELLIDA

The pigments of the small group of Polychztes
known as the Capitellidze are perhaps worthy of more
detailed notice. The Capitellide are the subject of
one of the large Naples monographs, and the author,
Professor Eisig, makes some observations on their
colours which have been very widely quoted. The
Capitellide are remarkable among Annelids in
having no closed blood-vascular system, the blood
being contained in the general body cavity. Ac-
cording to Eisig, it always contains hemoglobin, and
is of a bright red colour which directly affects the

v COLOUR-PHENOMENA IN WORMS 107

superficial coloration. The deposition of pigment
either in the cuticle or in the hypodermis is rare,
but in Capitella and Heteromastus granules and
droplets of yellow-brown pigment lie between the
cuticle and the hypodermis in patches. As the
nephridia contain similar granules, Eisig is of opinion
that these pigment patches are due to substances
excreted by the nephridia which do not reach the
surface but, lying in the skin, are got rid of at the
(hypothetical) moult. In Cafztella, in the head and
tail regions, the skin is of a red-yellow colour, which
is due to clusters of blood-discs containing excretory
particles again lying between the cuticle and hypo-
dermis. According to Ejisig these blood-discs, after
they have taken up excretory particles, lose their
power of circulating and stagnate at the areas in the
skin mentioned above. Further, Eisig considers that
the pigment is directly derived from the hemoglobin
of the blood, that it occurs in the nephridia in
association with guanin as one of the nitrogenous
waste products of the organism, and that it may
find its way to the skin and bristles and there be an
important agent in coloration. He therefore con-
cludes that pigment is always, or at least frequently,
in origin a waste product eliminated by the skin.
This is of course a position which has been advo-
cated or supported by many writers, and that such
a utilisation of waste product does occur has now
been abundantly proved, eg. in Lepidoptera ; that
it occurs in Annelids is eminently probable, and is
rendered more so by Graf’s observations on leeches,
to which we shall afterwards refer. At the same
time it is perhaps permissible to remark that it does

108 COLOUR IN NATURE CHAP.

not appear that any good end is served by the over-
hasty application of the term waste product to all
pigments occurring in the skin, even when such
pigments are periodically eliminated by means of a
moult. In human physiology the term waste pro-
duct has a perfectly definite meaning, and it is surely
desirable in the interests of scientific nomenclature
that a term implying a certain chemical composition
should not be loosely applied to unknown substances.
Then again the occurrence of a pigment in a struc-
ture which is periodically cast and renewed is not
absolute proof that the substance is useless or noxious.
Thus the cuticle of the Crustacea certainly contains
proteid, and, according to Krakow, also glycogen, and
glycogen is certainly used up in its formation, but
proteid and glycogen are not waste products.

THE COLORATION OF LEECHES AND THE ORIGIN
OF MARKINGS

With regard both to their pigments and to their
coloration, the leeches, in the case of the more
specialised forms at any rate, are sharply contrasted
with marine worms. The cuticle compared with that
of many worms is unspecialised, bristles are absent,
as are also structural colours. The smaller forms
may exhibit little pigment, but the differentiated
forms, like Hzvudo, are characterised by the develop-
ment of a large amount. This pigment is not
uniformly distributed, but is arranged, especially on
the upper surface, in definite lines and spots, which
give rise to a beautiful and complex style of colora-
tion. Any one who doubts the propriety of the word

v COLOUR-PHENOMENA IN WORMS 109

“beautiful,” as applied to it, is advised to examine the
plates illustrating Whitman’s Memozr on the Leeches of
Japan, where the native draughtsman has delineated
the colouring with a fidelity which Western artists
can only envy and admire.

The pigments of leeches are largely of the dark,
insoluble, and little-known type, but it is probable
that lipochromes are also often present. What part,
if any, the hemoglobin of the blood takes in colora-
tion is unknown.

On the origin of the pigment and markings of
leeches there is an exceedingly interesting paper by
Dr. Arnold Graf. According to this investigator
there are in leeches certain migratory cells, compar-
able to the yellow cells of the earthworm, which
arise from the endothelium of the body cavity,
receive waste products from the blood-vessels, and
carry these to the nephridia and so to the exterior.
The waste products received by these “excreto-
phores” are of the nature of fine dark granules,
which are capable of acting as a pigment. Graf
finds that all the excretophores do not reach the
neighbourhood of the nephridia, but that certain of
them penetrate the musculature of the body, and
come to lie immediately beneath the epidermis!
where the contained dark-coloured granules give rise
to surface coloration. He further states that the
number of the excretophores and the intensity of the
pigmentation increase with age, and when, as in
albino varieties, pigment is scarce, the excretophores
are also greatly reduced in number. The amount of
pigment appears to depend upon the intensity of
metabolism, being greatest in the most voracious

1IO COLOUR IN NATURE CHAP.

forms. When fed with food coloured with carmine
after a previous fast, it was found that the carmine
could be traced in the excretophores, in the nephridia,
and in the pigment cells beneath the skin.

In addition Dr. Graf makes some very ingenious
suggestions as to the origin of the characteristic
markings of the different species of leeches. Accord-
ing to him these depend primarily upon the arrange-
ment of the muscles of the body-wall; or, more
exactly, the visible coloration depends upon the
amount of resistance which the tissues offer to the
passage of the pigment-containing cells, the muscles
being the most important of the tissues concerned.
The muscles of the leeches are arranged. in three
layers, which from without inwards are the circular,
the diagonal, and the longitudinal layers. Each of
these layers consists of bundles of muscle-fibres, the
number of the muscles varying in different leeches.
According to Graf the pigment-containing cells can
pass outwards only in the spaces between the
bundles, so that the coloration depends upon the
number of these bundles, and this varies in the
different forms. Thus Wephelis quadrostriata has
five well-developed bundles of longitudinal fibres on
the dorsal surface, the circular and diagonal muscles
being less developed, and we find that it has four
well-marked longitudinal stripes, corresponding to
the spaces between these muscles. In Clepsine hol-
lensts, Whitman, on the other hand, the longitudinal
bundles are very numerous and relatively weak, while
the circular are well developed ; and here the surface
is spotted rather than striped, this being supposed
to be due to the stopping of the pigment cells at

v COLOUR-PHENOMENA IN WORMS III

regular intervals by the strong circular muscles, so
that the stripes which would be formed at the spaces
between the longitudinal muscles are interrupted.

These suggestions are exceedingly interesting and
ingenious, and of obvious importance in relation to
the origin of markings. In view of the origin of the
pigment from the blood, it would be very interesting
to know whether it is in any way derived from
hemoglobin, and also whether that transfer of pig-
ment from the gut to the surface which occurs when
the leech is fed with carmine, ever occurs during the
ordinary course of affairs.

The process described by Graf presents some
interesting analogies to that described by Ejisig for
the Capitellide. An inquiry into the causes which
determine the marked differences in colour between
the leeches and the Chetopoda would be very inter-
esting, but does not appear to have been attempted.

THE PIGMENTS OF THE POLYZOA, AND THE
ORIGIN OF PIGMENTATION

In connection with the colours of worms there
are a few points about the pigments of the Polyzoa
which are of interest.

The Polyzoa are aberrant worms, which form
colonies consisting of numerous polypides embedded
in cells or chambers, which, being united together,
constitute the ccencecium or substance of the colony.
The ccencecia of f/lustra, the sea-mat, are very
familiar objects on our own shores, where they are
often mistaken for seaweed. Each cell or chamber
with its contained polypide is known as a zocecium.

112 COLOUR IN NATURE CHAP.

As to structure, we recognise in each zocecium the
firm coating which surrounds and protects the con-
tractile polypide. The polypides themselves have a
ring of tentacles surrounding the mouth, and a well-
developed alimentary canal. The cavity of the
zocecium is largely filled up with the so-called
funicular tissue, which consists of a network of
branching cells, the meshes of the network contain-
ing numerous transparent connective-tissue cells,
which may be called leucocytes.

In certain forms, eg. in Bugula neritina, this
funicular tissue is coloured by pigment which varies
in tint and gives the colony a purple or yellowish-
brown colour. In other cases this tissue is quite
colourless. Krukenberg studied the pigments in the
case of Bugula neritina, and found that there was in
the first place an interesting colouring matter readily
soluble in cold water and glycerine, to which it gave
a rose-red tint. On account of the presence of this
pigment, dying specimens of this species, as was
observed by Cohn, coloured the water in which they
were found a deep purple. When the purple solu-
tions were kept for some time the tint changed to a
pale yellow, but the original purple could be restored
by shaking with air. The purple solution gave two
bands in its spectrum, which disappeared completely
when it turned yellow. On account of the readiness
with which it could be oxidised and deoxidised,
Krukenberg credited the pigment with respiratory
importance. Besides the red colouring-matter there
is also present in Bugula neritina a yellow pigment
apparently of lipochrome nature.

Besides these observations of Krukenberg’s on

v COLOUR-PHENOMENA IN WORMS 113

the characters of the pigments, there are some inter-
esting facts connected with the pigmentation disclosed
by a series of experiments made by Mr. Sidney F.
Harmer. Mr. Harmer’s experiments were made with
a view to determine the exact nature of the so-called
“brown bodies” of marine Polyzoa, which have been
supposed to be excretory:

These brown bodies arise roughly in the following
way. [Each polypide in the colony is only capable
of a relatively brief existence ; after a certain period
the organs, and especially the alimentary canal, be-
come degenerate and, fusing together into a mass,
form the so-called brown body which is found lying
inside the zocecium. The brown colour is due in
large part to masses of dark pigment which occur in
the cells of the alimentary canal. Contemporane-
ously with the formation of the brown body there
occurs in many forms a process of regeneration,
which results in the formation of a polypide bud,
and ultimately of a new individual. The brown
body is either eliminated by means of the gut of the
new polypide, or is simply stored up in the cavity of
the zocecium.

In studying these processes Harmer made a series
of experiments with solutions of various pigments,
such as indigo-carmine and Bismarck-brown. His
method was briefly as follows :—living colonies were
placed in sea-water containing the pigments in
solution ; after a short time they were transferred to
clean water and the distribution of the pigment in
the different tissues carefully studied. The results
obtained differed considerably according to the pig-
ment employed, but in general terms it was found

I

114 COLOUR IN NATURE CHAP.

that the leucocytes of the funicular tissue, the so-
called hepatic cells of the alimentary canal, and to a
less degree the cells of the funicular tissue itself, all
took up the pigment. When the polypides began
to degenerate, which perhaps occurred sooner on
account of the treatment, the leucocytes became
more or less aggregated round the brown body,
while the pigment of the alimentary canal took a
direct part in the formation of the brown body. The
new-formed polypide is entirely devoid of artificial
pigment, which is either stored up within the
zocecium or in part eliminated with the brown body.
The introduced pigments are thus eliminated from
the living tissues, the active agents being the three
sets of cells already named. The funicular tissue
took up pigment most readily in the case of
Bugula neritina, the species in which it is normally
pigmented.

Now as the alimentary canal of very young
individuals shows no pigment in its cells, and as
this pigment subsequently appears in gradually
increasing amount in cells which certainly excrete
introduced pigment, Harmer is of opinion that the
view that the formation of the brown body is an
excretory process is well founded, and that the
brown pigmentary substance is a waste product, or
at least a useless substance. The view is further
confirmed by the fact that indigo-carmine, when
introduced into the body cavity of other animals,
is excreted by cells which are certainly excretory in
nature.

It almost seems, however, as if we might go
farther than this. It appears that the leucocytes

v COLOUR-PHENOMENA IN WORMS 115

in Polyzoa have not any power of taking up pigment
from the so-called hepatic cells of the gut and carry-
ing it outwards, as they have, for example, in the
Capitellide, where carmine introduced into the gut
is carried outwards to the skin by the action of
leucocytes, Is it too daring a suggestion that the
mysterious process of regeneration in the Polyzoa is
due to the want of such a mechanism, and the con-
sequent choking of the important “hepatic” cells
with noxious substances? The drastic measure of
total reconstruction may thus be compelled by the
poisoning of essential cells.

This discussion may seem somewhat irrelevant
to our main subject, but it is in reality of much
importance in connection with theories as to the
origin of pigment. In Polyzoa the cells of the gut
are deeply pigmented with an apparently useless
substance, and there is no mechanism by which
leucocytes can carry away this pigment. In many
animals the cells of the gut, or the hepatic cells,
contain pigment, and the leucocytes have the power
of removing pigment from these cells to the skin or
to the exterior ; are we therefore justified in regard-
ing such skin pigments as waste products? This is
a question which we shall have to consider in some
detail later. The suggestion is one which has been
repeatedly made under various forms, and it is bound
up with some interesting questions in relation to
colour problems.

It should be further noticed that the purple
pigment of Bugula neritina disappears for a certain
period during the development of the young poly-
pides, and then reappears later. Harmer differs from

116 COLOUR IN NATURE CHAP, V

Krukenberg in considering that it is nutritive in
function, apparently regarding it as of importance
in the nutrition of the young individuals. This case
is of some interest, because it is so common to find
coexisting with the so-called excretory pigments
others whose function is unknown or doubtful, but
which are apparently not excretory.

It is also of interest to notice that the so-called
respiratory pigments, those which are capable of re-
peated oxidation and deoxidation, frequently occur
in organisms where it is exceedingly doubtful whether
they have really respiratory significance.

CHAPTER VI

THE COLOURS OF CRUSTACEA AND ECHINODERMA

Nature of Colours of Crustacea—Colour-variation—Pigments
of Crustacea: (1) Lipochromes, (2) Soluble Blues—Re-
lations of Two—General Characters of Pigments—The
Colours of Echinoderma : (1) Star-fishes, (2) Brittle-stars,
(3) Sea-urchins, (4) Crinoids, (5) Holothurians—Pigments
of Five Classes—Relations to those of Crustacea.

WE shall in this chapter discuss both the Echino-
derma and the Crustacea—not that there is any
morphological affinity between them, but because
there is considerable resemblance as regards pigments.
In both, moreover, there is a tendency for lime salts
to be associated with the coloured regions.

NATURE OF COLOURS OF CRUSTACEA

The coloration of the Crustacea presents many
points of very great interest; the beauty of the
colours, their adaptation to the surroundings, and
their great variety combine to render the subject
peculiarly attractive.

It is unnecessary to dwell upon the general

118 COLOUR IN NATURE CHAP.

characters of the Crustacea, but it may be well to
mention some points which are of importance in
reference to the coloration. In the first place, we
may note that the Crustacea, like all other Arthro-
pods, have a firm cuticle of chitin. This chitinous
coat is not infrequently impregnated with lime salts,
and is in most cases pigmented. The subjacent
epidermis almost always contains pigment, either in
special contractile chromatophores, or in solution in
the general cells, or more usually in both. In some
cases, as in the adult lobster, the epidermis is so
completely concealed during life beneath the thick
cuticle that it is of practically no importance as
regards coloration, while in other cases the cuticle
is thin and translucent, and it is the epidermis which
is important in the production of surface coloration.
The degree of calcification is the most important
factor in the production of opacity in the cuticle, so
that in general terms we may say that the shell
or cuticle tends to be transparent in small forms, in
those inhabiting fresh water, and in abyssal forms.

COLOUR-VARIATION IN CRUSTACEA

As aclass the Crustacea are remarkable for the
brilliancy of their colours, blue, green, shades of
orange, pink, red, and brown being all common
tints. Nor are the bright colours confined to the
higher forms; the tiny Daphnza forms vivid scarlet
patches on the surface of the sea, Diaptomus bacillifer
forms red patches in fresh-water lakes, and so on.
As a general rule, however, the colours harmonise
with the surroundings, this being especially true of

vt COLOURS OF CRUSTACEA AND ECHINODERMA 119

the smaller and more delicate forms. The resem-
blances between shrimps, sand-hoppers, shore-crabs,
etc., and the localities which they respectively haunt
are too patent to need emphasis, and phenomena of
this nature are exceedingly common in the group.
In many cases the colours vary in harmony with
the environment, but this can only occur in forms in
which the epidermis is the important agent in colora-
tion, and is then due to the sensitiveness of the
chromatophores. We find this in the shore-crab in the
young stage, and in many of the small sessile-eyed
Crustacea, such as /dotea and Caprella. The prawn
Hippolyte, according to M. A. E. Malard, is green on
green seaweed, brown on Fucus, red on red seaweed,
and transparent among Axtennularia and Sertularia.
Experiment in artificial environment showed that
this form is red in darkness, emerald-green in bright
light, and brown in semi-obscurity, which seems to
prove that it is the intensity of light rather than its
colour which is of importance in effecting colour-
change. Pouchet found that prawns (e.g. Leander
serrator) turned brownish-red in vessels with black
bottoms, and if then transferred to white vessels
became yellow, passing through a stage in which
they were bright blue. Pouchet stated that the
brownish-red colour was due to a combination of a
bright red colour due to chromatophores and a
soluble blue pigment. When the prawns were re-
moved to a white dish, the chromatophores slowly
contracted and allowed the blue colour to become
apparent. As the contraction proceeded the blue
pigment gradually disappeared, and the ultimate
yellow colour was produced by the contracted

120 COLOUR IN NATURE CHAP.

chromatophores only. This case is exceedingly
curious, but is confirmed in a striking manner by an
observation made on a prawn taken by the Adbatross
during deep-sea dredgings off the coast of Mexico.
This prawn (Benthesicymus tannerz) is usually of a
deep blood-red or crimson colour. In one specimen,
however, the abdomen was marked with spots of
blue on the second, third, and fourth abdominal
segments. The spot on the second segment was
partly blue and partly yellow, the line of demarcation
between the two colours crossing the segment
obliquely so as to produce a strikingly unsymmetrical
type of coloration. In view of Pouchet’s observations
mentioned above, Mr. Faxon in describing the
Crustacea of the Adbatross suggests, reasonably
enough, that this peculiar appearance is due to a
change induced during the passage from the ocean
depths to the surface. This suggestion leads on to
the curious fact that the blue and green pigments
are almost invariably absent from deep-sea Crustacea,
which are usually shades of red, often deep red, or
pink, but occasionally yellow or dead-white. When
blue or green colours occur they are almost always
confined to the eggs. As might be expected, many
have sought to explain this fact as due directly to
the absence of light at great depths. Without
entering at this stage into details on the subject, it
may be noted that such facts as that the fresh-water
crayfish occasionally appears in a full blue or in a
red variety, that the common lobster is sometimes
red in the living condition, that in Copepoda living
under similar conditions the eggs are sometimes red
and sometimes blue, and so on, suggest that the

vit COLOURS OF CRUSTACEA AND ECHINODERMA 121

phenomenon is not absolutely dependent upon a
single environmental factor for its manifestation.
The other suggestion that the absence of blue or
green colours among deep-sea Crustacea is due to
adaptation, because such colour would be invisible
in the dark abysses, is even less satisfactory as a com-
plete explanation of the common colour-variations
of the Crustacea, for there are many facts besides
those mentioned which suggest that there is a
necessary relation between the different colours.
The constant tendency visible in the group to
oscillate between red and blue can hardly be ex-
plained throughout by adaptation. As facts are
always more convincing than general statements,
we may add to the examples mentioned above an
account of the seasonal colour-change in Holopedzum
gibberum, one of the Daphnids, which shows clearly
the relation existing between red and blue.

This little form was studied by Professor Anton
Fritsch in the ponds of Bohemia, and was found to
be colourless in winter. Towards the end of May
the first trace of bright colour appeared in the shape
of a diffuse rose-colour, accompanied by a blue tint
in the neighbourhood of the mouth and two narrow
blue stripes at the sides of the abdomen. The diffuse
rose-colour was most common in dead individuals
lying at the bottom of the pond, but it also occurred
in living forms.

By the end of July brightly coloured individuals
were very numerous; in these the under surface of
the food-canal from the mouth to the end of the
abdomen was coloured blue, except at the base of the
third pair of legs, where there was a cluster of bright

122 COLOUR IN NATURE CHAP.

red cells. Only about 3 per cent had red spots on
the shells.

At the beginning of August the red spots on the
shell had completely disappeared, but the blue colour
with the two red patches still persisted. By the middle
of August the great majority of the individuals had
lost their bright colours, and by September the
minority was very much smaller. The few which
still retained any colour had the abdominal stripes of
a greenish-blue, with greenish spots at the bases of
the legs, at points which had previously been red.
The red spots on the shells in the early stage con-
sisted of red and blue cells intermixed. According
to Fritsch there is no reason to suppose that the
“decorative colours” have any sexual significance.

This description shows very clearly how often
blue and red colours are locked together, and how
red may disappear to be replaced by blue or green.
Quite similarly Dr. F. H. Herrick, in describing the
development of the American lobster, notes the
striking colour-variations among individuals, some
“being bright red, others greenish-blue, and others
pale blue or nearly colourless.”

Illustrations of this relation between red and blue
might be multiplied to any extent, and yet it should
also be noted that there is not infrequently much
constancy of coloration, at least in detail. The lower
surface of the penultimate segment of the chele in
Astacus nobilis is always a bright orange-red, although
the tints of the other parts of the body may show
considerable variation. Those who are in the habit
of dissecting Mephrops norwegicus must have been
struck by the constant bright red colour of the gullet,

v1 COLOURS OF CRUSTACEA AND ECHINODERMA 123

remarkable in an animal generally distinguished as
compared with its allies by a marked deficiency of pig-
ment. In general it may be said that the Crustacea
exhibit a marked tendency to vary in colour, and
especially to oscillate between shades of blue and
green on the one hand and of red and yellow on the
other; they at the same time often exhibit much
constancy in detail, and in deep-sea forms the blue
and green colours tend to disappear.

So far we have considered the two sets of colours
as if they were entirely distinct from one another, but
it is familiar to all that the lobster turns red when it
is boiled; in other words, the action of a large
number of agents, such as heat, alcohol, ether, dilute
acids, etc., upon the blue or green series is to convert
them into the red. The blue or cyanic series occur,
as we have seen, in solution, while the reds occur in
fixed anatomical elements——the chromatophores.
This difference in distribution has caused many to
contrast the two sets sharply, and to suppose that
the reddening of the lobster is due to the total
destruction of the blue pigment, which allows the red
to become visible. By others, and notably by Kruken-
berg, it has been maintained that the blue pigment
is a lipochromogen, which very readily undergoes
decomposition and then becomes converted into a
lipochrome. The following description is based on
observations of my own in the case of Astacus,
Hlomarus, and Nephrops ; for details reference should
be made to my paper on the subject.

124 COLOUR IN NATURE CHAP.

THE PIGMENTS OF CRUSTACEA

1. Zhe Lipochromes.—That the yellow, orange,
and red pigments of the Crustacea are lipochromes
was long since proved by the researches of Maly,
Krukenberg, and others. Further, the lipochromes
have been described as usually occurring in pairs—a
red and a yellow together, which can be separated by
the process of saponification.

A superficial examination of the colours of
Nephrops norwegicus, the Norway lobster, shows that
the epidermis is bright red, and the shell orange-red,
the colour of a boiled lobster. If, however, the shell
be decalcified with dilute acid, it entirely loses its
orange colour and becomes dull red—the colour of
the epidermis. The interest of this change lies in
the fact that while the boiled lobster, and those parts
of the lobster’s shell which are not blue during life,
are of a similar orange colour, the shells of deep-sea
Crustacea tend to show such colours as “ blood-red”
“deep crimson,” “crimson-red,” and so on, and are
usually not orange. Now the red lipochrome of the
lobster or crayfish when removed from the shell very
readily forms a combination with lime which is of
orange colour. The shells of the deep-sea Crustacea
contain little or no lime; it therefore seems to me
possible that the difference in colour between, say,
Nephrops and one of the deep-sea Crustacea is not
due to any difference in pigment, but only to the fact
that in the former the lipochrome occurs associated
with lime, and in the latter in the pure state. ‘This
is interesting, because Moseley during the voyage of

v1 COLOURS OF CRUSTACEA AND ECHINODERMA 125

the Challenger remarked on the fact that a bright
red pigment (which he called crustaceorubrin) should
occur in small surface forms like Daphnia, and
then reappear in the ocean depths. The probability
is, however, that this pigment is very widely dis-
tributed in the Crustacea, and that its apparent colour
depends upon the conditions under which it occurs.

As to the simultaneous occurrence of red and
yellow lipochromes in the Crustacea, there is no
doubt that by various agents, and especially in the
presence of heat, both red and yellow pigments can
be extracted from the skin and shell of the lobster
and crayfish. I am, however, much inclined to doubt
the existence of both these pigments in the living
condition. Certainly the yellow, if it exists, has
practically no effect upon the coloration. The epi-
dermis of the lobster yields to water a beautiful
bright red solution, with no trace of orange, and does
not on filtering leave behind any orange or yellow
pigment. Lipochromes as such are of course not
soluble in water, but the red ones dissolve very
readily in solutions containing albumen.

The red lipochromes, when in solution in water
containing proteid, are precipitated with the proteid
on boiling, but if alkali be added to the solution,
alkali-albumen is formed, and they are not precipitated
even on boiling. The fact that the red lipochromes
thus form compounds with alkalies which are soluble
in albuminous solutions, is one of considerable in-
terest to which we shall recur.

2. The Soluble Blue Pigments——These unstable
pigments constitute the cyanic series of Pouchet,
the lipochromogens of Krukenberg. They occur

126 COLOUR IN NATURE CHAP.

in the epidermis of Astacus but not of Homarus or
Nephrops, and in the shells of the two first but not
the last. From the epidermis of Astacus the blue
pigment can be readily dissolved by water, or better,
a dilute saline solution. From the shells of the
lobster or Astacus the blue pigment can be ex-
tracted by treatment with a dilute solution of
ammonium chloride, or better, with very dilute
hydrochloric acid. The solution from the epidermis
is a very bright Prussian blue; that from the shells
is usually paler, as it is more difficult to obtain a
concentrated solution in their case. We cannot
here discuss all the properties of this blue solution.
It may be sufficient to say that the blue colour is
very fugitive, and that the solution invariably
contains traces of albumen. Agents such as heat,
acid, alcohol, etc, which destroy the blue colour,
turn the solution pink. The pink colour is due
to the presence of the red lipochrome, which can
be so readily extracted from the epidermis. From
the reactions of the blue solution I have come to
the conclusion that the blue pigment is a compound
of the red lipochrome with a complex unstable
organic base perhaps derived from the muscle, and
possibly of the same nature as the so-called muscle
extractives. The usefulness of dilute saline solutions
in obtaining the blue pigment is probably due to the
fact that these solutions dissolve out some proteid,
and the lipochrome compound is soluble in these
solutions (see above). The compound is readily
destroyed by various reagents and then the red
lipochrome reappears.

Next as to the green pigments of the Crustacea.

v1 COLOURS OF CRUSTACEA AND ECHINODERMA 127

Green colouring - matters occur sometimes in the
shells of various Crustacea, but also very commonly
in the eggs both of shallow and deep water forms.
The ovarian eggs of the lobster are a bright green,
while the extruded eggs are a dark green. Both
yield a turbid green solution to water, which clears
up at once on the addition of ammonium chloride,
probably because this dissolves up proteid. If the
clear green solution be allowed to stand, it some-
times deposits orange-coloured drops of oil and then
becomes a clear blue. The blue solution gives all
the characters of that obtained from the carapace.
There is therefore reason to believe that the green
colour of the eggs is due to a combination of the
blue lipochrome compound found in the shell and a
yellow pigment dissolved in fat. The association
of this yellow pigment with yolk in eggs is, of
course, exceedingly common.

An interesting confirmation of the view here
propounded as to the origin of the green colour
of lobsters’ eggs is found in an observation by Mr.
Chiyomatsu Ishikawa. In studying the develop-
ment of Atyephira compressa, this author noticed
that the ova in the very young stages were pale
blue, but as the yolk developed they became green.
It would thus seem that we are justified in saying
that the green pigments of Crustacea are at least
in some cases produced by a mixture of the blue
lipochrome compound and a yellow pigment. From
the changes which the shell of the lobster undergoes
as the blue colouring- matter is removed, I am
further inclined to believe that the brown colours
which are not uncommon in the group are similarly

128 COLOUR IN NATURE CHAP.

produced by a mixture of the blue compound and
unaltered red lipochrome.

GENERAL CHARACTERS OF PIGMENTS

Looking at the pigments of Crustacea in general,
it is seen that the red lipochromes form the central
pigments of the group. There occurs in addition
a widely spread yellow pigment, which does not
apparently give the lipochrome reaction, and whose
relation to the red remains doubtful. It is also
uncertain whether some of those changes from red
to yellow which have been described, are or are not
due to an actual change of the red pigment into the
yellow. On the other hand, there appears no doubt
that the red pigment in itself or in its modifications
is instrumental in the production of most of the
colours of the Crustacea. When the shell contains
little lime or is very thin, the red pigment present
in the shell and in the underlying skin gives rise to
bright red or scarlet tints. When the shell contains
much lime it is often of an orange tint, and is then
possibly coloured by a combination of the red
lipochrome with lime. It is at least certain that
the lipochrome does form orange-coloured combina-
tions with lime, soda, etc, and the colour and
insolubility of the pigment in the orange-coloured
shell of, eg., Vephrops, suggest the presence of such
a combination there. This suggestion seems also
to explain the brightness of the tint in deep-sea
Crustacea where lime salts are virtually absent,
and in recently moulted specimens of the edible
crab before the development of the lime.

vi COLOURS OF CRUSTACEA AND ECHINODERMA 129

Again, the red lipochrome is apparently capable

of uniting with a complex organic base to form a
‘blue compound, readily soluble in solutions containing
albumen, which probably gives rise to the blue colours
of many Crustacea. When mixed with the yellow
pigment or with the red lipochrome, this blue
pigment gives rise respectively to green and brown
colours.

It does not seem as yet possible to explain the
absence of the blue and green colours from many
Crustacea, and especially from those inhabiting deep
water, but it may be noted that their continued
persistence in the eggs is not very remarkable. Yolk
is distinguished for the number of complex substances
which it contains, especially such bases as neurin ; it
may be that it is something of this nature which
forms the lipochrome compound.

From this description it is obvious that, in spite
of the multiplicity of tints in the Crustacea, there is
much uniformity of pigments—the lipochromes being
fundamentally important in coloration. It is of some
interest to note that the pigment called “ entero-
chlorophyll” occurs at most only in very small amount
and infrequently in connection with the digestive
gland of Crustacea. Dr. M‘Munn has described it
there in some cases, but seems to have never obtained
the complete spectrum ; in many cases it is certainly
absent, and is probably never of great importance.

THE COLOURS OF ECHINODERMA

The colours of Echinoderms are almost as
brilliant as those of the Crustacea, and are often of
K

30 COLOUR IN NATURE CHAP.

imilar tints. Some at least of the pigments are also
imilar, but those of the Echinoderms seem to be far |
nore numerous.

1. Starfishes—Among these yellow, orange, or
ed colours are exceedingly common. In an account
if the deep-sea Asteroidea collected by the “ Investi-
rator,” Dr. Alcock gives the colours in the fresh
ondition of 25 forms. Of these 18 were of some
hade of pink or red, 1 was jet black, 1 gray, I
wown, I orange, 2 red and yellow combined, 1
‘rellow and brown. The lipochrome colours thus
wedominate very largely. Out of the 25, 11 came
rom depths exceeding 1000 fathoms, and all of these
vere of some shade of red or pinkish-orange. There
s thus some evidence to show that in star-fishes, as
n Crustacea, the lipochrome colours are conspicuous
.t great depths, while shallow-water forms tend to
lisplay greater variability of tint. The same state-
nent can also apparently be made of brittle-stars,
nut the star-fishes do not usually extend to the depths
»ccupied by the former.

Among shallow-water star-fishes we may mention
Linckia levigata, in which the upper surface is a bright
Antwerp blue, the tube feet being chrome yellow.
This species inhabits the waters of the Barrier Reef
of Australia. Another species, ZL. mdlaris, also of
an intense blue colour, is found among coral reefs
n the Malay Archipelago. It is mentioned by
Kiikenthal as having upon it a parasitic Mollusc—
Capulus crystallinus—one of the bonnet-limpets, which
's of exactly the same blue colour. One can only
regret in these cases that exact chemical observations
on the spot were impossible.

v1 COLOURS OF CRUSTACEA AND ECHINODERMA 131

2. Ophiurotds—The brittle-stars resemble the
star-fishes in displaying very considerable brilliancy of
colour. According to Agassiz’s account the shallow-
water forms exhibit the greatest variety of colour,
being blue, green, red or yellow, while deep-sea forms
are more usually bright orange or red. Agassiz
(ii. p. 133) makes the interesting observation that
the colours of the deep-sea forms fade much more
rapidly in alcohol, than those of the denizens of
shallow water. This strongly recalls the conditions
already emphasised for Crustacea, and is probably
due in the same way either to a combination between
the pigment and lime, or to the want of penetration
of the alcohol into lime-containing tissues. The
brittle-stars not infrequently resemble their surround-
ings in colour; the common “Sand-stars” of our
own shores are good examples of this, and various
observers speak of the resemblances in colour between
the forms living among gorgonians and corals, and
their organic surroundings. This is, however, in
curious contrast with a statement made by Kiikenthal,
to the effect that forms living in the interstices of
coral-reefs are well protected by their surroundings,
do not need protective tints, and are therefore “mostly
of a blackish colour.”

3. Sea-urchins.—In these we have the same
difference in permanence between the colours of the
deep-sea and shallow-water forms as in the brittle- -
stars. The colours seem to be usually reddish, some
are violet or claret-coloured, others brown or orange.
A form (Dzadema setosa) which is found on coral-
reefs has five bright ultramarine blue spots arranged
round the aboral aperture, while Astropyga freuden-

132 COLOUR IN NATURE CHAP.

berg? has similar blue spots arranged in radiating
lines. In Dzadema the spots have been called eye-
spots. ‘

4. Crinoids—The Crinoids are remarkable in
showing great individual variation in colour. During
life the colours are usually brown, red, purple, violet,
green, yellow or white, but the curious form called
Flolopus is of a jet-black colour. Agassiz remarks
that the colours of the deep-sea and shallow-water
forms show remarkable similarity.

As to individual variation in colour Moseley notes
that during the voyage of the Challenger he found
specimens of Pentacrinus which were purple, and
others which were yellow or pale-coloured. Agassiz
noticed the same thing in both Pextacrinus and
Rhizocrinus, while Malard describes it in greater
detail for Comatula. Malard found a large number
of specimens of the rosy feather-star on a buoy near
La Hougue which were of three distinct colours,
violet-red, orange-red, and white and red with reddish
pinnules. Clinging to the feather-stars, Malard found
numerous specimens of the Crustacean Azppolyte,
and he states that “at least in the majority of
instances” the prawns resembled their neighbours
so much in tint that it was exceedingly difficult to
distinguish them. Malard regards this as an ordinary
case of colour resemblance, but it is also interesting
on account of the numerous analogies which exist
between the colours of the Echinoderms and the
Crustaceans. We find in the two groups not only
similar colours, but also a similar range of variation.
As we shall see, the variation in Crinoids is not
merely apparent, but is a question of pigment.

vi COLOURS OF CRUSTACEA AND ECHINODERMA 133

5. Holothuria—Of the external colouring of the
sea-cucumbers there is not much to say; as a rule
they tend to be dull and dark in tint. Black or
brown colours are not uncommon, while greenish-
violet or gray also occur. The relative dulness fof
the tints is curious in view of the fact that the
pigments are numerous and often brilliant.

THE PIGMENTS OF ECHINODERMA

1. The pigments of the star-fishes, as their colour
and their distribution indicate, are largely lipochromes,
and are probably very similar in nature to those
of the Crustacea. As in the Crustacea, the pig-
ments are not confined to the skin but occur also
in the internal organs and especially in the ovary.
M‘Munn (1883) describes hzematoporphyrin, on
spectroscopic grounds only, in the integument of
Astertas glactalis. He has also shown that entero-
chlorophyll occurs in considerable amount in the
digestive ceca. As to the nature of the blue and
violet pigments of star-fishes, there is much more
difficulty. Krukenberg describes a blue pigment in
Astropecten auranticus which is soluble in water, and
is readily turned red by the action of heat, alcohol,
and other reagents. The blue pigment does not
apparently affect the colour of the organism during
life, and the blue colour is only apparent when the
superficial lipochrome has been removed by alcohol.
Krukenberg regarded this blue pigment as being in
all probability identical with cyanein, the blue pig-
ment of jelly-fish. It does not seem to be the same
as the blue pigments of Crustaceans.

134 COLOUR IN NATURE CHAP.

2. As to the pigments of the Ophiuroids there is
even less to be said; they are probably for the most
part lipochromes. Points of interest about both
groups are first, as we have already noticed, the
tendency for the colours to be more intense in
deep-sea than in shallow-water forms, and second
the greater instability of these bright pigments.
There is reason to believe that this is due to the
same cause as the similar phenomenon in the
Crustacea, namely, that diminished power of secret-
ing lime which is characteristic of abyssal organisms.

3. The pigments of the sea-urchins are more
numerous and more difficult. Lipochromes probably
occur, but apparently do not markedly predominate
in the production of the surface coloration. The
pigment enterochlorophyll is apparently common
and present in considerable amount. Other pigments
have been described, but are somewhat imperfectly
known. As an example of the colour phenomena
to be seen in the group, we may take the common
Echinus esculentus. This is usually of a purplish
colour with green spines tipped with violet. The
perivisceral fluid and blood are a deep claret-colour,
and apparently contain both “ enterochlorophyll”
and a brown pigment called echinochrome. _ If this
claret-coloured fluid be exposed to the air it turns
bright green—the colour of the spines. The
change has been supposed to indicate the presence
of a respiratory pigment, but there is as yet no
certainty.

4. Among the Crinoids there is, as in the sea-
urchins, a marked tendency for the lipochromes to
be masked or replaced by more complex pigments.

vr COLOURS OF CRUSTACEA AND ECHINODERMA 135

Moseley discovered in several species of Pentacrinus
a complex pigment, which he called pentacrinin.
We have already mentioned the variations in colour
which his specimens presented. He found that the
purple specimens yielded a pigment which formed a
pink solution in acidified alcohol, the solution turning
blue-green with ammonia. Both solutions yielded
banded spectra differing in the two cases. Pale or
yellowish-coloured specimens, on the other hand,
yielded a green solution to alcohol, but the solution
turned pink with acid, and apparently contained the
same pigment as the purple specimens. Moseley
suggests that the reaction of these specimens was
probably alkaline during life, and that the pigment
was therefore present in its alkaline form. Curiously
enough Moseley obtained another specimen of Penta-
crinus in which the complex pigment was entirely
absent, and a lipochrome only was found. It is,
however, probable that the purple specimens con-
tained a lipochrome mixed with the pentacrinin.
The species of Aztedon show similar variations in
the nature of their pigments. In Axtedon (Comatula)
rosacea, Krukenberg describes yellow, red, and brown
pigments, all nearly related to one another, and all
soluble in water; they do not give banded spectra.
In another Amtedon Moseley found a purple pigment
which he calls antedontn. It formed in alcohol when
dilute a pink solution which turned orange with acid
and violet with ammonia, and gave banded spectra
differing in the different conditions. | These two
peculiar pigments, antedonin and pentacrinin, re-
semble in several respects the pigments of the
enterochlorophyll or chetopterin group, but have not

136 COLOUR IN NATURE CHAP.

been reinvestigated since Moseley’s work. Moseley
found antedonin also in a deep-sea Holothurian.

5. Among the Holothuria black or brown pig-
ments are widely distributed, and may be identical
with those occurring in the Crinoid Holopus and the
dark-coloured Ophiuroids. In addition, numerous
bright pigments occur, but are rarely important in
the production of surface coloration. Among them
lipochromes are probably common, especially in
internal organs. In Ocnzus brunneus, according to
M‘Munn (1889), the ovaries are pale blue, turning
red and yielding a lipochrome when treated with
alcohol and other reagents. This is an interesting
case, for it seems to be the only one mentioned in
the literature in which an identity with Crustacean
pigment can be definitely maintained.

RELATIONS TO PIGMENTS OF CRUSTACEA

Looking at the pigments of Echinoderms as a
whole, we must remark, first, the prevalence of lipo-
chromes, especially in the star-fishes and brittle-stars
where they seem to form the predominant pigments.
Again, the case just mentioned suggests that the blue
Crustacean pigments—the so-called lipochromogens
—are found also in Echinoderms, though there is as
yet no evidence to prove that they give rise to
external colours.

So far, there is an analogy with the Crustacea,
but we find further that the pigment called entero-
chlorophyll is common in the digestive czeca, in
the perivisceral fluid and probably elsewhere in
Echinoderms, while in Crustacea it is, at most, rare.

vt COLOURS OF CRUSTACEA AND ECHINODERMA 137

Associated probably with the presence of this
pigment we find numerous bright-coloured integu-
mentary pigments, which may be of the nature of
derivatives, as well as complex pigments like ante-
donin and pentacrinin whose relations are more
doubtful.

Another point of contrast with the Crustacea is
the prevalence of dark pigments, which, though most
characteristic of Holothurians, tend to appear more
or less frequently in the other groups.

CHAPTER VII

THE COLOURS OF THE LEPIDOPTERA

Insect Coloration and its General Relation to Physiology—
Pigments of Caterpillars—lIntrinsic and Derived Pigments
—Mr. Poulton’s Experiment—Meaning of Derived Pig-
ments—Other Characters of the Coloration of Caterpillars
—Colours of Butterflies, Pigmental and Optical Colours
—The Pigments of the Pieride—Pigments of other
Butterflies—Origin of the Pigments of Butterflies—Con-
trast between the Pigments of Butterflies and those of
Caterpillars—Pigments and Mimicry.

THE colours of insects are in many cases so con-
spicuous that they have always attracted much
attention, and of late years few biological subjects
have been more keenly debated than the meaning
and uses of the tints and markings of caterpillars
and butterflies, bees and flies. The frequent simi-
larity in colour between insects and their environment,
or between insects not nearly related, or on the
other hand the exceeding conspicuousness of some
well-protected insects, are facts which are obvious to
every one, and which have therefore attracted wide-
spread interest. Until very recently, however, this
interest has been chiefly confined to the external

cHaPp. vil THE COLOURS OF THE LEPIDOPTERA 139

aspect of the colours, and even now little is known
of their meaning to the organism in which they
occur.

