BIOLOGY
LIBRARY
G
.
PREFACE
The present volm brings together in brief form
fundamental priix ipl» >f biology for the college student
the general reader
It is well recogni/i that there is no adequate subst
for detailed laborator vork on the structure and physic
of representative ory -m> as a means of affording a
hand knowledge of ;| acts and methods of biology. I
ever, the author I ili/cd with increasing force thai
student's correlation the laboratory data from day to
and accordingly hi- preciation of the broader aspec
the subject are great enhanced by a synchronous 'run
account' of the um lying principles. The material
this volume has pro1 to be of great value for this pur
in a course* on (iencr Biology elected each year by se\
hundred Yale underg dilates.
The large problems f life are common to both zoology
botany, and therefoi both animals and plants have 1
drawn upon for illust tion and discussion. This inetho
presentation accords ith the author's conviction that
general biological vie point is the most favorable mean
approach both to a road knowledge of living phenon
as a part of a 'lilH'il1 education, and to more advai
studies in zoology ad botany which are prerequisite
FOUNDATIONS OF BIOLOGY
BY
LORANDE LOSS ^OODRUFF
PROFESSOR OF BIOLOGY IN YALE UNIVERSITY
J^eto gorfe
THE MACMILLAN COMPANY
1922
All rights reserved
FOUNDATIONS OF BIOLOGY
BY
LORANDE LOSS ^OODRUFF
PROFESSOR OF BIOLOGY IN YALE UNIVERSITY
THE MACMILLAN COMPANY
1922
All rights reserved
FOUNDATIONS OF BIOLOGY
BY
LORANDE LOSS
PROFESSOR OF BIOLOGY IN YALE UNIVERSITY
J^teto
THE MACMILLAN COMPANY
1922
All rights reserved
PRINTED IN THE UNITED STATES OF AMERICA
BIOLOGY
LIBRARY
6
COPYRIGHT, 1922,
BY THE MACMILLAN COMPANY
S«t up and electrotyped. Published June, 1922
PREFACE
The present volume brings together in brief form the
fundamental principles of biology for the college student and
the general reader.
It is well recognized that there is no adequate substitute
for detailed laboratory work on the structure and physiology
of representative organisms as a means of affording a first-
hand knowledge of the facts and methods of biology. How-
ever, the author has realized with increasing force that the
student's correlation of the laboratory data from day to day
and accordingly his appreciation of the broader aspects of
the subject are greatly enhanced by a synchronous ' running
account' of the underlying principles. The material in
this volume has proved to be of great value for this purpose
in a course on General Biology elected each year by several
hundred Yale undergraduates.
The large problems of life are common to both zoology and
botany, and therefore both animals and plants have been
drawn upon for illustration and discussion. This method of
presentation accords with the author's conviction that the
general biological viewpoint is the most favorable means of
approach both to a broad knowledge of living phenomena
as a part of a 'liberal' education, and to more advanced
studies in zoology and botany which are prerequisite for
rr o f i» rr •»
VI PREFACE
medicine, forestry, etc. As is natural, however, the zoolog-
ical aspect has been emphasized since it affords indispensable
data for the interpretation of Man himself. For courses in
general zoology, therefore, the book will be found adequate
in its treatment of animals, while chapters VIII and IX on
plants may readily be omitted, without breaking the con-
tinuity of the discussion.
The author is indebted, of course, to innumerable sources
for the facts and principles outlined. The content has
grown by accessions year by year. Nearly all the stand-
ard treatises have been drawn upon, but those which
have been most generally suggestive are listed in the bibli-
ographies of the respective chapters. Specific mention, how-
ever, should be made here of Professor Wilder's History of
the Human Body, Professor Conklin's Heredity and En-
vironment in the Development of Men, Professor Ganong's
Text-book of Botany, and Professor Coulter's Evolution of
Sex in Plants.
The author has availed himself of the constructive criticism
generously given by Professor B. W. Kunkel of Lafayette
College, Professor E. H. Cameron of the University of
Illinois, and his colleagues at Yale, Professors R. G. Harrison,
W. R. Coe, A. Petrunkevitch, F. P. Underbill, Henry
Laurens, G. A. Baitsell, W. W. Swingle, and Dr. J. W.
Buchanan, who have read the book either in manuscript or
in the mimeographed form in which it has been used by the
Yale classes. And Professor Baitsell's interest in the work
of the course has made it possible to impose upon him the
added task of reading the book at each stage of its develop-
ment. Miss Hope Spencer of the Yale Laboratory has as-
sumed with enthusiasm a considerable portion of the editorial
work involved in seeing the book through the press.
Finally, the author's indebtedness to the criticism and
PREFACE Vll
cooperation of his wife, Margaret Mitchell Woodruff, must
not remain unmentioned, though it cannot be adequately
expressed.
The original illustrations as well as those from other sources
which have been modified or merely redrawn are, with a
few exceptions, the work of Mr. R. E. Harrison, Yale, 1923.
In most cases these figures have been selected because of
their proved pedagogic value. Acknowledgments are due to
the authors and publishers of the following works, from which
illustrations have been reproduced by permission: Coulter,
Barnes, and Cowles' Textbook of Botany, Coulter's Plant
Life and Plant Uses (American Book Co.) ; Kellicott's Social
Direction of Human Evolution, Jordan and Kellogg's Evolu-
tion and Animal Life, Darwin's Life and Selected Letters,
Huxley's Life and Letters (D. Appleton & Co.); Folsom's
Entomology, Gager's Fundamentals of Botany (P. Blakis-
ton's Sons & Co.); Jennings' Behavior of the Lower Organ-
isms (Columbia University Press); Bergen's Foundations
of Botany, Bergen and Caldwell's Practical Botany, Bergen
and Davis' Principles of Botany, Densmore's General
Botany, Hough and Sedgwick's The Human Mechanism,
Linville and Kelly's General Zoology (Ginn & Co.); Kelli-
cott's General Embryology, Sedgwick and Wilson's General
Biology (Henry Holt & Co.); Morgan's Physical Basis of
Heredity (J. P. Lippincott & Co.); Romanes' Darwin and
After Darwin (Open Court Publishing Co.); Conklin's
Heredity and Environment in the Development of Men
(Princeton University Press) ; Conn and Budington's Physi-
ology and Hygiene (Silver, Burdett & Co.) ; Coulter's Evolu-
tion of Sex in Plants (University of Chicago Press) ; Abbott's
General Biology, Buchanan and Buchanan's Bacteriology,
Campbell's University Textbook of Botany, Ganong's Text-
book of Botanv for Colleges, Hegner's College Zoology, and
Vlll PREFACE
Introduction to Zoology, Holmes' Biology of the Frog, Hux-
ley's Physiology, Lankester's Treatise on Zoology, Lull's
Organic Evolution, Packard's Textbook of Entomology,
Parker's The Elementary Nervous System, Parker and Has-
well's Textbook of Zoology, Parker and Parker's Practical
Zoology, Scott's The Theory of Evolution, Shipley and
McBride's Zoology, Verworn's General Physiology, Walter's
Genetics, and The Human Skeleton, Wiedersheim's Compar-
ative Anatomy of Vertebrates, Wilson's The Cell (The Mac-
millan Co.). To The Macmillan Company is also due the
author's appreciation of the liberal attitude which they have
assumed in all the arrangements attendant upon the pub-
lication of the book.
L. L. WOODRUFF.
Yale University, ,
May, 1922.
CONTENTS
CHAPTER PAGE
I. THE SCOPE OF BIOLOGY 1
II. THE PHYSICAL BASIS OF LIFE 6
A. Protoplasm 7
B. Characteristics of Living Matter 10
1. Chemical Composition 11
2. Metabolism 15
3. Growth 16
4. Reproduction 17
5. Adaptation 17
6. Organization 18
III. ORGANIZATIONAL UNITS OF PLANTS AND ANIMALS . 21
A. The Cell 23
1. Cytoplasm 24
2. Nucleus 27
B. Origin of Cells 28
TV. METABOLISM OF GREEN PLANTS 30
A. Structure and Life History of Sphaerella ... 30
B. Metabolism in Sphaerella 34
1. Food Making 35
2. Respiration 37
V. METABOLISM OF ANIMALS 39
A. Structure and Life History of Paramecium . . 39
B. Metabolism in Paramecium 41
1. Food Taking 42
2. Respiration and Excretion 43
VI. METABOLISM OF COLORLESS PLANTS 44
A. The Bacteria 44
B. Cycle of the Elements in Nature 46
C. The Hay Infusion Microcosm 50
VII. THE MULTICELLULAR ORGANISM 54
VIII. THE PLANT BODY 61
A. Gross Structure 65
1. Root 65
2. Stem 69
3. Leaf . 71
X CONTENTS
CHAPTER PAGE
VIII. THE PLANT BODY — Continued
B. Histology 75
1. Root 78
2. Stem 81
3. Leaf 82
C. Physiology 84
1. Circulation Paths 85
2. Dynamics of Circulation 88
3. Food Utilization 89
IX. REPRODUCTION IN PLANTS 91
A. Spore Formation 92
B. Gamete Formation 94
C. Sex Differentiation 96
D. Reproductive Organs 98
E. Alternation of Generations 100
1. The Moss 100
2. The Fern 103
3. Higher Ferns 105
4. Flowering Plants 107
X. THE ANIMAL BODY 115
A. The Chief Groups of Animals . 116
B. Hydra 118
C. Earthworm 121
D. Crayfish 129
E. Vertebrates 135
1. Body Plan 136
2. Skin 138
3. Muscles 138
4. Coelom 140
5. Skeleton 140
F. Diagnostic Vertebrate Characters 146
XI. NUTRITION IN ANIMALS 154
A. The Alimentary Canal . 154
B. Digestion 157
XII. CIRCULATION AND RESPIRATION IN ANIMALS . . . 161
A. Circulation in the Lower Vertebrates . . . . 162
B. Respiration 168
C. Circulation in the Higher Vertebrates . . . . 170
XIII. EXCRETION IN ANIMALS 175
XIV. COORDINATION IN ANIMALS 181
A. Chemical Coordination 181
B. Coordination by the Nervous System .... 183
CONTENTS xi
CHAPTER PAGE
XIV. COORDINATION IN ANIMALS — Continued
C. Sense Organs 193
1. Cutaneous Senses 195
2. Sense of Taste 195
3. Sense of Smell 196
4. The Ear 196
5. The Eye 198
XV. REPRODUCTION IN ANIMALS 203
XVI. ORIGIN OF THE INDIVIDUAL 209
A. Origin of Life 209
B. Reproduction 212
C. Origin of the Germ Cells 223
1. Mitosis 224
2. Gametes 228
3. Spermatogenesis 230
4. Oogenesis 232
5. The Chromosome Cycle 233
6. Fertilization 237
D. Significance of Fertilization 242
1. Protista 243
2. Metazoa 249
E. Organization of the Zygote 251
XVII. HERITAGE OF THE INDIVIDUAL 261
A. Heritability of Variations 264
1. Modifications 265
2. Combinations 268
3. Mutations 269
B. Galton's 'Laws' 269
C. Mendelisrn 271
1. Monohybrids 272
2. Dihybrids 276
3. Trihybrids 280
4. General Principles 280
D. Neo-Mendelism 282
E. Mechanism of Mendelian Inheritance .... 287
1. Sex Determination 291
2. Linkage 293
F. Nature versus Nurture . . '.-•-• 296
G. Selection . . . -. . 299
Pure Lines . . 393
Summary 306
Xll
CONTENTS
CHAPTER
XVIII.
XIX.
ADAPTATION OF ORGANISMS
A. Adaptations to the Physical Environment
1. Adaptations Essentially Functional .
Food
Temperature
Pressure
2. Adaptations Essentially Structural .
Adaptive Radiation of Mammals .
Animal Coloration
The Legs of the Honey Bee
B. Adaptations to the Living Environment .
1. Communal Associations
2. Symbiosis
3. Parasitism
4. Immunity
C. Individual Adaptability
THE ORIGIN OF SPECIES
A. Evidences of Organic Evolution
1. Taxonomy
2. Comparative Anatomy
3. Paleontology
4. Embryology
5. Physiology
6. Distribution
B. Factors of Organic Evolution
EPOCHS IN BIOLOGICAL HISTORY
A. Greek and Roman Science
B. Medieval and Renaissance Science
C. The Microscopists
D. The Development of the Subdivisions of Biology
1. Taxonomy
2. Comparative Anatomy
3. Physiology
4. Histology
5. Embryology
6. Genetics
7. Organic Evolution
APPENDIX
I. CLASSIFICATION
A. Plants .
B. Animals
II. BIBLIOGRAPHY
III. GLOSSARY
PAGE
307
308
308
308
311
313
313
313
319
324
330
331
331
334
338
339
345
347
348
351
356
364
367
368
372
379
379
382
386
389
390
392
394
398
401
403
406
413
413
414
417
429
INDEX
457
LIST OF ILLUSTRATIONS
FIGURE TITLE PAGE
Charles Darwin Frontispiece
1 The divisions of biology 4
2 Amoeba proteus 9
3 Alveolar appearance of protoplasm 10
4 Cells 21
5 Section of a leaf 22
6 Section of Hydra 22
7 Types of cells 25
8 Diagram of a cell 26
9 Life cycle of Sphaerella 32
10 Paramecium calkinsi 40
11 Paramecium aurelia, dividing 41
12 Paramecia conjugating 42
13 Types of Bacteria 45
14 Types of flagellation in Bacteria 46
15 The carbon cycle 48
16 The nitrogen cycle 49
17 Spondylomorum 55
18 Volvox globator 56
19 Cleavage of Sea Urchin's egg 58
20 Section of Frog's intestine 59
21 Cross section of a plant stem 60
22 Spirogyra 61
23 Common Seaweed (Fucus) 62
24 Giant Kelp r 63
25 Gulf weed (Sargassum) 64
26 Types of roots O ... 65
27 Seasonal history of an annual plant 66
28 Seasonal history of a biennial plant 67
29 Aerial roots of English Ivy 67
30 Haustoria of Dodder 68
31 Strawberry runners 69
32 Hyacinth bulb 70
33 'Smilax' (Myrsiphyllum) 71
34 Leaf of a Flowering Plant 71
XIV LIST OF ILLUSTRATIONS
FIGURE TITLE PAGE
35 Winter buds 72
36 Onion leaf 72
37 Pitcher Plant (Sarracenia purpurea) 73
38 Sundew 73
39 Sensitive Fern (Onoclea sensibilis) 74
40 Floral parts of Azalea (Loiseleuria) 75
41 Ideal vertical section of a Flowering Plant 76
42 Generalized plant cell 77
43 Root tip 79
44 Root hair 80
45 Cross section of a stem 81
46 A bud of Elodea canadensis 82
47 Cross section of a leaf 83
48 Diagram illustrating the physiology of a plant .... 87
49 Ulothrix 95
50 Oedogonium, filamentous form 97
51 Oedogonium, zygote 98
52 A Brown Alga (Ectocarpus) 99
53 Life history of a Moss 101
54 Life history of a Fern 104
55 Decline of gametophyte and increase of sporophyte . . 105
56 Life history of a higher Fern 106
57 Megaspore and microspore 107
58 Typical flower 107
59 Transition between petals and stamens 108
60 Union of carpels to form the ovule case 108
61 The life history of a higher Flowering Plant 110
62 Seed of Violet Ill
63 Diagram of comparative morphology of plants . . . . 112
64 Hydra, longitudinal section 119
65 Hydra, transverse section 120
66 Earthworm, body plan 122
67 Dissection of Earthworm (Lumbricus terrestris) .... 123
68 Earthworm, transverse section . 124
69 Stages in development of the Earthworm 126
70 Structure of a primitive Arthropod 130
71 Dissection of Crayfish (Cambarus affinis) 131
72 Appendages of Crayfish 132
73 Nervous system of Earthworm and Crayfish .... 134
74 Sagittal section of an ideal Vertebrate 137
75 Cross section of an ideal Vertebrate 137
76 Section of human skin 139
77 Skeleton of a bony Fish (Perca fluviatilis) 141
LIST OF ILLUSTRATIONS XV
FIGURE TITLE PAGE
78 Relation of the notochord to vertebrae 142
79 A typical human vertebra 143
80 Plan of th3 Vertebrate limb skeleton 144
81 Skeleton of a Mammal (Felis domesticd) 145
82 Dissection of Yellow Perch (Perca flavescens) .... 148
83 Dissection of Green Frog (Rana clamitans) 149
84 Dissection of Pine Lizard. (Sceloporus undulatus) . . . 150
85 Dissection of domestic Pigeon (Columba livid] .... 151
86 Dissection of Gray Squirrel (Sciurus carolinensis) . . . 152
87 Sagittal section of human body 153
88 Human alimentary canal and derivatives 155
89 Chemical activities of the digestive tract 158
90 A gland 159
91 Vascular system of a Shark . . . 164
92 Plan of the circulatory system of a Fish, Amphibian, and
Mammal 165
93 Paths of absorbed food 168
94 Food and respiratory paths . . 170
95 Transformations of the aortic arches 171
96 Nephridium of an Earthworm 177
97 Evolution of the urogenital system 179
98 Section of human kidney and ureter 180
99 Simple receptor-effector system 184
100 More complex receptor-effector system 184
101 Simple reflex arc 184
102 Differentiation of nerve cells 185
103 Nervous organization of intestinal wall 186
104 Development of the Vertebrate brain 187
105 Types of Vertebrate brains 189
106 Nervous system of the Frog 190
107 Paths of nervous impulses 193
108 Differentiation of sense cells 194
109 Membranous labyrinth of the ear 197
110 Human ear 198
111 Development of the eye 200
112 Vertebrate eye (human) 201
113 Uterus (human) 205
114 Yeast cells 213
115 Reproductive cell cycles 214
116 Hydra, dividing 217
117 Flatworm, dividing 217
118 Obelia 218
119 Body plan of polyp and medusa 219
XVI LIST OF ILLUSTRATIONS
FIGUBE TITLE PAGE
120 Regeneration and grafting in Hydra 221
121 Regeneration and grafting in an Earthworm 222
122 Regeneration and grafting in a Flatworm 223
123 Typical stages in Mitosis 225
124 Period of chromosome reduction in animals and plants . 229
125 Spermatogenesis and oogenesis in animals 231
126 Chromosome cycle in an animal 235
127 Egg and sperm of Lamprey 236
128 Hen's egg 238
129 Human egg and sperm 239
130 Conjugation in Paramecium 245
131 Endomixis in Paramecium • . 248
132 Development of Dentalium 255
133 Comparison of the development of Dentalium and Amphioxus 257
134 Relation of cytoplasmic differentiation to development . 259
135 Continuity of the germ plasm 265
136 Alternative, mosaic, and blending inheritance .... 269
137 Law of filial regression 270
138 Inheritance of size in Peas 273
139 Mendelian monohybrid 275
140 Mendelian dihybrid 277
141 Inheritance of human hair characters 278
142 Possible types of zygotes in a dihybrid 279
143 Mendelian trihybrid 281
144 Color inheritance in the Four-o'clock (Mirabilis jalapa) . 284
145 Color inheritance in mulattoes 285
146 Chromosome cycle in an animal 289
147 Chromosomes or genes at fertilization and maturation . . 290
148 The X chromosome in fertilization 293
149 Inheritance of color-blindness from the male 294
150 Inheritance of color-blindness from the female .... 295
151 Crossing-over at synapsis 296
152 Nature and nurture 298
153 Population and pure lines in Beans 300
154 Model to illustrate the law of probability 301
155 Normal frequency curve 302
156 Selection and filial regression 303
157 Relation of pure lines to a population 305
158 Sulfur Bacteria (Beggiotoa) 309
159 Yeast 310
160 Spore formation in Bacteria 312
161 Adaptive radiation in Eutheriau Mammals 314
162 Gymnura .... 315
LIST OF ILLUSTRATIONS XVU
FIGURE TITLE PAGE
163 Foot postures of Mammals 316
164 Sloth (Choloepus hoffmanni) . 317
165 Mole (Talpa europea) 317
166 Porpoise (Phocaena communis) 317
167 'Flying Lemur' (Galeopithecus volans) 318
168 Bat (Vespertilio noctula) 318
169 Katydid (Microcentrum laurifolium) 319
170 Underwing Moth (Catocala lacrymosa) 320
171 Dead-leaf butterfly (Kallima paralecta) 321
172 Walking-stick (Diapheromera femorata) 322
173 Larva of a Geometrid Moth 322
174 Drone Bee and Bee Fly (Eristalis tenax) 323
175 Bees 325
176 Head of Bee 326
177 Legs of Bee 327
178 Foot of Bee 328
179 Formation of a Lichen 332
180 Ants and Aphids 333
181 Life cycle of a Malarial Parasite 335
182 Trypanosoma theileri 337
183 Avoiding reaction of Paramecium 340
184 Rotation of Paramecium 341
185 Vertebrate fore-limbs 352
186 Skeleton of Man and Gorilla 354
187 Vestigial hind-limbs of a Snake 355
188 Archaeopteryx 360
189 Evolution of the Horse 362
190 Evolution of the Camel 363
191 Comparison of embryos of Fish, Bird, and Man . . . 366
192 Phylogeny of the Elephants 370
193 Evolution of the head of Elephants 371
194 Varieties of domestic Pigeons 373
195 Aristotle 380
196 Theophrastus of Eresus 381
197 Andreas Vesalius 384
198 William Harvey 386
199 Antony van Leeuwenhoek 387
200 Marcello Malpighi 389
201 Carolus Linnaeus 391
202 Georges Cuvier 392
203 Thomas Henry Huxley 393
204 Stephen Hales 397
205 Matthias Jacob Schleiden 399
XVili LIST OF ILLUSTRATIONS
FIGUBE TITLE PAGE
206 Theodor Schwann 400
207 Karl Ernst von Baer 402
208 Gregor Johann Mendel 405
209 Comte de Buffon 407
210 Erasmus Darwin 408
211 Jean-Baptiste Lamarck 409
FOUNDATIONS OF BIOLOGY
FOUNDATIONS OF BIOLO.GY
CHAPTER I
THE SCOPE OF BIOLOGY
Science is, in its source, eternal; in its scope, unmeasurable;
in its problem, endless; in its goal, unattainable. — von Baer.
THE oldest and the most obvious classification of the
materials of our environment is into non-living and living;
and the accumulation of knowledge in regard to the former is
represented in the so-called physical sciences, while that of
the latter comprises the content of BIOLOGY, the science of
matter in the livine^state.. Biology, like all science, has as its
ultimate object the explanation of its phenomena in terms
of the basic concepts — matter and energy acting in space
and time; but it is needless to say that the realization of this
object is not imminent in any department of knowledge, and
least of all in the science of living things which exhibit a
condition of matter which altogether transcends the classi-
fications of physicist and chemist to-day — a condition which
expresses in its highest manifestations what we call 'our life.'
Whether the 'riddle of life' will ultimately be solved is a
question which every one would like to answer but only the
rash would attempt to predict. Suffice it to say that biolo-
gists who are on the firing line of progress to-day are directing
their attention solely to an attempt to elucidate life phenom-
ena in terms which the chemist and physicist offer. Our
present interest, however, is not in discussing the theoretical
goal of biology, but in drawing in bold strokes an outline
picture of the present-day knowledge of the subject which
1
2 FOUNDATIONS OF BIOLOGY
represents the cumulative results of the application, to
problems of life, of the Scientific method — a method which
is not peculiar to science but merely a perfected concentra-
tion of our human resources of observation, experimentation,
and reflection. Thus far this has been a most productive
method and certainly has given no evidence that its useful-
ness is being exhausted. To follow any other course would
be to abandon the method of science. "In ultimate analysis
everything is incomprehensible, and the whole object of
science is simply to reduce the fundamental incomprehensi-
bilities to the smallest possible number."
The foundations of the scientific study of living nature were
laid by Aristotle and Theophrastus over 2000 years ago. On
the basis of collecting, dissecting, classifying, and pondering
they reached generalizations, many of which have but recently
been put on a firm basis of fact. Indeed these pioneers
asked nearly all the broad questions which are fundamental
to-day; but from the Greeks until about the fifteenth century
there is little to record. There were many additions to the
body of knowledge during this long slumber period, but fact
and fancy were so amalgamated that the truth was obscured.
The feeling that though Man is of nature, he is still apart,
was expressed at the revival of learning in the broad classifica-
tion of all knowledge as history of nature and history of
Man; the former having as its content the record or "history
of such facts or effects of nature as have no dependence on
Man's will, such as the histories of metals, plants, animals,
regions, and the like"; the latter treating of the voluntary
actions of men in communities. Thus all record of facts
was either natural history or civil history. From this more
or less nebulous natural history the present-day sciences of
astronomy, physics, chemistry, geology, and biology were
thrown off as relatively independent bodies of facts as each
THE SCOPE OF BIOLOGY 3
gained content, clearness, and individuality. Astronomy,
physics or natural philosophy, and chemistry were emanci-
pated first owing to the fact that their material was more
readily susceptible to mathematical and experimental treat-
ment, thus leaving the histories of the Earth, animals and
plants, or so-called observational sciences, as the residue for
natural history. It is in this restricted sense that natural
history still lingers.
It remained, however, for Lamarck and Treviranus during
the opening years of the nineteenth century to attain a vision
of the unity of animal and plant life — the unity of ZOOLOGY
and BOTANY — and to express it in the term biology. But
biology is something more than a union of botany and
zoology under one name — for it endeavors, in addition to
describing the characteristics of animals and plants, to un-
fold the general principles underlying both.
Thus the biologist has as his field the study of living
things — what they are, what they do, and how they do
it. He asks, how this animal or that plant is constructed
and how it works — and this he attempts to answer.
He would like to ask, why it is so constructed and why
it works the way it does — but this is beyond the scope
of science.
These queries of the biologist reflect the two primary
viewpoints from which biological phenomena may be ap-
proached: the morphological in which interest centers;
upon the form and structure of living things, and the;
physiological in which attention is concentrated upon the
functions performed — the mechanical and chemical engin-
eering of living machines. Clearly, however, it is impossible
to draw a hard and fast distinction between morphology and
physiology because in the final analysis structure must be
interpreted in terms of function, and vice versa. But again,
4 FOUNDATIONS OF BIOLOGY
the fields of morphology and physiology naturally resolve
themselves into special departments of study, depending on
the level of analysis of structure or of function which is em-
phasized. Thus MOBPHOLOGY stresses the general form of
the animal or plant; ANATOMY, the gross structure of in-
dividual parts, or organs; HISTOLOGY, the microscopic
FIG. 1. — The chief divisions of Biology.
structure of organs, or tissues; CYTOLOGY, the component
elements of tissues, or cells, and the physical basis of life, or
protoplasm. Similarly, PHYSIOLOGY investigates the activi-
ties of animals and plants, the functions of organs, the
properties of tissues, the phases of cell life, and finally the
physico-chemical characteristics of protoplasm. So much
for the study of the adult individual animal or plant — but
this is not all. The origin and development of the individual,
THE SCOPE OF BIOLOGY O
GENETICS and EMBRYOLOGY; and the origin and develop-
ment of species, ORGANIC EVOLUTION, are other wide fields,
sciences in themselves, which must be approached from both
the structural and functional aspect if any real advance is
to be made toward a comprehensive appreciation of life.
(Fig. 1.)
Thus, just as the various physical sciences have expanded
and become specialized until they are beyond the grasp of
a single man, so biology and its subdivisions, or the BIO-
LOGICAL SCIENCES, are now distributed among many special-
ists. Although specialization results in a narrowing and
isolating of the fields of study, as deeper levels of investiga-
tion have been reached in all the sciences there has been a
tendency for the basic phenomena to meet on the common
ground of the fundamental sciences, physics and chemistry
-for in the last analysis the biologist must assume as a
working hypothesis that the properties of protoplasm are the
resultant of the properties and interrelationships of the
chemical elements which compose it. "In one direction,
supported by chemistry and physics, biology becomes bio-
chemistry and biophysics. In a contrary direction it forms
a connection with the psychical sciences which relate to
human nature, with psychology and sociology, with ethics
and religion."
CHAPTER II
THE PHYSICAL BASIS OF LIFE
Science never destroys wonder, but only shifts it, higher
and deeper. — Thomson.
THE old saying that the materials forming /£he human bod}7
change completely every seven years is a taciFrecognition that
lifeless material, in the form of food, is gradually transformed
into similar living matter under the influence of the body. In-
deed, just as a geyser retains its individuality from moment
to moment though it is at no two instances composed of the
same molecules of water identically placed, so the living
individual is a focus into which materials enter, play a part
for a time, .and then emerge to become dissipated in the
environment. But here the analogy stops. For in the
living organism the materials which enter as food, endowed
with POTENTIAL energy, are arranged and rearranged until
specific molecular aggregates result, which in turn are trans-
formed into integral parts of the organization of life itself.
However, to live is to work, and to work means expenditure
- the transformation of the potential into KINETIC energy -
with the result that materials in relatively simple form and
largely or entirely devoid of energy are returned to the
realm of the non-living.
Thus we reach a fact of prime importance: so far as we
know, living matter is merely ordinary matter which has
assumed, for the time being, a peculiar condition in which it
displays the remarkable series of phenomena which we
recognize as LIFE.
6
THE PHYSICAL BASIS OF LIFE 7
The body of Man in common with that of all animals and
plants is composed of living and non-living matter closely
associated, though totally distinct. For example, the
visible parts of hair and nails, a large part of bone and the
liquid part of blood is non-living material. But, the non-
living is not confined to gross structures, for the dead among
the living is still revealed until the resolving power of the
microscope fails us.
A. PROTOPLASM
Although there is a continuous stream of matter and
energy flowing through the living individual, nevertheless
the physical and chemical study of living matter from what-
ever source we take it — Mold or Elm, Amoeba or Man -
reveals a remarkable similarity in its fundamental factors,
and it is to a consideration of what the concept PROTOPLASM
holds for the biologist that we now turn.
As the finer structure of animals and plants came within
the range of vision through improvements in microscope
lenses, it was gradually recognized that the ultimate living \
part appeared to be a granular, slime-like material. Thus
Dujardin, in 1835, designated as sarcode the material forming
the bodies of microscopic animals. Purkinje, in 1840,
named the formative substance of the developing animal
protoplasm, and compared it with the granular material of
the growing region of certain plants. Six years later, von
Mohl similarly named the contents of the finer structural
units of plants. Confirmatory observations came from many
sources during the following decade and culminated in the
classical studies of Max Schultze and de Bary which estab-
lished the full physiological significance of protoplasm as the
essentially similar, fundamental, living material of both
animals and plants. This reduction of all life phenomena
8 FOUNDATIONS Ci~ BIOLOGY
to a common denominator was the final justification of the
prevision of the earlier workers in recognizing a life-science —
biology.
Although we speak of a common ' physical basis of life/ it
is of paramount importance to bear in mind that the proto-
plasm of no two animals or plants or, indeed, of different
parts of the same animal or plant is exactly the same.
Identity of protoplasm would mean identity of structure
and function — identity of life itself. The concept proto-
plasm merely emphasizes that, after allowances are made
for all the variations, we still have the similarities far
outnumbering the dissimilarities in the 'agent of vital
manifestations.'
The physical chemists tell us that matter in the living
state represents a type of COLLOIDAL CONDITION of matter
known as an emulsoid which, in turn, may exist either as a
sol — the apparently homogeneous liquid state of living
matter; or as a gel — the apparently amorphous semi-solid
state. Protoplasmic sols appear, as a rule, homogeneous be-
cause of the exceedingly small size of the molecular aggre-
gates which form them, while protoplasmic gels reveal either
a homogeneous or heterogeneous molar structure because of
the relatively large particles which set to form the gel. In
other words, living matter holds an intermediate position
between true solids and true liquids, and has many properties
of both, as well as many peculiar to itself.
But this leaves the reader without any clear conception
of the appearance of protoplasm. As a matter of fact it is
as difficult to describe the appearance of, as it is to define,
protoplasm. It must be seen under the microscope to be
appreciated. With a moderate magnification, protoplasm
presents a fairly characteristic picture, appearing like a
translucent, colorless, viscid fluid containing many minute
THE PHYSICAL BASIS OF LIFE 9
granules as well as clear spaces or vacuoles. (Fig. 2.)
If it is examined in water it exhibits no tendency to mix with
the surrounding medium, though investigations show that
osmotic interchanges are constantly going on. For this
reason it is impossible to consider protoplasm except in
connection with its surroundings whatever they may be —
FIG. 2. — A simple animal (Amoeba proteus) which consists ot a single unit mass
of protoplasm (highly magnified). 1, nucleus; 2, contractile vacuole; 3, pseudopodia;
4, food material in process of digestion (food vacuole); 5, sand particle or other indi-
gestible inclusion. (From Shipley and McBride, after Gruber.)
variations in its environment and variations in its activities
being reflected directly or indirectly in its appearance.
Under the highest magnifications, not only does the finer
structure of protoplasm differ in various specimens, but also
in the same living unit mass under slightly different physi-
ological conditions. At one time it presents the appearance
of a fairly definite net-like structure, or reliculum, the meshes
of which enclose a more fluid substance; at another, a frothy
10
FOUNDATIONS OF BIOLOGY
appearance in which the alveoli, or 'bubbles, ' represent a more
liquid substance emulsified in a less liquid medium. Again,
at other times, the denser portion seems to take the form of
minute rods, or fibers, distributed in
a somewhat fluid matrix. (Fig. 3.)
These appearances have given
rise to various theories which em-
phasize one or another as the
universal formula for the physical
structure of protoplasm, from
which the other appearances are
merely secondarily derived. But
the trend of recent work has been
to indicate that although the gen-
eral similarity of protoplasmic ac-
tivity, wherever we find it, might
lead us to expect to find also a
visible fundamental structural
basis, such does not exist within
the range of magnifications at our
command. Reticular, alveolar, and
fibrillar structures which our mi-
croscopes reveal are, as it were,
merely surface ripples from^ underlying physico-chemical
changes which, thus far, have proved unfathomable.
B. CHARACTERISTICS OF LIVING MATTER
Since the phenomena of life are without exception the
results of protoplasmic activity, it is obvious that we must
look to protoplasm for the primary attributes of living
matter. The properties which are absolutely diagnostic of
living matter are its l chemical composition, ^metabolism
including the power of waste and repair, growth by intus-
FIG. 3. — Alveolar appearance
of the protoplasm of a cell from the
skin (epidermis) of an Earthworm.
(From Verworn, after Butschli.)
THE PHYSICAL BASIS OF LIFE
^
susception, the power of reproduction, the power of adapta-
tion, and specific form and organization.
1. Chemical Composition
It is impossible to make an analysis of living matter
because the disturbance of its molecular organization by
chemical reagents kills it. Therefore our knowledge of its
chemical composition has of necessity been derived from a
study of dead protoplasm. However, since in the trans-
formation from the living to the non-living state there is
clearly no loss of weight, it follows that the complete material
basis of life is still present for examination. In other words,
the death of protoplasm is a result of disorganization.
Chemical analysis of protoplasm shows that it invariably
comprises the elements carbon, oxygen, hydrogen, nitrogen,
sulfur, and phosphorus; and usually also chlorine", potassium,
sodium, magnesium', calcium, and iron. Occasionally a num-
ber of other elements are found normally in the protoplasm
of certain parts of various species of animals and plants.
The average composition of the human body is about as
follows :
Oxygen 65.00%
Carbon 18.00
Hydrogen 10.00
Nitrogen 3.00
Calcium 2.00
Phosphorus. . 1.00
Potassium 0.35
Sulfur... 0.25
Sodium 0.15
Chlorine.. 0.15
Magnesium. 0.05
Iron 0.004
Iodine traces
Fluorine traces
Silicon.. traces
12 FOUNDATIONS OF BIOLOGY
At first glance there is nothing very striking about this
list of elements. They are all common ones with which the
chemist is familiar in the non-living world. But it is the
combination of the elements which is significant, and this
results from the capacity of carbon, hydrogen, and oxygen,
or carbon and hydrogen together, to form the numerous
complex compounds which in turn supply the basis for inti-
mate associations with other elements. As a matter of fact,
the bulk of protoplasm is composed of carbon, oxygen,
hydrogen, and nitrogen associated with each other in an
apparently infinite series of relationships, in which the
carbon seems to play the leading role. Some of these com-
pounds are relatively simple, such as water (H2O) which is
quantitatively the most important constituent of all proto-
plasm, but the majority consist of elaborate atomic arrange-
ments and not a few represent molecular complexes of hun-
dreds and even thousands of atoms.
The compounds of carbon which are characteristic of
protoplasm fall into three chief groups: proteins, carbohy-
drates, and fats.
PROTEINS invariably consist of the elements carbon,
oxygen, hydrogen, nitrogen, and sulfur, and frequently
phosphorus and iron; Examples are albumin of the white of
egg, casein of milk, gluten of cereals, and myosin of lean
meat. The nitrogen particularly distinguishes proteins
from the other compounds of the living complex and, as we
shall see later when considering the chemical processes in
animals and plants, is largely responsible for their command-
ing position as "the chemical nucleus or pivot around which
revolve a multitude of reactions characteristic of biological
phenomena." Study of the relationship of nitrogen to the
other chemical elements of proteins long since revealed the
fact that the protein molecule is a huge complex of linked
a -=?• K , v/ '-*
'
THE PHYSICAL BASIS OF LIFE ^H -{ 13
AMINO ACIDS — an amino acid being an organic acid in
which one hydrogen atom is replaced by the amino group,
NH2. But at the present time it is becoming increasingly
patent that the amino acids are, as it were, the nitrogenous
units with which organisms deal physiologically, rather than
the proteins themselves. An animal, for example, with
various proteins available in its food, chemically disrupts
these into their amino acid constituents, and then takes an
amino acid here and another there and synthesizes the
specific proteins it demands. And further, if individual
amino acids are supplied, the animal employs them. So it
seems highly probable that the specific structure of an or-
ganism depends upon the chemical specificity of its proteins.
Although the presence of proteins and the power of form-
ing them is the chief diagnostic chemical characteristic of
living matter, at the present stage of our knowledge it is
impossible to define proteins satisfactorily on the basis of
chemical or physiological properties. The most we can say
is that the biochemist describes proteins as "huge molecules,
complex in structure, labile in character, and therefore prone
to chemical change" —and the latter characteristic un-
doubtedly is closely associated with the perennial plasticity
and responsiveness of the protoplasmic system itself.
CARBOHYDRATES consist of various combinations of
carbon, hydrogen, and oxygen, the latter elements invariably
being present in the proportion found in water (H20).
Though more simple in chemical structure than proteins,
they range in complexity from the simple sugars, or monosac-
charids, such as glucose and fructose, to polysaccharids such
as dextrins, starches, and cellulose.
FATS are composed of the same elements as the carbo-
hydrates, but in quite different arrangements. The propor-
tion of oxygen is always less, and therefore they are more
14 FOUNDATIONS OF BIOLOGY
oxidizable and richer in potential energy. Fats represent
a synthesis of an acid (fatty acid) and glycerine. Examples
are butter and all oils of plant or animal origin.
Thus proteins, carbohydrates, and fats represent large
classes of substances which are distinctly characteristic of
living matter, not being found in nature except as the result
of protoplasmic activity; although biochemists now can
artificially synthesize certain fats and carbohydrates as well
as the amino acid constituents of some proteins. Proteins
undoubtedly play the most important part in the organiza-
tion of protoplasm, while the carbohydrates and fats contrib-
ute largely to the supply of available energy. However, it is
impossible to draw a hard and fast distinction in regard to
their respective contributions because, for example, as we
shall see later, carbohydrates form the foundation upon
which proteins are synthesized by green plants.
Proteins, carbohydrates, and fats are frequently referred
to as the foodstuffs, but it will be recognized that while, in a
way, they constitute the chief groups, all the constituents
of protoplasm must be available. Accordingly, inorganic
salts, water, and free oxygen are really foodstuffs. Further-
more, recent investigation has disclosed another class of
organic substances which are absolutely necessary for the
constructive phases of protoplasmic activity. These are
termed VITAMINES and must be classified as accessory food
substances, although as yet little is known in regard .to their
chemical structure or mode of action. And then, finally, on
the border line of food substances may be mentioned a great
group of organic catalyzers, called ENZYMES, which play a
major role in metabolism. But, when all is said, our knowl-
edge of the chemical complexities of protoplasm affords no
adequate conception of how they are related to the phe-
nomena of life. This is beyond present-day biology.
THE PHYSICAL BASIS OF LIFE 15
2. Metabolism
We have emphasized that living matter is continually
changing, and this fundamental fact is reflected in nearly all
attempts to define life. Aristotle described life as "the
assemblage of operations of nutrition, growth, and destruc-
tion"; deBlainville, as a " twofold internal movement of
composition and decomposition"; and Spencer, as "the
continuous adjustment of internal relations to external
relations."
This interaction consists of chemical and physical pro-
cesses in which combustion or oxidation plays the chief role.
Lavoisier and Laplace in 1780 showed that animal heat results
from a slow burning of the materials of the body, involv-
ing the consumption of oxygen and the liberation of carbon
dioxide; and further, that for a given consumption of oxygen
and liberation of carbon dioxide, about the same amount of
heat is produced by an animal as by a burning candle. This
was an important discovery, because it went far toward
establishing the fact that at least certain characteristic vital
phenomena are amenable to the laws which hold in the non-
living world.
But the processes involved in life are not so simple as
perhaps might be imagined from the results just mentioned.
Heat represents but one of the many energy transformations
within the organism. Indeed the living organism, like a
steam engine, is a machine for transforming energy — trans-
forming the potential energy stored in chemical complexes
of its own substance into the various vital processes of living
- into work performed. In these processes many complex
substances rich in potential energy, which have entered
as food and have in whole or part added to the protoplasmic
complex, are reduced to simpler and simpler conditions and
16 FOUNDATIONS OF BIOLOGY
finally, with their energy content nearly or entirely ex-
hausted, are eliminated as EXCRETIONS. This continual
waste must, if life is to persist, be counterbalanced by a
proportionate intake of food in order to renew the supply of
energy and afford the materials which, after preliminary
changes, are made into an integral part of the living organ-
ism. Thus in living the animal or plant is partially consum-
ing and rebuilding itself continually. This dual process is
METABOLISM. When constructive metabolism. ANABOLISM^
keeps pace with destructive metabolism. KATABOLISM^ the
individual remains essentially unchanged and this is the
normal condition of adult life. During youth the anabolic
phases are in the ascendency and growth occurs, while old
age is characterized by a predominance of katabolic processes.
3. Growth
The results of metabolism force themselves upon our
attention chiefly as growth, or permanent increase in
the size of the individual. As a rule growth in plants con-
tinues more or less rapidly throughout life, while in
animals it is confined mainly to the early part of the
individual's existence, or youth. Indeed, at birth a child
is about a billion times larger than the egg from which it
has developed.
Growth means that the organism makes over the materials
which it receives in the form of food from its environment
and fits them into the protoplasmic organization here and
there throughout as needed. This method 'of addition of
materials, which is termed growth by INTUSSUSCEPTION, is
highly characteristic of life. When growth occurs in the
non-living world, it is typically by accretion; as, for example,
in crystals where new material of the same kind is superim-
posed upon the surface. But protoplasm, with materials
THE PHYSICAL BASIS OF LIFE 17
and energy taken from its environment, constructs more
protoplasm and, if the available materials are adequate, the
specifically organized living substance tends to increase
indefinitely. Thus it is not only the method of growth
which is diagnostic of animals and plants, but also the fact
that when the individual body has reached a certain phys-
iological balance, or maturity, in which it ceases to increase
in size, under normal conditions it expresses the inherent
growth power of living matter by setting free certain living
units, which go through a cycle of growth phenomena that
result in re-productions of the parent individual.
4. Reproduction
So far as is known, living matter never arises except under
the direct influence of preexisting living matter. We have
seen that this transformation is continually going on in the
constructive phase of metabolism in the animal or plant,
and brings about repair and growth of the individual;
but it is in reproduction that what may be termed the over- .
growth of the individual results in the production of a new
one. A larger or smaller part of the parent generation is
detached and becomes the new generation, so that in ultimate
analysis reproduction is division. This is a highly unique
characteristic of living things which provides for the con-
tinuation of the race.
5. Adaptation
The discussion of metabolism has emphasized the close
interrelationship between the living complex and its sur-
roundings, and the dependence of life upon the interplay
and interchange between protoplasm and its environment.
As a matter of fact the plant or animal retains its individual-
ity — lives — solely by its powers of developing and main-
18 FOUNDATIONS OF BIOLOGY
taining exquisite adjustments to its surroundings. This
results from the IRRITABILITY of living substance : its inherent
capacity of reacting to environmental changes by changes
in the equilibrium of its matter and energy. The inciting
Changes, known as STIMULI, may be chemical, electrical,
/thermal, photic, or mechanical, but the nature of the response
is determined rather by the fundamental character of the
protoplasmic system itself than by the nature of the stimulus.
Muscle protoplasm contracts however it is stimulated. The
reaction of living matter by virtue of its intrinsic irritability
implies not only response to a stimulus but also conduction
so that the protoplasmic system as a whole is directly or
indirectly influenced. It responds as a coordinated unit —
an individual. It adapts itself structurally and functionally
to the exigencies of its existence. This power of adaptation,
as exhibited in active adjustment between internal and
external relations, overshadows every manifestation of life
and contributes, more than any other factor, to the " enor-
mous gap that separates even the lowest forms of life from
the inorganic world."
6. Organization
Finally, adaptation implies that living things are not
homogeneous, but exhibit reciprocal structural and physio-
logical organization. Accordingly animals and plants are
referred to as organisms. Indeed a major part of the present
volume is devoted to the organization of organisms.
The characteristics which we have described — chemical
composition, metabolism including waste and repair, growth
by intussusception, reproduction, adaptation, and specific
organization — individually and collectively are diagnostic
of living matter. It is possible, to be sure, to take exception
to one or another; e.g., to say that growth by intussusception
THE PHYSICAL BASIS OF LIFE 19
occurs in non-living things when a salt is dissolved in water;
but such formal objections only emphasize the unique condi-
tions which obtain in life.
The reader may be surprised to note that the power of
movement has not been mentioned as a characteristic of life,
but a moment's thought will make it apparent that visible
movement is not confined to living matter. Though this is
so, movement is one of the most obvious manifestations of
life and depends, of course, in every instance, upon molar
changes resulting from tumultuous ultramicroscopic chemical
changes of protoplasm itself.
And it is to these changes that, in the last analysis, we must
turn for the energy which brings about the visible movements
in animals and plants, such as the contraction of the muscles
of animals, the streaming movement (amoeboid movement)
of the simple animals known as Amoebae, the rotation and
circulation of the protoplasm in certain of the living units of
plants and, finally, the lashing of threads of cytoplasm (cilia)
which not only enables many a tiny plant and animal to swim,
but also aids in numerous ways in certain parts of the
bodies of higher organisms. The phenomena of life are quite
generally expressed in visible movements, but the latter are
not peculiar to living things.
In our discussion thus far we have endeavored to describe
the characteristics of matter in the living state on the basis
of the fundamental vehicle of life manifestations — proto-
plasm. We have not attempted formally to define 'life' or
' protoplasm ' because they are so unique that it is impossible
to resort to the lexicographer's trick of comparing them with
something else; and because the expressions 'protoplasm'
and 'life' are abstractions; one indicating that all individual
animals and plants have to a large extent a common organi-
zational foundation, and the other that they exhibit certain
20 FOUNDATIONS OF BIOLOGY
characteristic actions and reactions. The living organism
is a microcosm which exhibits a permanence and continuity
of individuality correlated with specific behavior, and this it
transmits to other matter which it makes a part of itself,
and to its offspring in reproduction.
CHAPTER III
ORGANIZATIONAL UNITS OF PLANTS
AND ANIMALS
Over the structure of the chemical molecule rises the struc-
ture of the living substance as a broader and higher kind of
organization. Over the structure of the cell rises again the
structure of plants and animals, which exhibit the yet more
complicated, elaborate combinations of millions and billions
of cells coordinated and differentiated in the most extremely
different ways. — -Hertwig.
/SINCE living matter is only known to us in the form of
individual animals and plants, individuals are the only
FIG. 4. — Cells, highly magnified, from the surface of the Frog's skin (A),
and a plant loaf (B) .
realities in living nature, and we turn now to a consider-
ation of the organization of the individual.
A thin slice of material from the surface of the skin of a Frog
or the leaf of a Buttercup shows under the microscope the
same general structure. Each appears to be a mosaic of in-
numerable small bodies, no two of which are exactly alike even
in the same piece, though all are similar enough to be one and
21
22
FOUNDATIONS OF BIOLOGY
the same type of unit. And if we extend our study to other
parts of the Buttercup or the Frog or, indeed, to any part of
FIG. 5. — Vertical section (highly magnified) of a leaf to show its cellu-
lar structure, a, guard cells, at opening (stoma) through epidermis;
b, cells containing chlorophyll; c, upper and lower epidermal cells.
(From Abbott, after Bailey.)
any familiar plant or animal, we find essentially similar units
of structure in every case. In fact, the bodies of all living
FIG. 6. — Transverse section (highly magnified) of a simple animal
(Hydra) to show the cellular structure. Outer layer, ectoderm; inner
layer, endoderm; central cavity, enteron. (After Shipley and
McBride.)
things either consist of a single organic unit or are congeries
of millions of essentially similar units called CELLS. (Fig. 4.)
ORGANIZATIONAL UNITS 23
Such being the case we reach another great generalization :
all organisms have the same elementary structure, just as
we have seen that all organisms are composed of a similar
fundamental life-stuff, protoplasm. Therefore a cell may
be described as a small mass of protoplasm, either living
apart as a microscopic plant or animal, or forming a building
block, as it were, in the architecture of one of the higher
organisms. Indeed, organisms are organisms because of
specific local differentiations in the living material — this
differentiation being possible largely because the protoplasm
is disposed in microscopic unit masses, or cells. (Figs. 5, 6.)
The appreciation of this dual similarity — protoplasmic
basis and cellular organization — of all living things, which
was finally attained about the middle of the last century,
firmly established the fact that all living nature is one, —
the corner stone of modern biology.
A. THE CELL
Having taken a general survey of the building materials
of living nature, microscopic unit masses of protoplasm,
termed cells, we are now in a position to consider in some
detail the structure of a typical cell. With the diversity
of gross structure of animals and plants in mind, one is not
surprised that there are considerable, even great, variations
in their component elements. In fact the characteristics of
an organism or part of an organism are determined by those
of its cells. But there are certain generalized cell characters
which are common to all cells — by virtue of which they
are cells — and it is important to emphasize these.
In its simplest form a cell is a small spherical mass of
protoplasm. Such are the eggs of various animals and the
complete body of some of the lowest plants and animals.
24 FOUNDATIONS OF BIOLOGY
Cells forming the units of multicellular organisms, however,
frequently exhibit more or less hexagonal surfaces on account
of stresses and strains incident to their position among other
cells, while specializations and differentiations, for one purpose
or another, produce forms which are characteristic of different
parts of the organism, as, for example, the long spindle-
shaped cells of smooth muscle, and the widejv_J^ranching
cells of parts of the brains of animals. Broadly speaking,
the greater diversity of cell form is found in animals, while
in plants, owing to the more general presence of rigid cell
walls about the protoplasm, the units more frequently present
symmetrical, angular outlines. (Figs. 7, 42.)
The term cell is a relic of the time when the cell wall was
regarded as the more important part, and its protoplasmic
contents, if observed at all, were considered as only of second-
ary importance, if not a waste product. Now we recognize
many cells which are merely naked masses of protoplasm,
such as certain types of blood cells. In other words, the
protoplasm is the essential living part — the cell wall
frequently being a non-living accessory which more or less
sharply delineates one unit mass of protoplasm from another
and lends rigidity and form to the group of cells as a whole.
1. Cytoplasm
The protoplasm of all typical cells is differentiated into
two parts: the CYTOPLASM, or general groundwork which
makes up the bulk of the cell; and the NUCLEUS, a restricted,
clearly defined area, usually situated near the center of the
cytoplasmic mass.
The cytoplasm may be considered the more generalized
protoplasm of the cell, and its appearance and other character-
istics are those which have been outlined in our discussion of
protoplasm. With that in mind, for the sake of definiteness,
ORGANIZATIONAL UNITS
A C
25
FIG. 7. — Various types of cells, highly magnified: A, female germ cell, egg of a Cat.
B, male germ cell, sperm of a Snake. C, three ciliated epithelial cells from the digestive
tract of a Clam . D, supporting tissue (cartilage) of a Squid. E, voluntary (striated) mus-
cle fiber from an Insect. F, involuntary (smooth) muscle fibers from the bladder of a Calf.
6, nerve cell from the brain of Man. (From Hegner, after Dahlgren and Kepner.)
26
FOUNDATIONS OF BIOLOGY
we may consider its basis as consisting of a meshwork, com-
posed of innumerable, minute granules which permeate an
apparently homogeneous hyaline ground-substance. Dis-
tributed throughout the cytoplasm are usually various lifeless
inclusions such as granules of food, droplets of water or oil,
vacuoles of cell sap, crystals, etc., representing materials
FIG. 8. — Diagram of a cell, a-d, nucleus; a, nucleolus; b, chromatin
'network'; c, linin meshwork; d, karyosome, or chromatin knot; e, meta-
plasmic inclusions in cytoplasmic meshwork; /, vacuole; g, plastida in
cytoplasm; h, centrosome, recently divided. (From Wilson.)
which are to be, or have been, a part of the living complex,
or are by-products of the vital processes. This passive
material is frequently referred to as METAPLASM, but it is
quite evident that such a term stands for no essential mor-
phological part of the cell, and we have no absolute criterion
to distinguish between some granules which are regarded as
.
ORGANIZATIONAL UNITS 27
metaplasmic in nature and others which are ordinarily con-
sidered active elements of the cytoplasm. Specialized living
cytoplasmic bodies, known as PLASTIDS, are sometimes also
present. Finally, within the cytoplasm in the vicinity of the
nucleus, there is frequently visible a differentiated area con-
taining a CENTROSOME, an important cell organ which is es-
pecially active during cell reproduction. (Fig. 8.)
The cytoplasm, since it forms the general groundwork, is
that part of the cell which comes most closely into relations
with the environment, and accordingly near the surface it is
frequently modified somewhat in texture and consistency so
that an outer region, or ECTOPLASM, may be distinguished
from an inner, or ENDOPLASM. Again, the cell may form
about itself a definite membrane or a heavy cell wall. Nearly
all gradations exist between highly differentiated cytoplasm
(ectoplasm) and definite membranes and cell walls. The
ectoplasm is certainly a part of the living protoplasm, while
certain types of membranes and cell walls must be regarded
as non-living, though in many cases they are direct trans-
formations of the living materials which grow and play an
important, indeed a crucial, part in controlling directly or
indirectly the flow of matter and energy to and from the cell
and its surroundings.
2. Nucleus
Within the cytoplasmic mass there is a restricted area
of clearly differentiated material, which typically has a
rounded form, bounded by a membrane, so that it appears
as a definite body of protoplasm called the nucleus. The
structural basis of the nucleus appears to be essentially
similar to that of the cytoplasm — the so-called LININ mesh-
work and KARYOLYMPH being comparable respectively to the
granular meshwork and hyaline ground substance of the
28 FOUNDATIONS OF BIOLOGY
cytoplasm. But superimposed upon this, as it were, is the
highly characteristic nuclear material, or CHROMATIN, which
takes various forms during different phases of cell activity
but generally, in a 'resting' cell, gives the appearance of a
network of tiny granules with one or more dense 'knots' of
chromatin (KARYOSOMES) . Later we shall describe some of
the important changes in chromatin arrangement, but it is
sufficient at this time to emphasize that the nucleus is a
differentiated area of the cell protoplasm which is the arena
of the chromatin. Frequently there is a conspicuous round
achromatic body within the nucleus known as the NUCLEOLUS.
Cytoplasm and nucleus, looked at from the functional view-
point, represent a physiological division of labor within the
confines of the cell. Experiments have shown that they are
mutually necessary for cell life; the removal of the nucleus
putting an end to anabolic processes — assimilation, repair,
and growth — and thus leading rapidly to death. Accord-
ingly the nucleus may be considered as the center of the
synthetic activities of the cell, and the cytoplasm, if not as
the area in which destructive processes are chiefly involved,
at least as a neutral region which translocates material
toward and away from the nucleus. All the evidence points
to the nucleus as the ''controlling center in cell activity, and
hence a primary factor in growth, development, and trans-
mission of specific qualities from cell to cell, and so from one
generation to another."
B. ORIGIN OF CELLS
With regard to the origin of life on the Earth absolutely
nothing is known. But all the evidence at hand tends to
show that, at the present time at least, living matter never
arises except under the influence of preexisting living matter.
That is, protoplasm grows — cells grow and, having attained
ORGANIZATIONAL UNITS 29
a certain size, reproduce by dividing into two more or less
equal parts. The process of cell division involves the divi-
sion of both cytoplasm and nucleus, and therefore we must
enlarge our conception of a cell as a small mass of protoplasm
differentiated into cytoplasm and nucleus, by adding that
both cytoplasm and nucleus arise through the division of the
corresponding elements of a preexisting cell.
We shall later have occasion to make a study of the details
of cell division, known as MITOSIS, but from what has been
said it must occur to the reader that, since cells arise only
by division, those of the present day, whether complete
free living organisms or units composing the bodies of higher
animals and plants, are actually lineal descendants in un-
broken series from the beginning of life on the Earth. The
bond of discontinuity between parent and offspring is typi-
cally a single cell division, (Fig. 123.)
CHAPTER IV
METABOLISM OF GREEN PLANTS
Matter and force are the two names of the one Artist who
fashions the living as well as the lifeless. — Huxley.
IT has been emphasized that life is only known to us in the
form of individuals, and we turn now to concrete examples
of unicellular plants and animals which present, in relatively
simple form within the confines of a cell, an epitome of all the
fundamental life processes which we shall later have occasion
to review in their complex expressions in the higher animals
and plants.
Unicellular green plants are distributed all over the world
and adapted to a great variety of conditions. We find them,
for example, forming green coatings on the bark of trees,
scums on puddles and ponds, or being blown about as dust
by winds. Of the many hundreds of species we select
Sphaerella lacustris because it can readily be obtained and
kept for observation, and because its life history has been
carefully studied.
A. STRUCTURE AND LIFE HISTORY OF SPHAERELLA
A single Sphaerella is invisible or barely visible to the naked
eye, but, like many another microscopic form, it makes up in
numbers for the small size of the individual, and sometimes
gives a stagnant pool of rain-water a bright green or red
color. Sphaerella has a complicated LIFE CYCLE, or series of
forms which it assumes under different conditions, chiefly
environmental. We shall take as the initial stage for descrip-
30
METABOLISM OF GREEN PLANTS 31
tion the so-called PORMANT FORM which may be assumed when
the water in which it has been living dries up. In this
condition the organism consists of a spherical mass of pro-
toplasm near the center of which is a rather large nucleus.
The protoplasm, which appears greenish or reddish for rea-
sons to be discussed later, is enclosed within a distinct, rigid
cell wall. This has been secreted by the cell and is com-
posed of CELLULOSE, a carbohydrate which is especially
characteristic of plant cells. It is evident that the organism
is a single cell. (Fig. 9.)
Sphaerella in this phase is able to withstand unfavorable
conditions for several years at least. All the life processes
of the protoplasm are reduced to the lowest ebb; so low that
it is difficult to demonstrate any chemical change whatsoever
going on. Life in a dormant condition is not peculiar to
Sphaerella, but is quite a characteristic phase in the life of
many animals and plants, being most familiar to us in the case
of plant seeds, some of which are known to retain their vital-
ity for nearly a century under proper conditions.
When dormant specimens of Sphaerella are placed in rain
water in the sunlight active life shortly is resumed. The cell
wall swells up and the protoplasm within divides twice, with
the result that four smaller but otherwise essentially similar
cells, known as SPORES, take the place of the original cell and
are set free by the rupture of its wall. The four daughter
cells soon become more or less oval in outline and secrete
cellulose walls of their own. The cell walls do not fit closely
about the body of protoplasm, termed the PROTOPLAST, but
are separated from it' by a liquid-filled space, or vacuole,
except where cytoplasmic strands extend through the vacuole
to the wall. But a more remarkable change occurs at the
same time — two long slender cytoplasmic strands are
developed from the more pointed end of the cell, and these,
32
FOUNDATIONS OF BIOLOGY
FIG. 9. — Life history of Sphaerella lacustris. a, b, c, d, asexual cycle; a, w,x, y, z,
sexual cycle, a, dormant cell enclosed within a protective cyst wall which has
ruptured to allow the enclosed protoplast to escape; 6, division of the protoplast to
form four spores (c) each of which grows, develops two flagella, and assumes the typical
'adult' free living form of Sphaerella (d). This may divide many times, but each cell
eventually assumes the dormant form (a) again. Under other circumstances the proto-
plast from the dormant form may divide until 32 or 64 small cells (w) are formed. These
make their escape and are gametes (x) since they fuse in pairs (j/). The composite cell
resulting from fertilization is a zygote (z) which soon forms a cyst wall and assumes the
dormant phase.
METABOLISM OF GREEN PLANTS 33
passing through the cellulose wall, extend for some distance
into the surrounding water. These threads of cytoplasm, or
FLAGBLLA, lash vigorously and pull the cell rapidly through
the water. The activity of the flagella is one expression of a
fundamental property of protoplasm, CONTKACTILITY, which
is exhibited in its most specialized form in the muscles of
the higher animals.
Returning now to the life-history of Sphaerella. The four
free-swimming individuals, which have arisen from the parent
dormant phase, may each divide many times so that instead
of four there may be, before long, thousands of flagellated
cells, all direct lineal descendants of the original resting cell.
If this number seems high, one only has to determine how
many cells there would be at, say, the twenty-fifth generation
by raising 2 to the 25th power. Sooner or later, however,
these active cells withdraw their flagella and again become
dormant forms.
But Sphaerella is still more versatile. Now and. then,
probably influenced by environmental conditions, the proto-
plasm within the wall of a spherical dormant form divides
rapidly into 32 or 64 relatively small cells which, when set
free, are termed GAMETES. These differ structurally from
the active form already described chiefly by the absence of
the prominent cell wall and vacuole. But it is the behavior
rather than the structure of these small cells which is char-
acteristic. After swimming about for a time by means of
their flagella, they come together in pairs, the two cells of a
pair completely fusing — nucleus with nucleus and cyto-
plasm with cytoplasm — to form a single cell, or ZYGOTE,
with four flagella. Soon the individual absorbs its flagella
and, secreting about itself a heavy cell wall, enters upon a dor-
mant stage with the characteristics and potentialities de-
scribed above*.
34 FOUNDATIONS OF BIOLOGY
It is clear that the various forms which follow one another
arise by cell division in every case, though this is interrupted
once by just the opposite process — the complete cytoplasmic
and nuclear fusion of two distinct cells to form one cell.
This is the process of FERTILIZATION, an expression of a funda-
mental phenomenon of protoplasm at the basis of sex and
sexual reproduction, which we shall consider at length later.
Such is the history of Sphaerella. It is apparent that the
sequence of diverse forms which arise from one another
constitute a life cycle, and although each individual cell in
the cycle is a Sphaerella, nevertheless the plant called
Sphaerella lacustris comprises all the forms assumed. From
one viewpoint we may look upon the cycle as forming an
individual of a different or higher order — an individual the
component cells of which are separate.
B. METABOLISM IN SPHAERELLA
We ,now turn our attention from the structure and life
history of Sphaerella to the point it was chosen especially to
illustrate — the metabolism of green plants. It may appear
to the reader that a tree or shrub might with more propriety
be taken as the example of a typical plant, but, since the
fundamental distinction between animals and plants is
chiefly a question of -metabolism, there are advantages in
studying it in a single cell, where one's attention is not dis-
tracted by root, stem, and leaf.
Since Sphaerella lives, grows, and multiplies in pools of
water exposed to sunlight, it is to this environment that we
must look for the materials which it turns into protoplasm,
and the energy by which it makes the transformation. And
further, since the organism is enclosed in a cell wall, its income
and outgo of materials must be in solution in ordei to pass
through.
METABOLISM OF GREEN PLANTS 35
1 . Food Making
In short, Sphaerella takes materials from its surroundings
in the form of simple compounds, as carbon dioxide,
water, and mineral salts, which are relatively stable and there-
fore practically devoid of energy, and, through the radiant
energy of sunlight, shifts and recombines their elements in such
a way that products rich in potential energy result. Sphae-
rella thus exhibits the prime diagnostic characteristic of green
plants — the power to construct- its own foodstuffs.
The key Fo this power of chemical synthesis by light —
PHOTOSYNTHESIS — resides in a complex chemical substance
called CHLOROPHYLL.1 This pigment, which is segregated in
special cytoplasmic bodies known as CHLOROPLASTIDS, gives
to Sphaerella during its active phases and to the foliage
of plants in general their characteristic green color. The
chlorophyll arrests and transforms a small part of the energy
of the sunlight, which impinges upon it, in such a way that
the protoplasm can employ this energy for food synthesis.
The first great step in the constructive process is a com-
bination of carbon with hydrogen and oxygen to form a
carbohydrate. Sphaerella gets these elements from carbon
dioxide and water by a process of molecular disruption. We
know that when charcoal, for instance, is burned, carbon and
oxygen unite to form carbon dioxide, and energy in the form
of light and heat is liberated. Obviously Sphaerella must
employ an equal amount of energy in separating the carbon
and oxygen of carbon dioxide; that is, in overcoming their
chemical affinity. And this kinetic energy which the plant
employs is then represented in the chemical potential which
exists between the oxidizable carbon and free oxygen — it is
1 A rough approximation of the formula of chlorophyll has been given as: (MgN4-
Cx-HaoOO) (COOCHs) (COOC2oH39). A slight chemical modification of chlorophyll
results in hematochrome, which gives at certain times the reddish tinge to Sphaerella.
36 FOUNDATIONS OF BIOLOGY
potential energy. Thus the plant in sunlight is continually
separating the carbon from the oxygen of carbon dioxide.
The oxygen is liberated as free oxygen while the carbon
which has been separated from the oxygen is combined with
molecules of water to form carbohydrates — grape sugar
(glucose) and fruit sugar (fructose).
The conventional equation for this reaction is:
6 C02 + 6H2O C6H12O6 + 6O2
(carbon dioxide) (water) (glucose or fructose) (free oxygen)
It should be emphasized, however, that the processes in-
volved are by no means so simple as is implied above; but
since there is little conclusive data in regard to the details,
the equation as stated affords a formal explanation which is
adequate for the present discussion.
The first great step in food synthesis, the formation of a
sugar, having been accomplished, the green plant trans-
forms the sugar and stores it as starch for future use as fuel
or ES the basis of further synthesis. Starch is the first visible
product of photosynthesis.
We have seen that the characteristic of proteins as com-
pared with carbohydrates (sugars, starches) is the presence
of nitrogen, and this element must be added to the CHO
basis already constructed as the next step toward protein
synthesis. The green plant not only can, but must employ
nitrogen in such simple combinations as nitrates, and this is
a fact of prime importance, for typically, as will appeal-
later, animals and most colorless plants require nitrogen
in more complex combinations. By the addition, then, of
nitrogen to the carbohydrate basis a very simple nitrogen
compound, such as an amine (e.g., asparagine = C^s^Oa), is
built up, which may be transformed into a protein by the ad-
dition of sulfur and other elements secured from sulfates,
phosphates, etc. Again, we do not know how this is done.
METABOLISM OF GREEN PLANTS 37
or, after it is done, how the protein becomes an intrinsic part
of the living material itself. So we attribute it to synthesiz-
ing ENZYMES. These are chemical bodies which are only
known as products of living protoplasm and are the activat-
ing agents (catalytic agents) for chemical transformations
in which, however, they themselves take no integral part.
The chemical composition and constitution of enzymes is
undetermined.
Sphaerella thus takes the raw elements, so to speak, of
living matter and by the radiant energy of sunlight, which
its chlorophyll traps, constructs carbohydrate, protein, pro-
toplasm. In other words, the green plant is a synthesizing
agent, building up highly complex and unstable molecular
aggregates brimming over with the energy received from
the Sun.
2. Respiration
So the green plant, whether Sphaerella or Elm, manu-
factures its own food and itself! But, as we have said before,
protoplasm is always at work — to live is to work — and
this means expenditure of energy, the same energy which
chlorophyll has secured for the plant and stored away in its
food. In other words, the food must be oxidized in order to
release the energy, and for this the plant must have available
a supply of free oxygen. Sphaerella obtains this oxygen dis-
solved in the water and, incidentally, in sunlight, from that
liberated through photosynthesis. The process involved,
for the sake of simplicity, may be represented by the equa-
tion:
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O
which, it will be noted, is the reverse of the equation for
photosynthesis. This intake of free oxygen by the cell and
outgo of carbon dioxide and water, the chief products of
combustion, is known as RESPIRATION. It is an interchange
38 FOUNDATIONS OF BIOLOGY
of gases between the living matter and its surroundings which
is not only characteristic of Sphaerella and all green plants,
but of all living things. Plants breathe just as truly as
animals, though the active life of most of the latter requires
a more or less elaborate respiratory apparatus in order
that an adequate gaseous interchange may be effected with
the necessary rapidity.
Thus the green plant may be regarded as a chemical
machine for the transformation of energy — the radiant
energy from the Sun — into life work; the matter and energy
which enters, forms, and leaves the organism obeying, to the
best of our knowledge, the fundamental laws of matter and
energy of the non-living world.
We have now obtained some idea of one living organism,
Sphaerella lacustris, a green plant reduced to the simplest
terms — a single cell provided with chlorophyll. And we
have seen that this chlorophyll is the key to the photo-
synthetic activity of the green plant. In other words, the
expression 'green plant' does not refer specifically to the
color of a plant (in some cases it may appear red, as in
Sphaerella under certain conditions), but to the fact that
there is present a complex pigment functionally similar to
chlorophyll by virtue of which the plant is a constructive
agent in nature. It has the power to manufacture its own
foodstuffs from relatively simple compounds largely devoid
of energy and, in particular, is able to utilize nitrogen in the
form of nitrates.
We pass now from the essentially constructive agents in
nature to the chiefly destructive; from the collectors of
energy to the energy dissipators; from the green plants to
animals and to 'colorless' plants.
CHAPTER V
METABOLISM OF ANIMALS
The most important discoveries of the laws, methods, and
progress of Nature have nearly always sprung from the exami-
nation of the smallest objects which she contains, and from
apparently the most insignificant enquiries. — Lamarck.
THERE is probably no better introduction to the study
of the biology of an animal than that afforded by PARA-
MECIUM, a common organism of ponds, ditches, and decaying
vegetable infusions. Paramecium is a representative of
some 10,000 kinds of single-celled animals, or PROTOZOA.
Members of this group are found in almost every niche in
nature and, like the PROTOPHYTA, as the unicellular plants
are sometimes called, are important because in numbers there
is strength.
A. STRUCTURE AND LIFE HISTORY OF PARAMECIUM
Paramecium is a giant among the Protozoa, though just
visible to the naked eye as a whitish speck if the water in
which it is swimming is properly illuminated. But to make
out the details of structure it is necessary to magnify it
several hundred times. This done, it appears as a more or
less cigar-shaped organism which one would not consider,
at first glance, a single cell because it shows highly differen-
tiated parts. However, careful study reveals the fact that the
organism really consists of a single protoplasmic unit differ-
entiated into cytoplasm and nucleus, though each of these
regions shows specializations. The nuclear material, instead
30
40
FOUNDATIONS OF BIOLOGY
of forming a single body as it does in most cells, in Parame-
cium is distributed in two parts: a larger body, or MACRO^T
NUCLEUS, and a smaller body, or MiCRONUCLEUs.1 Strictly\
speaking, the macronucleus and \
micronucleus together constitute \
the nucleus of the cell, and rep-
resent a sort of physiological,
division of labor in the chromatin y\
complex. But it is in the cyto-,
plasm that specialization is most
conspicuous. Not only are there
general differentiations into ecto-
plasm and endoplasm, but these
regions also have local speciali-
zations such as CILIA for loco-
motion, TRICHOCYSTS for defense,
PERISTOME, MOUTH, and GULLET
for the intake of solid food,
GASTRIC VACUOLES for digestion,
and CONTRACTILE VACUOLES for /
excretion. And withal, recent in-/
vestigations indicate that various'
parts of the cell are coordinated
FIG. 10. — Paramecium calkinsi.
Diagrammatic, a, contractile vac-
uole surrounded by radiating
canals; b, macronucleus; c, mouth;
d, undulating membrane extending
lengthwise in gullet; e, gastric
vacuole in process of formation, at
end of gullet; /, contractile vacuole,
fully formed; g, gastric vacuoles;
h, endoplasm; i, micronuclei; j,
peristome and peristomial cilia; k,
trichocysts in ectoplasm; /, cilia.
apparatus.
by a * neuromotor '
(Fig. 10.)
Paramecium, under favorable
conditions, grows rapidly and,
when it has attained the size limit characteristic of the
species, cell division takes place, with the result that from
the single large cell there are formed two smaller individuals
which soon become complete in all respects. These, in turn,
1 The several species of Paramecium differ in regard to micronuclear number; e.g.,
P. caudatum has one micronucleus, and P. aurelia and P. calkinsi have two micronuclei.
METABOLISM OF ANIMALS
41
grow and repeat the process in about ten hours so that, as
in the case of Sphaerella, within a few
days the original Paramecium has divided
its individuality, so to speak, among a
multitude of descendants. (Fig. 11.)
This process of multiplying by dividing
can go on indefinitely under optimum en-
vironmental conditions. But periodically
Paramecium undergoes an internal nuclear
reorganization process (ENDOMIXIS). Also
now and then individuals temporarily fuse
in pairs and interchange nuclear material
(CONJUGATION) — an expression of the
same fundamental sex phenomenon which
is exhibited in Sphaerella. (Figs. 12, 130,
131.)
B. METABOLISM IN PARAMECIUM
1 1 . — Parame-
cium aurelia, dividing.
N, N', macronucleus ;
n, n', the two dividing
micronuclei ; o, o',
mouth. (After Hertwig.)
Paramecium thus affords some idea of the complexities of
structure and function which a cell may exhibit when it forms
the whole animal organism. The Pro-
tozoa are the simplest, though by no
means simple, animals. But the great
structural differences between Sphaerella
and Paramecium, though to a certain
extent representative of plants on the
one hand and animals on the other, are
not essentially diagnostic, because, as we
have suggested before, in the last analysis
FIG. 12 —Position as- ^ *s a matter of metabolism. And it is
by conjugating largely for this reason that Sphaerella and
Paramecia.
Paramecium, organisms shorn of all, or
nearly all, non-essentials, have been selected as illustrations.
42 FOUNDATIONS OF BIOLOGY
1. Food Taking
The food of Paramecium is chiefly microscopic, colorless
plants known as BACTERIA which are present in countless
numbers in decaying vegetable infusions. As Paramecium
swims about by means of its cilia, a current of water laden
with Bacteria is whirled down the peristome on one side of
the animal and some passes through the mouth and gullet
into the endoplasm. Here the Bacteria, surrounded by a
droplet of water, form a gastric vacuole, into which the endo-
plasm secretes chemical substances (enzymes, etc.) which
gradually break down — that is, digest — the complex
proteins, carbohydrates, etc., of the plant cells. Finally,
this material which shortly before was Bacteria protoplasm
is incorporated into Paramecium protoplasm — matter and
energy is supplied and the animal lives and grows.
This is, in most regards, a strikingly different condition
from that which we have seen in Sphaerella. In Paramecium
solid particles of food — Bacteria — are taken into the cell,
and since the chief organic constituents of protoplasm are^
proteins, associated with carbohydrates and fats, it is clear
that the income of the animal organism is, unlike that of the
green plant, chiefly ready-made complex foodstuffs. In
other words, Paramecium, like all animals, requires
relatively complex chemical compounds rich in potential
energy: proteins, carbohydrates, and fats. Of these, pro-
teins or their constituent amino acids are absolutely indis-
pensable because it is only from this source that nitrogen
is available for the animal. But the green plant, through
its chlorophyll apparatus, is able to take materials largely
devoid of energy and to rearrange them and endow them'
with potential energy which it has received in the kinetic
form from sunlight.
METABOLISM OF ANIMALS 43
2. Respiration and Excretion
Of course, during life, the animal, like the green plant, is
continually breaking down its food and its own protoplasm
by a process of combustion which involves an intake of free
oxygen and the liberation of carbon dioxide and water.
Nitrogenous wastes, chiefly UREA, as well as inorganic salts,
are also excreted. So the animal, like the plant, returns to
its environment the elements in simple combinations which
are devoid or nearly devoid of energy. We have stated that
green plants are essentially constructive and animals es-
sentially destructive agents in nature. It is now apparent
that green plants are both constructive and destructive, while
animals are essentially destructive.
A little consideration of the income and outgo of green
plants and animals will show that, although the animals are
dependent on the plants for their complex foodstuffs, they do
not return, for example, the nitrogen to the outer world in
a form simple enough to be available for green plants. In
other words the urea, (NH2)2CO, which still has a little
energy left which the animal is unable to extract, must be
transformed into nitrates.
Furthermore, since plants die, which are not consumed by
animals, and animals die, which are not devoured by other
animals, large stores of matter and energy are locked up in the
complex compounds of their dead tissues. Clearly, there must
be some way of completing the cycle of the elements, for if
there were not, life, as we know it, could not have continued
long on the Earth. This gap is filled by the so-called COLOJI-^
LESS PLANTS, that is plants which, because chlorophyll is not
present, lack the power of photosynthesis and so in most
cases are dependent for food on more complex substances
than green plants demand, though not so complex as animals
require.
