LIBRARY
LEONARD WILLIAM
• BUCK-
UNIVERSITY OF CALIFORNIA
MEDICAL CENTER LIBRARY
SAN FRANCISCO
IN MEMORY OF
LEONARD W. BUCK, M.D.
A X
AN OUTLINE OF THE
THEORY OF ORGANIC EVOLUTION
Plate J
J
< V
Ncritina virglnea, variety minor.
PLATE i. — Frontispiece. Variation in color and in color pattern in Neritina virginca, variety
minor. Magnified two diameters.
Color pattern :
i, marked with a few heavy lines. From i to 6 these major lines become broken up into
small V-shaped loops. In the shells, a, accessory minor lines are added. In the shells, d, these
are more numerous.
Series 9 to 14 shows diversity in the pattern near the apex of the coil : 9 has a few very slightly
larger white dots near the coil ; 10 has larger dots here ; n has them very large ; in 12 they have
united to form a continuous white band ; in 13 and 14 this band is wider.
Series 15 to 24 shows diversity in the character of the equatorial light band. In 15 and 16
only the minor lines are interrupted or faint along the equator of the shell. In 17 the major lines
also are interrupted. In 18 the band is almost clear white. 19 and 20 show narrower bands. In
21, 23, and 24 the equatorial band is shown by a difference of color in or under the pattern. In 22
the equatorial line is faintly indicated in the pattern itself, being bordered above and below by
large, heavy, black loops.
Color shade :
The colored lines are black in I, 3, 5, 5^, 6, 6 a, 19, 20, and 22; purplish in 66 and II ; red in
7 a ; gray in 23 ; black and red in 2 ; the major lines are black and the minor lines red in I a, 3,
3 a, and 50; the major lines are black and the minor lines purple in 16 and 17.
These are a few shells selected from a large double-handful scooped up from the sand beach
of the "Salt Pond," near Port Henderson, Jamaica, W.I. The shells were so numerous as to
completely cover the beach for rods at the water's edge. Sixty-eight quite distinct varieties in
color or color pattern were found in this one pint of shells. It is possible to find a completely
intergrading series between any two of these shells, however divergent.
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AN OUTLINE OF THE THEORY
OF
ORGANIC EVOLUTION
WITH A DESCRIPTION OF SOME OF THE
PHENOMENA WHICH IT EXPLAINS
BY
MAYNARD M. ^METCALF, PH.D.
PROFESSOR OF BIOLOGY IN THE WOMAN'S COLLEGE OF BALTIMORE
r \
SECOND EDITION
REVISED
gorfc
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.
1906
All rights reserved
COPYRIGHT, 1904,
BY THE MACMILLAN COMPANY.
Set up and electrotyped. Published October, 1904.
Second edition July, 1906.
Norwood Press
J. S. Gushing & Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.
JFat|}cr
WHO LOVED AND HELD FRIENDLY INTERCOURSE
WITH NATURE
Tom to Mother Carey : " I heard, ma'am, that you were
always making new beasts out of old."
Mother Carey ; " So people fancy. But I am not going
to trouble myself to make things, my little dear. I sit
here and make them make themselves."
— CHARLES KINGSLEY'S "THE WATER BABIES."
PREFACE TO FIRST EDITION
THE lectures out of which this book has grown were
written for the author's students at the Woman's College of
Baltimore, and for others in the college not familiar with
biology who had expressed a desire to attend such a course
of lectures. The book is, therefore, not intended for biolo-
gists, but rather for those who would like a brief introductory
outline of this important phase of biological theory.
It has been the author's endeavor to avoid technicality
so far as possible, and present the subject in a way that will
be intelligible to those unfamiliar with biological phenomena.
The subject, however, is somewhat intricate, and cannot be
presented in so simple a manner as to require no thought
on the reader's part ; but it is hoped that the interest of the
subject will make the few hours spent in the perusal of this
book a pleasure rather than a burden.
In many instances matter that might have been elabo-
rated in the text has been treated in the pictures, which, with
their appended explanations, form an essential part of the
presentation of the subject. This method of treatment has
been chosen both for the sake of the greater vividness thus
secured and because it enables the book to be reduced to the
limits desired. Many of the illustrations have been obtained
from books with which the reader may wish later to become
familiar.
In his lectures upon evolution the author made no
attempt to avoid following the manner of presentation or
even the phraseology of prominent writers upon the subject,
Vlll PREFACE
and for this book little claim to originality can be made.
The author has attempted to present the subject in the way
that seemed simplest and most natural to him, realizing that
in so doing he would almost necessarily follow in large meas-
ure the authors who have influenced his thinking upon the
subject. He is especially indebted to his former instructor,
Professor W. K. Brooks, than whom there is no clearer
thinker in the field of evolution.
There are a number of very valuable books which treat
of the evolution theory. Most prominent among these are
the writings of Darwin and Wallace, and Romanes' Dar-
win and After Darwin. The author does not intend that
this volume shall be accepted by any reader as a substitute
for those more important books, but rather that it shall serve
as an introduction to the subject, giving a comprehensive
outline of the theory, with just sufficient illustration to invite
the reader to seek fuller knowledge of the great number of
most interesting phenomena which are related to the theory.
At the end of this book will be found a list of a few of the
more important volumes treating of the theory of evolution
and the phenomena which it explains.
In the preparation of this book, especially in securing or
preparing the pictures, the author has received much assist-
ance and many courtesies. It is a pleasure to him to
acknowledge his indebtedness:
For the gift or loan of photographs or material for illus-
tration, to the authorities of the United States National
Museum, the American Museum of Natural History, the
United States Department of Agriculture, the United States
Fish Commission, to A. Radcliffe Dugmore, Rev. Dr. John
T. Gulick, Mr. C. L. Allen, and especially to his friend,
Horace W. Britcher, whose untimely death has removed one
of our keenest students of living spiders ;
For assistance in the identification or in the preparation
of material for illustrations, to Dr. Harrison G. Dyar,
PREFACE ix
Dr. L. O. Howard, Dr. F. H. Chittenden, Dr. Charles W.
Richmond, Miss Mary J. Rathbun, Mr. Nathan Banks, and
Professor L. H. Merrill ;
For generously giving permission to copy certain figures,
to the Open Court Publishing Company, Macmillan & Com-
pany, D. Appleton & Company, Edward Arnold, Bradlee
Whidden, Swan Sonnenschein & Company, Smith, Elder &
Company, Charles Scribner's Sons, E. P. Dutton Company,
The Crowell Publishing Company, to Professor August
Weismann, Professor E. B. Poulton, Dr. and Mrs. G. W.
Peckham, Rev. Dr. H. C. McCook, Mr. A. R. Dugmore, Dr.
F. M. Chapman, President D. S. Jordan, Professor Vernon
L. Kellogg, and Hon. Addison Brown ;
For kindly selling the right to use certain figures, to
Doubleday, Page & Company, A. & C. Black, the Autotype
Company, and A. G. Wallihan ;
For assistance in revising certain paragraphs, to Dr. C.
Hart Merriam and Professor W. B. Clark ;
For assistance in revising proof of all of the illustrations,
to Mr. Max Broedel.
PREFACE TO SECOND EDITION
IN this second edition a few slight modifications of the
text have been made for the sake of greater clearness, several
inadvertent errors have been rectified, and mistakes have
been corrected in two of the plates (76 and 77) which
were borrowed without sufficient scrutiny. The author de-
sires to acknowledge with most cordial appreciation the
kindness of Professor E. B. Poulton, who pointed out the
errors in these plates. In a few instances proper credit was
not given for borrowed figures. These omissions have now
been supplied. Also a few titles have been added to the list
of books in the Appendix.
In the first edition of this book, the author suggested
very briefly that there might be inherent tendencies in
organisms, leading them to evolve in certain directions rather
than in others. In Appendix I to this edition, some further
evidence for this view has been given, and Weismann's sug-
gestion as to a possible explanation of these tendencies has
been briefly treated. It has seemed best, also, in Appendix I,
to discuss a little further the influence of individual plasticity
upon evolution.
For all the kindly comments, and especially for criticism,
upon this book, the author feels very grateful. He was at
first doubtful if the published lectures would be useful, and it
is a satisfaction to know that they have found a place and are
apparently proving helpful.
TABLE OF CONTENTS
PAGES
INTRODUCTION xix-xxii
PART I
THE THEORY OF ORGANIC EVOLUTION 3-83
Natural Selection 3-47
Heredity 3-10
Variation 7-10
The Struggle for Existence 10-18
Mutation 18-20
General Principles in the Operation of Natural Selection . . 20-28
Artificial Selection 28-31
Objections to Natural Selection as a Factor in Evolution . . 31-47
Sexual Selection 47-60
Objections to the Theory of Sexual Selection .... 56-60
Segregation ........... 60-67
The Inheritance of Parental Modifications 67-82
Summary of Part I 82-83
PART II
THE PHENOMENA EXPLAINED BY THE THEORY 87-163
Comparative Anatomy 88-96
Classification 88-92
Homology 92-93
Vestigial Structures ..... 0 ... 93-96
Embryology ........... 96-103
Paleontology 103-111
Geographical Distribution 111-116
Color in Animals 116-151
Protective Coloration and Resemblances ..... 117-125
Aggressive Coloration and Resemblances 125-127
xi
Xll TABLE OF CONTENTS
PAGES
Alluring Coloration and Resemblances 127-129
Warning Colors 130-134
Convergence in Warning Coloration 134
Mimicry . 135-146
Protective Mimicry . . 135-145
Aggressive Mimicry . . 145-146
Signals and Recognition Marks . . . . . . 146-147
Confusing Coloration 147-149
Sexual Coloration ......... 149-151
Summary of the Treatment of Color in Animals . . . .151
Color in Plants 151-163
MAN IN RELATION TO EVOLUTION 163-183
GENERAL CONSIDERATIONS 183-188
APPENDIX I. — Trends in evolution, germinal selection, organic selection 189-196
APPENDIX II. — A. few books which treat of organic evolution and phe-
nomena of special adaptation ..... 197-199
INDEX . 2oi-
212
LIST OF ILLUSTRATIONS
(!N THE ORDER OF THEIR INSERTION)
PLATE i. Variation in color and in color pattern in Neritina virginea,
FIG.
i. <
FIG.
2. '
PLATE
2.
PLATE
3.
FIG.
3- '
FIG.
4-
PLATE
4-
PLATE
4, a.
PLATES
5-7-
PLATE
8.
variety minor. (In color) .
Goose-barnacle ......
Gerarde's figure of " Barnacles producing geise"
Variation in Trillium gran diflomm
Varieties of Paludestrina protea .
" Bag-worm," Thyroidopteryx ephemeriformis
Honey-bees
Varieties of horses
The wild cabbage (Brassica oleraced)
Frontispiece
4
5
6
Following
Following
Following
Varieties of cabbage : Savoy cabbage, kale, broccoli, Brussels
sprouts, cauliflower, Swedish turnip, and kohlrabi . Following
Varieties of cabbage, etc., as figured in Gerarde's Herball, six-
(In color) . . "
B. The evolution of the
. Following
21
22
28
28
28
teenth century Following 28
PLATE 9. Varieties of turnips " 28
PLATE 10. Varieties of dahlias
PLATE 1 1 . " Cactus " type of dahlia
PLATES 12-15. Varieties of domestic chickens.
PLATE 16. A. Jungle fowl (Gallus bankiva),
game cock ....
PLATE 17. Japanese long-tailed cocks "
PLATE 18. A. "Frizzled fowls." B. Head of Breda cock. C. Head of
salmon faverolle Following
PLATE 19. A. Feather from a "silky fowl." B. Leg of Cochin cock.
C. " Cochin " bantams Following
PLATE 20. Varieties of domestic pigeons "
FIG. 5. Skull of Polish fowl
FIG. 6. Rock pigeon (Columba livid)
PLATE 21. Skeletons of various unicellular animals and plants . Following
PLATE 22. Male and female bobolink (Dolichonyx oryzivorus} . "
PLATE 23. Ruffed grouse (Bonasa umbellus}, male, female, and young "
PLATE 24. A. Male and female argus pheasant. B. Male and female lyre
bird ......... Following
PLATE 25. A. Male and female Nesocentor milo. B. Male and female pigeon
{Phlogcenas jobiensis) ...... Following
PLATE 26. Male and female humming-birds "
PLATE 27. Turkey cock " strutting " "
PLATE 28. Courting attitudes in hunting spiders .... "
xiii
28
28
30
30
30
30
30
30
30
3i
32
48
48
48
48
48
48
50
XIV
LIST OF ILLUSTRATIONS
PLATE 29. A. Male and female seventeen-year cicada. B. Staghorn beetle,
males and female Following 50
PLATE 30. Male and female Hercules beetle " 50
FIG. 7. Heads of male and female beetles 52
PLATE 31. Male, female, and larva of Chauliodes cor nut us . . Following 52
PLATE 32. Male and female fish : A. Callionymus lyra. B. Xiphophorus
heller ii ......... Following 52
PLATE 33. A. Male and female dragon-fly (Calopteryxmaculatd). B. Male,
female, and larva of crested newt {Triton cristatus) Following 52
PLATE 34. Males and females of different species of lizards " 52
FIG. 8. Secondary sexual characters in copepods . . . ; . -57
FIG. 9. Locusts from the Galapagos Islands 62
FIG. 10. Map of Oahu, Hawaiian Islands 64
FIG. ii. Viola cucullata 88
FIG. 12. Viola rostrata 89
FIG. 13. Solea concolor .......... 90
FIG. 14. Skeletons of the fore limbs of various vertebrates ... 92
FIG. 15. Vestigial bones of the hind limbs in a boa constrictor . . 94
FIG. 1 6. Skeleton of Greenland whale ....... 94
PLATE 35 . Apteryx australis Following 94
PLATE 36. Eyes of various vertebrates, showing the nictitating membrane
Following 94
PLATE 37. Hair tracts on the arms and hands of a man and a male chim-
panzee Following 94
FIG. 17. Muscles of the human ear 95
FIG. 1 8. Three fishes, showing stages in the loss of eyes and color . . 95
FIG. 19. Stages in the development of the pond snail (Lymri&us) . . 97
PLATE 38. Embryos of various vertebrates ..... Following 98
FIG. 20. Tadpole of salamander ........ 98
PLATE 39. American lobster ....... Following 98
PLATE 40. A. Central nervous system of crawfish. B. u Blue crabs11 " 98
PLATE 41. A. " My sis stage1' in the development of the American lobster.
B. My sis stenolepis. C. Leg of My sis stenolepis . Following 98
FIG. 21. Three stages in the development of a crab 100
FIG. 22. Hydra. A diagrammatic longitudinal section . . . . 101
FIG. 23. Gastrula of a coral polyp (Monaxenia darwinii) . . . .102
PLATE 42. Longitudinal sections of gastrulae of : A. frog, young. B. frog,
older. C. chick Following 102
FIG. 24. Longitudinal sections of gastrulas of various animals . . . 103
PLATE 43. Antlers of a stag, showing the addition of new branches in suc-
cessive years ....... Following 106
FIG. 25. Fossil deer antlers 107
FIG. 26. Successive forms of Paludina from the tertiary deposits of Slavonia 108
PLATE 44. Archcsopteryx lithographica Following 108
PLATE 45. Fossil skeletons of: A. Hesperornis regalis. B. Ichthyornis
victor. C. Phrodactylus spectabilis . . . Following 108
FIG. 27. Skeleton of a crow no
LIST OF ILLUSTRATIONS
XV
PLATE 46.
PLATE 47.
FIG. 28.
PLATE 48.
PLATE 49.
PLATE 50.
PLATE 51.
PLATE 52.
PLATE 53.
PLATE 54.
PLATE 55.
PLATE 56.
FIG. 29.
PLATE 57.
PLATE 58.
PLATE 59.
FIG. 30.
PLATE 60.
PLATE 61.
PLATE 62.
PLATE 63.
PLATE 64.
FIG. 31.
PLATE 65.
FIG. 32.
FIG. 33.
PLATE 66.
PLATE 67.
PLATE 68.
FIG. 34.
FIG. 35.
FIG. 36.
PLATE 69.
PLATE 70.
Fossil skeleton of Phenacodus primcevus . . . Following
Changes in foot-structure and teeth in fossil and recent species of
the horse family Following
Map of southeastern Asia, the East Indies, and Australia .
A. Bluefish. B, Sand flounder ..... Following
B. Quail • "
no
Cotton-tail " rabbit.
"5
118
118
118
118
120
A. Field sparrows.
Woodcock on nest .
A. Nighthawk. B. Humming-bird's nest
Tree lizards on oak bark
Protectively colored mammals. A.
B. Thirteen-striped spermophile .... Following 120
A. Cony (Otochona) among rocks. B. Moth on bark " 120
Protectively colored woods-moths. (In color) . . "• 120
Protectively colored caterpillars. (In color) . u 120
A straw-colored spider ( Tetragnatha grallator ) in its accustomed
position on a blade of dead grass 120
Snow grouse in winter, spring, summer, and fall plumage
Following 1 20
Grass porgy, showing changes in color occurring in a few
moments ........ Following 120
Color adaptation in pupae of Pieris rapes and Vanessa urticce.
(In color) Following 120
Twig-like caterpillar of the moth Selenia tetralunaria . . .122
Caterpillar of the moth Catocala amatri.r, on a poplar twig .
Following 1 22
A. " Walking sticks " on a twig. B. "Moss insect" . " 122
Leaf insects. A. Locust (Cycloptera). B. Mantis (PhylUuni)
C. Longicorn beetle (Mormolyce) .... Following 122
Logoa opercularis and L, crispata, adults, larvae, and cocoons
Following 122
Spiders whose color and shape render them difficult to see " 1 24
A crab (Cryptolithodes sitchensis*) which resembles a pebble . 124
Sargassum fish (Pterophryne histrio) in a tuft of floating seaweed
(In color) Following 124
A " sea-horse " {Hippocampus} 124
Tree-frogs whose backs resemble oak leaves in color and color
pattern 125
A. Tree-frog on bark. B. Common toads . . Following 126
Weasels in winter and in summer pelage ... " 126
A. Tiger. B. Jaguar u 126
Polar bear 126
Arctic fox, in winter and in summer pelage 127
A mantis (Hymenopus) which resembles an orchid blossom . 128
Warning form and coloration in: A. Two bugs (Prionotus and
Euchistus) . B. Lady-beetles. C. Colorado potato beetle .
Following 130
Warning coloration and mimicry in moths. (In color) 132
XVI
LIST OF ILLUSTRATIONS
PLATE 71. Inedible caterpillars, showing warning coloration. (In color)
Following 132
PLATE 72. A. Gila monster (Heloderma) . B. Species of skunks belonging
to the sub-genus Chincha ..... Following 132
FIG. 37. Salamander (Salamandra maculosa) . . . . . 133
PLATE 73. Inedible curculios and lady-beetles imitated by edible longicorn
beetles and grasshoppers Following 134
PLATE 74. Several species of flies and the bees and wasps which they
imitate Following 134
PLATE 75. A. Aggressive coloration in a spider (Misumena vatia).
B and C. " Tree-hoppers " which imitate leaf-cutting ants
with their bits of leaves. (In color) . . . Following 136
FIG. 38. A spider which imitates an ant 137
FIG. 39. Spiders which mimic ants 138
PLATE 76. Mimicry, and convergence in warning coloration, among butter-
flies. (In color) Following 138
PLATE 77. Convergence in warning coloration, and mimicry among butterflies.
(In color) Following 138
PLATE 78. Caterpillars which assume "terrifying attitudes" when startled.
(In color) Following 138
FIG. 40. Caterpillar of the large elephant hawk-moth .... 140
FIG. 41. A moth (Smerinthus ocellatd) in " terrifying attitude " . . 142
PLATE 79. Mimicry in snakes Following 142
FIG. 42. A moth from India {Attaciis atlas) at the tips of whose wings
are markings resembling those upon the head of a cobra . 143
PLATE 80. A " honey-sucker " or " friar-bird " which is imitated by an oriole
Following 144
FIG. 43. " Cottontail " rabbit, showing white patch under tail . . . 146
PLATE 81. Antelope showing " danger signal1' . . . .Following 146
PLATE 82. "Recognition marks" in: A. Kill-deer or ring-necked plover.
B. Nighthawk Following 146
PLATE 83. Confusing coloration in butterflies, moths, and grasshoppers. (In
color) Following 146
PLATE 84. Sexual coloration and mimicry in butterflies and moths. (In
color) Following 150
PLATE 85. Sexual coloration and protective coloration in spiders. (In color)
Following 150
PLATE 86. Diagrams of various flowers to show the arrangement of their
parts Following 152
FIG. 44. Fertilization in the rock-rose (Helianthemwn marifolium) . .152
PLATE 87. Plants whose pollen is carried by wind. A. New Jersey scrub
pine. B. Fescue-grass Following 152
FIG. 45. A bee, showing hairs on the head, body, and legs to which pollen
grains are clinging . . . . . . . .154
PLATE 88. Partridge-berry (Mitchelld) Following 156
PLATE 89. The fertilization of an orchid by a wasp ... " 156
PLATE 90. Flowers of Aristolochia sipho and Orchis militaris . " 156
LIST OF ILLUSTRATIONS
xvn
PLATE 91. A. Skeletons of man and various apes. B. Pelvis of man and
various apes . Following 164
PLATE 92. A. Teeth of man and gorilla. B. Cerebral hemispheres of man
and chimpanzee Following 164
PLATE 93. Hair tracts on the arms and hands of a man and a male chim-
panzee . ...... Following 164
PLATE 94. Ears of various Primates « 164
PLATE 95. A. Head of foetus of orang, showing pointed ear. B. Ear of a
man, showing a point on the recurved edge. C. Vestigial
tail muscles in man, abnormal .... Following 164
PLATE 96. A. Vestigial muscles of the human ear. B. Vermiform appen-
dices in orang, man, and human foetus . . . Following 164
PLATE 97. Eyes of various vertebrates, showing the nictitating membrane .
Following 1 66
PLATE 98. Embryos of various vertebrates .... 166
PLATE 99. Foot position and curvature of spinal column in gorilla, adult
man, and human infant ..... Following 166
PLATE 100. Foot position and strength of grip in human infants . " 166
PLATE 101. Development of Saccnlina carcini .... " 184
FIG. 46. Early development of Sacculina carcini 185
INTRODUCTION
IT is not my purpose to argue in favor of the theory of
evolution as opposed to the theory of special creation. The
time is past when such discussion would be profitable. It is
rather my wish to set forth in brief outline the evolution
theory and describe some of the phenomena which it
explains, and then to discuss the relation of mankind to
evolution.
The biological sciences have been the last to come to a
position of dignity as orderly, self-consistent explanations
of phenomena. Supernaturalism and anthropomorphic inter-
pretations once prevailed in the whole domain now claimed
by natural science. Gradually the so-called physical sciences
were emancipated from the superstitions that oppressed
them. Galileo, Kepler, Newton, and the more modern
physicists and chemists have shown that the phenomena of
nature are orderly and self-dependent, that the explanation
of natural phenomena is to be sought in other natural phe-
nomena. The stellar systems of the universe are held in
their proper places by that mutual influence they exert upon
one another which we call gravitation. Our own sun moves
along its appointed daily course not because of the guiding
reins of the charioteer Apollo, but under the control of this
same omnipresent force, gravitation. The mysteries of chem-
istry were not so much in the thought of men as were the
more patent physical phenomena, so we find less of supersti-
xx INTR OD UCTION
tion and unnatural interpretation in this field, yet the false
hopes of the alchemist and his unscientific methods show
that even chemistry has had to grow away from a mass of
ignorant belief that prevented its being worthy the name of
science.
But the biological sciences were still slower to come to
their true position as dignified science. Here was the last
stronghold of the supernaturalist. Thrust out from the field
of " physical science " it was in the phenomena of life that the
last stand was made by those who claim that supernatural
agency intervenes in nature in such a way as to modify the
natural order of events.1 When Darwin came to dislodge
them from this, their last intrenchment, there was a fight,
intense and bitter, but, like all attempts to stay the progress
of human knowledge, this final struggle of the supernatural-
ists was foredoomed to failure. The theory of evolution has
taken its place beside the other great conceptions of natural
relations, and largely through its establishment biology has
become truly a science with a large group of phenomena con-
sistently arranged and properly classified. The discussion
which followed the publication of Darwin's " Origin of Spe-
cies " lasted for nearly a generation, but it is now practically
closed, so far as any attempt to discredit evolution as a
true scientific generalization is concerned. Scientists are no
1 The author believes that all nature is controlled by an intelligent Providence,
and that every phenomenon of nature is either natural or supernatural, according to
one^ point of view. A book upon the philosophical bearing of the theory of evolu-
tion might treat of the supernatural aspects of nature. It is my purpose, however,
to discuss only the natural aspects. But it is important to insist that all our scien-
tific knowledge of natural phenomena points to the conclusion that these phenomena
are orderly and self-consistent, and that the supernatural and natural are never in
conflict ; in other words, that natural phenomena are capable of being studied and
classified.
INTR OD UCTION xxi
longer questioning the fact of evolution ; they are busied
rather with the attempt to further explore and more perfectly
understand the operation of the factors that are at work to
produce that development of animals and plants which we
call organic evolution.
But though the fact of organic evolution seems satis-
factorily established, we are still far from a satisfactory knowl-
edge of the factors which are at work to produce it, and
especially are we ignorant of the manner of their operation.
For many generations to come there will be in this field
abundant opportunity for profitable study. It is not my
purpose to enter into much discussion of the more doubtful
questions, but rather to give, as briefly as is consistent with
clearness, an outline of the apparently well established facts
as to the theory and some of its important corollaries.
By thus avoiding critical discussion as far as possible, I
would not create the impression that biologists are entirely
agreed upon all points of the theory. There is endless dis-
cussion of many phases of the subject. In three cases
where there is general difference of opinion upon a funda-
mental point I have tried to state the divergent opinions and
to show what seems to me to be the safest conclusion, with
the reasons for my opinion. Two of these much mooted
points are the degree of efficiency of natural selection, and
the inheritance of the effects of use and disuse. Another
much discussed factor in evolution is sexual selection. This
I have treated largely by pictures, showing some of the phe-
nomena about the explanation of which there is so much dif-
ference of opinion. But however much difference of opinion
there may be among biologists in regard to many subsidiary
points of the theory, there is substantial agreement upon the
xxil INTR OD UCTION
fact of evolution. Biologists do not doubt that evolution has
occurred and is continuing.
It has seemed best to develop some one subdivision of the
subject a little more fully than the rest. The author has
chosen the phenomena of color in animals for this fuller
treatment, being led to this choice chiefly by the fact that
these phenomena may readily be observed by any reader in
any locality.
We will speak first of the theory, then of some of the phe-
nomena which find their explanation in the theory ; we will
consider the relation of man to evolution, and finally will
refer to a few of the corollaries of the theory which are of
general interest.
PART FIRST
ORGANIC EVOLUTION
I. THE THEORY
NATURAL SELECTION
Heredity.
Every one knows that among both animals and plants
the offspring tend to resemble their parents. The young
of a horse is always a horse and never a zebra. Wolves do
not give birth to foxes. Sunflowers will not grow from
thistle seed. Each kind of animal and plant breeds true,
as we say. This was not always recognized, as is illustrated
by the ancient Greek conceptions of the origin of animals
from plants, not only supposed to have taken place in the
original creation of animals, but also thought to be of con-
tinued occasional occurrence. Similarly, the belief, preva-
lent during the Middle Ages, that the goose-barnacle (a
kind of crustacean, Fig. i) transforms into the barnacle-
goose (Fig. 2) is an indication that at that time the inde-
pendence of different species was not so clearly recognized
as now. Sylvester Giraldus, in his Relations concerning
Ireland, written in 1187, quaintly describes this remark-
able reputed process as follows : —
"Chap, n, Of Barnacles which grew from fir timber
and their nature.
" There are likewise here [in Ireland] many birds called
3
4 ORGANIC EVOLUTION
barnacles, which nature produces in a wonderful manner,
out of her ordinary course. They resemble the marsh
geese, but are smaller. Being at first gummy excrescences
from pine-beams floating on the water, and then enclosed
in shells to secure their free growth, they hang by their
FlG. I. — Goose-barnacles (Lepas an at if era) attached to a floating piece of wood. Natural
size. — From Brehm's Thlerleben.
beaks, like seaweeds attached to the timber. Being in pro-
cess of time well covered with feathers, they either fall into
the water or take their flight into the free air, their nour-
ishment and growth being supplied, while they are bred
in this very unaccountable and curious manner, from the
juices of the wood in the water. I have often seen with
my own eyes more than a thousand minute embryos of
NATURAL SELECTION
birds of this species on the sea-shore, hanging from one
piece of timber, covered with shells, and already formed.
No eggs are laid by these birds . . .; the hen never sits
on eggs in order to hatch them ; in no corner of the world
are they seen either to pair, or build nests. Hence, in
some parts of Ireland, bishops and men of religion make
no scruple of eating these birds
on fasting days, as not being
flesh, because they are not born
of flesh, but these men are curi-
ously drawn into error. For, if
any one had eaten part of the
thigh of our first parent, which
was really flesh, although not
born of flesh, I should think
him not guiltless of having eaten
flesh. Repent, O unhappy Jew."
Again, Sir Robert Murray,
in 1676, reports his observations
of these phenomena to the Royal
Society of England : —
" In many shells I opened, I
found a perfect Sea-Fowl ; the
little Bill like that of a Goose ;
the Eyes marked ; the Head, Neck, Breast, Wings, Tail, and
Feet, formed ; the Feathers everywhere perfectly Shaped, and
Blackish colored; and the Feet like those of other Water-
Fowl, to my best Rememberance. The biggest I found
upon the Tree, was but about the size of the Figure [an
inch long] ; nor did I ever see any of the little Birds alive,
nor meet with any Body that did ; only some credible Per-
FlG. 2. — Gerarde's figure of" Barnacles pro-
ducing geise." — From Gerarde's Herball.
6 ORGANIC EVOLUTION
sons have assured me that they have seen some as big as
their Fist."
This conception of the transformation of barnacles into
geese, remarkable as it was recognized to be, was still
accepted among scientific men for a long time. And why
should not we gather figs from thistles ? why should not
plants give rise to animals as the Greek philosophers be-
lieved? That they do not do so is really a remarkable
fact which no one without experience of nature could safely
have predicted.
We, however, have had sufficient experience of nature
to affirm with confidence that animals and plants do breed
true. The statement needs no proof to our minds.
We can go farther and say that not only do plants and
animals, when they reproduce, give rise to young which
belong to the same species as their parents; the young
resemble usually the particular individuals from which
they have sprung. This is a fact perfectly familiar to
breeders. Among domestic cattle, for example, the off-
spring resemble their parents in such qualities as size, form,
color, amount and quality of milk, in disposition, in fact in
all features which we can observe. The same is true of
all our domestic animals, and no less true of cultivated
plants, and of both plants and animals in their natural
habitat.
We can accept, then, without further discussion, the
statement that plants and animals (all living things) breed
true; that offspring tend to resemble their parents in both
specific characters and individual peculiarities. This rela-
tion between parent and offspring we have named heredity.
PLATE 2. — Variation in Trillium grandiflorum. [After BRITCHER.]
Mr. Britcher collected all these varieties at one time in a single very restricted area. Observe
that the plants differ in size of blossoms, color of petals (all white, A ; all green, C, D, ^; or of
green and white in varying proportions, B, E, F, G, H, /) ; shape of petals (sessile, A, B, C, H, I ;
or stalked with stalks of varying lengths, A E, F, G, J ; broad, A ; or slender, H) ; form of flower
bracts (sessile, A, B, C, D, E, G, H, I ; or with long, J, or short, F, stalks; broad, E, F,G,J\
or slender, A, B, C, H) ; position of stem leaves (arising from the base of the stem, G; or situ-
ated at different levels upon the stem, y, F, H, D, B, C \ often occurring just below the flower
bracts, A; in one case absent altogether, E) ; form of stem leaves (sessile, A, B, C, E; or with
petioles of varying lengths, D, F, I, H, J, G ; slender, //, or broad, A) ; number of stem leaves
(one, G; or three, A, B, C, D, F, I, J ; or none, E) ; number of stalks from a single bulb
(one, A, B, C, D, E, F, G, J ; two, //, / ; or in some cases, not shown, three may be found).
The stamens and pistils also vary in form and in size, Bt D, F, G, J. Probably no finer example
of variation in any plant has been described.
NATURAL SELECTION 7
Variation.
Yet however clearly we see that offspring tend to re-
semble their parents, it is no less evident that this resem-
blance is not an exact one. Among human kind we find
excellent illustrations of this principle. However strong
may be the family resemblance between the different mem-
bers of a family, still each has his or her own individual
peculiarities. No two are exactly alike. The children do
not exactly resemble each other or their parents. These
facts of individual differences we group under the one term,
variation. We say that while, under the influence of he-
redity, the young tend to resemble their parents, because of
variation this resemblance is more or less imperfect.
No one doubts the existence of variation. All about us
we constantly see illustrations of the principle. Yet few but
trained biologists realize how universal and how extensive is
variation. All species of organisms are always varying in
every characteristic and in almost all directions, and the
extent of the variation is very considerable in most species.
The individual plants of any species vary in size, in size of
the several parts, in shape of stem and roots and leaves, in
number of leaves and of blossoms, in color of petals, in num-
ber of seeds, and in hardiness, that is, in ability to resist
adverse conditions of heat or cold, of drouth or flood, and of
unfavorable soil. In all features, both structural and physio-
logical, we find the individuals of any species of plant will
differ from one another. Absolute uniformity is not found
in organic nature (Plate 2).
Study a thousand individuals of any species with regard
to any single character, and you will see how true this is.
Take the common trailing arbutus as an example. You will
8 ORGANIC EVOLUTION
find the greatest difference in the number of blossoms in a
single head ; the number of clusters of blossoms on a single
plant will vary greatly ; the number of seeds is very variable,1
so, also, is the proportion of these that will mature ; the size,
shape, and weight of the seeds vary ; within the seeds is a
variable amount of nutriment, and careful chemical analysis
would show that this nutriment is not absolutely constant in
character; the relative proportions of the parts within the
seeds are by no means constant, for in some seeds the embryo
will be relatively larger and the nutrient materials fill a
smaller space, while in other seeds these relations will be
reversed ; in the minute embryo which each seed contains
the relative proportions between the several parts, the minia-
ture stem and leaf and bud, are subject to much variation ;
examine still more closely, and you will find that in the cells
of which any portion of this minute embryo is composed
there is no uniformity in shape, size, or structure. The
analysis can be carried to any extent, and still it will be found
that every part of the organism is variable, and that this vari-
ation is not confined to a particular direction. The flowers
of the arbutus, for example, vary, not in a single regard.
They vary in number, size, shape, number of petals, length of
petals, breadth of petals, thickness of petals, color of petals,
in the size of the nectaries upon the petals, in the abun-
dance of the nectar secreted, in its strength of fragrance, in
its quality of fragrance, etc. I have developed this point to
the extent perhaps of wearying the reader, for it has not
usually been sufficiently prominent in the minds of those
who are thinking of the processes of evolution, and much
confusion and false thinking can be avoided if we remember
1 In many localities the trailing arbutus rarely matures seed.
13 14
16 17
PLATE 3. — Varieties of Paludestrina protea. [After STEARNS.]
NATURAL SELECTION 9
that almost all sorts of variations are always present among
the individuals of every species. We have taken illustrations
from the plants ; of course the same phenomena are found
among animals (Plate 3).
Not only is variation universal, affecting all organisms
and all parts of every organism ; we find also that the degree
of divergence is really very great. In some of our common
birds, for example, the length of wing varies to the extent of
one quarter of the average for the species. So also with the
length of tail, the proportion between length of wing and
length of tail, the size of beak, the proportions of the legs,
feet, toes, and claws, and many other characters. Mr. J. A.
Allen, in his memoir On the Mammals and Winter Birds
of East Florida, says, " The facts of the case show that a
variation of from fifteen to twenty per cent in general size,
and an equal degree of variation in the relative size of
different parts, may be ordinarily expected among specimens
of the same species and sex, taken at the same locality, while
in some cases the variation is even greater than this."
Animals and plants do not all show an equal amount of
variation. Among animals the domestic goose is a good
example of a species in which variation is comparatively
slight. Partly as a result of this stability, domestication has
resulted in the establishment of but few breeds of geese.
But even in those species of animals and plants in which
there is the least variation the differences between individ-
uals are still readily noticed upon careful observation.
As an example of variation in color and color pattern
notice the frontispiece, which shows thirty-five shells of
Neretina virginea variety minor selected from a thousand,
most of which were gathered by the author in the Salt Pond
10 ORGANIC EVOLUTION
near Port Henderson, Jamaica. Among the shells there col-
lected were sixty-eight distinct varieties, as indicated by the
color and the pattern of their markings. I know of no finer
example of variation in color and color pattern than is
shown in these little shells.
Remembering now these facts of heredity and variation,
let us observe the conditions under which organisms live, and
see how these operate to cause and guide their evolution.
The struggle for existence.
As we go about unobservant through the woods and
fields, glancing carelessly at the bright flowers and the birds
busily seeking their food or singing in apparent contentment,
or as we look over the ocean and think of the fish darting
swiftly through the clear water, it all seems to us an idyl
of perfect happiness, full of ease and play. We rarely think
of the constant struggle for food and life in which all these
trees and flowers, all the fish and birds and other animals,
are engaged. We fail to see that for them life is one
continual struggle ; that the gathering of food, that resist-
ance to the unfavorable conditions of climate, cold, drouth,
flood, and storm, that rivalry in marriage and the effort
to rear their young when born, absorb the energy of animals
and plants alike ; and that, despite the strenuous efforts
put forth, the result, in the great majority of cases, is
failure and death. Yet this is by far the truer picture
of organic nature. Everywhere is starvation and death,
failure to reach success in their own lives or in rearing
their young. To some this aspect of nature may not
seem so pleasant to contemplate, yet a moment's consid-
eration will show its truth.
NATURAL SELECTION II
The never ending, ever stressful struggle for life is a
direct and necessary result of two facts: first, that the
amount of food and the space to be occupied on the earth
by animals and plants are limited ; and, second, that the
processes of reproduction, if unhindered by any adverse
circumstances, would give a geometrical ratio of increase
of plants and animals. Let us look a moment at the sec-
ond of these two propositions. The first, that the earth
is capable of supporting only a limited number of living
things, is, of course, understood without illustration, but the
facts of geometrical ratio of increase in animals and plants,
unless opposed by unfavorable conditions, are worth illus-
trating. Our common American animals and plants give
us as good examples as we could wish.
The common robin raises annually one to three broods,
of three to six young in each brood. Say that the yearly
offspring of each pair of birds is four on the average,
which is surely a low estimate, then a single pair of robins
would have in the first generation four young. The second
year they would have four more young, and their young
of the first year, mating, would have eight young, four for
each of the two pairs. If for ten years the original pair
and all of their offspring were to live and reproduce at
the assumed rate, four young a year for each pair of adults,
then at the end of the tenth year there would be over one
hundred thousand robins, all descendants of the first pair.
(See Table.)
Adults Young
One pair of adult robins .... 2
Fiist year, their young ..... 4
Second year ....... 6 12
Third year 18 36
12 ORGANIC EVOLUTION
Adults Young-
Fourth year ...... 54 108
Fifth year ...... 162 324
Sixth year . . . . . . 486 972
Seventh year , . . . . 1,458 2,916
Eighth year . .. . 4,374 8,748
Ninth year . ... . . . 13,122 26,244
Tenth year . . . . . . 39,366 78»732
End of tenth year . ' . . . 118,098
End of twentieth year .... 20,913,948,846
We see at once that the earth could not support the
animals of even a single species that would arise were not
the natural increase of the species held in check.
As a matter of fact, the number of animals or plants of
any given species remains about constant. There are usu-
ally no great fluctuations from year to year. To return,
then, to our illustration of the robin, we can say cnat more
birds (including eggs and young) die every year than live.
If the whole number remains constant from year to year
and if each pair of robins have four young yearly, of
course four robins die every year for each two that sur-
vive. That is, the death-rate is twice as great as the total
permanent population.
This death-rate is greatly surpassed by that of many
species both of animals and plants, which have a much
larger yearly birth-rate. Among mammals the average
birth-rate would perhaps be no greater than it is among
the robins, but among birds are many which have twice
or three times or even four times as many young each
year as do the robins; e.g. the whole grouse tribe, includ-
ing the pheasants, the partridges, and the quail, also the
wild jungle-fowl from \vhich our domestic chickens have
been derived. Snakes, turtles, lizards, and most reptiles
NATURAL SELECTION 13
have a yearly birth-rate at least as great as that of the
more prolific birds. Frogs and other Amphibia have an
immensely larger number of young each season, often
several hundred for each pair. Many of the fishes lay
half a million eggs for each mature female, so that here
we have an example of a yearly death-rate two hundred
and fifty thousand times as great as the permanent popu-
lation, since on the average only one male and one female
out of this half-million of young survive to take the place
of their parents and keep the number of individuals in the
species up to its usual mark. A starfish may lay a million
eggs each season, and, as the number of adult starfish
remains about constant from year to year, we see that for
every starfish living nearly half a million die each year.
The birth-rate among the Mollusca, worms, jellyfish,
sponges, and the Protozoa, like that of the starfish, is
enormous. Taking animals as a whole, it would be safe
to say that hundreds of thousands die every year for each
one that lives.
%
Among plants the figures are no less startling. The
higher flowering plants reproduce much more slowly than
most of the lower plants, yet among them the death-rate
is very large. The common marguerite daisy, which
grows so abundantly in eastern America, is a fair ex-
ample. It is a moderate estimate to say that one of
these daisies of ordinary size, blooming as it does for
about two months, would have one hundred and twenty-
five heads of bloom each year. Each head of blossoms
would have about five hundred seeds, making a total of
sixty-two thousand five hundred seeds for each plant each
year. Of this number sixty-two thousand four hundred
14 ORGANIC EVOLUTION
and ninety-nine, all but one, are destined, on the average,
to die, even assuming that the parent plant dies, which is
by no means always the case. Very many of our flower-
ing plants form more seeds than this annually, yet their
numbers do not materially increase under ordinary con-
ditions.
Fern spores are much more numerous than the seeds
of flowering plants, and the lower cryptogams, the Fungi,
especially the Bacteria, breed with a rapidity which is far
beyond our comprehension. Under favorable conditions
a single bacterium might produce a million bacteria in a
day. If this rate of increase should continue, we would
have at the end of a week a million million million million
million million million bacteria, all derived from the single
individual with which we started.
If all living things tend to reproduce with such aston-
ishing rapidity, and yet we find that their numbers do not
materially increase, but remain about constant, what is it
that holds them in check ? What kills the excess ? Many
things, unfavorable conditions of all sorts. Starvation
claims probably the largest share of victims ; heat and cold
kill many; floods, drouth, and storms destroy others ; multi-
tudes perish to feed their enemies ; disease takes its share.
Nature is fertile in expedients for killing. Life is not
easy. Success is not the rule but the rare exception.
For every one which lives and succeeds in rearing off-
spring, thousands and thousands perish. Competition is so
keen that no unhealthy or imperfect individual can endure
it. The weak fall first, leaving the field to their stronger
brethren, who in turn fight it out among themselves, till
finally only the strongest and finest survive. In a struggle
NATURAL SELECTION 15
so severe any advantage, however slight, of greater vigor,
or better structure, may be decisive and turn the scale.
In these three sets of phenomena, heredity, variation,
and the strenuous struggle for existence, we have the
basis for progress, for evolution, by the survival of the
most perfect individuals. Let us illustrate.
Among the existing individuals of any species of
animal or plant there will be found, at any time, a great
variety of more or less divergent forms. Take as an
example the common rabbit of eastern America. Some
when full grown are larger, some smaller; some are swifter,
some run less swiftly ; some are darker colored, some
lighter colored ; some are grayish, some more brownish ;
some are more shy than the average, some more bold than
their fellows ; some are more observant, some less so ;
some have greater endurance, some diverge .to the other
extreme. So we might go on. Whatever character we
choose to observe, we will find it more strongly developed
in some individuals than in the rest, and conversely in
some it will be developed to less than the average degree.
The larger number of individuals in the species will usu-
ally pretty closely agree in the extent to which any par-
ticular character is developed, but a considerable number
will be found who diverge toward either extreme.
Suppose now there be introduced into the region where
these rabbits live some predatory enemy swifter and more
sly than those to which the rabbits are now exposed.
The first result would be the extermination of those
rabbits which are less swift and less cautious and observ-
ant. Most of those of average swiftness and alertness
also might be caught and killed. There would soon be
1 6 ORGANIC EVOLUTION
left, then, only the individuals in which these valuable
qualities are most highly developed. They would persist,
and, escaping their enemies, would succeed in rearing
young, to which, according to the principles of heredity,
they would hand down their good qualities, so that the
young, like the parents, would be swift and keen. Thus,
by the elimination of the less perfect individuals of the
species, there will have been developed a race of rabbits in
which the qualities which aid in escape from a swift, keen
enemy are more highly marked than in the former race.
This is evolution by the elimination of the unfit, or by
the survival of the fittest, the process which is called
natural selection, meaning the selection or retention of the
individuals most perfectly adapted to the environment in
which they live.
The new race referred to in the illustration chosen
might be especially characterized not only by the two
qualities mentioned, swiftness and keenness, but also very
likely by other qualities that would aid in escaping the
new enemy, such, for example, as more perfect conformity
in color to the environment, provided its conditions of
life had been so easy that perfect color resemblance to the
environment had not been previously a necessity. Several
of the desired qualities could probably be perfected at the
same time, since the variations from which to select would
not appear separately in different individuals, but would
often be present in the same individual at one time.
Thus there would be found among our Eastern rabbits
some which were at once more swift, more keen-sighted,
more observant, more shy, more perfectly like the environ-
ment in color, and perhaps marked by special development
NATURAL SELECTION 17
of other desirable qualities. Variation is much more exten-
sive than we usually think, and such divergence in many
qualities at once might readily be found.
Illustrations of this principle of natural selection might
be indefinitely multiplied. The environment presses upon
the animal or plant at all points, and the whole organism
is capable of adaptive response, since the whole organism
varies, giving favorable peculiarities for selection. Any
feature, of structure or of function, may be perfected when-
ever it becomes desirable to have it emphasized. The only
things necessary are that the useful character shall be pres-
ent year after year as a variation in some individuals, and
that it shall be of sufficient importance to aid its possessors
to win in the struggle for life in which they are constantly
engaged. This struggle is so severe that only the most
perfectly endowed can hope to win ; so that an advantage,
though very slight, may determine survival, or, as Romanes
puts it, be " of selection value."
There are two quite different methods used by both
plants and animals to enable the several species to persist
and not be destroyed in the battle of life. The first is
the one already illustrated, namely, the gradual establish-
ment, by selection of the most perfect individuals, of a
condition of more perfect adaptation of the individuals of
the species to the environment in which it lives. The sec-
ond is to so greatly increase the number of the offspring
by great development of the reproductive functions, that
from very numbers they will have more chance of survival.