We do not propose here to consider in detail the
characters of the colours of insects in their relations
to the habits of the species. The subject has been
most fully worked out for the Lepidoptera, and the
facts of the case, as well as the conclusions drawn,
will be found in Mr. Poulton’s Colours of Animals
and Mr. Wallace’s Darwinism. Here we are con-
cerned more with the proximate origin of colour
than with its ultimate justification.

The colours of insects are of especial importance
to the comparative physiologist on account of the
general tendency of the group to exhibit a life-history
divided into two sharply-contrasted stages: the
larval stage in which growth and nutrition are at
their maximum, and an adult stage in which these
are almost at a standstill, while the activities are
directed to the maintenance of the species.

As might be expected the colours of the two
stages are often very sharply contrasted. In the
majority of cases the colours of the larval stage tend
to be sober as compared with the often bright colours
of the adult, just as generally speaking the larva
may be called sedentary as compared with the active
imago. Further, since the cuticle of the larva is
usually little differentiated as compared with that of
the adult, and we have already considered the relation
existing between a differentiated cuticle and the

1 For an exceedingly interesting discussion of this and other points
connected with the physiology of insects, the reader should consult
Dr. David Sharp’s ‘‘ Insects” in the Cambridge Natural History.

140 COLOUR IN NATURE CHAP.

development of optical colours, it is almost unnecessary
to add that optical colours are almost always absent
from the larva, however brilliant they may be in
the adult. These statements refer especially to the
Lepidoptera where the contrast between the two
parts of the life-history is very marked. It may also
be noted that caterpillars are free living forms, more
or less completely exposed .to air and sunlight, and
we know that, whatever be the explanation of the
fact, this mode of life tends to favour the develop-
ment of pigment. We are thus justified in selecting
the Lepidoptera as a suitable group with which to
begin the study of the colour phenomena of insects.
Both stages of the life-history frequently display
beautiful coloration, so that the contrasts between
the pigments of larve and adults present themselves
here in the most vivid form.

PIGMENTS OF CATERPILLARS

Butterflies and caterpillars are such familiar forms
that we need not discuss their characters, either of
structure or of coloration, but may pass at once to
consider their pigments.

It is remarkable that in spite of the attention
which has been directed to the colours of the Lepi-
doptera very little is certainly known of the chemical
nature of the pigments of caterpillars, and this in
spite of the fact that, as has been already mentioned,
there are several painstaking researches on the
colouring - matters of butterflies’ wings. | What
information we have is mostly due to Prof. E. B.
Poulton, who has experimented especially on the

VII THE COLOURS OF THE LEPIDOPTERA 141

relations existing between larve and their environ-
ment. Mr. Poulton divides the colours of larve into
two classes: (1) colours due to pigments derived
from the food, and (2) colours due to pigments
formed by the larve. Of the chemical nature of the
pigments belonging to the second group we un-
fortunately know nothing; they appear to be usually
though not invariably dark in colour, and are de-
posited in the cuticle or in the epidermis. Their
stability and their insolubility in alcohol suggest the
possibility that they may belong to the same group
as the dark pigments found in the adult.

The pigments belonging to the first group are
green, yellow, or brown and are described by Mr.
Poulton as ‘“ modified chlorophyll” and xanthophyll.
Xanthophyll, as we have already explained, is the
lipochrome pigment which can be obtained from the
solutions of chlorophyll formed by steeping green
leaves in alcohol. The chlorophyll was recognised
in the larvz solely by certain resemblances between
the spectroscopic characters of the larval pigment
and those of green leaves. These pigments occur
primarily in the digestive tract, whence they seem to
reach the blood; while from the blood they may be
deposited in the subcutaneous connective tissues. In
most cases it is only when they occur in the last
position that they are important in producing
coloration. In his first paper (1885) Mr. Poulton
supported his position that these pigments were
derived from the food by a number of arguments,
as well as by the results of spectroscopic examina-
tion. More recently (1893) he has been able to
demonstrate experimentally that the pigments are

142 COLOUR IN NATURE CHAP.

absent in the larve when they do not occur in the
food.

Mr. POULTON’S EXPERIMENT

For the purpose of this experiment Mr. Poulton
obtained a large number of the eggs of Zxyphena
pronuba, the Yellow Underwing. Immediately after
hatching, the larvz were divided into three sets. The
first set was furnished with the yellow etiolated
leaves from the heart of a cabbage, the second with
the white midribs from the same leaves after the
removal of the whole of the blade, and the third with
the ordinary green outer leaves of the cabbage. All
the specimens were kept in the dark, in order to
avoid the risk of the conversion of the etiolin into
chlorophyll, and were only brought to the light to be
examined and fed. The larvz in the first and third
sets developed well and showed almost identical
colouring. In theearly stages they were mostly pale
green, but as development proceeded the colour
deepened to dark green, and before maturity was
reached most had turned dark brown. In addition
to the conspicuous green or brown pigment in the
connective tissue, or in the epidermis, the cuticle
showed spots and patches of dark pigment, which
were most conspicuous in the regions where the cuticle
was especially thick, such as the head, the true legs,
etc. This pigment is called by Mr. Poulton “true”
pigment, and as is shown by the condition of the
second set of larve is not dependent for its formation
upon the presence of pigment in the food.

The larvz in this second set developed badly and
out of a large number only one reached complete

vil THE COLOURS OF THE LEPIDOPTERA 143

maturity. This was probably due to the fact that
the cut mid-ribs dried up rapidly and rendered it
difficult for the larve to obtain sufficient food,
especially in the early stages when the mandibles
were weak. The larvze in this set were throughout
of a pale cream colour with no trace of green tint,
but with the cuticular markings quite distinct ; the
cuticular pigment being as usual most conspicuous
in the parts of the body where the cuticle was
thickest. The single larva which attained maturity
had, therefore, cuticular pigment developed as fully
as usual, but was otherwise quite colourless.

From this experiment Mr. Poulton draws the
natural conclusion that the green or brown ground-
colour of these larve is produced by modified pig-
ments derived from the food, and is dependent for
its production upon the presence of these pigments
in the food, while the dark cuticular pigment is not
so dependent. Further, as the colour of the larve
fed on yellow leaves is the same as that of larve fed
on green leaves, it would seem that the larve can
transform the yellow pigment into a green one.
The exact nature of this green pigment is still
uncertain ; it is unlikely that it is chlorophyll. The
brown colour is probably the result of an oxidation
of the green pigment. By developing control speci-
mens in the light, Mr. Poulton found that the only
difference produced by the absence of light was the
assumption by the larve of a brown ground-colour
before reaching maturity, while larve developed in
light usually remained green till maturity.

Although there has been relatively little experi-
mentation, there is, according to Mr. Poulton,

144 COLOUR IN NATURE CHAP.

considerable evidence to show that yellow, green,
and brown ground-colours in vegetarian larve are
all due to pigments derived from the food. The
pigments frequently colour the blood very markedly ;
it will be recollected that the blood in insects has
probably no connection with respiration, but is
hemolymph, primarily concerned with nutrition.
The blood of the pupz and adults is often likewise
green and coloured with these derived pigments,
but as yet there seems to be no instance described
in butterflies or moths in which these pigments
assist in coloration (a possible exception will be
discussed when we consider butterflies). They are
retained within the body, however, and not in-
frequently colour the eggs and thus the newly-
hatched larve. The break between larval and
adult coloration seems in Lepidoptera to be complete.
But lipochrome pigments at least are not always
absent in adult insects, for Zopf describes red and
yellow pigments belonging to this group in several
small leaf-eating beetles. In Lzxa populz, for example,
which lives on the leaves of the poplar, a red
lipochrome colours the wing-covers and part of the
abdomen, it is found in the secretion of the salivary (?)
glands, and occurs associated with oil-drops in the
ova. One is tempted to suppose that its presence
here is associated with the prolongation of the larval
diet into adult life.

MEANING OF DERIVED PIGMENTS

We must now proceed to consider what the
abundant occurrence of derived pigments in insects

VII THE COLOURS OF THE LEPIDOPTERA 145

may mean in the physiology of the individual.
Perhaps the following may not be thought to
transgress the bounds of legitimate speculation. It
is well known that the fats of different animals
are different both in their physical and chemical
characters. Now it has been found by experiment
that if an animal A be fed to excess with the fat
of another animal B, and the body of A be subjected
to subsequent examination, then the fat deposited in
it will not exhibit the normal characters of the fat
peculiar to the animal, but will partake more or less
of the characters of the fat of the food. In other
words, an animal is unable to impress its own
individuality on the fat of its food, if this be ingested
in excessively large quantity. Now we have seen
that in all probability the derived pigments consist,
at least in great part, of lipochromes, and we know
that in most cases the lipochromes tend to occur in
association with fats ; in chlorophyll the presence of
fat has indeed been directly affirmed. Is it not
possible that in the caterpillar—a notably voracious
feeder—a process occurs quite similar to that described
above for mammals? That is, may not caterpillars,
which have a practically unlimited food-supply, be
unable to completely assimilate all the fat ingested,
but yet have the power of storing up in their tissues
this extra fat, and with it the pigment with which it
is associated in the food? The process would thus
be very similar to that ‘which occurs in the salmon,
where, as we have seen, both fat and pigment are
transferred from the muscles to the ovaries during
the period of fasting. If the assertion that in the
salmon the pigment is derived from the food is correct,
L

146 COLOUR IN NATURE CHAP.

the two processes would be very closely analogous.
According to this theory, food containing lipochrome
pigment is richer diet than food without it, and leads
to the deposition of extra reserves in the tissues and
indirectly to additional pigmentation. The facts
observed by Mr. Poulton that the pigments are
frequently found associated with fat, and tend to
occur in the connective tissues, seem to support this
suggestion. It is also not inconsistent with the
observed fact that the pigments remain within the
body of the moth, and may be used in the formation
of the eggs and so passed on to the second genera-
tion. As nutrition in the butterfly is unimportant,
there is reason to believe that the caterpillar must
provide the nutritive substance subsequently employed
in the formation of the yolk. Yolk is usually remark-
able for the considerable amount of fat which it
contains, and the lipochrome may be simply trans-
ferred with the fat.

OTHER CHARACTERS OF THE COLORATION OF
, CATERPILLARS

As to the other characters of larval coloration,
Mr. Poulton describes cases in which colourless
bands of fat give the appearance externally of white
stripes. It is also of interest to note that the blood
of the larve, when exposed to air, oxidises and
becomes dark-coloured or black. It is possible that
some such oxidation process accounts for the deepen-
ing in tint of the true or cuticular pigment when the
larve are exposed to the air. ,

We do not propose here to consider the question

VII THE COLOURS OF THE LEPIDOPTERA | 147

of variable coloration, or the nature of the adjust-
ments by which it is rendered possible. Those who
are interested in the question will find it discussed
in numerous other popular books besides those
already mentioned. It is, however, of some interest
for our purpose to note that, according to recent
research, it is not so much the colour of the environ-
ment which directly affects the larve as the intensity
of the light (see Garbowski). In other respects also
some of the first crude statements made on the
subject are being modified. It may, indeed, be
accepted as a general truth in biology that when-
ever a series of phenomena seem to be susceptible
of an extremely simple and beautiful explanation,
that explanation is wrong and founded on an im-
perfect acquaintance with the phenomena in ques-
tion. All recent progress in biological theory has
shown that life is not simple but complex, and has
involved a substitution of extremely complex theories
for simple ones.

THE COLOURS OF BUTTERFLIES

In butterflies the colour-phenomena are much
more complex than in caterpillars, for while in the
latter the colour is a direct pigmental effect, in the
former the two factors of structure and pigment occur
in combination. In both kinds of colour in butter-
flies the important elements are the scales of the
wings. These are outgrowths of the chitinous
cuticle, and consist of a double membrane ; the outer
membrane is frequently much differentiated, and
may display rows of blunt projections, which are

48 , COLOUR IN NATURE CHAP.

thought to be of importance in colour-production.
The two membranes are connected by bridges of
chitin, and pigment granules may be deposited both
in these bridges and in the outer membrane.

Having regard to the small size of the scales, it
will be readily understood that there is frequently
great difficulty in determining whether a particular
colour is due to pigment or structure; the fact that
a scale showing structural colour frequently contains
pigment in addition further complicates the matter.
We thus find that there is much difference of opinion
among observers as to the cause of particular colours.
This occurs especially in the case of dzchrozc scales,
those displaying one colour by transmitted light and
the complementary colour by reflected light. Some
regard this effect as purely structural, others main-
tain that the colours are due to dichroic pigments.
Making due allowance for difficulties of observation,
it seems, however, certain that blue in butterflies is
always a structural colour, while green, black, and
white are at least usually due to structure. Other
of the structural colours are readily recognised by
their metallic brilliancy and changing glow, which
give to the butterflies possessing them an appear-
ance of surpassing beauty. Many will recall Mr.
“Wallace’s description of how his heart beat fast
and his brain reeled when his perseverance was
rewarded, and he captured with his own hands
one of the finest of these living gems in the Malay
Archipelago.

In connection with these colours Urech (1893)
notices one little point of some interest. He found
that in some cases the scales display under the

VII THE COLOURS OF THE LEPIDOPTERA 149

microscope a wonderful play of colours which is
quite invisible to the naked eye. He suggests
reasonably enough that it is in no respect im-
probable that these colours may be visible to other
insects although not to us except by the aid of
optical instruments, and that therefore they may
quite alter the appearance of the insects as seen by
other butterflies. Such observations are of interest
as tending to exercise a check upon the purely sub-
jective treatment of questions of colour-resemblance.

THE PIGMENTS OF THE PIERIDZ

As an introduction to the discussion of the pig-
ments of butterflies we may take Mr. F. Gowland
Hopkins’s laboriously patient work on The Pigments
of the Preride.

The Pieridz include a large number of butterflies,
among which the most familiar are our common
Cabbage Butterflies or Garden Whites; they are
very widely distributed, and are said to exhibit in
a striking manner the phenomena of mimicry. In
America especially they are said to mimic very
closely the (by hypothesis) well-protected Heliconide.
The Pieridz exhibit a relatively simple plan of colora-
tion ; optical colours are absent or little developed ;
and we shall see later that this greatly limits the
possible colouring. In fact we find that the colours
are mainly white, yellow, and black. The yellow,
in accordance with a rule which is exceedingly pre-
valent among butterflies, may deepen into orange or
red, this occurring most frequently in species from

150 COLOUR IN NATURE CHAP.

warm climates. The Heliconide exhibit similar
types of colour-schemes.

In the Pieride the pigments, according to Hop-
kins (1896), occur in three ways—(q) first uniformly
distributed through the chitin, apparently of the
outer layer of the scale: the black and brown pig-
ments occur in this position ; (4) in granules between
the two layers of scales, according to Hopkins—that
is, probably in the bridges of chitin already described :
white, yellow, and red pigments occur in this posi-
tion ; (¢) between the two chitinous lamelle of the
wing: a green pigment is described in this position.

The black and brown pigments, as elsewhere in
butterflies, are characterised chiefly by their insolu-
bility and their stability ; they are not attacked by any
agents which do not attack chitin. They are possibly
the same as the dark cuticular pigments of the larvae,
and may have some connection with the substances
which occur in the blood and oxidise rapidly when it
is exposed to the air. In the Pieride they are im-
portant in directly giving rise to the dark patches
and markings on the wing, but in many butterflies
they probably aid in the production of structural
colours.

The white pigment of the Pieridz is, as we have
seen, uric acid itself. It can be extracted from the
tissues by dilute alkaline solutions, which yield a
copious precipitate on acidification. The precipitate,
if treated with nitric acid and heated, leaves a
residue which turns purple with ammonia — the
murexide reaction, so characteristic of the uric-acid
group of substances.

The yellow or orange pigment is readily soluble

VII THE COLOURS OF THE LEPIDOPTERA I51

in hot water, forming a bright yellow solution with
a green fluorescence. The pigment is insoluble in
organic solvents, is acid in reaction, precipitates on
cooling the watery solution, and gives the murexide
reaction. It thus belongs to the uric-acid group, and
has been called lepidotic acid by Hopkins. As
already stated, it occurs as one of the ordinary
waste products of the organism, at the time of the
emergence from the pupa. It is a derivative of uric
acid, and can be artificially produced by heating uric
acid with water in sealed tubes to a high temperature.
It is somewhat interesting to note that by heating
the yellow pigment with dilute acids in a water-bath,
Hopkins obtained a purple colouring-matter, yielding
a spectrum with two bands. This pigment does not,
however, appear to occur naturally, unless the purple
tips of the fore-wings of Axthocharis zone be due to it.
Although uric and lepidotic acids frequently occur
in the same wing, Mr. Hopkins believes they never
occur in the same scale; pale yellow patches are
due to a small amount of pigment in the scales and
not to an intermixture of the two acids. Orange
patches are due to a large amount of the yellow
pigment, but red ones are due to the presence of a
red pigment. They yield their pigment to hot
water, but the aqueous solution is yellow. On
evaporation this solution yields, however, a red
residue. The residue gives the murexide reaction,
and also yields the purple substance, lepidoporphyrin,
just as the yellow does. Mr. Hopkins (1892) is of
opinion that the difference between the red and
yellow is either purely physical or due to the
association of the yellow acid body with a base.

152 COLOUR IN NATURE CHAP,

The only. other pigment found in the Pieride is
the green one descrtbed by Mr. Hopkins as occurring
between the laminew of the wing. This is soluble
in cold water, contains iron, and gives a spectrum
with a single well-marked band in the red; it is
perhaps a blood pigment, and is common among
the Pieride. It, however, rarely affects the surface
coloration, except in the male of Nepheronzca lutescens,
where the blue ground-colour is said to be due to
this pigment shining through the colourless scales.

In general, therefore, the pigments of the Pieride
are few in number, and the colours are usually due
to simple pigmental effects. The pigments are uric
acid, lepidotic acid and that modification of it which
forms the red pigment, the dull black and brown
pigments, and the rare green pigment. These pig-
ments rarely occur intermixed, and structural colours
are typically absent.

PIGMENTS OF OTHER BUTTERFLIES

With regard to the pigments of other butterflies
there is much less certainty. Hopkins has not
succeeded in obtaining the murexide reaction outside
the limits of the Pieridze, and is inclined to think
that the pigments of the uric-acid group are confined
to this family. Urech, on the other hand, assumes
that the pigments of butterflies in general are modi-
fied waste products, which Mr. Hopkins also admits
as possible. The fact that almost all the bright-
coloured pigments are soluble in hot water, as well
as some other reactions, seems to support this view.

Among butterflies in general the following colours

VII THE COLOURS OF THE LEPIDOPTERA 153

are wholly or in part due to pigments deposited in
the tissues :—white (in part), yellow, red (at least in
part), very rarely green, brown, black (in part).
White as a pigmental colour is probably rare outside
the Pieride ; it often exhibits a changing iridescence
like that of mother-of-pearl, and is then structural.

In the Marbled White (Avge galathea), an insect
with a simple colour-scheme in black and yellowish
white, a white pigment occurs which turns bright
yellow with alkalies. Of yellow pigments in butter-
flies not included in the Pieridz very little is known ;
none give the murexide reaction. Many red pig-
ments, on the other hand, give a reaction the same
as that displayed by the red pigment of the Pieride.
That is to say, it has been noticed by several
observers that in many cases scales of a bright red
or scarlet colour yield a yellow solution when treated
with hot water, but if the solution be evaporated to
dryness a ved residue remains. Similarly, the red
colour is turned yellow by the application of acid,
but the red colour may be restored by the addition
of ammonia. This is the so-called “reversion effect ”
of Mr. Perry Coste, and strongly recalls the relation
described by Mr. Hopkins as existing between the
red and yellow pigments of the Pieride. Red
pigment showing this peculiar character occurs, for
example, in Dezlephila elpenor. The phenomenon is
of common though not universal occurrence, some
reds being quite unchanged by acids.

Green in butterflies presents many difficulties.
It may be entirely structural, and arise by surface
markings or by the superposition of scales, as in
the species of ematozs (Spuler). Again, from the

154 COLOUR IN NATURE | CHAP.

green scales of Papilio eurymedes, Urech extracted a
yellow pigment which was almost insoluble in water,
but which dissolved readily in hydrochloric acid.
He adds as a note, however, that the scales retained
their green colour after treatment with acid and
ammonia. It is almost impossible to doubt that in
this case the green colour is structural, the part
played by the yellow pigment being uncertain.
Further, he found that the green scales of Thecla
vubt are yellow by transmitted light, and almost
colourless when the light falls from the base of the
scale upwards, while to hydrochloric acid they yield
a yellow pigment. These two cases seem to suggest
that in butterflies, as in birds, green may be pro-
duced by a combination of a yellow pigment and
a structural modification. On the other hand, from
the green scales of Sphinx nevec Urech extracted a
pigment which was slightly soluble in water and
readily soluble in acid and ammonia. Of the three
solutions the first was greenish-yellow, the second
orange-yellow, and the third green. The addition of
ammonia to pigment turned yellow by acid restored
the green colour. This fact would suggest that the
green pigment of Sphimx merei is derived from a
yellow, in much the same way as is the red pigment
of Detlephila. These facts suggest the possibility
that a yellow acid body in some way related to
lepidotic acid occurs in butterflies outside the Pieride,
capable of acting itself as a pigment and also of
giving rise to red (or green?) pigments.

Urech (1890) has made numerous observations on
the relation between the colour of the urine in butter-
flies and the prevalent colours of the scales. His

VII THE COLOURS OF THE LEPIDOPTERA 155

table seems on the whole to confirm the idea that a
relation exists between the two, and therefore that
the pigments are waste products, but observations on
colour alone are perhaps not of very great value.

The dark pigments, as we have already seen, are
probably identical in all butterflies; a pure black
colour is, however, in most cases due to the sculptur-
ing of the surface.

In general, although the evidence is still incom-
plete, we seem justified in believing that the pigments
of butterflies are either modified waste products or
the dark pigments. From the scales of a large
number of butterflies Urech did not succeed in a
single instance in obtaining any pigments which were
soluble in alcohol, benzol, or the other common
organic solvents. That is, if we may trust his
results, lipochromes are absent in the adult. This
is interesting, not only because of the prominence
of these pigments in the larve, but because they are
so important in coloration in animals nearly related
to the insects. In Crustaceans, for example, as we
have seen, the lipochromes colour the shell, the skin,
and the ova. We have Mr. Poulton’s authority for
saying that even in Lepidoptera lipochromes still
colour the ova, though here they are probably
derived pigments and no longer native, but they
have in the adult entirely disappeared from the
cuticle and from the skin so far as external colora-
tion goes, being replaced by modified waste products.
As a probable exception we may notice the green
pigment found by Hopkins in the Pieride ; this must
surely be one of Mr. Poulton’s derived pigments !

156 COLOUR IN NATURE CHAP.

ORIGIN OF PIGMENTS OF BUTTERFLIES

Let us now consider the meaning of this utilisa-
tion of waste products, and first of all their immediate
origin. Urech suggests that they arise from the
breaking down of the substance known as nuclein,
which has recently been rising in importance in the
eyes of physiologists. Nuclein isa complex substance
forming a considerable part of the nuclei of all living
cells ; itis derived from albumen, from which it chiefly
differs in containing a large amount of phosphorus.
According to Urech the nuclein of the leucocytes in
butterflies undergoes a process of breaking down,
yielding the nuclein bases, that is, such waste products
as uric acid, guanin, adenin, hypoxanthin, etc., and
also phosphoric acid and albumen. The nuclein
bases then become further modified into the pigments.
The different colours of the pigments Urech describes
as due to an increase in the molecular weight of the
substances, the yellow pigment being thus simpler
chemically than the red one.

Mr, A. G. Mayer gives a somewhat different
account of the development of the pigment of the
scales in butterflies. We may consider first his
account of the development of the scales them-
selves, which is interesting in several respects, and
is illustrated by some very good figures. These
show clearly that the scales develop from prolonga-
tions of. certain formative cells of the epidermis,
which project outwards beyond the level of the other
cells, The scales at first contain protoplasm, which
is continuous with the protoplasm of the formative

vil THE COLOURS OF THE LEPIDOPTERA 157

cell, but as development proceeds the protoplasm
is withdrawn, leaving the scale empty; the forma-
tive cell also undergoes a process of degeneration.
The scale itself, of course, consists of chitin formed
apparently at the expense of the original protoplasm.

Mr. Mayer’s account of the formation of the
pigments is as follows:—After the protoplasm is
withdrawn from the scale, this for a time is entirely
empty and contains nothing but air; a little later
the hemolymph of the pupa enters it and gives it
an ochre-yellow colour. The fresh hemolymph is
of an amber colour, but when shed it speedily turns
a turbid ochre-yellow colour, the same colour as that
manifested by the lymph of the scales. After re-
maining for about twenty-four hours in the ochre-
yellow stage, the scale begins to acquire gradually
the characteristic colours. Mayer is of opinion that
this development proves that the pigments are formed
from the degenerating hemolymph contained in the
scale. He endeavours to prove his position by
recounting some observations on the reactions of the
hemolymph to chemical agents. Thus “warm con-
centrated nitric acid” gives a “ chrome-yellow ” colour
changing to “reddish-orange” with ammonia, the
colour being very like that of a pigment-band on the
wing of the moth experimented with (Sama cecropfia).
In what respect, however, the colour differs from that
always given by proteids when treated with these
reagents—which of course constitute the xantho-
proteic reaction—is not mentioned. Although the
reagents employed by Mr. Mayer were all too
powerful for us to lay any stress on the chemical
side of his work, yet on the histological side there

158 COLOUR IN NATURE CHAP.

are several points of great interest. Thus he notices
that in the case of the larger scales it is not infre-
quent for leucocytes to enter the cavity of the scale
and there disintegrate. These leucocytes apparently
originate from the cells of the fatty body, a structure
which has probably much to do with excretion in
insects. This passage of “ wandering cells” outward
to be deposited in the superficial structures is a
phenomenon which we shall frequently have to
mention in connection with pigmentation. Not
infrequently, as in the leech, the cells are themselves
pigmented, and deposit their pigment in the skin.
It would be a fact of some interest if Urech’s
hypothesis is correct, and these cells by their
disintegration be found to give rise to the peculiar
pigments of butterflies. It is of course quite possible
that Mayer is also right, and that the hemolymph
does give rise to certain of the pigments, especially
those of relatively dull tints. As, however, it is
the bright-coloured waste products which are most
important in butterflies, we must return to our
consideration of these. Urech’s suggestion is of
much interest so far as it goes, but we require to
know why this decomposition of nuclein should go
on in the adult and not in the larva. In attempting
to answer this question we must consider the differ-
ences between larva and adult, and the physiology
of the state which lies between the two.

VII THE COLOURS OF THE LEPIDOPTERA 159

CONTRAST BETWEEN PIGMENTS OF BUTTERFLIES
AND CATERPILLARS

We have already considered some of the
characters of the caterpillar—how it eats far in
excess of its immediate requirements, how it is rela-
tively sluggish, and remarkable for its rapid growth.
There comes a period, however, when the caterpillar
ceases to eat, ceases to grow, becomes absolutely
sedentary, and passes into the pupa stage. Here
within its pupal covering fundamental changes take
place ; there is first an extraordinarily thoroughgoing
destruction of tissue and then an equally thorough-
going reconstruction. Now if we turn to higher
forms whose physiology is well known, we find that
such extensive changes in tissues are associated with
a large and increased production of nitrogenous waste
products. It is natural to conclude that this also
occurs in the pupa. The butterfly is built up by
the aid of the reserves stored by the caterpillar,
but in the course of the reconstruction there must
be an enormous production of waste substances.
Unfortunately we know relatively little of the
physiology of insects, but there is at least one
interesting research by M. L. Cuénot. This
naturalist worked at the Orthoptera and found
that besides the Malpighian tubes, the familiar
excretory organs of insects, certain cells of the
fatty body stored up waste products. In the
Orthoptera — cockroaches, locusts, grasshoppers,
etc.—waste products are apparently not employed
in coloration; and Cuénot found that throughout

160 COLOUR IN NATURE CHAP.

life the fatty bodies contained deposits of urates—
that is, salts of uric acid——-which were apparently
never eliminated, not even at the moment of
emergence from the pupa. In the Lepidoptera, on
the other hand, which are more perfectly organised
animals, we find that these useless waste products
are perhaps in part eliminated at the time of the
emergence from the pupa state, and in part utilised
as pigments. The wings are non-vital parts of the
body, at a distance from the essential organs; we
can therefore readily understand that associated
with the advance in structure from the Orthoptera
to the butterflies we should have a removal of
waste products from the fatty body, to be in part
eliminated, in part stored up in the wings. Further,
if in vertebrates some of the most important advances
are associated with the development of a more efficient
kidney, if the steps from fish to snake and from
snake to bird are accompanied by a diminished
deposition of waste products in the skin, is it not
possible that a similar process occurs in butterflies ?
That is, may not the occurrence of a large amount of
bright pigment in the wings of a butterfly indicate
a relatively low degree of specialisation? We have
seen that in the Pieridz structural colours are
practically absent, while pigmental colours are
vivid and striking. When, however, we pass to
such families as the Lycenide and the Apaturide,
we find that pigmental colours decrease in im-
portance, and structural colours are responsible
for the major part of the effect. Is it not possible
that this marks a genuine advance?

VII THE COLOURS OF THE LEPIDOPTERA 161

PIGMENTS AND MIMICRY

Mr. Hopkins notes in relation to the question
of mimicry that the pigments of the Heliconide
are not the same as those of the Pieride, for though
soluble in water they do not give the murexide
reaction. He regards this as evidence against the
views of those who hold that the resemblances of
mimicry may be due to relatioriship between the
forms. We do not propose to discuss the question
of mimicry here, but may just note that mimicked
species usually display what are known as “ warning
colours,” which apparently, in butterflies at least, are
always pigmental colours, or the simple types of
optical colour like white and black. Now as there
are in butterflies, apart from structural colours, only
about three pigments to choose from (yellow, red,
brown, and rarely green), the range, if we may
so speak, is somewhat limited. Apart from the
question whether the presence of the yellow and
red pigments in considerable amount is not a sign
of little specialisation, the frequent absence of
structural colour from these “warning” forms is at
least probably such a sign. Instead therefore of
supposing that the Heliconide have, in Mr. Wallace’s
words, “acquired lazy habits” and a slow flight
because they are uneatable, and the Pieride because
they resemble the Heliconide, may we not rather
suppose that the slow flight and “warning” colours
in both cases are due to the same cause, the
relatively low organisation which renders pigmenta-
tion by waste products possible, which makes brilliant

M

162 COLOUR IN NATURE CHAP. VII

optical colours impossible? The resemblance between
“mimicking” and “mimicked” forms is, of course,
not wholly explained by the small number of pig-
mental colours which can appear in a butterfly devoid
of optical colours, for the resemblance is often as
much in markings as in actual colour; but if we deny
the “mimicry,” we must, apparently, fall back upon
those “laws of growth” which many, perhaps justly,
find so unsatisfactory.

CHAPTER VIII

THE COLOURS OF INSECTS IN GENERAL AND OF
SPIDERS

Optical and Pigmental Colours in Insects—Colours of the
different Orders of Insects—Colours and Pigments of
Orthoptera—Optical Colours of Neuroptera—Coloration
of Beetles—Pigments of Hemiptera—Colour Resem-
blances between Diptera and Hymenoptera—Contrast
between Coloration of Lepidoptera and other Insects—
General Aspects of Insect Coloration — Variation in
Colour: (1) Natural, (2) Artificially produced — The
Colours of Spiders : (1) Optical, (2) Pigmental—Develop-
ment of Colour—Variation in Colour—Colours of the
Sexes—Markings of Spiders.

AMONG insects other than butterflies the colours
are often striking and beautiful, and are again
divisible into pigmental and optical. Probably,
as a rule, when an individual exhibits brilliant
colouring, this is mainly due either to pigment
or to structure, the two types of colouring being
rarely both conspicuous. at the same time. But
this is true only of the bright pigments, the dark
being common in insects displaying much structural
colour. This is due to the fact that the dark pig-
ments are present in excess only when the cuticle

164 COLOUR IN NATURE CHAP.

is well differentiated, and this is also the condition
necessary in the general case for the development
of optical colour. Mr. Poulton speaks of larval
Lepidoptera exhibiting the dark cuticular pigments
in the places where the cuticle is thickest decause the
epidermal pigments would be useless in these spots,
the cuticle being too thick to allow them to shine
through ; a comparative survey, however, makes it
more probable that there is a direct association
between the pigment and the thickened cuticle.
An excellent example is afforded by the dark
colours of many beetles with their thickened elytra,
which form a marked contrast to the browns or greens
of the Orthoptera with their relatively little developed
cuticle.

Another important point about the colours of
insects is the relation of food to colour. Mr. Poulton’s
experiments on caterpillars have shown that the pig-
ments of the food may find their way to the blood
or to the tissues, and owing to the thinness of the
cuticle be instrumental in producing the typical
coloration. The prevalence of green colours among
herbivorous forms with little developed cuticle, ey.
the Aphides or plant-lice, certainly suggests that
this also occurs elsewhere. The question has been
considerably confused by the common habit of
calling such derived pigments chlorophyll. In the
Crustacea green pigments occur, eg. in Vzerbius,
which there is little doubt are merely combinations
of lipochromes with other substances, perhaps bases,
and there seems no reason why similar combinations
should not occur in insects, with the aid of the
derived pigments. At the same time there is no

VIII THE COLOURS OF INSECTS IN GENERAL 165

apparent reason why insects should not themselves
produce lipochromes, and why such _lipochromes
should not occur in the cuticle as in the Crustacea.
We have seen that lipochromes occur in the elytra
of Lina populi ; according to Krukenberg they perhaps
also colour the red lady-birds (Coccionellz) and some
other red beetles, but whether these are intrinsic or
derived pigments remains undecided.

COLOURS OF THE DIFFERENT ORDERS OF
INSECTS

It is interesting to observe the types of colouring
prevalent in the more important orders of insects.
Thus in the ORTHOPTERA optical colours seem to be
practically absent, and deep black is also rare; shades
of brown, green, and red are perhaps the commonest
colours, as notice the brown “stick” insects, the
brown and green (or sometimes red) grasshoppers
and locusts, and so on. Associated with these
peculiarities of colouring we notice the fact that the
cuticle is in most cases but slightly developed. The
relatively sober tints do not, however, prevent the
Orthoptera often displaying great beauty of colour-
ing. Thus Dr. D. Sharp describes the female of
Pneumora scutellaris as being “one of the most

remarkably coloured of insects.” “She is of a gay
green, with pearly - white marks, each of which is
surrounded by an edging of magenta; . . . the face

has magenta patches and a large number of tiny
pearly-white tubercles, each of which when placed
on a green part is surrounded by a little ring of
mauve colour.” The male is much plainer, being

166 COLOUR IN NATURE CHAP.

“of a modest, almost unadorned green colour”
(Insects, p. 302).

It is impossible to speak of the colours of the
Orthoptera without mentioning the remarkable leaf-
insects (Phyl/ium). As is well known these insects
exhibit an extraordinary resemblance to leaves both
as to structure and colour. They are, however, when
hatched not green but red; after a few days, during
which they feed greedily on leaves, the colour be-
comes yellow, and after the first moult it is greenish.
After this the green becomes more and more intense
after every moult (Becquerel et Brongniart), but it is
asserted that when dying the colour changes, exhibit-
ing the hues of a fading leaf. In dried insects the
colour is very fugitive. All these characters suggest
an origin from the pigment of the food, and MM.
Becquerel and Brongniart have sought to prove this
spectroscopically. They found that the pigment
occurs in amorphous granules in the subcutaneous
connective tissues, and that it gives a spectrum
closely resembling that of the green leaves upon which
the insects feed. As, however, the whole insect was
employed for spectroscopic purposes, without appar-
ently any effort being made to remove the gut or its
contents, the observations are not very conclusive.
The frequency in the order of green colours associated
with herbivorous habit may suggest that derived pig-
ments are common and important in coloration, but,
on the other hand, we have cases like that of the
species of Mantzs where a green colour is not infre-
quent, and yet all are purely carnivorous. There
seems also nothing intrinsically improbable in the
idea that the green colour may be produced by a yellow

VIII THE COLOURS OF INSECTS IN GENERAL 167

pigment in combination with some simple effect of
structure, and that progressive colour-change, like that
of Phyllium, may have more relation to structure, eg.
thickened cuticle, than to pigment.

Besides the uniform brown and green colours in
Orthoptera there is not infrequently a development
of beautiful markings and patterns, especially in
browns and yellows. Dr. Sharp mentions as a very
peculiar case that of Corydia petiveriana, where the
wing-covers are spotted, but not symmetrically.
In the resting position, however, the two overlap in
such a way that the markings then appear com-
pletely symmetrical—a case of some theoretical
interest.

On the whole there is reason to believe that the
colours of the Orthoptera are to a considerable extent
due to lipochrome pigments, whether intrinsic or
derived is still uncertain.

In the NEUROPTERA, as exemplified by the
dragon-flies and may-flies, the colours are in large
part optical The gauzy wings frequently display
beautifully changing tints, but in other cases instead
of being transparent they show graceful patterns and
markings in shades of brown (e.g. species of AZyr-
meleon). The body also often displays bright optical
colours, especially shades of blue. These colours are
associated with the development of a considerable
amount of dark pigment, and are mostly very fugitive.
Bright coloured pigments are relatively rare, and in
general terms the colours are due to the dark pig-
ments, or are optical colours produced by the differ-
entiation of the cuticle. The optical colours may
be associated with a thickening and pigmentation

168 COLOUR IN NATURE CHAP.

of the cuticle as in the body, or may be simple inter-
ference colours like those of the wings.

In the large order of beetles (COLEOPTERA) many
varieties of coloration are displayed, some species
being notably dull while others exhibit a brilliance
rivalling that of the Lepidoptera. Black and deep
brown pigments are very common, especially in the
elytra or wing-covers, which are of course the con-
spicuous features in most beetles. We have already
noticed the relation between the thickened cuticle in
this region and the pigment. The colour may be
simply a dull, dead black as in many Carabidae, or it
may be a vivid metallic green or blue as in the rose
beetle ; between the two there are many intermediate
stages ; but the metallic tints, which are of course
optical, are very common. Then there may be
patterns and markings in brown and black, as in
the very beautiful Macropus longimanus which is
elaborately marked with shades of pink and gray.
Finally, we find the development of bright red, orange,
and yellow (lipochrome, Krukenberg) pigments, as
in species of Coccinella and Chrysomela, producing
patterns and markings when blended with black
and brown. As a whole, however, probably optical
colours are most common in beetles. A consider-
able number of beetles are furnished with scales, but
these are of less importance in coloration than in the
Lepidoptera.

Among the pigments of beetles few have been
investigated, but as to the minute details of colouring
there are some facts of interest. Thus in Luciola
italica, the Italian glow-insect, as described by Emery,
the male has the prothorax coloured a clear red, while

VIII THE COLOURS OF INSECTS IN GENERAL 169

that of the female is paler and more yellow. The
cuticle is, however, similarly coloured in both sexes,
and the difference is due to the different colouring of
the fatty bodies in the two. In both these contain
concretions of urates, but in the female they are
chalk-white, in the male a delicate rose colour. In
the related Lampyris, on the other hand, both sexes
have rose-coloured concretions. In Luczola the testes
are also coloured red by concretions deposited in
the connective tissue. The association between the
urates and pigment is interesting, but Emery says
nothing as to the characters of the pigment. In
Luciola the adult does not eat, reproduction being
the only object of existence at this stage. It is a
point of some interest that we have here an indirect
utilisation of the pigment of the fatty body in colora-
tion, a condition which suggests a transition towards
the complete utilisation of coloured excretory pro-
ducts seen in some of the Lepidoptera.

The HEMIPTERA display great variety in their
coloration, and apparently contain many peculiar
pigments. Optical colours are not very common
but are far from being unknown; thus Eurymela
shows a faint metallic sheen, and Lzbyssa signata is
marked by alternate bands of vivid metallic green
and black.

Among pigmental colours we have not only
uniformity of colour but also frequently beautiful
patterns and markings. Our own familiar Bishops’
Mitres (Zvopicoris) exhibit considerable beauty of
marking, while many of the large Indian and
American forms have most beautifully marked and
spotted wings. In these cases the fore-wings are

170 COLOUR IN NATURE CHAP.

not infrequently different from the hind, and the
type of coloration suggests that seen in butterflies.

Among the pure pigmental colours brown, red,
yellow, and green are common. A brilliant red
colour is far from uncommon, and is displayed for
example on the surface of the abdomen in Wega
rubra.

Among the pigments of the Hemiptera we must
assign the first place to the carmine of Coccus cacti,
not on account of its importance in coloration but
because its commercial value has caused it to be
somewhat fully investigated.

Carminic acid occurs in nature in the Coccide,
especially Coccus cacti, and, according to Sorby, also
in various species of Aphzs. It is a glucoside, yielding
a sugar when treated with dilute acid.

In the case of Coccus cactz the pigment occurs in
large amount in the female (26 to 50 per cent of the
body weight according to Krukenberg) and in less
amount in the male; to the dried insects at least it
gives a dull red-gray colour rather than a pure full
red. According to Mayer it occurs in drops near the
periphery of the cells of the fatty body, the drops
being less numerous in the case of the male. It
occurs also in the yolks of the eggs and in the diffuse
fatty body in new-hatched larve. Mayer says ex-
pressly that the pigment does not occur in the gut,
but in another place he states that it markedly
colours the feces ; the anomaly is, however, nowhere
explained. It is possible that the meaning is that
the pigment is not found in the anterior part of the
gut, but is introduced into it by the Malpighian
tubules.