CHAPTER VI
METABOLISM OF COLORLESS PLANTS
Nature, which governs the whole, will soon change all things
which thou seest, and out of their substance will make other
things and again other things from the substance of them, in
order that the world may be ever new. — Marcus Aurelius.
As representative of the diverse types of colorless plants
which, lacking chlorophyll or a functionally similar pigment,
are without the power of photosynthesis, we select the vast
group known as the BACTERIA. For reasons that will appeal-
later, it is not practical to focus attention on one particular
species of Bacteria, as we have just done in considering green
plants and animals. Instead we shall discuss in very general
terms the group as a whole, referring now and then to special
kinds of Bacteria to illustrate particular points.
A. THE BACTERIA
The wide distribution of the Protozoa is exceeded by the
Bacteria. Representatives are literally found everywhere:
floating with dust particles in the air; in salt and fresh water:
in the water of hot springs; frozen in ice; in the upper layers
of the soil; and in the bodies of plants and animals. Bacteria
have received a considerable notoriety under the names of
' microbes ' and 'germs,' owing to the fact that certain types
get a living within the human body as parasites and bring
about disturbances, chiefly chemical, which we interpret as
disease. But aside from these forms, which are relatively
few in number, human life and life in general on the Earth
44
METABOLISM OF COLORLESS PLANTS
45
could not long continue without their services. It is this
aspect of the Bacteria which concerns us at present.
Among the Bacteria are the smallest organisms known.
Some species are less than one fifty-thousandth of an inch
in length and much less in breadth. None of the typical
forms comes within the range of unaided vision, — indeed
there is room and to spare for thousands of millions of Bac-
teria to live in a thimble-full of sour milk. The small
size and similarity of structure of many of the Bacteria
render their study particularly difficult, and accordingly
0®
CD
FIG. 13. — Chief types of Bacteria. A, cocci; B, bacilli; C, spirilla; D, branched
filamentous form. (From Buchanan.)
they are grouped and classified largely on the basis of chem-
ical changes which they produce, rather than on structural
characteristics. However, there are three chief morphologi-
cal types: the rod -like forms or BACILLI; the spherical forms
or cocci; and the spiral forms or SPIRILLA. Bacilli or cocci
may be associated in linear, branching, or plate-like series,
or grouped together in colonies. (Fig. 13.)
The individual bacterium is generally regarded as a single
cell though in most species there is no definite nuclear body;
the chromatin material being distributed in the form of
granules throughout the cytoplasm. A cell wall chemically
similar to protein is usually present. Some forms show
active movements by means of prolongations of the cytoplasm,
46
FOUNDATIONS OF BIOLOGY
or flagella, as in the case of the common Spirillum of decay-
ing vegetable infusions. (Fig. 14.)
Reproduction is by a process of cell division which, under
favorable conditions, may occur as often as every half hour.
The vast multitude of cells thus produced before long exhaust
the food supply and contaminate with excretion products
the medium in which they are living, so that further growth is
inhibited. Under these circumstances the protoplasm within
the cell wall ordinarily assumes a spherical form and secretes
FIG. 14. — Types of flagellation in Bacteria. 1, without flagella (at richou.s forms);
2, 3, 4, 5, with flagella (trichous forms). (From Buchanan.)
a protecting coat about itself, and thus enters upon a resting
state. In this spore form the Bacteria can withstand drying
and variations in temperature to which in the active state
they would readily succumb, and thereby the organisms tide
over periods of unfavorable conditions and are ready to start
active life again when the opportunity presents itself. (Fig.
160.)
B. CYCLE OF THE ELEMENTS IN NATURE
We have seen that carbon dioxide is the source from which
green plants derive the carbon which they synthesize into
carbohydrates, fats, and proteins. Animals directly or in-
METABOLISM OF COLORLESS PLANTS 47
directly feed on plants so that the ultimate source of the
carbon of animals is likewise the carbon dioxide of the atmos-
phere. Although both plants and animals by their respir-
atory process are continually returning to the outer world
some of this carbon as carbon dioxide, it is evident that
relatively enormous amounts of carbon are nevertheless being
taken out of circulation and locked up in the bodies of the
plants and animals. For example, it has been estimated
that about one half the weight of a dried tree trunk is con-
tributed by carbon.
The same general segregation is going on in regard to
nitrogen. The green plants take it in the form of nitrates,
for instance, and store it away in the proteins; and again
animals get their nitrogen from plant proteins, so that the
ultimate source of the animal nitrogen is the same. In a
somewhat similar manner we might trace the fate of the
other chemical elements necessary for protoplasm, but that
of carbon and nitrogen is particularly striking and instructive,
and is sufficient to illustrate the fact that although both
green plants and animals are continually taking elements
from and returning them to their environment, nevertheless
more is taken away than is returned. (Figs. 15, 16.)
The agents which restore to the inorganic world the ele-
ments removed by green plants and animals are the colorless
plants, chief among which are the Bacteria. As we know,
when an animal or plant dies, decay sets in almost immedi-
ately; that is, the complex chemical compounds are slowly
but surely reduced to simpler and simpler forms until ' dust '
remains. Although undoubtedly many of these compounds
would automatically, so to speak, tend to simplify, never-
theless this is not only hastened, but chiefly carried out by
organisms of decay such as the Bacteria. Through enzymes,
or ferments, which they form, FERMENTATION occurs. The
48
FOUNDATIONS OF BIOLOGY
carbohydrates and fats are resolved into carbon dioxide and
water, and the proteins are reduced to carbon dioxide, water,
and ammonia (NH3) or free nitrogen, while the nitrogenous
waste (urea, etc.) of animals is similarly broken down. Prac-
tically all of these long series of chemical reactions are carried
on by different kinds of Bacteria. Most green plants, how-
ever, take their nitrogen chiefly in the form of nitrates and
• Dead
Organisms
Living
Animals
Bacterial
Decay
Carbohydrates,
Proteins, Fats,
in Green Plants
\
Fermentation
and Animal
Respiration
Intermediate
Decomposition
Products
FiG. 15. — The Carbon Cycle. A schematic representation of the circulation
of carbon in nature.
accordingly we find that another type of Bacteria (NITRITE
BACTERIA) acts upon the ammonia and transforms it into
nitrous acid (HNO2). After certain chemical reactions in
the soil, forming, e.g., potassium nitrite or ammonium nitrite,
still another type (NITRATE BACTERIA) oxidizes the nitrites
into nitrates (e.g., KNO3 or NH^Oa), so that again this
nitrogen is in a form which is available for green plants.
But, still confining our attention to the nitrogen, it is
obvious that there is a leak from this cycle, since some of the
METABOLISM OF COLORLESS PLANTS
49
nitrogen in the form of ammonia or free nitrogen escapes
to the atmosphere. The greatest loss, however, is brought
about by a group of DENITRIFYING BACTERIA whose activities
are largely spent in changing nitrates into gaseous nitrogen
which escapes into the air, and so putting it beyond the reach
of green plants and animals. Fortunately there is also a
special group of NITROGEN-FIXING BACTERIA which rescue
.Animal,
Proteins
Proteins
of Green Plants
itrite Ammonia
itrites^±£i^7
Denitrifying
Bacteria
FIG. 16. — The Nitrogen Cycle. A schematic representation of the circula-
tion of nitrogen in nature.
the nitrogen from the atmosphere and return it to the cycle
of elements in living nature. These organisms inhabit
the soil or little nodules which they produce on the
rootlets of leguminous plants, such as beans, clover, and
alfalfa; and this accounts for the fact, long known but not
understood, that these plants when plowed under are par-
ticularly efficient in enriching the soil. In brief, there is a
cycle of the elements in nature through green plants and
animals and back again to the inorganic world through the
50 FOUNDATIONS OF BIOLOGY
Bacteria and other colorless plants. Such is the reciprocal
nature of the nutritive processes of living organisms.
It is hardly necessary to state that the chemical changes
produced by the Bacteria are either the direct results of, or
are incidental to, the process of nutrition in these organisms.
Therefore the material taken as food by certain groups is
relatively complex, for example, by those which bring about
the early putrefactive changes in proteins; while that em-
ployed by others is very simple since they find adequate
chemical combinations less complex than those needed by
green plants. Indeed, it is now known that one group of
Bacteria is able to utilize carbon dioxide and water just as
do green plants. But instead of obtaining energy for the
synthesis from sunlight, these Bacteria derive it from chemi-
cal energy liberated by the oxidation of substances in their
environment. This process is referred to as CHEMOSYN-
THESIS, in contrast with photosynthesis, and although it is
apparently restricted to a relatively small group of organ-
isms, may well represent the most primitive method of
nutrition from which all the others have been derived in the
evolution of life.
C. THE HAY INFUSION MICROCOSM
The importance of the complex nutritional interdependence
of organisms in general and the cosmical function of green
plants — the link they supply in the circulation of the ele-
ments in nature — may be emphasized and summarized by
a brief description of a 'hay infusion.1
Probably nowhere is the 'web of life' more conveniently
or convincingly exhibited than in the kaleidoscopic sequence
of events — physical, chemical, and biological — which are
initiated when a few wisps of hay are added to a beaker of
water. Apparently the chief components of a hay infusion
METABOLISM OF COLORLESS PLANTS 51
are hay and water, but these merely supply the matter and
energy for the interplay of various forms of life. Most of
these are beyond the scope of unaided vision though chiefly
responsible for the obvious changes which occur from day to
day in their environment.
Ordinary tapwater, for instance, contains free oxygen and
various inorganic salts in solution, and not infrequently
different species of Bacteria, unicellular green plants, and
Protozoa. The hay soaking in the water contributes soluble
salts, carbohydrates, proteins, etc. It also supplies many
microscopic animals and plants which have adhered to it in
dormant form and are only awaiting suitable surroundings
to assume active life again.
A microscopical examination of an infusion when it is first
made shows very few active organisms, but within a day or
so, depending largely on the temperature, it reveals countless
numbers of Bacteria which have arisen by division from the
relatively small number of dormant and active specimens
originally present. At first the Bacteria are fairly evenly
distributed in the infusion, but as conditions change, largely
through the chemical and physical transformations which
they themselves bring about, those species which can employ
oxygen in combined form (that is, in chemical compounds)
find existence possible and competition less keen at the bot-
tom of the beaker, while those types of Bacteria which are de-
pendent upon free oxygen gather nearer the surface where the
supply is being replenished constantly from the atmosphere.
Up to this point the life of our microcosm is largely bac-
terial — unicellular SAPROPHYTIC plants which employ as
food the complex decomposition products of the proteins,
etc., of the hay. The process is essentially destructive and
the simplified products are represented in the relatively
simple excretions of the Bacteria.
52 FOUNDATIONS OF BIOLOGY
But during bacterial ascendency another factor has been
gradually intruding itself almost imperceptibly into the
drama. This is the microscopic animal life which has
been multiplying with increasing rapidity as conditions
became more favorable, and forthwith assumes the dominant
life phase in the infusion. Among the animal forms, the
first to appear are exceedingly minute flagellated Protozoa,
known as Monads, many species of which absorb products
of organic disintegration brought about by the Bacteria,
while others ingest solid food — the Bacteria themselves.
Then tiny ciliated animals, close relatives of Paramecium,
appear in untold numbers and feed upon the Bacteria. The
dominance of these smaller ciliates is brought to an end
after a few days by the ascendency of larger ciliates, which,
though feeding to a certain extent upon the already greatly
depleted bacterial population, obtain most of their food by
eating the smaller ciliates. And so the cycle of life continues
— saprophytic forms gradually being replaced in dominance
by herbivorous and these in turn by carnivorous organisms.
But obviously this chain of events must sooner or later
come to an end through the dissipation of energy brought
about by the metabolic processes of the colorless plants and
animals. Sooner or later the supply of potential energy
stored up in the chemical compounds of the hay will have
become nearly or completely exhausted — transformed
into the kinetic form and expressed in the life activities of the
plant and animal population.
Thus, after a few weeks, the hay infusion world has reached
a standstill — extermination faces the population and
inevitably occurs unless microscopic green plants, possibly
Sphaerella, find their opportunity to develop in the energy-
exhausted environment and proceed to entrap the kinetic
energy of sunlight, store it up in carbohydrates and proteins,
METABOLISM OF COLORLESS PLANTS 53
and thus restore energy in the potential form to the hay
infusion.
If this occurs, the hay infusion world is a microcosm
indeed — green plants, colorless plants,and animals gradually
become reciprocally adjusted so that a self -perpetuating con-
dition of practically stable equilibrium subvenes; in other
words, what is termed a ' balanced aquarium.' The cres-
cendo and diminuendo of teeming populations, made possible
by the rapidly changing environmental conditions which the
bringing together of hay and water initiated, is replaced by
an apparently harmonious interdependence of organisms
demanding different food conditions, such as we are familiar
with in the world at large.
CHAPTER VII
THE MULTICELLULAR ORGANISM
The student of Nature wonders the more and is astonished
the less, the more conversant he becomes with her operations;
but of all the perennial miracles she offers to his inspection,
perhaps the most worthy of admiration is the development of
a plant or animal from its embryo. — Huxley.
IT has been pointed out that all organisms consist of one
free living cell or of many cells, and some idea has been
gained of unicellular forms from Sphaerella, Paramecium, and
the Bacteria which were selected to illustrate various types
of nutrition. We are now in a position to consider the origin
and organization of the individual in the METAZOA and
METAPHYTA, as the multicellular animals and plants are
sometimes called.
Every individual, with exceptions to be noted later, begins
its existence as a single cell which has been set free as such
from the parent; or which has been formed at fertilization
by the fusion of two cells, or GAMETES, each typically de-
rived from a separate parent individual. The former is
known as UNIPARENTAL, or ASEXUAL, reproduction and the
latter as BIPARENTAL, or SEXUAL, reproduction. Both
asexual and sexual methods are widespread among plants and
animals, frequently alternating in regular sequence in the
same species to give what is termed an ALTERNATION OF
GENERATIONS.
The most remarkable fact about the reproductive cells is the
inherent power of each to develop into a replica of the parent
54
THE MULTICELLULAE ORGANISM
55
species from which it has separated. Both the spore and the
zygote (fertilized egg) are set, one may say, to go through a
series of changes which transform an apparently simple cell
into an obviously complex multicellular plant or animal with
all the tissues and organs characteristic of the species. It* is
important, at this point, to review the general method by
which the development of the adult is accomplished.
Briefly, the modus operandi of development is cell division
accompanied by differentiation. The
spore (asexual) or the fertilized egg
(sexual) by a succession of cell divi-
sions, termed CLEAVAGE, passes
from the single-cell stage to a two-
cell stage and then, with more or
less regularity, to four-cell, eight-
cell, sixteen-cell stages, etc. If these
cells separated after each divi-
sion, the same general condition
would obtain here which has been
seen in the Protophyta and Proto-
zoa, where each organism is a com-
plete free-living cell. Or again, if
cleavage merely resulted in a group
of so many exactly similar cells, there would arise a colony of
unicellular individuals rather than a multicellular organism.
Such colonial forms are, in fact, numerous among the lower
plants and animals, and show nearly all grades of complexity
from simple associations of a few identical cells, as for example
in Spondylomorum, to groups of many thousands in which
some of the individuals are specialized for certain functions.
(Fig. 17.) Volvox affords an instructive example of the
latter condition. The majority of the cells, ten thousand or
more, which form the relatively large spherical colony are
FIG. 17. — A simple colony of
unicellular organisms (Spondylo-
morum) each of which carries on
all the functions of nutrition and
reproduction. Highly magnified.
(From Hegner, after Oltmanns.)
56
FOUNDATIONS OF BIOLOGY
flagellated individuals each of which lives a practically in-
dependent existence in organic union with its fellows. The
chief contribution of each of these cells to the economy of the
FIG. 18. — Volvox globator, a large colony of flagellated unicellular or-
ganisms in which the various cells have become organically connected,
and certain cells specialized for reproduction. A, mature colony (highly
magnified) showing sperm, $ , and eggs, $ , in various stages of devel-
opment. B, four cells (more highly magnified) showing the connections
between three 'somatic' cells, and the early differentiation of a repro-
ductive cell, rp; cv, contractile vacuole; st, 'eyespot'or stigma. (From
Hegner, after Kolliker.)
whole results from the lashing of its flagella, which helps to
propel the colony through the water. But, under certain
conditions, some of the cells become specialized for repro-
duction and form new colonies which sooner or later are set
THE MULTICELLULAR ORGANISM 57
free. Thus we have a foreshadowing of that differentiation
and physiological division of labor between cells which is
the most characteristic feature of the Metaphyta and
Metazoa. (Figs. 18, 115.)
However, in the developing multicellular organism cleav-
age results, sooner or later, in a body composed of cells ^
which possess differentiations of one kind or another that
adapt them for the special part they are destined to play in
the economy of the individual. Thus cell division, involving
differentiation, is the keynote of development in the higher
plants and animals.
Among animals, for example, the cells which arise from
the cleaving egg frequently become arranged so that they
form the surface of a hollow sphere of cells known as a BLAS-
TULA. All the cells at first appear essentially similar, but
soon those at one side of the blastula become invagi-
nated until the central cavity, termed the BLASTOCOEL, is
largely obliterated. Accordingly there results the GASTRULA
stage, which may be roughly compared to a sack, with an
opening to the exterior termed the BLASTOPOBE, composed of
two layers of cells. The outer layer is known as the ECTO-
DERM, and the inner, which lines the gastrula cavity (ENTERIC
CAVITY), as the ENDODERM. The ectoderm comprises cells
which are already somewhat differentiated among themselves
for special purposes, but which, as a whole, form a primary
tissue with general functions of its own, chiefly sensory and
locomotor. Similarly the endoderm consists of cells which,
as a group, form the nutritive cells of the embryonic animal.
(Fig. 19.)
In the gastrula stage of most animals, a third layer of cells
arises typically from the endoderm and becomes disposed
between the ectoderm and endoderm. This new middle
layer is the MESODERM. In this way the so-called three
58
FOUNDATIONS OF BIOLOGY
PRIMARY GERM LAYERS are established which are characteris-
tic of the developing animal, and from these the specialized
tissues which compose the various systems of organs of the
I)
G
II
— b
FIG. 19. — Early stages in the development of the egg of a Sea Urchin. A-F, cleavage
and formation of the blastula; G, section of blastula showing the beginning of gastrula-
tion; H-I, early and later gastrula stages, a, ectoderm; b, endoderm; c, blast ocoel;
d, blastopore, leading into the enteric cavity; e, cells, arising from the endoderm,
destined to form the mesoderm.
adult are derived. For example, the ectoderm by cell divi-
sion and differentiation gives rise to the outer skin and central
nervous system; the mesoderm to muscular, connective, and
supporting tissues and the blood vascular system; while the
THE MULTICELLULAR ORGANISM
59
endoderm forms the layer of cells which lines the alimentary
canal of the adult organism.
This grouping of more or less similar cells into functional
systems, or tissues, is at the basis of the architecture of
multicellular organisms, and thus we have now reached
another level in the analysis of their structure. Although
the unit of organization is the cell, these are associated in
groups, or tissues, which represent a morphological unit of a
FIG. 20. — Portion of a cross section of the small intestine of the Frog,
highly magnified to show cellular differentiation and tissues. bl, blood
vessels; aj, unicellular glands; ep, ordinary absorptive epithelial cells
lining the intestine; es, connective tissue; m.c., circular muscle cells;
TO. L, longitudinal muscle cells; pe, peritoneum. (From Holmes, after
Howes.)
higher order. A TISSUE may be defined as a group of
essentially similar cells specialized to perform a certain func-
tion. Examples are bone, muscle, and nerve in animals;
and wood and bark in plants. (Figs. 20, 21.)
Since the similar cell components of multicellular organisms
are grouped to form tissues, it follows that the major working
units, or ORGANS, of the animal or plant body as a whole are
formed of tissues. In other words an organ is a complex of
60 FOUNDATIONS OF BIOLOGY
tissues which has assumed a definite form for the perform-
ance of a certain function: for example, the human hand
composed of bone, muscle, nerve, etc.; or the plant leaf with
its chlorophyll-bearing tissue, epidermal covering, etc.
As one would naturally expect, among the lowest Meta-
phyta and Metazoa there are forms in which the body is
relatively simple, without highly specialized tissues and
organs, but in most animals specialization is carried still
another step forward by the grouping of organs devoted to
FIG. 21. — Portion of a cross section of a young plant stem, magnified
to show cellular differentiation and tissues. ca, cambium; co, cortex; e,
epidermis; p, pith; ph, phloem; x, xylem. (From Gager.)
the performance of some one general function into an ORGAN
SYSTEM. An animal has many muscles, each of which is an
organ but which collectively constitute a working unit, the
muscular system; or it has stomach, intestine, liver, etc., form-
ing the digestive system. On the other hand, even in the
highest plants, differentiation has proceeded neither in just
the same way, nor so far, since the body is composed of
TISSUE SYSTEMS rather than organ systems. This point will
be clear when the structure of the plant and animal body
has been considered.
CHAPTER VIII
THE PLANT BODY
The evidence seems to show beyond question that our present
species of plants have descended by gradual evolution from
simpler and fewer species which formerly existed — back, it
is possible, to a single kind which throve in remotest antiquity.
— Ganong.
NEARLY all stages exist between the simplest unicellular
plant body such as is exhibited by Sphaerella, and the highly
complex condition which obtains in the familiar FLOWERING
PLANTS, technically known as SEED PLANTS or SPERMA-
TOPHYTES. A simple type is found among the filamentous
green Algae commonly called pond scums. In forms such as
>*" -"•--,
FIG. 22. — Spirogyra. Portion of a filament, highly magnified, showing cell wall,
cytoplasm, nucleus, vacuole of cell sap, and spiral chloroplastid. (From Coulter.)
Spirogyra the body of the plant consists of a series of similar
cells placed end to end and, therefore, from one point of view,
may be looked upon as a colony of cells since the individual
cells of the filament are essentially independent. (Fig. 22.)
In Ulothrix, another simple form, the filament instead of
floating free is attached by a more or less specialized cell
devoid of chlorophyll. (Fig. 49, A.) Still more common
are plant bodies composed of branching filaments of cells.
The branches may all be similar, or there may be a chief axis
61
62
FOUNDATIONS OF BIOLOGY
with lateral branches of different form. Frequently the
branches, though still composed merely of filaments of cells
placed end to end, may show, for example, larger chloro-
plastids and thus be more active in photosynthesis. This is
essentially the same general division of labor that occurs
between the stem and
leaves of higher plants,
but without the attend-
ant structural differen-
tiation of the parts into
tissues. (Cf. p. 413.)
The next specializa-
tion we find is in regard
to the character of the
growth. Whereas, in
simple unbranched fila-
mentous forms, growth
takes place as a rule by
the division of all of the
cells composing it, in
the branched types this
is usually restricted to
one or more cells near
the end of each filament.
Thus, depending on the
character of the growth
from the apical cells,various complex forms of massive branch-
ing structures arise as, for instance, in many of the Red
Seaweeds. In these plants the chloroplastids are chiefly de-
veloped in the cells on the surface; which again indicates the
physiological division of labor referred to above and suggests
that many, if not all, of the modifications of the simple plant
bodies thus far considered are provisions for the purpose of
FIG. 23. — A common Seaweed (Fucus). One
of the Brown Algae, showing comparatively sim-
ple thallus structure. (From Coulter.)
THE PLANT BODY
63
bringing about the most favorable exposure to light of the
photosynthetic apparatus.
Another method of attaining the same object is found in
other Seaweeds. In the common Sea Lettuce (Ulva) and
the Rockweed (Fucus) the plant body takes the form of a
plate of cells, as a result of cell division occurring in two
planes, and then this THALLUS usually becomes thicker by
division of the cells in a third plane also. As a result of
further modifications of the thallus, the single attaching cell
of the simple filamentous types is replaced in the larger Sea-
FIG. 24. — The Giant Kelp. A marine Alga
which may attain more than 200 feet in length.
A thallus plant exhibiting distinct leaf-like and
stem-like structures, and holdfast. (From
Ganong.)
weeds by massive HOLDFASTS which anchor them securely
to rocks. Still there is no marked differentiation in the
cellular components of the holdfasts because they perform
only this one function of the roots of higher forms; the ab-
sorption of food materials dissolved in the water being
carried on by the individual cells of the whole plant. Al-
though among the most complex Seaweeds, for example in
the Kelps and Gulf weed, the form of the thallus is highly
modified into divisions which serve certain of the func-
tions of root, stem, and leaf of higher plants, still none of
the fundamental tissue differentiations so characteristic of
the higher forms occur. Similarity of function has given
rise to ANALOGOUS structures. (Figs. 23, 24, 25.)
64 FOUNDATIONS OF BIOLOGY
It is among the so-called VASCULAR PLANTS — the Ferns
and Flowering Plants — that the most highly specialized
plant body occurs. As will be explained in more detail later,
these plants exhibit in their life history an alternation of
generations: a sexual plant (GAMETOPHYTE) bearing gametes
gives rise to a non-sexual spore-bearing plant (SPOROPHYTE)
FIG. 25. — Gulfweed (Sargassum) showing the 'stem,' 'leaves,' and
the berry-like floats of the thallus. (From Coulter, Barnes, and
Cowles.)
which in turn produces a gametophyte. These two genera-
tions exhibit marked differences in structure. The game-
tophyte body is relatively very simple, consisting merely
of a few cells, the main function of which is to develop male
and female gametes. On the other hand, the sporophyte is
composed of a number of specialized tissues and organs, and
is the conspicuous generation which is recognized by everyone
as a 'Fern' or a 'Flowering Plant.'
THE PLANT BODY
65
A. GROSS STRUCTURE
The body of the sporophyte of a typical Flowering Plant is
clearly differentiated into two parts, ROOT and SHOOT. The
root is the organ of attachment, as well as of absorption of
food material in solution. The shoot consists of STEM and
LEAVES. The stem is largely a passive structure and forms
the connecting link between the root and the photosynthetic
FIG. 26. — A, tap root of the Dandelion; B, fibrous roots of a grass; C, clustered and
fleshy roots of the Dahlia. (From Bergen and Davis.)
apparatus of the leaf. The reproductive organs (SPORANGIA)
are usually developed as appendages of modified leaves
(SPOROPHYLLS) .
1. Root
The PRIMARY root of a young plant, which is usually a
continuation downward from the shoot, may persist through-
out the life of the plant as the chief root and merely give off
laterally small secondary roots. Such a root system, known
as a TAP root, is common in many herbs, as for example the
Dandelion. More often the primary root is entirely replaced
66
FOUNDATIONS OF BIOLOGY
by the SECONDARY roots which radiate and branch in all direc-
tions from the main axis of the plant until they form a com-
plex underground structure. This may equal in size the
part of the plant body which is developed above the surface
by the shoot system.
In plants which live through two years (BIENNIALS) , often
Spring Seedling
germination growth
Seed (winter rest)
Adult plant
FIG. 27. — The seasonal history of an annual plant, a Bean.
(From Densmore.)
the first year is spent in storing up food. Sometimes this is
in the roots, in which case they are greatly enlarged to form
a reservoir of material, at the expense of which during the
second season the plant rapidly develops flowers and seeds.
These storage roots may be tap roots as in the Turnip, or
lateral roots as in the Dahlia and Sweet Potato. (Figs. 26, 27, 28.)
Although the contact of the plant with its environment
through its roots is ordinarily underground, tropical plants
in particular frequently develop AERIAL roots from the stem or
THE PLANT BODY
67
First summer
(Photosynthesis
and storage)
Second Summer
(Photosynthesis
and reproduction)
Second winter
(Death)
FIG. 28. — The seasonal history of a biennial plant, White Sweet Clover (Melilotus).
(From Densmore.)
its branches. Roots which rise from such unusual places are
called ADVENTITIOUS roots. In certain species the aerial
roots hang free in the" air and absorb moisture from the at-
mosphere. In addition, such roots
may develop chlorophyll and so
perform the characteristic func-
tion of leaves. In the Fig tree the
aerial roots grow from the branches
down to the earth where they
become attached and eventually
form a stout trunk which functions
as a stem. Comparable to these
roots are the so-called Burxsess
roots of some Palms and of the
familiar Indian Corn. (Fig. 29.)
Many plants depend chiefly or FIG 29._English Ivy> showing
entirely on other plants for their the aerial roots which enable it to
,. -. • i mi r i clinS to walls. (From Ganong,
lOOd materials. The rOOtS Of SUch aftcr LeMaout and Decaisne.)
68
FOUNDATIONS OF BIOLOGY
parasitic species frequently grow into the tissues of the host,
and become more or less modified into suckers, or HAUSTORIA.
FIG. 30. — Dodder, a parasitic Flowering Plant, entwined about the stem of
its host, a Golden Rod. A, cross section of stem of host to show its penetra-
tion by the Dodder roots (haustoria). C, several Dodder seedlings growing in
the soil before attachment to a host, h, stem of host; I, scale-like_leaves;
r, haustoria; s, seedlings. (From Bergen and Davis.)
In the Dodder and Mistletoe, the haustoria enter the tissues
of the stem of the host, while in many of the false Foxgloves
(Gerardia) they enter the tissues of the roots. In some
THE PLANT BODY
69
aquatic and parasitic plants roots are absent, their function
being taken over by other parts of the body such as stem or
leaves. (Fig. 30.)
Without multiplying examples, it is clear that the part of
a plant which the botanist calls a root, and which typically
anchors the plant to the earth and takes water with food
materials in solution from the soil, frequently is highly modi-
FIG. 31. — Propagation of the Strawberry plant by runners. A, parent plant; B, young
plant; 6, modified leaf; r, runner or stem. (From Bergen and Caldwell.)
fied and even assumes the duties of other organs in certain
plants which are adapted for special places in the economy
of nature.
2. Stem
The stem of the vascular plants is the axis of the shoot and
has two primary functions. First,* to support and raise the
leaves into a position of vantage with respect to light; and,
second, to act as the medium of communication between the
absorbing organs, or roots, and the photosynthetic organs,
or leaves. But, like the root, it may be modified and diverted
70
FOUNDATIONS OF BIOLOGY
from its typical structure and take over more or less of the
functions of other parts.
For the purpose of propagation, creeping stems occur
such as the surface RUNNERS of the Strawberry, and the
underground RHIZOMES of many Sedges, Grasses, and
common Ferns. Sometimes the stem to a large extent re-
places the root system, but more often
it acts as an underground reser-
voir in which food material is stored
up during the short growth period for
the rapid development of the flower-
ing shoot. This is well seen in some
of the early spring Flowering Plants of
New England such as Bloodroot and
Trillium. (Figs. 31, 39.)
Again, the stem is greatly short-
ened to form a BULB or a CORM; types
particularly common in plants adapted
to dry soil. (Fig. 32.) Extremely arid
regions are characterized by plants,
such as the Cacti, in which the leaves
are completely suppressed to prevent
rapid evaporation ; their function being
taken over by the stem which is pro-
vided with well-developed chlorophyll-
bearing tissue. Sometimes parts (branches) of the stem
may superficially resemble a leaf by being flattened or other-
wise modified, as in the Prickly Pear, the apparent leaves of
the so-called Smilax (Myrsipmfllum) , and the filamentous
'leaves' of Asparagus. (Fig. 33.) Finally, the versatility of
the stem is illustrated by the thorns of the Honey Locust,
the twining tendrils of the Grape, and the tuber of the
Potato which is essentially a 'concentrated rhizome.'
FIG. 32.— Bulb of a Hya-
cinth, in section, showing
roots, stem, bases of leaves
of previous year stored with
food, and new foliage leaves
about the flower cluster.
(After Figurier.)
THE PLANT BODY
71
FIG. 33. — Stem of Smilax (Myrsiphyllum). d. leaf-like branch, or cladophyll, situated
in the axil of a leaf; I, leaf; ped, flower stalk. (After Bergen.)
3. Leaf
Although the leaves of the higher
plants exhibit much diversity in
form, they agree in their essential
features. The fundamental func-
tions of leaves are to expose to
the sunlight the chlorophyll appa-
ratus and to afford a surface for
evaporation and the exchange of
gases with the environment. Ac-
cordingly the principal part of a
typical leaf is a broad blade, or
LAMINA, which affords the optimum
conditions for exposure. The leaf
is usually attached to the stem by
a stalk, or PETIOLE, which is some-
FIG. 34. — Leaf of a Flowering
Plant, showing blade, or lamina,
petiole, and two stipules at the
leaf base. (From Ganong, after
Gray.)
72
FOUNDATIONS OF BIOLOGY
what modified at the point of union with the stem into a
LEAF BASE from which arise leaf-like appendages, or STIP-
ULES. When the petiole is absent the lamina of such a sessile
leaf appears to arise directly from the stem.
(Fig. 34.)
The leaf, like the root and the stem, ex-
hibits numerous modifications in adaptation
to other functions. The chlorophyll-bearing
tissue may be nearly or completely sup-
pressed, as in the SCALES which
enclose winter buds in a pro-
tective case. These are con-
spicuously developed in the
Horse Chestnut and the Hick-
ory. (Fig. 35.) Or the scale
leaves, in addition to affording
protection, may act as reser-
voirs in which food materials
are stored, an example of
which is the familiar Onion.
(Fig. 36.) All transitions be-
tween scale leaves and typical
foliage leaves may frequently
FIG. 35. — Shoot . . <• i T i r
of Horse chestnut be seen in an unfolding leaf
sea
bud. Still more marked de-
FIG. 36— Onion
leaf, cut longitudi-
nally, bl, blade;
int, hollow interior
of blade; s, thin
sheath of leaf;
showing winter
buds enclosed by
thick scale leaves; partures from the usual leaf-
k, small axillary r ,i e <snrnp sea, thickened base
bud; x, scar of leaf IOrm are
of previous season. climbing plants such as the
(From Campbell.) ° '
Sweet Pea, the SPINES of the
Barberry and the Thistle, and the 'insect traps' of Pitcher-
plants and Sundews which capture small living animals.
(Figs. 37, 38.)
Leaf modification in another direction occurs in the spore-
of leaf. (From
Bergen and Davis,
after Sachs.)
THE PLANT BODY
73
FIG. 37. — Common Pitcher-plant (Sarracenia purpurea). At the right, one
of the pitcher-like leaves is shown in cross section. (From Bergen.)
FIG. 38. — Leaves of Sundew during digestion of captured prey. The one at
the left has all the tentacles closed ; the one at the right, only half of them closed
over the prey. (From Bergen.)
74
FOUNDATIONS OF BIOLOGY
FIG. 39.— The Sensitive Fern (Onoclea
sensibilis), showing vegetative leaf, and
spore leaf, or sporophyll, arising from the
rhizome. (From Bergen and Davis.)
bearing structures of Ferns
and Flowering Plants. In
some Ferns the spore cases
(sporangia) are borne upon
typical leaves, while in others
they arise on special leaves
with chlorophyll-bearing
tissue partly or completely
suppressed. Such leaves
which are given over to the
production of spores, as in
the Sensitive Fern, are known
as SPOROPHYLLS. (Fig. 39.)
In the Flowering Plants, the
FLOWER is a group of sporo-
phylls, known as CARPELS
and STAMENS, associated in
most cases with certain sterile
leaf structures, termed SE-
PALS and PETALS, which af-
ford protection to the sporo-
phylls and offer attraction to
insect visitors. (Figs. 40, 58.)
We shall consider the
structure of the flower in
more detail in discussing re-
production in plants, but it
is essential now, having con-
sidered the fundamental di-
visions (root, stem, and leaf)
of the body of vascular
plants, and some of the adap-
tive modifications of these
THE PLANT BODY
^Corolla
75
FIG. 40. — The Floral parts of the Alpine Azalea (Loiseleuria). Collectively
the sepals constitute the calyx, and the petals, the corolla. The pistil repre-
sents several united carpels. (From Bergen and Caldwell, after Miiller.)
parts that fit plants for different modes of life, to obtain
some insight into the tissue organization, or HISTOLOGY, of a
typical plant.
B. HISTOLOGY
As we have seen, the functions of organisms are performed
by their protoplasm which constitutes the structural units, or
cells. The cells, when specialized for a particular duty in
the economy of the organism, are usually associated in more
or less homogeneous groups, or tissues. Tissues, in turn, are
grouped to form tissue systems and organs; that is, major
divisions of the body which allow the tissues and, therefore,
the cells devoted to a special function to play their part under
the most suitable relations to internal or external conditions.
It is important, however, as we resolve the individual plant
(or animal) into its component cells, tissues, or organs, not to
lose sight of the fact that these parts are all at work for the
good of either the individual or the race. The many dif-
ferent kinds of work which are being carried on by the organ-
ism, whether it is simple or complex, must provide in the
final analysis for two things: the support or nutrition of the
76
FOUNDATIONS OF BIOLOGY
individual and the production of
other similar individuals, reproduc-
tion.
In order to obtain a mental picture
of the essential working plan of the
tissue distribution in the body of a
higher Flowering Plant, we shall con-
FIG. 41 . — Ideal vertical section through a generalized
plant, showing the typical distribution of the systems
of tissues. The central cylinder, or stele, comprises the
pith (coarse dotted), xylem (diagonally lined), phloem
(cross lined), with cambium (fine dotted)- between and
extending to growing points of root and shoot. The
cortical system (crosses) forms the 'hollow' cylinder
which surrounds the central cylinder and is, in turn,
enclosed by the outer hollow cylinder (double lined),
or dermal system. (From Ganong.)
sider first an ideal vertical section
through a generalized plant. (Figs.
41, 43, a.)
The root and shoot system together
constitute a continuous body, which
may be regarded as forming a long
narrow cylinder of cells, the bottom
of which is the growing point of the
primary root, and the top, the grow-
ing point of the shoot. This primary
cylinder, in turn, is made up of a solid
central cylinder of cells, surrounded
by two concentric 'hollow1 cylinders.
The inner of these concentric hollow
cylinders surrounds the central cyl-
inder, and in turn is surrounded by
THE PLANT BODY 77
the outer cylinder. The latter forms the surface of the prim-
ary cylinder, or the outer layer of cells of the plant. These
three cylinders comprise the primary tissue systems.
The central cylinder, known as the STELE, runs continu-
ously throughout root and stem, and sends bifurcations into
the branches by which certain of its elements enter the
leaves to form the VEINS. It provides the PITH, or primary
axial tissue, and the VASCULAR BUNDLES. The latter include
the food-conducting tissue (PHLOEM), the water-conducting
tissue (XYLEM) , and between them the actively growing tissue
- WALL
-PLASTID
--SAF^CAVITY
— NUCLEUS
J NUCLEOLU9
- CYTOPLASM
FIG. 42. — Optical section (highly magnified) of a generalized plant cell.
(From Ganong.)
(CAMBIUM). The cambium becomes continuous with the
tissues at the growing points of stem and roots, and together
these embryonic tissues, called MERISTEM, may be regarded
as the growth system of the plant.
The hollow cylinder immediately surrounding the solid
central cylinder comprises the CORTICAL system which pro-
vides the CHLORENCHYMA, or chlorophyll -bearing tissue of the
young stem and of the leaves, and also the CORTEX of bark
and root.
The outside cylinder forms the DERMAL system which
supplies the hair layer of the surface of the young root and
the protective EPIDERMIS covering the stem and leaf.
78 FOUNDATIONS OF BIOLOGY
With this diagrammatic arrangement of the tissue systems
of the plant in mind, we are in a position to consider the
histology of a typical root, stem, and leaf of the higher
Flowering Plants; in other words, to resolve the cylinders or
tissue systems of the plant into their component parts by
the study of transverse and longitudinal sections cut at
various levels, and so to determine the general character
and distribution of the cells as seen under the microscope.
1. Root
An examination of the tip of a root shows that it is covered
with a large number of cells which form the ROOT CAP.
These cells are gradually rubbed away as the root works
through the soil and continually replaced by new ones from
the GROWING POINT which is immediately above. The nu-
merous, small, densely-packed cells constituting the growing
point represent the region of cell formation for the entire root
tip, since near the center is a group of cells from which
smaller cells are divided off, and these in turn absorb food
materials and attain the normal size. It will be recalled
that the growing point is continuous with the cambium
region above, and it thus represents the growth system
(meristem) at the root tip. (Fig. 43.)
Just above the growing point is the GROWTH ZONE which
includes cells recently formed by the tip in its growth down-
ward. In this region the cells enlarge rapidly, especially in
length, and at the same time retain relatively thin cell walls.
The cytoplasm of these cells, by the development and coales-
cence of large vacuoles of cell sap (water, sugar, and other
substances in solution) , soon forms merely a lining closely ap-
plied to the wall ; a condition characteristic of many plant cells
in contrast with those of animals. In the growth zone also
is clearly seen on the surface the protective layer, or epidermis,
THE PLANT BODY
79
and, just within, the cortex made up of several layers of cells.
Still further toward the center of the root, the central cylinder
appears, which shows differentiating vascular bundles.
Passing now to a point a little above, we find the growth
zone merging imperceptibly into a region in which many of the
C
FIG. 43. — Cell division and tissue differentiation in a growing root tip. Dia-
gram of root (a) shaded to show the several regions from which the highly mag-
nified sections (b, c, d, d', e) are taken. (From Densmore.)
epidermal cells on the outer surface of the root are modified
into ROOT HAIRS. It has been emphasized above that the
primary function of the root is the intake of certain of the
elements of food in solution. This function is performed
almost entirely by osmosis through the extensive area af"
forded by the surface of the root hairs, and accordingly cells
80
FOUNDATIONS OF BIOLOGY
of this type form the vital point of contact between the root
and its environment. The root hairs exhibit the selective
power of protoplasm to a remarkable degree. For example,
Red Clover plants and Barley plants when burned yield about
the same proportion of mineral matter as ash. But the
Barley ash contains nearly
twenty times as much
silica as the Clover, while
the latter contains nearly
six times as much lime as
the Barley. (Fig. 44.)
In the zone in which
root hairs are present the
central cylinder of the
root shows still more cell-
ular specialization. The
young vascular bundles
are differentiated into the
phloem tissue, character-
ized by its small angular
ducts; while the develop-
ing xylem, with its large
FIG. 44. — Root hair (very highly magnified) ducts, has obliterated the
showing its relation to adjoining cells of the root ... ,
and to particles of the soil, a, vacuole filled primitive ground tlSSUC, Or
with cell sap; b, cytoplasm (dotted); c soil ith Between the Xylem
particles; d, nucleus within the cytoplasm lining *
the ceil wail. and the phloem appears
the developing cambium, but this begins its characteristic
growth contribution somewhat above the hair zone. Indeed,
as we pass upward from the region where the root hairs are
developed, the cellular structure becomes more and more
similar to that of the stem; the older woody roots of trees
and shrubs being, from the standpoint of both structure and
function, stems.
THE PLANT BODY
81
2. Stem
Just as cell division in the meristem tissue of the growing
tip of the root proceeds in such a way that the direction
of growth is typically downward, so in the similar region at
the growing point (BUD) of the shoot (stem and leaves) cell
multiplication results in progress upward. In other words,
from the region where root and shoot merge, the growth of
the plant is in opposite
directions.
The general character
of the embryonic tissue
is essentially the same
in both the root and
shoot regions, being
composed of densely
packed, more or less
cubical cells which are
completely filled with
protoplasm. At this
stage the cells have not
become modified for
special functions by the
formation of vacuoles of cell sap, or in other ways. But all
the tissues of the stem are derived from the cells of the
growing point, so that slightly below this region three areas
are to be noted in which differentiations are in progress.
These result in the formation of the outer cylinder (epider-
mis), the intermediate cylinder (cortex), and the central
cylinder (stele) in which the ground tissue (pith) is gradually
encroached upon by the developing vascular bundles. The
general arrangement of these fundamental tissue systems
may be seen in a transverse section of the young stem of a
Bean which affords an excellent working plan of stem anat-
FIG. 45. — Portioft of a cross section of the stem
of a young plant (Ricinus). ca, cambium; co, cor-
tex; e, epidermis; p, pith; ph, phloem; x, xylem.
(From Gager.)
82
FOUNDATIONS OF BIOLOGY
omy in one of the main divisions (Dicotyledons) of Flower-
ing Plants. (Fig. 45.)
3. Leaf
The embryonic cells forming the growing point, or bud, of
the shoot comprise, as we have seen, the fundamentals of
both stem and leaves; that is to say, stem and leaves arise
together in buds. The method of formation of stem and
leaves is well seen in the buds of a common water plant,
FIG. 46. — A bud, of unusually elongated form, of Elodea canadensis, in
exterior view and section, showing the development of leaves; X 150.
(From Ganong, after Kny.)
Elodea. Here the rounded end of the stem is composed of
the characteristic embryonic tissue. The rudiment of each
individual leaf is first visible on the surface as an enlarged cell
which by division and differentiation gradually develops into
a flat projection of epidermal, cortical, and vascular tissue,
constituting the fully formed leaf. (Fig. 46.)
Bearing in mind that the leaf is the chief organ for the in-
take and utilization of the energy of sunlight and for the
interchange of gases, it is evident that it forms the second of
the two chief points of contact of the plant with its sur-
roundings. This intimate relationship is manifest in the in-
THE PLANT BODY
83
numerable adaptations in the form of leaves, but the funda-
mental structure of all can be reduced to a common plan which
may be illustrated by a transverse section of a leaf. (Fig. 47.)
The essential features of a leaf consist of upper and lower
limiting membranes (epidermis) which are continuous at the
edges of the blade, and thus enclose the supporting and con-
ducting tissues consisting of vascular bundles (veins), and
FIG. 47. — Cross section of a typical leaf, highly magnified, a, air spaces; b, vein; e,
e', upper and lower epidermis; p, palisade layer of chlorenchyma; s, stoma; sp,
irregularly arranged 'spongy' chlorenchyma cells. (From Bergen and Davis.)
the chlorophyll-bearing cells (chlorenchyma) which carry on
the work of photosynthesis.
The walls of the epidermal cells are impervious to water
and gases, and therefore the epidermis is perforated with tiny
pores (STOMATA) which lead into air spaces among the chlo-
renchyma cells. It is estimated that the number of stomata
in the epidermis of common leaves averages about 500 per
square millimeter, so there is ample provision for the exchange
of water vapor, carbon dioxide, and oxygen with the atmos-
phere. Each stoma is enclosed by two specialized epidermal
cells, termed GUARD CELLS, which regulate the size of the
opening according to varying internal and external conditions.
84 FOUNDATIONS OF BIOLOGY
The veins, as we have seen, are merely the extensions into
the leaf of the chief elements of the vascular bundles of the
stem. They form the framework of the leaf, as well as the
system of ramifying highways for the transportation of
materials between the blade as a whole and the stem.
In cross-section the larger veins show the essential features
of the vascular bundles seen in the stem, lacking, however,
the cambium.
The chief tissue of the leaf is the chlorenchyma, consisting
of chlorophyll-bearing cells. Immediately under the upper
epidermis these cells are arranged in a definite layer known as
the PALISADE LAYER. Below this region the cells are more or
less irregularly disposed so that there are larger and smaller
AIR SPACES between them. These air spaces form a prac-
tically continuous system of passages and thereby facilitate
the interchange of oxygen, carbon dioxide, and water vapor
between the leaf cells and the outer world through the
stomata.
The cytoplasm within the thin walls of the chlorenchyma
cells forms merely a lining in which are situated the
nucleus and numerous specialized, disc-shaped, cytoplasmic
bodies, the chloroplastids, which bear the chlorophyll and
therefore appear green. It will be recalled that these are the
essential agents of photosynthesis. The center of the cell
is occupied by a large vacuole of cell sap. This sap is usually
under considerable pressure, which accounts for the close
application of the cytoplasm to the inside of the cell wall and
produces the turgor characteristic not only of the chloren-
chyma cells, but also of many other types of plant cells as well.
C. PHYSIOLOGY
We have now outlined the essential structure of a general-
ized Flowering Plant, with the exception of the parts modi-
THE PLANT BODY 85
fied for reproduction. Before turning to the flower which
in its function has to do with the race rather than the
individual, it is important to consider the organism as a
whole — how the various cells, tissues, and organs cooperate
in the nutrition of the living plant; for nutrition, it will be
recalled, is the function of primary importance to the indi-
vidual.
The essentials of nutrition were readily described in the
simple green plant Sphaerella, because the whole organism
comprises but a single cell which directly interchanges matter
and energy with its environment. But with the establish-
ment of the complex plant body, an organization of many
millions of highly specialized cells, the intricate interrelation-
ship of these various parts to the nutrition of the whole —
the mechanical engineering of the plant — becomes a problem
in itself.
1. Circulation Paths
The green plant, as we know, takes in the raw materials
and builds them up into its foodstuffs. In the case of the
higher plants, water in large amounts is taken in through the
root hairs. Dissolved in this water are various substances —
nitrates, phosphates, sulfates, etc. — supplying most of the
elements that are necessary for the make-up of protoplasm.
The leaves admit carbon dioxide through the stomata. Thus
the substances which are to be built up into foodstuffs enter at
the opposite ends of the plant, and must be brought together
in a chemical laboratory, as it were, in order that their union
may be effected. The organ in which food construction takes
place is the leaf, and, specifically, in the chloroplastids of the
chlorenchyma cells. Accordingly we must consider the high-
ways which bring the raw materials from the root hairs to
the leaves and those which distribute the finished products
to the various parts of the plant for their use. (Fig. 48.)
86 FOUNDATIONS OF BIOLOGY
The water which enters the root hairs is given a start up
the stem by 'root pressure' due to osmotic phenomena in the
multitude of cells of the root. This pressure in an actively
growing tree in spring may be nearly forty pounds to the
square inch. Passing from the xylem of the root the
ascending water enters the similar region of the stem. Here
the conducting vessels are of two kinds: namely, greatly
elongated single cells, known as TRACHEIDS, and DUCTS.
The latter are really tubes which have been formed by the
absorption of the contiguous walls of many long cells ar-
ranged end to end. The mature tracheids and ducts, though
originally derived from living cells, are devoid of protoplasm,
and form a series of non-living tubes extending up the stem
to the leaves, through which they are distributed in the veins.
This is the fluid-conducting highway from root to leaf.
The supply of carbon which the plant needs is obtained
from carbon dioxide which enters the leaves through the
stomata. The water, containing various salts in solution,
which has been taken in by the roots, meets the carbon diox-
ide in the chlorenchyma cells; and it is here that these
raw materials are manufactured into food. Therefore, the
leaves are the organs specialized for assembling the materials
of the inorganic world and forming from them new chemical
compounds of such a character that they can be utilized as
building material for the plant body and as sources of energy
for carrying on the vital functions. In other words, the new
compounds are the food of the plant. As we know, through
the radiant energy of light a complicated series of chemical
reactions are initiated by which carbon, oxygen, and hydrogen
are united to form a sugar. In this process, free oxygen is
evolved which may be used in respiration or liberated through
the stomata. Part of the sugar thus formed is directly util-
ized by the plant as fuel, and part is employed as the basis for
THE PLANT BODY
87
the manufacture of proteins and the living material itself by
the addition of nitrogen and various other chemical elements.
In every case all types of food built up in the leaves must
be distributed to the organism as a whole. This occurs
Transpiration Respiration Photosynthesis
Foods
Cortex
— \Phloem (food path)
— \XyUm (water path)
Pith
Absorption
Water
Salts-'"^'''
Oxygen'
\Respiratfon
FIG. 48. —Diagrammatic presentation of the chief physiological activities of a Flower-
ing Plant (Bean). The first leaves (seed leaves, or cotyledons) are richly stored with
food but contribute only slightly to photosynthesis. (From Densmore.)
chiefly by diffusion in the form of soluble carbohydrates (e.g.,
grape sugar) or in soluble nitrogenous form (e.g., amines or
soluble proteins) to the smallest veinlets and then on to
larger and larger veins, which finally deliver it to the stem.
88 FOUNDATIONS OF BIOLOGY
In the stem, the course taken by the food depends upon the
immediate needs of the plant. It may pass either up or down
through the phloem, or some may be transferred to the xylem
and carried to the growing tip or the developing flower and
fruit for immediate use. When growth is not active, most
of the food passes downward through the phloem to supply
the cambium and to be stored, chiefly as starch, in stem and
root. In brief, all the living cells of the plant directly or
indirectly draw upon the supply of food circulating through
the phloem, so we may look upon the phloem as primarily a
food-distributing system from the leaf, just as we have seen
that the xylem is the system for carrying water and solutes
from root to leaf. The raw materials pass up through the
wood and the finished products pass down through the bark.
2. Dynamics of Circulation
The question will naturally arise in the mind of the reader:
what is the force which brings about the circulation of all
these fluids in the plant body? We have mentioned that
water containing solutes enters the root hairs and passes to the
cortical cells and ducts of the xylem by the physical process
known as osmosis. It is now believed that osmosis in the
leaves draws water from the ducts of the stem into the cells
of the leaf. It is also thought that osmotic forces operating
in leaf cells are adequate to lift the water to the tops of the
tallest trees, where, in turn, the water is removed from the leaf
cells by evaporation through the stomata.
The outgo of water by evaporation is termed TRANSPIRA-
TION, and is brought about by heat energy from the sur-
rounding atmosphere. In the last analysis, if the explanation
suggested above is correct, the energy of heat, resulting in
evaporation from the leaves, is chiefly responsible for the
movement of the column of water which is continually pass-
THE PLANT BODY 89
ing through the plant — entering the root with various sub-
stances in solution and emerging through the stomata as
water vapor. The fact that much more water usually is
evaporated from a forest than from an equal area of a lake,
affords some conception of the part played by vegetation not
only in returning water to the atmosphere but also in 'con-
suming' heat energy — cooling the summer air. The dynam-
ics of the circulation through the xylem, however, are prob-
ably by no means so simple as might appear from the theory
just outlined; and, moreover, there is no satisfactory explana-
tion of the causes of food distribution in the phloem, further
than that osmotic phenomena play an important role.
3. Food Utilization
The food which the plant has constructed and distributed
to the various parts of its body must be employed by the
individual cells in supplying the material and energy for their
life processes. It is important not to lose sight of the cell in
the larger organization of which it is a part, for, in the final
analysis, the life of the individual plant is but the life of the
multitude of units which cooperate toward its make-up.
Although the cells suppress their individuality in the cor-
porate whole which they form, the life of the plant is as truly
the life of the protoplasmic units which form it as is the life
of a human community resident in the individual citizens.
The cells select from the food stream not only the materi-
als essential for their individual life, but in addition those
which they require for the performance of their particular part
in the economy of the whole. But doing this implies work,
and work means expenditure of energy — the same energy of
sunlight which was stored in the food during its construction
by the chlorenchyma cells of the leaf. In order to release
this energy RESPIRATION must occur. Carbohydrates, fats,
90 FOUNDATIONS OF BIOLOGY
and proteins must be oxidized, that is, burned, and conse-
quently free oxygen transmitted throughout the plant to the
various cells, and carbon dioxide carried away. This is
effected by an intercellular system of air spaces which rami-
fies throughout the plant and communicates with the sur-
rounding atmosphere chiefly by way of the stomata.
We have now considered, in such detail as the scope of the
present work requires, the structure and functions of a typical
higher plant as a whole, and have indicated how the organ-
ism is specialized for the chief function which primarily
concerns the individual; that is, nutrition, or the trans-
formation of matter and energy into life and work. Since,
however, the duration of the existence of the individual is
relatively limited, it is obvious that some provision must
exist for the continuation of the race. In other words new
individuals must be formed. This brings us to the second
great function of the organism, reproduction.
CHAPTER IX
REPRODUCTION IN PLANTS
The synthetic act by which the organism maintains itself is
fundamentally of the same nature as that by which it repairs
itself when it has undergone mutilation, and by which it multi-
plies and reproduces itself. — Bernard
AMONG the lowest members of the plant kingdom the body
consists of but a single cell; the individual and the cell are
identical. As has been seen irr Sphaerella, all the life pro-
cesses essential to the individual are exhibited in relative
simplicity and without obviously complicated machinery.
Moreover, the continuation of the race is provided for by the
individual cell dividing to form two new cells. Neglecting
for the time being the mechanism of cell division, it is clear
that reproduction in Sphaerella, since it is not complicated by
specialized organs for its performance, is a comparatively
simple process.
We have considered briefly the gradual increase in com-
plexity of the plant body from the unicellular condition,
through colonies of essentially similar cells and the thallus
type, to that of the higher vascular plants, placing emphasis
on organs directly or indirectly associated with nutrition.
It is necessary now to review in a similar manner the speciali-
zations of structure and function which exist in the plant
kingdom for the multiplication of individuals.
It may be well to reiterate here that reproduction and
growth are phenomena which are intrinsically the same —
both are the result of a preponderance of the constructive
91
92 FOUNDATIONS OF BIOLOGY
phase of metabolism. The single cell, whether a whole
organism or a single unit of a complex body, increases in
volume up to a certain limit and then divides. In the former
case two"Tie^wnMividuals replace the parent cell; in the
latter, liie^complelTbody has been increased to the extent of
one— celL- In both "cases cell division has resulted in cell
reproduction. Thus cell division is always reproduction,
though it is customary and convenient to:restrict the term
reproduction to cell divisions which result in the formation of
new individuals — single cells or groups of cells which sooner
or later separate from the parent organism.
It will be recalled that during the life cycle of Sphaerella
there is associated with the reproductive act of cell division,
the formation of cell individuals which exhibit in simple form
the fundamental characteristics of spores and gametes. We
shall now see that the development and specialization of
these is at the basis of the elaborate reproductive processes of
the higher plants.
A. SPORE FORMATION
As already emphasized, cell division among unicellular
plants results in the formation of new individuals, and, among
multicellular plants, in the growth of the single individual.
This is well illustrated by the familiar pond scums in which
the plant body consists of a series of similar cells placed end
to end to form a long thread-like body. In such cases, cell
division results merely in an increase in the length of the fila-
ment constituting the plant body, unless the newly formed
cell becomes detached from the parent plant. As a matter
of fact, however, under certain conditions the protoplasmic
content actually does make its escape from the cell wall and
swims about in the surrounding water. This independent
PROTOPLAST is a spore.
REPRODUCTION IN PLANTS 93
Moreover, this spore now begins a series of cell divisions
which result in a new filamont, or individual. It will be noted
that the potentialities of the spore and the protoplasts which
continue to retain their stations in the parent body are in-
trinsically the same, but the opportunity of the spore is dif-
ferent. In brief, the fact that the spore has separated from
the parent stock appears to be^the^r^ason^why it reproduces.
Therefore a SPORE may be defined as a cell, or the essential
part of a cell, the protoplast, which has separated from one
plant body and is capable of producing another plant body.
This statement might, at first glance, seem to indicate that
spore formation is restricted to plants with multicellular
bodies, whereas we have seenjthat s^ore formation occurs
in the life cycle of Sphaerella. This apparent contradiction is
cleared away when we recall that in the latter the cell divi-
sions which produce the spores do not involve the cell wall;
merely the protoplast within divides and the daughter cells
make their escape. (Fig. 9.)
Therefore spore formation is not a necessary result of the
establishment of a multicellular body, but an inheritance
from unicellular forms which makes possible one of the two
effective types of asexual reproduction in the Metaphyta.
The other type is FRAGMENTATION, which consists essentially
in the separation from the body of larger or smaller parts,
which later reproduce the whole plant. It is a familiar fact
that, under proper conditions, cuttings, buds, bulbs, and
sometimes pieces of leaves may reproduce or, as it is some-
times stated, REGENERATE a complete plant. This is just an
expression of the same power which the spore, though a single
cell, exhibits. It regenerates, as it were, a plant body similar
to the one from which it has separated.
94 FOUNDATIONS OF BIOLOGY
B. GAMETE FORMATION
In the life cycle of Sphaerella it was noted that under cer-
tain conditions the so-called dormant cell, instead of dividing
twice to form four spores, divides five or six times and forms
32 to 64 small cells called gametes. Now it is not the struc-
ture but the behavior of the gametes which particularly dis-
tinguishes them from spores. From the standpoint of their
origin, gametes may be regarded as spores which have de-
veloped the habit of fusing to form a zygote. Moreover,
the origin of gametes is the origin of SEX, so that sexuality
arose in plants when spores, instead of reproducing, devel-
oped the habit of pairing and thus became gametes. The
act of fusing is FERTILIZATION and the cells which unite are
sex cells.
A concrete example may emphasize this important point.
The body of a filamentous Alga, Ulothrix, is composed of
a linear series of cells all of which are essentially the same in
structure and function. Under favorable conditions the
cells divide and the plant grows in length. New individuals
are not formed by this process, although the mechanical
breaking of the filament into two parts, owing to the sim-
plicity of the body, gives two individuals. When conditions
become less favorable for vegetative growth, some of the
cells cease to contribute to the elongation of the filament.
Instead, the protoplasts begin to divide within their cell
walls, and thus each forms from 2 to 64 or more spores of
different sizes, depending upon the number of divisions the
parent protoplast undergoes. (Fig. 49.)
The largest spores are provided witfci four, and the smallest
with two, flagella by means of which they swim actively in
the water when discharged from the parent plant body.
However, the number of flagella is apparently of no im-
REPRODUCTION IN PLANTS
95
portance since the cells of intermediate size may have either
two or four. Nevertheless the behavior of the spores of dif-
ferent sizes is characteristic and significant. The largest
spores spon settle down and, attaching themselves by the
flagellated end, begin to develop into new filaments. The
spores intermediate in size likewise form new individuals,
\\
FIG. 49. — Vlothrix, a filamentous Green Alga. A, modified cell for attach-
ment at the base of a filament. B, cells of a filament which have formed
spores. From three cells the spores have been liberated. C, part of a filament
liberating spores (below), and gametes (above) which pair to form zygotes.
(From Coulter.)
but the process is much less rapid; while the smallest spores
not only germinate very slowly, but give rise to dwarf fila-
ments with vigor below the normal. As a matter of fact very
few of the smallest spores germinate at all. Instead, they
unite in pairs, each pair fusing to form a large single cell. It
is apparent that the small spores by fusing, instead of feebly
germinating, perform the sex act and, therefore, are gametes,
while the product of this process of fertilization is a zygote.
96 FOUNDATIONS OF BIOLOGY
Nothing could indicate more clearly the primary relationship
of gametes to spores than the origin of sex and sexual repro-
duction through the assumption by certain spores of the habit
of pairing to form a zygote before germination.
It should be noted that sexual reproduction is not a differ-
ent kind of reproduction, but merely reproduction preceded
by the formation of a zygote; a fact very readily lost sight
of in the higher forms where accessory phenomena connected
with sexuality obscure the essential features, but quite ap-
parent in Ulothrix because here the zygote does not form
directly a new filament. Instead, after passing a longer or
shorter time in a dormant condition protected by a heavy
wall, the protoplast (zygote) within divides to form a number
of spores, each of which then germinates into a new indi-
vidual. Thus in Ulothrix, as in Sphaerella, reproduction is
solely by spores, "sexual 'reproduction' not reproducing, but
only protecting a spore-forming protoplast."
C. SEX DIFFERENTIATION
So far we have seen that sex cells, the gametes, arose with
the establishment of the habit of reduced spores uniting in
pairs. This is obviously a statement of fact rather than an
explanation of sex. Although the two cells which fuse show
no morphological characters by which they can be distin-
guished from each other, there is certainly a physiological
basis of sex which induces them to swim toward each other,
to become oriented so that fusion begins at the flagellated
ends, and to melt into a single cell, which culminates in a
reorganized cell with the complicated structural and physio-
logical equipment of the two cells which entered into its
make-up. The zygote thus is a cell which combines the
characteristics of both the contributing gametes, and to this
REPRODUCTION IN PLANTS
97
significant fact must be attributed the profound importance
of sex phenomena in the life history of plants as well as of
animals.
Although sexuality is fundamentally a physiological dif-
ference between gametes which
leads to their characteristic
behavior (zygote formation) ,
even among the lower plants
structural differentiations ap-
pear. In fact, a series of
plants can be arranged show-
ing a gradual transition from
gametes which are morpholog-
ically identical to those which
differ so widely that they ap-
pear to have little in com-
mon. Oedogonium, an un-
branched filamentous Alga,
will suffice as an example,
since it affords an excellent
illustration of an intermediate
stage in gamete differentiation.
One form of Oedogonium
gamete, representing an entire
protoplast of a greatly enlarged
cell, is richly supplied with
food materials and chloroplas-
tids and remains motionless
within the cell wall. The
other type develops in pairs in small cells with greatly re-
duced chloroplastids and food content. Instead of being
motionless, each cell is provided with a circlet of cilia by
which it leaves its place of origin, swims actively in the
FIG. 50. — Oedogonium, a filamentous
Green Alga. A, young filament. B, por-
tion of a filament forming gametes (egg
and sperm) . Below are two sperm which
have just been liberated; above is a large
egg with a sperm just coming into con-
tact with it to form a zygote. (From
Coulter.)
FOUNDATIONS OF BIOLOGY
water and, entering a cleft in the wall surrounding a large
gamete, fuses with it to form a zygote. (Figs. 50, 51.)
In short, one gamete, designated the EGG, is a large non-
motile cell stored with food materials, while the other
gamete, or SPERM, is a small active cell largely devoid of
food. This is typical of the conditions which are at the foun-
dation of gamete differentiation throughout the plant and
animal kingdoms — eggs and sperm expressing a physiologi-
cal division of
labor which en-
tails structural
specialization
in opposite di-
rections.
In Oedogo-
nium sexuality
is apparent both
in the behavior
and in the
structure of the
gametes, so
that it is pos-
sible to identify
the sex cells as MALE gametes, or sperm, and FEMALE gametes,
or eggs. It will be noted that this is not the origin of sex,
for sex arose when spores by their behavior became gametes.
In other words, the sex act is the fusion of two cells which
reorganize as a single cell; and all modifications of these cells,
which enable them to function as gametes, are secondary.
D. REPRODUCTIVE ORGANS
Hand in hand with the specialization of spores and gametes
there is a progressive modification of the cells or groups of
FIG. 51. — Oedogonium; A, zygote emerging from cell of
parent filament. B, division of zygote into four spores. C,
mature spores ready to escape and develop into new fila-
ments. Note that the zygote does not directly give rise to
a filament, but to spores. (From Coulter.)
REPRODUCTION IN PLANTS
99
cells which produce them, until highly developed REPRODUC-
TIVE ORGANS arise. The asexual reproductive cells are formed
in SPORANGIA, which may be merely vegetative cells in which
the protoplast becomes transformed into a spore, or elaborate
multicellular structures set aside for this one function. Simi-
larly, with the origin of sexuality, the sex cells arise in GAME-
TANGIA, which later are
distinguished as ANTHE-
RIDIA, or sperm-produc-
ing, and ARCHEGONIA, or
egg-producing organs.
Moreover, although the
terms male and female
are strictly applicable
only to the sperm and
eggs respectively, the an-
theridia and archegonia
are called male and fe-
male organs; while a
plant body which bears
only male reproductive
organs is designated as a
male plant and one which
bears female reproduc-
tive organs is known as
a female plant. In short, the sexuality of the gametes is
reflected back, as it were, to the organs and then to the
individual which bears them; although actually the gametes
are the only sex cells. If this is kept clearly in mind it will
obviate confusion in considering the remarkably specialized
secondary features which sexuality imposes on the bodies of
higher plants and animals. (Fig. 52.)
We may now recapitulate before proceeding to further
FIG. 52. — A Brown Alga, Ectocarpus. A,
portion of a filament with a sporangium and a
liberated spore; B, portion of a filament with
a gametangium and a liberated gamete. (From
Coulter.)
100 FOUNDATIONS OF BIOLOGY
complications. Beproduction, divested of its specialized
features, is merely growth expressed in cell divisions. This
primary potentiality of all cells may exist side by side
with the development of cells specialized for asexual repro-
duction (spores) and sexual reproduction (gametes). In
either case the products become separated from the parent
body and develop new bodies. Furthermore, spores which
at first are developed from any of the vegetative cells of the
plant body, later arise in asexual reproductive organs
(sporangia), while gametes are produced in sexual reproduc-
tive organs (gametangia) . With the morphological differen-
tiation of gametes into sperm and eggs, a further specializa-
tion of the gamete-forming organs results in male and female
reproductive organs (antheridia and archegonia). When
sporangia and gametangia are borne by separate individuals,
asexual plants (SPOROPHYTES) and sexual plants (GAMETO-
PHYTES) result. Finally, the sperm and eggs may be borne
on separate gametophytes, in which case male and female
gametophytes result.
E. ALTERNATION OF GENERATIONS
From the standpoint of the evolution of the higher plants
the most significant fact stated above is that sporangia and
gametangia may be borne by separate individuals, for this
clearly involves an asexual, spore-bearing generation, and a
sexual, gamete-bearing generation. We shall outline this
alternation of generations in the life history of a typical Moss
and Fern as an introduction to the problem of reproduction
in the higher Flowering Plants.
1. The Moss
The common Mosses of woods, hillsides, and fields are a
relatively inconspicuous but nevertheless an important part
REPRODUCTION IN PLANTfc
101
of our flora, since they form heavy growths or carpets of vege-
tation which hold back much of the rainfall so that it sinks
into the soil. Although there are over 8000 species which
botanists include in the order Bryales of the Phylum BRYO-
Fia. 53. — The life history of a Moss, chiefly Polytrichum. a, the entire plant
(gametophyte and sporophyte), XI. b, median vertical section of the capsule in
which spores are formed, X 6, with spore (c) and germinating spore (d), X 300; e,
spore germinated to a protonema with a bud which forms leafy plant (gametophyte),
X 75; /, tip of gametophyte with two archegonia, X 2; g, archegonium in section
showing egg, X 16; h, tip of gametophyte with antheridia, X 2; i, antheridium, X 16;
j, a single liberated sperm, X 600; k, gametophyte with sporophyte developing in
enlarged and transformed archegonium. (After Ganong, Dodel-Port and others.)
PHYTA, a general description of a typical common Moss, such
as Polytrichum communae, will suffice for the purpose at
hand. (Fig. 53.)
The shoot of a moss plant is differentiated into stem and
102 FOtJNDATiONS OF BIOLOGY
leaves which are of very simple construction compared with
those of the Flowering Plant we have studied. True roots are
not present, but their function is performed by filamentous
outgrowths called RHIZOIDS. At the top of the leafy moss
plant, inconspicuous reproductive organs are developed.
Some species bear both antheridia and archegonia on the
same plant, while others have only one type. The leafy moss
plant is thus a sexual individual, or gametophyte. When the
reproductive organs are mature, sperm escape from the
antheridia and, swimming about in moisture which has col-
lected on the leaves, are attracted to the archegonia contain-
ing the eggs, apparently by a chemical substance secreted
within these organs. A single sperm which has made its way
down into an archegonium, fuses with the egg to form a
zygote. The fertilized egg retains its position in the arche-
gonium and germinates. The result is a rod-shaped embryo
which grows not only upward through the archegonium and
so out into the world, but also downward into the tissues of
the gametophyte, from which it secures practically all of its
food materials.
The essentially parasitic nature of the new individual
renders the development of leaves superfluous, so it consists
of a simple upright stalk at the top of which reproductive
organs are borne. These are sporangia and accordingly the
individual is a sporophyte. The ripe spores are liberated and,
falling to the ground, each forms a filamentous outgrowth
called a PROTONEMA. Soon a bud arises on the protonema
which develops into a leafy moss plant.
A common Moss thus exhibits in its life history an alterna-
tion of sexual and non-sexual generations. The leafy moss
plant, with antheridia and archegonia, produces gametes
and is the gametophyte. The leafless generation, which
develops from the fertilized egg in the archegonium, produces
REPRODUCTION IN PLANTS 103
spores and is the sporophyte. The gametophyte arises
asexually, but is itself sexual; the sporophyte arises sexually
but is itself asexual. The dominant generation from the
viewpoint of both structure and nutrition — the plant one
thinks of as a 'moss' — is the gametophyte.
2. The Fern
The common Ferns comprise the largest group of one of the
major divisions of the plant kingdom known as the PTERIDO-
PHYTA. Although the forms of different species are remark-
ably varied, the ensemble of characters and in particular
the foliage is quite distinctive, so that one would recognize
practically any member of the group as a 'fern.' The stems
may be short and close to the ground, or upright as in the
Tree Ferns, though creeping and underground stems
(rhizomes) are more common. The leaves, known as FRONDS,
either arise in clusters from the tip of the stem (Tree Ferns),
or are distributed along the creeping and underground stems.
Roots bring the stem into intimate contact with the food
materials of the soil, though rhizomes function to a certain
extent as roots. An examination of the cellular structure of
a common Fern, such as Aspidium marginale, shows that it
is much more complex than a Moss, the tissues of stem and
leaves being essentially like those we have seen in the Flower-
ing Plants, and accordingly Ferns and Flowering Plants are
frequently referred to as vascular plants.
The leafy fern plant bears, on certain of its fronds, repro-
ductive organs which are sporangia. These, of course, pro-
duce spores and therefore the plant commonly recognized as
a Fern is a sporophyte. The spores when ripe are liberated
from the sporangia and fall to the ground, where they germi-
nate. From the spore arises a tiny body, about a quarter of
an inch in diameter, called a PROTHALLUS, which is essentially
104
FOUNDATIONS OF BIOLOGY
a plate of chlorophyll-bearing cells with rhizoids attaching
it to the ground. On its lower surface are developed repro-
ductive organs, antheridia and archegonia, which form gam-
etes. The prothallus therefore is a gametophyte. (Fig. 54.)
Sperm are liberated from the antheridia and swim in the
moisture from dew or rain to the archegonia. A single sperm
FIG. o4. — The life history of a common Fern, chiefly Aspidium. a, the entire
sporophyte, X A; b, portion of a leaf showing groups of sporangia (sori), X 3; c, a
sorus showing sporangia, X 10; d, a sporangium, X 50; e, a single spore, X 100;
/, ventral view, X 3 and g, a median section, X 6 of a prothallus showing rhizoids, anthe-
ridia, and archegonia; h, antheridium liberating sperm, X 120; i, single sperm still at-
tached to a remnant of 'mother cell', X 300; j, open archegonium with sperm passing
down to egg, X 120; k, young sporophyte developing from zygote. (After GanongJ
works its way down an archegonium and fuses with the egg
to form a zygote. Then the zygote, which remains in the
archegonium, proceeds to divide and forms at first a small
plant, with stem and leaf which grows upward and root
which seeks the soil. During the process of root and shoot
development the plant retains its attachment to the parent
REPRODUCTION IN PLANTS
105
prothallus from which its food is secured. Later, when direct
communication with the environment has been established
by its own root and leaf, the new generation becomes entirely
independent of the prothallus, which then degenerates and
dies. The young plant gradually grows into the typical
asexual leafy fern plant, which itself in due time produces
spores.
It is clear that in the Fern, as in the Moss, there is an al-
ternation of generations. The leafy fern plant (sporophyte)
gives rise to the prothallus (gametophyte) . The leafy fern
Ferns Flowering Plants
Fio. 55. — Diagram to illustrate the decline of the gametophyte generation
and the advance of the sporophyte generation. (From Coulter.)
arises sexually, but is itself asexual; the prothallus arises
asexually, but is itself sexual. The significant fact, however,
is that the conspicuous leafy moss plant is a gametophyte,
while the large leafy fern plant is a sporophyte; or, one
may say, the 'moss' is a sexual plant and the 'fern' is an
asexual plant. This ascendancy in dominance of the asexual
and suppression of the sexual generation, which is so charac-
teristic of the fern as compared with the moss life history, is
carried still further in the higher Ferns and finally culminates
in the Flowering Plants. (Fig. 55.)
3. Higher Ferns
As we have seen, the sporophyte of the common Ferns pro-
duces spores on ordinary vegetative fronds or, more rarely, on
specialized sporophylls. (Figs. 39, 54.) In either case but one
106
FOUNDATIONS OF BIOLOGY
kind of spore is formed. Among the higher Ferns, however,
spores of two kinds occur which, since they differ greatly in
size, are called MICROSPORES and MEGASPORES. The produc-
tion of two kinds of spores is
known as HETEROSPORY and
leads to the differentiation of
the sporophylls into MICRO-
SPOROPHYLLS and MEGASPO-
ROPHYLLS. Moreover, the
microspores on germination
form gametophytes which
produce sperm, and therefore
are called MALE GAMETO-
PHYTES, while the megaspores
develop into gametophytes
bearing eggs, and accordingly
are known as FEMALE GAME-
TOPHYTES. Finally, in these
heterosporous forms, the
gametophytes are no longer
even small independent
plants, such as the prothallus
of the common Ferns, but
both male and female gameto-
phytes are so greatly reduced
that they practically remain
permanently in the parent
microspore and megaspore,
respectively, which supply
them with food. This, it will
be noted, is just the reverse of the condition which exists in
the Moss, where it is the sporophyte which is the dependent
generation. (Figs. 56, 57.)
FIG. 50. — Stages in the life history of
a higher Fern (Marsilia). A, micro-
spore, enclosing the male gametophyte
with two groups of sperm mother cells,
and prothallial cells (p); B, sperm;
C, megaspore, enclosing food material
(starch grains), and female gametophyte
comprising a single archegonium (with
egg) at one end of the spore; D, a week-
old embryo sporophyte, still attached to
the megaspore, with first leaf (I) and
root (r). (From Bergen and Davis.)