We can hardly say that a million starfish eggs have a mill-
ion times more chance of survival than would one, but
surely a starfish that lays a million eggs has much more
1 8 ORGANIC EVOLUTION
likelihood of leaving descendants than would one which
laid but few eggs, other things of course being equal.
Most animals and plants adopt both methods, being very
prolific and being well adapted to their environment.
Now, evolution is brought about by the occurrence,
among the individuals of a species, of certain ones which
are better fitted for the life they are to live than are the
others of the species ; by the survival of these favored
ones; and by the transmission of their valuable qualities
from parent to offspring generation after generation. The
appearance of the desirable quality is an example of varia-
tion : the survival of those individuals which possess these
qualities is secured by natural selection : and the perpetua-
tion of the useful qualities is secured by heredity. It would
seem necessary that, given these three factors, variation, natu-
ral selection, and heredity, evolution should be the result.
Later we will take up some of the most frequently urged
objections and see if there is any flaw in this argument.
Mutation.
Recently De Vries has shown by a very careful and
very extensive series of observations of wild and cultivated
plants, chiefly of the species CEnothera lamarckiana, that
there may be two somewhat different types of variation —
(1) "fluctuating variation," by which a species varies in
greater or less degree and in almost all directions, and
(2) " mutation," by which the whole character of the species
is changed and a new species established at one leap. The
new species thus established by mutation will show, as did
the former species, a series of fluctuating variations. Every
species of animal and plant with its numerous fluctuating
NATURAL SELECTION 19
variations still shows a certain rather definite "species
mean," to which most of the variants rather closely conform,
but from which some considerably diverge. Mutation, ac-
cording to De Vries, establishes a new species with a new
species mean and a new series of fluctuating variations
gathered about the new mean.
Similar phenomena have long been known to florists
and breeders of animals, the divergent individuals of the
new type having been called sports. To De Vries, how-
ever, belongs the credit of having studied these phenomena
in many thousands of individuals through many genera-
tions. Yet, careful and extensive as has been De Vries'
work, we cannot yet be assured that the appearance of
discontinuity in variation, by which new types are suddenly
established, is not due to insufficient observation, and that
the study of a still larger series of individuals would not
show forms completely bridging over the gap between the
old and new types. At present we can say only that De
Vries' work has shown the likelihood of there being a
real distinction between fluctuating variations and muta-
tions. Other features of De Vries' observations will be re-
ferred to later.
The individuals of new character, arising by mutation,
must be subject to natural selection, and therefore those
which are not well adapted to their environment will be
destroyed, as in the case of divergent individuals arising
by fluctuating variation.
Referring again to the illustration given above, ob-
serve that all the rabbits in the given region are sub-
ject to natural selection and the more perfectly adapted
individuals will be preserved. It makes no difference
20 ORGANIC EVOLUTION
whether they obtained their useful character through fluc-
tuating variation or through mutation. This does not
affect the fact of their survival being determined by natu-
ral selection.
Before referring to the objections to the theory of natu-
ral selection, let us notice a few general principles in the
operation of this factor in evolution.
Observe that in the process of evolution by natural
selection the welfare of the individual is conserved only
so far as it contributes to the welfare of the race. It is
necessary that the more perfect individuals should survive
long enough to breed and hand down to their young their
useful qualities, but, having done this, their further life is a
matter of indifference, so far as the processes of evolution
are concerned. In case an animal or plant has several
breeding seasons during its normal life period, of course
its preservation until the completion of all these reproduc-
tive processes may be an important advantage to the spe-
cies, and, if so, will tend to be secured ; but in the case of
a species whose members have but a single reproductive
period in a lifetime, as is the case with many insects for
example, their persistence after the completion of the pro-
cesses of reproduction would be even disadvantageous to
the species, since they would consume food and occupy
space needed for the younger individuals which are to
continue the species by reproduction. It is natural to
find, then, as we do among the insects, the adults usually
dying after the breeding season is over. The same thing
is true, of course, of all annual plants. Among some kinds
of animals the parents care for the young after birth, and
NATURAL SELECTION
21
in these cases it is easily seen that the life of the parent
will naturally be continued until the completion of the
period of parental care over the offspring. In the case of
animals which form communities, it may be advantageous
to these communities to have their members continue to live
even after their reproductive activity ceases, since they may
aid the community in other ways than by reproduction.
Let us see a
few concrete il-
lustrations of this
principle that in
the processes of
natural selection
the welfare of the
race and not of
the individual is
sought. Very
commonly seen
on our trees are
the egg-cases of
the bag-worm
(Fig. 3), a moth,
the female of which never comes to complete development,
in fact, never leaves the cocoon, but is fertilized by the male
and lays her eggs without ever emerging into a free life as
an active, flying adult. More than this, not only is the
active life of the adult female suppressed: her body disin-
tegrates in the process of laying the eggs, so that ovulation
and the death of the female are simultaneous. Here we
see the continued existence of the adult female after the
eggs are laid is of no value to the species, and she is
FlG. 3. — The "bag-worm," Thyroidopteryx ephemeriformis.
a. Larva, b. Pupa. c. Adult female (wingless), d. Adult male.
e. Longitudinal section of a cocoon showing the degenerate female
full of* eggs. f. One of the larvae, showing the covering of silk and
twigs in which the posterior part of the body is enclosed, g. Young
larvae, natural size.— By the courtesy of the United States Depart-
ment of Agriculture.
22
ORGANIC EVOLUTION
allowed to die. The male in this same species is an
active, flying moth, flight being necessary in order that he
may seek the female and fertilize the ova.
Another example of a similar sort is found among the
bees. Here the males die in the process of fertilizing the
eggs. The males in the beehive take no active share in
the work of the community, except to fertilize the eggs, so
that when this function
is performed their con-
tinued life would be of
no profit to the com-
munity, in fact would
be a positive disadvan-
tage, since they would
use food and space
which could better be
given to those indi-
viduals who were of
present value to the
community.
Still another ex-
ample from the bees.
The beehive contains three sorts of individuals (Fig. 4) :
the males, or drones, whose only function, as just stated,
is to fertilize the eggs ; the perfect female, or queen,
which lays all the eggs, usually only one adult queen at a
time being present in a normal hive ; and the workers,
sterile females, who perform all the labor of the hive and
show the remarkable instincts so well known among the
bees. The workers generally keep on hand a number of
queen larvae, so that if anything should destroy the old
FIG. 4. — Honey-bees and a piece of honeycomb.
a. Male bee, or drone. b. Worker-bee, a sterile
female, c. Queen bee, a fertile female. — From Brehm's
Thierleben.
NATURAL SELECTION 23
queen they can rear another queen ; but they do not allow
these larvae to hatch so long as the old queen is still in
the hive and in good condition, unless swarming is about
to occur. The queens have the bitterest antipathy for one
another, and should a new queen be allowed to hatch
there would at once be a mortal duel between her and her
mother, the old queen. As this would not be conducive
to the welfare of the hive, the workers allow the old queen
to approach the cells of the young queens, just as these
are ready to hatch, and permit her to sting them to death
before they hatch. Now these young queens are partially
encased in an outer envelope which is not easily pierced
by the sting of their would-be destroyer ; but as it is advan-
tageous for the hive that these unhatched queens should
be put to death, we find that in forming this envelope
around themselves they have left the posterior part of their
bodies naked, so that the sting of the adult queen can
readily penetrate and kill them, death being certain when
once they are stung. In this case we see that the queen
larvae provide in their own structure for their own destruc-
tion, since this is for the advantage of the communities in
which they live. The welfare of the race, not of the indi-
vidual, is secured.
As an example of communal forms in which the con-
tinued life of the individual members of the community is
advantageous to the community, even though these indi-
viduals be not active in reproduction, we can again instance
the bees. The worker-bees are not usually able to repro-
duce ; they are sterile females, generally incapable of laying
eggs. Yet these workers are the most valuable members
of the community, carrying on all the wonderful activities
24 ORGANIC EVOLUTION
of the hive, making the honeycomb, gathering and storing
the honey, rearing the young, guiding the queen in the
performance of her duties, expelling the males when the
breeding season is over, in fact running the whole hive.
In this case it is not the individual worker which is the unit,
but the community in which it lives, the hive. It is the
whole hive, with all its mutually helpful members, that enters
the struggle for existence, and natural selection determines
which hives, just as much as which individual bees, shall
survive. There is selection here of communities as well as
individuals for survival, and an individual useful to the
community for some other reason than breeding will be pre-
served because of this other value.
Among human beings we have excellent illustration of
the fact that their helpfulness to the young or to the
community as a whole may make the continued life of the
parents of value, though they bear no more children. The
human child is very imperfectly developed at birth ; it is
dependent on the parent's care; should the parent die
the child would suffer. The life of the parent cannot be
allowed, then, to cease with the birth of the child. More
than this, the family is in a very real way a unit in the
struggle for existence, and the continued life of its members
helps the family to succeed, so that when the children of
the family shall begin to rear families of their own, they
shall have an advantageous start in their new, semi-inde-
pendent life. Again there is a rivalry between communi-
ties of a larger sort. Different industrial centres enter into
competition with one another, and nation contends with
nation and race with race. As the continued life of the
individual beyond the close of the reproductive period is
NATURAL SELECTION 25
of advantage to these larger communal units, we find the
length of life is not determined by the close of the time of
functional reproduction among men, as it is among so many
of the lower forms. Still, among men, as among other
animals, it is the advantage of the race and not the welfare
of the individual which determines the length of life.
This fact, that among men the welfare of the race is the
thing secured even at the sacrifice of the good of the indi-
vidual, is clearly seen when the two come into conflict. It
is not well for the individual that he die in battle, yet,
when the national welfare demands it, thousands so perish,
and there has even been developed among men a passion
for such death for the good of their country. When a man
has so indulged his evil impulses that he has become a
menace to the communal welfare, he is restrained by a fine,
or is deprived of his liberty, or may even be killed, and no
conditions of his personal welfare are allowed to interfere.
Even those who oppose capital punishment do so chiefly
because they believe it hurtful to the community as a whole.
Altruistic self-sacrifice is in line with the great principle in
accordance with which nature seeks the welfare of each
species as a whole, with no hesitation because of any hard-
ship to individuals which may be involved.
Let us give attention to one other corollary of the
theory of natural selection. The struggle for existence is
most severe between those animals or plants which seek to
occupy the same place in nature. Plants which live in
moist valleys may come into very severe competition with
one another, but they do not come into rivalry with the
plants which like the dry hills or the barren rocks. The
26 ORGANIC EVOLUTION
individuals of a single species, fitted as they are for life
under the same conditions, enter into the most constant and
the most severe rivalry. We may state this fact in another
form by saying that the struggle for existence is most severe
between near relatives. Now see what is the effect of this.
We have a group of individuals belonging to the same
species. Between them the competition is more severe than
is the rivalry between themselves and any other forms. If
now there arise among them individuals that diverge, so as
to fit them to occupy a place slightly different from that
occupied by the parent stock, this will allow the divergent
forms to withdraw a little from the place where competition
is most severe, and so will give them a better chance for sur-
vival. We see the tendency is constantly toward divergence,
since divergence lessens the severity of the competition
for life. Variations which arise, if they enable their possess-
ors slightly to change their habit of life, will tend to be
preserved, even though the place to which the divergent
individuals migrate is, in itself, no better than the one they
leave. This, we see, may materially affect the result of the
process of evolution, causing forms to survive which other-
wise would not be chosen.
Evolution, so far as it is dependent upon natural selec-
tion, is more rapid while the environment is changing than
it is under stable environmental conditions. By the con-
tinued action of natural selection animals and plants become
so well adjusted to their environment that while this remains
unchanged they undergo comparatively little modification ;
but when the environment is changing the plants and ani-
mals must change with it, if they are to be well adapted to
NATURAL SELECTION 2?
their surroundings. Under changing environmental con-
ditions, especially if the changes be rapid and considerable,
the more plastic species, and those in which the largest
degree of variation is present, will have a decided advantage
over their less readily modified neighbors, and those species
which do not so greatly vary. Many of the less plastic and
less variable species may be destroyed because of their in-
ability to keep pace with the changes in their surroundings.
The plasticity of the organism and its variation are, there-
fore, important elements, and the degree to which they are
developed in any given species may have an important
bearing upon the fate of that species. Lloyd Morgan, J.
Mark Baldwin, and H. F. Osborn have emphasized the im-
portance of plasticity, showing very clearly that the ability
of the individuals of a species each so to change its habit or
structure as to adapt itself to new disadvantageous conditions
may preserve its life and so prevent the rapid extermination
of the species when environmental conditions change for the
worse. In this way a plastic species may be tided over a
period of hurtful environmental changes until natural selec-
tion shall have time to secure the fundamental adaptation
of the species to its new conditions of life, after which the
individuals will be born in a condition so suitable to their
surroundings that they will not need to change their struc-
ture or natural habits in order to survive. In a species which
withstands unfavorable environmental conditions through
the plasticity of its individual members, each individual will
need to be educated into harmony with the environment.
Such individuals of the species as vary toward greater
natural adaptation will need less education. Of course
innate adaptation is more advantageous than adaptation
28 ORGANIC EVOLUTION
through education, since it is immediate, no period of dis-
advantage appearing in the early life of the individual. The
death-rate of the individuals which become adapted through
education may be greater than that among the individuals
with more perfect innate adaptation. Thus in time innate
adaptation may be established for the species as a whole.
Mankind are in all intellectual features more plastic than
animals of any other species. By education, to which they
readily respond, they learn to so adapt themselves to un-
favorable conditions as to escape from much of the stress
of the struggle for existence. They have learned to protect
themselves from cold and inclement weather, from hunger
and from disease, and from many other dangerous elements
in their environment. Man's great individual adaptability
has secured his survival, but at the same time, has greatly
hindered his evolution. This will be discussed later. It is
desirable here merely to observe that plasticity (educability)
in any species of organism hinders its evolution by lessening
the destruction which lack of conformity to the environment
would cause. If the plasticity is very marked, as among
human kind, it may almost prevent evolution through natu-
ral selection. (Cf. Appendix I.)
Artificial selection.
Before leaving the subject of natural selection it would
be well to refer to the similar phenomena of artificial selec-
tion. Florists and breeders of animals use methods that
very closely parallel natural selection. We are familiar with
the remarkable results which have been obtained in the
rearing of domestic animals and plants. The many kinds
.of horses in use (Plate 4) are widely different from the origi-
E V
PLATE 4. — VARIETIES OF HORSES.
A. Thoroughbred mare. B. Shire horse and Shetland pony. C Arab horse. D. Hackney
mare and foal. E. Iceland pony. F. New Forest pony stallion. — From Hayes' Points of the
Horse.
PLATE 4, a. — Brassica oleracea, L., the wild species from which the many varieties of
domestic cabbage, kale, cauliflower, Brussels sprouts, Savoy cabbage, and Swedish turnip have
been derived, i. Part of a flowering and fruiting specimen, two years old, gathered on the
rocks near the sea, Great Orme's Head, Wales, September, 1892. 2. Another specimen, found
at the same place and at the same time, probably three years old, branched and bearing
many leafy shoots. Both specimens were photographed on the spot. — From Errera and
Laurent, Planches de Physiologic vi'getale.
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10 ylirriBl bi£Jai;M sriJ to ladmsm B zl rioiriw ,»^»t^\Q »^uiatft. aaioaqg bJiw srlt moil
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,gnol ",^BoI iBgua " .sgfiddfiD .^ .bsarf on ".nBohsmA b9S£l§-n99i§ " .ggBdd^D .£
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naad asrf bnuoig avod£ jfljsJa ariJ ; idfiilrioJS .81
PLATES 5, 6, 7. — Different varieties of cabbage, Savoy cabbage, kale (borecole), broccoli,
Brussels sprouts, cauliflower, Swedish turnip, and kohlrabi ; all of which have been derived by
cultivation from the wild species Brassica oleracea, which is a member of the Mustard Family or
Crucifera.
Plate 5. — I. Cabbage, dark red, early, pointed head. 2. Cabbage, " Schweinfurt," spherical
head, large. 3. Cabbage, "green-glazed American," no head. 4. Cabbage, "sugar loaf," long,
oval head. 5. Cabbage, " Rennes early," small. 6. Savoy cabbage, " Frankfurt," long, oval head.
Plate 6. — 7. Savoy cabbage, "extra early midsummer." 8. Savoy cabbage, "Tours." (6,
7, 8, differ in size, shape of head, and degree of curling or crinkling of leaves.) 9. Kale, curled,
dwarf, sometimes called " German green." 10. Kale, tall, curled, n. Kale, " marrow-stemmed."
12. Kale, very tall, "cow or tree-kale." (In kale the leaves are highly developed, but are not
compacted into heads.)
Plate 7. — 13. Brussels sprouts, dwarf ; many small heads along the stalk. 14. Broccoli, purple
sprouting; leaves and blossoms both used. 15. Cauliflower, Sicilian, " purple Cape broccoli."
16. Cauliflower, dwarf, early " Chalon." (In the cauliflower the blossoms are greatly developed,
forming a compact head.) 17. Swedish turnip. In this type, which is said to have been derived
from the same wild species as the cabbage, the underground portion of the stalk has been enlarged.
18. Kohlrabi ; the stalk above ground has been enlarged.
PLATE 5.
PLATE 8. — Varieties of cabbage, or " colewort," in the latter part of the sixteenth century.
a. " White cabbage cole " (red cabbage also was known at that time), b. " Open cabbage
cole " (head less compact), c. "Savoy cole." d. " Curled Savoy cole" (leaves and flowers both
developed ; head of flowers almost like cauliflower), e. " Cole-florie." f. " Garden colewort "
(kale), g. " Curled garden cole." h. " Parsley colewort." *'." Swollen colewort." /."Round
rape cole" (kohlrabi). — From Gerarde's Herball. Comparison with Plates 5, 6, and 7 shows
something of the extent of modification in the last three hundred years.
,
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nsqO " .^ .(ami) leilt Ja nwonjf enw oals s^ddfio bai) " sloo sgBddip sJiriW" .^
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" Mowtiioo nabiJsO " \ ".aitoR-a'^ I/BO ajfil t -.ft to busri ; byqobvab
bnuoH " \ ".nowaloj nsliowS " ,i
aworie ^ bn£ ,d ,£ Eatal*! ritiw nc
,rifisv bdifa/mrf ssifit J2£l sffj ni noiteaftibom 'to JaaJxa srii^o
v*a
PLATE 9. — Varieties of turnips, all of which have been derived by cultivation from the wild
species, Brassica rapus, L., a member of the Mustard Family.
I. " Early stone or stubble," green top. 2. " Chirk Castle black stone," dark purple. 3. " Long,
white Meaux or cowhorn," pale green top. 4. " Early, white, strap-leaved American," all white.
5. " Early Vertus or Jersey." Observe how these varieties differ in form.
PLATE 10. — VARIETIES OF DAHLIAS.
a. Types of single dahlias, b. " Clifford W. Bruton," a large, yellow dahlia, c. " A. D. Livoni,"
pink, pompon type. d. A modern form, red. — From Country Life in America, by permission of
Doubleday, Page and Co.
PLATE n. — A new " cactus" type of dahlia. This particular variety is called " Kriemhilde.'
— From Country Life in America, by permission of Douhleday, Page and Co.
NATURAL SELECTION 29
nal stocks from which they were derived.1 Our domestic
chickens have been much modified from the jungle fowl,
their ancestor. Sheep, cattle, hogs, canary birds, pigeons,
and other kinds of domesticated animals show similar
modifications of the original stock. Among plants we
have more numerous instances ; for example, most of our
garden vegetables, the many varieties of the cabbage (Plates
5-8), the several sorts of potatoes, peas, lettuce, turnips,
etc. (Plate 9). Other instances are furnished by the numer-
ous kinds of roses, chrysanthemums, pansies, tulips, sweet-
peas, asters, hollyhocks, dahlias (Plates 10 and n), and a host
of others of our common flowers which show many varieties.
Now, as just stated, the methods used by breeders to
produce these varieties of the different species of domestic
animals and plants are closely similar to the chief method
adopted by nature in the evolution of natural species. The
breeder, whether of plants or animals, finding in his stock an
individual or several individuals which show some desirable
quality, chooses these individuals to breed from, and when,
among their offspring, he finds some in which the useful
quality is especially pronounced, these again are chosen for
breeding. The desired character can be intensified by choos-
ing, generation after generation, those individuals in which
it is most strongly developed, and rejecting the others. The
breeder rejects the individuals in which the important quality
is weakly developed. So also does nature in the process of
natural selection. The resemblance between the two pro-
cesses is very close, and the results are similar. In the case
of natural selection we get modification of the original stock
1 It is probable that domestic horses have been derived from several wild
species.
30 ORGANIC EVOLUTION
in such a .way as to give more perfect conformity to the
environmental conditions ; while in artificial selection the
modification is such as to make the altered form more per-
fectly suit the uses to which man wishes to put it. The
results of artificial selection are usually more quickly seen ;
for the selection for breeding purposes of individuals with
the desirable qualities is generally more rigid than in nature,
where the more and the less adapted forms will for a time
breed side by side, the more perfect gradually predominat-
ing more and more.
The extent of the
modification produced by
artificial selection is very
great in many cases.
Notice the common do-
mestic chickens, in which
FIG. 5.— skuii of Polish fowl, showing the pe- the different breeds differ
culiar knob that has been developed in front of the
brain case. — From Wright's New Book of Poultry, from One another tO SLlch
by the courtesy of Cassell & Company.
a degree that if they
occurred in nature the several kinds would be referred not
only to different species, but to different genera (Plates 12-19
and Fig. 5). Compare the slender "game" (Plate 12, A\
1 6, B], which most closely of all resembles the ancestral
"jungle fowl" (Plate 16, A\ with the heavy " Brahma" (Plate
15, C, D) or "Cochin-china" (Plate \$,A,B\ 19, B\ or with
the long-tailed "Japanese" cocks (Plate 17), or with the little
"bantam" (Plate 14, D\ 19, C\ Or notice the varieties of
pigeons, as shown in another illustration (Plate 20 and Fig. 6).
These races differ from one another anatomically and
in disposition as much as do natural species, yet in one
important particular they fail to resemble natural species.
A. Malay cock.
B. Colored Dorkings.
f. *
C White Dorking. D. Spanish.
PLATE 12.— VARIETIES OF DOMK.STIC CHICKENS. [Alter TEGETMEIER.]
A. Silver Polish.
B. Houdanj
C. La Fleche. D. White and game bantams.
PLATE I^ — VARIETIES OF DOMESTIC CHICKENS. [Alter TEGETMEIER.]
A. Partridge Cochins.
B. Buff Cochin hen.
C. Dark Brahmas. D. Light Brahmas.
PLATE 15. — VARIETIES OF DOMESTIC CHICKENS. [After TEGETMEIER.]
1870.
1900.
PLATE 16. — A. Jungle fowl, cock and hen (Gallus bankiva), a wild species found in southern Asia,
from which our domestic chickens have been derived. — From mounted specimens in the United States
National Museum. B. The evolution of the game cock. — From Wright's New Book of Poultry, by
the courtesy of Cassell and Company.
PLATE 17. -Japanese long-tailed cocks. -From Romanes' Darwin and After Darwin, by the
courtesy of The Open Court Publishing Company.
II-
PLATE 18. — A. "Frizzled fowls." Many different kinds of the ragged-feathered chickens, both
bantam and larger varieties, have been bred. [After TEGETMEIER.] B. Head of Breda cock. — From
Wright's New Book of Poultry, by the courtesy of Cassell and Company. C. Head of salmon foverolle,
showing the peculiar development of the feathers beneath the eyes and the bill. — From Wright's New
Book of Poultry, by the courtesy of Cassell and Company.
PLATE 19. — A. A single feather from a "silky fowl." Almost any breed can be obtained
with this type of feathers. [After TEGETMEIER.] B. Leg of Cochin cock. All the feathers
shown are upon the leg. — From Wright's New Book of Poultry, by the courtesy of Cassell and
Company. C. " Cochin " bantams. [After TEGETMEIER.]
PLATE 20. — VARIETIES OF DOMESTIC PIGEONS.
i. Wild blue-rock pigeon (Columba livia) . 2. Homing pigeon. 3. Common mongrel pigeon.
4. Archangel. 5. Tumbler. 6. Bald-headed tumbler. 7. Barb. 8. Pouter. 9. Russian trumpeter.
10. Fairy swallow. n. Black-winged swallow. 12. Fantail. 13. Carrier. 14 and 15 Bluetts.
The bird between 14 and 15 is a tailed turbit. — From a photograph of an exhibit in the United
States National Museum.
NATURAL SELECTION
They will often freely intercross in breeding, while, as a
usual thing, natural species will not do so. This brings us
V
FIG. 6. — The rock pigeon (Columba livla) of northern Africa, from which the different vari-
eties of domestic pigeons have been derived by artificial selection. — From Brehm's Thierleben.
to a discussion of some of the objections urged against
natural selection as a widely effective factor in evolution.
Objections to natural selection as a factor in evolution.
To Huxley the inability of artificial selection to produce
races which are sterile when crossed, seemed the strongest
objection to the certainty of effectiveness in natural selec-
tion to produce true species, which in nature are so generally
characterized by inability to breed together, or at least by
infertility in their hybrid offspring, in cases in which hybrids
can be obtained. Doubtless mutually infertile races could
be produced by artificial selection if breeders should care-
32 ORGANIC EVOLUTION
fully observe relative degrees of fertility and select as pro-
genitors for the several races individuals which would not
readily breed with others than those of their own race. As
a matter of fact breeders have not cared to produce infertile
races and have not done so. There seems little doubt that
they could have done so.
Mutual infertility between certain individuals may often
in nature have been the starting-point in the divergence
which has resulted in the establishment of new species.
This point will be discussed farther on.
Another objection which has been urged against the
efficiency of natural selection as a factor in evolution is the
fact of the apparent uselessness of some of the character-
istics of different species, both animals and plants. If a
character is useless how can it have been developed by
natural selection, which operates only to perpetuate char-
acters which aid their possessors in the struggle for exist-
ence? First let us ask, are useless characters really found?
Apparently they do occur, but much less frequently than
we would at first thought suppose. Careful study often
shows that structures or habits apparently useless are of
real value to their possessors. One would find it difficult
to give an instance of an organ or characteristic which he
is sure is of no value to the plant or animal in which it is
found. Yet we could probably find such instances. Many
are familiar with the beautiful markings on the shells of
diatoms, a group of microscopic Algcz, or with the beauti-
fully regular skeletons of many other microscopic animals
and plants. These shells and their markings are often
of elaborate pattern (Plate 21); they are regular in their
I — ,v\ -^**"ov-ZH
!' T 5 1
13
NATURAL SELECTION
33
arrangement, and this arrangement is constant for the
species. They are, then, true specific characters. Of what
possible use can these minute ridges and furrows upon the
shell, or the particular arrangement of skeletal spicules, be
to these little plants and animals ; or why are they more
useful if regularly arranged according to a particular pat-
tern ; or why is it important that each species should have a
pattern peculiarly its own ? We cannot satisfactorily answer
these questions. We know comparatively little about the
details of the life of these species. If we knew more it is
possible the explanation of these skeletal characters might
appear and we see that they are useful. Much of our
inability to show the utility of the apparently useless char-
acters of animals and plants is probably due to our ignorance
of the life habit of these organisms.
Yet we may, for the present, grant that certain struc-
tures and habits are useless. We must, however, remem-
ber that natural selection is not the only factor of evolu-
tion, and that, while it develops directly none but useful
characters, the other factors give rise to characters that are
not necessarily useful. This point will come out more
clearly after we have described the action of these other
factors.
But, setting this point aside, natural selection may indi-
rectly give rise to features of organization or disposition
that are not useful to their possessors. An organism is a
very complex thing, with its parts most intimately related to
each other. No single structure in the body is independent
of the rest. One part acts upon another in ways most
remarkable. The intimacy of this interrelation of parts and
the complex way in which they react upon and influence
34 ORGANIC EVOLUTION
one another we have lately been able to appreciate more
than ever before. There seems to be some reason to believe,
though it is not yet proven, that every organ and cell in the
body so acts upon every other as to affect its behavior.
This is well illustrated by the effects of extirpation of
organs. We do not know what effect the thyroid glands
have on the other organs of the human body, but if they be
removed or become badly diseased, we find there results a
profound disturbance of the functions of other parts of the
body, showing that the thyroid glands when present and nor-
mal probably exert some influence the absence of which from
the body is disastrous. There are many other organs whose
functions we do not understand, whose extirpation is seriously
injurious. Their influence upon other organs of the body
must be very important. The changes which follow the
destruction of the organs of reproduction are of especial
interest in this connection. In the common domestic
chickens the destruction of the testes in a young male pre-
vents the comb and wattles and spurs reaching their normal
size, the habit of crowing is given up, the characteristic
combative disposition of the male is lost. Likewise the
destruction of the ovaries in a young hen makes the comb
and wattles enlarge, the habit of crowing may be acquired,
and the disposition becomes more pugnacious. Here we
have a clear indication that the presence or absence of the
reproductive organs influences organs which seemed to
casual observation to be unrelated to them, namely the brain
(change of disposition), the comb and wattles upon the head,
and the spurs on the feet. Probably many other organs of
the body are equally influenced in ways not so readily ob-
served.
NATURAL SELECTION 35
Now if the organs of the body are so intimately con-
nected with one another that what affects one may affect
also the others, we see at once that changes produced by
natural selection in any organ of the body because of the
usefulness of such change, might very likely bring about
correlated changes in other organs, though these latter
changes be not in themselves useful. The secondary modi-
fications would not be directly due to natural selection
and so would not necessarily have to be useful. Their
connection with a useful modification would be enough
to account for their presence. This principle of correlation
is undoubtedly of great importance, but it is often difficult
to understand the details of its operation in particular cases,
since the nexus between the different organs, postulated by
this principle, may be so intimate and subtle as to be ex-
ceedingly difficult to study.
As a very evident example of correlation think for a
moment of the great weight of the antlers of an elk and
the great strength required in the ligamentum nuchcz, the
ligament which stretches from the top of the skull along
the back of the neck to the vertebras between the shoul-
ders. The strength of this ligament must have increased
as the weight of the antlers which it supported increased,
the two being correlated. In this instance it is easy to
see the nature of the connection between the two struc-
tures, and that natural selection has probably produced the
correlation. In many cases, however, it is very difficult
to understand the relation between correlated structures,
as in the case of the reproductive organs and the organs
affected by their extirpation in the domestic fowl. Wallace,
in his delightful book, Darwinism, says : " In Paraguay,
36 ORGANIC EVOLUTION
horses with curled hair occur, and these always have hoofs
exactly like those of a mule, while the hair of the mane
and tail is much shorter than usual. Now, if any of
these characters were useful, the others correlated with it
might be themselves useless, but would still be tolerably
constant because dependent on a useful organ. So the
tusks and bristles of the boar are correlated and vary in
development together, and the former only may be useful,
or both may be useful in equal degrees." If, in case of the
boar, the conditions of life became such that increase in
the size of the tusks would be useful, there might be de-
veloped a race of boars with larger tusks, and at the same
time the length and coarseness of the bristles would
probably increase, not because better developed bristles
are needful in themselves, but because of the correlation
between large tusks and coarse, long bristles, a correlation
the reason for which we are unable to understand.
In the case of the regular patterns in the skeletons of
many unicellular animals and plants, to which we have re-
ferred, it is possible, I will not say probable, that the regular-
ity of their arrangement may be due to the constitution of
the protoplasm of the cells which form them, to some regular
arrangement of the constituent particles of this protoplasm,
especially as regards its chemical activity, so that the
skeletons will be regular, not because of any utility in their
regularity, but because they are each formed by a bit of
protoplasm so constituted that, if it is to form a skeleton
at all, it must form a regular skeleton. Thus the regular-
ity of the diatom shell may be due to correlation with a
kind of protoplasmic structure which is itself useful.
But, though natural selection is a factor in evolution,
NATURAL SELECTION 37
and even if it were, as it is not, the only factor, why should
all characters of animals and plants be useful to their
possessors? Would not many chance variations be pre-
served whether they were useful or not? Hurtful char-
acters, of course, would be eliminated, but why should not
certain neutral characters persist without reference to natu-
ral selection ? It is truly a remarkable fact, and one hardly
to have been anticipated, that so large a proportion of the
habits and structures of organisms are useful to their pos-
sessors. On page 66 et seq. is shown one way in which
useless characters may be preserved. [Physiological seg-
regation.]
A third objection urged against the importance of the
agency of natural selection in evolution is that certain
organs which are useful in their present condition could
hardly have been so when beginning to form in the past,
or, at least while as yet very slightly differentiated, could
hardly have been sufficiently useful to be of " selection
value," i.e. to secure the survival of the animals or plants
possessing them. This is really a modification of the
objection last mentioned. In reply we may say, as we did
in the last case, that it is difficult to say what might be
the usefulness of the lowly developed organs from which
the at present clearly useful organs have come by modifica-
tion. If it is difficult to determine the usefulness of an
organ in a living animal which we can study, how much
more difficult it must be to decide as to the usefulness of
an organ in an extinct animal, and the early stages in the
evolution of organs at present useful were generally passed
through in animals or plants of a kind no longer found
38 ORGANIC EVOLUTION
on the earth. Also the principle of correlation between
organs is important here. Organs not useful in them-
selves may be correlated with other organs of great value
and be developed and perfected along with these until
they reach a degree of development that renders them
themselves useful.
There is another important principle that helps us under-
stand the beginnings in the evolution of useful structures and
habits. If some organ is to be developed to meet some new
need, it is rarely, if ever, formed from a previously undiffer-
entiated part of the organism, but is rather formed by modi-
fication of some organ already present, the change in this
organ fitting it for a different use, fitting it to meet the new
need. Similarly if a new habit needs to be acquired, it is
likely to arise as a modification of some previous habit.
The different stages in the evolution of an organ may each
be useful for a different purpose. In fact it is probable that
the organ in its several conditions will serve somewhat
different purposes. One can hardly mention an organ in
the human body, for example, which has not in this way
been changed in its function. The heart was once a simple
blood vessel, serving for the carriage of blood, not for its
propulsion ; the lungs were, in the fishes, the swim-bladder,
which became changed into an air-breathing organ as the
terrestrial habit was acquired ; the limbs in the early aquatic
vertebrates probably were used as guides and balancers in
swimming and as swimming paddles, but, later, as the terres-
trial habit was acquired, they assumed a form adapted for loco-
motion on land. Change of function and change of structure
go hand in hand, so that the different stages in the evolution
of an organ do not all serve the same purpose. Hair was
NATURAL SELECTION 39
derived from delicate cuticular sense organs. The internal
ears were probably once represented by minute bristle-like
organs in the skin, which probably were organs of touch
or for the perception of pressure. Remembering this most
important principle of change of function, we find that many
apparent difficulties in the way of understanding the origin
of structures in the body disappear.
But the chief apparent force of the objection that in their
beginnings organs could not have been of use lies in the
misconception that variation is very slight and that therefore
any organ would first appear as a very slight modification
and would progress by minute stages toward a condition in
which it could be of use. In reality variation is very con-
siderable, so that a structure at its first appearance may
be sufficiently developed to be of real importance to its pos-
sessor. What has been said of organs would apply as well
to instincts and other mental characters.
Individuals which diverge to a very considerable degree
from the species average are often called sports. De Vries
and some others are inclined to believe that most species
have arisen as sports which breed true, handing down to
their offspring their own peculiar characters. If this be true,
natural selection will still be operative to determine which of
these new species shall survive, only those persisting which
advantageously conform to the environmental conditions.
The derivation of new species from sports has been called
by De Vries, "mutation."
A fourth objection, which is related to the latter two, is
that in our study of the fossil remains of extinct animals we
sometimes find that as we pass from the older to the more
40 ORGANIC EVOLUTION
recent species there is a progressive series of modifications
of one or more organs, showing that there has been a grad-
ual, steady change in a particular direction, the several steps
in this change being very slight. In the fossil remains
which give us the history of the evolution of the horse
(Plates 46 and 47) we see the gradual loss of the outer toes,
and a corresponding increase in size of the middle toe, a
gradual increase in length of the molar teeth, and a gradually
increasing complexity of the ridges on their grinding sur-
faces. It has been claimed that the several steps in these
modifications are not of enough importance to have given
their possessors decided advantage in the struggle for exist-
ence, and that their progressive development in these par-
ticular directions must indicate an inherent tendency to
become modified in these directions. If this progressive
modification in the ancestors of the horse be due to some
inherent tendency rather than to natural selection acting
on a great number of all sorts of variations, selecting only
the useful ones, then this casts doubt on the importance of
the role of natural selection in other cases. May not much
of the evolution of which we have evidence be due to similar,
not understood, inherent tendencies? (Cf. Appendix I.)
The last and by far the most important objection, which
we will mention, to the idea of evolution by means of natural
selection is this : It is well known, of course, that, in general,
the offspring of any pair of parents tend to be somewhat
intermediate in character between the two parents.1 Now if
1 This statement needs slight modification, as will appear later when we come
to the mention of Mendel's laws in their relation to the persistence of variations
'(page 44)-
NATURAL SELECTION 41
a certain favorable variation arise in but a few individuals of
a species, it seems improbable that these divergent individ-
uals will breed with one another rather than with the much
more numerous non-divergent members of the species. If,
however, a divergent individual crosses with a non-diver-
gent individual, the useful character which has appeared in
the divergent individual will be less marked in the offspring.
In the following generations it would be still more dimin-
ished by the same process, until finally it will be entirely lost.
This swamping of variations by interbreeding has seemed to
some to make the development of new characters by natural
selection improbable.
The force of this objection is great. Doubtless many
divergent characters are swamped by their possessors inter-
breeding with those individuals of the species in which these
characters do not appear. If it were not for this fact evo-
lution might be much more rapid. Evolution is slow, and
the swamping effect of interbreeding may largely account
for the slowness of the process. But while evolution may
be retarded by intercrossing, we have no indication that it
is prevented.
Two individuals of different species ordinarily will not
breed together in a state of nature, though occasionally they
will do so ; and in those rare cases in which species do
cross, the offspring only very rarely are fertile. Nature, by
this infertility, has provided against promiscuous interbreed-
ing between species, and has thus prevented the species
already developed from being lost by confusion with one
another. Does she in some similar way prevent promis-
cuous intercrossing between the individuals of a single
species, and thus secure the perpetuation of favorable varia-
42 ORGANIC EVOLUTION
tions that may arise ? There are ways in which she might
do so. In what ways may free intercrossing between the
individuals of the same species be prevented ?
In the first place, self-fertilization is a most effective bar
to promiscuous intercrossing and must serve to perpetuate
many variations that otherwise might be swamped. This
would be more common among plants than among the
higher animals, but it could occur among the lower animals,
many of which are bisexual. As a rule, however, at least
occasional cross-fertilization is advantageous and is often
secured either by a reluctance on the part of the sperm to
fertilize the ova of the same individual, as is the case, for
example, in most flowering plants, or by the sperm ripening
either before or after the eggs of the same individual, so that
self-fertilization cannot occur. Yet self-fertilization does fre-
quently occur among both animals and plants, and when it
does occur it may allow certain variants to persist which
would be likely to be swamped by cross-fertilization.
Interbreeding between near relatives is another thing that
serves to perpetuate and intensify new characters which may
appear. This is the same thing which among domestic ani-
mals and plants is called " breeding in and in " and is a most
effective method in artificial selection. Similar interbreeding
between near relatives among undomesticated forms will often
be helped by the fact that the individuals of any species in a
limited locality are likely to be closely related. An insect,
for example, lays its eggs on a certain food plant. When
these hatch it is very probable that the males and females in
the brood will mate together and so hand down unimpaired
to the offspring of the second generation the characteristics
they received from their parents. Among sedentary ani-
NATURAL SELECTION 43
mals and plants, and among those that are restricted to a
limited locality, breeding in and in, or breeding between near
relatives, must be frequent or even usual. An occasional
cross with some individual less closely related will be suffi-
cient to avoid deleterious effects from the close inbreeding.
The influence of locality will sometimes serve to hinder
swamping of variations by free intercrossing. The environ-
mental conditions are frequently not uniform throughout the
whole range of a species. Take as an example a species of
plant which spreads over a wide area, part of which is moist
bottom-land, and part drier upland. If the individuals of the
species vary in their adaptability to conditions of moisture
and drouth, as they almost surely would do, some being
better fitted for life where moisture is abundant, others for
life in drier soil, then natural selection would, in each gener-
ation, eliminate from the bottom-lands a large proportion of
the plants best fitted for dry soil, and, conversely, would
destroy on the dry hills a large proportion of the individuals
adapted to wet soil. Thus in each locality, in each genera-
tion, the chances would be greater of like breeding with like
than with unlike. Natural selection, acting on each genera-
tion separately, would in this way raise a bar to free inter-
crossing of all variants in the species and would create a
probability of like breeding with like that would materially
increase the cumulative effect of natural selection from gen-
eration to generation.
Variations in the time of breeding act as a direct bar
to free intercrossing between the members of a species,
those which mature their reproductive products at differ-
ent times being, of course, by this fact, prevented from
interbreeding. In this way differences in breeding season
44 ORGANIC EVOLUTION
might soon become definitely established in two groups of
the species, making a constant distinction which might
become a specific character. Now, as no part of an organ-
ism varies independently of the rest, there would doubt-
less, in establishing the two groups which differ in time
of breeding, also be established as constant certain other
characters associated with the difference in breeding time.
Among some of the higher animals sexual selection,
that is, the exercise of choice in mating, prevents promis-
cuous intercrossing and so may serve to preserve from
swamping certain divergent characters which may be asso-
ciated with such choice. To this point we will refer again.
Anything which divides a species into groups will be
likely to prevent free intercrossing, and so tend to pre-
serve characters associated with the different groups. We
will come back to this point soon.
The recently rediscovered work of Mendel has a bear-
ing upon the question of the persistence of variations.
Mendel showed a half-century ago, and recent workers
have more fully established, certain facts of heredity in
the case of hybrids between distinct species, and crosses
between widely divergent varieties of the same species.
Castle's work in breeding mice, which closely agrees with
Mendel's observations, shows the point clearly. Castle
bred white mice and common gray mice together and got
the following results. The offspring developed from the
first cross were all apparently normal gray mice. When,
however, a male and female from this first lot of young
were bred together very interesting results were obtained.
Three-fourths of the young of this second lot appeared to
be normal gray mice, but one-fourth were found to be
NATURAL SELECTION 45
pure white mice. If two of these white mice were bred
together they had white offspring, and the same was true
in breeding again from their young, generation after gen-
eration, showing that they were of pure strain without
admixture from the gray variety, though the original
parents in the first cross were one gray and one white.
It is of great interest to note that, in spite of the cross-
ing of the two varieties, there appeared in the later gen-
erations certain individuals which were of pure blood,
showing no trace of the admixture which we would expect
to find resulting from the cross. Extensive experiments
in breeding showed that the results were to be interpreted
as follows : a gray mouse, G, bred with a white mouse,
W, gave offspring which seemed to be all gray, but were
really a mixture of gray and white, the gray character
being dominant and the white character obscured, or
" recessive," as Mendel called it. That is G x W gave
G ( W\ G ( W\ G ( W}, etc., the parenthesis indicating that
the white character was recessive. This hidden complex
nature of the second generation (the young from the first
cross) was clearly indicated when they were bred together.
It was found that their offspring were of three sorts, and
that these three kinds were in definite and constant
numerical proportions. G ( W} x G (W} gave offspring
i G + 2 G ( W) + i W, one-fourth being pure gray, one-
fourth pure white, and one-half apparently gray, but really,
as further breeding showed, gray and white, the white
character being recessive and obscured. These numerical
proportions held true for an extensive series of experiments
in the case of white mice, as they had done in the experi-
ments of Mendel upon certain plants.
46 ORGANIC EVOLUTION
We do not care here to discuss in detail the Mendelian
laws, their cytological explanation, and the exceptions to
them, though these subjects are most interesting and im-
portant. We are chiefly interested, in the present connec-
tion, in the fact that if the individuals crossed be sufficiently
divergent the result is not a mere admixture of the qualities
of the two parents in the young, but that individuals of
pure strain, showing no admixture, appear in the third
generation and in succeeding generations. Very divergent
individuals which arise by variation are commonly called
" sports." It is easy to see that if a single brood of sports
arose which were especially well adapted to their environ-
ment, although they might breed with non-divergent indi-
viduals of the species, yet among the offspring of the third
generation there would be individuals like the original
sports. It might, therefore, be possible for natural selection
to change the character of the species from the old type
to that of the sport, by preserving the sports and allowing
them by competition to destroy the individuals of the old
type. Should the sports prove to be more fertile when
crossed with one another than when crossed with individ-
uals of the old type this would increase the probability of
the new type becoming predominant.
It may be that less divergent characters also may be
preserved from immediate swamping by intercrossing, but
it is too early in our study of the Mendelian phenomena
for us to be able to say. We do not know whether the
Mendelian laws apply at all to ordinary varieties or only to
sports. If they apply to ordinary varieties of course the
possible effect upon evolution would be greater.
We should also note that in the experiments of Mendel,
SEXUAL SELECTION 47
and of others who have followed him, the results stated above
were not without exception. For example, Castle found that
a certain proportion of the mice resulting from the first cross
of a gray with a white mouse were not gray, as we would
have expected according to Mendel's laws, nor yet white, but
were a dappled gray and white. In such a case there was a
true mingling of the characters of both parents in the young,
neither set of characters predominating.
Enough has been said to show that interbreeding be-
tween the different individuals of a species is not promis-
cuous and wholly indeterminate, and therefore the favorable
varieties preserved by natural selection from among the indi-
viduals of any generation will not necessarily be swamped
when these divergent forms come to breed. We will return
to this subject again. The phenomena of organic nature
seem to indicate very clearly that evolution has taken place,
and the evidence points strongly to natural selection as a
real factor and apparently the chief factor in this evolution.
But natural selection is not the only factor in evolution.
Reference has already been made to sexual selection and
segregation, and besides these there is still another important
factor, the inheritance of parental modifications. Let us
consider these.
SEXUAL SELECTION
By sexual selection, as we will use the term, is meant the
exercise of choice in mating.1 Among plants and lower
animals, if cross-fertilization occur at all, propinquity at the
1 Those familiar with Darwin's writings will recognize that I use the phrase
sexual selection in a more limited sense than does Darwin, following rather the
usage of Wallace, Lloyd Morgan, and others. For example, Darwin includes under
48 ORGANIC EVOLUTION
time of reproduction is usually the thing that determines
which individuals shall mate with one another. Of prefer-
ence or choice, of course, there is nothing. But among some
of the higher animals there is evidence that individual choice
is exercised in the selection of mates. Breeders of domestic
animals find that the females sometimes prefer certain mates
rather than others. To quote Lloyd Morgan : " Professor
Low, one of the greatest authorities on our domestic animals,
says, ' The female of the dog, when not under restraint, makes
selection of her mate,' and again, ' The merino sheep and the
heath sheep of Scotland, if two flocks are mixed together,
each will breed with its own variety.' Mr. Darwin has
collected many facts illustrating this point. One of the chief
pigeon fanciers in England informed him that, if free to
choose, each breed would prefer mating with its own kind.