VIII THE COLOURS OF INSECTS IN GENERAL 171

As to function, Mayer has no suggestion to offer,
but is of opinion that the pigment is undoubtedly
intrinsic and not derived. Krukenberg believed that
it functions as a reserve, basing his opinion especially
on the ground that the large amount present in the
female is inexplicable on any other hypothesis, and
that it is of glucoside nature. It may be pointed
out, however, that chitin itself is a derivative of
a carbohydrate, and often occurs in considerable
quantity, and yet no one has suggested that it is
a reserve product. It seems as yet impossible to
decide the question of the function of carmine, but
we may note that as we have already seen in the
case of Luczola, the association of pigment with the
cells of the fatty body is not unknown among other
insects. Mayer describes colourless, crystallised
bodies in the cells containing carmine, but does not
discuss their nature; they may of course again be
urates. If so the contrast between the pigmentation
of the sexes in Luczola and Coccus is striking in the
extreme.

As to the other pigments of the Hemiptera, Sorby
investigated those of Aphides, and described a pig- '
ment found in them as respiratory, but, according to
Krukenberg, his results were due to an admixture of
carmine and lipochromes. The remaining pigments
do not appear to have been much investigated ; the
characters of the bright red one found in various
forms would be of great interest in a comparative
study of insect pigments. The red pigment of
Pyrrhocoris apterus is, according to C. Phisalix, closely
related to carotin, and therefore a lipochrome.

Among other orders of insects the HYMENOPTERA

t72 COLOUR IN NATURE CHAP.

and DIPTERA may be mentioned as exhibiting a
striking parallelism of coloration. In various instances
(eg. Volucella, Eristalis, etc.) this has of course been
described as protective mimicry on the part of the
flies, but it is difficult to look at collections of the
two orders side by side without being struck by the
similarities of colouring. In both cases the body is
frequently covered with hairs which are important
factors in coloration, but the hair may be completely
absent and the surface may be then brilliantly
metallic or black and polished-looking. Among
pigmental colours in both cases black, dull brown,
and yellow are the commonest. There seem to be
no investigations on the pigments in either case.

CONTRAST BETWEEN COLORATION OF LEPI-
DOPTERA AND OTHER INSECTS

In comparing the colours of other insects with
those of the Lepidoptera, we have to notice that in
the former the occurrence of much beauty or special-
isation of colour in the larve is relatively rare. The
larve not infrequently are of the form popularly
known as maggots or grubs, and they are usually
colourless or almost so. Similarly, outside the
Lepidoptera, it is not very common to find the
nutritive and reproductive stages sharply contrasted
in large groups, many of the adults being not in-
frequently both. Directly associated with this fact,
or as the indirect result of the diminished specialisa-
tion which makes food-taking possible to the adults,
we find that the adults may display that type of
coloration which is larval for the Lepidoptera, that is

VIII THE COLOURS OF INSECTS IN GENERAL 173

especially coloration by lipochromes. When, how-
ever, the adults, as in the beetles, are remarkable for
the great development of the cuticle, the coloration
may be due either to the presence of a large amount
of dark pigment or to structural colours. Coloration
by waste products has not yet been described outside
the Lepidoptera, at least in detail.

GENERAL ASPECTS OF INSECT COLORATION

It is impossible to conclude this section on the
colours of insects without touching, be it never so
slightly, on some of the interesting questions which
cluster around the subject. As most of the current
theories of colour have been in large part founded on
insects, it is not surprising that the literature of the
subject has already reached enormous dimensions, so
that to attempt to abstract even the more important
papers would be impossible. We have therefore
confined our attention very closely to the physio-
logical side, and the result is of course professedly
incomplete. Many of the omitted subjects, such as
the colours of the sexes, the relation between food
and colour, variable coloration, and so on, are, how-
ever, already in their rough outlines familiar to most
people, and their physiological side has been so little
investigated, that little more can be profitably said.
There are, however, one or two points which, on
account of their general interest, are worthy of more
extended treatment.

174 COLOUR IN NATURE CHAP.

VARIATION IN COLOUR

1. Natural. With regard to those variations of
colour in insects, especially butterflies, which occur
in nature there are some interesting observations.
Much of their interest is of course due to the
fact that butterflies show such a marked tendency
to develop into geographical varieties distinguished
primarily by their colour. Mr. Bateson, in his
Materials for the Study of Variation, pp. 44, 45,
describes several cases having especial reference to
yellow, orange, and red pigments. Thus Colas hyale,
the Pale Clouded Yellow (Pieridz), is usually sulphur-
coloured but may occur in white varieties, or may
be of a “rich sulphur colour” with apical marginal
patches of a red colour. A more interesting case is
that of Gonepteryx rhamnz, a British Pierid, in which
the male is of sulphur-yellow colour, while a South
European species, G. cleopatra, is very similar but has
more orange on the fore-wings. Not only have
forms closely resembling G. cleopatra been found in
Britain, but intermediate forms have also been found,
and also forms in which the fore-wings were broadly
suffused with scarlet instead of orange. These facts
are of interest not only on account of the relation
which we have already noticed as existing between
yellow, orange, and red pigments, but also on
account of their importance in regard to questions
of mimicry and of the effect of environment on
colour. We shall see in the chapter on mimicry
that such colour variations often occur simultaneously
in very different genera, and so give rise to what are

Vul THE COLOURS OF INSECTS IN GENERAL 175

known as cases of protective mimicry. Again, when
such variations arise in the course of experiments on
the rearing of larve, many would regard them as
directly produced by variations in temperature. The
fact that a British species may be yellow, orange,
or red suggests that the pigment produced depends
upon the chemical condition of the cells, and that
this may be only indirectly influenced by the state
of the temperature. There can at least be no doubt
that such variations are of much importance in
considering the bearing of the colours of Lepidoptera
upon the general problems of evolution.

Mr. Bateson also mentions interesting cases of
variations from red to blue or from blue to red in
insects, e.g. in the wings of butterflies and the tibiz
of locusts. This is especially interesting because
blue in insects seems to be always(?) an optical
colour, so that the variation in this case must be
associated with the development or suppression of
surface sculpturing. It is somewhat curious to note
that in the same paragraph Mr. Bateson gives as other
examples of alternations between blue and red that of
various Copepods and of flowers, ¢.g¢. pimpernel, in both
these cases the colours are produced by pigments,
which exist in red and blue forms (see pp. 68, 126).

2. Artificially produced—Another question which
has attracted much interest in insects is the relation
existing between the normal colours of insects
showing much variation of tint, and the colours
which may be produced by subjecting the developing
organism to varying environmental conditions. The
subject has of course been especially discussed in the
case of the Lepidoptera, and although the whole

176 COLOUR IN NATURE CHAP.

question of artificially produced colour-change is too
vast to be entered into in detail here, this chapter
would be incomplete without some reference to it.
As the observations of Mr. Poulton and others have
rendered the adaptive changes of caterpillars tolerably
familiar in this country we shall not discuss them,
but rather note some points in connection with the
changes produced in the butterflies of the genus
Vanessa by subjecting the pupz to varying condi-
tions of temperature ; these being not quite so well
known.

The occurrence of seasonal dimorphism in the
butterfly formerly described under the two specific
names of Vanessa prorsa and V. levana is well known,
and the fact that similar colour-variation may be
produced in other Vanesse by subjecting the pupa
to varying temperatures is also familiar through the
researches of Weismann, Fischer, and others. The
interpretation of the results obtained has always,
however, been a matter of considerable difficulty.
Weismann’s conclusion in the case of Vanessa levana-
prorsa is, or was, that each pupa contained within it
the potentialities of both forms, and that the tempera-
ture was not the efficient cause of the one produced,
but merely the stimulus which set in motion the
necessary pre-existing mechanism. Fischer made
an extended series of observations on the same
subject, and in the main agrees with Weismann,
while Urech (1896) dissents very strongly from this
position. We cannot here enter into detail on the
subject of Urech’s paper—it is largely theoretical in
nature—but may mention a few of his points. His
main object is to prove that the differences between

VIII THE COLOURS OF INSECTS IN GENERAL 177

the forms are not due to the action of unknown
forces—Weismann’s ids—but are dependent upon
simple chemical and physical causes, which have a
direct action upon the organism.

In the first place, Urech draws a sharp distinction
between pigmental and optical colours in the Vanesse ;
according to him the bright blue or violet colours only
occur in scales which are almost devoid of pigment,
so that an actual decrease in amount of pigment
may produce an increase in colour-brilliancy. This
of course assumes that any scale which is devoid
of pigment may display optical colours, and _ this
remains to be proved. Urech also believes that
artificially- produced colour-variations are not ac-
companied by variation in the amount of pigment
present in the wings, but only by variations in the
distribution of the pigment, resulting in alterations
in the extent of the blue patches. For details of this
position reference must be made to the original paper.

Urech further makes an interesting suggestion as
to the nature of the association between red and
yellow pigments. He believes that the pigments are
complex organic compounds which owe their colour
to the presence of a certain radicle ; successive sub-
stitutions might then produce pigments of deepening
colour. The production of red and yellow pig-

ments from an orange one might, he suggests, occur
R

in the following way :—Let MER be an orange pig-
ment, R being the colour-producing radicle, then
R R
Te eal oe,
M—-R+M—
a = = oe <3

178 COLOUR IN NATURE CHAP.

that is, two molecules of the orange pigment might
combine, and form one molecule of red and one of
yellow pigment. The chief interest of this suggestion
lies in the fact that such a splitting of colours has
been described in the males as compared with the
females in both birds and butterflies. It may quite
well be that this is in reality due in some such way
to a chemical change. In butterflies, as we have
already seen, there is certainly a strong tendency for
colours like yellow, orange, and red to show oscillation
in amount.

Into the numerous other problems connected
with the colours of insects space does not permit us
to enter here. Many of them are indeéd subjects
which can only be adequately treated, by ento-
mologists, and reference should be made to the works
of Wallace, Poulton, Weismann, Meldola, Beddasd,
and others for details. It is, however, hoped that
the foregoing summary will be of use in calling
attention to aspects of the subject which are less
familiar, but are undoubtedly of great importance.

THE COLOURS OF SPIDERS

Among other Arthropods the colours of spiders
merit at least a brief notice. In them the pigments
do not seem to have been investigated at all, and
the structural colours only to a very slight degree,
but there are some interesting observations on the
colours themselves.

As compared with insects, the characteristics of
spiders which are important for our purpose are the
practical absence of obvious (segmentation in the

VUI THE COLOURS OF SPIDERS 179

abdomen, and the delicacy of the cuticle which
covers that region. The whole surface of the body
is not infrequently covered with a dense coating of
hairs, which are analogous to the scales of Lepido-
ptera, and are often important in coloration. Accord-
ing to Dr. Henry M‘Cook (American Spiders), the
cuticle only contains a relatively small amount of
pigment, this being chiefly confined to the “soft
skin” ( = epidermis ?). )).

(1) Optical Colours.—Opinions differ greatly as to
the prevalence of bright colours in the group, non-
specialists being generally inclined to regard spiders as
dull-coloured, while enthusiasts like M‘Cook and the
Peckhams speak of them as rivalling insects and
birds. It is at least certain that metallic colours do
occur in various species. The commonest of these
colours appears to be a metallic or silvery white due
to the structure of the hairs, but in some cases these
hairs or scales give rise to brilliant iridescence, or to
pure blue and green colours. In certain instances,
as for example in the mandibles of Phzdippus morst-
tans, the cuticle itself is, as in many beetles, finely
ridged, and so gives rise to a brilliant green colour.
In this connection it is interesting to note that the
Peckhams remark that the bright colours of the male
are most conspicuous in the anterior region. This
they ascribe. to sexual selection, but if the colours are
structural they can probably only occur in those
regions where the cuticle is markedly differentiated,
and these regions are especially the appendages of
the mouth and the neighbouring parts.

(2) Pigmental Colours—Apart from structural
colours, those due to pigment seem to be bright and

180 COLOUR IN NATURE CHAP.

varied; black, red, green, yellow, brown, etc, are
apparently all common. Although, there is not the
sharp division of the life-history into two contrasted
stages which is so obvious in many insects, yet there
is in many cases a marked development of colour in
ontogeny. The following facts are taken from Dr.
M‘Cook’s book quoted above.

Development of Colour—Almost all very young
spiders are light yellow or greenish-white ; as develop-
ment proceeds the colours deepen and yellows and
browns appear, but it is not until the spiders begin
to weave webs on their own account that the colours
characteristic of the species develop. The character-
istic patterns may be present in the young, but differ
in colour from those of the adult; thus Argzope
cophinaria has pure white markings in the situations
in which the adult has yellow markings. A general
deepening of tint is. markedly characteristic of the
passage from youth to maturity ; and this may take
place to such an extent that the markings which
are at first distinct may gradually disappear. As
illustrations of the deepening of colour we may-take
Epeira trifolium, which is at first white, then becomes
yellow, at full maturity displays brilliant and very
variable colouring, and then after laying her eggs
becomes a dull, dark colour, a change which immedi-
ately precedes death. A somewhat similar series of
colour-changes is displayed by Zegenaria medicinalis,
which is first pale, then deep yellow, and finally
blackish. In marked contrast to these cases, however,
there are a few forms, like Epezva strix, which are
deep black at the time of hatching. The last-named
species is very variable in colour, but the adults are

VIII THE COLOURS OF SPIDERS 181

usually yellowish with red bands, or sometimes
whitish ; the colour-development is thus in marked
contrast to the cases already described.

Before passing on to discuss the beautiful and
complex markings seen in many spiders, we may
dwell for a little on some cases of colour-variation
more complex than those merely due to development,
and on the colours of the sexes.

Variation in Colour—wWe have already described
the colour-changes occurring during development in
Lpeira trifolium, but the adults themselves show
an extraordinary variability, often changing colour
markedly in captivity. The specific name is founded
upon the resemblance of certain markings on the
dorsal surface of the abdomen to the leaves of trefoil,
but it is only in certain cases that these markings
are at all distinct. The following list is compiled
from Dr. M‘Cook to illustrate the common colour-
variations :—

Colours of Body. Colours of Legs.

1, White with faint black Brown and white.
markings.

2 Do. do. Black and white.

3. Orange to crimson -red Dark brown and white.
with yellow markings.

4. Bright red with yellow Yellow and red-brown.
markings.

5. Yellow to brown with white Orange and brown.
markings.

6. Yellow to orange. Yellow and brown.

These interesting variations, which show such a close
connection between white, yellow, red, and brown,
suggest strongly that there is a relation between the
pigments corresponding to that which exists in the

182 COLOUR IN NATURE CHAP.

Lepidoptera between white, yellow, and red pigments,
but exact observations have still to be made.

Sexual Coloration.—As to the colours of the sexes,
the careful observations of Mr. and Mrs. Peckham
have familiarised us with the fact that the males are
not infrequently more brightly coloured than the
females, the bright colours being especially marked
on those regions of the body which are most fully
displayed during the antics of courtship. The males
are not, however, invariably brighter than the females.
Thus in Mephila plumipes the male is dull brown and
small, the female has a jet-black cephalothoracic
region largely covered with silvery hairs, while the
abdomen is an olive-brown colour tending to light
yellow above and with yellow and white spots and
stripes. The legs are yellow with dull red rings,
several of the joints being furnished with plumose
tufts. This is an interesting case, because if the
colours and ornamental tufts had been present in the
male, they would have been certainly ascribed to
sexual selection. As it is, male spiders are asserted
on all sides to prefer amiability to beauty, so that
such an explanation is hardly valid.

Markings of Spiders——The last characteristic of
the coloration of spiders which we must consider is
the beauty and complexity of the markings. These
markings tend especially to occur on the abdomen,
where they are usually very complex ; the cephalo-
thorax is usually more uniform, but the legs not in-
frequently display bands of colour of a deeper tint
than that of their ground-colour. The abdominal
markings often take a leaf-like Shape, as, for example,
is well seen in the “folia” which give its specific

VIII THE COLOURS OF SPIDERS 183

name to Epeira trifolium, The relative uniformity
of the cephalothorax is perhaps in part associated
with the uniform thickness of its cuticle. The mark-
ings of the abdomen are, as we shall see later in more
detail, probably associated with the tendency of the
epidermal pigment to become aggregated round the
little pits which indicate the points of attachment of
the muscles. M‘Cook.considers that the variation
in colour and markings which is so marked in the
abdomens of many spiders, may be due in part to
variations in the amount of distention in this region
due to the presence of eggs and so on.

Dr. M‘Cook’s volumes contain many other interest-
ing observations on the relations of colour to habitat
in spiders, on instances of so-called mimicry, etc., for
which reference must be made to the original. There
are various observations in the literature of the subject
tending to prove that spiders in some instances can
vary their tints according to their environment ; that
harmony with the colours of the environment which
is known as “ protective coloration” seems also to be
relatively common.

CHAPTER IX

THE COLORATION OF MOLLUSCA AND OF
INVERTEBRATES IN GENERAL

Colours of the Mollusca: (1) of the Mantle, (2) of the Shell,
(3) of the Secretions—Pigments of the Mollusca—Green
Oysters—Characters of Coloration of Invertebrates—
Patterns and Markings of Organisms ; Plants, Ccelentera,
Leeches, Arthropoda, Mollusca, Vertebrates— Meaning
of Patterns.

THE Mollusca are especially remarkable for the
number and brilliancy of their pigments; unfor-
tunately, however, these have been relatively little
investigated. The pigments occur especially under
three circumstances—in the mantle, in the shell, and
in the secretions poured out by many, of which the
far-famed fluid of Purpura may be taken as a
type.

1. Colours of the Mantle.—The mantle-folds
themselves, or the general body wall, are not infre-
quently highly coloured, and this whether the coloured
regions are visible during life or not. Thus Mr.
Saville Kent describes the mantle of Zvidacna com-
pressa, the Frilled Clam, as being during life resplen-

cH. 1x THE COLOURS AND PIGMENTS OF MOLLUSCA 185

dent with the most gorgeous colours. This giant
clam occurs freely among the corals of the Australian
Barrier Reef, and during life the valves gape widely,
disclosing the frilled mantle-folds which give the
species its popular name. These are often coloured
with shades of blue varying from “palest turquoise
to the richest ultramarine,” or are green variegated
with spots and markings of black. Sometimes the
ground colour is purple or rich brown spotted and
streaked with bright blue or green. The shell in
this clam is a pale yellow, and the organisms are
exceedingly conspicuous on the reefs. In many
Molluscs the pigments of the mantle are confined to
special spots, as in the case of the simple “eyes”
of many Lamellibranchs, but cases of uniformly
diffused colour are far from uncommon. The
delicate pink colouring of the fringed mantle-folds of
Lima is familiar to all those who have done coast
dredging. In Patella vulgata, the common limpet,
the dorsal body wall is of a deep blue-black colour,
while in the little tortoise-shell limpet (Acmea
testudinals), common on Scotch coasts, the same
region is a delicate blue-green. In the shell-less
Gasteropods, such as the sea-slugs (Doris, etc.) and
the terrestrial slugs Lzmaz, etc., the whole of the body
wall is usually coloured, often with bright pigments,
but this is much less inexplicable than the occurrence
in the mantle of shelled forms of bright pigments
having no obvious relation to the pigments of the
shell.

As to the colours of the Cephalopoda there is
little need either for description or eulogy ; most
people must have seen and admired the lovely

186 COLOUR IN NATURE CHAP.

changing tints of the hapless cuttles which the storms
of spring cast upon the beach. To those who have
not, no learned talk of chromatophores will suggest
the delicate rosy flush which comes and fades as one
touches the sensitive skin.

The cuttles of our own shores are chiefly remark-
able for their changing tints rather than for their
brilliancy, but some tropical forms display very
bright colours. Thus Octopus pictus is of a yellowish
colour with spots “suffused with shades of the
richest ultramarine blue, each spot having a light
centre and a dark annular border” (Saville Kent).
According to Agassiz the deep-sea cuttles are usually
brown or purplish-brown.

2. Colours of Shells—The fact that most shells
are distinguished by their rich and beautiful colour-
ing is familiar enough ; their variety and beauty of
marking has endeared them alike to the savage, the
child, and the artist. Brightness of colour is most
common among the shallow-water forms, especially
of the tropics; deep-sea forms tend to be pale in
colour, but often possess an iridescent sheen absent
from those living in shallow water (Agassiz). Many
shells are coloured on the inner surface as well as the
outer, and the colouring-matter is rarely uniformly
diffused, being most frequently arranged to form
patterns and markings. According to Dr. Woodward,
“those which are habitually fixed or stationary (like
Spondylus and Pecten pleuronectes) have the upper
valve richly tinted whilst the lower one is colourless.”
In some cases the mantle is coloured and marked in
the same way as the shell. In /anthina the upturned
base of the shell is coloured by the violet pigment

1x THE COLOURS AND PIGMENTS OF MOLLUSCA 187

which is present in the well-known secretion of the
animal. Among the bright colours of shells shades
of red, orange, and yellow are perhaps the most
common, but blue and green also occur.

3. Coloured Secretions—The third way in which
conspicuous colours occur in the Mollusca is in con-
‘nection with the secretions poured out by many.
Among these may be mentioned the ink of Cephalo-
pods, the violet fluids of /anthina and Aplysia, the
colourless fluids of species of Murex and Purpura
which in air undergo a series of changes rendering
them ultimately violet-coloured, the purple fluid of
species of Scalaria, and soon. Many of these colour-
ing-matters are, or were, of commercial value, and it
is somewhat interesting to note that in animals, as in
plants, it is usually the pigments which are incon-
spicuous during life which can be utilised as dyes.

PIGMENTS OF THE MOLLUSCA

To pass now to the pigments themselves: we find
that these are exceedingly numerous and varied ; in
this respect they are sharply contrasted with the
Crustacea, whose pigments are very uniform through-
out large groups. Many of the pigments are, however,
little known. The Mollusca agree with the Crustacea
in their blood pigments, and in the presence of a
limy exoskeleton, frequently impregnated with lipo-
chrome pigments. In both groups the blood may
contain hemocyanin or hemoglobin. The latter, rare
in the Crustacea, is distributed in an exceedingly
capricious way throughout the Mollusca, sometimes
occurring in the hemolymph, sometimes only in

188 COLOUR IN NATURE CHAP.

certain muscles; but in the Mollusca, as else-
where in Invertebrates, it demands further study.
Hemocyanin, on the other hand, is very widely
distributed in both groups; the question whether it
can function as a pigment strictly so called is a
somewhat difficult one. It will be recollected that
it differs from a pigment like hemoglobin in dis-
playing colour only in the oxidised condition—
reduced hzmocyanin is quite colourless. It is in
consequence difficult to believe that hemocyanin can
be of permanent value as a pigment, except in delicate
organs freely exposed to sea-water. Under such
conditions, however, there is nothing intrinsically
improbable in the idea that it may give rise to
brilliant colour. In dissecting recently killed sea-
slugs the bright blue colour of the abundant blood is
very noticeable. It seems quite possible that the
bright blue tints seen in the transparent papille of
many Eolids may be due to the oxidised blood
shining through. The colours are at least exceedingly
fugitive after death.

The fate of hemocyanin in the body is unknown,
but there is no reason to suppose that it can give
rise to series of pigments in any way resembling those
which arise in vertebrates from the breaking down
of hemoglobin. Derivatives of hemoglobin have
on the other hand been described in various mol-
luscs. Dr. M‘Munn, for example, describes haemato-
porphyrin in the slugs Lzmazx and Arion, and so on.
According to Krukenberg the shells of some species of
Ffaliotis, Trochus, and Turbo are coloured by biliverdin,
one of the bile pigments of Vertebrates, while other
species of the first and last genera have their shells

1X THE COLOURS AND PIGMENTS OF MOLLUSCA 189

coloured by turbobrunin, a red pigment easily con-
verted into biliverdin. Krukenberg regards this as
evidence that the bile pigments can arise inde-
pendently in Molluscs, without the intervention of
hemoglobin, but the whole subject is still obscure.

As colouring agents of shells the ubiquitous
lipochromes also occur; they have been found in
such shells as the gaily coloured Pectens and in
Littorina ; it is probable that here, as in Crustacea,
they form compounds with the lime of the shells, and
are most stable in those cases where the shell con-
tains the largest amount of lime. Numerous other
pigments have been described as colouring the shells
of Molluscs, but as most of them are very imperfectly
known, it is unnecessary here to mention the names
which have been given them. The relation between
the pigments of the mantle and the shell, a subject
of great interest, does not appear to have been
investigated at all, Although in some cases the two
structures are similarly coloured, yet in others there
is no apparent relation. It is quite probable that this
may often be due to the fact that the pigments of
the shell are compounds or oxidised products, but
as yet nothing seems known on the subject.

In connection with this subject we may mention
the peculiarly vivid pigment of the tortoise-shell
limpet. This pigment does not seem to have been
described, in spite of its striking tint. It is entirely
confined to the cells of the ciliated epithelium which
lines the shell, is turned in to cover the dorsal
surface of the foot, and also covers the mantle-fold.
During life the colour is very inconspicuous, being
only visible in the mantle-fold and in small specimens

190 COLOUR IN NATURE CHAP.

as a faint green line down the centre of the foot,
produced by the coloured dorsal epithelium shining
through. In the cells the pigment is present in
small granules, varying in colour from light green
to greenish-blue or even light blue. At times it
has a black or brownish tint, the extreme edge of
the mantle-fold being always brown. The pigment
is soluble in water, or better in alcohol, but in solution
the colour rapidly disappears. Alkalies destroy the
colour at once, but it returns upon acidification ; dilute
acids give the pigment a blue tint. The shell of
this little limpet is a warm brown colour, and in view
of the tendency of the green pigment to become
brown, it is not unreasonable to suppose that during
the process of shell-formation the green pigment
becomes converted into the brown.

This green pigment is of some interest ; it varies,
as already seen, from blue to green, and is perhaps
somewhat widely distributed among Invertebrates.
The one just described certainly resembles closely
that found in the eggs of Eulalia viridis, and in
Thalassema. The origin of the pigment is quite
unknown, but it is not impossible that it arises from
the “enterochlorophyll” of the digestive gland.

As to the glandular secretions which are so
common in the Mollusca, we find that their pigments
exhibit very diverse characters. The ink of Cephalo-
pods, for example, contains a black or brown pig-
ment, which is nitrogenous and is said to have a
chemical composition almost identical with that of
the black pigment of crows’ feathers. According to
the older analyses of Girod it is free from ash, but
Dr. Emil André considers it identical with melaine,

1x THE COLOURS AND PIGMENTS OF MOLLUSCA 191

the pigment of Limncga stagnalis, which he states is
iron-containing. André regards melaine as a waste
product stored up in the tissues. When we consider
the almost universal presence of pigment in the shell
of shelled forms, it seems not unnatural to conclude
that the large amount of pigment in the ink of
Cephalopods is associated with the suppression of
the shell, which has thus, as it were, forced the
organism to get rid of its pigment by some new
means.

The beautiful purple colouring-matter of the fluid
of /anthina has always attracted much attention, but
in this case, as in those of the chromogen of Murex
and the pigment of Aflysza, there have been few
exact investigations. The colour of the secretion
of Aplysia has been asserted to be due to natural
aniline dyes, but this, according to Krukenberg, is
incorrect. Moseley found it to be soluble in alcohol
with a purple colour, turning violet with acid;
it was very unstable and gave two sets of spectra
according as it was or was not acidified. The pig-
ment of /anthina, on the other hand, dissolved in
alcohol to form a violet or pinkish-blue solution,
acid turning it alight pale blue. It gave a spectrum
with three bands. It would be interesting to know
if these curious pigments occur elsewhere in Mollusca,
and if they have any connection with the blue pig-
ments of shells like the common mussel.

In connection with the pigments of the secretions
we may note the peculiar pigment enterochlorophyll,
which is so widely spread in the digestive glands of
the Mollusca. In Patella I find that it occurs in
the epithelial cells lining the alimentary canal, in the

192 COLOUR IN NATURE CHAP.

digestive gland and its secretion, intermingled with
the contents of the gut, and finally in unaltered form
in the faeces. It thus acts like a true bile pigment.
We have already frequently spoken of the peculiar
group of pigments to which enterochlorophyll belongs,
and of the facility with which they can be made to
yield bright-coloured derivatives. It is evidently
here a useless substance, and it is very probable that
in some cases, instead of being eliminated unaltered
in the feces, it may undergo modification into other
pigments which may be deposited in the mantle or
shell. We have already noticed such a suggestion
in the case of Acmea, and it is possible that many
of the bright pigments of Mollusca arise in this
way.

The yellow, orange, or black pigments of naked
forms such as the slugs, Doris, etc, are probably
due either to lipochromes or the dark - coloured
“ melaines.”

It seems probable that the pigments of Cephalo-
poda are chiefly of the dark-coloured nitrogenous
type, though they have not been fully investigated.
The beautiful changing colours are in large part due
to the movements of the chromatophores, which as
they expand or contract alter the whole appearance
of the animal. So long ago as 1852 Briicke watched
the colour-changes of Sepiola rondeletii, and noting
how the tints varied in the order of the spectrum
from blue to green, yellow, and red, came to the
conclusion that the colours were optical. He thought
that they were due to the colours of thin plates, but
according to Krukenberg they are due to fine ridges
in the surface of the cells.

1x THE COLOURS AND PIGMENTS OF MOLLUSCA 193

GREEN OYSTERS

In connection with the colours of Mollusca, we
may mention the vexed question of the cause of the
colour of Green Oysters. Prof. E. R. Lankester,
who studied the question in 1886, described the
green pigment as occurring especially in amoeboid
cells, which he. speaks of as “crawling over the
surface of the gills.’ He found the pigment to be
very refractory to solvents, to be of a blue-green
colour, and not to contain iron or copper. As
a pigment of similar nature occurs in Mavicula
ostrearta, a diatom found in the tanks in which
Marennes oysters occur, and used by them as food,
he regarded the pigment (marennin) of the green
oysters as a derived pigment, which was removed
from the gut by amcebocytes and ultimately trans-
ported to the gills and there eliminated. MM.
Chatin and Muntz in 1894 investigated the distribu-
tion of iron in the oysters, and found that the green
parts contained about twice as much iron as the
colourless parts, and that the amount of iron varied
with the intensity of the pigmentation. More
recently Dr. Carazzi, in reinvestigating the whole
subject, has come to the following conclusions. The
green pigment occurs in the branchial epithelium
and in amcebocytes included in this epithelium, but
not in the “gland-cells” of Lankester. It is not
transported by amcebocytes from the gut, is not due
to Navicula, but is a special product of metabolism
in'the oyster, perhaps nutritive, and is a peculiar
organic compound containing iron. If these con

10)

194 COLOUR IN NATURE CHAP.

clusions be correct, then marennin must be added to
the list of the pigments of Mollusca. In this con-
nection it is interesting to note its extreme stability,
a character which is rare among green pigments in
Mollusca or elsewhere ; the extreme vividness of the
tint is also very striking.

Carazzi’s statements, if well established, will over-
throw the view that the green oysters are coloured
by derived pigment, which is a point of some
importance. Of course there are other instances,
eg. that of Mr. Poulton’s caterpillars, which seem to
be well established and to prove the possibility of
pigment transference from one organism to another,
but we have still no means of knowing whether or
not this is of common occurrence in the animal
kingdom. The question is interesting, because
suggestions as to pigment transference have been
very freely made to explain cases of resemblance
between organisms and their surroundings.

The subject of “Green Oysters” has given rise
to so many heated discussions that, although a
little apart from our main theme, we may pause
to note a very interesting paper by Drs. Boyce and
Herdman on an abnormal green pigmentation. Part
of the interest taken in the general subject has been
due to the fact that there have repeatedly occurred
instances of poisoning from the eating of green
oysters, where the symptoms have been declared
to be those of copper - poisoning. On the other
hand, many examinations of coloured oysters have
failed to demonstrate the presence of copper. The
researches of the authors mentioned have shown that
the dispute presents a strong analogy to that historic

IX THE COLOURS OF INVERTEBRATES 195

one waged over the question whether lobsters are
black or red! They find that, altogether apart from
the famous Marennes oysters, there occurs in diseased
oysters a green colour due to the presence of a large
amount of copper. Drs. Boyce and Herdman do
not believe that the large increase in the amount
of copper normally present is necessarily the result
of the presence of copper in the food, but “are
inclined to suggest that it may be due to a disturbed
metabolism, whereby the normal copper of the hemo-
cyanin, which is probably passing through the body
in minute amounts, ceases to be removed, and so
becomes stored up in certain cells.” There is every
reason to believe that such “green oysters” are
highly poisonous, while the Marennes oysters are of
course quite harmless. The paper thus settles a
time-honoured controversy in the most satisfactory.
manner.

CHARACTERS OF THE COLORATION OF
INVERTEBRATES

With the Mollusca we end our study of the colours
and colouring-matters of the Invertebrates. As a
whole the Invertebrates are characterised by the
number and diversity of their pigments, which in the
simple forms occur in connection with internal organs
and are often very variable ; by the frequent beauty of
their structural colours ; and in the case of the more
differentiated forms in certain groups, by the beauty
of their patterns and markings. In Vertebrates the
pigments are less numerous, often less vivid, and the
beauty is usually due either to structural colour or

196 COLOUR IN NATURE CHAP.

to the markings. It is fitting, therefore, that at this
point, before passing from the Invertebrates to the
Vertebrates, we should consider for a little the mean-
ing and characters of patterns. We have again and
again encountered such types of coloration among
the Invertebrates, but when we pass to Vertebrates
we shall find that absolute uniformity of colour is
relatively rare, and that in most cases the markings
or patterns are the conspicuous features. We pro-
pose, therefore, to consider here first some of the
characters of the colour-patterns naturally occurring
in organisms, and then some suggestions as to their
meaning.

THE PATTERNS AND MARKINGS OF ORGANISMS

Plants —Among plants colour-markings occur
most conspicuously in flowers, for the artistic value
of foliage depends in the general case upon simple
colour contrasts between stem and leaves and upon
beauty of form rather than upon complexity of
marking. In flowers, as is obvious upon a moment’s
reflection, the markings depend greatly upon the
form and character of the parts. Flowers in which
the floral envelopes consist of similar parts have these
parts similarly marked, as, for instance, the stripes or
markings on the petals of tulips, of geraniums, snow-
drops, and so on. Slight irregularities of colour are
associated with correspondingly slight variations in
structure, as seen in some of the so-called honey-
guides, such as the spot of dark colour in the flower
of the rhododendron. On the other hand, the more
complicated colour-markings are usually confined to

1X THE MARKINGS OF ORGANISMS 197

one region of the corolla, and are associated with
irregularity of parts. This is well seen in the mark-
ings of orchids, of the foxglove, of pentstemon, and
many others, in which cases the parts of the corolla
are irregular, and the markings are confined to certain
of them. In general, in flowers the markings tend
to be similar in homologous parts, but variations in
the structure of homologous parts tend to be accom-
panied by variation in the colour-patterns.
Celentera.—The colours of Protozoa and sponges
are too simple to show any arrangement into patterns,
or perhaps it would be more accurate to say that the
structure is too simple. In any case, we do not find
the phenomenon markedly displayed before we reach
the Ccelentera. In them it exhibits itself with the
same simplicity seen in plants. The organisms have
the radial symmetry so characteristic of flowers, and
again we find that homologous parts show similar
colouring. One tentacle is in structure and develop-
ment like another, and the colours, whether uniform
or arranged in simple banded patterns, tend to be
identical throughout. As we have already seen,
structural differentiation and the development of new
colours tend to be associated; thus knobbed or
capitate tentacles usually have the terminal knob of
a different colour from the rest of the tentacle—a
fact which may be susceptible of a simple physical
explanation. Again, when the tentacles are greatly
reduced in size, they may cease to behave as in-
dividuals in the general colour-scheme, which has
then as its unit groups of tentacles. This occurs in
the giant anemones (Dzscosoma), and is of interest
because the subordination of the individual coloured

198 COLOUR IN NATURE CHAP.

structures to the pattern of the whole is so common
among higher organisms.

Leeches—Among worms, as we have already seen,
the leeches are distinguished par excellence by their
markings and patterns. There are few facts more
striking to one interested in colour than the change
seen in passing from the Nemerteans and the marine
Annelids, as one finds them on the shore, with their
bright, uniform, and fugitive colours, to the leeches,
with their dark persistent tints and their beautiful
markings. That the colours of, for example, the
medicinal leech show a considerable amount of
variation, no one accustomed to handling specimens
can deny, but at the same time there is sufficient
constancy to admit of the dark spots being employed
as a ready means of counting the segments. The
occurrence of a constant and elaborate scheme of
colour-markings at such a low grade in evolution is
very interesting.

We may notice also that the development of
elaborate patterns in the leeches, as compared with
the marine worms, is associated with the develop-
ment of a large amount of dark pigments. This is
interesting because it is perhaps universally true that
elaborate patterns are dependent, at least in part,
upon dark pigments, while the bright pigments tend
as a rule to be more uniformly distributed. It is
difficult to avoid coming to the conclusion that the
fact is associated with the insolubility of the dark
pigments, which will render them on the whole less
readily diffused than the more soluble bright-coloured
pigments. Lipochromes, for example, dissolve readily
in solutions containing albumen or in fat, which

IX THE MARKINGS OF ORGANISMS 199

seems to account in some degree for their usual uni-
form distribution in the tissues or structures in which
they occur. We may note here that in the only
case in which a dark (melanin) pigment has been
subjected to a thorough examination, it was found
that the insolubility was not intrinsic, but was due to
intermixed substances which acted as mordants; if,
however, such substances tend always to occur in
association with the pigment, the practical result is
the same. The question whether the numerous
dark pigments found in the animal kingdom are
related to one another is an interesting one, but has
not yet been properly attacked.

It should be noticed in leeches that in general
terms each segment tends to repeat the colour, as it
does the structure, of the others. The segments are
here the similar parts which tend to resemble one
another ; we have already (p. 108) considered the in-
genious theory which seeks to correlate the colour
with the detailed structure of the muscles.

Arthropoda—iIn the Arthropoda we have again
the same contrast between uniformly, often brightly,
coloured organisms on the one hand, and on the
other those showing wonderful arrangements of
colour-markings, with black or brown as a basis. In
a rough way the contrast again lies between uni-
formly coloured marine forms—the Crustacea, and
patterned terrestrial forms—the Insects. The con-
trast is only roughly true, however, for we have
already seen how many insects there are, especially
in the less specialised orders, which are uniformly
coloured in greens or browns.

As to the patterns and markings of insects, if we

200 COLOUR IN NATURE CHAP.

take caterpillars, dragon-flies, bees, or other cases
where the segmentation of the body is obvious, we
find that, just as in leeches, the coloration bears a close
relation to the segments, as is seen in the banding of
many caterpillars, of the abdomen of dragon-flies, and
so on. But since the Arthropods, as compared with
worms, show a subordination of the individuality of
the segments to the needs of the whole—a synthesis
of segments—so we find that there is a constant
tendency for the pattern to dominate the whole
organism instead of being the result of the patterns
of the segments. The whole is symmetrical round a
median line, but is in most cases not merely due to
the repetition of the patterns of the segments. This
is especially well seen in caterpillars in the colouring
of, for example, the head. This is a specialised
region of the body often with much thickened cuticle,
and accordingly we find that it often differs in
coloration from the rest of the body. When we
pass from caterpillars to forms like dragon-flies or
bees, we find that, while the relation of the pattern
to the segments is obvious in the relatively un-
specialised abdomen, it is lost in the much modified
thorax. In other bees again, as in some of the
humble bees, the coloration has largely lost, even in
the abdominal region, any direct relation to the
segmentation. In this connection the coloration of
spiders offers some points of great interest. As we
have already seen, the markings, especially on the
abdominal region, are often exceedingly complex.
According to M‘Cook, the markings tend to adopt a
leaf-like shape, and these /folia appear to be related
to the little pits on the surface of the abdomen which

Ix THE MARKINGS OF ORGANISMS 201

mark the internal attachment of the muscles. These
spots are “centres for aggregation of pigment,” and
have been supposed to indicate the segmentation of
the body, and thus, as Dr. M‘Cook says, the patterns
probably bear a direct relation to the segmentation.
The same facts are observable to a much less degree
in the cephalothorax, and M‘Cook suggests that the
annuli round the legs, which occur especially in the
neighbourhood of the joints, may be determined in a
similar way by the arrangement of the muscles. All
this is exceedingly interesting and suggestive, but
from a study of other forms, eg. insects, one would
expect to find that the gradual disappearance of
marked segmentation in spiders was associated with
a profound modification of the coloration. If, as
there is reason to believe, coloration and structure are
directly associated, then the new structural characters
of the body should be reflected in the markings.
It seems probable that this really occurs. Thus
Argiope argyraspis has on the abdomen two longi-
tudinal yellow lines and thirteen unbroken transverse
black lines as well as other incomplete ones. Now
as the embryos in spiders are commonly supposed to
have ten to twelve abdominal segments at most, and
as the posterior ones degenerate during development,
it is at least unlikely that these transverse bars have
a direct relation to segmentation. It seems more
probable that we have another instance of the fact
that in specialised organisms the process of integration
which has so profoundly modified the segmentation
has been accompanied by changes in the colour-
markings.

In butterflies, beetles, and other insects with large

202 COLOUR IN NATURE CHAP.

and conspicuous wings, and small or concealed
abdomen, the colour-markings occur on the wings or
wing-covers, and have thus no relation to segmenta-
tion. In this case the coloured structures are lateral
organs and not parts arranged in linear series, but
they conform again to the rule that organs occupying
similar positions display similar coloration ; the two
fore-wings display similar colours, and the hind agree
with one another and may agree with the fore-wings.
On these points there are some interesting observa-
tions by Mr. Scudder, whose statements have been
confirmed and amplified by Mr. A. G. Mayer. In
discussing the butterflies of North America, Scudder
showed that the number of instances in which similar
markings appear in the same areas in the two pairs
of wings is too large to be due merely to coincidences.
The process is most readily traced in the case of
ocelli, which usually tend to be similar in size and
position, and to be situated between the same
branches of homologous veins. In summing up his
own and Scudder’s observations, Mayer (1897) lays
down the following statements with regard to colour-
patterns in butterflies :— °

1. “Any spot found upon the wings tends to be
bilaterally symmetrical both as regards form and
colour, the axis of symmetry being a line passing
through the centre of the interspace in which the spot
is found, and parallel to the direction of the longi-
tudinal nervures.”