REPRODUCTION IN PLANTS
107
Fig. 57. — A microspore and
megaspore of a 'higher Fern',
Selaginella, magnified and drawn
to the same scale. (From
Coulter.)
4. Flowering Plants
Passing to the Flowering Plants,
we find that these are heterosporous
sporophytes, and the FLOWER rep-
resents a greatly modified stem
(branch), the leaves of which are
specialized as sporophylls and ac-
cessory structures. In order to
make this clear it is necessary to
review the structure of a typical
flower. (Figs. 40, 58.)
A complete flower consists of four whorls of modified
leaves. These arise near together at the tip of a PEDUNCLE,
representing the floral branch, which connects the flower
proper with the main tissue systems of the plant as a whole.
The outer and lower circle of leaves
(CALYX) is composed of several
parts (SEPALS) which usually are
green and retain a leaf-like appear-
ance. Just within and above the
calyx is the second circle (COROLLA)
formed of larger leaves (PETALS)
which are usually brightly colored.
The calyx and corolla together form
the PERIANTH, or floral envelope
which surrounds the primary floral
organs, the STAMENS and CARPELS.
The stamens represent the third
circle of leaves, but are so highly
modified that their leaf origin is not
—diately apparent. Each con-
6, calyx; c, corolla; d, stamens; gists of a slender FILAMENT at the
e, pistil formed of fused carpels. ,. . . . . ,
(Modified from Gager.) apex of which is a small case known
108
FOUNDATIONS OF BIOLOGY
as the ANTHER. Within the anther POLLEN GRAINS are
formed. The pollen grains are microspores and, therefore, it is
FIG. 59. — Transition between petals and stamens in a Water Lily. (After Gray.)
apparent that
FIG. 60. — Dia-
gram to illustrate
the method of union
of three carpels
( megasporophylls )
to form the ovule
case of a pistil
(compound) . The
edges which unite
form the point of at-
tachment of the ov-
ales. (After Gray.)
the pollen sacs of the anthers are MICROSPORAN-
GIA and the stamens are microsporophylls.
Finally, just within the circle of stamens
is the fourth whorl of floral leaves, the car-
pels, in which specialization has gone so far
that practically no suggestion of leaf struc-
ture remains. Each carpel consists of three
parts: a lower, expanded portion termed
the OVULE CASE, merging above into the
elongated, slender STYLE, the tip of which is
the STIGMA. Such a fully developed carpel
is known as a PISTIL and when, as frequently
happens, the various carpels fuse to form a
composite structure, this is termed a com-
pound pistil. Within the ovule case are de-
veloped the reproductive bodies known as OV-
ULES which are essentially MEGASPORANGIA,
for within each is formed a megaspore, com-
monly known as an EMBRYO SAC. A carpel,
therefore, is a megasporophyll. (Fig. 60.)
REPRODUCTION IN PLANTS 109
So far it is clear that a flower is a group of sporophylls
which produce microspores and megaspores. Since, how-
ever, such reproductive bodies always form male and female
gametophytes, their development must now be considered.
The first fact to have clearly in mind is that the megaspore
is never liberated by the megasporangium. And further that
the latter remains just where it arose in the ovule case of the
pistil. Consequently the megaspore germinates within the
pistil, and it forms there a female gametophyte composed of
only a few cells, including the female gamete, or egg. Thus
the female gametophyte generation of Flowering Plants is in-
visible except with the microscope.
The pollen grain is a typical microspore, a single cell en-
closed within a protective wall. Germination starts, while
the pollen is still in the anther, by the division of the spore
nucleus into two, one of which divides again. Further devel-
opment does not occur unless the pollen is transferred in
some way, usually by insects or the wind, to the stigma of
the pistil. The stigma secretes fluids suitable for the germina-
tion of the ripe pollen grain, which bursts its rigid wall and
puts forth a cytoplasmic tube. This grows down through the
tissues of the pistil until its tip enters the ovule case, and
carries with it the nuclei, two of which represent sperm. The
pollen has now completed its development and thus the con-
tents of the pollen grain plus the tube itself constitute a
greatly reduced male gametophyte.
By the time the pollen tube reaches the ovule case, the
megaspore within has formed, as already described, the
female gametophyte with its egg. One of the sperm nuclei
unites with the egg and forms a zygote, which remains just
where it is, surrounded by the tissues of the pistil base, and
proceeds to divide to form an embryo sporophyte with rudi-
mentary root, stem, and leaf. Concurrently, the ovule case
110
FOUNDATIONS OF BIOLOGY
and associated tissues of the base of the pistil undergo more
or less profound changes ('ripen') and become transformed
into a FRUIT. The young sporophyte within, together with
food material for its further development, is hermetically
sealed up in a special packet — it has become a SEED.
FIG. 61. — The life history of a higher Flowering Plant, from various species, a,
shoot of a Flax with flowers, X J; b, vertical section of a flower, XI; c, an anther
cut to show four microsporangia containing pollen grains (microspores), X6; d, an
ungerminated pollen grain, and one, e, which has formed tube (male gametophy te) ,
X 110; /, longitudinal sections of an ovule enclosing megaspore and its contents
(female gametophyte) , X 20; g, an ovule transformed into a seed with young embryo
sporophyte and endosperm, X 10; h, a mature seed, X 5; i, youjig sporophyte from
the germination of the seed, X 5. (After Ganong.)
In this form the new generation is prepared not only to
leave the parent plant and withstand adverse conditions for
a long time, but also to continue rapidly its development into
an adult sporophyte when it falls upon favorable soil. Inci-
dentally, it may be mentioned that the establishment of seed
formation is probably chiefly responsible for the dominant
position which the Flowering Plants hold in the flora of to-day.
(Fig. 61.)
REPRODUCTION IN PLANTS 111
Thus it is clear that the gametophyte generation of Flower-
ing Plants is reduced to practically its lowest terms — a few
nuclear divisions sufficient to form the gametes. The whole
generation is telescoped, as it were, within the flower of the
previous sporophyte generation, so that sporophyte seems
to produce sporophyte; whereas, as a matter of fact, three
distinct generations contribute directly to the formation of
the seed. A seed is really a highly modified megasporangium
with its contents. The seed coat comprises tissue from the
megasporangium of the parent sporophyte bearing the flower
(first generation). Certain nutritive tissues (endosperm)
represent the female gametophyte (second generation). The
product of the fertilized egg is
a young sporophyte (third
generation). (Fig. 62.)
The great reduction of the
gametophyte generation in
Flowering Plants is accom-
panied by a transference of
some of the phenomena associ-
ated With Sexuality tO the FIG. 62. — Seed of a Violet. At the left
sporophyte, so that the latter,
though intrinsically asexual, gametophyte) enclosing the embryo, or
i M • young sporophyte. (From Coulter.)
comes secondarily to exhibit
certain sexual characters, chiefly in the flower. Thus,
although the stamens and pistil (carpels) are actually
sporophylls of the non-sexual generation, they are frequently
referred to as the male and female organs of the flower.
Likewise POLLINATION, or the transference of the pollen
grains from anther to stigma, is often called the fertiliza-
tion of the flower; whereas, as we have seen, it is merely a
preliminary step which makes it possible for gametophytes
to meet on common ground so that the sperm, which them-
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112
REPRODUCTION IN PLANTS 113
selves have suffered reduction and lost their motility, can
reach the egg and perform the act of fertilization. Frequently
this sexual differentiation extends to the flower as a whole,
since some flowers bear only stamens and others only carpels
and are known as male and as female flowers respectively.
Moreover, male and female flowers may be borne on separate
plants, in which case the plants themselves are called male
and female. In brief, the terminology which rightly is appli-
cable only to the sexual generation is, for convenience, trans-
ferred to the asexual generation, in consequence of the fact
that vicarious sexual functions are reflected back to it through
the almost complete suppression of the actual sexual genera-
tion itself.
If we glance back over the reproductive processes in plants,
we are impressed with the fact that reproduction is, to a
very large extent, asexual. The great masses of thallus
vegetation, represented by Seaweeds and their allies, in-
crease in bulk chiefly by vegetative cell division, and new
individuals are formed in profusion by fragmentation and
spore formation. In the Mosses, where the sexual generation
is prominent, beds of moss are developed chiefly by the
asexual budding of the sexual plant, while spore formation
holds a prominent place in the increase and dissemination of
individuals. In the Ferns and Flowering Plants, which are
to all intents and purposes asexual plants, since the sexual
phase is relegated to an increasingly obscure position in the
life history, reproduction is not only by spores, but also by
cuttings, bulbs, fragments of leaves, etc. In brief, reproduc-
tion, unaccompanied by sexual phenomena, is apparently
amply sufficient for the propagation of plants.
However, it will be noted that sexuality has persisted from
its simple origin when spores of unicellular plants performed
the sexual act and became gametes. The gametophyte
114 FOUNDATIONS OF BIOLOGY
generation in the life history which it provoked wanes in im-
portance as we proceed from the lower to the highest plants,
but in spite of this the sex act itself is retained and shows its
modifying powers even in the asexual generation. Obviously
some advantages must be gained in the long run by fertiliza-
tion, other than the establishment of another generation
in the life history, or such devious methods culminating
in the flower would not be elaborated for its preservation.
We shall leave this large problem until we take up the ques-
tion of sex in animals, since sexuality is a fundamental attri-
bute of both plants and animals which profoundly affects
their morphology and physiology.
CHAPTER X
THE ANIMAL BODY
If we contemplate the method of Nature, we see that every-
where vast results are brought about by accumulating minute
actions. — Spencer.
THE most obvious characteristic which distinguishes fa-
miliar plants and animals is the power of locomotion of the
latter. This criterion, however, fails among the lowest forms;
for example, Sphaerella, as we have seen, swims as actively as
Paramecium. Moreover, among multicellular animals there
are innumerable sessile forms, such as the typical stages of
the Sponges, Hydroids, Barnacles, etc. Although the power
of locomotion is not a diagnostic character of animals as com-
pared with plants (this, as has been explained, being chiefly a
matter of metabolism), it is a fact that, taken by and large,
the great dissimilarity between the bodies of multicellular
plants and animals is a direct or indirect result of the loss by
plants and the development by animals of the primitive power
of locomotion which most unicellular organisms possess. At
the basis of this difference is probably the fact that early in
the evolution of plants comparatively rigid cell walls of
cellulose were established, which directed the development of
the body along relatively fixed lines. On the other hand,
animal cells, unhampered by the limitations imposed by rigid
confining walls, were free to respond in more ways to environ-
mental conditions, and this made possible the extremely
diverse forms of mobile bodies characteristic of the animal
kingdom. This greater plasticity of the animal in compari-
115
116 FOUNDATIONS OF BIOLOGY
son with the plant is reflected again in the fact that the body
of the higher plants is essentially a combination of a series
of tissue systems and organs, each of which plays a particular
part in the economy of the whole, while that of the higher
animal is a cooperating series of organs, or ORGAN SYSTEMS.
The organ systems of animals may be classified as the
INTEGUMENTARY AND SKELETAL SYSTEMS which Constitute the
covering and the framework of the individual; the ALIMEN-
TARY, RESPIRATORY, CIRCULATORY, and EXCRETORY SYSTEMS
which directly or indirectly are concerned with nutrition;
the NERVOUS SYSTEM which, in cooperation with the system
of SENSE ORGANS, the MUSCULAR SYSTEM, etc., not only coordi-
nates the various parts of the individual, but also orients the
whole with respect to its environment; and, finally, the RE-
PRODUCTIVE SYSTEM which makes possible the continuation
of the race. The fundamental life processes for which
these systems provide must be carried on by all animals, and
the chief differences in the structure of animals, from the
lowest to the highest, is a resultant of the means adopted to
serve these essential functions under different exigencies im-
posed by the environment and mode of life.
A. THE CHIEF GROUPS OF ANIMALS
The animal kingdom may be divided into two main
groups ; on the one hand, thft nnip.ft11nlfl.r ariiTy^-Hnr "PROTOZOA
comprising about ten thousand known kinds, nearly ^all
of which are microscopic, such as Amopha,
their allies^ and on the other hand, multicellular forms, or
METAZOA. The latter division includes animals ranging in
size from those which are so small that hundreds can sport
in a drop of water, to the present-day Whales and the Dino-
saurs of the past. Although the actual stages in the transi-
tion from the Protozoa to the Metazoa are unknown, among
THE ANIMAL BODY 117
the more complex colonial Protozoa there are forms, as al-
ready noted, in which the various cells become organically
connected so that a primitive sort of body results, and, fur-
thermore, certain cells are set aside for reproduction. In other
words, cooperation involving a physiological division of labor
takes place between the individuals of a group of cells, and
this results in the establishment of an individual body of
somatic cells associated with germ cells. (Fig. 18.)
The Metazoa proper may be divided into two large groups
known as INVERTEBRATES and VERTEBRATES. The former
group, frequently referred to as the lower animals, comprises
some five hundred thousand living species and exhibits an
enormous variety of form and complexity of structure ranging
from the Sponges and Hydroids to the Molluscs, Crustacea,
and Insects. On the other hand, the Vertebrates, or higher
animals, form a relatively homogeneous group of about thirty-
five thousand species, including the FISHES, AMPHIBIA, REP-
TILES, BIRDS, and MAMMALS. The Birds and Mammals in
contrast with all other animals are commonly referred to as
warm-blooded, because their body temperature is practically
constant and usually above that of their surroundings.
The highly complicated and varied organization of animals
renders it impossible to present a concise and adequate plan
of a typical animal body, and it is therefore necessary in the
present work to select one group of animals as the basis of
study and then to compare with this, in so far as comparisons
are possible without confusion, a few of the most significant
morphological and physiological variations presented by
other groups. We naturally select the group of Vertebrates
for chief consideration not only because its relative homo-
geneity renders it the most available, but because it includes
Man. However, even before we focus attention on the
Vertebrates, it is necessary to make a brief preliminary sur-
118 FOUNDATIONS OF BIOLOGY
vey of certain morphological principles as exhibited among
the Invertebrates — selecting as types the Hydra, Earth-
worm, and Crayfish — in order to afford a background for
the consideration of Vertebrate structure and function.
B. HYDRA
In discussing the development of animals, it was pointed
out that the dividing egg typically forms a blastula which, in
turn, becomes transformed by the invagination of its wall at
one pole into the gastrula stage. This early gastrula is essen-
tially a sac composed of two layers of cells : an outer or ecto-
derm and an inner or endoderm layer. Although no adult
animal retains this simple gastrula form, the animals com-
posing the group known as the COELENTERATES are to all
intents and purposes permanent gastrulae since their bodies
are built on the plan of a two-layered sac. This is well ex-
hibited in Hydra, an almost microscopic, fresh -water Coelen-
terate which is commonly found attached to submerged
vegetation or stones in brooks and ponds. (See p. 414.)
The body of Hydra somewhat resembles a tube closed at one
end, constituting the FOOT, and open at the other, forming the
MOUTH. Surrounding the mouth is a circle of outpocketings
of the body wall termed TENTACLES. The main axis of the
body extends from foot to mouth, and every plane passing
through this axis divides the body into symmetrical halves.
In other words, the parts of the body are symmetrically disr
posed about, or radiate from, the main axis, and so Hydra
affords an example of RADIAL SYMMETRY. (Fig. 64.)
The tubular body wall of Hydra is composed of two dis-
tinct cell layers, ectoderm and endoderm, separated by a thin
non-cellular supporting layer of jelly-like material (MESO-
GLOEA) secreted by the cells of both ectoderm and endoderm.
Hydra thus illustrates a simple type of Metazoan structure
THE ANIMAL BODY
119
in which but two primitive tissues exist; such specializations
as are necessary for the performance of the essential life func-
tions being confined to the more or less isolated cells of these
layers. The majority of the cells of the endoderm which
FIG. 64. — Hydra. Longitudinal section, magnified. 1, mouth; 2, tentacles;
3, early stage in budding; 4< older bud; 5, ectoderm; 6, endoderm; 7, enteric
cavity; 8, testis; 9, ovary. (From Linville and Kelly, after Parker.)
line the ENTERIC CAVITY are concerned with the digestion of
solid food taken in through the mouth, while those of the
ectoderm are variously modified for protection, and the other
relations of the individual to its surroundings, as well as for
reproduction.
In short, in the organization of Hydra the primary tissues
(ectoderm and endoderm) have not become differentiated
120
FOUNDATIONS OF BIOLOGY
into secondary specialized tissues (muscular tissue, nerve
tissue, etc.) for one function or another — the simple life
processes of Hydra are adequately provided for by the
specialization of isolated cells or small cell groups within
ectoderm and endoderm. (Fig. 65.)
The bodies of all animals above the Coelenterates are built
up of three primary layers, which, as development of the
Fia. 65. — Hydra. Transverse section, highly magnified. Outer layer of cells,
ectoderm; inner layer, endoderm. Between, mesogloea, represented by a line. (After
Shipley and McBride.)
individual proceeds, give rise to the secondary tissues and
thereby form a relatively complex body. This third primary
layer, known as the mesoderm, typically is developed, as we
have described earlier, from the endoderm and comes to
occupy the position held by the mesogloea of Hydra; that is,
between the ectoderm and the endoderm.
The development of the mesoderm is the key to the ad-
vance in body organization of higher animals, because it
makes possible a radical change in plan that involves the
establishment of a body cavity, or COELOM, in which are dis-
THE ANIMAL BODY 121
posed many of the chief organs and organ systems. Accord-
ingly the Coelentrates, since they lack the coelom, are often
referred to as ACOELOMATBS, and the animals above the
Coelenterates, since they possess the coelom, are known as
the COELOMATES. The difference in structure can best be
made clear by comparing the body plan of a higher Inverte-
brate, such as the common Earthworm, with that of Hydra.
C. EARTHWORM
Whereas the Hydra body is essentially a single tube com-
posed of two layers of cells surrounding the enteric cavity,
the body of the Earthworm is built on th^e plan of a tube
within a tube — the outer tube forming the body wall, and
the inner, the wall of the ALIMENTARY CANAL. The walls of
these tubes become continuous, or merge into each other at
both ends, and thus together they enclose a space, the coelom.
Or, to state it another way: the outer tube, or body wall,
surrounds a space, the coelom: through the coelom runs a
second tube, the alimentary canal, which opens to the ex-
terior at either end forming the mouth and anus. (Fig. 66.)
The coelom of the Earthworm is divided by a large number
of transverse partitions, called SEPTA, which extend from the
inner surface of the body wall to the outer surface of the
alimentary canal. The result is that the worm's body cavity
is not a continuous space running from one end of the ani-
mal to the other, but consists of a linear series of chambers
through the center of which runs the alimentary canal. The
limits of these chambers are indicated on the outside of the
worm by a series of grooves which encircle the body wall.
In short, the body is made up of a series of essentially similar
units known as METAMERES, and thus affords a simple exam-
ple of METAMERISM, which is a characteristic of all the higher
animals. (Fig. 67.)
122
FOUNDATIONS OF BIOLOGY
Many of the chief organs of the Earthworm are developed
as outgrowths from the walls enclosing the coelom, so that
it is in this cavity that we find, for example, the main parts
of the organ systems devoted to circulation, excretion, and
reproduction, as well as the nervous system. Moreover, the
71
FIG. 66. — Diagrams of the body plan of the Earthworm. A and C, longitudinal
sections; B, transverse section, a, aortic loops of the blood vascular system; al, ali-
mentary canal; an, anus; e.g., brain (cerebral ganglion) ; coe, coelom; cv, blood vessels
(parietal) to body wall; ds, partitions (septa) between the segments; d.v., dorsal blood
vessel; ra, mouth; n, nephridia; o, ovary; o.d., oviduct; s.i., ventral blood vessel.
{From Sedgwick and Wilson.)
organs are symmetrically disposed with respect to the long
axis of the body which passes from mouth to anus. For
instance, the chief blood vessels and the nerve cord lie in the
long axis and extend from end to end, while the organs of the
excretory and reproductive systems are disposed in pairs on
either side of this axis. Thus there may be passed through the
main axis a single plane which divides the body into sym-
metrical halves, each of which is a 'mirror picture' of the
123
124
FOUNDATIONS OF BIOLOGY
other. The main axis, therefore, extends from the mouth
(ANTERIOR END) to the anus (POSTERIOR END) , and the plane
which divides the body into right and left sides passes through
the upper (DORSAL) and lower (VENTRAL) side. This general
dors, v
typh
neph
hep
FIG. 68. — Transverse section through the middle region of the body of the Earth-
worm, circ. mus, circular muscle fibers; coel, coelom; cut, cuticle; dors, v, dorsal blood
vessel; epid, epidermis; ext. neph, external opening of nephridium; hep, gland cells;
long, mus, longitudinal muscles; neph, nephridium; nephrost, internal opening of ne-
phridium; n. co, nerve cord; set, setae; sub. n. vess, subneural vessel; typh, typhlosole;
•cent, v, ventral vessel. (From Parker and Haswell, after Marshall and Hurst.)
disposition of organs is known as BILATERAL SYMMETRY and
is characteristic of all higher animals.
The body of the Earthworm is radically different from that
of Hydra, exhibiting as it does such essential features as
coelom, bilateral symmetry, and metamerism, which are
adopted by higher animals as the basic plan of organization.
THE ANIMAL BODY 125
It is important in this connection to understand how these
modifications are related to the third primary germ layer, or
mesoderm, which, as we have stated, plays a part in the
development of all forms above Hydra. For the sake of con-
creteness we shall describe the development of the Earth-
worm from the fertilized egg to the establishment of the general
body plan, though it must be borne in mind that in no two
species of animals is the process of development identical.
After fertilization, the egg of the worm proceeds to divide
first into two cells, then four cells, eight cells, and so on, with
more or less regularity, until a condition is attained in which
many relatively small cells are arranged about a central
cavity. This stage of the embryo will be recognized as the
blastula. (Fig. 69.)
The various cells of the blastula appear essentially the
same except that those at one end are somewhat larger than
at the other. The larger cells now sink into and nearly ob-
literate the central cavity of the blastula, thus forming a
typical gastrula stage composed of two layers of cells, ec-
toderm on the outside and endoderm on the inside. The
infolded endoderm pouch (ENTERIC POUCH) enclosing the en-
teric cavity eventually becomes the main part of the alimen-
tary canal of the worm, its present opening to the exterior
(BLASTOPORE) forming the mouth. The ectoderm is destined
to form the skin, or outer layer of the worm's body.
While these two primary germ layers are being established,
the developing embryo shows the rudiments of the third
primary germ layer (mesoderm) in the form of two cells
(POLE CELLS) which leave their original position in the wall
of the embryo and take up a place between the ectoderm
and endoderm ; that is, in the remnant of the cavity of the
blastula which the invagination process during gastrulation
has not completely obliterated. Here the pole cells, by di-
FIG. 69. — Diagrams of stages in the development of the Earthworm. A, blastula (surrounded
by a membrane) ; B, section of a blastula showing blastocoel and one of the primary cells (pole
colls) of the mesoderm; C, later blastula with developing mesoderm bands; D, start of gastrula-
tion; E, lateral view of gastrula showing invagination, which as it proceeds leaves the mesoderm
bands on either side of the body as indicated by the cells represented with dotted outline;
F, section of E, along the line »S-<S to show pole cells, mesoderm bands, and enteric cavity.
G, later stage showing cavities in the mesoderm bands, H, the same (G) in cross section; 7, dia-
gram of a longitudinal section of a young worm after formation of mouth and anus; J, the same in
cross section; K, later stage in cross section, nl, alimentary canal; an, anus; ar, enteric cavity;
coe, coelom; ec, ectoderm; en, endoderm; m, primary mesoderm cells or pole cells; m2, mesoderm;
mh, mouth; n, nerve cord; s, cavity of segment; sc, blastocoel; sm, somatic layer of meso-
derm which with the ectoderm forms the body wall; splm, splanchnic layer of mesoderm which
THE ANIMAL BODY 127
vision, form on either side of the enteric pouch a linear
series, or band, of mesoderm cells. These MESODERM BANDS
gradually increase in size and spread out until finally they
unite above and below, that is encircle, the enteric pouch.
Thus they form a continuous mesoderm layer between
ectoderm and endoderm. Simultaneously with the growth
of the mesoderm bands to form a definite middle layer, a
linear series of spaces appears in each band which presages
the future segmentation of the worm's body. These cavities
increase in size and, when the bands unite around the enteric
pouch, the corresponding cavities of each band also become
continuous in the same regions.
In this way the mesoderm becomes divided up into what
are essentially two cellular layers, an outer, or SOMATIC LAYER,
next to the ectoderm, and an inner, or SPLANCHNIC LAYER, in
contact with the endoderm. The space between these layers
of the mesoderm is the body cavity, or coelom. The coelom,
however, is not a continuous cavity from one end of the em-
bryo to the other, because the mesodermal cells which sepa-
rated the linear series of cavities in the respective mesodermal
bands persist. These cells form a regular series of connecting
sheets of tissue between the two mesoderm layers and thus
divide the body of the worm into a series of essentially similar
segments, or metameres, the limits of which are indicated on
the outside by a series of grooves which encircle the worm's
body.
While these processes are transforming the two-layered
gastrula into an embryo composed of three primary layers,
and exhibiting metameric segmentation, coelom, etc., — in
short, the 'tube within a tube' body-plan characteristic of
higher forms — the embryo is gradually increasing in size and
elongating. The mouth, representing the blastopore, remains
at one end, which is therefore designated as anterior, while
128 FOUNDATIONS OF BIOLOGY
growth is chiefly in the opposite direction or 'toward the pos-
terior. At this end (the blind end of the enteric pouch formed
at gastrulation) an opening to the exterior, the anus, is formed
so that the enteric pouch now communicates with the ex-
terior at both ends and becomes the alimentary canal. Thus
antero-posterior differentiation is clearly established.
A cross section perpendicular to the main axis of the devel-
oping worm at this stage presents the appearance of a circle
within a circle. The smaller circle surrounds the enteric
cavity and is the wall of the alimentary canal. It is separated
by a space, the coelom, from the larger circle, or body wall.
Moreover, each of these circles is composed of two tissue
layers: the alimentary canal, formed internally of endoderm
and externally of mesoderm; and the body wall, internally
of mesoderm and externally of ectoderm. Thus the coelomic
cavity is entirely enclosed by mesoderm.
It is from these four layers of cells (ectoderm, somatic and
splanchnic mesoderm, and endoderm) that all of the tissues
and organs of the adult worm arise through thickenings,
foldings, outgrowths, etc. For example, the nervous system
is formed by the ingrowth of a thickened region of the ecto-
derm; the blood vascular system develops by a specialization
of cells throughout the mesoderm; while the reproductive
system first appears as thickenings of the somatic mesoderm
which, as development proceeds, becomes largely separated
from it as independent organs in the coelom. In general, itt
may be said that in all the higher animals the ectoderm forms
the outer skin and nervous system; the endoderm supplies
the lining membrane of the major part of the alimentary
tract; while the mesoderm contributes muscles, blood vessels,
reproductive organs, and the membrane lining the coelom. i
This similarity in origin of the organ systems throughout
the animal series above Hydra and its allies is of the highest
THE ANIMAL BODY 129
significance, because it indicates a basic structural identity in
the body plan of all these forms. It is exhibited in the
developmental process in each generation, even though the
adult body in the various groups differs widely in form and
arrangement of organs. Such a state of affairs clearly sug-
gests a genetic relationship throughout the whole animal
series — the origin of the diverse forms by evolution.
4 D. CRAYFISH
Bearing in mind the general plan of organizatioi^and
development of the body of the Earthworm, we must next
consider briefly the main principle underlying the changes
in this plan which give rise to many of the diverse forms
among the higher Invertebrates. This principle appears to
be chiefly a specialization of the individual segments so that
the body, instead of consisting of a large number of essen-
tially similar metameres, is formed of a linear series of meta-
meres, many of which are quite different from the rest.
Moreover, by the partial or complete fusion of two or more
metameres and the suppression of segmentation, definite
regions of the body are delineated. This principle is well
illustrated by animals of the group known as the ARTHRO-
PODA, or 'jointed-footed' Invertebrate^ such as Lobsters, In-
sects, Millipedes, and Spiders. Altogether the Arthropoda
comprises nearly half a million living species.
The body of a primitive Arthropod differs from that of the
Earthworm chiefly in the reduction of |,hp number of seg-
ments and the development of paired jointed appendages as^
^outgrowths from the body in each sepnerit. (Fig. 70.) From
such a type all the multitude of diverse forms of Arthropod
bodies can be derived. For instance, in the CRAYFISH, which is
essentially a fresh-water Lobster, the body consists of nine^
to 5 together form the
130 FOUNDATIONS OF BIOLOGY
HEAD; segments 6 to 13. the THORAX:, and segments 14 to 19,
the ABDOMEN. In other words, by the coalescence or com-
plete fusion of certain segments, the body has become divided
jnto more or less distinct regions. (Fig. 71.) Also, the primi-
tive locomotor appendages of the respective segments have
become modified into organs for the performance of widely
different functions: those of the head, as sensory organs,
jaws, etc.; those of the thorax, as organs for grasping, offense
and defense, and walking; and those of the abdomen for
swimming, etc. Thus change in structure has gone on hand
FIG. 70. — Diagrammatic representation of the structure of a primitive Arthropod
in which very little specialization of the segments has occurred. A, eye; D, digestive
tract; F, antenna; G, jointed appendages; H, dorsal blood vessel; M, mouth append-
ages; N, ventral nerve cord with ganglia; S, mouth; Sk, chitinous exoskeleton; OS,
cerebral ganglion; Us, suboesophageal ganglion. (After Schmeil.)
in hand with change in function, so that although there is no
superficial resemblance between the jaws of the Crayfish and
the legs employed for swimming, nevertheless a study of their
development shows beyond doubt that they owe their origin
to modifications of one primary type. Accordingly thp, vari-
ous appendages are said to be HOMOLOGOUS, signifying a
fundamental similarity of structure based on descent from
ji common antecedent form. .(Fig. 72.)
On the other hand, organs of dissimilar fundamental
structure, which nevertheless perform the same function, are
called ANALOGOUS. In the group of the Arthropods known
as the Insects, the series of head appendages and the legs are
homologous with those of the primitive Arthropod type,
131
132
FOUNDATIONS OF BIOLOGY
while the wings are new, unrelated structures and not modi-
fications of the primitive serial appendages of the ancestral
form. However, as we shall see later, the wing of a Bird and
5.**M..HI. 6.lfM«,lliped 7>r,MM.llhp.
** 8. y.d Leg
O.CopuUrory Organs 10. Swimming Poor
FIG. 72. — Typical appendages of a Crayfish. All have been derived from a simple
biramous appendage similar to the swimming foot (10). Protopodite, endopodite, and
exopodite are homologous throughout the series, en, 1-5, parts of endopodite; ep,
epipodite; ex, exopodite; fl, parts of antennule; g, gill; pr, 1-2, parts of protopodite.
(From Parker and Haswell, after Huxley.)
the arm of Man are homologous, while the wing of an Insect
and the wing of a Bird are analogous structures. One of the
chief tasks of the branch of biology known as COMPARATIVE
ANATOMY is to discover the various parts of plants or of animals
THE ANIMAL BODY 133
which are homologous and to study the modifications which
are associated with change of function. (See p. 353.)
We have considered the principle of specialization and
fusion of the segments of the higher Arthropods in so far
as it affects external structures, but profound modifications
of the internal organs also occur. In the first place, the
partitions between the various segments which are present
in the Earthworm have disappeared in the Crayfish. Again,
the alimentary canal of the Earthworm is a nearly straight
tube extending through the coelom, with relatively slight
modifications in certain segments for the elaboration of the
food material as it passes along from mouth to anus ; while in
the Crayfish we see the accentuation of such modified regions,
and the development of large outpocketings which are spe-
cialized for the formation of chemical substances to DIGEST
the food material. That is, to change the food into a soluble
form so that it can pass through the cellular membrane which
lines the digestive tract and thus actually pass to the circula-
tory system for distribution to the tissues of the animal.
As a final illustration we may take the nervous system.
In the Earthworm this consists of a nerve cord which runs
along the body in the mid-ventral line below the digestive
tract. At the anterior end, it bifurcates into commissures
which encircle the digestive tract and unite above in a rela-
tively large body of nervous tissue which constitutes the
cerebral ganglion, or BRAIN. In each segment the nerve cord
also is somewhat enlarged to form masses of nerve tissue
(GANGLIA) from which nerves pass to the organs in the vicinity.
The nervous system of the Crayfish exhibits the same general
plan as that of the Earthworm, but certain modifications have
been brought about by the coalescence of segments in the
region of the head and thorax. This process has resulted in
the union of the segmental ganglia in this region into larger
134
FOUNDATIONS OF BIOLOGY
ganglionic masses. The brain of the Crayfish, for example,
comprises the primitive ganglia of the segments which have
coalesced to form the head. (Fig. 73.)
FIG. 73. — Diagram of the general plan of the anterior portion of the central
nervous system of an Earthworm and a Crayfish, o, brain (cerebral, or supraoesopha-
geal, ganglion); b, nerve commissures, encircling the pharynx (shown in section); c,
suboesophageal ganglion; d, ganglia of the ventral nerve cord, with nerves emerging.
We have now considered the fundamental body plan of
Hydra, Earthworm, and Crayfish. These Invertebrate types
afford an excellent background for a proper understanding
of the body structure of the Vertebrate groups. Hydra
exhibits the simple two-layered condition (ectoderm and
endoderm) which is a transient phase in the early develop-
THE ANIMAL BODY 135
ment of higher forms. The Earthworm is of particular
value since it illustrates bilateral symmetry, an alimentary
canal opening to the exterior by an anterior mouth and a
posterior anus, metameric segmentation, coelom, definite
organ systems for various functions, and, finally, the part
played in development by the mesoderm. The Crayfish
shows, in simple form, certain general principles underlying
the modification of the Earthworm type, which involve the
specialization of various regions in connection with the change
of functions of the parts to fulfil more complex life conditions.
The reader, however, must be cautioned against supposing
that there is a sort of progression through all the series of
lower animals up to the Vertebrates. We have selected from
the groups of Invertebrates certain types which illustrate
several of the fundamental structural principles which are
to be found in the Vertebrate body, but there are other In-
vertebrate groups that exhibit body plans which depart
widely from the types described. The consideration of the
morphology of the groups which comprise such forms as
the Tapeworms, Rotifers, Sea Urchins, Oysters, etc., would
but tend to obscure those principles which are requisite for a
proper interpretation of the structure and functions of th<
Vertebrates, including Man.
E. VERTEBRATES
The Vertebrates form one of the most clearly defined divi-
sions of the animal kingdom and include all the larger and
more familiar animals — Fishes, Amphibians, Reptiles, Birds,
and Mammals — so that in the popular mind the words
animal and Vertebrate are essentially synonymous. (Figs.
82-87.)
A Fish, as every one knows, is an aquatic backboned animal
which breathes by means of gills and moves by fins. An
136 FOUNDATIONS OF BIOLOGY
Amphibian may be thought of as a Fish which early in life
— at the end of the tadpole stage — discards its gills, devel-
ops lungs, substitutes five-toed limbs for fins, and takes up a
terrestrial existence. In the same general way, a Reptile may
be pictured as an Amphibian which has relegated, as it were,
the tadpole stage to the egg, and therefore emerges with limbs
and lungs. Birds and Mammals may be regarded as deriva-
tives of the reptilian stock which have transformed the scales
of the reptile into feathers and hair respectively, and have
developed a special care for their young; the Birds by incu-
bation of the eggs and the Mammals byretention of the young
essentially as parasites within the body of the female until
birth occurs. It will be appreciated, of course, that other
important characteristics — some of which will be apparent
as we proceed — delineate these chief Vertebrate groups;
but there is, in fact, less diversity in structure among the
Vertebrates as a whole than is present, for example, in the
one subdivision of the Arthropods, the Crustacea, of which
the Crayfish is a member. Accordingly we shall confine our
attention largely to a description of the structure and physi-
ology of an 'ideal' Vertebrate, and mention incidentally, so
far as possible, the chief modifications of general significance
which appear in the different groups.
1. Body Plan
The ideal Vertebrate body is more or less cylindrical in
form, and is bilaterally symmetrical with respect to a plane
passed vertically through the main axis which extends from
the anterior to the posterior end. Three regions of the body
may be distinguished, HEAD, TRUNK, and TAIL. The head
forms the anterior end and contains the brain, eyes, ears, and
nostrils, as well as the mouth and throat. On either side of
the head is a series of openings, or GILL SLITS, leading into the
THE ANIMAL BODY
137
SPINAL CORD
NEURAL CANAL
NOTOCHORD
BRAIN
ORAL CAVITY /
GILL SLITS H
COELOM
CLOACA
SPLEEN URINARY BLADDER
BILE DUCT
PANCREAS
Fia. 74. — Diagrammatic longitudinal section of an ideal Vertebrate (female).
(From Hegner, after Wiedersheim.)
sp.c
CTl
FIG. 75. — Diagrammatic transverse section through the trunk of an ideal
Vertebrate, en, centrum of vertebra; coel, coelom; crd. v, cardinal vein; d.ao,
dorsal aorta; d.f, dorsal fin; d.m, dorsal muscles; f.r, fin-ray; gon, gonad; int,
intestine; l.v, lateral vein; mes, mesentery; ms.n.d, mesonephric duct; ms.nph,
mesonephros; na, neural arch; p.n.d, pronephric duct; pr, peritoneum, parietal
layer; pr', peritoneum, visceral layer; r, subperitoneal rib; r', intermuscular rib;
sp.c, spinal cord; t.p, transverse process; v.m, ventral muscles. (From Parker
and Haswell.)
138 FOUNDATIONS OF BIOLOGY
throat, which, however, in air-breathing Vertebrates disap-
pear before the adult condition is attained. The trunk forms
the body proper and its cavity, or coelom, contains the ali-
mentary canal, opening to the exterior by the anus, as well
as the chief circulatory, excretory, and reproductive organs.
The tail comprises the region posterior to the coelom and
anus. (Figs. 74, 75.)
In aquatic forms thin extensions from the trunk and tail
form median and paired FINS, the latter comprising the
PECTORAL fins, situated near the junction of head and trunk,
and the PELVIC fins, just lateral to the anus. The pectoral
and pelvic fins, or the fore-limbs and hind-limbs which re-
place them in all forms above the Fishes, are the only lateral
appendages found in Vertebrates.
2. Skin
The surface of the body which comes in direct contact
with the environment is covered by an integument, or SKIN,
which, though primarily protective and sensory in function,
takes part to a greater or less degree in respiration, excretion,
and secretion. Scales, feathers, claws, horns, hoofs, nails,
teeth, etc., are derivatives of the skin. The skin, unlike that
of the Invertebrates, is formed of two layers; an outer EPI-
DERMIS derived from the ectoderm, and an inner DERMIS from
the mesoderm of the embryo. (Fig. 76.)
3. Muscles
The body wall proper is chiefly composed of MUSCULAR
TISSUE, commonly spoken of as 'flesh,' which varies in thick-
ness in different regions of the body. In the mid-dorsal re-
gion it 'surrounds the CENTRAL NERVOUS SYSTEM and the
axial supporting structure (NOTOCHORD), while ventrally it
forms the wall of the coelom. In the lower Vertebrates and
THE ANIMAL BODY
139
the embryonic stages of higher forms the muscular layer is
composed of segments known as MYOTOMES. But in the
adult stage of the latter this evidence of Vertebrate seg-
mentation largely disappears, since the muscular tissue for
the most part assumes the form of highly complex longi-
tudinal bands, extensions from which pass into the paired
appendages.
A muscle consists of a very large number of muscle cells
bound together by connective tissue and permeated with
FIG. 76. — Vertical section of human skin, highly magnified, to show its com-
posite structure. Co, dermis; SM, Malpighian layer of epidermis; Se, outer
layer of epidermis; G, Gp, blood vessels; H, hair with sebaceous glands (D) ;
N, nerves; NP, sensory endings of nerves; SD, sweat glands with ducts opening
atSD1. (From Wiedersheim.)
blood vessels and nerves. The muscle cells themselves have
in a highly developed and specialized form a primary attri-
bute of all protoplasm, contractility, which they exhibit by
shortening and broadening when stimulated by impulses
reaching them through the nervous system. Muscles, such
as those attached to the bones, in which contraction can be
brought about at will, are termed VOLUNTARY muscles, while
those which cause most of the movements of the viscera are
known as INVOLUNTARY muscles. (Fig. 7, E, F.)
140 FOUNDATIONS OF BIOLOGY
4. Coelom
The Vertebrate coelom, in contrast with the condition in the
Earthworm, essentially comprises only two chambers — a large
ABDOMINAL cavity which contains most of the chief viscera,
and a small, anterior, PERICARDIAL cavity in which the heart
is situated. In the Mammals, including Man, however, the
anterior chamber, known as the THORAX, contains the heart
and lungs and is separated from the abdominal cavity by
a muscular partition, or DIAPHRAGM. The lining membrane
of the coelom is known as the PERITONEUM and forms the
innermost layer of the body wall. (Figs. 74, 82-87.)
5. Skeleton
The form of the Vertebrate body is maintained by a system
of supporting and protecting structures, termed the SKELE-
TON. Although various outgrowths of the skin, such as scales,
feathers, and hair, form a part of the skeletal system known as
the EXOSKELETON which is comparable to the protective
coverings of the Invertebrates, it is a bony ENDOSKELETON
which is characteristic of the higher animals. This internal
skeleton which is largely mesodermal in origin exhibits
such great diversity and complexity that its study, known as
OSTEOLOGY, forms a most important subdivision of compara-
tive anatomy. In the lower Fishes the endoskeleton is com-
posed of a firm elastic tissue, CARTILAGE, or gristle, but from
the 'bony' Fishes to Man most of the cartilage becomes ossi-
fied: that is, impregnated with lime salts and transformed
into BONE. The human skeleton is formed of about 200
separate bones, but the number varies at different periods
of life, because some bones which at first are distinct later
become fused. (Figs. 77, 81, 186.)
While it is true that the bones constitute the main support-
THE ANIMAL BODY
141
142
FOUNDATIONS OF BIOLOGY
ing framework of the body, they are entirely inadequate
to knit together the organism into a working unit. We find
therefore various kinds of CONNECTIVE TISSUE interwoven
between the integral parts of the body. These tissues form
sheaths about most of the organs and also supply the con-
necting links between muscle and muscle, muscle and bone,
and bone and bone. Skeletal tissues, of which bone, cartilage,
and connective tissue form the chief groups, are distinguished
from the other body tissues by the development of large
amounts of non-living material in or between the component
cells themselves; the character of the skeletal tissue being
determined chiefly by the
Notochordal sheath r , , . , .
invading cartilage nature of thls matrlx'
The primitive axis of
the skeleton consists of
a cylindrical cord or rod
of cells (NOTOCHORD),
which lies in the mid-
dorsal line of the body
wall just below the dorsal
nerve tube (SPINAL CORD)
and above the coelom. In most Vertebrates, however,
the notochord in its original form is only a temporary struc-
ture, being partially or completely replaced during later
development by a linear series of cartilaginous or bony ele-
ments, known as VERTEBRAE, which form the VERTEBRAL
COLUMN, or backbone. This is the most characteristic struc-
ture of Vertebrates as compared with Invertebrates, or back-
boneless animals. (Figs 74, 78.)
A typical vertebra of the higher animals consists of a basal
portion, known as the CENTRUM, and a NEURAL ARCH which
it supports. These form a protecting ring of bone about
the spinal cord. From various parts of the vertebra as a
Extent of one vertebra
FIG. 78. — Diagram of a longitudinal section
through a developing vertebral column to show
the invasion of the notochord by cartilage to
form the centra of the vertebrae. (From
Walter.)
THE ANIMAL BODY
143
ns
whole arise PROCESSES for movable articulation with its
neighbors, the attachment of muscles, etc. Between the
vertebrae of the Mammals are
cushions of cartilage which ab-
sorb shock. (Fig. 79.)
In some forms, RIBS are at-
tached to the transverse pro-
cesses of certain vertebrae.
These extend outward and down-
ward within the body wall, and
become attached in the mid-
ventral line to the breast bone
(STERNUM). Thus, in the adult
of thp hip-hpr Vprtphrafps thp FlG- 79-~A tvPical human ver-
eS' ' tebra (tenth thoracic) viewed from
Series Of Centra Of the Vertebrae the dorsal surface. C, centrum; lam,
. . . ped, neural arch; 7i.s, neural spine;
COme tO OCCUpy the position prez> anterior articulating process;
formerly held by the notochord; tr' transverse Process; *• neural
canal through which the spinal cord
While above, the neural arches passes. (From Walter, after Spalte-
encircle the NEURAL CANAL con-
taining the spinal cord; and below, the transverse pro-
cesses, ribs, and sternum surround the anterior portion of
the coelom. (Fig. 75.)
The Vertebrate head, containing the anterior end of the
alimentary and neural canals, the brain, and the chief sense
organs, is protected in the lower Fishes by a case of cartilage.
In higher forms the cartilage is replaced by a bony SKULL
which articulates with the first vertebra of the backbone.
JAWS, or supporting structures of the rnouth, are attached
to the skull.
The skull, vertebral column, ribs, and sternum together
comprise the AXIAL skeleton, from which is suspended the
APPENDICULAR skeleton, or bony frame-work of the paired
appendages. This is relatively simple in the anterior (pec-
144
FOUNDATIONS OF BIOLOGY
toral) and posterior (pelvic) paired fins of Fishes, which merely
act as paddles; but when these are modified into paired limbs
for progression on land, the mechanical problems involve the
development of complex limb skeletons to support the body,
and to act as levers for the limb muscles to move in locomo-
SCP
HU
PU
dsb.S
mils. 5
JS
FIG. 80. — Diagram of the plan of the Vertebrate limbs. A, fore limb and pectoral
girdle; B, hind limb and pelvic girdle; actb, socket for femur; CL, clavicle (collar
bone) ; en, 1-2, middle row of carpals and tarsals; COR, coracoid; dst, 1-5, distal row of
carpals and tarsals; FE, femur (thigh bone) ; FI, fibula; fi, fibulare (a tarsal) ; gl, socket
for humerus; HU, humerus (upper arm bone); IL, ilium; int, intermedium (a tarsal);
IS, ischium; mlcp, 1-5, metacarpals; mtts, 1-5, metatarsals; p.cor, procoracoid; ph,
phalanges; PU, pubis; RA, radius; ra, radiale (a carpal); SCP, scapula; TI, tibia;
li, tibiale (a tarsal); UL, ulna; ul, ulnare (a carpal). (From Parker and Haswell.)
tion. In response to this need an elaborate series of bones is
developed which, in all cases, however, may be referred to a
common plan, known as the PENTADACTYL LIMB in allusion
to the five digits (FINGERS and TOES) in which it usually
terminates. The limbs are attached directly or indirectly to
THE ANIMAL BODY
145
jjll'fcr
Tills;-
146 FOUNDATIONS OF BIOLOGY
the axial skeleton by groups of bones which form respectively
the PECTORAL and PELVIC GIRDLES. (Figs. 80, 81, 185, 186.)
F. DIAGNOSTIC VERTEBRATE CHARACTERS
As a summary of this general outline of the structure of the
Vertebrate body, we may emphasize three characters which
are of prime diagnostic importance.
In the first place, whereas the skeletal structures of Inver-
tebrates typically consist, as in the Crayfish, of an exoskeleton
of hard non-living materials deposited on the surface of the
body, the chief function of which is protection, the Verte-
brate skeleton is primarily a living endoskeleton. It is an
organic part of the organism which, although it affords pro-
tection for delicate parts, provides adequately for support
and supplies muscle levers, and thus makes practicable the
relatively large bodies of the higher animals. The notochord
is at once the foundation and axis of the Vertebrate internal
skeleton and either persists throughout life as such, or simply
long enough to function as a scaffolding about which the
vertebral column is built. In recognition of the prime im-
portance of the notochord, the Vertebrates and their nearest
allies (e.g., the Tunicates and Amphioxus) are technically
known as CHORDATES (cf. pp. 415, 416)..
Glancing back at the Earthworm and Crayfish, it will be
recalled that the central nervous system consists of a ventral
nerve cord running along in the coelom below the digestive
tract, except at the anterior end where it encircles the
pharynx to form the brain above. The position of the Verte-
brate brain is similar, though the spinal cord is not a 'cord'
but a nerve tube, which lies in the neural canal imbedded in
the muscles of the body wall above the digestive tract and,
of course, outside of the coelom. Thus the spinal cord itself
and its location are highly characteristic.
THE ANIMAL BODY 147
A third fundamental peculiarity is a series of perforations
or slitr, through the throat and body wall. In the lower
forms the gill slits provide an exit for the current of water
entering by the mouth and, being richly supplied with blood,
afford the chief means of respiratory interchange between the
animal and the surrounding medium. In the higher Verte-
brates the gill slits are present merely during a transient
phase in the development of the individual since the function
of aerating the blood is taken over by the lungs. (Figs. 74,
75.)
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FIG. 87. — Diagrammatic median section of the human body. 1, cranium; S, nos-
trils (external nares); 3, mouth; 4, internal nostrils; o, pharynx; 6, 'Adam's apple'; 7,
trachea (to lungs); 8, oesophagus; 9, sternum (breast bone) imbedded in body wall;
10, lungs; 11, heart; 12, thoracic cavity (part of coelom) ; 13, diaphragm; 14, ab-
dominal cavity (part of coelom) ; 15, liver; 16, stomach; 17, large intestine; 18, kidney;
19, small intestine; 20, ureter; 21, vermiform appendix of large intestine; 22, urinary
bladder; 23, pubis of pelvic girdle; 24, urethra; 25, anus; 26, coccyx; 27, posterior
part of neural canal; 28, centrum of vertebra; 29, neural spines of vertebrae; 30,
spinal cord; SI, medulla; 32, cerebellum; S3, cerebral hemisphere.
153
CHAPTER XI
NUTRITION IN ANIMALS
The living body is the theatre of many chemical and physical
operations in line with those of the inorganic domain.
— Thomson.
WE have now considered the form and supporting struc-
tures of the body wall of a typical Vertebrate; in other
words, the outer tube which surrounds and contains, the
viscera. Through this outer tube, just as in the case of the
Earthworm and Crayfish, there runs from mouth to anus a
second or inner tube, the alimentary canal, which has been
mentioned incidentally in describing the various regions of
the Vertebrate body.
A. THE ALIMENTARY CANAL
The entrance to the alimentary canal is the mouth, a trans-
verse ventral aperture near the anterior end of the head,
which leads into the BUCCAL CAVITY supported by the jaws.
The buccal cavity merges into the PHARYNX, or throat, which
in turn leads by a narrow passage, the OESOPHAGUS, to the
STOMACH. (Figs. 74, 82-88.)
In the aquatic Vertebrates the region of the alimentary
canal from the mouth to the oesophagus acts as a common
food and respiratory passage. The food passes on through the
oesophagus to the stomach, while the water makes its exit by
a series of perforations, or gill slits, through the pharynx and
body wall directly to the exterior. During this passage the
respiratory interchange of gases takes place. Among the
154
NUTRITION IN ANIMALS
155
air-breathing Vertebrates the gill slits persist merely as
transient embryonic reminders of evolutionary history, their
FIG. 88. — Diagram of the alimentary canal and its derivatives in Man. a,
mouth; b, salivary glands; c, pharynx, showing the embryonic position of five
pairs of gill pouches, the second pair probably giving rise to the tonsils, and the
third and fourth to the thym us glands; d, thyroid gland ; e, trachea; /, thymus
gland; g, lungs; h, oesophagus; i, diaphragm — the muscular partition between
the thorax and abdomen; j, liver; k, gall bladder; I, stomach; TO, pancreas;
n, small intestine; o, large intestine; p, vermiform appendix; q, rectum leading
to exterior by the anus.
function being taken over by an outpocketing of the ventral
wall of the pharynx into the body cavity, which forms the
LUNGS. Thus, even in Man, the respiratory membrane which
156 FOUNDATIONS OF BIOLOGY
lines the lungs is, from the standpoint of development, a
specialized part of the epithelium of the alimentary canal.
(Fig. 94.)
The STOMACH is the first stopping place of food which has
been swallowed and where the work of the digestive tract
(alimentary canal) essentially begins by the dissolving action
of chemical substances (enzymes) secreted by its walls. The
stomach leads by a constriction (PYLORIC VALVE) into a long
and usually convoluted INTESTINE. The anterior portion of
this is known as the SMALL INTESTINE, and it is here that the
major part of digestion is accomplished directly or indirectly
by means of chemical secretions supplied by its walls and by
the PANCREAS and LIVER. In the small intestine ABSORPTION
also begins; that is, the passage of the soluble food materials
through the wall of the digestive tract into the body proper.
The soluble proteins and carbohydrates are taken up
directly by the blood vascular system and conveyed to the
liver, while the fats enter the lymph vessels which later
deliver it to the blood. A constriction marks the origin of
the LARGE INTESTINE which continues the absorption of
water and carries the undigested material, or FAECES, to the
exterior through the anus. This either opens into a terminal
sac, the CLOACA, in which also are situated the orifices of
the urogenital ducts, or directly on the ventral surface, as
in Man. (Fig. 93.)
The wall of the alimentary canal consists of three chief
cellular layers: a lining epithelium, a connective tissue layer,
and a muscular layer. The epithelium, however, together
with its derivatives is the digestive tract proper in the sense
that it is of prime functional importance; the other layers
performing accessory functions such as support, conduction
of blood vessels, and movements of the canal. (Figs. 20,
103.)
NUTRITION IN ANIMALS 157
B. DIGESTION
Among the single-celled animals such as Paramecium,
digestion is reduced to its simplest terms. The food material
enters the cell and is acted upon directly by substances
formed by the protoplasm (endoplasm) in its vicinity. In
Hydra a special layer of cells, the endoderm, is largely de-
voted to digestion. Although some of the endoderm cells
actually engulf small particles of food and digest them within
the cell (INTRACELLULAR DIGESTION), the major part of
digestion is brought about within the enteric cavity by secre-
tions from the endoderm cells. Digestion of the latter type
(INTERCELLULAR) is characteristic of all higher animals and
reaches its full development in the Vertebrates.
The alimentary canal is essentially a tubular chemical
laboratory which passes the food on by its own muscular
activity, known as PERISTALTIC CONTRACTIONS, from one com-
partment to another. Each of these compartments, in turn,
supplies the chemical reagents which it uses for changing
the food into a soluble form so that it can pass through the
walls to be taken up by the circulatory system and finally
distributed to the cells of the organism as a whole. The
complex food materials which enter the human mouth run
the gauntlet of a whole series of digestive fluids. The sali-
vary glands in the mouth secrete an enzyme which chemically
modifies the starches; the gastric glands of the stomach
supply the gastric juice containing enzymes which act on
proteins, and free hydrochloric acid which renders the
stomach contents acid in reaction; while glands in the intes-
tinal walls, and the pancreas collectively supply other enzymes
which act on proteins, carbohydrates, and fats in a medium
made alkaline chiefly by certain substances from the liver.
Turning now to the origin of the chemicals which bring
158
FOUNDATIONS OF BIOLOGY
LOCATION
SECRETION
ENZYMES
SUBSTANCES
CHANGED
INTER-
MEDIATE
PRODUCTS
PRODUCTS
READY FOR
ABSORPTION
Mouth
Saliva
Ptyalin
Starch
Maltose
Stomach
Gastric
juice
Pepsin
Protein
Proteoses
and Pep-
tones
Small
intestine
Pancreatic
juice
Amylopsin
Starch
Maltose
Lipase
Fats
Fatty acid
and
Glycerine
~\*'
Intestinal
juiee
Trypsin
Maltase
Sucrase
Lactase
Proteins
Maltose \
Cane sugar \
Milk sugar )
Amino
acids
Simple
sugars
Erepsin
Proteoses
and Pep-
tones
Amino
acids
FIG. 89. — Chemical activities of the human digestive tract.
about the solution of the food materials. Every cell of the
body receives .^rom the circulatory system the materials nec-
essary for its own life, but some cells take in addition sub-
stances which they do not need and, after transforming them
chemically, contribute the product as a SECRETION for the
good of the whole organism. Such cells may act more or less
independently as UNICELLULAR GLANDS, but generally, for
economy of space and adequate blood supply, many cells are
grouped together to form MULTICELLULAR GLANDS. This is
usually brought about by sinking the glandular area below
the level of the membrane to which it really belongs. Such
is the origin of complex glands as the liver and pancreas,
which are outpocketings of the wall of the digestive tract;
the sole remaining connection in each case being a narrow
tube, or DUCT, which delivers the products of the glands to
the intestine. (Fig. 90.)
NUTRITION IN ANIMALS
159
Other glandular derivatives of the digestive tract in Man
are the SALIVARY glands? of the mouth, the THYROID and
THYMUS glands near the anterior end of the oesophagus, and
the GASTRIC and INTESTINAL glands imbedded in the wall of
the stomach and intestine respectively. As a matter of fact
Duct
Alveoli
Secreting Cells
Capillary Network
FIG. 90. — Diagram of a gland, in section, together with the surrounding connective
tissue and blood vessels. Highly magnified. (From Hough and Sedgwick.)
the thymus degenerates, while the thyroid loses all connection
with the alimentary canal and contributes its products directly
to the blood. Accordingly the thyroid as well as a number of
other similar glands, are known as DUCTLESS, or ENDOCRINE,
glands, and their products as INTERNAL SECRETIONS. (Fig. 88.)
At first glance the complicated digestive system of the
Vertebrate may seem to have little in common with that of ^ ^ ;
the Earthworm, but as a matter of fact the fundamental plan
is the same. The differences which are present are the re-
sult of an increase of the working area of the alimentary
160 FOUNDATIONS OF BIOLOGY
canal, not only to afford greater secretive and absorptive
surface and a larger variety and amount of digestive sub-
stances, but also to prolong the length of time the food is
subjected to treatment. This increase in area has been
effected by folds and elevations of the inner surface of the
tract; by outpushings of limited areas of the tube to form
large glands which in most cases contribute their products to
their point of origin through ducts; and by increasing the
length of the inner tube as compared with the outer tube, or
body wall, which results in throwing the intestine into vari-
ous convolutions within the body cavity. Thus is met the
increasingly complex nutritional demands of more highly
organized animals.
CHAPTER XII
CIRCULATION AND RESPIRATION IN ANIMALS
I finally saw that the blood, forced by the action of the left
ventricle into the arteries, was distributed to the body at large,
and its several parts, in the same manner as it is sent through
the lungs, impelled by the right ventricle into the pulmonary
artery, and that it then passed through the veins and along
the vena cava, and so round to the left ventricle . . . which
motion we may be allowed to call circular. — Harvey, 1628.
THE crucial points of contact between the higher animal
and its environment, in so far as the intake of matter and
energy is concerned, are the membranes which line the
digestive tract and a large diverticulum from it, the lungs.
Through the former must pass all the materials which are to
be assembled as integral parts of the organism and the fuel
which is to supply the energy for the vital processes, while
through the latter must pass the oxygen which is to release
this energy. Only when these membranes have been passed
are the materials really within the body and at its disposal
for distribution by the CIRCULATORY SYSTEM to the individual
cells of the various organs which are to use them. In addition
to carrying the fuel and the oxygen, the circulatory system
must remove the waste products of metabolism from the cells
and deliver them to the proper excretory organs, such as the
lungs or kidneys, to be passed to the outside world. The cir-
culatory system is therefore the essential connecting link be-
tween the points of intake, utilization, and outgo of materials
— a distributing system which in cooperation with the nerv-
ous system unifies the organs into an organism.
161
162 FOUNDATIONS OF BIOLOGY
A. CIRCULATION IN THE LOWER VERTEBRATES
In the higher plants the movement of water and food in
solution through the conducting systems is effected chiefly
by physical forces which are, to a certain extent, independent
of, though directed by, the activity of the plant cells. In
the higher animals, on the other hand, circulation is brought
about by an active system which forces as well as conducts
throughout the body what is to all intents and purposes a
fluid tissue.
Many stages in the evolution of this elaborate circulatory
system can be traced from the lowest coelomate Inverte-
brates — in which it consists merely of a single cavity or
several connected cavities filled with a fluid containing vari-
ous types of cells — through forms in which more and more of
the spaces are replaced by definite tubes for the conduction
of the fluid. With the establishment of closed vessels, the
contractions of various organs and the movements of the
body as a whole can no longer be entirely depended on for
the movement of the fluid, and accordingly, in certain regions,
a muscular layer is developed in the walls of the tubes, which
by rhythmic pulsation forces the fluid along. Thus, for exam-
ple, in the Earthworm there is a fluid (coelomic fluid) within
the body cavity which is forced about by the movements of
the worm and bathes most of the internal organs; and also
a system of vessels, a part of which contracts rhythmically
and distributes the blood to the individual cells. (Figs. 66, 67.)
In the higher forms a closed vascular system gradually takes
the ascendency and becomes what one ordinarily has in mind
when speaking of 'the circulatory system/ but the primitive
type of open system still functions as an auxiliary of no mean
importance even in Man. The highly developed Vertebrate
circulatory system, therefore, really consists of two parts.
CIRCULATION AND RESPIRATION IN ANIMALS 163
First, a closed system of vessels containing BLOOD. Blood is
a lifeless liquid PLASMA in which float detached cells, the red
and the white blood CORPUSCLES. Second, a series of spaces,
channels, and vessels, closely associated with the blood vas-
cular system, which is filled with LYMPH. Lymph consists
of some of the liquid plasma of the blood, with some white
corpuscles, which has passed through the thin walls of the
smallest blood vessels and bathes the individual tissue cells.
The lymphatic system really acts as an intermediary between
the blood and the tissues. It supplies the milieu of the cells,
and finally returns the materials again to the blood vascular
system.
The essential elements of the blood vascular system are
first, a muscular organ for propulsion of the blood, the HEART,
which lies, as has been mentioned, near the mid-ventral line
in the anterior part of the coelom; and second, tubes which
convey the blood to the heart, the VEINS, and away from the
heart, the ARTERIES. The arteries divide and subdivide to
form smaller and smaller vessels (ARTERIOLES) which finally
merge into exceedingly delicate tubes (CAPILLARIES) that per-
meate the tissues of the body. The capillaries, in turn, de-
liver the blood to VEINLETS which pass it on through larger
and larger veins to the heart. Consequently the blood flows
in a circle from heart to heart again, through a closed system
of vessels. (Figs. 91, 92.)
The heart represents that part of the vascular system in
which the power of rhythmic contraction has concentrated,
and can be regarded as a blood vessel whose walls have
become highly modified by an excessive development of the
muscular layer. In the lowest Vertebrates and in em-
bryonic stages of higher forms the heart consists typi-
cally of two chief chambers, an AURICLE and a VENTRICLE,
fitted with muscular flaps, or VALVES, which allow the blood
CIRCULATION AND RESPIRATION IN ANIMALS 165
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166 FOUNDATIONS OP BIOLOGY
to flow in one direction only; that is, from auricle to ventricle.
An enlargement, the SINUS VENOSUS, connects the veins
(VENOUS SYSTEM) with the auricle, and there is frequently
another, called the CONUS ARTERIOSUS, in a similar position at
the arterial end. The heart is thus essentially a linear series
of chambers. The sinus venosus and auricle function mainly
as reservoirs to fill rapidly the especially muscular ventricle.
The latter, acting both as a suction and force pump, passes
the blood on to the conus arteriosus and from there to the
ARTERIAL SYSTEM as a whole. For our purposes, however, we
may consider the heart in the lowest Vertebrates (Fishes) as
composed of the two chambers, auricle and ventricle. (Fig. 91.)
The arterial system is the distributing system of vessels
which carries the blood to all regions of the body. Soon after
its origin at the heart the circuit in the aquatic forms is tempo-
rarily interrupted to allow the blood to pass through the GILLS
and exchange carbon dioxide for a supply of oxygen. To
facilitate this gaseous interchange, the arteries (AFFERENT
BRANCHIAL) as they enter the gill membrane break up into
smaller and smaller vessels which finally are of microscopic
calibre and consist of but a single layer of cells. These capil-
laries, in turn, merge into larger vessels (EFFERENT BRAN-
CHIAL ARTERIES) which finally lead into the chief artery of
the body, the DORSAL AORTA. This extends along the median
dorsal line of the body, just below the vertebral column, and
sends branches to the various organs.
The branches of the dorsal aorta, on reaching the location
which they supply with arterial blood, break up into capil-
laries similar to those in the gills, and pass to the tissues
the blood carrying food and oxygen. The blood receives in
return various waste products of metabolism, including car-
bon dioxide and, in certain cases, absorbed food materials
from the intestine, and special secretions chiefly from ductless
CIRCULATION AND RESPIRATION IN ANIMALS 167
glands. The fine capillaries lead into veinlets and these into
veins of constantly increasing calibre which sooner or later
complete the circuit by returning the blood to the heart.
The return current, however, is not quite so simple as
would appear from the above statement because, just as all
the outgoing stream is interrupted for the respiratory inter-
change in the gills, so a part of the return current is tempora-
rily side-tracked through the liver. The veins returning blood
from the digestive organs merge to form the PORTAL VEIN
which proceeds to the liver, where it resolves into capillaries
to allow that organ to regulate certain of the blood constit-
uents — in particular, to store up sugar after a meal and
later dole it out to the blood as needed. These capillaries
then pass the blood into the HEPATIC VEIN, which conveys
it toward the heart. Thus the liver receives blood from two
sources: an artery providing blood primarily for the use of
the organ itself and a vein (portal vein) delivering blood con-
taining a large amount of food material solely to receive
special treatment before being sent back to the heart and then
all over the body. This special arrangement for a venous
blood supply to the liver is known as the HEPATIC PORTAL
SYSTEM. Moreover, in Vertebrates lower than the Birds, the
venous blood from the posterior part of the body makes a
detour through the capillaries in the kidneys on its way back
to the heart. This constitutes what is termed the RENAL
PORTAL SYSTEM. Therefore in these forms the kidneys as
well as the liver receive blood from two sources, an artery
and a vein. It will be noted that both the hepatic portal vein
and the renal portal vein arise in capillaries and terminate in
capillaries. (Figs. 91-93.)
Such is the general plan of the blood vascular system of the
lower Vertebrates. The modifications of this which occur in
higher forms are related chiefly to changes in the respiratory
168
FOUNDATIONS OF BIOLOGY
mechanism necessitated by abandoning the aquatic for the
terrestrial mode of life, with the consequent dependence on
FIG. 93. — Diagram of paths of absorbed food from the digestive tract.
Proteins and carbohydrates by] veins (in solid black) ; Fats by lymphatics
(dotted). (From Conn and Budington.)
the free oxygen of the atmosphere instead of that dissolved
in the water.
B. RESPIRATION
As we have seen, the essential factor of respiration is an
interchange of gases between protoplasm and the environ-
CIRCULATION AND RESPIRATION IN ANIMALS 169
ment: an intake of free oxygen for combustion, and an outgo
of the waste products, chiefly carbon dioxide. In the unicel-
lular organisms, such as Sphaerella and Paramecium, and in
simple multicellular animals like Hydra, this appears to be a
relatively simple process since an elaborate mechanism is not
necessary to facilitate the interchange. But with the estab-
lishment of a highly differentiated multicellular body, fewer
and fewer cells are in direct contact with the aerating medium
and so various provisions are necessary to transfer the gases
to and from the outer world and the individual cells them-
selves. In all forms the skin functions to some extent; in
the Earthworm, in fact, it acts as the chief respiratory
membrane since a profuse supply of blood vessels to the
moist surface of the body effects a sufficiently rapid gaseous
interchange for the relatively inactive life of the organism.
The Crayfish meets the problem of respiration by finger-form
out-pocketings of the body wall, the gills: a method of bath-
ing a large area of the respiratory membrane in the respiratory
medium, the surrounding water. The Insects, on the other
hand, instead of bringing the blood to the surface, develop a
network of tubes which ramify throughout the body and
conduct the air directly to the various tissues. Among the
lower Vertebrates, as has been indicated, the anterior end of
the digestive tract functions as a common food and respira-
tory passage. In Fishes, the respiratory water current which
enters the mouth makes its exit by way of the gill pouches
and gill slits; the lining of the pouches — outpocketings of
the lining of the alimentary canal — functioning as the res-
piratory membrane. (Fig. 94.)
Among the air-breathing Vertebrates tnere are the added
problems of protecting and keeping moist the greatly in-
creased respiratory surface which their active metabolism
demands. Accordingly the gill pouches are replaced by a
170
FOUNDATIONS OF BIOLOGY
huge outpocketing from the alimentary canal into the
anterior portion of the coelom, which constitutes the
lungs. This entails, in turn, a complex respiratory mechanism
so that the air within the lungs may be changed at frequent
intervals. As a matter of fact
one ordinarily thinks of the move-
ments involved in the renewal of
the air in lungs as respiration, but
from what has been said it is
clear that neither the respiratory
movements involved in inhala-
tion and exhalation, nor the inter-
change of gases between blood and
air through the lung membrane is
respiration proper. The essential
feature of respiration takes place
throughout the body when the
blood trades its oxygen supply for
carbon dioxide with the tissue cells. Thus respiration in
the final analysis is the securing of energy from food.
C. CIRCULATION IN THE HIGHER VERTEBRATES
But to return to the blood vascular system, which neces-
sarily undergoes far-reaching modifications as a result of the
substitution of lungs for gills. In the first place the series of
paired branchial arteries, which formerly supplied the gills,
no longer break up into capillaries, but instead lead directly
into the dorsal aorta, and accordingly are termed AORTIC
ARCHES. Thus Fishes bequeath, as it were, to higher forms a
series of pairs of aortic arches which, though they are no
longer of use in their former capacity, appear in the develop-
mental stages. Some disappear at that time and others are
modified and diverted to various uses in the adult. (Fig. 95.)
FIG. 94 — Diagram of a verti-
cal section through the head
region of Fish (above) and Reptile
or Bird (below) to show the paths
of the respiratory currents (a) and
food (6). See Fig. 87.
CIRCULATION AND RESPIRATION IN ANIMALS 171
For our purpose it is sufficient to emphasize that in Man's
body one branchial arch continues to carry blood directly
from the heart to the dorsal aorta, while parts of another
deliver blood from the heart to the lungs and back again to
D E F
FIG. 95. — Diagram to show the transformation of the six pairs of primitive gill slit
arteries (aortic arches) in the ascending series of Vertebrates. A, primitive condition,
embryonic; B, Fish; C, Amphibian (Frog); D, Reptile; E, Bird; F, Mammal, a,
dorsal aorta; b, ventral artery from heart; c, internal carotids; d, external carotids;
e, e', right and left aortic arches; /, pulmonary arteries; g, g', subclavian arteries to
fore limbs.
the heart. Thus there is established a second current of blood
through the heart, which necessitates a median partition in
both the auricle and ventricle in order to keep the two cur-
rents separate. In this way a four-chambered heart arises
172 FOUNDATIONS OF BIOLOGY
which consists of right and left auricles and ventricles. The
RIGHT AURICLE receives blood from the venous system of
the body and passes it through the TRICUSPID VALVE into the
right ventricle to be pumped through the PULMONARY ARTERY
to the lungs. After traversing the capillaries of the lungs the
blood is returned by the PULMONARY VEIN to the LEFT AURI-
CLE, thence through the MITRAL VALVE into the LEFT VENTRI-
CLE, which forces it into the AORTA and so on its way about
the body as a whole. To all intents and purposes, the higher
Vertebrates have two hearts which act in unison — a right, or
pulmonary, heart receiving non-aerated blood from the entire
body and pumping it to the lungs, and a left, or systemic, heart
receiving aerated blood from the lungs and delivering it to
the body as a whole. (Fig. 92, C.)
In this way the blood vessels of the primitive aquatic res-
piratory apparatus are transformed by gradual additions and
subtractions into the pulmonary system of the higher Verte-
brates, including Man — the most convincing evidence that
nature, whenever possible, turns to structures at hand to
construct what is to be, and thereby weaves in the woof and
warp of higher forms a record of their lowly origin.
The blood vascular system of the higher Vertebrates, in
spite, shall we say, of its makeshift origin, is a highly efficient
apparatus. Day in and day out throughout life the human
heart, beating rhythmically at an average rate of 70 times
per minute, does about 175,000 foot-pounds of work. This
power is expended in moving the weight of the blood, in
imparting to it the velocity of its motion, and in raising the
pressure in the aorta and pulmonary artery.
The RATE of flow is greatest when the blood leaves the heart
and gradually diminishes until, in the capillaries of both the
pulmonary and systemic systems, it is reduced to a minimum.
On the return trip from the capillaries through the veins the
CIRCULATION AND RESPIRATION IN ANIMALS 173
rate of flow gradually increases though it reenters the heart
at a slower rate than it departed. Thus of the 23 seconds
which it takes a unit of blood to make the complete circuit
in Man, about two seconds are spent in the capillaries — a
relatively long time when it is realized that the average length
of the capillary path is about one fiftieth of an inch. The
principle underlying the change in rate is simple. The blood,
driven throughout its course by the same force — the heart
beat — varies in rate with the width of the bed through
which it is flowing. Although the area afforded individually
by the arteries and veins is greater than that by the single
capillaries, nevertheless the total area afforded by the capillary
system is enormously greater than that by either the arterial
or venous system.
Moreover, since a liquid in a closed system of tubes must
flow from a region of high to one of low pressure, the blood
PRESSURE continuously diminishes from heart back to heart
again. But, it should be noted, that although the pressure in
the capillaries of any region as a whole is greater than in the
veins which they supply, nevertheless the pressure in a single
capillary is very low, as is demanded by its delicate wall.
Thus it is apparent that in the capillaries the blood moves
very slowly under low pressure — • for it is here that the blood
does its work. All the rest of the vascular system — heart,
arteries, and veins — is arranged to give the blood just this
opportunity in the capillaries.
It is in the capillaries that the blood vascular system turns
over the work of distribution to the lymphatic system. As
has been said, lymph to. all intents and purposes consists of
plasma and white corpuscles from the blood which have
passed through the thin capillary walls, carrying along food
materials, oxygen, etc., to exchange for the various waste
products of metabolism of the cells which it bathes. Thus
174 FOUNDATIONS OF BIOLOGY
there is a continuous drainage of lymph from the capillaries
into intercellular lymph spaces. Some of the fluid, with waste
products, etc., passes immediately into the capillaries again,
but the excess passes from lymph spaces into small lymph
vessels, and from these into large lymph vessels. The latter,
in turn, empty into the venous system and so restore the
materials to the blood. (Fig. 93.)
So much for the path and the duties of the liquid tissue
which circulates through the body. But clearly some provi-
sion must exist for regulating the blood flow in order to meet
the varying local demands of the*~oTgsrHs of the body under
different physiological conditions. This is attended to chiefly
by nerve impulses which are conducted by a system of VASO-
MOTOR nerves and bring about the dilation or contraction of
the smaller blood vessels (arterioles) leading to an organ, and
thus increase or decrease the volume of the blood which it
receives. The elaborate mechanism in homothermal animals,
which maintains a practically constant body temperature,
is largely dependent upon heat distribution, loss, and con-
servation by the blood vascular system. Since the total
volume of blood in the body is practically constant, an extra
supply to one part obviously necessitates a reduced supply
to another. So it happens, for instance, that after a hearty
meal more blood is concentrated where digestion is actively
going on, leaving less for the other organs — the reduced
supply to the brain resulting in the proverbial drowsiness
at such times.
CHAPTER XIII
EXCRETION IN ANIMALS
The ihathematically accurate end-reaction of a chain of
known and unknown causes and effects. — Noyes.
PROVISIONS for eliminating from the organism the waste
products of metabolism are only second in importance to
those for supplying the matter and energy by which the vital
processes are carried on. Accordingly we find the kidneys
devoted solely to excretion; the gills or the lungs, largely to
excretion ; and the skin and liver acting in subsidiary capaci-
ties. In nearly every case the essential parts of the excretory
organ are gland cells which select from the blood supply at
their disposal one or another waste product. This material
they secrete in more or less changed form so that it eventually
leaves the body as an excretion. There is therefore an essen-
tial distinction between an EXCRETION, which represents
chemical waste from the vital processes, and the major part
of the material which is eliminated from the digestive tract
as FAECES. The latter is almost entirely indigestible material
taken in with the food which has not directly contributed to
the metabolic processes of the organism. Accordingly the
digestive tract is not included in the list of excretory organs,
though as a matter of fact certain waste products excreted by
the liver reach the outside world with the faeces.
We have already emphasized the elimination of carbon
dioxide by the GILLS or the LUNGS. Here the cells of the RES-
PIRATORY MEMBRANES play essentially a passive role in excre-
tion, since the carbon dioxide, which is under higher tension
175
176 FOUNDATIONS OF BIOLOGY
in the blood than in the water or air, follows the physical laws
of diffusion of gases and passes from the blood. In addition to
carbon dioxide, the blood of warm-blooded animals (Birds
and Mammals) loses a large amount of water and heat; the
amount depending on the temperature and moisture of the air
which enters the lungs. When the air is exhaled its tempera-
ture is essentially that of the body and it is saturated with
water vapor.
The SKIN in some of the lower Vertebrates, for instance the
Frog, is an exceedingly important excretory organ, because
more carbon dioxide is eliminated through the skin than
through the lungs ; but in higher forms, including Man, excre-
tion by the skin is confined to the SWEAT GLANDS. These take
from the blood, in addition to large quantities of water, traces
of nitrogenous waste or urea, fatty acids, and salts, which form
a residue on the surface of the skin when the PERSPIRATION
evaporates. (Fig. 76.)