Darwin was informed by the Rev. W. D. Fox that his
flocks of white and Chinese geese kept distinct." Many
other instances of preferential mating could be mentioned
among domestic animals. To some further illustrations we
will refer in connection with the description of segregation.
Among wild animals, also, choice of mates can be observed.
Phenomena which are often explained by sexual selection
are found in some kinds of insects, among spiders, and
among fishes, Amphibia, reptiles, birds, and mammals.
Among humankind sexual selection is, of course, an impor-
tant factor in evolution.
The birds give us some of the best examples of sexual
sexual selection the fighting between the males for the possession of the female,
though this may have no connection with any exercise of choice on the part of the
female. I would include this rather under natural selection, restricting the term
sexual selection to the voluntary choice of mates by either the female or the male.
PLATE 22. — Male and female bobolink (Dolichonyx oryzivorus). — From a photograph provided
by the American Museum of Natural History.
I
K
^B*'
.
• JB
F-a
I*
Bl
HL.X,
•<-
£ o
PLATE 25. — A. Male find female Nesocentor milo. B, Male and female pigeon (Phlogcenas
jobiensts). — From Gould's Birds of New Guinea.
PLATE 27. — Turkey cock " strutting." — From a mounted specimen in the United States National Museum.
SEXUAL SELECTION
49
selection. The males are usually more brilliant in plumage
and have more highly developed voices than the females
(Plates 22-27). At the mating season they parade their
fine plumage before the females and use all their charms
of voice to render themselves attractive to their desired
mates. They often go through the most remarkable court-
ing antics, and there seems to be sufficient evidence from
observation that these antics and the brilliant voice and fine
plumage influence the female in her choice, that they act
as a sexual excitant. The strutting of the rooster or the
turkey cock (Plate 27) is a good example of courting habits
among birds that is familiar to all (cf. also Plates 23 and 24).
Under the influence of the courting instinct the behavior of
many of our birds changes its whole character. The Ameri-
can woodcock is one of the most retiring birds we have.
Few but sportsmen have ever seen it in its native woods.
(See Plate 50.) By day it stays close in the thickets, feeding.
It rarely flies except at night. It has no calls or song. But
at the beginning of the breeding season even this shy bird
loses his sedate character and lightly turns his fancy to
thoughts of love. During the morning and evening twilight
a male and female may come day after day to the same spot
at the edge of the woods, where the male will go through a
series of performances wholly foreign to his usual quiet habit.
Chapman, in his Handbook of Birds of Eastern North
America, thus describes the courting of the woodcock:
" How many evenings have I tempted the malaria germs of
the New Jersey lowlands to watch the woodcock perform his
strange sky dance ! He begins on the ground, with a formal,
periodic peent, peent, an incongruous preparation for the wild
rush that follows. It is repeated several times before he
50 ORGANIC EVOLUTION
springs from the ground and on whistling wings sweeps out
on the first loop of a spiral which may take him three hun-
dred feet away from the ground. Faster and faster he goes,
louder and shriller sounds his wing song; then, after a
moment's pause, with darting, headlong flight he pitches in
zigzags to the earth, uttering as he falls a clear, twittering
whistle. He generally returns to near the place from which
he arose, and the peent is at once resumed as a preparation
to another round in the sky."
In most birds the males are colored more conspicuously
than the females, and in many species the males show
special development of certain feathers, or of spurs, or comb
and wattles, which are less marked or wholly absent in the
females.
Wallace has called attention to the fact that natural
selection could hardly allow the females of the birds, which
are chiefly occupied in brooding the eggs and caring for
the young, to be conspicuously colored because of the dan-
ger to the nest and young that would thus result. It has
also been suggested that brilliant coloration in the male
may aid him to serve as a decoy to distract attention from
the female and the nest. Unfortunately for both of these
suggestions, some brilliantly colored males help the female
in brooding the eggs and caring for the young.
Among the spiders also are seen good examples of
certain courting colors and habits (Plate 28).1 Here the
males of many species have brilliantly colored legs or have
other portions of the body brightly colored. The eyes also
are like splendid little jewels of different shades of red
and green and blue. As the diminutive male approaches
the often much larger female, he advances with a swaying,
1 Compare also Plate 85.
H I
PLATE 28. — Courting attitudes in hunting spiders. [After G. W. and E. G. PECKHAM.]
A. Marptusa familiaris. Left-hand figure, female ; right-hand figure, male. B. Ic ius mit> atus,
male dancing before female. C, D. Habrocestum howardii, front view and side view of male in
courting attitude. The first legs in the male are " a delicate, light-green color, with a fringe of
white hairs along the outer side." The patella (second joint) of the third leg is enlarged, and its
anterior face is white, with a black spot. The eyes are brilliant. Observe that the male assumes
a position which shows all of these features to best advantage. E. Salt is pule x, male in his court-
ing dance. He bends the legs, first of one side, then of the other, scurrying back and forth before
the female, moving always toward the side on which the legs are bent. F. Astia vitiata, variety
nigra, position of male approaching female. G, H, /. Synageles picata, male dancing before
the female. His first pair of legs are " of a brilliantly iridescent steel-blue color."
PLATE 2g. — A. Male (upper figure) and female (lower figure) of seventeen-year cicada (Cicada
septende dm), often inaccurately called "seventeen-year locust." x. Stridulating organ of the male.
It is absent in the female. B. Males and female (middle figure above) of staghorn beetle (Lucanus
damd). These figures illustrate not only the difference between the sexes, but also the variation in
size among the males.
PLATE 30. — Male (upper figure) and female of the "Hercules beetle" (Dynastes hercules).
— Fro m Br eh m ' s Th ierleben .
SEXUAL SELECTION 51
teetering gait, the bright-colored portions of the body being
displayed to the most advantage. It behooves him to be
discreet in his courtship, for, if he fails to charm the
female, he is likely to be seized and devoured by her. Dr.
and Mrs. Peckham, of Milwaukee, who have been the most
careful observers of the hunting spiders, the group of
spiders in which courting colors and courting habits are
perhaps most developed, are fully convinced that the
female is influenced by the display of his charms made
by the male, and that his success is often determined by
this stimulus.
Among insects are found many instances of structures
present in the males and wanting in the females of the
same species. Stridulating organs for the production of
sounds are common among the grasshoppers, crickets, and
cicadas (Plate 29, A). The males of many beetles have
enlarged jaws of a form not useful for fighting (Plate 29, B),
or hornlike appendages on the head or thorax, which are
not seen in the females (Plate 30; Fig. 7). In many species
of butterflies the males are decidedly more brilliant than
the females (Plate 84). Bates, speaking of the butterflies
on the upper Amazon, says: "They were of almost all
colors, sizes, and shapes. I noticed here altogether eighty
species, belonging to twenty-two different genera. It is a
singular fact that, with a few exceptions, all the individuals
of the various species thus sporting in sunny places were
of the male sex; their partners, which are much more
soberly dressed and immensely less numerous than the
males, being confined to the shades of the woods."1 (Italics
mine.) Again, speaking of the butterflies of the whole
1 The Naturalist on the River Amazon.
ORGANIC EVOLUTION
Amazon region, Bates says : " It is almost always the males
only which are beautiful in colors." (See also Plates 31
and 33, A.)
The males of many kinds of fishes are more brilliantly
colored than the females, and in some species the males
have ornamental ap-
pendages which are not
found, or are less de-
veloped, in the females
(Plate 32). Apparently
these characters are to
be referred to sexual
selection, for the colors
are generally more
brilliant at the breeding
season, and the behavior
of the male in the pres-
ence of the female is
such as to show off to
the best advantage the
brightly colored parts
of his body, or the orha-
D mental appendages.
In some of the Am-
phibia the males are
more conspicuous than
the females during the breeding season. Darwin says, in
his Descent of Man: "With our common newts (Triton
punctatus and cristatus) a deep, much indented crest is
developed along the back and tail of the male during the
breeding season, which disappears during the winter (Plate
FIG. 7. — Heads of male and female beetles. The
left-hand figures show the males. [After DARWIN.]
A. Copris isidis. B. Phanasus faunus.
cus cantori, D. Onthophagus rangifer.
C. Dipeli-
PLATE 31. — Male, female, and larva of Chatdiodes cormitus, a relative of the dragon-flies.
The upper figure shows the male.
PLATE 32. — A Callionymus fyra, male and female. [After DARWIN.] The upper figure
shows the male. The lower figure is more reduced than the upper. B. Xiphophorus hellerii,
male and female. [After DARWIN.] The upper figure is the male.
PLATE 33. — A. Male (a) and female (b) dragon-fly (Calopteryx maculata). The wings of the
male are a rich lustrous green, almost black. The wings of the female are very pale green, almost
colorless. The male is much more conspicuous. B. Triton cristatus, male, female, and larva. The
upper figure is the male. — From Brehm's Thierleben.
A
B
C
PLATE 34. — Males and females of various species of lizards. [After DARWIN.]
A. Sitana minor, male. B. Ceratophora stoddartii, male and female. C. Chameleo bifurcus, male and
female. D. Chameleo owenli, male and female.
SEXUAL SELECTION 53
33, B\ Mr. St. George Mivart informs me that it is not
furnished with muscles, and therefore cannot be used for
locomotion. As during the season of courtship it becomes
edged with bright colors, there can hardly be a doubt that
it is a masculine ornament. In many species the body
presents strongly contrasted, though lurid tints, and these
become more vivid during the breeding season. The male,
for instance of our common little newt (Triton punctatus),
is * brownish gray above, passing into yellow beneath, which
in the spring becomes a rich bright orange, marked every-
where with round dark spots.' The edge of the crest is
then tipped with bright red or violet. The female is usually
of a yellowish brown color with scattered brown dots, and
the lower surface is often quite plain."
The males of some kinds of lizards have certain por-
tions of the body, especially about the head and neck,
brightly colored, and sometimes there are in these regions
brilliantly iridescent folds of skin which may be distended
and in this way made more showy (Plate 34). It is possible
that these are used in attracting the female.
The mane of the lion, the antlers of the male deer, the
proud carriage of the male in many species of mammals,
may be instances of structures and habits used in courtship
and developed, in part, through sexual selection, though the
former two may be due partly to natural selection also,
since they are of use in fighting, the lion's mane as a pro-
tection, the deer's antlers as weapons.
Referring once more to the birds, observe how the use
of these special characters and habits in the male is indi-
cated by the following facts (I quote from Romanes):
" (a) Male secondary sexual characters of an embellishing
54 ORGANIC EVOLUTION
kind are, as a rule, developed only at maturity, and most
frequently during only a part of the year, which is invariably
the breeding season ; (b) they are always more or less seri-
ously affected by emasculation ; (c) they are always, and only,
displayed in perfection during the act of courtship ; (d) then,
however, they are displayed with the most elaborate pains ;
yet always, and only, before the females ; (e) they appear, at
all events in many cases, to. have the effect of charming the
females into " accepting the male. These statements are
perhaps a little too emphatic, yet they indicate clearly the
reasons for believing in sexual selection. Remembering
the facts of individual preference in choice of mates ob-
served among domestic animals by their breeders, the real-
ity of sexual selection seems well established.
Groos 1 has pointed out that the coyness of the females, in
those groups of animals in which sexual selection occurs,
may be developed through natural selection. He says : " As
the sexual impulse must have tremendous power, it is for
the interest of the preservation of the species that its dis-
charge should be rendered difficult. This result is partly
accomplished in the animal world by the necessity for great
and often long-continued excitement as a prelude to the
act of pairing. This thought at once throws light on the
peculiar hereditary arts of courtship, especially on the indul-
gence in flying, dancing, or singing by a whole flock at
once. But the hindrance to the sexual function that is
most efficacious, though hitherto unappreciated, is the
instinctive coyness of the female. This it is that necessi-
tates all the arts of courtship, and the probability is that
seldom or never does the female exert any choice. She is
1 The Play of Animals, Preface.
SEXUAL SELECTION
55
not an awarder of a prize, but rather a hunted creature. So,
just as the beast of prey has special instincts for finding his
prey, the ardent male must have special instincts for subdu-
ing feminine reluctance, and just as in the beast of prey the
instinct of ravenous pursuit is refined into the various arts
of the chase, so, from such crude efforts at wooing, that
courtship has finally developed in which sexual passion is
psychologically sublimated into love." Groos is very likely
correct in his belief that the importance of the act of pair-
ing has led, through natural selection, to the development
of coyness in the female, in order that more ardor may be
necessitated in the male and the act of pairing effectually
performed. This belief, however, does not diminish at all
the reasons for recognizing that the females do exercise
choice. This choice is probably not so much a conscious
choice between rival males as a choice between accepting
a certain mate and refusing to pair at all with him. But,
under this conception, it will be those males which most
successfully stimulate the sexual instincts of the females
which will secure mates. It has been observed by Dr. and
Mrs. Peckham that often a male hunting spider may fail
to win the female. In observing the courtship of butterflies
I have found the male unsuccessful after more than an hour
of pursuit, until finally he has abandoned his quest. There
seems no doubt that the females of many groups of animals
do exercise choice, accepting or rejecting certain mates.
Now observe what is the effect of sexual selection on
evolution. Natural selection secures the preservation of
characters which help their possessors to survive in the
struggle for existence.1 Sexual selection, on the other hand,
1 This statement is not quite accurate, as we will see later (page 82), but it will
serve for the present use.
56 ORGANIC EVOLUTION
secures the perpetuation of those characters in the male which
make him attractive to the female, irrespective of any advan-
tage or disadvantage in the struggle for existence. Those
males which are attractive will, because of their attractiveness,
get mates and have offspring, while many of the less attrac-
tive males will fail to find mates. In time, then, through the
action of this preference on the part of the females, there
will be developed a race whose males show the characters
which are attractive to the females. The results of sexual
selection are different from those produced by natural selec-
tion, and may often be opposed to the latter. For example,
it is of advantage to most birds to be inconspicuously col-
ored, so that they may more readily escape their enemies.
Natural selection, therefore, will tend to produce protec-
tively colored forms. Sexual selection, on the other hand,
in the case of many species, tends to produce brilliantly
colored males. The two tendencies are thus often opposed
to one another, sometimes one, sometimes the other, pre-
dominating.
Important objections have been urged against the theory
of sexual selection. Many species of animals which show
bright colors or ornaments in the male that are not found in
the female are forms in which we have observed no court-
ing habits by which these adornments are displayed before
the female ; and many of these are forms in which we would
not expect to find the females exercising choice on the basis
of the ornamentation of the male. Note, for example, the
beetles (Plates 29 and 30, and Fig. 7) and certain lowly Crus-
tacea (Fig. 8, A). If the peculiar adornment of the males
in these species is due to something other than sexual selec-
SEXUAL SELECTION
57
tion, it is distinctly possible that sexual selection may not be
the cause, or at least the sole cause, of the adornment of the
males among butterflies, spiders, fishes, Amphibia, lizards,
and birds, in all of which courting has been observed.
FIG. 8. — Secondary sexual characters in copepods.
A. Male of Calocalanus plumulosus. B. Female of Calocalanus pavo. C. Male of the same
species. [From MORGAN, after GIESBRECHT.]
Wallace believes that the greater brilliancy of the male
or his possession of finer voice or special ornamental ap-
pendages is due to his greater vigor and vitality, which is
associated with his greater ardor.
Groos has suggested that the coyness of the female
necessitates greater ardor in the male and that this secures
58 ORGANIC EVOLUTION
greater effectiveness in the act of pairing, and that this
difference in mental character in the two sexes has been
brought about by natural selection because of its usefulness,
and has not been developed through the females choosing
the more ardent males. (Compare page 54.)
Sometimes it is the female and not the male which
shows the greater development of secondary sexual charac-
ters (Fig. 8, B and C). In these forms we have no evidence
of the exercise of choice by the male or of ardent courtship
by the female. These cases, however, are rare, and we do
not know what may be the use of the special appendages.
Wallace urges that for sexual selection to produce the
results claimed the less ornamented males must fail to find
mates, and, he says, we have no evidence that the less
adorned males do fail to obtain mates, but that, on the con-
trary, the less adorned as well as the highly ornamented
have offspring.
This statement of Wallace's is not surely true. If there
is a correlation between vigor and high development of the
ornamental sexual characters, as there is between vigor and
high development of other structures, then, though the less
ornamented males may obtain mates, they are less vigorous
and will have less vigorous offspring. If it be also true that
the more vigorous females are more sought after by the
males than are their less vigorous sisters, then they will have
first choice of the males, choosing the most highly orna-
mented, which are at the same time the more vigorous.
Thus the vigorous, highly ornamented males will mate with
the vigorous females, having vigorous offspring, while the
less ornamented and less vigorous males will mate with the
less vigorous females and have less vigorous offspring. Nat-
SEXUAL SELECTION 59
ural selection will then preserve the vigorous offspring of
the vigorous parents, and the males among these will be
highly ornamented like their fathers. This is but conjec-
ture. The relations suggested have not been established by
observation. It is clear, however, that Wallace's statement
is not self-evident.
Morgan keenly suggests an interesting objection. He
says, " If in order to bring about, or even maintain, the
results of sexual selection, such a tremendous elimination1
of individuals must take place, it is surprising that natural
selection would not counteract this by destroying those
species in which a process, so useless for the welfare of the
species, is going on." ... " If, in nature, competition be-
tween species takes place on the scale that the Darwinian
theory of natural selection postulates, such forms, if they are
much exposed, would be needlessly reduced in numbers in
the process of acquiring these [ornamental] structures " in
the male. This objection of Morgan's is based upon the
same assumption as that of Wallace which is criticised in
the preceding paragraph.
Prolonged and careful observation, on a large scale, of
the courting and mating of animals is needed to give us a
sound basis for judging of the reality and degree of impor-
tance of sexual selection. We do not even know from obser-
vation whether the highly ornamented males are more suc-
cessful in finding mates than are their less adorned fellows.
Such observation is very difficult, for it involves keeping
large numbers of individuals under as nearly natural condi-
tions as possible, and observing them continuously, as well
as keeping complete records of the mating and offspring.
1 Elimination from the breeding process.
60 ORGANIC EVOLUTION
It is not surprising, in view of these difficulties, that the
statistical records are very scant.
There is no doubt of the reality and great importance of
sexual selection among mankind, and to the author its opera-
tion seems probable at least among birds, fishes, and spiders.
SEGREGATION
Natural selection and sexual selection, and also the
inheritance of parental modifications which we will discuss
later, are primary factors in evolution. Segregation, to
which we have already made some reference, is not a pri-
mary factor in the development of species, but, acting in
connection with the primary factors, it greatly modifies the
results produced by these. Anything which divides a
species into groups which do not freely interbreed is said to
segregate the members of the species into these subdivisions.
In connection with one of the objections urged against the
effectiveness of natural selection we spoke of some of the
things that may cause segregation within a species. It is
well to treat the subject a little more fully.
Segregation may be due to any of a number of causes.
If only anything operates to prevent free interbreeding
between any of the individuals of a species, it is a true
cause of segregation.
The cause of segregation may be geographical. A
species of wide distribution is likely to be divided into
groups, which do not habitually interbreed, by the inter-
vention of rivers, or mountain ranges, or deserts, or oceans
between the different groups. The foxes of Europe differ
from those of America, and probably this divergence from
SEGREGATION 6 1
their common ancestral condition was somewhat influenced
by the fact that the foxes east of the Atlantic Ocean were
unable to breed with their relatives on this continent.
The Rocky Mountains have been a most effective cause
of segregation in this country, and to their presence is
due probably a considerable part of the difference between
eastern and western forms with common ancestry. The
fauna and flora of some of the islands off the west coast
of South America give us fine examples of the effects of
isolation. We find the species distinct from those on the
continent, but closely related to the latter. It is hardly
possible that the island forms are not different from what
they would have been if they had not been so separated
from the continental members of the species that inter-
breeding with the latter was impossible. Even the species
of the several islands within the Galapagos group are
different, as is well illustrated by the locusts (Fig. 9). The
divergence of these allied species has not been due to
segregation alone. The environmental conditions in the
different areas being different, natural selection must have
been constantly at work to produce differences between
the individuals residing in the two regions. But, though
natural selection may have been the cause of divergence,
we can readily see that its results must have been mate-
rially affected by segregation. Segregation operates in
conjunction with the other factors of evolution.
Another cause of segregation is climate. Conditions of
drouth or of excessive humidity, of heat or cold, often
raise effective barriers to the migrations of both animals
and plants, and so segregate widely distributed species into
groups which are separate from one another so far as
62
ORGANIC EVOLUTION
reproduction is concerned. The faunas and floras of east-
ern Asia and of our west coast give us possibly the best
example of the segregating effect of climate. At one time
the climate of Siberia and that of Alaska was semi-tropical,
being considerably more mild than the present climate of
Baltimore. Of this we have abundant evidence in the
fossil remains of semi-tropical plants and animals over
FlG. 9. — Locusts taken on the Galapagos Islands, Pacific Ocean. All descended from a
common ancestor, but now scattered over the various islands and differing in size and markings.
a. Schistocerca melanora (Charles Island), b. S. intermedia borealis (Abingdon and Bindloe
Islands), c. S. intermedia (Duncan Island), d. S. literosa (Chatham Island). e. S. melanora
lineata (Albemarle Island), f. S. melanora immaculata (Indefatigable Island.) The species
intermedia is probably a hybrid between the other two species. — From Jordan and Kellogg's
Animal Life, by the courtesy of the authors and of D. Appleton & Co.
this whole area. During the continuance of the warm
climate many species crossed from Asia to America and
vice versa across Behring straits. As the cold increased,
culminating in the extreme cold of the glacial period, there
was formed a most effective barrier to further migration
from one continent to the other, resulting in the complete
segregation into two groups of each species which had
representatives in both regions. We now find, as we
SEGREGATION 6^
\j
would expect from these conditions, that the Siberian
and western American faunas and floras, while having
many forms which are closely similar because of common
descent, are still distinct, having very few species in com-
mon. (Of common genera, of course, there are many.)
Natural selection, aided by segregation, has had time to
produce great changes.
Diversity in soil conditions produces segregation among
plants, and local differences in food conditions thus aris-
ing must cause segregation among animals, different
groups of a single species being found in the separate
localities where the suitable conditions of soil or food
exist.
One of the finest examples of extreme segregation within
a limited area is furnished by the land shells of Oahu, one of
the Hawaiian Islands. Along the northeastern shore of the
island is a high mountain range whose sides have by erosion
been cut into deep valleys (Fig. 10) with high and steep
ridges between. The soil in the lower ground of each valley
is rich and bears a profusion of tropical trees, shrubs, ferns,
and other plants. The tops of the main ridge, however, and
also the tops of the lateral ridges, are barren, being denuded
of their soil by the heavy rains. Several genera of land
snails, which feed upon the foliage of the trees, shrubs, and
herbs, are very abundant along this whole series of valleys,
and it is interesting to observe that each of the several
species (or varieties ?) of snails is confined to a single valley
or to two or three adjacent valleys. Their proper food and
the necessary shade being absent on the tops of the ridges,
the snails do not cross from one valley to the next. Such
spreading as has occurred has probably been due to the
64 ORGANIC EVOLUTION
snails being transported by birds or by some different means
other than their own powers of locomotion. Gulick, who
first called attention to the importance of segregation as a
FiG. 10. — Map of Oahu, one of the Hawaiian Islands.
factor in evolution, was led to his conclusions by his study
of the remarkably restricted range of each of the many
species of land snails in these Oahu mountain gorges.
The difficulties in the way of migration over great dis-
tances must tend toward segregation among both plants and
animals. The individuals at the extremes of the area occu-
pied by any species cannot intercross directly unless the area
be very limited in extent or their powers of migration very
considerable. And even the birds, whose powers of migra-
SE GRE GA TION 6 5
tion are so well known, usually breed year after year in the
same localities, the same individuals returning each spring to
the same spot and often occupying the same nest that was
left the year before. Of course, as those individuals of the
species which occupy the intermediate area will breed freely
with those nearer the two extremes, the segregation of the
latter is but partial, yet it must be sufficient to affect
evolution.
Natural selection, sexual selection, and segregation all
mutually interact, as we can readily see. Sexual selection,
the exercise of choice in mating, causes reproductive segrega-
tion, and this, in turn, may affect natural selection. Let me
again quote Lloyd Morgan : " Among the wild horses in Para-
guay those of the same colour and size associate together;
while in Circassia there are three races of horses which have
received special names, and which, when living a free life,
almost always refuse to mingle and cross, and will even
attack one another. In one of the Faroe Islands, not more
than half a mile in diameter, the half-wild native black sheep
do not readily mix with imported white sheep. In the Forest
of Dean and in the New Forest the dark and pale-coloured
herds of fallow deer have never been known to mingle;
and even the curious ancon sheep, of quite modern origin,
have been observed to keep together, separating themselves
from the rest of the flock when put into enclosures with other
sheep. . . . This preference of animals for their like, even
in the case of slightly different varieties of the same species,
is evidently a fact of great importance in considering the
origin of species by natural selection, since it shows us that,
so soon as a slight differentiation of form or colour has been
effected, isolation will at once arise by the selective action
66 ORGANIC EVOLUTION
of the animals themselves." This is a good statement of the
case except that Lloyd Morgan should have said isolation
may at once arise, not " will " at once arise.
Romanes 1 has called attention to a factor in segrega-
tion which has as yet been insufficiently studied, but which
may prove of the greatest importance. He has called it
physiological selection. It has been observed that certain
individual animals of the same species, when crossed with
each other, are infertile, whereas either one, if crossed with
a different mate, might have been normally fertile. There
exists some insufficiently understood bar to fertility between
those two individuals. This is a restraint upon the perfect
freedom of intercrossing, a sort of negative segregation, and
must have a real effect on evolution. It seems quite pos-
sible that further observation and experiment may show
this factor in segregation to be more common and impor-
tant than, in our present ignorance of the actual facts, we
can assert. The reproductive function is very delicate and
liable to disturbance from apparently slight causes. Many
wild animals, however well kept, are barren in captivity or
are less fertile than when unrestrained. Transportation to
a strange locality sometimes interferes with reproduction.
Again, there are some observations which suggest that
variation in structure in any of the different organs of the
body may be correlated with such disturbance of the repro-
ductive functions as to decrease the fertility of crosses be-
tween the individuals which diverge from the species type
and those which do not so diverge. This point, however,
needs much more study before we can determine the im-
portance of its influence in producing physiological segre-
1 Darwin and After Darwin, Volume III, "Isolation and Physiological Selection."
INHERITANCE OF PARENTAL MODIFICATIONS 67
gation. If it be true that closely related individuals, when
bred together, are more fertile than are distant relatives,
as seems under some circumstances to be true, this fact
also is of great importance. The whole subject of physio-
logical selection needs much more study. It is surely of
some importance as a cause of segregation ; it may be of
great importance.
Segregation might cause the perpetuation of divergent
characters, though these were of no use and so not subject
to the preserving action of natural selection. This, how-
ever, would not produce adaptation to the environment,
which is the striking character of animals and plants.
Segregation, therefore, unaided by natural selection, cannot
have been an important factor in that evolution of animals
and plants which we find has taken place, bringing them
into harmony with their environment. Segregation becomes
important when it acts in connection with the other factors
of evolution, natural selection, sexual selection, and, possibly
also, among lowly forms, in connection with the inheritance
of parental modifications.
THE INHERITANCE OF PARENTAL MODIFICATIONS
One more factor in evolution needs careful discussion,
namely, the inheritance of parental modifications, that
which Weismann has called the " inheritance of acquired
characters." For this factor the largest claims are made
by some biologists. It probably exerts a powerful influence
on the evolution of some of the lower forms of plants and
animals. Its influence upon higher forms is much more
doubtful.
68 ORGANIC EVOLUTION
It is well known that both animals and plants change
constantly during their whole lives as a result of the effects
on them of the environment, and through the reaction upon
themselves of their own activity. Use strengthens a muscle
and disuse allows it to waste away. Some claim that, as
a matter of course, any such effect produced in one indi-
vidual will be handed down to his descendants, and that
here we have a most potent cause of evolution in the trans-
mission to the offspring of the modifications produced in
the parent. Favorable or poor conditions of nutrition pro-
duce great effects on individual plants and animals ; so also
do climatic conditions. Are these effects upon the indi-
viduals of one generation transmitted to their offspring of
the next generation ? If so, this inheritance of parental
modifications must have the greatest influence upon evolu-
tion. The matter needs careful scrutiny.
Among the lower forms of living things, the unicellular
forms, many of which are so lowly that we cannot determine
whether they be animals or plants, among these lower forms
the inheritance of parental modifications is undoubtedly a
fact, as a single illustration will suffice to show. An Amoeba
is a lowly animal of microscopic size, consisting of a bit of
protoplasm with a single nucleus. It has no highly differ-
entiated organs, but the whole body takes part in the per-
formance of each function. When this animal reproduces, it
merely divides into two (or more) little Amoeba, each of
which eats and grows again to the characteristic adult size,
when the process of division is repeated. The offspring are
merely parts of the original parent, and of course show in
themselves the features of organization characteristic of this
parent. We can readily see that modifications of the parent
INHERITANCE OF PARENTAL MODIFICATIONS 69
may affect the offspring, which are but detached portions
of the parent. Even the effects of injuries to the parent
may be inherited by the offspring. Parental modifications
among the unicellular animals and plants must, then, often
be inherited. This is probably an important factor in their
evolution, perhaps as important as any other, though this
is doubtful, natural selection seeming even here to be the
chief factor. Among these lowly forms the whole body
shares in the process of reproduction. There are no special
parts of the body set aside for this function, while the rest of
the body functions as bone and muscle and gland and nerve.
The whole body divides, leaving no residue, so that any
modification in the parent may pass directly to the offspring.
But how is it with more highly organized animals in
which the body is differentiated into different portions, each
with its special function, — bone, muscle, nerve, digestive
organs, renal organs, and a great number more of special
organs and tissues ? In these higher animals, and in the
higher plants as well, the function of reproduction is not per-
formed by the body as a whole, but is given over to special
groups of cells, the germ cells, constituting the ovaries and
testes. It is these cells, and these only, which under ordinary
conditions give rise to new individuals. Under such circum-
stances the problem of the inheritance of parental modi-
fications is not so simple. How can the enlargement of a
muscle, due to exercise, so affect the germ cells, which lie
perhaps at a distance from the muscle in question, as to
cause the new individual, which shall arise from these germ
cells, to have the corresponding muscle in its body enlarged ?
The question, we see, is not a simple one.
The germ cells in the body are the only ones which
ORGANIC EVOLUTION
under ordinary conditions have any descendants in the fol-
lowing generation. The whole body of the offspring comes
from two united germ cells, — an egg from one parent and a
spermatozoon from the other parent. No bone cell, or muscle
cell, or any other body cell, in either parent, gives rise to any
part of the offspring. Weismann has used the term soma to
include all the cells of the body which are not germ cells,
that is, the muscle cells, bone cells, nerve cells, etc. . . .
This distinction between the germ cells, from which the
young are derived, and the soma cells, which ordinarily have
no offspring in the next generation but are destined to die, is
a very important one, and upon it must be based the discus-
sion of the inheritance of parental modifications.
As a fertilized egg is developing into an adult organism
it divides into a number of portions called blastomeres,
certain of which will form the germ cells of the new organ-
ism, while the remainder will become its soma. The germ
cells of one generation are thus derived almost directly from
the germ cells of the preceding generation.
The accompanying diagram may make the matter clearer.
Generation A,
Generation B,
Generation C,
Generation D,
Germ cells
Germ cells
Germ cells
Germ cells
Soma.
Soma.
Soma.
Soma.
In the diagram the lines indicate lines of descent. Both the
germ cells and soma cells of any generation are derived from
the germ cells alone of the preceding generation. The
INHERITANCE OF PARENTAL MODIFICATIONS 71
soma cells have no descendants.1 They die without off-
spring. Moreover, apparently no germ cell has ever been
anything but a nascent germ cell. It has never been a
muscle cell or a nerve cell. Muscle cells, or any other highly
differentiated soma cells, do not change into germ cells.
We can leave out of account the processes of asexual
reproduction (fission, budding, reproduction by asexual
spores), for, while modifications of the soma of the parent
could pass from parent to offspring by these processes of
asexual reproduction, the modifications, if unable to be inher-
ited through sexual reproduction, would be lost whenever,
perhaps after several asexually produced generations, sexual
reproduction should intervene ; and we know of no species
of multicellular animal, or higher plant, which reproduces
indefinitely by asexual methods. Sexual reproduction inter-
venes sooner or later. The fact that asexual reproduction
occurs does not, then, alter the general argument in regard
to the inheritance of parental modifications.
The phenomena of regeneration would be of some inter-
est in this connection, if we knew well-authenticated instances
of animals regenerating their reproductive organs, forming
from soma cells the new germ cells to take the place of those
which had been lost. We do not know, however, that such
regeneration is customary, or even possible, in any group of
animals. Certainly it is not of sufficient frequency to be
taken into account as a means by which soma cells might
impress their character upon germ cells and thus secure the
inheritance of parental modifications.
1 As the diagram shows, the body (soma) of the " parent " and the body (soma)
of the " child " are in the relation of uncle and nephew, being related only through
the germ cells of the parent's parents.
72 ORGANIC EVOLUTJON
The relation of soma and germ cells in plants and the
relation of the germ substance to the processes of regenera-
tion in plants are more obscure than the similar relations in
animals. It does not seem best to attempt to discuss them
here.
The modifications of the soma, to which we must refer,
are of two- sorts, first, those produced by the effect of the
environment upon the organism, and, second, those resulting
from the reaction upon itself of the activity of the animal
or plant. Let us illustrate each.
The direct influence of food and climate is often of such
a nature as to produce changes in the individual. For exam-
ple, plants, if grown in a warm moist climate and in rich soil,
may be larger than if grown under less favorable circum-
stances. Will these plants have larger offspring as a result
of inheritance of the increased size ? Is the direct effect
of the favorable environment (increased size) handed down
to the offspring ? If the offspring be large, as they probably
will be, is their large size due to the fact that their parents
became large under the favorable conditions in the midst of
which they grew, or to the fact that the offspring themselves
grow under the same favorable conditions as their parents
and so, for this reason, are large ? That is, is their size
due to the inheritance of the increased size of their
parents, or to the same favorable soil and climate that made
their parents large ? Is there at all any inheritance of
increased size ? How can we tell ? We have at least one
test which we may apply. When plants are taken from
unfavorable conditions and are grown under the most favor-
able conditions, do they only gradually assume increased size,
or are those of the first or second generation as large as
INHERITANCE OF PARENTAL MODIFICATIONS 73
those of the third or fourth or tenth or fiftieth ? If parental
modifications be inherited, the plants of the later generations
should be larger than those of the first, the inherited effect
of increased size accumulating from generation to genera-
tion. We do not, however, find this to be the case. It is
not by this method that large plants have been produced
by the gardeners. They have been produced by selecting
the larger plants to breed from and continuing this process
from generation to generation, the same process of selection
that goes on in nature.
Let us look at an illustration of the reputed inheritance
of the effects of use and disuse and see if we can accept this
influence as a factor in the evolution of the higher animals
and plants. We have referred to the increase in size that
follows the use of a muscle, and the decreased size that
results from its disuse. Are these effects inherited by the
offspring ? Does the man who is strong because he leads
an active life have stronger children than he would have
if he led an inactive life? Notice this: The fact that he
does develop strong muscles as the result of exercise shows
that he must have had an innate capacity for developing
strong muscles by exercise. If he inherited from his parents
the ability to develop strong muscles under the stimulus of
an active life, his offspring in turn will inherit from him the
same ability. A blacksmith has a son who becomes an office
clerk and takes no exercise. Does the son have any stronger
right arm than he would have had if his father had been
an office clerk ? Of course the son will have the same
capacity for developing a strong arm that his father had
before him, but will the fact that the father developed this
capacity and became strong give the son any greater strength
74 ORGANIC EVOLUTION
than he would have had if the father, through inactivity,
had allowed his capacity for strength to lie undeveloped ?
There is little, if any, carefully scrutinized and carefully
recorded evidence in favor of an affirmative answer.
How can we test the case ? It is very difficult. Experi-
mentation has failed to show inheritance of the effects of use
and disuse among the higher plants and animals, and we
have practically no evidence in its favor except its apparent
plausibility. But, when carefully scrutinized, is it as plausible
as it seems at first thought ? How can the use of the biceps
muscle in the arm of the parent so affect the offspring that
he will be not only stronger, but stronger in the biceps
muscle, the particular part affected in the parent ? The child
is not the child of the biceps muscle of the parent, but the
child of the germ cells of the parent, and these germ cells
have little to do with the parent's biceps muscle. They are
separated by a great space, and they do not, so far as we
know, have any special mutual relation. If the increased
strength gained by the biceps muscle of the parent is to be
handed down to the offspring, the increase in size in the
parent's biceps must in some way produce an effect upon
the parent's distant germ cells from which the child is to
develop ; and this effect upon the germ cells must be of so
particular and definite a kind as to produce not a general
effect upon the offspring but a particular effect, namely,
greater strength, and not only greater strength, but greater
strength in a particular portion of the body, the biceps muscle
of the right arm. The hypothesis, apparently so simple at
first glance, is seen, when scrutinized, to involve a connection
between muscle cells and the distant germ cells so intimate
and so definite as to be marvellous beyond almost any known
INHERITANCE OF PARENTAL MODIFICATIONS 75
fact of biology. No greater assumption has ever been made
as the basis of any biological theory, and it is pure assump-
tion, for as yet we have no evidence of any such mechanism
connecting muscle or nerve or bone cells with the germ cells.
In the absence of evidence in favor of the inheritance of
parental modifications among highly organized forms, and in
the presence of the tremendous assumption upon which this
hypothesis rests, I think it unsafe to accept this principle
even as a working theory. We may get definite evidence
sometime that will lead us to a different conclusion. The
phenomena of biology are wonderful, and even this great
assumption may yet be proven. It has not yet been proven
or been shown to be probable.
Let us direct our attention to two further points in con-
nection with this part of the discussion. Many of the most
remarkable phenomena of nature we are sure have been
developed without the aid of the inheritance of parental
modifications, so we do not need the help of this hypothesis
because natural phenomena are " too wonderful to be ex-
plained without it." The color of flowers is useful to attract
insects. It has served its purpose when an insect has seen
the color and has responded. The plant lies passive ; the
insect actively responds. How can the reactionary effect of
the active response in the insect be inherited by the offspring
of the plant ? Or another equally absurd case : Many
animals, rabbits for example, are protectively colored. This
protective color serves its purpose, i.e. is used, when the fox
fails to see the rabbit. How can the failure of the fox to see
the rabbit produce such an effect on the germ cells of the
rabbit that the offspring of the rabbit shall be still more pro-
tectively colored ? Again : many seeds have spines or hooks
76 ORGANIC EVOLUTION
on their outer surfaces, which become entangled in the
wool of animals or the clothing of men, and so secure the
scattering of the seeds at a distance. These hooks dry up
by the time the seeds are ripe, and are nothing but dead hard
tissue incapable of receiving any impression. They cannot,
then, hand down the effects of their use to the next genera-
tion. This is all the more true, since, at the time of their
use, they are separated from the plant of which they were a
part, and so, of course, can have no effect on the germ cells
of that plant. Of course, the dry seed coats can have no
vital relation to the little embryo they enclose.
Again, the instincts of the bees, to which we have already
referred, are wonderful. The worker-bees, which are the
ones with the remarkable instincts, build the honeycomb,
gather and store the honey, feed the young, control the
queen, manage the whole hive in fact, with an intelligence,
or in accordance with instincts, of the highest order. It is
the workers alone who have these wonderful instincts, but the
workers are practically sterile, very rarely having offspring;
so, apparently, the instincts of the workers cannot have been
developed through the inheritance of the effects of use. The
workers have no offspring to whom they could hand down
their instincts. The workers come from eggs laid by the
queen, and it seems to have been natural selection, choosing
for survival those hives in which the workers are most intel-
ligent, or have the most perfect instincts, that has produced
the complex activities of the present beehive. This has
been urged by Weismann and others as an example of great
development of instinct or intelligence which natural selec-
tion alone can have produced.
Now, while I believe that the remarkable instincts of the
INHERITANCE OF PARENTAL MODIFICATIONS 77
worker-bees have been developed through natural selection,
I would suggest that the argument stated above is hardly
conclusive. The sterility of the worker-bees is a character
acquired within comparatively recent times. Their compli-
cated instincts (or high degree of intelligence) may have
been acquired before they became sterile. This possibility
is suggested by the fact that the fertile females of certain
wasps have most remarkable instincts, almost, if not fully,
as wonderful as those of the worker-bees. Among the
solitary wasps the fertile females never cease to exercise
their special instincts. Among some of the social wasps,
on the other hand, we find species in which the fertile
females exercise these instincts for a time and later cease
to use them. Dr. and Mrs. Peckham say of the genera
Vespa and Polistes: " In the autumn the queens, having
mated with the drones, creep away into crevices and shel-
tered corners, where they pass the winter. In the spring
they may be seen seeking for suitable nesting places, and
forming, from the fibres of weather-beaten wood, which are
scraped off and chewed up, the first layer of cells. So
much being accomplished, the queen deposits her eggs, one
in each cell, and when these develop into grubs she feeds
them, until at the end of a week or ten days they spin their
cocoons and become pupae. In from eight to ten days the
perfect wasp is formed and emerges from its cell ready to
assume its share of responsibility in the work of the nest.
These first wasps are always neuters, and hereafter all the
duties which the queen has been obliged to perform, with the
single exception of egg-laying, fall upon them." The neuters
of these social wasps die when winter comes on. Should
they live through the winter, there would be no need of the
78 ORGANIC EVOLUTION
fertile females retaining their special instincts of nest-build-
ing and caring for the young. These activities might then
be left wholly to the neuter workers, which would give us
the condition found among the bees at present. It seems
not improbable that this has been the general course of the
development of the instincts of the worker-bees. I have
given Weismann's argument because it is one so often
quoted, though it is not conclusive. There are, however,
many classes of phenomena whose development can be ex-
plained by natural selection but not by the inheritance of
parental modifications, and these phenomena are as remark-
able as any we have to explain. We do not need the hypoth-
esis of the inheritance of parental modifications to explain
nature because of natural phenomena being " too wonderful
for any other explanation."
Finally, the inheritance of parental modifications, even
if it occurred, would be wholly inadequate to explain the
most fundamental feature of the phenomena of organic
nature ; namely, the adaptation of the organism to its
environment. Adaptation is the key-note of organic nature,
and it is exactly the thing natural selection secures, for those
individuals which are not adapted to their environment are
destroyed in the struggle for existence, leaving only the well-
adapted forms alive. The inheritance of parental modifica-
tions, on the other hand, could not produce adaptation to the
environment, unless the influence of the environment upon
each individual organism and the reaction of the organism
itself were such as to produce adaptation of each individual
to its environment, and we are far from having sufficient
evidence that the direct changes produced in each individual
by the influence of the environment are thus adaptive. For
INHERITANCE OF PARENTAL MODIFICATIONS 79
example, animals living in cold countries have thicker fur
than tropical species. This might readily be brought about
by natural selection, but we have little to indicate that the
direct effect of cold upon each individual is such as to cause
increased thickness of hair.
One more question naturally presents itself. If changes
in the offspring are not produced by changes in the body
(soma) of the parent, how do variations come to appear in
the offspring? Variations arise in the germ cells and are
transmitted from them to their offspring. Changes in the
internal constitution of the germ cells will cause changes to
appear in the young which arise from these germ cells. The
character of every animal or plant is dependent upon the
character of the germ cell from which it comes.1 A new-
laid egg of a chicken almost exactly resembles a new-laid egg
of a duck. The most careful study of the two would not
show any trace of the differences which are to appear as the
eggs develop ; yet it must be that the two eggs differ in their
constitution and that to this difference in structure is due the
difference between the birds which will hatch from the two
eggs. The character of the adult is predetermined by the
character of the egg. Of course, then, anything which
causes changes in the character of the egg may cause
correlated changes in the adult which is developed from
the egg.
But what can cause such changes in the egg or spermato-
zoon ? It lies inside the body of the animal or plant which
1 This is equally true whether we believe with Weismann that every organ of the
future adult is represented by a corresponding differentiated though minute particle
in the germ, or with Hertwig that the germ cell is more nearly homogeneous, differ-
entiation appearing as growth proceeds.
8o ORGANIC EVOLUTION
bears it, and is to a considerable degree protected from con-
tact with the outer world. Why, then, does the egg change
its constitution ?
Those who are at all familiar with biological phenomena
know that all living things and all parts of their bodies are
constantly changing. No bit of living protoplasm is ever
at rest. It liberates the energy used in its different life
activities only by the destruction of some of its sub-
stance, and this constant waste has to be constantly
repaired. For this repair food is needed and is digested and
assimilated, being built up into new protoplasm to take the
place of that which was destroyed. Changes in nutrition
may cause changes in the constitution of the organism which
is being nourished. The constitution of the germ cells may
thus vary with the changing conditions of nutrition, and such
changes in the structure of the germ cells may register them-
selves in changes in the organisms which arise from these
germ cells. Variation in animals and plants may therefore
be due to the conditions of nutrition of the germ cells from
which they came.
Germ cells receive their nutriment from the blood or
lymph in all higher animals. The blood may contain other
substances than food which will affect the character of the
germ cells. Changes in the blood other than those con-
nected with nutrition may therefore cause changes in the
germ cells, producing variation in the offspring. Such
changes in the constitution of the blood may be due to
the action of the somatic cells, since their waste products
and secretions find their way into the blood. One can
readily conceive, for example, that imperfect action of the
renal cells (perhaps due to disease), resulting in impure
INHERITANCE OF PARENTAL MODIFICATIONS 8 1
blood, might so affect the germ cells as to cause the offspring
which arise from them to diverge somewhat from the usual
character. It is hard to see how this somewhat indefinite
effect of soma upon germ could be avoided. We have, how-
ever, no evidence that the substances given off by the several
sorts of soma cells into the blood affect the germ cells in
such a way that when they give rise to new organisms these
will repeat in their own bodies those peculiar modified
somatic activities of their parents which gave into the blood
the substances which caused the modification of the germ
cells. So, while we recognize the probability that germ cells
are constantly affected by changes in the blood due to the
activity of soma cells, and while recognizing also that we may
have here a real cause of variation, we still have no evidence
that these somatic influences upon the germ are of such a
nature as to cause the offspring to inherit the adventitious,
accidental, or secondarily acquired somatic characters of the
parent. We have here a probable cause of variation, but
not a means for securing the inheritance in kind of modifica-
tions of the parental soma.