2. “Spots tend to appear not in one interspace
only, but as a row occupying homologous places in
successive interspaces.”

It is thus seen that the wings and their homo-

IX THE MARKINGS OF ORGANISMS 203

logous parts tend to behave in the colour scheme as’
do the successive segments in little differentiated
forms. So close, however, is the relation between
structure and colour, that differential growth of the
wings resulting in the overlapping of parts is also
accompanied by corresponding change of pattern.
This is well seen in many butterflies where the fore-
wings overlap the hind. We have already noticed
the peculiar case described by Sharp in which the
fore-wings of an orthopterous insect differed from one
another in their colouring, but the irregularity was
corrected in the position of rest by the overlapping
of parts. It is a curious example of that detailed
relation which exists between colour-markings and
structure.

In spite of the conspicuousness of colour-markings
in many insects, there seems as yet no certainty as
to the proximate cause or meaning of the patterns.
Over the markings of caterpillars, their meaning and
their evolution, it is true that many fierce controversies
have raged; but the questions as to whether longi-
tudinal or transverse markings are the most primitive,
the number of the possible longitudinal stripes, their
use, and the kindred questions seem to be decided so
much by the caprice of the individual investigator,
that a non-specialist may be permitted to retire from
the field until the parties concerned have been able
to patch up some sort of a truce.

Mollusca—In the Mollusca colour-patterns are,
except in the cuttles, confined to the shells, in which
banding is very common, and is well seen in common
snails (Helix). More elaborate schemes of ornamenta-
tion are not uncommon, and are not infrequently

204 COLOUR IN NATURE CHAP.

associated with a sculpturing of the shell. Accord-
ing to the Countess Maria von Linden, both the
sculpturing and the colours show a definite orderly
progression in which phylogeny and ontogeny run
parallel, In the garden snails, Helix nemoralis and
H. hortensis, there is an extraordinary amount of varia-
tion in the bands, but nothing is known of the reason.

Vertebrates—So far we have seen that colour-
patterns are best developed in animals in which
there is a distinct segmentation of the body, that in
unspecialised forms they show a direct relation to
the segments, but that as specialisation proceeds,
they tend to lose this simple and direct relation.
When we pass upwards to Vertebrates, we find that
an apparent relation to segmentation is completely
lost. It is true that in the case of the ringed snake,
as we shall see, there appears to be a direct relation
between the ornamentation and the arrangement of
the cutaneous blood-vessels, and these may well have
a segmental significance, but at the same time the
statement on the whole is true that a direct relation
between the markings and the segmentation of the
Vertebrate body is no longer obvious. In the general
case the pattern in a Vertebrate dominates the whole
organism and is not produced by the repetition of
one colour scheme. This is perhaps an advance due
to the perfect synthesis of the body, just as the
frequent striking difference between the fore and
hind wings of a butterfly is an advance when com-
pared with the segmental patterns of caterpillars ;
or it may be an indication that the segmentation of
a Vertebrate is in essence a different thing from that
of a worm or an insect.

IX THE MARKINGS OF ORGANISMS 205

Though there is no successional repetition in a
Vertebrate, the rule that parts occupying similar
positions tend to be similarly coloured holds good
here ‘as in Invertebrates. The similarity seems,
however, here to have relation to position with refer-
ence to the median axis of the body only. A brief
study of birds’ feathers affords very convincing proof
of this. Thus, though the secondary quills of one
wing in a bird showing complicated feather-markings
resemble those of the other wing, it will be found
that the quills are not all absolutely the same, but
form a graduated series, of which the first and last
members may differ very considerably. The usual
difference between the markings of the two sides
of the vane may be explained in wing-quills as due
to the unequal development of the two sides, but,
except in the case of the central rectrices, it is quite
as common in the tail-quills, in which the two sides
are equally developed. In short, it is a general fact
that in Vertebrates, where the coloration depends
upon the colours of the epidermal outgrowths, the
pattern has reference to the body as a whole, the
colours of the individual structures being completely
subordinated to that whole.

As to the nature and meaning of patterns, there
can be little doubt that in simple organisms they are
closely related to segmentation, are expressions of
the same process. It has been dimly suggested by
various authors, but nowhere with such force and
eloquence as by Mr. Bateson, that both patterns and
segmentation may be purely mechanical phenomena,
and that “the perfection and symmetry of the pro-
cess, whether in type or in variety, may be an ex-

206 COLOUR IN NATURE CHAP. 1X

pression of the fact that the forms of the type or the
variety represent positions in which the forces of
division are in a condition of mechanical stability ”
(Variation, p. 71). It can hardly be doubted that
this explanation has an important bearing on those
simple forms of patterns which are directly related
to segmentation, but even the approximate ex-
planation of the complicated colour-markings of
Vertebrates seems as yet impossible. “Forces of
division” seems too vague an expression to be of
much aid in their case. For some other explanations
of markings in Vertebrates, reference should be made
to Mr. Wallace’s Darwindsm.

CHAPTER X

THE COLOURS OF FISHES

The Colours of the Lower Vertebrates—General Characteristics
of Vertebrate Coloration—Colours of Fishes, their Char-
acters and Distribution in Tropical, Abyssal, and Temper-
ate Forms—Coloration of Flat-fishes—The Pigments and
Structural Colours of Fishes.

OF Vertebrates below fishes little need be said as to
colour or pigments. The Tunicates alone display
any marked colouring, and are often bright and varied
in tint. The pelagic forms are mostly transparent
and almost colourless, and, as we have seen, they are
often brilliantly phosphorescent. Those of sedentary
habit, on the other hand, often show a great range
of colour. Krukenberg describes the species of the
genera Didemnum and Botryllus, in the neighbour-
hood of Trieste, as exhibiting the following colours :
—blue, violet, yellow, yellowish-green, orange, brown,
and black, and observation on the shore confirms
this impression of prevalent brightness of tint.
Krukenberg made a few observations on the pig-
ments which are of interest. The red and yellow
colours are apparently due as usual to lipochrome

208 COLOUR IN NATURE CHAP.

pigments. The blue and violet colouring-matters
are very unstable, turning brown with most reagents,
including water and dilute alkalies. Curiously
enough acid restores the blue colour. Certain of the
yellow pigments are not lipochromes, but singularly
unstable substances of quite different characters.
Thus the lymph of Ascidéa fumigata is coloured by
a yellow pigment which is very soluble in alcohol
and ether, and slightly soluble in water. On stand-
ing in air, however, the solutions rapidly become
dark coloured, and ultimately black. This is, accord-
ing to Krukenberg, due to ferment action; the in-
terest of it is, however, that the tunic of this species
is marked with black, the pigment being probably
the same as that which arises from the alteration
of the yellow pigment. It will be remembered that
in many lepidopterous larve the blood contains a
green pigment which darkens in a similar way on
exposure to the air, and that this is possibly the
origin of the dark pigments both of larve and adults.
These curious pigments—uranidines of Krukenberg
—are worthy of further investigation.

M‘Munn (1889) confirms Krukenberg’s state-
ment that most of the pigments of Ascidians are of
lipochrome nature.

GENERAL CHARACTERISTICS OF VERTEBRATE
COLORATION

Before passing on to consider separately the
divisions of the higher Vertebrates, it may be well to
mention some of the peculiarities of the colours as a
whole. In the first place, in spite of the varieties of tint

x THE COLOURS OF FISHES 209

and colouring there is a remarkable uniformity of
pigments. With a few exceptions, turacin being per-
haps the most important, the pigments seem to be all
either lipochromes or melanins, the lipochromes pre-
dominating in fishes, amphibians, lizards, and birds,
the melanins in snakes and mammals (Krukenberg).
As the melanins are often regarded.as derivatives
of hemoglobin, and the lipochromes of fats, some
would say that there are two kinds of pigments in
Vertebrates—effete blood pigments and modified fats
(see Kiikenthal). Secondly, in many cases the
general coloration is due either to structural effects
or to the patterns and markings rather than simply
to the pigments themselves. The development of
structural colours is associated with the degree of
development of the cuticular outgrowths, and reaches
perhaps its greatest height in birds. In mammals
bright structural colours are rare, and the frequent
beauty of colour is due solely to the unequal
distribution of the melanin pigments which gives
rise to bands or spots. Eimer has endeavoured to
prove that in all Vertebrates longitudinal markings
are the most primitive form of coloration, and that
spots, transverse stripes, or uniform coloration are
secondary derivatives. His work on the Markings
of Animals shows, at least, that there is an
extraordinary constancy in the markings which run
through orders, as is, for example, well seen in the
Carnivora among mammals.

Sexual dimorphism of colour is another char-
acter which is exceedingly common among Verte-
brates. This may manifest itself simply in the
greater brightness or purity of tint in the male as

P

210 COLOUR IN NATURE CHAP.

in some fishes and many birds; or in increased
growth and development of the cuticular structures
in the male, associated usually with a great develop-
ment of optical colours and of pigment; or as in
many mammals there may be simply a special
development of the exoskeleton, not associated with
a great increase of pigment, or any special brilliancy
of tint. Beauty in mammals is, however, usually due
rather to form than to bright colour.

THE COLOURS OF FISHES

1. Tropical Fishes—Among the colours of fishes,
those of the forms inhabiting the neighbourhood
of the coral reefs of warm seas must be especially
mentioned. We have already seen how brilliant are
the tints of the coral polypes themselves, and the
same brilliancy seems to occur almost universally
among the inhabitants of the waters round the reef.
In fishes, however, the brilliant colours are probably
produced not by pigments but by structure. The
colours fade with extraordinary rapidity after death
or removal from the water, and according to, those
who have seen the fish in their natural condition,
the coloration is at best quaint and striking rather
than beautiful. Generally speaking, the tints seem
to be very vivid and hard, with a lack of light and
shade ; the occurrence of sharply contrasted bands
or spots on a bright ground is exceedingly common.
Bright green, blue, red, and yellow seem to be among
the commonest colours, and there are some striking
instances of sexual dimorphism. Thus in Ostracion
ornata the male has a ground colour of grass-green,

x THE COLOURS OF FISHES 211

with spots and stripes of brilliant blue, while the
female, often classed as a separate species under the
name of O. aurita, is of a pale yellow or flesh-colour,
with brown markings; a so-called hermaphrodite
specimen was obtained by Mr. Saville Kent in
which the two sides displayed respectively the two
types of coloration. One of the parrot - fishes
also, Pseudoscarus rivulatus, displays marked sexual
coloration, the female being blue and yellow and the
male green and red, the prevailing tints during life
being respectively blue and green—an interesting
case because it recalls the conditions seen in some
parrots. In connection with the coloration of the
parrot-fishes there has been noticed a case of so-
called “mimetic” resemblance in the case of a goby
(Gobtus douglas?), which is green banded with red,
the usual colours of the parrot-fish, The giant
anemones already mentioned have frequently small
commensal fishes living in their central cavities ; thus
Discosoma kenti is inhabited by Amphiprion percula,
a small red fish with broad white bands, the bands
being separated from the red ground colour by a
black margin; Dzscosoma haddonz, on the other
hand, is inhabited by Amphiprion bicinctus, which is
closely similar in colouring but is without the black
margins to the white bands. Both sea-anemones
show great colour variation, but in all cases their
colours are sharply contrasted with those of their
commensals.

It is interesting to note that the brilliancy of tint
is entirely confined to the fishes which actually live
among the brightly coloured corals. Those living
in the lagoons which are floored with coral sand and

212 COLOUR IN NATURE CHAP.

have a scant fauna and flora are all dull of tint.
This has been explained on the usual assumption of
protective coloration, but it seems difficult to see
that any explanation is necessary beyond that of
the simple fact that in those situations the conditions
of life are relatively unfavourable, that therefore -
dominant or brightly coloured forms are not likely
to occur.

2. Deep-sea Fishes.—With the colours of these
reef-fishes we may contrast the type of coloration
seen in deep-sea forms. These are usually remark-
able for their uniformity of colouring, bands, spots, or
stripes being rare. Dark brown or black colours
seem on the whole to predominate, and it is not
infrequent for the mouth and gill cavities to be very
darkly pigmented with black. After brown and
black colours come violet or yellowish tints, usually
dull-coloured, although a few examples are known
of deep-sea fishes showing considerable brilliancy of
tint.

3. Temperate Fishes.— Jt is unnecessary to
describe the colours of the fishes found at moderate
depths in temperate waters. The silvery iridescence
of the lower surfaces of many, the frequency of dark
spots or stripes, the not uncommon occurrence of
red or orange pigments in the skin or muscle are
familiar to all.

In general, fishes display considerable beauty of
colour, but usually of a somewhat fugitive type.
Under the most favourable conditions the beauty is
in large part lost during any process of preparing
the skins for mounting; and it is probably to this
rather than to any other cause that the prevalent

x THE COLOURS OF FISHES 213

impression that fishes are dull-coloured as compared
with birds is due. Sexual dimorphism of colour is
common both in those from temperate and from
tropical seas, and so-called protective coloration is
not infrequent.

The foregoing remarks, it should be noticed,
refer almost exclusively to the modern dominant
bony fishes or Teleosteans. Sharks and rays, as
typical of the older Elasmobranchs, are dull in
colour without the silvery sheen of modern fishes ;
and although they may display a certain amount of
beauty of marking, the colours are relatively dull
and sombre.

The colours of fishes are due to the structure of
the dermis or to colouring-matter contained in it.
The colouring-matter is in part at least deposited
in contractile pigment cells—chromatophores—the
result being that many fishes, like Amphibians, are
capable of a considerable amount of colour change,
varying according to a familiar observation with the
colour of the ground upon which they lie. The
mechanism is set in motion through the eye, the
phenomenon not being observable in blind fishes.
Mr. Cunningham (1893, b) states that in the case of
the flounder (Pleuronectes) deficiency of oxygen or
alarm causes the chromatophores to contract, and so
diminishes the intensity of the colour ; exclusion of
light causes them to expand and so deepens the
colour. Probably, as in the case of the chameleon,
the colour depends in part upon the psychological
state of the individual. Direct mechanical stimula-
tion also causes the pigment cells to contract.

214 COLOUR IN NATURE CHAP.

COLORATION OF FLAT-FISHES

It is impossible to speak of the coloration of
fishes without touching, however briefly, on the
classical problems connected with the coloration of
flat-fishes. As is well known, these fishes are much-
modified forms, being flattened from side to side,
and having the upper surface, that is one of
the sides, coloured usually in relatively dull tints,
while the actual lower surface, strictly the other
side, is of a silvery white colour without pigment.
According to many naturalists, the colours of the
upper surface are protective, the lower surface is
without pigment, because it is usually concealed in
these ground fishes, and therefore colour, if present,
would be useless. Others, notably Mr. Cunningham,
regard this explanation as totally inadequate, and
hold that the absence of colour on the lower surface
is due directly to the absence of light, or rather was
so due in the first instance. To use Mr. Cunning-
ham’s own words (1893, a): “The disappearance of
the pigment from the lower side in the normal flat-
fish is an hereditary character, not due to the with-
drawal of the action of light in the individual. .
On the other hand, the fact that the pigment, after
prolonged action of the light, actually reappears, is
strong evidence that originally, in the beginning of
the evolution, the pigment disappeared, in conse-
quence of the withdrawal of the lower sides from
the action of the light. If so, an acquired character
has become hereditary.” We cannot here discuss
the main question of the inheritance of acquired

x THE COLOURS OF FISHES 215

characters; it is unfortunately not one which is
likely to be settled by direct experiment, but the
careful observations to which the dispute has given
rise are worthy of all attention.

In most Pleuronectide the upper surface is
coloured with black and yellow pigment, the lower
is pure silvery white; the peritoneum also is darkly
pigmented on its upper surface and white on the
lower, and this in spite of the fact that the upper
part does not shine through the body walls at all
and the lower very little. Two cases have been
described, however, in which the lower surface of
the skin is normally pigmented. In Pleuronectes
cynoglossus the lower surface is gray and not white,
the colour being due to the presence of black but
not yellow pigment. This form “has been taken
at all depths up to 7oo fathoms.” The upper
surface contained both yellow and black pigment,
but was not so darkly pigmented as in the case of
shallow-water forms. It will be noted that this
appears like an approximation to the uniform colour-
ing of deep-sea forms. The other case is that of
Engyophrys sanctilaurenti, in which the blind side
has five or six dusky bands of colour occurring in
the anterior half of the body ; the posterior half is
colourless.

Mr. Cunningham has made a series of careful
experiments on the artificial production of pigment
on the lower surface of flounders. The method
adopted was to keep the young flounders in tanks
under such conditions that the lower surface could
be artificially illuminated by means of mirrors. The
result of the experiments was to show that the

216 COLOUR IN NATURE CHAP.

amount of pigment developed increased with the
amount of exposure, and that although pigment was
developed more rapidly when the larve were very
young, yet the power of developing pigment was
not confined to any particular age. This is interest-
ing as showing that the abnormal pigmentation was
not a purely larval phenomenon as, for example, was
the peculiar colour change shown by Fischel’s sala-
manders (see Amphibians, p. 233). The illumination
of the lower surface during the process of metamor-
phosis did not in any way interfere with the ordinary
course of development, but throughout the whole of
the experiments Mr. Cunningham noticed that the
larve all had what he calls the “objectionable
habit” of clinging to the sides of the tank so as to
avoid as far as possible the illumination of the lower
surface. This suggests that the light had some
powerful and unpleasant influence upon the nervous
system. The pigmentation of the lower surface was
apparently in no case so marked as that of the
normal upper surface, but was always the same in
kind, and showed the same variations of colour due
to contraction or expansion of the chromatophores
as the upper surface. Artificially produced pigment
always appeared first in the middle region of the
body on each side of the lateral line, and last in the
head and tail regions.

Besides these artificially produced abnormalities
in flat-fish, many cases have been described in
which the lower surface is more or less coloured
under natural conditions. In some cases the colour
is due to mere ill-defined patches of pigment, but in
others the lower or blind surface shows in whole or

x THE COLOURS OF FISHES 217

in part a type of coloration in all respects similar
to that of the upper surface, the similarity being
carried out into the details of spots and markings.
Such forms are called “double” or “ambicolorate.”
This peculiarity has been described in the turbot
(Rhombus maximus), the brill (R. /evis), the flounder
(Pleuronectes fiesus), the plaice (P. platessa), the
merry sole (P. microcephalus), and perhaps occurs
in others. An exceedingly interesting point about
the variation is that in the turbot, the brill, and the
flounder it is when marked always associated with
a peculiar malformation of the head, due to a grow-
ing forward of the dorsal fin. In the plaice it is
occasionally so associated, in the merry sole rarely
if ever. In the turbot the correlation is so exact
that, according to Mr. Cunningham, “if we draw an
imaginary line through the preopercular bone in the
turbot, pigmentation may extend over the whole
of the lower surface behind this line without any
structural malformation being present, but when
pigment is also present on the lower side in front of
this line the characteristic structural malformation
occurs also; on the other hand, the structural mal-
formation has never been observed in any speci-
mens in which the lower side was unpigmented or
pigmented to a less extent than that defined above.”
The meaning of the structural malformation is not
quite clear, though it has been suggested that it is
due to a delay in the shifting of the eye from the
blind side; to explain its association with the pig-
mentation of the lower surface, it has been further
suggested that the delay allows a “continuation of
the power of receiving visual sensations from this

218 COLOUR IN NATURE CHAP.

side” (Bateson). This hardly, however, explains
the detailed correlation seen in the turbot, and the
whole subject is still imperfectly understood.

As to the meaning of the similarity between the
upper and lower surfaces in the ambicolorate speci-
mens, both Mr. Bateson and Mr. Cunningham point
out that this cannot be due to reversion, for there is
no reason to suppose that the upper and lower
surfaces of the ancestral Pleuronectidz were identical
in colour. Mr. Bateson regards it as a case of
what he calls homceotic variation. “In the flat-
fish the right side and the left have been differ-
entiated on different lines, as the several appendages
of an arthropod have been, but on occasion the one
may suddenly take up all or some of the characters,
whether colour, tubercles, or otherwise, in the state
to which they have been separately evolved in the
other” (Materials for the Study of Variation).

In summarising the points which give their
special interest to the coloration of flat-fish, we may
note the usual absence of pigment from one side,
right or left as the case may be, the occurrence of
two forms which normally possess traces of pigment
on this side, the facility with which pigmentation of
the lower surface may be produced by artificial
illumination, the occurrence of ambicolorate forms
as a frequent variation in nature, and finally, the
extreme difficulty of accounting for this variation on
any hypothesis of reversion.

x THE COLOURS OF FISHES 219

PIGMENTS AND STRUCTURAL COLOURS OF FISHES

As to the detailed characters of the coloration
of fishes, there are several points of great interest.
We have already mentioned the occurrence of
crystals of guanin in the skin, which are important
factors in the production of the iridescence and the
silvery appearance of many fishes. The exact effect
of these crystals on the coloration has not, however,
been sufficiently determined, and it remains un-
certain how far they are instrumental in producing
the gorgeous colours of many tropical fishes. There
is every reason to believe that these are optical in
nature, but exact investigations into the mechanism
of production are still required. Owing to the
fugitive nature of the colours, the investigation
would need to be conducted on the spot where the
fishes are found, so we must look to some tropical
biological station of the future for the complete
solution of the question.

The most important recent work on the colours of
fishes is that of Mr. Cunningham and Dr. M‘Munn,
who investigated especially the colours of the Pleuro-
nectidz or Flat-fishes. The first point is to realise
the position of the coloured structures. We_have
already noticed that fishes possess some power of
adapting their colours to their surroundings, and this
proves at once that the colouring-matter is deposited
in living cells and is not a cuticular product. In
fact, it is the dermis which contains the elements
giving rise to the coloration. It will be remembered
that the scales of fishes are, for the most part, dermic

220 COLOUR IN NATURE CHAP.

outgrowths, and that in fishes in general the epidermis
is little specialised ; in accordance with this we find
that the epidermis is usually almost entirely devoid
of pigment, or when, as in the flounder (Pleuronectes
jiesus), some pigment is present, it has no effect on
the visible coloration. In birds, on the other hand,
where the epidermis gives rise to greatly specialised
outgrowths, the feathers, the pigment of the body
is found in these epidermal structures. There is,
however, a general consensus of opinion that in all
cases the pigment originates either in the dermis or
in deeper connective tissue cells and migrates out-
wards to the epidermis, in the cases where it is
found there. Many would regard this as a proof
that pigments are in essence waste products, and
that this migration outwards is to be interpreted as
a process of excretion, the connective tissue cells or
amcebocytes carrying the waste pigment from the
essential organs of the body outward to the inert
skin. For an elaborate hypothesis of this kind the
reader may be referred to a paper by Mr. H. E.
Durham; it is doubtful, however, whether such
generalisations are not as yet premature,

To return from this digression to the immediate
subject of the skins of fishes, we find that, accord-
ing to Cunningham and M‘Munn, the elements im-
portant in coloration occur in two layers in the skin,
and the two layers differ considerably in the upper
and lower surfaces of the fish. In most cases the
outer layer occurs in relation to the scales, while the
inner layer lies close to the surface of the muscles,
but in the mackerel the loose scales contain no
colouring elements. The outer layer consists of

x THE COLOURS OF FISHES 221

often branched and contractile chromatophores, con-
taining different pigments, and of polygonal cells
of peculiarly silvery appearance called by Pouchet
iridocytes. The silvery iridescent appearance is
due to the presence of crystals of guanin. The
inner layer consists of the same elements. On the
lower surface of the skin the outer layer contains
iridocytes but no chromatophores, the inner consists
of an argenteum—that dead-white opaque layer
which clings to the surface of the muscles and is
familiar to all who have examined fishes. Though
no histological elements can be found in the argen-
teum in the adult, development shows, as might be
expected, that it is formed from the fusion of irido-
cytes. The silvery or dead-white appearance is
again due to crystals of guanin. The distribution
of the colouring elements of the skin may be sum-
marised in the following table :=+

Upper Surface. Lower Surface.

Outer layer | Ividocytes and Chromatophores | Iridocytes only
Inner layer | Iridocytes and Chromatophores | Argenteum

This table is true for the flounder and, stato
mutandis, for other fishes as well. The points of
importance are the presence of a large amount of
reflecting tissue, especially on the lower surface, and
the absence of pigment cells on that surface. The
relative development of the parts varies of course
greatly in different fishes, but the distribution
appears to be fairly constant. The two layers of
reflecting tissue seem to have different functions in

222 COLOUR IN NATURE CHAP,

some cases at least, for while the outer layer may
exhibit brilliant iridescence, the inner “presents
either a chalk-white opaque surface or an evenly
bright silvery surface.”

The pigments contained in the chromatophores,
so far as they are at present investigated, appear
to be always either lipochromes or dark melanins.
The lipochromes are apparently very widely dis-
tributed and exhibit many different shades of colour.
They are chiefly known by their spectroscopic char-
acters. In some cases, as in many of the flat-fishes,
the yellow and black pigment cells produce a direct
effect upon the coloration—that is, the combination of
the two pigments produces a brownish colour tend-
ing either towards yellow or black, according as the
one or the other pigment is most abundant. Simi-
larly in the gurnards some species like Trzgla lyra
are red, and others like 7. gurnardus are gray,
the difference being, according to Cunningham and
M‘Munn, merely due to the relative development
of black and red pigment. There are, however,
other cases of greater difficulty in which the colour
is not obviously due to the combination of two
pigments. Thus the beautiful green colour of the
mackerel is produced not by a green pigment, but
apparently by a blending of black and yellow
chromatophores. In the black bands the black
chromatophores are much more numerous than the
yellow, while in the green bands the two kinds are
equally abundant. The statement made above, that
the green is the result of the “ blending ” of the two
colours, is due to Mr. Cunningham and Dr. M‘Munn,
but it is a little difficult to believe that there may

x THE COLOURS OF FISHES 223

not be in addition some assisting optical effect.
A similar but even more interesting case is that
of the green pipe-fish (Szphonostoma typhle), often
quoted as an instance of protective resemblance.
In this form a similar combination of yellow and
black pigment occurs, but there is some doubt
whether the green colour is not in part due to the
action of the reflecting tissue. This pipe - fish,
though usually green, may appear in a brown form,
while the common pipe-fish (Syngnathus acus) is
always brown. The common pipe-fish also contains
yellow and black pigments, but the yellow is said by
the authors to be more orange-coloured than that
of the preceding form. It is, however, curious to
note that the three pigments, that of the mackerel
(Scomber scomber) and those of the two pipe-fishes,
yield solutions whose spectroscopic characters are
almost the same; this certainly suggests that the green
is primarily a result rather of structure than of pig-
ment. True green pigments in fishes were, however,
described some time ago by Mr. G. Francis in certain
of the Wrasses (species of Odax and Labrichthys),
but the observations have never been repeated.
The Mediterranean members of the family are said
by Krukenberg to owe their green and blue colours
to structural effects.

In general, we may say of the colours of fishes
that they are either pigmental or structural, and that
the only pigments which have been described with
certainty are lipochromes or melanins. In most
cases both kinds of pigments occur simultaneously,
but in some, as in the gold-fish (Cavasszus auratus),
the melanin pigments are entirely absent. The

224 COLOUR IN NATURE CHAP. X

presence of a green colour is frequently associated
with a combination of yellow and black pigments,
but the green colour is probably always in part
produced by the structure of the tissues. The
silvery whiteness of the lower surface of most fish,
and the gleaming iridescence of the other parts of
the body in some, are both due to the abundant
deposits of guanin in the skin, but the relation of
this reflecting tissue to the brilliant evanescent
colours of many tropical fishes is unknown. Spots
or bands produced by the irregular distribution of
the colouring elements are common, but the reason
of this irregularity of distribution is quite unknown.
Usually the lateral line is marked externally by a
distinct band of pigment, but in some cases, as in
Atherina presbyter, it is marked instead by a white
band due to a special development of reflecting
tissue (Cunningham).

The colours of the lipochrome pigments vary
from yellow to red, but the yellows are apparently
the most common. In some instances the skin is
sufficiently transparent to allow the subjacent
muscles to shine through, and these may be so
coloured with hemoglobin as to have a distinct
effect upon the coloration.

According to Mr. Beddard (Anzmal Coloration, p.
11), the green colour of the bones of the fishes
Belone, Protopterus, and Lepidosiren is due to the
presence of the mineral vivianite, but no reference is
given.

CHAPTER XI
THE COLOURS OF AMPHIBIANS AND REPTILES

Colours and Pigments of Amphibia—The Colours of the
Larvee—Development of Colour—The Relation of the
Larval and Adult Coloration—Colour Variation in Larvee
—Colours of Reptiles, especially Lizards and Snakes—
The Origin of Markings in Snakes.

In Amphibia the colours are not infrequently sober
and incline towards the so-called protective tints, but
in some cases they are bright and conspicuous. The
skin is smooth, soft, and scaleless, and the power of
colour-change not infrequently well marked. As a
whole, the prevailing colours are dull brown or
black, shades of green, or vivid reds and yellows,
thrown up against a dark background. As ex-
amples of forms showing dull coloration we may
mention the Mexican axolotl and the common
newt. A green colour is well shown by the tree-
frogs (Hyla), often kept as pets in ferneries. The
spotted salamander (Salamandra maculata), with its
vivid colouring in black and yellow, and Mr. Belt’s
famous blue and red frog show, however, that more
brilliant types of colouring are far from being absent

Q

226 COLOUR IN NATURE CHAP.

in Amphibians. The power of colour-change is
familiar in the case of the common frog. A very
little experimentation will show that in it the
general colour is dull and dark against a back-
ground of earth or peat, and bright yellow-green
among fresh herbage. In spite, however, of this
power of colour-change, and of individual colour
variation, there is much constancy of marking. In
our common British frog there are two longi-
tudinal stripes running down the sides of the body
which seem to be absolutely constant, and which
appear in the larva at the commencement of the
metamorphosis. All those who have kept tadpoles
in confinement must have noticed this fact, and
learnt to regard it as a sign that their pets will
shortly require a complete change of environment—
from water to land. Markings of this kind occur
constantly both in Amphibians and Reptiles, and are
of much importance. Their origin has been investi-
gated in one case only, in a snake, and it has been
found that, as might be expected, they are closely
related to internal structures. Further investigations
on the same lines would be of much interest. That
they are constant throughout large groups and
dependent upon “laws of growth” has long been
maintained upon theoretical grounds by Professor
Eimer and his school.

As to the details of the mechanism of colour, the
slight development of the epidermis, and the power
of colour-change, show at once that the elements
important in coloration must occur in the true skin
or dermis. The epidermis does contain a small
amount of pigment granules, but these are unim-

XI THE COLOURS OF AMPHIBIANS 227

portant as compared with the contractile pigment
cells of the dermis. The colour-changes are due to
variations in the degree of contraction and expan-
sion of these pigment cells, or of their protoplasmic
contents.

The pigments, where they have been investigated,
have been found, as in the case of fishes, to be either
lipochromes or melanins. Guanin is also present in
the skins of Amphibians, but the amount is smaller
than in fish, and the effect on coloration much less
obvious. The lipochromes and melanins, occurring
separately or combined, produce such colours as
black, brown, yellow, orange, and so forth, while in
association with optical effects they produce such
colours as blue and green. The skin of the frog
when steeped in alcohol loses its green colour owing
to the fact that the lipochrome dissolves out. It is,
however, an old observation that the colour will
return if the grayish skin be covered over with wet
yellow tissue paper (Krukenberg).

THE COLOURS OF THE LARVA

The Amphibia display in general a very marked
metamorphosis during the course of development,
and there are often notable differences in the colour
of the larval and adult stages. The larve are
peculiarly susceptible to environmental influences
as regards colour, but they differ from the adults in
apparently being more sensitive to heat than to light,
and also in the relative stability and permanence of
the effect produced. There is some evidence to show
that the elements producing coloration are more

228 COLOUR IN NATURE CHAP.

uniformly distributed in the larve than in the adults,
and that development is accompanied by a re-
distribution of pigment as well as by changes in
amount. It is interesting to note that in Am-
phibians as in Fishes colour-changes, whether natural
or artificially produced, are not confined to the skin
but occur also in internal structures, eg. the peri-
toneum.

In studying the pigmentation of larve we may
take first the case of the spotted salamander (Sada-
mandra maculata), where the coloration has been
described in detail by Fischel. This salamander is
often kept in confinement, and the vivid black and
yellow colouring of the adult is very familiar. The
larve under normal conditions when nearly full-
grown are almost entirely black, the black pigment
occurring in spots on a somewhat lighter ground. A
histological examination shows that the colour is due
to the action of four factors, which are as follows :—

(1) Pigment granules lying in the cells of the
epidermis.

(2) Branched pigment cells lying between the
epidermal cells, sending their processes into the
intercellular spaces.

(3) Similar branched pigment cells lying beneath
the epidermis in the true skin or dermis.

(4) Pale yellow cells similar in character and
position to (3) but in the normal condition largely
concealed by these.

In the first three cases, the pigment is melanin of
dark brown or black colour, in the fourth it is yellow
lipochrome. The pigment cells of the dermis—
black and yellow—are by far the most important

XI THE COLOURS OF AMPHIBIANS 229

factors in coloration, the epidermic pigment having
relatively little effect. In the larva, the dermis over
its whole area contains both black and yellow cells,
but the latter are more abundant at certain regions,
corresponding to the future yellow spots of the adult.
The peritoneum contains also both yellow and black
cells, while in the adult there is no trace of yellow
cells in it.

Fischel’s observations on the coloration were made
in connection with some experiments on artificially
produced colour-change, and he did not therefore
carry them beyond this point. Before proceeding,
however, to consider the theoretical bearing of the
facts given above, we may supplement them by some
notes made by Dr. Bedriaga on the development of
colour in the larve of newts. Bedriaga did not
investigate the histology of his specimens, but in
view of the similarity of colour, we are probably
justified in applying Fischel’s statements throughout.

We may take the case of Molge montana, which
is described in some detail, as typical of one set. In
this form, larve 10-15 mm. in length are uniformly
yellow beneath, and yellowish-white with black spots
above. Larve 20-25 mm. in length show these
black spots spreading over the light ground, especially
on the upper surface. From 25 up to 40 mm. the
larvee show a gradual increase of the black pigment
which now becomes the ground colour, the light pig-
ment appearing in the shape of small spots, but
forming also a median yellow stripe. Larve of 40-
45 mm. length appear when taken from deep water
to be perfectly black, but if placed in shallow vessels
they become much lighter, and then exhibit very

230 COLOUR IN NATURE CHAP.

numerous black spots on a dull gray or yellow-gray
ground. This type, in which the very young larve
have dark spots on a pale ground on the upper
surface and a pale unspotted lower surface, is ex-
ceedingly common among species of Molge. During
the course of development the upper surface tends to
darken until it presents the appearance of a few light
spots on a dark ground, while the primitively uniform
lower surface becomes spotted with dark colour.
Development, therefore, seems to be accompanied by
an increase of dark pigment, and a decrease of yellow
or lipochrome pigment. This type, though common,
is not universal among the species of Molge; some,
like the common frog, are dark at first and gradually
grow lighter. Thus very young larve of Molge
alpestris occur in two varieties. In the one the
upper surface is covered with dark-brown spots
closely connected together so as to form a network
whose meshes are pure brown, while the lower surface
is clear and unspotted. The other variety is similar
except that the meshes of the network are grayish-
yellow instead of brown. As the larve increase in
size, the meshes of the network grow larger and paler
in colour until they become grayish-brown or yellow,
the network itself at the same time growing paler in
colour. Throughout, however, the distinction between
pale and dark larve is preserved, and there is evidence
to show that the dark are the future males, and the
light the future females. ’

The larve of Molge alpestris \ike many other
larval Amphibians may become sexually mature
before metamorphosis. In that case the colours
differ slightly from those of the normal adult, the

XI THE COLOURS OF AMPHIBIANS 231

difference expressing itself especially in diminished
brightness of tint and in the absence of the cuticular
modification which gives rise toa blue colour.

RELATION OF LARVAL AND ADULT COLORATION

It is unfortunate that the paucity of observations
makes us unable to say much on this interesting
subject, but the statements given above suggest one
or two points.

First as to the position of the pigment and its
meaning. We have seen that the dark pigment,
both in adult and in larva, occurs to a slight extent
in the epidermis, and much more markedly in special
pigment cells in the dermis or true skin. The
epidermis, with its contained pigment, is periodically
cast and renewed, so that there must be a slow
elimination of pigment from the body in this way.
We have already seen that it is most probable that
the pigment originates in the dermis or deeper tissues,
so that there must be a continual migration of pig-
ment from the dermis to the epidermis. Such a
state of affairs occurs very frequently in vertebrates,
and is, as noted above, often regarded as a proof that
the coloration is the result of an excretory process.
The development of the coloration, as already
described, supports this theory in so far as it tends
to show that the dark pigment usually increases in
amount during growth, and that it tends to be more
abundant in males than in females, in normal indi-
viduals than in those which are precociously sexual.
The proof would, however, be more cogent were it
not for the presence of the lipochromes. These can

232 COLOUR IN NATURE CHAP.

hardly be regarded as of excretory nature, they
apparently tend to decrease during the process of
development, but, on the other hand, they are
abundant in adults showing specialised colouring, as
in the case of the spotted salamander, and often
increase in amount during the breeding season in
individuals which exhibit a seasonal change. Reinke
(Arch. f. mtkrosk. Anat. Bd. 43, 1894) has sought
to solve some of those difficulties, and especially the
disappearance of the yellow cells from the peritoneum,
by the supposition that the yellow pigment may be
converted into the black. This is strongly opposed
by Fischel, and the question apparently does not at
present admit of decisive settlement. It thus seems
that the simultaneous existence of both black and
yellow pigments prevents us here, as in so many cases,
from accepting in its entirety the theory that pigment-
ation is the result of an excretory process.

Although our present knowledge does not enable
us to make any statement as to the origin of the
lipochromes in Amphibia, yet the developments
described above disclose some interesting facts in
connection with their distribution. The statements
of Fischel and Bedriaga taken together show clearly
that in the early stages of development the two sets
of pigments are closely intermixed, and that the
growth of the larve or the transition from larval to
adult life, tends to be accompanied by a process of
segregation, the yellow pigments accumulating at
certain spots and the black at others. The process
is an interesting one because it seems to occur
frequently in birds, in the males as compared with
the females. Its external result is to greatly increase

XI THE COLOURS OF AMPHIBIANS 233

the purity and brilliancy of the colouring, as well
as to give rise to markings. An inquiry into the
causation of the process would be of great interest.

COLOUR VARIATION IN LARVA

As we have already seen, the larve of Amphibians
are very sensitive to environmental influences, their
colour changing according to the conditions under
which they are found. This fact was noticed both
by Fischel and Bedriaga, and the former made some
extensive experiments on the subject. He and one
of his colleagues obtained a number of larve of
Salamandra maculata. Walf were put in a fish-
hatching apparatus, and the other half in standing
water in a dish. The former displayed the ordinary
type of coloration, the latter were distinctly lighter
without displaying any other abnormality. The
ground colour was a light yellowish-white, and the
ordinary black spots were only represented by slight
traces of dark pigment, occurring especially at the
posterior end of the body. As well as displaying
little pigment, the larve are said to have had a
peculiar glassy and transparent appearance. The
larve were not all uniform, but showed considerable
variation, those in the standing water were, however,
all much lighter than those in the running water.

On making a histological examination of the skin
of the pale larvz, it was found that the cells of the
epidermis contained a much smaller amount of
pigment than usual, while the branched pigment cells
of this layer and of the underlying dermis were
represented by dark-coloured oval or rounded bodies.

234 COLOUR IN NATURE CHAP.

A careful examination of a series of specimens showed
that these structures represented the normal branched
pigment cells, whose processes are here completely
contracted. The yellow cells of the dermis were, how-
ever, not contracted, and now displayed themselves
as a delicate network of branched pigment cells ; it is
to these that the pale larve owe their colour. The
difference in tint between the two sets of larve is
thus ultimately due to the differing susceptibilities
of the pigment cells of the skin. The peritoneum
exhibited the same variation in colour as did the skin.

As to the cause of the difference, experiment
convinced Fischel that it was due to the difference
in temperature of the water in which the larve were
reared ; that of the standing water being 9° to 11°
higher than the running water. This was confirmed
by removing dark larve from the cold water and
placing them in warm water, when it was found that
in the course of a day or two they had become
notably lighter, and at the end of at most a fort-
night were completely converted into the pale form.
Although the pale larvae so produced were quickly
reconverted into the dark form, it was not found so
easy to render the original light forms dark. A long
exposure to the influence of warm water apparently
renders it difficult for the pigment cells to become
fully expanded when exposed to cold, no doubt also
these larva were actually deficient in dark pigment.
The colour-change could only be produced in young
larve, older ones having apparently lost the power of
reacting to heat or cold. The effect of prolonged
exposure to heat or cold is, therefore, to produce a
stable unalterable type of coloration.

XI THE COLOURS OF AMPHIBIANS 235

The effect of light on the developing larve was
also tested, and it was found that light has but little
effect, though on the whole the larve are darker in
light than in darkness. This is interesting because
the same thing has been noticed in tadpoles of the
frog, which become pale and transparent in darkness,
while adult frogs, many fishes, reptiles, etc., become
pale in strong light. Thus at least in the frog the
larva and the adult react differently to the stimulus
of light.

As an interesting commentary upon these obser-
vations of Fischel’s, we may note that Bedriaga in
describing the larve of Molge aspersa, remarks that
while the colour is usually an olive-brown with
yellowish spots, larve taken from deep water are
much darker in tint, and the yellow spots are reduced
both in size and intensity. On the other hand,
larve from sunny shallow situations are pale in
colour and very distinctly spotted with yellow. As
Fischel found that light as a whole tended to darken
the colours, we must suppose that in this case it is
the temperature of the water in the two situations
which is important in affecting coloration.