The LIVER, in addition to its various other functions, aids
in no small way in excretion. On the one hand, the liver
removes deleterious compounds of ammonia from the blood
and transforms them into urea. Then it secretes the urea
into the blood from which it is later removed by the kidneys.
On the other hand, the liver collects other waste products etc.,
from the blood, which form the bile. This passes to the GALL
BLADDER for temporary storage or directly to the intestine.
The KIDNEYS are, in a way, the chief excretory organs o£
Vertebrates, and any serious interference with their activity
rapidly leads to a poisoning of the body with its own waste
products. Certain cells of the kidneys remove the urea from
the blood stream which reaches them, while water and various
solutes are drained from the blood. Aside from their func-
tional importance, the kidneys are of considerable interest
to the comparative anatomist because of their complicated
EXCRETION IN ANIMALS
177
evolutionary history — indeed the structure of the human
kidneys is intelligible only in the light of the relatively simple
excretory organs of Invertebrates, such as the Earthworm,
and of lower Vertebrates. (Figs. 66, 67, 96.)
The chief excretory organs of the Earthworm consist of
pairs of coiled tubes, or NEPHRIDIA, segmentally arranged in
the coelom on either side of the alimentary canal. Each
nephridium begins as an open funnel in the coelom of one seg-
ment, passes through the
partition to the next posterior
segment and there, after coil-
ing, passes to the ventral sur-
face and opens to the exterior
by a pore. Thus, reduced to
its simplest terms, a nephri-
dium is a tube communicat-
ing between the coelom and
the outer world, and afford-
ing a path of egress for the
waste matter in the coelomic
fluid. But the closed blood
vascular system of the worm
collects various waste products in addition to the carbon
dioxide which it delivers to the skin. Nitrogenous waste,
inorganic salts, etc., are carried to the coiled part of the
nephridial tube where gland cells take them from the blood
and deliver them to the interior of the tube to be passed out
of the body. Now strange as it may seem, although the
primitive segmentation of the coelom has disappeared in the
Vertebrates, nevertheless there are good grounds for believ-
ing that the archaic, segmentally arranged nephridia have
been taken over, as it were, and made the basis of the essen-
tial excretory elements of the kidneys.
FIG. 96. — Diagram to show the gen-
eral structural plan of a nephridium of
an Earthworm, anterior end toward the
right, a, internal opening of nephridium;
b, external opening; c, capillary network
about the coiled, glandular portion.
178 FOUNDATIONS OF BIOLOGY
In the lowest Vertebrates the primitive type of kidney, or
PRONEPHROS as it is called, consists of a series of segmentally
arranged nephridia in the dorsal part of the anterior end of
the coelom. These, however, instead of opening independ-
ently to the exterior, discharge their products into a common
tube (PRONEPHRIC DUCT) which passes them to the outside.
In higher forms the pronephros disappears, and its function
is taken over by another series of nephridia which appear in
the coelom posterior to the pronephros. This series consti-
tutes the MESONEPHROS, and opens into the pronephric duct
which accordingly now is called the MESONEPHRIC DUCT. Fi-
nally, in still higher Vertebrates this second urinary organ is
replaced by a third, the kidney proper (METANEPHROS) and
its special duct, the URETER. Thus as we ascend the Verte-
brate series three distinct kidney systems appear, in each
case by the development and grouping of a number of ne-
phridia into a definitive organ. In this process the primitive
communication of the individual nephridia with the body
cavity is lost and the activity of the glandular portion in-
creased, until, in the higher forms, all the waste products are
taken solely and directly from the blood. (Fig. 97.)
It is therefore apparent that each of the relatively large,
compact kidneys of the higher Vertebrates, including Man,
is to all intents and purposes a large group of nephridia-like
elements, the tubules, bound together by connective tissue
and covered with a protective coat. The tubules within the
kidney deliver the materials taken from the blood to the
pelvis of the kidney, from which it passes down the ureter
and on to the URINARY BLADDER and finally to the exterior.
(Fig. 98.)
Such, in broad outline, is the historical viewpoint from
which the kidneys of Man must be interpreted. As a matter
of fact, however, the evolutionary transformation is still
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180
FOUNDATIONS OF BIOLOGY
further complicated by anatomical, though not physiological,
relations with the reproductive system. As will be pointed
out later, this neighboring system now and again foists, as it
were, some of its accessory responsibilities upon parts of the
FIG. 98. — Longitudinal section of the human kidney. Ct, cortex, or region in
which the essential nephridial elements, the tubules, come into functional asso-
ciation with the capillaries; M, medullary portion, through which the tubules
extend to open on the summits of the pyramids (Py); P, pelvis of kidney; Ra,
renal artery; U, ureter. (After Huxley.)
excretory (urinary) system, and even takes over portions and
makes them integral parts of its own when they have been
permanently abandoned by the urinary system in its evolu-
tionary development.
CHAPTER XIV
COORDINATION IN ANIMALS
It seems that Nature, after elaborating mechanisms to meet
particular vicissitudes, has lumped all other vicissitudes into
one and made a means of meeting them all. — Mathews.
SINCE a primary attribute of protoplasm is irritability — •
the power of responding to environmental changes by changes
in the equilibrium of its own matter and energy — it is not
strange that the cells of an organism mutually modify each
other's activities and reciprocal interrelationships have been
established during their long evolutionary history. The
various cells, tissues, organs, and organ systems are unified
into an organism by what may be called the chemical inter-
play between its various parts, which is made possible by the
facilities for distribution afforded by the circulatory system;
and also by the directing influence of the nervous system
which supplies a central station with lines for instantaneous
intercommunication with every part of the body.
A. CHEMICAL COORDINATION
It is only with the recent increase in knowledge of the
general problem of metabolism that the far-reaching impor-
tance of the chemical control of bodily processes has grad-
ually been brought to the fore. Although we may properly
think of the various chemical regulators, or HORMONES, as
forming a coordinating system in so far as their collective
action has such a result, in the present stage of our knowledge
it is possible to cite only the specific action of individual hor-
181
182 FOUNDATIONS OF BIOLOGY
mones as examples of the general method of chemical regu-
lation which their study, ENDOCRINOLOGY, is revealing.
Certain hormones are secreted by organs whose sole func-
tion is their production, such as the various endocrine glands
which pour their secretion directly into the blood stream.
Others are elaborated by special cells imbedded in organs,
such as the pancreas and reproductive organs, of which they
physiologically form no part. As a concrete example of an
endocrine gland we may select the THYROID which, as has
been seen, arises as an outpocketing of the digestive tract in
the neck region and finally loses all connection with its point
of origin and becomes a ductless gland. (Fig. 88.)
The general effect of the thyroid hormone on metabolism
is a regulation of the rate of oxidation in the body. An excess
of the substance results in such vigorous fuel consumption
that no surplus remains in the body to be stored as fat ; while
a deficiency in the glandular secretion results in a tendency
toward fat formation. Accordingly the administration of
thyroid extract is often an efficient means of reducing flesh
by increasing the oxidative processes of the body. A defi-
ciency of the hormone during adult life frequently results in
a type of mental deterioration called MYXEDEMA. Children
in whom the development of the thyroid is suppressed be-
come dwarfish idiots known as CRETINS, while overdevelop-
ment of the gland induces increased nervous activity and
mental disorders. Feeding with thyroid material sometimes
prevents the development of cretinism and cures myxedema,
while a surgical removal of part of the gland may cure the
nervous instability and other symptoms due to an excessive
amount of the hormone. Goitre is a pathological enlarge-
ment of the thyroid.
Finally, as a further indication of the nicety of the recip-
rocal adjustments within the organism, it may be mentioned
COORDINATION IN ANIMALS 183
that the thyroid gland itself is subject to regulating stimuli
reaching it through the nervous system, as well as by a hor-
mone derived from the PITUITARY BODY which is another
endocrine gland situated in conjunction with the lower part
of the brain. Glimpses of such interrelationships are being
gradually afforded as one hormone after another is discovered.
But chemical coordination, indispensable as it is as a means
of regulating many of the processes of the organism, espe-
cially the slower ones such as growth, is entirely inadequate
for the instantaneous correlation of diverse parts of an animal
and also for the adjustment of the whole animal to its sur-
roundings. The nervous system supplies this need by a com-
plicated arrangement of cellular elements in which irritability
and conduction are highly developed. (See p. 206.)
B. COORDINATION BY THE NERVOUS SYSTEM
In some unicellular organisms certain portions of the
protoplasm are especially differentiated for receiving and
conducting stimuli, and others for making effective such
stimuli by contractions of the whole or parts of the cell.
It is in the lower Metazoa, such as Hydra and its allies,
however, that we find the establishment of definite NERVE
CELLS some of which are specialized for receiving stimuli and
others for conducting the excitation to cells specialized
for contracting (muscle cells), etc. Thus a simple RECEPTOR-
EFFECTOR system arises which may be regarded as the basis
for the development of the elaborate NEURO-MUSCULAR
MECHANISM of higher forms. Although from the functional
point of view it is impossible to differentiate between the re-
ceiving and conducting elements and those which make them
effective (muscular system) in the economy of the organism,
from the standpoint of anatomy the former constitutes a
definite entity, the NERVOUS SYSTEM proper. (Figs. 99, 100.)
184
FOUNDATIONS OF BIOLOGY
The structural elements of the nervous system of all
animals consist of cells known as nerve cells, or NEURONS.
FIG. 99. — Diagram of a
simple type of receptor-ef-
fector system, found in some
Hydra-like animals. It com-
prises receptors (6), or sense
cells, reaching to the body
surface (a), with basal nerve
net (c) connecting with mus-
cle cells (d). (Slightly modi-
fied, after G. H. Parker.)
FIG. 100. — Diagram of a
more complex type of recep-
tor-effector system, found in
some Hydra-like animals. It
comprises, in addition to the
receptor (6) with nerve net
(c) and the muscle cells (e),
another nerve (ganglion) cell
(d) interpolated in the nerve
net. a, body surface. (After
G. H. Parker.)
In the lower forms these cells are permanently united so that
they form NERVE NETS which surround and permeate the
tissues which they stimulate to action. In more highly
FIG. 101. — Diagram of a primary sensory (s/) and motor (mf) neuron of the ventral
nerve cord of an Earthworm, showing their connections with the skin (ep) and the
muscles (Im) to form a simple reflex arc. cm, circular muscles; ep, epidermis; Im,
longitudinal muscles; me, motor neuron cell body (in a ventral ganglion), with fiber
(mf); se, sensory neuron cell body with fiber (sf) entering ganglion to form synapses
with processes of motor neuron. See Fig. 68. (After G. H. Parker, and Rrtzius.)
COORDINATION IN ANIMALS
185
\
developed animals the net arrangement is relegated to the
control of relatively subsidiary functions (Fig. 103), while
the main nervous system consists of neurons arranged
in groups, or GANGLIA, and prolongations of the neurons,
or nerve FIBERS, bound together
into cables, or NERVES. The
neurons, which are imbedded in
protective sheaths of connective
tissue in the ganglia, are in
physiological continuity one with
another by 'transmitting con-
tacts/ or SYNAPSES, but each
neuron, it is believed, preserves
its structural integrity. (Figs.
101, 102.)
It will be recalled that the
first great structural differentia-
tion during the development of
a multicellular animal establishes
FIG. 102. — Diagram of stages in
an OUter ectoderm and inner the differentiation of nerve cells
endoderm, and thus segregates <">' Jj; £tXSE
the functions Of protection and animals; B, motor neuron of the
i . . ., . Earthworm; C, a primary motor
general reactions to the environ- neuron of a vertebrate, in B and
ment from that of nutrition. It c thenerve imPulse passes from be-
low upward. (After G. H. Parker.)
is natural therefore that the
ectoderm should become the seat of those specializations
which have evolved into the nervous system and sense
organs. Such is the case in all forms from the lowest to
the highest and thus the development and comparative an-
atomy of the nervous system of Vertebrates, in particular,
affords the most cogent evidence of the genetic continuity
of the whole series, including Man.
In the development of a Vertebrate the first evidence of
A
II
186
FOUNDATIONS OF BIOLOGY
the nervous system is a longitudinal groove in the ectoderm
along the dorsal surface, which soon becomes converted into
a tube by the apposition and,
finally, the fusion of its edges.
This NEURAL TUBE then becomes
separated from and sinks below
the surface ectoderm, and in time
forms the CENTRAL nervous sys-
tem consisting of the brain and
spinal cord. As development pro-
ceeds, outgrowths from the central
nervous system establish the
PERIPHERAL aild the AUTONOMIC
(SYMPATHETIC) nervous systems,
so that structurally as well as
physiologically the whole nervous
system represents a unit; a single
organ, as it were, which seconda-
rily becomes closely identified
here and there with sense organ,
muscle, or gland, as the case
may be.
The first marked structural
9-
FIG. 103. — Diagram of a section
(highly magnified) of the wall of
the intestine of a Vertebrate to
show its intrinsic nervous organiza-
tion which brings about the move-
ments of the tube. The two plexuses
consist essentially of simple neurons modifications in the developing
central nervous system of Verte-
brates are two constrictions of the
enlarged anterior end of the neural
tube, which delineate the three
primary brain vesicles: FORE-
BRAIN, MID-BRAIN, and HIND-BRAIN. Thus very early in
embryonic development, one end of the neural tube is
molded into the brain, leaving the rest to form the spinal
cord. (Fig. 104.)
arranged as nerve nets. a, food
absorbing surface of the intestine;
b, mucous layer; c, plexus of neu-
rons (submucouS) ; d, circular mus-
cle; e, plexus of neurons (my en-
teric); /, longitudinal muscle; g,
serous layer. (From Parker, after
Lewis.)
COORDINATION IN ANIMALS
187
The three- vesicle brain now becomes transformed into one
of five vesicles by a hollow outpocketing from the anterior end
FIG. 104. — Diagrams to illustrate the general method of transformation of the
anterior end of the neural tube into the brain. A, B, C, median vertical sec-
tions; D, dorsal view of C. a, fore-brain; b, mid-brain; c, hind-brain; d, prosen-
cephalon; e, diencephalon; /, cerebellum; g, medulla oblongata; h, cerebral hemi-
spheres; i, olfactory lobes; j, pineal body; k, inf undibulum.
of the fore-brain and a dorsal outpocketing of the hind-brain.
In some of the lower Vertebrates the brain throughout life
188 FOUNDATIONS OF BIOLOGY
consists of these divisions, known as PROSENCEPHALON,
DIENCEPHALON, mid-brain or MESENCEPHALON, EPENCEPHA-
LON Or CEREBELLUM, and METENCEPHALON Or MEDULLA
OBLONGATA, the latter merging into the spinal cord. Usually,
however, the prosencephalon gives rise to a pair of
PARENCEPHALA, Or CEREBRAL HEMISPHERES, which are deS-
tined gradually to overshadow in development all the other
parts of the brain and to become the seat of consciousness
as well as of the higher mental life in general.
Finally, the development from the prosencephalon, or
from the cerebral hemispheres when present, of a pair of
RHiNENCEPHALA,or OLFACTORY LOBES, completes the establish-
ment of the chief brain chambers. The further changes
which transform the more or less linear series of vesicles
into the increasingly complex and compact brain of higher
forms are due to bendings, or FLEXURES, and to unequal
thickenings and outgrowths of the chamber walls. For in-
stance, the upper and lower surfaces of the diencephalon give
rise to the PINEAL BODY and the INFUNDIBULUM respectively,
while from similar regions of the mesencephalon are de-
veloped the OPTIC LOBES arid CRURA CEREBRI. Hand in hand
with these changes the primary cavities (VENTRICLES) of the
chambers undergo a gradual constriction, but throughout all
there persists at least a remnant of the original tubular cavity
which is continuous with that of the spinal cord. (Fig. 105.)
The brain and spinal cord lie, as we know, imbedded in
the muscles forming the dorsal part of the body wall, and
are protected and isolated by a cartilaginous or bony tube
formed by the skull and neural arches of the vertebrae.
The sole paths of nervous communication between the
central system and the rest of the organism and its sur-
roundings are a series of pairs of CRANIAL and SPINAL NERVES.
These arise at fairly regular intervals from one end of
COORDINATION IN ANIMALS
189
8
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II
CLi <£
II
^ i
if
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cu ,=5
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1*
Q Q
190
FOUNDATIONS OF BIOLOGY
the brain and cord to the other, and pass out through
openings in the skull and between or through the vertebrae
to constitute the peripheral
nervous system. (Fig. 106.)
It is usually considered
that the primitive segmen-
tal condition of the Verte-
brate body is well exhibited
in the arrangement of the
cranial and spinal nerves,
and that the origin of the
cranial nerves from the
brain affords a partial in-
dex to the primary series
of metameres which ap-
parently have been merged
to form the Vertebrate
head. Conditions as they
exist at the present time
can perhaps be most readi-
ly understood by imagining
a simple, ancestral, seg-
mented worm-like form in
which the dorsal neural
tube gives off a pair of
nerves to each segment of
the body. As the result of
a gradual shifting forward
and a consequent coales-
cence and fusion of certain
segments near the anterior
end, there is brought about
the delineation of a head
FIG. 106. — Ventral view of the nervous
system of the Frog. Br, second and third
spinal nerves (brachial plexus); Js, sciatic
nerve leading from the sciatic plexus; O, eye;
Ol, olfactory nerve; Op, optic nerve; Sg 1-10,
ten ganglia of autonomic system; Spn 1,
first spinal nerve; Sp 4, fourth spinal nerve;
Vg, Gasserian ganglion; Xg, ganglion of 10th
cranial nerve (vagus). (After Ecker.)
COORDINATION IN ANIMALS 191
region, with its brain, battery of sense organs, and
skull, from a trunk region with its spinal cord, vertebral
column, paired limbs, etc. This naturally involves a cor-
responding shifting and modification of the primitive con-
dition of the paired nerves; especially since the innervation
of a group of cells in normal development is apparently
rarely changed — a nerve following the part which it origi-
nally supplied through many of the transformations and
even migrations of the latter.
If this point of view is accepted, the cranial and spinal
nerves are, historically considered, similar structures. But
the former, synchronously with the changes in the head
region, have departed somewhat widely from their ancestral
condition and have even been augmented by nerves of diverse
origin. The spinal nerves, on the other hand, continue to
issue from the cord at about equal intervals and in metameric
arrangement as indicated by muscle segments and skeletal
structures, although those of certain regions unite in the body
cavity to form PLEXUSES for the adequate innervation of the
appendages.
From the standpoint of function the nerves are of two
classes, SENSORY and MOTOR. The former are distributed
mainly to the skin and sense organs of the head, and are the
paths over which excitations (NERVOUS IMPULSES) due to
external stimuli are conducted to the cord and brain. The
motor nerves, on the other hand, are the media for distribut-
ing impulses from the central organ to muscle cells, gland
cells, etc., and thus induce the response of the organism.
In discussing nerves, it must be kept in mind that a nerve
is actually a bundle of nerve fibers; the fibers themselves in
turn being prolongations of nerve cells, the cell bodies of
which are usually situated in groups or GANGLIA. Moreover,
nerve impulses are not transmitted by nerves as a whole, but
192 FOUNDATIONS OF BIOLOGY
by one component cell process, the nerve fiber; that is, by
way of a definite cell path through the nerve. The same is
equally true of the cord and the brain, which differ from
nerves largely in the circumstance that they comprise more
cell processes and also the cell bodies themselves. In other
words, the brain and cord comprise the elements of both
ganglia and nerves.
A given nerve may conduct impulses both to and from the
central organ if it contains afferent and efferent cell paths, or
fibers. As a matter of fact all the peripheral nerves primarily
are mixed nerves, because typically they arise by two roots
from the central organ; the DORSAL ROOT containing only
sensory (afferent) fibers and the VENTRAL ROOT only motor
(efferent) fibers. This condition is preserved by the spinal
nerves of higher forms since each arises by two roots. But
some of the cranial nerves, in response to the profound modi-
fications which have been wrought in the head region, have
only one root, and so are either solely sensory, as those to the
sense organs, or only motor, as those innervating the muscles
which move the eye. (Fig. 107.)
So far we have considered the central system — the brain
and spinal cord — and its lines of communication with the
body as a whole, the peripheral system, or cranial and spinal
nerves. In point of fact, however, the peripheral system
gives rise to an auxiliary series of ganglia and nerves which
are charged with the innervation of certain of the internal
organs, particularly the alimentary canal and arteries, which
are not directly under voluntary control. This AUTONOMIC
SYSTEM in the higher Vertebrates consists essentially of a
double nerve chain situated chiefly within the coelom just
ventral to the spinal column. It communicates with the
central system by way of the sensory roots of the spinal and
some of the cranial nerves. (Fig. 106.)
COORDINATION IN ANIMALS
193
Such in essence are the ramifications throughout the body
of the nervous system which, although it arises as an infolding
of the ectoderm and therefore is primarily external, eomes to
be internal and so chiefly dependent upon more or less iso-
lated groups of sensory cells for the reception of stimuli.
Some of these, termed EXTERNAL RECEPTORS, remain at the
surface to receive stimuli from the outer world, while others,
d.e
C.COT*
FIG. 107. — Diagram of a section of the spinal cord to show the paths of nerve im-
pulses, c.c, central canal; col, collateral fibers; c. corl, cells of the cortex of the cerebral
hemispheres of the brain; c.g, smaller cerebral cells; d.c, cells in dorsal part of gray
matter; d.r, dorsal root of spinal nerve; g, ganglion of dorsal root; g.c, cell body of
sensory neuron; g.m, gray matter of cord; M, muscle; m.c, nerve cell in medulla; m.f, fi-
ber (axon) of motor neuron ; s, sensory surface ; s.f, fiber of sensory neuron ; spc, spinal cord ;
v.c, cells in ventral part of gray matter; v.r, ventral root of spinal nerve; w.m, white
matter of cord. The arrows indicate the direction of the impulses. (After Parker and
Parker.)
known as INTERNAL RECEPTORS, are situated within the
body for the reception of stimuli arising there. The ex-
ternal receptors are what one ordinarily thinks of as sense
organs.
C. SENSE ORGANS
Although among some of the Protozoa certain regions of
the cell are specialized so that they are more sensitive to
one or another kind of stimulation, the great majority show
no trace of sense organs. Nevertheless all forms, in common
with all protoplasm, possess the power of receiving and re-
194
FOUNDATIONS OF BIOLOGY
spending to environmental changes. Thus Paramecium re-
acts to mechanical, thermal, chemical, and electrical stimula-
tion: the en-tire surface of the cell is sensitive to stimuli, and
the excitations are conducted from
one part to another essentially by
the protoplasm as a whole. In some
Invertebrates, such as Hydra and
the Earthworm, the whole surface
of the body is still depended upon
as a receiving organ for all kinds of
stimuli, and only simple sense
cells are developed. In the major-
ity of animals, however, although all
the cells' retain to some extent their
pristine power of irritability, envi-
ronmental changes exert their influ-
ence chiefly upon complex receptors,
which are specialized to respond
most readily to particular forms of
energy. The energy, for example of
heat or light, is transformed by ap-
propriate mechanisms into the ener-
gy of a NERVE IMPULSE, and accord-
ingly the sense organs constitute
the outposts of the nervous system.
Since we necessarily gain our knowledge of the outside world
solely through the data afforded by our sense organs, it fol-
lows that we judge the capacity of the sense organs of other
animals merely by analogy with our own. This is a safe pro-
cedure only in the case of sense organs which more or less
correspond in structure to those which we possess. In the
Crayfish, for example, we find complex sense organs which,
without doubt, are eyes, and others which are ears, or at least
ABC
FIG. 108. — Diagram of stages
in the differentiation of sense
cells. A, primitive sensory neu-
ron of Hydra-like animals ; B, sen-
sory neuron of a Mollusc; C,
primary sensory neuron of a
Vertebrate. In each case the
sensory surface is represented
below, and therefore the nerve
impulse passes upward. (After
G. H. Parker.)
COORDINATION IN ANIMALS 195
perform one of the functions of our ears, equilibration; while
some of the head appendages are particularly adapted to
receive sensations of touch. The senses of smell and taste are
also probably present, but here we are on less certain ground.
It is possible, perhaps probable, that environmental changes
which are without effect on the sense organs of the human
body, and so play no recognizable part in the 'world' of
Man, may stimulate receptors in lower organisms.
The simplest form of sense organ in Vertebrates is a single
epithelial cell for the reception of stimuli, connected with a
nerve fiber for the conduction of the nerve impulse to a sen-
sory center. Usually, however, many associated cells are
arranged to respond and are aided by accessory structures for
intensifying the stimulus, protection, etc , so that the whole
forms a highly complex sense organ. (Figs. 108C, 112.)
1. Cutaneous Senses
Confining our attention to the Vertebrates we find that
practically the entire surface of the body constitutes a sense
organ, because the skin is permeated with a network of sen-
sory nerves. Certain regions are supplied with special tactile
organs, which may take the form of a regular system of sense
organs, such as the LATERAL LINE ORGANS of Fishes and Am-
phibians, or of groups of TACTILE CORPUSCLES as in Man. In
addition to pressure receptors, the whole surface of the human
body is provided with PAIN, HEAT, and COLD SENSE SPOTS.
2. Sense of Taste
In the higher Vertebrates the sense of taste is restricted
to the cavity of the mouth, particularly to the tongue, where
special receptors known as TASTE BUDS are in communication
with' the brain by two of the cranial nerves; but in some Fishes
they are scattered quite generally, so that the whole body
surface is sensitive to such qualities as sweet, sour, and salt.
196 FOUNDATIONS OF BIOLOGY
3. Sense of Smell
The special sense organs of smell, or OLFACTORY BUDS,
reside in the membrane which lines a pair of invaginations of
the anterior end of the head, termed OLFACTORY POUCHES.
The buds are in communication with the brain by the olfac-
tory, or first pair of cranial nerves. The pouches constitute
relatively simple sacs in the lower Vertebrates, but in the air-
breathing forms, and especially in the Mammals, the walls of
the pouches are thrown into folds, ridges, and secondary
pouches. This is necessitated by the concentration of the
olfactory surface to the air passages of the nose which lead
to the lungs. On the other hand, in Man the olfactory appa-
ratus has fallen somewhat from the complexity which it attains
in the lower Mammals, as is attested not only by its structure
in the adult but also by transient remnants in the human em-
bryo.
4. The Ear
The ears, or organs of hearing and equilibration, arise as
paired depressions of the ectoderm of the head, which, in
all Vertebrates above the lower Fishes, lose their connection
with the exterior and form the so-called INNER EAR, or LABY-
RINTH. This becomes divided into two chief parts, the SAC-
CULUS and the UTRICULUS from which are developed three
SEMICIRCULAR CANALS, one in each plane of space. The sac-
culus is largely devoted to the reception of vibrations of the
surrounding medium, that is to hearing in the usual sense of
the word. Accordingly the sacculus becomes progressively
differentiated as we ascend the Vertebrate scale — a complex
derivative in the mammalian ear being the COCHLEA. On the
other hand, the utriculus and the semicircular canals provide
for sensations of loss of equilibrium, or orientation of the
body in space, and show far less change. It is probable
COORDINATION IN ANIMALS
197
that equilibration is the chief function of the entire labyrinth
in Fishes, as it is of the so-called auditory organs of many
Invertebrates, such as the Crayfish. With the progres-
sive specialization of the labyrinth, the essential sensory
cells, which are in communication with the brain by the
eighth, or AUDITORY NERVE, become
limited to a few definite areas.
These sensory cells are provided
with auditory hairs which project
into the cavity of the labyrinth and
so are stimulated by movements of
the fluid which fills it. (Fig. 109.)
The ears of Fishes lie immediately
below the skull roof, where they are
readily accessible to vibrations
transmitted by the water. But
with the substitution of air for water
as the surrounding medium, there
arises the necessity of a more deli-
cate method for conducting and
also for collecting and augmenting
the sound waves. The result is
that, in ascending the Vertebrate
series, we find the ear proper receding farther and farther
below the surface.
Soon, between the inner ear and the surface of the head, a
simple resonating chamber is added which is provided with a
vibrating TYMPANIC membrane, or EAR DRUM, situated just
under the skin. Then this is improved by the development
of a bony transmitting mechanism between the tympanic
membrane and the inner ear. This consists of a single bone
until we reach the Mammals, when two more bones are added
by being diverted from their earlier function of articulating
Fio. 109. — Semidiagram-
matic figure of the left mem-
branous labyrinth of a lower
Vertebrate to show the sacculus
(s, I), utriculus (u, rec), and the
three semicircular canals (aa, ca;
ae, ce, and ap, cp). I, lagena, a
derivative of the sacculus which
becomes the cochlea in higher
Vertebrates; cus, utriculo-saccu-
lar canal; de, se, endolymphatic
duct and sac ; ass, sp, ss, utricular
sinuses. (After Wiedersheim.)
198
FOUNDATIONS OF BIOLOGY
the jaws with the skull! Finally, the resonating (tympanic)
chamber recedes farther below the surface and becomes the
FIG. 110. — Front view of the human organ of hearing, right side, a, pinna of outer
ear; b, bone of skull; c, d, I, transmitting mechanism of three bones — 'malleus, incus,
and stapes; e, one of the three semicircular canals; g, vestibule; h, auditory nerve; i,
cochlea; j, Eustachian tube leading to the throat; k, tympanic chamber or middle ear;
/, stapes; m, tympanic membrane; n,» external auditory passage, or outer ear; o,
cartilage.
MIDDLE EAR to which sound waves are conducted through a
tubular passage, the OUTER EAR. In some forms, as in Man,
there is an external funnel-like collecting appendage, the
PINNA. (Fig. 110.)
5. The Eye
The organs of sight are the most complex sense organs of
animals and reach a very high degree of specialization even
in some of the Invertebrate forms. Among the latter the
essential sensory element (RETINA) of the eye usually arises
by the invagination of a limited area of ectoderm, the cells
of which become differentiated for receiving the photic
stimuli that produce impulses to be transmitted to the central
nervous system. Among Vertebrates the sensory cells are also
of ectodermic origin, but only secondarily so, since the OPTIC
COORDINATION IN ANIMALS 199
VESICLES arise as lateral outpocketings directly from the fore-
brain. (Fig. 111.)
A retina alone such as exists in some of the lower Inverte-
brates can afford no visual sensations other than light and
darkness, and perhaps in some cases the ability to distinguish
light of one color from that of another. In order that not
merely degrees of the intensity of light may be perceived, but
that objects may be seen, many of the higher Invertebrates
have developed various kinds of complicated apparatus for
bringing the rays from a given point to a focus at one point
on the retina, culminating on the one hand in the mosaic
vision of the Arthropods, and on the other hand in the camera
eye of the Cuttlefish. In the latter case the mechanism is
quite similar to that found in the Vertebrates, but since it
occurs in the group of Molluscs which cannot be considered
in the direct evolutionary line of the Vertebrates, it affords
an example of similar responses of different organisms to
similar needs giving rise to analogous structures. (Fig. 112.)
In the development of the Vertebrate eye, the hollow out-
growth or optic vesicle (one of which arises from either side
of the diencephalon) gradually extends toward the outer sur-
face of the head, where it becomes associated with an in-
pocketing of the ectoderm. The latter gradually becomes
separated from the surface ectoderm as a sac, the very thick
walls of which almost completely obliterate its cavity. This
sac is destined to become the LENS, and as it enlarges it comes
in contact with the optic vesicle, which now is connected
with the point of origin from the diencephalon by a narrow
isthmus (OPTIC STALK). Apparently under the influence of
the developing lens, the optic vesicle is invaginated and there-
by transformed from a single-layered structure into a double-
layered cup (OPTIC CUP). These two layers form the retina,
the inner layer becoming differentiated into the essential
200
FOUNDATIONS OF BIOLOGY
a — £
h —
— c
I)
FIG. 111. — Diagrams illustrating the method of formation of the eye of an
Invertebrate (A) and a Vertebrate (B, C, D, E, — successive stages). Note that
the opposite surface of the retinal cells is exposed to the light rays in the Verte-
brates as compared with the Invertebrate Eye. a, ectoderm; b, retinal area; c,
future position of optic nerve; d, cavity of the diencephalon; e, optic vesicle; /,
stalk of optic vesicle later replaced by the optic nerve; g, vitreous chamber with-
in optic cup; h, developing lens.
COORDINATION IN ANIMALS
201
visual elements (RODS and CONES) of the eye, while the outer
supplies the PIGMENTED LAYER. The nerve cells of the retina
develop fibers which proceed to the brain through the path
FIG. 112. — The Vertebrate eye (human). A, vertical section of the eye in situ.
B, horizontal section to show relation of optic nerve to fovea centralis through which
the optical axis passes, a, eyelash; b, lid; c, bony orbit; d, superior rectus, one of the
six muscles which revolve the eyeball; e, muscle to upper lid; /, optic nerve (bundles
of fibers cut obliquely); g, inferior rectus muscle of eyeball; h, anterior chamber filled
with aqueous humor; i, pupil, opening to posterior chamber, also filled with aqueous
humor, between iris and lens; j, conjunctiva, a transparent membrane, continuous with
the lining of the eyelid; k, cornea; I, iris; m, lens; n, suspensory ligament of lens;
o, retina; p, choroid coat; q, sclerotic coat; r, muscles to ligament suspending lens;
s, vitreous chamber containing vitreous humor; t, point of entrance of optic nerve
('blind spot'); u, fatty connective tissue; x, fovea centralis at posterior end of axis
of eyeball.
occupied by the optic stalk and so give rise to the OPTIC
NERVE.
To the optic cup and lens, the former indirectly and the
latter directly of ectodermal origin, other portions largely of
mesodermal origin are added — e. g., the CORNEA, CHOROID
202 FOUNDATIONS OF BIOLOGY
and SCLEROTIC COATS, the IRIS, and the VITREOUS HUMOR -
all of which contribute to the make-up of the eye-ball. The
eye of Vertebrates is an optical apparatus which may be com-
pared roughly with a camera. Light waves which pass
through an outer transparent protective coating and an
opening (PUPIL) in a regulating diaphragm (iris) reach the
lens and are brought to a focus on the retina. The sensory
stimulation thus brought about is transmitted by the optic
or second cranial nerve to the brain.
A broad survey of the sense organs of Vertebrates impresses
one with the fact that, taken by and large, the improvements,
though considerable, are not so marked as one might expect
when the great development of the nervous, system, and the
brain in particular, is considered. And so we must look
chiefly to the cumulative influence of the sensory stimuli
themselves for the underlying factor in the development of
the brain during its long evolutionary history — the brain,
in turn, being enabled to make more out of the same stimuli
and create in Man the higher mental life with all that it im-
plies.
CHAPTER XV
REPRODUCTION IN ANIMALS
So careful of the type . . .
So careless of the single life. — Tennyson.
IN addition to the organs devoted to the life of the individ-
ual animal, the Vertebrates in common with all forms of life
necessarily are provided with means for the continuation of
the life of the race. Reproduction, it will be recalled, is,
in the last analysis, division; the setting free by the organism
of cells with the power of going through a complex series of
changes, involving cell division and differentiation, by which
the relatively simple germ cell becomes transformed into the
obviously complex individual, similar to the parent. In most
plants and animals this process is complicated at the start by
the fusion of two germ cells, the male and female gametes,
to form the fertilized egg, or zygote. Disregarding for the
time being the ultimate origin of the germ cells in the body,
we find in the Metazoa special organs in which the germ cells
reside and undergo changes preparatory to their liberation.
Such reproductive organs, or oojiAps, ordinarily contain germ
cells of one kind, and accordingly are either OVARIES (egg-
producing organs) or TESTES (sperm-producing organs) .
In many of the simpler animals, the gonads are merely
temporary structures which appear during certain seasons of
the year when conditions favor sexual reproduction. Fre-
quently also the same individual produces both eggs and
sperm, in wrhich case the sexuality of the germ cells is not
reflected back, so to speak, to the organism as a whole, which
203
204 FOUNDATIONS OF BIOLOGY
accordingly is known as a HERMAPHRODITE. Such is the
condition in Hydra, where the testes appear as small swellings
in the ectoderm a little below the circle of tentacles; and the
ovary, which is usually single, is a somewhat larger projection
near the opposite end of the animal. Both the testis and the
ovary at first appear to be a heap of ectoderm cells, which in
one case gives rise to many sperm and in the other to a single
egg. The mature sperm are set free from the testis and swim
about in the water. Sooner or later one enters the now rup-
tured covering of the ovary and fuses with the egg. With the
conclusion of fertilization the zygote begins to divide and
forms an embryo, which at an early stage becomes detached
from the parent. Thus in Hydra there is no complicated
apparatus for sexual reproduction; merely now and again
the temporary development of the primary sex organs,
ovaries and testes. (Fig. 64.)
The complex bodies of most animals, however, demand
more or less permanent gonads as well as means for trans-
ferring the gametes directly or indirectly to the exterior.
This is brought about by the fact that in coelomate animals
the gonads come to lie, not on the outside of the body, but
within the coelom. In the Earthworm, which also is her-
maphroditic, the testes and ovaries are permanent organs
attached to the partitions between certain somites. The
sexual products are set free in the coelom, where they are
taken up by SPERM DUCTS and OVIDUCTS and carried to the
outside. Although each Earthworm possesses both male
and female reproductive organs, two worms copulate and
exchange sperm which are stored in the respective seminal
receptacles. Later, when the eggs pass to the exterior, the
'foreign' sperm are shed on them. Thus cross-fertilization is
insured in this hermaphroditic form. In the Crayfish the
sexes are represented by separate individuals, and the appen-
REPRODUCTION IN ANIMALS
205
dages of the first and second abdominal segments of the
male are modified into copulatory organs for the transfer of
the sperm to the body of the female, where they are retained
FIG. 113. — Diagrammatic section of the human uterus with developing embryo.
The embryo (h) is suspended in a fluid-filled cavity (c) surrounded by the foetal mem-
branes (e) and by tissue (/) from the uterus itself. The sole path of communication
between embryo and mother is by blood in vessels passing up through the umbilical
cord (i), spreading out into capillaries in the placenta (6) and there coming into close
relations with the maternal blood supply. The openings of the oviducts (d) into the
uterus become closed during the development of the embryo, a, dorsal wall of uterus;
b, placenta; c, fluid-filled cavity of amnion; d, openings of oviducts (Fallopian tubes);
e, foetal membranes; /, uterine tissue; g, uterine cavity; h, embryo; i, umbilical cord.
until egg-laying. In most terrestrial Vertebrates, including
Man, fertilization occurs while the eggs are still within the
oviducts, the copulatory organs transferring the sperm
directly to the terminal portion of the ducts from which they
make their way up to meet the descending eggs. (Figs. 67,
71, 72, 86.)
206 FOUNDATIONS OF BIOLOGY
When fertilization occurs within the body, the egg may
soon pass to the exterior, usually after being wrapped up
in nutritive and protective coats secreted about it during its
passage down the oviduct. Or, as is the case sporadically
among lower forms and the rule among the highest Verte-
brates, the Mammals, the egg on reaching the lower part
of the oviduct may become attached to the wall of an en-
largement of the oviduct, or of a chamber formed by the
union of the two oviducts, called the UTERUS. Here the
embryo derives nourishment from the maternal blood sup-
ply, and proceeds far along in development before it is ex-
pelled to the exterior, or born. (Fig. 113.)
Thus, except in the simplest animals, there is a special
REPRODUCTIVE SYSTEM; a series of organs connected with
the reproductive function. But it must be emphasized
that the essential organs are the gonads themselves and all
the rest are accessory. Furthermore, in relation to the sexual
differentiation of male and female individuals, many so-called
SECONDARY SEXUAL CHARACTERS arise which are not directly
connected with the reproductive organs, but nevertheless
depend very largely for their development upon hormones
liberated by the gonads. For .example, early castration of
the Stag inhibits the growth of a distinctive male secondary
sexual character, the antlers; while if performed later when
the antlers are full grown, they are shed and abnormal ones
take their place. Similarly, the development and functioning
of the mammary glands during pregnancy in the human fe-
male is induced by hormones produced, not by the ovary it-
self, but by its product, the developing embryo within the
uterus. Here at least two hormones are involved; one
directly stimulates the development of the glands, while
another inhibits their active functioning until it is removed
by the birth of the offspring. (See p. 181.)
REPRODUCTION IN ANIMALS 207
Throughout all the chief Vertebrate groups the sexes are
distinct, although in rare instances abnormal hermaphro-
ditic individuals occur. The definitive primordial germ cells
first appear as localized areas of the coelomic epithelium, on
either side of the vertebral column. As the germ cells develop
they become associated with connective tissue, blood vessels,
and nerves and form the paired gonads. In the most primi-
tive Vertebrates a condition more simple than in the Earth-
worm is found, for both male and female germ cells when ripe
merely break out of the gonads and find their way to the ex-
terior by a pair of minute ABDOMINAL PORES. In -higher
forms, however, the labor of conducting the products out of
the body is foisted upon the urinary system, as was
suggested when that system was under discussion. We
now turn to a statement of the structural inter-relations
of these two systems to form the UROGENITAL SYSTEM.
It has been pointed out that the nephridia, which combine
to form the kidneys, in some of the lower Vertebrates retain
their funnel-like openings into the coelom and therefore afford
a direct exit for waste material in the coelomic fluid. It is
some of these nephridia which are employed in the lower
Fishes for the transfer of the germ cells to the outside. The
testes of the male, which lie close to the kidneys, become con-
nected with the nephridia (mesonephros) by a series of short
delicate tubes. Through these tubes the SPERMATIC FLUID,
containing the sperm from the testes, is transferred to the
nephridia and by them to the kidney (mesonephric) ducts
and so to the exterior with the urinary waste. In this way,
during the period of sexual activity of the male, the kidney
tubules satisfactorily perform two functions, and the mesone-
phric ducts become UROGENITAL CANALS. (Fig. 97, C.)
Turning to the female, we find that the ovaries, which are
situated in about the same position with relation to the kid-
208 FOUNDATIONS OF BIOLOGY
neys as the testes in the male, do not enter into communica-
tion with a set of nephridia of the kidneys (mesonephros) ;
probably because the eggs are too large to pass through the
tubules. Instead, what appears to be the coelomic opening,
or NEPHROSTOME, of a single nephridium on either side (which
fails, so to speak, to enter the kidney complex) enlarges and
becomes the funnel which connects up with a new duct open-
ing into the cloaca. Thus there arises from the female urinary
system a pair of entirely distinct OVIDUCTS. An egg, liberated
from the ovary into the coelom, finds its way into one of the
oviducts and descends directly to the outside, or into an en-
largement (uterus) of the terminal portion of the duct where
development proceeds until birth occurs. (Fig. 97, D.)
The female reproductive system, though derived from the
mesonephric system, has become entirely independent of it.
Accordingly the disappearance of the mesonephros and duct
in higher Vertebrates, when it is replaced by the metanephros
and the ureter as the functional urinary system, has little
effect on the female reproductive system. As a matter of
fact the abandoned mesonephros and duct degenerate and
disappear in the female, while in the male the mesonephric
duct remains and becomes completely appropriated by the
reproductive system. The sperm now pass directly into the
former mesonephric duct, which thereby becomes solely a
sperm duct. Such is the historical origin of the foundations
of the reproductive system as it occurs in the Reptiles, Birds,
and Mammals. Naturally each of these groups, building on
this foundation, has developed modifications and additions
demanded by its special lines of evolution; (Fig. 97, E, F.)
CHAPTER XVI
ORIGIN OF THE INDIVIDUAL
Owing to the imperfection of language the offspring is termed
a new animal, but is in truth a branch or elongation of the
parent. — Erasmus Darwin, 1794.
A GENERAL background of biological facts and principles
has now been established and we are therefore in a position
to take up from an advantageous viewpoint some of the
broad questions relating to the origin of life and the origin
of species, that is the origin of individuals since life and
species are merely concepts, and individuals are the realities
in living nature.
A. ORIGIN OF LIFE
It must seem strange to the reader, with some of the com-
plexities of organisms before him, that the best minds up to
the seventeenth century saw nothing more incongruous in
the spontaneous origin of plants and animals of all kinds from
mud and decaying matter, than does the boy of to-day who
believes that horse hairs soaked in water are transformed into
worms. As a matter of fact, we find that even Aristotle, who
laid such broad foundations for the science and philosophy of
the organism, believed that certain of the Vertebrates, such
as Eels, arose spontaneously.
Naturally, with the increase of knowledge, the idea of
SPONTANEOUS GENERATION was gradually restricted more and
more to the lower forms. It remained, however, for Redi
during the latter half of the seventeenth century to question
seriously the general proposition and to substitute direct
209
210 FOUNDATIONS OF BIOLOGY
experimentation for academic discussion and hearsay. By
the simple expedient of protecting decaying meat from con-
tamination by flies, he demonstrated that these insects are
not developed from the flesh and that the apparent trans-
formation of meat into maggots is due solely to the develop-
ment of the eggs deposited thereon by flies.
But the time-honored doctrine of spontaneous generation
was not overthrown by this experiment nor the long series
which Redi made. The presence of parasites within certain
internal organs of the higher animals , as in the brains of
sheep, baffled Redi himself. Also, the improvements of the
microscope revealed an unknown microcosm whose origin
seemed plausibly explained as spontaneous. BIOGENESIS, or
all life from preexisting life, was placed on a secure foundation
only within the past half-century by the working out of the
remarkably complex life histories of internal parasites, which
showed that they all arise from parents like themselves,
and by the classical demonstrations of Pasteur and others
that microorganisms are not the result, but the cause of de-
cay. The latter fact is at the basis of, and is attested by, the
methods now universally used in food preservation and
aseptic surgery — to mention but two instances.
At the present time, we may consider it as established that
all known forms of life arise from preexisting life by reproduc-
tion. But if we accept the testimony of astronomer and
geologist, the Earth was at one time in a condition in which
life as we know it could not exist, and so we are face to face
with the problem of how it came to be established on the
Earth in the past — the remote past, since the geological
record affords convincing proof that life has existed continu-
ously on the Earth for some hundreds of millions of years.
Unless one is willing to ascribe life's origin to SPECIAL
CREATION — which at once removes it from the sphere of
ORIGIN OF THE INDIVIDUAL 211
science and so beyond the present discussion — or to AN-
OTHER PLANET from whence it was transferred through
space to the Earth — which removes it to a " conveniently
inaccessible place where its solution is impossible" -there
remains but one alternative: life arose through the gradual
evolutionary complexification of matter when, ages ago,
Earth conditions became favorable. Such living matter must
have been relatively simple compared with protoplasm as
we know it today; so simple, in fact, that we would not
recognize it as such, because protoplasm as we see it even in
the simplest organisms has had a long evolutionary history.
Of course it is not, a priori, impossible that such simple life
is even at the present time arising spontaneously under spe-
cial environmental conditions, perhaps in the ocean depths,
but is unable to come to fruition in competition with existing
protoplasm of ancient pedigree and evolutionary specializa-
tion.
However that may be, during the past quarter century
some biologists have now and then thought they were on the
verge of artificially creating life in the test tube, only to leave
the problem, like the alchemists of old, with more respect for
the complexities of its organization and the " enormous gap
which separates even the simplest forms of life from the in-
organic world." And so we may more profitably turn to a
consideration of the present-day manifestations of life in the
reproduction of organisms, and dismiss the insolvable prob-
lem of the origin of life on the Earth with the conservative
statement penned over forty years ago by Huxley:
" Looking back through the prodigious vista of the past, I
find no record of the commencement of life, and therefore I
am devoid of any means of forming a definite conclusion as
to the conditions of its appearance. Belief, in the scientific
sense of the word, is a serious matter, and needs strong
212 FOUNDATIONS OF BIOLOGY
foundations. To say, therefore, in the admitted absence of
evidence, that I have any belief as to the mode in which exist-
ing forms of life have originated, would be using words in a
wrong sense. But expectation is permissible where belief is
not; and if it were given to me to look beyond the abyss of
geologically recorded time to the still more remote period
when the Earth was passing through physical and chemical
conditions, which it can no more see again than a man can
recall his infancy, I should expect to be a witness of the
evolution of living protoplasm from not living matter. . . .
That is the expectation to which analogical reasoning leads
me; but I beg you once more to recollect that I have no right
to call my opinion anything but an act of philosophical faith."
Since so far as is known all life now arises from preexisting
life and has done so since matter first assumed the living state,
it apparently follows that the stream of life is continuous
from the remote geological past to the present and that all
organisms of to-day have an ancient pedigree. It is to the
establishment of this as the reasonable conclusion from the
data accumulated during recent years, that from now on our
attention is somewhat more particularly directed; and ac-
cordingly it is necessary first of all to consider in some detail
the genetic connection of present-day forms as exhibited in
reproduction.
B. REPRODUCTION
The power of producing new individuals specifically similar
to the parent is, as has been seen, one of the most important
characteristics of living in contrast with lifeless matter, and
is exhibited in its simplest form in the unicellular plants and
animals. In Paramecium the nucleus and cytoplasm divide
into two parts, so that by cell division, here called BINARY
FISSION, the identity of the parent organism is merged into
ORIGIN OF THE INDIVIDUAL 213
the two new cells. Simple as this seems, the fission of Para-
mecium, for instance, involves considerably more than the
halving of the original cell, because, as a matter of fact, each
half must reorganize into a complete new individual with all
parts characteristic of the parent. (Fig. 11.)
Among some unicellular organisms (e.g., Sphaerella) the
parent cell, instead of merely forming two cells by binary
fission, becomes resolved into from four to several hundred
cells by a series of practically simultaneous divisions known
as MULTIPLE FISSION, or SFORULATioN. This is usually pre-
ceded by a considerable growth of the parent cell and its
A B C D
FIG. 114. — Yeast cells, very highly magnified. A, cell showing granular
cytoplasm and a large vacuole; B, showing nucleus; C, cell budding; D, mother
cell and bud after division is completed.
enclosure in a protective covering, or CYST, which ruptures
to liberate the spores. Other unicellular forms, such as the
Yeasts — colorless plants chiefly responsible for alcoholic
fermentation — exhibit a modified form of fission, in which the
parent cell forms one or several outgrowths, 'or BUDS, which,
gradually assuming the characteristic adult structures, are
usually detached as complete similar individuals. (Fig. 114.)
In a considerable number of instances, however, the cells
arising by multiple fission or budding remain closely asso-
ciated or organically connected so that they form a COLONY.
In some colonial organisms the component cells are all alike
and each retains its individuality, while in others certain cells
are restricted more or less in their functions, so that a phys-
iological division of labor is established which involves the
214
FOUNDATIONS OF BIOLOGY
shifting of individuality from the cells to the colony as a
whole. This specialization is exhibited chiefly with regard
to reproduction and reaches its highest expression among
colonial PROTISTA (Protozoa and Protophyta) in VOLVOX,
where among ten thousand or so cells, perhaps a score are
specialized for reproduction and the rest are vegetative.
Usually each of the reproductive cells (germ cells) divides
A. Paramecium.
CELL DIVISION
TEMPORARY
CELL DIVISION
CELL DIVISION
(Binary Fission)
CONJUGATION
(Period of Uo-
(Binary Fission)
An indefinite number
(Fertilization)
construction)
An indefinite
of generations
Each cell fer-
Each fertilized
number of gen-
tilizes the other.
cell gives rise to
erations,
four typical
etc.
animals.
B. Volvox.
CELL DIVISION CELL DIVISION CELL DIVISION PERMANENT CELL DIVISION
(Colony for- (Asexual Re- (Gamete for- CJN.IUGA- (Colony forma-
mation) production) mation) TION tion)
Zygote (z) de- Germ cells (g.c.) Certain germ (Fertilization) Zygote develops
velops into a give rise to new cells produce One sperm into a colony,
colony. colonies. eggs (e) ; others fuses with one etc.
sperm, (sp.) egg, forming
a zygote (z).
ORIGIN OF THE INDIVIDUAL
215
C. Hydra.
ec.en.g.c.
CELL DIVI-
BUDDING
CELL DIVISION
PERMANENT
CELL DIVI-
SION
(Asexual Re-
(Gamete for-
CONJUGA-
SION
(Embryological
production)
mation)
TION (Embryological
development)
Part of animal
Certain germ
(Fertiliza-
develop-
Zygote (z)
separates from
cells produce
tion)
ment)
produces ani-
parent and
one egg (e) and
One sperm
Zygote (z)
mal contain-
leads separate
polar bodies
unites with
produces ani-
ing germ cells
existence.
(pb.) ; others
one egg, form-
mal, etc.
(g.c.) and two
produce many
ing a zygote
layers of spe-
sperm (sp.).
GO
cialized somat-
ic cells, the
ectoderm (ec.)
and endoderm
(en.).
D. Earthworm
2
g.c.
pb.e
z
1
"2§iP$§ic*N.
.xfiSSI^fcSv.
i &
i
PERMANENT CELL DIVISION
CONJUGATION (Embryological
(Fertilization) development)
One sperm unites Zygote (z) pro-
with one egg, duces animal,
forming a zygote etc.
(z).
CELL DIVISION CELL DIVISION
(Embryological (Gamete forma-
development) tion)
Zygote (z) pro- Certain germ
duces animal cells produce one
containing germ egg (e) and
cells (g.c.) and polar bodies
three layers of (pb.); others
specialized produce many
sojnatic cells, sperm (sp.).
the ectoderm
(ec.), mesoderm
(ms.), and endo-
derm (en.).
FIG. 115. — Diagrams to illustrate the general reproductive cell cycle in (A) a uni-
cellular organism (Paramecium) ; (E) a colony of cells (Volvox); (C) a simple Metazoon
(Hydra); and (D) a more complex Metazoon (Earthworm). (From Hegner.)
216 FOUNDATIONS OF BIOLOGY
to form a group which is set free as a miniature colony; but
in certain cases some of the reproductive cells become trans-
formed into male and others into female gametes. After
fertilization of the eggs, usually by sperm from another
colony, the zygotes develop into new colonies which even-
tually are liberated from the parent colony. (Fig. 18.)
As has been previously suggested, the physiological divi-
sion of labor in the colonial Protista, involving, as it does, a
segregation of reproductive from vegetative structures,
affords a logical transition from the unicellular condition to
that characteristic of the multicellular forms. These, to all
intents and purposes, may be considered highly complex
colonies of cells in which specialization, no longer confined
merely to demarking germinal and vegetative regions, has
transformed the latter into a complex of tissues and organs,
the body (SOMA) of the individual, while the germinal tissue
(GERM) is confined to the essential reproductive organs.
It is customary, therefore, to draw a more or less sharp
distinction between the soma and germ — to consider the
soma the individual which harbors, as it were, the germ des-
tined to continue the race. This theory of GERMINAL CON-
TINUITY, which is chiefly associated with the name of Weis-
mann, recognizes that the germ contains living material
which has come down in unbroken continuity ever since the
origin of life and which is destined to persist in some form as
long as life itself. On the other hand, the soma may be said
to arise anew in each generation as a derivative or offshoot of
the germ; and, after playing its part for a while as the vehicle
of the germ, to pass the germ on at reproduction, and then
die. The germinal continuity concept has altered the attitude
of biologists toward certain fundamental questions in heredity
and evolution, as will be apparent when these subjects are
considered. (Figs. 115, 135.)
ORIGIN OP THE INDIVIDUAL
217
FIG. 116. — Hydra reproducing asexu-
ally by dividing lengthwise. (After
Koelitz.)
Though Volvox and other colonial forms afford a glimpse
of the conditions which probably prevailed when the evolu-
tionary bridge from unicellu-
lar to multicellular organ-
isms was crossed, the varied
methods of reproduction of
the latter by no means in-
dicate the early establish-
ment of a hard and fast
boundary between soma and
germ. Many of the In-
vertebrates, such as Hydra
and various types of worms,
reproduce not only by germ cells, but also by strictly asexual
processes which are known as FISSION and
BUDDING. These processes are comparable
merely in a superficial way with the similarly
named methods in the Protista. In some
forms the whole complex body divides into two
or more parts, each of which reforms —
REGENERATES — what was lost and so becomes
a complete though a smaller individual. In
other forms, as well as in Hydra itself, buds
arise as outgrowths from the body and develop
into replicas of the parent either before or after
becoming detached. (Figs. 116, 117.)
In many of the nearest allies of Hydra the
buds remain permanently attached so that
eventually a large colony of organically con-
nected hydra-like individuals (HYDRANTHS) is
formed. (Fig. 64.) This condition leads to a
physiological division of labor between the various hydranths
which may become more or less modified in structure so that,
FIG. 117.— An
unsegmented
worm (Flat-
worm) in pro-
cess of fission.
(After Child.)
218
FOUNDATIONS OF BIOLOGY
for instance, feeding, protective, and reproductive individuals
are established, and thereby the HYDROID COLONY exhibits
what is termed POLYMORPHISM. Our present interest is confined
FIG. 118, — Life history of Obelia. A, portion of a colony: 1, ectoderm; 2, endo-
dsrm; 3, mouth; 4, enteric cavity; 5, stalk of colony; 6, 7, and 10, exoskeleton; 8~,
reproductive hydranth (blastostyle) ; 9, medusa bud. B, free swimming medusa:
1, mouth; 2, tentacles; S, reproductive organs; 4< radial canals; 5, sense organ.
C, ciliated larva of closely related species. (From Hegner, after Parker and Haswell,
Shipley and MacBride, and Allman.)
to the reproductive hydranths, which in many of the Hydroids
are so modified that they are dependent upon the colony
as a whole for all the necessities of life and are merely bodies
which form by budding other individuals known as MEDUSAE.
ORIGIN OF THE INDIVIDUAL
219
The medusae, which become detached and swim away,
usually bear no superficial resemblance to any of the other
individuals of the colony on which they arose, but a study
of their structure shows that they are built on the same
fundamental plan and are, to all intents and purposes, free-
swimming sexual hydranths, some of which produce sperm
and others eggs. The medusae liberate their sexual products
in the water where fertilization occurs, and the zygote gives
FIG. 119. — Diagrams to show the fundamentally similar structure of Hydra or
of a hydranth of Obelia (A) and of a medusa (B). circ, circular canal; ect, ectoderm;
end, endoderm; ent. cav, enteric cavity; hyp, mnb, region of mouth (mth); msgl, meso-
gloea; nv, nvl, nerve rings; rad, radial canal; v, velum. (From Parker.)
rise to a free swimming embryo (LARVA) . This soon becomes
attached to some submerged object and develops into a Hy-
droid colony. (Figs. 118, 119.)
Thus the common Hydroids, such as OBELIA, exhibit two
distinct phases, or generations, in their life history — the
fixed, polymorphic colony of hydranths, or polypes, which is
produced sexually but is itself asexual; and the free-swim-
ming medusae which are produced asexually but are them-
selves sexual. The asexual and sexual generations alternate
with each other in regular sequence, so that an alternation of
220 FOUNDATIONS OF BIOLOGY
generations known as metagenesis occurs, which, thougn it
differs from, recalls the conditions which obtain in plants.
Alternation of asexual and sexual methods of reproduction,
attended by more or less difference in structure of the indi-
viduals of the generations, is fairly widespread among the
Invertebrate groups, particularly in forms which have
adopted a parasitic mode of life. Frequently the life his-
tories are exceedingly complicated: several asexual, sexual,
and parthenogenetic generations succeeding one another in
response to the exigencies imposed by adaptation to a life
within another animal or series of animals.
It is clear from such life histories that the conception of
special germ cells early set aside, as it were, from the somatic
cells must not be taken too literally. The same point is
emphasized by the power exhibited by plants and animals
in restoring parts lost by mutilations of one kind or another.
Among many plants, pieces of the root, stem, or, in special
cases, of the leaf may give rise to individuals complete in every
respect. Until the middle of the eighteenth century this was
considered a property peculiar to plants, and accordingly
soon after Hydra was discovered experiments were made to
determine whether the organism was a plant or an animal.
Specimens were cut into several pieces and it was found that
each piece developed into a complete Hydra. This result,
from the ideas of the time, should have led to the conclusion
that Hydra is a plant, but additional characteristics were ob-
served which outweighed all other considerations. Accord-
ingly Hydra was recognized as an animal with the power of
replacing lost parts. (Fig. 120.)
Since the classic work on Hydra the power of regeneration
has come to be recognized as a fundamental property of all
animals. It is exhibited to the greatest degree among the
lower animals while in the higher Vertebrates it is confined
ORIGIN OF THE INDIVIDUAL 221
chiefly to the replacement of cells which especially suffer from
wear and tear, such as those forming the outer layer of the
skin. It will be recognized that regeneration is but one
phase of a fundamental property of protoplasm, namely
growth, whether it consists in restoring a part of a Parame-
cium, transforming a bit of a Flatworm into a complete
animal, or replacing half of an Earthworm, the head of a
• •ft I
FIG. 120. — Regeneration and grafting in Hydra. A, an individual with
seven ' heads' as a result of lengthwise cuts. B, stages in the regeneration of a
complete individual from a small piece. C, Portions of two individuals grafted
together. (From Hegner; A, after Trembley; B, after Morgan; C, after King.)
Snail, the claw of a Crayfish, or the leg of a Salamander. But
the experimental study of regeneration phenomena has
opened up a new vista of the regulatory powers of living
things from Protist to Vertebrate and from egg to adult, and
has afforded a means of approach to some fundamental bio-
logical problems. And withal it has a practical value. The
surgeon now knows more of the regeneration of tissues in
general and nerves in particular in wound healing, and the
oysterman knows — or should know — that his attempt to
destroy Starfish by tearing them up and throwing the pieces
222
FOUNDATIONS OF BIOLOGY
overboard, serves merely to increase many fold this enemy
of the oyster. (Figs. 121, 122.)
The power of fragments of distinctively somatic tissue, as
in the Earthworm and many plants, to form a complete
organism including the reproductive organs and germ cells,
indicates that we must postulate at least a potential supply
i\
\j
FIG. 121. — Regeneration and grafting in the Earthworm. A, regeneration of re-
moved anterior segments by the posterior piece. B, regeneration of posterior seg-
ments by the posterior part, so that the worm has a 'tail' at either end. C, regenera-
tion of removed posterior end by the anterior piece. D, three pieces grafted together to
make a long worm; E, two pieces grafted to form a worm with two 'tails'; F, short
anterior and posterior pieces grafted together. Regenerated portions are dotted.
(From Hegner, after Morgan.)
of the germ residing in the somatic tissue, which can make
good the definitive germ cells when they are lost. At first
glance this may seem to be a far cry to save an idea, but it is
a fact that there is a continuity of the nuclear complex (GERM
PLASM) whether the germ cells are set aside early in individual
development, or later by the transformation of what seem to
be typical somatic cells. That this is really the crux of the
question will be appreciated after the details of cell division
have been described.
ORIGIN OP THE INDIVIDUAL
223
C. ORIGIN OF THE GERM CELLS
Among the Vertebrates, as we know, the germ cells reside
during adult life in definite organs, the ovaries and testes, and
upon these cells the power of reproduction of the individual
is solely dependent. It seems clear, however, that the
I
B'
I
t
$&
C'
FIG. 122. — Regeneration of a Flatworm (Planaria maculata). A, normal
worm; cut across at line indicated. B, B', and C, C', regeneration of an-
terior and posterior parts of A to form complete worms. D, piece cut from a
worm; Dl, Z>2, D3, Z>4, successive stages in the regeneration of D. E, 'head' from
which rest of animal has been cut off. E1, E2, Es, successive stages in the re-
generation by E of a complete body. F, similar experiment to E, but a new
'head' in reversed position is regenerated instead of a body, Fl. (From Hegner,
after Morgan.)
primordial germ cells do not arise as such by division in the
tissues which during development form the ovaries and testes.
Just when the germ cells are set aside in Vertebrates is un-
certain, but it would seem to occur very early in embryonic
life, perhaps during the cleavage of the egg. Then by
shif tings of the tissues during growth, and possibly also by
amoeboid movements of the germ cells themselves, they
finally reach definite positions in the epithelium lining the
224 FOUNDATIONS OF BlOLOGi
dorsal wall of the coelom, which becomes an integral part of
the gonads as development proceeds.
With regard to the fate of the PRIMORDIAL GERM CELLS,
once they have reached testis or ovary, we are on surer ground
and can trace with considerable exactness their divisions and
transformations which give rise to the gametes, sperm and
eggs. In the first place the primordial germ cells proceed to
multiply in the testis and ovary so that they produce a large
number of relatively small germ cells known as SPERMATO-
GONIA and OOGONIA respectively.
1. Mitosis
Before taking up the origin of the gametes from the sper-
matogonia and oogonia, it will be necessary to describe in
some detail the complicated internal process involved in all
typical cell divisions, known as MITOSIS, which was dismissed
when considering the origin of cells until the reader would be
in a position to appreciate to the full its significance.
Reduced to its simplest terms, a typical resting cell, that
is one which is not dividing, consists of a mass of cytoplasm
surrounding a nucleus; the latter with its chromatin dis-
tributed so that it presents a net-like appearance. In addi-
tion to the nucleus, it will be recalled that there is present
another important cell organ, the CENTROSOME, which ap-
pears like a tiny granule situated in the cytoplasm near the
nucleus of the resting cell. For all practical purposes we may
consider the cytoplasm as the arena in which mitosis takes
place, the centrosome as the dynamic agent, and the nucleus,
or more specifically its chromatin, as the essential element
which the complicated process is particularly designed to
distribute with nicety to the daughter cells which are in pro-
cess of formation. With this in mind we may proceed to an
outline of the chief stages of mitosis, first cautioning the reader
ORIGIN OF THE INDIVIDUAL
225
to remember that variations in the details are as numerous as
the different types of cells, and that any general account can
do no more than present the fundamental plan of operations.
FIG. 123. — Diagrams of typical stages in mitosis. A, resting cell with chromatin
presenting a net-like arrangement within the nuclear membrane; c, centrosome divided;
B, Prophasc (early): centrosomes, asters (a), and spindle; most of the chromatin
material seems to assume the form of a long thread (spireme); C, prophase (later)
involving the disappearance of the nuclear membrane, and the separation of the chro-
matin of the spireme stage into discrete bodies (chromosomes); D, prophase (final)
with chromosomes arranged in the equatorial plate (ep); E, metaphase; each chromo-
some splitting lengthwise; F, anaphase: the daughter sets of chromosomes moving
toward the asters; if, 'inter-zonal fibers'; G, H, early and later telophase involving the
gradual loss of visibility of chromosomes as they spin out into the resting net-like
arrangement of the chromatin; division of the cytoplasm; n, nucleolus. (After Wilson.)
Broadly speaking, mitosis can be divided into four chief
stages: PROPHASE, METAPHASE, ANAPHASE, and TELOPHASE,
during each of which characteristic changes take place in
the nucleus, cytoplasm, and centrosome. (Figs. 8, 123.)
226 FOUNDATIONS OF BIOLOGY
At the beginning of the prophase, or earlier, the centrosome
divides to form two, each of which becomes surrounded by
what appears to be a halo (ASTER) of radiating fibers which
are possibly cytoplasmic currents — the visible expression of
physico-chemical forces. The centrosomes and asters now
proceed to move apart, take up positions at opposite sides
of the nucleus, and the astral fibers between lengthen until
they form a CENTRAL SPINDLE. While these changes are
going on, the nucleus is not inactive. The nuclear membrane
gradually disappears and the chromatin granules, originally
in a net-like arrangement, seem to become rearranged in a
more or less continuous thread of chromatin called the
SPIREME. This, however, actually represents a number of
definite chromatin entities, termed CHROMOSOMES, which
gradually by chromatin concentration become distinctly in-
dividual. The number of chromosomes varies greatly in
different species, but is typically an even number and the
same for all the cells of a given species.
When the chromosomes have assumed definitive form, the
preliminary events which constitute the prophase of mitosis
are brought to a close by the chromosomes being drawn to
the center of the spindle. Here they are arranged in a plane
at right angles to the long axis of the central spindle, midway
between the two centrosomes, and form the EQUATORIAL
PLATE.
And now the stage is set for what is apparently the climax
of mitosis, designated the metaphase. Each of the chromo-
somes separates into two parts along the line of a longitudinal
split, in such a manner that each of the thousands of chromatin
granules which make up a chromosome is equally divided.
Two sets of similar daughter chromosomes are thus formed.
With chromosome division consummated, the metaphase
merges into the anaphase which is devoted to a shifting of a
ORIGIN OF THE INDIVIDUAL 227
daughter set of chromosomes along the fibers to either end
of the spindle. In this way each centrosome becomes asso-
ciated with one set of daughter chromosomes.
The last stage, or telophase, is one of nuclear reconstruc-
tion and division of the cytoplasm. The chromosomes be-
come indistinct as they spin out to form the net-like arrange-
ment of the chromatin in the nucleus of each daughter cell;
a nuclear membrane arises; and the nucleus again assumes
the form of a definite spherical body characteristic of the
resting cell. It must be emphasized, however, that although
the chromosomes usually disappear from view as definitive
entities in the resting nucleus, nevertheless the individuality
of each persists and the same chromosomes emerge from the
nuclear complex at the next division period.
Simultaneously with these nuclear changes, and before the
spindle and asters — the machinery of mitosis — disappear,
the division of the cytoplasm is initiated as indicated by an
indentation of the cell wall at the equator of the cell. This
gradually extends through the cytoplasm in the same plane
which the equatorial plate formerly occupied, until the cyto-
plasm is cut into two separate masses, each containing one
of the daughter nuclei and centrosomes. And one cell has
merged its individuality into two daughter cells by mitotic
division.
A little thought will convince the reader that whereas the
mitotic process apparently results in merely a mass division
of the cytoplasm, the chromatin material is rearranged and
distributed in a manner which makes it possible for each cell
to receive a very definite share. Indeed this seems to be the
primary object of mitosis. For in many cases there is a very
great difference in the size of the resulting cells, but the num-
ber of chromosomes in each is the same. This, and other
evidence which will presently appear, has clearly established
228 FOUNDATIONS OF BIOLOGY
the chromosomes as the chief factors in the transmission of
characters from cell to cell, and therefore in inheritance.
2. Gametes
Returning now to the origin of gametes. The spermato-
gonia and oogonia in the reproductive organs are, together
with all the cells forming the body proper, direct descendants
by mitotic cell division from the fertilized egg which gave rise
to the individual organism. This is equally true of the chro-
mosomes themselves and accordingly every cell of the animal
has the same number of chromosomes as the fertilized egg.
Fertilization, as we now know, always consists of the
fusion of two gametes, whether it is in plants or animals; a
fusion of nucleus with nucleus and cytoplasm with cytoplasm
to form a zygote, which therefore is one cell reconstructed from
two. Such being the case, one of two things must happen at
fertilization. Either the fertilized egg must have double the
chromosome number, that is a set contributed by both egg
arid sperm; or some method must exist by which the chromo-
somes of the gametes are reduced in number to one half that
characteristic of the somatic cells.
As a matter of fact a reduction in the number of chromo-
somes always takes place sometime during the life history.
In plants such as the Mosses, Ferns, and Flowering Plants, it
occurs at the formation of the spores. Thus it follows that
the gametophyte contains half as many chromosomes as the
sporophyte, and the sporophyte number is restored by the
union of the gametes. It must be borne in mind, however,
that the familiar plants are sporophytes which, for all prac-
tical purposes, directly produce sporophytes because the
gametophyte is reduced almost to the vanishing point. The
chromosome number of the parent sporophyte and the
sporophyte in the seed is the same. But we cannot digress
ORIGIN OF THE INDIVIDUAL
229
ADULT ANIMAL
Primordial
Germ
Cells
Somatic
Cells
Diploid
ADULT GAMETOPHYTE
Somatic
Cells
Primordial
Germ
Cells
Haploid
ADULT SPOROPHYTE
Primordial
Spore
Cells
Somatic
Cells
Diploid
FIG. 124. — Schematic representation of the life history of an animal (A) and of a
plant, e.g., Fern, (B) from the standpoint of the diploid and haploid condition of the
chromosomes.
230 FOUNDATIONS OF BIOLOGY
to elaborate the details of the chromosome cycle associated
with alternation of generations in plants — attention must
be concentrated on the conditions as they exist in animals,
in which the somatic number of chromosomes is reduced one
half at the formation of the gametes. From the standpoint
of chromosome number, the sporophyte is comparable to
the animal soma and the gametophyte is represented by
merely a couple of cell generations during the formation of
the gametes in animals. (Fig. 124.)
The MATURATION or 'ripening' of the germ cells of animals
involves two cell divisions by which each spermatogonium
gives rise to four sperm, and each oogonium to one functional
egg and three tiny, abortive eggs known as POLAR BODIES;
each and all with one half the number of chromosomes of the
somatic cells and of the germ cells up to this point in their
developmsnt. Consequently these two divisions, termed
MATURATION DIVISIONS, must be examined in some detail if
we are to appreciate the nicety of the process by which the
chromosome number is reduced one half without impairing
the chromatin heritage from cell to cell. We shall describe
first the origin of the sperm which, though it is fundamentally
the same as that of the egg, is somewhat simpler to under-
stand.
3. Spermatogenesis
A given SPERMATOGONIUM, with, let us say, eight chromo-
somes characteristic of the species, proceeds to increase in
size preparatory to the first maturation mitosis, and is desig-
nated a PRIMARY SPERMATOCYTE. At the close of the growth
period, when this cell is preparing to divide, the chromosomes
are arranged in pairs by a process termed SYNAPSIS. The
number of such pairs will obviously be half that of the chro-
mosome number. The synaptic pairs are then distributed in
the equator of the spindle exactly as the single chromosomes
ORIGIN OF THE INDIVIDUAL
231
FIG. 125. — Diagram of the general plan of spermatogenesis and oogenesis in animals.
The somatic, or diploid, number of chromosomes (duplex group) is assumed to be eight.
Male, to the left; female, to the right. A, primordial gerrn cells; B, spermatogonia and
oogonia, many of which arise during the period of multiplication; C, primary spermato-
cyte and oocyte, after the growth period, with chromosomes in synapsis; D, secondary
spermatocytes and oocytes, with haploid number (simplex group) of chromosomes,
which have arisen by the first maturation (reduction) division; E, spermatids (which
become transformed into sperm) and egg and three polar bodies which have arisen by
the second maturation (equation) division; F , union of sperm and egg (fertilization)
to form zygote with diploid number (duplex group) of chromosomes; C, chromosome
complex of cells after first division of the zygote, and of all subsequent somatic cells,
and germ cells until maturation.
232 FOUNDATIONS OF BIOLOGY
are in ordinary mitosis. But, and this is the crucial point,
in the early anaphase the members of each pair are separated,
one sy nap tic mate going to each pole of the spindle. Thus
each of the daughter cells — SECONDARY SPERMATOCYTES —
receives half the total number of chromosomes that were
present in the primary spermatocyte or the somatic cells.
The essential difference between this type of mitosis (REDUC-
TION DIVISION) and that involved in other nuclear divisions
(EQUATION DIVISIONS) lies in the separation of entire chromo-
somes (synaptic mates) instead of the splitting of each chro-
mosome. Both the secondary spermatocytes now divide by
typical mitosis, thus distributing to each of the resulting cells
(SPERMATIDS) half the somatic number of chromosomes. The
spermatids are presently transformed into sperm and thus
each spermatogonium with eight chromosomes gives rise to
four sperm with four chromosomes apiece. (Fig. 125.)
4. 0 agenesis
The maturation of the egg, as already intimated, follows
the same plan as that of the sperm, and the reduction of the
chromosomes is the same. Such modifications as occur are
related to the fact that the egg is usually a relatively large
passive cell stored with nutritive materials for use during the
developmental process, while the sperm is among the smallest
of cells — essentially a nucleus surrounded with a delicate
envelope of cytoplasm. Accordingly it is only necessary to
emphasize that the growth period of egg formation, in which
the OOGONIUM becomes transformed into the PRIMARY
OOCYTE, is characterized by a much greater increase in size
than is the case in the corresponding period in spermato-
genesis; and that both of the ensuing cell divisions (one a
reduction and the other an equation division) involve very
unequal divisions of the cytoplasm. Thus one SECONDARY
ORIGIN OF THE INDIVIDUAL 233
OOCYTE is very large, while the other is a tiny cell termed the
FIRST POLAR BODY. Both the large secondary oocyte and
first polar body now divide again; the former giving rise to
a large cell, the mature EGG, and a tiny SECOND POLAR BODY;
while the first polar body divides equally to form two polar
bodies. In this way arise the four cells, comparable to the
four sperm in spermatogenesis, each with half the somatic
number of chromosomes. But only one of these, the egg,
functions as a gamete. The three polar bodies, although
possessing a similar chromosome complex, are sacrificed in
providing one cell with its special cytoplasmic equipment.
The polar bodies get just enough cytoplasm to be regarded
as cells, and soon degenerate and disappear.
Such is the outline of the essentials of spermatogenesis and
oogenesis in animals; processes which involve at one stage a
modification of ordinary mitosis to give each gamete half the
somatic number of chromosomes characteristic of the species.
It is clear that this is not merely a mass reduction of chromatin
material, but is a separating of definite chromatin entities,
the chromosomes, so that the gametes receive the reduced
number.
5. The Chromosome Cycle
Throughout the animal kingdom, wherever sexual repro-
duction occurs, phenomena which can be interpreted as
nuclear reduction have been observed in the formation of
gametes. In some of the Protozoa this seems to be merely
an extrusion of a certain amount of chromatin, but since
whenever chromosomes can be observed and counted the
process has been found to follow in principle essentially the
same lines described above, we have every reason to believe
that it is never a haphazard mass reduction, and that the ripe
gametes emerge with a definite chromatin heritage, relatively
simple as this may be in the lowest forms.