Again observe that wrhen a spermatozoon unites with an
egg in the process of fertilization, there are mingled germ
cells from two different ancestors, each with its own he-
reditary potentialities. The organism resulting from the
development of this compound cell will naturally be dif-
ferent from either of its parents, the hereditary tendencies
received from one parent being modified by those from the
other parent. For a proper understanding of the possi-
bilities of variation which are involved in this fact of the
union of two germ cells in the process of fertilization one
needs to be familiar with some of the most intricate
82 ORGANIC EVOLUTION
phenomena of cell structure and physiology, which it is not
appropriate to describe here.
In closing this exposition of the theory of organic evolu-
tion it is well to call attention to one important point. The
whole process of evolution centres in the processes of repro-
duction. Natural selection is the selection of the individuals
who are to perpetuate the species, and not merely of the indi-
viduals who are to live out their own lives. Sexual selection is
the selection of mates in breeding. Segregation is the preven-
tion of free intercrossing in the breeding processes. Parental
modifications can produce an effect upon the evolution of the
species only when they are handed down by reproduction to
the following generations. The offspring of the next gen-
eration, and not the adults of the present generation, are
the goal in all the processes of evolution. Much inaccu-
rate thinking has been due to the failure to clearly grasp
this fundamental conception. Lloyd Morgan sums the
matter up in the phrase, " To breed or not to breed.
That's the question."
SUMMARY
In the foregoing rapid review we have noted the manner of operation
of these
Factors of evolution :
Natural selection :
Heredity (Offspring tend to resemble their parents) :
Variation (This resemblance is far from exact) :
The destruction, in the struggle for existence, of the individuals
which are not adapted to their environment, resulting in a
more and more perfect adjustment of organisms to the con-
ditions in the midst of which they have to live.
SUMMARY 83
Sexual selection :
The exercise of choice in mating, observed among spiders,
insects, and vertebrates. It results in the developing of
courting habits, of conspicuous colors, ornamental append-
ages, beauty (?) of voice, etc., which tend to make the
individuals of one sex (usually the males) attractive to those
of the other sex.
Segregation :
By which the individuals of a species are divided into different
groups which do not freely interbreed. The causes of seg-
regation are various : geographical, climatic, physiological,
aesthetic, etc.
Inheritance of parental modifications :
This is probably an efficient cause of evolution among unicellu-
lar organisms, but apparently is not effective among higher
animals and plants.
And we have seen that all of the processes of evolution necessarily centre
in reproduction.
PART SECOND
II. THE PHENOMENA EXPLAINED BY THE
THEORY
WE have reversed the natural order in our treatment
of the theory of evolution. It was the phenomena, to which
we wish now to direct our attention, which first suggested
the theory, and it was only by prolonged study of these
phenomena that the theory was tested and established.
For the sake of brevity in the presentation of the subject,
we have chosen first to develop the theory and then to
apply it to the phenomena upon which it bears.
For the purposes of our treatment the phenomena to
which we wish to direct attention may be classified as
follows : the phenomena of comparative anatomy ; the phe-
nomena of comparative embryology ; the phenomena of
paleontology ; the phenomena of geographical distribution ;
and the phenomena of color in animals and in the blos-
soms of plants. A complete discussion of these subjects
would still be but a partial treatment of the phenomena
which have a bearing upon the theory. Many points of
physiology, the phenomena of sterility, hybridization, in-
stinct, habit, etc., etc., would still be omitted. We shall
attempt but a very brief treatment of some of the phe-
nomena of the several types mentioned in the classification
given above. Do not, then, be under the impression that
we shall have reviewed, even in outline, the whole subject.
87
88
ORGANIC EVOLUTION
The phenomena of comparative anatomy in their bearing
upon the theory of evolution.
Classification.
All are familiar with the fact that animals and plants are
of very many different sorts, and that the different kinds show
very different degrees of
complexity in their or-
ganization. We give ex-
pression to these facts in
our classification of ani-
mals and plants. Forms
which are closely similar
almost to the point of
identity we call members
of the same species. For
example, while hardly
any, if any, two robins
are so similar that we
cannot detect some dif-
ferences between them,
still all robins quite
closely conform to the
same type, and their mutual differences are so slight that
without hesitation we group them together in one species.
We see the same thing among plants. Such of our
common blue violets as have rounded, heart-shaped, slightly
pointed leaves, and scentless blue flowers of large size,
having also very much shortened stems, we class under
the one species Viola cucullata (Fig. u). (There are other
characters of the species besides those mentioned by which
FIG. ii. — Viola cucullata. — From Britton and
Brown's Illustrated Flora of the Northern States and
Canada, by the courtesy of the authors and of Charles
Scribner's Sons.
COMPARATIVE ANATOMY
89
it can be recognized.) But we have other plants whose
blossoms in their form so closely resemble those of the
common Viola cucullata, and which in their whole appear-
ance are so similar, that we conclude they are connected
with this species ; yet the differences are sufficiently great
for us to be unable to assign them to this species. One
kind of these violets have
smaller blossoms with a
much longer spur. Their
stems are highly developed
and branching, while their
leaves are smaller and are
borne upon shorter petioles
(Fig. 12). These we classify
as Viola rostrata, indicating
the difference between the
two types by the different
specific names, but at the
same time calling attention
to the resemblance between
the two forms by giving
them both the same genus
name, Viola. There are
a dozen or more species of the genus Viola found around
Baltimore. In this same region is found an apparently
very different plant with tall and branching stems, with
coarse leaves and small greenish blossoms, a coarse, weed-
like plant (Fig. 13). This form has been named Solea
concolor. Now, great as are the superficial differences be-
tween this species and our violets, careful study shows that
the blossoms of both are made up on the same plan, and
FlG. 12. — Viola rostrata. — From Britton and
Brown's Illustrated Flora of the Northern Stales
and Canada, by the courtesy of the authors and
of Charles Scribner's Sons.
9o
ORGANIC EVOLUTION
that there are important fundamental resemblances between
Solea and the members of the genus Viola. This funda-
mental resemblance in the midst of more superficial differ-
ences we indicate by classifying both Solea and Viola in
a common larger group which we call the family, in this
case the violet family or the Violacecz. As we have sev-
eral genera within the one family Violacecz, so we have
many different families of
plants, — the daisy family
or Composite, the prim-
rose family or Primulacea,
the rose family or Rosacea,
and so on. Now all these
families mentioned have
certain general resem-
blances to one another,
such as the presence of
blossoms and seeds. Many
other kinds of plants are
without either blossoms or
j r i
SCCdS \ lemS ana mOSSCS,
r -\ TTT -i • ,•
for example. We distin-
guish the former as flowering plants or phanerogams, and
the latter as flowerless plants or cryptogams. Thus we
have different grades in the classification to indicate dif-
ferent degrees of resemblance and divergence.
Moreover, as we study the different groups of plants, we
find them very different in the complexity of their organ-
ization, in the extent to which their organs and tissues are
developed. Some, like the flowering plants, are highly
organized, showing very elaborate structure, while others
FlG. 13. — Solea concolor. — From An Illustrated
Flora of the Northern States and Canada, by the cour-
tesy of the authors and of Charles Scribner's Sons.
COMPARATIVE ANATOMY 91
of the lower, flowerless plants, such as the yeast plant, or
the Algcz, are very simple in comparison. In the same
way, among animals we find the lowly organized Amoeba
and its protozoan relatives, the more complex sponges and
jellyfishes, the still more developed flatworms, the annulated
worms, the Crustacea, the spiders, the insects, the Mollusca
(snails, clams, oysters, etc.), the starfishes, and the verte-
brates, including the fishes, Amphibia, lizards, birds, and
mammals.
Now, what is the meaning of all this diversity of form
and the various degrees of complexity ? It is the theory of
evolution which interprets these phenomena, showing us
that the different degrees of resemblance and divergence
between these forms indicate different degrees of relationship.
Descent from common ancestors, with divergence under
the influence of natural selection and the other factors of
evolution, is the key to these phenomena. The taxonomic
system, or the system of classification of animals and plants
into varieties, species, genera, families, orders, subclasses,
classes, subkingdoms, and kingdoms, is but an expression
of relationships, the erection of a genealogical tree, in which
the animal and plant kingdoms would be the two great
branches, the lesser subdivisions corresponding to the
smaller branches and the twigs. The several species of
violets resemble one another because they are the descend-
ants of common ancestors, and that is what we mean when
we class them in the same genus Viola. Viola and Solea
in turn have a still more remote common ancestor, a fact
we express by placing the two genera in the same family,
the Violacecz. At some very much more remote period
the flowering plants were derived from the flowerless plants,
92 ORGANIC EVOLUTION
and we give expression to this fact when we establish the
two major divisions of the plant kingdom ; namely, Phanero-
gamia and Cryptogamia. These phenomena of taxonomy
or classification were unintelligible until the theory of evolu-
tion gave us the talismanic word relationship.
Homology.
There are other phenomena of comparative anatomy
fully as important to the student of evolution. The phe-
a b c d
FIG. 14. — Skeletons of fore limbs of various vertebrates,
a. Wing of a bird. b. Fore leg of a dog. c. Arm of man. d. Wing of bat.
nomena of homology are of great interest. The wing of a
butterfly and that of a bird serve the same purpose and
are built on the same mechanical principle, but they are
fundamentally different in their structure. On the other
hand, the wing of a bird and the fore leg of a dog, while
used for very different purposes and appearing superficially
to be very different, are in reality very much alike in their
fundamental structure (Fig. 14). Each has four chief divi-
sions,— upper arm, fore arm, wrist, and hand, — and in each
we find the same bones, except that the number of fingers has
COMPARATIVE ANATOMY
93
been reduced in the bird's hand. We find the explanation
of this resemblance when we recognize that the bird and
the dog are descended from common ancestors in which the
leg was used for walking; that the dog has perfected the
limb for walking, while the bird has modified and adapted it
for the very different use, flying. The two organs are funda-
mentally alike because they are modifications of the same
thing. They are superficially different because they are
used for very different purposes. This fundamental re-
semblance founded on common descent is called homology,
and the phenomena of homology, no less than those of
taxonomy, lend much support to the evolution theory, being
intelligible in the light of that theory, while without this
theory they have no meaning to us. We might multiply
almost indefinitely illustrations of homology based on ge-
netic relationship ; the illustration given, however, will
show the line of evidence as well as is needed for our
purpose.
Vestigial structures.
&
Among the most interesting of the anatomical evi-
dences of evolution are the vestigial organs found in so
many animals and plants, organs once normally developed
and functional, but now reduced, and, so far as we can
judge, functionally insignificant. Certain snakes have very
slightly developed hind limbs, reminding us of the fact that
they are descended from forms which had well-developed
limbs, their present limbless condition being secondary
(Fig. 15). Whales also have vestiges of hind limbs, in the
form of certain small bones lying beneath the skin and not
in any way functional (Fig. 16). They are vestiges of the
94
ORGANIC EVOLUTION
functional hind limbs possessed by the terrestrial ancestors
of the whales. Similarly, the Apteryx of New Zealand,
which has no functional wings, has vestiges of wings, recall-
ing the typical bird
condition (Plate 35).
All these vestigial
structures are with-
out much meaning
until we recognize
that they point us to
the ancestral forms in
which they were im-
portant functional
organs. We might
give many illustra-
tions of such vestigial organs. I will merely mention a few
found in man : the muscles which move the skin, but in
most persons are too weakly developed to do so except in
FlG. 15. — Part of the skeleton of a boa constrictor,
showing the vestigial bones of the hind limbs. — From a
specimen in the United States National Museum.
FIG. 16. — Skeleton of Greenland \vhnle, showing the vestigial pelvic bones near the base of
the tail. [From ROMANES, after FLOWER.]
the region of the face; the muscles that should move the
ears but usually are not functional (Fig. 17); the nictitating
membrane, vestigial in man, but well developed as a third
eyelid in reptiles and birds (Plate 36); the hair of the body,
PLATE 35. — Apteryx australis.
The upper figure from a stuffed specimen in the Smithsonian Institution ; the lower figure
from a skeleton in the museum of The Woman's College of Baltimore. A piece of black
cardboard has been placed behind the skeleton of the diminutive wing.
JAfiff
PLATE 36. — Eyes of various vertebrates, showing the nictitating membrane, indicated by the
letter N. In some reptiles and birds the nictitating membrane can be drawn over the whole front
of the eyeball. — From Romanes' Darwin and After Darwin, by the courtesy of The Open Court
Publishing Company.
PLATE 37. — Hair tracts on the arms and hands of a man and a male chimpanzee. Drawn
from life. Observe that in the corresponding regions the direction of the slope of the hairs is the
same. — From Romanes' Darwin and After Darwin, by the courtesy of The Open Court Publish-
ing Company.
COMPARATIVE ANATOMY
95
FIG. 17. — Muscles of the human ear. —
From Gray's Anatomy.
reduced to a mere vestige of
what we see in the apes, the
nearest relatives we have (Plate
37)-
The eyes of some cave-
dwelling animals are among
our best examples of vestigial
structures. In Mammoth Cave,
for example, there is an under-
ground river of considerable
size in which are found fish
and Crustacea whose eyes are
in different stages of degenera-
tion (Fig. 1 8). Of course, liv-
ing in total darkness as these
animals do, they can have no use for eyes. The presence
of eyes in a vestigial condition is an indication of the fact
that these cave-dwelling
species are descended from
forms which once lived in
the outer world. As eyes
are useless to animals living
in the dark, natural selection
of course no longer will keep
the eyes perfect, and the
degeneration begun by the
withdrawal of natural selec-
FlG. 18. — Three fishes, showing stages in the ,• ••\-\ .MI r j_i
loss of eyes and color. A. Dismal Swamp fish tlOH Will gO Still further,
(Chologaster avetus), thought to be the ancestor •• •, • ••• i •
of the blind fish. B. Agassiz's cave fish (Cholo- DCCaUSC it IS a pOSltlVC Q1S-
gaster agassizi) . C. Cave blind fish ( Typhlich- 1
thys subterraneus}.- From Jordan and Kellogg's advantage tO any SpCClCS tO
Animal Life, by the courtesy of the authors and . • i
of D. Appieton & GO. waste nutriment on useless
96 ORGANIC EVOLUTION
organs : thus in time the eyes will become mere vestiges of
their former selves. Weismann's theory of germinal selec-
tion also may apply here.1
The great variety of forms among animals and plants,
their different degrees of complexity, the phenomena of
homology and of vestigial structures, are readily explained
by the theory of evolution, though without the aid of this
theory they are apparently meaningless to us.
The phenomena of embryology as related to the theory of
evolution.
In the study of the anatomy of different plants and
animals we find, as already stated, that they are of very
different degrees of complexity. We judge in general that
the simpler species are the more primitive and that the more
elaborate have been evolved from simpler forms, perhaps from
forms more or less like some we find living to-day. The
study of embryology gives us additional evidence of the truth
of this conclusion. We find that complexly organized ani-
mals and plants arise each from a single cell, the fertilized
egg, and gradually acquire new organs and a more compli-
cated structure, till finally the adult condition is reached
(Fig. 19). The series of stages of increasing complexity,
seen in the development of one of these higher forms, reminds
us of the taxonomic series in our classification of plants and
animals, in which we found all gradations in complexity from
1 For a brief statement of the essentials of the theory of germinal selection
see Appendix I. 2 lines short
EMBRYOLOGY
97
the lowly Protozoa and Protophyta to the vertebrates and
flowering plants.
Not only do we find that there are these two kinds of
series, the anatomical and the embryological, but we find
that the two series often correspond to a remarkable degree.
Take an illustration. Among the vertebrates, fishes are the
4-.
FIG. 19. — Stages in the development of the pond snail {Lymnceus). [After HAECKEL.]
simplest on the whole. The Amphibia are in general some-
what more modified in their organization. The reptiles and
birds are still more so, and the mammals are in some re-
gards the most highly developed of all. Now, as we study
the embryology of the Mammalia, we find that in some
features of their general organization and in the character of
many of their separate organs the different stages in their
98 ORGANIC EVOLUTION
development correspond to the conditions seen in the lower
vertebrates (Plate 38). There is a stage when the mam-
malian embryo has gill-slits like a fish, also a simple tubular
heart and a blood circulation much more fish-like than is the
adult mammalian circulation. This we interpret as a remi-
niscence of the time when the ancestors of the mammalia were
aquatic animals. Birds and reptiles show in their embry-
ology a similar stage resembling the fish in many important
regards. The frog and other terrestrial Amphibia are
actually aquatic in early life, their tadpoles being very
fish-like (Fig. 20).
In these different stages in the
embryology of an animal we read
FIG. 20. — Tadpole of salamander (/*»»- the history of its evolution from
blystoma) , magnified 2j times. .
simpler forms to its present state.
We say that the development of the individual tends to
recapitulate the evolution of the race, and in studying
embryology from this standpoint we are studying the
racial history.
Many examples of the interpretation of race histories
from the study of embryology might be given among both
plants and animals. I will give but one more, chosen from
the higher Crustacea. The Decapoda, the highest group
of the Crustacea, includes among many others several forms
familiar to us all: the lobster, the crawfish, and the crab.
The lobster (Plate 39) has the posterior part of the body
long and well developed, using it in swimming, and by
its aid the lobster is able to leap through the water to con-
siderable distances. We call this portion of the body the
abdomen. It is filled with powerful muscles, and is divided
into seven parts, or segments, which move freely upon one
PLATE 39. — Lobster (Homarus americanus) , two-filths natural size.
PLATE 41. — A. " Mysis stage" in the development of the American lobster. Each leg is seen
to have two branches. [After HERRICK.] B. Mysis stenolepis. [From GLAUS.] C. A single
leg of Mysis, showing its two branches. [From LANG.]
EMBRYOLOGY
99
another. In six of these segments are ganglia of the ner-
vous system, controlling the action of the muscles of the
several segments (Plate 40, A). The crab appears to be
very different (Plate 40, B). There does not at first sight
seem to be any abdomen at all, but turn the crab on its
back, and we see on the under side a small structure cling-
ing close to the under side of the body, which when care-
fully examined shows the same divisions into segments that
we observed in the abdomen of the lobster (Plate 40, B, c).
It is the abdomen of the crab, but much reduced in size, and
almost functionless. It contains no nervous ganglia and is
very different apparently from the abdomen of the lobster.
But when we come to study the embryology of the crab we
see that it passes through a stage when it has an elongated
abdomen with ganglia in six of its seven somites (Fig. 21).
This lobster-like stage in the development of the crab is a
reminder of the fact that the crab is descended from ancestors
resembling the lobster. Let us go a little farther. The
lobster has legs like those of a crab, consisting of a linear
series of joints. In the embryology of the lobster, however,
we find a stage when the legs are double, not single, each
leg having two branches (Plate 41, A). In this regard the
lobster larva resembles another member of the group Deca-
poda, namely My sis, a small animal with which many may
not be familiar (Plate 41, B and C). We call the stage in
the development of the lobster when its legs are biramous the
Mysis stage, and conclude that it is an indication that the
lobster is descended from Afysis-\\ke. ancestors. Some crabs
have larvae with biramous legs. Of course conclusions are
not drawn from a single indication like the above, but the
whole condition of the organism is studied. For the sake of
100
ORGANIC EVOLUTION
simplicity we have noticed in each case but a single feature
of the comparison.
The embryological repetition of the race history is gen-
erally much distorted by secondary modifications which
FlG. 21. — Three stages in the development of a crab (Cancer pagur us}. [After HUXLEY.]
A. A newly hatched larva. B. An older larva. C, D. Much older larvae. In all of these the
elongated abdomen is shown. In the two earlier stages some of the legs are seen to be biramous.
cause all stages in the life history to become more perfectly
fitted for their life conditions, but underneath these sec-
ondary modifications we can often see indications of the
EMBRYOLOGY
101
character of the ancestral forms to which the several em-
bryonic stages correspond.
The phenomena of homology are as evident in the study
of embryology as in anatomy. Many structures in the
embryo can be properly understood only after comparison
with similar organs in other forms to which they are related.
Another class of structures, which we may call nascent
organs, appears in the embryology of very many forms.
These are organs which
begin to appear during
the development of the
animal or plant, but
which never become
fully developed or nor-
mally functional, and
soon disappear before
the adult condition is
reached. They recall
some ancestral condition
in which these organs
were important, and are
OI interest aS Showing B, The single aperture (mouth anus) by which the
the racial history, but, digestive cavity °pen
so far as we now can judge, the weakly developed rudiments
of these structures are of little importance to their present
possessors. Numerous examples might be given. I will
mention but one.
The jellyfishes and their relatives have but a single open-
ing into their alimentary canal, which serves both for the inges-
tion of food and the egestion of wastes (Fig. 22). Most of the
FIG. 22. — Hydra. A diagrammatic longitudinal
section.
IO2
ORGANIC EVOLUTION
higher animals when adult have two apertures into the diges-
tive tract, the mouth and the anal aperture, but in their devel-
opment they pass through a stage when like the jellyfishes they
have only the one opening (Figs. 23 and 24). This single em-
bryonic aperture is called the blastopore and is a reminiscence
of the jellyfish mouth. In certain of the lower vertebrates,
the frog for example, we find the blastopore present in the
embryo and well formed and functional (Plate 42, A and B\
FIG. 23. — Gastrula of a coral polyp (Monaxenia darwinii) . [After HAECKEL.]
a. A surface view. b. A longitudinal section.
Later it closes and disappears. In the higher vertebrates,
on the other hand, the blastopore does not become functional
at any time during the embryonic life (Plate 42, C). It is
a nascent organ. It begins to appear, but never reaches
normal development, and later disappears without ever hav-
ing come to its typical condition. Its presence is of no use
to its possessor, so far as we can see, but the fact that it is
there in a rudimentary condition agrees with our principle
that the development of the individual tends to recapitulate
the evolution of the race. The ancestors of the vertebrates,
PLATE 42. — A section of a gastrula embryo of a frog. bp. Blastopore. — From Marshall's
Vertebrate Embryology, by the courtesy of Smith, Elder and Co. B. A diagrammatic longitudinal
section of an older embryo of a frog. b. Blastopore. e. A layer of cells which will become
the lining of the alimentary canal. n. A rod of cells (the notochord) which later is replaced
by the vertebral column, p. The so-called primitive streak where the notochord and the lining
of the alimentary canal fuse with the outer layer of the embryo, forming a plug of cells through
which opens the blastopore. The thickened part of the outer layer of the embryo, on the upper
side, will form the brain and spinal cord. C. A diagrammatic longitudinal section of the upper
portion of an embryo of a bird. Reference letters as in Fig. B, with which this figure should be
compared. The blastopore is very imperfectly developed and does not open. It is indicated only
by a thin spot in the primitive streak, which soon disappears.
PALEONTOLOGY
103
we believe, had, like the jellyfishes, but a single opening into
the alimentary canal. The lower vertebrates repeat this con-
dition in the course of their
embryonic development. The
higher vertebrates no longer
use the blastopore even while
embryos, but they retain it as a
transient rudiment. Of facts
like these we have no satis-
factory explanation except the
theory of evolution, with its
corollary that the development
of the individual tends to be
a recapitulation of the race
history.
The relation of the phenom-
ena of paleontology to the theory
of evolution.
In the phenomena of com-
parative anatomy and compara-
tive embryology we see much
that is intelligible only with
FIG. 24. — Longitudinal sections of gas-
the aid Of the theory Of eVO- trulae of various animals. [After HAECKEL.]
Intirm Tn the nhpnnmprm of A- Ofaworm.&^/^a. B. Of a star fish,
1UUOn- Uraster. C. Of a crustacean. D. Of a
T-»o1^/™f/-0/~»rrw Anf^ VIOTT^ fV>^ o/- snai1' LymntBus. E. Of Amphioxus, a lowly
paleontology we have the ac- relative of the vertebrates> A Digestive
tual record of this evolution in cavity- '• Blast°P°re-
the remains of the animals and plants which have lived in
the past. The record is very imperfect, to be sure, but so
far as it goes it is an actual record. Only very unusual
104 ORGANIC EVOLUTION
circumstances will secure the preservation of any animal or
plant as a fossil. An organism, or portion of an organism,
to be so preserved usually must be hard ; it must be buried
beneath soil of the proper kind, and when buried must be
impregnated with mineral salts or in some other way pre-
served from disintegration. When once converted into a
fossil it must escape destruction at the hands of those
agencies that are constantly destroying the rocks, heat, press-
ure, the disintegration that comes from exposure to the
atmosphere, abrasion by ice, and especially erosion by water.
The character of whole continents has been repeatedly
changed by these agencies. No wonder, then, since fossiliza-
tion is rare and the destruction of fossils when once formed
so easy, that our record of past faunas and floras is so scant.
It is a cause for congratulation that we have so much of
a record as we do possess. Thousands of species of fossil
plants and animals are known, and as yet but a small portion
of the earth has been searched. We will give attention to
but a few illustrations of the kind of record we find in the
fossil-bearing rocks, choosing naturally records that are quite
complete.
Let us first look at a table showing the order of for-
mation of fossil-bearing rocks. At the bottom of the table
are named the oldest of all the rocks in which fossils are
known to be found, the Cambrian formation, about 24,000
feet, four and one-half miles, in thickness. In these rocks
we find fossil remains of many different types, jellyfish,
sponges, Polyzoa, brachiopods, echinoderms, Mollusca, and
annulated worms, but no vertebrates. Numerous types are
represented, but they were simple organisms in comparison
with the representatives of the same types found in the
PALEONTOLOGY
105
PALEOZOIC MESOZOIC C^ENOZOIC
Epochs and Formations
Faunal Characters
PLEISTOCENE.
PLIOCENE, 3,000 ft.
MIOCENE, 4,000 ft.
OLIGOCENE, 8,000 ft.
EOCENE, 10,000 ft.
Man. Mammalia principally of living species.
Mollusca exclusively recent.
Mammalia principally of recent genera — liv-
ing species rare. Mollusca very modern.
Mammalia principally of living families; ex-
tinct genera numerous; species all extinct.
Mollusca often of recent species.
Mammalia with numerous extinct families and
orders; all the species and most of the gen-
era extinct. Modern type shellfish.
CRETACEOUS, 12,000 ft.
Chalk.
JURASSIC, 6,000 ft.
Oolite.
Lias.
TRIAS, 5,000 ft.
New Red Sandstone.
Dinosaurian reptiles; pterodactyls (flying rep-
tiles) ; toothed birds; earliest snake; bony
fishes; crocodiles; turtles; ammonites.
Earliest birds;, giant reptiles (ichthyosaurs,
dinosaurs, pterodactyls); ammonites; clam
and snail shells very abundant; decline of
brachiopods; butterfly.
First mammalian (marsupial) ; 2-gilled cephal-
opods (cuttle-fishes, belemnites); reptilian
footprints.
PERMIAN, 5,000 ft.
CARBONIFEROUS, 26,000 ft.
Coal.
DEVONIAN, 18,000 ft.
Old Red Sandstone.
SILURIAN, 33,000 ft.
CAMBRIAN, 24,000 ft.
Earliest true reptiles.
Earliest amphibian (labyrinthodont) ; extinc-
tion of trilobites; first crayfish; beetles;
cockroaches; centipedes; spiders.
Cartilaginous and ganoid fishes; earliest land
(snail) and freshwater shells; shellfish
abundant; decline of trilobites; May-flies;
crab.
Earliest fish; the first air-breathers (insect,
scorpion) ; brachiopods and 4-gilled cephal-
opods very abundant; trilobites; corals;
graptolites.
Sponges, jellyfish, annulated worms, Mollusca,
brachiopods, Polyzoa, echinoderms — no ver-
tebrates.
From Romanes' Darwin and after Darwin, slightly modified.
106 ORGANIC EVOLUTION
rocks of more recent formation. Take any group and com-
pare a number of Cambrian fossils of this group with a num-
ber from the younger rocks and we find the younger fossils
decidedly higher in their organization. In the rocks formed
during the Silurian age, which succeeded the Cambrian
period, we find the vertebrates, fishes, beginning to appear,
and the earliest air-breathing animals, insects and scorpions,
also animals of the same groups that we found represented
in the Cambrian rocks, but of a more elaborate structure.
In the Devonian period cartilaginous and ganoid fishes and
terrestrial and fresh-water shells are among the most inter-
esting forms. In the next younger rocks, the Carboniferous,
appear the earliest Amphibia as well as more highly organ-
ized representatives of the several groups of invertebrates.
The earliest reptiles appear in the Permian rocks, which
follow the Carboniferous. Mammals and birds are found
in the rocks of the succeeding two periods, and all of the
groups of vertebrates and invertebrates continue to be repre-
sented by progressively more highly differentiated species,
many of the more lowly types disappearing, until we come
to the present age, commonly called the age of man. This
general sequence of fossils, the simpler giving way to the
more complex as we come down to the younger rocks, is
a most impressive thing, and is one of the chief evidences
that evolution has taken place.
Turning to a few illustrations of the origin of particu-
lar species or organs, we find the same principle of grad-
ual increase in complexity as we come from the older to
the younger geological formations. Our record of the evo-
lution of branching antlers in the deer is fairly complete
(Fig. 25). The first deer in the early Miocene had no
PLATE 43. — Antlers of a stag, showing the addition of new branches in successive years. —
From Romanes' Darwin and After Darwin, by the courtesy of The Open Court Publishing
Company.
PALEONTOLOGY 107
antlers at all. In the middle Miocene we find deer with
two-pronged antlers of small size (Fig. 25, A and B\ In
the upper Miocene and lower Pliocene are found three-
pronged antlers somewhat larger (Fig. 25, C and D]. In
the later Pliocene we meet four-pronged and five-pronged
antlers and still larger (Fig. 25, E). In the Pleistocene
clays we see arborescent antlers like those of the modern
deer (Fig. 25, F}. It is especially interesting to see that
B c D E F
FIG. 25. — Fossil deer antlers. [From ROMANES, after GAUDRY.]
A and B. Cervus dicrocerus. C. C. Matheronis. D. C. paradinensis. E. C. issiodorensis.
F, C. sedgwickii.
the antlers of our deer, as the animal grows older, pass
successively through the several stages we find in the
series of fossils just referred to, new branches being added
each year (Plate 43), thus again illustrating the fact that
the development of the individual tends to recapitulate
the history of the evolution of the race.
In Fig. 26 are shown drawings of seventeen different
varieties of fossil Paludina shells, all from the same local*
ity in Slavonia. Paludina is a fresh-water snail, and indi-
viduals similar to the variety figured in the last drawing
io8
ORGANIC EVOLUTION
are living to-day in the lakes of Slavonia. These lakes
have been gradually filled up by the silt brought into them
by their tributary streams. Careful study of the deposits
FIG. 26. — Successive forms of Paludina from the tertiary deposits of Slavonia. — From
Romanes' Darwin and after Darwin, by the courtesy of The Open Court Publishing Company.
thus formed has brought to light a remarkably complete
series of fossil Paludina shells. The uppermost of these,
those nearest the surface and last deposited, are identical
with the forms now living in the same region. As we go
PALEONTOLOGY
109
lower we find shells of a gradually simpler and simpler
form, less corrugated and with less irregular aperture and
less elongated from mouth to apex. We have here in these
fossils a most complete record of the several steps in the
evolution of the irregular, rugose shells of this species of
pond snail. Such a series points almost indisputably to
the theory of descent with modification for its explanation.
There are many indications of close resemblance between
birds and reptiles, but the descent of the former from the
latter is most clearly shown by the numerous fossil forms
which bridge the gap between the two groups. Notice the
accompanying drawings of three of these intermediate forms :
Archczopteryx (Plate 44); Hesperornis (Plate 45, A]\ and
Ichthyornis (Plate 45, B\ Compare these drawings with
Plate 45, C, which represents the skeleton of one of the
ancient flying reptiles, and with the skeleton of a modern
bird as shown in Fig. 27. The intermediate forms first fig-
ured so approach the character of the flying reptiles as to
strongly indicate that they are descended from the latter,
but they are true birds. The fact of the development of
the birds from the reptiles is very clearly indicated in the
discovered fossils which are intermediate in structure be-
tween the two types.
One further illustration will be sufficient. The record
of the origin of the horse, worked out by American paleon-
tologists from American fossils, is probably the best example
of paleontological evidence of evolution. The horse is
especially peculiar in the character of its feet and teeth, and
we will direct our attention to these points as shown in the
accompanying illustrations. In the lower Eocene rocks
we find an animal, Phenacodus, about the size of a fox,
no
ORGANIC EVOLUTION
having five well-developed toes on each foot, and with short
and but moderately corrugated teeth (Plate 46). This is one
of the simplest known relatives of the hoofed mammals ; and
FIG. 27. — Skeleton of a crow (Corvus americana}. Observe that there are no teeth in the
jaws, that the fingers are reduced in number and partially fused together, and that the skeleton
of the tail is short, ending in an enlarged bone (to which the chief tail feathers are attached).
from forms something like Phenacodus must have been
developed the elephant, rhinoceros, hog, sheep, camel, and
all the other hoofed mammals, including the horse and its
long line of ancestors. Observe the steps in the transfer-
Equus : Qua-
ternary and
Recent.
Pliohippus :
Pliocene.
Protohippus :
Lower Plio-
Miohippus :
Miocene.
Mesohippus :
Lower Mio-
cene.
Orohippus :
Eocene.
PLATE 47. — Diagrams illustrating gradual changes in foot structure and pattern of ridges on
the crowns of the molar teeth in fossil and recent species of the horse family. [After MARSH.]
a. Bones of the fore foot. b. Bones of the hind foot. c. Bones of the fore arm (radius and
ulna), d. Bones of the lower leg (tibia and fibula), e. side view of molar tooth, f, g. grinding
surfaces of upper and lower molar teeth, showing the grinding ridges.
GEOGRAPHICAL DISTRIBUTION in
mation of the five-toed limb of a form like Phenacodus into
the one-toed limb of the horse (Plate 47). Notice also the
increasing complexity of the ridges on the grinding surface
of the teeth of the same species from which the illustrations
of foot structure are taken. We have here a very complete
paleontological record of a profound change of structure,
giving us the actual history of the evolution of the horse.
Geographical distribution.
The comparison of the phenomena of paleontology,
anatomy, and embryology seems to point us very clearly
to the theory of evolution as the solution of the problem
of origin. It is interesting also to find that the distribution
of animals and plants over the earth is such as this theory
would lead us to expect. We find the character of the
fauna and flora decidedly different in different regions of
the earth, and these differences are not due solely to differ-
ences of climate and soil and other conditions of the envi-
ronment. Similar environmental conditions do not produce
similar animals and plants if the regions compared be sepa-
rated from each other by sufficient distances or by barriers
that prevent free migration and interbreeding. The phe-
nomena of distribution, as we find them, agree with the
hypothesis that the different species of animals and plants
have each arisen at some particular place and have spread
from that spot, becoming modified to a greater or less
extent during their wandering.
In general, we may say that the degree of intimacy in
relationship between the faunas and floras of any two
regions is in inverse ratio to the degree to which barriers
112 ORGANIC EVOLUTION
are present between these two areas to prevent free pas-
sage from one to the other. There is also a correlation
between the kinds of barriers present and the kinds of
animals and plants held in check by them. Aquatic ani-
mals and plants are restricted by the intervention of land
areas. Terrestrial organisms are held back by the presence
of large bodies of water. Animals and plants adapted to
warm climates may be unable to cross high mountain ranges
whose summits will have a cold climate. Dry regions will
check organisms which are adapted to life in fertile areas.
Desert species will not readily pass a forest barrier or a
region of marshes.
Observe the conditions on some of the islands off the
west coast of South America. Their faunas and floras,
while different from those of the mainland because of their
isolation and different environment, are still quite closely
related to those of the mainland, presenting just the con-
ditions we would expect on the supposition that they are
descended from forms which migrated from the mainland at
some remote period, migration having since been suspended.
Similarly we explain .the resemblance between the fauna
and flora of the west coast of North America and those of
eastern Asia by the fact that at one time, when the climate
of Alaska was mild, migration across Behring Straits was
possible, and by our belief that the Asiatic forms once estab-
lished in this country and American forms once having
crossed into Asia, communication having then been broken
off, the forms thus separated would diverge by evolution.
The flora of the higher altitudes in the White Mountains
of New Hampshire shows a remarkable resemblance to that
of Labrador. This suggests that the White Mountain flora
GEOGRAPHICAL DISTRIBUTION 113
is a remnant of the arctic flora which was spread over New
England during the later glacial period, and that, as the ice
melted and the arctic flora retreated northward, some species
persisted in more southern latitudes by ascending the moun-
tains, the cold of whose higher altitudes resembles the arctic
climate to which these species are adapted.
Certain cases of distribution which at first glance seem to
be anomalous are found on careful scrutiny to support our
hypothesis. For example, the opossums of North and South
America are very different from all the other mammals of the
same region, so different as to be properly placed in a distinct
subclass, the Marsupialia. In no other region are similar
animals found except in Australia and its adjacent islands. In
Australasia, however, there are, with two exceptions, no indig-
enous mammals except those belonging to the same subclass
as the opossum. It seems at first sight absurd to postulate any
communication between Australasia and America by which
one may have become peopled from the other. It looks as if
the opossum type must have arisen independently in the two
areas, a thing which would be contrary to our knowledge of
the ways of evolution. Paleontology here comes to our aid.
The fossil fauna of America is rich in species of the opossum
type, the opossums being the only living representatives of an
at one time very extensive marsupial fauna. The marsupial
type is more primitive than that of the other Mammalia.
There is evidence that at one time, before the higher Mamma-
lia came into existence, the marsupials were spread over the
whole eastern and western hemispheres, and that as the higher
mammals arose they exterminated the mammals of the more
primitive marsupial type, except that in Australia the earlier
forms persisted and in America the opossums remained.
114 ORGANIC EVOLUTION
Why the opossums were preserved in spite of the compe-
tition of the more perfect higher Mammalia we cannot say,
but we do know probably how the marsupials of Australia
managed to persist. There is reason to believe that the con-
tinent of Australia, or the chain of islands to the north of it,
was once connected with the Malay Peninsula, so that the
mammals of that time, which we believe were marsupials,
could readily pass from one region to the other. At this
time apparently much of the earth was peopled by the Mar-
supialia. When, however, Australasia was separated from
southeastern Asia by the formation of the deep straits south-
east of Sumatra (Fig. 28), communication between the two
continents was cut off and the marsupials of Australasia
were thus protected from competition with the higher mam-
mals which soon arose upon the larger continent. The
mammals of the higher type spread over Asia, Europe,
Africa, and North and South America, and replaced the
marsupial forms. The peculiar distribution of the Mar-
supialia, therefore, instead of arguing for the independent
origin of the marsupials in two regions, is a beautiful exam-
ple of the support given to the theory of evolution by the
phenomena of geographical distribution when studied in con-
nection with the phenomena of paleontology, geology, and
comparative anatomy. Other striking examples might be
quoted, but this will suffice to show the general relation of
these phenomena of distribution to the theory of evolution.
The fact that great weight is given by students of zoology
and palaeontology to the phenomena of geographical distri-
bution is evidenced by a belief which is becoming more
general among paleontologists ; namely, that there was at
one time a great Antarctic continent connecting South Africa,
GEOGRAPHICAL DISTRIBUTION
1 1
South America, New Zealand, and perhaps Australia. This
belief is based upon the close resemblance in many remark-
FlG. 28. — Map of southeastern Asia, the East Indies, and Australia. The heavy black line
southeast of Bali, Borneo, and the Philippine Islands indicates the deep-water straits which sepa-
rate the Asiatic fauna from the Australasian fauna. The sharp contrast between the terrestrial
faunas in these two regions makes it probable that this line of demarcation is an ancient one.
able particulars between the fossil faunas of these several
southern regions, no connecting links between which are
I 1 6 OR GANIC E VOL UTION
found among the fossils of the northern hemisphere. It
would at first thought seem preposterous to postulate the
former presence of such a connecting continent with no more
evidence in its favor than the resemblance between these
fossil faunas. Yet this line of evidence has proven so trust-
worthy in other instances that some of our most conservative
paleontologists are inclined to accept the evidence in this
case and to believe that such a continent once existed.
Color in animals.
The phenomena of color in both animals and plants are
among the most remarkable and interesting in the whole
realm of nature. It is not so much the way in which the
color is produced, whether by pigments or by refraction, that
interests us in this connection, as it is the uses to which the
colors are put. Let us first refer to the colors of animals.
According to the uses to which colors of animals are
put, we may classify them, for purposes of description, as
follows : * —
Indifferent colors, not useful, so far as we can judge;
Colors of direct physiological value ;
Protective colors and resemblances ;
Aggressive colors and resemblances ;
Alluring colors and resemblances ;
Warning colors ;
Mimetic colors and resemblances ;
A, Protective,
B, Aggressive ;
1 In the main I have followed the classification used in Poulton's delightful book
The Colors of Animals.
COLOR IN ANIMALS 1 1 7
Signals and recognition marks ;
Confusing coloration ;
Sexual coloration.
We are not interested, in this connection, in non-
useful colors, or in the direct physiological value of colors.
The other uses of color, however, present a diverse series
of phenomena very significant in the light of the theory
of evolution.
Protective coloration and resemblances.
We referred in the early pages of this book to the seventy
of the struggle for existence and to the importance of any
structure or character which enables its possessor to escape
destruction. Carnivorous animals are so common and so
voracious that, as we would naturally expect to find, their
prey have adopted various means of defence. Among these
some of the most important have to do with color. Ani-
mals which closely resemble their environment in color
will escape the notice of their enemies and thus be pre-
served, while their less protectively colored neighbors will
be seen, captured, and devoured. Natural selection will
thus tend to produce protective coloration. The principle
must be sufficiently clear. Let us observe a number of
instances of such coloration.
Many animals which live at the surface of the open
ocean are transparent, so as to be distinguished only with
difficulty from the water itself. This is true of many of
the jellyfishes and their relatives the ctenophores and
siphonophores, of most pelagic Crustacea and worms, of
the pelagic tunicates, and many other less familiar forms,
and of almost all marine larvae. This invisibility must be
Il8 ORGANIC EVOLUTION
a most effective means of protection to these transparent
forms.
Fish are commonly dark-colored above and light-colored
below. To any enemy, such as a sea-gull, looking down
upon them from above, their dark color would cause them
to harmonize with the dark appearance of the water, while
another fish looking at them from below or from the side
would be less likely to detect them than if they were
dark-colored instead of light-colored beneath. Were the
lower surface as dark-colored as the dorsal surface it would
appear to be much darker still, because of its being in shadow.
The light-colored sides and belly of most fish, when the
light comes upon the fish from above, are shaded, and
being in shadow appear about as dark as the dorsal sur-
face. If the sides and ventral surface were actually dark-
colored the added shadow would make them seem very
dark and would make the fish conspicuous. The accom-
panying photograph of a bluefish, taken while the fish
was swimming in an aquarium with the light coming from
above, shows the really brilliant white sides and belly ap-
parently as dark as the steel-blue back, because of their
being in shadow (Plate 48, A). The color of most fish
resembles that of their environment. The flatfish and others
which live upon or near the bottom often closely resemble
the bottom in color (Plate 48, £}.
Most birds are so colored as to conform to the sur-
roundings in which they live. Think for a moment of the
sparrows, streaked and speckled browns and grayish browns
like the grasses and bushes among which they are com-
monly found (Plate 49, A]\ of the whole grouse tribe, the
quail (Plate 49, B], the pheasants, the ruffed grouse (Plate
PLATE 48. — A. Bluefish (Pomatomtis saltatrix}. — From a photograph from life by A. R. Dugmore,
published in Jordan and Evermann's American Food and Game Fishes. By permission of Doubleday,
Page and Co. D. Photograph of a living flat-fish, "sand flounder" (Paralichthys dentata). It is lying
upon clean white sand. Against an ordinary sand bottom its mingled grays, browns, and greens would
render it almost indistinguishable. It is interesting to observe that the circular markings with dark
centres closely resemble shadows of bubbles. The much darker " mud-flounders " are almost equally
well protected by their resemblance in color to the dark mud against which they lie.
.
PLATE 50. — Woodcock
{Philohela minor} on nest. —
by A. R.
PLATE 51. — A. A nighthawk (dead) upon an oak log. B. A humming-bird's nest upon a pin?
branch. — From an exhibit in the United States National Museum.
COLOR IN ANIMALS 119
23), and the jungle fowl from which our domestic fowl
are descended (Plate 16, A), all of which are colored more
or less like the sparrows and have a similar habitat. Think
of the snipe tribe, including the shore birds like the sand-
pipers, the curlew, and the woodcock. The woodcock in
its native haunts is almost invisible (Plate 50). I have
shot scores of them, yet have never but once seen one of
them upon the ground, and this too in spite of the fact
that I have had a dog with me on all of my shooting trips,
and he would stand pointing the bird, often for a long time
before the bird would rise.
The bright green color of some tropical birds, like cer-
tain of the parrots, is to them a most effective protection.
In Jamaica there is a small bright green bird, the "green
tody." While spending a summer in zoological study in
Jamaica I wanted to shoot one and bring home its skin
to show as an illustration of protective color. Often when
out with my gun I heard the faint piping whistle of one
of these little fellows and searched carefully for him, but
always without success. They rarely fly when one is near
them, seeming instinctively to rely for protection upon their
color while they remain motionless among the green leaves.
Once I thought I was at last to be successful, for I located
a tody in a drooping branch of a tree where I could walk
all around him and thoroughly inspect the whole branch.
Yet, though I came within six feet of the branch, peering
among the leaves in every part, I could not recognize the
bird. Finally I drew away about five rods and fired into
the branch, but the bird escaped, for I fired too high. He
had been within six feet of my eyes during the whole of
my closest search. (See also Plate 51.)
120
ORGANIC EVOLUTION
Most snakes, lizards, and frogs are protectively colored.
Our common eastern tree-lizard, which is found often on the
gray, lichen-covered bark of the scrub pines, is a mottled
greenish gray and is hardly distinguishable from the bark
(Plate 52). Most snakes, living as they do upon the ground,
are dull colored, gray or brown, or dull
blackish, like the shadows among the
bases of the grass stalks. One beauti-
ful little snake, found throughout the
eastern United States, is a bright green,
and at first thought it seems very con-
spicuously colored, but it is a climber,
living a large share of the time in the
branches of low shrubs, where its color
renders it inconspicuous among the
green leaves. It is interesting to note
that when disturbed this snake is very
likely to seek safety by flight into the
bushes rather than along the ground.
Deer, rabbits, antelope, wild sheep,
and goats, and most other mammals, are
dull-colored and resemble the region in
which they live (Plates 53 and 54, A).
Most insects show protective colora-
tion (Plates 55 and 56), and so do crabs, lobsters, crawfish,
and most other Crustacea. This is true also of the spiders,
most of which are inconspicuously colored. Most species
are dull brown or gray, like the dead leaves, bark, or lichens
upon which they are found (Fig. 29) ; some are green, like
living foliage (Plate 85, D\ The members of one family,
which live usually within the blossoms of flowers, are
FIG. 29. — A straw-colored
spider ( Tetragnatka grallator}
in its accustomed position on
a blade of dead grass. — From
a specimen given by H. W.