Similarly, Karl Knauthe found that adult Am-
phibians (Bufo variabilis and B. vulgaris, Pelobates
Jfuscus, and others) turned dark, often gray or black,
when exposed to the influence of intense cold. It
would appear from this observation that the larve
and adults react similarly to variations in temperature.

236 COLOUR IN NATURE CHAP.

COLOURS OF REPTILES, ESPECIALLY LIZARDS
AND SNAKES

Among Reptiles the colours are most conspicuous,
and have been most studied among snakes and
lizards. In lizards black, gray, brown, and green
are common colours, while most display very distinct
and constant markings. According to Eimer the
typical ground plan of the dark markings consists of
seven longitudinal bands. The pigments seem to
consist of the dark “ melanins” and lipochromes, the
green colour of, ¢g., the green lizard being not due
to a green pigment but to the structure of the skin,
and to the contained yellow and black pigments.
According to Krukenberg lipochromes are rare or
perhaps absent in snakes, while melanins largely
predominate. Yellow pigments do occur freely in
snakes but their nature is undetermined, probably
they are merely altered lipochromes ; for the peculi-
arities which Krukenberg enumerates may be due to
impurities or to decomposition. Lizards frequently
exhibit the type of coloration known as protective,
and, as is well known, have often considerable power
of colour-change, best exemplified in the chameleon.
It is interesting to note that the aberrant poisonous
lizard Heloderma diverges markedly in colour from
other lizards, being vividly marked with black and
yellow, instead of showing the sober greenish hues
of most lizards with their beautiful but unobtrusive
striping. In its histological characters the skin of
lizards shows much similarity to that of the frog, the
power of colour-change, when present, being again due

XI THE COLOURS OF REPTILES 237

to the varying susceptibilities of the chromatophores
to the action of light. For an elaborate discussion of
the coloration of the wall-lizard (Lacerta muralis)
the reader may be referred to Eimer’s well-known
papers.

The colours of snakes are often dull, and: due in
large part to obscure mottlings, but in some cases,
as in the deadly coral snake (Z/afs), there is a con-
spicuous black and red banding. As compared with
lizards, snakes seem to be characterised by an in-
creasing predominance of epidermic pigments which
are got rid of and renewed when the slough is cast.
Many would regard this again as a proof that the
pigments are waste products, in process of being
eliminated by means of the skin. The skin of
snakes, like that of most reptiles, contains guanin,
which is said by Leydig to give rise in some cases
to white and yellowish patches.

ORIGIN OF MARKINGS IN SNAKES

In discussing the coloration of leeches we have
seen that an attempt has been made to explain this
on mechanical grounds, by associating the stripes or
spots with the development and arrangement of the
muscles—it is interesting to note that in snakes the
coloration seems to bear a similar relation to the
arrangement of the blood-vessels. Herr Jonathan
Zenneck finds that in the case of the ringed snake
(Trophidonotus natrix) the three longitudinal rows of
spots in the adult correspond to three red lines in
the embryo which mark the course of subcutaneous
blood-vessels. Zenneck found that in the adult the

238 COLOUR IN NATURE CHAP.

black spots were produced by accumulations of black
pigment in the cutis of these regions, in other re-
spects the skin showed no special peculiarities. He
therefore set himself the problem to find out what
anatomical or developmental necessities produced
these aggregations of pigment.

The ringed snake is marked by three paired
longitudinal rows of black spots, which correspond
to the longitudinal lines described by Eimer in the
wall-lizard, except that in the lizard there is, in ad-
dition, an unpaired median line on the dorsal surface.
In the embryo at an early stage the surface is marked
by three pairs of red lines, and a slender median
unpaired one. The lines are produced by the sub-
jacent blood-vessels and are not necessarily perfectly
continuous, being sometimes broken into red spots ;
they are connected with numerous transverse vessels,
the junctions being marked by distinct swellings of
bright colour. Of the three lateral lines the median
is the most distinct, and extends forward in front of
the eye. A little later the scales begin to develop
from before backwards, and as they become distinct
the red lines disappear before them, and, at the same
time the first traces of pigmentation appear as a
longitudinal row of pigment spots at each side,
developed in the position previously occupied by the
median red line already mentioned. The extension
of this line in front of the eye is now marked by a
distinct spot of pigment, which forms a marked feature
in the adult. The formation of this middle row of
spots is followed a little later by the appearance of
two other rows corresponding to the upper and lower
of the lateral red lines. The first spot of the middle

XI THE COLOURS OF REPTILES 239

row at a later stage fuses with the first of the upper
row at each side, and the two large spots lying close
together form the black part of the “collar” of the
adult. The author does not state what becomes
of the unpaired median red line, it presumably dis-
appears without being replaced by pigment spots,
but its presence is interesting in view of the fact that
the lizard has a black line in this region.

An examination of the red lines, by means of
sections, shows that they are due to superficial
blood-vessels connected at regular intervals with the
deeper vessels of the body. The median red line
of the three laterals corresponds to a vein called the
epigastric which receives blood brought back from
the vessels of the skin, and transmits it vz@ a series
of transverse vessels to the deeper veins, eg. the
cardinal vein. The first appearance of pigment,
except in the choroid, is in connective tissue cells
lining the body cavity, and these appear to spread
round about the epigastric, though this is uncertain.
At any rate the epigastric vein loses its endothelial
lining, becomes filled with connective tissue cells, and
is gradually obliterated, the obliteration being pre-
ceded by a formation of direct connections between
the vessels of the skin and the transverse vessels, so
that the epigastric vein is thrown out of the circula-
tion. At this period the pigment spots are developed,
and they occur in the cutis opposite the points where
the transverse vessels formerly entered the epigastric.
A similar relation exists between the other rows of
spots and blood-vessels.

Herr Zenneck leaves undecided the question
whether the pigment originates entirely in the con-

240 COLOUR IN NATURE CHAP.

nective tissue cells of the body cavity, and migrates
outwards through the blood-vessels in the wandering
cells which fill up the cavities of these, or whether it
May arise in part in the epidermis zz sz¢u, and con-
fines himself to emphasising the fact of the relation
between the pigment spots and the obliterated open-
ings of the transverse vessels. When we remember,
however, that the places where blood stagnates are
especially liable to become deeply pigmented, it is diffi-
cult to avoid the conclusion that the pigment directly
originates from the degenerating blood which must
accumulate at these points. The question has some
bearing upon theories of the origin of colour and
colour patterns, but is obscured by the usual diffi-
culty of distinguishing between fost and propter.

In connection with this paper of Zenneck’s we
may note an observation by Mr. J. Loeb on the rela-
tion between the blood-vessels and the coloration of
the yolk-sac in the embryos of the fish Pundulus.
The yolk-sac has here a peculiarly tiger-like colour
due to a combination of black and red chromato-
phores. At the time of their first appearance the
pigment cells are practically uniformly scattered,
but as the blood-vessels develop and the blood
begins to circulate, the chromatophores begin to
migrate to the surface of the vessels, and ultimately
pigment is only visible as a covering to the vessels.
By an ingenious experiment with a heart poison
(potassium chloride), Loeb convinced himself that
the migration only occurred when the blood was
actually circulating in the vessels of the embryo.
He concludes that the coloration of the yolk-sac
in Fundulus is due to a specific irritability of the

XI THE COLOURS OF REPTILES 241

chromatophores, comparable to the irritability dis-
played by many leucocytes, which forces the chroma-
tophores to migrate to the surface of the blood-
vessels. This observation affords an interesting
parallel to those of Zenneck’s, except that in the
latter case the chromatophores congregated about
degenerating blood-vessels, and in Loeb’s case about
living vessels. Loeb considers that the typical
coloration of the embryo is similarly related to the
distribution of its blood-vessels. He promises
further observations on the subject.

List describes the pigment of the embryos of
bony fishes as originating from the debris of the
yolk. It is then taken up by wandering cells and
carried to different tissues. The relation between
blood-vessels and pigment cells described by Loeb
for Fundulus, List describes as universal for verte-
brates. He considers that the surface of the blood-
vessels forms the main track outwards for the
pigmented leucocytes, which tend continually to
migrate from the deeper tissues to the epidermis.
The two investigators do not appear to have been
aware of each other’s work.

The interest of these three sets of observations
is that they all correlate the production of pigment
and the development of markings with the physi-
ology of the developing embryo, and suggest that
we may yet be able to similarly explain colour-
phenomena in general.

CHAPTER XII

THE COLOURS OF BIRDS

General Characters of the Coloration of Birds—Sexual Colora-
tion—Distribution of Colour and Colour Variation—Food
and Colour—Pigments of Birds, their Characters and
Distribution— Pigments of Birds’ Eggs— Markings of
Feathers.

THE colours of birds often rival or surpass those of
insects in brilliancy and variety, and have attracted
quite as much attention. Indeed, the greater size of
birds and their greater specialisation give to their
colouring a variety and a charm which many fail to
find in insects; this widespread admiration has un-
fortunately been in many cases singularly fatal to
the birds.

Birds resemble insects in displaying both types
of colour—the optical and the pigmental—to great
perfection ; but in marked contrast to the conditions
which prevail in the Lepidoptera, we find that the
bright pigments are usually lipochromes, never so
far as is known waste products of the uric acid
group, Further, although there are several instances
described of birds whose colours can be heightened
or altered by the employment of special kinds of

CHAP. XII THE COLOURS OF BIRDS 243

food, there is at present no reason to doubt that
under ordinary circumstances the lipochromes of birds
are self-produced and not derived.

Let us first consider the distribution of colour in
birds. The bright colours are largely, though by
no means exclusively, confined to the exposed parts
of the feathers. They may, however, occur on the
beak, the feet, and legs, the bare patches on the
head and neck of many birds, and even the parts of
the body which are covered with feathers, and the
mouth cavity. The colours in general fall into
three sets :—Those due to lipochromes, those due to
the dark melanins, and those which are structural.
The lipochromes when present tend to be uniformly
diffused, and are probably always in solution in the
abundant fats. It is an old observation that the
intensity of the red colour in the flamingo depends
upon the amount of oil contained in the feathers.
Besides occurring in the feathers, bill, feet, etc., lipo-
chromes usually colour the deposits of fat in the
body and the yolk of the eggs.

The dark melanin pigments are very widely dis-
tributed in birds as in all Vertebrates. They occur
in the form of minute amorphous granules in the
epidermis or the cuticular structures, and not in-
frequently give rise to brilliant structural colour or
to very elaborate patterns and markings; in some
cases they are uniformly distributed and give rise to
plain gray, brown, black, and related tints, Struc-
tural colours in birds are abundant, and include, in
addition to all metallic colours, blue, green, some
yellows, white, and in part the glossy black colours.
Blue, whether it occurs on feathers as in the jay, or

244 COLOUR IN NATURE CHAP.

on the beak, or the skin of the head, as in some
birds of paradise, is always structural. The naked
patches of skin in birds indeed exhibit the same
tendencies with regard to colouring as are visible in
the feathers.

SEXUAL COLORATION

Before giving a detailed account of the colours
and colouring-matters of birds we may summarise
some of the general characters of the coloration.
One of the most marked characters of the group is
the great prevalence of sexual differences in colour,
usually though not invariably of such nature that
the male excels the female in brilliancy, or at least
in intensity of colour. The colour differences are of
many kinds. Thus, as in the humming-birds and
the peacock, the male may display brilliant struc-
tural colour, absent or feebly developed in the
female; or as in some of the birds of- paradise, the
plumage of the male may be coloured by special
pigments which are absent in the female. Again,
as in our own blackbird, the male may, as compared
with the female, merely exhibit a greater intensity
of colour. The same fact is illustrated in a curious
way in the case of the satin bower bird (Pélono-
rhynchus violaceus); here the female has a grayish
somewhat thrush-like plumage exhibiting through-
out faint but distinct metallic tints, so that the
whole bird has a delicately iridescent appearance.
The male, on the other hand, is a deep glossy black,
with a hard metallic lustre—a more specialised, if
rather less beautiful colour. Another very interest-

XII THE COLOURS OF BIRDS 245

ing case is that of the genus Pericrocotus, in which,
according to Professor Newton, the males are gener-
ally black and rose-colour and the females gray and
saffron; this is probably again due to increased
amount of pigment in the male. So far the sexual
differences we have noted have depended on the
development of structural colour in the male, on the
development of new pigment, or on the increased
amount of existing pigments. There is, however,
another difference often marked, and that is the
relatively greater purity of tint in the male, and the
frequent presence of contrasting colours. Thus in
the male blackbird the glossy plumage contrasts
sharply with the bright yellow beak. In Seréculus
melinus, one of the regent birds of Australia, the
female is dull grayish-brown and speckled, while the
male is black, with brilliant patches of bright orange.
In the beautiful orioles (/cterus) of North America
the females are olive-green, the males black and
yellow ; the true orioles of the genus Oriolus show
the same sexual difference even more distinctly. In
the North American jays the colours of the males
are frequently blue, white, and black, with bars and
spots, while the females and some unspecialised
species are gray. Facts of this kind are of very
common occurrence, and have been much insisted
upon by Mr. Charles Keeler, who regards them
as tending to prove that the general ground colour
of the females or of unspecialised species is due
to a mixture of pigments, while the separation of
the pigments gives rise to the pure colours of the
specialised males. The force of Mr. Keeler’s argu-
ments is diminished by the lack of precise dis-

246 COLOUR IN NATURE CHAP.

crimination between pigmental and optical colours ;
but the facts are at least interesting, and it is quite
possible that the dull olive colours of many un-
specialised birds may be due to a mixture of lipo-
chrome and melanin, which, when separated, give:
rise to vivid orange and black colours in the males.
We have already noticed facts of similar nature in
larval Amphibians as compared with the adults

(see p. 232).

DISTRIBUTION OF COLOUR IN GENERA AND
COLOUR VARIATION

Closely associated with sexual colour differences
is the question of the distribution of colour in the
species of a genus, and in this connection a few
examples of the relation between yellow and red are
worth quoting from Mr. Keeler. That such a rela-
tion should exist at all is interesting, because it
‘presents some sort of parallelism to the relation
between red and yellow which exists in the
Lepidoptera. In that group there is some reason
to believe that the red pigment bears to the yellow
a direct chemical relation, but the reds and yellows
of birds are usually due to lipochromes, and the
relation between the red and yellow lipochromes is
still very obscure.

Mr. Keeler lays down the general rule that
wherever red is present in a genus, yellow will also
be present. Thus in the grosbeaks (Hadia) the
male of H. Judoviciana has a breast patch of bright
red, some of the wing-coverts being of the same
tint, in A. melanocephala these parts are lemon-

XII THE COLOURS OF BIRDS 247

yellow, while in the females of both species they are
a pale yellow. The brilliant red of the scarlet
tanager (Pyranga erythromela) is replaced in the
western tanager to a large extent by yellow, while
the females and young of both are yellow. So also
in the American redstart (Setophaga ruticilla) the areas
which in the female are yellow are orange-red in
the male. In none of these cases do the lipochromes
appear to have been investigated ; it is most probable
that in some at least the difference in colour is due
merely to the amount of pigment present in the
coloured parts, or it may be in part due to structural
differences.

The question as to whether it is possible to
speak of a geographical distribution of colour is an
interesting one, but one which it does not appear as
yet possible to decide. That tropical birds tend
to be brilliant, and Arctic birds white, appear to be
as yet almost the only certainties on the subject.
White as a general ground colour is of considerable
interest in birds; it is certainly most common among
the birds of cold climates, but is there often very
slowly acquired; the gannet, for example, takes
several years to acquire the pure white adult plum-
age, and so with many others in which the adults
are pure white. The existence of pure white birds
is complicated by the frequent occurrence of albinos
among many species normally coloured; “ white”
blackbirds are, for example, of common occurrence.
This albinism may be complete, affecting even the
eye, or it may be confined to special feathers or
regions of the body. Now the natural whiteness of
many birds is often compared to these cases of

248 COLOUR IN NATURE CHAP.

partial or complete albinism, although there is some
doubt how far the comparison is legitimate. White
patches are certainly often a sign of specialisation,
and are not infrequently confined to the males. In
the male of the king paradise bird (Cictnnurus
vegius) the ventral surface of the body is a pure
white, adding greatly to the beauty of the colora-
tion ; but the white feathers are a deep ashy-gray at
their bases, and there can be little doubt that this
is merely one of those cases of sifting of colour
which are so characteristic of many male birds. The
physiological meaning of cases like that of the
gannet, however, where there seems to be a complete
disappearance of colour, is very obscure ; to suggest
that it is due to a constitutional change does not
appear to advance the question much.

In connection with the subject of albinism it may
be well to mention that the converse variation, that
of excess of pigment, is said to occur both in the
case of melanin and lipochromes, giving rise to
melanism or erythrism. The fact that neither of
these can be associated with the functional dis-
abilities which are usually believed to result from
total albinism puts them upon a somewhat different
plane, but the whole question is difficult and doubt-
ful. The same may be said of some other facts
relating to colour, such as the tendency to melanism
said to be exhibited by birds occurring in islands.

Another question of some interest is whether a
change of colour can be effected in the plumage
without a previous moult. This is strongly sup-
ported by some authorities and as strongly denied
by others. Géatke, in his Heligoland as an Ornitho-

XII THE COLOURS OF BIRDS 249

logical Observatory, gives numerous instances of this
change occurring in birds, and distinguishes between
cases in which the colour-change is associated with
a change in feather-structure, and those in which it
is not so associated. Thus in the linnet and mealy
redpole, the surface of the barbs is said to peel off,
exposing the fresh and bright colours beneath. In
the case of the guillemot and little auks, on the
other hand, he describes an increase in the amount
of pigment in the feather without any textual
change. Gatke enumerates numerous other instances
of colour-change produced in this way by an in-
creased amount of pigment, or by a rearrangement
of pigment, as when black or blackish-brown replaces
white or gray, or when black and white replace
gray.

A change of colour accompanied by a shedding
of part of the feather seems not inexplicable, but
the mechanism of a colour-change without this
is difficult to understand, and the fact has been
strenuously denied by many. A recent summary
by Schenkling gives an account of the various
opinions which have been held on the subject.
Schenkling himself strongly inclines against the
view that a notable colour-change can occur without
a moult or a shedding of portions of the feathers.
He believes that the confusion has arisen from the
fact that the same moult has not identical effects
upon all the individuals of a species, or even upon all
the feathers of an individual. It is thus possible
to obtain specimens displaying feathers apparently
characteristic of successive moults. Such specimens
have been described as birds showing colour-change

250 COLOUR IN NATURE CHAP,

without moult, but it can be demonstrated that the
feathers possess their peculiar colouring before they
leave their sheaths, These statements are true only
of birds which display complex colour-patterns, and
which require years to completely attain the typical
plumage. Schenkling is of opinion that a more
careful study of such cases will greatly modify
existing views as to the rigid limits of each moult.

FooD AND COLOUR

It may perhaps be well to mention here for the
sake of completeness the colour-changes which may
be produced in some birds by supplying coloured
food. An account of these changes will be found
in Mr. Beddard’s Axzmal Coloration. In the case of
the canary it is well established that the addition of
cayenne pepper to the food will change the colour
from yellow to deep orange, or flame colour. Mr.
Beddard also cites the artificially produced change
from green to yellow in Brazilian parrots as another
instance of the effect of food on colour, but there
seems some doubt whether the change in this case
is not produced by direct local application to the
young feathers. In the case of the canary the
change can only be produced in very young birds,
which is so far evidence against the view that
mature feathers can change colour. In the canary
the change is produced by means of the intervention
of a fat,—a point of some interest because the
association of introduced pigments with fats is so
common—is perhaps universal. Similarly the Rajah
Lori is said to attain its brilliancy from a diet of

XII THE COLOURS OF BIRDS 251

fish fat (cf. the statement as to the flamingo
above).

THE PIGMENTS OF BIRDS, THEIR CHARACTERS
AND DISTRIBUTION

As to the details connected with the pigments of
birds, we may notice first that practically nothing is
known of the melanin pigments. They are widely,
perhaps universally, distributed in birds, give rise
to all the dull and sober tints, colour all feathers
displaying any beauty of marking, and are usually
associated with feathers displaying structural colour.
In birds as in mammals they form the groundwork
of the coloration. Of their characters little is known.
They may possibly be the same as the dark pig-
ments of mammals; an origin from hemoglobin has
been suggested here as elsewhere in Vertebrates, but
is not supported by very cogent evidence.

The lipochromes are numerous and diverse, and
have been chiefly investigated by Krukenberg.
The most familiar is probably the red pigment,
called by Bogdanow zoonerythrin, and by Wurm
tetronerythrin, which colours the red feathers of the
flamingo (Phenicopterus antiqguorum), of the cardinal
bird (Cardinalis virginianus), and of many others,
the red wattles in the male pheasant, etc. etc. The
only other red pigment certainly of lipochrome
nature which Krukenberg describes, is one which
he calls “araroth,” found in the red feathers of
certain parrots, and differing only slightly from
zoonerythrin.

Of yellow lipochromes, on the other hand, he

252 COLOUR IN NATURE CHAP.

describes a considerable number, differing from one
another by their spectra, their solubilities, or their
reaction to light. It does not appear that any good
purpose would be served by giving a list of these
pigments, at least in our present state of ignorance
as to the relations of the lipochromes. _ It is sufficient
to note that yellow lipochromes occur very frequently
in the skin, the fat, the yolks of the eggs, and the
feathers of birds; that these yellow lipochromes
not infrequently occur mingled with zoonerythrin,
that yellow feathers may contain two different
yellow pigments, and that these yellow pigments
may also be present in feathers which contain too
much dark pigment for the yellow colour to be
visible. The appearance of the feather is thus no
certain criterion of the presence or absence of lipo-
chrome pigment.

An exceedingly curious instance of this is afforded
by Krukenberg’s researches on the colouring-matters
of Eclectus polychlorus. In this interesting parrot the
male is chiefly green, with patches of red and blue;
the female is chiefly red, with patches of yellow and
blue ; while, according to Meyer, the young of both
sexes are red. The blue and green feathers are
grayish-black on their lower surfaces and appear
dull-coloured by transmitted light; the yellow and
red do not change colour by transmitted light. The
pigments present are one or more dark-coloured
melanins, a yellow lipochrome, zoofulvin, and red
“araroth”; the colour differences are in part due to
structure, in part to the varying amounts of the pig-
ments in the two sexes. This is shown in the
‘following table :—

XII THE COLOURS OF BIRDS 253

Green feathers contain zoofulvin and melanin.
Male Red ‘is >» *Sararoth,”
Blue $5 > melanin,
Red 3 >» **araroth.”
Female <{ Yellow. ,, »» zoofulvin and ‘‘araroth.”
Blue 55 > Melanin.

It would appear from this that the melanin
pigments are more abundant in the male, and the
lipochrome in the female and the young. The case
is a very interesting one, but it is doubtful how far
it is safe to build conclusion upon it.

Besides the lipochrome and melanin pigments in
birds, there are a few other isolated colouring-matters
of some importance. , Of these the best known is
turacin, with which the name of Professor A. H.
Church is so closely associated. Turacin is a
reddish-purple pigment occurring in patches on
some of the primary and secondary wing-quills of
various of the Musophagidz, or plantain-eaters, such
as the type genus and Corythaix. The pigment is
soluble in water, is said to be in part washed out of
the feathers by heavy rain, and also to colour the
water in which specimens kept in confinement are in
the habit of bathing. It is present in exceedingly
small quantities in the species in which it occurs,
and is absent from some of the genera of the family.
According to Church, it is absent in the species of
Schizorrhis, in which the parts of the feathers which
are in other genera coloured by turacin are here
marked by white patches destitute of pigment. The
great interest of the pigment is that it contains
copper and not iron, but presents many interesting
analogies to hemoglobin. There is, however, no
evidence that it can exist in both the oxidised and

254 COLOUR IN NATURE CHAP.

the reduced condition. The pigment, like carmine,
behaves as an acid, being readily soluble in dilute
alkalies, but insoluble in acids.

Turacin is generally supposed to be confined to
the plantain-eaters, but it has been also described
by Krukenberg in one of the cuckoos (Dasylophus
superciliosus). The use, meaning, and origin are
alike unknown ; its importance in coloration appears
to be relatively slight, the feathers in which it occurs
frequently showing bluish structural colour in addition
to the red colour due to turacin. It is somewhat
interesting to note that the family of the plantain-
eaters is an exceedingly small one of very limited
distribution—it occurs in Africa only.

If turacin be boiled for a long time in air, it loses
its red colour and becomes green, the change, accord-
ing to Krukenberg, indicating the conversion into a
new pigment. This new pigment he describes as
being devoid of copper, but containing a considerable
amount of iron; its spectrum shows a single band,
instead of the two of turacin itself. This green
pigment was found by Krukenberg in the green
feathers of Coryth@ola cristata, one of the Musopha-
gide in which turacin is absent, and of Corythaix
albicristata, one in which it is present. This seems
therefore to be one of those interesting cases of
chemical relations existing between the different
pigments of allied genera—a subject of which we
know only too little. Church is, however, inclined
to doubt the existence of an independent green
pigment.

Another interesting pigment of similarly restricted
distribution is the red colouring-matter to which the

XII THE COLOURS OF BIRDS 255

red feathers of the male king paradise bird (Czczn-
nurus regius) owe their brilliancy. This pigment,
called zoorubin by Krukenberg, occurs freely in
various species of the birds of paradise, chiefly in
the males, and has also been found in one of the
Indian trogons (Pyrotrogon diardi) in the male,
in the great bustard (Otis tarda), and in certain
varieties of the common fowl.

Zoorubin is soluble only in dilute caustic soda,
from which it is precipitated by the addition of
acid as a dull brownish mass. Its solutions show
no bands, but give two well-marked reactions. If
cold concentrated sulphuric acid be poured cautiously
into a test tube containing the solution, a blue or
green ring forms at the junction of the liquids.
Again, if the solution be rendered feebly acid and a
trace of copper sulphate added, a bright cherry-red
colour is produced. The pigment does not appear
to contain.iron or copper.

This list almost exhausts the known pigments of
birds, and its two most striking features are, on the
one hand, its uniformity, and on the other, the occur-
rence of peculiar and rare pigments like turacin in
exceptional cases. It may, of course, be suggested
that the impression of general uniformity is due
to ignorance, and that many families of birds may
contain peculiar and as yet undescribed pigments.
There is, of course, no proof that this is not so, but
at the same time the observations which have been
made by Krukenberg and others tend to prove at
least the very wide distribution of lipochromes and
melanins, while they have failed to disclose any pig-
ments of the uric acid group. The presence of

256 COLOUR IN NATURE CHAP.

melanin pigment is perhaps explicable enough in
view of the great prevalence of these pigments in
Vertebrates, but what are we to say of the lipo-
chromes? Is their presence in the feathers in some
families and apparent absence in others a sign of
the greater primitiveness of the first or not? If
their presence in feathers is associated with the
amount of oil in these structures, why are they
absent from the hair of mammals, which is also very
oily? We have also to consider the curious fact
that, while the muscles of fishes may be coloured by
red lipochromes, those of birds are not so coloured,
and the fat of birds is apparently always (?) coloured
with yellow, and not with the red lipochromes, Are
the reds formed from the yellow during the process
of the development of the feathers? These and
many other similar questions are suggested by the
study of the pigments of birds, and some at least
might be answered by a careful study of the pig-
ments even of the species of a genus. To say that
the coloration is in each case produced by natural
selection obviously helps us little, for it can hardly
be supposed that the insignificant colour patches pro-
duced by turacin can have been of such supreme
importance as to determine the development of a
new pigment, while similarly in the birds of paradise
a red colour is sometimes due to zoorubin and some-
times not.

PIGMENTS OF BIRDS’ Eacs

The colours of the egg-shells in birds are, as is
well known, often beautiful and varied. Rare as

Xu THE COLOURS OF BIRDS 257

blue pigments usually are among animals, blue and
green tints are exceedingly common among birds’
eggs, while various shades of brown, red, and yellow
also occur. According to Professor Alfred Newton
(Dictionary of Birds, article “ Eggs”), there is some
reason to believe that for a time the eggs increase
in brilliancy of colouring with each season until a
maximum is reached, after which the brilliancy may
again begin to decline.

The pigments of the egg-shells of birds have been
investigated by several authors. The important
points upon which all agree are first, that the colour-
ing is due to definite pigments; and second, that
these are derived directly or indirectly from hzmo-
globin—results of much theoretic importance. The
interesting point is not that derivatives of hemoglobin
should be used in coloration, but why, if vivid and
beautiful colouring-matters do arise in this way, they
should not be employed in the coloration of the
feathers. It seems also generally admitted that even
the ingenuity of that highly esteemed person, the
field naturalist, is unequal to the task of explaining
the colours of all birds’ eggs upon the hypothesis of
usefulness, so that from the theoretical point of view
these pigments are of quite special interest.

Of pigments colouring eggs, Mr. H. C. Sorby
describes seven with the following names and pro-

perties :—

1. Oorhodeine—a red-brown pigment of very common
occurrence and great permanence.

i losely rel
2. Oocyan "s pigments probably closely related,

3. Banded oocyan of which the second only yields a

banded spectrum.
S

258 COLOUR IN NATURE CHAP,

4. Yellow ooxanthine—a bright yellow pigment giving rise
when mixed with oocyan to the bright permanent
green so familiar in the eggs of the emu.

5. Rufous ooxanthine—a reddish-yellow pigment perhaps

peculiar to the eggs of the tinamon.

. A substance giving a banded spectrum but otherwise

little known.

7. Lichenoxanthine—a brick-red pigment, possibly due to
the growth of minute fungi.

On

As to the nature of the pigments, Krukenberg
regards the blue and green colours as due to modi-
fications of the bile- pigment biliverdin, and the
brown and red colouring-matters as closely allied to
iron-free hzematin (hamatoporphyrin), A more recent
observer, Wickmann, regards all the pigments as
originating directly from haemoglobin. According
to him, the pigments originate from the blood which
fills up the corpus luteum. This blood stagnates
and undergoes retrogressive metamorphoses which
result in the formation of the pigments. He com-
pares the process to that occurring in mammals,
where there is a formation of hzematoidin crystals in
the corpus luteum, the difference may, perhaps, be
explained by the diminished outflow of blood in
mammals consequent on the greatly reduced size of
the ova. According to Wickmann, the pigments
formed in this way within the ovary are shed into
the oviduct, and mingled with the materials of the
shell in its uterine portion. If his observations are
correct, they perhaps help to explain the facts noticed
by Professor Newton (of. cz#), that when a bird lays
only two eggs, it not infrequently happens that all the
available pigment is deposited on one, while the other
maybecolourless. Professor Newton gives the Golden

XII THE COLOURS OF BIRDS 259

Eagle as an examplé of this. Wickmann further
explains the differences in the pigments of the eggs
of different birds as the result of differences in the
composition of the blood. It is well known that in
mammals the blood varies in different species, as is
shown by the differences in the shape of the crystals
of hemoglobin, the colour of the plasma, and so on ;
similar differences may express themselves in birds
as differences in the products of decomposition.

For some criticisms of and additions to these
statements of Wickmann, reference may be made to
papers by Taschenberg and Von Nathusius.

If the pigments of the shell are iron-free deriva-
tives of hemoglobin, then the question of the fate of
the iron thus set free becomes interesting. Kruken-
berg is of opinion that it may be used to colour the
feathers in some cases ; he speaks of finding a large
amount of iron oxide in the feathers of the lammer-
geier, the feathers losing their dark brown colour
after the removal of the iron.

MARKINGS OF FEATHERS

We have already touched upon the interesting
questions connected with the markings of birds’
feathers, but a general survey of the colour phenomena
of birds would be incomplete without some further
reference to them. It is unfortunate that there is
so little certainty on the subject.

First, as to the origin of markings, and the
simplest form of marking. On this point there are
many suggestions, unfortunately, however, in most
cases only suggestions. Hacker, in an interesting

260 COLOUR IN NATURE CHAP,

paper on the subject, adopts the view that longi-
tudinal striping is the most primitive condition, that
this tends to develop into a spotted condition by
the suppression of portions of the stripe, and that
the fusion of spots gives rise to cross-striping.
Kerschener, on the other hand, regards cross-striping
as the primitive condition from which spots are
derived. In point of fact, the distinction is perhaps
less important than it seems, for Hacker’s conception
of waves of pigmentation passing down the shaft
might equally be regarded as resulting in longi-
tudinal striping, or in a very primitive form of cross-
barring. Hicker’s observations were made chiefly
upon nestlings of thrushes and chats (Turdide and
Saxicolinz), and also upon certain of the Limicoline
birds. His researches lead him to regard the most
primitive form of colouring as that seen in some of
the downs of the Limicole, such as Podiceps rubri-
collzs, where there is merely a little pigment collected
at the tip of an otherwise colourless down. This is
his ~r7mary pigmentation. Most downs, however,
show, on the other hand, in addition to this terminal
patch of pigment, a basal pigmented area due to the
process of secondary pigmentation. In this way is
produced the characteristic appearance of the feathers
of young thrushes, where there is a pigmented downy
area, and then a clear colourless area defined by a
terminal pigmented band. Besides occurring in the
young of the thrushes and their allies, this type of
coloration is found in the adults in the simplest
feathers, such as those of the cheeks, the chin, etc.
The primary pigment may form a dark spot at the
apex of the feather, giving the plumage a spotted

XII THE COLOURS OF BIRDS 261

appearance, or it may spread out to form an apical
band which then gives the plumage a cross-barred
appearance ; of the two the first is the more primitive.
Beginning with these simple types of pigmentation
common to the downs both of the Limicole and the
Turdide, Hacker seeks to prove that the coloration
of the adult thrushes can be derived from this primi-
tive type by various processes, especially the in-
creasing importance of the secondary pigmentation.
Thus if the secondary pigmentation increase greatly
in importance, it may encroach upon the colourless
median area and, uniting with the primary apical
pigment, produce a uniformly coloured feather.
Again, the primary pigmentation may disappear,
and the colourless median area form a border to
the secondarily coloured feather, and so on. It is
unnecessary to carry the consideration of Hiacker’s
theories beyond this point. There is apparently no
doubt that the spotted appearance of the plumage in
young thrushes is a primitive condition, and the
nature of the pigmentation of the feathers in them is
therefore of great interest, but when the attempt is
made to derive more complex forms of marking
from these simple ones, there is great difficulty and
uncertainty. A point of some interest is the question
whether there is any relation between the structure
of special regions of feathers and the characteristic
pigmentation of these regions. It is at least certain
that there is much constancy in the association of
special types of colour with special regions of the
feather. The nature of the association we shall
consider in the next chapter in corinection with the
colours of certain families of birds.

CHAPTER XIII

THE COLOURS OF BIRDS (Continued)

The Structure of Feathers—Relation between Structure and
Colour—The Colours of Sun-birds, Humming-birds, and
Birds of Paradise, their Distribution and Characters—
Markings of Kingfishers—General Characters of the
Colours of Birds—Meaning of Colour in Birds.

HAVING in the previous chapter considered some
general aspects of the colours of birds, we shall now
proceed to study the coloration of special families in
detail. In order to make the descriptions readily
comprehensible, it will be first necessary to briefly
revise the structure of feathers.

Feathers are outgrowths of the epidermis, formed,
like all such outgrowths, of the substance keratin.
They differ according to their function, and the part
of the body in which they occur. Thus there are
the quill-feathers which occur in wings and tail, the
general contour-feathers which cover the surface of
the body, and the downs or soft under feathers,
which are often abundant on the breast. All these
either contain pigment or are filled with bubbles of
air and so display a white colour. Before proceed-
ing to describe the distribution of pigment in these

CHAP. XIII THE COLOURS OF BIRDS 263

feathers, we shall consider their characteristics in
detail.

We shall take first a quill, such as one of the
primaries of the wing (see Fig. 1). Such a feather
consists of a central axis or stem bearing on its upper.
portion a large number of lateral growths—the barbs.
The lower naked part of the axis forms the quill,
while the whole of the remainder of the feather is
known as the vane and consists of a central rachis

Fic. 1.—Feathers of sun-birds to show relation between
colour and shape. The quill-feather was uniformly coloured
except for a slight edging of metallic colour at one side; the
short feather shows three zones—a terminal metallic zone, a
median dark-coloured and slightly V-shaped zone, and a
downy basal zone.
and lateral barbs. The barbs of the vane are closely
connected together, and on pulling them gently
apart, it is possible to see that they bear on either
side innumerable small processes, the barbules. The
barbs cling together because the barbules are locked

to one another in a manner presently to be described.

264 COLOUR IN NATURE CHAP,

Each barb bears two rows of barbules, and one of these
rows points to the tip of the feather, and the other
to its outer or inner edge. The latter is called the
proximal, and the former the distal row. Each
barbule consists of a flattened process which appears
to be twisted upon itself at about the middle of its
length. Its proximal part has therefore the appear-
ance of a flattened lamina and its distal of a filament,
as owing to the twist the edge only is in the plane
of the lamina. Now in the barbules of the distal
series, the filamentous region bears a series of
hooklets and slender processes which fit into a
groove and notches developed in the lamina of the
proximal barbules. Each set of distal barbules is
thus hooked into a set of proximal barbules, so that
each barb is locked to its neighbour. When the
uniform surface of the feather vane is destroyed by
forcibly separating the barbs, the hooklets are pulled
out of the groove in which they lie. When the
feather is restored to its original condition by
smoothing with the fingers, the hooklets are slipped
back into their original position.

The barbs just described constitute the greater
part of the vane of a quill-feather, but at the base of
the vane there will usually be found a number of
barbs of very different appearance. These are the
downy barbs, and they are characterised by the fact
that they are quite unconnected, and that their
barbules are usually very long and slender, so as to
be far more conspicuous than the barbules of the
vane proper. These barbules bear no hooklets, the
twisting is less obvious, and the appearance of length
is given by the great development of the filamentous

XIII THE COLOURS OF BIRDS 265

region. Barbs bearing barbules of this type are
the only ones present in down feathers; these are
further characterised by the shortness of their axis,
and are rarely important in coloration, except in the
‘young.

The small feathers which cover the surface of the
body differ in several respects from quills. They
are much shorter, the quill region is practically
absent, the rachis is reduced in length and thickness,
and the downy region tends to be more fully de-
veloped. The result of this shortening of the axis
is that the barbs tend to radiate from a common
point, while in feathers with elongated axis they are,
roughly speaking, parallel to one another. This has
an important bearing upon the coloration, for it is
obvious that if the barbules standing near the ends
of the barbs tend to exhibit special colours, then
the colour will form a transverse band on short
feathers, a longitudinal band on long feathers.
Similarly the median barbules will form a trans-
verse band on short feathers, a V-shaped marking
on long feathers; both of these actually occur (see
Fig. 1).

RELATION BETWEEN STRUCTURE AND COLOUR

We have already seen that there are three great
sets of colour phenomena displayed by the feathers
of birds:—(1) The feathers may show beautiful
and complex markings in brown, gray, and black ;
(2) they may display vivid optical colours; or
finally (3) may contain brightly coloured pigments,
usually of the nature of liprochromes. Of these

266 COLOUR IN NATURE CHAP.

three, the first occur equally in long quill-feathers
and in the short contour-feathers; they have no
obvious relation to the structure of the coloured
parts, and as already seen, little is known of their
meaning or course of evolution.

(2) The optical or structural colours are divided
into subjective and objective. The changing sub-
jective colours occur only in the barbules, and require
the presence of a large amount of dark pigment for
their full manifestation. Objective colours like green
and blue are confined to the barbs and do not occur
in the barbules, and (3) the bright pigments occupy
the same position. In general terms, therefore, we
may say the barbules always contain a certain amount
of dark pigment, and when this is in excess and the
structure is modified metallic colours arise. The
barbs, on the other hand, may contain dark pigment,
may show objective optical colours, or may contain
bright pigments. The variations which produce these
colour phenomena are much commoner in the general
feathers of the body than in quills; they do not
usually occur simultaneously, and the appearance of
any one set of colours is associated with an increased
development of the special region of the feather with
which the colour is associated, as of the barbs, a
portion of the barbules, and so on. It may be that
this is in part the explanation of the fact that, apart
from the development of markings, quill-feathers are
slow to vary in colour, and are rarely brilliant.
Colour brilliancy is associated with a special de-
velopment of some individual region of the feather,
and it is essential for the purposes of flight that there
should be a harmonious development of all the parts

XIII THE COLOURS OF BIRDS 267

of the quills, and no specialisation of particular areas.
Therefore any tendency to the development of
brilliant colouring in the wing-quills would be checked
by the resulting injury to flight, and so to the well-
being of the species.

It will be noted that in the following descriptions
red and yellow colours are always ascribed to the
presence of lipochrome. We have already seen that,
according to Gadow, yellow may at times be an
optical colour; in the cases discussed, however, the
presence of yellow lipochrome has either been
directly proved, or is assumed from the simultaneous
occurrence of a red colour, which is always due to
lipochrome pigment. The thesis here put forward
as to the relation existing between brilliant colouring
and variation in feather structure, we propose to
develop by a consideration of the colour phenomena
in sun-birds, humming-birds, and birds of Paradise.

COLOURS OF SUN-BIRDS

The Nectariniidz or sun-birds are a family of
mostly small birds often with brilliant colours,
inhabiting Africa, and India, and the Malay, where
they seem to replace the American humming-birds.
Their beauty and their habit of frequenting flowers
have caused them to be frequently confounded with
true humming-birds, but they are not in any way
related to the latter. The bright colours are almost
entirely confined to the males, and are by them
acquired with extreme slowness, so that birds are
said to be not infrequently seen mated while the
male is still in a sort of hybrid plumage. Birds in

268 COLOUR IN NATURE CHAP.

this condition are peculiarly ugly, as the bright
metallic feathers occur scattered among the dull
youthful plumage. If the colours of the male
exercise as important an influence on the choice of
the female as is commonly asserted for birds, the
female sun-bird must also be assumed to be possessed
of much faith and foresight. Some at least of the
ornamental feathers of the male are cast almost as
soon as the breeding season is over.