234 FOUNDATIONS OF BIOLOGY
We have now surveyed the germ cell cycle from fertil-
ized egg through the germ plasm in the adult to the
gametes again, but before proceeding to consider the details
of the fusion of egg and sperm — the fertilization process —
it may clarify matters to glance back to the chromosome
condition in the fertilized egg at the beginning of the cycle
which has just been considered. Obviously this fertilized egg
(zygote) contained chromosomes, half of which belonged to
the egg and therefore may be termed MATERNAL, and half of
which were derived from the sperm and thus are PATERNAL.
When the zygote divided by mitosis to form the body and germ,
every cell received a set of chromosomes, directly derived
from this original set in the zygote. It logically follows,
and all observations indicate, that each and every cell, both
of the soma and of the germinal tissue, possesses a set of chro-
mosomes, half of which are of maternal and half of paternal
origin — in other words are direct lineal descendants of the
combined set formed at fertilization. So it happens, that
each body cell really has a double set (DUPLEX GROUP, DIPLOID
NUMBER) of homologous chromosomes — and the same is
true of the germ cells until maturation. Then at synapsis
maternal and paternal chromosomes pair and, after the re-
duction division, the secondary spermatocytes and oocytes
and the gametes themselves have a single set (SIMPLEX
GROUP, HAPLOID NUMBER). (FigS. 124A, 126.)
Thus far we have emphasized chromatin and, in particular,
chromosome reduction as the main purpose of the compli-
cated maturation phenomena. The question now arises: Is
this chromatin distributed so that all the gametes receive the
same heritage?
All the evidence at hand indicates not only that chromo-
somes differ qualitatively one from another but also that
the various parts (CHROMOMERES) of each chromosome are
ORIGIN OF THE INDIVIDUAL
235
ABCD
JL
JOL
AaBbCcDd
eiBCd.
AbcD
FIG. 126. — Diagram of the chromosome cycle of an animal. Somatic (diploicl)
chromosome number assumed to be eight. Paternal chromosomes (from sperm) =
A B C D; •maternal (from egg) = a b c d. I, union of nuclei of gametes, each with a sim-
plex group (haploid number) of chromosomes, in the zygote nt fertilization to form a
duplex group (diploid number) of chromosomes. II, III, IV, somatic divisions or divi-
sions of germ cells before maturation (duplex groups of chromosomes). V, synapsis,
involving pairing of homologous paternal and maternal chromosomes to give the
haploid number of paired chromosomes. VI, reduction division — separation of pairs
into single chromosomes again. VII, two gametes, with simplex groups (haploid
number) of chromosomes; there are 14 more possible combinations of the chromosomes,
or types of gametes, which are not shown. (After Wilson, slightly modified.)
236
FOUNDATIONS OF BIOLOGY
qualitatively distinct. And further that these qualitative
differences are the physical basis of inheritance — the de-
terminers (GENES) of characters which will be realized in the
individual or the race to which the cell containing them con-
tributes. Such being the case, the chromosomal complex of
the nuclei which arises after synapsis — that is, the nuclei
FIG. 127. — A, section through the egg of a primitive Vertebrate, the Lamprey. B
sperm of the same species, drawn to scale, d.en, dense endoplasm; i.m, inner membrane
o.m, outer membrane; p, granular 'polar' cytoplasm; v.en, vacuolated endoplusm
T.ex, vacuolated ectoplasm; /, first polar body; II, spindle for second polar body
(From Kellicott.)
of the gametes — depends on how the various chromosomes
happen to be distributed during the two maturation divisions.
As a matter of fact all the chromosomal combinations occur
which are mathematically possible with the available num-
ber of chromosomes in a given species, but with one limita-
tion: every cell must receive one member of each synaptic
pair of chromosomes, so that each and every gamete receives a
complete simplex group of chromosomes, but rarely the same
ORIGIN OF THE INDIVIDUAL 237
groups (maternal and paternal) which existed before matura-
tion. For example, if the somatic (diploid) number of chro-
mosomes is eight, sixteen different types of gametes are
possible. In Man with 48 somatic chromosomes, and
after synapsis 24 pairs of paternal and maternal chromosomes,
there are 224, or about twenty million possible types of
gametes in each sex, and since these combine at random at
fertilization, the number of possible different types of zygotes
from one parental pair mounts far up in the trillions. No
wonder the children of a family differ — there is variation!
In a way, therefore, fertilization is not consummated, so
far as its influence on the race is concerned, until the matura-
tion of the gametes in the new generation. We must defer
until later the consideration of the significance of these facts
in biparental inheritance, and take up now some necessary
details of the gametes themselves and of how they unite to
form the zygote.
6. Fertilization
The gametes, while exhibiting in certain cases peculiar
adaptations to special conditions, are remarkably similar in
general structure throughout the animal series. It is possible
in animals, just as in plants, to arrange a series of lower forms
which shows various stages in sex differentiation. Beginning
with animals in which both gametes are structurally similar,
we pass by slow gradations to others in which the egg is a
relatively large, passive, food-laden cell and the sperm a
minute, active, flagellated cell. As a matter of fact the egg
is subject to somewhat more variation in size and general
appearance than the sperm, for after fertilization it must
be adapted to meet the special conditions of development
peculiar to the species. Thus, for instance, the actual size
of the egg in both plants and animals is determined chiefly by
238 FOUNDATIONS OF BIOLOGY
whether the developing embryo is in the main dependent upon
food stored in the cytoplasm of the egg itself, or upon some
outside source, such as the sea water in which it floats, or the
tissues. of the parent. The first case is well illustrated by a
Bird's egg in which the so-called YOLK is the egg cell proper,
hugely distended by stored food, and surrounded by nutritive
FIG. 128. — Diagram of the egg of the domestic Fowl, before incubation, a, air
space between two layers of shell membrane; alb, ch, dense albumin (chalaza); alb',
more fluid albumin (white of egg) ; bl, point of cytoplasmic concentration from which
embryo arises (blastoderm) ; sh, shell; shm, shell membrane. (After Marshall.)
and protective envelopes consisting of the ' white of the egg/
shell membranes, and shell which are formed by secretions
from the walls of the oviduct during the passage of the egg to
the exterior. On the other hand the eggs of Mammals, for
instance of the Rabbit and Man, are very small — the human
egg being less than 1/1 25th of an inch in diameter — since
their essentially parasitic method of development in the uterus
renders superfluous the storage of any considerable amount
of food material in the egg cytoplasm. (Figs. 127-129.)
ORIGIN OF THE INDIVIDUAL
239
With the specialization of the egg along lines which render
it non-motile, it has devolved upon the sperm to assume the
function of seeking out the egg for fertilization. It does this
FIQ. 129. — Human egg cell, X 415, and sperm cell, X 2000. A, Egg just
removed from the ovary, surrounded by follicle cells of the ovary and a clear
membrane. The central part of the egs; contains metaplasmic bodies and the
large nucleus. Superficially there is a clear ectoplasmic region. (After Wal-
deyer.) B, two views of the human sperm, c, centrosome; h, 'head' consisting
of the nucleus surrounded by a cytoplasmic envelope; m, ne, middle piece; t,
tail or flagellum. (After Retzius.)
in most cases by active lashing of its flagellum. This necessi-
tates a fluid medium in which the sperm can swim, and such
is provided by the environment in which the organism lives
or, in the case of most higher animals, where fertilization
240 FOUNDATIONS OF BIOLOGY
takes place within the oviduct, by special fluids secreted for
the purpose. In the highest plants, however, it will be
recalled that the characteristic motility of the sperm is lost
in the excessive specialization attendant upon gametophyte
reduction — the sperm nucleus reaching the egg by the
growth of the pollen tube down through the tissues of the
style.
A question of much interest is how the actual meeting of
the gametes is brought about, In many cases it is un-
doubtedly merely by chance; the random swimming of the
sperm sooner or later bringing one in contact with an egg.
In other cases the movements of the sperm seem to indicate
some definite attraction by the egg. It has been shown,
for example, that the sperm of some Mosses and Ferns are
attracted by exceedingly dilute solutions of cane sugar and
malic acid respectively, traces of which are secreted by the
tissues in the vicinity of the egg. Also the sperm of some of
the lower animals are attracted by substances eliminated by
the egg during maturation. In such instances there can be
but little doubt that chemical stimulation of the sperm by
specific substances plays a part in bringing the gametes to-
gether. This is an example of CHEMOTAXIS: a phenomenon
of considerable importance, especially in the behavior of
free-living cells.
Once a single sperm has come into functional contact with
the egg, it initiates a chain of events which constitutes fer-
tilization. Although, as might be expected, the variations in
details are legion, they do not obscure the main facts. The
first reaction on the part of the egg is to prevent the entrance
of other sperm and thereby to insure a free field for the opera-
tions of the first arrival. In some of the lower plants this is
accomplished by secreting instantly a chemical substance
which repels other sperm. Frequently among animals a
ORIGIN OF THE INDIVIDUAL 241
jelly-like layer is formed about the egg, or, if a membrane
is already present, this may be rendered impermeable or
still another formed. In cases where the egg is surrounded
originally by a dense and resistant wall, the tiny opening
provided for the entrance of the sperm is closed. How-
ever, the accessory wrappings about certain eggs, such as
those of Birds, have no relation to the present subject,
since they are secreted, not by the egg itself, but by glands
in the wall of the oviduct some time after fertilization has
occurred.
The reactions of the egg cytoplasm that exclude accessory
sperm are overshadowed in importance by others which upset
the stable equilibrium of the egg and render its surface
permeable, so that extensive osmotic interchanges take place
between the cytoplasm of the egg and its surroundings. Most
often this is visible merely in a shrinkage of the cytoplasm
due to loss of water, but sometimes contractions, amoeboid
movements, or flowing of special cytoplasmic materials to
definite regions of the egg are visible. In any event it is cer-
tain that the cytoplasm undergoes profound changes —
its organization as a gamete gives place to a reorganization
which establishes and determines the general outlines of its
subsequent development as an individual. (Fig. 132, A, B.)
Turning now to the nuclei, known as male and female
PRONUCLEI, the union of which to form the single fertiliza-
tion nucleus (SYNKARYON) is the climax of fertilization. Dis-
regarding the flagellum of the sperm, which disappears as it
enters the egg, we find that the sperm nucleus moves through
a quite definite path toward the center of the egg where it is
met by the egg nucleus. Both the pronuclei now become re-
solved into chromosomes which lie free in the cytoplasm,
while a pair of centrosomes, surrounded by asters, appear and
take up positions on either side of the chromosomes to form a
242 FOUNDATIONS OF BIOLOGY
typical mitotic figure. The two sets of chromosomes form an
equatorial plate at the center of the spindle, thus establishing
at once not only the mitotic apparatus for the first division
of the egg, but also the intimate association on equal terms
of chromosomes, with their potentialities from the two
parents, to form a common structure — the nuclear complex
of the new individual. (Fig. 126, I, II.)
Such are the outstanding facts of fertilization which a host
of investigators have brought to light chiefly within the past
century. It was not until 1839 that Schwann, with the es-
tablishment of the 'cell theory/ recognized the egg as a cell,
and sixteen years more before the sperm was similarly under-
stood; while the first realization that fertilization is an
orderly amalgamation of two cells to form one came during
the seventies of the past century. Then it became evident
that in sexual reproduction each individual contributes to
the formation of the offspring a single cell, in which must be
sought the solution of the problems of sex, fertilization, de-
velopment, and inheritance. However, the concentration of
attention on the cell has not simplified the solution of these
fundamental problems; but rather it has contributed to an
ever-increasing appreciation of the complexities of cell phe-
nomena and the difficulties of formulating them in general
terms.
With a realization of the intricacies of the phenomena
involved and that they are cell phenomena, we may turn to
a consideration of the significance of fertilization.
D. SIGNIFICANCE OF FERTILIZATION
It may be emphasized again that fertilization is not repro-
duction. Reproduction, in the final analysis, is division -
cell division or the detachment of a portion of the substance
of a living organism to constitute another. Rather is fer-
ORIGIN OF THE INDIVIDUAL 243
tilization a phenomenon associated with reproduction — so
closely associated in nearly all organisms that the two pro-
cesses, evolving together from relative simplicity to great
complexity, have reciprocally influenced one another until in
higher forms they seem to be related as cause and effect, and
reproduction becomes dependent on fertilization. With this
somewhat didactic statement of our viewpoint, we may con-
sider some of the salient features of the almost endless discus-
sion of the significance of fertilization.
Quite naturally the original view, emphasized by Harvey
and a long series of successors, was that fertilization funda-
mentally is a reproductive process, and echoes of this idea
are preserved in certain present-day hypotheses, on the basis
of such facts as the following. The mature egg pauses in de-
velopment and usually comes to naught unless fertilized —
the entrance of the sperm affording a necessary stimulus for
the resumption of cell division which is to transform the egg
into the adult. . Again, the egg typically contains only half
the somatic chromosome complex (simplex group, haploid
number) and most of the cytoplasm, while the sperm con-
tributes a reciprocal haploid set of chromosomes; in short,
seemingly transforms what is essentially a half into a whole.
However, it does not necessarily follow from these facts
that fertilization is primarily a reproductive process. The
evidence against this conclusion is derived largely from the
relations of fertilization and reproduction in the Protista, and
trom the development, in certain cases, of eggs without
fertilization, or by PARTHENOGENESIS. A single example of
each class of facts will suffice.
1 . Protista
The life histories of nearly all Protozoa and Protophyta
which have been carefully studied include a period in which
244 FOUNDATIONS OF BIOLOGY
fertilization, or SYNGAMY, occurs. Under usual conditions,
Paramecium, for instance, reproduces by fission two or three
times a day so that in a remarkabty short period the one cell
is replaced by a host of descendants. Sooner or later, how-
ever, the individuals exhibit a tendency to unite temporarily
in pairs, or CONJUGATE. In this process complicated changes
take place in the nuclei of the cells, during which, after so-
called maturation phenomena, two pronuclei are established
in each individual of the pair of conjugants. Then one of the
pronuclei in each conjugant migrates over and fuses with the
stationary pronucleus of the other to form a synkaryon, or
fertilization nucleus, in each cell. After this the two Para-
mecia separate, reconstruct their characteristic vegetative
nuclear apparatus, and proceed to reproduce by division as
before. (Fig. 130.)
This is fertilization in Paramecium, and it is generally con-
ceded that the primary significance of synkaryon formation
must be sought among unicellular forms of which this is an
example. Accordingly a large amount of experimental
breeding has been carried out on Paramecium and its allies.
The earlier results seemed to demonstrate conclusively that
Paramecium can divide only a limited number of times, say
a couple of hundred, after which the cells die from exhaustion
or SENILE DEGENERATION, unless conjugation takes place.
In other words, it was believed that periodic REJUVENATION
by fertilization is a necessity for the continuance of the life
of the race. And therefore, so the natural conclusion ran,
protoplasm is unable to grow indefinitely; there is an in-
herent tendency for the destructive phases of metabolism
to gain ascendency over the constructive, and fertilization
serves to maintain or restore the youthful condition and
thus secure the continuance of the race.
In this connection, the life history of Paramecium from one
ORIGIN OF THE INDIVIDUAL
245
B
1)
G
FIG. 130. — Diagram of the nuclear changes during conjugation in Paramecium
aurelia. A, union of two individuals along the peristomal region; B, degeneration of
rnacronucleus and first division of the micronuclei; C, second division of micronuclei;
D, seven of the eight micronuclei in each conjugant degenerate (indicated by circles)
and disappear; E, each conjugant with a single remaining micronucleus; F, this nucleus
divides into a stationary micronucleus and a migratory micronucleus — the gametic
or pronuclei. The migratory micronuclei are exchanged by the conjugants and fuse
with the respective stationary micronuclei to form the synkarya. This is fertilization.
G, conjugants, with synkarya, separate (only one is followed from this point); //, first
division of synkaryon to form two micronuclei; I, second reconstruction division;
/, transformation of two micronuclei into macronuclei; K. division of micronuclei ac-
companied by cell division; L, typical nuclear condition restored.
246 FOUNDATIONS OF BIOLOGY
conjugation to the next is often compared to the life of a
multicellular organism from its origin as the fertilized egg
through youth and adult life to old age. The striking dif-r
ference is that, in the case of Paramecium, the products
of division of the animal which has conjugated (EXCONJU-
GANT) separate as so many independent cells, all of which are
alike and, in later generations, capable of conjugation; while
all the products of division of the fertilized egg of multi-
cellular forms remain together as a unit and become differ-
entiated for particular functions in the individual, except a
few, the germ cells, which retain the power of forming new
individuals. Pushing this comparison a little further, if
somewhat fancifully, it is stated that after conjugation in
Paramecium we have the period of greatest cell vigor, or
youth, followed by maturity when the cells are ripe for con-
jugation again, and in the absence of conjugation — and only
then — the onset of old age, and death. Thus death has no
normal place in the life history of Paramecium, for all the cells
at the period of maturity are capable of conjugation. On the
other hand, in multicellular forms only some of the cells, the
germ cells, retain this power — the somatic cells have paid
the penalty of specialization and must die. Thus death of
the individual except by accident does not occur among uni-
cellular forms because fertilization 'rejuvenates' the cell, and
the cell and the individual are one and the same. With the
origin of multicellular forms, involving the segregation of
soma from germ, death became possible, and was established —
it is the 'price paid for the body.7 (Figs. 115, 135.)
Suggestive as is this comparison and contrast — and it is
not without some justification — the cardinal fact remains
that recent work has demonstrated that Paramecium, under
favorable environmental conditions, can continue reproduc-
tion indefinitely; at least for fourteen years and some ten
ORIGIN OF THE INDIVIDUAL 247
thousand generations, without conjugation and without any
signs of degeneration. In other words fertilization is not a
necessary antidote for inherent senescence, and this, taken
in connection with other data wrhich point in the same direc-
tion, such as the fact that in many plants sexual propagation
is seldom if ever resorted to, renders it fairly safe to make the
general statement that the need of fertilization is not a pri-
mary attribute of living matter. Now, reproduction is such
an attribute and therefore the evidence at hand indicates
that reproduction and fertilization are intrinsically separate
processes which, however, have become closely associated,
especially in higher forms.
So far our conclusion is entirely negative — fertilization
is not reproduction and is not intrinsically necessary for re-
production. What then is its significance? Though ferti-
lization may not be necessary in the life of simple organisms
under favorable conditions, this does not indicate that it
may not be a stimulus to protoplasmic activity when it does
occur — perhaps a very important factor under special en-
vironmental conditions. Indeed there is no doubt that con-
jugation in certain cases directly results in stimulating all the
vital processes of the cell including reproduction. But it
would seem that the essential factor in this stimulation is
not the essence of fertilization — • synkaryon formation. In
Paramecium, for example, an internal nuclear reorganization
process known as ENDOMIXIS occurs periodically. Although
endomixis is carried on by each cell without the cooperation
of another cell, and therefore without synkaryon formation,
nevertheless it apparently effects a physiological stimulation
similar to that which follows synkaryon formation during
conjugation. This is the special aspect which the discussion
of the 'dynamic' effect of fertilization in Protista has recently
assumed. (Fig. 131.)
248
A
B
C
D
E
F
G
H
FOUNDATIONS OF BIOLOGY
FIG. 131. — Diagram of the nuclear changes during endomixis in Paramecium aurelia.
A, typical nuclear condition; B, degeneration of macronucleus and first division of
micronuclei; C, second division of micronuclei; D, degeneration of six of the eight
micronuclei; E, division of the cell; F, first reconstruction micronuclear division:
G, second reconstruction micronuclear division; H, transformation of two micronuclei
into macronuclei ; /, micronuclear and cell division; J, typical nuclear condition
restored.
ORIGIN OF THE INDIVIDUAL 249
2. Metazoa
Turning from Paramecium and its allies, we may consider
some evidence among higher forms in regard to the 'dynamic'
influence of fertilization. Although fertilization is usually
necessary for the resumption of the series of cell divisions
which paused after the maturation divisions, and which are
to transform egg into adult, there are many exceptional but
entirely normal cases where the egg proceeds to divide of
its own accord. Such parthenogenetic eggs are formed like
other eggs, though sometimes without synapsis and there-
fore without chromosome reduction. Thus the eggs of the
Honey Bee, to cite the most interesting case, develop either
with or without fertilization — fertilized eggs forming fe-
males and unfertilized eggs, males. Certain species of Roti-
fers and Round Worms apparently reproduce solely by
parthenogenesis, males not being known. Leaving out of the
question the effect on the chromosome complex, it is at once
apparent that the mere fact that an egg divides without the
influence of a sperm indicates clearly that, in such cases at
least, neither structural additions nor physiological influences
of the sperm are necessary to initiate development.
It may with justice be urged, however, that such cases of
normal parthenogenesis are special adaptations to peculiar
conditions in which the egg has usurped, as it were, the usual
sperm function, and that therefore the evidence is of little
weight in determining the primary significance of fertiliza-
tion. Accordingly the data from so-called ARTIFICIAL
PARTHENOGENESIS are particularly cogent. Within recent
years it has been found that the eggs of a considerable num-
ber of Invertebrates and even of Vertebrates, such as some
Fishes and Frogs, which normally require fertilization, can
be induced to start development 'parthenogenetically' by
250 FOUNDATIONS OF BIOLOGY
various artificial means such as subjection to certain chemi-
cals, unusual temperature changes, shaking, or the prick of a
needle — the effective stimulus varying with different species.
Just what happens in the egg as a result of such treatment
is open to discussion, but for our purposes it is sufficient to
know that the egg begins to divide in normal fashion. This
shows conclusively that even eggs which normally require
fertilization are intrinsically self-sufficient at least to start to
develop, and therefore this strongly indicates that an inci-
dental and not the main function of fertilization is to stimu-
late cell division.
Restating the evidence in its bearings on the meaning of
fertilization, we may say that conjugation, under suitable
environmental conditions, is not fundamentally an indis-
pensable event in the life history of the Protozoa, and further
that whatever stimulus is associated with fertilization is
also provided by endomixis which does not involve synkaryon
formation. Similarly in the Metazoa, both normal and arti-
ficial parthenogenesis indicate that the egg itself comprises
a mechanism which is capable of initiating and carrying on
development. From this viewpoint, fertilization may be
satisfactorily interpreted as a means of insuring under special
or unfavorable environmental conditions in unicellular or-
ganisms, and under usual conditions in the eggs of multicellu-
lar forms, a suitable stimulus which otherwise might be un
available at the proper time.
Granting then that one aspect of fertilization is 'dynamic,'
what is its main significance? Many lines of evidence at
present are slowly but surely converging toward the view that
the opportunities which fertilization affords for changes in
the complex of the germ are of paramount importance.
Fertilization establishes new duplex groups of hereditary
characters by combining diverse simplex groups from the
ORIGIN OF THE INDIVIDUAL 251
two gametes. Careful studies show, in Paramecium for in-
stance, that variation is greater after than before fertilization,
and therefore that the chief significance of the process is to
afford new combinations, some of which will more effectually
meet — be better adapted to — the exigencies of the environ-
ment, and so have a survival value for the organism in the
struggle for existence. So whatever the primary meaning of
fertilization may be, its importance in establishing the essen-
tially dual nature of every sexually produced organism is
settled beyond dispute, and it is the cardinal fact of heredity.
It may seem strange that such a fundamental phenomenon
and one so generally distributed throughout the animal and
vegetable kingdom should so long have eluded solution. The
truth probably is that therein lies the secret of the difficulty.
Whatever fertilization may have been originally, it is no
longer a simple process, but has undergone evolutionary
specialization hand in hand with that of other functions and
with the structure of organisms. To-day one or another of
its various aspects — rejuvenation, stimulus to development,
control of variation, or basis of biparental inheritance — •
may assume the chief role or, at any rate, loom largest in the
mind of the student. The popular idea that fertilization is
reproduction is solely due to the fact that in higher organ-
isms, if fertilization is to occur at all, it must take place at
that period in the life history when the individual is but a
single cell detached from the parent — that is, at repro-
duction.
E. ORGANIZATION OF THE ZYGOTE
The new individual, established by the orderly merging of
a cell detached from each parent in sexually reproducing
species, has before it first of all the problem of assuming the
adult form by a complicated developmental process. As we
252 FOUNDATIONS OF BIOLOGY
have shown, this involves cleavage of the egg, followed, in the
Metazoa, by blastula and gastrula stages during which the
primary germ layers are established — the fundament out
of which the definitive form, organs, and organ systems of the
adult are evolved. The description and comparison of these
processes in different organisms constitute the content of one
aspect of EMBRYOLOGY. We must be satisfied merely with the
realization of the fact that animal development, though it
varies widely in producing the immensely diverse body forms,
exhibits throughout a thread of similarity in its broader
fundamental features. (Figs. 19, 69.)
But embryology is something more than the description
of the kaleidoscopic series of stages which seem to melt one
into the other as development progresses. It attempts, espe-
cially at the present time, to look below and beyond structure
to the processes involved, and to determine how the sequence
of events is brought about. This is but a repetition of the
stages of progress in all science; a passage from the descrip-
tive to the experimental. Although many of the results thus
far secured are necessarily largely tentative, they have gone
far toward placing the science of biology as a whole on an
experimental basis.
From what the pioneer students of embryology during the
seventeenth and eighteenth centuries saw, or thought they
saw, with simple lens and newly invented compound micro-
scope, there were gradually formulated two opposing views
of development which, though long since swept aside in their
original form as a result of the increase of knowledge, raised
the problem of problems that is still before the embryologist
to-day. In brief, one view virtually denied development by
maintaining that the adult organism is nearly or completely
formed within the germ, either in the egg or the sperm, which
merely by expansion, unfolding, and growth gives rise to the
ORIGIN OF THE INDIVIDUAL 253
new generation. In this first crude form the PREFORMATION
theory demanded the 'encasement' of all future generations
one within another in the germ of existing organisms, so that
when it was computed that the progenitor of the human race
must have contained some two hundred million homunculi
(a conservative estimate, to say the least) the reductio ad
absurdum was irresistible.
But careful studies on the transformation of the Hen's egg
into the chick soon made it clear that the chick is not pre-
formed in the egg. The embryo arises by a gradual process
of progressive differentiation from an apparently simple
fundament — it is a true process of development or EPI-
GENESIS. So the upholders of epigenesis versus preforma-
tion were before long beyond their depth and in danger of
attempting to get something out of nothing — lost in the
miraculous!
A statement in such succinct form tends to accentuate the
crudities of these two conflicting views — " pref ormation ex-
plaining development by denying it and epigenesis explaining
development by reaffirming it" — and it may be well to re-
mark that the early embryologists with the means at their
command faced a stupendous task of which only recent work
has brought a full appreciation.
The path to progress cleared by the realization that adult
structures are not preformed as such in the egg, and that
development is not an expansion but the formation — the
'becoming' — by an orderly sequence of events of structures
of great complexity out of apparent simplicity, the problem
of the embryologist was to determine what the egg structure
is and how related to that of the adult. To trace the develop-
ment of these studies would involve the history of embryology
since the formulation of the cell theory. We must confine
ourselves to the bare statement of the new guise in which the
254 FOUNDATIONS OF BIOLOGY
old theories of preformation and epigenesis confront us to-
day as a result of recent research.
The reader already recognizes the fertilized egg as a cell,
with its nucleus comprising a complex of quite definite ele-
ments — the chromosomes — contributed jointly by the two
gametes. To this extent, then, the nucleus and therefore the
egg exhibits a ready-formed structural basis which (as we
have alreadjr suggested, and will have occasion to elaborate
later) seems to be definitely related to characters which
appear in the offspring.
Turning to the egg cytoplasm, we are confronted with
conditions which are not so uniform but nevertheless highly
suggestive. In the first place, before fertilization the egg
possesses a definite polarity, expressed, for example, in the
position of the nucleus and the distribution of food material
(yolk), pigment granules, and vacuoles. This polarity is
traceable, in part at least, to the polarity of the oogonia, and
through them to the germinal epithelium. In brief, the egg
as a whole is organized', the invisible organization of the
fundamental matrix of the cytoplasm being revealed, in part,
by the disposition of various elements of the cell. Now in
some cases this cytoplasmic organization remains essentially
undisturbed at fertilization, and persists as that of the
zygote, while in others it is superseded sooner or later by a
reorganization which establishes that of the new organism.
Herein, apparently, is to be sought the explanation of the
difference in behavior — in potentialities — of various types
of eggs during cleavage stages. Clear-cut examples of the
two chief types will serve to bring the main facts before us.
The first type is well illustrated by the egg of a Mollusc,
Dentalium, and a primitive Chordate, Cynthia. The egg of
the latter shows at the first division five clearly differentiated
cytoplasmic regions. For the sake of simplicity these may
ORIGIN OF THE INDIVIDUAL
255
G
Fia. 132. — Development of a Mollusc (Dentalium) , after removal of the 'polar
lobe'. A, egg, shortly after being extruded and before maturation is completed, show-
ing three differentiated regions. B, section through an egg after fertilization, showing
cytoplasmic rearrangement involving the segregation of clear ' polar lobe ' at p. C, nor-
mal eight cell stage with 'polar lobe' in cell D; D, normal sixteen cell stage, with materi-
als of polar lobe now in cell X; E, sixteen cell stage, from an egg with the 'polar lobe'
removed at the first division; F, normal larva at end of twenty-four hours; G, larva
(abnormal) of same age developed from egg from which 'polar lobe' was removed;
H, normal larva of seventy-two hours. /, abnormal larva of same age from'lobeless*
egg. (From Kellicott, after Wilson.)
256 FOUNDATIONS OF BIOLOGY
be described as hyaline, light and dark gray, and light and
dark yellow. As cleavage proceeds, these substances are
distributed with great regularity to definite cell groups,
which in turn form special organs or organ systems of the
animal. Thus cells which receive the hyaline region form
the ectoderm; those which receive the dark gray, the endo-
derm; while the cells with light or dark yellow form meso-
dermal structures, 'and so on. And further, the experimental
removal of a cell or cell group in which a certain substance
is segregated results in an embryo deficient in the very
structures which this normally forms. In other words, the
egg cytoplasm seems to be a mosaic of 'organ-forming sub-
stances,' which either themselves directly, or through more
fundamental conditions of which they are but the visible
expression, have a causal relation to definite adult structures.
Just in so far as this is true, the adult is predelineated in bold
lines, though not actually preformed, in the egg. (Fig. 132.)
Passing now to the second type, represented by the eggs of
Amphioxus and the Sea Urchins, the results which we obtain
seem to be diametrically opposite. Although in the egg
of the Sea Urchin more or less clearly differentiated cyto-
plasmic regions appear to exist, the removal of a part of the
egg before division, or of one or more cells during cleavage,
blastula, or gastrula stages, has no permanent effect on the
structural integrity of the developing embryo. Experi-
ments show that each of the cells, even as late as the sixteen-
cell stage, has the power to develop into an embryo complete
in every respect, but smaller than the normal. Or, to put it
another way: at the sixteen-cell stage, a single cell which
normally forms, let us say, one-sixteenth of the embryo, if
isolated with two other cells, will form one third of a normal
embryo; if isolated with three other cells, will form one
quarter; and so on. What now has become of the egg
ORIGIN OF THE INDIVIDUAL
257
organization? Or, if we lean toward a mechanistic inter-
pretation of development or life, what kind of a 'machine'
is it which has such potentialities? (Fig. 133.)
At first glance the behavior of these two classes of eggs
seems to afford results which are irreconcilable — the former
A B
Fia. 133. — • Diagram to show development of whole eggs and isolated cells of the
two cell stage. A, Dentalium; at the left, development of the whole egg; at the right,
development of the first two cells, when separated, into two abnormal larvae. B, Am-
phioxus; identical experiment at the two-cell stage resulting in two perfect small larvae.
(From Wilson.)
supporting in a refined form the perennial doctrine of pre-
formation, and the latter its antithesis, epigenesis. But an
explanation is not far to seek. The difference apparently
depends upon the time when differentiation of the egg cyto-
plasm is chiefly established. If this occurs before or at
fertilization, so that the early divisions give rise to dissimi-
258 FOUNDATIONS OF BIOLOGY
larly organized cells, then each of the cells is not equipotent
and the mosaic type of development results; but if the initial
differentiation is delayed until later, or is relatively slight so
that the cells of the early stages are all essentially similar,
then during this period each cell is totipotent — the whole
forms an equipotential system — as exhibited by the early
stages of the Sea Urchin. Thus we may bring under one
viewpoint the apparently paradoxical behavior of the two
classes of eggs, for it turns out to be reducible to the common
factor, differentiation. In one case this has progressed
further than in the other during the early embryonic stages.
In both cases, therefore, development is epigenetic in its
obvious features. (Fig. 134.)
However, since cytoplasmic differentiation is a fact
whether it appears early or late, we have merely pushed the
solution of the problem further back and the question be-
comes: Is there a primary differentiation and, if so, where?
It is not possible to present here the specific evidence on
this point, but the reader's knowledge of the nucleus, and
particularly its definite chromosomal architecture, will lead
him to anticipate that modern research tends more and more
to emphasize the chromosome as representing a material
configuration — a packet of chemicals, may we say — which
is transmitted, in a way, 'preformed' from generation to
generation and determines the cytoplasmic characteristics of
the cells. As to how the specific physical basis of inheritance,
constituting the chromosomes, is related to cytoplasmic
organization and to characters which arise later, we can offer
no satisfactory explanation or even guess. We must be
content with a discussion, in the next chapter, of some of the
salient facts of heredity and their definite association with
certain chromosome arrangements.
But in so far as the nucleus possesses an organization
ORIGIN OF THE INDIVIDUAL
259
which is definitely related to differentiations of the cyto-
plasm, 'organ forming substances,' or characters of embryo
and adult, we may look upon the chromatin to this extent as
D E
FIQ. 134. — Diagram of zones of cytoplasmic differentiation and their distribution
at the first division of the egg. A, immature egg, assumed to have no definite segrega-
tion of cytoplasmic stuffs; B, mature egg, with cytoplasmic zones established; C, first
division of egg; D and E, two types of two-cell stages; D, Dentalium or Cynthia type,
with one cytoplasmic zone entirely distributed to one of the cells, and therefore each of
the two cells, if separated, gives rise to an abnormal larva; E, Echinoderm or Amphioxus
type, with equal distribution of the zones to both cells, and therefore, if separated, each
of the two cells gives rise to a normal larva. (After Wilson.)
representing a sort of primary preformation which is real-
ized by a process of building up — epigenesis — as one char-
acter after another becomes established in the development
of the individual. This is the guise in which the old problem
of preformation versus epigenesis faces the biologist to-day.
260 FOUNDATIONS OF BIOLOGY
The early embryologists were right when, watching the egg
develop into the chick, they maintained that development is
development and not merely an unfolding of an organism
already fashioned in more or less definite adult form. But
it took two centuries of research to reveal the fact that,
below and beyond its superficial aspects, there is a germ of
truth in the principle of preformation hidden in the nuclear
architecture — that the origin of the individual, though
obviously through epigenesis, is fundamentally from a sort of
preformed basis. We no longer bother ourselves with the
old conundrum as to which is more complex, the hen or the
egg, but recognize the fact that each is complex in its way—
the simplicity of the egg being more apparent than real as
is attested by every endeavor to analyze cytoplasm, nucleus,
chromosomes, chromatin, and beyond.
CHAPTER XVII
HERITAGE OF THE INDIVIDUAL
The entire organism may be compared to a web of which
the warp is derived from the female and the woof from the
male. — Huxley.
THE old adage that ' like begets like ' expresses the general
fact of HEREDITY. Every one recognizes that parent and
offspring agree in their fundamental characteristics or 'be-
long' to the same 'species.' And every one realizes that the
resemblance may be strikingly exact even in details of form
or behavior. Family traits crop out. The mere statement
of striking resemblances among the individuals of a family
is a tacit admission that no two individuals are exactly alike;
in other words heredity is "organic resemblance based on
descent " - inheritance of the characters exhibited by the
parents is not complete, there is VARIATION. Indeed "varia-
tion is the most invariable thing in nature," but one must
guard against the impression that there is an antithesis
between heredity and variation. "Living beings do not
exhibit unity and diversity, but unity in diversity. In-
heritance and variation are not two things, but two imperfect
views of a single process."
We must now address ourselves to the problems of heredity
and variation which are at the basis not only of what organ-
isms have been in the past and are at the present, but also
of whatever the future may have in store for them. Varia-
tions are the raw materials of evolutionary progression or
regression. From a broad point of view, the origin of
261
262 FOUNDATIONS OF BIOLOGY
species and the origin of individuals are essentially the same
question. If we can solve the relations of parent and off-
spring, the origin of species will largely take care of itself. As
a matter of fact, historically the question of species origin was
approached first, and through the work of Darwin became
of paramount interest in the latter half of the nineteenth
century. The twentieth century finds the individual — the
genetic relation of parent and offspring — the center of
investigation, and it forms the science of genetics. OR-
GANIC EVOLUTION established the general fact that all or-
ganisms are related by descent; GENETICS attempts to show
how specific individuals are related.
Even further has the pendulum swung from the general
to the particular. To-day the most intense investigation is
centered not on the heritage of the individual as a whole,
but on particular characters of the individual. The concept
has arisen from recent experimental work that, for practical
purposes, the individual may be regarded as congeries of
UNIT CHARACTERS, both structural and physiological, which
are more or less stable, and which are inherited as units.
But the analysis does not stop even at this level. There
seems to be good reason to believe that each so-called unit
character is represented in the chromosomes of the germ ceils
by a definite factor, determiner, or, as it is now usually
termed, GENE; and whether or not a given character will be
present in a tree or a man depends upon whether the gene for
this particular character entered into the nuclear complex of
the fertilized egg which formed the individual. Therefore,
geneticists are busy plotting the relative positions which
these genes occupy on certain chromosomes and how they
may ' cross-over' from one chromosome to the other of a
synaptic pair.
Although at present we are apparently at the threshold of
HERITAGE OF THE INDIVIDUAL 263
great advances, in knowledge of the underlying factors of
heredity, the data already accumulated are so vast that we
can attempt no more than to indicate the character and
promise of the principles already discovered.
We may survey the field before us by a concrete example.
A score of years ago, just at the opening of the modern con-
centrated attack on genetic problems, an association of Brit-
ish millers awoke to the fact that some active means must be
taken to offset the increasingly great deficiency in quantity
and quality of the wheat yield. Accordingly they com-
missioned a specially trained biologist to investigate the mat-
ter. He collected many different varieties of domestic and
foreign wheat, each known to have one or more good qualities,
and studied how these were inherited. Making use of the
data thus secured, in the course of a few years he produced a
wheat which combined the good qualities of several varieties;
including high content of gluten, beardlessness, immunity to
rust, and large yield. And this 'made to order' wheat has
proved successful in the British Isles. But with the opening
up of new territory in western Canada another obstacle was
encountered: the growing season was too short for the finest
varieties of wheat. This contingency was quickly met by
transferring the quality of early ripening from an inferior
grade of wheat to a wheat possessing several valuable charac-
ters.
In a similar fashion, a host of workers have performed the
impossible of a few years ago. Corn of desirable percentage
content of starch or sugar; cotton with long fibers of exotic
varieties and quick maturing qualities to escape insect
ravages; sheep combining choice mutton qualities of one
breed with the fine wool of another and the hornlessness of
a third, and so on almost ad infinitum. Furthermore, there
is no end in sight of the new stable races of plants and
264 FOUNDATIONS OF BIOLOGY
animals which are forthcoming as the principles already
known are applied, and subsidiary ones are discovered. And
last but not least, Man has begun to study himself as a prod-
uct of breeding and the process of evolution — to determine
the distribution of characters in the family, and the conse-
quences of their combinations in the physical and mental
make-up of the individual.
A. HERITABILITY OF VARIATIONS
What then are the basic principles of heredity which are
to-day at the command of the scientific breeder? To answer
this question it is necessary to go into some details because
no real appreciation of the underlying principles involved is
otherwise forthcoming. Most of these details have been ac-
quired through patient investigations made from the standpoint
of so-called pure science — one more proof of the indebtedness
of the 'practical man of affairs' to the biological laboratory.
In the Protista the problems of heredity confront us
in their simplest, though by no means simple, form. Para-
mecium, as we know, divides into two cells which through
growth and reorganization soon are to all intents and
purposes replicas of the parent cell. The parent has merged
its individuality into that of its offspring. Thus stated, one
does not wonder that parent and offspring are alike — each
is composed of essentially the same protoplasm. But when
we come to multicellular forms in which reproduction is
restricted to special germ cells which involve fertilization,
confusion is apt to arise unless one keeps clearly in mind -
and perhaps exaggerates for the sake of concreteness — the
distinction between germ and soma which has been
previously discussed. Since in higher forms, to which brevity
demands that our attention be confined, the sole connection
between parent and offspring is through the germ cells, it
HERITAGE OF THE INDIVIDUAL
265
follows that this must be the sole path of inheritance. In
other words, whatever characters the body actually inherits
must have been represented by genes in the fertilized egg
Germplasm \ Somatoplasm
FIG. 135. — Scheme to illustrate the continuity of the germplasm. Each
triangle represents an individual composed of qermplasm (dotted) and somato-
plasm (clear). The beginning of the life cycle of each individual is at the
apex of the triangle where both germplasm and somatoplasm are present. In
biparental (sexual) reproduction the germplasms of two individuals become
associated in a common stream which is the germplasm and gives rise to the
somatoplasm of the new generation. This continuity is indicated by the heavy
broken line and the collateral contributions at each succeeding generation by
light broken lines. (From Walter.)
from which it has arisen: and conversely, any characters
which the individual can transmit must be represented in
its germ cells. (Figs. 115, 135.)
1 . Modifications
Every individual organism — a man, for instance — is a
mosaic not only of inherited characters but also of MODIFICA-
TIONS of the soma produced by external conditions during
266 FOUNDATIONS OF BIOLOGY
embryonic development or later. The individual's environ-
ment, food, friends, enemies, the world as he finds it, on the
one hand, and on the other his education, work, and general
reactions to this environment, all have their influence on
body and mind arid determine to a considerable extent the
realization of the possibilities derived from the germ —
what he makes of his endowment. He acquires, let us say,
the strong arm of the blacksmith, the sensitive fingers of
the violinist, or the command of higher mathematics. In
other words, what he is depends on his heritage and what he
does with it. Now, if he does develop an inherited capacity,
.can he transmit to his offspring this talent in a more highly
developed form than he himself received it? Or, must his
children begin at the same rung of the ladder at which he
started and make their own way in the world? This is the
old question of the inheritance of modifications, or so-called
ACQUIRED CHARACTERS. Is the length of the* Giraffe's neck,
to take a classic though crude example, due to a stretching
toward the branches of trees during many successive
generations, with the result that a slight increment has been
gained in each generation and inherited by the following?
We cannot enter into a discussion of the problem here, but
must simply assure the reader that the general consensus of
opinion of biologists is certainly to the effect that modifica-
tions, or changes in the individual body due to nurture, use
and disuse, are not transmitted as such. This conclusion is
held chiefly because there is no positive and much negative
evidence forthcoming, and also because there is no known
mechanism by which a specific modification of the soma can
so influence the germ complex that this modification will be
reproduced as such or in any representative degree. How-
ever, it should be emphasized that biologists in general recog-
nize the potent influence of environment and the organisms'
HERITAGE OF THE INDIVIDUAL 267
reactions to the environment on the destinies of the race,
even though they see, at present, no grounds for a belief that
any specific modification can enter the heritage and so be
reproduced.
In this connection the question of the inheritance of disease
will undoubtedly arise in the reader's mind. But this is really
not a special case. If the disease is the result of a defect
in the germinal constitution, it may be inherited just as any
other character, physiological or morphological. But if the
disease is a disturbance set up in the body by some exigencies
of life or through infections by specific micro-organisms, be-
fore birth or later, inheritance does not occur; though it is
well known that susceptibility or immunity to disease-pro-
ducing organisms — the 'soil' for their development — may be
inherited. It may, however, be suggested in passing that
from the standpoint of the individual born malformed, struc-
turally or mentally, as a result of parental alcoholism or other
obliquities, it probably will not appear of the first moment
that the sins have been visited otherwise than by actual
inheritance.
The whole question of the nonheritabilit}^ of modifications
or acquired characters is a relatively new point of view which
has been fostered by an ever-increasing appreciation of the
details of the chromosome mechanism of inheritance, and the
realization of the essential truth of Weismann's contrast of
the soma and germ. Indeed, Lamarck did not question the
inheritance of acquired characters and made it the corner-
stone of his theory of evolution, while some have even gone
so far as to say that either there has been inheritance of ac-
quired characters, or there has been no evolution. But the
question is not so serious as that, as will be seen later on;
though it obviously is profoundly important from many
viewpoints, biological, educational, and sociological.
268 FOUNDATIONS OF BIOLOGY
2. Combinations
Turning from modifications, which are useless to the geneti-
cist, and concentrating attention on characters which repre-
sent an expression of germinal factors, we see that, in the
final analysis, heredity is germinal resemblance among
organisms related by descent — a consequence of the con-
tinuity of cells by division/ Hereditary differences which
appear in offspring are either COMBINATIONS of ancestral char-
acters — apparently new characters which owe their origin to
recombinations of the germinal factors of old characters — or
MUTATIONS due to fundamental changes in the germinal con-
stitution, possibly in the factors, or genes, themselves. ./
For didactic purposes we may somewhat arbitrarily classify
the obvious hereditary differences following fertilization
which are the .result of recombinations of parental characters
represented in the egg and sperm: that is, cases in which
nothing is apparent which is not clearly related to the condi-
tions expressed in the ascendants. In the first place the off-
spring may exhibit a character, eye color let us say, of one
parent to the exclusion of that of the other — the character
appearing unmodified. This may be termed ALTERNATIVE
inheritance. Or the offspring may seem to be a sort of mosaic
of the characters of its progenitors. Here each parent con-
tributes a certain character but without the exclusion of that
of the other and without blending — the offspring exhibits
MOSAIC inheritance. Sometimes the parental traits seem to
fuse so that the progeny exhibit a more or less intermediate
and different condition, as in the color of the skin of mulat-
toes. Such a result is known as BLENDING inheritance. Or
again, certain characters are transmitted from males solely
to female offspring. This is an example of SEX-LINKED in-
heritance. In still other instances characters of grandparents
which are invisible, or 'latent/ in the parents appear again in
HERITAGE OF THE INDIVIDUAL 269
the progeny. This has long been known as ATAVISM. Finally,
characters of still more remote ancestors may crop out, and
constitute REVERSIONS. (Fig. 136.)
3. Mutations
But quite different results now and then occur. Characters
which have no place in the ancestry appear and are trans-
mitted to the descendants. Sometimes these new inherited
variations are only slight departures from the parental condi-
tion, while in other instances they are quite abrupt. However,
the studies of deVries and others
have led to the realization that
there is no fundamental difference
between the two classes — it is
chiefly one of degree — • and so
we speak of all heritable varia-
tions, which are not the result of
FIG. 136. — Diagram to illustrate
recombinations, as mutations, three types of inheritance which fol-
and contrast combinations and ZZTZttXttZZ
mutations Sharply With modifica- C< blending. (From Conklin, after
Walter.)
tions which are not transmitted to
the offspring and are the results of environing conditions on
the soma during embryonic development or later. The im-
portance of this distinction can hardly be overemphasized
because it makes comprehensible many of the inconsistencies
in earlier work on genetics, as will immediately appear.
B. GALTON'S 'LAWS'
The studies of Galton, a cousin of Darwin, on the inheri-
tance of definite characters open the modern era of scientific
investigations in genetics. In particular, his work on the
inheritance of characters in Man, such as stature and intel-
lectual capacity, is a biological classic judged by the momen-
tous consequences which followed from the discussion it
270
FOUNDATIONS OF BIOLOGY
evoked. As a result of the statistical treatment of data,
Galton formulated two principles of heredity which may be
briefly stated as follows:
Law of Ancestral Inheritance. The two parents contribute
between them, on the average, one half of each inherited
Inches
73 J
72
71
70
69
68
67
64
Mean height ofaU parents rjt '
X
FIG. 137. — Scheme illustrating Galton's law of filial regression, as shown
in the stature of parents and children. The circles represent the height of
graded groups of parents and the arrow heads show the average heights of
their children. The length of the arrows indicates the amount of ' regression '
toward mediocrity. (From Walter.)
faculty; each of them contributing one quarter of it. The
four grandparents contribute between them one quarter, or
each of them one sixteenth; and so on.
Law of Filial Regression. On the average any deviation
of the parents from the racial type is transmitted to the
progeny in a diminished degree; the deviation from the racial
mean being two thirds as great as that of the progenitors.
(Fig. 137.)
HERITAGE OF THE INDIVIDUAL 271
These so-called laws taken by and large undoubtedly
express general truths — offspring inherit much more from
their immediate than from their remote ancestors; and off-
spring of gifted or deficient parents, judged by the average
standard of a mixed population, regress toward mediocrity.
But the ' laws ' are not particular^ helpful in arriving at the
fundamental principles involved in heredity because the data
upon which they are founded include indiscriminately
both heritable variations and modifications. The individ-
ual's somatic characters, which form the data, belie in many
cases the underlying germinal constitution — what will be
transmitted to the progeny. Thus, for instance, experiments
show that when the germinal make-up of all the mem-
bers of a population is the same, the regression is com-
plete, no matter how far the particular parents may diverge
somatically from the population average. The somatic
divergence represents chiefly modifications which are not
inherited. Conversely, when the divergence of the parents
from the population average is due to characters which
represent expressions of their germinal constitution, then
there is no regression.
C. MENDELISM
It was reserved for Mendel to apply statistical methods to
facts observed in the progeny derived from carefully con-
trolled experiments in breeding. In other words, to substi-
tute for 'ancestral generations,' controlled pedigrees — to look
forward as well as backward and thus largely to remove
the unknown and unknowable quantity which rendered the
materials of Galton somewhat delusive. Mendel's studies
actually were made a score of years before Galton's, but failed
to reach the attention of the biological world engrossed in the
evolution theory; in fact were never known to Darwin to
272 FOUNDATIONS OF BIOLOGY
whom they would have meant so much in his work to secure
experimental data in heredity. To-day Mendelism is
essentially a science in itself, with its own vocabulary of
technical terms. We can attempt no more than to make
clear its fundamental features by a few concrete examples;
the first from Mendel's own work.
Mendel chose seven pairs of contrasting, or alternative,
characters which he found were constant in certain varieties
of edible Peas, such as the form and color of the seeds, whether
round or wrinkled, yellow or green; and the length of the
stem, whether dwarf or tall: and these he studied in the
HYBRIDS. One ordinarily thinks of a hybrid as a cross be-
tween two species or, at least, two characteristically distinct
varieties of animals or plants; but as a matter of fact the off-
spring of all sexually reproducing organisms are really hybrids
because two parents seldom, if ever, are exactly the same in
all of their germinal characters. Consequently the offspring
are hybrids with respect to the characters in which the par-
ents differ.
1. Monohybrids
Mendel found, for example, in the cross between the tall
and dwarf varieties of Peas, that all of the progeny in the
FIRST FILIAL (Fi) generation were tall like one parent, there
being no visible evidence of their actual hybrid character.
Accordingly tallness was designated a DOMINANT (D) and
dwarfness a RECESSIVE (d) character. His next step was to
follow the behavior of these characters in succeeding genera-
tions. Therefore the tall hybrids (Fi) were inbred (self-fer-
tilized) and their offspring, the SECOND FILIAL (F2) generation,
were found to be tall and dwarf in the proportion of three to
one (3D :ld). This is now the broadly established MENDELIAN
RATIO. Of course in dealing with a small number of individ-
HERITAGE OF THE INDIVIDUAL 273
138. — Inheritance of size in a cross between a tall and a dwarf race of garden Peas.
(After Morgan.)
274 FOUNDATIONS OF BIOLOGY
uals this proportion is merely approximate; the greater the
number of offspring, the closer it is approached. In this par-
ticular case Mendel obtained 787 dominant and 277 recessive
individuals. (Fig. 138.)
Continuing the work, Mendel found that the dwarfs (reces-
sives) when inbred gave only recessives generation after
generation, and accordingly were 'pure', or EXTRACTED RECES-
SIVES. On the other hand, the tall plants (dominants) when
inbred proved to be of two kinds, one third pure EXTRACTED
DOMINANTS which bred true indefinitely, and two thirds
hybrids like their parents, giving when inbred the same ratio
of three dominants to one recessive in the THIRD FILIAL (F3)
generation.
Aside from his masterly foresight in realizing that success
depended on simplifying the problem by dealing with definite
contrasting characters, Mendel's claim to fame lies chiefly
in his discovery of a simple principle by which the results
may be explained. Since the hybrids when inbred always
give rise to hybrids and also to each of the parental types in a
pure form, it must be that the factors (genes) which deter-
mine the characters in question are SEGREGATED in the germ
cells. That is, some germ cells bear one gene and other
germ cells the other, but one cell never bears both. If we
assume that the germ cells contain genes which determine
the size of the plant — those of the original tall parent con-
taining the gene for tallness (S) , and those of the dwarf parent
the gene for dwarf ness (s) — then the hybrids will arise from a
zygote which combines both genes (Ss), and since tallness is
dominant over dwarf ness all will be tall. Further, when
the germ cells of this hybrid (Ss) mature, if these genes
segregate so that, as a rule, half of the gametes bear S and
half bear s, then when such plants, each with this germinal
constitution, are inbred there will be equal chances for
HERITAGE OF THE INDIVIDUAL
275
gametes bearing the same and for gametes bearing different
genes to meet in fertilization.
The zygotes are 1 SS : 2 Ss: 1 ss. But, since S is dominant,
the resulting organisms
will be in the proportion
of 3 tall to 1 dwarf, which
is the familiar 3: 1 Men-
delian ratio of dominants
to recessives in the F2
generation. The import-
ant point, however, is
that these tall organisms,
although they all appear
alike or, as we now say,
belong to the same
PHENOTYPE, are different
with respect to their germ-
inal constitution; because
one third bear germ cells
all of which contain the
gene S, . and two thirds
bear germ cells half of
which contain S and
the other half s. Conse-
quently the phenotype is
composed of two GENO-
TYPES which are distin-
guishable only by what
they produce. (Fig. 139.)
It is thus apparent why
the pure tall plants (ex-
tracted dominants) al-
ways breed true, and why
FIG. 139. — Diagram of a Mendelian mono-
hybrid. Results of crossing large size (S) and
small (s) Pea plants. The circles represent the
zygotes and the characters of the soma (pheno-
type); the letters within the circles, the ger-
minal constitution (genotype). The letters out-
side the recombination square represent the gam-
etes. Note that each of the parents (P) represents
a different phenotype and genotype; all the FI
(one shown) belong to the same phenotype and
genotype ; while the F2 represent two phenotypes
and three genotypes. The relative number of
individuals composing the Fa phenotypes is 3 : 1 .
276 FOUNDATIONS OF BIOLOGY
the pure dwarfs (extracted recessives) do the same — all the
germ cells of one bear S and those of the other, s. The plants
are, as we say, HOMOZYGOUS with respect to the characters in
question. It is also, clear why the hybrids give rise to
hybrids and extracted dominants and recessives — an equal
proportion of the germ cells bear S and s. The plants are
HETEROZYGOUS.
The real difference then between the F2 hybrids (Ss) and
the extracted dominants (SS) is that the former are heterozy-
gous and the latter are homozygous. In order to tell which is
which, since they are phenotypically the same, it is necessary
to breed them. When self-fertilization can be practiced, as
in the case of most plants, we get the result directly; that is
an individual's progeny are either all dominants or dominants
and recessives in 3 : 1 ratio, and thus the garnet ic constitution
of the parent is immediately known. However, in the case
of animals, where self-fertilization is impossible, the deter-
mination can be made by mating the dominants with reces-
sives, for a homozygous dominant then will give all dominants
while a heterozygous dominant will give half dominants and
half recessives. Thus:
Gametes = D\ /D Dx /d
I \/ I
Gametes dx xd dx xd
Possible zygotes = 100% Dd 50% Dd+50% dd
So far we have considered the inheritance of one pair of
alternative characters — the resultant of a pair of genes
termed ALLELOMORPHS — but if the reader has grasped the
principles involved, we may pass rapidly over cases where
two, three, or more pairs are concerned; that is DIHYBRIDS,
TRIHYBRIDS, and POLYHYBRIDS.
2. Dihybrids
Mendel found the solution to heredity in dihybrids by
HERITAGE OF THE INDIVIDUAL
277
YR
YR
Yr
yR
yr
FIG. 140. — • Diagram of a Mendelian dihybrid — • results of crossing yellow
round seeded (YR) Peas with green wrinkled seeded (yr). The circles
represent the zygotes and the characters of the soma (phenotype) ; the letters
within the circles, the germinal constitution (genotype). The letter groups out-
side the recombination square represent gametes. The hybrids of the Ft
generation are all yellow round seeded since green and wrinkled are recessive.
The FI plants form four types of gametes which affords sixteen possible
types of zygotes, representing four phenotypes (shown graphically) and nine
genotypes (numbered). There is one pure (extracted) dominant (1) and one
pure (extracted) recessive (9). The zygotes numbered 4 are identical with the
FI generation. Four are homozygotes (1, 7, 8, 9) and the rest are heterozy-
gotes. The relative number of individuals composing the phenotypes is
9:3 :3 : 1.
278
FOUNDATIONS OF BIOLOGY
crossing, for example, a Pea producing yellow round seeds
with one producing green wrinkled seeds. The plants in the
Key to Symbols
• = Dark
O = Light
3 = Curly
• = Straight
FIG. 141. — Scheme to illustrate the heredity of human hair characters. Mendelian
dihybrid. Dark and curly, dominant characters; light and straight, recessive charac-
ters. The arcs represent somatic cells of four individuals. The dominant characters are
placed on the outer side of the cells, since they represent the visible characters (pheno-
type). The gametes are placed within the arcs (cf. Fig. 142). (From Walter.)
FI generations bear only yellow round seeds, and therefore
yellow and round are each dominant characters when paired
with green and wrinkled. After self-fertilization such
hybrid plants produce offspring (F2) with seeds showing all
the possible combinations of the four characters, and in the
HERITAGE OF THE INDIVIDUAL
279
proportion of 9 yellow round to 3 yellow wrinkled to 3 green
round to 1 green wrinkled. (Fig. 140.)
This logically can only be interpreted as indicating that
one of the original parent plants bore germ cells all contain-
ing the genes for yellow and for round peas (YR), while the
other parent plant bore cells
all containing the genes for
green and for wrinkled (yr) .
Such being the case, the re-
sulting zygote is YRyr, and
the hybrid which it forms
develops germ cells with all
the possible combinations
of these genes (except, of
course, Rr an.d Yy) which
are YR, Yr, yR, and yr.
Now, in turn, at fertiliza-
tion there are sixteen possi-
ble combinations of germ
cells, since there are four
different kinds of sperm and
four different kinds of eggs
with respect to the char-
acters in question. Accord-
ingly the F2 generation, .
which is produced by the union of these gametes, is repre-
sented by one extracted dominant (YRYR), one extracted
recessive (yryr) , four (including the former two) homozygotes
and twelve heterozygotes. These sixteen individuals form
nine genotypes but, since only the dominant character is
expressed when dominant and recessive genes combine, they
are resolvable into four phenotypes (YR, Yr, yR, yr) in the
ratio 9 YR : 3 Yr : 3 yR : 1 yr. Thus the 9 : 3 : 3 : 1
Number
in each
class
Genotype
Phenotype
Number
in each,
class
4
fi^\
Dark curly
9
2
©
2
®
1
©
1
©
Dark straight
3
2
©
1
®
Light curly
3
2
/Os\
1
<§)
Light straight
1
16
16
FIG. 142. — Diagram classifying the six-
teen possible types of zygotes, shown in the
middle of Fig. 141, according to genotypes
(nine) and phenotypes (four) . (From Wal-
ter.)
280 FOUNDATIONS OF BIOLOGY
Mendelian ratio for two pairs of contrasting characters is
merely the monohybrid 3 : 1 expanded. Both rest on the
same fundamental assumption that there is an independent
assortment of the genes and that those for alternative char-
acters segregate — both members of a pair of allelomorphs
can never occur in the same gamete. (Figs. 141, 142.)
3. Trihybrids
Similarly, Mendelian trihybrids, for example the cross
between tall Peas bearing yellow round seeds and dwarfs
bearing green wrinkled seeds, give in the F2 generation 27
genotypes and 8 phenotypes; the relative number of indi-
viduals in each phenotype being in the proportion 27 : 9 : 9 :
9:3:3:3:1. Of course, in nature there are few instances
in which parents and offspring differ by only one, two, or three
characters, but since characters arising from each pair of
allelomorphs can usually be treated singly, expediency
demands that the analysis be made with respect to one
or two pairs at a time, which accordingly is the usual
method of procedure. (Fig. 143.)
4. General Principles
Before passing to certain modifications and extensions of
Mendelian principles, it may serve to clarify the subject if we
restate in slightly different form and then summarize the
essential facts thus far discussed on the basis of Mendel's
own work.
Every cell of the soma of an individual bears a pair of
genes for each 'unit' character (e.g., size in the case of the
garden Pea), one member of each pair having been derived
from each gamete which contributed to the individual's make-
up. When both genes are identical (e.g., either SS or ss) they
are expressed in the soma (e.g., the plant is tall or dwarf).
The individual is homozygous with respect to size. But when
HERITAGE OF THE INDIVIDUAL
281
SYR sYR SyR syR SYr
sYr
Syr
syr
SYR
sYR
SyR
syR
SYr
eYr
Syr
syr
FIG. 143. — Diagram of a Mendelian trihybrid. Results of crossing tall Peas bearing
yellow round seeds (SYR) with dwarf Peas bearing green wrinkled seeds (syr). The
circles represent the zygotes and the characters of the soma (phenotype) ; the letters
within the circles, the germinal constitution (genotype). The letter groups outside the
recombination square represent the gametes. The Fi hybrids form eight types of game-
tes, giving sixty-four possible types of zygotes, representing eight phenotypes (shown
graphically) and twenty-seven genotypes. There is one pure (extracted) dominant
(upper left corner) and one recessive (lower right corner). Eight are homozygotes
(diagonal from upper left to lower right corner) and the rest are heterozygotes. The
zygotes in the diagonal from upper right to lower left are identical with the ^i generation.
The relative number of individuals composing the phenotypes is27:9:9:9:3:3:3:l.
282 FOUNDATIONS OF BIOLOGY
the two genes are not identical (e.g., S and s), then one, the
dominant (S), is expressed in the soma (the plant is tall),
while the other, the recessive (s), is not expressed. The indi-
vidual is heterozygous with respect to the character in ques-
tion (e.g., size).
At the maturation of the germ cells of the individual, an in-
dependent assortment, or segregation, of the genes occurs so
that the gametes bear only one gene (e.g., either S or s) for
each unit character. Thus the gametes of homozygous indi-
viduals are all alike with respect to the gene in question (e.g.,
all bear S or s) , while the gametes of heterozygous individuals
are of two numerically equal classes (e.g., half bear S and the
other half bear s).
UNIT CHARACTERS. From the standpoint of heredity an
individual organism may be regarded as comprising a com-
plex of single characters, each of which, broadly speaking,
behaves essentially as a unit.
DOMINANCE. When the determining genes (allelomorphs)
for each of a pair of alternative characters are present in the
zygote, one (the dominant) is expressed in the resulting indi-
vidual; although the other (the recessive) is also present in all
of its somatic and in one half of its mature germ cells. In
other words, the recessive is not expressed unless it is present
in duplicate.
SEGREGATION. The genes for each of a pair of alternative
characters are never both present in the same gamete. There-
fore the ripe germ cells of hybrids fall into two numerically
equal classes : in one the gene of the dominant character and
in the other the gene of the recessive character is segregated.
This is the so-called purity of the germ cells.
D. NEO-MENDELISM
It so happens that, as data accumulate, it becomes more
HERITAGE OF THE INDIVIDUAL 283
and more apparent that exceptions which prove the rule,
make it necessary to revise somewhat our ideas regarding the
unity of unit characters and the dominance of dominants, and
to accentuate the principle of segregation as the prime Men-
delian contribution. A few examples will serve to bring the
main facts before us.
The seven pairs of contrasting characters in Peas which
Mendel studied showed essentially complete dominance of
one character in each pair, and therefore, quite naturally, he
laid stress on this principle. As a matter of fact we may
say that dominance is hardly the rule because there are in-
numerable cases in which the hybrid (Fi) shows a different
condition from either of the parents. For instance, on cross-
ing homozygous red and white races of the Four-o'clock, all
the progeny in the heterozygous (Fi) generation bear pink
flowers, or, we may say, flowers intermediate in color between
the two parents. Neither red nor white is dominant. But in-
breeding these give an F2 of 1 red, 2 pink, and 1 white. Thus
the typical Mendelian 3 : 1 ratio is, so to speak, automati-
cally resolved into the 1:2:1 ratio which, when one character
is dominant, is only patent on further breeding. (Fig. 144.)
In the case of the Four-o'clock, only the hybrids are inter-
mediate; segregation occurs as usual and the homozygous
progeny show the original parental characters unmodified.
But sometimes, with the apparent lack of dominance, segre-
gation seems not to take place. The cross between white and
black races of Man is a typical example.
The mulatto (Fi) is intermediate in skin color between the
parental types and even in the F2 and later generations rarely
gives pure white or black offspring. But an adequate Men-
delian explanation is not far to seek. It has been found that
both white and black are really composite characters, each
made up of varying amounts of black, yellow, and red pig-
284
FOUNDATIONS OF BIOLOGY
ments. Now, assuming that the full-blooded Negro of Africa
bears two sets of genes for black (AABB) which are absent
(aabb) in the white race; then, since in the germ cells single
genes segregate, the cross of white and black would give only
a single set of genes for black (AaBb) and the hybrid (Fi)
Fi<3. 144. — Diagram to illustrate the results from crossing white and red flowered
races of Four-o'clocks (Mirabilis jalapa). The somatic condition (phenotype) is shown
graphically; the small circles represent the genes which are involved.
would be neither black nor white, but intermediate. Again,
the progeny of these mulattoes, that is the F2 and subsequent
generations, should show different degrees of color, as they
actually do, owing to varying combinations of genes; except
in the small number of cases of extracted dominants (black)
and extracted recessives (white). Therefore the intermedi-
ate color of the offspring of black-white crosses is reasonably
HERITAGE OF THE INDIVIDUAL
285
explained, if we regard the character black as the expression
of at least two pairs of genes, neither of which alone gives
black but only when reinforced by the other. The infrequent
appearance of pure whites or blacks in the F2 and later gen-
erations is not due to lack of segregation, but to the fact that,
since the parental characters have a multiple gene basis, the
chances are slight that in segregation all the separate genes
A B
A b
a B
a b
A B
Ab
a B
a b
A B
A b
a B
a b
A B
A B
A B
A B
A B
A b
a B
a b
A b
A b
A b
A b
A B
A b
a B
a b
a B
a B
a B
a B
A B
A b
a B
a b
a b
a b
a b
a b
FIG. 145. — Recombination square showing the result of mating two
mulattoes, each having the color factors AB and their absence ab — the latter
from their respective white parents. The color of the offspring varies from
black (upper left corner) to white (lower right corner). Compare Fig. 140.
(After Conklin.) ,
will be brought together in a single gamete and further
that such a gamete at fertilization will meet one similarly
endowed. (Fig. 145.)
From experiments with several races of Locusts which
breed true for color pattern, it has been found that the hy-
brids between any two show the entire pattern of each parent,
one superimposed upon the other. Thus, again merely by
inspection, it is possible to determine the parental compo-
nents, and since such hybrids give progeny showing the
1:2:1 ratio, it is evident that the mosaic, instead of blended,
result is due merely to the fact that each of the 'alternative'
characters completely expresses itself.
286 FOUNDATIONS OF BIOLOGY
From these few examples, selected almost at random from
the wealth of data at hand, it is clear that some cases of blend-
ing and mosaic inheritance, as well as alternative inheritance,
can be satisfactorily interpreted on fundamental Mendelian
principles. It is merely necessary to bear in mind that when
speaking of unit characters, we mean that the germinal
physical basis of characters, that is the genes which condition
their development, behave as units, for now we know that
some characters are determined by single genes, and some by
multiple genes. And further, that dominance is a relation
between a pair of genes rather than between their expressions,
characters. Therefore blending inheritance may be merely
an expression of the action of several pairs of genes, each
gene displaying dominance for one member of a pair; while
mosaic inheritance may represent the extreme where each
gene's influence is exhibited to the full in the hybrid.
Within the past few years geneticists have been able by
the MULTIPLE FACTOR hypothesis to bring into line with the
Mendelian interpretation the inheritance of a large number
of characters, especially in the higher animals. Thus stature,
proportions of the parts of the body, build, as well as nearly
all of the physiological and mental characteristics in Man,
are evidently dependent upon multiple genes. This seems so
generally true in the higher animals and plants as to suggest
that their characters arc genetically relatively complex as
compared with those of many of the lower organisms.
So it happens, as is usually the case, the more a
problem is studied the more complex it appears to become.
Suffice it to say that, although our idea of 'unit characters/
'dominance/ and even 'segregation' is to-day somewhat
broader than Mendel conceived on the basis of his classic
experiments, it is evident that he supplied us with fundamen-
tal principles which are affording a common denominator for
HERITAGE OF THE INDIVIDUAL 287
an ever-increasing number of facts in genetics. Only the
future can determine whether they are universal.
E. MECHANISM OF MEND ELIAN INHERITANCE
With this general outline of the Mendelian principles
before us, it is now necessary to bring them into relation with
the facts so far known in regard to the structure of the germ
cells. In other words, we have assumed germinal factors,
or genes, segregation, etc., but has the actual study of cells
(cytology) given any evidence of the physical basis of genes
and of a segregating mechanism? The reader will at once
answer this in the affirmative on the basis of our discussion
of the origin and structure of the germ cells and their behavior
in fertilization. But all, or nearly all, of these cardinal facts
were unknown when Mendel worked and this makes still
more remarkable his prevision in interpreting his results in
the terms he did.
The essential facts may now be restated from a slightly
different viewpoint. The egg and sperm each carry a definite
number of chromosomes and consequently after fertilization
the zygote contains a double set. For each chromosome con-
tributed by the sperm there is a corresponding, or homolo-
gous, chromosome contributed by the egg. In other words,
there are two chromosomes of each kind which may be con-
sidered as pairs. When division of the zygote takes place each
chromosome splits into two chromosomes, so that each
daughter cell receives a daughter chromosome derived from
each of the original ones. Since all the cells of the organism
are lineal descendants by similar mitotic cell divisions, all of
its cells contain the double set of chromosomes — half paternal
and half maternal; and since the primordial germ cells have
a similar origin, they also have a double set of chromosomes.
But during the maturation process synapsis occurs: that is,
288 FOUNDATIONS OF BIOLOGY
homologous chromosomes of paternal and maternal origin
unite in pairs — the process of fertilization which gave rise
to the individual being consummated in the ripening of its
own germ cells. But this union is only temporary; a suc-
ceeding mitosis, instead of dividing each chromosome as
usual, separates the maternal and paternal chromosomes of
each synaptic pair and delivers one of each (though rarely all
of the same maternal or paternal set) to the two arising cells.
Thus each mature germ cell contains one member of every
chromosome pair and the number of chromosomes is reduced
one half.1 (Fig. 146.)
Mendel postulated that the genes for alternative charac-
ters segregate in the formation of the germ cells of hybrids
so that a single gamete bears one and not both genes of a
pair of allelomorphs. That is the genes, which come together
in the zygote which forms the hybrid, separate again in the
formation of its own germ cells. This is just what cytological
studies show. Chromosome behavior exactly parallels the
typical behavior of the Mendelian gene, because in the matu-
ration of the germ cells each chromosome of paternal origin
separates from the corresponding chromosome of maternal
origin. The genes similarly situated on homologous paternal
and maternal chromosomes are allelomorphs and are segre-
gated during maturation. And further, in considering Men-
delian dihybrids we found, for instance, that genes for yellow
and round, and green and wrinkled seeds were inherited in a
fashion which indicated that yellow and round, let us say, are
segregated independently of each other, because all possible
combinations with green and wrinkled occur. This clearly is
1 It will be recalled that in plants exhibiting an alternation of generations, the chromo-
some reduction occurs at the formation of the spores. (Fig. 124.) A little thought
will convince the reader that this difference is of no importance from the standpoint
of the present discussion, because we are interested in inheritance from sporophyte to
sporophyte and can neglect the gametophyte which intervenes.
HERITAGE OF THE INDIVIDUAL 289
oo
ABCD
0°
JL
aBCd.
AbcD
AaBbCcDd
V
FIG. 146. — Diagram of the chromosome cycle of an animal. Somatic (diploid)
chromosome number assumed to be eight. Paternal chromosomes (from sperm) =
ABCD; maternal (from egg) = abcd. T, union of nuclei of gametes, each with a simplex
group (haploid number) of chromosomes, in the zygote at fertilization to form a duplex
group (diploid number) of chromosomes. II, III, IV, somatic divisions or divisions of
germ cells before maturation (duplex groups of chromosomes). V, synapsis, involving
pairing of homologous paternal and maternal chromosomes to give the haploid num-
ber of paired chromosomes. VI, reduction division — separation of pairs into single
chromosomes again. VII, two gametes, with simplex groups (haploid number) of chro-
mosomes; there are 14 more possible combinations of the chromosomes, or types of
gametes, which are not shown. See Fig. 147. (After Wilson, slightly modified.)
290
FOUNDATIONS OF BIOLOGY
fully accounted for, provided the gene for yellow and the gene
for round are not borne by the same chromosome, since in
maturation the gametes secure one of each pair of homologous
FIG. 147. — Diagram to show the union of simplex groups of either the chromosomes
or of the genes of the gametes to form the duplex condition of the zygote and animal
body; and then their pairing at synapsis, and segregation in the gametes. With four
pairs of chromosomes or of genes (Aa, Bb, Cc, Dd) there are sixteen possible types of
gametes. (After Wilson.)
chromosomes (a simplex group), but not necessarily all of
maternal or paternal origin. (Fig. 147.)
In short, when two gametes unite they each contribute
to the zygote two corresponding, simplex groups of genes with
the result that the offspring is of a double, or duplex gene,
constitution. Similarly, the gametes contribute two simplex
chromosome groups so that the zygote is of a duplex chromo-
some constitution. Thus both the chromosomes and the
HERITAGE OF THE INDIVIDUAL 291
characters (genes) are in the simplex condition in the gametes
and duplex in the zygote. This close parallelism of gene
and chromosome behavior affords the most cogent evidence
that the chromosomes supply the physical basis of inheritance,
and that Mendelian segregation and related phenomena are
facts. For all practical purposes A, B, C, D, and a, b, c, d,
in figures 146 and 147 may be interpreted either as chromo-
somes or as characters.
Turning now to the inheritance of characters whose genes
are borne by the same chromosome: these would seem to
be indissolvably linked together; and since the chromosome
number is usually not large — there are twenty-three or
twenty-four in the gametes of Man — compared with that of
heritable characters, we would expect sometimes to find
characters linked together. That is, not separately in-
herited as are yellow and round in our example. In reality
many cases are known in which characters are inherited
in groups. The inheritance of sex and sex-linked characters
will make the main point clear, and at the same time serve
to bring before us the essential facts in regard to the
determination of sex.
1 . Sex Determination
The reader will recall that in the general description of cell
structure it was stated that every cell of an organism contains
a definite even number of chromosomes. As a matter of fact,
in most instances the body cells of one sex, usually the male,
have one more functional chromosome than the 'regular' set,
and therefore an odd number. This extra chromosome,
which is commonly designated the X, or SEX CHROMOSOME,
has no mate at synapsis, remains undivided in the reducing
maturation division, and passes entire to one of the daughter
cells. Thus two classes of sperm are formed, one with and
292 FOUNDATIONS OF BIOLOGY
the other without the X chromosome — half of the sperm
contain an X chromosome.
Furthermore, in species in which the male has the X
chromosome, the female has two of them. The female
therefore has one more chromosome than the male. Thus
during oogenesis the X chromosomes pair in synapsis just
as the other homologous chromosomes, and then one is dis-
tributed to each of the daughter cells, so that all of the eggs
contain an X chromosome. For instance, in Man the somatic
number of chromosomes apparently is forty-seven in males,
or forty-six plus the X chromosome; while the female somatic
number is forty-eight, or forty-six plus two X chromosomes.
Half of the sperm contain 23 and half 24 chromosomes; all
the eggs contain 24 chromosomes.
Since there are equal numbers of sperm with and without
the X chromosome, on the average as many eggs will be
fertilized by one class of sperm as the other, with the result
that half of the zygotes will contain one X and half two X
chromosomes. Obviously the former will develop into
males and the latter into females, since the somatic cells
of males have the X chromosome and therefore the 'sex
gene' in simplex condition, and similar cells of females have
the duplex condition. So it is possible — it has been accom-
plished in several species — to ascertain the sex of an
embryo by counting the chromosomes in its cells. (Fig. 148.)
Thus there is good cytological evidence that sex inheri-
tance follows the Mendelian formula. The male carries one
sex gene (on the single X chromosome) and the female two
sex genes (one on each of the X chromosomes) . At matura-
tion these segregate so that the male is heterozygous and the
female is homozygous in regard to sex, and therefore all
possible combinations of gametes result in the 1 : 1 ratio of
males to females. In passing, we may emphasize that this
HERITAGE OP THE INDIVIDUAL
293
shows that the sex of an individual is usually determined
at the time of fertilization, and not subsequently as most of
the well-known theories contend. But obviously we must
guard against thinking of either the X chromosome or the
'sex gene' as 'producing' sex. Sex is a complex character
whose full development is undoubtedly conditioned by 'sex
MatureEgg Oogonium
sSpermatogon ium
FIG. 148. — Diagram to show the relation of the two classes of sperm in fertilization.