Britcher.
PLATE 52. — TREE LIZARDS (Sceloporus undulatus) ON OAK BARK.
PLATE 53.—^. Common "cotton-tail" rabbit under a sage bush. B. Spermophile (Spermo-
philus tridfcemlineatus} at the mouth of its burrow. — From photographs by E. R. Warren.
A B
PLATE 54. — A. A "cony" or "pika" ( Otochona prlnceps) among rocks. —From a photo-
graph by E. R. Warren. B. A protectively colored woods-moth (Homoptera edusa) on a piece
of bark.
'
PLATE 55. — PROTECTIVELY COLORED WOODS-MOTHS. [After PACKARD and KAPPEL AND
KIRHY.]
A. Sphinx convolvuli. B. Leucania l-album. C. Phorodesmia sm argdaria. D. Smerinthus
tillce. E. Dasychira pudibunda 9. F. Eriopus purpureofasciata. G. Dianthcecia compta.
H. Panthia coenobita. 1. Ichthyura inclusa, var. inversa. J, Cidaria galiata. K. Cidaria ocel-
lata. L. Aplecta occulta. M. Hete> ocarnpa pulverea.
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PLATE 58. — Grass porgy (Calamus arctifrons) , showing changes in color occurring in a few
moments. — From photographs by A. R. Dugmore in Jordan and Evermann's American Food and
Game Fishes, by permission of Doubleday, Page and Co.
PLATE 59. — Color adaptation in pupoe of Pieris rapes and Vanessa urticce. [After POULTON.]
a. Imago of Pieris rapce. b-k. Pupae of Pieris rapes, l-q. Pupae of Vanessa urticce. r. Imago
of Vanessa urticce.
The color of these pupae has been determined by placing the caterpillars, when nearly ready to
pupate, in boxes lined with different colored papers (black, red, yellow, green). Each pupa con-
forms more or less closely to the color of the lining of the box in which it was formed.
COLOR IN ANIMALS 121
brightly colored like the blossoms, their color rendering
them inconspicuous (Plate 75, A). Spiders are exposed to
the attacks of enemies, especially of certain wasps which
capture them, paralyze them by stinging them, and then
use them to provision their nests, the young wasps feeding
upon the living spiders. They therefore need protection.
Of special interest are the protective seasonal changes
of color, seen in some northern animals; for example, sev-
eral species of ptarmigan and the New England and Cana-
dian hare, which are white in winter, resembling the snow,
are grayish or brownish in summer like the dead leaves and
the rocks among which they are found, while in the spring
and fall, while shedding their feathers or hair, they are a
spotted gray and white or brown and white, bringing them
into color harmony with their environment, in which patches
of snow are scattered among the rocks or leaves (Plate 57).
Some animals are able rapidly to change their color,
thus keeping them in harmony with the varying color of
their surroundings as they move from place to place. The
chameleon, the little Anolis of our southern states, some
frogs, and many kinds of fishes, especially tropical fishes,
have this ability (Plate 58).
It is well known that the pupae of most butterflies
are colored to correspond to their environment. Professor
Poulton, experimenting upon certain species of butterflies,
has shown that by placing the full-grown caterpillars in
boxes lined with different colored paper, pupae of colors
corresponding to that of the paper with which they were
surrounded can be obtained (Plate 59).
There are many instances of special resemblance, in
color or in form or in both, between a species of animal
122
ORGANIC EVOLUTION
K
m
and some particular object, the animal escaping detection
because of this resemblance. Often the animal has peculiar
habits which make the resemblance more perfect. Among
insects these special resemblances are not uncommon. One
of the best examples is the caterpillar of the brimstone moth,
which resembles a twig, and which remains motionless in
just the position to make this resemblance most perfect. In
color, shape, and habit-
ual position the resem-
blance is very exact.
The caterpillars of many
other species of moths
show a similar resem-
blance to twigs (Fig. 30
and Plate 60). Some
caterpillars resemble
the ragged edges of the
leaves of their food-
plant, both color and
shape making a striking
resemblance (Plate 56).
Other caterpillars are
green with brown spots,
conforming closely in color and color pattern to the fungus-
spotted leaves upon which they are found (Plate 56). Some
adult insects resemble sticks ; for example, the common
"walking-stick" (Plate 61, A}. In Nicaragua there is
found a walking-stick in which the deception is carried still
farther by certain excrescences on the body and legs which
cause it to resemble a bit of moss (Plate 61, B). Belt, its
discoverer, says it is found on moss. Many insects resemble
FIG. 30. — Twig-like caterpillar of the moth Selenia
tetralunaria, on a spray of birch. [After WEISMANN.]
K. The head,
bling a bud scar.
F. The feet. M. A mark resem-
PLATE 60. — Caterpillar of the moth Catocala amatrix, on a poplar twig.
A. Indicates its head. B. Its posterior end. The bark of the young twigs of this tree is of a
peculiar purplish gray color. The caterpillar not only imitates this color to perfection, but it also
has the habit of so flattening itself against the twig as to appear a part of the twig itself. This
caterpillar on a leafy spray, while alive, was handed at different times to four biologists with the
remark, " Isn't that a fine example of protection ? " and none of them saw the caterpillar.
PLATE 61. — A. Three "walking-sticks" on a twig. The two larger ones are of the species
Diapheromera femorata. — From an exhibit in the United States National Museum. B. An
insect which lives upon moss and which closely resembles the moss in form and color (green) .
[After BELT.]
PLATE 62. — A. A green locust which resembles a leaf. It is probably a species of Cycloptera.
[After BEDDARD.] B. A leaf-like mantis (Phy Ilium sicci/oiium). — From Brehm's Thierleben.
C, A longicorn beetle (Mormolyce phyllodes} . — From Brehm's Thierleben.
PLATE 63. — Logoa opercularis and Logoa crispata. About natural size.
A. Cocoon of L. opercularis. R. Larva of L. opercularis. C. Dorsal view of larva of L.
crispata. D. Side view of larva of L. crispata. E. Cocoon of L. crispata, with moth emerging.
F. Imagines of L. opercularis : upper figure, male ; lower figure, female. B, C, D, and E drawn
from specimens lent by the United States National Museum.
COLOR IN ANIMALS 123
leaves. We have leaf-like grasshoppers, leaf-like Mantides,
leaf beetles (Plate 62), leaf moths, and leaf butterflies (Plate
83, B, D, E, K}. There are a number of the latter in this
country, but the finest example is Kallima inachis, found in
India. In this species the resemblance to a dead leaf is almost
perfect when the wings are closed (Plate 83, A and B\
In the pupa stage of many insects we find remarkable
special resemblances. Perhaps the finest example is fur-
nished by the cocoon of the "waved-yellow moth," Logoa
opercularis. The pupa of this moth lies inside a cocoon
which in color and apparent texture closely resembles the
bark of the alder and other twigs on which it is found
(Plate 63, A\ At the top of the cocoon is a trap-door
not noticeable until it opens to free the adult insect. At
the middle of the cocoon there is a peculiar depression
with rough elevated edges, giving an appearance almost
identical with that of the winter buds of the alder twigs.
Another species of the same genus (L. crispata) has a
cocoon of quite different character (Plate 63, E), for, since
it is found underground, there is no need of its having the
peculiarities which so perfectly protect the cocoon of L.
opercularis. The caterpillars of these same moths are
also protected by great numbers of yellow or brown hairs.
In L. opercularis the hairs so completely conceal the body
of the caterpillar that one would not suspect its real nature
(Plate 63, B]. In L. crispata the hairs, while present, are
less thickly set, allowing the form of the caterpillar to be
seen (Plate 63, C and D}. Both in its larval stage and in
the chrysalis L. opercularis is more perfectly protected
than is JL. crispata.
The examples of special resemblance thus far cited
124
ORGANIC EVOLUTION
have all been taken from the insects. Examples could be
found in other groups. Along our eastern coast is a small
FlG. 31. — A crab (Cryptolithndes sitchensis) which resembles a pebble. Its color is a bluish
gray, resembling a piece of slate. — From a specimen collected in Puget Sound.
spider found very frequently on the little roadside rush,
Juncus bufonius, which so closely resembles the buds of
the rush in color and shape
that the most careful observer
could be excused for not detect-
ing the imposition (Plate 64, A\
Many other spiders show special
protective resemblances (Plate
64). One of the crabs found in
Puget Sound is so exactly like
the pebbles of the bottom along
shore that no one would recog-
nize it as a crab until he saw it
in motion (Fig. 31). In the
tufts of floating seaweed, so
abundant in the Sargassum Sea,
there are small fishes of two
FIG. 32. -A "sea-horse -{Hippocampus species which in color are pe-
mohnikei). a fish which is highly modified to i* i 1*1 ^i i *j_ i£
resemble the seaweed attached to which it CUliarly like the Seaweed itSClf
lives. [After JORDAN, in the Proceedings /T»I ^ /- \ T'l J
of the United States National Museum.] (Plate 65). The Seaweed IS
PLATE 6^. — SPIDERS WHOSE COLOR AND SHAPE RENDER THEM DIFFICULT TO SEE.
A. Epeira stellata upon a rush (Juncus bufonius), natural size; from a specimen given l>y
H. W. Britcher. When this spider rests with its legs folded, its resemblance to a seed pod of the
rush is very close. D. Ariamnes attenuata, which resembles a stick. [From G. W. and E. G.
PECKHAM, after CAMBRIDGE.] C. A spider which resembles a seed pod, natural size. D and
E. Cccrostris mitralis, which resembles a knot on. a twig (magnified). [From G. W. and E. G.
PECKHAM, after VINSON.] F. Epeira prompta on a lichen-covered branch. [After G. W. and
E. G. PECKHAM.] G. Uloborus plumipes, with its cocoon in. its web on a twig of larch. [After
G. W.and E. G. PECKHAM.]
PLATE 65.— SARGASSUM FISH (Pterophryne histrio} IN A TUFT OF FLOATING SEAWEED.
The white spots on the fish resemble the spots of Bryozoa upon the seaweed. The fins of the
fish are frayed out and irregular, resembling somewhat the fronds of the seaweed. Two pairs of
the fins are modified to form clasping organs, by means of which the fish clings to sprays of the
seaweed.
COLOR IN ANIMALS 125
mottled light and darker brown with small white blotches,
and these colors are reproduced in the fishes and with the
characteristic irregularity seen in the seaweed. (See also
Fig. 32.) Many other examples might be cited, but enough
has been said to emphasize the remarkable nature and
the prevalence of phenomena of protective color and re-
semblance.
Aggressive coloration and resemblance.
Let us next look at some instances of aggressive color-
ation and resemblance. Here we have phenomena very
FIG. 33. — Tree-frogs whose backs resemble oak leaves in color and color pattern. [From
BEDUARD.]
similar to those just illustrated, but the use of the color or
resemblance is just the opposite to that which we have
seen. Instead of enabling its possessor to escape its enemies
the color or resemblance enables it to capture its prey.
Anything which will render a predaceous animal less con-
126
ORGANIC EVOLUTION
spicuous will aid it in stalking its prey, or, as it lies in wait,
to capture it. Often the same color which protects an
animal from its own enemies will also aid it in its search
for food, so that the same characters will be both protective
and aggressive. The dull color of the field sparrow (Plate
49, A) will enable it to escape the view of the hawk, but
also it will enable it unobserved to approach its insect prey.
Many of the color characters already referred to probably
have this double use; e.g. think of the insect-eating birds in
general, the lizards (Plate 52), the frogs and toads (Fig. 33 and
Plate 66), the snakes, the
leaf mantis, which is a pre-
daceous form feeding upon
small insects (Plate 62, B] ;
think of the numerous un-
obtrusively colored spiders
(Plates 64 and 85, D\ of the
pebble-like crab (Fig. 31),
and the Sargassum fish
(Plate 65). While the color
FIG. 34. — Polar bear (Ursus maritimus) . —
From a block obtained from the New York Zoo- of the animal often has this
logical Society.
double significance, there
are many instances in which the color is purely aggressive.
To this class belong the colors of the polar bear, white like
the snow (Fig. 34) ; of the arctic fox, white in winter and
grayish brown in summer (Fig. 35) ; of the weasel (Plate 67)
and of the snowy-owl, both of which show a similar seasonal
change ; of the wolf, the fox, the lion ; of the tiger, tawny
with dark stripes, resembling the vertical shadows of the
reeds among which it lies in wait for the antelopes as
they come to the waterside to drink (Plate 68, A}\ of the
m^?
4«?S3fc -*^**%iiiar ^
^ i*i
-- -^--^ttk. - " —
^ 1^*2^
Wv^^
- -^^ *^ir*
PLATE 66. — A. Common toad (Rufo lentiginosus}. B. Tree-frog, "tree-toad" (Hylaversicolor),
on a pine tree. — From a photograph obtained from the New York Zoological Society.
PLATE 67. — WEASELS (Putorius ermineus) .
A. In winter. B. In summer pelage. — From photographs of exhibits in the American Museum
of Natural History. Photographs given by the Museum.
PLATE 68. — A. Tiger (Felis tigris). — From Flower and Lydekker's Mammalia, by permission
of A. and C. Black. B. Jaguar (Felis onca). — From a photograph by Gambier Bolton, by permis-
sion of the Autotype Company.
COLOR IN ANIMALS
127
jaguar, a forest species and a tree climber, the blotches on
whose skin resemble the confused shadows among the trees
(Plate 68, B\
FIG. 35. — Arctic fox, in winter and in summer pelage. [After BEUDARU.]
Alluring colors and resemblances.
There are a few examples of a still more remarkable
use of color and resemblance. In India there is a Mantis
which in shape and color resembles an orchid blossom
(Fig. 36). It deceives butterflies and other insects, which
it captures as they approach the seeming flower. In Java
there is a spider which resembles a bit of bird-excrement
upon which butterflies are so apt to light. This resem-
blance enables it to capture the butterflies upon which it
feeds. Forbes, in his interesting book, A Naturalist's Wan-
derings in the Eastern Archipelago, thus describes his
discovery of this peculiar spider: "I had been allured
into a vain chase after one of those large, stately flitting
butterflies (Hestia) through a thicket of prickly Padanus
128
ORGANIC EVOLUTION
horridus, to the detriment of my apparel and the loss of
my temper, when on the bush that obstructed my further
pursuit I observed one of the Hesperidce at rest on a leaf
on a bird's dropping. I approached with gentle steps but
ready net. ... It permitted me to get quite close and
x^, c\ even to seize it
between my fin-
gers ; to my sur-
prise, however,
part of the body
remained be-
hind, . . . adher-
ing,as I thought,
to the excreta.
I looked closely
at, and finally
touched with the
tip of my finger,
the excreta, to
find if it were
glutinous.
FIG. 36. — A mantis (Hymenopus bicornis), which resembles an
orchid blossom. By courtesy of Crowell Publishing Company.
To
my delighted
astonishment I
found that my
eyes had been most perfectly deceived, and that the excreta
was a most artfully colored spider lying on its back, with
its feet crossed over and closely adpressed to the body.
" The appearance of the excreta rather recently left on
a leaf by a bird or lizard is well known. Its central and
denser portion is of a pure white chalklike color, streaked
!here and there with black, and surrounded by a thin
COLOR IN ANIMALS 129
border of the dried-tip more fluid part, which, as the leaf is
rarely horizontal, often runs for a little way toward the
margin. The spider, which belongs to a family, the Tho-
misidtz, possessing rather tuberculated, thick, and prominent
abdomened bodies, is of a general white color ; the underside,
which is the one exposed, is pure chalk-white, while the lower
portions of its first and second pairs of legs and a spot on
the head and on the abdomen are jet black.
" This species does not weave a web of the ordinary kind,
but constructs on the surface of some prominent dark leaf
only an irregularly shaped film, of the finest texture, drawn
out toward the sloping margin of the leaf into a narrow
streak, with only a slightly thickened termination. The spi-
der then takes its place on its back on the irregular patch I
have described, holding itself in position by means of several
strong spines on the upper sides of the thighs of its anterior
pair of legs thrust under the film, and crosses its legs over its
thorax. Thus resting with its white abdomen and black legs
as the central and dark portions of the excreta, surrounded
by its thin web-film representing the marginal watery portion
become dry, even to some of it trickling off and arrested in a
thickened extremity such as an evaporated drop would leave,
it waits with confidence for its prey, — a living bait so artfully
contrived as to deceive a pair of human eyes even intently
examining it."
In Algiers is found a lizard which has at the corners of its
mouth bright red folds of skin which are of the same color and
shape as the blossoms of one of the desert plants. Insects
are deceived and come to feed upon the nectar and pollen, but
serve themselves as food for the lizard. These are examples
of what we may call alluring coloration and resemblance.
130 ORGANIC EVOLUTION
Warning colors.
Warning colors are another important class. Many ani-
mals are dangerous because of some means of defence, or are
noxious or nauseous as food, and many such are conspic-
uously colored, as if advertising their dangerous or disagree-
able nature. Many insects show conspicuous colors of this
sort. Many of the bees, wasps, hornets, and yellow-jackets
are conspicuously banded with yellow or white, or have a
brilliant metallic lustre, like the blue wasps (Plate 74).
That this conspicuous coloration is an actual protection to
these stinging insects is readily shown by experiment. Very
few insect-eating birds, lizards, frogs, toads, or mammals will
eat these insects. Apparently they have learned that they
are unpalatable. By experimenting with young birds which
have never before seen bees or wasps we get evidence that
the noxious character of the insect has to be learned, but it
is learned with astonishing rapidity, and when once learned,
seems not to be forgotten. Lloyd Morgan describes feed-
ing a young chick with flies among which he placed a
wasp. The chick took the wasp, was stung, and sho\ved
great agitation, wiping its bill and scratching it. Several
days later, while again feeding the little fellow with flies, he
offered it another wasp. The chick looked at the wasp,
turned away from it, and began wiping its bill, apparently
remembering the disagreeable sensations which followed its
former attempt to eat a wasp. Hundreds of experiments
show a similar ability in other birds, in lizards, frogs, toads,
and monkeys, to rapidly learn the unpalatable character of
conspicuous insects. If the stinging Hymenoptera * were
less conspicuously colored, they would often be mistaken
for edible forms and either be eaten or at least be grasped
1 Ants, bees, and wasps.
- • "^,
A
c
PLATE 69. — A. Two bugs (Prionotus cristatus on the left and Euchistus servus on the right)
whose odor and flavor are disagreeable to insect-eating birds, lizards, toads, and frogs, and whose
shape is easily recognized, causing them to be avoided. B. Lady beetles (Hippodamia convergens,
Megilla maculata, Adalia bipunctata}. — By the courtesy of the United States Department of Agri-
culture. C. Colorado potato beetle (Doryphnra decemlineata); a, eggs; b, larva; c, adult. — By
the courtesy of the United States Department of Agriculture.
COLOR IN ANIMALS 131
and injured, even if finally rejected without being eaten.
Their conspicuous color is readily remembered, and, as it is
associated in the minds of their enemies with their dis-
agreeable character, it must serve to save many from injury
or destruction. The coloration, therefore, is properly called
warning coloration.
Often such warning coloration is associated with peculiar
shape or distinctive habits which make the insect still more
easy to recognize. The bees, wasps, yellow-jackets, and hor-
nets have a peculiar buzzing flight, and when standing, they
commonly teeter the abdomen up and down in a way that
always suggests to us their excitable disposition. Apparently
these habits produce much the same effect upon their bird,
lizard, and frog enemies that they do upon us. The slender
waist of the Hymenoptera is also a conspicuous feature.
As further examples of warning coloration we might call
attention to the Hemiptera, the bugs, many of which have a
very- disagreeable taste and equally disagreeable odor. These
insects are frequently conspicuously colored, and they gener-
ally have a very characteristic and readily recognized body
form (Plate 69, A). Many of the beetles are very tough and
some are disagreeable in flavor; accordingly we find many
conspicuously colored beetles. Perhaps the best example is
the common Colorado potato-beetle, the adult of which is con-
spicuously marked with longitudinal stripes and whose larva
is also bright-colored and conspicuously spotted (Plate 69, C).
Both the adults and the larvae are unpalatable to birds, lizards,
frogs, and toads. Other examples among the beetles are the
goldenrod-beetle and the lady-beetles, commonly miscalled
ladybugs (Plate 69, B). Many conspicuously colored butter-
flies are inedible ; for example, the common yellow and white
132 ORGANIC EVOLUTION
forms, Pierid&\ found so frequently about wet places in the
roads (Plate 59, A\ and most of the swallow-tailed butterflies,
Papilionidce, which are our most conspicuous North American
forms (Plate 76, D\ Some moths show warning color (Plate
70, A-K\ The larvae of many moths and butterflies are
inedible, and these also are conspicuously colored (Plate 71).
Wallace, in his Darwinism, says : " These uneatable
insects are probably more numerous than is supposed,
although we already know immense numbers that are so
protected. The most remarkable are the three families of
butterflies — Heliconidce [Plate 77, A-D~\, Danaidcz [Plate
76, A, E, and Plate 84, E and F~\, and Acrczidcz [Plate 76,
G, /, and 77, /, Z] — comprising more than a thousand spe-
cies, and characteristic respectively of the three great tropical
regions: South America, Southern Asia, and Africa. All
these butterflies have peculiarities which serve to distinguish
them from every other group in their respective regions.
They all have ample but rather weak wings, and fly slowly.
They are always very abundant ; and they all have con-
spicuous colors or markings, so distinct from those of other
families that, in conjunction with their peculiar outline and
mode of flight, they can usually be recognized at a glance.
Other distinctive features are, that their colors are always
nearly the same on the under surface of their wings as on the
upper ; they never try to conceal themselves, but rest on the
upper surfaces of leaves or flowers ; and, lastly, they all have
juices which exhale a powerful scent, so that when one kills
them by pinching the body, the liquid that exudes stains the
fingers yellow, and leaves an odor that can only be removed
by repeated washings.
" Now there is much direct evidence to show that this
PLATE 70. — WARNING COLORATION AND MIMICRY IN MOTHS. [After KAPPEL AND KIRBY.]
A-K. Inedible moths, showing warning coloration. A. Zygcena tnfolii. B. Callimorpha
dominula. C. Zyg&na epialtes. D. EmydiajacobecB. E. Callimorpha hera, F. 7yg&na achillece,
G. Z,ygcena minor. H. Arctia caja. I. Z,ygcena fausta. J. Zenzera cssculi. K. Abraxas glos-
sulariata. L-O. Mimicry of bees and wasps by moths. /.. Sesia culiciformis. M. Sesiatipuli-
formis. N. Trochilium apiforme. O. Macroglossia bombyliformis. P-S. Moths closely related
to L- O, which do not imitate bees or wasps. P. Macroglossia stellatarum ; cf. O. Q. Pterogon
proserpina. R. Ino pruni. S. hw statices.
PLATE 71.— INEDIBLE CATERPILLARS, SHOWING WARNING COLORATION. [After KAPPEL
AND KlRBY.]
A. Papillio machaon. B. Arctia caja. C. Orgya antiqua. D. Acronycta alni. E. Acronycta
psi. F. Parnassius apollo. G. Melitcea cinxia. H. Leucoma salicis.
--•v-
I
•
^p3pfC«*^-;-'5l?*3
PLATE 72. — A Gila monster (Heloderma horriduni}. B. Some varieties of North American
skunks of the sub-genus Chincha : I. Chincha mesomelas (Louisiana) ; 2. (7. mephitis (Keewatin) ;
3. C. estor (Arizona); 4. C. putida (Massachusetts); 5. C. elongata (Florida); 6 and 7.
C. macroura milleri (Northern Mexico). Figures of skunks from A. H. Howell's Revision of
the Skunks of the Genus Chincha {North American Fauna, No. 20), by the courtesy of the United
States Department of Agriculture.
COLOR IN ANIMALS 133
odor, though not very offensive to us, is so to most insect-
eating creatures. Mr. Bates observed that, when set out to
dry, specimens of Heliconidce were less subject to the attacks
of vermin ; while both he and I noticed that they were not
attacked by insect-eating birds or dragon-flies, and that
their wings were not found in the forest paths among the
numerous wings of other butterflies whose bodies had been
devoured."
Among the Amphibia the frogs are edible and are pro-
tectively colored. Toads are distasteful, but show a dull
color which is probably aggressive, aiding them in capturing
their insect prey (Plate
66, B}. The salaman-
ders, on the other hand,
are night feeders and
do not need to be ag-
gressively colored, and
we frequently find them
very conspicuously
^
Spotted, Since they are FIG. 37. — Salamander (Salamandramaculosa). —
vi i /T-" \ From Brehm's Thierleben.
inedible (rig. 37).
Lizards, almost without exception, show dull colors, or
colors that are in harmony with their environment, their col-
oration being both protective and aggressive (Plate 52). It
is, therefore, especially interesting to find that the only known
poisonous lizard, the Gila monster of our southwestern states,
is a conspicuously colored form, salmon-pink with broad
irregular black bands and blotches (Plate 72, A).
The Mammalia as a rule show aggressive or protective
coloration in harmony with their surroundings ; the skunk,
however, which is so effectively protected by the foul-smelling
134 ORGANIC EVOLUTION
secretion of its scent glands, advertises its disagreeable char-
acter by its conspicuous black-and-white color (Plate 72, B\
There are a number of similar instances among the Mam-
malia. The black-and-white color of the skunk probably
renders it inconspicuous when hunting its prey on moonlight
nights, the black resembling shadows, and the white marks
blotches of light. Its color, therefore, is probably both
aggressive and warning coloration, aggressive by night,
warning by day.
Similar phenomena of warning coloration are found
among the different groups of marine invertebrates, but,
as the forms are less familiar, we will not refer to them.
Convergence in warning coloration.
One very interesting feature is observed in the warning
coloration of the inedible butterflies. Different inedible
species, belonging to distinct genera or even to distinct
families, in many instances show the closest similarity in
color and in color pattern, and often also in shape (Plate
77, A-F). This was for a long time a puzzle to stu-
dents of color phenomena, until the German naturalist,
Fritz Mullet, suggested that this convergence in coloration
among unrelated inedible butterflies must decrease consider-
ably the number of experiments necessary to teach young
birds and lizards the evil character of the butterflies, since
they are all of one pattern, and so save from destruction
many individuals which would be sacrificed did their enemies
need to learn a separate pattern for each inedible species.
This suggestion seems plausible. It is, at least, the best we
have yet found.
PLATE 73. — A. Inedible curculios and lady-beetles imitated by edible longicorn beetles and
grasshoppers. All from the Philippine Islands.
a. Doliops sp., edible longicorn beetle which imitates b. Pachyrhynchus orbifa, a hard curculio.
c. Doliops curculionides, a longicorn beetle which imitates d. Pachyrhynchus sp., a hard curculio.
e. Scepastus pachyrhynchoides, a grasshopper which imitates /. Apocyrtus sp., a hard curculio.
g. Doliops sp., a longicorn beetle which imitates h. Pachyrhynchus sp., a hard curculio. i. Pho-
raspis, a grasshopper which imitates k. Coccinella, an inedible lady-beetle. [From WALLACE.]
The resemblance is exact in color as well as color pattern.
B. A wasp (a. Mygnimia aviculus) which is imitated by a longicorn beetle (b. Coloborhombus
fasciatipennis) . [From WALLACE.]
3, 4
t I >
5,6
, 10
ii
12, 13
t
PLATE 74. — SEVERAL SPECIES OF FLIES, AND THE BEES AND WASPS WHICH THEY
RESEMBLE.
I. Mydas clavatus. 2. Pompilus atrox. 3 and 5. Apis mellifera. 4 and 6. Eristalis tenat.
7. Spilomyia hamifera. 8. Vespa occiden tails. 9-11. Bombus vancouverensis. 12-14. Volucella
facialis.
COLOR IN ANIMALS 135
Mimicry.
Some of the instances of protective, alluring, and warning
coloration that have been described are sufficiently remark-
able, but the phenomena of mimicry are even more surpris-
ing. Many animals which are not protected by stings, or
disagreeable odors or flavors, and are really palatable to
predaceous species, are protected from the attacks of such
predaceous enemies by their resemblance to species which
are inedible. Instances of such mimicry are very numerous
among the insects, and are found also in other groups. Let
us see some examples.
Many beetles are inedible, either because of their very
hard outer shell, or because of some nauseous flavor, and we
find many such forms to be conspicuously marked with
strongly contrasted colors; e.g. the lady-beetles and curculios
(Plate 73, A, b, d, f, h, /£). There are edible beetles which
mimic some of these warning-colored inedible forms (Plate 73,
A, a, c, g). The hard and unpalatable curculios are imitated
also by grasshoppers (Plate 73, A, e\ Certain grasshoppers
also imitate the evil-flavored lady-beetles (Plate 73, A, i}.
Wasps, bees, hornets, and yellow-jackets are armed with
stings which make them dangerous to attack, and their dan-
gerous character is usually advertised by their conspicuous
coloration. As we would naturally expect, we find that they
are frequently imitated by other insects. We have longi-
corn beetles which mimic wasps (Plate 73, B]. Very many
flies mimic bees and wasps (Plate 74). One common kind of
fly imitates the honey-bee so closely that one would hesitate
to handle it even after being told that it is harmless. Other
flies mimic bumble-bees in appearance and in manner of
flight. In all of these cases, the resemblance is enhanced by
136 ORGANIC EVOLUTION
the habits of the imitating form. The drone-fly, for example,
which imitates a honey-bee, has the same kind of buzzing
flight, and, when standing, occasionally teeters its abdomen
up and down, as is characteristic of the bees and wasps.
Some of these mimicking flies even protrude and withdraw
the tip of the abdomen, as does an angry bee or wasp,
making the imitation in habit as well as in form and color
as perfect as possible.
At Wood's Holl one summer, while collecting insects
from the blossoms of the common milkweed, I was struck by
the resemblance of a moth to the large metallic blue wasp.
When the moth was at rest upon the milkweed blossoms,
this resemblance was not marked, but as one approached at
all near, the moth sprang into the air, flying with a peculiar
buzzing flight that seemed at once to transform it into a
wasp. The blue wasps were common upon the same blos-
soms, and the deception was very perfect. As these moths
are keen-sighted and easily startled, they must rarely be cap-
tured while at rest, and when flying they are likely to be let
alone by insect-eating birds and dragon-flies. In the Alle-
ghany Mountains I have found a large, blue-back longicorn
beetle, which when in flight closely resembles one of the blue
wasps. We have an American moth which similarly resem-
bles a bumble-bee, only in this case the resemblance is almost
as noticeable when the moth is at rest as when it is in flight
(Plate 70, O). The body has the same shape, is banded with
yellow, and is covered with similar long yellow hairs ; the
wings also are very different from those of most moths,
having lost most of their scales and being transparent, like
the wings of a bumble-bee. Many other moths mimic the
.stinging Hymenoptera (Plate 70, L, M, N}.
COLOR IN ANIMALS
137
The ants, another group of the Hymenoptera, are hard,
gritty little insects, with an acid flavor, and are not esteemed
as food by insect-eating birds. Some even have stings, like
their relatives the bees and wasps. In the tropics certain
species of ants are in the habit of gathering bits of leaves
from the trees and taking them to their nests to fertilize
their fungus gardens. These leaf-cutting ants are often
seen in great abundance, marching in procession from the
tree which is being denuded to their nest, each with a piece
of green leaf held in
his jaws and hang-
ing back over his
shoulder. Among
some of these leaf-
cutting ants in the
Amazon basin, Mr.
T-»*ncr1ic;h
£
an
FIG 38 _An ant (a) which in size> spread of legs> glossy
black character of abdomen, and in general appearance at a
little distance, is imitated by a spider (b) which lives in the
nV»Qpr\7-p»rl same nest. Both are quite small. It is very difficult for one
observing them closely to detect the spiders among the ants.
an insect belonging ~ From sPecimens sent b? H-w- Britcher-
to a different order, a " tree-hopper," one of the Homoptera,
which mimicked the ant with its leaf (Plate 75', B}. Its
body was brown below, like the ant, and above was drawn
up into a narrow longitudinal ridge, irregular in outline on
the upper edge and colored a bright green, giving the whole
insect almost the exact appearance of an ant carrying a bit
of green leaf. The ants being unpalatable, the bug which
imitated them was protected from attack by insect-eating
birds. Ants are also mimicked by spiders (Figs. 38 and 39).
Many species of edible butterflies imitate the appearance
of some of the ill-flavored butterflies. One of the best
examples is found throughout the whole of eastern North
138
ORGANIC EVOLUTION
America. The color and color pattern of the inedible
Danais archippus is imitated by the edible Limenitis disip-
pus (Plate 76, A, B, C). I have several times found these
two butterflies flying together, and the first time I captured
any of them I did not see until I reached home that I had
two species, instead of one as I thought. The edible form is
slightly smaller than the ill-flavored one, so that when once
distinguished they can again be recognized without diffi-
culty, but I much doubt if our insect-eating birds would
detect the difference.
The inedible Helico-
nidce of South and
Central America are
imitated by edible spe-
cies of other families
(Plate 77, G, H, B\
The inedible Acr&idcz
of Africa are imitated
[From by edible butterflies
(Plate 77, Z, M). One
of the most remarkable cases of mimicry is that of the
imitation of three different inedible species by three varie-
ties of females in the less distasteful though somewhat pro-
tected Papilio merope (Plate 76, D-J\ As Papilio merope
is itself distasteful, it might be better to call these condi-
tions an illustration of convergence in warning coloration.
Euplcea midamus, an inedible butterfly, is mimicked by
Calamesia midama, a moth (Plate 84, C, D, E, F}. The male
and female butterfly differ in color and in the pattern of
their markings, and it is interesting to see that the male
moth imitates the male butterfly and the female moth copies
the female butterfly.
FIG. 39. — Spiders which mimic ants.
a. Synageles picata. b. Synemosyna formica.
G. W. and E. G. PECKHAM.]
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PLATE 77- — A-D.— Four inedible butterflies belonging to four different genera and three different
families, but all showing the same type of warning coloration ; an example of convergence in warning
coloration. E-F. I-K and L- M also show convergence in warning coloration. G H illustrate mimicry.
[After WEISMANN, with modifications.]
A. Lycorea halia, inedible. B. Heliconiuseucrate, inedible. C. Melincea ethra, inedible. D. Mtcha-
nitis lysimnia, inedible. E. Perhybris pyrrha (male), which shows but slight resemblance to the
inedible Heliconid?e. F. Perhybris pyrrha (female), which closely resembles the Heliconidse. G. and
H. Male and female Dismorphia astynome, edible ; both sexes imitate the Heliconidre. 7. Acrcra egina,
of the family Acrseidae. J Papilio ridleyanus (female), of the family Papilionidse. K. Psendacnra
bnisduvalii, of the family Nymphalidae. J and K, which are inedible, depart from the type of colora-
tion charcteristic of their families and resemble the inedible Acrzea ( /). L. Acrera gea, inedible, which
is imitated by M. Elymnias phegea, an inedible species.
COLOR IN ANIMALS 139
There are instances in which insects are supposed to be
protected by an apparent resemblance to certain vertebrates.
Let me quote from Professor Poulton's delightful book The
Colors of Animals.
" Mr. Bates describes a South American caterpillar which
startled him, and every one to whom he showed it, by its
strong resemblance to a snake, and it even possessed the
features which are characteristic of a poisonous serpent.
" Equally interesting examples are to be found among
our British caterpillars. The brown (or occasionally green)
mature larva of the large elephant hawk moth (Chrczo-
campa elpenor) generally hides among the dead brown leaves
on the under parts of the stem of its food-plant, the great
willow herb (Epilobium hirsutuni) (Plate 78, A]. In this
position it is difficult to see, for it harmonizes well with the
color of its surroundings. It possesses an eyelike mark on
each side of two of the body rings (the first and second
abdominal segments), but these markings do not attract
special attention when the animal is undisturbed.
" As soon, however, as the leaves are rustled by an
approaching enemy, the caterpillar swiftly draws its head and
the first three body rings into the next two rings, bearing
the eyelike marks. These two rings are thus swollen and
look like the head of an animal upon which four enormous,
terrible-looking eyes are prominent (Fig. 40). The effect is
greatly heightened by the suddenness of the transformation,
which endows an innocent looking and inconspicuous animal
with a terrifying and serpentlike appearance. I well remem-
ber the start with which I drew back my hand as I was going
to take the first specimen of this caterpillar I had ever seen."
A good many different species of caterpillar show " terri-
fying " attitudes and motions. Poulton thus describes the
140
ORGANIC EVOLUTION
behavior of the caterpillar of the puss moth : " The larva
of the puss moth (Centra vinula) is very common upon pop-
lar and willow. The circular domelike eggs are laid either
singly or in little groups of two or three, upon the upper
side of the leaf, and being of a reddish color strongly suggest
the appearance of little galls or the results of some other in-
jury. The youngest larvae are black, and also rest upon the
upper surface of the leaf, resembling the dark patches which
FIG. 40. — Caterpillar of the large elephant hawk-moth (Ch&rocampa elpenor). [After WEIS-
MANN and POULTON.]
a. In normal position when feeding, b. In " terrifying attitude." Compare Plate 79, Fig. A,
which shows the same caterpillar in natural colors.
are commonly seen in this position. As the larva grows, the
apparent black patch would cover too large a space, and
would lead to detection if it still occupied the whole surface
of the body. The latter gains a green ground-color which
harmonizes with the leaf, while the dark mark is chiefly con-
fined to the back. As growth proceeds the relative amount
of green increases, and the dark mark is thus prevented from
attaining a size which would render it too conspicuous. In
the last stage of growth the green larva becomes very large,
COLOR IN ANIMALS 141
and usually rests on the twigs of its food-plant. The dark
color is still present on the back but is softened to a purplish
tint, which tends to be replaced by a combination of white
and green in many of the largest larvae (Plate 78, D\ Such
a larva is well concealed by general protective resemblance,
and one may search a long time before rinding it, although
assured of its presence from the stripped branches of the
food-plant and the fceces on the ground beneath.
" As soon as the larva is discovered and disturbed it with-
draws its head into the first body ring, inflating the margin,
which is of a bright red color. There are two intensely
black spots on this margin in the appropriate position for
eyes, and the whole appearance is that of a large flat face
extending to the outer edge of the red margin (Plate 78, D).
The effect is an intensely exaggerated caricature of a verte-
brate face, which is probably alarming to the vertebrate ene-
mies of the caterpillar. The terrifying effect is therefore
mimetic. The movements entirely depend upon tactile
impressions: when touched ever so lightly a healthy larva
immediately assumes the terrifying attitude, and turns so as
to present its full face toward the enemy ; if touched on the
other side or on the back it instantly turns its face in the
appropriate direction.
" The effect is also greatly strengthened by two pink
whips which are swiftly protruded from the prongs of the fork
in which the body terminates. The end of the body is at
the same time curved forward over the back (generally much
further than in the figure), so that the pink filaments are
brandished above the head."
Experiment showed that the terrifying attitude and mo-
tions were effective in frightening away enemies. I suspect
142 ORGANIC EVOLUTION
that the suddenness of the change from one condition to the
other when irritated has as much to do with scaring away
enemies as does the reputed resemblance to the front part
of a snake, for most insect-eating birds and lizards are very
wary and easily startled.
This description is quoted in full, for it gives a remark-
able instance of the combination of general protective
resemblance, terrifying attitude, terrifying motions, with
special appendages and mimicry. Two other caterpillars in
"terrifying attitudes"
are shown on Plate
78, and in Fig. 41
is shown a moth in
what is said to be its
" terrifying attitude."
Another reputed
instance of mimicry
FIG. 41. - " Terrifying attitude " of a moth (Smerinthus SOmetimCS mentioned
ocellata}. [After WEISM ANN.] . .-, f , n , .
is that of the marking
on the tips of the wings of some of the large moths, which
very closely resembles the head of a cobra with its expanded
hood, even the spectacle-like marks on the back of the hood
being reproduced (Fig. 42). I know, however, of no experi-
ments which test the effect of this appearance upon insect-
eating animals, and without such experiments we have no
right to regard the fancied resemblance as significant.
There are examples of mimicry among the vertebrates.
Several venomous species of Elaps, the coral snake, are
conspicuously banded with red and black, or with red and
black and yellow, and these venomous species are each
imitated by other species of harmless snakes, belonging to
PLATE 79. — Mimicry in snakes.
group B are harmless.
The snakes in group A have poisonous fangs ; those in
a. Elaps dumerili, New Granada, b. Elaps lenmiscatus, Brazil, c. Elaps semipartitus, New
Granada, d. Elaps psyche, Brazil, e. Elaps corallimus, Brazil, Central America. /. Ophibolus
doliatus, Southern North America and Central America. g. Pliocercus elapsides, Mexico.
h. Oxyrrhopus trigemimis, Brazil, i. Pliocercus euryzonus, New Granada. /. Erythrolampms
escnlapii, Brazil. k. Cemophora coccinea, Southern United States. /. Erythrolamprtts venus-
tissimus, Brazil, Central America. [After COPE.]
COLOR IN ANIMALS
143
different genera (Plate 79). Many of our common Ameri-
can snakes imitate poisonous serpents in one peculiar habit,
though not in exact color. Poisonous serpents when cor-
nered and irritated have the habit of flattening their heads
FIG. 42. — Moth from India (Attacus atlas}, at the tips of whose wings are markings resembling
those upon the head of a cobra.
so that they become even more triangular than when at rest,
and they show a pugnacity that is very forbidding. Most
of our little harmless snakes, when cornered, will behave in
much the same manner, flattening the head and making it
triangular, and by their hissing and striking they seem to
suggest that they are dangerous.
144 ORGANIC EVOLUTION
There are a few examples of mimicry among birds. Let
me quote from Wallace's Darwinism a description of prob-
ably the best example. " More perfect cases of mimicry
occur between some of the dull-colored orioles in the
Malay Archipelago and a genus of large honey-suckers, the
Tropidorhyncki or 'friar-birds' (Plate 80). These latter are
powerful and noisy birds which go in small flocks. They
have long, curved, and sharp beaks, and powerful, grasping
claws; and they are quite able to defend themselves, often
driving away crows and hawks which venture to approach
them too nearly. The orioles, on the other hand, are weak
and timid birds, and trust to concealment and to their retir-
ing habits to escape persecution. In each of the great
islands of the Austro- Malayan region there is a distinct
species of Tropidorkynchus^ and there is always along with
it an oriole that exactly mimics it. All the Tropidorhyncki
have a patch of bare black skin around the eyes, and a ruff
of curious, paler, recurved feathers on the nape, whence their
name of friar-birds, the ruff being supposed to resemble the
cowl of a friar. These peculiarities are imitated in the
orioles by patches of feathers of corresponding colors ;
while the different tints of the two species in each island are
exactly the same. Thus in Bourru both are earthy brown ;
in Ceram they are both washed with yellow ochre ; in Timor
the under surface is pale and the throat nearly white, and
Mr. H. O. Forbes has recently discovered another pair in
the island of Timor Laut. The close resemblance of these
several pairs of birds, of widely different families, is quite
comparable with that of many of the insects already
described. It is so close that the preserved specimens have
even deceived naturalists, for, in the great French work,
COLOR IN ANIMALS 145
Voyage de r Astrolabe, the oriole of Bourru is actually
described as a honey-sucker, and Mr. Forbes tells us that,
when his birds were submitted to Dr. Sclater for description,
the orioles and the honey-suckers were, previous to close
examination, considered to be the same species."
Well-authenticated examples of mimicry among mam-
mals, or other vertebrates than the birds and reptiles, are
/iot numerous. Among the invertebrates, outside the classes
3f the insects and the spiders, there are some instances
known, but as they are not very frequent, and, as they are
seen in forms which are less generally known, we will not
refer to them.
Wallace mentions five conditions which are always ful-
filled in cases of mimicry. Let me quote his statement
of these.
" i. The imitative species must occur in the same area
and occupy the very same station as the imitated.
" 2. The imitators are always the more defenceless.
" 3. The imitators are always less numerous in individuals.
" 4. The imitators differ from the bulk of their allies.
" 5. The imitation, however minute, is external and visible
only, never extending to internal characters or to
such as do not affect the external appearance."
The instances thus far mentioned are all of protective
mimicry. Of aggressive mimicry there are but very few
instances known. Some of the hunting spiders are very
like the flies on which they prey; possibly also the ant-
like spiders can more readily approach their prey because
of their resemblance to ants which may not be so much
avoided by small flies (Figs. 38 and 39). Certain insects,
146
ORGANIC EVOLUTION
•« '*••.
whose larvae are parasitic upon other insects, closely
resemble the form upon which their larvae are parasitic,
being enabled thus to escape detection when approaching
to lay their eggs in the nests of the species whose members
will become infested with their larvae. These parasites
live chiefly upon different kinds of bees.
Signals and recognition marks.
Signals and recognition marks are seen in many animals.
Birds and mammals especially display these. Our com-
mon rabbit, when startled,
lifts his tail as he runs,
the white on the under
surface and on the flanks
under the tail showing
almost like a flash of
white light. This brill-
iant white patch is sup-
posed to serve as a signal
to other rabbits, especially
the young, to seek in
flight safety from some
impending danger (Fig. 43). Our common eastern deer
have a similar white spot on the under surface and below
the tail, which serves the same purpose. Some of the
western American antelopes have upon the flanks a much
larger patch of long white hairs, which when expanded by the
contraction of the skin muscles and the consequent erec-
tion of the hairs, flashes out as a white signal visible on
the plains for miles (Plate 81). Similar white rump patches
are found in some of the African gazelles.
FIG. 43. — Common "cottontail" rabbit, which is
startled and about to run. The tail is lifted enough
to show a part of its white under surface and the white
rump patch. — From a photograph from life by E. R.
Warren.
PLATE 81. — Antelope showing danger signal. — From Wallihan's Camera Shots at Big Game, by permis-
sion of Mr. Wallihan and of Doubleday, Page and G».
PLATE 82. — A. Killdeer, or ring-marked plover (sEgialitis vocifera). — From an exhibit in
the United States National Museum. B. Nighthawk (Chordeiles virginianus) spread out on a
log in such a way as to show the white marks on the wings and tail.
PLATE 83. — CONFUSING COLORATION.
A and B. Kallima inachis. C and D. Grapta sp. E and F. Hebomoia glaucippe. G and
H, Catocala concumbens. I and y. Junonia sp. K. Phyllodes verhuellis. L. Dissosteira Caro-
lina. M. Hippiscus tuberculatus.
PLATE 84. —SEXUAL COLORATION AND MIMICRY IN BUTTERFLIES AND MOTHS.
A. Ornithoptera priamus, female. B. Ornithoptera priamus, male. C, Calamesia midama,
male, imitates E. D. Calamesia midama, female, imitates F. E. Euplaea midamus, male.
