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A X 


Plate 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. 

ni malteq ioloo ni bnB ioloo ni noilRirsV 

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l bariingjsM 

averf ^srfa si ni 

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aaariJ ,$> .aliade arit nl .babb aus aanil lonim yioaaaooB ,& .allarie 3rlJ ni .aqool b3qsri2-V 

.eiroiamjjn aiora SIB 

: lioo srfJ ^o xoqr. ^:!t i9n msJJtq fjfil ni vligiavtb eworl?. fi ol Q zs'nsfi 
isv mariJ ax>il u ; yiarf alofa isgii,! aiul 01 ;Ii<x> sri) liisn ?Job slirfw isgiel 
:// ?.l Lncd f.irfj fi bnfi i ni ; bnnd sliriv/ auounilnoo K wol ot byJinu 
di bns gi ni .bnsd Jrfgil Ifihotsupa srit lo istojsierio grit ni ytiaisvib aworla ^s o) ji ashag 
aanrl loi^m aril \i ni Jlarfa ariJ lo loJBiipa irit gnot^ InitA 10 baiqunatni SIB eanil tonim srl; yino 
ni .abn^d isworiBn worla os fanfi 91 .aliriw iB3lo JaomlB ai bnfid aril 81 ni .baJqmiatni SIB oals 
as ni .maJtsq arli isbnu 10 ni loloo lo aonaiafttb B vd nv/orla ai bnf;rf iBhotsupa aril fs bns ,2 ,is 
^d woia^-baB avodB baisbiod ^niad .llaaJi msJJBq sriJ ni bslBoibni xlJni^ ^ 


. sViJi^ ^ 

ni bf*T ; n bne ^,d rri ffailqiuq ;ss bn ,os ,91 .to 6 ,d ,*J . . - 1 n ' ^ofiid SIB s^nii bs'ioloo ariT 
, ,ai ni bai aanil lonim arit bns M-u.Id yns asnif -io(Bm aril ;s ni bai fan A ! la 1^2 ni VB-I ;'o ^ 
.^i fans di ni slqinq a^nil nonim srif bnB jlor.ld SIB ssnti To[rn -i.i) ; & g bne ,ug 
rfoBsd bnsa srft moil qu bsqoooa lulbnBff-alduob 9iBl B mo-rt bsJoalgg allaria wa^ B 9iB aaariT 
oJ as auoianum oa 9iaw allaria 9riT .I.W .BoiBmB^ .noaisbnaH lio*! iB9n ".bnoS Jlfi8 " arit lo 
ni ashai'iBV JoniJaib aJiup Jrfgia-YlxiS .9]gb9 a'laJew ad) IB aboi iol rloisad arit lavoo ylatalqnioo 
i(Ial3iqnioo bnn ot aidiaaoq zi Jl .allarig to Jniq ano airft ni bnuo^ aiaw maJlBq 10(00 10 loloo 

isvawori ,allaria aeori) lo owl x nj na9Wiad aanaa 








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All rights reserved 


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. 



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." 



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 

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, 


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, 


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. 


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. 






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 



Comparative Anatomy 88-96 

Classification 88-92 

Homology 92-93 

Vestigial Structures ..... ... 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 




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 



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 . 2 oi- 




PLATE i. Variation in color and in color pattern in Neritina virginea, 


i. < 


2. ' 






3- ' 






4, a. 





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 


Varieties of horses 

The wild cabbage (Brassica oleraced) 




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 




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 .... " 










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 crabs 11 " 98 

PLATE 41. A. " My sis stage 1 ' 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 



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 " 


Cotton-tail " rabbit. 



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 



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 signal 1 ' . . . .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 



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 


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 

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- 


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 

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 


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 


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. 





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 



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 


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. 


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 

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, B t D, F, G, J. Probably no finer example 
of variation in any plant has been described. 



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 


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.] 


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 


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. 


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 


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 7 8 73 2 

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 


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 


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- 

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 


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 


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 


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 


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. 


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 


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 


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 



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. 



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 

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 


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 


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 


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 


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 


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 


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 


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 

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. 

,iloooo7d .(alooaiod) alfid ,asddo yov2 .agfiddfio to aabahfiv JnaigfliQ .\ ,6 , 83TAJ1 
yd bavngb nggd svBrl rioiriw \o Us ; idsilrioji bn ,qimul riaibgwS ,igwonilij0 .aJtroiqa algaein9 
10 ylirriBl biJai;M sriJ to ladmsm B zl rioiriw ,^t^\Q ^uiatft. aaioaqg bJiw srlt moil 

hsohariqa ".JiulniawrioS " ,9SddfiD .2 .beari balnioq .yliea .bsi jhfib . 

,gnol ",^BoI iBgua " .sgfiddfiD .^ .bsarf on ".nBohsmA b9Sl-n99i " .ggBdd^D . 