The formation of gametes in the male is shown at the left, in the female at the right;
fertilization, producing the male or female zygote, in the center. X chromosome in
black (After Wilson.)
hormones/ etc., but since the X chromosome is the differen-
tial in the sexes, it is to that extent 'sex-determining.'
2. Linkage
Since sex is regulated by an internal mechanism which
appears to be the same as that which determines the dis-
tribution of characters in Mendelian inheritance, it might be
supposed that the genes of other characters as well are
carried by the X chromosome. As a matter of fact the
behavior in inheritance of certain characters is such that it
can only reasonably be explained on this assumption. Ac-
cordingly such characters are known as SEX-LINKED. This
294
FOUNDATIONS OF BIOLOGY
brings us again to the point at which we digressed to consider
sex — the discussion of genes associated on the same chromo-
some. One example must suffice to bring out the main
facts.
The common form of color-blindness known as Daltonism,
in which the affected individual is unable to distinguish red
from green, has long been known to be inheritable, but in a
F,
xo
d
XX
9
K
XO
X
XX
XO
XO
d
FIG. 149. — Diagram to show the inheritance of color-blindness from the male.
A color-blind male (shown in black) transmits the character to half of his grandsons.
•fc indicates the 'sex' chromosome with the gene for color-blindness. (After Morgan.)
peculiar crisscross way. The condition is transmitted from
a color-blind man through his daughters, who are normal,
to half of his grandsons; and from a color-blind woman to all
of her sons and none of her daughters. This behavior is
readily accounted for if we assume that the gene for color-
blindness is associated, when present, with the gene for sex
on the X chromosome, and that color-blindness develops in
males, just as 'maleness,' when it is simplex or from one
parent, and develops in females when it is duplex, or from
both parents. (Figs. 149, 150.)
HERITAGE OF THE INDIVIDUAL 295
Color-blindness thus serves to illustrate the association
of genes of different characters on the same chromosome and
the association later of their respective characters in the
adult. But the presence of separate genes on the same
chromosome by no means indicates that the genes must
always be distributed together, for there is considerable
evidence that during synapsis genes may reciprocally cross-
XO XX
X K9
XX XO
9 d1
HI
XX XX XO XO
9 9 tf d
FIG. 150. — Diagram to show the inheritance of color-blindness from the female.
4 color-blind female (shown in black) transmits the character to all of her sons, and to
half of her grandsons, and to half of her granddaughters. (After Morgan.)
over from one synaptic mate to the other and thus become
separated from their former gene associates on the same
chromosome. This CROSSING-OVER removes the limitations
which, at first glance, would seern to confine the possible
number of characters capable of independent segregation in
Mendelian inheritance to that of the chromosome number,
and renders invalid any objections to the universality of
Mendelism which are based on the chromosome mechanism
as at present understood. And further, the crossing-over
gives an opportunity to determine the relative positions of
different genes on a chromosome — if it is assumed that the
296
FOUNDATIONS OF BIOLOGY
distance between two genes is proportional to the percentage
of crossing-over which these genes show. (Fig. 151.)
F. NATURE versus NURTURE
From one viewpoint, then, the individual may be considered
as a composite of very many unit characters which behave in a
definite way in inheritance. ''Expressed otherwise, and
Ha
JLc
FIG. 151. — Diagram to show a possible mechanism of crossing-over during
synapsis of homologous paternal and maternal chromosomes. The segments
indicate the assumed linear arrangement of the genes with allelomorphic genes
opposite each other. I, pair of chromosomes which have entered and emerged
from the synaptic state without any crossing-over; I la, chromosomes winding
about each other at synapsis; 116, separation of these chromosomes, involving
breaking at the points of crossing; He, their emergence from synapsis with the
members of the pairs of allelomorphic genes interchanged. (After Wilson.)
somewhat fancifully, individuals are simply temporary
kaleidoscopic combinations of the various determiners (genes)
belonging to the species; the act of reproduction, especially
the reduction division and subsequent fusion, providing the
new turn of the kaleidoscope." But since the life of an
organism is one continuous series of reactions with its sur-
roundings, it follows that nurture plays an immensely im-
portant part in molding the individual on the basis of its
heritage. This is especially true in the case of Man. Devel-
opment is a form of behavior, and how a child develops
HERITAGE OF THE INDIVIDUAL 297
physically and mentally is determined not by its heritage
alone nor by its environing conditions alone, but by both
in intricate combination. Although apparently we do
not inherit the effects on our forebears of their surroundings
and training, nevertheless we are the heirs to their mores,
which entails added responsibilities as well as opportunities
for each succeeding generation. Thus 'social heredity' bids
fair to outstrip our conservative and essentially unchanging
inherited nature. The EUTHENIST emphasizes nurture, the
EUGENIST emphasizes nature. As is so often the case, how-
ever, when doctrines are opposed, the truth combines both;
though we cannot doubt, knowing what we know of the
genetic constitution of organisms, that from the standpoint
of permanent advance — racial rather than individual -
the path to progress is through EUGENICS, the science of
being well born. "This distinction between heritage and
acquirements leaves a fatalistic impression in many minds,
and to some extent this is justified. We cannot get away
from inheritance. On the other hand, although the organism
changes slowly in its heritable organization, it is very modi-
fiable individually; and this is Man's particular secret — to
correct his internal organic inheritance by what we may call
his external heritage of material and spiritual influences."
(Thomson.) (Fig. 152.)
It is therefore clear that the problem of human improve-
ment has two aspects: in the first place, the effects of culture
on the individual which, though not inherited, are cumulative
from generation to generation through training; and secondly,
racial betterment through breeding the best. But the
reader may well ask: What is the possibility of anything
much better than the present best if heredity is essentially a
recombination of the characters of our forebears — a turn of
the kaleidoscope?
298 FOUNDATIONS OF BIOLOGY
Although we are wofully ignorant of the cause of variations,
the difficulty is more apparent than real and arises from our
absolute ignorance of what genes really are. We may
conceive them to be chemical molecules, and if so they can
change only by an alteration of their chemical constitution.
And for all we know, this may occur. Or, without any
change in the genes themselves, their expression — the
\
\
\
\
\ * / V \
\ «, v \ \
HERITAGE
FIG. 152. — Scheme to illustrate the contributions of nature and nurture to the make-
up of the individual. The triangles represent various types of individuals which may
be produced by the same germ cells (heritage) if environment and training are variable.
The foundation of the "triangle of life" is heritage. (After Conklin.)
chemical effects which they produce — may change by the
alteration of other substances with which they react. If we
interpret such phenomena as recombinations, they are
profoundly more subtle and far-reaching than are called to
mind by our simile of a kaleidoscope. They may be essen-
tially infinite in number and infinite in potentialities for varia-
tions in the germ plasm and therefore for heritable variations
expressed in the soma. Again it is possible, perhaps probable,
that inheritable variations are often the result of chromosomes
'accidentally' losing or gaining one or more genes during
HERITAGE OF THE INDIVIDUAL 299
synapsis. That is, one member of a pair of synaptic mates
leaves with the other member certain genes for which it gets
none in return: only half of the crossing-over process occurs.
Such a phenomenon would probably profoundly modify the
constitution of both chromosomes involved and accordingly
the organisms to which they contribute. And all such types
of mutations must be important raw materials for evolution.
G. SELECTION
For more than half a century selection has been something
to conjure with — a sort of creative principle to explain the
progressive changes in plants and animals,. It was assumed
that the SELECTION of a certain type of individual for breeding
would result in a gradual and continuous transformation of
the race or species in the direction of the selection. But
Darwin recognized that selection in itself can produce nothing
- its efficacy depends on the materials afforded by variation.
He did not and, in fact, could not make the modern sharp
distinction between modifications, combinations, and muta-
tions, but accepted all variations as at the disposal of selec-
tion. But recent work indicates that selection of certain
types of variations effects only an apparent and not a real
change. An example will make this clear. (Fig. 153.)
Take, say, a quart of beans and sort them into groups ac-
cording to the weight of each bean. Then put each group
into a separate cylinder and arrange the cylinders in a series
according to the weight of the enclosed beans. Now if we
imagine a line connecting the tops of the bean piles in each
cylinder, it takes the form of a typical curve of probability, or
frequency polygon. A similar figure would be obtained by
the statistical treatment of nearly all fluctuating characters
among the members of any large group of organisms, or of the
size of the grains in a handful of sand, or the deviations of
300
FOUNDATIONS OF BIOLOGY
shots from the bull's-eye in a shooting match. Therefore the
variations with respect to a given character very closely ap-
Pure Line
population
FIG. 153. — Diagram to illustrate a population of beans and its five compo-
nent pure lines. The beans are assorted according to weight. Tubes containing
beans of the same weight are placed in the same vertical row. The population
represents the quart of beans discussed in the text. (From Walter, after
Johannsen.)
proximate the expectation from the mathematical theory of
probability, or chance, and the reasonable conclusion is that
the FLUCTUATIONS are a resultant of a large number of factors
HERITAGE OF THE INDIVIDUAL
301
each of which contributes its slight and variable quota to the
expression in a given individual. (Figs. 154, 155.)
The question is, what results are obtained by breeding from
individuals which exhibit such a fluctuating variation to,
let us say, a greater degree than that of the mean of a mixed
population? The reader with Galton's theory of filial regres-
i
\
A B
FIG. 154. — Model to illustrate the law of probability, or chance. A, shot held in
the funnel at the top of the board; B, the shot, released by opening the mouth 4F the
funnel, have fallen through the series of hazards (pins), and bejen deflected by 'chance'
into the vertical compartments at the bottom. The curve connecting the tops of the
columns of shot is the normal probability, or frequency, curve. (After Kellicott.)
sion in mind will naturally expect, and rightly, that the off-
spring usually will exhibit the character to a less degree than
the parents but to a greater degree than the population. The
top (mode) of the curve will have moved, so to speak, slightly
in the direction of selection. Now, by continuing generation
after generation to select as parents the extreme individuals,
is it possible, with due allowance for some regression, to take
one step after another indefinitely, or until the character in
question is expressed to a degree which did not exist previ-
302
FOUNDATIONS OF BIOLOGY
ously? The experience of practical breeders gives a partial
answer, since the continual selection of the best animals for
mating and the best plants for seed has been a profitable
procedure. But it has long been known that after a certain
amount of selection has been practiced it ceases to be so
\
\
\
Inoktf 64 55 56 57 58 59 §0 61 62 63 64 65 66 67 68 69 10 7J
Pergont 3 3 7 18 34 80 135 163 183^163 115 78 41 ' IB 6 5 &
FIG. 155. — Normal frequency curve. Plotted measurements of the height of
1,052 women. The height of each rectangle is proportional to the number of individuals
of each given height. (Cf. Figs. 153, 154.) (From Kellicott, after Pearson.)
effective, and thenceforth serves chiefly to keep the character
at the higher level attained. (Fig. 156.)
The crux of the matter is in regard to exactly what the
fluctuations are. Modifications (non-heritable) and fre-
quently combinations (heritable) give a normal variability
curve, and both may be included in fluctuations. This mix-
ture of heritable and non-heritable variations is what makes
confusion. If we rule out combinations, by inbreeding
or by self-fertilization of homozygous individuals — establish
PURE LINES — then the fluctuations are all modifications and
selection is ineffectual with characters which are not inherited.
HERITAGE OF THE INDIVIDUAL 303
Pure Lines
The importance of this point was discovered by careful
experiments on the inheritance of characters in single pure
lines; particularly those of Johannsen on inheritance in a
brown variety of the common garden Bean. For example, by
keeping the progeny of each individual bean separate from
that of all the rest, he was able to isolate a number of pure
lines which differed in regard to the average weight of the
FIQ. 156. — Schematic representation of the effect of selection from the viewpoint
of Galton's 'law of filial regression.' (/) Mode before selection; 2, 3, 4, new (successive)
modes, the results of selections of individuals at #', 3', 4'- The mode has been shifted
in the direction of selection (toward the right) . But there has been each time an amount
of regression indicated by the length of the arrows.
beans. Selection did nothing but resolve the species, or the
bean 'population' with which he began, into its constituent
'weight types/ or lines, each of which exhibited a characteris-
tic variability curve of its own with a mode departing more
or less from that of the population. But when Johannsen
selected within a pure line (ruled out combinations) nothing
at all resulted; he was unable to shift the mode because he
was dealing with nonheritable characters. In other words,
the effect of selection is one of isolation and not creation. As
a rule it sorts out pre-existing pure lines (lines with homo-
geneous germinal constitution) from a population and then
stops — though if selection is stopped the isolated lines usually
soon merge again into the original population. A mutation
304 FOUNDATIONS OF BIOLOGY
must occur in a pure line for selection to be effective — and
then, ipso facto, the single pure line becomes two. (Fig. 153.)
The trend of present work certainly seems to indicate
that these conclusions are of general application and that the
explanation of the long-accepted feeling that selection is
'creative' is to be found in the fact that variations are of
three sorts: modifications which are not heritable and com-
binations and mutations which are heritable. Most of the
variations within pure lines apparently are the result of en-
vironmental influences recurrent in each generation, but the
germ plasm is homogeneous. The variability within a popu-
lation is the composite variability of its component pure lines,
but the germ plasm is not the same in all individuals — these
may be segregated into groups, the pure lines. Thus, very
liberally interpreted, the pure line concept is a formal expres-
sion of the fact that most of the variations which we recog-
nize are either somatic or the result of recombinations of
diverse parental genes. Accordingly when the genes of the
gametes are identical (as in pure lines) the latter source of
variation does not exist, and selection is powerless except
when comparatively rarely mutations occur. (Fig. 157.)
However, some recent work indicates that under certain
conditions selection appears to be effective, at least to a
limited degree, within a pure line. We have previously seen
that certain characters are the expression of multiple genes.
In some such cases one gene is, so to speak, the determining
gene for the character as a whole, while associated with this
gene there is a galaxy of modifying genes which themselves
do not find expression without the presence of the determin-
ing gene, but merely serve to alter the character expression
of the latter. Under such conditions it is possible to modify
the character by selection — to add or subtract or otherwise
change the relationships of the modifying genes to the pri-
HERITAGE OF THE INDIVIDUAL
305
mary gene. It would seem however that the effectiveness of
selection of this sort should be relatively limited in any par-
ticular case, and, in any event, the data thus far secured do
FiQ. 157. — Curves illustrating the relation between pure lines and popu-
lations or species. A, a population or 'species' curve, comprising three pure
lines; B, the separate elements (pure lines) of A, each with its own average
and variability. (After Kellicott.)
not fundamentally alter the general importance of the pure
line concept.
When all is said, it is clear that the realization of certain
categories of variations, taken in connection with the pure
line concept, has given new content to the problem of selec-
tion. The appreciation of its limitations has but accentuated
its possibilities. Selection is not shorn of its importance
either practical or theoretical. Artificial selection is useful in
306 FOUNDATIONS OF BIOLOGY
separating one line from another, as is attested by practical
breeders everywhere, and in taking advantage of mutations
when they occur. Most of the 'new creations' in horticulture
and animal breeding are the result of hybridization and the
rigid selection of individuals exhibiting desirable new com-
binations and sometimes mutations which hybridizing seems
to induce. Natural selection, in a quite similar manner, may
act as a 'sieve' and sort out new combinations and mutations
presented — leave the fit and eliminate the unfit — and so
afford a natural explanation of the adaptation of organisms
to their environing conditions. (Fig. 194.)
SUMMARY
Before leaving the subject a brief summary of the most
important general principles which the study of genetics has
thus far afforded may be helpful. In the first place, it appears
clear that the basis of inheritance is in the germinal rather
than in the somatic constitution of the individual. A charac-
ter to be inherited must be innate in the germ cells, and there
is no satisfactory evidence that modifications of the body,
'acquired characters,' can be transferred to the germ and so
inherited. Secondly, characters or groups of characters are
usually, if not universally, inherited as definite units. These
follow Mendelian principles of segregation and recombination
in the formation of the germ cells of an individual, so that
paternal and maternal contributions are readjusted in all the
combinations which are mathematically possible. And
finally, the germinal factor basis (genes) of unit characters is
remarkably constant. Selection is apparently powerless to
alter it, but merely sorts out what is already there, or, taking
advantage of such changes (mutations) as do occur, deter-
mines their survival value for their possessor in the struggle
for existence.
CHAPTER XVIII
ADAPTATION OF ORGANISMS
Every creature is a bundle of adaptations. Indeed, when
we take away the adaptations, what have we left?
— Thomson and Geddes.
ORGANISMS are systems dependent for their maintenance
and operation upon energy liberated by chemico-physical
processes in protoplasm, and therefore any and all influences
which induce changes in the structure or functions of an
organism must initially modify the underlying phenomena
which are responsible therefor. In a word, organic response
is a problem of metabolism. Although it is highly important
that this cardinal fact be clearly grasped, the science of
biology to-day is not in a position to interpret the responses
of organisms in these fundamental terms, and we shall merely
present some representative instances to illustrate the fact
that the response of organisms, as exhibited in active adjust-
ment — adaptation — of internal and external relations,
overshadows in uniqueness all other characteristics of life
and at one stroke differentiates even the simplest organism
from the inorganic.
Overwhelmingly striking as is the fitness of organisms to
their physical surroundings, we must not lose sight of the fact
that the environment itself presents a reciprocal fitness. This
results from the "unique or nearly unique properties of water,
carbonic acid, the compounds of carbon, hydrogen, and oxy-
gen. ... No other environment consisting of primary con-
stituents made up of other known elements, or lacking water
307
308 FOUNDATIONS OF BIOLOGY
and carbonic acid, could possess a like number of fit charac-
teristics, or in any manner such great fitness to promote com-
plexity, durability, and active metabolism in the organic
mechanism which we call life." (Henderson.)
A. ADAPTATIONS TO THE PHYSICAL ENVIRONMENT
In any consideration of the reciprocal relations which must
exist between organisms and their surroundings, of first im-
portance is the inconstancy~of the latter. Uncertainty is the
one certainty in nature and accordingly the response of living
things — their adaptability to environmental exigencies —
is at once the most striking and indispensable adaptation.
1. Adaptations Essentially Functional
Although the changes of the environment are almost in-
conceivably complex — witness the kaleidoscopic series of
events exhibited in the hay infusion microcosm — there are
certain general conditions which every environment must
supply, and without which life cannot exist. These are food,
including water and oxygen, certain limits of temperature
and pressure.
FOOD. As we know, food represents the stream of matter
and energy which is demanded for the metabolic processes
of living matter. And each and every element which forms
an integral part of protoplasm must be available. Since
all protoplasm consists chiefly of a dozen chemical elements,
these, of course, must be present; and further, since proto-
plasm is a colloidal complex in which water plays a funda-
mental role, life processes without water are impossible.
But the old adage that what is food for one is another's
poison has a broader content than is immediately apparent.
Although it is true there are general 'food-elements' which
all life demands, it is equally true that the combinations in
ADAPTATION OF ORGANISMS
309
which these elements must be presented to the organism, in
in order to be available for its metabolic processes, are sub-
ject to the widest variation.
We have emphasized and contrasted the nutrition of a
typical animal, green plant, and colorless plant, and have
seen the reciprocal part which they play in the circulation
of the elements in nature, so it is only necessary, with these
facts in mind, to cite special cases in order to illustrate the
adaptation of special groups of organisms to special condi-
FIG. 158. — Portion of filaments of Begyiotoa alba (a), and two cells of Beggiotoa
mirabilis (6) showing enclosed sulfur granules. (From Buchanan.)
tions of existence. The demands of the so-called Sulfur
Bacteria and the Yeasts are in point.
The Sulfur Bacteria (Beggiotoa) live in water containing
sulfuretted hydrogen, from which, by oxidation, they obtain
energy and store up within the protoplasm free sulfur in the
form of tiny granules. And then by further oxidation they
transform the sulfur into sulfuric acid and excrete it. Thus
a gas which is poisonous to nearly all organisms is for Beg-
giotoa a necessary life condition. (Fig. 158.)
The Yeasts include a host of microscopic colorless plants
which play an important part in the simplification of organic
compounds. (Fig. 159.) Being devoid of chlorophyll,
Yeasts of course lack photosynthetic powers, though like
many other colorless plants they are not dependent upon
310 FOUNDATIONS OF BIOLOGY
proteins for nitrogen but obtain it in less complex forms.
But the essential fact of interest at present is the chemical
changes associated with Yeast metabolism — the transforma-
tion of a large proportion of the sugar content of the medium
in which they live into alcohol and carbon dioxide. This
process of alcoholic fermentation may be approximately
expressed by the formula:
C6H12O6(sugar)+yeast=2 C2H5OH (alcohol) +2 CO2
The explanation is not far to seek. Deprived of an adequate
supply of air, Yeasts resort to the energy released when, with
the decomposition of the sugar, the carbon and oxygen unite
FIG. 159. — Yeast cells, very highly magnified. A, cell showing granular
cytoplasm and a large vacuole; B, showing nucleus; C, cell budding;
D, mother cell and bud after division is completed.
as C02. The formation of alcohol by the remnants of the
sugar molecules is, from the standpoint of the Yeasts, a mere
incidental factor which is, so to speak, unavoidable. On the
other hand, from the broad viewpoint, the waste products
of the action of the Yeast plants' enzymes represent an impor-
tant phase in the general simplification of organic compounds
in nature. And Man turns to account in numerous ways both
products of the Yeasts' destructive powers — the alcohol
and the carbon dioxide.
Thus the Yeasts are practically independent of free oxygen
and in this they agree with many kinds of Bacteria as well as
some animals, chiefly parasitic worms, which are able to
secure the necessary oxygen by the rearrangement of the
atoms within a molecule or the disruption of the molecule
ADAPTATION OF ORGANISMS 311
itself. Indeed, certain species of Bacteria not only do not
need free oxygen at all, but are killed when it is present in
any considerable amount. All such organisms are termed
ANAEROBES. A common example is Bacillus tetani which
inhabits garden soil and street dust and produces tetanus,
or lockjaw, in Man and certain domesticated animals when it
gains entrance to the tissues.
TEMPERATURE. Although protoplasmic activity is re-
stricted to ranges of temperature which do not seriously
interfere with the chemico-physical processes involved, it is
a commonplace that various species are adapted to different
degrees of temperature. The great majority of organisms,
however, find their optimum temperature between 20° C.
and 40° C., though species inhabiting the polar and tropical
regions show adaptations to the temperature extremes of
their surroundings. As a matter of fact, it is not possible to
state the upper and lower limits beyond which active life is
suspended, but some Algae and Protozoa are known to
multiply in the water of hot springs, certainly at temperatures
higher than 50° C., and others in water until freezing actually
occurs.
But many of the lower forms of life, such aos the Bacteria
and Protozoa, have the power of developing, particularly
under unfavorable surroundings, protective coverings of
various sorts about themselves and of assuming a resting
condition in which all the metabolic processes characteristic
of active life are reduced to the lowest ebb. (Fig. 160.)
In this spore or encysted state they are immune to extremes
of temperature and of desiccation to which they readily
succumb during vegetative life. Thus some types of Bacteria
can successfully withstand a temperature of nearly —200° C.
for six months, and about — 250° C. for shorter periods, which
is a temperature approaching closely that at which no
312
FOUNDATIONS OF BIOLOGY
chemical reactions are known to occur. Again, the spores
of other Bacteria can endure a temperature as high as 120° C.
for a short time.
It is clear that the great majority of organisms are at the
mercy of environmental temperatures. This is true of all
except the higher Vertebrates, the Birds and Mammals.
These so-called warm-blooded, or HOMOTHERMAL, animals
possess a highly complex mechanism which maintains their
FIG. 160. — Spore formation and germination in Bacteria. A. B, C, a
pair of rods forming spores, drawn at one hour intervals; D, a five-celled
rod, with three fully-formed spores, which was allowed to dry for several
days and then placed in a nutrient medium; E, F, the same spores at
one and three hours later; G, a pair of typical vegetative rods. (From
Sedgwick and Wilson, after De Bary.)
body temperature practically constant; e.g., in Man at 37° C.
(98.6° F.).
The heat regulatory mechanism represents, so to speak,
the culmination of the assembling and elaborating, during
Vertebrate evolution, of elements, the genesis of which is
found among the Fishes. In the Mammals it comprises
insulating material in the skin, a closed blood vascular
system, power of rapid oxidation, endocrinal and other
glandular products, evaporation surface of the lungs and
skin, 'trophic' and 'temperature' nerves, coordinating
centers, etc., — the whole complex rendering its possessors
largely independent of the surrounding temperature and
making possible a carrying on of the various bodily functions
with such nicety as the life of these forms demands.
ADAPTATION OF ORGANISMS 313
PRESSURE. The metabolism of organisms, in common
with chemical processes in general, is influenced by the
surrounding mechanical pressure. Therefore it is evident
that the pressure of either the water or air plays an important
part in the carrying on of the life functions. We find organ-
isms adapted to the greatest depths of the ocean where the
water pressure is several hundred atmospheres — so great
that some forms burst when rapidly brought to the surface;
while others are adapted to live at high altitudes where the air
pressure is relatively low. And again, the higher Vertebrates
present an adaptive mechanism which renders them less
dependent on a constant barometric pressure.
These few examples must suffice to emphasize the general
environmental conditions which are necessary for life, as we
know it, to exist, and to suggest that within these broad
limits organisms are adapted to special environmental condi-
tions so that there is scarcely a niche in nature untenanted.
2. Adaptations Essentially Structural/
We may now broaden our view of the plasticity of organ-
isms by a brief consideration of adaptations which are
essentially structural . But here as elsewhere it is absolutely
impossible to divorce structure and function which, ob-
viously, are only reciprocal aspects of the fitness of living
creatures.
ADAPTIVE RADIATION OF MAMMALS. In the group of
Eutherian Mammals, forms are to be found which are ex-
traordinarily modified in adaptation to the most diverse
environmental conditions. From a more or less primitive
type, or focus, there radiate, as it were, types which are
specialized for different habitats and modes of life. (Fig.
161.) We may select a small Malayan insectivorous animal
known as Gymnura, which is allied to the Hedgehogs, as
314
FOUNDATIONS OF BIOLOGY
most similar among living Mammals to the generalized or
focal type of terrestrial Mammal. Gymnura has relatively
short pentadactyl limbs with the entire palms and soles
Cursorial-
Unguligrade
Volant
(Aerial)
Cursorial-Digi tigrade
(Terrestrial)
Scansorial
(Arboreal)
Ambulatory
(Terrestrial)
Natatorial
(Amphibious )
Short-limbed, plantigrade,
pentadactyl, unguiculate
stem
Fossorial
(Subterranean)
(Aquatic)
FIG. 161. — Diagram of the adaptive radiation of Eutherian Mammals as exhibited
in limb structure. (From Lull.)
resting flat upon the ground (PLANTIGRADE) and therefore
essentially adapted for comparatively slow progression.
(Fig. 162.)
Radiating from this focus, adaptations for rapid running
(cursorial adaptations) are chiefly evident in a lengthening
ADAPTATION OF ORGANISMS 315
of the limbs. Thus, for example, in the Dogs, Foxes, and
Wolves, the effective limb length is increased by raising
the wrist and heel from the ground and walking merely upon
the digits (DIGITIGRADE) ; ' while in Antelopes, Horses, and
hoofed runners in general, the chief limb bones themselves
are lengthened, subsidiary ones are suppressed, and the wrist
and ankle are raised still further from the ground, so that
merely the tips of one or two digits of each limb support
the animal (UNGULIGRADE) . Thus the typical cursorial
FIG. 162. — Gymnura. (From Lull, after Horsfield and Vigors.)
forms represent the culmination of Mammalian adaptation
to plains and steppes; regions in which long distances must
frequently be traversed in quest of food, and safety is to the
swift. (Fig. 163.)
Another line of adaptive radiation is presented by the
tree dwellers : arboreal forms which make their own the /
world of foliage high above the ground. Such are, for
instance, the Sloths (Fig. 164), which are really tree climbers
that walk and sleep upside down suspended from branches;
the Man-like Apes that swing among the boughs chiefly
by their arms; and the Squirrels that scamper along the
branches. Some Squirrels and the so-called Flying Lemurs
316
FOUNDATIONS OF BIOLOGY
FIG. 163. — Foot postures of Mammals.
A, plantigrade; B, digitigrade; C, unguli-
grade. (From Lull, after Pander and D'Alton.)
take long soaring leaps
supported by wide folds
of skin between the sides
of the body and the ex-
tended limbs. (Fig. 167.)
But the Mammals have
not left the air untenanted,
for truly volant forms are
represented by the Bats in
which the fore limbs with
greatly elongated fingers
form the framework of
true wings. (Fig. 168.)
Passing below the sur-
face of the earth, fossorial
animals are found such as
the Woodchucks, Gophers,
and especially the Moles,
which are adapted to a
subterranean existence by
bodily modifications which
facilitate digging. (Fig.
165.) The gap between ter-
restrial arid aquatic Mam-
mals is bridged by the
Muskrats, Beavers, Otters,
and Seals which are more
or less equally at home on
land and in the water.
The truly aquatic Mam-
mals are the Porpoises and
Whales which have com-
pletely abandoned the
ADAPTATION OP ORGANISMS
317
land of their ancestors of the geological past and to-day
approach, in adaptations to a marine life, the general contour
of the primitively
adapted aquatic Ver-
tebrates, the Fishes.
(Fig. 166.)
Thus the various
lines of adaptive ra-
diation of the Mam-
mals from a general-
ized terrestrial type,
such as Gymnura,
have provided Mam-
mals fitted for all sorts and conditions of the environment
— representatives are competing with members of other
FIG. 164. — A Sloth, Choloepus, walking suspended
from a branch. (After Allen.)
FIG. 165. — Skeleton of a Mole, Talpa europaea.
Pander and D'Alton.)
(After
FIG. 166. — Skeleton of a Porpoise. The vestigial pelvic bones are shown
imbedded in the flesh. (After Pander and D'Alton.)
groups beneath, on, and above the earth and in the water.
Somewhat similar adaptative radiations are traceable in other
animal and plant groups, though there seems no doubt that
318 FOUNDATIONS OF BIOLOGY
FIG. 167. — 'Flying Lemur,' Galeopithecus volans. (After Lull.)
FIG. 168. — A Bat, Vespertilio noctula. (After Lull.)
ADAPTATION OF ORGANISMS 319
the adaptability of the Mammal stock — its potential of
evolution — is in no small degree responsible for the
dominant position which the Mammals hold in the animal
world of to-day. Man is a Mammal.
ANIMAL COLORATION. L Perhaps the most generally strik-
ing characteristic of organisms is their color and color
pattern. Among plants this applies chiefly to the flowers
and fruit of the higher forms, though here and there through-
FIQ. 169. — The common green Katydid (Microcentrum).
(After Riley.)
out the plant series the typical green color is replaced or
rendered inconspicuous by others. But the absence of photo-
synthetic pigments in animals and their relatively active life
have permitted more latitude in body color, and accordingly
it is in the animal world that color adaptations are more
numerous and varied. Some colors and color patterns are,
of course, merely incidental to the chemical composition of
the whole or parts of the body. Others, however, irresistibly
arouse our interest and seem to demand a less simple ex-
planation because they are apparently of special service to
their possessors. A few examples will serve to bring the
problem before us and indicate the class of facts involved.
The color and color patterns of many animals are such that
they harmonize or fuse with the usual surroundings of the
creatures and render them practically indistinguishable from
their immediate environment. Every frequenter of the open
320
FOUNDATIONS OF BIOLOGY
knows innumerable instances. The song of the green Katy-
did readily guides one to its immediate vicinity, but it is quite
another matter to distinguish its leaf-green wings among the
foliage of its retreat. (Fig. 169.) One is attracted by the
FIG. 170. — Catocala lacrymosa; A, wings expanded, exposing the highly
colored hind-wings; B, resting on bark. (From Folsom.)
striking colors of an Underwing Moth (Catocala) while in
flight, but is at a loss to find the insect when scarlet or orange
is obscured by the overlapping grayish-mottled fore-wings
blending with the tree trunk where it has come to rest. (Fig.
ADAPTATION OF ORGANISMS
321
The white of the Foxes, Hares, and Owls of alpine and
arctic regions; the green color of foliage-dwelling Insects
and Frogs; the tendency toward fawn and gray of desert
Insects, Reptiles, Birds, and Mam-
mals; the olive upper surface of
the bodies of brook Fishes; the
steel gray above and white below
of sea Birds which harmonize with
sea and sky when viewed from
above and below respectively -
the number of such cases is legion.
Gazelles living on the lava fields
of volcanic regions are dark gray,
while those of the great stretches
of sand plains are white — the
same species exhibiting regional
variations in color which blend
with the surroundings. Further-
more, the same individual may
vary in color with the seasonal
changes in its environment, or
present different color schemes
in different localities. Thus the
summer coat colors of the Arctic Fox and the Weasel har-
monize with the browns of rocks; and the winter coat of
white with snow-clad nature. And the Chameleons are by
no means unique in their ability to change color very rapidly
in response to that of their immediate surroundings.
But confusion is worse confounded when to harmonizing
color is added harmonizing form, striking examples of which
are the Dead-leaf Butterfly (Kallima) of the East Indian
region, the familiar Walking-sticks (Diapheromera), and the
caterpillars of Geometrid Moths. (Figs. 171. 172, 173.)
FIG. 171. — Dead-leaf Butterfly,
Kallima paralecta. (After Weis-
mann.)
322 FOUNDATIONS OF BIOLOGY
Although the general tendency in nature is for sympathetic
coloration — indeed, it is frequently possible to infer from
FIG. 172. — A Walking-stick Insect, FIG. 173. — Larva of a Geometric! Moth
Diapheromera femorata, on a twig. resting extended from a twig. (From Jordan
(From Jordan and Kellogg.) and Kellogg.)
the color of an animal its habitat — there are numerous cases
in which the colors and color schemes seem to be in striking
ADAPTATION OF ORGANISMS 323
contrast with the animal's usual background. Sometimes,
however, the contrast which is so striking with the bird in
the hand, proves to be 'obliterative' with the bird in the
bush — a conspicuous color pattern, expressing gradations
of light and shadow, and counter shading, fuses with a
background of light and shadow afforded by foliage.
But examples of color patterns which by the most liberal
stretch of the imagination cannot be interpreted as harmoni-
FIG. 174. — 'Protective Mimicry.' A, drone Honey Bee; B, a Bee-fly,
Eristalis tenax. (From Folsom.)
ous with the animal's usual surroundings are not far to
seek. Brilliant yellows and reds render, for instance, many
Wasps, Bees, Butterflies, and various species of Snakes actu-
ally conspicuous. And it is suggestive that very many of
these forms are provided with special means of defense, such
as poison glands and formidable jaws, or special secretions
which render them unpalatable. Moreover, what is still
more interesting, many animals possessing this 'protective
conspicuousness' which renders them easily identified and
advertises that they are to be avoided by their foes, are
frequently 'mimicked' in color pattern and form by defenseless
creatures. Thus commonly associating with the various
species of Bees hovering about flowers are defenceless Flies
which are so bee-like in appearance that they are usually mis-
324 FOUNDATIONS OF BIOLOGY
taken for Bees, and avoided accordingly by human and
presumably by other enemies also. (Fig. 174.)
Now, what is the significance of such phenomena of animal
coloration and form which are so universal in nature? The
problem appears by no means so simple to-day as it did a
generation ago, and biologists are not so ready to interpret
individual cases as 'protective,' 'aggressive,' 'alluring,'
'confusing,' or 'mimetic.' But it is beyond dispute that no-
where else is the plasticity — adaptability — of organisms
better illustrated, and that, taken by and large, such adap-
tations are of crucial importance in the life and strife of
species. Whatever may be the origin of adaptive variations,
natural selection is undoubtedly responsible for their ac-
cumulation and preservation.
THE LEGS OF THE HONEY BEE. From time immemorial
the Honey Bee (Apis mellifica) has been the subject of wonder
and study, and to-day there is no more interesting and instruc-
tive example of adaptation than that exhibited by the Bee in
relation to the highly specialized community life of the hive.
An average hive comprises some 65,000 Bees of which one
is a QUEEN, several hundred are DRONES, and the rest WORK-
ERS. The queen is the only fertile female and accordingly
she is the mother of nearly all the other members of the hive.
Throughout her life of about three years she is tended and
fed by her numerous offspring. The drones, or males, con-
tribute nothing to the life of the hive in which they live, but
at the swarming of the Bees, one of them mates with a
virgin queen, which thenceforth becomes the queen of a new
hive. Thus the queen and the drones represent an adapta-
tion of the colony to communal life — a physiological di-
vision of labor in the hive which involves a specialization
of a class solely for reproduction, while the daily work and
strife of the colony devolves upon the workers. The latter
ADAPTATION OF ORGANISMS 325
are sexually undeveloped females which do not lay eggs but
spend their time carrying water, collecting nectar and pollen,
secreting wax, building the comb, preparing food, tending
the young, and cleaning, airing, and defending the hive.
(Fig. 175.)
The worker is a 'bundle of adaptations' for its varied
duties. Indeed, when we take away the adaptations there is
little left! The primitive insect appendages have become
specialized in the worker Bee so that collectively they con-
stitute a battery of tools adapted with great nicety to the
MALE FEMALE WORKER
FIG. 175. — The Honey Bee, Apis mellifica. (After Shipley and MacBride.)
uses for which they are employed. This applies to all of the
appendages of the insect's body, but we shall neglect those of
the head (Fig. 176) and consider only the specializations of
the three pairs of legs. These, as in all Insects, arise from
the THORAX; the anterior pair from the first segment of the
thorax (prothorax); the second, or middle, pair from the
second thoracic segment (mesothorax) ; and the posterior
pair from the third and last thoracic segment (metathorax) .
A typical insect leg consists of several parts: the COXA,
which forms the junction with the body, followed in order by
the TROCHANTER, FEMUR, TIBIA, and five-jointed TARSUS, or
foot. (Fig. 177.)
The worker Bee's PROTHORACIC LEGS show the following
specializations. The femur and tibia are covered with long,
326
FOUNDATIONS OF BIOLOGY
branched FEATHERY HAIRS which aid in gathering pollen
when the Bee visits flowers: the tibia, near its junction with
the tarsus, bears a group
of stiff bristles (POLLEN
BRUSH) which is used to
brush together the pol-
len grains that have
been dislodged by the
- «< hairs °f the Upper leg"
? segrnents> Qn the oppo-
site side of the leg is a
composite structure, the
ANTENNA CLEANER,
formed by a movable
plate-like process
(VELUM) of the tibia
which fits over a circu-
lar notch in the upper
end of the tarsus. The
notch is provided with
a series of bristles which
form the teeth of the
antenna COMB. The
antennae, or 'feelers/
which are important sense organs of the head, are cleaned
by being placed in the toothed notch and, after the velum
is closed down, drawn between the bristles and the edge
of the velum. On the anterior face of the first segment
of the tarsus is a series of bristles (EYE BRUSH) which is used
to remove pollen and other particles adhering to the hairs on
the head about the large compound eyes and interfering with
their operation.
The terminal segment of the tarsus of each leg is provided
Fio. 176. — Head of a worker Honey Bee.
a, antenna; 6, bouton; g, epipharynx; I, hypo-
pharynx; Ip, labial palpus; m, mandible; mx,
maxilla; mxp, maxillary palpus. (After Cheshire.)
ADAPTATION OF ORGANISMS
327
FIG. 177. — Legs of the worker Honey Bee. A, outer side of metathoracic leg: p,
metatarsus (first segment of tarsus) ; t, tarsus; ti, tibia. B, inner side of metathoracic
leg: c, coxa; p, metatarsus; t, tarsus; ti, tibia; tr, trochanter; wp, pecten and auricle.
C, prothoracic leg: b, pollen brush; eb, eye brush; p, metatarsus; t, tarsus; ti, tibia;
v, velum. D, mesothoracic leg: lettering as in C; s, pollen spur. E, joint of prothoracic
leg: lettering as in C. Fj teeth of antenna comb. G, transverse section of tibia through
pollen basket: a, antenna; fa, pollen; h, holding hairs; n, nerve. H, antenna in process
of cleaning: o, antenna; c, antenna comb; I, section of leg; s, scraping edge of v, velum.
(From Hegner, after Cheshire.)
328 FOUNDATIONS OF BIOLOGY
with a pair of notched CLAWS, a sticky pad (PULVILLUS)
and TACTILE HAIRS. (Fig. 178.) When the Bee is walking
up a rough surface, the points of the claws catch and the
pulvillus does not touch, but when the surface is smooth,
so that the claws do not grip, they are drawn beneath the
foot. This change of position applies the pulvillus, and it
clings to the smooth surface. Thus the character of the
surface automatically determines whether claw or pul-
villus shall be used. But there is another adaptation equally
FIG. 178. — Foot of the Honey Bee in the act of climbing, showing the 'automatic'
action of the pulvillus. A, position of foot on a slippery surface, fh, tactile hairs; pv,
pulvillus; t, last segment of tarsus; a n, claw. B, position of foot in climbing on a rough
surface, an, c, claw. C, section of a pulvillus just touching a flat surface; cr, curved
rod. D, the same applied to the surface. (From Packard, after Cheshire.)
remarkable. "The pulvillus is carried folded in the middle,
but opens out when applied to a surface; for it has at its
upper part an elastic and curved rod, which straightens as
the pulvillus is pressed down. The flattened-out pulvillus
thus holds strongly while pulled along the surface by the
weight of the Bee, but comes up at once if lifted and rolled
off from its opposite sides, just as we should pull a wet
postage stamp from an envelope. The Bee, then, is held
securely till it attempts to lift the leg, when it is freed
at once; and, by this exquisite yet simple plan, it can fix
and release each foot at least twenty times per second."
(Cheshire.)
The characteristic structures of the middle (MESOTHORACIC)
legs of the Bee are a small POLLEN BRUSH and a long spine, or
ADAPTATION OF ORGANISMS 329
SPUR, which is employed in removing the pollen from the
pollen baskets on the metathoracic legs, and also in cleaning
the wings.
The METATHORACIC LEGS exhibit four remarkable adapta-
tions to the needs of the insect known as the POLLEN COMBS,
PECTEN, AURICLE, and POLLEN BASKET. The pollen combs
comprise a series of rows of bristle-like hairs on the inner sur-
face of the first segment of the tarsus: the pecten is a series
of spines on the distal end of the tibia which is opposed by a
concavity, the auricle, on the proximal end of the tarsal seg-
ment; while the pollen basket is formed by a depression on
the outer surface of the tibia which is arched over by rows of
long curved bristles arising from its edges.
Thus the worker is fully equipped. Flying from flower to
flower for nectar, the Bee brushes against the anthers laden
with pollen, some of which adheres to the hairs on its body
and legs. While still in the field, the pollen combs are first
brought into play to comb the pollen from the hairs, while
the pectens scrape the pollen from the combs. Then the
auricles are manipulated so that the accumulating mass of
pollen is pushed up into the bristle-covered pollen baskets.
This process is repeated until the baskets are full and then
the insect returns to the hive, where the contents of the pollen
baskets are removed by the aid of the spurs with which the
mesothoracic legs are provided.
Moreover, the structural adaptations of the worker Bee
are but one aspect of a reciprocal fitness. Many of the
flowers which the Bee visits show remarkable adaptations
for the reception of the Bee and for dusting it with
pollen, because Bees are effective agents in transfer-
ring pollen from flower to flower and thus insuring cross-
fertilization.
330 FOUNDATIONS OF BIOLOGY
B. ADAPTATIONS TO THE LIVING ENVIRONMENT
We have now discussed the close reciprocal relationship be-
tween organism and environment, putting emphasis upon
adaptations to the non-living surroundings, and must turn
more specifically to some striking interrelations of organism
with organism, in order to make possible an appreciation of
the devious means to which they have recourse — to what ex-
tent the strands of the web of life become entangled — in
the competition for a livelihood.
The mutual biological interdependence of organisms is. in
the final analysis, the result of the primary demands of all
creatures — proper food, habitat, reproduction, defense
against enemies and the forces of nature. The web of life
is an expression of the cooperation, jostling, and strife of
individual with individual, and species with species for these
primary needs; and the activities which follow from them
form the foundations of life in the lowest as well as the high-
est. There is a struggle for existence. A common food Fish,
the Squeteague, captures the Butter-fish or the Squid, which
in turn have fed on young Fish, which in their turn have fed
on small Crustacea, which themselves have utilized micro-
scopic Algae and Protozoa as food. Thus the food of the
Squeteague is actually a complex of all these factors, and
such a ' nutritional chain' is no stronger than its single links.
Circumstances which modify or suppress the food and there-
by reduce the abundance of the microflora and microfauna of
the sea are reflected in correlative changes in the abundance
of economically important food Fishes. And this same prin-
ciple is true throughout living nature, though only occasion-
ally is it possible to trace it. "Nature is a vast assemblage
of linkages."
ADAPTATION OF ORGANISMS 331
1. Communal Associations
Perhaps the simplest organismal associations are repre-
sented by GREGARious~"animals, such as Wolves which hunt
in packs, and Buffaloes and Horses which herd for protection.
Here the association is more or less temporary and there is
no division of labor between the members, other than leader-
ship by one animal.
COMMUNAL animals, however, exhibit highly complex asso-
ciations in which the members merge, as it were, their indi-
viduality in that of the community. This is well exhibited,
for example, among the Ants, in which all of the various spe-
cies, about 5000 in number, are communal, and in the Wasps
and Bees in which all gradations exist from solitary to hive-
dwelling species. And, as has been mentioned in the case of
the Bees, the division of labor has developed to the extent
that structural differentiations have given rise to classes of
individuals specially adapted for the performance of certain
functions in the economy of the hive.
It is in Man, however, that we find the highest expression
of communal cooperation, because increased intelligence, in
particular, makes flexible the stereotyped life as exhibited in
the lower forms — the human individual being adaptable to
the various community tasks.
But associations are not confined to members of the same
species, nor are all an expression of cooperative adaptations.
All gradations occur from those which are mutually beneficial
to the parties in the pact, to those in which one member
secures all the advantage at the expense of the other.
2. Symbiosis
The most intimate associations in which the organisms
involved are mutually benefited, if not absolutely necessary
332 FOUNDATIONS OF BIOLOGY
for each other's existence, are termed SYMBIOTIC. A familial-
illustration is the common green Hydra (Hydra viridis)
which owes its characteristic color to the presence of a large
number of unicellular green plants which live in its endoderm
cells. The products of the photosynthetic activity of the
plant cells are at the disposal of the Hydra and the latter
FIG. 179. — The formation of a Lichen, Pkyscia paratina, by the combination of an
Alga and a Fungus. A, germination of a Fungus spore (sp), whose filaments are sur-
rounding two cells (a) of the unicellular Alga, Cystococcus humicula. B, later stage in
which spores have formed a web of filaments (mycelium), enveloping many algal cells.
Magnified about 400 times. (From Abbott, after Bonnier.)
in return affords a favorable abode and the material neces-
sary for the life of the plant.
A far more striking example of symbiosis is afforded by
Lichens which represent intimate combinations of various
species of Fungi and Algae. (Fig. 179.) In each case the
Fungus supplies attachment, protection, and the raw mate-
rials of food, while the green Alga performs photosynthesis.
Each can live independently under favorable conditions, but
ADAPTATION OF ORGANISMS
333
in partnership, they are superior to vicissitudes with which
many other plants cannot cope, and thus sonic forms become
the vanguard of vegetation in repopulating rocky, devas-
tated areas.
From the practical standpoint of agriculture the symbiotic
nitrogen-fixing Bacteria are of first importance. It will be
FIG. 180. — Rose Aphids visited by Ants. (After Kellogg.)
recalled that these Bacteria form small tubercles on the
rootlets of higher plants and make atmospheric nitrogen
available to the latter. Thus in return for an abode and cer-
tain food elements, such nitrogen-fixing Bacteria render their
symbiotic associate largely independent of soil nitrogen.
Again in the higher animals, including Man, evidence is
accumulating which indicates that certain kinds of Bacteria
334 FOUNDATIONS OF BIOLOGY
find their normal habitat in the digestive tract, where, inci-
dental to getting their own living, they bring about chemical
changes in the food of their host which is an important factor
in the digestive processes of the latter.
Still another type of association in which both partners
profit is represented by the relation that occurs between Ants
and Aphids. The defenseless Aphids are protected, herded
and 'milked' by the Ants to supply their demand for honey-
dew, a secretion of the Aphids which the Ants greedily de-
vour. (Fig. 180.)
3. Parasitism
But associations in which one organism, the PAKASITE,
secures the sole advantage, and in most cases at the expense
of the helpless second party, the HOST, are far more numerous
— it has been estimated that nearly half the animal kingdom
are parasites. And these are particularly forced upon our
attention because many human diseases are the result of
Man's unwilling partnership in such associations. Indeed,
PARASITOLOGY has become an important subdivision of bi-
ology, both practical and theoretical. Practical, as a corner
stone of public health; and theoretical, because many of the
most remarkable functional and structural adaptations are
FIG. 181. — Diagram illustrating the life history of a Malarial Parasite. The stages
above the line of dashes occur in human blood; those below, in the body of a Mosquito.
I-V and 6-10 show asexual multiplication (schizogony) in human red blood corpuscle
following introduction of a parasite (XIX) by a Mosquito. This may continue by the
parasites (10) entering other corpuscles until a large number of the latter are destroyed.
Sooner or later sexual forms arise. VI-XIII, the sexual generation involving the
differentiation of male ( $ ) and female ( 2 ) gametes which unite (XI) to form a zygote
(XII). The zygote becomes motile (XIII), works its way into the wall of the stomach
of the Mosquito, and encysts (XIV). Within the cyst a number of small cells (XVI,
sp.bl.) arise by division, and these, in turn, give rise to a multitude of motile cells
(XVIII) termed sporozoites. The sporozoites are liberated (XIX) from the cyst,
make their way to the salivary glands of the Mosquito where they are ready to be
inoculated into the human body, and so gain entrance to a red blood corpuscle (I).
The production of the sporozoites from the zygote is known as sporogony. n, nucleus
of the narasite; p, pigment and waste prorlunts of the parasites; fl, long slender male
gametes. (From Minchin, in Lankester's Treatise.)
336 FOUNDATIONS OF BIOLOGY
exhibited by parasites in becoming fitted for this apparently
highly successful method of gaining a livelihood, and by the
hosts in bearing the burden with the least outlay. Generally
speaking, the effect on the parasite consists in a simplifica-
tion of the various organs of the body devoted to food-
getting, locomotion, etc., since their duties are foisted upon
the host; while the organs and methods of reproduction are
highly specialized and elaborated, owing to the necessity of
producing enough offspring to compensate for the hazards
involved in reaching a proper host. For in the majority of
cases a parasite is adapted to live in a specific host, and death
ensues if this is not attained at the proper time.
Probably the most generally interesting example of para-
sitism is the cause of the disease known as MALARIA. Man is
subject to at least three types of malaria, each the result of
infection by a different malarial organism. The malarial
parasites are all unicellular animals, Protozoa, with compli-
cated life histories which are adaptations to the specific exi-
gencies of their parasitic existence. (Fig. 181.) One part
of the life history, the asexual, is passed in the red blood
corpuscles of Man; while the other, the sexual, occurs in the
digestive tract of certain species of Mosquitoes. A single
parasite inoculated into the human system by the bite of an
infected Mosquito enters a red blood corpuscle and multi-
plies. The progeny, liberated from the destroyed corpuscle,
similarly attack other corpuscles and multiply until a very
large number of blood corpuscles are destroyed. And the liber-
ation of poisonous products of the life processes of the parasites
provoke the chills and fevers characteristic of the disease.
But the parasites must make their escape before the human
host successfully combats the toxic substances, kills the
parasites by taking quinine, or succumbs to them. The get-
away is accomplished, if at all, by a Mosquito biting the host
ADAPTATION OF ORGANISMS 337
and taking with the blood certain sexual stages of the para-
site which can develop in the cold-blooded insect. And now
the Mosquito is the host. In its stomach the sexual phase of
the life history of the malarial parasite takes place, fertiliza-
tion occurs, and finally the numerous products of the zygote
work their way to the mouth parts of the Mosquito, where
they await an opportunity to enter the human blood.
The life history of malarial parasites exhibits a continuous
series of adaptations to parasitic life: the nicety of the ad-
FIG. 182. — A trypanosome (Trypanosoma theileri) from the blood of cattle. Magni-
fied about 3000 times. (After Liihe.)
justment being especially well illustrated at the transfer
from Man to Mosquito, since all the parasites which enter
the stomach of the latter are digested except those sexual
forms which are ready to initiate the sexual part of the cycle
in the new host.
But the acme of parasitic associations is only attained
when the adaptations of parasite and host have become so
complete that the latter 'pays the price' without any un-
toward results. Thus the Antelopes and similar Mammals
of certain regions of Africa harbor in their blood various
species of Protozoan parasites, known as TRYPANOSOMES,
338 FOUNDATIONS OF BIOLOGY
without any apparent discomfort. But if the intermediate
hosts, which are biting Flies, transfer for example Trypano-
soma brucei to imported Horses or Cattle, a serious disease
results which is usually fatal. Indeed, the opening up of cer-
tain regions of Africa has been greatly retarded by the
ravages of this Trypanosome in new hosts to which it is not
adapted. Generally speaking, pathogenic species may be
regarded as aberrant forms which are not yet adapted to
their hosts or are not in their normal hosts. And these are
the parasites which are forced upon our attention, though
there are few organisms without their specially adapted para-
sites — the parasites themselves not excepted.
4. Immunity
At best, however, the part played by the host cannot be
regarded as ideal, and devious types of adaptations against
parasites exist which, insofar as they are effective, bring
about IMMUNITY. Usually among the higher animals, includ-
ing Man, immunity to pathogenic Bacteria seems to have its
foundations in specific chemical substances in the blood,
termed ANTIBODIES. These either modify the activities of
certain cells of the body, chiefly the white blood corpuscles,
or act directly upon the invaders themselves and the poisons
(TOXINS) which they produce. The white blood corpuscles
have been called the 'policemen of the body7 because, under
the influence of invading organisms and of certain antibodies
called OPSONINS, some of them make their way through the
walls of the capillaries in the region of the infection and, in
amoeboid fashion, engulf and digest the intruders. When
acting in this capacity the corpuscles are referred to as
PHAGOCYTES. L
Among the various classes of antibodies are also the ANTI-
TOXINS which neutralize the poisonous products of Bacteria,
ADAPTATION OF ORGANISMS 339
and the CYTOTOXINS which actually destroy the foreign cells.
Various specific antibodies may be naturally present in the
blood — a part of the heritage — so that an individual is
immune to certain diseases due to pathogenic organisms.
Or the antibodies may be produced in response to the para-
sites themselves, and the individual acquires immunity only
after undergoing the disease. Finally, immunity may be
artificially acquired by various means, such as VACCINATION,
which stimulate the production of antibodies so that the
individual is fore-armed, as it were, in the event of an infec-
tion. But the subject of immunity has become a science in
itself within the past few years — a science which has as its
basis the exploitation of the marvelous power of adaptation
of protoplasm as exemplified in coping with disease-producing
parasites.
C. INDIVIDUAL ADAPTABILITY
We may now turn to a survey of the highest expression
of adaptation evolved by nature, which is revealed in rela-
tively simple form in the behavior of the lower organisms,
gains definiteness and content as we ascend the animal series,
and becomes the basis of the intelligence and all that the
mental life of Man involves. It is the adaptation which
renders Man essentially superior to adaptation — enables
him to a large extent to control, instead of being controlled
by, his environment. "It seems that nature, after elaborat-
ing mechanisms to meet particular vicissitudes, has lumped
all other vicissitudes into one and made a means of meeting
them all" - the nervous mechanism.
That organisms respond to environmental changes, we are
well aware. Life itself is the result of — in fact, is — a con-
tinuous flow of physico-chemical actions, interactions, and
reactions with the surroundings. But by the behavior of
340 FOUNDATIONS OF BIOLOGY
the organism we refer specifically to the reactions of the or-
ganism as a unit, rather than to the internal processes in the
economy of its life. And surveyed from a broad viewpoint,
there is discernible in the behavior of animals, just as in their
structure in general and in their nervous system in particu-
lar, from the lowest to the highest, a gradual increase in the
complexity of behavior. The behavior of Amoeba or Para-
mecium is an expression of the primary attributes of proto-
FIG. 183. — Diagram to illustrate the avoiding reaction of Paramecium. A, a
solid object or other source of stimulation. 1-6, successive positions taken by the
animal. The rotation on its long axis is not indicated. See Fig. 184. (After Jennings.)
plasm — irritability, conductivity, and contractility. So is the
behavior of Hydra and Earthworm, in which special cells
constitute a definite coordinating, or nervous, system. And
so is the complex behavior of the higher animals, including
Man, with their elaborate series of sense organs and highly
developed sensorium, or brain.
"Let us now try to form a picture of the behavior of Par-
amecium in its daily life under natural conditions. An indi-
vidual is swimming freely in a pool, parallel with the surface
and some distance below it. No other stimulus acting, it
begins to respond to the changes in distribution of its internal
contents due to the fact that it is not in line with gravity.
ADAPTATION OF ORGANISMS
341
4
It tries various new positions until its anterior end is directed
upward, and continues in that direc-
tion. It thus reaches the surface film.
To this it responds by the avoiding
reaction (Fig. 183), finding a new
position and swimming along near
the surface of the water. . . . Swim-
ming forward here, it approaches a
region where the sun has been shining
strongly into the pool, heating the
water. The Paramecium receives
some of this heated water in the
current passing from the anterior
end down the oral groove. (Fig.
184.) Thereupon it pauses, swings
its anterior end about in a circle,
and finding that the water coming
from one of the directions thus
tried is not heated, it proceeds for-
ward in that direction. This course
leads it perhaps into the region of a
fresh plant stem which has lately
been crushed and has fallen into the
water. The plant juice, oozing out,
alters markedly the chemical consti-
tution of the water. The Parame-
cium soon receives some of this
altered water in its ciliary current.
Again it pauses, or if the chemical
was strong, swims backward a dis-
t
FIG. 184. — Diagram to show the rotation on the long axis, and the spiral path of
Paramecium. 1-4, successive positions assumed. The dotted areas with small arrows
represent the currents of water drawn from in front. (After Jennings.)
342 FOUNDATIONS OF BIOLOGY
tance. Then it again swings the anterior end around in a
circle till it finds a direction from which it receives no
more of this chemical; in this direction it swims for-
ward. . . .
"In this way the daily life of the animal continues. It
constantly feels its way about, trying in a systematic way all
sorts of conditions, and retiring from those that are harmful.
Its behavior is in principle much like that of a blind and
deaf person, or one that feels his way about in the dark.
It is a continual process of proving all things and hold-
ing to that which is good." (Jennings.)
The behavior of Paramecium leaves one with the impres-
sion that the animal is largely at the mercy of its surround-
ings — that the environment rather than the organism itself
is the dominant factor, but this is true only to a limited
degree. Paramecium is not merely an automaton.' Its be-
havior is modifiable and, in the long run, is adapted to the
usual changes of its surroundings. That the reactions are
adequate for the simple life and methods of reproduction of
Paramecium is attested by the fact that it is one of the most
common and widely distributed animals.
In such simple beginnings, then, must be sought the largely
automatic responses of animals to the exigencies of external
conditions, known as REFLEXES and INSTINCTS. Both are the
result of inherited nervous structure and therefore may be
regarded as inherited behavior — just as truly characteristics
of the organism as form of body or method of reproduction.
And increase in the complexity of life processes has involved
a synchronous increase in the number and complexity of in-
stincts. The primitive reflexes and instincts of Hydra lead
it to seize with its tentacles small organisms within reach
and pass them to its mouth: the Earthworm, to swallow
decaying leaves as it burrows through the soil: the Crayfish,
ADAPTATION OF ORGANISMS 343
to grasp its prey with its large claws, tear it into pieces by
means of certain appendages about the mouth which are
adapted just for the purpose — and so on to the higher Verte-
brates where the feeding instincts reach their maximum of
complexity. The marvelous behavior of Ants and Bees is
essentially a complex of instincts. Turn the hive around and
the homing instinct of the Bees proves abortive — they can-
not find the entrance. Moreover, instincts of fear, self-
defense, play, care of the young, etc., render a considerable
part of the behavior of even the higher organisms more
'automatic' than is perhaps, at first thought, apparent. (Fig.
101.)
But just as the behavior of Paramecium and its allies is
modifiable, so instincts which seem the most stereotyped
show at least a slight degree of adaptability to unusual condi-
tions. And it is this ever-present modicum of modifiability,
which is in Man called /choice,' that leavens the whole and
becomes the dominant factor in the behavior of the highest
animals; while reflex action and instinct are relegated to a
subsidiary though by no means unimportant role.
The power of such more or less conscious 'choice' of re-
sponses to external conditions affords a gradual and ill-
defined transition from instincts to intellectual processes, or
reason. The foundations of both are to be sought in simple
reflex actions and oft-repeated voluntary actions which
gradually become habits — relegated to the level of reflex
actions. Indeed a large part of the education of Man con-
sists in establishing adaptive reflexes which relieve the
conscious life of innumerable simple factors of behavior, and
leave it more or less free for the higher intellectual processes.
Although it is necessary to emphasize that mind and intelli-
gence, in the biological sense, are expressions for that inte-
gration of nervous states and actions which makes possible
344 FOUNDATIONS OF BIOLOGY
a nicety of adaptation of behavior to environmental condi-
tions that otherwise would be impossible — that it is our
chief means of adaptation — • "it is a grave mistake to mini-
mize the importance of the great gulf between Man's nature
and that of the most highly developed of the lower animals.
In no respect are these differences more marked than in the
various forms of learning that, taken together, form the
.means of education." (Cameron.)
Thus it is clear that, with all the variations in structure and
function, organisms all possess irritability in common: they
all exhibit adaptive responses which enable them to exist in
spite of surrounding changes. " Adaptability appears to be
the touchstone with which nature has tested each kind of
organism evolved; it has been the yard-stick with which she
has measured each animal type; it has been the counter-
weight against which she had balanced each of her produc-
tions . . . the general course of evolution has been always
in the direction of increasing adaptability or increasing per-
fection of irritability." (Mathews.) The individual's heri-
tage affords the cumulative result of the adaptations of the
race — including adaptability.
CHAPTER XIX
THE ORIGIN OF SPECIES
Thoughtful men, once escaped from the blinding influences of
traditional prejudice, will find in the lowly stock whence Man
has sprung, the best evidence of the splendor of his capacities;
and will discern in his long progress through the Past, a reason-
able ground of faith in his attainment of a nobler Future.
— Huxley.
EVERYONE recognizes not only that there are many kinds
of animals and plants, but also that many individuals are
essentially the same. Groups may be formed of individuals
which differ less among themselves in the sum of their char-
acters than they do from the members of any other group
of individuals. And further, the members of a group produce
other individuals which are essentially similar. Such a
group of similar individuals is termed by the biologist a
SPECIES. It will be noted, therefore, that a species is merely
a concept of the human mind — the only reality in nature
is the individual, and an understanding of the differences be-
tween individuals gives us the key to the differences between
species. This seemingly obvious point of view has but
relatively recently been clearly emphasized by biologists, and
the species rather than the individual has loomed large
in the discussions of how plants and animals came to be what
they are to-day.
From the time of the Greek natural philosophers there
always have been men who have sought a naturalistic expla-
nation of the origin of the diverse forms of animals and plants,
345
346 FOUNDATIONS OF BIOLOGY
and who have suggested that the present ones arose from ear-
lier forms by a process of descent with modification, or EVO-
LUTION. But with the revival of natural history studies after
the Middle Ages, the Mosaic account of creation led the
majority, perhaps almost unconsciously, to assume that there
are as many kinds of organisms as issued from the Ark. And
this is not so strange, as might at first glance appear, when
one considers that all of the important facts which we have
reviewed in the preceding pages were then absolutely un-
known, and that the number of known kinds of animals
totalled but a thousand or so, instead of upward of a million,
as to-day.
The pioneer work of the early Renaissance naturalists
consisted principally of collecting and describing animals
and plants. This involved making a catalog of the different
kinds — classifying them in some way — and consequently
some basis of classification was sought. Thus attention was
focused on the kinds of species and for practical, if for no
other, reasons, the species assumed a prominence which over-
shadowed the individuals which composed it. As a matter of
fact during the eighteenth century the greatest student of
plant and animal classification, Linnaeus, emphasized the
idea that each species represents a distinct thought of the
Creator and that the object of classification is to arrange
species in the order of the Creator's consecutive thoughts.
This viewpoint is somewhat whimsically expressed by the
old naturalist who, finding a beetle which did not seem to
agree exactly with any species in his collection, solved the
difficulty by crushing the unorthodox individual under his
foot. (See page 391.)
We may consider that the consensus of opinion up to the
middle of the last century was overwhelmingly on the side
of SPECIAL CREATION and FIXITY OF SPECIES, and there-
THE ORIGIN OF SPECIES 347
fore against the idea occasionally advanced by men, as it now
appears, ahead of their times, that DESCENT WITH MODIFICA-
TION is the explanation of the origin of the diverse forms
of plants and animals. But, as nearly every one knows, a
complete reversal of opinion has occurred since 1860 — to-
day professional scientists and most educated laymen
accept ORGANIC EVOLUTION. And we have accepted it in the
preceding sections of this work; but if this appears to have
been prejudging the question, the explanation is that the
genetic connection of organisms is the guiding principle of all
biology — and the mere fact that an unbiased presentation
of the data seems to prejudge the question is the most cogent
presumptive evidence for evolution. It is true that there are
wide differences of opinion among biologists in regard to the
factors which have brought about the evolutionary change —
but there are none in regard to the fact of evolution itself. It
will be convenient, therefore, first to summarize the evidences
of evolution and then to discuss modern views in regard to
the methods of evolution. (See Glossary, 'evolution.')
A. EVIDENCES OF ORGANIC EVOLUTION
To one who has thoughtfully followed the preceding pages
there must immediately occur many facts which are readily
and reasonably interpreted from the point of view of descent
of one species from another, but which are entirely enig-
matical from that of the special creation of species. For
instance, one will recall the cellular structure of all organisms;
the method of origin and the fate of the germ layers in ani-
mals; the interrelationship of the urinary and reproductive
systems in the Vertebrates; the comparative anatomy of
the vascular and skeletal systems of Vertebrates; the simi-
larity of the physical basis of inheritance in animals and
plants; the gradual dominance of the sporophyte over the
348 FOUNDATIONS OF BIOLOGY
gametophyte from the lower to the highest plants; and
so on.
In general, such is the nature of the data which support
the evolution theory. Although the evidence, from the
nature of the case, must be indirect, it is none the less cogent
chiefly because the facts for evolution are from such diverse
sources and all converge toward the same conclusion. The
theory of evolution reaches the highest degree of probability,
since in every branch of botany and zoology all the data
are most simply and reasonably explained on the basis of
'descent with modification/ and not a single fact points to-
ward special creation. It is a cardinal principle of science
to accept the simplest conceptions which will embrace all
the facts.
We may now summarize some of the most striking evi-
dence from taxonomy, comparative anatomy, paleontol-
ogy, embryology, physiology, and distribution of ani-
mals. But, as will soon appear, it is impossible to arrange
the facts in hard and fast groups under these headings
-the evidence from one merges into that from another,
and in the final analysis nearly all are based on compara-
tive anatomy in the broadest sense of the term.
1. Taxonomy
When the serious study of classification was well under
way, biologists found increasing evidence of the similarity,
or affinity, of various SPECIES of animals and plants. Not
only is it possible to arrange animals, for example, in an
ascending series of increasingly complex forms, but also in
many cases it is difficult or impossible to decide just where
one species ends and the next begins. That is, the most aber-
rant individuals within a given species frequently approach
those of a closely similar species. There are intergrades.
THE ORIGIN OF SPECIES 349
Again, it is found that species themselves can be naturally ar-
ranged in more comprehensive groups to which the name
GENUS is applied. For example, the common Gray Squirrel
represents the species carolinensis, and the Red Squirrel, the
species hudsonicus. Both are obviously Squirrels, and there-
fore both species are grouped under the genus Sciurus. Ac-
cordingly, each animal is given a name composed of two
words: the first, generic and the second, specific. The Gray
Squirrel is Sciurus carolinensis and the Red Squirrel is Sciurus
hudsonicus. Thus to give a scientific name to an animal or
plant is really to classify it, because the first word of the
name indicates that it possesses some fundamental char-
acteristics in common with the other species of the genus —
in fact, is more like them than it is like any other group of
organisms.
But again, the members of the genus Sciurus have many
characteristics in common with other animals which obvi-
ously are not true squirrels. The Chipmunks or Ground
Squirrels, for instance, differ not only in certain obvious
features, but in the possession of internal cheek pouches,
etc. This dissimilarity and similarity is expressed by placing
them in a different genus, Tamias, but in the same FAMILY,
Sciuridae. The familiar eastern Chipmunk is Tamias
striatus.
Moreover, while the Beaver (Castor americana) differs
still more from the Squirrels than do the Chipmunks, and
therefore is placed in a distinct family, the Castoridae, it
nevertheless agrees with both in many fundamental ways, so
that it is placed in the ORDER Rodentia, which also includes
the Squirrels and Chipmunks, as well as many other families
and genera. Other orders, such as the Ungulata (Horses,
Cattle, etc.) and the Carnivora (Cats, Dogs, Bears, etc.),
while they differ widely from the Rodents, still agree with
350 FOUNDATIONS OF BIOLOGY
them in possessing hair, and milk glands for suckling the
young. This basic likeness is expressed by including all un-
der the CLASS Mammalia.
The Mammals in turn are readily distinguished from Birds,
Reptiles, Amphibians, and Fishes (each of which forms a sepa-
rate class), but nevertheless are constructed on the same
fundamental plan, comprising a dorsal central nervous sys-
tem surrounded by skeletal elements forming the skull and
spinal column. Therefore, all are comprehended in the larger
group Vertebrata, in contrast with the Invertebrate groups
which include Hydra, Earthworm, Crayfish, etc. (See pp. 116,
146, 414.) The classification of the Gray Squirrel, Stiurus
carolinensis, (Fig. 86.) may be outlined as follows:
SUBPHYLUM — Vertebrata.
CLASS — Mammalia.
ORDER — Rodentia.
FAMILY — Sciuridae.
GENUS — Sciurus.
SPECIES — S. carolinensis.
This classification of the Gray Squirrel, although it
incidentally serves to illustrate the general method of classi-
fication of all organisms, is important because it places con-
cretely before us the fact that organisms show such funda-
mental similarities with obvious dissimilarities. In short,
the mere fact that animals and plants naturally arrange
themselves, as it were, in classes, orders, families, genera,
species, etc., raises the question of the origin of species. Is
special creation implying fixity of species, or is descent with
modification the more plausible explanation?
The unavoidable answer is, descent with modification —
evolution — because the principle in accordance with which
the groups of increasing comprehensiveness are formed is
THE ORIGIN OF SPECIES 351
solely the greater or less similarity in the structural features
of the organisms. It its much more reasonable to assume
that the thread of fundamental similarity which runs through
all the Vertebrates, for instance, is the result of inheritance,
while the differences of orders, families, genera, etc., are
due to changes brought about under different unknown
conditions, than it is to assume that each is the result of
a special creative act. Especially so when we realize that in
a very large number of cases it is difficult or impossible to
decide the limits of a species, owing to variations among
the individuals comprising it, and it is necessary to resort
to subspecies and varieties in classification. And further,
among genera, intergrading forms demand subgenera; among
orders, suborders; among classes, sub-classes; and so on. If
we admit the origin by descent with modification of the sub-
species and varieties, there is no logical reason for denying
the same origin of species, orders, and higher groups. The
difference is one of degree and not of kind. Before the recog-
nition of evolution classification was a groping after an
elusive ideal arrangement which naturalists felt but were
unable to express except in artificial form and in transcenden-
tal terms. Under the influence of the evolution theory classi-
fication became the natural expression of biological pedigrees.
2. Comparative Anatomy
The evidence from taxonomy is, as has just been seen, really
evidence from comparative anatomy, since modern classifi-
cations are based chiefly on anatomical characters. The
various groups — classes, orders, families, genera, species,
etc. — are founded not on a single difference, nor on several
differences, but on a large number of similarities. For in-
stance, the differences exhibited throughout the five classes
of the Vertebrates are relatively slight in comparison with
352
FOUNDATIONS OF BIOLOGY
THE ORIGIN OF SPECIES 353
the fundamental resemblances. This similarity in dis-
similarity is brought out by the science of comparative
anatomy. A few concrete examples, some of which we are
already familiar with, will serve to bring the main facts clearly
before us.
The fore-legs of Frogs and Lizards, the wings of Birds, the
fore-legs of the Horse, and the arms of Man are built on the
same basic plan. (Figs. 80, 81, 185, 186.) The same is true of
the hind-limbs. Clearly all are homologous structures, such
variations as exist being brought about chiefly by the modi-
fication or absence of one part or another. In short, all the
chief parts of both the fore-limbs and the hind-limbs are
homologous throughout the series. All are composed of
the same fundamental materials disposed in practically
the same way — nearly all the bones, muscles, blood vessels,
and nerves are homologous. Or compare the digestive sys-
tems of the same forms, or the excretory and reproductive
systems. One has but to recall that, on an earlier page, it
was possible to describe in general terms these systems as
they exist throughout the Vertebrate series — in forms as
obviously different as Fish and Man. They are all funda-
mentally the same. (Figs. 82-87, 97.)
Turning to the Invertebrates, we may remind the reader
that all the appendages of the Crayfish are built on the same
simple biramous plan as exhibited in the swimming legs
(swimmerets) of the abdomen. The highly specialized
walking legs, great claws, jaws, and feelers (antennae and
antennules) are all reducible to modifications of the simple
swimmeret type. (Fig. 72.) In short, all are homologous
structures, though differing widely in function. This is a
most striking example of SERIAL HOMOLOGY, though we have
seen the same principle exhibited in the Vertebrates where
the fore-limbs and the hind -limbs of each animal are homolo-
354
FOUNDATIONS OF BIOLOGY
Fio. 186. — Skeletons of Man and of Gorilla. (From Lull.)
THE ORIGIN OF SPECIES
355
gous. Moreover, the appendages of the Crayfish are not only
serially homologous among themselves, but are also homolo-
gous with the appendages of all the other members of the
class Crustacea — just as the limbs of one Vertebrate are
homologous with those of all other Vertebrates.
Another class of facts presented by comparative anatomy
is derived from the so-called VESTIGIAL organs. In Man
there are nearly a hundred structures which at best are
useless and sometimes are harmful. One thinks at once of
the VERMIFORM APPENDIX of the large intestine, a remnant
of an organ which serves a useful purpose in the vegetable-
feeding (herbivorous) Mammals. (Fig. 88.) But equally
suggestive are the muscles of the ear, which in some indi-
viduals are sufficiently developed to move the external ear;
the so-called third eyelid at the inner
angle of the eye which corresponds
to the lid (NICTITATING MEMBRANE)
that moves laterally across the eye
in Bird and Frog; or the terminal
vertebrae (COCCYX) of the human
spinal column which are remnants
of the tail of lower Vertebrates.
(Fig. 87.)
Other animals are likewise replete
with such structures. Porpoises
possess vestiges of hind-limbs en-
closed within the body, and cer-
tain species of Snakes have tiny use-
less hind-legs. The 'splint bones' of
the Horse are remnants of lost toes.
Among plants, it will suffice to mention the functionless
remnant of the pistil which sometimes is present in 'male'
(staminate) flowers. (Figs. 166, 187, 189.)
FIG. 187. — Vestigial hind-
limbs of a Snake, Python, f,
femur or thigh bone; il, ilium
or hip bone. (From Romanes.)
356 FOUNDATIONS OF BIOLOGY
In another class of cases, the organs, or remnants of organs,
of a lower form are altered or completely made over, as it
were, into new organs of the higher form. During the embry-
onic life of all Vertebrates there are gill slits, all of which soon
vanish except one, which remains as an opening (EUSTACHIAN
TUBE) connecting the middle ear with the pharynx. (Fig.
110.)
Gill arches, which function as supports for the gills in the
aquatic Vertebrates, persist in highly modified form as skele-
tal structures associated with the tongue and entrance to the
lungs (LARYNX) in terrestrial forms. The milk glands of
Mammals are transformed sebaceous glands of the skin,
while the poison glands of Snakes are specialized salivary
glands of the mouth. Finally, in this connection the reader
will recall the transformations of the blood vessels in the
Vertebrates which occur with the substitution of lungs for
gills, and also the variations and interrelationships of the
excretory and reproductive systems in the ascending series
of Vertebrate classes. (Figs. 95, 97.)
One may, of course, conclude from all these facts that Fish,
Frog, Lizard, Bird, and Man have each been independently
created according to the same preconceived plan — and like-
wise all the great numbers of orders, families, genera, species,
etc., of each of the five classes that these forms represent.
Or, one may conclude, that all have arisen by descent with
modification from a primitive Vertebrate organism which
possessed the fundamental similarities exhibited from Fish
to Man. The latter is the conclusion accepted unreservedly
by biologists to-day.