F. Euplaea midamus, female.
COLOR IN ANIMALS 147
Wallace interprets some of the very distinct marks on
different birds, such as the white outer tail feathers which
show in flight, and the streaks and spots about the head
and neck, as recognition marks, by which the individuals of
the same species recognize each other, often at consider-
able distances. Such marks are seen in our common kill-
deer and in the night-hawk (Plate 82). Probably this is a
true explanation of one use of such marks.
Confusing coloration.
Dr. C. Hart Merriam has suggested another use for
certain color markings that have sometimes been described
as signals or recognition marks. All must have noticed
that many of the butterflies have the upper surface of
the wings brightly colored, while the under surface is dull,
and that these forms, when at rest, close the wings, dis-
playing the protectively colored under surface. This is
markedly true of the beautiful leaf-butterfly, Kallima inachis,
(Plate 83, A and B\ These insects are very noticeable
when in flight, but when they light and close the wings,
their sudden disappearance is most startling and confusing,
greatly increasing the difficulty of observing their resting-
place. Many of the moths, which, when at rest, hold the
posterior wings covered by the front wings, show a very
similar condition, the back wings being brilliantly colored
above, while the front wings are dull (Plate 83, G, //, and K\
These moths do not fly by day, unless disturbed, and will be
well protected by their dull color. In flight, however, the
bright color of their posterior wings is very noticeable and
serves to make their disappearance more disconcerting when
they alight. The yellow or red under-wings of grasshoppers
148 ORGANIC EVOLUTION
and their noisy, jerky flight render them very conspicuous
when on the wing (Plate 83, L and M). This makes their
sudden disappearance upon alighting all the more startling
and confusing. Any one who has attempted to catch the
common brown, roadside grasshoppers will, I am sure, ac-
cept this explanation of one use of the conspicuous color
of their hind wings. When at rest they can be seen only
by the keenest attention and closest observation, but when
in motion they are seen by the most careless observer. The
sudden mental change from careless observation of the
brilliant color and noisy flight to the close scrutiny necessary
to detect these grasshoppers when quiet is very difficult, and
is a change one does not succeed in making without much
practice.
Some birds which are in general inconspicuously colored
have white or some other bright color upon the wing or tail
feathers, which becomes visible in flight. Examples are the
night-hawk, the Junco, the vesper-sparrow. The night-hawk
is so colored as to be observed only with great difficulty
when at rest upon a log or upon the ground (Plate 51, A\
It often lies quiet, trusting to its inconspicuousness, until one
nearly steps upon it. When flushed, however, it flies away
with a jerky, zigzag flight, showing in the most conspicuous
manner its clear white spots upon the wings and tail (Plate
82, B\ The great contrast between its conspicuousness in
flight and its almost invisible character when at rest renders
it very difficult to find when it has alighted.
Merriam would give a similar explanation of the use of
the conspicuous bands seen upon the hips and tails of
the desert Kangaroo rats and of the white under tail of the
antelope, squirrel, cottontail rabbit, and some of the jack
COLOR IN ANIMALS 149
rabbits, all of which markings are invisible when the ani-
mals crouch. Some of the desert lizards are conspicuously
banded on the under surface of the tail, which they elevate
and arch over the back when startled, running with great
rapidity for a short distance, then suddenly crouching, until
only the protectively colored back is visible, or rather in-
visible, among the rocks and sand.
These animals, which show such confusing coloration,
generally run or fly in an irregular course, and just before
they come to rest they cover the conspicuous color and fre-
quently dodge to one side, so that they lie unnoticed at some
distance to one side of the spot where they were last seen
by the observer.
It is of course possible that in many cases the same
markings may serve the double purpose of recognition marks
or signals and of increasing the startling effect of the sudden
disappearance of their possessors when they come to rest.
Sexual coloration.
We have already had occasion, in connection with the
discussion of sexual selection, to refer to the differences in
the appearance of the males and the females of many species.
These differences are often largely differences in color, and
should be mentioned in any treatment of the phenomena of
color in animals. The use of these sexual colors is,
apparently, to render the male attractive to the female and
secure her as his mate. In our discussion of sexual selection
we said that these brilliant colors of the male are seen
among birds, lizards, fishes, spiders, in many species of
butterflies, and in some insects. We will stop here to men-
tion but a few instances from these groups. Among birds
150 ORGANIC EVOLUTION
think of the brilliant colors of the male and the more modest
coloring of the female in the peacock, the common chickens
(Plates 12-15), the argus pheasant (Plate 24, A), the birds of
paradise, the oriole, cardinal, and bobolink (Plate 22), the
bluebird, American goldfinch, and the indigo-bird. Even the
robin and the common grackle, or blackbird, show brighter
colors in the male than in the female. The humming-birds
also are good illustrations (Plate 26).
The brilliant bronze-green-and-blue neck of the males of
our common eastern tree lizards is an instance of sexual
coloration. Other finer examples could be mentioned
among tropical lizards. In many species of fish the males
are much brighter colored than the females, and they
display the brilliant colored parts of the body to full
advantage when approaching the females in the breeding
season.
Greater brilliancy of color in the male than in the female
is a quite general rule among fishes, and it is important to
note that, in those cases in which the courting habits of
species with bright-colored males have been observed, the
male has the habit of displaying to the greatest advantage
his bright colors when he approaches the female.
Not only do we find differences in color between the
sexes among the fishes ; we also find instances of differ-
ences in form, the males having certain ornamental append-
ages not seen in the females, or the fins of the males being
larger (Plate 32).
See also Plates 84 and 85 for illustrations of differences
in coloration between the sexes in butterflies, moths, and
spiders.
PLATE 85. SEXUAL COLORATION AND PROTECTIVE COLORATION IN SPIDERS.
A. Habrocestum splenrfens, male. B. Habrocestum splendens, female. C. Phidippus cardinalis,
male. /; Tetragnatha laboriosa. \A, B and C after G. W. and E. G. PECKHAM. D. From specimens
given by H. W. HRITCHER.]
COLOR IN PLANTS 151
Summary
In recapitulation, then, we may say that, aside from their direct physio-
logical value, many colors of animals are useful to their possessors in relation
to their environment or to their special life habits. Such colors we may
class as —
Protective, causing their possessors to harmonize in color with their envi-
ronment, and so escape their enemies ;
Aggressive, rendering their possessors inconspicuous, and so enabling
them to capture their prey ;
Alluring, serving to attract the prey of the forms which show the alluring
coloration ;
Warning coloration, conspicuous, and rendering dangerous, noxious, or
ill-flavored species readily recognizable, thus saving them from attack ;
Mimetic coloration, by which an edible species is protected from its
enemies by its resemblance to a dangerous, noxious, or ill-flavored species
(protective mimicry); or by which a species is brought to resemble its
habitual prey or some species of which its prey is not afraid (aggressive
mimicry) ;
Signals and recognition marks, by which individuals of a species may
recognize their fellows or may warn them of impending danger ;
Confusing coloration, which disconcerts an enemy by the startling differ-
ence between the conspicuousness of the individual when in motion and its
inconspicuous character when at rest ; and
Sexual coloration.
Color in plants.
The color phenomena in plants are as interesting as
those in animals, and are as intimately connected with
the theory of evolution. They are, however, not so well
understood in some of their aspects. We will consider
the colors of plants, chiefly of plant blossoms, only as
related to insects. It seems to be wholly probable that
the colors of blossoms have been developed in connection
with insects. The bright colors serve to attract insects
and the insect visits are an advantage to the plants.
152
ORGANIC EVOLUTION
All are familiar with the general structure of plant
blossoms (Plate 86). Within the brightly colored floral
leaves are found two sets of reproductive organs: an inner
set, female, called carpels, or when united as they com-
FlG. 44. — Fertilization in the rock-rose (Heliantkemum marifoliuni). [After K.ERNER ]
i. A single flower, natural size. 2. A flower, stripped of its sepals and petals, showing the
pistil in longitudinal section. The pollen grains are seen upon the stigma, and their tubes are
seen passing down the stalk of the pistil to reach the ovules. The tubes are indicated erroneously
as going direct to the openings at the tips of the ovules; actually they follow a more devious
course, first down the inside wall of the chamber of the pistil and then up to reach the apertures
in the ovules ; ov. = ovule, stg. = stigma. 3. A more enlarged drawing of the tip of the pistil, show-
ing the pollen grains and the sprouting pollen tubes. 4. A dry pollen grain. 5. A moistened
pollen grain developing its tube. 6. An ovule, showing the opening at its tip through which the
pollen tube enters to effect fertilization.
monly are, together composing the pistil ; and an outer
whorl of stamens, or male organs. The ovules, or imma-
ture seeds, are formed within the pistil (Fig. 44, ov\ while
the pollen, by which the ovules are to be fertilized, is
formed in the anthers at the tips of the stamens. To
PLATE 86. — Diagrams of various flowers to show the arrangement of their parts. [Aftei
KERNF.R.]
A. Flower of Butomus umbellatus, in which all the parts are distinct. B. Flower of Phytolacca
decandra, in which the five carpels are somewhat united to one another. C. Flower of Gagea
lutea, in which the three carpels are united to form a single pistil with one style but a three-parted
stigma.
a, anther; c, carpel (the carpels when fused to form a single structure are called a pistil);
p, .petal (the petals taken together compose the corolla) ; ps, pistil ; s, sepal (the sepals as a
whole compose the calyx) ; st, stamen, at the tip of which is the anther, which bears the pollen;
stg, stigma, the tip of the pistil (it is adapted to receiving the pollen in fertilization).
If
COLOR IN PLANTS 153
produce a seed which will grow and give rise to a new
plant, pollen from a stamen must be deposited on the
stigma, or tip of the pistil ; here it will sprout and send
down a tube within the pistil to reach and fertilize an
ovule (Fig. 44, 2, j, and 5), which then becomes a seed
capable of producing a new plant. Now, it has been
observed over and over again that if a pistil is impreg-
nated with pollen from another plant the new plants com-
ing from the seeds thus fertilized will often be stronger
and more vigorous than if they had been developed from
seeds fertilized by pollen from the same plant that formed
the seeds. Cross-fertilization, as it is called, is advanta-
geous. Self-fertilization does occur, but it is important for
most species that cross-fertilization should come in every
few generations at least.
Different methods of fertilization are adopted by differ-
ent kinds of plants. The flowerless plants have their own
methods, and the flowering plants usually different ones.
We are here interested only in the means of securing fer-
tilization adopted by the flowering plants. Some of these,
like the pines and other evergreen trees, have an enormous
amount of pollen which is cast out into the air in great
clouds and is carried by the winds to the female cones,
there to fertilize the ovules (Plate 87, A}. There are many
such wind-fertilized plants, the palms and grasses, as well
as the cone-bearing trees, being familiar examples. These
do not use insects to aid in carrying pollen to fertilize
their ovules, and so, as every one knows, they have no
brilliantly colored blossoms (Plate 87, ^).
By far the larger number, however, of our common
flowering plants are aided in securing fertilization by the
154
ORGANIC EVOLUTION
insects which visit their blossoms. The petals of the
flowers usually secrete nectar, which is attractive to insects,
and many blossoms have an odor which also serves to
attract insects. The nectar is a sweet fluid secreted by
small glands, or nectaries, on the bases of the petals. It
is this nectar which bees gather and make into honey.
The odors of blossoms are caused by the presence of
volatile oils usually also secreted by the petals. These
odors may be
such as are agree-
able to our nos-
trils, as are the
odors of the rose,
the sweet violet,
the trailing arbu-
tus, or they may
be to us disa-
greeable, like the
odors of the car-
skunk-cabbage ;
but, whether agreeable to us or not, they serve to secure the
visits of insects, and it is apparently because of this attrac-
tiveness to insects, and the advantage of cross-fertilization
in which the insects aid, that these odors and the nectar
have been developed. Many insects also seek the pollen
in the blossoms, using it as food, and most plants form more
pollen than is needed to fertilize their ovules, thus having
a surplus supply upon which insects may draw without much
or any injury to the plant.
The insects which come to the blossoms to gather pollen
FlG. 45. — A bee, showing the hairs on the head, body, and legs.
Pollen grains are shown caught in the hairs on the legs.
COLOR IN PLANTS 155
or nectar, as they go from plant to plant, will carry with
them pollen dust clinging to their heads and legs and bodies
(Fig. 45), and by means of the pollen thus carried the later
plants visited will secure cross-fertilization. One might per-
haps think that the insect visitor would scatter pollen from
one plant on the pistils of the same plant, and thus cause
self-fertilization as a general rule, but there are three chief
ways in which this is commonly prevented.
Frequently the pollen and the ovules of a single plant
do not mature at the same time, so that self-fertilization
is prevented.
Many plants have the parts of their blossoms so ar-
ranged that the visiting insect will go to the nectar first,
without coming into contact with the pollen until he is
about to depart, when he will become dusted with the
pollen and carry it away on his visit to the next blossom.
Here, on the way to the nectar, he will brush against the
tip of the pistil and give to it some of the pollen he has
brought from the first plant, thus providing a means of
cross-fertilization. Blossoms are often remarkably modified
in form and structure to prevent in this way self-fertiliza-
tion. In a moment we will consider a few instances of
such modification.
The third and almost universal method of preventing
self-fertilization is a physiological one, the pollen from any
given plant being considerably slower to sprout on a pistil
of the same plant than it is upon the pistil of another
plant; thus, even though the pistil of any blossom be
dusted first with pollen from the same plant, if, later, pollen
from another plant be brought to the blossom, the later-
received pollen is likely to be that which will effect fertili-
156 ORGANIC EVOLUTION
zation, because of its more prompt sprouting and the more
rapid growth of its pollen tube.
Let us now observe a few illustrations of special adap-
tions in the form of blossoms, by which plants secure cross-
fertilization.
The flowers of Mitchella, the beautiful little partridge
berry of our woods, are adapted to secure cross-fertilization
by insects through having their stamens and pistils of dif-
ferent lengths (Plate 88). In all the blossoms of one plant
the stamens will be long and the pistils short, while in all the
blossoms of another plant these relations will be reversed, the
pistils being long and the stamens short. An insect visiting
these blossoms will have one part of its body dusted with
pollen from the short stamens and another part with pollen
from the long stamens. As it passes from blossom to blos-
som it will carry pollen from the short stamens of one flower
to the short pistils of other flowers, and the pollen from the
long stamens will be carried to the long pistils. In this way
cross-fertilization will be secured, since long stamens and
long pistils do not occur on the same plant, nor are short
stamens and pistils found on the same plant.
Many orchids show an interesting method of using insects
to secure cross-fertilization. In these species the stigma is
in the centre of the flower, while the anther with its two
pollen masses lies above the stigma (Plate 89, A, 2 and j).
The two pollen masses protrude a little, and at their protu-
berant ends are attached to a sticky "rostellum" (Plate 89,
A, 4). The corolla of the flower is so developed as to form
a flat landing-stage for the visiting bee or other insect (com-
pare Plate 90, C), and the rostellum protrudes into the
centre of the flower, above this landing-place, in such a way
B C
PLATE 88. — PARTRIDGE-BERRY (Mitchella rcpens).
A. Plant with blossoms and fruit. — From Goodale's Wild Flowers of America, by the courtesy
of Bradlee Widden. B. Blossoms with long pistil and short stamens. C. Blossoms with long
stamens and short pistil. — B and C from Bastin's College Botany, by the courtesy of G. P. Engel-
hard and Co.
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toward the mtr-
*irdbyat.
PLATE 89. — A. The fertilization of an orchid by a wasp. [After K.ERNER.]
I. Flowering spike of the broad-leaved helleborine {Epipactys latifolia) upon which a wasp is
alighting. 2. Flower of the same seen from the front. 3. Side view of the same flower, with the
half of the perianth toward the observer cut away. 4. The two pollen masses joined by the sticky
rostellum. 5. The same flower being visited by a wasp which is licking honey and at the same
time detaching with its forehead the tip of the rostellum together with the pair of pollen masses.
6. The wasp leaving the flower with the pollen masses cemented to its head ; the pollen stalks are
erect. 7. The wasp visiting another flower and pressing its forehead with the pollen masses (which
in the meantime have bent down) against the stigma, i, natural size ; the other figures, x 2.
B. Fertilization of Salvia by a bumblebee. [After K.ERNER.]
I. Part of an inflorescence of Salvia glutinosa ; the right-hand flower is being visited by a
bumblebee, and the pollen-covered anther is in the act of striking the insect's back. 2. Another
part of the same inflorescence with three open flowers in different stages of development; the left-
hand flowers are slightly more mature than the right-hand flower; one of the flowers is being
visited by a bumblebee which carries on its back pollen from a younger flower and is rubbing it
off on the deflexed stigma. 3. A single stamen, showing the hinge (K). 4. A vertical section
through a blossom ; the arrow indicates the direction through which bumblebees advance
toward the interior of the flower. 5. A similar section, showing how the anther is bent down-
ward by a bumblebee pushing against the bottom of the stamen.
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COLOR IN PLANTS
157
that the bee in entering the flower to reach the nectar
will press its head against the rostellum (5). When the
bee withdraws, the rostellum, with its two pollen masses,
sticks to its forehead (6\ and the pollen masses are thus
carried to the next blossom visited. At first the pollen
masses stand erect upon the forehead of the bee (5 and 6),
but, as the bee flies through the air, the stalks of the pollen
masses dry slightly and bend downward (7), so that, when
the bee enters another flower, the pollen masses are pressed
against the stigma. Thus cross-fertilization must be fre-
quently effected, the bees carrying the pollen not only from
blossom to blossom of the same plant, but also from one
plant to another.
The flowers of Salvia have adopted another and equally
interesting method of reaching the same result. In these
blossoms the stamens are hinged, and the lower end of the
stamen, below the hinge, is so placed that a bee, in entering
the blossom, will push against it, and in doing so will cause
the other end of the stamen with its pollen to drop down and
dust the back of the bee with pollen (Plate 89, B, /, j, ^, and
5). This pollen will be carried to the next flower visited by
the bee. Frequent cross-fertilization is secured by another
simple character of these blossoms. The pollen is mature by
the time the blossom bud opens, but at this time the pistil is
short and lies arched in the upper part of the blossom (/).
As the flower grows older, the pistil elongates and bends
downward, so as now to come into contact with any insect
which may visit the flower with its load of pollen, thus secur-
ing fertilization (2). As the pollen and the pistil are not
ready for fertilization at the same time, no blossom will be
self-fertilized ; and, as the insects pass frequently from plant
158 ORGANIC EVOLUTION
to plant, as well as from blossom to blossom of the same
plant, cross-fertilization between plants must frequently
result.
Aristolochia sipho illustrates another method of secur-
ing cross-fertilization through insects. The flowers of this
species are in the form of a bent tube with a flaring end,
something like a trumpet (Plate 90, A]. Flies enter the
opening of this tube, but find their egress prevented by a
mass of hairs (Plate 90, B] which fills the tube, pointing
toward its base, allowing the flies to enter but not to
depart. The stigma of these blossoms is a large top-shaped
structure, nearly filling the base of the tube. Behind it,
and inaccessible to the flies, are the three biscuit-shaped
anthers with their pollen. The swollen stigma shrinks as
the flower grows older, and if the flies which have entered
have brought pollen with them and have fertilized the
stigma its shrinking is hastened. After the stigma has
shrivelled, the flies, as they wander about their prison, can
reach the pollen and will become well dusted with it. Now
the hairs which have prevented their departure dry and
shrivel and the flies are set free to seek another blossom
and fertilize its ovules.
Each of these general methods of securing cross-fertili-
zation which we have illustrated is used by a considerable
number of plants, and there are scores of other devices to
which we have not space to refer. Many of these are vividly
described, with good pictorial illustrations, in Kerner's The
Natural History of Plants, the English translation of which,
by Oliver, is published by Henry Holt & Company.
Enough has been said to emphasize the importance to
the plant of insect visits. We have seen that by the secre-
COLOR IN PLANTS 159
tion of nectar and odoriferous oils, and by the formation of a
surplus supply of pollen, plants invite the visits of insects,
and that they sometimes adopt remarkable means to secure
cross-fertilization by the aid of the visiting insects. Careful
experiments have been made by numerous competent
students to determine if color in itself is recognized by
insects of different sorts. These have established the fact
that color is recognized by insects of many kinds, and that
to certain species of insects different colors have different
degrees of attraction. Also it has been shown that the most
attractive color is not always the same for two species of
insects.
Lord Avebury's experiments upon bees are worth our
attention for a moment, as an illustration of the methods
which have enabled us to draw these conclusions. In a
very brief summary of an extended series of experiments
Lord Avebury says : " I placed slips of glass with honey
on papers of various colours, accustoming different bees to
visit special colours, and when they made a few visits to
honey on paper of a particular colour, I found that if the
papers were transposed the bees followed the colour."
Describing another series of experiments, he says : " I took
slips of glass of the size generally used for the microscope,
viz. three inches by one, and pasted them on slips of paper
coloured respectively blue, green, orange, red, white, and
yellow. I then put them on a lawn, in a row, about a foot
apart, and on each put a second slip of glass with a drop of
honey. I also put with them a slip of plain glass with a
similar drop of honey. I had previously trained a marked
bee to come to the place for honey. My plan then was,
when the bee returned and had sipped for about a quarter of
160 ORGANIC EVOLUTION
a minute, to remove the honey, when she flew to another
slip. This then I took away, when she went to a third ; and
soon. In this way — as bees generally suck for three or
four minutes — I induced her to visit all the drops succes-
sively before returning to the nest. When she had gone to
the nest I transposed all the upper glasses with the honey,
and also moved the coloured glasses. Thus, as the drop of
honey was changed each time, and also the position of the
coloured glasses, neither of these could influence the selec-
tion by the bee.
" In recording the results I marked down successively the
order in which the bee went to the different coloured
glasses. For instance, in the first journey from the nest, as
recorded below, the bee lit first on the blue, which accord-
ingly I marked i ; when disturbed from the blue, she flew
about a little and then lit on the white, which I marked 2 ;
when the white was removed, she settled on the green, which
was marked 3 ; and so on successively on the orange, yellow,
plain, and red. I repeated the experiment a hundred times,
using two different hives — one in Kent and one in Middle-
sex— and spreading the observations over some time, so as
to experiment with different bees and under varied circum-
stances. Adding the numbers together, it of course follows
that the greater the preference shown for each colour the
lower will be the number standing against it.
" The following table gives the first day's observations in
extenso : —
COLOR IN PLANTS
161
"JOURNEYS
BLUE
GREEN
PLAIN GLASS 1 ORANGE
RED
WHITE
YELLOW
I
,
3
6
4
7
2
5
2
5
4
7
6
I
2
3
3
i
4
7
6
5
3
2
4
2
4
6
7
5
i
3
5
I
4
7
2
6
5
3
6
I
2
3
6
5
4
7
7
2
I
4
7
3
5
6
8
3
4
6
2
7
5
i
9
5
i
7
4
6
3
2
10
i
6
7
5
3
2
4
ii
4
6
5
2
7
3
i
26
39
65
51
55
35
37 "
The order of preference here indicated is, we see, be-
ginning with the most favored, blue, white, yellow, green,
orange, red, and the plain glass. A much larger number
of experiments by the same method gave the following
figures: blue 275, white 349, yellow 405, red 413, green
427, orange 440, plain glass 491. We may say, then, that
bees show a strong preference for blue, that they like
white next, and that yellow, red, green, and orange are
about equally attractive, and are all preferred to uncolored
objects.
Other experiments by Lord Avebury show that wasps
have a decided color sense and are able to distinguish
vermilion, orange, blue, white, yellow, and green, but that
they do not show a very decided color preference. Similar
results have also been obtained by Dr. and Mrs. Peckham.
Experiments upon most other insects are more difficult
to perform, for they do not have nests in which they live
together and to which they return after each hunting trip,
or in which they store honey, returning time after time to
1 62 ORGANIC EVOLUTION
the flowers for nectar. Most insects eat their fill and
then fly away and do not return. It is possible, though, by
observation of flowers in nature to determine what kinds
of insects are their most frequent visitors. In this manner
we can determine that " white flowers are especially visited
by small flies ; that flowers which depend upon beetles
for fertilization are frequently yellow ; that those which
especially bid for the favor of bees and butterflies," the
nectar gatherers par excellence, "are usually red, purple,
lilac, or blue."1
Since the visits of insects are so valuable to plants in
securing cross-fertilization, it is easy to see that natural
selection would be likely to bring about the bright colora-
tion of flowers ; and, as insects of different kinds have
different color preferences, the color of any sort of flower
is likely to be such as to attract the kind of insect best
adapted to secure its cross-fertilization. And, in general,
we may say that the observations upon the colors of flowers
agree with these conclusions.
The most assiduous honey gatherers are the bees and
the butterflies, and it is interesting to observe that the
most highly specialized flowers in the different families of
plants are usually red or purple or blue, being thus espe-
cially attractive to these insects whose preference is for
these same colors.
Much has been written about other principles in the
coloration of blossoms, their original color, the order of
development of the several colors, the way in which new
colors arise, the parts of the petals upon which these new
colors are most likely to appear, the meaning of variega-
1 Grant Allen, The Colours of Flowers.
EVOLUTION OF MAN 163
tion in the colors of petals, the colors of degenerate
blossoms, and many other subjects of much interest; but,
as the conclusions to be drawn from the great number
of observations are still somewhat in dispute, it seems
unwise for us to attempt further discussion along these
lines.1
There is one further thing in this connection to which
it is well to call attention. Many highly specialized flowers
have developed unusual shapes so as to cause the visiting
insects to enter the blossoms by the path most likely to bring
them into contact with the pistil and the pollen in such a
way as to insure cross-fertilization, and have provided
special lighting spots or platforms for their visitors (Plate
90, C\ compare Plate 89), and these are often spotted and
streaked in such a way as to make them conspicuous. More
interesting still is the fact that these streaks are usually so
arranged as to point the way to the nectaries, guiding the
insect along the right path, the pistil and the anthers being
so placed as to come into contact with the body of the insect
in the most advantageous manner as it passes along this
prescribed way.
MAN IN RELATION TO EVOLUTION
Naturally the subject of the relation of humankind to
evolution is one of particular interest to us. Study of
human anatomy shows mankind to be probably a single
species, belonging to the Primates, a group of the Mam-
malia, including, besides man, the lemurs, and the apes and
1 The reader will find Grant Allen's The Colours of Flowers, which treats of
these subjects, a most interesting and suggestive book.
1 64 OR GANIC E VOL UTION
monkeys of the eastern and western hemispheres. Man is
most nearly related to the Simiuke, the tailless apes of
Asia and Africa, including the gibbon, the orang, the
chimpanzee, and the gorilla. It is usual to place human-
kind in a distinct family of Primates, Hominidce. It is now
the general consensus of opinion that we should recognize
but a single species and distinguish as subspecies the sev-
eral races of men.
As an illustration of some of the reasons for asserting
that men are primates and are closely related to the Simi-
idcz, glance at the illustration of the skeletons of representa-
tives of four genera of Simiidtz and of man (Plate 91, A).
Part for part the skeletons are the same in all fundamental
regards. Look at but a single group of bones, those com-
posing the pelvis (Plate 91, B). The larger bones, the
sacrum, and the coccyx show the closest resemblances in
man to what we see in the gorilla. The relative size and
shape is slightly different, and man has lost one of the
coccygeal bones still seen in the gorilla, but in all essential
features the two sets of bones are closely similar. Similar
comparisons with a similar result might be made between
the hands, feet, sterna, ribs, spinal columns, teeth (Plate 92,
A\ bones of the skull, etc.
But let us turn to structures other than the skeleton.
Passing by the close resemblance between the vital organs,
the muscles, and the other important organs (Plate 92, B\
observe again some of the remarkable similarities in certain
minor details, to some of which we have before referred.
We think of the hairiness of the apes as distinguishing
them rather sharply from man, but in reality the whole of
the human body is covered with hair, save the palms of the
ftft&ra.
PLATE gi. — A. Skeletons of man and four apes. [After HUXLEY.] i. Man. 2. Gorilla.
3. Chimpanzee. 4. Orang. 5. Gibbon. B. Pelvis of man, gorilla, and gibbon. [After
HUXLEY.]
Man.
GcriUa.
Chimpanzee.
B
PLATE 92. — A. Teeth of man and gorilla. [After HUXLEY.] B. Cerebral hemispheres of man
and chimpanzee. [After HUXLEY.]
PLATE 93. — Hair tracts on the arms and hands of a man and a male chimpanzee. Drawn
from life. Observe that in the corresponding regions the direction of the slope of the hairs is the
same. — From Romanes' Darwin and After Darwin, by the courtesy of The Open Court Publish-
ing Company.
PLATE 95. — A. Head of foetus of an orang-outang; observe the pointed ear. [After DAR-
WIN.] B. A human ear in which a point is present upon the recurved edge. [After DARWIN.]
C. Front and back view of an adult human sacrum, showing an abnormal persistence of vestigial
tail muscles. — From Romanes' Darwin and After Darwin, by the courtesy of The Open Court
Publishing Company.
FCETAL
PLATE 96. — A. Muscles of the human ear. — From Gray's Anatomy. B. Vermiform appen-
dices of orang, man, and human foetus. — From Romanes' Darwin and After Darwin, by the
courtesy of The Open Court Publishing Company.
EVOLUTION OF MAN 165
hands, the soles of the feet, and the backs of the terminal
joints of the fingers and toes ; and these same portions are
naked in the apes. Not only does hair clothe the whole
human body, the slant of the hair in the several regions of
the body is the same that we observe in the apes (Plate 93).
Therefore, even to minute details, the apes and man can be
compared as to the presence and slope of hair; the only
considerable difference in the condition of the hair in the
two being in the length and the coarseness of the indi-
vidual hairs.
Observe another minute characteristic, one often seen
in human ears (Plate 94). In many monkeys the ears are
pointed and do not show any recurved edge such as is seen
in the ears of apes and men (ear of Barbary ape, Plate 94).
On the recurved edge of the human ear and that of apes
there is often a portion slightly more developed than the
rest, showing as a wider place (Plate 94), or even a point
(Plate 95, A and B] on the reflected edge. This corre-
sponds to the point seen in the ears of the lower monkeys,
only in their ears the point is erect, the edge of the ear not
being folded over.
The apes and man have the tail greatly reduced, it
being represented merely by the coccyx, a reminiscence of
the ancestral condition when functional tails were present.
It is interesting to know that there have been instances in
which a human being has retained in an abnormally highly
developed condition the muscles which represent the func-
tional muscles of this ancestral tail (Plate 95, C). In a
similar manner, while our ears are slightly, if at all, movable,
we retain in a vestigial condition the muscles which in some
ancestor must have served to move the ears (Plate 96, A}.
1 66 ORGANIC EVOLUTION
The vermiform appendix is less developed in man than
in the apes, and in an adult man is relatively smaller than
in the human foetus (Plate 96, B\
At the inner angle of the human eye is a fold of tissue
called the plica semilunaris. This is a remnant of that
third eyelid which in many lower vertebrates, notably the
birds, is greatly developed and can be drawn over the whole
eyeball, inside the outer eyelids (Plate 97).
These vestigial structures in man have little or no mean-
ing until in them we recognize the traces of an earlier con-
dition through which our ancestors have passed.
In human embryology there is every indication that we
must regard man as closely related to the rest of the ani-
mal kingdom. A little study of the illustrations of the
embryos of man and a number of other vertebrates will
bring out this resemblance in their embryology, and the
fact that the human embryo, in the earlier stages of its
growth, has many features which are a reminiscence of its
fishlike early ancestors (Plate 98). In the later develop-
ment of the human child, after birth, there are .a number
of things that are instructive in this connection. In a baby
the spinal column has a single curve, as it does in the apes
and monkeys, instead of the S-shaped curve seen in the
adult human being (Plate 99). The feet are held in a
position characteristic of the apes (Plate 100). For a few
weeks after birth, the child has a remarkably strong finger-
grip, recalling the strength with which the young apes grasp
the mother's hair, as she climbs with them among the trees.
The young human baby is able to sustain its own weight
by its hands, and, when hanging thus, shows often a posi-
tion of the legs which is strikingly apelike (Plate 100, B\
PLICA
SEM-ILUNARIS
PLATE 97. -Eyes of various vertebrates, showing the nictitating membrane (third eyelid),
indicated by the letter N. — From Romanes' Darwin and After Darwin, by the courtesy of
The Open Court Publishing Company.
PLATE 99. — A and B. Diagrams illustrating the curvature of the spinal column in a human
infant (A) and an adult man (B). The curvature of the spinal column in an ape (C) resembles
that in the human infant. (Compare the upper figure in cut C of this plate.) C. A group of
gorillas, male, female, and young ; observe the position of the feet in the female and in the young
gorilla. — From Brehm's Thierleben.
PLATE 100. — A. Foot position of a human infant. — From Romanes' Darwin and After Dar-
win, by the courtesy of The Open Court Publishing Company. B. Two human infants, ten and
thirteen days old respectively, supporting their weight by their hands. — From a photograph by
Dr. Louis Robinson, by the courtesy of The Open Court Publishing Company.
EVOLUTION OF MAN 167
The position of the legs after birth is, however, probably
largely due to the prenatal folded position of the legs.
We might develop to an indefinite extent these points
of anatomical and embryological resemblance between man
and other vertebrates. The character of the evidence, how-
ever, has been sufficiently illustrated. I know of no scien-
tific reason for separating man from the rest of the animal
kingdom as regards the processes of evolution. His whole
structure shows that he has arisen by differentiation from
lower vertebrates. We do not understand all the stages
by which his body has been thus evolved, nor do we know
in detail by what steps his mental faculties have arisen from
the lower condition of mind seen in other vertebrates; yet
we have, apparently, no reason for believing that the method
of their evolution has been different in any fundamental
regard from the methods by which the minds and bodies
of other animals have been developed. Comparative psy-
chology is as yet in its infancy, and we are not at all pre-
pared to discuss the relations between the mind of man
and the minds of lower animals, much less to attempt to
describe the steps in the evolution of the human mind. We
must wait a good many years before our curiosity in this
regard can be satisfied. There appears, however, no suffi-
cient reason for believing that the development of man's
mind has been anything other than natural and in accord-
ance with the principles that apply in the development of
the minds of other species. So far as we can judge, man
is the result of the same processes and factors that have
produced the bees with their wonderful instincts and the
tiger with his superb physique.
Not only has man been produced under the influence
1 68 ORGANIC EVOLUTION
of the factors of evolution, he is still subject to them and
is still being modified by them to-day. Disease and unfavor-
able climate kill those who are unable to resist them, while
the stronger survive. Men fail in the struggle for existence
and become submerged and disappear. Natural selection
is constantly removing those who are unable to resist the
pressure of the adverse conditions of life. This is the same
process we have seen among the lower animals and the
plants, and has the effect of making man more fit for his
surroundings by eliminating the less adapted.
Sexual selection also is operative, more so among man-
kind than in any other group of animals. There is closer
scrutiny and more careful choice is exercised in human
marriage than in the mating of any of the lower animals.
There is an important difference to notice. Among human-
kind, at least among more highly civilized men, choice in
marriage is based more largely upon intellectual and moral
attractions and less upon physical attractions than is the case
among lower animals. Among lower forms sexual selection
secures chiefly ornamentation or fine voice. Among men it
is more those of good intellect, of pleasing disposition, of
right character, who are chosen; sexual selection thus serving
to increase and perpetuate these characteristics.
Segregation also is an important factor in human evolu-
tion. The fact that the Chinese live in Asia and the negroes
in Africa, has prevented intercrossing between these two
races, which, if it had taken place, would have changed the
character of both races. In any community there are many
important segregating factors. There is in America a well-
nigh universal distaste toward marriage between negroes and
Caucasians, and this has had an important effect upon the
EVOLUTION OF MAN 169
development of the two races. Intermarriage between those
of different social strata is unusual, culture and wealth thus
effecting segregation. Religious belief has had an important
effect in causing segregation in marriage. It would be im-
possible to enumerate all the efficient causes of segregation
among humankind.
Let us look a little further at man's relation to natural
selection and sexual selection. First as to natural selec-
tion : While man, like all other animals, is subject to natural
selection, he is less so than any other species, so far as physi-
cal factors are concerned. Our great intellectual develop-
ment enables us to escape from many phases of the struggle
for existence. We build houses which protect us from the
inclemency of the weather. We have fires to protect us
from the cold of winter. We cook our food, thus largely
escaping the internal parasites which so commonly infest the
lower animals. Wre have physicians who enable us to sur-
vive diseases which otherwise would destroy us. By cultiva-
tion of the soil and by raising flocks and herds we increase
the productiveness of the earth, making it support a far
greater population than would otherwise be possible. When
crops fail in certain localities, whole nations are saved from
extermination by the great development of our means of
transportation, which bring food from distant regions to save
the starving. In thousands of ways we are relieved by our
greater intelligence from much of the stress of the struggle
for existence. Natural selection plays a less prominent part
among men than among plants and the lower animals.
Of course this partial elimination of natural selection is a
very great advantage, producing inestimable good to man,
170 ORGANIC EVOLUTION
yet there are disadvantages as well. By means of our well-
warmed houses we protect ourselves from rain and cold, and
thus save from death many delicate ones who would other-
wise perish. But by preserving these weaker ones we allow
them to hand down to the next generation their weak consti-
tution, and so the race will average less robust than it would
be if the weak ones had been allowed to succumb to the cold
and so had never had offspring. Similarly the physician
saves from death many a weakling whose children bring
down the average of physical efficiency in the next genera-
tion. Physical deterioration has resulted from the partial
elimination of natural selection. Invalids are rare among
the lower animals : they are rare among savage races. How
common they are among us ! The invention of spectacles
has allowed our eyes to deteriorate without putting us at a
serious disadvantage. The skill of the dentist has tended
toward unsound teeth for civilized man. Such instances
might be multiplied.
One point here should be clearly seen. Natural selection
seeks the highest efficiency of the species as a whole, and to
this end sacrifices innumerable defective individuals, lest they
and their children bring down the average of efficiency.
We, on the other hand, seek the welfare of the individual
and preserve and cherish the weak, though we know that by
so doing we in the end decrease the vigor of the race. Be-
cause of our charitable and altruistic tendencies we preserve
also the intellectually and morally weak, and thus cause a
certain intellectual and moral deterioration in the race aver-
age. I believe this is very largely compensated for by other
considerations, yet the deterioration is no less real.
A good illustration of the effect of natural selection in
EVOLUTION OF MAN 171
connection with disease is seen in the relation of savage
peoples to certain mild diseases prevalent among civilized
races. Measles is not very serious in civilized communities.
It has long been a common disease. Those, in the past,
who were unable to resist this disease have died ; and, as
it is mostly a disease of children, they have died before
reaching adult life and becoming parents. They have,
therefore, not transmitted to the next generation their consti-
tution with its slight powers of resistance to this disease.
Many of those children, on the other hand, who have
been strong enough to survive attacks of measles have
reached maturity and have handed down to their children
something of their natural ability to resist its attacks. There
has thus been developed among civilized peoples a consider-
able degree of power to throw off this disease.
But among savage races, the North American Indians,
for example, measles has often been a fearful scourge. It
has not been prevalent among them for many generations,
as among the white peoples, and they have not acquired
through natural selection the ability to resist it. Therefore,
it was but natural that when introduced among them it
should wipe out whole communities, slaying adults as well
as children.
Were vaccination now to be universally given up, it is
possible that small pox would be more dangerous than it
used to be before Jenner found a way to save us from its
ravages ; though perhaps vaccination has not been used
long enough to allow much deterioration in the power of
resistance to small pox which was to a degree acquired dur-
ing those centuries when the disease had free course.
How far will the deterioration which results from par-
172 ORGANIC EVOLUTION
tially freeing ourselves from the action of natural selection
go? It cannot go on indefinitely. Natural selection still
eliminates those who are physically very defective ; so also
sexual selection will operate against the perpetuation of
physical deformity and great physical weakness. We need
not fear the extermination of the race through freeing our-
selves from the action of natural selection. I think, how-
ever, that we must anticipate a still further physical
deterioration of humankind, not only in such minor points
as our teeth and eyes, but in all regards, invalidism becom-
ing more and more prevalent as medical skill advances.
There is another profitable inquiry as to our relation to
natural selection. What is the nature of our environment
to which we must conform in order to survive and prosper
and succeed in giving our children favorable opportunities ?
The environment of lower animals and plants is made up
of many elements that have a bearing upon their lives —
climate, food and drink, enemies, disease, etc. We have
the same elements in the physical environment to which
we have to relate ourselves, but in addition we have another
factor, perhaps as important as any, namely, public opinion.
Unless we conform to a certain standard of intelligence,
moral character, and good taste we find ourselves at a dis-
advantage in life, and have to struggle hard to maintain
ourselves and care for our children. The man who in any
or in all of these ways is far in advance of his fellows, or
the one who falls much below popular standards, feels the
pressure of life more than he who conforms to the popular
ideas of right character and good taste. Conformity to
public opinion is of great importance if one desires the
E VOL UTION OF MAN I 7 3
best chance of survival for himself and family. Public
opinion is a vitally important part of our environment.
It is not only important as regards natural selection ;
it is perhaps even more important in relation to sexual
selection. A man or woman, to be desired as a husband
or wife, must, in general, be one whose ideas of right living
conform to those of the community, one whose character
and disposition are such as to command respect. These
characteristics have more influence upon choice in marriage
than do merely physical characteristics.
It may be worth our while to ask one further question.
Under present conditions, how is the race to make desirable
progress ? How can we influence the evolution of the race,
so that it shall take the right direction ? Notice, first, that
the very asking of this question indicates an interesting con-
dition. We can, to a considerable extent, control our own
evolution. The lower animals cannot do so. They lack
the intelligence which gives us this power.
How shall we secure the evolution of the race in desir-
able directions ? Before attempting to discuss this question
it is important to distinguish clearly between human evolu-
tion and social progress. By evolution, as we here use the
term, we mean a change in innate character. Social progress
may be secured by training the individuals of each succeed-
ing generation to higher and higher standards of living,
even while no change in the innate character of the race
has been brought about.
The distinction we would emphasize can be easily illus-
trated. If a savage should receive some suggestion that
should cause him to improve his standard of living, his
174 ORGANIC EVOLUTION
whole family would be benefited. The son born into this
family would receive by education the knowledge of the
better way of living. He would, naturally, during his own
lifetime, learn still more, making the life of his family a little
more comfortable than was that in his father's home. His
son would therefore be born into a more favorable family
environment than that in which he passed his own early
life. Thus from generation to generation, through experi-
ence, the results of which would be handed on by education,
the standard of living would be improved in the families of
the descendants of this savage. Great progress might be
thus made without any change in the inborn nature of the
children from generation to generation.
Continuing the illustration, we may suppose a child of
the tenth (or thousandth) generation to be stolen from its
parents at birth and removed from the improved family en-
vironment, to be taken to a primitive savage home similar
to that of his savage ancestor with whom our illustration
started. We have no reason to believe that under these
circumstances the higher culture of his ancestors for nine
generations would cause him to lead any better life than
if his ancestors had remained primitive savages. The nine
generations of advancing culture secured by education need
not have produced any change in innate character in the
descendants. The social progress may have been secured
without any real evolution.
Social progress and evolution may, therefore, be very
different things. The former is secured chiefly through the
transmission by education of the knowledge and moral tone
reached through experience, and by the summation genera-
tion after generation of these increments of progress. Evo-
E VOL UTION OF MAN I 7 5
lution of the race, on the other hand, is a fundamentally
different thing. It will be secured by the same methods
which are operative to produce evolution among the lower
animals, i.e. through natural selection and sexual selection,
influenced of course by segregation. We have seen that it
is, to say the least, very doubtful if parental modifications
are inherited. We have no reason to believe that the
progress in culture, secured by education in one generation,
will directly improve the innate character of the children
of the next generation.
Were the effects of education inherited, human evolution
should be rapid, but it has been slow ; how slow perhaps few
of us realize. We speak with pride of the advance in human
civilization, of our progress in the arts and in useful knowl-
edge, of the improvement in morals and the growth of altru-
ism, and this all makes us blind to the fact that since the
dawn of history there has been no very great real evolution
of mankind. We reach larger results in the problem of life
than did our progenitors five thousand years ago, but we are
able to do so because we build upon their experience and
that of all the generations between.
Have we much greater innate powers ? Are we at birth
endowed with characters having much higher possibilities
and much higher tendencies physically, intellectually, and
morally ? Have we to-day men of much greater physical
prowess than the ancient conquerors of the world, than the
builders who constructed the monuments of Egypt ? Have
we more adventurous spirits or more successful explorers than
the Phoenicians, who without compass sailed the ancient seas,
reaching the whole Atlantic coast of Europe and the British
Isles, also passing southward even around the tip of Africa ?
1 76 ORGANIC EVOLUTION
Are there among us to-day men of keener inventive genius
than the one who first used fire, or the inventor of the lever
or of the wheel, or than the man who first made bronze or
smelted ore ? Our modern engines have been invented
screw by screw by successive builders, each building upon
the others' work. Have we to-day men of much larger legal
and social understanding than the ancient lawgivers who
forged the legal systems which still are the basis of our most
enlightened governments ? Have we poets whose genius
greatly transcends that of Homer or of the authors of the
books of Job and Ruth ? In aesthetic appreciation and in
the power of artistic expression in sculpture and architecture
we are degenerate compared with the Greeks.
Even in innate moral character have we greatly advanced ?
We are learning the lesson of altruism, but are we born with
a sturdier moral sense? If we could take a hundred thousand
infants from London or Chicago and, turning back the wheel
of time, place them in the homes of ancient Babylon, would
they reach a higher standard of righteousness or of altruism
than their neighbors ? How little evidence we have of real
evolution of mankind since^ the first emergence of the race
from the darkness of prehistoric times !
Whether or not we believe that man has advanced in
innate character during the last five or ten thousand years,
we can certainly say that the advance has not been rapid.
The zoologist thinks of the problems of evolution in periods
of geologic time, not in years. He sees decided change in
the ancestors of the horse, when he compares the Eocene
and Miocene fossil faunas. He would hardly expect to find
great progress in evolution indicated in the fossils found in
the last few feet, say, of the Miocene strata, which would
EVOLUTION OF MAN 177
represent a period of time equal to the five to ten thousand
years of human history.
Is it then hopeless ? Is there no probability of securing
real advance for man in innate character ? Must we content
ourselves with merely a veneering of civilization over the
fundamental savage nature ?
The questions asked in the last few paragraphs force
themselves upon the attention of any candid student of
human evolution. The author does not claim to be able to
furnish a complete answer to them, but he would make a
few suggestions.
Setting aside the inheritance of parental modifications,
of which we have no evidence, and whose reality seems so
improbable, we have the two factors — natural selection and
sexual selection, aided by segregation. From the action of
natural selection we in considerable measure escape. (Com-
pare pages 169 to 172.) Even from the action of public
opinion, one of the most important elements in our environ-
ment, we in part escape by our adaptability. One whose
innate character is unsound may be trained to so conform, at
least outwardly, to the standards of the community that he
will be held in esteem and will succeed in rearing his family
in conditions of comfort. On the other hand, a boy of natu-
rally more desirable character may, by wrong training, be
brought into such relation to the community that he will be
destroyed. Survival in the struggle for existence among
humankind is influenced not by innate character alone, but
by what this character comes to be through training. This
greatly complicates the problem of securing, through survival
of the best adapted, an advance in innate character, i.e. true
evolution. The plasticity, or educability, of the human
178 ORGANIC EVOLUTION
being preserves him from destruction in the struggle for life.