.b9ri Ifivo ,nol ".nifbInBi'5 " ,9Sfidd0 yovBg .6 .llama ",yliB9 aanna^ " .sgBddBO .g, .b3ri lvo 

ovfig .8 ".i9mmijabirn yhfia BiJxa" ,9BddBO 
to gniblnho 10 gnihuo lo 99i9fa bnfi ,b9rf lo 
.11 .bghuo ,Il} ,9l^ .01 ".nggig nmi9O " 

Jon 9i Jud .baqolsvgb xlrigirf 9i aavBgl aril alfial ni) ".9l3i-99it 10 woo" ,Iit x 13 


afqiuq .ilooooia .^i .^iBta arit ^nolB abBari IlBma ynsm ;hfiwb .atuoiqa alsaamS .i 
".ilooooid sqBD glqiuq " ,nfiilioi2 ,i9wonilu3 .gi .baati ri)od amoaaold bns aavBal ; gniJuoiqa 
,b9qoi9V3fa ylJ3ig 9i amoaaold grit lawoftilufio grit ni) ".noUriO " yh9 ,"hwb ,iawoniIuD .61 
baviiab naad 9Vri ot biBa ai rioiriw .gqyf airiJ ni .qimut rfaibawS .TJI (.bBari JoBqmoo nirmol 
.bagiBlng naad ari ^IsJa aril lo noirtoq bnuoigiabnii aril ,9^Bddo grit a aaiogqa bliw amea aril moil: 

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 

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 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. 


dinsaJxia sriJ lo tisq isttfil arft ni ".tiowaloo " 10 .agsddfio lo zaiterusV .8 3TA Jl 

nsqO " .^ .(ami) leilt Ja nwonjf enw oals s^ddfio bai) " sloo sgBddip sJiriW" .^ 
rftod : : .[) "slew ^-.-. ' .V> ".sloo Yov*8" ^ .(toBqrriop aasi bfi'jrfj " aloo 

" 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 J2l sffj ni noiteaftibom 'to JaaJxa srii^o 


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. 


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. 


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 


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. 


A. Silver Polish. 

B. Houdanj 

C. La Fleche. D. White and game bantams. 


A. Partridge Cochins. 

B. Buff Cochin hen. 

C. Dark Brahmas. D. Light Brahmas. 




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. 


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.] 


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. 


They will often freely intercross in breeding, while, as a 
usual thing, natural species will not do so. This brings us 


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- 


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 \ -^**"o v -ZH 

!' T 5 1 




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 

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 


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- 


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, 


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, 


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- 

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 


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 


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 


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)- 


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- 


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- 


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 


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 


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. 


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, 


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. 


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 


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. 










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. 



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 


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 

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 2 g. 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 . 


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. 


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. 




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. 


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 


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. 



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. 


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- 

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- 



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 


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- 


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 elimination 1 
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. 


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. 


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 


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 



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 



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 

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 


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- 


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 

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 


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." 


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. 


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 


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 


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 


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 





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 


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. 


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 

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 


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 


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 


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 


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 


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 


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 


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. 


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 


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 w r hen 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 


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." 


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. 


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. 




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. 




The phenomena of comparative anatomy in their bearing 
upon the theory of evolution. 


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. 



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. 

9 o 


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. 


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 

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, 


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. 


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 



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 

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 



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. 


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. 



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 


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 


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 

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 



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 


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 


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.] 



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 



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 



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 



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. 



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 

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 


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 

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 




Epochs and Formations 

Faunal Characters 

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. 

JURASSIC, 6,000 ft. 


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 

PERMIAN, 5,000 ft. 

CARBONIFEROUS, 26,000 ft. 

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; 

Earliest fish; the first air-breathers (insect, 
scorpion) ; brachiopods and 4-gilled cephal- 
opods very abundant; trilobites; corals; 

Sponges, jellyfish, annulated worms, Mollusca, 
brachiopods, Polyzoa, echinoderms no ver- 

From Romanes' Darwin and after Darwin, slightly modified. 


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 


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 



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 



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, 



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 

Pliohippus : 

Protohippus : 
Lower Plio- 

Miohippus : 

Mesohippus : 
Lower Mio- 

Orohippus : 

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. 


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 


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 


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. 


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, 


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 


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. 


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 


a most effective means of protection to these transparent 

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. 


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.) 



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. 

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. 




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. 


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 





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. 


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 



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 /TI ^ /- \ T'l J 

of the United States National Museum.] (Plate 65). The Seaweed IS 


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.] 


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 


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- 

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 


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- 



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 


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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. 



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 



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 


FIG. 36. A mantis (Hymenopus bicornis), which resembles an 
orchid blossom. By courtesy of Crowell Publishing Company. 


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 


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. 


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. 

- "^, 



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. 


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 


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 


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. 



A. Papillio machaon. B. Arctia caja. C. Orgya antiqua. D. Acronycta alni. E. Acronycta 
psi. F. Parnassius apollo. G. Melitcea cinxia. H. Leucoma salicis. 




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. 


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 

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 


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 > 


, 10 


12, 13 




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 



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 


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}. 



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. 



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 
nVQpr\7-prl 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 s P ecimens 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 



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.] 