3. Paleontology
Huxley once said that if zoologists and embryologists had
not put forward the theory of evolution, it would have been
THE ORIGIN OF SPECIES 357
necessary for paleontologists to invent it. What then are the
main facts offered by the study of the fossil remains of extinct
animals and plants?
In the first place it must be made clear that geologists are
able to determine, with remarkable accuracy in most cases,
the sequence in time, or CHRONOLOGICAL SUCCESSION, of the
rock strata composing the Earth's surface. The main outline
of this scheme of geological chronology was understood long
before the evolution of organisms was a crucial question; so
that we may consider the evidence which it affords of the
chronological succession of the fossil remains exhibited by
the various strata, as impartial testimony to the order of ap-
pearance on the Earth of the different types of animals and
plants.
The following geological time-table summarizes the
panoramic succession of life as it is seen by the paleontolo-
gist. It is useless to attempt to state the absolute duration
of geologic time, because we have little more than guesses to
depend on, though there are fairly reliable data in regard to
the relative length of the various eras. Perhaps the conserva-
tive estimate of 500,000,000 years — at least half of which
was before the Permian period — will serve to spell the
Earth's unfathomable past.
THE GEOLOGICAL TIME-TABLE1
PRESENT TIME.
PSYCHOZOIC ERA. AGE OF MAN OR AGE OF REASON.
Includes the present or 'Recent time/ and the time during
which Man attained his highest civilization, estimated to be
probably less than 30,000 years.
GEOLOGIC TIME.
CENOZOIC ERA. AGE OF MAMMAL DOMINANCE.
Glacial or Pleistocene time. Last great ice age.
Late Cenozoic or Pliocene and Miocene time. Primates changing
into Apes and Man.
Early Cenozoic or Oligocene and Eocene time. Rise of higher
Mammals, including Primates.
MESOZOIC ERA. AGE OF REPTILE DOMINANCE.
Cretaceous period. Rise of primitive Mammals.
Comanchian period. Rise of Flowering Plants and higher In-
sects.
Jurassic period. Rise of Birds and flying Reptiles.
Triassic period. Rise of Dinosaurs, and Mammalian stock.
PALEOZOIC ERA. AGE OF FISH DOMINANCE.
Permian period. Rise of Reptiles. Another great ice age.
Pennsylvanian period. Rise of Insects and first time of marked
coal accumulation.
Mississippian period. Rise of marine Sharks.
Devonian period. First known marine Fishes, and Amphibians.
Silurian period. First known land floras.
Ordovician period. First known fresh-water Fishes.
Cambrian period. First abundance of marine fossils, and
dominance of Trilobites.
PROTEROZOIC ERA. AGE OF INVERTEBRATE DOMINANCE.
An early and a late ice age.
ARCHEOZOIC ERA. ORIGIN OF PROTOPLASM AND OF SIMPLEST LIFE.
COSMIC TIME:
FORMATIVE ERA. BIRTH AND GROWTH OF THE EARTH OUT OF THE
SPIRAL NEBULA OF THE SUN.
Beginnings of the atmosphere and hydrosphere, and of con-
tinental platforms and oceanic basins. No known geological
record.
1 From The Earth's Changing Surface and Climate by Professor Charles Schuchert.
See Bibliography.
358
THE ORIGIN OF SPECIES 359
Even a casual survey of this history - — natural history —
of the Earth and its inhabitants cannot but impress one with
the fact that, taken all in all, there has been a continuous,
though not always a uniform, advance in the complexity of
organisms from the most ancient times, and that the older
types seem gradually to melt into modern forms as the
remoter geological eras merge into the more recent. "Only
the shortness of human life allows us to speak of species as
permanent entities." Invertebrates appear in the Protero-
zoic Era; Fishes and Amphibia in the Paleozoic; Reptiles,
Birds, and Primitive Mammals in the Mesozoic; higher
Mammals and Man in the Cenozoic. Mosses and Ferns arise
before Conifers and the latter before the familiar Flowering
Plants. "Just in proportion to the completeness of the
geological record is the unequivocal character of its testimony
to the truth of the evolutionary theory." For the sake of
concreteness we may select two examples from the wealth of
material offered by the paleontologist.
At first glance there seems to be little but contrasts
between a typical Reptile and a typical Bird; between a
cold-blooded, scaly-skinned Lizard, let us say, and a warm-
blooded, feathered Pigeon. And yet the zoologist is con-
vinced that Birds have evolved from a reptilian stock,
because, in spite of superficial dissimilarities, there are funda-
mental structural resemblances not only between adult
Reptiles and Birds, but also between their developmental
stages. And further, because, the fossil remains of a very
primitive Bird, Archaeopteryx, have been found which form,
in many ways, a connecting link between the Reptiles and
Birds as we know them to-day.
Archaeopteryx was undoubtedly a bird about the size of
a Pigeon, but one with jaws supplied with many small teeth;
with a long lizard-like tail formed of many vertebrae, each
360
FOUNDATIONS OF BIOLOGY
bearing a pair of quill feathers; w,ith a four-fingered rep-
tilian hand; and so on. In brief, just such a creature as the
imagination of an evolutionist would picture for a primitive
Bird has been disclosed by the lithographic stone quarries of
Fia. 188. — Reptilian Bird, Arckaeopteryx, (A), compared with Pigeon,
Columba livia (B). (From Lull.)
Bavaria, representing the later Jurassic period. (Fig. 188.)
The ancestry of the modern Horse has been the most im-
pressive 'fossil pedigree/ ever since Professor Marsh collected
the famous series of fossil skeletons from the western United
THE ORIGIN OF SPECIES 361
States and arranged them in the Yale Museum. The essen-
tial facts are these. Horse-like animals probably arose
from an extinct group known as the Condylarthra- which had
five toes on each foot and a large part of the sole resting
on the ground. However, the first unquestionably horse-
like form found in North America is a little animal less than
a foot in height, known as Eohippus, from rocks of the Eocene
age. The fore-foot of Eohippus has four complete toes
(digits 2, 3, 4, and 5) and a vestige of the first digit in the
form of a splint bone. The hind-foot has three toes (digits
2, 3, and 4) with a remnant of the fifth digit. Later in the
Eocene we find Protorohippus with the same functional
digits but lacking the vestiges. Coming to the Oligocene,
Mesohippus appears. This animal is about the size of a
sheep and still has three toes (digits 2, 3, and 4) on the hind-
foot, but only three complete toes (digits 2, 3, and 4) and the
vestige of a fourth (digit 5) on the fore-foot. Also the
middle toe (digit 3) is now much larger than the side toes,
which barely touch the ground. Then during the late
Miocene and early Pliocene we find Protohippus, an animal
about three feet tall, with three toes on each foot, but with
only one reaching the ground, and with no vestiges of other
digits. Finally, toward the end of the Pliocene, appears
the genus Equus which includes the modern horse, Equus
caballus, with one functional toe (digit 3) on each foot and
the remnants of two more (digits 2 and 4) in the splint bones.
(Fig. 189.)
In this outline of what must be interpreted as the fossil
ancestors of the Horse of to-day, we have merely selected
several representative forms to emphasize changes in foot
structure. But the reader will realize that many other
equally significant changes were involved in the transforma-
tion of an Eohippus type into that of Equus. This much
362
FOUNDATIONS OF BIOLOGY
THE ORIGIN OF SPECIES
363
EVOLUTION OF THE CAMELS
•fe
fe
Recent
3leistocene
Auchenio
(Llama)
Skutl
Feet
1
Pliocene
Procamelus
Miocene
<
fc
^
d
Poebrotherium
Ohgocene
Protylopus
Eocene
1
Mesozoic or Age of Reptiles
Hypothetical five-toed Ancestor
FIG. 190. — Graphic presentation of the evolution of the Camel.
(From Lull, after Scott.)
364 FOUNDATIONS OF BIOLOGY
appears certain to the biologist: "In early Eocene times
there lived small five-toed hoofed quadrupeds of generalized
type, that the descendants of these were gradually specialized
throughout long ages along similar but by and by divergent
lines, that they lost toe after toe till only the third remained,
that they became taller and swifter, that they gained longer
necks, more complex teeth and larger brains. So from the
short-legged splay-footed plodders of the Eocene marshes
there were evolved light-footed horses running on tiptoe
on the dry plains. " (Thomson.)
4. Embryology
If evolution is a fact, one would expect to find evidences
of the genetic relationships of organisms in their individual
development from egg to adult, that is in ontogeny. Under
former headings we have incidentally mentioned embryo-
logical data which point toward evolution, so that now
attention may be confined to an attempt to make clear a fact
of first importance — the history of the individual (ONTOGENY)
frequently corresponds in broad outlines to the history of the
race (PHYLOGENY) as indicated by evidence from comparative
anatomy, etc. If we have in mind the earlier discussion of
Vertebrate anatomy, one or two examples will suffice to
suggest the type of evidence which supports this so-called
RECAPITULATION THEORY, Or BIOGENETIC LAW.
Lower Vertebrates, such as the Fishes, have a heart com-
posed of two chief chambers : an auricle which receives blood
from the body as a whole and a ventricle which pumps it to
the gills on its way to supply all parts of the body. Among
the members of the next higher group, the Amphibia (Frogs,
Toads, etc.), the auricle is divided into two parts, while the
ventricle remains as before. Thus these forms have a three-
chambered heart. Passing to the Reptiles, we find that
THE ORIGIN OF SPECIES 365
most of the Lizards, Snakes, and Turtles have the ventricle
partially divided into two chambers, while the more special-
ized Crocodiles and Alligators have a complete partition and
therefore a four-chambered heart. This is the condition in
all adult Birds and Mammals, but the significant fact is that,
in the development of the heart of the individual Bird and
Mammal, embryonic stages succeed each other which parallel
in a general though remarkable way this sequence from a
two-chambered to a four-chambered condition as exhibited
in the adults of the lower Vertebrates. (Figs. 91, 92.)
Or take the development of the brain in the Vertebrate
series. Even in the human embryo the fundament of the
brain arises by simple transformations of the anterior end of
the neural tube, which at first are nearly indistinguishable
from the conditions which exist in the lowest Vertebrates.
Then the changes become progressively more complex along
lines broadly similar to those occurring from Fish to Mammal,
until finally the complex human brain is formed. (Figs.
104, 105.)
The same picture is presented by a study of the develop-
ment of the excretory system, the reproductive system
(Fig. 97), the skull, and so on. One cannot avoid the fact
that the organs of higher animals pass through develop-
mental stages which correspond with the adult condition
of similar organs in lower forms. The correspondence is not
exact, to be sure, but it is not an exaggeration to say that
embryological development is parallel to that which ana-
tomical study leads us to expect. A knowledge of the
anatomy of an animal actually gives a sound basis of facts
from which to predict in broad outlines its embryological
development. (Fig. 191.)
What are the bearings of these facts on the evolution
theory? It is perfectly logical to conclude that it is an
366
FOUNDATIONS OF BIOLOGY
'architectural necessity,' let us say, for the four-chambered
heart to arise from a two- and three-chambered condition —
and undoubtedly if this were the only example of 'ontogeny
repeating phylogeny' the conclusion might be justified.
But when one considers the widespread general correspond-
FIG. 191. — Embryos in corresponding stages of development. A, Fish (Shark) ;
B, Bird; C, Man. g, gill slits. (From Scott.)
ence of the developmental stages in higher forms with con-
ditions as they exist in the adults of lower forms, the facts
almost overwhelmingly force us to go further and conclude
that the similarity has its basis in inheritance, in actual
blood relationship between the higher and lower forms, in
descent with modification — evolution.
THE ORIGIN OF SPECIES 367
5. Physiology
Fundamental structural similarities throughout a series
of organisms implies fundamental physiological similarities —
structure and function go hand in hand, each being an expres-
sion of -the other. But the physiological evidence is less
readily presented in brief form, so we may confine attention
to one striking example on the borderline.
It has been known for a long time that there are important
chemical differences — not determinable by ordinary chemical
analysis — between the blood even of closely related species,
because the transfusion of the blood of one species into
another is usually attended by physiological disturbances
and often by death. It has been found by innumerable
transfusions and also by so-called precipitation tests of
the blood in vitro, that is outside the body, that the degree
of the ' reaction ' is in many cases proportional to the degree
of relationship of the species involved, as indicated by their
classification on the basis of anatomical criteria.
Thus, as one would expect, human blood shows closer
chemical relationships with the blood of the Man-like Apes
than it does with that of the Old World Monkeys; closer
relationships with the blood of the latter than it does with
that of the New World Monkeys; and closer with the blood of
these than with that of the Lemurs; and so on. Or, descend-
ing to the Reptiles: paleontology indicates that there is a
close relationship between Lizards and Snakes and also
between Turtles and Crocodiles, while the reptilian ancestor
of the Birds was probably more closely allied with the latter
than the former groups. These same relationships are
indicated by blood tests.
Thus aside from a few startling exceptions, which further
study perhaps may bring into line, all the data warrant the
conclusion that the chemical similarities of the blood are
368 FOUNDATIONS OF BIOLOGY
almost as constant as the structural similarities of the blood
vessels, or, in evolutionary terms, "a common property has
persisted in the bloods of certain groups of animals through-
out the ages which have elapsed during their evolution from
a common ancestor." Blood relationship is a fact.
6. Distribution
Every one recognizes that the fauna and flora are not the
same in all regions of the Earth. There is a characteristic
life on mountain, plain, and seashore, and in the sea — as well
as in pond and puddle — and also in the Arctic, Temperate,
and Torrid zones. But the problem of animal and plant
distribution is by no means so simple as this statement
might seem to imply, because the study involves the in-
vestigation of both the relations of the various organisms to
the general environing conditions, and also the interrelations
of the species with each other. It forms a part of the sciences
of plant and animal ECOLOGY.
Confining attention merely to the geographical distribu-
tion of animals — which forms the science of ZOOGEOG-
RAPHY— let us take a couple of clear-cut examples and see
whether special creation or evolution is the more reasonable
explanation of the facts.
At the present time a characteristic family of Mammals,
known as the Tapirs, is represented by distinct species in two
widely separated regions, Central and South America and
Southern Asia and adjacent islands. But paleontological
studies show that in the Pliocene period Tapirs were distrib-
uted over nearly all of North America, Europe, and Northern
Asia, and thereafter gradually became extinct so that by the
close of the Pleistocene period the remnants were distributed
as we find them to-day. In brief, the present discontinuous
distribution represents the remnants of a world-wide Tapir
THE ORIGIN OF SPECIES 369
population, and the differences between the existing species
are such as one might expect to find among the members of a
genus long isolated in different environments by geographical
barriers. We know, for example, that a litter of European
Rabbits was introduced on the small island of Porto Santo
during the fifteenth century and by the middle of the last
century its descendants had become so distinct from the
parent form that it was described as a 'new species/
As a matter of fact the characteristic fauna of islands
was what impressed Darwin with the need of some interpre-
tation other than special creation. During his famous three
years' voyage around the world on the "Beagle," he stopped
at the Galapagos Islands, situated about 600 miles off the
west coast of South America, and was astonished to find that
although the fauna as a whole resembled fairly closely that
of the mainland, nevertheless the species for the most part
not only were different, but even those of the separate islands
were distinct — the islands nearest to each other having
species most similar. Darwin wrote, "My attention was
first thoroughly aroused by comparing together the numerous
specimens, shot by myself and several others on board, of
Mocking Thrushes, when, to my astonishment, I discovered
that all those from Charles Island belonged to one species
(Mimus trifasdatus) ; all from Albemarle Island to M. par-
vulus] and all from James and Chatham Islands (between
which two other islands are situated as connecting links)
belonged to M . melanotis."
Darwin's observations of such facts as these have been
corroborated in the Galapagos and extended to isolated
island faunas and floras all over the world. And further, his
explanation of the phenomena is the most plausible extant.
Continental islands secure their life from the mainland before
they are cut off, and Oceanic islands after their formation by
370
FOUNDATIONS OF BIOLOGY
Loxodon
Africa
Elephas
Asia
o
Extinct Nf
Elephas
Extinct
Elephas Mastodon
?
Extinct
Dibelodon
Extinct
Meritherium
Africa
Meritherium
Africa
..t
Proboscideo-Sirenian
ancestor
FIG. 192. — Chart of the phylqgeny of the Elephants, showing their geological
and geographical Hiytribution. (After T,ulM
THE ORIGIN OF SPECIES
371
FIG. 193. — Evolution of the head and molar teeth of Elephants. A, A', Elephas,
Pleistocene; B, Stegodon, Pliocene; C, C', Mastodon. Pleistocene; D, D', Trilophodon,
Miocene; E, E', Palaeomastodon, Oligocene; F, F', Moeritherium, Eocene. (After
Lull.)
372 FOUNDATIONS OF BIOLOGY
volcanic action alone or aided by coral growth. In either
event the organisms are isolated from the main stock of the
species, and in proportion to the length of time and the degree
of isolation the insular forms diverge until separate races and
species arise. Each species peculiar to each isolated island
has not arisen by a special act of creation but by descent with
modification.
B. FACTORS OF ORGANIC EVOLUTION
We have now summarized a few concrete examples of the
chief types of evidence that organisms — species — have
come to be what they are to-day through a long process of
descent with modification. This evidence, taken with that
presented, so to speak, on and between the lines throughout
this work, should- place the reader in a position to form a more
or less independent judgment of the question. It is only nec-
essary to remind him again that, since the evidence, from the
nature of the case, must inevitably be indirect, its cogency is
tremendously increased by its amount. And the overwhelm-
ing impressiveness of all the concordant evidence for organic
evolution the reader, with only a very limited amount of
the data before him, cannot appreciate.
Taking for granted the fact of evolution — as we have had
to do throughout — what are the factors which have brought
evolution about? That is quite a different question, but one
which has often brought confusion to the popular mind.
Biologists are not so sure to-day as they were a generation
ago that they know just what the factors are. And the lay-
man has mistaken their questioning of one factor or another
for a questioning of the fact.
No purpose will be served by a long historical account of
the origin of the present-day point of view. Suffice it to say
that the evolution idea is a generalization which has crept
THE ORIGIN OF SPECIES
373
FIG. 194. — A few varieties of domestic Pigeons. Over one hundred and fifty
different breeds have been derived by selection from the wild Blue-rock Pigeon, some
of which "differ fully as much from each other in external characters as do the most
distinct natural genera." (Darwin.) 1, Blue-rock Pigeon, Columba livia, ancestral
form; 2, homing; 3, common mongrel; 4, archangel; 5, tumbler; 6, bald-headed
tumbler; 7, barb; 8, pouter; 9, Russian trumpeter; 10, fairy swallow; 11, black-
winged swallow; 12, fantail; 13, carrier; 14, 15, bluetts; bird between 14 and 15, a
tailed turbit. (From photograph of an exhibit in the United States National Museum.)
374 FOUNDATIONS OF BIOLOGY
from science to science — from Astronomy to Geology, from
Geology to Biology. The idea in one form or another is as
old as history, but for all practical purposes the biologist
Lamarck, during the early part of the nineteenth century,
formulated the first consistently worked out theory of organic
evolution. But the evidence he presented was in many cases
neither happily selected nor convincingly presented and it
was laughed out of court by biologists and laymen alike.
Lamarck's evolution factor was essentially the change of the
organism through the use and disuse of parts, the physiologi-
cal response of the organism to new needs offered by new con-
ditions of life. And these changes, somatic in origin, he be-
lieved were transmitted to the progeny. As we know, to-
day little or no value is placed on such somatic changes
as evolution factors, because there is no evidence that they
are heritable. But this weak point was not the one which
caused the rejection of the theory by Lamarck's contempo-
raries. The various antagonistic influences can be summed
up by saying, the time was not ripe for evolution.
Then a generation later appeared Charles Darwin in
England. With a better background prepared for him through
the headway being made by the evolution theory in geology,
he did two things. He presented an overwhelming mass of
data which could be explained most reasonably by assuming
the origin of existing species by descent with modification
from other species. And he offered as an explanation of the
origin of species the theory of "NATURAL SELECTION, or the
preservation of favoured races in the struggle for life." It
was the combination of the facts and the theory to account for
the facts which won the thinking world to organic evolution.
What, in brief, was the theory? In the first place, without
discussing the cause of variations, Darwin showed the great
amount of variation in nature. And any and all kinds of
THE ORIGIN OF SPECIES 375
variation were, broadly speaking, equally important — he
made no sharp distinction between somatic and germinal.
The universality of variations established, Darwin empha-
sized the fact that the power of reproduction of organisms far
exceeds space for them to live in and food for them to eat.
Some recent facts will illustrate this point. A single micro-
scopic Paramecium possesses the potentiality to eat, grow,
and reproduce — to transform the materials of its environ-
ment into Paramecium protoplasm — at the rate of 3000
generations in five years. And all the descendants (if they
actually existed) would equal 2 raised to the 3000th power,
or a volume of protoplasm approximately equal to 101000
times the volume of the Earth !
Something must inhibit the inherent power of each species
to overpopulate the Earth, and Darwin emphasized the
struggle for existence between the individuals of species.
Since the struggle is so keen, a variation, however slight,
which fits — adapts — an individual better to its surroundings
than its neighbors are adapted, will, more often than not, give
its possessor an advantage in the struggle, and accordingly
the latter will survive to pass on the favorable variation to
its progeny. Thus is brought about, in Spencer's phraseology,
"the survival of the fittest" -the survival of those indi-
viduals, and therefore species, which are adapted to the pe-
culiar conditions of their environment and mode of life. And
note, this offers an explanation of the fact of adaptation itself
- the most striking phenomenon which organisms exhibit.
This is all so simple from one point of view and so confus-
ingly complex from others that it may well be restated in a
couple of sentences by Darwin himself: "As many more
individuals of each species are born than can possibly survive,
and as, consequently, there is frequently recurring struggle
for existence, it follows that any being, if it vary however
376 FOUNDATIONS OF BIOLOGY
slightly in any manner profitable to itself, under the complex
and sometimes varying conditions of life, will have a better
chance of surviving, and thus be naturally selected. From
the strong principle of inheritance any selected variety will
tend to propagate its new and modified form."
Nothing succeeds like success, and once started Darwin's
theory gradually swept all opposition away, and some of its
exponents out-Darwined Darwin. Then, as was to be ex-
pected, the reaction came. Acquired characters are not
heritable; variations are swamped by interbreeding; large
variations and not small fluctuating variations are crucial;
and so on. But it is not necessary to obscure the main issue
by entering into these controversies. What is the status of
the theory of natural selection to-day?
Evolution is not a closed book — an event which has been
completed in the past — but a process which is actively going
on now. "Nothing endures save the flow of energy and the
rational order that pervades it." And there is every reason
to believe that the factors involved in present evolution are
the same as those which have operated in the past. The
uniformitarian doctrine has proved productive in explain-
ing the evolution of the Earth, and there is every reason to
think that this viewpoint will prove — is proving — equally
valuable in understanding the origin of the diverse inhabit-
ants of the Earth. We have come to realize that evolution
is a bird's-eye view of the results of heredity, since the origin
of life — the facts of inheritance hold the key to the factors
of evolution. Therefore in a previous chapter we discussed
the relations of recent discoveries in genetics to the evolution
problem, some of which may be restated now with special
reference to the origin of the fitness of organisms.
In the first place we have seen that though variations are
the rule and not the exception, some are of importance for
THE ORIGIN OF SPECIES 377
evolution and some are not. All the evidence indicates that the
effective variations are germinal and not somatic. Changes
arising in the soma — acquired characters — are unable
so specifically to modify the germ that they are 'born again'
and evolution must be brought about by the evolution of the
germ itself. Accordingly selection must operate to eliminate
the 'unfit' germ plasm rather than the unfit soma, though as
a matter of fact the fitness of an individual is determined
largely by its somatic characters. This apparently is the
crux of the matter and presents a complication of the mental
picture of the operations of selection which did not exist
when we thought of soma producing germ and germ produc-
ing soma again. Since individuals frequently belie their
germinal condition — what they will pass on to their progeny
— selection has, so to speak, a more devious though not less
sure path.
Secondly, how does the germ plasm change? Developed
characters are the result of the activities of one or more genes.
Of course, a gene is not a character. It is not even an unde-
veloped character. Characters in many cases arise from the
interaction of several genes, though, since one gene deter-
mines whether the gene complex will give rise to a certain
character, it is really the determining factor — for example,
the so-called sex gene on the so-called sex chromosome. Such
being the case, characters may be changed by alterations of
the gene complex. (Cf. p. 298.) This may be from the
influence of changes within the soma itself or from the en-
vironment of the organism, but here particularly we are
on debated ground. On the whole, it may be said that muta-
tions— germinal changes other than those arising from
recombinations — seem to be infrequent compared with
non-heritable changes of somatic origin.
These facts from genetics, taken in connection with
378 FOUNDATIONS OF BIOLOGY
the data from geographical distribution, the succession of
types in the geologic past, and the great diversities in 'breeds'
in nature, etc., give us the modern background for attempt-
ing to form an opinion of the method of evolution. The
consensus of opinion seems to be that natural selection in
some form is the guiding principle in the establishment of the
'adaptive complexes' of organisms. Evolution is the result
of germinal variations, largely independent of environing
conditions. Many of these variations give rise to characters
which neither increase nor decrease the adaptation of the
organism, and consequently are neutral from the standpoint
of its survival. With regard to such characters natural
selection is essentially inoperative. Other germinal variations
arise which produce adaptive structures and here natural
selection is effective — it sifts them out, as it were, from the
unadaptive and neutral variations and in this way makes
possible their survival value in the struggle for existence.
So, it will be noted, this is essentially a clarified Darwinism.
Instead of all variations being heritable — some are inherited
and some are not. Instead of all heritable variations being
important — some are and some are not. The important
ones are the heritable adaptive variations and these form
the raw materials for natural selection. Natural selection
is still the only natural explanation of that coordinated
adaptation which pervades every form of life, but it is prob-
able— indeed, positive — that there are more factors involved
than are dreamt of in our philosophy.
CHAPTER XX
EPOCHS IN BIOLOGICAL HISTORY
History must convey the sense not only of succession but
also of evolution.
SOME knowledge of hunting, agriculture, and husbandry
was one of the early acquirements of prehistoric Man, and
at the dawn of history, nearly 5000 years ago, systems of
medicine apparently found a place in Egyptian and Babylo-
nian civilizations. So, on the practical side, biology has a very
ancient beginning. But biology as the science of life in
which emphasis is transferred to the philosophical — to the
study of vital phenomena for their own sake — really begins
with the Greeks.
Science reaching Greece from the South and East fell upon
fertile soil, and in the hands of the Hellenic natural philoso-
phers was transformed into coherent systems through the
realization that nature works by fixed laws — a conception
foreign to the Oriental mind and the corner-stone of all
future scientific investigation. It is not an exaggeration
to say that to all intents and purposes the Greeks laid the
foundations of the chief subdivisions of natural science and,
specifically, created biology.
A. GREEK AND ROMAN SCIENCE
ARISTOTLE (384-322 B.C.), the most famous pupil of
Plato and dissenter from his School, represents the high-
water mark of the Greek students of nature and is justly
called the Father of Natural History. Although Aristotle's
379
380 FOUNDATIONS OF BIOLOGY
contributions to biology are manifold, perhaps of most
significance is the fact that he took a broad survey of the
existing data and welded them into a science. He did this
by relying, to a considerable extent, on the direct study of
organisms and by insisting that the only true path of advance
lies in accurate observation and description. The observa-
FIG. 195. — Aristotle.
tional method and its very modern development, the labora-
tory method of biological study, find their first great exponent
in Aristotle. But mere observation without interpretation
is not science. Aristotle's generalizations based on the facts
accumulated and his elaboration of broad philosophical
conceptions of organisms give to his biological works their
perennial significance.
While Aristotle's biological investigations were devoted
EPOCHS IN BIOLOGICAL HISTORY 381
chiefly to animals, his pupil and co-worker, THEOPHRASTUS
(370-286 B.C.), made profound studies on plants. Theo-
phrastus not only laid the foundations but also gave sug-
gestions of much of the superstructure of botany; an achieve-
ment which entitles him to rank as "the first of real botanists
in point of time."
Before leaving the Greeks we must mention HIPPOCRATES
FIG. 196. — Theophrastus of Eresus.
(460-370 B.C.) , the Father of Medicine. Writing a generation
before Aristotle, at the height of the Age of Pericles, Hippo-
crates crystallized the knowledge of medicine into a science,
dissociated it from philosophy, and gave to physicians "the
highest moral inspiration they have."
The history of medicine and of biology as a so-called pure
science are so inextricably interwoven that consideration of
the one involves that of the other. Indeed the physicians
form the only bond of continuity in biological history be-
382 FOUNDATIONS OF BIOLOGY
tween Greece and Rome. The chief interest of the Romans
lay in technology, and it is but natural that the practical
advantages to be gained from medicine should ensure its
advance. As it happens, however, two Greek physicians
were destined to have the most influence: Dioscorides, an
army surgeon under Nero, and Galen, physician to the
Emperor Marcus Aurelius.
DIOSCORIDES wrote the first important treatise on applied
botany. This was really a work on the identification of
plants for medicinal purposes but, gaining authority with
age, it became the standard 'botany' for fifteen centuries.
GALEN (131-201) was the most famous physician of the
Roman Empire and his voluminous works represent both a
depository for the anatomical and physiological knowledge
of his predecessors, rectified and worked over into a system,
and also a large amount of original investigation. Galen
was at once a practical anatomist and the first experimental
physiologist, inasmuch as he described from dissections and
insisted on the importance of vivisection and experiment.
Galen gave to medicine its standard 'anatomy' and 'physi-
ology' for fifteen centuries.
Any consideration of the biological science of Rome would
be incomplete without a reference to the vast compilation
of fact and fancy indiscriminately mingled made by PLINY
the Elder (23-79). It was aside from the path of biological
advance, but long the recognized Natural History, passing
through some eighty editions after the invention of printing.
B. MEDIEVAL AND RENAISSANCE SCIENCE
For all practical purposes we may consider that biology
at the decline of the Roman Empire was represented by the
works of Aristotle, Theophrastus, Dioscorides, Galen, and
Pliny. Even these exerted little influence during the Middle
EPOCHS IN BIOLOGICAL HISTORY 383
Ages, being saved from total loss for future generations
chiefly by Arabian scientists, and the monasteries of Italy
and Britain. In so far 'as science was taught at all, it was
from small compilations of corrupt texts of ancient authors
interspersed with anecdotes and fables. Under theological
influence there arose the oft-quoted PHYSIOLOGUS, found in
many forms and languages, which is at once a collection of
natural history stories, and a treatise on symbolism and the
medicinal use of animals. The centaur and phoenix take
their place with the Frog and Crow in affording illustrations
of theological texts and in pointing out more or less evident
morals.
So low had science fallen that the scientific Renaissance
may be said to owe its origin to the revival of classical learn-
ing — to the translation and study of the writings of Aristotle,
and other authors we have mentioned. These were so
superior to the existing science that, in accord with the spirit
of the time, Aristotle and Galen became the bible of biology.
The first works were merely commentaries on the writings
of these authors, but as time went on more and more new
observations were interspersed with the old. In short, the
climax of the scientific Renaissance involved a turning away
from the authority of Aristotle and an adoption of the
Aristotelian method of observation and induction.
Botany was the first to show visible signs of the awakening,
probably because of the dependence of medicine on plant
products. "All physicians professed to be botanists and
every botanist was thought fit to practice medicine." In
the HERBALS published in Germany during the sixteenth
century we can trace the evolution of plant description
and classification from mere annotations on the text of
Dioscorides to well-illustrated manuals of the flora of
western Europe.
384
FOUNDATIONS OF BIOLOGY
During the same century zoology made abortive attempts
to emerge as a science, but the less immediate utility of the
subject, combined with the difficulty of collecting material
and therefore the necessity of more dependence on travelers'
tales, contributed to retard its advance. One group of natu-
ralists, the ENCYCLOPAEDISTS, so-called from their endeavor to
FIG. 197. — Andreas Vesalius.
gather all available information of living things, attempted
the impossible. Gleaning from the ancients and adding such
materials as they could gather, led to the publication of huge
volumes of fact and fiction whose value bore no just propor-
tion to the vast expenditure of labor — even in the case of
the best, Gesner's History of Animals.
Although GESNER (1516-1565) of Switzerland was without
doubt the most learned naturalist of the period and probably
the best zoologist who had appeared since Aristotle, the direct
EPOCHS IN BIOLOGICAL HISTORY 385
path to progress was blazed by men whose plans were less
ambitious. Contemporaries of Gesner, who confined their
treatises to special groups of organisms which they themselves
investigated, really instituted the biological monograph which
has proved to be the effective method of scientific publica-
tion.
While the herbalists, encyclopaedists, and monographers
at work in natural history were making brave endeavors to
develop the powers of independent judgment, which were
oppressed to such an extent during the Middle Ages that the
very activity of the senses seemed stunted, the emancipator
of biology from the traditions of the ancients appeared in the
Belgian anatomist, ANDREAS VESALIUS (1514-1564). Dis-
gusted with the anatomy of the time, which consisted almost
solely in interpreting the works of Galen by reference to crude
dissections made by barbers' assistants, Vesalius attempted
to place human anatomy on the firm basis of exact observa-
tion. The publication of his great work On the Structure of
the Human Body made the year 1543 the dividing line be-
tween ancient and modern anatomy, and thenceforth ana-
tomical as well as biological investigation in general broke
away from the yoke of authority and men began to trust
their own eyes.
The work of Vesalius was on anatomy, and physiology was
treated somewhat incidentally. The complementary work on
the functional side came in 1628 with the publication of the
epoch-making monograph on the Motion of the Heart and
Blood in Animals by WILLIAM HARVEY (1578-1657) of Lon-
don. No rational conception of the economy of the animal
organism was possible under the influence of Galenic physi-
ology, and it remained for Harvey to demonstrate by a
series of experiments, logically planned and ingeniously
executed, that the blood flows in a circle from heart back to
386
FOUNDATIONS OF BIOLOGY
heart again, and thus to supply the background for a
proper understanding of the dynamics of the organism as a
whole. With the work of Vesalius and Harvey, biologists had
again laid hold of the great scientific tools — observation,
FIG. 198. — William Harvey.
experiment, and induction — which since then have not
slipped from their grasp.
C. THE MICROSCOPISTS
Even while the marshalling of accurate descriptions of
plants and animals was getting under way, and the study of
macroscopic anatomy and physiology was making rapid
strides forward, an event occurred which was destined to
make possible modern biology. This was an adaptation of
the principle of the spectacles — the invention, probably by
Roger Bacon, of the simple microscope. Then, came the
EPOCHS IN BIOLOGICAL HISTORY
387
compound microscope as a development of the telescope
at the hands of Galileo about 1610, and by the middle of the
century simple and compound microscopes were being made
by opticians in the leading centers of Europe.
The earliest clear appreciation of the importance of study-
ing nature with instruments which increase the powers of
the senses in general and of vision in particular is found in
FIQ. 199. — Antony van Leeuwenhoek.
a remarkable book, by HOOKE (1635-1703) of London, pub-
lished in 1665. Using his improved compound microscope,
Hooke clearly observed and figured for the first time the
"little boxes or cells" of organic structure, and his use of the
word cell is responsible for its application to the protoplas-
mic units of modern biology.
Microscopical work was a mere incident among the varied
interests of Hooke, while LEEUWENHOEK (1632-1723) of
Holland spent a long life studying nearly everything which
388 FOUNDATIONS OF BIOLOGY
he could bring within the scope of his simple lenses. With an
unexplored field before him, all of his observations were dis-
coveries. Bacteria, Protozoa, Hydra, and many other
organisms were first revealed by his lenses. But Leeuwen-
hoek's discovery of the sperm of animals created the most
astonishment. His imagination, however, outstripped his
observations for he thought he saw evidence of the organism
preformed within the sperm and so came to regard it as the
true germ which had only to be hatched by the female.
The patience of Leeuwenhoek would have been strained
to the breaking point by the studies on insect anatomy made
by SWAMMERDAM (1637-1680) of Holland. Instigated
largely by the desire to refute the current notion that insects
and similar lower animals are without complicated internal
organs, Swammerdam spent his life in studies on their struc-
ture and life histories. Revealing, as he did, by the most deli-
cate technique in dissection, the finest details observable
with his lenses, Swammerdam not only set a standard for
minute anatomy which was unsurpassed for a century, but
also dissipated for all time the conception of simplicity of
structure in the lower animals. He thus, quite naturally,
added one more argument to those of the Italian REDI (1626-
1698) and others against spontaneous generation.
Malpighi of Bologna and Grew of London, contemporaries
of Hooke, Leeuwenhoek, and Swammerdam, may be con-
sidered as the pioneer histologists. GREW (1641-1712)
devoted all his attention to plant structure, while MALPIGHI
(1628-1694), in addition to botanical studies which paralleled
Grew's, made elaborate investigations on animals.
The versatility as well as the genius of Malpighi is shown by
his studies on the anatomy of plants, the function of leaves,
the development of the plant embryo, the embryology of the
chick, the anatomy of the silkworm, and the structure of
EPOCHS IN BIOLOGICAL HISTORY 389
glands. Skilled in anatomy but with prime interest in physi-
ology, his lasting contribution lies in his dependence upon the
microscope for the solution of problems where structure and
function, so to speak, merge. This is well illustrated by his
ocular demonstration of the capillary circulation in the lungs,
which is not only his greatest discovery but also the first
FIG. 200. — Marcello Malpighi.
of prime importance ever made with a microscope, since
it completed Harvey's work on the circulation of blood.
D. THE DEVELOPMENT OF THE SUBDIVISIONS OF BIOLOGY
The microscopists taken collectively created an epoch in
the history of biology, so important is the lens for the ad-
vancement of the science. Broadly speaking, we find that
its development along many lines during the eighteenth and
particularly the nineteenth century went hand in hand with
improvements in the compound microscope itself and in mi-
croscopical technique. Again, the microscopists in. general
and Malpighi in particular opened up so many new paths of
390 FOUNDATIONS OF BIOLOGY
advance that from this period on it is not possible, even in
the most general survey, to discuss the development of biol-
ogy as a whole. The composite picture must be formed by
emphasizing and piecing together various lines of work, such
as taxonomy, comparative anatomy of animals, embry-
ology, physiology of plants and animals, genetics, and evolu-
tion.
1. Taxonomy
Taxonomy has as its object the bringing together of or-
ganisms which are alike and the separating of those which are
unlike; a problem of no mean proportions when a conserva-
tive estimate to-day shows upward of a million species of
animals and plants — leaving out of account the myriads
of forms represented only by fossil remains.
Naturally the earliest classifications were utilitarian or
more or less physiological, but as knowledge increased em-
phasis was shifted to the anatomical criterion of specific dif-
ferences, and thenceforth classification became an important
aspect of natural history — a central thread both practical
and theoretical. Practical, in that it involved the arranging
of living forms so that a working catalog was made which
required nice anatomical discrimination, and therefore the
amassing of a large body of facts concerning animals and
plants. Theoretical, because in this process botanists and
zoologists were impressed, almost unconsciously at first, with
the 'affinity' of various types of animals and plants, and so
were led to problems of their origin.
From Aristotle, who emphasized the grouping of organisms
on the basis of structural similarities, we must pass over some
seventeen centuries, in which the only work of interest was
done by the herbalists and encyclopaedists, to the time of
RAY (1628-1705) of England and LINNAEUS (1707-1778) of
Sweden. Previous to Ray the term species was used some-
EPOCHS IN BIOLOGICAL HISTORY
391
what indefinitely, and his chief contribution was to make the
word more concrete by applying it solely to groups of similar
individuals which seem to exhibit constant characters from
generation to generation. This paved the way for the great
taxonomist, Linnaeus.
Linnaeus was first and foremost a botanist who gave plant
students at once a practical classification of Flowering Plants,
m
FIG. 201. — Carolus Linnaeus.
based chiefly on the number and arrangement of the stamens ;
and at the same time insisted on brief descriptions and the
scheme of giving each kind of organism a name of two words,
generic and specific, thereby establishing the system of
BINOMIAL NOMENCLATURE. Linnaeus' success with botani-
cal taxonomy led him to extend the principles to animals and
even to the so-called mineral kingdom: the latter showing
at a glance his lack of appreciation of any genetic relation-
ship between species. Although the terms genus and species
to Linnaeus expressed a transcendental affinity, since he
392
FOUNDATIONS OF BIOLOGY
believed that species, genera, and even higher groups repre-
sented distinct acts of creation, nevertheless his greatest
works, the Species Plantarum and Systema Naturae, are of
outstanding importance in biological history and by common
consent the base line of priority in botanical and zoological
nomenclature.
2. Comparative Anatomy
Owing to the less marked structural differentiation of
plants in comparison with animals, plant anatomy lends
FIG. 202. — Georges Cuvier.
itself less readily to descriptive analysis, so that an epoch in
the study of comparative anatomy is not so well defined in
botany as in the sister science, zoology. Therefore, we shall
confine our attention to the comparative anatomy of animals.
Comparative anatomy as a really important aspect of
zoological work, in fact as a science in itself, was the result
of the life-work of CUVIER (1769-1832) of Paris. It is true
EPOCHS IN BIOLOGICAL HISTORY 393
that some of his predecessors had reached a broad viewpoint
in anatomical study, but Cuvier's claim to fame rests on the
remarkable breadth of his investigations — his grasp of the
comparative anatomy of the whole series of animal forms.
And not content merely with the living, he made himself the
FIG. 203. — Thomas Henry Huxley.
first real master of the anatomy of fossil Vertebrates as was
his contemporary, Lamarck, of fossil Invertebrates.
Cuvier's grasp of anatomy was due to his emphasizing, as
Aristotle had done before him, the functional unity of the or-
ganism: that the interdependence of organs results from the
interdependence of function: that structure and function are
two aspects of the living machine which go hand in hand.
Cuvier's famous principle of correlation — "Give me a
tooth," said he, "and I will construct the whole animal"
is really an outcome of this viewpoint. Every change of
394 FOUNDATIONS OF BIOLOGY
function involves a change in structure and, therefore, given
extensive knowledge of function and of the interdependence
of function and structure, it is possible to infer from the
form of one organ that of most of the other organs of an
animal. But Cuvier undoubtedly allowed himself to exagger-
ate his guiding principle until it exceeded the bounds of facts.
Among Cuvier 's immediate successors, OWEN (1804-1892)
of London perhaps demands special mention. Owen spent a
long life dissecting with untiring patience and skill a remark-
able series of animal types, as well as reconstructing extinct
forms from fossil remains. Aside from the facts accumu-
lated, probably his greatest contribution was making con-
crete the distinction between homologous and analogous
structures, which has been of the first importance in working
out the pedigrees of plants as well as of animals; though Owen
himself took an enigmatical position in regard to organic
evolution — quite different from that of his great English
contemporary comparative anatomist, HUXLEY (1825-1895).
3. Physiology
The functions of organisms were discussed by Aristotle
with his 'usual insight, though, as might be expected since
physiology is more dependent than anatomy upon progress
in other branches of science, with less happy results. Simi-
larly Galen was hampered in his attempt to make physiology
a distinct department of learning, based on a thorough study
of anatomy, and the corner stone of medicine; though fate
foisted upon uncritical generations through fifteen centuries
his system of human physiology. The worst of it was not
that it was nearly all wrong, but that to question Galen's
physiology or anatomy became little less than sacrilege until
the studies of Vesalius and Harvey brought a realization that
Galen had not quite finished the work.
EPOCHS IN BIOLOGICAL HISTORY 395
Neither Vesalius nor Harvey made an attempt to explain
the workings of the body by appeal to so-called physical
and chemical laws; and for good reason. Chemistry had
not yet thrown off the shackles of alchemy and taken its
legitimate place among the elect sciences, while during
Harvey's lifetime, under the influence of Galileo, the new
physics was born. But by the end of the seventeenth
century both physics and chemistry had forced their way into
physiology and split it into two schools. The physical school
was founded by BORELLI (1608-1679) of Italy, who, employ-
ing incisive physical methods, attacked a series of problems
with brilliant results; while the chemical school developed
from the influence of FKANCTSCUS SYLVIUS (1614-1672) of
Holland as a teacher rather than as an investigator.
This awakening brought a host of workers into the field
and the harvest of the century was garnered and enriched
by HALLER (1708-1777) of Geneva. In a comprehensive
treatise which at once indicated the erudition and critical
judgment of its author, Haller established physiology as a
distinct and important branch of biological science. It was
no longer a mere adjunct of medicine. Perhaps the most
significant advance in Haller's century consisted in setting
the physiology of nutrition and of respiration — both of
which awaited the work of the chemists — well upon the way
toward their modern form.
REAUMUR (1683-1757) of Paris and SPALLANZANI (1729-
1799) of Pavia may be singled out for their exact studies of
gastric digestion, which showed solution of the food to be
the main factor in digestion — though it was not clear how
these changes differ from ordinary chemical ones. It was
left for nineteenth-century investigators to establish the fact
that food in passing along the digestive tract runs the gauntlet
of a series of complex chemical substances, each of which has
396 FOUNDATIONS OF BIOLOGY
its part to play in putting the various constituents of the food
into such a form that they can pass to the various cells of
the body where they are actually used.
On the side of respiration, a closer approach was made
toward a true understanding of the process. In France
LAVOISIER (1743-1794) made it clear that the chemical
changes taking place in respiration involve essentially a
process of combustion, and it only remained for later work
to show that this takes place in the tissues rather than in
the lungs.
Enough perhaps has been said to indicate the trend of
physiology away from the maze of Galenic "spirits" in
which science lost itself, toward the modern viewpoint of
science which assumes as its working hypothesis that life phe-
nomena are an expression of a complex interaction of physico-
chemical laws which do not differ fundamentally from the
so-called laws operating in the inorganic world, and that the
economy of the organism is in accord with the law of the
conservation of energy — probably the most far-reaching
generalization attained by science during the past century.
Most of the firm foundation on which the physiology of
animals rests to-day has been built up by the work on Verte-
brates. But since the middle of the nineteenth century,
when the versatile MULLER (1801-1858) of Germany empha-
sized the value of studying the physiology of higher and lower
animals alike, there has been an ever-increasing tendency
to focus evidence, in so far as possible, from all forms of life
on general problems of function. This Jias culminated in the
science of COMPARATIVE PHYSIOLOGY.
The less obvious structural and functional differentiation
of plants retarded progress in plant physiology as it did in
plant anatomy. Probably of most historical and certainly
of most general interest is the development of our knowledge
EPOCHS IN BIOLOGICAL HISTORY 397
of the nutrition of green plants. Aristotle's notion that
the plant's food is prepared for it in the ground was still
prevalent during the seventeenth century when Malpighi,
from his studies on plant histology, gave the first hint of
supreme importance — the crude sap enters by the roots
and is carried to the leaves where, by the action of sunlight,
evaporation, and some sort of a fermentation, it is elaborated
and distributed as food to the plant as a whole.
Fio. 204. — Stephen Hales.
It is STEPHEN HALES (1677-1761) of England, however, to
whom the botanist looks as the Harvey of plant physiology,
because in his Vegetable Statics (1727) he laid the foundations
of the physiology of plants by making " plants speak for
themselves" through his incisive experiments. For the
first time it became clear that green plants derive a con-
siderable part of their food from the atmosphere, and also
that the leaves play an active role in the movements of
fluids up the stem and in eliminating superfluous water by
evaporation. Still the picture was incomplete, and so it
398 FOUNDATIONS OF BIOLOGY
remained until the biologist had recourse to further data
from the chemist, in 1779, PRIESTLEY (1733-1804) of Eng-
land, the discoverer of oxygen, showed that this gas under
certain conditions is liberated by plants. This fact was
seized upon by a native of Holland, INGENHOUSZ (1730-
1799), who demonstrated that carbon dioxide from the air
is reduced to its component elements in the leaf during
exposure to sunlight. The plant retains the carbon and
returns the oxygen — this process of carbon-getting being
quite distinct from that of respiration in which carbon
dioxide is eliminated. It remained then for DE SAUSSURK
(1767-1845) in Geneva to show that, in addition to the
fixation of carbon, the elements of water are also employed,
while from the soil various salts, including combinations of
nitrogen, are obtained. But it was nearly the middle of
the last century before the influence and work of LIEBIG
(1803-1873) at Giessen led to a clear realization of the
fundamental part played by the chlorophyll of the green
leaf in making certain chemical elements available to animals.
The establishment of the cosmical function of green plants —
the link they supply in the circulation of the elements in
nature — is a landmark in biological progress.
4. Histology
Studies on the physiology of plants and animals natural^
involved the progressive analysis of the physical basis of
the phenomena under consideration, but the Aristotelian
classification of the materials of the body as unorganized
substance, homogeneous parts or tissues, and heterogeneous
parts or organs, practically represented the level of analysis
until the beginning of the eighteenth century. In fact it was
not .until the revival of interest in embryology early in
the last century that the cell became a particular object of
EPOCHS IN BIOLOGICAL HISTORY 399
study, and attention began gradually to shift from more or
less superficial details to cell organization. This culminated in
the classic investigations of two German biologists, the
botanist SCHLEIDEN (1804-1881) and the zoologist SCHWANN
(1810-1882), published in 1838 and 1839. Together these
studies clearly showed that all organisms are composed of
FIG. 205. — Matthias Jacob Schleiden.
units, or cells, which are at once structural entities and the
centers of physiological activities. And further that the
development of animals and plants consists in the multi-
plication of an initial cell to form the multitude of different
kinds which constitute the adult. Unquestionably the cell
concept represents one of the greatest generalizations in
biology, and it only needed for its consummation the full
realization that the viscid, jelly-like material which zoolo-
400
FOUNDATIONS OF BIOLOGY
gists interpreted as the true living matter of animals, and the
quite similar material which botanists considered the true
living part of plants are practically identical. This viewpoint
was crystallized in the early sixties by SCHULTZE (1825-
1874) of Germany in the formulation of the protoplasm
concept and thenceforth not only morphological elements —
FIG. 206. — Theodor Schwann.
cells — but also the material of which they are composed —
protoplasm — were recognized as fundamentally the same in
all living beings. Indeed, the realization of a common physi-
cal basis of life in both plants and animals — a common
denominator to which all vital phenomena are reducible —
gave content to the term biology and created the science of
life in its modern form.
EPOCHS IN BIOLOGICAL HISTORY 401
5. Embryology
The enunciation of the cell theory came, as we have seen,
from combined studies on the adult structure and on the
development of plants and animals from the germ or egg, and
accordingly implies that the science of embryology has a
history of its own. As a matter of fact, Aristotle discussed
the wonder of the beating heart in the hen's egg after three
days' incubation, but there the subject rested until FABRICIUS
( 1537-1 6 19) at Padua, early in the seventeenth century, pub-
lished a treatise which illustrated the obvious sequence of
events within the hen's egg to the time of hatching. This be-
ginning was built upon by a pupil of Fabricius, the cele-
brated Harvey, who added many details of interest, though
little progress in embryology was possible without the micro-
scope. This was first turned on the problem by the versatile
Malpighi in two treatises published in 1672, and at one step
animal development was placed upon a plane so advanced
that for over a century it was unappreciated. One conclusion
of Malpighi, however, was seized upon by contemporary
biologists. Apparently, unbeknown to him, some of the eggs
which be studied were slightly incubated, so that he thought
traces of the future organism are preformed in the egg. This
error contributed to the formulation of the preformation
theory, which gradually became the dominant question in
embryology.
As a matter of fact the time was not ripe for theories of
development. The preformationists were wrong, but so were
Aristotle, Harvey, and others who went to the opposite ex-
treme and denied all egg organization and therefore tried to
get something out of nothing. It remained, as we know, for
the present generation of embryologists to work out many of
the details of the origin and organization of the germ cells,
402
FOUNDATIONS OF BIOLOGY
and to reach a level of analysis deep enough to suggest how
"the whole future organism is potentially and materially im-
plicit in the fertilized egg cell" and thus that "the preforma-
tionist doctrine had a well-concealed kernel of truth within
its thick husk of error."
The next great advance came in the accurate and compre-
hensive studies of the Russian, VON BAER (1792-1876), pub-
FIG. 207. — Karl Ernst von Baer.
lished in the thirties of the last century. Taking his material
from all the chief groups of higher animals, von Baer founded
COMPARATIVE EMBRYOLOGY. Among his achievements may be
mentioned : the clear discrimination of the chief developmental
stages, such as cleavage of the egg, germ layer formation, tis-
sue and organ differentiation; the insistence on the importance
of the facts of development for classification; and the dis-
covery of the egg of Mammals. His observations on the
EPOCHS IN BIOLOGICAL HISTORY 403
origin and development of the germ layers, which afforded
the key to many general problems of the origin of the body-
form (morphogenesis) , and his emphasis on the resemblance of
certain embryonic stages of higher animals to the adult stages
of lower forms, were crystallized by his successors, under the
influence of the evolution theory, as the germ layer theory
and the recapitulation theory.
From every point of view von Baer created an epoch in
embryology synchronous with the formulation of the cell
theory by Schleiden and Schwann, and it thenceforth became
the problem of the embryologist to interpret development in
terms of the cell. It is unnecessary to follow historically the
establishment of the fact that the egg and the sperm are
really single nucleated cells; that fertilization consists in the
fusion of egg and sperm and the orderly arrangement of their
chief nuclear contents, or chromosomes; that the new genera-
tion is the fertilized egg, since every cell of the body as well as
every chromosome in every cell is a lineal descendant by
division from the zygote, and so from the gametes which
united at fertilization to form it. Such, however, are the
chief results of cytological study since von Baer. But em-
bryologists have not been content to employ merely the de-
scriptive method, and the dominant note of the most modern
research is physiological — the experimental study of the
significance of fertilization, the dynamics of cell division, the
basis of differentiation, the influence of environmental
stimuli, and so on.
6. Genetics
The study of inheritance could be little more than a grop-
ing in the dark until embryology, under the influence of the
cell theory, afforded a body of facts which clearly indicated
that typically the fertilized egg is the sole bridge of continuity
404 FOUNDATIONS OF BIOLOGY
between successive generations. Indeed, the present science
of genetics has a history largely confined to this century.
Although clearly intimated by a number of workers, the
conception of the continuity of the germ cells was first forced
upon the attention of biologists and given greater precision
by WEISMANN (1834-1914) of Germany in a series of essays
culminating in 1892 in his volume entitled The Germ Plasm.
He identified the chromatin material which constitutes the
chromosomes of the cell nucleus as the specific bearer of
hereditary characters, and emphasized a sharp distinction
between germ cells and somatic cells.
While this viewpoint had been gradually gaining content
and precision, the science of genetics had been advancing not
only by exact studies on the structure and physiology of the
germ cells, but also by statistical studies of the results of
heredity — the various characters of animals and plants as
exhibited in parents and offspring. The studies of this type
which first attracted the attention of biologists were made by
G ALTON (1822-1911) of England. In the eighties and nine-
ties of the last century, he amassed a great volume of data
in regard to, for example, the stature of children with refer-
ence to that of their parents, and formulated his well-known
'laws' of inheritance. But the work which eventually
created the science of genetics was that of GREGOR MENDEL
(1822-1884) of Austria. Mendel combined in a masterly
manner the experimental breeding of pedigree strains of
plants and the statistical treatment of the data thus secured
in regard to the inheritance of sharply contrasting characters,
such as the form and color of the seeds in Peas. Mendel's
work was published in 1863 in an obscure natural history
periodical, and he abandoned teaching and research to be-
come the Abbot of his monastery. Thus terminated pre-
maturely the productive work of one of the epochmakers of
EPOCHS TN BIOLOGICAL HISTORY 405
biology, and the now famous Mendelian laws of inheritance
were unknown to science until 1900, when other biologists,
coming to similar results, unearthed his forty-year-old paper.
We have already seen that the fundamental principle of the
segregation of the genes of 'alternative' characters in the
germ cells, which Mendel's work indicated, has been ex-
FIG. 208. — Gregor Johann Mendel.
tended to other plants and to animals, and that instead of
being, as at first thought, a principle of rather limited ap-
plication, has come to be the key to all inheritance. And
the present results are extremely convincing because cyto-
logical studies on the architecture of the chromosome com-
plex of the germ cells keep pace with and afford a picture
of the physical basis of inheritance — the mechanism by
which the segregation and distribution of characters by the
406 FOUNDATIONS 6F BIOLOGY
Mendelian formula takes place. Such is the deeply hidden
modicum of truth in the old preformation theories!
7. Organic Evolution
A question which has interested and perplexed thinking
men of all times is how things came to be as they are to-day.
The historian of human affairs attempts to trace the sequence
and relationship of events from the remote past to the pres-
ent. Similarly, the geologist endeavors to formulate the
history of the Earth; and the biologist, the history of plants
and animals on the Earth. All recognize that the present is
the child of the past and the parent of the future, and that
past, present, and future, though causally related, are never
the same. It was the Greek natural philosophers who pro-
jected this idea of history into science and attempted to
substitute a naturalistic explanation of the Earth and its
inhabitants for the established theogonies, and thus started
the uniformitarian trend of thought which culminated in the
establishment of organic evolution during the past century.
Aristotle held substantially the modern idea of the evolu-
tion of life from a primordial mass of living matter to the
higher forms, and placed Man at the head of animal creation.
"To him belongs the God-like nature. He is preeminent by
thought and volition. But although all are dwarf -like and
incomplete in comparison with Man, he is only the highest
point of one continuous ascent." And evolution is still going
on — the highest has not yet been attained. In looking for
the effective cause of evolution Aristotle rejected the hy-
pothesis of EMPEDOCLES (495-435 B.C.), which embodied in
crude form the idea of the survival of the fittest, and substi-
tuted secondary natural laws to account for the apparent
design in nature. This was a sound induction by Aristotle
from his necessarily limited knowledge of nature, but had he
EPOCHS IN BIOLOGICAL HISTORY 407
accepted the idea of the survival of the fittest to account for
adaptations in organisms, he would have been "the literal
prophet of Darwinism."
The thread of continuity in evolutionary thought is not
broken from Aristotle to the present, but from the strictly
biological viewpoint two Frenchmen, Buffon and Lamarck,
Fia. 209. — Comte de Buffon.
and two Englishmen, Erasmus Darwin and his grandson,
Charles Darwin, stand preeminent.
BUFFON (1707-1788) was a peculiarly happy combination
of entertainer and scientist who found expression in each
new volume of his great Natural History. And it was largely,
so to speak, between the lines of this work that Buffon's
evolutionary ideas were displayed; beyond the reach, he
hoped, of the censor and dilettante. It is not strange, there-
fore, that it is often difficult to decide just how much weight
is to be placed on some of his statements; though certainly
it is not exaggerating to ascribe to him not only the recogni-
408 FOUNDATIONS OF BIOLOGY
tion of the factors of geographical isolation, struggle for
existence, artificial and natural selection in the origin of
species, but also the propounding of a theory of the origin
of variations — that the direct action of the environment
brings about alterations in the structure of animals and
plants and these are transmitted to the offspring.
When Buffon's influence had passed its zenith, ERASMUS
DARWIN (1731-1802) expressed consistent views on the
FIG. 210. — Erasmus Darwin.
evolution of organisms, in several volumes of prose and poetry,
which lead biologists to-day to recognize him as the antici-
pator of the Lamarckian doctrine that somatic variations
arise through the reaction of the organism to environmental
conditions. " All animals undergo transformations which are
in part by their own exertions, in response to pleasures, and
pain, and many of these acquired forms or propensities are
transmitted to their posterity."
EPOCHS IN BIOLOGICAL HISTORY 409
LAMARCK (1744-1829) developed with great care the first
complete and logical theory of organic evolution and is the
one outstanding figure in biological uniformitarian thought
between Aristotle and Charles Darwin. "For nature," he
writes, "time is nothing. For all the evolution of the Earth
and of living beings, nature needs but three elements, space,
time, and matter." In regard to the factors of evolution,
FIG. 211. — Jean-Baptiste Lamarck.
Lamarck put emphasis on the indirect action of the environ-
ment in the case of animals, and the direct action in the
case of plants. The former are induced to react and so
adapt themselves, as it were; while the latter, without a
nervous system, are molded directly by their surroundings.
And, so Lamarck believed, such changes, somatic in origin —
acquired characters — are transmitted to the next generation
and bring about the evolution of organisms.
410 FOUNDATIONS OF BIOLOGY
Through the relative weakness of Lamarck's successors
the French school of evolutionists dwindled to practical
extinction; while in Germany, GOETHE (1749-1832), the
greatest poet of evolution, and TREVIRANUS (1776-1837)
"brilliantly carried the argument without carrying convic-
tion," for the man and the moment must agree. Then in
England the uniformitarian ideas of HUTTON (1726-1797),
elaborated by LYELL (1797-1875) in his Principles of Geology
(1830-1833), established evolution in geology, and the way
was paved for CHARLES DARWIN (1809-1882) to do the same
for the organic world. It is true that "the idea of develop-
ment saturated the intellectual atmosphere — nevertheless
the elaborate and toilsome labor of thinking it through for
the endless realm of nature was to be done" and Darwin
did it in his Origin of Species which appeared in 1859. By
his brilliant, scholarly, open-minded, and cautious mar-
shalling of the facts pointing toward the universality of varia-
tions and the mutability of species; and by the theory of
natural selection on the basis of slight adaptive variations
resulting in the survival of the fittest in the struggle for
existence — which, strange to say, Darwin and WALLACE
(1822-1913) reached simultaneously and independently -
Darwin "made the old idea current intellectual coin."
To-day, as we know, no representative biologist questions
the fact of evolution — "evolution knows only one heresy, the
denial of continuity" -though in regard to the factors
involved there is much difference of opinion. It may well
be that we shall have reason to depart widely from Darwin's
interpretation of the effective principles at work in the origin
of species, but withal this will have little influence on his
position in the history of biology. The great value which he
placed upon facts was exceeded only by his demonstration
that this "value is due to their power of guiding the mind to a
EPOCHS IN BIOLOGICAL HISTORY 411
further discovery of principles." The Origin of Species
brought biology into line with the other inductive sciences,
recast practically all of its problems, and instituted new ones.
Darwin beautifully and conservatively expressed this new
outlook on nature in the historically important concluding
paragraph of his epoch-making work:
"It is interesting to contemplate a tangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other,
and dependent upon each other in so complex a manner, have
all been produced. by laws acting around us. These laws,
taken in the largest sense, being Growth with Reproduction;
Inheritance which is almost implied by reproduction; Varia-
bility from the indirect and direct action of the conditions of
life, and from use and disuse: a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to Natural
Selection, entailing Divergence of Character and the Extinc-
tion of less-improved forms. Thus, from the war of nature,
from famine and death, the most exalted object which we are
capable of conceiving, namely, the production of the higher
animals, directly follows. There is a grandeur in this view
of life, with its several powers, having been originally breathed
by the Creator into a few forms or into one; and that, whilst
this planet has gone cycling on according to the fixed law
of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being
evolved."
APPENDIX
I. A BRIEF SYNOPTIC CLASSIFICATION
OF PLANTS AND ANIMALS
A. PLANTS
Phylum 1. THALLOPHYTA: Thallus plants.
Series of the ALGAE. (1500 species.)
Class I. CYANOPHYCEAE : Blue-green Algae. Oscillatoria.
Class II. CHLOROPHYCEAE : Green Algae.
Order 1. Protococcales: Unicellular Green Algae. Pleuro-
coccus, Sphaerella.
Order 2. Confervales: Confervas and Sea Lettuces. Ulothrix,
Oedogonium, Ulva.
Order 3. Conjugates: Pond Scums, Desmids, and Diatoms.
Spirogyra, Closterium, Navicula.
Order 4. Siphonales: Tubular Algae. Vaucheria.
Order 5. Charales: Stoneworts. Chara.
Class III. PHAEOPHYCEAE : Brown Algae. Kelps and Rock-
Weeds. Laminaria, Fucus, Sargassum.
Class IV. RHODOPHYCEAE : Red Algae. Rhodomela.
Series of the FUNGI. (65,000 species.)
Class V. SCHIZOMYCETES: Bacteria.
Class VI. PHYCOMYCETES: Alga-like Fungi. Molds.
Class VII. ASCOMYCETES: Sac Fungi. Mildews, Morels, Truffles,
Yeasts, (Lichens).
Class VEIL BASIDIOMYCETES: Basidia Fungi. Smuts, Rusts,
Toadstools, Mushrooms.
Phylum 2. BRYOPHYTA: Liverworts and Mosses. (17,000
species.)
Class I. HEPATICAE: Liverworts. Marchantia.
413
414 APPENDIX
Class II. Musci: Mosses.
Order 1. Sphagnales: Peat Mosses. Sphagnum.
Order 2. Bryales: Common Mosses. Polytrichum, Bryum.
Phylum 3. PTERIDOPH YTA : Ferns and their allies. (4500
species.)
Class I. FILICINEAE: Common Ferns and Water Ferns.
Aspidium, Marsilia.
Class II. EQUISETINEAE : Horsetails. Equisetum.
Class III. LYCOPODINEAE : Lycopods. Selaginella.
Phylum 4. SPERMATOPH YTA : Seed Plants. Flowering Plants.
Subdivision 1. GYMNOSPERMAE: Cycads and Conifers.
Pines. (600 species.)
Subdivision 2. ANGIOSPERMAE: The familiar 'flowering
plants/
Class I. MONOCOTYLEDONEAE : Grasses, Palms, Lilies, Or-
chids. (25,000 species.)
Class II. DICOTYLEDONEAE : Elms, Buttercups, Pitcher Plants,
Roses, Beans, Flax, Cacti, Daisies. (110,000 species.)
B. ANIMALS
Phylum 1. PROTOZOA. (10,000 species.)
Class I. SARCODINA: Amoeba, the Forarninifera.
Class II. MASTIGOPHORA: Flagellates. Euglena, Volvox, Try-
panosoma.
Class III. SPOROZOA: Plasmodium malariae.
Class IV. INFUSORIA: Paramecium, Vorticella.
Phylum 2. PORIFERA: Sponges. (2500 species.)
Phylum 3. COELENTERATA. (4500 species.)
Class I. HYDROZOA: Hydra, Obelia, Gonionemus.
Class II. SCYPHOZOA: Jellyfish.
Class III. ANTHOZOA: Sea Anemones, Corals.
Class IV. CTENOPHORA: Sea Combs.
Phylum 4. PLATYHELMINTHES: Flatworms. (5000 species.)
Class I. TURBELLARIA: Planaria.
Class II. TREMATODA: Liver Flukes,
Class III. CESTODA: Tape Worms.
CLASSIFICATION 415
Phylum 5. NEMATHELMINTHES: Round Worms. Ascaris,
Trichina. (1500 species.)
Phylum 6. TROCHELMINTHES: Rotifers. (500 species.)
Phylum 7. MOLLUSCOIDA: Polyzoans and Brachiopods. (2000
species.)
Phylum 8. ECHINODERMATA: (4000 species.)
Class I. ASTEROIDEA: Starfishes.
Class II. OPHIUROIDEA: Serpent Stars.
Class III. ECHINOIDEA: Sea Urchins.
Class IV. HOLOTHUROIDEA: Sea Cucumbers.
Class V. CRINOIDEA: Feather Stars, Sea Lilies.
Phylum 9. ANNELIDA. Segmented Worms. (4000 species.)
Class I. ARCHIANNELIDA : Polygordius.
Class II. CHAETOPODA: Earthworms, Clamworms.
Class III. HIRUDINEA: Leeches.
Phylum 10. MOLLUSCA. (60,000 species.)
Class I. LAMELLIBRANCHIATA : Oysters, Clams, Scallops,
Shipworm.
Class II. AMPHINEURA: Chiton.
Class III. GASTROPODA: Snails.
Class IV. SCAPHOPODA: Dentalium.
Class V. CEPHALOPODA: Squid, Octopus, Nautilus.
Phylum 11. ARTHROPOD A.
Class I. CRUSTACEA: Barnacles, Crayfishes, Lobsters, Crabs,
Trilobites (extinct). (16,000 species.)
Class II. ONYCHOPHORA: Peripatus.
Class III. MYRIAPODA: Centipedes, Millipedes.
Class IV. INSECTA: Locusts, Bugs, Flies, Butterflies, Beetles,
Ants, Bees, Wasps. (400,000 species.)
Class V. ARACHNIDA: Scorpions, Spiders. (16,000 species.)
Phylum 12. CHORDATA.
Subphylum A. ENTEROPNEUSTA: Dolichoglossus.
Subphylum B. TUN 1C AT A: Tunicates. Cynthia. (1500
species.)
Subphylum C. CEPHALOCHORDA: Amphioxus.
416 APPENDIX
Subphylum D. VERTEBRATA.
Class I. CYCLOSTOMATA : Lampreys.
Class II. ELASMOBRANCHII: Sharks. Dogfish.! (15,000
Class III. PISCES: Cod, Trout, Perch. J species.)
Class IV. AMPHIBIA: Frogs, Toads, Salamanders. (1400
species.)
Class V. REPTILIA: Lizards, Snakes, Tortoises, Turtles,
Crocodiles, Dinosaurs (extinct). (3500 species.)
Class VI. AVES: Birds. (13,000 species.)
Subclass 1. Archaeornithes: Archaeopteryx (extinct).
Subclass 2. Neornithes.
Division A. Ratitae: Apteryx, Ostrich.
Division B. Carinatae: All familiar birds.
Class VII. MAMMALIA. (3500 species.)
Subclass 1. Prototheria: Duck-bill, Echidna.
Subclass 2. Metatheria: Opossums, Kangaroos.
Subclass 3. Eutheria: Sloths, Whales, Porpoises, Horses,
Tapirs, Camels, Cats, Hedgehogs, Bats, and the Primates
including Monkeys, Apes, Man.
II. BIBLIOGRAPHY
Some easily available works in English which are suitable for
reference and collateral reading.
CHAPTER I
COLTON, H. S. A List of Selected Readings for Students in Elementary
College Zoology. University of Pennsylvania, 1915.
GREGORY, R. A. Discovery, or the Spirit of Service of Science. The
Macmillan Co., 1919.
HENDERSON, I. F. and HENDERSON, W. D. A Dictionary of Scien-
tific Terms: Pronunciation, Derivation, and Definition of Terms
in Biology, Botany, Zoology, Anatomy, Cytology, Embryology,
Physiology. Oliver & Boyd, 1920.
HUXLEY, T. H. " Educational Value of the Natural History
Sciences." Collected Essays, Vol. Science and Education. D.
Appleton & Co.
HUXLEY, T. H. "On our Knowledge of the Causes of the
Phenomena of Organic Nature." Collected Essays, Vol. Dar-
winiana.
HUXLEY, T. H. "On the Study of Biology." Collected Essays,
Vol. Science and Education.
MILLS, JOHN. Realities of Modern Science. Introduction for the
Modern Reader. The Macmillan Co., 1919.
PEARSON, KARL. The Grammar of Science. 3d edition. A. & C.
Black, 1911.
SANFORD, FERNANDO. The Scientific Method: Its History and
Its Value. The Macmillan Co., 1921.
THOMSON, J. A. An Introduction to Science. H. Holt & Co.,
1911.
WESTAWAY, F. W. Scientific Method. Blackie & Son, 1912.
417
418 APPENDIX
CHAPTER II
BAYLISS, W. M. Principles of General Physiology. 3d Edition.
Longmans, Green & Co., 1921.
EULER, HANS. General Chemistry of the Enzymes. John Wiley &
Sons, 1912.
HARROW, BENJAMIN. Vitamines: Essential Food Factors. E. P.
Button & Co., 1921.
HUXLEY, T. H. "On the Physical Basis of Life." Collected Essays,
Vol. Method and Results. D. Appleton & Co.
LOEB, JACQUES. The Dynamics of Living Matter. Columbia
University Press, 1906.
SHERMAN, H. C. Chemistry of Food and Nutrition. 2d Edition.
The Macmillan Co., 1918.
SLOSSON, E. E. Creative Chemistry. The Century Co., 1920.
TAYLOR, W. W. The Chemistry of Colloids and some Technical
Applications. Longmans, Green & Co., 1915.
UNDERBILL, F. P. Physiology of the Amino Acids. Yale Univer-
sity Press, 1915.
CHAPTER III
AGAR, W. E. Cytology, with Special Reference to the Metazoan
Nucleus. The Macmillan Co., 1920.
DONCASTER, L. An Introduction to the Study of Cytology. Cam-
bridge University Press, 1920.
SHARP, L. W. Introduction to Cytology. McGraw-Hill Book Co.,
1921.
THOMPSON, D'ARCY W. On Growth and Form. Cambridge
University Press, 1917.
WILSON, E. B. The Cell in Development and Inheritance. Columbia
University Press, 1900.
CHAPTER IV
DENDY, ARTHUR. Outlines of Evolutionary Biology. D. Appleton
& Co., 1911.
DUGGAR, B. M. Plant Physiology, with Special Reference to Plant
Production. The Macmillan Co., 1911.
BIBLIOGRAPHY 419
GANONG, W. F. Textbook of Botany for Colleges. The Mac-
millan Co., 1917.
PEEBLES, FLORENCE. "Life History of Sphaerella lacustris." Central-
blattfilr Bakteriologie, 1909.
THATCHER, R. W. The Chemistry of Plant Life. McGraw-Hill
Book Co., 1921.
CHAPTER V
CALKINS, G. N. Protozoology. Lea and Febiger, 1909.
HUXLEY, T. H. "On the Border Territory between the Animal
and Vegetable Kingdoms." Collected Essays, Vol. Discourses
Biological and Geological.
MINCHIN, E. A. Introduction to the Study of the Protozoa. Arnold,
1912.
SEDGWICK, W. T. and WILSON, E. B. General Biology. Henry
Holt & Co., 1895.
CHAPTER VI
BUCHANAN, E. U. and BUCHANAN, R. E. Bacteriology. Revised
Edition. The Macmillan Co., 1921.
FROST, W. D. and MCCAMPBELL, E. F. Textbook of General Bac-
teriology. The Macmillan Co., 1910.
MUIR, ROBERT and RITCHIE, JAMES. Manual of Bacteriology.
4th Edition. The Macmillan Co., 1907.
CHAPTER VII
BOHM, A. A. and VON DAVIDOFF, M. A Textbook of Histology,
Including Microscopic Technic. Edited by G. Carl Huber.
2d Edition. W. B. Saunders Co. 1914.
CHAMBERLAIN, C. J. Methods in Plant Histology. 3d edition. Uni-
versity of Chicago Press, 1915,
DAHLGREN, ULRIC and KEPNER, W. A. Principles of Animal Histol-
ogy. The Macmillan Co., 1908.
GUYER, M. F. Animal Micrology. Practical Exercises in Zo-
ological Micro-technique. 2d edition. University of Chicago
Press, 1917.
KELLICOTT, W. E. General Embryology. H. Holt & Co., 1913.
420 APPENDIX
STEVENS, W. C. Plant Anatomy from the Standpoint of the Develop-
ment and Functions of the Tissues. • P. Blakiston's Sons & Co.,
1911.
CHAPTERS VIII and IX
BERGEN, J. Y. and CALDWELL, 0. W. Practical Botany. Ginn &
Co., 1911.
BERGEN, J. Y. and DAVIS, B. M. Principles of Botany. Ginn &
Co., 1906.
CAMPBELL, D. H. A University Textbook of Botany. The Mac-
millan Co., 1907.
COULTER, J. G. Plant Life and Plant Uses. American Book Co.,
1913.
COULTER, J. M. The Evolution of Sex in Plants. University of
Chicago Press, 1914.
COULTER, J. M., BARNES, C. R. and COWLES, H. C. Textbook
of Botany. American Book Co., 1910.
DENSMORE, H. D. General Botany. Ginn & Co., 1920.
GAGER, C. S. Fundamentals of Botany. P. Blakiston's Son & Co.,
1916.
GANONG, W. F. Textbook of Botany for Colleges. The Mac-
millan Co., 1917.
GRAY, ASA. Manual of Botany. 7th edition. American Book
Co., 1908.
STRASBURGER'S Textbook of Botany. 5th English Edition. The
Macmillan Co., 1921.
CHAPTERS X-XV
BEDDARD* F. E. Earthworms and their Allies. Cambridge Uni-
versity Press, 1901.
Cambridge Natural History. Ten volumes. S. F. Harmer and
A. E. Shipley, Editors. The Macmillan Co., 1895.
CONN, H. W. and BUDINGTON, R. A. Physiology and Hygiene.
Silver, Burdett & Co., 1909.
DREW, G. A. Invertebrate Zoology. 3d edition, revised. W. B.
Saunders Co., 1920.
BIBLIOGRAPHY 421
HEGNER, R. W. Introduction to Zoology. The Macmillan Co.,
1913.
HEGNER, R. W. College Zoology. The Macmillan Co., 1914.
HOLMES, S. J. Biology of the Frog. The Macmillan Co., 1914.
HOUGH, T. and SEDGWICK, W. T. The Human Mechanism. Re-
vised edition. Ginn & Co., 1918.
HOWELL, W. H. Textbook of Physiology. W. B. Saunders Co.,
7th edition. 1920.
HUXLEY, T. H. The Crayfish. 1880.
HUXLEY, T. H. Lessons in Elementary Physiology. 6th edition.
The Macmillan Co., 1915.
HYMAN, L. H. A Laboratory Manual for Comparative Vertebrate
Anatomy. University of Chicago Press, 1922.
KEITH, ARTHUR. The Engine of the Human Body. J. B. Lippin-
cott Co., 1920.
KINGSLEY, J. S. Comparative Anatomy. P. Blakiston's Sons
& Co., 1912.