NATURE OF BRIGHT COLOURS

The bright colours of the sun-birds are due either
to lipochrome pigments or to metallic structural
colours, belonging to Gadow’s group of subjective
structural colours.

1. Pigmental Colours——The colours due to lipo-
chrome pigments are either yellow or bright scarlet-
red. Brilliant patches of red or yellow feathers
frequently occur on the throat or on the ventral
surface, or dorsally at the root of the tail. The
feathers so coloured are always short contour-feathers
and not quills. The bright colour is confined
to the apical part of the feather, the base being
grayish or white, and the pigment occurs as usual only
in the barbs. The barbules, if present, are grayish,
but most frequently they are rudimentary or absent,
so that the visible part of the feather consists of the
diverging naked barbs, containing a considerable
amount of bright pigment.

2. Structural Colours—The metallic colours of
the sun-birds occur on feathers arranged in special
patches on the head and throat, or as transverse

XIII THE COLOURS OF BIRDS 269

bands near the tip of the general contour-feathers, or
as longitudinal bands at the edges of the quill-feathers.

é ad

Fic. 2.—Barb and barbules from feathers of sun-birds,
magnified to show the peculiar structure of metallic barbules.
ais a barb bearing both proximal and distal barbules, the
lower barbules being partially, the upper completely metallic.
4, non-metallic distal barbules from a tail-quill of a sun-
bird, showing lamina, hooklets, and filamentous region.
c, partially metallic distal barbules ; the colour is confined
to the enlarged filamentous region, but the lamina and
hooklets persist unaltered. d@, completely metallic barbules,
with no trace of hooklets or lamina.

In tint they are usually green, blue, violet, or reddish-
violet, yellow or red ‘structural colours being absent.

270 COLOUR IN NATURE CHAP.

The colours are produced by a modification of the
barbules of the metallic feathers. We have already
described the general structure of barbules and
noticed that each is divided into two regions—a
proximal flattened region which may be called the
lamina, and a distal slender region which, from its
appearance, may be called the filamentous region.
Now as in sun-birds the metallic colours are usually
confined in quill-feathers to a lateral stripe, it is
obvious that it is possible to obtain a single barb
which bears both metallic and non-metallic barbules.
If we examine microscopically a non-metallic barbule,
we shall find that it exhibits the ordinary structure
of a barbule, and shows quite distinctly the division
into two regions separated by a twist (see Fig. 2, 0).
The metallic barbules (2), on the other hand, are of
quite different appearance, being broad, flattened, club-
shaped bodies supported on a short stalk, and con-
taining abundant dark pigment. Close examination
of the barb(a)shows that the metallic and non-metallic
barbules are not perfectly sharply defined, but tend
to pass into one another. Thus, as we follow the
non-metallic barbules upwards, we find that the
lamina diminishes in size, while the filamentous region
becomes flattened, broader, and larger, at the same
time losing its slender processes (c). Finally, the
lamina becomes so much reduced as to form only the
short stalk of the metallic barbules, while the distal
region becomes modified into the club-shaped body,
and is then completely devoid of hooklets or pro-
cesses (cilia). These club-shaped barbules further
exhibit a series of cross bars which, according to
Gadow, are a series of compartments overlapping

XIII THE COLOURS OF BIRDS 271

like the tiles of a roof. The ultimate causation
of the physical colour Gadow ascribes to the trans-
parent sheaths of keratin covering these compart-
ments, which he thinks act like a series of prisms.
An important point in connection with these metallic
barbules is, that they are so modified that both
hooklets and folds are completely lost, and therefore
there is no connection between the barbules or the
barbs. Metallic feathers of this type have therefore
a peculiar looseness of texture which is, for example,
very obvious in the ornamental feathers of the
peacock; the solidity of the flattened metallic
barbules gives, however, to such feathers an appear-
ance quite different from that of ordinary downy
feathers, in which also the barbs are unconnected.
The unconnected nature of the barbs is of especial
interest, because it would render the feathers quite
unfitted for purposes of flight if the variation were
to occur in quill-feathers. In sun-birds it is usually
the contour-feathers which are metallic, rarely the
tail-quills, and apparently never the wing-quills.
Development of Colours—The types of coloration
already described in the sun-birds are seen in the
specialised feathers, especially of the male. In the
unspecialised feathers, such as the general contour-
feathers of the female, we find what may be regarded
as the primitive condition. These feathers are of a
dull olive colour, and are divided into three regions—a
basal downy region usually of an ashy colour, a median
slightly V-shaped region in which the barbules have
a very close texture and are of a brown colour, an
apical region in which the barbules are unconnected,
slightly modified, and faintly pigmented with dark

272 COLOUR IN NATURE CHAP.

pigment, while the barbs stand out as being of a dull
yellow colour, apparently produced by a mixture of
lipochrome and melanin. This type is seen in most
of the females and in the males of the inconspicu-
ously coloured species. Taking such a feather as a
starting-point, we may have divergence in two direc-
tions. In the first case, the lipochrome pigment
may increase greatly in amount, and colour the barbs
very deeply, while the dull barbules do not become
brightly coloured, but tend to become rudimentary
and disappear ; thus we get the bright red or yellow
patches formed. On the other hand, the lipochrome
may get swamped by the development of a large
amount of melanin, which occurs not only in the
barbs, but also in the barbules. At the same time,
the modified barbules near the ends of the barbs
progress further in the direction in which they have
begun to develop, become converted into completely
metallic barbules, and thus give rise to the band of
metallic colour seen at the ends of the general
contour - feathers of many males, eg. Mectarinia
famosa. When, as in Cinnyris frenatus, the ventral
surface of the male has both metallic colour and
bright pigmental colour, it is possible to find in-
dividual feathers displaying both tendencies—that is,
with naked yellow barbs at the tip, then metallic
barbules placed on a dull-coloured region of the-
barb, and then the covered unspecialised region of
the feather.

In quill- feathers the tendencies as to colour
evolution seem slightly different. The quills of the
females, of the unspecialised species, and of the
wings of perhaps all species are a dull brownish

XIII THE COLOURS OF BIRDS 273

colour, with a tendency to exhibit a longitudinal
edging of olive colour in which the barbules are
pale-coloured, unconnected, and slightly modified.
This pale band is sometimes replaced in the greater
wing-coverts of the male by a dark brown band ; in
the tail-quills of the male usually by a metallic
band which, in the case of the central rectrices, may
invade the whole vane. The development of lipo-
chrome colour or of transverse bands of colour does
not occur in the case of quill-feathers. The latter
is due to the fact that in sun-birds it is only the
barbules which stand near the distal end of the barb
which tend to become metallic, the result being the
formation of transverse stripes of bright colouring
on short feathers, and longitudinal stripes on long
feathers, the type developed having a definite rela-
tion to the length of the feather (see Fig. 1). This
peculiarity is apparently the result of the fact that
in metallic barbules the lamina tends to disappear,
and this seems to occur typically only in downy
barbules or in the barbules standing near the apices of
the barbs. Downy barbules never become metallic,
so that it is only the apical barbules which can
become metallic, and give rise to a band of colour.

From the above description it is obvious that the
development of brilliant colouring in sun-birds is
certainly associated with modifications of feather
structure which cause the feathers to deviate more
or less completely from the primitive type of feather
structure.

274 COLOUR IN NATURE CHAP.

COLOURS OF HUMMING-BIRDS

Turning now to humming-birds, we find that
here pigmental colours are of relatively little im-
portance, while structural colours attain an extra-
ordinary beauty and brilliancy. Further, we find
that the place of the pigmental colours of sun-birds
in contrasting with and showing up the metallic
colours is taken in humming-birds by black and
white. White especially is often of great import-
ance in producing the general effect of beauty.

Fic. 3.—Metallic feather from the throat of a humming-
bird. The colour is confined to the central pigmented
patch and is there exceedingly bright. The barbs are con-
tinued beyond the metallic region, but are then without
barbules. The figure at the side is a section through the
barb to show the method of insertion of the barbules which
produces strongly marked ridges on the surface, so that the
barbs lie at the bottom of a trough.

In humming-birds metallic tints occur in both
sexes, but are usually more brilliant in the male.
They very frequently occur on the general contour-
feathers, the colour being then often a bronze-green,
which is not sharply confined to a transverse band,
but fades away gradually behind. The metallic

XI THE COLOURS OF BIRDS 275

colours which are especially characteristic of hum-
ming-birds, however, occur, as is well known, in
patches of extraordinary brilliancy either on the
head as a crest, or on the lower surface, especially of
the throat. The feathers forming these patches are
peculiarly modified, and may display any of the
colours of the spectrum including ruby-red and
golden-yellow—the colours which are so markedly
absent from the metallic feathers of sun-birds. The
rectrices of humming-birds not infrequently display
metallic colour, which may be distributed over the
whole feather, or may be limited to a transverse band
near the tip. Longitudinal bands of metallic colour
like those of the sun-birds do not seem to occur.

Pigmental colours among humming-birds are not
remarkable for brightness of tint, being usually shades
of gray or dull brown. The only marked exception
is the colour called by systematists “rich chestnut”
or “ cinnamon,” which is often limited to the males,
as, for example, in Eustephanus fernandensis. In
this connection it may be noticed that not only
are metallic tints almost invariably absent from the
wings, but where, as in the above species, the male
as compared with the female is characterised. by the
development of a special pigmental colour, this pig-
ment is entirely absent from the wing-quills, though
present in the wing-coverts.

As an exception to the general rule that the
humming-birds display great brilliancy, we find that
the so-called “hermit” forms which live in the deep
shades of the forests are only soberly tinted, with
little metallic colour; of these the genus Phethornis
may be taken as a type.

276 COLOUR IN NATURE CHAP.

Structure of Metallic Feathers. — The brilliant
metallic feathers of the head region of many male
humming-birds are in several respects very peculiarly
modified. They are very short, much rounded, and
overlap one another ; the surface is strongly metallic
and marked with deep ridges (see Fig. 3). A further
point of interest is that the barbs, quite devoid of
barbules, are prolonged as a delicate fringe beyond the
apex of the feather. While for further details I must
refer to my paper on the subject, we may simply notice
that the metallic colouring is here not produced by a
modification of the distal portion of the barbules, but
by a deeper pigmentation and a structural change in
the proximal region. The result of this is that the
metallic colour in humming-birds tends to appear
first in the middle region of the body feathers—that
is, the region where the barbules tend to attain their
maximum development, and not at the tip of the
feathers as in sun-birds. This primitive condition is
well seen in the breast feathers of the female of
Eustephanus fernandensis, and of both male and
female of Z. galerttus. Here we have white or dull-
coloured feathers, with a central spot deeply pig-
mented, and displaying a varying amount of metallic
colour. The increasing specialisation of the metallic
region is accompanied by a gradual retrogression of
the apical region which is eventually represented
only by the slender naked barbs,

The metallic modification of the feathers in
humming-birds is therefore not accompanied by
any change which affects the locking together of
the barbules, and so the adaptability for purposes
of flight ; it differs in this respect sharply from the

XU THE COLOURS OF BIRDS 277

modification seen in the sun-birds where the metallic
barbules are entirely unconnected. We can thus
understand how it is that the quills of humming-
birds may display structural colour without their
efficiency being in any way impaired. The fact is
also readily explicable on structural grounds, if we
recollect that it is the mid region of the feather
which tends to become metallic, and it is this region
which is most fully developed in quills. In humming-

Fic. 4.—Metallic barbs from feathers of humming-birds,
magnified. a@ is from a feather belonging to a female of
Eustephanus fernandensis, the barbules at the base are
metallic, but the barb also bears rudiments of barbules at
its tip. 4, a barb from the feather shown in Fig. 3; all the
barbules are metallic and the tip of the barb is naked.

birds the colour differences between the male and
female, or between specialised and unspecialised
species, are thus largely the result of an increased
amount of melanin pigment in the brilliant forms,
accompanied by a process of structural modification.

The type of metallic colour seen in the humming-
birds is of much interest, because it has not been
described outside of the family. In other cases we

278 COLOUR IN NATURE CHAP.

find that metallic colours in birds are of the type
described in sun-birds, z.e. are due to the conversion
of the distal portion of the terminal barbules into a
club-shaped body consisting of a series of overlap-
ping compartments, the process being accompanied
by the total suppression of cilia and hooklets. Such
a modification of structure is apparently of very
common occurrence in birds, but does not give rise
to metallic colour unless there is a simultaneous
development of a large amount of black pigment.

COLOURS OF THE BIRDS OF PARADISE

We may now for a little pass to the consideration
of the birds of Paradise, which on account of their
greater size afford more obvious illustrations of’some
colour problems. The birds of Paradise as a group
exhibit an extraordinarily specialised type of colora-
tion, the specialisation being visible alike in colour
and in structure. Though probably nearly allied to
the crows, the development of melanin pigment is
here less remarkable than the display of bright pig-
mental colours. These are in part due to lipochromes,
but in part, as we have already seen, to the pigment
zoorubin, which is almost confined to the group.
Further, we have not only the display of tufts and
crests of additional feathers, as in humming-birds,
but we find that these feathers are modified in every
conceivable way, sometimes being reduced to mere
wires, and at other times displaying brilliant metallic
colours. Among many of the birds of Paradise the
metallic colours are of somewhat limited distribution,
contrasting with the pigmental colours rather than

XIII THE COLOURS OF BIRDS 279

forming the basis of coloration. In the nearly related
rifle-birds, on the other hand, the pigmental colours
have disappeared, and the great predominance of the
melanins is associated with the development of the
most gorgeous metallic colour, set off by the velvety
blackness of other feathers ; a similar type of colora-
tion occurs in the genera Parotia and Astrapia among
the true birds of Paradise; in these the speckled
plumage of the female is very noticeable. As usual
throughout the beauty of colouring is largely confined
to the adult males, the females and young males
being relatively unadorned.

The birds of Paradise, as is well known, inhabit
the Malay Archipelago, and a full description of the
family, including an account of the native methods
of obtaining them, will be found in Mr. Wallace’s
account of his travels in that region.

The great bird of Paradise, called Paradzsea
apoda by Linnzus, who described it from a specimen
preserved after the native method, and therefore
without its feet, may be chosen as an example of
one of the prevalent types of coloration. In this
bird the quill-feathers of tail and wing, and the
feathers which cover the greater part of the back, are
a dull brown colour, showing little specialisation of
colour. In the feathers of the back the barbs are
devoid of barbules near their apices, but show no
other specialisation. At the sides of the body are
the beautiful erectile tufts of feathers, which give the
bird half its beauty. These consist of long drooping
feathers, pinkish-white in colour, with a tuft of short
bright yellow feathers at their base. The elongated
feathers have muchelongated barbules,astructure some-

280 COLOUR IN NATURE CHAP.

what resembling that seen in downy feathers, and like
many downy feathers they have very little pigment.
Among these long feathers are the so-called “ wires,”
which are feathers with all the parts save the rachis
suppressed. The yellow feathers have no barbules,
and the barbs are smooth, dilated, and brightly coloured
with lipochrome pigment. The same pigment and
the same feather structure is found in the bright yellow
feathers forming the crest. Round the base of the
beak and extending over the throat there is a band
of green metallic colour produced by very much
shortened feathers, in which the barbules have under-
gone the same modification as those of the metallic
feathers of sun-birds. Speaking generally, we may
say that this bird shows more tendency to develop
additional plumes than any great brilliancy of colour,
but when bright colours are developed, their develop-
ment is associated with a tendency to suppression of
the barbules, and to dilatation of the naked barbs.
The king Paradise bird (Czcznnurus regius), on the
other hand, shows less tendency to develop additional
tufts but much greater brilliancy of tint. The female
is a dull grayish tint, with a speckled breast, the male
is brilliant red on the back, with a metallic green
band separating the red head from the pure white
of the ventral surface. The tail contains two much
elongated wires displaying a brilliant green colour at
their curled tips. The red feathers are coloured by the
peculiar pigment zoorubin, which is practically absent
from the female. The feathers containing it have as
usual naked barbs, which are smooth, dilated, and
polished, so that as compared with the general
contour-feathers of the preceding species they are

XIII THE COLOURS OF BIRDS 281

modified both as to structure and pigmentation.
The quill-feathers are mostly dull in colour, but a
close examination shows that this relative dulness is
due to the fact that, while the barbs are as before
coloured with bright red zoorubin, the barbules contain
a dull brown pigment, the result being to greatly
diminish the brilliancy of colour. Certain of the
quills have bright scarlet longitudinal bands at their
edges; this is due to the fact that here the barbules
are absent and the bright red barbs have their full
effect.

It would be tedious to go on to discuss in detail
the coloration of the rifle-birds, but we may briefly
notice that here, associated with the development of
a large amount of melanin and the loss of the lipo-
chromes, we have also the loss of the tendency to
suppression of the barbules; here these tend to become
greatly specialised, and to develop metallic colour.
In the elongated metallic feathers of the throat of
eg. Ptilorts magnifica, we have further the develop-
ment of those V-shaped bands of which we have
already spoken.

The examples given above have been taken from
birds exhibiting bright pigmental colours or subjective
structural colours ; perhaps we may be allowed further
to give some illustrations with regard to the objective
structural colours, like blue and green. A blue colour
is always entirely confined to the barbs of feathers,
and is often associated with a suppression of the
barbules; it only appears on exposed parts of
feathers.

282 COLOUR IN NATURE . CHAP.

MARKINGS OF KINGFISHERS

Blue and green structural colours are admirably
displayed in the family of the kingfishers, which
show also a gradual progression of colour. Thus in
Ceryle rudis the feathers are dark brown or black,
more or less irregularly spotted with white, but with
the white showing a distinct tendency to form a
transverse band at the tip of the feather. In Ceryle
guttata the feathers are regularly cross-barred with
dull blackish-brown and white. In Carcinentes
melanops in the wing-coverts the covered part of the
feather is striped black and white, but the terminal
bar of white is replaced by blue. In the tail-quills
the under surface is usually black and white and the
upper surface blue and black. Where there is partial
overlapping of the quills, one side of the vane may
be black and white and the other exposed side black
and blue. The blue patches occur in positions
corresponding to the white ones, but are larger and
show a tendency to fuse together. In the case of
quill-feathers, the blue is confined to the barbs but
the barbules are still present, and their dull colour
somewhat diminishes the brilliancy of the blue. On
the general contour-feathers, on the other hand, the
development of the blue colour is associated with a
suppression of the barbules, while the barbs as usual
tend to become dilated and polished.

While in many kingfishers blue and black are the
dominant colours, in some the blue is replaced by
green. Thus in Halcyon lindsayi a yellow colour is
common on various parts of the feathers, and where

XII THE COLOURS OF BIRDS 283

structural colour occurs, it is here green and not blue.
Many of the feathers of the back are black, cross-
barred with yellow, and here the terminal cross-bar
is wholly or in part replaced by green.

GENERAL CHARACTERS OF COLOURS OF BIRDS

These illustrations of colour phenomena in birds,
if they do not explain the development of bright
colour, may perhaps at least shed some light on
the problem. They show that the development of
brilliant colour and structural modification go hand
in hand ; that brilliant pigmental colours tend to be
confined to the barbs and are often associated with
the suppression of the barbules; that melanin pig-
ments may be present in large amount in both barbs
and barbules ; and that their presence in the latter is
often associated with a structural modification which
gives rise to optical colours; that the closeness of
the association between the deposition of pigment in
any region of a feather and the special development
of that region is such as to prevent in the general
case the feathers of flight acquiring great brilliancy
of colour. Facts of this kind surely tend to prove
the definiteness of variation ; they should, at least,
be allowed for by those who discuss the questions
connected with the origin of colour.

MEANING OF COLOUR IN BIRDS

As to the meaning of the various types of colour
in the physiology of birds we can say very little.

284 COLOUR IN NATURE CHAP.

Those associated with the presence of melanin pig-
ment are here as elsewhere less inexplicable than
those due to lipochromes. They are certainly far
more conspicuous in the males than in the females,
and may therefore be ascribed more or less directly
to the greater vitality of the male, which expresses
itself in more rapid metabolism and increased pro-
duction of pigment. But with the lipochromes the
case is different ; their constant association with fats
makes it difficult to regard them as products of
destructive metabolism. Again, though in many
cases they are only conspicuous in coloration in the
males, yet the peculiar case of the green and red
parrots (p. 252) seems to show that in some instances
they may be more abundant in the females. It may,
of course, be suggested that the lipochromes in the
female are largely used up in the colouring of the
yolk, while in the male they may colour the plumage ;
but it is still difficult to account for the virtual
absence of lipochrome from many families, as the
crows, the humming-birds, and so on—families cer-
tainly highly specialised in other respects. Nor can
we regard the presence of lipochrome as indicating
want of specialisation in view of the fact that, as
in the birds of Paradise, they may be absent or
present within the limits of a family, without obvious
differences in the amount of specialisation. In view
of the absence of lipochrome from the cuticular
structures of mammals the question is of much
interest. The absence of derivatives of waste
products in the cuticular structures of birds as
compared with insects is, of course, probably to
be associated with the well-developed, excretory,

XIII THE COLOURS OF BIRDS 285

and vascular systems in a bird as compared with
those of an insect. The great differentiation
has now rendered it impossible for nitrogenous

waste products to be directly employed in colora-
tion.

CHAPTER XIV

THE COLOURS OF MAMMALS AND THE ORIGIN
OF PIGMENTS

Coloration of Mammals—Pigments of Mammals—Colour of
‘the Hair and Skin in Man, and its Bearing on General
Problems— Origin of Melanin—Pigment and Waste
Products——Experimental Evidence—Conclusions—Criti-
cism of these Conclusions.

WE have already remarked on the familiar fact that
mammals are rarely remarkable for brilliant pig-
ments, the prevailing colouring-matters being the
dull-coloured melanins. The statement is, of course,
true only of the colours of the skin, for bright pig-
ments do occur in the tissues; thus hemoglobin
colours not only the blood but most of the muscles,
and yellow lipochromes occur often in quantity in
the fat, in the plasma of the blood, in the muscles,
and so on. Of these the hemoglobin of the blood
is an important factor in coloration in the white
races of mankind, and when associated with certain
peculiarities of the structure of the epidermis, gives
rise to the bright tints of the callosities of many
monkeys, of the face of the mandrill (Cynocephalus
maimon), and so on. Under normal conditions

CHAP. XIV COLOURS OF MAMMALS 287

the decomposition products of hzmoglobin, unless
melanin be one of these, are of no importance in pro-
ducing external colour ; nor do the lipochromes ever
appear to occur in the epidermis or cuticle. Optical
colours except white are rare in mammals, but true
metallic colours occur in the Cape golden mole
(Chrysochtoris). In this little insectivore the fur
especially of the upper surface displays “a brilliant
metallic lustre, varying from golden-bronze to green
and violet of different shades” (Flower). The exact
causation of the colour appears to be unknown.

In mammals generally the beauty of the colour-
ing is dependent upon the unequal distribution of
the melanin pigments, which are very frequently so
arranged as to produce the effect of stripes or spots.
There are several papers upon the origin, relations,
and ‘meanings of these markings, but all are too
purely theoretical to demand detailed notice here.
An account of them will be found in the works of
Wallace, Eimer, Bonavia, and others.

Among the general colour characteristics of
mammals, we should notice the tendency of certain
variations to recur constantly in many different
orders ; such are the deepening of the tint (melan-
ism), the disappearance of the pigment (albinism),
the prevalence of a sandy colour in mammals in-
habiting deserts, and so on. Melanic varieties are
seen not infrequently in the leopard (Fels pardus),
especially in Southern Asia; they seem to occur
quite sporadically. A very interesting point about
these black leopards is that, in certain lights, the
markings characteristic of the leopard can be seen
on the black ground like the pattern on “ watered

288 COLOUR IN NATURE CHAP.

silk.” This shows that these markings are not
wholly determined by the amount of pigment pre-
sent in the hairs, there must be also some additional
cause.

Albino varieties occur occasionally as a sport,
especially under domestication, but many mammals
are naturally white. As is well known, certain
Arctic animals, eg. the polar bear, are always white,
others only turn white in winter, eg. the Arctic fox.
The change of colour in these cases is associated
with the development of numerous air-bubbles in the
hair. It would seem that in some cases this is not
accompanied by a destruction of the pigment, which
is merely concealed by the air-bubbles.

For further particulars as to the characters of the
colours in mammals, reference should be made to
the text-books, and for the markings to Eimer’s
papers.

The pigments of mammals have been relatively
little investigated, but there is probably great uni-
formity throughout the group. Leydig describes
uric acid compounds as occurring in the skin of
Chrysochloris, and regards them as factors in the
coloration, but in general the colours are apparently
due only to the melanins,

COLOUR OF THE HAIR AND SKIN IN MAN

In connection with the pigments, a few remarks
upon the colour of the skin and hair in our own
species may not be out of place, especially as the
questions connected with it have considerable bear-
ing upon general problems. As is well known, the

XIV COLOURS OF MAMMALS 289

dark races of mankind owe the colour of their skin
to a black pigment deposited in the deeper layers of
the epidermis—a pigment which is practically absent
in white-skinned people. The varying tints of the
hair are also due to the varying amounts of the same
dark pigment deposited in it. That differences in
skin-colour often correspond to profound racial dif-
ferences is familiar in a rough way to every one, but
there are some interesting facts which tend to show
that even apparently slight differences in intensity of
pigmentation may correspond to relatively vast con-
stitutional differences.

We propose to confine our study of the question
to variations in the colour of the hair and eyes in
the white-skinned peoples, where the data are most
easily obtainable. The first point of interest is the
curious fact shown by Galton’s statistical researches
that among ourselves there is little tendency for the
dark and fair strains to mingle, “to be swamped by
intercrossing,” in the current phrase. The children
of parents of whom one is dark and the other fair
will as a rule either have dark oy light eyes, only
rarely will they have eyes of medium colour (Matural
Inheritance, p. 139). In Mr. Bateson’s words, the
variations are discontinuous.

The next point is that, according to Dr. Beddoe’s
prolonged observations, “the colour of the hair is so
nearly permanent in races of men as to be fairly trust-
worthy evidence in the matter of ethnical descent, and
nearly as much may be said for the colour of the iris ”
(The Races of Britain, p. 269). His observations
further show that the dark-haired people correspond
roughly to the Gaelic and Iberian stocks, while the fair-

U

290 COLOUR IN NATURE CHAP.

haired belong to the Teutonic races ; in other words,
the difference in hair-colour corresponds to all those
profound mental and moral differences which separate
Celt from Saxon. That the mental and moral dif-
ferences are associated with physical ones hardly
needs proof to the biologist, but there are fortunately
some exact observations. During the course of an
extended series of observations on the specific gravity
of the blood, Dr. E. Lloyd Jones found that this was
markedly greater in dark-eyed persons than in light-
eyed ones, and he is of opinion that the difference is
fundamentally a racial one. Further, there is reason
to believe that the dark-haired people are better able
to stand prolonged dosing with drugs like mercury
than the fair-haired ones; and, according to Beddoe,
the dark-haired persons in Britain are more prone to
phthisis than the fair. It thus seems that just as the
phthisical tendency and the other characters tend to
eliminate the dark people from cold climates, so ap-
parently the fair people are less fitted to-survive, or
at least less likely to become dominant, in hot
countries. Facts of this kind have probably an
important bearing upon the coloration of mammals
in general. The constancy of the coloration, and
the closeness of its connection with the constitution,
are at least of much interest in relation to the
general question.

THE ORIGIN OF MELANIN

As to the direct relation of the amount of pig-
ment to the general metabolism, many would say
that the pigment is directly derived from the

XIV ORIGIN OF PIGMENTS 291

hemoglobin of the blood, and that, therefore, its
amount is a direct measure of the rapidity of the
degenerative changes occurring in the hemoglobin.
From some recent work it would, however, appear
that there is not this direct relation between the
pigment and hemoglobin. Drs. John Abel and
Walter Davis, in the course of a laborious investiga-
tion on the pigments of the negro’s skin and hair,
found that the pigment granules of the epidermal
cells contained a substratum of non-pigmentary
substance, apparently of the nature of a highly
resistant proteid. When this proteid is removed the
pigment is readily soluble in dilute alkalies, from
which it may be precipitated by acids. It contains
carbon, nitrogen, oxygen, hydrogen, and sulphur, but
in the pure state very little iron. It is the presence
of sulphur without any considerable amount of iron
which, in the opinion of the authors, makes an origin
from hemoglobin very doubtful. The proteid which
is also present in the pigment granules contains a
considerable amount of iron as well as of other in-
organic constituents. Floyd showed in 1876 (Chem.
News, vol. xxxiv. p. 179) that the skin of the negro
contains about twice as much iron as the white skin,
but this is apparently due to the proteid and not
to the actual pigment itself’ The investigators are
of opinion that the pigment originates from some
proteid of the blood or “parenchymatous juices.”
Similarly Dr. Sheridan Delépine considers that
melanin is elaborated out of the plasma of the blood
and is not a derivative of hemoglobin. On the
other hand, he is of opinion that hemoglobin itself
is perhaps manufactured from some “antecedent,

292 COLOUR IN NATURE CHAP.

variety, or derivative of melanin,’ a somewhat inter-
esting conclusion.

PIGMENT AND WASTE PRODUCTS

The question as to whether the pigments of
mammals are to be regarded as products of destruc-
tive metabolism is one which has considerable
bearing on the general question. In considering
particular cases of pigmentation we have again and
again come across suggestions to the effect that the
pigments of organisms are effete substances incapable
of serving directly useful purposes, which may be
stored up in the cutaneous tissues, and so give rise
to coloration. In considering these suggestions in
a little more detail, we may, in the first place,
provisionally exclude cases like that of the Lepi-
doptera, where the pigments, in some cases at least,
are definitely excretory substances. Our immediate
concern is not with these, but with the numerous
kinds of pigment which are different from the
ordinary waste products of the organism in which
they occur, and which have not been proved to havea
genetic connection with these. Such pigments have
not infrequently been described as waste products,
and it is necessary for us to consider how far this is
justifiable.

EXPERIMENTAL EVIDENCE

In the first place, it is interesting to note that
the suggestions have .been usually made in connec-

XIV ORIGIN OF PIGMENTS 293

tion with physiological experiments on leucocytes.
The modern doctrine of the physiological and patho-
logical importance of leucocytes and phagocytes,
with which the name of Metschnikoff is so honour-
ably associated, has been largely founded on results
obtained from the injection of foreign substances in
suspension or solution into the body. The injected
substances are usually colouring - matters for con-
venience of observation, and the result has been to
prove that they are systematically removed by
leucocytes from the general cells of the body ; and
either eliminated through the excretory organs or
stored up in various parts of the body, where they
may give rise to artificial coloration. Now we have
already frequently seen that natural pigmentation
may result from the emigration of pigmented con-
nective tissue cells from the deeper tissues outwards
to the skin. This occurs, for example, in the leech,
and, according to Kolliker, is true for all Vertebrates.
This being so, it is eminently natural that the
physiologists should draw a parallel between these
natural pigmented “wandering cells” and the pig-
mented leucocytes found after injection of colour-
ing-matter, and regard the former as active agents in
eliminating the normal waste products. The necessity
for finding a physiological justification for the con-
tinued production of pigment being so obvious, the
suggestion once made has been eagerly adopted by
many.

The simplest case is that in which the introduced
pigment is injected into the alimentary canal, and
its subsequent fate compared with that of the
pigments normally occurring in the cells connected

294
with this.

COLOUR IN NATURE

CHAP.

The following table shows the results
obtained from some of these experiments :—

PIGMENTS INTRODUCED INTO ALIMENTARY CANAL

ituations in which in-
Organisms, troduced Pigments are bee Observer,
Polyzoa. In ‘‘hepatic cells” | Brown pigment | Harmer.
of gut. of these cells.
Crustacea. |In cells of hepatic | Pigment nor- | Cuénot.
ceca, ultimately in | mally found in
feeces. hepatic cells
and feces.
Capitellida. | In cells of gut, ulti- | Pigment nor- | Eisig.
‘ mately in skin. mally —occur-
ring in cells
of gut and in
skin.
Oligocheta |In the so-called | (?) Pigment of | Cuénot.
(Zubifex). | chloragogenous cells | chloragogenous
covering the intes- | cells and of
tine, ultimately in | skin.
the skin. .

In these cases the Capitellida are the only forms
in which the introduced pigments come to have a
marked effect on the coloration. Another case in
which the introduced pigment is important in this
way is perhaps the case of caterpillars, where the
pigment introduced with the food reaches the
connective tissues and so the skin. In the general
case, however, it would seem that the pigments
which normally result from the activity of the liver
or “hepatic cells,” as well as pigments artificially
introduced into the gut, are usually directly eliminated
by means of it, and do not become important in

XIV ORIGIN OF PIGMENTS 295

coloration. It may be that worms are an exception
to this rule. It is commonly stated that in the
earthworm carmine introduced into the gut is re-
moved by the yellow cells, which then go free and
pass out with their burden by the nephridia. In
the Capitellidz, and in Txdzfex, on the other hand,
the pigment is not wholly eliminated, but is in part
stored in the skin. These facts may show that in
worms the products of the metabolism of “liver” cells
are not readily eliminated by the gut itself, and so
may in some cases be important in coloration. It
seems possible that in Bonelléa also the colour of the
skin is due to a modification of a pigment occurring
in the gut (?), or in the cells lining the body cavity.
Further, we have seen (p. 191) that in Mollusca the
peculiar pigment enterochlorophyll, at least in some
cases, colours the feces; that it occurs both in the
cells of the gut and in the digestive gland; and
finally, that it is possible that in some cases it may
give rise to the pigments colouring the mantle, and
ultimately to those of the shell. Unfortunately this
is as yet uncertain.

When we come to the fate of pigments intro-
duced into the body cavity, and their natural
analogues, the matter is much more difficult and
complicated. In the following table the natural
analogues column must be almost left blank :—

296 COLOUR IN NATURE CHAP.

PIGMENTS INTRODUCED INTO BODY CAVITY

Situations in which in-
Organisms, troduced Pigments are Plea Observer.
found.
Polyzoa. In the interior of Harmer.

leucocytes which re-
main ‘within the

zocecium.
Echino- In wandering cells | Similar wander- | Durham.
derms. which may leave the | ing cells which

body at any point, | contain pig-
and which give rise | ment, and may
to temporary pig- | give rise to pig-

mentation. mentation.
Dytiscus In ameeboid blood Durham.
(Insecta). corpuscles = which

later formed patches
at various points in
the skin.

Crustacea. | In excretory organs. Cuénot.

CONCLUSIONS

Although certain analogies between the fate of
introduced pigments and the natural occurrence of
pigment in the tissues are thus rare, yet suggestions
as to analogy have been very freely made. Eisig
considers that the pigment which occurs somewhat
sparingly in the skin of the Capitellide is an effete
product temporarily stored there, and further regards
this as a widespread origin of pigment. Mr. Durham,
basing his view largely on his own researches on
Echinoderms, regards colouring - matters as either
waste products or effete respiratory pigments which,
when eliminated by means of amceboid cells, may
give rise to coloration of the skin.

XIV ORIGIN OF PIGMENTS 207

Of all suggestions of this kind, those of List for
Vertebrates are the clearest and most definite, and
may be summarised here.

In the first place, List accepts without reservation
the view that pigment does not originate in epi-
dermal structures, but is carried to them by wander-
ing leucocytes. He believes that the pigment
originates within the blood-vessels by the degenera-
tion of red blood corpuscles; that it is taken up by
leucocytes ; and that these with their burden follow
the track of the blood-vessels outwards from the
corium to the sub-epithelial layer. The pigment
granules are to be looked upon as excretory pro-
ducts, which are in part taken up by the epithelial
cells and gradually eliminated as these degenerate.
In bony fish, and apparently in Amphibia, pigment
arises in the embryos from the degeneration of the
yolk, but the pigment which is produced later prob-
ably arises from blood pigment.

Similarly the migration outwards of pigmented
cells in the leech is often regarded as a process of
excretion.

CRITICISM OF THESE CONCLUSIONS

As to the whole question, it is probably too soon
to attempt to draw conclusions, but one or two
points may at least be touched on. In the first
place, an obvious difficulty in the way of regarding
all pigments as waste products, or as derivatives of
respiratory pigments, is that the great majority of the
researches hitherto carried on have almost entirely
omitted to consider the pigments soluble in alcohol.

298 COLOUR IN NATURE CHAP.

Most have been conducted by the method of sections,
and during the course of preparation of the objects,
the lipochromes, and the numerous other unstable or
soluble pigments,are completely removed or destroyed,
so that of these the investigators have nothing to
say. It is obvious, however, that it is these brightly
coloured substances which give rise to the most
striking of the phenomena of coloration. Practically
any substance occurring in opaque granules may give
rise to dull brownish colours, and so may be termed
a pigment, but does this help us as to the origin of
the bright blue of many jelly-fish, the gorgeous red
of some Crustacea, the bright colours of fishes and
birds? These may be “waste products,” but there
is yet no proof of it; they may be reserves; they may
be comparable to the production of aniline dyes in
the coal-gas industry, ze. by-products (Durham), but
there is as yet little certainty. It is possible that
some of the difficulties may be solved by a careful
study of the chetopterin group of pigments, for the
members of it are widely distributed, tend to occur
in connection with endodermic (digestive) organs, and
under artificial conditions give rise to brightly coloured
derivatives, but the investigations have still to be
made.

Again, the method of study by means of injec-
tions has obviously its limitations as a method of
determining the physiological value of pigments.
Thus Cuénot found that introduced pigments in the
case of the Crustacea were eliminated by the excre-
tory organs, or by the hepatic cells and the feeces ;
they were never stored up in the epidermal tissues,
and yet the Crustacea are remarkable for the pro-

XIV ORIGIN OF PIGMENTS 299

fuse pigmentation of the epidermis and cuticle, and
there is certainly a marked elimination of pigment
in the shell at the moult. The elimination of intro-
duced pigment by the skin in the Capitellide is
regarded as evidence that the pigment naturally
occurring there is a waste product, but the applica-
tion of the same principle to the Crustacea is fraught
with obvious difficulties. If conclusions are to be
drawn from the fate of introduced pigments, then
the pigment of the cuticle in Crustacea is not a waste
product ; if, on the other hand, it is the elimination
of pigment by a moult which is the criterion, then
the pigment is a waste product.

As a whole, therefore, it would seem that while it is
impossible for a physiologist to conceive of pigment
being produced in the organism in the haphazard
fashion some would have us believe, yet it is at
present also impossible to give a universal physio-
logical explanation of its origin ; it probably arises in
many different ways. As yet the classification of
pigments given in the second chapter cannot appar-
ently be simplified.

CHAPTER XV

THE RELATION OF FACTS TO THEORIES

General Summary—Theories as to Origin of Colour: Poulton,
Wallace, Eimer, Cunningham, Simroth — Criticism of
Natural Selection—Criticism of Other Theories—Con-
clusion.

WE have now completed our general survey of the
colours and colouring-matters of organisms. We
have seen that these colours are due either to definite
pigments deposited in the tissues, or to optical effects
produced by the structure of these tissues. We have
discussed the chemical, and, so far as is known, the
physiological nature of some of the chief pigments,
and described the appearances presented by the most
striking optical colours. Finally, we have rapidly
surveyed the colour phenomena presented by the
most familiar plants and animals. That the survey
as a whole is halting and incomplete must be obvious
to all. We have seen that it is as yet impossible to
give a definite physiological explanation of the origin
of pigment; that it is practically impossible to classify
pigments in a logical manner; that most of the
problems connected with the subject are entirely

cH. xv THE RELATION OF FACTS TO THEORIES 301

unsolved. What is the meaning of the great series
of lipochromes in the economy of animals? How
do they arise, and why are they sometimes introduced,
and in other cases synthetically formed? Why
should they so frequently occur in pairs, and what is
the relation between the red and the yellow series?
These are only a few of the unanswered questions
which make one at times doubtful whether it is not
still too soon to attempt a synthetic survey of the
biological aspects of colour. It is, however, notice-
able that if the physiology of pigments and colour is
still in an embryonic condition, yet the speculative
side of the subject has attained rank and rapid
growth. It is impossible to conclude a work of this
description without some reference to theories, but
we should pass to the consideration of these with a
full consciousness of the blanks in knowledge.

THEORIES AS TO ORIGIN OF COLOUR IN
ORGANISMS

1. The Darwinian Theory.—So far we have con-
sidered organisms as if they were isolated objects,
uninfluenced by their surroundings; it is, however, one
of the most striking characteristics of modern scientific
thought that organisms are no longer looked upon
as independent creations, but as linked to one another
by the closest of relations. Their colours are often
their most striking external features; we must ask
what effect these colours have upon their relations to
other organisms. Now it is a matter of common
observation that the colours of some animals corre-
spond so closely to the colours of the objects among

302 COLOUR IN NATURE CHAP.

which they live, that they can only be distinguished
with difficulty. If the enemies of such species are
psychologically similar to ourselves, the colouring
must render them less conspicuous to these enemies,
and must thus be protective. Therefore it may be said
that, however the colour in these cases first arose, it
must always have been, other things being equal,
useful to the species ; therefore the forms displaying
these colours would tend to persist, the others would
tend to be eliminated ; therefore we may say that the
colour arose by Natural Selection, which weeded out
all those not possessing it.