Natural selection secures the preservation of the more plas-
tic, and this, in turn, makes it still more difficult to secure
advance in innate character.
Likewise sexual selection, choice in marriage, among
humankind is based not alone on innate character, but upon
what the character has become through training. This again
hinders advance in innate character through sexual selection.
But however powerful training may be in determining
the character of the adult man or woman, still the innate
character does count, and in the long run both natural selec-
tion and sexual selection should tend to modify it. The
child with weak body may by training become a strong
man, yet, in general, it is true that the strong children
make the strong men. So also a child of inferior intel-
lectual endowments may by proper culture become a man
of considerable intellectual development, yet on the whole
it is true that men of high mental power were probably
boys of good intellectual capacities.
We know less about innate moral character, still it seems
to be true that men differ greatly in their innate moral sound-
ness and moral sensitiveness. There is much evidence in
favor of the belief that one of mediocre moral endowments
may by proper training become a man of moral power, yet
here again it seems to be true that, in general, innate moral
capacities are correlated with high moral attainments.
If, therefore, there is such a general correlation between
innate capacities and attainments, whether physical, intellec-
tual, or moral, it must follow that, in so far as natural and
sexual selection operate, they will tend gradually to modify
innate character in these three aspects.
EVOLUTION OF' MAN 179
Believing then that, in spite of all deterrent influences,
both natural selection and sexual selection do operate slowly
to produce modification in innate character, let us ask again
the question : Can we so control this evolution that it will be
in desirable directions, and, if so, how can it be controlled?
Let us elevate the standards of public opinion by every
means in our power, and then natural selection and sexual
selection, which are greatly influenced by public opinion,
will secure the evolution of the race. The progress will be
slow, painfully slow, but it will be real. This does not mean
that we shall cease trying to improve individuals. Each
individual, who is led to a more desirable attitude toward
life, will act as leaven in the community in which he lives,
raising somewhat the standards of the whole community. I
believe that in the continued influence of Jesus we find the
greatest force tending to the improvement of the individual
character and to the elevation of public opinion, and so to
the evolution of mankind in desirable directions.
Improvement in social conditions, even though reached
through improved education, generation after generation,
rather than by advancing the innate qualities of the race, is
of course a most worthy object for which to labor, and it is
comforting to find that there is hope that such efforts may,
in the course of thousands of years, improve also the innate
fibre of the race through the effect which the advance of
public opinion will have upon natural and sexual selection.
To those who have faith in immortality, work for the
improvement of the individual assumes added importance
irrespective of its relation to evolution.
We have referred to the relative importance of sexual
selection, choice in marriage, in the evolution of mankind.
l8o ORGANIC EVOLUTION
This point deserves practical emphasis. In choosing a wife
a man is selecting the mother of his children as well as a
companion for himself, and he should think as much and
more of those qualities that tend to make a good mother
as of those which will make an agreeable companion. A
woman in accepting the responsibilities of marriage should
look forward to her children's welfare and think as much of
the father she is giving to her children as of the husband she
is accepting for herself. I believe that love is the chief con-
sideration, and that it would be a serious misfortune to have
this relegated to the background, as it is among so many
peoples. Fortunately this seems unlikely ever to occur in
America. Yet all important as is love, the essential foun-
dation in marriage, it is not the only thing. The welfare
of the coming generation is bound up in the choices in
marriage of the present generation, and this fact should
never be forgotten. There are those who because of physi-
cal, intellectual, or moral disability should not be parents,
and there is need of a general public sentiment which will
recognize it as a sin against society for such to seek their
own happiness in marriage when unable properly to meet
the responsibilities of marriage, of which the bearing and
rearing of children are a vital part. In spite of the senti-
ment in much of our poetry, our novels, and the drama that
love is supreme and therefore all else should be sacrificed for
it, it is really selfish and evil to regard only present happi-
ness and forget the coming generation.
I believe that gradually this ideal of responsibility to the
race will work its way more and more into the social mind,
and a larger thoughtfulness before entering into marriage
will result. It will come first in our great literature, but it
EVOLUTION OF MAN l8l
will leaven all society in time. More strict statutory limita-
tions upon marriage may ultimately be wise, but these will
not now secure the desired result. This will be reached
only through a larger general recognition of the responsi-
bilities in marriage, and the worthiness and beauty of
unselfishness here as everywhere else. Thus, in time, choice
in marriage may do much to counteract the hurtful influence
of having freed ourselves from the stress of the struggle for
existence.
One good influence upon choice in marriage is being
felt through a comparatively recent change in the lives of
women, outdoor sports and outdoor life in general having
become so much more popular. Riding, tennis, golf, the
bicycle, bird study, nature observation, and the love of nature,
all are tending to take more and more women into the open
air. These things are perceptibly changing the ideas of
what constitutes attractiveness in a woman. It is now
somewhat the case, and seems likely to be more largely true,
that the girl who, because of physical incapacity, cannot
share in this vigorous, healthful, outdoor life, will be at a
social disadvantage. This is but one way of saying that
considerations of physical vigor will increasingly influence
choice in marriage, and this, of course, will be for the wel-
fare of the race.
It is interesting to think what might be the result if there were started a
sect in which careful choice in marriage, under the advice of those most able
to discern hereditary tendencies, should be regarded as a sacred obligation,
looking toward an increasing perfection of the race in all respects, physical,
intellectual, and moral. It would probably be easy thus to raise human
stature to eight or nine feet or more, to very greatly increase muscular power
and agility, to very largely do away with invalidism, to increase the mental
capacity to an indefinite extent, and at first thought it would seem easy to
1 82 ORGANIC EVOLUTION
secure a race with finer and firmer moral fibre. Yet there would be serious
difficulties in the way. This, which is the logical goal of socialism, would be
likely to mar the beauty of family life, which is dependent upon a peculiar
mutual attraction between individuals, that cannot be dictated. The time
may possibly come when individuals will so cordially recognize their responsi-
bility for the advancement of the race that choice in marriage will look to the
welfare of the race as a whole, rather than to that of the family, as the chief
goal ; but this will not come in our day or before there has been wrought in
men a most far-reaching change in life ideals. We have reached the stage in
which there is more or less general recognition of the fact that in marriage
the welfare of the family rather than that of the individual should be sought
by all intelligent and right-minded persons ; but it seems impossible that the
welfare of the race can ever be secured at the sacrifice of the beauty of the
family life ; and it is a question whether the advancement of the race physi-
cally, intellectually, and morally, by choice in marriage, directed chiefly to
that end, can be secured without lessening the beauty of family life. The
elevation of general standards of opinion as to what constitutes attractiveness
in a man or woman, so that these shall include physical, intellectual, and
moral soundness and beauty, will cause choice in marriage to operate for the
perfection of the race along these lines, desire and duty combining to pro-
mote the progress of the race. It is apparently hopeless to accomplish much
in this direction by cultivating the sense of duty at the expense of love. A
family founded upon the sense of duty and not upon love would not be the
best soil in which to cultivate the most beautiful elements of character.
An objection might be made to the idea of evolution
among men through the action of sexual selection similar to
that which was made to the effectiveness of sexual selection
among lower animals, — namely that, to secure evolution
in the desired direction, public opinion must be so strong
that few but those possessing the desirable qualities shall
succeed in marrying, a condition of whose coming we see no
present signs. But this objection is really without weight.
If men of fine stamina, physically, intellectually, and morally,
seek to marry and are accepted by women of similar charac-
ter, their children will in the end predominate over the off-
GENERAL CONSIDERATIONS 183
spring of the physically, intellectually, and morally weak, no
matter how many of the latter may marry, or how large be
their families. Comparatively few people are living to-day
who will have any descendants a thousand years from now,
and these are men of vigor and soundness, not only physi-
cally, but intellectually and especially morally, for nothing
will more surely bring a line of descendants to its close
than moral unsoundness. If the best among us should
marry the best, and generation after generation keep the
strain free from taint of weakness, real evolution in desirable
directions would be much more rapid. We need a more
wholesome ideal of character, so that we shall delight
in real strength, delight in men and women who in each
phase of their character have stamina and power. Strength-
ening this ideal and spreading it among men is the hope of
evolution into larger manhood.
SOME GENERAL CONSIDERATIONS
In closing this discussion of evolution let us emphasize
three general considerations. First, we should remember
that natural selection, the great factor in evolution, produces
adaptation to the conditions of the environment, and that
this does not by any means always imply an advance in com-
plexity of organization in plants and animals, or greater
development of the mind in animals. On the contrary,
degeneration, in the sense of simplification, often results
from the action of natural selection. To make this point
more vivid, let us look at an example of extreme degenera-
tion, so far as complexity of structure is concerned, and see
184 ORGANIC EVOLUTION
how, by its changed character, the animal in question is
more perfectly adapted to the environment it has chosen,
and is thus benefited.
Among the simpler Crustacea, in the same group with
the common ship-barnacles and goose-barnacles, there is a
genus of parasitic forms called Sacculina. These are fre-
quently parasitic upon the common crab. When seen upon
the crab they appear to be little more than soft bags full
of eggs, and no one would suppose that they were in reality
Crustacea and related to the crab itself (Plate 101, C. Sacc.}.
They show no hard outer covering, such as is seen in all
normally developed Crustacea, and from which the group
derives its name. They have no jointed legs as do other
Crustacea. There is nothing in their adult anatomy to
suggest that they are Crustacea. No one would think for
a moment of so classifying them, were it not for their
embryology, which clearly shows that they are descended
from forms which closely resemble goose-barnacles. In the
course of their embryology we see a larva, which is like that
usually found in the Crustacea, the so-called Nauplius
(Plate 101, A). This is followed by another stage in
which we see the animal resembles Cypris, one of the
Ostracoda, a group of lowly developed Crustacea (Plate
101, B\ Soon the little Sacculina larva passes through
this stage and comes to a higher condition when it is practi-
cally a little goose-barnacle. Now it leaves its independent,
free-swimming life and becomes attached to a crab, or
occasionally some other animal (Fig. 46, A\ Living at-
tached to the crab, as it does, the parasite has no use for
legs or any locomotor organs, and these are cast off. Sense
organs are not needed, and these are lost. There being no
PLATE 101. — Sacculina.
A. Its nauplius larva. #. The Cypris stage in its development. C The adult Sacculina para-
sitic upon a crab, to the under side of whose abdomen it is attached, and whose body is pene-
trated in all directions by the root-like processes of the Sacculina. [From WEISMANN, after
DEL AGE.] D. A larva which has crawled into the interior of the body of a crab where it is
rapidly growing as it feeds from the blood of the crab ; it is now an almost shapeless mass of
cells. E. A section through a mature Sacculina. Most of its body has been pushed out from
the inside of the crab and now protrudes to the exterior. There are no appendages or sense
organs, and the nervous system (g) is greatly reduced. The body contains little but the ovaries
(pv.) and testes (/.) full of eggs and spermatozoa. [After DELAGE.j
GENERAL CONSIDERATIONS
185
sense organs and no muscles to be controlled, the useless
nervous system becomes very much simplified (Fig. 46).
Apparently because of the protection thus afforded, the
Sacculina penetrates now within the tissues of the crab,
becoming an internal parasite instead of an external parasite
as at first (Plate 101, D]. While thus parasitic it gets its
food from the blood of the crab, which of course contains
much digested food ready to be assimilated. As digested
food is supplied for its use, the Sacculina has no need of
A B C
FlG. 46. — Development of Sacculina carcini.
A, Larva which has just become attached to the base of a hair (ti) on the surface of a crab.
It is throwing off its legs and part of its body. B, C, D. Further stages in the degeneration of
the Sacculina larva while attached to the outer surface of the crab. [After DELAGE.]
digestive organs of its own, and consequently these dis-
appear. Here, within the tissues of its host, relieved of all
need of gathering or digesting its own food, and freed from
the necessity of moving about from place to place by its own
energy, it has an abundant amount of energy to devote to its
growth and to the formation and maturing of its reproductive
elements.
The Sacculina soon becomes little more than a bag of
1 86 ORGANIC EVOLUTION
eggs and spermatozoa held together by a little soft tissue
which surrounds these germ cells. In this condition, appar-
ently to allow of its growth to still larger size, it begins to
protrude from the body of the crab, becoming in the end a
bag of considerable size held to the crab by root-like pro-
cesses that penetrate through the shell and into the body of
the crab, and take up nourishment from its blood (Plate
101, E). Soon the Sacculina bursts and the eggs are set free,
and each starts upon a new cycle of development similar to
that described.
Life under the conditions of parasitism is very easy, and
it is no wonder that many animals and plants have been
adapted to such life. Since many organs essential to the
welfare of self-dependent animals are useless to parasitic
forms, we find that parasitism is usually associated with the
loss of these useless organs ; or, in other words, we can say
that parasitism results in simplification. We have quoted
an extreme instance of simplification. There are other
cases of as great simplification of structure, but in most
instances the degeneration is not so pronounced. Phe-
nomena of degeneration, however, are not observed only in
parasitic forms but are very general, and animals which as a
whole are not degenerate, usually have some of their organs
degenerate. In our own bodies are many such degenerate
organs. (Skin muscles, except over the face ; ear muscles,
Plate 96; tail, coccyx, Plate 91 ; third eyelid, Plate 97; hair
of body, Plate 93 ; vermiform appendix, Plate 96, B ; and a
hundred others.) Many phenomena of simplification are just
as much the result of natural selection as are the phenomena
of increasing complexity of structure. Natural selection
brings about more perfect adaptation to the conditions of
GENERAL CONSIDERATIONS 187
life, no matter whether this more perfect adaptation be
secured through simplification or through elaboration.
Change in its conditions of life may render certain struc-
tures in an organism useless, so that natural selection will
cease to keep the structures up to their former highly devel-
oped condition. Simplification may therefore be due either
to cessation of the action of natural selection when an organ
o
has become useless or to the direct action of natural selec-
tion in cases in which simplification is advantageous.
A second principle of great importance, and one we have
already emphasized, is that natural selection secures the wel-
fare of the species and not that of the individual, unless the
welfare of the individual happens to be promoted by that
which brings about the welfare of the species. Nature is
socialistic, not individualistic, in the processes of evolution,
and this statement applies to her relations to humankind as
well as to her relations to plants and the lower animals.
Those races whose ideals of life are such as to bring men
into the most advantageous relations to their environment
will in the end prevail. But, by the most advantageous
relations to the environment, we mean such relations as will
most effectively secure the perpetuation and increase in num-
bers of the race, and do not mean to imply any moral signifi-
cance. It is interesting, however, to observe that nothing
promotes the preservation and increase of mankind more
than good morals, the foundation for which is, in great part,
respect for the general welfare.
A third general consideration : There are two great
factors in the processes of organic evolution, — first, the
1 88 ORGANIC EVOLUTION
nature of the organism ; and, second, the character of the
environment and its relation to the organism. Of the lat-
ter, the character of the environment and its relation to
the organism through the struggle for existence and in
other ways, we know much. Of the intimate nature of the
organism, however, we as yet know but little. We do not
even know whether the life processes are conducted in
accordance with the ordinary principles of chemistry and
physics, or in conformity to some more subtle " vital " prin-
ciples. There are many questions which we are unable to
answer because we do not understand the intimate nature
of living things. Are there inherent tendencies in the or-
ganism, leading it to evolve in certain directions rather
than in others, as St. George Mivart contended, or is its
evolution controlled by the needs created by the character
of the environment ? Such questions are as yet beyond
our ken, and we have no present prospect of soon being
able to answer them. It is possible that our knowledge of
evolution may very materially advance when our knowledge
of the life processes of living things becomes more intimate.
APPENDIX I
TRENDS IN EVOLUTION
THE possibility of the existence of definite trends in certain species leading
them to evolve in certain directions rather than in others is indicated by at
least two sets of phenomena.
I have referred (page 40) to the fact that we have some quite complete
series of fossils in which are seen gradual modification of structure, the several
steps of the modification being so slight as to be of doubtful " selection value."
Plate 46 shows such a series in the fossil horses, and Fig. 26, page 108, shows
an even more instructive series of fossil Paludina shells. It is difficult to be-
lieve that the gradual transformation of the latter was due to some advantage
from the possession of a rugose shell, an advantage sufficient to cause the
" selection" of each slightly more corrugated variety. This series of shells
seems to suggest an inherent trend toward greater rugosity.
Recent studies of variation have shown that inherent tendencies toward
modification in particular directions do exist in at least one species. De Vries
in Amsterdam, and MacDougal, at the New York Botanical Gardens, in
their careful and extensive experiments in rearing an evening primrose ((Eno-
thera lamarckiana) , found that the mutants which arose were of certain definite
types and that these same types appeared generation after generation in con-
siderable numbers (compare page 18). De Vries found seven mutants;
MacDougal, fourteen.
In the Amsterdam garden the mutant albida appeared in four generations
from lamarckiana parents, previous to 1902, 15 albida appearing in one gen-
eration, 25 in another, n in another, and 5 in another. The mutant nanella
appeared 5 times in one generation, and in other generations, respectively,
189
190 APPENDIX
3, 60, 49, 9, n, and 21 times. The mutants lata, oblonga, rubrinervis, and
scintillans appeared frequenty.
In the fourth generation along with 14,000 lamarckiana plants there ap-
peared 41 gigas, 15 albida, 176 oblonga, 8 rubrinervis, 60 nanella, 63 lata, and
i scintillans, all bred from lamarckiana seed. In the fifth generation, simi-
larly bred from pure lamarckiana seed, among 8000 lamarckiana plants were
found 25 albida, 135 oblonga, 20 rubrinervis, 49 nanella, 142 toz, and 6 scin-
tillans. In the fourth generation one plant in 80 was oblonga. In the fifth
generation one plant in 60 was oblonga. De Vries himself says, "A [par-
ticular mutation] therefore, is not born only a single time, but repeatedly,
in a large number of individuals and during a series of consecutive years."
The mutant oblonga differs from the parent species, lamarckiana, not in
a single feature, but in an elaborate complex of characters. The other mutants
likewise are distinguished from lamarckiana by a complex of characters rather
rhan by a single feature.
The mutations can hardly be entirely fortuitous if, for several generations,
out of every thousand offspring of pure lamarckiana parents, there appear
more than ten plants marked by the particular complex group of characters
which designate oblonga. Were oblonga demarcated from lamarckiana by
but a single character it would be remarkable to find it appearing repeatedly
and in such numbers. When we remember that it is defined by an extensive
series of characters differentiating it from lamarckiana and from all other
mutants observed, are we not led to the conclusion that mutation in QLno-
thera lamarckiana is not wholly fortuitous, but is to a degree predetermined,
that there is some tendency to the production of the oblonga and other types
in numbers much greater than would be secured by purely fortuitous and
indeterminate mutation ?
It seems of much interest that the evidence from paleontology, so long
emphasized by Osborn and other American students, in favor of determinate
variation (or mutation) should be borne out by such careful observations as
those of De Vries in so different a field of research.
It is possible that (Enothera lamarckiana is a hybrid and that its mutation
is due to its hybrid character. I know of nothing, however, to indicate that
this is the case.
These observed phenomena of determinate mutation suggest an explana-
APPENDIX 191
tion of such a series of fossils as we see in the horse or the Slavonian Paludince.
If variations (or mutations) tend to occur more in certain particular directions
than in others, then unless these variants are of a disadvantageous character,
so as to be destroyed by natural selection, there will likely ensue a modi-
fication of the species in the direction of these variants. It makes no difference
whether or not we understand the nature and cause of such a tendency to
variation in particular directions; the fact that such tendencies do exist, if
it be a fact, must affect evolution.
I believe that certain phenomena of paleontology and a few observations
of mutation indicate the existence in some species of such trends to modifica-
tion in particular directions. It is by no means probable that such trends
exist in all species. For all we know, they may arise in a species and persist
for a time and then disappear. We greatly need careful, extended, tabulated
observations upon the variation (and mutation) of many species to see if
variation is always fortuitous, occurring equally in all directions, or if, on the
other hand, the variations tend to group themselves and to be more numer-
ous in certain directions than in others. The observations upon (Enothera
lamarckiana are very suggestive, but are hardly extensive enough to give a
secure foundation to a theory of inherent trends in evolution.
GERMINAL SELECTION
Weismann, in some of his more, recent writings, has urged that such trends
do exist, and by his theory of " germinal selection" he has endeavored to
explain their persistence. Weismann believes that all the organs of the adult
are represented in the egg and spermatozoan by minute protoplasmic par^
tides which, as development proceeds, grow up each into its corresponding
organ. The evidence in favor of this conception is necessarily theoretical
more than observational, and can hardly be stated in the space at our disposal.
Those interested can find Weismann's own treatment of the subject in his
book The Germ Plasm and his essay Germinal Selection, also in his new
work The Evolution Theory.
Having postulated this high degree of organization in the germ cells, each
part representing a particular future organ, Weismann proceeds to attribute
to these several determinants, as he calls them, an active struggle for food.
192
APPENDIX
He says with Wilhelm Roux that just as animals contend with other animals
for food, so the organs in the body of any one animal contend with each other
for food, each taking what it can get, the stronger organs (nutritionally)
getting most, the weaker faring more poorly. He carries this principle even
further and says that the parts of a single cell are engaged in a similar rivalry
for food and that in the germ cells the determinants thus struggle with each
other for nutrition. Finally he suggests that when any determinant acquires
an advantage in this contest for food its success will give it added vigor, en-
abling it to become a still more successful rival to its neighboring determinants.
The effect will be cumulative and generation after generation the favored
determinants will continue to increase in vigor. Now, as each determinant
gives rise to some particular portion of the adult, that part of the adult will
be modified step by step as its determinant becomes more favored. The effect
upon the favored determinant in the germ tends to be cumulative, its success
increasing the more vigorous it becomes, and similarly the modification of the
adult will steadily increase. In this way, Weismann believes, the suggested
trends in evolution have arisen and persisted.
The theory is not so fanciful as this bald statement would make it seem.
It is certainly well worth consideration from any one who has genuine interest
in evolutionary problems.
PLASTICITY, "ORGANIC SELECTION"
If such trends in evolution exist, they suggest an interesting consideration
in connection with the plasticity of the individuals of certain species. We
have already seen (pages 27, 28, and 177) that the ability of organisms to adapt
themselves during their lifetime to conditions of disadvantage may enable
them partially to escape from the stress of the struggle for existence and per-
sist when, if less plastic, they would be destroyed. I cannot quite agree
with Morgan, Osborn, and Baldwin in the emphasis they have laid upon
this accommodation of the individual as a guide to the course of evolution
by natural selection. But if it be true that trends to evolution in particular
•directions occasionally arise in certain species, it is conceivable that the adapta-
bility of the individual members of a species might tide the species over a period
•of disadvantageous environmental conditions, giving time for some new and
APPENDIX 193
advantageous trend to appear. Such an effect is not only conceivable ; it seems
not unlikely that in some instances it may have been important.
I have said above that I cannot quite agree with Morgan, Osborn, and
Baldwin in the emphasis they have laid upon the accommodation of the in-
dividual as a guide to the course of evolution by natural selection. Before
commenting further on this suggestion, let me quote in part Professor Conn's *
statement of the principle of "organic selection," as this factor in evolution
has been called : —
" The essence of the theory of organic selection is, that these acquired
variations will keep the individuals in harmony with their environment, and
preserve them under new conditions, until some congenital variation happens to
appear of a proper adaptive character. The significance of this conception is
perhaps not evident at a glance. It may be made clear by considering, for
illustration, the problem of the development of habits and organs adapted to
each other. . . ,
" Perhaps a concrete case may make this somewhat obscure theory a little
clearer. Imagine, for example, that some change in conditions forced an early
monkey-like animal that lived on the ground to escape from its enemies by
climbing trees. This arboreal habit was so useful to him that he continued
it during his life, and his offspring, being from birth kept in the trees, acquired
the same habit. Now it would be sure to follow that the new method of
using their muscles would soon adapt them more closely to the duty of climb-
ing. Changes in the development of different parts of the body would in-
evitably occur as the direct result of the new environment, and they would
all be acquired characters. The children would develop the same muscles,
tendons, and bones, since they, too, lived in the trees and had the same influ-
ences acting upon them. Such acquired characters would enable the ani-
mals to live in the trees, and would thus determine which individuals should
survive in the struggle for existence, for those modified individuals would
clearly have the advantage over those that stayed on the ground, or did not
become properly adapted to arboreal life by acquired habits. All this would
take place without any necessity for a congenital variation or the inheritance
of any character which especially adapted the monkey for life in the trees.
"But in the monkeys thus preserved, congenital variations would be ever
1 The Method of Evolution, p. 308 et seq.
O
1 94 APPENDIX
appearing in all directions. It would be sure to follow that after a iime there
might be some congenital variation that affected the shape of the hands and
feet. These would not be produced as the result of the use of the organs or
as acquired variations, but simply from variations in the germ plasm. There
might be thousands of other variations in other parts of the body in the mean-
time. The miscellaneous variations, however, would not persist. But as
soon as variations appeared which affected the shape of the hands and feet,
the fact that the animal had continued to climb trees would make these varia-
tions of value, and therefore subject to natural selection. Selection would
follow, and thus in time the monkeys might be expected to inherit hands and
feet well adapted for climbing. The acquired variations, in such a case,
had nothing to do with producing the changes directly, but they did shield
the animal from destruction until congenital variations appeared. Acquired
variations have determined that the individuals shall live in trees, and this
life has determined what congenital variations will be preserved. Indirectly,
therefore, acquired variations guide evolution."
On page 28 I wrote: "In a species which withstands unfavorable environ-
mental conditions through the plasticity of its individual members, each
individual will need to be educated into harmony with the environment.
Such individuals of the species as vary toward greater natural adaptation will
need less education. Of course innate adaptation is more advantageous than
adaptation through education, since it is immediate, no period of disadvan-
tage appearing in the early life of the individual. The death-rate of the in-
dividuals which become adapted through education may be greater than that
among the individuals with more perfect innate adaptation. Thus, in time,
innate adaptation may be established for the species as a whole."
When the innate adaptation is by means of a character similar to that ac-
quired by the plastic individuals through education, the only advantage which
the innately adapted will have will be from the fact that they pass through
no stage in their youth when, being as yet insufficiently modified, they are not
well adapted to their environment. This is a real advantage and, in a species
whose individuals become modified slowly or imperfectly, the advantage
to the innately adapted may be of selection value. But if the ontogenetic
adaptations are prompt and sufficient, the innately adapted individuals will
have but little advantage. That is, when plasticity is very marked, it will
APPENDIX 195
greatly lessen, and may almost remove, natural selection so far as these par-
ticular adaptive qualities are concerned. A high degree of plasticity hinders
the development of innate qualities by selection, because it diminishes the
selection. Plasticity obstructs selection. As an example, think of humankind.
(Compare page 177.)
This is true if we are considering innate adaptations of the same sort as
those produced by education. But it is not so true if we consider different
types of modification in the two cases. An ontogenetic modification, such as
a forced change in habit, leading to a change in habitat, may enable the in-
dividuals of a species to escape destruction. (Compare Conn's illustration
of the acquired arboreal habit.) Some individuals may later be born with
an innate taste for tree-climbing, but this would hardly give them great ad-
vantage over the other individuals which took to the trees generation after
generation because they had to, rather than because they wanted to. It is
doubtful if the innate instinct to climb trees would be promptly established
by natural selection through the extermination of the more reluctant tree-
climbers. The forced habit of tree-climbing would not in this case cause
.the prompt evolution of an innate tree-climbing instinct.
But in Conn's illustration of the acquired arboreal habit, it was not the
instinct of tree-climbing, but foot and hand structure suitable for climbing,
which were evolved. The arboreal habit adopted by the several individuals ,
generation after generation, brought the animals into a new environment,
and here new structural features became advantageous and were evolved.
The ontogenetically acquired habit did not cause the evolution of a similar
innate habit, but caused the evolution of something very different, namely,
special foot and hand structure. The character acquired through plasticity
(tree-climbing habit) did not serve as a close guide to evolution, but as a
general influence toward the production of a different type of adaptation to
arboreal life. We are thus led to the conclusion that the plastic response of the
individual is not a close guide to the course of evolution. In a species whose
individuals are highly plastic, the ontogenetic modifications will usually be of
a different sort from the adaptive innate characters which may arise later.
We come, then, to this general result: In a species whose members
are but slightly plastic, or slowly responsive to modifying influences, innate
characters similar to those ontogenetically acquired may be evolved; but,
196 APPENDIX
in a species whose members are highly plastic and rapidly responsive, the
adaptive innate characters, which may later be produced, will probably be
of a type different from that of those ontogenetically acquired. In other
words, the greater the plasticity, the less intimate will be its guidance of the
course of evolution, for a rapidly acquired and highly developed ontogenetic
adaptation is almost as beneficial as an innate adaptation of the same type.
Note further that it is in cases of change in the environment, or change in
the habitat of the species, that the chief influence of plasticity upon evolution
is felt. When the environment remains unchanged, evolution is less rapid
and the influence of plasticity is also less.
Were the author to state dogmatically his belief as to the role of plasticity
in evolution, he would say : The accommodation of the individual to adverse
conditions is of great importance in enabling the species to survive during
a period of temporary disadvantage; it may serve in a general way to guide
the course of evolution, but this guidance is not intimate and exact; in the
case of species whose members are highly plastic, it is an important hindrance
to the evolution by selection of qualities of similar use to those in which the
plasticity is shown.
Of course plasticity is itself a very useful quality under many conditions
and will be developed through natural selection.
APPENDIX II
A FEW BOOKS WHICH TREAT OF ORGANIC EVOLUTION
AND PHENOMENA OF SPECIAL ADAPTATION
DARWIN : The Origin of Species. Presents the theory of natural selection
with a wealth of description of phenomena bearing upon it.
The Descent of Man. Treats especially sexual selection.
WALLACE: Darwini m. Gives, on the whole, the best statement of
natural selection; treats variation well; is interesting in its criticism of sexual
selection; suggests the use of colors for signals and recognition marks; does
not adequately treat segregation; claims that natural selection is insufficient
to account for the evolution of the human mind.
Island Life. Gives a good statement of the phenomena of geographical
distribution in their bearing upon evolution.
ROMANES: Darwin and After Darwin, three volumes. Vol. I, Natural
and Sexual Selection and the natural phenomena which bear upon them;
very clearly stated, many good illustrations. Vol. II, Heredity and Utility:
in part a discussion of the inheritance of parental modifications. Vol. Ill,
Isolation and Physiological Selection: the best statement of the influence
of segregation upon evolution.
WEISMANN : Essays upon Heredity and Kindred Biological Problems. A
very valuable and stimulating book in which is developed the theory of the
continuity of the germ plasm and the non-inheritance of parental modifications.
The Germ Plasm. A fuller statement of Professor Weismann's theory
of the continuity of the germ plasm : somewhat intricate.
Germinal Selection. Supplementary to The Germ Plasm.
The Evolution Theory. Translated by J. Arthur Thompson. A sum-
mary of Professor Weismann's contributions to the theory of evolution, written
for general readers as well as special students.
197
198 APPENDIX
CONN : The Method of Evolution. A readable statement of the theory,
including its more modern phases.
HUXLEY: Man's Place in Nature. Giving comparisons between man
and the apes.
Many of Huxley's essays deal with the theory of evolution, especially
those collected in the two volumes Darwinians and Evolution and Ethics.
LLOYD MORGAN: Animal Life and Intelligence and Animal Behaviour.
Morgan is a very discriminating thinker in problems of heredity and evolu-
tion, and his writings are very helpful as well as very readable.
LUBBOCK: The Origin of Civilization, also a second volume, supple-
mentary to this, entitled Prehistoric Times. Very interesting volumes, but
by many regarded as unsound.
WESTERMARCK: The History of Human Marriage. Largely a reply to-
Lubbock's Origin of Civilization.
T. H. MORGAN: Evolution and Adaptation. Contains an interesting
criticism of the theory of sexual selection ; gives a good statement of the theory
of mutation; and attempts to minimize the importance of natural selection
by advocating the belief that evolution may occur through mutation unaided
by natural selection.
There are many books upon the theory of evolution, but those mentioned
are perhaps as important as any for one who is not familiar with the subject.
The author knows of no satisfactory presentation of evolution from the stand-
point of those who believe in the inheritance of parental modifications. COPE'S.
Origin of the Fittest and The Factors of Organic Evolution are two of the most
important books written from this standpoint, but they are very difficult
reading, almost unintelligible in parts. LE CONTE'S Evolution and its Rela-
tion to Religious Thought is written from this point of view, but it is uncritical,
assuming rather than discussing the inheritance of parental modifications.
There are also many books dealing with the phenomena of adaptation r
which have such an intimate relation to the theory of evolution. COULTER'S
Plant Life and JORDAN and KELLOGG'S Animal Life are written from the
point of view of evolution, and are not only valuable for the information they
convey, but are very readable and entertaining. KERNER'S Natural History
of Plants, translated by Oliver, is a great storehouse of information as to special
adaptations seen in plants. It is an expensive, four-volume work, but should
APPENDIX 199
be found in all libraries. POULTON'S The Colors oj Animals gives the best
treatment of this interesting subject. GRANT ALLEN'S The Colours oj Flowers
suggests very interesting conceptions as to the evolution of the colors of blos-
soms. Its contentions are not fully admitted by botanists, but it is well worth
reading.
If any of the readers of this Outline are interested to read further in regard
to evolution, the author would suggest that ROMANES' Darwin and Ajter Dar-
win, Vols. I and III and WALLACE'S Darwinism, followed by WEISMANN'S
Essays upon Heredity, would probably be the best books to read first, and with
these COULTER'S Plant Life and JORDAN and KELLOGG'S Animal Lije.
INDEX
(Italicized page numbers and plate numbers indicate illustrations of the subject mentioned.)
Abraxas gloss ularia, PI. 70.
Acquired characters, 67.
Acr&a egina and gea, PI. 77.
Acraida:, 132, 137, PL 76, PL 77.
Acronycta alni and psi, PL ?l.
Adalia bipunctata, PI. 69.
Adaptation, innate, 27; not explained by
the theory of the inheritance of parental
modification, 78, of the individual, 27,
28, 177, 192.
ALgialitis vocijera, PL 82.
Agassiz's cave fish, 95.
Aggressive coloration and resemblance,
125-127; mimicry, 145.
Alaska, former mild climate of, 62, 112.
Alchemy, Intro, ix.
Alga, 32, 36, 91, PL 21.
Algiers, alluring color in lizard, 129.
Allen, Grant — colors of flowers, 162, 198.
Allen, J. A. — variation in Florida birds, 9.
Alluring colors and resemblances, 127-129.
Altruism, 25, 176, 180, 182.
Amauris echeria and niavius, PI. 76.
Amazon Valley — butterflies, 5 1 ; leaf-
cutting ants and tree-hoppers, 137,
PI- 75-
Amblystoma, tadpole, 98.
"American Food and Game Fishes"
(Jordan and Evermann), PL 48, PL 58.
Amoeba, inheritance of parental modifica-
tions, 68; reproduction, 68; simple or-
ganization, 91.
Anatomy, comparative, 87, 88-96.
Ancon sheep — segregation, 65.
"Animal Behaviour" (Lloyd Morgan), 198.
"Animal Life" (Jordan and Kellogg),
62, 95, 198, 199.
"Animal Life and Intelligence" (Lloyd
Morgan), 198.
Antarctic continent, former existence of,
114.
Antelope, confusing coloration, 148, PL 81 ;
protective color, 120; signalling, 146,
PL 81.
201
Antlers — of deer, 106, 707, PL 43; of
elk — correlation with ligamentum nu-
chtz, 35.
Ants — antlike spiders, 737, 138, 145 ;
mimicked by tree-hoppers, 137, PL 75 >
unpalatable, 137.
Apatura iris, PL 56.
Ape — related to Hominida, 163; ear
of Barbary ape, PL 94; nictitating
membrane, PL 36.
Apis mellijera, PL 74.
Aplecta occulta, PL 55.
Apocyrtus, PL 73.
Appendix, vermiform, man and orang, 166,
PL 96.
Apteryx, vestigial wings, 94, PL 35.
Arbutus, trailing, 7, 154.
Archaopteryx lithographica, 109, PL 44.
Arctia caja, PL 70, PL 71.
Arctic fox, 126, 127.
Argus pheasant, PL 24.
Ariamnes attenuata, PL 64.
Aristolochia sipho, 158, PL 90.
Artificial selection, 28-31.
Asexual reproduction and inheritance of
parental modifications, 71.
Astia vittata, var. nigra, PL 28.
"Astrolabe, Voyage de 1'," 145.
Attacus atlas, 143.
Attractiveness, criteria in choice in mar-
riage, 181, 182, 183.
Australasia — fauna, 113; limits of fauna
(map), 115.
Avebury, Lord, color sense in insects,
159 et seq., 198.
Bacteria, rate of increase, 14.
Bagworm, 21.
Baldwin, J. Mark, 27, 192.
Bali — Lombok strait (map), 115.
Barnacle goose, 3, 5; goose barnacle, 3,
4, 5; sacculina, 184, 185, PL 101.
Barriers to migration, 112.
Basilarcha (Limenitis) disippus, 138, PL 76.
2O2
INDEX
Bastin, E. S., PL 88.
Bat — skeleton of wing, 92.
Bates, H. W. — butterflies of Amazon
Valley, 51; terrifying attitude in cater-
pillar, 139.
Bear, polar, 126.
Beddard, F. E., 125, 127, PI. 62.
Bee, carrying pollen, 154; color prefer-
ence, 159, 162; evolution of instincts,
76; honey-bee, three types. 22; mim-
icked by flies, PL ^4; mimicked by
moths, PL jo; parasites, 146; pro-
tected by stings, 130, 135 ; sacrifice
of individuals for benefit of hive, 22;
sterility of workers, 23; warning color,
130, PL 74.
Beetle, Colorado potato-beetle, 131, PL 69;
color preference, 162; curculio, 134,
PI- 73 >' golden rod-beetle, 131; Her-
cules beetle, 51, PI. 30, lady-beetle,
131, 135, PL 69, PI. 73; leaf beetle,
123, PL 62; mimicry, 135, 136, PL 73,
sexual divergence, 51, 52; staghorn
beetle, 51, PL 2Q; warning color, 131.
Behring Straits, migration across, 62, 112.
Belt, moss insect, 122, PL 61.
Biology, Intro, ix, x.
Bird, confusing coloration, 148; females
protectively colored, 50; gastrula, PL
42; males serve as decoys, 50; mim-
icry, 144, PL 80; noxious insects,
130; protective color, 118, PL 49-51 ;
recognition marks, 147, PL 82; segre-
gation, 65; sexual coloration, 149, 150;
sexual phenomena, 49-50; sexual selec-
tion, 53; skeleton, 92, no.
"Birds of Eastern North America, Hand-
book of " (Chapman), 49.
"Birds of New Guinea" (Gould), PL 25,
PL 26.
Birth-rate, n, 12; relation to struggle for
existence, 17.
Blackbird, sexual coloration, 150.
Blastomere, 70.
Blastopore, 101, 102, 103, PL 42.
Blind fish, 95.
Blossoms, see Flowers.
Bluebird, sexual coloration, 150.
Blue crab, PL 40.
Bluefish, 1 1 8, PL 48.
Boa constrictor, vestigial limbs, 94.
Boar, correlation between tusks and bristles,
36.
Bobolink, sexual coloration, 150, PL 22.
Bolton, Gambier, PL 68.
Bombus vancouverensis, PL 74.
Bonasa umbellus, PL 23.
Borecole, PL 6.
Bourru, friar-bird and oriole, 144, 145.
Brassica oleracea, Pis. 4, a.,-8 ; rapus, PI. 9.
Breeding, in and in, 42 ; methods used, 29 ;
mice (Castle), 44; breeding-time and
segregation, 43.
Brehm, 4, 22, 31, 133, PL 24, PL 30, PL 33,
PL 62, PL 99.
Britcher, H. W., 137, PL 2, PL 64, PL 85.
Britton, N. L., 88, 89, 90.
Broccoli, PL 7, PL 8.
Bronze, invention of, 176.
Brown, Addison, 88, 89, 90.
Brussels sprouts, PL 7.
Bujo lentiginosus, PL 66.
Bugs, noxious character and warning
color, 131, PL 69.
Butomns umbellatus, PL 86.
Butterfly, color preference, 162; confusing
coloration, 147, PL 83; courting, 55;
leaf butterflies, 123, PL 83; mimicry,
137, PL 76, PI. 77; sexual color,
ation, 51, PL 84; warning color, 131,
PL 59, PL 76, PI. 77, PL 84.
Cabbage, varieties of, 29, Pis. 4, a,-8.
Calamesia midama, 138, 150, PI. 84.
Calamus arctijrons, PL $8.
Calf embryo, PL 38.
Callimorpha dominula and hera, PI. 70.
Callinectes hastatus, PI. 40.
Callionymus lyra, PI. 32.
Calocalanus plumulosus and pavo, 57.
Calopteryx maculata, PL 33.
Cambrian fossils, 106.
Cambridge, PL 64.
"Camera Shots at Big Game" (Wallihan),
PL 81.
Cancer pa gurus, 100.
Capital punishment, 25.
Carboniferous fossils, 106.
Cardinal, sexual coloration, 150.
Carrion flower, 154.
Castle, W. E., mendelian phenomena, 44.
Caterpillars, protective color, PI. 56; terri-
fying attitude, 139, 140, PL 78 ; warning
color, PI. 71.
Catocala amatrix, PL 60; concubens, PI. 83,
Cauliflower, PL 7, PL 8.
Cave-dwelling animals, eyes, 95.
Cemophora coccinea, PL 79.
INDEX
203
Ceram, friar-bird and oriole, 144.
Ceratophora stoddartii, PI. 34.
Cerebral hemispheres, man, 164, PI. Q2.
Cerura vinula, 140, PI. ?8.
Cervus, antlers, 107, PI 43.
Ch&rocampa elpenor, 139, 140, PL 78.
Chameleo bijurcus and owenii, PI. 34.
Change, in environment makes evolution
rapid, 26; of function in organs, 38.
Chapman, Frank M., 49.
Chauliodes cornutus, PL 31.
Chemistry, Intro, ix.
Chemistry, relation of life processes to, 188.
Chickens, breeds of, 29, 30, Pis. 12-19 >'
embryos, PI. 38 ; reject noxious insects,
130 ; sexual coloration, 150, Pis. 1 2-1 5.
Chimpanzee, 164, Pis. 91-94.
Chincha, various species, PL 72.
Chinese, segregation, 168.
Choice in mating, 48; in marriage, 180.
See Sexual selection.
Chologaster, 95.
Chordeiles virginianus} PI. 82.
Christianity, effect on evolution, 179.
Cicada septemdeeim, 51, PL 29.
Cidaria cucullata, PL $6; galiata and ocel-
lata, PL 55.
Classification, 88.
Claus, C, PI. 41.
Climate, cause of segregation, 61 ; of
Alaska and Siberia, 52.
Cobra, imitated by moth, 142, 14 3.
Coccinella, PL 73.
Coccyx, man and apes, 164, 165, PL 91.
Coerostris mitralis, PL 64.
Colewort, wild, PL 4, a ; varieties of, Pis. $-8.
Colinus, virginianus, PL 49.
"College Botany" (Bastin), PI. 88.
Coloborhombus jasciatipennis, PL 73-
Color, adaptation in butterfly pupae, 121,
PL 59; aggressive, 125-127; alluring,
127-129; animals, 116-151; change,
121, PL 58 ; classification of color phe-
nomena, 116; confusing, 147-149; con-
vergence in warning color, 1 34 ; flowers,
151-163; insects and color of flowers,
159 et seq. ; mimicry, 135-146; recogni-
tion marks, 146-147; seasonal, 121,
126, 127, PL 57; sexual, 52, 149-151;
signals, 146-147; use and disuse and
color of flowers, 75; warning, 129-134.
Colorado potato-beetle, 131, PL 69.
"Colors of Animals, The" (Poulton),
116, 139, 198.
"Colors of Flowers, The" (Grant Allen),
162, 163, 198.
Columbia livia, 31, PI. 20.
Community, length of life in communal
animals, 2 1 ; unit in struggle for exist-
ence, 24.
Complexity, increases during growth of
an organism, 96; in more recent fos-
sils, 1 06; varying degrees of, 90, 97.
Composite, 90.
Confusing coloration, 147-149, PL 83.
Conn, H. W., 192, 198.
Convergence in warning coloration, 134.
Cony, protective color, PL 54.
Cope, E. D., PI. 46, 198.
Copepoda, 57.
Coral polyp, gastrula, 102.
Correlation of organs, 33, 34, 38; between
innate characters and attainments, 178;
between vigor and secondary sexual
characters, 58.
Corvus americanus, no.
Cotton-tail rabbit, confusing coloration,
148; protective coloration, PL 53; sig-
nals, 146.
Coulter, J. M., 198, 199.
Courtship, birds, 49, PL 23, PL 24, PL 27;
fish, 52, 150; Groos on relation of
courtship to natural selection, 54; not
observed in some forms which show
divergence in secondary sexual charac-
ters, 56; observation difficult, 59;
spiders, 50, PL 28.
Crab, 99, 100; blue crab, PL 40; protec-
tive color, 120; sacculina parasitic upon
crab, 184, PL 101 ; which resembles a
pebble, 124^ 126.
Crawfish, nervous system, PL 40; protec-
tive color, 1 20.
Creation, Theory of Special, Intro, ix.
Cross-breeding swamps varieties, 41.
Cross-fertilization, 42, 153-158, Pis. 88-90.
Crustacea, development of higher, 98,
Pis. 39-41; pelagic Crustacea trans-
parent, 117; protective color, 120.
Cryptolithodes sitchensis, 124.
Ctenophores, transparency of, 117.
Curculio, 135, PL 73.
Curlew, protective color, 119.
Cycloptera, PL 62.
Cypris, 184.
Dahlia, varieties of, 29, PL 70, PL II.
Daisy, marguerite — rate of increase, 13.
2O4
INDEX
Danaida, 132, 138, PL 76, PL 84.
Danais archippus, 138, PL 76; chrysippus,
PI. 76.
Darwin, Charles, 47, 48, 52, 197, PL 32,
PI- 34, PL 95-
"Darwin and After Darwin" (Romanes),
105, 197, 199, PL 17, PL 36, PL 37, PL 44,
PL 75, Pis. 93-^97, PI 99.
"Darwiniana" (Huxley), 198.
"Darwinism" (Wallace), 35, 132, 144,
197, 199.
Dasychira pudibunda, PL 55.
Datana ministra, PL 75.