S^IS^ 1 

= 5g5: 

12 So 

X .- - 1WST 5 .2 


, |H"i 

M? ^ P-t ^ ^ '" 

'a x" 1 " = = 

?- rf" 'Z ^ " 

" c ^ 

a. ^ p 5 rt r 

^2; gca" 


C/3 VH -2 sI^^-C W * O 

I Plllli ^ 

P4 ** C cu 1^ Q' 

5: b' J 3N' L 'w < <j2 </! 
M ^ ?. " -'S'O S 6 
55 bi :3| <iS < " a *" 




>-3ri X 1} 



- M ^ 'C -Ss 

53 ^ g^ g-'S'r 7! 

2 ^ rt ;?,&, o' 

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. 


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 



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- 


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, 


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 


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.] 



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. 


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, 


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, 




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. 

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. 


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. 


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. 


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 


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 

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 


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 


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 

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 


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.] 



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. 



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 

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 

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). 



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 



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. 


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- 


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- 

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. 




f Y Illl |MIJM> 1 

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f <kttdnf wfc 



--Jajh t bins 
toward the mtr- 


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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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 


to plant, as well as from blossom to blossom of the same 
plant, cross-fertilization between plants must frequently 

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- 


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 

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 


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 : 








































































































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 

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 


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. 


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. 


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. 


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 


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 





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. 


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. 


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}. 


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\ 



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. 


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 


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 


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. W r e 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, 


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 


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- 


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 


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 


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- 


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 ? 


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 


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 


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. 


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. 


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 


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 

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 


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- 


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. 


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 


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 



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 

The Sacculina soon becomes little more than a bag of 


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 


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 


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 


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. 



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, 



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 
r han 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- 


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. 


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. 



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. 


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 


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. 


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 


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, 


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. 



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. 



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 


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 

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. 


(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, 

Antelope, confusing coloration, 148, PL 81 ; 
protective color, 120; signalling, 146, 
PL 81. 


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. 



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, 

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. 



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. 



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, 


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, 


Euckistus servus, PL 69. 
Euplcea midamus, 138, 150, PL 84. 
Evermann, Barton G., PL 48, PL 58. 
"Evolution and Adaptation" (Morgan), 


"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, 




"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 onca t 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, 

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. 

Galeus t 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, 

"Germ Plasm, The" (Weismann), 191, 

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, 

Goodale, William, PI. 88. 

Goose, barnacle, 3, 4, 5; mating of white 
and Chinese, 48; variation slight, 

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. 



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, 

"Heredity and Utility" (Romanes), 197. 

Herrick, F. H., PL 41. 

Hesperid(p t 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 

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 I 75> T 7 8 > !79- l8 3- 

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. 



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. 


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. 



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, 


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- T 45- 
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, 


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), 

"Origin of Species, The" (Darwin), Intro. 

x, 197. 
"Origin of the Fittest, The" (Cope), 198, 



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, 

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. 

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, 

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 i5 6 - 

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, 


Protohipptis, PL 47. 
Providence, Intro, ix. 
Pseudacrcea boisduvalii, PL 77. 



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), 


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, J 68; see also 42-47, 48. 

Selection, artificial, 28-31; germinal, 96; 
natural, 3-47; organic, 27, 28, 177, 192; 
physiological, 66; "selection value," 17, 


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. 



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, 


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, go y 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, 


Supernaturalism, Intro, ix, x. 
Superstition, Intro, ix. 
Survival of the fittest, 15, 18. 
Swamping of varieties by cross-breeding, 


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 - J 9> P1 - 2 4- 
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. 



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. 

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, 


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. 







De Costa Professor of Zoology, Columbia University, 



Professor of Zoology, Columbia University. 



Cloth. 8vo. 259 pages. Illustrated. Price, $2.00 net. 


" 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." 

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." 


in Nature, London. 



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- 

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. 



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." 

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." 


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." 

in Natural Science, London. 



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." 


" 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. 

" One of the very best scientific manuals published 
in America. ... A noteworthy characteristic of the 
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. 



Professor of Zoology, Johns Hopkins University. 
8vo. Cloth, viii + 339 pages. Price, $2.50 net. 

" A book that will live as a permanent addition to 
the common sense of science. It belongs to literature 
as well as to science. It belongs to philosophy as 

much as to either, for it is full of that fundamental 
wisdom about realities which alone is worthy of the 
name of philosophy." Science. 



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 
exhaustive description. It is intended for students and for general readers who wish to know what the Pro- 
tozoa are, and what their relations are to current biological problems. In the first few chapters of the book 
the Protozoa are treated as a phylum of the animal kingdom. A short historical sketch leading up to the 
present systems of classification is followed by a general description of the group, touching upon some of the 
more special subjects, such as mode of life, motion, excretion, respiration, reproduction, colony-formation, 
encystment, etc., and this is followed by more general subjects dealing with the Protozoa in relation to man 
and other animals; eg. their sanitary aspects, parasitism, symbiosis, etc. 

In the final chapter the Protozoa are dealt with from the standpoint of phylogeny. Theories as to the 
origin of life, spontaneous generation, and the relations of the classes of Protozoa to one another are con- 
sidered, and the volume ends with a discussion of the various views regarding the origin of the Metazoa from 
ihe Protozoa. 



Professor of ftiology, Bryn Mawr College, 


THE MACMILLAN COMPANY, 66 Fifth Avenue, New York. 


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