This is in essence the explanation of colour
phenomena given by a great number of naturalists at
the present time. Colour they say is originally non-
significant, a result of the chemical or physical
properties of substances ; its appearance in the super-
ficial tissues may render the organism better fitted
to survive in the struggle for existence, and therefore
is encouraged and maintained by Natural Selection.
The various types of coloration presumed to be of
use have been classified under the headings of
Protective Resemblance, Mimicry, Warning Colours,
and so on; their use is supposed to be to protect
the organism from its enemies, to enable it to steal
unperceived on its prey, or to warn its enemies that
it is unpalatable or dangerous and must be avoided.
Beside these, however, there is another series of
colours to be considered. We have already seen
that in birds the males are frequently far more
brilliantly coloured and ornamented than the females.
As these colours do not fall into any of the divisions
already mentioned, many naturalists have adopted

xv THE RELATION OF FACTS TO THEORIES 303

Darwin’s view that they are due to the persistent
choice by the females of the most ornamental males,
and therefore to Sexual Selection. So that to the
general statement that the colours of animals are due
to the action of Natural Selection, we must add,
except in the case of the bright colours of males,
which are due to the action of Sexual Selection.
This explanation of the colours of animals is sub-
stantially that given by Prof. Poulton in his Colours
of Animals. Mr. Poulton is indeed one of the most
thoroughgoing of all the adherents to the doctrine of
Natural Selection, as the following example taken
from his book may serve to show. He describes the
buff-tip moth (Pygera bucephala) as exhibiting a
very marked resemblance to a broken piece of
lichen-covered stick, and then come the following
sentences :—“ A friend has raised the objection that
the moth resembles a piece of stick cut cleanly at both
ends, an object which is never seen in nature. The
reply is that the purple and gray colour of the sides of
the moth, together with the pale yellow tint of the
parts which suggest the broken ends, present a most
perfect resemblance to wood in which decay has
induced that peculiar texture in which the tissue
breaks shortly and sharply, as if cut, on the applica-
tion of slight pressure or the force of an insignificant
blow” (Colours of Animals, p. 57). These state-
ments, whatever else they do, certainly display a
most profound faith in the efficiency of Natural
Selection as a factor in evolution. The efficiency in
this case seems almost excessive; one cannot help
wondering whether a protective resemblance which
was a little less laboured would not have served the

304 COLOUR IN NATURE CHAP.

purpose. Another example of a similar elaboration
of protective resemblance may be also quoted from
Mr. Poulton’s pages. The insect in this case was
found by Mr, W. L. Sclater in Tropical America. In
the place where it was found the leaf-cutting ants are
extremely numerous, and are constantly seen carrying
pieces of leaves “about the size of a sixpence held
vertically in the jaws.” The insect found by Mr.
Sclater, though not an ant, resembled one; and,
moreover, had an anterior, thin, flat expansion which
imitated the leaf carried by the ants, so that, as a
whole, in Mr. Poulton’s words, the insect “ mimicked
the ant, together with its leafy burden” (zbid. pp.
252,253). Now, as it is only the homeward-bound
ants which carry pieces of leaves, it seems in
this case also that the protective resemblance is
unnecessarily laborious ; something less might surely
have served.

Apart from this, however, the examples show how
some naturalists attack the problems of colour. It
is unnecessary here to go into further detail as to the
various applications of the theory ; most of these have
now become completely popularised.

2. Mr. Wallace’s Theory.—We shall next pass on
to consider the modification of this theory which is
supported by Mr. A. R. Wallace. Mr. Wallace, in his
book on Darwinism, expresses his general belief in
the theory of colour production implicit in such terms
as Mimicry, Warning Coloration, etc., and dissents
only from the theory of the origin of the bright
colours of males by Sexual Selection. In point of
fact, however, his dissent in reality carries him further
than this, and to some extent at least shakes the

xv THE RELATION OF FACTS TO THEORIES 305

whole theory of the origin of colour as a result of the
action of Natural Selection. Mr. Wallace, as is well
known, gives up Sexual Selection on the ground that
there is no evidence that the females do exercise
such a selection ; while if they did, the effect of their
choice would be neutralised by the action of Natural
Selection. The fact that the males in most animals
are more brightly coloured than the females, Mr.
Wallace ascribes in general terms to the “greater
vigour and excitability of the male”; if the colour
and ornamentation be an expression of abundant
vitality, its persistence and increase is easily accounted
for apart from the choice of the female. The
hypothesis of Sexual Selection is therefore as needless
as it is unproved.. Mr. Wallace then sums up his
theory of the origin of colour in five theses, of which
the following is a brief abstract :—-Colour arises as a
necessary result of the complex chemical constitution
of animal tissues ; it becomes more conspicuous and
intense as external tissues become more complicated
in structure ; it is probable that colour development
takes place according to definite laws of growth ;
finally, “the colours thus produced, and subject to
much individual variation, have been modified in
innumerable ways for the benefit of each species.” It
is in this way that Protective Coloration, Mimicry, etc.
have been produced. Again, in the higher forms the
male as compared with the female exhibits brilliant
colours due to his greater vigour, while his mate has
been kept plain by Natural Selection.

Now all this is very different from the statements
made by Mr. Poulton. Both certainly begin by
saying that colour is originally non-significant ; but

x

306 COLOUR IN NATURE CHAP.

Mr. Wallace speaks of laws of growth as determining
the progressive changes seen in the development of
feather-markings ; while Mr. Poulton tells us that
although pigments tend to occur in animals, it is by
no means certain that they would have appeared on
the surface apart from Natural Selection, and that
they tend to disappear from the surface directly they
cease to be useful.

Thus, according to the school which is usually
known as the Darwinian, colour, wherever seen, is
due to the favouring influence of Natural Selection,
and is in some way useful to the species. In the
view of the popularisers of the subject, it therefore
becomes the main object of the naturalist to invent
as ingenious an explanation as possible of the way
in which it is useful. If the naturalist’s powers
of invention fail, though this happens but rarely,
then the colour is non-significant, or better still, the
animal has recently changed its habitat, and is no
longer perfectly adapted to its environment. The
theory is, therefore, perfectly complete and coherent,
and persons refusing to accept it are at once stigma-
tised as laboratory-made scientists, ignorant of nature,
and unworthy of the name of naturalist.

Mr. Wallace’s modified views, if less capable of a
reductio ad absurdum, are apparently less completely
logical. As noticed by Professor Geddes and Mr.
Thomson, in their Evolution of Sex, the denial of
Sexual Selection has a considerable bearing upon
Natural Selection in general. To illustrate this, we
may take an example from humming-birds. The
genus Eustephanus includes the species &. galeritus
and &. fernandensis, in both of which the sexes

xv THE RELATION OF FACTS TO THEORIES 307

differ considerably from one another. The sexual
differences are especially well marked in the latter,
in which the male is of a bright brown-red colour, a
tint comparatively rare among the humming-birds.
The female exhibits colours of a more usual type,
but is remarkable in possessing a brilliant metallic
crest, an ornament which is very rare among female
humming-birds. The male of the other species is
very like the female of E. fernandensis, except that
his crest is red instead of green, while his own female
is very plain, and without acrest. Now, if Mr. Wallace
admits that the brilliancy of the male £. fernandensis,
as compared with his mate, is due to his greater
vigour and vitality, surely it is not unreasonable to
conclude that the general greater brilliancy of this
species, as compared with £. galerctus, is due to its
greater vigour and vitality. In other words, bearing
in mind that the male of &. galerztus is hardly more
brightly coloured than the female of E. fernandenszs,
may we not say that the two species bear to each
other, as regards vitality, the same relation as the
male and female of &. fernandensis bear to one
another? If this be granted, then surely, other things
being equal, the coloration of a species bears some
relation to its vitality—that is, it is primarily deter-
mined by the physiological condition of the organism
and not forced upon it by the stress of environmental
conditions (see the Evolution of Sex). Again, if this
be so, much of the elaborate treatment of colour
phenomena in the early part of Mr. Wallace’s book
seems needless. If we may account for the colours
of many birds as the incidental consequences of
physiological conditions, then surely we need no

308 COLOUR IN NATURE CHAP.

elaborate discussion of the possible uses of colour,
for we see that it arises apart from usefulness, and
ergo may persist apart from usefulness. This is the
view put forward in the Evolution of Sex, where the
colours of organisms are regarded as expressions of
the constitution of the individual.

3. Mr. Cunningham and Professor Eimers
Theortes—Although the theories as to the origin of
colour, adopted on the one hand by Mr. Poulton,
and on the other by Mr. Wallace, are widely accepted
among biologists, dissentients are not wanting, and
are probably on the increase. Among the older
theories, that dependent upon the acceptance of
Lamarck’s factor of an inheritance of acquired
characters, has been vigorously maintained by Mr.
Cunningham in this country, and Professor Eimer
and a numerous school abroad. Professor Eimer’s
theories of the origin of colours and markings involve
especially the conception that in this, as in other
respects, evolution is a progression along definite lines
determined by laws of growth which are the accumu-
lative result of environmental stimuli; the emphasis
is, however, so laid upon the laws of growth that the
fact that these involve an inheritance of the effects
of environmental influence is apt to be lost sight of.
The difference between this and the preceding
theories is best indicated by a concrete example.
We may take the vexed question as to the reason
for the absence of pigment in cave-inhabiting animals.
According to Mr. Poulton, animals which live in
darkness are pale, because pigment would not be
visible in these situations and is consequently no
longer of any use to them: it is, therefore, no longer

XV THE RELATION OF FACTS TO THEORIES 309

maintained by Natural Selection, and cherefore it dis-
appears,—the last therefore being one of the great
points of dispute. Mr. Cunningham, on the other
hand, considers that pigment is, or at least was
primarily produced by the action of light on the
skin, and that cave-inhabiting animals are pale-
coloured because there is no light to stimulate the
development of pigment. According to him, light
and pigment are directly related; according to others,
light is not the cause of pigmentation, it only puts in
motion the machinery produced in the organism by
Natural Selection. We have already seen by what
beautiful experiments Mr. Cunningham has endea-
voured to support and prove his position as to the
relation between light and colour.

4. Dr. Simroth's Theory.—Mr. Cunningham’s
position may be taken as typical of that taken up
by those who refer variation to the inherited and
cumulative effect of environmental influences, but as
an elaboration of the same principle we may take up
a paper recently published by Dr. Heinrich Simroth.
Dr. Simroth is well known, not only by his concrete
researches, but as an ingenious and fertile theorist,
and his present paper, though vague and mystical,
has yet considerable interest, and to some extent
may serve as a type of many of the most recent
theories as to colour production. Dr. Simroth’s
theory is, however, remarkable in displaying an
absolute indifference to the facts of chemistry, which
even among biologists is relatively rare. As papers
of this kind are exceedingly difficult to interpret,
it may be well to state clearly that although the
following is an attempt to give a purely objective

310 COLOUR IN NATURE CHAP.

summary, yet it may quite well be that the subjective
element is far from being absent.

So far, then, as I understand Dr. Simroth, he
refers all pigments back to a prime substance which
is closely united to primitive protoplasm, and which
has evolved along with primitive protoplasm, pro-
ducing the simple spectral colours in the order of
the spectrum, beginning at the red end. That is,
red pigments are simpler in composition than those
of green or violet colour, and tend to appear earlier,
and to be particularly prominent in simple organ-
isms. We may thus speak of an evolution of pig-
ments corresponding to an evolution of organisms,
and the red or yellow pigments correspond to the
simpler organisms. These red and yellow pigments
have a simple chemical composition and a small
molecular weight, and as we pass upwards and find
the colours of the pigments changing, so also we find
the chemical composition growing more complex and
the molecular weight increasing in amount.

As to the causation of this evolution of pigment,
Dr. Simroth refers primarily to the effects of light
and warmth, but makes the following detailed sug-
gestion as to the determining cause of the actual
direction of evolution.

In the first place, he suggests that at an early
stage in the world’s history the atmosphere was so
saturated with watery vapour, that it at first only
allowed the red rays of the sun’s light to pass through,
and then, as the vapour gradually cleared away, the
other rays, in the direction from the red to the violet,
were able to penetrate.

Secondly, he believes that protoplasm is so con-

XV THE RELATION OF FACTS TO THEORIES 311

stituted, that it responds differently to the varying
stimuli of the separate rays. Thus it responds to
the rays of long wave-length by the formation of
simple pigments, and to those of short wave-length
by the formation of more complex pigments, so that
there is a relation between the molecular weight of
the pigments produced and the wave-lengths of the
rays producing them. If we combine this statement
with the previous one as to the relation existing
between the colour of a pigment and its molecular
weight, it would seem that red light produces red
pigments of simple composition ; violet light, violet
pigments of complex composition, and so on.
Further, the previous suggestion as to the gradual
appearance of the rays, accounts for the order in
which the pigments appear.

Before proceeding further with Dr. Simroth’s
theory, we may note that so far it is in its details
largely an adding together of the suggestions of
others. Thus, the suggestion that the action of red
light on organisms is to cause them to produce red
pigment, is merely the suggestion as to the photo-
graphic sensibility of living beings which has already
been made in various quarters. As every one
knows, the essence of the process of photography
lies in the fact that certain chemical substances are
extremely sensitive to the action of light. When
the photographer exposes a plate to light in his
camera, the sensitive substance with which it is
covered is rapidly decomposed by the action of the
light, and dark-coloured substances are produced.
So great is this sensitiveness that the brightest rays
of the incident light correspond to the darkest parts

312 COLOUR IN NATURE CHAP.

of the “ negative ” produced, and all the developments
of modern photography are rendered possible. Now,
if it is possible to obtain inorganic substances which
are so extraordinarily sensitive to light, it is surely
not impossible that organic substances, in their
ordinary position within the organism, may display
a similar sensitiveness, and therefore that pigment
production may be the result of exposure to light.
Further, as every one knows, one of the great objects
of recent photographers has been to discover a
method of photographing in colours—that is, of find-
ing substances which react in such a manner to
different rays of light as to themselves build up
compounds having the same colour as the inci-
dent light. According to Herr Otto Wiener, certain
compounds of silver chloride will do this; and he
suggests that organic substances may possess the
same property, and that thus “ protective” coloration
may be accounted for. A caterpillar may be like
its environment, because its skin photographs that
environment by means of the sensitive compounds of
its own tissues. So far, therefore, Simroth’s theory
is largely based upon Wiener’s suggestion, though he
carries it much further.

Again, Simroth’s suggestion as to a relation
between the colour of a pigment and its chemical
composition has been made on a smaller scale by
Urech, whose researches on the pigments of butter-
flies we have already quoted. Urech, in comment-
ing on the fact that in the butterflies of the genus
Vanessa the wings are at first white and the colours
then develop in the order of the spectrum (yellow,
orange, red, brown, black), suggests that there is a

xv THE RELATION OF FACTS TO THEORIES 313

relation between the molecular weight of these pig-
ments and their respective colours, and that this
gradual development of colour in the history of the
individual corresponds to the evolution of colour in
the history of the race.

We must now return to a more detailed consider-
ation of Simroth’s paper. He supports his central
thesis as to the origin of all pigments showing simple
spectral colours from a prime substance by three
arguments, which are not, however, very sharply
differentiated from one another.

His first argument is based upon the modifica-
tions of the retinal purple in Vertebrates. As is well
known, the rods of the retina of most Vertebrates
contain a purple pigment known as rhodopsin or
“sehpurpur,” which, when exposed to light, under-
goes a series of changes—becoming red, orange,
yellow, and finally colourless. These modifications
Simroth, so far as the author understands him,
regards as evidence that all pigments are genetically
related, and that one can be derived from another.
He also lays especial stress upon the fact that red
pigment is usually associated with the eye-spots of
simple organisms, and that such organisms seem
never to possess dark-coloured pigments. This he
regards as evidence that pigments belonging to the
less refrangible end of the spectrum tend to appear
first.

The second argument is based upon the modi-
fications of the lipochromes of plants. Simroth
regards chlorophyll as the result of the modification
of a lipochrome, a view for which, as we have seen,
there is practically no evidence. He also believes

314 COLOUR IN NATURE CHAP.

that lipochromes are of great importance both as
reserve stuffs and as oxygen carriers in the process
of assimilation. When the metabolism of the cell is
active, oxygen is withdrawn from the yellow pigment,
and it becomes converted into green chlorophyll.
When metabolism diminishes, the green chlorophyll
becomes oxidised and is converted into a lipochrome,
and thus the colours of autumnal leaves, of fruits, and
of flowers are produced. The fact that chlorophyll
is commonly regarded as a nitrogenous compound,
which the lipochromes are certainly not, is nowhere
alluded to. Lipochromes, Simroth regards as pig-
ments of relatively great simplicity, especially char-
acteristic of plants as the simpler organisms. When
they occur in animals, they are to be looked upon
as evidences of a primitive condition, though they
may be utilised for purposes of warning coloration,
mimicry, etc., such colours, according to the author,
being always of simple nature. If lipochromes are,
however, evidences of a primitive condition, it is
difficult to understand why they should be so frequent
in birds ; but the author does not touch upon this.

The third kind of evidence upon which Simroth
bases his thesis is the order of appearance of the
colours which either belong to the right half of the
spectrum, or are not pigmental colours at all. Such
pigments are characterised by their chemical com-
plexity, and are associated with complex tissues.
Thus the greater intensity of animal life expresses
itself in the nature of animal pigments; the masses
of simple colours, like red, yellow, and green, which
are so common in plants, being rare in animals (but
Crustacea ?).

XV THE RELATION OF FACTS TO THEORIES 315

This is in outline Simroth’s theory as to the
origin of colour. His paper contains also numerous
other suggestions which we cannot well discuss here.
In general, he appears to believe that pigments are
all related, that their development follows a definite
order, and that their origin is due to the properties
of protoplasm and the inherited stimuli of external
conditions, such as light and warmth. On these,
which he calls “inorganic” factors, he is inclined to
lay much stress, especially in such cases as the
colours of shells, in which he says there can be no
question of adaptation.

The theory is interesting in spite of its vagueness,
and is included here because it is in many respects
typical of recent theories. As neither it nor the
other suggestions similarly based upon an inheritance
of the effects of environmental stimuli, have the ex-
tensive following possessed by the Natural Selectionist
theory, we shall return, before going further, to a
detailed discussion of the latter, beginning with the
familiar subject of mimicry.

CRITICISM OF NATURAL SELECTION

The existence of colour resemblances between
widely separated organisms, and that explanation
of it which is implicit in the term Mimicry, have
recently become almost universally familiar. The
term in its present use was first employed by Mr.
Bates, and his suggestions were adopted by Darwin,
Wallace, and others, and have since been widely
accepted. Criticism has, however, never been want-
ing, and in a recent paper M. M. C. Piepers brings

316 COLOUR IN NATURE CHAP.

forward some new facts of great interest in this
connection. Piepers opposes altogether the idea of
the action of Natural Selection in the matter, and
remarks that the idea that the phenomenon is main-
tained by the accruing practical profit to the organism
is one “essentially English.”

1. Mimicry—As is well known, the doctrine of
mimicry among butterflies involves primarily the
hypothesis that birds are the great enemies of
diurnal butterflies, that certain families of butterflies,
notably the Heliconide, the Danaide, and the
Acreidz, are not attacked by birds, and that there-
fore wherever these butterflies occur they are
mimicked by non-protected butterflies. | Piepers
attacks the prime proposition that birds are the
great enemies of butterflies, and then discusses in
detail some of the so-called examples of mimicry.

As to the first point, it is admitted on all hands
that the night-flying Lepidoptera are constantly eaten
by birds, but with regard to the diurnal forms the
question is different. Observations as to the actual
pursuit of butterflies by birds are exceedingly few,
although Bates and Wallace speak of finding scattered
wings in the forest. M. Piepers, during more than
thirty years’ observation in India and the Malay, saw
one or two isolated cases only, and he quotes other
observers (Pryer, Skertchly, Scudder) as being equally
or more unfortunate. As a whole, he concludes that
although some birds may occasionally eat diurnal
butterflies, there is as yet no evidence of that habitual,
unvarying persecution which the theory of mimicry
demands—a conclusion which is somewhat surprising
to the outsider.

xv THE RELATION OF FACTS TO THEORIES 317

The author then proceeds to discuss various cases,
of which the first is an example rather of protective
coloration than of mimicry proper. It is the case of
the colour-change of the caterpillar in the Sphingides
from green to brown just before pupation. The
caterpillar feeds among green leaves and is then
green, but it forms a chrysalis in earth, and the
brown colour has been held to be of protective
importance during the period when the caterpillar is
in search of a retreat. Piepers, however, observes
that the period which elapses between the cessation
of feeding and the formation of the chrysalis is
exceedingly short, in some cases not more than some
minutes ; that the brown is constant, while the tint of
the earth varies and is often quite different ; and that
as the caterpillar is necessarily moving all the time,
a protective colour can hardly be of much avail.
Finally, the colour-change is exceedingly common in
the larve of Lepidoptera at this stage, and is probably
due to a discoloration resulting from the drying up
of the skin preparatory to its being shed. It occurs
also in Sphinx larvee which are brown to begin with
and not green, though here the change is so slight as
to be little noticed.

The next case taken up is that difficult one of
the occurrence of several different forms among the
females of certain Papz/iones of India and the Malay,
which was discovered and discussed by Mr. Wallace
(Trans. Linn. Soc. xxv. ; see also Contrzbutions to the
Theory of Natural Selection, London, 1871). The case
is a somewhat difficult one, in part because the names
used involve in themselves an interpretation of the
facts. So far as it is possible for one who is not an

318 COLOUR IN NATURE CHAP.

entomologist to judge, the following is an impartial
statement of the case :—The genus Pagzlio in India
and the Malay shows a marked tendency to develop
varieties (species) of definitely limited geographical
distribution, so that, for example, a variety (species)
occurring in India is replaced by a closely related
variety (species) in the Malay. Further, in many
cases there occurs a marked polymorphism in the
females, which expresses itself in the fact that they may
resemble the males or may display large spatulate
appendices to their hind wings which are entirely absent
from the males, as well as other peculiar characters.
This, however, occurs as a specific character in both
males and females of other series of Pafzlzones, the
result being that some of the females of one series
may resemble (mimic) both sexes of other series.
The following table will perhaps make this plain, the
brackets indicate that the forms are “ geographical
species” (replacing species), the habitat being indi-
cated at the left :—

SERIES A.

Java P. Memnon,

Sumatra

Polymorphic.

India and | P. Androgeos.
Polymorphic.

SERIES B.

{ P. Coon.

One female ‘‘ mimics ” Bt
(Sexes similar).

»| &. Doubledayz.

One female ‘‘ mimics ae
(Sexes similar).

SERIES C. SeriEs D.
‘ P. Theseus. ebacctic tin cy {Pe Hector:
ine Polymorphic. Onedemal ee esinaice (Sexes similar).
Mala | P. Pammon. One female “ mimics ») £. Diphilus.
y Polymorphic. Another ,, 5 £. Antiphus.
(Sexes similar).

Mr. Wallace explained the case as a typical one of

XV THE RELATION OF FACTS TO THEORIES 319

mimicry, displaying itself only in the females, and
not in all of these. According to M. Piepers, it is
to be explained on the ground that the polymorphic
forms represent successive stages in the transition
between one monomorphic species and another. He
is of opinion that the Papilionide of the Malay are,
for the most part, descended from ancestors with
large spatulate appendices to the hind wings, but
that many have lost or are losing these. In the two
series P. Memnon-Androgeos and P. Theseus-Pammon,
the species are undergoing this transition ; the males
and some of the female forms display the new type
of structure, while certain of the female forms display
in part ancestral traits. The resemblance between
these female forms respectively and the other two
series of butterflies (P. Coon-Doubledayi and P. Hector-
Diphilus-Antiphus) is not so great in the field as in
the study, and is merely due to the fact that these
two series display a more primitive type of structure
and coloration, one nearer to that displayed by the
hypothetical ancestor of the Eastern Papilionide.
The whole question is considerably complicated
by the great variability of all the Papz/iones, which
makes it practically impossible to distinguish be-
tween species and varieties ; while, on the other hand,
the nomenclature employed has a considerable bear-
ing on the question of mimicry, at least when repre-
sented in tabular form. The mimicking females of
P. Memnon and P. Androgeos are exceedingly alike,
so much so that they were formerly classed together
as one species. The chief difference is that the
Javan form has yellow spots, while the Indian form
has reddish. In the species P. Coon and P. Double-

320 COLOUR IN NATURE CHAP.

dayt the same contrast of colour is observed—yellow
in the Javan P. Coon, and red in the Indian P.
Doubledayz. Wallace regards this as a proof of
mimicry, the mimicking forms varying as the
mimicked forms vary; Piepers regards it as a
response to similar geographical conditions, and
denies specific value to the forms Coon and Double-
dayi, as also to Memnon and Androgeos. The case
shows considerable resemblance to the one which he
next considers, and which may be briefly noticed.
Among the Satyridz there are two closely related
species, Pavaga Egeria and P. Megera, both common
in Western Europe, of which the former frequents
shady woods and the latter exposed places, especially
the neighbourhood of walls heated by the sun. The
colour of the first is a dull brown, with dull yellow
spots, of the second a bright reddish-orange. When
traced southwards, however, the tint of P. Hgeria
deepens and approaches more closely to that of P.
Megera. In Java there are two species of /unonia,
/. Erigone, and /. Asterig, belonging to the Nymph-
alidze and certainly not closely related to the above,
which have respectively similar habits and colora-
tion. Here, then, is one of those cases of duplex
“mimicry,” so dear to the hearts of many, spoilt
only by the trifling circumstance that the two sets
are separated by the distance of nearly half the
globe!

This paper has been quoted at such length, not
because it is the only detailed criticism of mimicry
extant, but because of the care with which it is done,
and the apparent strength of its criticisms. Very
few of the instances of mimicry have been subjected

xv THE RELATION OF FACTS TO THEORIES 321

to such authoritative criticism; and the fact that
those here submitted have not stood the ordeal
furnishes a strong presumption that a large number
of the cases contained in the literature of the subject
are likewise valueless. The term mimicry is applied
indiscriminately to all cases of colour resemblance ;
many of these can certainly not be so explained,
therefore we are justified in saying that at the
present time the explanation of the facts of colour
resemblance implicit in the use of the term “ mimi-
cry” is insufficient, or to use Mr. Sedgwick’s term,
inadequate. Mr. Sedgwick’s observations in regard
to the cell-theory seem indeed eminently appli-
cable throughout to the theory of mimicry. “A
theory to be of any value must explain the
whole body of facts with which it deals. If it
falls short of this, it must be held to be in-
sufficient or inadequate; and when, at the sare
time, it is so masterful as to compel men to look at
nature through its eyes, and to twist stubborn and
unconformable facts into accord with its dogmas,
then it becomes an instrument of mischief, and
deserves condemnation, if only of the mild kind
implied by the term inadequate” (“Remarks on
the Cell Theory,” Compie Rendu, 3me, Cong. Zool.
Leyde, 1896, p. 121).

2. Protective Resemblance-——Although mimicry is
commonly said to be merely a special case of pro-
tective colour resemblance, it is in some respects
more difficult to understand than the latter, and it
is not perhaps necessary to suppose that the two
stand or fall together. It is, of course, to be clearly
understood that the existence of resemblances between

Vv

322 COLOUR IN NATURE CHAP.

organisms and their surroundings or between un-
related organisms is denied by no one; it is the
explanation involved in the use of the terms “ pro-
tective” and “mimicry,” which is doubtful.

It would lead us too far to enter in detail into
all the arguments which have been advanced as
tending to prove that the resemblance between
organisms and their surroundings has been acquired
and is maintained by the aid of Natural Selection ;
the following summary of the facts in the case of
the Lepidoptera, taken from Weismann, is sufficient
for our purpose here.

Weismann states the case as follows :—“ Im-
mune” butterflies, such as the MHeliconide, the
Danaidez, the Acrzidz, the Euplocide, have usually
both surfaces of their wings coloured alike, and never
resemble their surroundings in the resting position ;
unprotected butterflies, such as the Nymphalidae, are
in the great majority of cases protectively coloured
on their lower surface. Further, the coloration of
this lower surface bears a close relation to the
position of the wings in repose—that is, if in this
position the hind wings overlap the fore, it is only
the exposed tip of the fore-wing which is protectively
coloured ; while if, as in Kalima, there is no over-
lapping, the whole under surface of the fore-wings
displays’ this type of coloration. Again, the special
type of protective coloration which consists in
resembling a leaf, is exceedingly common among
wood-inhabiting butterflies whether related or not.
This coloration bears no definite relation to the
structure of the wing, but “die Flache behandelt als
eine tabula rasa, auf der man zeichnen kann, was

xv THE RELATION OF FACTS TO THEORIES 323

man will,” the “man” in question being apparently
the all-compelling force of Natural Selection.

The facts so stated are certainly sufficiently
remarkable and seem at first sight at least to
warrant Weismann’s conclusion that they are only
explicable by the action of Natural Selection, but
detailed reflection shows many difficulties. The
prime assertion of the immunity of the Danaide, etc.
is denied by many, the persecution of the pro-
tectively coloured butterflies is also, as we have seen,
doubted by field entomologists. The relation be-
tween the colour of the wings and the position taken
up by them in repose seems a very striking fact,
but I have noticed a somewhat similar occurrence in
the feathers of birds. In the humming-bird Cyno-
lesbia gorgo the tail is forked and the tail-feathers
overlap one another ; the tips of the feathers are of a
gorgeous metallic colour, but this is confined to a
simple band at the exposed part, the part of the
feather which is overlapped being a deep black.
Owing to the forking of the tail, the overlapping is
such that in each quill more of the vane is covered
on one side than on the other, the distribution of
black and metallic colour corresponds exactly to
this overlapping, so that the metallic colour extends
farther back on one side of the rachis than on the
other. Here is a case very like that of the butter-
flies’ wings, and yet it is almost impossible to believe
that it can have been produced and maintained by
its utility (see also p. 167).

We have confined our study of protective colora-
tion and mimicry to the Lepidoptera, because they
are admitted on all hands to exhibit the phenomena

324 COLOUR IN NATURE CHAP.

most strikingly. The two papers which have been
chosen to represent the two positions in regard to
the matter illustrate at least the main fact that both
parties are somewhat stronger in attack than in
defence. It would be easy to multiply references
almost indefinitely, but this would in large part
involve mere repetition. The advocate of the
Allmacht of Natural Selection reiterates in many
tones the well-established facts of colour resemblance,
and the insufficiency of laws of growth, of correla-
tion of parts, and the rest to account for these.
His opponent returns to the charge again and again,
well armed with the lack of evidence, the absence of
experimental verification, the disproof of particular
cases; there are weak places in the walls of both
citadels, but both parties are strong in attack; all
the clamour has not, however, as yet caused the
walls of either Jericho to fall.

To drop the metaphor, it must be obvious from
the above discussion that there are great difficulties
in the acceptance of Natural Selection as the most
important factor in the evolution of colour, and that
there is little doubt that its aid has been invoked in
far too reckless a fashion. At the same time, it
must be confessed that there is not as yet in the
field a complete and cogent theory which is capable
of dispensing with Natural Selection ; whether this is
due to ignorance of physiology, or to the real import-
ance of this factor must be left to the future to
decide.

XV THE RELATION OF FACTS TO THEORIES 325

CRITICISM OF OTHER THEORIES

Theories which attempt to minimise Natural
Selection seem always sooner or later to assume
an inheritance of acquired characters, and of this
there is little evidence. They also assume that
environmental influences have a direct effect upon
the organism, and of this Weismann’s work has
made many doubtful. Or rather, Weismann has
endeavoured to prove that those apparently direct
responses to environmental stimuli which are facts
of experience, can be interpreted also as the result
of adaptation, and this, if proved, is fatal to
theories like that of Simroth. It is, however,
to be noticed that in the case of the artificially
produced variations in the colours of butterflies,
competent entomologists (¢,¢. Garbowski) are of
opinion that the new colours have little or no phylo-
genetic importance, and that as yet it is impossible
to correctly interpret them. There is, indeed, much
evidence to show that in the case of butterflies the
colours can be influenced by their surroundings in a
way of which the mechanism is at present unknown.

Much of this is, however, apart from our main
object, which is merely to show that in spite of the
fluency with which so many people talk of the
meaning of colour in organisms, the subject is as in-
complete on the theoretical as on the physiological
side. It seems reasonable to believe that the two
deficiencies are related, and that a little more
physiology will arm the theorists with better
weapons, In the meantime, we cannot end a book on
Colour more fitly than by an appeal for more facts.

REFERENCES

1. General Works of Reference. —For general facts and theories the
following among others may be consulted :—

BEDDaRD, F, E.—Animal Coloration (London, 1892).

Ermer, G. H. T.— Organic Evolution, as the Result of the Inherit-
ance of Acquired Characters, according to the Laws of Organic Growth.
Trans. by J. T. Cunningham (London, 1890).

GepbEs, P., and THomson, J. A.—TZhe Evolution of Sex (Con-
temporary Science Series, London, 1889).

Poutton, E. B.—The Colours of Animais (International Science
Series, London, 1890).

Wa ttace, A. R.—Darwintsm (London, 1889).

Reference should also, of course, be made to the works of Darwin,
especially Zhe Descent of Man, and Selection in Relation to Sex
(London, 1871), and to the numerous books of travel which will be
found cited in the above.

2. Spectal Questions.

ABEL, J., and Davis, W.—‘‘The Pigments of the Negro’s Skin
and Hair,” Jour. Exper. Med. vol. i. No. 3 (1896), pp. 361-400
1 pl.

F Aeiaserd, A.—The Cruise of the Blake, Bull. Museum Compar.
Zool. Harvard College, U.S.A., 1888. Many Observations on Colours,
of Marine Organisms.

ALcock, H.—‘‘ The Asteroidea of the Indian Marine Survey,”
Ann. and Mag. Nat. Hist. vol. xi. (1893), pp. 73-121, 2 pls.
Colours of Deep-sea Forms.

ANDRE, E.—‘‘ Le Pigment mélanique des Limnées,””’ Revue Sudsse
de Zool, iii. (1895), pp. 429-431.

BATESON, W.—WMaterials for the Study of Variation, London,
1894. Especially for Colour Variation, and for Colours of Pleuro-
nectidze.

BECQUEREL, HENRI, et BRONGNIART, CHARLES—‘‘ La matiére
verte chez les Phyllies,” C. R. Ac, Sez. cxviti. No. 24 (1894), pp-

1299-1303.

328 COLOUR IN NATURE

BepriaGa, J. voN—* Mittheilungen iiber die Larven der Molche,”
Zool, Anzeig. xiv. (1891), pp. 295-300, 301-308, 317-323, 333-341,
349-355, 373-378, 397-404; and xviii. (1895), pp. 153-157. Develop-
ment of Colour.

Bocanpow, ANDRE—“ Etudes sur les causes de la coloration des
oiseaux,” Compt. Rend. xlvi. (1858), pp. 780, 781.

Boyce, R., and HERDMAN, W. A.—‘‘ On a Green Leucocytosis in
Oysters associated with the Presence of Copper in the Leucocytes,”
Proc. Roy. Soc. \xii. (1897), pp. 30-38.

BrUcKe—(Colours of Octopus), S¢tzungsb. d. Akad. Wiss. Wien.
viii. (1852), p. 196. ‘Structural Colour.

BUrcer, O.—Dze Nemertinen des Golfes von Neapel, 22 Mono-
graphie, Berlin, 4to, xvi +743 pp. 31 pls. For Colours.

BuTscHLI, O.— ‘* Protozoa,” Bronn’s lassen uw. Ord. d.
Thierreichs, Leipzig, 1889. Pigments of Different Forms.

Carazz1, D.—‘‘Contributo all’ istologia e alla fisiologia del
Lamellibranchi 1. Ricerche sulle ostriche verdi,” A/étthezl. Stat.
Neapel, xii. (1896), pp. 381-431, 1 pl. See also letter in Mature, vol.
lii. pp. 643, 644.

CuaTIn, A., and Muntz, A.—‘“ Etude chimique sur la nature et
les causes du verdissement des Huttres,” C. R. Ac. Sez. vol. cxviii.
(1894), pp. 17-23 and 56-58.

CuurcH, A. H.—‘‘ Researches on Turacin, an Animal Pigment
containing Copper,” Chem. Mews, vol. xix. (1869), p. 265. ‘* Re-
searches on Turacin, an Animal Pigment containing Copper,” Phz/.
Trans. Roy. Soc. vol. clix. (1870), pp. 627-636. ‘*Lecture on
Turacin,” reported in /Vatzre, vol. xlviii. (1893). ‘‘ Researches on
Turacin,” Phil, Trans. Roy. Soc. 183a, pp. 511-530.

Coste, F. H. Perry—‘ Insect Colours,” Vature, vol. xlv. pp.
513-517, 541, 542, and 605, 1892.

Cusnor, L.—‘‘ Etudes physiologiques sur les crustacés Decapodes,”
Arch, Biol. xiii. (1894), pp. 245-303. Elimination of Introduced
Pigments. ‘Etudes physiologiques sur les Orthoptéres,” Arch. Biol,
xiv. (1895), pp. 293-341, 2 pls. Functions of Fatty Body, and
Nature of Pigments.

(a) CUNNINGHAM, J, T.—“ Researches on the Coloration of Flat-
fishes,” J. Mar. Biol. Ass. iii. (1893), pp. 111-118.

(4) CUNNINGHAM, J. T., and MacMunn, C. A.—‘* On the Colora-
tion of the Skins of Fishes, especially of Pleuronectidee,” Phil. Trans.
oy. Soc. London, vol. clxxxiv., B. pp. 765-812, 3 pls., 1893.

DELEPINE, SHERIDAN—“ Proceedings of Physiological Soc.,”
Journ. Physiol, xi. (1890), p. 27. Origin of Melanin.

DuruaM, HERBERT E.—‘‘ On Wandering Cells in Echinoderms,”
Q. J. M.S. vol. xxxiii, (1891), pp. 81-121, I pl. Origin of Pigment.

EnRING, C.— Ueber den Farbstoff der Tomate (Lycopersicum
esculentum), Ein Beitrag zur Kenntnis des Carotins, Miinster, 1896,

pp: 1-35-

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EIMER, THEODOR—Uebver das Variiren der Mauereidechse, Berlin,
a Ueber die Zeichnung der Thiere, Humboldt, 1885, 1886, 1887,
1888.

E1sic, H.—Die Capitelliden, Naples Monograph, 1887. Pig-
ment and Waste Products.

Emery, C.—‘ Untersuchungen iiber Zuctola italica,” Zeitschr.
wiss. Zool. xl. (1884), pp. 338-354, 1 pl. (Luminosity of Luciola
italica), Bull. Soc. Entomol. Ital. xviii. (1885), pp. 351-355, I pl.

ENGELMANN, W.—‘‘ Farbstoff von Hematococcus,” Bot. Zeit.
xxxix. 1882. ‘‘Farbe und Assimilation,” Botan. Zezt. 1883, pp.
1-17, and 17-29. ‘Die Purpurbakterien und ihre Beziehung zum
Licht,” Bot. Zeit. 1888, pp. 42-46.

Faxon, W.—‘‘ The Stalk-eyed Crustacea,” Mem. Mus. Harvard,
xviii. (1896), pp. 1-292, 67 pls. Chapter on Colour.

FIscHER, E.—Zransmutation der Schmetterlinge tnfolge Tempera-
tuvdnderungen. Lxperimentelle Untersuchungen uber die Phylogenese
der Vanessen, Berlin, 8vo, 36 pp., 1895.

FLEMING, W.—‘ Ueber den Einfluss des Lichts auf die Pigmen-
tirung der Salamander-larve,” Arch. AGikr. Anat. xviii. (1896), pp.
369-374.

Francis, G.—‘‘ Pigments in Fishes’ Skins,” Wazzre, vol. xiii. p. 167.

Fritscn, ANron—‘‘ Uber Schmuckfarben bei Holopedium gib-
berum,” Zoolog. Anzeig. xiv. (1891), pp. 152, 153.

Gapow, H.—‘‘The Coloration of Feathers as affected by Struc-
ture,” Proc. Zool. Soc. 1882, pp. 409-421, 2 pls. See also Bronn’s
Thierreich, vi. 4, pp. 575-584.

GaRBowskI, T.—‘‘ Descendenztheoretisches iiber Lepidopteren,”
Biol, Centralbi, xv. (1895), pp. 657-672. Summary and Criticism of
Experiments of Fischer and Others.

GATKE, H.—Zeligoland as an Ornithological Observatory: the
Result of Fifty Years? Experience. Trans. by R. Rosenstock, Edin-
burgh and London, 8vo, pp. x. +599, 1895. For Change of Colour
without Moult.

GEDpDES, PaATRICK—‘‘ Further Researches on Animals containing
Chlorophyll,” Mature, 1882, pp. 303-305. ‘‘On the Nature and
Functions of the Yellow Cells of Radiolarians,” Proc. Roy. Soc.
Edinburgh, vol. xi. (1881-1882), pp. 377-396.

GessarD—‘‘ Bacillus pyocyaneus,” An. d. PInstitut Pasteur, 1891,
p- 65. Blue Colouring-matter.

GIESBRECHT, W.—‘ Mittheil. ii. Copepoden,” Mitthecl. Stat.
Neapel, xi. (1895), pp. 631-694, i Fig. For Luminosity.

Girop, P.—C. R. Ac. Sez. xciii. p. 96; and Arch. Zool. exp.
et gen. ix. (1882), pp. 1-100. Analysis of Melanin.

GraF, ARNoLD—“ Uber den Ursprung des Pigments ii. der Zeich-
nung bei den Hirudineen,” Zool. Anzeig. xviii. (1895), pp. 65-70.

GREENWOOD, M.—‘‘ The Process of Digestion in Aydra fusca,”
Journ. Physiol. ix. Meaning of Brown Pigment.

330 COLOUR IN NATURE

H&cker, V.—‘ Untersuchungen iiber die Zeichnung der Vogel-
federn,” Zool. Jakrbiich, iii. (1888), pp. 309-316, 1 pl.

HALiBuRTON, W. D.—‘‘On the Blood of Decapod Crustacea,”
Jour. Physiol. vi. (1883), pp. 300-335, 1 pl. Pigment of Hypoderm
of Lobster.

Harmer, SIDNEY J.—‘‘ On the Nature of the Excretory Processes
in Marine Polyzoa,” Q. J. MZ. S. vol. xxxiii. (1891), pp. 123-167, 2
pls. Formation of Brown Body.

Herpman, W. A.— Note on a New British Echiuroid Gephyrean,
with Remarks on the Genera Zhalassema and Hamingia,” Q. J. Al. S.
xl. (1898), pp. 367-384, 2 pls. &e Green Pigment.