Dean, Forest of, segregation of deer in,
65-
Death, at close of reproductive period, 20,
22, 23, 24; bagworm moth, 21; capital
punishment, 25; causes, 14; drone
bees and young queen bees, 22, 23;
of individual for general welfare, 20-25 >
rate, 12.
Decapoda, development of, 98, Pis. 39-41.
Decoy, male birds serve as, 50.
Deer, antlers, 106, 107, PL 43; protective
color, 120; secondary sexual charac-
ters, 53; segregation, 65; signals, 146.
Degeneration, 183-187.
Delage, Yves, PL 101, 185.
"Descent of Man" (Darwin), 52, 197.
Deterioration due to escape from struggle
for existence, 170; due to parasitism,
184 et seq.
Devonian fossils, 106.
De Vries, Hugo — mutation, 18, 19, 39,
189, 190.
Dianthcecia compta, PL 55.
Diapheromera jemorata, PL 61,
Diatoms, skeletons of, 32, PL 21.
Dimorphism, in flowers of Mitchella, 156,
PL 88; sexual, see Sexual selection.
Discontinuous variation, 19.
Disease, 169, 171.
Dismal Swamp fish, 95.
Dismorphia astynome, PL 77.
Dissosteira Carolina, PL 83.
Distribution, geographical, 87, 111-116.
Divergence, degree of, in variation, 9;
from species type, an advantage in
struggle for existence, 26; in relation
to mendelian phenomena, 46; sexual,
see Sexual selection ; swamped by cross-
breeding, 41.
Dog, mating, 48 ; skeleton of fore limb, 92.
Dolichonyx oryzivorus, PL 22.
Doliops sp. and D. curculionides, PL 73.
Doryphora decemlineata, PL 69.
Dragon-fly, PL 31, PL 33.
Drone-fly, imitates bee, 136, PL 74.
Drouth as cause of segregation, 43.
Dugmore, A. R., PL 48, PL 50, PL 58.
Dugong, nictitating membrane, PL 36.
Dynastes hercules, 51, PL 30.
Eagle — nictitating membrane, PL 36.
Ear, change of function, 39 ; man and apes,
165, Pis. 94-96; vestigial muscles of
human, 94, 95.
Education (plasticity), 27, 192; suscepti-
bility to, hinders Evolution, 177.
Egypt, 175-
Elaps, 142, PL 79.
Elephant hawk moth, 139, 140, PL 78.
Elk — correlation between antlers and
ligamentum nucha, 35.
Elymnias phegea, PL 77.
Embryology, comparative, 87, 96-103;
of higher Crustacea, 89, Pis. 39-41 ; of
man, 166, PL 98; of vertebrates, 97,
PL 38, PL 98.
Emydia jacobece, PL 70.
Environment, and nature of organism, 188 ;
change in — makes evolution rapid, 26;
inheritance of direct effects of, 72;
nature of man's, 172.
Epeira prompta and stellata, PL 64.
Epilobium hirsutum, 139.
Epipactys latijolia, PL 89.
Equus, PL 47.
Eriopus purpureofasciata, PL 55.
Eristalis tenax, PL 74-
Errera, PL 4, a.
Erythrolamprus esculapii and venustissi-
mus, PL 79.
"Essays upon Heredity and Kindred Bio-
logical Problems" (Weismann), 197,
199.
Euckistus servus, PL 69.
Euplcea midamus, 138, 150, PL 84.
Evermann, Barton G., PL 48, PL 58.
"Evolution and Adaptation" (Morgan),
198.
"Evolution and Ethics" (Huxley), 198.
"Evolution and its Relation to Religious
Thought" (Le Conte), 198.
"Evolution, The Factors of Organic"
(Cope), 198.
"Evolution, The Method of" (Conn), 192,
198.
INDEX
205
"Evolution Theory, The" (Weismann),
191, 197.
Extirpation of organs, effects of, 34.
Eye, deterioration of human, 170; of cave-
dwelling animals, 95; of various ver-
tebrates, PL 97.
Factors in evolution, Intro, xi, 82, 188.
"Factors of Organic Evolution, The"
(Cope), 198.
Family, 24, 182.
Faroe Islands, segregation of sheep, 65 .
Faunas and floras, 112, 113.
Felis tigris and oncat PI. 68.
Fertilization, cause of variation, 81; cross
and self, 42 ; of flowering plants, 152-163 ;
by wind, 153; by insects, 154 et seq.
Fescue-grass, 153, PI. 87.
Field sparrow, P . 49, 126.
Fish, birth-rate, 12; blind fish, 95; blue-
fish, 118: PL 48; embryos, PL 38;
flatfish, 1 1 8, PL 48; protective colora-
tion, 1 1 8, PL 48; sexual coloration, 52,
150; sexual divergence, PL 32.
Flatfish, 118, PL 48.
Flies, drone-fly, mimics bee, 136, PL 74;
fertilization of Aristolochia, 158; fer-
tilization of white flowers, 162; mimi-
cry of bees and wasps, 135, PL 74.
Flora, 112, 113.
"Florida, On the Mammals and Birds of
East" (Allen, J. A.), 9.
Flounder, protective color, PL 48.
Flower, W. H., 94, PI. 44, PI. 68.
Flowers, diagrams of various, PL 86;
insect visitors, 162; landing place for
insects, 163, PL 89, PL 90; of Aris-
tolochia, 158, PL 90; of Mitchella, 156,
PL 88; of orchid, 156, PL 89, PL 90;
of Salvia, 157, PL 89; of wind-fer-
tilized plants, 153.
Forbes, H. O., 127, 144, 145.
Fossils, conditions for the formation and
preservation of, 104; table of fos-
siliferous rocks, 105; marsupials of
America, 113.
Fowl, see Chickens.
Fox, aggressive coloration, 126; Arctic, 126,
127 ; segregation, 60.
Fox, Rev. W. D., mating of Chinese geese,
48.
Friar-bird, 144, PL 80.
Fr°g» aggressive color, 126; gastrula,P/. 42;
noxious insects, 130 ; protective color, 120.
Gagea lutea, PL 86.
Galapagos Islands, segregation of locusts,
60, 62.
Galeust nictitating membrane, PL 36.
Galileo, Intro, ix.
Game cock, evolution of, PL 16.
Gastrula, coral polyp, 102; various ani-
mals, /oj; vertebrates, 102, PL 42,
Gazelle, white rump patch, 146.
General considerations, 183-188.
General principles in operation of natural
selection, 20.
Geographical distribution, 87, 111-116.
Gerarde, 5.
Germ cells, 69; and variation, 79; nutri-
tion of, 80; organization of, 191,
Germinal selection, 96, 191.
"Germinal Selection" (Weismann), 191,
197.
"Germ Plasm, The" (Weismann), 191,
197.
Gibbon, 164, PL 91.
Giesbrecht, 57.
Gila monster (lizard), 133, PL 72.
Giraldus, Sylvester, 3.
Goat, protective color in wild, 120.
Goldfinch, American, sexual coloration,
150.
Goodale, William, PI. 88.
Goose, barnacle, 3, 4, 5; mating of white
and Chinese, 48; variation slight,
8.
Gorilla. 164, PL 91, PL 92.
Gould, PI. 25, PI. 80.
Government, progress in, 176.
Grackle, sexual coloration, 150,
Grapta, PL 83.
Grass, fertilization, 153.
Grasshopper, confusing coloration, 147,
PL 83; leaf, 123, PL 62; mimicking
beetles, 135, PL 73.
Grass porgy, 121, PL 58.
Gravitation, Intro, ix.
Gray's "Anatomy," 95, PI. 96.
Greeks, 176; conception of origin of ani-
mals from plants, 3, 6.
Greenland whale, skeleton, 94.
Grip, strength of, of human infant, 166,
PL ioo.
Groos, courtship, 54, 57.
Grouse, birth-rate, 12; ruffed grouse, 118,
PL 23; snow grouse, 121, PL 57.
Gulick, John T., segregation of land snails
of Oahu, 63, 64.
206
INDEX
Habrocestum howardii, PL 28; splendens,
PI. 85.
Haeckel, Ernst, 102, PL 21, PL 38, PI. 98.
Hair, change of function, 38; of man and
ape, 94, 164, PL 37, PL 93.
"Handbook of Birds of Eastern North
America" (Chapman), 49.
Haswell, W. A., PL 44.
Hawaiian Islands, segregation in land
snails, 63.
Hawk moth, elephant, 139, 140, PL 78.
Hayes, PL 4.
Heart, change of function, 38.
Hebomoia glaucippe, PL 83.
Helianthemum marijolium, 152.
Heliconidae, 132, PL 77.
Heliconius eucrate, PL 77.
Heloderma horridum, PL 72.
Hemiptera, warning color and shape, 131,
PL 69.
"Herball, The" (Gerarde), 5.
Hercules beetle, 51, PL 30.
Heredity, 3, 10, 18; inheritance of paren-
tal modifications, 67, 68, 175.
"Heredity and Kindred Biological Prob-
lems, Essays upon" (Weismann), 197,
199.
"Heredity and Utility" (Romanes), 197.
Herrick, F. H., PL 41.
Hesperid(pt 128.
Hesperornis regalis, 109, PL 45,
Hestia, 127.
Heterocampa biundata, PL $6.
Heterocampa pulverea, PL 55.
Hippiscus tuber culatus, PL 83.
Hippocampus mohnikei, 124.
Hippodamia convergens, PL 69.
"History of Human Marriage, The" (Wes-
termarck), 198.
Hog, embryos, PL 38.
Homarus americanus, PL 39, PL 41.
Homer, 176.
Hominidce, 164.
Homology, 92, 93, 101.
Homoptera, — mimicking ants, 137, PL 75;
edusa, PL 54.
Honey-bee, 22, 23.
Honeysucker, imitated by oriole, 144, PL
80.
Hornet, 130, 135, PL 74.
Horse, breeds of, 28, PL 4; correlation be-
tween hair and hoofs, 36; evolution of
feet and teeth, 109, PL 46, PL 47;
gradual change in horse family, 40, 189,
191, nictitating membrane, PL 36; seg-
regation in Paraguay, 65.
"Horse, Points of the" (Hayes), PL 4.
Human evolution, 163-183; how controlled,
179, 183; sexual selection, 168, 169, 172,
!73» I75> T78> !79-l83-
Humming-bird, — nest, PL 51 ; sexual
coloration, 150, PL 26.
Huxley, T. H., 100, 198, PL 40, PL 91, PL 92.
Hydra, 101.
Hyla versicolor, PL 66.
Hymcnoptera, mimicked by other insects
and spiders, 135, 136, 737, 138; pecul-
iar form, 131; protected by stings, 130,
136; warning color, 130, PL 73, PL 74.
Hymenopus bicornis, 128.
Ichthyornis victor, 109, PL 45.
Ichthyura inclusa, var. inversa, PL 55.
Icius mitratus, PL 28.
Immortality, 179.
Improvement of human race, 173.
In -breed ing, 42.
Indians, North American, measles, 171.
Indigo-bird, sexual coloration, 150.
Individual sacrificed for welfare of species,
20, 187.
Infant, human — foot position, 166, PL 100;
spinal curve, 166, PL 99; strength of
grip, 1 66, PL 100.
Infertility, domestic races not infertile
when crossed, 31; of crosses between
species, 31; of hybrids, 31; starting-
point in evolution, 32.
Inherent tendency, in variation, 40; in
evolution, 188.
Inheritance of parental modifications,
Intro, xi, 67, 175.
Injury — effects of — inherited among uni-
cellular organisms, 69.
Innate adaptation vs. acquired adaptation,
27; character vs. training, 127 et seq.
Ino pruni and statices, PL 70.
Insects, and color of flowers, 151 et seq.;
and plant fertilization, 154 et seq.;
protective color, 120.
Instinct, 39.
Internal factors in evolution, 188.
Invalidism, 170, 172, 181.
Inventive genius, 176.
"Ireland, Relations concerning" (Giral-
dus), 3.
Island faunas and floras, 112.
"Island Life" (Wallace), 197.
INDEX
207
Isolation, see Segregation.
"Isolation and Physiological Selection"
(Romanes), 197.
Jack-rabbit, confusing coloration, 148.
Jaguar — aggressive coloration, 127, PL 68.
Jamaica — Neritina, 10.
Java — spider which resembles bird excre-
ment, 127.
Jellyfish, mouth, 101; transparency, 117.
Jenner, 171.
Jesus, influence upon evolution, 179.
Job, 176.
Jordan, D. S., 62, 95, 124, 198, 199, PL 48,
PL 58.
Juncus bufonius, seedpods imitated by a
spider, 124, PI. 64.
Jungle-fowl, 29, 119, PI. 16.
Junonia, PI. 83.
Kale, PL 6, PI. 8.
Kallima inachis, 123, 147, PI. 83.
Kangaroo rat, confusing coloration, 148.
Kappel and Kirby, PL 56, PL 70, PL 71,
PI. 76, PL 77, PI. 78^
Kellogg, V. L., 62, 95, 198, 199.
Kepler, Intro, ix.
Kerner, A., 152, 158, 198, PL 86, PL 89.
Killdeer, recognition marks, 147, PI. 82.
Kirby, see Kappel.
Kohlrabi, PI. 7, PI. 8.
Lady-beetle, noxious character and warn-
ing coloration, 131, 135, PL 69, PI.
73-
Lagopus leucurus, PI. 57.
Landing-place for insects in plant blos-
soms, 163, PI. 89, PL go.
Lang, Arnold, PL 41.
Larvae, transparency of marine, 118.
Laurent, PL 4, a.
Leaf-cutting ants, mimicked by tree-hop-
pers, 137, PL 75.
Leaf-like insects, 122, 123, PL 56, PL 62,
PL 83.
Le Conte, Joseph, 198.
Lemur, 163.
Lepas anatifera, 4.
Leucania l-album, PL 55.
Leucoma salicis, PL 77.
Lever, invention of, 176.
Life, length of, 20, 23, 24 ; processes, chem-
istry and physics, 188.
Limenitis (Basilarcha) disippus, 138, PL
76; sibylla, PL 56, PL 76; populi,
PL 78.
Lion, aggressive color, 126; secondary
sexual characters, 53.
Lizard, aggressive coloration, 126, PL 52;
alluring coloration, 129; confusing
coloration, 149; gila, 133, PL 72;
protective coloration, 120, PL 52;
rejects noxious insects, 130; sexual col-
oration, 150; sexual divergence, PL 34.
Lobster, 98, 120, PL 39-41.
Locust — Galapagos Islands, 62; leaf,
122, PL 62.
Logoa, 123, PL 63.
Lophornis adorabilis, PL 26.
Love, foundation in marriage, 180.
Low, Professor, mating of domestic ani-
mals, 48.
Lubbock, John, see Lord Avebury.
Lucanus dama, PL 29.
Lungs, change of function, 38.
Lycorea halii, PL 77.
Lydekker, Richard, PL 68, PL 72.
Lymncnis, development of, 97.
Lyre bird, PL 24.
McCook, H. C., PL 75.
MacDougal, 189.
Macroglossa bombylijormis and stellatarum,
PL 70.
Mammalia, development, 97, PL 38; fossils,
105, 106.
"Mammals and Winter Birds of East
Florida, On the" (Allen), 9.
Mammoth Cave, blind animals in, #5.
Man, embryos, PL 38; evolution, 163-183;
nictitating membrane, PL 36; one
species, 163; plasticity, 28, 177; sexual
selection, 168, 169, 172, 173, 175 178,
179-183; skeleton of arm, 92; slow
evolution, 175; social progress vs.
evolution, 173.
"Man's Place in Nature" (Huxley), 198.
Mantis, alluring color and form, 127, 128;
leaf mantis, 123, 126, PL 62.
Marguerite, daisy, rate of increase, 13.
Marptusa jamiliaris, PL 28.
Marriage, human, 168; choice in, see
Man, Sexual selection; laws, 181;
responsibility in, 180.
"Marriage, The History of Human"
(Westermarck), 198.
Marsh, O. C., PL 45, PL 47.
Marshall, A. M., PL 42.
208
INDEX
Marsupialia, geographical distribution, 113.
Mating, preferential, see Sexual selection.
Mean, species, 19.
Measles, among savage races, 171.
Mechanitis lysimnia, PL 77.
Meg-ilia maculata, PL 69.
Melincea eihra, PL 77.
Melitcea cinxia, PL Jl.
Mendel, 40, 44.
Menura superb a, PL 24.
Mephitis mephitica, PL 72.
Merriam, C. Hart, 147 148.
Mesohippus, PL 47.
"Method of Evolution, The" (Conn), 192,
198.
Mice, Mendelian phenomena, 44.
Migration, 64.
Mimicry, aggressive, 145, 146; conditions
fulfilled in, 145; in insects, 135, 137,
PL 70, PL 73, PL 74, PL 76, PL 77;
in snakes, 142, 143, PL 97; protective,
i35-T45-
Mind, 39; development of human, 167;
training of human, 178.
Miohippus, PL 47.
Misumena vatia, PL 75-
Mitchella repens, 156, PL 88.
Mivart, St. George, 188.
Modification, inheritance of parental, 67-82,
175 et seq.
Monaxenia darwinii, 102.
Monkey, ears, 165; reject noxious insects,
130; relationship to man, 163.
Moral character, effect on evolution, 187;
growth in innate, 176; improvement
through sexual selection, 182; training
of, 177, 178.
Morgan, Lloyd, 27, 47, 48, 65, 130, 192,
198.
Morgan, T. H., 57, 59, 198.
Mormolyce phylloides, PL 62.
Moss insect, 122, PL 61.
Moths, confusing coloration, 147, PL 83;
elephant hawk, 139, 140, PL 78 ; leaf,
123, PL 83; mimicry, 136, 142, 143,
PL 70, PL 78; protective color, PL 54,
PL 55; sexual coloration, PL 84;
terrifying attitude, 142; warning color,
132, PI. 70; waved-yellow, 123, PL 63.
Miiller, Fritz, 134.
Murray, Sir Robert, 5.
Muscles, of human ear, 94, 165, PL 96;
of human skin, 94 ; vestigial — of tail
in man, 165, PL 95.
Mutation, 18-20, 39; determinate, 189.
Mydas clavatus, PL 74.
Mygnimia aviculus, PL 73.
Mysis stage in development of lobster,
PL 41; stenolepis, 99, PL 41.
Nascent organs, 101.
"Natural History of Plants, The" (Kerner),
158, 198.
Natural selection, 3-47, Intro, xi; man,
1 68, 169; sacrifices individual for wel-
fare of race, 170.
"Naturalist's Wanderings in the Eastern
Archipelago, A" (Forbes), 127.
" Naturliche Schopjungsgeschichte "(Haeck-
el), PL 21.
Nauplius, 184, PL 10 1.
Nectar, 154, 155, 163.
Negro, segregation, 168.
Nerice bidentata, PL 56.
Neritina, virginea, var. minor, Frontis-
piece, 9.
Nesocentor milo, PL 2$.
New Forest — segregation of sheep, 65.
"New Guinea, Birds of" (Gould), PI. 25,
PI. 26, PI. 80.
New Jersey scrub pine, PL 87.
Newton, Intro, ix.
Nictitating membrane, 94, 166, PL 36, 97.
Nighthawk, 147, PL 57, PL 82.
Nutrition of germ cells, 68.
Nymphalidce, PL 77.
Oahu — land shells, 63 ; map, 64.
Objections to natural selection, 31-47; to
sexual selection, 56-60.
Odor of warning-colored butterflies, 132;
of flowers, 154.
(Enothera lamarckiana, 18, 189.
Ophibolus doliatus, PL 70-
Opossum — geographical distribution, 113.
Orang, 164, PL 91, PL 94-96.
Orchid — cross-fertilization, 156, PL 89.
Orchis militaris, PL 90.
"Organic Evolution, The Factors of,"
(Cope), 198.
Organic selection, 27, 192.
Orgya antiqua, PL 71.
Origin of animals from plants, 3.
"Origin of Civilization, The" (Lubbock),
198.
"Origin of Species, The" (Darwin), Intro.
x, 197.
"Origin of the Fittest, The" (Cope), 198,
INDEX
209
Oriole, mimicry, 144, PL 80; sexual
coloration, 150.
Oriolus decipiens, PI. 80.
Ornithopiera (Papilio) priamus, 150, PL 84.
Orohippus, PI. 47.
Osborn, H. F., 27, 190, 192.
Ostracoda, 184.
Otaria, PI. 36.
Outdoor life and sexual selection, 181.
Owl, nictitating membrane, PL 36; snowy,
126.
Oxyrrhopus trigeminus, PL 79.
Pachyrhynchus, PL 73.
Packard, A. S., PI. 55, PI. 56, PI. 78.
Paleontology, 87, 103-111.
Palms, fertilization, 153.
Paludestrina protea, PI. 3.
Paludina, fossil shells, 107, 108, 189, 191.
Panolis piniperda, PI. 56.
Panthia ccenobita, PL 55.
Papilio echerioides, PL 76; machaon, PL
71; merope, PL 76; mimicry, 138;
priamus, PL 84; ridleyanus, PL 77.
Papilionidce, 132, PL 76.
Paradise, bird of, sexual coloration, 150.
Paraguay, segregation of wild horses, 65.
Paralichthys dentata, PL 48.
Parasitism, bee parasites, 146; effects of,
186; sacculina, 183.
Parental care and length of life, 20.
Parental modifications, inheritance of, 67,
68, 175.
Parker, T. J., PI. 44.
Parnassius apollo, PL 71.
Parrot, protective color, 119.
Partridge berry, 156, PL 88.
Peacock, sexual coloration, 150.
Peckham, G. W. and E. G., spiders, sexual
selection and sexual divergence, 55,
PL 28, PL 64; wasps, breeding habits,
77; wasps, color sense, 161, PL 85.
Pelagic animals, transparent, 117.
Pelvis, man and apes, 164, PL 91.
Perhybris pyrrha, PL 77.
Permian fossils, 106.
Phanerogamia, 92.
Pheasant, Argus, 150, PL 24; protective
color, 118.
"Pheasants" (Tegetmeier), PL 24.
Phenacodus, 109, PL 46.
Phidippus cardinalis, PI. 8$.
Philemon plumegenis, PL 80
Philohela minor, PL 50.
P
Phlogcenas jobiensis, PL 2$.
Phoenicians, 175.
Phoraspis, PL 73.
Phorodesmia smargdaria, PL 55.
Phy ilium sicci folium, PL 62.
Phyllodes verhuellis, PL 83.
Physics and life processes, 188.
Physiological selection and segregation, 66,
197.
Phytolacca decandra, PL 86.
Pieridce, 132, PL 59.
Pieris brassica, PL 56; rapce, color adap-
tation in pupae, 121, PL 59.
Pigeon, Phlogcenas, PI. 25; rock, 31, PL 20;
varieties of domestic, 30, PL 20.
Pika, protective color, PL 54.
Pine, 153, PL 87.
Pinus mops, PL 87.
"Plant Life" (Coulter), 198, 199.
Plasticity, 27, 28, 192; of man, hinders
evolution, 177.
"Play of Animals, The" (Groos), 54.
Plica semilunaris, see Nictitating membrane.
Pliocercus elapsides and euryzonus, PL 79.
Pliohippus, PL 47.
Plover, ring-necked, PL 82.
Polar-bear, aggressive color, 126.
Polish fowl, skull, 30.
Polistes, breeding habits, 77.
Pollen, food of insects, 154; masses of
orchid, 156, PL 89; slow to sprout on
stigma of same plant, 155; tube, 152,
i53» i56-
Pomatomus saltatrix, PL 48.
Pompilus atrox, PL 74.
Pond snail, development, 97.
Porgy, change of color in grass, 121, PL 58.
Potato beetle, 131, PL 69.
Poulton, E. B., Preface to 2d Ed., x, Intro.
xi, 116, 139, 198, PL 59, PL 75.
" Poultry, New Book of " (Wright), 30,
PL 16, PL 18.
"Prehistoric Times" (Lubbock), 198.
Primates, 163, 164.
Primulacea, 90.
Principle, general principles in operation
of natural selection, 20.
Prionotus cristatus, PL 69.
Progress of human race, 173.
Protective coloration and resemblance, 116,
117-125.
Protohipptis, PL 47.
Providence, Intro, ix.
Pseudacrcea boisduvalii, PL 77.
210
INDEX
Psilura monacha, PL 56.
Psyche unicolor, PI. 56.
Ptarmigan, seasonal color change, i2i,P/. 57.
Pterodactylus spectabilis, PL 45.
Pterogon proserpina, PL 70.
Pterophryne histrio, 124, 126, PL 65.
Public opinion, a part of man's environ-
ment, 172, 179.
Pupa, color adaptation, 121, PI. 59; Logoa,
123, PI. 63; protective color, PL 56.
Putorius ermineus, PI. 67.
Quail, protective color, 118, PL 49.
Rabbit, confusing coloration, 148; em-
bryos, PL 38; illustration of mutation,
19; illustration of natural selection, 15;
protective coloration not explicable by
the inheritance of effects of use and dis-
use, 75; signal, white rump patch, 146.
Radiolaria, skeletons, PL 21.
Recognition marks, 146-147.
Regeneration and inheritance of parental
modification, 71.
Relationship, key to classification, 91.
"Relations concerning Ireland" (Giraldus),
3-
Religion, a cause of segregation, 169.
Reproduction, among unicellular organ-
isms, 68, 69; and length of life, 20, 22,
23, 24; asexual, and inheritance of
parental modifications, 71; birth-rate,
n, 12; easily disturbed, 66; effects of
destroying organs of, 34; evolution
centres in, 82 ; germ cells in higher
organisms, 69; increase in, aids in
struggle for existence, 17; organs of,
in flowering plants, 752, 153, PL 86;
regeneration of organs, 71.
Ring-necked plover, recognition marks,
PL 82.
Robin, illustration of species, 88; rate of
increase, n; sexual coloration, 150.
Robinson, Louis, PL 100.
Rock-rose, 752.
Rocky Mountains, cause of segregation, 61.
Romanes, G. ]., 17,94, 10=5, 197, 199, PI- 17,
PL 36-38, PL 43, PL 44, PI- 75, PI-
93-99; sexual selection in birds, 53;
physiological selection, 66.
Rosacea, 90.
Rostellum of orchid, 156, PL 89.
Roux, \Vilhelm, 192.
Royal Society of London, goose barnacle, 5.
Ruffed grouse, PL 23.
"Ruth," 176.
Sacculina, 184, 185, PL 1 01.
Sacrum, human, with tail muscles, PL 95;
of man and apes, 164, PL 91.
Saitis pulex, PI. 28.
Salamander, embryos, PI. 38; tadpole,
98; warning color, 133.
Salamandra maculosa, 133.
Salvia, 157; glutinosa, PL 88.
Sand-flounder, PL 48.
Sandpiper, protective color, 119.
Sargassum fish, 124, 126, PL 65.
Savage, illustration of social progress, 173
et seq.
Savoy cabbage, PL 6, PL 8.
Sceloporus undulatus, PI. 52.
Scepastus pachyrhynchoides, PL 73.
Schistocerca, 62.
Sclater, mimicry of leaf-cutting ants, 137,
145, PI- 75-
Sea-horse, 124.
Sea-lion, nictitating membrane, PL 36.
Seasonal change of color, 121, 126, 129,
PL 57, PL 67.
Secondary sexual characters, see Sexual
selection ; more developed in female, 58.
Seeds, spiny, not evolved through inherit-
ance of the effects of use, 76.
Segregation, 60-67, J68; see also 42-47, 48.
Selection, artificial, 28-31; germinal, 96;
natural, 3-47; organic, 27, 28, 177, 192;
physiological, 66; "selection value," 17,
37;
Selenia tetralunaria, 122.
Sesia culicijormis and tipulijormis, PL 70.
Seventeen-year cicada, 51, PL 29.
Sexual coloration, 149-151.
Sexual selection, Intro, xi, 47-60, 168, 169,
172, 173, 175, 178, 179-183; a cause
of segregation, 44; objections to, 56-60.
Sheep, Ancon, segregation, 65 ; Faroe
Islands, segregation, 65 ; merinos and
heath sheep do not interbreed, 48;
protective color of wild, 120.
Shore birds, protective color, 119.
Siberia, former warm climate, 62.
Signals and recognition marks, 146-147.
Silurian fossils, 106.
Simiid(F, 164.
Simplification ("degeneration"), 183-187.
Siphonophores, transparency, 117.
Sitana minor, PL 34.
INDEX
211
Skeletons of unicellular organisms, 32,
PL 21 ; of arm of vertebrates, 92.
Skin muscles in man, 94.
Skunk, 133, 134, PI- 12.
Skunk-cabbage, 154.
Slavonia, fossil Paludina, 107.
Small pox, 171.
Smelting ore, 176.
Smerinthus tilice, PI. 55; ocellata, 142.
Snail, development of pond, 97.
Snails of Oahu, segregation, 63.
Snakes, aggressive color, 126; behavior of
poisonous, 143; hind limbs, 93, 94;
mimicry, 139, PL 79; protective color,
120.
Snipe — protective color, 119.
Snow grouse — seasonal color change, 121,
PL 57.
Snowy owl, 126.
Socialism, control of marriage, 182; nature
socialistic, 187.
Social progress, an end in itself, 176;
vs. evolution, 173-177.
Soil, relation to segregation, 63.
Solea concolor, 89, goy 91.
Soma, distinguished from germ cells, 70;
relation to processes of reproduction, 74.
Sparrow, aggressive color, 126; protective
color, 1 1 8, PI. 49.
Spathura solstitialis, PI. 26.
Species, mean, 19 ; meaning of, 88 ; preser-
vation of species, not of individual,
secured by natural selection, 187.
Spermophile, protective color, PL 53.
Sphinx convolvuli, PI. 55.
Spiders, aggressive coloration, 126, PL 7$;
aggressive mimicry of ants, 145 ; ene-
mies of, 12 1 ; protective coloration, 120,
PI. 75, PI. 8$; protective resemblances,
124, PL 64; resembling bird excre-
ment, 127; sexual coloration, PL 85;
sexual selection, 50, PL 28.
Spilomyia hamifera, PI. 74.
Spinal column, curvature in man and apes,
1 66, PL 99.
Spizella, pusilla, PL 49.
Sports, 19, 39, 46.
Sprouts, Brussels, PL 7.
Squirrel, confusing coloration, 148.
Stability of certain species, 9.
Staghorn beetle, 51, PL 29.
Starfish, birth-rate, 13, 17.
Stearns, PI. 3.
Sterility, domestic races not mutually
sterile, 31; of crosses between certain
individuals, 66; of crosses between
species, 31; of hybrids, 31, 41; of
soma cells, 71; of worker bees, recently
acquired, 77; starting-point in forma-
tion of species, 32.
Stick-like insects, 122, PL 61 ; spider, PL 64.
Stridulating organs, 51, PL 29.
Struggle for existence, 10-18; between near
relatives, 25 ; man, 169.
Summary, of Part I, 82 ; of color in animals,
151-
Supernaturalism, Intro, ix, x.
Superstition, Intro, ix.
Survival of the fittest, 15, 18.
Swamping of varieties by cross-breeding,
40-47.
Swedish turnip, PL 7.
Synageles picata, PL 28.
Tadpole, of salamander, 98.
Tail, vestigial muscles in man, 165, PL 95.
Taxonomy, 88.
Teeth, deterioration of human, 170; man
and gorilla, 164, PL 92.
Tegetmeier, PI. 12-15, P1- l8> P1- J9> P1- 24-
Tendency, inherent, in evolution, 188; in
variat'on, 40.
Terrifying attitude, 139, 140, 142, PI. 78.
Tetragnatha gr dilator, 120; laboriosa, PL 85.
"Thierleben" (Brehm), 4, 22, 31, 133, PI.
24, PL 30, PI. 33, PI. 62, PI. 99.
Thomisidce, 129.
Thompson, J. A., 189.
Thyroidopteryx ephemeriformis, 21.
Tiger, aggressive coloration, 129, PI. 68.
Timor, friar-bird and oriole, 144.
Timor Laut, friar-bird and oriole, 144.
Toad, aggressive coloration, 126, 133,
PI. 66; rejects noxious insects, 130.
Tody, green, protective coloration, 119.
Tortoise, embryos, PL 38.
Trailing arbutus, odor, 154; variation, 7.
Transparency of pelagic animals, 117.
Tree-frogs, protective coloration, 125, PL 66.
Tree-hopper, mimicry of leaf-cutting ants,
i37, PI- 75-
Trends in evolution, 40, 188, 189, 192.
Trillium grandiflorum, PI. 2.
Triton cristatus, 52, PL Jj; punctatus, 52.
Trochilium apiforme, PL 7®-
Tropidorhynchus, 144.
Tunicates, transparency of pelagic, 117.
Turkey cock, 49, PL 27.
212
INDEX
Turnip, 29, PL 9; Swedish, PL 7.
Turtle, embryos, PL 38; nictitating mem-
brane, PI. 36.
Twig-like caterpillars, 122, PI. 60.
Typhlichthys, 95.
Uloborus plumipes, PL 64.
Unicellular organisms and inheritance of
parental modifications, 68.
Unselfishness in marriage, 180.
Ursus maritimus, 126.
Use and disuse, Intro, xi; effects of, 68;
inheritance of effects of, 72, 73.
"Utility" (Romanes), 197.
Utility, and segregation, 67; uselessness
of certain specific characters, 32; use-
lessness of organs in their beginnings, 37.
Vaccination, 171.
Vanessa, c-album, PL 56; io, 121; urticce,
PL 56, PL 59.
Variation, 7-10, 18, 39; advantageous
when environment changing, 27 ; causes
of, 79, 80, 8 1 ; degree of divergence,
o, 39 ; determinate, 189; fluctuating, 18;
in Neritina, 9, Frontispiece; in Palu-
destrina, PI. 3; in trailing arbutus, 7;
in Trillium, PL 2; mutation, 18; un-
equal in different species, 9.
Varieties, of domestic animals and plants,
29; of horses, 28, PL 4; swamped by
intercrossing, 41.
Vermiform appendix, man and orang, 166,
PL 96.
"Vertebrate Embryology" (Marshall), PI.
42.
Vertebrates, development of, 97, 98, PL 38,
PL 98 ; varying degrees of complexity, 97.
Vespa, breeding habits, 77; occidentals ,
PL 74^
Vestigial structures, 93-96; in man, 164,
PL 93-97.
Vigor, correlated with secondary sexual
characters, 58.
Vilmorin, PL 87.
Vinson, PL 64.
Viola cucullata, 88; rostrata, 89.
Violacece, 90.
Voice, in birds, 49.
Volucella facialis, PL 74.
Vries, de, Hugo, 18, 19, 39, 189, 190.
Walking stick, 122, PL 61.
Wallace, A. R., 35, 47, 50, 58, 132, 144, 197,
199, PL 73.
Wallihan, A. G., PL 8r.
Warning coloration, 129-134; convergence,
134-
Warren, E. R., 146, PL 53, PL 54, PL 57.
Wasps, color sense, 161 ; enemies of spiders,
121 ; fertilizing orchid, 157, PL 89;
mimicked by other insects, 136, PL 70,
PL 73; protected by stings, 130, 135;
warning color, 130, PL 73, PL 74.
Waved-yellow moth, 123, PL 63.
Weasel, PL 67.
Weismann, August, acquired characters,
67; germinal selection, 96; also, 122,
142, 197, 199, PL 76, PL 77, PL 101.
Westermarck, 198.
Whale, hind limbs, 93, 94.
Wheel, invention of, 176.
White Mountains, flora, 112.
"Wild Flowers of America" (Goodale),
PL 88.
Wing, of bird and of butterfly, 92; fore
limbs of vertebrates, 92.
Wolf, aggressive coloration, 126.
Woodcock, protective coloration, 49, 119,
PL 50.
Wright, Lewis, 30, PL 16, PL 18.
Xiphophorus hetterii, PL 32.
Yellow-jacket, warning color, 130, 135,
PL 74-
Zenzera asculi, PL 70.
Zittel, K., PL 44, PL 45.
Zygsena, PL 56, PL 70.
COLUMBIA UNIVERSITY BIOLOGICAL SERIES.
DESIGNED FOR INDEPENDENT READING AND AS TEXT-BOOKS
FOR LECTURE AND LABORATORY COURSES
OF INSTRUCTION.
EDITED BY
HENRY FAIRFIELD OSBORN, Sc.D., LL.D.,
De Costa Professor of Zoology, Columbia University,
AND
EDMUND B. WILSON, Ph.D., LL.D.,
Professor of Zoology, Columbia University.
VOL. L FROM THE GREEKS TO DARWIN.
THE DEVELOPMENT OF THE EVOLUTION IDEA.
By HENRY FAIRFIELD OSBORN, Sc.D., LL.D.
Cloth. 8vo. 259 pages. Illustrated. Price, $2.00 net.
OPINIONS OF THE PRESS.
" Professor Henry Fairfield Osborn has rendered
an important service by the preparation of a concise
history of the growth of the idea of Evolution. The
chief contributions of the different thinkers from
Thales to Darwin are brought into clear perspective,
and a just estimate of the methods and results of each
one is reached. The work is extremely well done,
and it has an added value of great importance in the
fact that the author is a trained biologist. Dr. Os-
born is himself one of the authorities in the science
of Evolution, to which he has made important con-
tributions. He is therefore in a position to estimate
the value of scientific theories more justly than would
be possible to one who approached the subject from
the standpoint of metaphysics or that of literature."
— President DAVID STARR JORDAN,
in The Dial, Chicago.
" A somewhat new and very interesting field of in-
quiry is opened in this work, which is devoted to
demonstrating that the doctrine of Evolution, far
from being a child of the middle of the nineteenth
Century, of sudden birth and phenomenally rapid
growth, as it is by many supposed to be, has really
been in men's minds for ages. It appears in the
germ in the earliest Greek philosophy; in vigorous
childhood in the works of Aristotle ; in adolescence
at the closing period of the last century; and reaches
full-grown manhood in our own age of scientific
thought and indefatigable research."
— New Science Review.
" This is a timely book. For it is time that both
the special student and general public should know
that the doctrine of Evolution has cropped out of the
surface of human thought from the period of the
Greek philosophers, and that it did not originate
with Darwin, and that natural selection is not a
synonym of Evolution. . . . The book should be
widely read, not only by science teachers, by biologi-
cal students, but we hope that historians, students of
social science, and theologians will acquaint them-
selves with this clear, candid, and catholic statement
of the origin and early history of a theory, which not
only explains the origin of life-forms, but has trans-
formed the methods of the historian, placed philoso-
phy on a higher plane, and immeasurably widened
our views of nature and of the Infinite Power work-
ing in and through the universe."
— Professor A. S. PACKARD,
in Science, New York.
"This is an attempt to determine the history of
Evolution, its development and that of its elements,
and the indebtedness of modern to earlier investi-
gators. The book is a valuable contribution; it will
do a great deal of good in disseminating more accu-
rate ideas of the accomplishments of the present as
compared with the past, and in broadening the views
of such as have confined themselves too closely to
the recent or to specialties. ... As a whole the
book is admirable. The author has been more im-
partial than any of those who have in part anticipated
him in the same line of work." — The Nation.
" But whether the thread be broken or continuous,
the history of thought upon this all-important subject
is of the deepest interest, and Professor Osborn's
work will be welcomed by all who take an intelligent
interest in Evolution. Up to the present, the pre-
Darwinian evolutionists have been for the most part
considered singly, the claims of particular naturalists
being urged often with too warm an enthusiasm.
Professor Osborn has undertaken a more compre-
hensive work, and with well-balanced judgment
assigns a place to each writer."
— Professor EDWARD B. POULTON,
in Nature, London.
FIRST EDITION PUBLISHED OCTOBER, 1894.
VOL. H. AMPHIOXUS AND THE ANCESTRY OF
THE VERTEBRATES.
By ARTHUR WILLEY, Sc.D., Balfoitr Student of the University of Cambridge.
316 pages. 135 Illustrations. Price, $2.50 net.
" This important monograph will be welcomed by
all students of zoology as a valuable accession to the
literature of the theory of descent. More than this,
the volume bears internal evidence throughout of
painstaking care in bringing together, in exceedingly
readable form, all the essential details of the structure
and metamorphosis of Amphioxus as worked out by
anatomists and embryologists since the time of Pallas,
its discoverer. The interesting history of the changes
it undergoes during metamorphosis, especially its sin-
gular symmetry, is clearly described and ingenious
explanations of the phenomena are suggested. Most
important, perhaps, are the carefully suggested homol-
ogies of the organs of Amphioxus with those of the
embryos of the Vertebrates above it in rank, especially
those of the Marsipobranchs and Selachians. Though
the comparisons with the organisms next below Am-
phioxus, such as Ascidians, Balanoglossus, Cepha-
lodiscus, Rhabdopleiira, and the Echinoderms,
will be found no less interesting. In short, the book
may be commended to students already somewhat
familiar with zoological facts and principles, as an
important one to read. They may thus be brought
to appreciate to what an extent the theory of descent
is indebted to the patient labors of the zoologists of
the last forty years for a seen re foundation in observed
facts, seen in their correlations, according to the com-
parative method. . . . The present work contains
everything that should be known about Amphioxus,
besides a great deal that is advantageous to know
about the Tunicata, Balanoglossus, and some other
types which come into structural relations with Am-
phioxus."
— Professor JOHN A. RYDER,
in The American Naturalist, Philadelphia.
" The observations on Amphioxus made before the
second half of the present century, amongst which
those of Johannes Miiller take a foremost place, showed
that this remarkable animal bears certain resemblances
to Vertebrates ; and since then its interest in this re-
spect has gradually become more apparent. ... A
consecutive history of the more recent observations
was, therefore, greatly needed by those whose oppor-
tunities did not permit them to follow out the matter
for themselves, and who will welcome a book written
in an extremely lucid style by a naturalist who can
speak with authority on the subject."
— Professor W. NEWTON PARKER,
in Nature, London.
VOL. IIL FISHES, LIVING AND FOSSIL.
AN INTRODUCTORY STUDY.
By BASHFORD DEAN, Ph.D., Adjunct Professor of Zoology, Columbia University.
300 pages. 344 Illustrations. Price, $2.50 net.
This work has been prepared to meet the need of the general student for a concise knowledge of the living
and extinct Fishes. It covers the recent advances in the comparative anatomy, embryology, and palaeontology
of the five larger groups of Lampreys, Sharks, Chimseroids, Teleostomes, and Dipnoans — the aim being to
furnish a well-marked ground plan of Ichthyology. The figures are mainly original and designed to aid in prac-
ntrasts in the development of the principal organs through the five groups.
work. The suggestions here offered may be of use
for another edition. That another may be called for,
we may hope. For the work as it is, and for the care
and thought bestowed on it, our thanks are due."
— THEODORE GILL,
in Science, New York.
tical work as well as to illustrate the contrasts in tl
"The intense specialization which prevails in
zoology at the present day can lead to no other result
than this, that a well-educated zoologist who becomes
a student of one group is in a few years quite left
behind by the student of other groups. Books,
therefore, like those of Mr. Dean are necessary for
zoologists at large."
— The AthencEum, London.
" Dr. Bashford Dean is known to zoologists, first,
as the author of exhaustive and critical articles in the
publications of the United States Fish Commission,
on the systems of oyster culture pursued in Europe,
and, secondly, as an embryologist who has lately been
doing good work on the development of various Ga-
noid fishes and the comparison that may be instituted
with Teleostei. His recent addition to the well-known
' Columbia University Biological Series,' now being
brought out by The Macmillan Company, under the
editorship of Professor H. F. Osborn, is an interesting
volume upon fishes, in which considerable prominence
is given to the fossil forms, and the whole subject is
presented to us from the point of view of the evolu-
tionist. This is the characteristic feature of the book.
From the very first page of the introduction to the
last page in the volume, preceding the index, which
is a table of the supposed descent of the groups of
fishes, the book is full of the spirit and the language
of evolution." — Professor W. A. HERDMAN,
in Nature, London.
" The length to which this review has extended
must be evidence of the importance of Dr. Dean's
" L'ouvrage de M. Bashford Dean nous parait fait
avec soin; les illustrations sont excellentes et tres
nombreuses, et il merite le meilleur accueil de la part
des zoologistes."
— CH. BRONGNIART,
in Le Revue Scientifique, Paris.
" For the first time in the history of Ichthyology,
students are now provided with an elementary hand-
book affording a general view of the whole subject. . . .
The last sixty pages of the volume are devoted to
a list of derivations of proper names, a copious bibli-
ography, and a series of illustrated tabular statements
of the anatomical characters of the great groups of
fishes. These sections bear signs of having been
prepared most carefully and laboriously, and form an
admirable appendix for purposes of reference. There
will be much difference of opinion among specialists
as to the value of some of the tables and the judgment
pronounced by the author; but we have detected a
very small proportion of errors for so bold an enter-
prise, and students of the lower Vertebrata are much
indebted to Dr Dean for an invaluable compendium."
— ARTHUR SMITH WOODWARD,
in Natural Science, London.
VOL. IV. THE CELL IN DEVELOPMENT AND
INHERITANCE.
By EDMUND B. WILSON, Ph.D., LL.D.,
Professor of Zoology, Columbia University.
142 Illustrations. Price, $3.50 net.
371 pages
" In the highest degree fascinating. ... It is a
matter for congratulation that Professor Wilson has
given us in our own speech a book which is second
to none in the clear and comprehensive manner in
which the facts of cell structure and division are set
forth, and the masterly way in which the principal
theories are stated and criticised." — Nature.
" It certainly takes rank at once among the most
important biological works of the period."
— Science.
" We heartily recommend this book. There are
many practitioners who have neither time nor disposi-
tion to read the larger treatises on botany or histology
in which the modern views on the structure and func-
tions of the cell are to be found in detail. ... In
the present volume they will find an admirable expo-
sition of the knowledge that has been acquired during
the last: twenty years." — London Lancet.
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book is its thoroughness. . . . Students and inves-
tigators of biology, in whatever department they may
be working, ought to be familiar with this important
work." — New York Nation.
VOL. V. THE FOUNDATIONS OF ZOOLOGY.
By WILLIAM KEITH BROOKS,
Professor of Zoology, Johns Hopkins University.
8vo. Cloth, viii + 339 pages. Price, $2.50 net.
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VOL. VI. THE PROTOZOA.
By GARY N. CALKINS, Ph.D.,
Instrtictor in Zoology, Columbia University
8vo. Cloth. 365 pages. Price, $3.00 net.
The object of this volume is to set forth the main characteristics of the Protozoa without undertaking an
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sidered, and the volume ends with a discussion of the various views regarding the origin of the Metazoa from
ihe Protozoa.
VOL. VII. REGENERATION.
By THOMAS HUNT MORGAN,
Professor of ftiology, Bryn Mawr College,
Author of "THE DEVELOPMENT OF THE FROG'S EGG."
VOL. VIE. AN INTRODUCTION TO COMPARA-
TIVE NEUROLOGY.
By OLIVER S. STRONG, Ph.D.
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