Hopkins, F. GowLaND—‘ Pigment in Yellow Butterflies,”
Abst. Proc. Chem. Soc. vol. v. p. 117 (1889). See also Mature, vol.
xl. p. 335, and letter in Mature, vol. xlv. pp. 197, 198 (1891).
“¢ Pigments of Lepidoptera,” Mature, vol. xlv. p. 581 (1892). ‘‘ The
Pigments of the Pieridae,” Proc. Roy. Soc. London, vol. lvii. pp. 5, 6
(1894) ; and PAzl. Trans. clxxxvi. (1896), pp. 661-682.

Jounson, H. P.—‘*A Contribution to the Morphology and
Biology of the Stentors,” Journ. Morphol. viii. (1893), pp. 468-552,
4 pls. Pigment of S. cerzlea.

Jones, E. Ltoyp—*‘ Observations on the Specific Gravity of the
Blood,” Journ. Physiol. vol. xii. (1891), pp. 299-346, 4 pls.
Relation to Colour of Skin and Iris.

KARAWAIEW, W.— ‘Uber <Aulacantha scolymantha,” Zoolog.
Anzeig. xviii. (1895), pp. 293-301. Function of Phzeodia.

KEEBLE, F. W.—‘‘ The Red Pigment of Flowering Plants,” Scz.
Progress, New Series, vol. i. (1897), pp. 406-423. Abstract of Obser-
vations of Stahl and Others.

KEELER, CHARLES—‘ Evolution of the Colours of North American
Land-birds,” P. Caf. Ac. iii. (1893), xii +361 pp. 19 pls.

KERSCHNER, Lupwic—‘ Zur Zeichnung der Vogelfeder,” Zeztschr.
Wiss. Zool, (1886).

KNAUTHE, Kart—‘‘Erfahrungen iiber das Verhalten von Am-
phibien u. Fischen gegeniiber der Kalte,” Zool. Anzezg. xiv. (1891),
pp. 109-115. Change of Colour in Cold.

KGLLIKER, A.—‘‘ Ueber die Entstehung des Pigments in den
Oberhautgebilden,” Zezt. Wess. Zool. xlv. (1887), pp. 713-717, 2 pls.

KRUKENBERG, C. Fr. W.—Vergleichend-phystologischen Studien,
Heidelberg, 1880-1882. Contain numerous Papers on Animal
Pigments. Grundszdige einer vergleichenden Physiologie der Farbstoffe u.
der Farben, Heidelberg, 1884, pp. 85-184. With copious Bibliography.

KUKENTHAL, WILLY—“ Ergebnisse einer Zoologischer Forsch-
ungsreise in den Molukken u. in Borneo,” Abkand. Senckenberg. Nat.
Ges. Frankfurt a. M. xxii. (1896). Re Ueber die Farbung der Tiere
unter spezreller Bertichsichtigung der tropischen Formen, pp. 53-62.

LANGERHANS, PAUL—‘‘Die Wurmfauna von Madeira,” iv., Zed¢.
Wiss. Zool. vol. x1. (1884), pp. 247-285, 3 pls. Colours of Various Forms.

REFERENCES 331

LANKESTER, E. Ray—‘‘ Report on the Spectroscopic Examina-
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Cheetopterus,” Q. J. AZ. S. xl. (1897), pp. 447-468, 4 pls.

Leyp1c—“‘ Die Pigmente der Hautdecke u. der Iris,’ Verhandl. d.
Phys. Med. Gesell. Wiixzburg, xxii. (1888).

LINDEN, Maria v.—‘‘Die Entwicklung der Skulptur u. der
Zeichnung bei den Gehause Schnecken des Meeres,” Zeztschr. f. Wiss.
Zool. \xi. (1896), pp. 261-317, I pl.

List, H. J.—‘‘ Ueber die Herkunft des Pigmentes in der Ober-
haut,” Biolog. Centralbl. x. (1891), pp. 22-32.

Logs, J.—‘‘A Contribution to the Physiology of Coloration in
Animals,” Journ. Morphol. viii. (1893), pp. 161-164.

MacaLitum, A. B.—‘‘On the Distribution of Assimilated Iron
Compounds, other than Hemoglobin and Heematins, in Animal and
Vegetable Cells,” Q. 7. AL S. xxxviii. (1895), pp. 175-274, 2 pls.,
P. R. Soc. London, lvii. pp. 261, 262.

MacIntosu, W. C.—‘‘ A Monograph of British Annelida,” Roy.
Soc. vol. xxxiv. For Colours of Nemertines.

M‘KENDRICK, J.—‘‘ On the Colouring-matter of Medusz,”’ Jozrz.
of Anat. and Physiol. vol. xv. (1881), pp. 261-264.

MacMunn, C. A.—‘‘Studies in Animal Chromatology,” Proc.
Birm. Philos. Soc. vol. iii. (1883). ‘* Observations on the Colouring-
matters of the so-called Bile of Invertebrates,” Proc. Roy. Soc. London,
xxxv. (1883), pp. 370-403, 1 pl. ‘* The Chromatology of the Blood
of some Invertebrates,” Quart. Journ. Jicr. Sct. vol. xxv. pp. 469-
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“* Observations on the Chromatology of the Actiniz,” Zrans. Roy. Soc.
London, clxxvi. (1885), pp. 641-663, 2 pls. ‘‘ Further Observations on
Enterochlorophyll and Allied Pigments,” Zyazs. Roy. Soc. London,
clxxvii. (1) 1886, pp. 235-266, 2 pls. ‘‘ Contributions to Animal
Chromatology,” Q. /. AL S. xxx. (1889), pp. 51-96, I pl.

Mararp, A. E.—‘‘ The Influence of Light on the Coloration of
the Crustacea,” Aull. de la Soc. Phil. de Paris, 8, iv. (1892), pp. 24-
30. Translated in dv. and Alag. Nat. Hist. vol. xi. pp. 142-149.

Maty, R.—‘‘ Ueber die Dotterpigmente,” dad. der Wess. Wien.
Ixxxiii. (1881).

Mayer, A. G.—‘‘ The Development of the Wing Scales and their
Pigments in Butterflies and Moths,” Bul. ALus. Comp. Zool. Harvard,
vol. xxix. (1896), pp. 209-236, 7 pls. ‘On the Colour and Colour-
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332 COLOUR IN NATURE

xxvii. (1897), pp. 243-330, 10 pls. See also Bull. Mus. Comp. Zool.
Harvard, vol. xxx. (1897), pp. 169-256, 10 pls.

Maver, P.—‘ Ueber das Farben mit Carmin, Cochenille u.
Hamatein Thonerde,” A@z. Zool. Stat. Neapel. x. (1892), pp. 480-
504; and “Zur Kenntnis von Coccus cacti,” Ibid. pp. 505-518, 1 pl.

Meyer, A, B.—<“‘ Ueber einen bemerkenswerthen Farbenunterschied
der Geschlechter bei der Papageien-Gattung Zclectus,” Verh. d. k. k.
Zool-bot. Ges. Wien, 1874.

Mouiscu, H.—‘‘ Ueber den Farbenwechsel anthokyanhaltiger
Blatter bei rasch eintretendem Tode,” Bot. Zeit. xlvii. (1889),
pp. 17-23. ‘Das Phycocyan, ein krystallisibar Eiweisskorper,”
Botan. Zeit, vol. liii. (1895), p. 131.

MosE.LeEyY, H. N.—‘‘ On the Colouring-matter of Various Animals,”
Q. J. M. S. xvii. (1877), pp. 1-23, 2 pls.

NaTuHusius, W. von—‘‘ Ueber Farben der Vogeleier,” Zool.
Anzeig. xvii. (1894), PP--440-445, and 449-452, 2 Figs.

NENCKI, M.—‘‘ Ueber die biologischen Beziehungen des Blatt u.
Blutfarbstoffs,” Ber. de Chem. Gesell. Bd. xxix. p. 2877.

Newsicin, M. I.—‘‘The Pigments of Animals,” Mat. Scz. viii.
(1896), pp. 94-100 and 173-177. ‘‘Observations on the Metallic
Colours of the Trochilidze and the Nectarintide,” Proc. Zool. Soc.
(1896), pp. 283-296, 2 pls. ‘‘An Attempt to Classify Common Plant
Pigments, with some Observations on the Meaning of Colour in Plants,”
Trans. and Proc. Bot. Soc. Edinburgh, lix. (1896), pp. 534-550.
“The Pigments of the Decapod Crustacea,” Jour. Physiol. xxi.
(1897), pp. 237-257. ‘*The Pigments of the Salmon,” Refort on
Salmon, Fishery Board for Scotland (1898), pp. 159-164. ‘‘ On Cer-
tain Green (Chlorophylloid) Pigments in Invertebrates,” Q. /. 47. S.
xli. (1898).

Nussiin, O.—‘‘ Ueber einige neue Urthiere aus dem Herrenwieser
See im badischen Schwarzwalde,” Zez¢. wiss. Zool. xl. (1884), pp.
697-724, 2 pls.

PANCERI, PauL—‘‘On the Luminous Organs and Light of
Pyrosoma,” Q. J. M. S. vol. xiii. (1873), pp. 45-51. ‘On the Light
of Phyllirrhoé bucephala,” Ibid. pp. 109-116.

PHISALIX, C.—‘‘ Recherches sur la matiére pigmentaire rouge de
Pyrrhocoris apterus,” C. R. Ac. Sct. cxviii. pp. 1282-83.

Piepers, M. C.—‘‘ Mimétisme,” C. R. Third Zool. Congress,
Leyden (1896), pp. 460-476, 2 tables.

Pou.ton, E. B.—‘‘The Essential Nature of the Colouring of
Phytophagous Larve (and their Pups); with an Account of some
Experiments upon the Relations between the Colour of such Larve
and that of their Food-plants,” Proc. Roy. Soc. London, vol. xxxviii.
pp. 269-314, 1 Fig. (1885). ‘*The Experimental Proof that the
Colours of Certain Lepidopterous Larvz are largely due to Modified
Plant Pigments derived from Food,” Proc. Roy. Soc. London, vol. liv.

Pp. 417-430, 2 pls. (1893).

REFERENCES 333

ROSTAFINSKI — ‘“* Ueber den rothen Farbstoff einiger Chloro-
phyceen, etc.,” Bot. Zedt. (1881), p. 463.

SCHAUDINN, Fie quien arenilega, nov. gen. et spec.,”
Zeitschr. wiss, Zool, wii. (1893), pp. 17-31, 1 pl. Describes brilliant
Red Pigment.

SCHENKLING, C.—‘‘ Mutmasslicher Farbenwechsel der Vogelfeder
ohne Mauser,” Biolog. Centraibl. xvii. (1897), pp. 65-79.

ScHuBERG, A.—‘*Zur Kenntniss des Stentor ceruleus,” Zool.
Jahrb. iv. (1890), pp. 197-238. For Blue Pigment.

SCUDDER— The Butterfizes of the Eastern United States and
Canada with Special Reference to New England, Cambridge, three
vols. pp. 24 + 1958, 89 pls. Colour and Markings.

Semon, R.—‘‘ Entstehung u. Bedeutung der embryonalen Hiillen u.
«Anhangsorgane der Wirbelthiere,” C. 2. Thzrd Zool. Congress, Leyden
(1895). For Relation between Progress and Elimination of Nitrogen-
ous Waste.

SrmRoTH, HEINRICH—‘“ Ueber die einfachen Farben im Thier-
reich,” Biolog. Centralbl. xvi. (1896), pp. 33-51.

SLaTEr, C.—‘*On a Pigment-forming Bacterium,” Q. 7. AZ S.
xxxii. (1891), pp. 409-416, I pl.

Sorsy, H. C.—‘*On the Colouring-matter of Bonellia viridis,”
Q. J. M.S. xv. (1875), pp. 166-172. ‘* The Colours of the Eggs of
Birds,” Proc. Zool. Soc. (1875), pp. 351-365.

SPULER, A.—‘* Beitrage zur Kenntnis des feinen Baues und der Phy-
logenie der Fliigelbedeckung der Schmetterlinge,” Zool. Jahrbiich. Abth.
Anat. vol. viii. pp. 520-543, Ipl.(1895). Includes numerous references.

StauL, E.—‘‘Uber bunte Laubblatter,” Zxt. Ann. Jardin
Botanique de Buztenzorg, vol. xiii. pp. 137-216, 2 pls. Abstract in
Bot. Zeit. xiv. (1896), pp. 209-215.

SWINHOE, C.— ‘“ On the Mimetic Forms of Certain Butterflies of the
Genus Hypolimnas” (Abstract), P. &. Soc. London, liii. (1893), p. 47.

TASCHENBERG, O.—‘‘ Einige Bemerkungen gegen H. Wickmann,”
Zool, Anzeig, xvii. (1894), pp. 304-309. Colours of Birds’ Eggs.

UReEcuH, F.—‘‘ Chemisch-analytische Untersuchungen an lebenden
Raupen, Puppen u. Schmetterlingen u. an ihren Secreten,” Zool.
Anzeig. vol. xiii. pp. 254- -260, 272-280, 309-314, and 334-341,
(1890). ‘‘Beobachtungen iiber die verschiedenen Schuppenfarben u.
die zeitliche Succession ihres Auftretens (Farbenfelderung) auf den
Puppenfliigelchen von Vanessa urtéce u. to,” Zool. Anzezg. vol. xiv.
pp. 466-473 (1891). ‘‘ Beitrage zur Kenntnis der Farbe von Insek-
tenschuppen,” Zeztschr. wissenschaft. Zool. vol. lvii. pp. 306-384,
(1893). ‘‘Beobachtungen von Compensationsvorgangen in der
Farbenzeichnung bezw. unter dem Schuppenfarben an durch ther-
mische Einwirkungen enstandenen Aberrationen und Subspecies einiger
Vanessa-Arten. Erwagungen dariiber u. iiber die phyletische Recapit-
ulation der Farbenfelderung in d. Ontogenese,” Zool Anszeig. xix.
(1896), pp. 163-174 and 177-185.

334 COLOUR IN NATURE

VANHOFFEN—‘‘ Das Leuchten von Metridia longa,” Zool. Anzeig.
xviii. (1895), pp. 304, 305.

WALTER, B.—Die Oberflachen oder Schillerfarben, Braunschweig,
pp. vi. and 122, 8 Figs., 1 pl. (1895).

WEIsMaNnNn, AuGUST—“‘ Ueber Germinal-Selektion,” C. R. Third
Zool. Congress, Leyden (1895), pp. 35-70. Protective Coloration in
Butterflies.

Wickmann, H.—Dize Furbung der Vogeleier, Miinster, 64 pp.
(1893).

WIENER, OTTO—‘‘Farbenphotographie durch Korperfarben u.
mechanische Farbenanpassung in der Natur,” Azz. Phys. u. Chem. lv.
(1895), pp. 225-281.

ZENNECK, J. —‘‘Die Anlage der Zeichnung u. deren physiologische
Ursache bei Ringelnatter-embryonen,” Zedtschr. wiss. Zool. iii.
(1894), pp. 364-393, I pl.

ZopF, W.—Seitrage zur Physiologie u. Morphologie niederer
Organismen, Leipzig, Heften i. ii. iii. (1892-93). ‘‘ Ueber Pilzfarb-
stoffe,” Bot. Zeit. xlvii. (1889), pp. 85-92. lbzd. Ber. Deutsch. Bot.
Gesell. ix. (1891), pp. 22-28, I Fig,

INDEX OF

Abel, Dr. John, 291

Agassiz, Mr. A., 17, 24, 94, 132,
186

Alcock, Dr. H., 130

Allman, Prof., 16

André, Dr, Emil, 190, 191

Bates, Mr., 315, 316

Bateson, Mr. W., 5, 174, 175, 205,
218, 289

Becquerel, H., et
Charles, 166

Beddard, Mr. F. E., 5, 104, 178,
224, 250

Beddoe, Dr., 289

Bedriaga, J. von, 229, 232, 233,

Brongniart,

235
Belt, Mr. Thos,, 225
Bogdanow, André, 251
Bonavia, 287
Boyce, Dr. R., 194, 195
Brandt, 74
Briicke, 192
Birger, O., 99
Biitschli, O., 76

Carazzi, Dr. D., 193, 194
Carpenter, Dr., 15 °
Chatin et Muntz, 193

Church, Prof. A. H., 49, 253,
254
Cohn, 112

Coste, Mr. F. H. Perry, 153
Crookes, Sir W., 49

AUTHORS

Cuénot, L., 159, 294, 296, 298
Cunningham, Mr. J. T., 213-220,
222, 224, 308, 309

Darwin, Charles, 2-4

Davis, Dr. Walter, 291

Delépine, Dr. Sheridan, 291
Durham, Mr. H. E., 220, 296, 298

Ehring, Carl, 39, 72

Eimer, Prof. Th., 2, 209, 226, 236,
238, 287, 288, 308

Eisig, Prof. H., 106, 107, 111, 294

Emery, Prof. C., 21, 22, 24, 168,
169

Engelmann, W., 52, 58, 74

Faxon, Mr. W., 120

Fischel, 216, 228, 229, 232, 233-
235

Fischer, E., 176

Flower, Sir William, 287

Floyd, 291

Francis, Mr. G., 223

Fritsch, Prof. Anton, 121

Gadow, Dr. H., 11, 12, 14, 267,
268, 270, 271

Galton, Mr. F., 289

Garbowski, T., 147, 325

Gatke, H., 248, 249

Geddes, Prof. Patrick, 74, 89, 306

Gessard, 52

336

Giesbrecht, W., 20

Girod, P., 190

Graf, Arnold, 107, I0g-I1I
Greeff, 77

Greenwood, Miss M., 47

Hacker, V., 259-261

Harmer, Mr. Sidney F., 113-115,
294, 296

Herdman, Prof. W. A., 100, 194,
195

Herrick, Dr. F. H., 122

Hickson, Prof. Sidney, 46, 82, 88

Hopkins, Mr. F. Gowland, 36, 149,
I5I-153, 155, 16r

Ishikawa, Chiyomatsu, 127

Johnson, Mr. Herbert, 76, 77
Jones, Dr. E. Lloyd, 290

Karawaiew, W., 75

Keeler, Mr. Charles, 245, 246

Kent, Mr. Saville, 82-87, 184, 186,
211

Kerner, Prof. A., 58

Kerschener, Ludwig, 260

Knauthe, Karl, 235

Kolliker, A., 293

Krakow, 108

Krukenberg, C. Fr. W., 79, 89, 91,
92, 100, IOI, 105, 112, 116, 123,
125, 133, 135, 165, 168, 170,
I71, 189, 191, 192, 207-209, 223,
g2y, 236, 253, B50, 2h4, 285,
258

Kiikenthal, Willy, 130, 131, 209

Langerhans, Paul, 105

Lankester, Prof. E. Ray, 51, 53, 76,
93, 100, 105, 193

Leydig, 237, 288

Linden, Grifin Maria von, 204

List; Hi Jia 24%, BOF

Loeb, Mr. J., 240, 241

Macallum, Dr. A. B., 29, 32, 33,
72

M'Cook, Dr. Henry, 179-181, 183,
200, 201

COLOUR IN NATURE

M'Intosh, Prof. W. C., 97, 98, 99

M‘Kendrick, Prof. J., 93 :

MacMunn, Dr. C. A., 34, 90, 92,
129, 133, 136, 188, 208, 219,
220, 222

Malard, A. E., 129, 132

Maly, R., 124

Mayer, Mr. A. G., 156-158, 202

Mayer, P., 170, 171,

Meldola, Prof., 178

Metschnikoff, 293

Meyer, A. B., 252

Molisch, H., 43, 62-64

Moseley, H. N., 18, 19, 22, 92,
97, 124, 132, 135, 136, 191

Nathusius, W. von, 259
Nencki, N., 33

Newton, Prof., 245, 257, 258
Niisslin, Dr. O., 77

Panceri, Prof. Paul, 19, 24

Peckham, Mr. and Mrs., 182

Phisalix, C., 171

Piepers, M. C., 315-317

Pouchet, 119, 120, 125, 221

Poulton, Prof. E. B., 4, 41, 44,
139-143, 146, 155, 164, 176,
178, 194, 303-306

Pryer, Mr., 316

Reinke, 232
Rostafinski, 73

Schaudinn, Fritz, 75

Schenkling, C., 249, 250

Schuberg, A., 77

Schunck, Dr. E., 33

Sclater, Mr. W. L., 304

Scudder, Mr., 202, 316

Sedgwick, Mr., 321

Semon, Dr. R., 38

Sharp, Dr. D.,
203

Simroth, Dr. Heinrich, 309-315

Skertchly, Mr., 316

Slater, Mr. C., 52

Sorby, Mr. H, C., 45, 100, 170,
I7I, 257

Spuler, A., 153

Stahl, E., 61, 62, 64

¥aq, 165, 167,

INDEX OF

Taschenberg, O., 259
Thomson, Mr. J. A., 306

Urech, Dr. F., 36, 148, 152, 154-
156, 158, 176, 177, 312

Vanhoeffen, 20
Wallace, Dr. A. R., 3, 139, 148,

161, 178, 206, 279, 287, 304-
307, 315-318, 320

Li

AUTHORS 337

Walter, B., 11

Weismann, Prof. August, 176-178,
322, 323, 325

Wickmann, H. 258, 259

Wiener, Otto, 312

Whitman, Prof., 109, 110

Woodward, Dr., 186

Wurm, 251

Zenneck, Jonathan, 237, 239-241
Zopt, W., 39-41, 52, 54, T44

INDEX OF

-4cm@a, 185, 189, 190, 192
Acquired characters, inheritance of,
214, 215, 308, 309, 325

Acrzeidze, 316, 322
wlctinia, 85
.dctinodendron, 86
vEcidium, 54
-Holosoma, 104
.4garicus, phosphorescence of, 16
Albinism, 247, 248, 287, 288
Albinos, 247, 248, 288
Alcyonaria, phosphorescence of, 17,
22, 24.
colours of, 85, 86
Algae, colours and pigments of,

57-59

Alpine plants, colours of, 62

Amphibians, 209, 224-235, 246,
297

-Amphiporus, 98, 99

Amphiprion bicinctus, 211

percula, 21
-lmphizonella vielacea, 77
_tutmal Coloration, quoted, 5,

224, 250

Antedon, 132, 135

Antedonin, 135, 137

Anthea cereus, 84, 89, 90, 92

Anthocharis, 151

Anthocyan, 42, 43, 47, 60-72

Apaturida, 160

Aphides, 171

Aphis, 170

Aphrodite, colours of, 14, 102
hzemoglobin in, 30

Aplysia, 44, 187, TOL

SUBJECTS

Aplysinidz, 79

Araroth, 251-253

Arge, 153

Argenteum, 221

Argiope, 180, 201

Arion, 188

Ascidia, 208

Asparagus, pigment of fruit, 69
Astacus, 122, 123, 126
Aslerias, 133

Asteroidea, 130

Astrapia, 279

Astropecten, 133

Astropyga, 131

Atherina, 224

Atyephira, 127

Auk, 249

Aurelia, 92

Autumnal leaves, 65-67, 314
Axolotl, 225

Bacteria, phosphorescence of, 16
pigments of, 43, 51-53

Bacterium egregium, 52

Beggiatoa, pigment of, 34, 52

Belone, 27, 94, 224

Benthesicymus, 129

Biliverdin, 188, 189, 258

Birds' eggs, pigments of, 35, 44,

256-259

markings of feathers of, 205,
259-261

pigments of, 220, 242 ef svq,
298, 314

of Paradise, 13, 244, 255,

256, 267, 278-281, 284

INDEX OF SUBJECTS

Blackbird, 244, 245, 247

Black pigments, 47, 48, 109, 136,
I4I-143, 146, 149, 150, 152,
I55, 163, 164, 190-192, 208.
See also Melanin

Blue pigments, 41-44, 76-78, 94,
97, 125-129, 133, 136, 175,
Ig0, I91, 208, 257, 258. See
also Anthocyan, Cyanein, and
Heemocyanin

Bonellia, 90, 99, 100, 295

Bonellin, 45, 90, I00, IOI, 105

Boraginaceze, pigments of, 68

Botryllus, 207

Branchiomma, 104

Brasilin, 72

Brightly coloured leaves, 61, 62

Brill. See Rhombus

Brittle-stars. See Ophiuroids

Brown bodies of Polyzoa, 113, 114

Brown seaweeds, 59

Buccinum, heemoglobin in, 30

Bufo, 235
Bugula, 112, 114, 115
Butterflies. See Lepidoptera

Campanula, 68
Canary, 250
Capitella, 107
Capitellidze, 106-108,
294-296, 299
Caprella, 119
Capulus, 130
Carassius, 223
Carcinentes, 282
Carmine, 26, 170, 171, 254
as a reserve, 39
spectroscopic characters, 49, 50
Carotin, 39, 72, I71
Caterpillars, pigments of, 41, Ir4o-
I47, 200, 203, 294, 317
Cave-inhabiting animals, 308, 309
Ceryle guttata, 282
vudis, 282
Chzetopoda, rarity of dark pigments
in, 47
pigments of, ror-108, 111, 198
Cheetopterin, 45, 90, 91, 135, 298
Chetopterus, phosphorescence of, 17
pigment of, 90, 105
Chameleon, 213

DT TEs,

339

Chara, 59
Chlorocruorin, 105
Chlorophyll, 32, 33
in animals, 44, 73, 74, 76, 78,
79, 88-90, 96, 141-143, 145,
166
in plants, 55, 56, 60, 63-67, 313,
314
Chondrosia, 79
Chromoleucites, 68
Chromoplasts, 68, 99
Chrysaora, 93
Chrysochloris, 287, 288
Chrysomela, 168
Cicinnurus, 248, 255, 280
Cinnyris, 272
Cirratulus, 103
Classification of pigments, 27 ef
S€q., 299, 310, 311
in plants, 71, 72
Clepsine, 110
Coccionellze, 165, 168
Coccus, 170, 171
Ceelentera, colours of, 45, 79-95, 82
phosphorescence of, 17, 19, 22, 24
pigments of, 43, 46
Coleoptera, 168, 169
Coleus, leaves of, 63

Colas, 174

Colour-change, 65-69, 73, 74, 77)
84, 93, 118-123, 142, 146,
147, 166, 175, 183, 213, 226,
233-236

Colours of Animals, quoted, 4, 139,
303, 304

Comatula. See Antedon

Convoluta, 96
Copepoda, colours of, 75, 120, 175
phosphorescence of, 20
Coral polypes, pigments of, 34.
See also Coelentera
Corydia, 167
Coryth@ola, 254
Corythaix, 253, 254
Crayfish. See Astacus
Crinoids, 132, 134-136
Crows, 278, 284
Crustacea, colours of,
199
cuticle of, 108
phosphorescence of, 18, 20

117-123,

Z2

340

Crustacea, pigments of, 44-47,124-
129, 133, 136, 137, 155, 164,
165, 187, 189, 294, 296, 298,
299, 314

Cyanea, 92

Cyanein, 92, 93, 133

Cynocephalus, 286

Cynolesbia, 323

Daffodils, 69

Dahlia, 69

Danaidz, 316, 322, 323

Daphnia, 118, 125

Darwinism, quoted, 3, 139, 161,
304-308

Dasylophus, 254

Detlephila, 153, 154

Delesseria, 57

Dendrophyllia, 87, 88, 94

Development of colour in spiders,
180, 181

Diadema, 131, 132

Diaptomus, 118

Didemnum, 207

Diptera, 172

Discosoma, 197, 211

Doris, 185, 192

Dytiscus, 296

Earthworm. See Lumbricus
Echinochrome, 134
Echinoderms, colours of, 42, 43,
129-133
phosphorescence of, 18
pigments of, 133-137, 296
Echinus, 134
Eclectus, 252, 253, 284
Elaps, 237
Engyophrys, 215
Enterochlorophyll, 34, 45, 90, 91,
105, 129, 133-136, 190, 295
Eolids, 188
Epeira strix, 180
trifolium, 180, 181, 183
Eristalis, 172
Erythrism, 248
Etiolin, 56, 142
Euchone, 103
Euglena, 73, 74
Eulalia, 103, 105, 190
Euphyliia, 87

COLOUR IN NATURE

Euplocidz, 322
Eurymela, 169
Eustephanus fernandensis, 275-277,
306, 307
galeritus, 276, 306, 307
Evolution of Sex, quoted, 306-308

Fishes, 297, 298

Flamingo, 243, 251

Flat-fishes. See Pleuronectide

Florideze, or Red Seaweeds, 57

Floridines, 101

Flounder. See Pleuronectes

Flowers, colours of, 67-71, 314

Flustra, 111

Food, effect on colour of, 40, 41,
75, 98, 103, 250

Foraminifera, 75

Forget-me-not, 68

Fox, Arctic, 288

Foxglove, markings of, 71, 197

Frog, 225-227, 230, 235, 236

Fruits, colours of, 59, 67-72, 314

Fucus, 59

Fundulus, 240, 241

Fungi, pigments of, 53-55, 72

fungia, the mushroom coral, 87

Gannet, 247
Geoplana, 97
Globigerina, 74, 75
Glycera, heemoglobin in, 30
Gobius, 211
Golden eagle, 258, 259
Gonepteryx cleopatra, 174
rhamni, 174
Gordiodrilus, 103
Gorgonidee, 94
Green oysters,
193-195
Pigments, 44-66, 79, 88-92,
I00, IOI, 105, 126, 127, 129,
Iq4O-144, 152-154, 164, 166,
188-190, 193-195, 223, 254.
See also Bonellin, Cheetopterin,
Chlorocruorin, Chlorophylland
Enterochlorophyll
Guanin, in fishes, 37, 219, 221, 224
in amphibians, 227
in snakes, 237
Guillemot, 249

pigment of, 4o,

INDEX OF SUBJECTS

Habia, 246

Hematococcus, 73

Heematoporphyrin, 133, 188, 258

Hezematoxylin or logwood, 26, 72

Heemerythrin, ror

Heemocyanin, 34, 101, 187, 188

Heemoglobin, 28-32, 47, 49, 50,
Ior, 106, 107, 109, 111, 187-
189, 209, 224, 251, 254, 257-
259, 286, 291

Halcyon, 282

Haliotis, 188

Hamingia, 101

Heliconidz, 149, 150, 160, 316

Fleliopora, 85, 94

Hlelix, 203, 204

Heloderma, 236

Hemiptera, 169-171

Heterodactyla, 85, 86

Heteromastus, 107

Hippocampus, 30

Hippolyte, 119, 132

Hircinia variabilis, 79, tor

Hirudinea, 47, 108-111, 198, 199,
257

Hirudo, 108, 198, 199, 293, 297

Hlolopedium, 121

Folopus, 132, 136

Holothuria, 133, 136

Homarus, 122-127

Honeysuckle, pigment of fruit, 69

Humming - birds, 13, 244, 267,
274-278, 284

Hyacinths, 68

Hydra fusca, pigment of, 47

viridis, chlorophyll in, 44
Hyla, 225
Hymenoptera, 171, 172

Janthina, pigment of, 42, 44, 186,
187, 191

Icterus, 245

Idotea, 119

Ink of Cephalopods, 190, 191

Insects, 43-45, 47, 48, 138 e¢ seg.,
199, 200-203, 284, 285, 296

Introduced pigments, 40, 41, 74,
75, 140-146, 155, 193-195,
293-296, 299

Iridocytes, 221

Tridogorgia, 94

341

Jays, 245

Jelly-fish. See Medusze

Jonquils, 69

Junonia Asterie@, 320
Erigone, 320

Kallima, 322
Kingfishers, 282, 283

Labrichthys, 223

Lammergeier, 259

Lampyris, 169

Leandor, 119

Leaves, colours of, 56, 61-67

Leeches. See Hirudinea
Hirudo

Leopard, 287

Lepidoporphyrin, 151

Lepidoptera, 36, 46, 47, 138-162,
172, 173, 175, 201-203, 205,
246, 292, 303, 312, 316-323,
325

Lepidosiren, 27, 224

Lepidotic acid in the Pieridze, 36,
45) 151, 152, 154

Libyssa, 169

Lichenoxanthine, 258

Lilies, 9, 69

Lima, 185

Limax, 185, 188

Limicolze, 260, 261

Limnea, 191

Lina, 144, 165

Linckia levigata, 130

milaris, 130
Lineus, 98, 99

and

Linnet, 249

Lipochromes, 33, 39, 40, 45, 46,
52-54, 56, 59-76, 94, 112,
124-129, 133-136, 141, 144-
rgG, 165, B65, 167, 268, 271,
173, 187, 189, 198, 207-209,
222-224, 227-236, 242, 243,
246-248, 251-253, 255, 256,

265, 267, 268, 272, 273, 278,
28o, 281, 284, 30%, 313, 314.
Lipochromogens, 125, 136
Litmus reaction, 42, 60
Littorina, heemoglobin in, 30, 31
lipochromes in, 189
Lizard, 236, 237-239

342

Lobster. See Homarus
Cuciola, colours of, 168, 169, 171
phosphorescence of, 21, 22, 24
Lumbricus, colours of cuticle, 13,
IOI, 106
heemoglobin in, 30, 31
pigment of, 103, 104, 295
Lung-wort. See Pulmonaria
Cuvarus, 30
Lyczenidze, 160

Mackerel, 220, 222
Uacropus, 168 A
Madrepora, or stag’s horn coral, 83,
84
Mammals, 284, 286-292
pigment of hair, 46, 288-292
Mantis, 166
Marennes oysters. See Green Oysters
Marennin, 193, 194
Markings of plants and animals,

7o, 7%, 108-111, 182, 196-
206, 209, 237-241, 259-261,
287

Mealy redpole, 249
Medusze, colour and pigments of,
42, 43, 88, 92, 93, 133, 298
phosphorescence of, 17, 20
Megalactes, 86
Melaine, 190-192
Melanin, in Vertebrates, 35, 199,
209, 222, 223, 227-236, 238,
243, 246, 248, 251-253, 255,
256, 272, 277-279, 281, 283,
286-288
origin of, 290-292
Melanism, 248, 287
Melon, 69
Metridia, phosphorescence of, 20
Micrococcus, pigment of, 51, 52
Milkwort, 68
Mimicry, 161, 162, 183, 302, 304,
395, 314, 315-323
Molge, 225, 229, 230, 235
Mollusca, colours and pigments of,
42, 45, 46, 184-195, 203, 204,
295
phosphorescence of, 18
Murex, pigment of, 25, 26, 187,
I9I i
Musophagidz, 253, 254

COLOUR IN NATURE

Myrmeleon, 167
Myxotheca arenilega, 75

Nassula, 43, 77

Natural Inheritance, quoted, 289

Natural Selection, 3-5, 66, 302,
303, 305, 306, 309, 315-317,
322-325

Naturalist in N. Celebes, quoted,
82, 88, 89

Navicula, 193

Nectarinia, 272

Nectariniidae. See Sun-birds

Negro, pigment of, 291

Nematois, 153

Nemerteans, 30, 75, 97-99, 198

Nemertopsis, 99

Nepa, 179

Nephelis, 110

Nepheronica, 152

Nephila, 182

Nephrops, 122-124, 126, 128

Nereis, 102

Neuroptera, 167, 168

Newt. See Molge

Nitella, 59

Noctiluca, phosphorescence of, 16

Nymphalidz, 320, 322

Ocnius, 136

Octopus, 186

Odax, 223

Oligochzeta, 104, 106

Oocyan, 257

Oorhodeine, 257

Ooxanthine, 258

Ophiuroids, 131, 134, 136

Orange, pigment of, 69

Orchids, markings of, 71, 197

Oriolus, 245

Orthoptera, 159, 160, 164-167

Oscillatoria, 77

Ostracion, 210, 211

Otis, 255

Oxalis, 70

Papilio, 154

Papilionidee, 317-320

Paradisea, 279

Paraga egeria, 320
megera, 320

Parotia, 279

INDEX OF SUBJECTS

Parrot, 250-252, 284

Patella, 185, 191

Patterns. See Markings

Peach-coloured Bacterium, 51-53

Peacock, 8, 244, 271

Pecten, 186, 189

Pectinaria, 102

Pelobates, 235

Pennatula, phosphorescence of, 19

Pentacrinin, 135, 137

Pentacrinus, 132, 135

Pericrocotus, 245

Pesziza, 54 .

Pheeodellze, 75

Pheeodia, 75

Phethornis, 275

Phascolosoma, 10%

Pheasant, 251

Phidippus, 179

Pholas, phosphorescence of, 18

Phoronis, heemoglobin in, 30

Phosphorescence, 15-24

Photo -bacterium phosphorescens,
phosphorescence of, 16

Phycocyan, 43

Phycoerythrin, 57, 58

Phyllirhoé, phosphorescence of, 18-
20

Phyllium, 166, 167

Physalia, phosphorescence of, 17

Pieridze, pigments of, 36, 48, 149-
155) 174

Pigmental colours, definition of, 7

Pilobolus, pigment of, 40, 41

Pleuronectes, 213, 215, 217, 220,
221

Pleuronectidze, 214-219, 222

Pneumora, 165

Podiceps, 260

Polar bear, 288

Polychzeta, 102

Polygala vulgaris, 68

Polyperythrin, 92

Polyzoa, 111-116, 294, 296

Protective coloration, 225,
305, 312, 317, 321-324

Protopterus, 27, 224

Protozoa, pigments of, 43, 59, 73-78

Pseudoscarus, 211

Ptilonorhynchus, 244

Ptiloris, 281

302-

343

Pulmonaria, 7%
Purple pigments, 43, 44, 52, 112,

II5, 134, 191, 208. See also
Antedonin, Pentacrinin, and
Turacin

+ Purpura, pigment of, 26, 184, 187

Pygera, 303

Pyranga, 247

Pyrosoma, phosphorescence of, 18,
19

Pyrotrogon, 255

Pyrrhocoris, 171

Races of Britain, quoted, 289

Radiolaria, 74, 75

Red Algze, pigment of, 34, 57, 58,79

Red fruits, 59

pigments, 46, 47, 51-53, 69,

73-76, 94, 97, 135, IST, 153;
75, 205, @24. See also
Anthocyan, Araroth, Carmine,
Carotin, Crustaceorubrin,
Heemerythrin, Hzemoglobin,
Phycoerythrin, Tetronerythrin,
Zoonerythrin, Zoorubin

Respiratory pigments, 29-32, 34,
297

Retinal purple, 313

Rhiszocrinus, 132

Rhizostoma, 92, 93

Rhombus, 217, 218

Rhynchodemus, 97

Rifle-birds, 279, 281

Roses, 68, 69

Sabella, 103-105
Sagartia abyssicola,
cence of, 17
Salamander, 216, 225, 228, 232,

233
Salmon, pigments of, 39, 40, 145
Samia, 157
Sand-stars. See Ophiuroids
Saxifraga sarmentosa, 63
Scalaria, 187
Scales of butterflies, 147-150, 156-
1s8, 1771 178
Schisorrhis, 253
Scomber, 223
Sea-cucumbers. See Holothuria

phosphores-

urchins, 131, 134

344

Sepiola, 192

Sericulus, 245

Setophaga, 247

Sexual coloration, 99, 182, 209, 210,
2II, 213, 244-246, 252, 253,
267, 268, 274-277, 279, 284,
302-307, 318, 319

selection, 303-306

Siphonostoma, 104, 223

Sipunculus, TOL

Spectroscope and pigments, 48-50

Sphinx, 154, 337

Spiders, 178-183, 200, 201

Spirographis, 104

Spondylus, 186

Sponges, colours of, 78, 79

Spongilla, 44, 79

Star-fishes. See Asteroidea

Stentor, 43, 76, 77

Structural colours, 8, 14, 15, 98,
99, 139, 140, 147-149, 153)
167-169, 179, 192, 209, 223,
243-245, 266-283

Structure of feathers, 262-267

Sun-birds, 267-275, 278

Symphyllia, 87

Syngnathus, 223

Tannin, 48, 61, 67, 71
Tegenaria, 180
Terebella, 103
Tetronerythrin, 79, 251
Thalassema, 100, 101, 105, 190
Thalassicola, phosphorescence of, 16
Thecla, 154
Thrushes. See Turdidze
Tomato, pigment of, 69, 72
Tortoise-shell limpet. See Acm@a
Tridacna, 44, 184
Tridacophyllia, the lettuce-coral, 87
Trigla gurnardus, 222

‘lyra, 222
Trochus, 188
Trophidonotus, 237-241
Tropicoris, 169
Tryphena, 142
Tubifex, 30, 103, 294, 295

COLOUR IN NATURE

Tunicata, colours of, 42, 207
phosphorescence of, 18

Turacin, 49, 50, 209, 253-256

Turbellarians, heemoglobin in, 30
pigments of, 96, 97

Turbo, 188

Turbobrunin, 189

Turbot. See Rhombus

Turdidze, 260, 261

Uranidines, 208
Uric acid in butterflies, 48, 150-152
in mammals, 288

Vanessa, 176, 177, 312

Variation in colour, 174-178, 181,
182, 325

Vellela, 92, 93

Virbius, 164

Vivianite in bones, 27, 224

Volucella, 172

Vorticella, 74

Wandering cells, rog, 110, 158,
220, 231, 241, 293, 297

Warning colours, 302, 304, 314

Waste products as pigments, 36-38,
107-I11, 114-116, 156-160,
169, 220, 232, 237, 292-299

White pigment, 48, 150

structural colour, 9, 10, 62, 71,

247, 248, 288

Wood-sorrel, markings of petals, 70

Xanthophyll, 56, 57, 65, 72, 141

Yellow Cells of Radiolaria, 74
of sea-anemones, 89-91
Yellow pigments, 46, 69, 79, 124,
125, 192, 208. See also Lepi-
dotic Acid, Lipochromes, and
Ooxanthine

Zoo-chlorellz, 76

Zoofulvin, 252, 253

Zoonerythrin, 251, 252
Zoonomyxa violacea, 77

Zoorubin, 255, 256, 278, 280, 281

Printed by ®. & R. Cuark, Limitep, Edinburgh.

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