KINGSLEY, J. S. Vertebrate Zoology. H. Holt & Co., 1899.
LANKESTER, E. R. (editor), Treatise on Zoology. Eight volumes.
The Macmillan Co.
LILLIE, F. R. "The Free-Martin; A Study of the Action of Sex
Hormones in the Foetal Life of Cattle." Jour. Exp. Zool., Vol.
23, 1917.
LINVILLE, H. R. and KELLY, H. A. Textbook in General Zoology.
Ginn & Co., 1906.
MARSHALL, F. H. A. The Physiology of Reproduction. Longmans,
Green & Co., 1910.
MARTIN, H. N. The Human Body. 10th edition. H. Holt &
Co., 1917.
NEWMAN, H. H. Vertebrate Zoology. The Macmillan Co., 1920.
OSBORN, HERBERT. Economic Zoology. An Introductory Text-
book in Zoology. The Macmillan Co., 1912.
PARKER, G. H. The Elementary Nervous System. J. B. Lippincott
Co., 1919.
PARKER, T. J. and HASWELL, W. A. Textbook of Zoology. 3d
edition. The Macmillan Co., 1922.
422 APPENDIX
PETKUNKEVITCH, ALEXANDER. Morphology of Invertebrate Types.
The Macmillan Co., 1916.
PRATT, H. S. A Manual of the Common Invertebrate Animals,
Exclusive of Insects. A. C. McClurg & Co., 1916.
REYNOLDS, S. H. The Vertebrate Skeleton. 2d edition. Cam-
bridge University Press, 1913.
SHIPLEY, A. E. and MACBRIDE, E. W. Zoology. The Macmillan
Co., 1901.
WALTER, H. E. The Human Skeleton. The Macmillan Co., 1918.
WARD, H. B. and WHIPPLE, G. C. Fresh-Water Biology. John
Wiley & Sons, 1918.
WILDER, H. H. History of the Human Body. H. Holt & Co., 1909.
CHAPTER XVI
CALKINS, G. N. Biology. H. Holt & Co., 1917.
CHILD, C. M. Senescence and Rejuvenescence. University of
Chicago Press, 1915.
CONKLIN, E. G. Localization of Morphogenetic Substances in the
Egg. J. B. Lippincott Co., 1922.
DRIESCH, HANS. Science and Philosophy of the Organism. Gifford
Lectures, 1907-08. A. & C. Black.
GEDDES, P. and THOMSON, J. A. Sex. H. Holt & Co., 1914.
HEGNER, R. W. The Germ-cell Cycle in Animals. The Mac-
millan Co., 1914.
HUXLEY, T. H. " Biogenesis and Abiogenesis." Collected Essays,
Vol. Discourses Biological and Geological.
JENNINGS, H. S. Life and Death, Heredity and Evolution in Uni-
cellular Organisms. Gorham Press, 1920.
KELLICOTT, W. E. Chordate Development. H. Holt & Co., 1913.
LILLIE, F. R. Problems of Fertilization. University of Chicago
Press, 1919.
MORGAN, T. H. Regeneration. Columbia University Press, 1901.
WEISMANN, AUGUST. The Germ Plasm. Chas. Scribner's Sons, 1892.
WILSON, E. B. "The Problem of Development," Science, 1905.
WOODRUFF, L. L. "The Origin of Life," in the Evolution of the
BIBLIOGRAPHY 423
Earth and its Inhabitants, R. S. Lull, editor. 3d edition. Yale
University Press, 1922.
CHAPTER XVII
BABCOCK, E. B. and CLAUSEN, R. E. Genetics in Relation to Agri-
culture. McGraw-Hill Book Co., 1918.
BATESON, WILLIAM. Materials for the Study of Variation. The
Macmillan Co., 1894.
BATESON, WILLIAM. Problems of Genetics. Yale University
Press, 1913.
CASTLE, W. E. Genetics and Eugenics. Revised edition. Har-
vard University Press, 1921.
CONKLIN, E. G. Heredity and Environment in the Development of
Men. 4th edition. Princeton University Press, 1922.
COULTER, J. M. and COULTER, M. C. Plant Genetics. University
of Chicago Press, 1918.
CUNNINGHAM, J. T. Hormones and Heredity. The Macmillan
Company, 1922.
DAVENPORT, C. B. Heredity in Relation to Eugenics. H. Holt
& Co., 1911.
EAST, E. M. and JONES, D. F. Inbreeding and Outbreeding; their
Genetic and Sociological Significance. J. B. Lippincott Co., 1919.
GALTON, FRANCIS. Natural Inheritance. 1889.
GODDARD, H. H. The Kallikak Family. A Study in the Heredity
of Feeble-mindedness. The Macmillan Co., 1912.
GUYER, M. F. Being Well-born. Bobbs Merrill Co., 1916.
JENNINGS, H. S. "Heredity and Personality." Science, 1911.
KELLICOTT, W. E. The Social Direction of Human Evolution.
D. Appleton & Co., 1911.
MORGAN, T. H. Heredity and Sex. Columbia University Press, 1913.
MORGAN, T. H. The Physical Basis of Heredity. J. B. Lippin-
cott & Co., 1919.
MOTT, F. W. Nature and Nurture in Mental Development. London,
1914.
PEARL, RAYMOND. Modes of Research in Genetics. The Mac-
millan Co., 1915.
424 APPENDIX
POPENOE, P. and JOHNSON, R. H. Applied Eugenics. The Mac-
millan Co., 1918.
PUNNETT, R. C. Mendelism. 6th edition. The Macmillan Co., 1919.
THOMSON, J. A. Heredity. 2d edition. Henry Holt & Co., 1916.
WALTER, H. E. Genetics. 2d edition. The Macmillan Co., 1922.
CHAPTER XVIII
ADAMS, C. C. A Guide to the Study of Animal Ecology. The
Macmillan Co., 1913.
CAMERON, E. H. Psychology and the School. The Century Co.,
1921.
CHANDLER, A. C. Animal Parasites and Human Disease. John
Wiley & Sons, 1918.
CHESHIRE, F. R. Bees and Bee-Keeping. London, 1886.
CRILE, G. W. Man — An Adaptive Mechanism. The Macmillan
Co., 1916.
DARWIN, CHARLES. The Fertilization of Orchids. The Various
Contrivances by which Orchids are Fertilized by Insects. 1862.
HALDANE, J. S. Organism and Environment. Yale University
Press, 1917.
HENDERSON, L. J. The Fitness of the Environment. The Macmillan
Co., 1913.
HENDERSON, L. J. The Order of Nature. Harvard University
Press, 1917.
HOLMES, S. J. The Evolution of Animal Intelligence. H. Holt
& Co., 1911.
JENNINGS, H. S. Behavior of the Lower Organisms. Columbia
University Press, 1906.
LLOYD, R. E. What is Adaptation? Longmans, Green and Co.,
1914.
LOEB, JACQUES. The Organism as a Whole. New York, 1916.
LOEB, JACQUES. Forced Movements, Tropisms, and Animal Conduct.
J. B. Lippincott, 1918.
LONGLEY, W. H. "Studies upon the Biological Significance of
Animal Coloration." I. Journ. of Exper. Zoology, Vol. 23, 1917.
II. American Naturalist, Vol. 51, 1917.
BIBLIOGRAPHY 425
MACE, H. A Book about the Bee. E. P. Button & Co., 1921.
MORGAN, T. H. Evolution and Adaptation. The Macmillan Co.,
1903.
NEEDHAM, J. G. and LLOYD, J. T. Life of Inland Waters. Corn-
stock Publishing Co., 1916.
ROOSEVELT, THEODORE. "Revealing and Concealing Coloration
in the Birds and Mammals." Bull. Amer. Museum Nat. Hist.,
XXX, 1911.
SUMNER, F. B. "Adaptation and the Problem of 'Organic Purpose-
fulness.' " American Naturalist, 1919.
Symposium, on Adaptation. Papers by M. M. Metcalf, B. E.
Livingston, G. H. Parker, A. P. Mathews and L. J. Henderson.
American Naturalist, Vol. 47, 1913.
THAYER, G. H. Concealing Coloration in the Animal Kingdom.
The Macmillan Co., 1909.
THOMSON, J. A. The Study of Animal Life. 4th edition. John
Murray, 1917.
THOMSON, J. A. The System of Animate Nature. H. Holt & Co.,
1920.
VAN BENEDEN, P. J. Animal Parasites and Messmates. D.
Appleton & Co., 1876.
WASHBURN, M. F. The Animal Mind. A Textbook of Compara-
tive Psychology. The Macmillan Co., 2d edition, 1917.
ZINSSER, HANS. Injection and Resistance, 2d edition. The Mac-
millan Co., 1918.
CHAPTER XIX
ALLEN, J. A. "The Geographical Distribution of Mammals."
Bulletin U. S, Geological Survey, 1878.
BARRELL, JOSEPH. "The Origin of the Earth," in the Evolution of
the Earth and its Inhabitants. R. S. Lull, editor. 3d edition.
Yale University Press, 1922.
BERGSON, HENRI. Creative Evolution. English translation, 1911.
CAMPBELL, D. H. Plant Life and Evolution. H. Holt & Co., 1911.
CONKLIN, E. G. Direction of Human Evolution. Chas. Scribner's
Sons, 1921.
426 APPENDIX
CRAMPTON, H. E. The Doctrine of Evolution, its Basis and its Scope.
Columbia University Press, 1911.
DARWIN, CHARLES. Voyage of the Beagle. (A Naturalist's Voy-
age.) London, 1839.
DARWIN, CHARLES. The Origin of Species. London, 1859. 6th
edition, 1880.
DARWIN, CHARLES. The Descent of Man. London, 1871.
DARWIN, CHARLES. Variation in Animals and Plants under Domes-
tication. London, 1868.
Fifty Years of Darwinism: Modern Aspects of Evolution. Cen-
tennial Addresses in honor of Charles Darwin before the Ameri-
can Association for the Advancement of Science, 1909.
GADOW, HANS. The Wanderings of Animals. Cambridge Univer-
sity Press, 1913.
GEDDES, P. and THOMSON, J. A. Evolution. H. Holt & Co., 1911.
HARDY, M. E. An Introduction to Plant Geography. Oxford
University Press, 1913.
HOLMES, S. J. The Trend of the Race. Harcourt, Brace & Co.,
1921.
JOHNSTONE, JAMES. The Philosophy of Biology. Cambridge
University Press, 1914.
JORDAN, D. S. and KELLOGG, V. L. Evolution and Animal Life. D.
Appleton & Co., 1907.
KELLOGG, V. L. Darwinism To-day. H. Holt & Co., 1907.
LULL, R. S. Organic Evolution. The Macmillan Co., 1917.
NEWMAN, H. H. Readings in Evolution, Genetics, and Eugenics.
University of Chicago Press, 1921.
NUTTALL, G. H. F. Blood Immunity and Blood Relationships.
Cambridge University Press, 1904.
OSBORN, H. F. The Origin and Evolution of Life. Chas. Scribner's
Sons, 1917.
REICHERT, E. T. and BROWN, A. P. "The Differentiation and
Specificity of Corresponding Proteins and Other Vital Sub-
stances in Relation to Biological Classification and Organic
Evolution. The Crystallography of Hemoglobins." Carnegie
Institution of Washington, Publication 116, 1909.
BIBLIOGRAPHY 427
SCHUCHERT, CHARLES. "The Earth's Changing Surface and Cli-
mate," in the Evolution of the Earth and its Inhabitants, R. S.
Lull, editor. 3d edition. Yale University Press, 1922.
SCOTT, W. D. The Theory of Evolution. The Macmillan Co., 1911.
DEVRIES, HUGO. Species and Varieties. Their Origin by Muta-
tion. 3d edition. Open Court Publishing Co., 1912.
WALLACE, A. R. Darwinism. 3d edition, The Macmillan Co.,
1905.
WALLACE, A. R. The Geographical Distribution of Animals. Lon-
don, 1876.
WALLACE, A. R. Island Life. 2d edition, The Macmillan Co.,
1892.
WELLS, H. G. The Outline of History, Chapters I-XII. The Mac-
millan Co., 1920.
Yale Sigma Xi Lectures: Evolution of the Earth and its Inhabitants,
3d edition, 1922; Evolution of Man, 1922. Yale University
Press.
CHAPTER XX
BUTLER, SAMUEL. Evolution Old and New. Revised edition, E. P.
Button & Co., 1911.
FOSTER, MICHAEL. History of Physiology during the 16th, 17th, and
18th Centuries. Cambridge University Press, 1901.
GARRISON, F. H. History of Medicine. 3d edition. W. B.
Saunders Co., 1921.
GREEN, J. R. History of Botany, 1860-1900. Oxford University
Press, 1909.
HUXLEY, T. H. "The Progress of Science, 1837-1887." Collected
Essays, Vol. Methods and Results. D. Appleton & Co.
JUDD, J. W. The Coming of Evolution. The Story of a Great Rev-
olution in Science. Cambridge University Press, 1910.
LOCY, W. A. Biology and Its Makers. 3d edition. H. Holt & Co.,
1915.
MERZ, J. T. History of Scientific Thought in the Nineteenth Century.
W. Blackwood & Sons, 1903-1914.
MIALL, L. C. History of Biology. G. P. Putnam's Sons, 1911.
428 APPENDIX
OSBORN, H. F. From the Greeks to Darwin. An Outline of the
Development of the Evolution Idea. Columbia University
Press, 1894.
VON SACHS, JULIUS. History of Botany, 1530-1860. (English
translation.) Oxford University Press, 1890.
THOMPSON, D'ARCY W. On Aristotle as a Biologist. Oxford Uni-
versity Press, 1913.
THOMSON, J. A. The Science of Life. An Outline of the History of
Biology. Blackie & Son, Ltd., 1900.
WHITE, A. D. A History of the Warfare of Science with Theology.
D. Appleton & Co., 1896.
WOODWARD, H. B. History of Geology. G. P. Putnam's Sons,
1911.
Yale Gamma Alpha Lectures: History of the Sciences. Yale Univer-
sity Press, 1922.
III. GLOSSARY
ABIOGENESIS. The abandoned idea that living matter may arise
from non-living without the influence of the former. See Bio-
genesis.
ABSORPTION. The passage of nutritive and other fluids into living
cells.
ACOELOMATE. Not possessing a coelom, or body cavity. E.g., Hydra.
ACQUIRED CHARACTER. A modification of body structure or func-
tion which arises during individual life as a result of environ-
mental influences.
ADAPTATION. The reciprocal fitness of organism and environment;
a structure or reaction fitted for a special environment; the
process by which an organism becomes fitted to its surroundings.
ADRENALS. Suprarenal bodies. Ductless glands situated near
the kidneys. Secretion supplies a hormone known as adrenin.
ADVENTITIOUS. Not in the usual position, e.g., aerial roots.
AEROBE. An organism requiring free oxygen. See Anaerobe.
AFFERENT ROOT. Dorsal, or posterior, root of certain cranial and
all spinal nerves through which sensory nerve impulses enter the
brain and spinal cord. See Efferent Root.
ALGAE. A heterogeneous group of lower plants in which the body
is unicellular or consists of a thallus; e.g., Sphaerella, Spirogyra,
Seaweeds.
ALIMENTARY CANAL. The digestive tract.
ALLELOMORPHS. Genes similarly situated on homologous chromo-
somes which produce 'alternative/ or 'contrasting/ characters.
ALTERNATIVE INHERITANCE. Typical Mendelian inheritance.
AMINO ACID. Components of proteins. Organic acids in which one
hydrogen atom is replaced by the amino group (NH2). Mono-
amino acids, e.g., Glycine (CH2NH2.COOH). Diamino acids,
e.g., Lysine (H2NCH2.CH2.CH2.CH2.CHNH2.COOH).
429
430 APPENDIX
AMOEBOID. Usually applied to the flowing movements of a cell, as
in the Protozoon, Amoeba.
AMPHIMIXIS. The mingling of the germ plasm of two gametes in the
zygote.
ANABOLISM. The constructive phase of metabolism. See Katabo-
lism.
ANAEROBE. An organism not requiring free oxygen; e.g., certain
Bacteria and parasitic Worms. See Aerobe.
ANALOGY. Structural resemblance due to similarity of function.
See Homology.
ANAPHASE. Period in mitosis during which the daughter chromo-
somes move toward the respective centrosomes. See Telophase.
ANATOMY. The structure of organisms, especially as revealed by
dissection.
ANTHER. The part of the stamen which contains the pollen sacs
(microsporangia) in Flowering Plants.
ANTHERIDIUM. The organ in plants, such as the Mosses and Ferns,
in which the male gametes arise.
ANUS. Terminal orifice of the alimentary canal. Opening of the
large intestine either on the surface of the body (Man) or into
the cloaca (Frog).
AORTA. A great trunk artery carrying blood away from the heart.
See Dorsal Aorta.
AORTIC ARCHES. Arteries arising from the ventral aorta and
supplying the gills in aquatic Vertebrates. Undergo many
modifications in the ascending series of air-breathing Vertebrates.
APHIDS. Small sucking Insects; e.g., the green 'Plant Lice' of
garden shrubs.
ARCHEGONIUM. The organ in plants, such as the Mosses and Ferns,
in which the female gamete (egg) arises.
ARTHROPODA. Phylum of Invertebrates. Includes the Crustacea,
Insecta, Arachnida, etc.
ASTER. Radiations surrounding the centrosome during cell divi-
sion.
ATAVISM. Appearance of grandparental characters in an individual.
See Reversion.
GLOSSARY 431
AUTONOMIC SYSTEM. System of outlying ganglia and nerves which
communicates with the central nervous system via the roots of
the spinal and cranial nerves. Innervates chiefly the involuntary
muscles of blood vessels, digestive organs, etc. Sympathetic
system.
AXON. A nerve fiber conducting impulses away from the cell body.
Dendrites conduct toward the cell body. See Neuron.
BAST. The phloem portion of a vascular bundle.
BIENNIAL. A plant which completes its life history in two years,
usually reproducing in the second.
BILE DUCT. Tube which conveys the secretions (bile) of the liver
to the small intestine. Usually unites with the pancreatic duct
to form a common duct which enters the intestine.
BINARY FISSION. The division of a cell, especially a unicellular
organism, into two daughter cells; e.g., in Paramecium.
BINOMIAL NOMENCLATURE. The accepted scientific method of
designating organisms by two Latin or Latinized words, the
first indicating the genus and the other, the species. E.g., the
Dog, Canis familiaris; Man, Homo sapiens.
BIOGENESIS. The established doctrine that all life arises from pre-
existing living matter. See Abiogenesis.
BIOLOGY. The study of the manifestations of matter in the living
state.
BIPARENTAL. Derived from two progenitors, male and female, e.g.,
in sexual reproduction. See Uniparental.
BLASTOCOEL. The cavity within the blastula. Segmentation cavity.
BLASTOPORE. The opening to the exterior from the enteric pouch
of a gastrula.
BLASTULA. The stage following cleavage when the cells are ar-
ranged in a single layer to form a hollow sphere.
BLENDING INHERITANCE. Apparent fusion of parental characters
in the offspring so that a more or less intermediate condition
arises. E.g., skin color of mulattoes.
BLOOD CORPUSCLES. Detached cells present in the fluid plasma of
the blood. Two principal kinds, red and white.
BUCCAL CAVITY. Mouth cavity.
432 APPENDIX
BUD. Growing point of shoot. An undeveloped branch. Leaf
buds form stem and leaves; mixed buds, both leaves and flowers;
flower buds, flowers only.
CALCIFEROUS GLANDS. Glands opening into the oesophagus of the
Earthworm which secrete calcium carbonate, probably to neu-
tralize acidity of food.
CALYX. The outer whorl of modified leaves composing a typical
flower. Usually green.
CAMBIUM. Layer of actively dividing cells which, in the highest
Flowering Plants, is situated between xylem and phloem of vas-
cular bundles, and forms a thin cylinder between wood and bark.
CARBOHYDRATES. Compounds of carbon with hydrogen and oxygen^
the hydrogen and oxygen being in the same proportion as in
water (H20).
CARPEL. One of the innermost whorl of floral leaves which bear
the megaspores. A simple pistil or an element of a compound
pistil. A megasporophyll.
CATALYSIS. The acceleration of a chemical reaction by a substance
which itself remains unchanged (e.g., an enzyme).
CELL. A structural and physiological unit mass of protoplasm,
differentiated into cytoplasm and nucleus.
CELL SAP. Water, with solutes, under pressure in a large vac-
uole in the cytoplasm of certain types of plant cells. Effects
cell turgor.
CELLULOSE. A carbohydrate which characteristically forms the
walls of plant cells.
CENTROSOME. A minute body situated in the center of the aster
and active during cell division.
CHELIPED. The first thoracic appendages, or walking-legs, in the
Crayfish and its allies. The 'pincer/
CHEMOSYNTHESIS. Manufacture (synthesis) of food material from
water and carbon dioxide, through energy derived from chemical
changes involving oxidation instead of directly from sunlight.
Restricted to special groups of Bacteria.
CHEMOTAXIS. Movements of cells (e.g., Paramecium) in response
to chemical stimuli.
GLOSSARY 433
CHLORENCHYMA. The chlorophyll-bearing tissue of plants.
CHLOROPHYLL. The characteristic green coloring matter of plants
through which photosynthesis takes place.
CHLOROPLASTID. The special protoplasmic bodies in which
chlorophyll, or functionally similar pigments, resides.
CHORD ATE. An animal whose primary axial skeleton consists tem-
porarily or permanently of a notochord. All Vertebrates are
Chordates.
CHROMATIN. A deeply staining substance characteristic of the
nucleus, forming chromosomes, etc. See Germ Plasm.
CHROMOMERE. A chromatin granule of the linear series which con-
stitute a chromosome.
CHROMOSOME. One of the deeply staining bodies into which the
chromatic network of the nucleus becomes visibly resolved
during mitosis. See Germ Plasm.
CILIA. Delicate protoplasmic projections from a cell, which lash
in unison and propel the cell in the water (e.g., Paramecium), or
move particles over the cell surface (e.g., cells lining various tubes
in multicellular forms) .
CLASS. In classification, a main subdivision of a phylum. See
Order.
CLEAVAGE. The divisions which transform the egg into the blastula
stage during development.
CLOACA. A cavity at the posterior end of the Vertebrate body, into
which the intestine, urinary, and reproductive ducts open. Not
present in most Mammals.
COCHLEA. The portion of the ear, in communication with the sac-
culus, which is the essential organ of hearing ill the higher
Vertebrates.
COELOM. The body cavity, lying between the digestive tract and
the body wall. Lined with mesodermal tissue.
COELOMATE. Possessing a coelom, or body cavity; as in all the
chief groups of animals above the Coelenterates. The latter are
acoelomate.
COELOMIC EPITHELIUM. See Peritoneum.
COLLOID. A state of matter in which a substance is finelv divided
434 APPENDIX
into particles larger than one molecule and suspended in another
substance.
COLONY. An aggregation, or intimate association of several or
many individuals to form a superior unit.
COMBINATION. Heritable variation due to recombinations of genes
at maturation or fertilization.
CONJUGATION. The temporary union of two cells during which
sexual phenomena occur; e.g., in Paramecium. See Endomixis.
CONSERVATION OF ENERGY. The ' law ' that the total energy of the
universe is constant, none being created or destroyed but merely
transformed from one form to another.
CONTRACTILE VACUOLE. A reservoir in unicellular organisms (e.g.
Paramecium) in which water and waste products of metabolism
collect and are periodically expelled to the exterior.
CORM. A solid bulb-like expansion of a plant stem below the surface
of the ground. A bulb is an underground storage leaf bud.
COROLLA. The whorl of modified leaves immediately within the
calyx of a flower. The petals collectively.
CORTEX. The cylinder between the outer and central cylinder in
root and stem of the higher plants.
COTYLEDON. A seed leaf. The first leaf (in monocotyledons) or
pair of leaves (in dicotyledons) of the young sporophyte within
the seed.
CRANIAL NERVES. Nerves which arise from the brain.
CRANIUM. The protective case enclosing the brain.
CROSSING-OVER. The rearranging of linked characters as a result
of the exchange of genes during synapsis of chromosomes.
CRURA CEREBRI. Thickenings of ventral surface of mid-brain.
CRUSTACEA. A group of Arthropoda, including Crayfish, Crabs, etc.
CUTICLE. The outermost lifeless layer of organisms.
CYST. A resistant envelope formed about an organism (e.g., many
Protozoa) during unfavorable conditions or reproduction.
CYTOLOGY. The science of cell structure and function.
CYTOPLASM. The protoplasm of a cell exclusive of the nucleus.
DECAY. Chemical decomposition involving putrefaction or other
types of fermentation.
GLOSSARY 435
DENITRIFYING BACTERIA. Types of Bacteria which break down
compounds of nitrogen and set free the nitrogen.
DERMAL. Pertaining to the skin. The dermis is the inner layer of
the Vertebrate skin. See Epidermis.
DIFFERENTIATION. A transformation from relative homogeneity to
~ heterogeneity r involving; the production of specific substances or
parts from a general substance or part. Specialization.
DIHYBRID. The progeny of parents differing in regard to two given
characters.
DIPLOID. The maximum or full (duplex) number of chromosomes
which occurs during the life-history of a given species. See
Haploid.
DIVISION OF LABOR. Allocation of special functions to special
parts which cooperate toward the unity of the whole.
DOMINANT CHARACTER. One of a pair of alternative characters
which appears to the exclusion of the other (recessive) character.
DORSAL AORTA. Chief artery distributing pure blood to the body.
Ventral aorta carries blood from heart to gill-arteries in Fishes.
DUCTLESS GLAND. An organ whose function is to elaborate and se-
crete a hormone directly into the blood. An endocrine gland.
ECOLOGY. The study of the relations of the organism to environing
conditions, organic and inorganic.
ECTODERM. The primary tissue comprising the surface layer of cells
in the gastrula; its derivatives in subsequent stages forming the
outer part of the skin, nervous system, etc. See Germ Layer.
ECTOPLASM. Modified surface layer of cytoplasm of a cell. See
Endoplasm.
EFFERENT ROOT. Ventral, or anterior, root of certain cranial
and -all spinal nerves through which motor nerve impulses leave
the brain and spinal cord. See Afferent Root.
EGG. The female gamete. Ovum.
EMBRYOLOGY. The study of the early development of individual
organisms.
EMBRYO SAC. Megaspore of the Flowering Plants.
EMULSOID. A state in which one liquid is divided into very fine drop-
lets and suspended in another liquid with which it is immiscible.
436 APPENDIX
ENCYSTMENT. The formation of a resistant covering, or cyst wall,
about an organism.
ENDOCRINE GLAND. See Ductless Gland.
ENDODERM. The primary tissue comprising the inner layer of cells
in the gastrula, and in subsequent stages forming the lining of
the essential parts of the digestive tract and its derivatives. See
Germ Layer.
ENDOMIXIS. A nuclear reorganization process in Protozoa, e.g.,
Paramecium, which does not involve the cooperation of two
cells (as in conjugation) and therefore is without synkaryon
formation.
ENDOPLASM. The inner cytoplasm surrounding the nucleus; e.g.,
in Paramecium. See Ectoplasm.
ENDOPODITE. The inner of the two distal parts of the typical bira-
mous Crustacean appendage. See Protopodite and Exopodite.
ENDOSKELETON. An internal living skeleton affording support
and protection, as well as levers for the attachment of muscles.
Characteristic of Vertebrates.
ENDOSPERM. A tissue, containing reserve food materials, formed
within the embryo sac.
ENTERIC CAVITY. The digestive cavity of the gastrula stage, and of
simple Metazoa, e.g., Hydra.
ENZYMES. Complex chemical substances of organisms which bring
about by catalytic action many of the chemical processes of the
body; e.g., digestion.
EPIDERMIS. The outer cellular layer of the skin.
EPIGENESIS. Development from absolute or relative simplicity to
complexity. See Preformation.
EPITHELIUM. A layer of cells covering an external or internal sur-
face, including the essential secreting cells of glands.
EQUATION DIVISION. A typical division of the nucleus involving
division of the chromosomes. See Reduction Division.
EQUATORIAL PLATE. The equator of the spindle with its group of
chromosomes during the metaphase of mitosis.
EUGENICS. The system of improving the human race by breeding
the best. "The science of being well bom." See Euthenics.
GLOSSARY 437
EUSTACHIAN TUBE. Passage connecting the Vertebrate middle ear
with the pharynx. Remnant of the most anterior gill slit, rep-
resented in present-day Sharks by the 'blow-hole,' or spiracle.
EUTHENICS. The system of improving the human race by good
environment. Sep. Eugenics.
EUTHERIA. The highest of the three subclasses of Mammals,
including all the familiar forms-. See Appendix I, Classification.
EVOLUTION, ORGANIC. The accepted theory that present-day
organisms are the result of descent with modification, or change,
from those of the past. The word 'modification' is not used in
the technical sense employed in genetics. See Modifications.
EXCRETION. The elimination of waste products of metabolism.
The waste products themselves. See Secretion.
EXOPODITE. The outer of the two distal parts of the typical, bira-
mous, Crustacean appendage. See Protopodite and Endopodite.
EXOSKELETON. A non-living external skeleton chiefly for protec-
tion. The characteristic skeleton of Invertebrates, e.g., Cray-
fish.
EXTERNAL RECEPTORS. Sense organs upon the surface of the body.
See Internal Receptors.
EXTRACTED DOMINANT. A homozygous individual, exhibiting
the dominant character, derived from heterozygous (hybrid)
parents.
EXTRACTED RECESSIVE. An individual exhibiting the recessive
character, necessarily homozygous, derived from heterozygous
(hybrid) parents.
FAMILY. In classification, a main subdivision of an order. See
Genus.
FATS. One of the chief groups of foodstuffs. Organic salts con-
sisting of the glycerol radical (C3H5), the basic part, combined
with a fatty acid. E.g., mutton tallow is chiefly the fat Stearin
(CsyHnuOe) = Glycerin plus Stearic acid.
FERMENTATION. The transformation of organic substances chiefly
through the activity of ferments, or enzymes, derived from
living organisms. See Putrefaction.
FERTILIZATION. The union of male and female gametes, especially
438 APPENDIX
their nuclei (pronuclei), by which the chromatin complex of each
is arranged to form the composite nucleus of the zygote.
FLAGELLUM. A whip-like prolongation of the cytoplasm, the move-
ments of which usually effect the locomotion of the cell; e.g.,
Sphaerella.
FLOWER. A group of sporophylls and accessory structures, as in the
Flowering Plants.
FLUCTUATIONS. Relatively slight variations always found in organ-
isms; may be either 'modifications or combinations.
FOETAL MEMBRANE. The embryo of the higher Mammals before
birth lies in "the uterus of the mother enclosed in a series of
membranes the outer one of which is in intimate contact with
the uterine wall at one or more points to form the placenta.
FOVEA CENTRALIS. A slight depression at the posterior end of
the optical axis of the eyeball. The center of distinct vision.
FROND. Fern leaf, usually both vegetative and spore-producing.
FRUIT. The ripened ovule case and contents, together with any
structures which by adhesion become an integral part of it.
GALL BLADDER. Receptacle near the liver for the temporary
storage of "bile.
GAMETANGIUM. A gamete-producing organ, especially in the lower
plants.
GAMETE. A cell which unites with another at fertilization to form
a zygote. Egg or sperm.
GAMETOPHYTE. The sexual, gamete-bearing generation in plants.
GANGLION. A .group of nerve cells, chiefly the cell bodies, with
supporting cells.
GASTRIC VACUOLE. A droplet of fluid enclosing ingested food, in
which digestion occurs; e.g., in Paramecium.
GASTROLITHS. Calcareous bodies found at certain times in the
lateral walls of the stomach of the Crayfish. Probably represent
the storage of material for the exoskeleton.
GASTRULA. A stage in animal development in which the embryo
consists of a two-layered sac, ectoderm and endoderm, enclosing
the enteric cavity which opens to the exterior by the blastopore.
GEL. A colloid which is more or less rigid.
GLOSSARY 439
GENE. A factor or element in the chromosomes of the germ cells
which conditions a character of an organism.
GENETICS. The science of heredity.
GENOTYPE. The fundamental hereditary constitution of an organ-
ism or group of organisms. The gene complex of an organism.
See Phenotype.
GENUS. In classification, a main subdivision of a family. See
Species.
GERMINAL CONTINUITY. The concept of an unbroken stream of
germ plasm from the beginning of life, from which each genera-
tion is derived.
GERM LAYER. A primary tissue (ectoderm, endoderm, or meso-
derm) in the embryo from which the tissues and organs of the
adult animal develop.
GERM LAYER THEORY. The doctrine that the germ layers are
fundamentally similar throughout the Metazoa and that homolo-
gous structures in various animals are derived during ontogeny
from the same germ layer.
GERM PLASM. The physical basis of inheritance. The chromatin
which forms the specific bond of continuity between parent and
offspring. Contrasted with soma or somatoplasm.
GILL SLITS. Paired lateral openings leading from the anterior end
of the alimentary canal to the exterior for the exit of the respira-
tory current of water. Permanent or embryonic characters
of Vertebrates. Branchial clefts.
GLAND. One cell or a group of many epithelial cells which elaborate
certain materials and then secrete the product for the use of the
organism.
GLOTTIS. The opening from the pharynx into the tube (trachea)
leading to the lungs.
GONAD. An organ in which the germ cells develop. Ovary or
test is.
GREEN GLANDS. Excretory organs (nephridia) of the Crayfish
and its allies.
HAPLOID. The reduced (one-half) number (simplex group) of
chromosomes. See Diploid.
440 APPENDIX
HAUSTORIA. Sucker-like absorbing organs of parasitic plants; e.g.,
Dodder.
HEPATIC PORTAL SYSTEM. Non-oxygenated but food-laden blood
from digestive tract to the liver via hepatic portal vein. Oxygen-
ated blood reaches liver via hepatic artery. All leaves via
hepatic vein. Thus there is a double blood supply to liver in
all Vertebrates.
HEREDITY. The transmission of characters from parent to off-
spring through the germ cells.
HERMAPHRODITE. An organism bearing both male and female
reproductive organs; e.g., Hydra and Earthworm.
HETEROSPORY. The condition of producing two kinds of spores,
megaspores and microspores, as in the higher plants.
HETEROZYGOUS. Producing gametes which fall into two numeri-
cally equal classes with respect to the genes (allelomorphs) for a
pair of alternative characters. See Homozygous.
HISTOLOGY. The science which treats of animal and plant tissues.
Microscopic anatomy.
HOLOPHYTIC. Type of nutrition involving photosynthesis. Char-
acteristic of green plants. See Holozoic and Saprophytic.
HOLOZOIC. Type of nutrition involving the ingestion of solid food.
Characteristic of animals. See Holophytic and Saprophytic.
HOMOLOGOUS CHROMOSOMES. The members of a pair of chromo-
somes, of a duplex group, one paternal and the other maternal in
origin, which bear the same or allelomorphic genes. See Synap-
tic Mates.
HOMOLOGOUS GENES. Genes similarly situated on homologous
chromosomes. See Allelomorph.
HOMOLOGY. Fundamental structural similarity, regardless of func-
tion, due to descent from a common form.
HOMOTHERMAL. Animals provided with a mechanism which main-
tains the body at a practically constant temperature, usually
higher than that of the environment. E.g., the 'warm-blooded'
animals, or Birds and Mammals.
HOMOZYGOUS. Producing gametes all of which bear the gene for
one of a pair of alternative characters. See Heterozygous.
GLOSSARY 441
HORMONE. An internal secretion, usually from a ductless gland,
which is distributed by the blood and influences the activities
of one or more parts of the body.
HYALINE. Pellucid or glassy.
HYBRID. The progeny of parents which differ in regard to one or
more characters.
HYDROIDS. A group of animals allied to Hydra, exhibiting alterna-
tion of generations.
IMMUNITY. Resistance of the body to infection by disease-produc-
ing organisms. Exemption from disease.
INFUNDIBULUM. A funnel-like outgrowth from the ventral wall of
the diencephalon. See Pituitary Body.
INTERCELLULAR DIGESTION. Digestion by the secretion of enzymes
into a digestive cavity; e.g., in Earthworm and Man. See Intra-
cellular digestion.
INTERNAL RECEPTORS. Sense organs within the body. See Exter-
nal receptors.
INTERNAL SECRETION. See Hormone and Ductless Gland.
INTESTINE. Portion of the alimentary canal from pyloric end of
stomach to anus. Divided into small and large intestine.
INTRACELLULAR DIGESTION. Digestion of food within the cell it-
self; e.g., in Paramecium and to some extent in the endoderm
cells of Hydra. See Intercellular digestion.
INTUSSUSCEPTION. Interstitial growth by the addition of new
particles throughout the whole mass of protoplasm. Contrasted
with growth by accretion, or the deposition of particles on the
surface as in crystals.
INVAGINATION. Sinking or growing in of a portion of the surface of
a hollow body; e.g., during transformation of blastula to gastrula.
INVERTEBRATE . An animal without a notochord or a vertebral column.
IRRITABILITY. The power of responding to stimuli, exhibited by all
protoplasm.
KARYOLYMPH. The more fluid material of the nucleus in contrast
with the linin and chromatin.
KARYOSOME. An aggregation of part of the chromatin material
within the nucleus. A 'net-knot7. See Nucleolus.
442 APPENDIX
KATABOLISM. The destructive phase of metabolism. See Anabo-
lism.
KINETIC ENERGY. Energy possessed by virtue of motion. E.g.,
union of C with 02 transforms chemical potential energy into
kinetic energy, i.e., heat, etc. See Potential Energy.
LAMINA. The blade of a leaf.
LARVA. An immature stage in the life history of certain animals,
usually active arid differing widely in appearance from the adult.
E.g., caterpillar of a Butterfly, tadpole of Frog.
LENTICELS. Openings on the outer surface of the bark which per-
mit a slight amount of gaseous interchange. Arise as stomata in
the young shoot.
LININ. The material of the reticulum of the nucleus, upon and
through which the chromatin appears to be distributed in the
resting cell. The representative within the nucleus of the gen-
eral cytoplasmic reticulum.
LINKAGE. Tendency for certain characters to be inherited in
groups, probably because the genes for the characters are closely
associated on the same chromosome.
LYMPH. Essentially plasma and white blood corpuscles which have
passed through the capillary walls to supply the milieu of the
tissue cells.
MACRONUCLEUS. The large 'vegetative' nucleus in Infusoria with
dimorphic nuclei; e.g., in Paramecium. See Micronucleus.
MANDIBLES. Jaws. The third pair of appendages of the head of
the Crayfish.
MATURATION. Final stages in the formation of the germ cells, in-
volving chromosome reduction.
MAXILLIPEDS. The three posterior pairs of appendages of the
head of the Crayfish.
MECHANISM. The doctrine that the phenomena of life are inter-
pretable in terms of the laws of matter and energy which hold
in the realm of the non-living. See Vitalism.
MEDUSA. Sexual, gonad-bearing generation of hydra-like animals,
the Hydroids.
MEGASPORANGIUM. A sporangium which bears megaspores.
GLOSSARY 443
MEGASPORE. The large spore which in heterosporous plants forms
a female gametophyte.
MEGASPOROPHYLL. A modified leaf of a heterosporous sporophyte
which produces megaspores. A carpel.
MERISTEM. Formative tissue with rapidly dividing cells, as in cam-
bium and growing points of plants.
MESODERM. A primary tissue, or germ layer, of animals which
develops between ectoderm and endoderm. See Germ Layer.
MESOGLOEA. The non-cellular layer between ectoderm and endo-
derm in Hydra and other Coelenterates.
MESOPHYLL. Tissue of the leaf, between upper and lower epider-
mis, exclusive of the vascular bundles (veins).
METABOLISM. The sum of the chemical processes in organisms,
involving the building up and breaking down of the living matter.
See Anabolism and Katabolism.
METAGENESIS. Alternation of generations, as in Obelia.
METAMERE. One of the series of similar parts, or segments, of the
body; e.g., in the Earthworm and Crayfish and, in highly modi-
fied form, throughout the Vertebrates.
METAMORPHOSIS. A more or less abrupt transition from one devel-
opmental stage to another. E.g., in Insects.
METAPHASE. Climax of mitosis involving the separation of the
halves of the longitudinally split chromosomes arranged in the
equatorial plate. See Anaphase.
METAPH YTA . Multicdli.il ar plants .
METAPLASM. Lifeless inclusions in cytoplasm; e.g., yolk granules,
etc.
METAZOA. Multicellular animals.
MICRONUCLEUS. The small 'germinal' nucleus in Infusoria with
dimorphic nuclei; e.g., Paramecium caudatum has one, and P.
aurelia and P. calkinsi have two micronuclei. See Macro-
nucleus.
MICROSPORANGIUM. A sporangium which bears microspores; e.g.,
pollen sacs in anther of stamen.
MICROSPORE. The small spore, of heterosporous plants, which
forms a male gametophyte. A pollen grain.
444 APPENDIX
MICROSPOROPHYLL. A modified leaf, of a heterosporous sporo-
phyte, which produces microspores. A stamen.
MITOSIS. The typical process of cell division.
MODIFICATIONS. In genetics: changes in the soma due to environ-
mental influences; so-called acquired characters are modifica-
tions. In evolution : signifies ' change ' ; no technical connotation.
MONOHYBRID. The progeny of parents differing in regard to one
given character.
MORPHOGENESIS. The origin of the form and structure of an organ-
ism during ontogeny.
MORPHOLOGY. The science of the form of animals and planter
MOSAIC INHERITANCE. Inheritance of a character in part from each
parent but without blending.
MUTATION. A heritable variation due to a fundamental change in
the constitution of the germ plasm, independent of the normal
processes of segregation and crossing-over.
MYOTOMES. Muscle segments in body wall of lower Vertebrates
and embryos of higher forms.
NATURAL SELECTION. The processes occurring in nature which
result in the " survival of the fittest" individuals and the elimi-
nation of those less adapted to the conditions imposed by their
environment and mode of life.
NEPHRIDIUM. An excretory organ; e.g., in Earthworm.
NEPHROSTOME. Coelomic opening or funnel of a nephridium.
NERVE. Essentially a group or cable of parallel nerve fibers bound
together. See Axon.
NEURAL CANAL. The tube in which the brain and spinal cord lie.
Formed by the neural arches and centra of the vertebrae.
NEURAL TUBE. A tube derived from the ectoderm and forming
the brain and spinal cord in Vertebrates.
NEURON. A nerve cell, comprising cell body and cytoplasmic pro-
cesses. See Axon.
NITRIFYING BACTERIA. Types of Bacteria which, in the process of
their nutrition, change ammonia (NH3) into compounds with the
N(>2 radical (nitrites), and change nitrites into compounds with
the N03 radical (nitrates.)
GLOSSARY 445
NITROGEN-FIXING BACTERIA. Types of Bacteria which take free
atmospheric nitrogen and combine it with oxygen so that nitrates
available for green plants are formed. Found in the soil and
in tubercles on rootlets of various leguminous plants.
NOTOCHORD. An axial cord of cells about which the backbone is
formed. Gradually replaced by the centra of the vertebrae in
the ascending series of Vertebrates.
NUCELLUS. The megasporangium of Flowering Plants. See Ovule
and Embryo sac.
NUCLEOLUS. A spherical body of achromatic material within the
nucleus. Plasmosome. See Karyosome.
NUCLEUS. A specialized protoplasmic body in all typical cells.
Most characteristic element is chromatin.
OESOPHAGUS. Narrow tube leading from pharynx to stomach.
OLFACTORY. Relating to the sense of smell.
ONTOGENY. The developmental history of the individual. See
Phylogeny.
OOCYTE. The ovarian egg before maturation.
OOGENESIS. The development of the mature egg from a primordial
germ cell.
OPTIC LOBES. Thickenings of the dorsal surface of the mid-brain.
ORDER. In classification, a main subdivision of a class. See Family.
ORGAN. A complex of tissues for the performance of a certain func-
tion; e.g., the heart.
OSMOSIS. Diffusion of dissolved substances through a semi-perme-
able membrane. Osmotic pressure may be considered as a result
of the inhibited power of diffusion of a dissolved substance — in-
hibited because the membrane is semi-permeable. The physical
phenomena of diffusion and osmosis are complicated in living
cells by the fact that their limiting surfaces may function now as
permeable and again as semi-permeable membranes, i.e., per-
mitting water but not the substance in solution to pass
through.
OSTEOLOGY. The study of the Vertebrate skeleton.
OVARY. The definitive female reproductive organ in which "fcfoe
gametes (eggs) develop.
446 APPENDIX
OVULE. The body which after fertilization of the egg becomes a
seed. The ovule consists of protective envelopes (integument)
enclosing the nucellus (megasporangium) with the embryo sac
(megaspore) .
OVULE CASE. The base of the pistil in which ovules arise.
"Ovary."
OVUM. Egg. Female gamete.
OXIDATION. The combination of any substance or its constituent
parts with oxygen.
PALEONTOLOGY. The science of extinct animals and plants repre-
sented by fossil remains.
PARASITE. An organism which secures its livelihood directly at the
expense of another living organism, on or in whose .body it lives.
PARTHENOGENESIS. Development of an egg without fertilization.
PATHOGENIC. Disease-producing, especially in regard to the rela-
tion of a parasite to its host.
PEDUNCLE. Stalk of a flower; represents the floral branch.
PENTADACTYL. Having five fingers or toes; typical Vertebrate
limb.
PERIANTH. Collective term for calyx and corolla.
PERICARDIUM. Peritoneum lining the pericardial cavity containing
the heart.
PERISTALSIS. Rhythmical contractions of the wall of the alimen-
tary canal which forces the food along.
PERITONEUM. Membrane lining coelom of Vertebrates. Consists
of an outer layer of connective tissue next to the muscles of body
wall and an inner layer of coelomic epithelium which forms the
innermost layer of body wall.
PETAL. One of the leaves of the corolla of a flower.
PETIOLE. A leaf stalk.
PHARYNX. Region of alimentary canal between buccal cavity, or
mouth, and oesophagus. Throat.
PHENOTYPE. The somatic, or expressed, characters of an organism
or group of organisms irrespective of those potential in their germ
cells. See Genotype.
PHLOEM. The outer part of a vascular bundle. 'Inner bark.'
GLOSSARY 447
PHOTOSYNTHESIS. Process by which complex compounds are built
up from simple elements through the energy of sunlight absorbed
by chlorophyll, or a functionally similar pigment.
PHYLOGENY. The ancestral history of the race. See Ontogeny.
PHYLUM. In classification, a main subdivision of the animal or
plant kingdom. See Class.
PHYSIOLOGY. The study of the functions of animals and plants.
The mechanical and chemical engineering of organisms.
PINEAL BODY. An outgrowth from the upper wall of the diencepha-
lon. The vestige of an additional pair of eyes possessed by the
ancestors of existing Vertebrates. Fcssibly functions as an en-
docrine gland in Mammals. Brow-spot of Frog.
PISTIL. Organ of the flower, composed of ovule case, style, and
stigma. See Carpel.
PITH. Middle part of the central cylinder of a plant shoot. Func-
tions largely for the storage of water and food.
PITH RAYS. Extensions of the pith which radiate between the
vascular bundles to the bark. Medullary rays.
PITUITARY BODY. An ingrowth of the ectodermal tissue above the
mouth and the tip of the infundibulum from the ventral wall
of the diencephalon unite to form a gland-like structure (pitui-
tary body or hypophysis) .
PLACENTA. A Mammalian organ adapted for the interchange of all
nutritive, respiratory, and excretory materials between the
embryo (foetus) and mother. It also serves as an organ of
attachment. In the higher Mammals it is composed of both
foetal and maternal tissues. See Umbilical Cord.
PLASMA. Liquid portion of the blood.
PLEXUS. Intercommunication of the fibers from one nerve with
those of another to form a network of nerves; e.g., branchial and
sciatic plexus.
POLAR BODIES. Tiny abortive cells arising, by division, from the
egg during maturation.
POLE CELLS. Two cells which give rise to the mesoderm in the
development of the Earthworm and its allies.
POLLEN. The microspores of Flowering Plants.
448 APPENDIX
POLLINATION. The transference of pollen to the stigma of the pis-
til in higher Flowering Plants.
POLYHYBRID. The progeny of parents which differ in regard to
more than three given characters.
POLYMOEPHISM. Occurrence of several types of individuals during
the life history, or composing a colony; e.g., in some Hydroids.
POTENTIAL ENERGY. Energy possessed by virtue of stresses, i.e.,
two forces in equilibrium. Criterion is work done against any
restoring force; e.g., kinetic energy of sunlight through agency
of chlorophyll separates C02 into C and 02 and thereupon is
represented by an equal amount of chemical potential energy.
Restoring force is here chemical affinity. Similarly a raised
weight possesses gravitational potential energy in amount
equal to kinetic energy expended in raising it. See Kinetic
Energy and Conservation of Energy.
PREFORMATION. The abandoned doctrine that development is es-
sentially an unfolding of an individual ready-formed in the germ.
See Epigcnesis.
PRONEPHROS. Primitive kidney of Vertebrates.
PRONUCLEI. The nuclei of the male and female gametes ready to
unite at fertilization.
PROPHASE. Preparatory changes during mitosis leading to the dis-
position of the chromosomes in the center of the cell (equatorial
plate) ready for division. See Metaphase.
PROSTOMIUM. A lobe which projects from the first segment of the
body of the Earthworm and forms an upper lip.
PROTEIN. A class of complex chemical molecules, containing nitro-
gen, which form the chief characteristic constituent of proto-
plasm.
PROTHALLUS. The gametophyte of Ferns.
PROTISTA. Protophyta and Protozoa; all unicellular organisms.
PROTONEMA. A filamentous growth from a Moss spore which gives
rise to the leafy Moss plant.
PROTOPHYTA. Unicellular plants. See Protista.
PROTOPLASM. The physical basis of life. Living matter.
PROTOPLAST. The cell exclusive of the cell wall, especially in plants.
GLOSSARY 449
PROTOPODITE. The basal portion of the typical Crustacean ap-
pendage from which arise the endopodite and exopodite.
PROTOZOA. Unicellular animals.
PURE LINE. A group of individuals bearing identical genes,
derived from a common homozygous ancestor.
PUTREFACTION. The simplification of nitrogenous compounds,
such as proteins, chiefly through the action of enzymes of living
organisms. See Fermentation.
PYLORIC VALVE. Muscular constriction between stomach and
small intestine.
RECAPITULATION THEORY. Doctrine that individual development
(ontogeny) repeats in abbreviated and modified form the develop-
ment of the race (phylogeny). So-called biogenetic law.
RECESSIVE CHARACTER. See Dominant character.
REDUCTION. The halving of the chromosome number during
maturation. Transformation of duplex into simplex group.
REDUCTION DIVISION. The division during spermatogenesis and
oogenesis which separates synaptic mates and reduces the
chromosome number one half. The mechanism of segregation.
REFLEXES. Relatively simple and essentially automatic responses.
Merge into instincts which are the most complex reactions
made without learning.
REGENERATION. The power of replacement of parts which have
been lost through mutilations or otherwise.
RENAL PORTAL SYSTEM. Blood ('impure') from posterior part of
the body to kidneys via renal portal vein. Oxygenated blood to
kidneys via renal artery. Thus in animals with the renal portal
system there is a double blood supply to the kidneys. Present
in Fishes, Amphibians, and Reptiles; vestigial in Birds; absent
in Mammals.
REPRODUCTION. The power of living matter to reproduce itself.
Protoplasmic growth resulting in cell division.
RESPIRATION. Essentially the securing of energy from fnodr involv-
_ ing the exchange of carbon dioxide for oxygen by protoplasm.
RESPONSE. Any change in the activity oi protoplasm, and therefore
of an organism as a whole, as the result of a stimulus.
450 APPENDIX
RESTING CELL. One which is not undergoing mitosis.
RETINA. Actual percipient part of the eye by virtue of a sensory
layer which is stimulated by light rays.
REVERSION. The appearance of a distant ancestral character in an
individual. See Atavism.
RHIZOID. A root-like filament in lower plants; e.g., in Mosses and
prothallus of Ferns.
RHIZOME. Prostrate underground stem; e.g., in sporophyte of com-
mon Ferns.
ROOT HAIRS. Prolongations of epidermal cells just above the grow-
ing point of roots which afford surface for intake of water and
solutes.
ROSTRUM. The anterior pointed extension of the exoskeleton of the
Crayfish and its allies.
ROTIFERA. Microscopic, aquatic, multicellular animals. Wheel
animalcules.
RUSTS. Fungi which are destructive parasites of the higher plants;
e.g., the Wheat Rust.
SACCULUS. The anterior sac of the labyrinth of the ear, a derivative
of which becomes the cochlea in higher Vertebrates.
SAPROPHYTIC. Type of nutrition involving the absorption of com-
plex products of organic decomposition; e.g., in many groups of
Bacteria and other Fungi, as well as various species of lower
animals. See Holozoic and Holophytic.
SEBACEOUS GLANDS. Glands which elaborate a fatty substance
(sebum) and secrete it in the hair follicles.
SECRETION. A substance elaborated by glandular epithelium; or
the process involved. See Gland and Excretion.
SEED. An embryo sporophyte supplied with food and protective
envelopes.
SEGREGATION. The distribution of contrasting genes (allelomorphs)
to separate cells during the maturation of the germ cells in a
heterozygous individual (hybrid).
SEMICIRCULAR CANALS. Portion of the Vertebrate ear devoted to
equilibrium.
SEMINAL RECEPTACLES. Globular sacs within the body cavity of
GLOSSARY 451
the Earthworm, which receive the sperm from another worm
and retain them until fertilization is to occur.
SEPAL. A leaf of the calyx of a flower.
SEPTA. The partitions which divide the coelom of the Earthworm
into a series of chambers, or metameres.
SERIAL HOMOLOGY. Homology of a structure of an organism with
another of the same organism; e.g., appendages of the Crayfish,
fore- and hind-limbs of Vertebrates.
SETAE. Bristle-like structures which protrude from the body wall
of the Earthworm and aid in locomotion.
SEX CHROMOSOME. The odd, X, or accessory chromosome which
bears the differential gene for sex.
SEX-LINKED CHARACTERS. Characters represented by genes on
the sex chromosomes.
SHOOT. Stem and leaves as contrasted with the root.
SIMPLEX CHARACTER. The result of a determiner, or gene, from one
parent only.
SOL. A colloid which is highly fluid.
SOMA. Body tissue (somatoplasm) in contrast with germinal tissue.
SPECIAL CREATION. Abandoned doctrine that. each species was
specially created. Implies fixity of species. See Evolution.
SPECIES. In classification, the main subdivision cf a genus. A
group of individuals which do not differ from one another in
excess of the limits of "individual diversity," actual or as-
sumed.
SPERM. Male gamete. Spermatozoon.
SPERMATID. Male germ cells after the final maturation division
but before assuming the typical form. of the ripe sperm.
SPERMATOCYTES. Cells arising from the spermatogonia. Primary
spermatocyte arises by growth from the last generation of
spermatogonia. Primary divides to form two secondary
spermatocytes.
SPERMATOGENESIS. The development of the sperm from a primor-
dial germ cell.
SPERMATOPHYTES. Plants bearing true seeds. Seed Plants. Flow-
ering Plants. Phanerogams.
452 APPENDIX
SPINDLE. The 'fiber-like' apparatus between the centrosomes
during mitosis.
SPIREME. The linear arrangement of the chromosomes frequently
observed during mitosis.
SPLEEN. A vascular ductless organ of most Vertebrates, usually
situated near the stomach, which produces certain changes in
the blood.
SPONTANEOUS GENERATION. See Abiogenesis.
SPORANGIUM. A spore-producing structure on a sporophyll.
SPORE. A cell, liberated from the parent, which gives rise without
fertilization to a new individual. The resistant phase assumed
by certain unicellular organisms; e.g., Bacteria.
SPOROPHYLL. A leaf which bears sporangia.
SPOROPHYTE. Spore-bearing (asexual) generation in plants exhibit-
ing alternation of generations.
SPORULATION. Occurrence of several simultaneous divisions by
which a unicellular organism is resolved into many smaller cells.
STAMEN. The pollen-bearing organ in Flowering Plants. A micro-
sporophyll. See Anther.
STELE. The central cylinder of root and stem, formed of united vas-
cular bundles, in the highest Flowering Plants.
STIGMA. The tip of the pistil adapted to receive the pollen and pro-
vide for its germination.
STIMULUS. Any condition which calls forth a response from living
matter.
STIPULES. Pair of appendages frequently occurring at the point
(leaf base) where the petiole joins the stem.
STOMATA. Openings through the epidermis of a leaf for the inter-
change of gases and exit of water vapor. The 'stomatic ap-
paratus' comprises the stoma and its guard cells.
STYLE. An elongation of a pistil which bears the stigma.
SYMBIOSIS. The association of two species in a practically obliga-
tory and mutually advantageous partnership; e.g., Lichens.
SYMPATHETIC NERVOUS SYSTEM. See Autonomic.
SYNAPSE. The contact of one nerve cell with another, which makes
possible the conduction of a nervous impulse from cell to cell.
GLOSSARY 453
SYNAPSIS. The pairing of h^molp4ous chromosomes during matu-
ration of the germ cells.
SYNAPTIC MATES. Homologous chromosomes of maternal and
paternal origin paired in synapsis.
SYNGAMY. The union of gametes to form a zygote.
SYNKARYON. The composite nucleus formed by the union of the
nuclei of two gametes. Male and female pronuclei united to
form the fertilization nucleus. See Zygote.
TAPIR. A large herbivorous Mammal, having short stout limbs and
flexible proboscis with the nostrils near the end. New World
species are brownish-black, those of the Old World are black and
white.
TAXONOMY. The science of classification.
TELOPHASE. Final phase of mitosis during which the two daughter
nuclei are reformed and cytoplasmic division is completed.
See Prophase.
TESTIS. The definitive male reproductive organ in which the
gametes (sperm) develop,
THALLUS. A relatively simple plant body, not differentiated into
root, stem, and leaf ; e .g . , in Seaweeds and other multicellular Algae
THORAX. The anterior chamber of the coelom in Mammals, con-
taining lungs and heart. The middle portion of the body in the
Arthropoda; e.g., in all Insects. In the Crayfish the head and
thorax are fused to form the cephalothorax.
THYMUS. A glandular structure in the pharyngeal region of
Vertebrates. Disappears during early life in Man. Function
unknown.
THYROID. A glandular structure in the pharyngeal region of
Vertebrates. Supplies an important hormone.
TISSUE. An aggregation of similar cells for the performance of a
certain function. See Organ.
TRACHEIDS. Elongated cells which form water-conducting vessels
in the vascular bundles of higher plants.
TRANSPIRATION. The exhalation of water vapor, particularly
through the stomata of higher plants.
TRICHOCYSTS. Minute bodies, arranged in the outer part of the
454 APPENDIX
ectoplasm of certain Infusoria (e.g., Paramecium), each of which
upon proper stimulation is transformed into a thread-like process
protruding from the cell surface. Apparently defensive struc-
tures.
TRIHYBRID. The progeny of parents differing in regard to three
given characters.
TRILOBITES. Crustacea dominant during the early Paleozoic era.
Extinct.
TURGOR. Outward pressure of the cell, largely due to the absorp-
tion of water, which distends the cell wall. The turgidity of the
individual cells results in the semi-rigid position of many plants.
Wilting results from a lowering of the turgidity of the cells.
TYPHLOSOLE. A median dorsal invagination along the entire length
of the intestine of the Earthworm. Increases the area of the di-
gestive and absorptive surface.
UMBILICAL CORD. A Mammalian structure, commonly known as
the navel cord, by which the embryo is attached to the placenta.
The blood vessels from the embryo to the placenta pass through
it. See Placenta.
UNGUICULATE. Provided with claws.
UNIFORMITARIAN DOCTRINE. An interpretation of the present con-
dition of the Earth on the assumption of similarity of factors at
work during past ages and to-day.
UNIPARENTAL. Derived from a single progenitor; e.g., in asexual
reproduction. See Biparental.
UNIT CHARACTERS. Characters which behave more or less as units
in heredity.
UREA. Nitrogenous waste product of animal metabolism. Formed
as such in the liver, removed from the blood by the kidneys and
eliminated from the body chiefly in urine.
URETER. A tube carrying urine from kidney to the cloaca or to the
urinary bladder.
UROGENITAL. Relating to the urinary and reproductive systems.
UTERUS. Lower portion of the oviduct (or oviducts) modified for
the retention of the eggs temporarily (Frog) or until develop-
ment has proceeded a considerable way and 'birth' occurs (Man) .
GLOSSARY 455
UTRICULUS. The posterior sac of the labyrinth of the ear into
which the semicircular canals open.
VASCULAR BUNDLE. Composite of xylem, cambium, phloem, and
bundle sheath. Except for the cambium, essentially a system
of tubes for conducting water and food. A fibro-vascular
bundle. See Stele.
VASOMOTOR NERVES. Nerves which regulate the calibre of small
arteries by bringing about relaxation or contraction of the
muscular layer of their walls.
VERMIFORM APPENDIX. Blind outpocketing of the large intestine
near its origin from the small intestine. Vestigial end of the
caecum. Found only in Apes and Man.
VERTEBRA. One of the series of elements forming the backbone,
or vertebral column.
VERTEBRATE. An animal with a backbone, or vertebral column.
VITALISM. The doctrine which attributes at least some of the phe-
nomena of life to an interplay of matter and energy which tran-
scends the so-called laws operable in the inorganic world. See
Mechanism.
VITAMINES. Indispensable accessory food substances whose im-
portance has but recently been realized. Chemical composition
is as yet practically unknown.
WORKING HYPOTHESIS. A basic assumption to guide the study of
a subject, and to be proved or disproved by facts accumulated.
X CHROMOSOME. The 'accessory' or 'sex-chromosome.'
XYLEM. The inner woody part of a vascular bundle.
YEAST. A group of unicellular colorless plants (Fungi) which are
chiefly responsible for alcoholic fermentation.
YOLK. Food material stored within the cytoplasm of an egg. See
Metaplasm.
ZOOGEOGRAPHY. The science of the geographical distribution of
animals.
ZYGOTE. The composite cell formed by the union of male and
female gametes. See Synkaryon.
INDEX
[Figures in italics designate pages on which illustrations occur.]
Abdomen, 130, 158
Abdominal cavity, 140
Abdominal pores, 207
Abdominal vein, 165
Abiogenesis, 209, 210, 388
Absorption, 87, 156, 158
Accessory chromosome (see X
chromosome)
Acoelomates, 121
Acquired character, 266, 297, 306,
377, 408, 409
Adam's apple, 153, 356
Adaptability, individual, 339-344
Adaptation, 11, 17, 18, 307-344,
375; functional, 308-313; to
living environment, 330-339;
physical environment, 308-329;
structural, 313-329
Adaptive radiation of Mammals,
313-319
Adaptive variation, 378
Adrenal body, 152
Adventitious roots, 67
Aerial roots, 66, 67
Afferent nerve, 192
Air bladder, 148
Air spaces, 83, 84
Alcoholism, 267
Algae, classification, 413
Alimentary canal, 121, 123, 131,
137, 148-156; derivatives, 160
Alimentary system, 116
Allelomorphs, 276, 280, 282, 288
Alligator, brain, 189
Alternation of generations, jj4?
100-114, 218-220, 229
Alternative inheritance (see In-
heritance)
Amines, 36, 87
Amino acids, 13, 42, 158
Amoeba, 9, 19, 116, 340
Amphibian, 117, 136, 149, 189 (see
Frog)
Amphioxus, 146, 259, 415; devel-
opment, 257] egg, 256
Anabolism, 16
Anaerobe, 311
Analogous structures, 63, 130, 199
Anaphase, 225
Anatomy, 4\ comparative, 132,
351-356; history, 393, 394
Ancestral inheritance, law of, 270
Animal body, 115-153; versus
plant body, 115
Animal, chief groups, 116, 117;
circulation, 161-174; classifica-
tion, 116, 117, 414-416; colora-
tion, 319-324; coordination,
181-202; excretion, 175-180;
metabolism, 39-43; nutrition,
154-160; reproduction, 203-
208; respiration, 161-174; ses-
sile, 115; unicellular, 39
Annual plant, 66
Antenna, 131, 132] cleaner, 326;
comb, 326
Antennule, 131, 132
Anther, 108, 110, 112
Antheridium, 101, 104
457
458
INDEX
Antibody, 338
Antitoxin, 338
Antlers, 206
Ants, instincts, 343; associated
with Aphids, 333
Anus, 121, 153
Aorta (see Dorsal and Ventral
aorta)
Aortic arches, 171
Aphids and Ants, 333
Apis (see Bee)
Aqueous humor, 201
Arabian scientists, 383
Archaeopteryx and Pigeon, 360
Archegonium, 101, 104
Aristotle, 2, 15, 209, 379, 380, 383,
390, 394, 398, 401, 406
Arterial system, 166
Arteries, 163-165; pulmonary,
150, 151, 152
Arterioles, 163
Arthropoda, 129; classification,
415; structure of primitive, 130
Artificial parthenogenesis, 249
Ash, 80
Asparagine, 36
Asparagus, 70
Aspidium, 103, 104
Associations, communal, 331
Aster, 225
Atavism, 269
Auditory capsule, 145
Auditory nerve, 197, 198
Aurelius, Marcus, 44, 382
Auricle, 163, 172 (see Circulation)
Autonomic nervous system, 186,
191, 192
Azalea, 75
B
Babylonian science, 379
Bacillus tetani, 311
Bacon, R., 386
Bacteria, 44-53; discovery, 388;
chief types, 45 ', denitrifying, 49 ;
as food, 42; nitrate, 48; nitro-
gen-fixing, 49, 333; nutrition,
50; reproduction, 46; sulfur,
309; types of flagellation, 46
von Baer, 1, 402
Balanced aquarium, 53
Barberry, 72
Bark, 88
Barley, 80
Barnacle, 115
de Bary, 7
Bat, 318; wing skeleton, 352
Beagle, voyage of, 369
Bean, 66; inheritance in, 300;
section of stem, 81
Beaver, 349
Bee, 325, 415; head, 326; in-
stincts, 343; legs, 324-329;
parthenogenesis, 249; pollina-
tion by, 330
Bee-fly, 323
Beggiotoa, 309
Bernard, 91
Bibliography, 417-428
Biennial plant, 67
Bilateral symmetry, 124
Bile duct, 137, 148, 151, 152, 155
Binomial nomenclature, 391
Biochemistry, 5
Biogenesis, 210, 388
Biogenetic law, 364, 403
Biological sciences, 5
Biology, 1; divisions of, 4] his-
tory, 379-411; and medicine,
381; scope of, 1-5
Biophysics, 5
Biparental inheritance, 251 (see
Inheritance)
Biramous appendage, 132
Bird, 117, 136; brain, 189; circu-
lation, 165; dissection of, 151;
INDEX
459
egg, 238} embryo, 366} versus
Reptile, 359; skeleton of wing,
352
Birth, 208
de Blainville, 15
Blastocoel, 57
Blastoderm, 238
Blastopore, 57, 58, 127
Blastostyle, 218
Blastula~ 57, 58, 126, 252
Blending inheritance, 268, 283, 286
Blood, 163; capillary circulation
discovered, 389; circulation
demonstrated, 385; corpuscles,
163, 338; pressure, 173; rate
of flow, 172; relationships, 367;
specific differences, 367; trans-
fusion, 367
Body, animal, 115-153; plant,
61-90
Body plan of Earthworm, 122
Body plan of Vertebrates, 136-138
Body temperature, 174, 176, 312
Bone (see Skeleton)
Borelli, 395
Botany, 3, 4
Brain, 134, 137, 148-152} evolu-
tion of, 365; human, 153} ven-
tricles, 188
Branchial arteries, 165, 166
Branchial clefts, 164 (see Gill slits)
Bryales, 101, 414
Bryophyta, 101, 112, 413
Bud, 81, 82} winter, 72
Budding, 113, 119, 213
Buffon, 407
Bulb, 70
Buttress root, 67
Cactus, 70
Calciferous gland, 123
Calyx, 75, 107
Cambarus (see Crayfish)
Cambium, 60, 76, 81
Camel, evolution of, 363
Cameron, E. H., 344
Capillaries, 163, 173; of lungs,
165} network, 159, 389
Carbohydrates, 13, 14, 35, 42, 157
Carbon cycle, 48
Carnivora, 349
Carotid artery, 164, 165, 171
Carpal, 144, 145
Carpel, 75, 107, 108, 112
Cartilage, 25, 140
Castor, 349
Catalyzer, 14
Cat, brain, 189; skeleton, 145
Catocala, 320
Caudal artery and vein, 164
Cell, 21; ciliated, &J; defined, 23;
diagram of, 26} discovery, 3J7;
division, 29, 225, 227; doctrine,
399; epithelial, 59} forms of.
23-29; nerve, 25} origin, 28;
plant, generalized, 77; sap, 26,
80, 84; theory, 242; wall, 27,
77
Cell cycle, 214, 215
Cellulose, 13, 31
Cenozoic era, 358
Central cylinder, 76, 79
Central nervous system, 186 (see
Nervous system)
Central spindle, 226
Centrosome, 26, 27, 225
Centrum, 141, 143, 145
Cephalothorax, 131
Cerebellum, 153, 187, 189
Cerebral ganglion, 128, 131, 134
Cerebral hemispheres, 153, 187,
188, 189
Chameleon, 321
Characters, acquired, 266, 297
306, 377, 408, 409; alternative,
460
INDEX
405; dominant, 272; linked,
268, 291, 293; recessive, 272;
unit, 262, 280, 282, 286
Cheliped, 131
Chemical coordination, 181-183
Chemistry, origin of, 395
Chemosynthesis, 50
Chemotaxis, 240
Cheshire, F. R., 328
Chipmunk, 349
Chlorenchyma, 77, 83
Chlprophyll, 22 ', chemical compo-
sition of, 35
Chlorbplastid, 35, 84
Choloepus, 317
Chordate, 146, 415
Choroid, 201
Chromatin, 28; knot, 26', net-
work, 26
Chromomere, 234
Chromosome, 225, 242, 296', ac-
cessory, 292; combinations,
290; diploid number, 235, 289;
distribution, 236; division, 226;
duplex groups, 231, 235, 289;
haploid number, 235, 289;
homologous, 234, 235, 287, 289;
individuality, 227; in Man, 237,
291, 292; maternal, 234, 235,
287; pairs, 236; paternal, 234,
235, 287; reduction, 228; segre-
gation, 290; sex, 292, 295, 377;
simplex groups, 231, 235, -289;
synapsis, 285] X, 282-295, 377
Chromosome -cycle, 233-237; in
animals, 229, 289; diagram of,
235; in plants, 229, 288
Cilium, 19, 25, 40
Circulation, in animals, 116, 122,
131, 161-174; in Flowering
Plants, 85-88
Class, 350
Classification, animals, 116, 117,
414-416; Algae, 413; Arthro-
poda, 415; Eutherian Mam-
mals, 416; Ferns, 414; Flower-
ing Plants, 414; Fungi, 413;
history of, 390; Mammals, 416;
Mosses, 413, 414; plants, 413,
414; Protozoa, 414; Verte-
brates, 416
Clavicle, 141, 144
Claws, 138
Cleavage, 55
Cloaca, 137, 149, 156
Clover, 80
Clustered roots, 65
Coccyx, 153, 355
Cochlea, 196
Coelenterata, 118, 414 (see Hydra
and Obelia)
Coeliac artery, 164
Coelom, 120, 137, 140, IBS
Coelomate, 121
Coelomic fluid, 162, 177
Collar bone, 144
Colloidal, 8
Colony, 213
Coloration, animal, 319-324
Color-blindness, inheritance of,
294, 295
Colorless plants, 43-53
Columba (see Pigeon)
Combinations, 268, 269, 302, 377
Comparative anatomy (see Anat-
omy)
Conduction, 18, 183
Condylarthra, 361
Conjugation, 41, 214, 244, 250;
diagram of, 245
Conjunctiva, 201
Connective tissue, 142
Conservation of energy, 396
Contractile vacuole, 9, 40
Contractility, 33
Conus arteries us, 165, 166
INDEX
461
Coordination in animals, 181-202;
chemical, 181-183; by nervous
system, 183-193
Copulatory organs, 132, 152, 205
Coracoid, 141, 144
Corm, 70
Cornea, 201
Corolla, 75, 107
Correlation of structure and func-
tion, 393
Cortex, 60, 77, 79, 81
Cortical system, 76
Cotyledons, 66, 87
Cranial nerves, 188, 190
Cranium, 148-153
Crayfish, 129-135, 169, 415; ap-
pendages, 132, 353; circulatory
system, 131; copulatory organs,
132, 205; dissection of, 131;
feeding instincts, 342
Cretin, 182
Crop, of Earthworm, 123; of Bird,
151
Crossing-over, 262; mechanism
296
Crura cerebri, 188
Crustacea, 117, 415 (see Crayfish)
Cursorial, 314
Curve of probability, 299-305
Cutaneous senses, 195
Cuvier, 3£>£-394
Cyclostomes, 179, 416
Cynthia, 254, 259, 415
Cytology, 4, 287, 403
Cytoplasm, 24-27
Cytoplasmic differentiation, 257,
258, 259; organization, 254;
zones, 259
Cytotoxin, 339
D
Dahlia, 65
Daltonism, 294
Dandelion, 65
Darwin, C., 262,271, 299,369, 374-
376, 378, 410, 411, frontispiece
DarwiA, E., 209, 408
Darwinism, present status, 378
Dead-leaf Butterfly, 321
Dentalium, 254, 259, 415; devel-
opment, 257
Dermal system, 76
Dermis, human, 139
Descent with modification (see
Evolution)
Dextrin, 13
Diapheromera, 322
Diaphragm, 140, 153, 155
Dicotyledons, 82, 414
Diencephalon, 187, 188
Digestion, 42, 87, 157-160, 395
Digitigrade, 316
Digits, 144, 361
Dihybrid, 276-280
Dinosaur, 116, 416
Dioscorides, 382
Diploid number, 229, 234, 235
Disease, inheritance of, 267
Distribution, 368-372; discon-
tinuous, 368
Division of labor, physiological,
28, 57, 117, 324
Dodder, 68
Dogfish, 164, 416
Dominance, 272, 282, 286; in-
complete, 283; lack of, 283
Dorsal^orto, 137/150, 151, 166
Dorsal roo^ 192 /
Ductless glands, 159
Ducts, of glands, 151, 155, 158;
in plant stem, 86
Dujardin, 7
Duplex group, 234
E
Ear, 196-198
Earth, age of, 357
462
INDEX
Earthworm, 121-129, 159, 162,
415; body plan, 122; circula-
tory system, 122; dissection of,
123; excretion, 177; feeding
instinct, 342; nerve cells, 194;
nerve cord, 184; reflex arc, 184;
regeneration and grafting, 222;
reproductive organs, 204; sen-
sory and motor neurons, 184;
transverse section, 124
Echinoderm, 259, 415 (see Sea
Urchin)
Ecology, 4, 368
Ectoderm, 22, 57, 119, 126, 185,
215
Ectoplasm, 27, 40
Education (see Man)
Efferent nerve (see Nerve)
Egg, 101, 104, 106, 112, 236, 253;
of Cat, 25; changes at fertiliza-
tion, 241; human, 239; Mam-
mal, 238, 239, 402; membrane,
241; organization of, 236, 254,
255
Egyptian science, 379
Elements, cycle of, 43, 46-50, 309
Elephants, evolution of, 371; geo-
logical and geographical dis-
tribution, 370
Elodea, 82
Embryo, 112; Fish, Bird, Man,
205, 366
Embryology, 4, 252, 364-366;
comparative, 402; of Earth-
worm, 125-129; experimental,
255, 403; history of, 401-403
Embryo sac, 108, 112
Empedocles, 406
Emulsoid, 8
Encyclopaedists, 384
Endocrine, 312; glands, 159 (see
Thyroid and Chemical coordi-
nation)
Endocrinology, 182
Endoderm, 22, 57, 119, 126, 215
Endomixis, 41, 247, 250; nuclear
changes, 248
Endoplasm, 27, 40
Endopodite, 132
Endoskeleton, 140 (see Skeleton)
Endosperm, 110, 111, 112
Energy, conservation of, 396;
from sun, 37, 38; transforma-
tion of, 15, 38 (see Kinetic and
Potential energy)
Enteric cavity, 57, 119
Enteron, 22
Environment, fitness of, 307; in-
fluence of, 266, 267, 290-298,
409 (see Adaptation)
Enzymes, 14, 37, 156, 158, 310
Eohippus, 361, 362
Epencephalon, 188 (see Cerebel-
lum)
Epidermis, 22; 60, 77, 79, 81, 139
Epigenesis, 253, 257, 259, 401
Epithelium, 156
Epochs in biological history, 379-
411
Equation division, 231, 232
Equatorial plate, 225, 226, 242
Equipotent, 258
Equus (see Horse)
Eristalis, 323
Eugenics, 297
Eustachian tube, 149, 150, 198,
356
Euthenics, 297
Evaporation, 89
Evolution, 4, 129, 185, 251, 262,
267, 345-378; of Camel, 363;
of Elephant, 371; evidences
of, 347-372; factors of, 372-
378; and heredity, 376; of
Horse, 361, 362; history of,
406-411
INDEX
463
Exconjugant, 245
Excretion, 16; in animals, 13,
175-180
Excretory system, 116; evolu-
tion of, 179, 365
Exopodite, 132
Exoskeleton, 140, 146
Experimental biology, 252
External receptor, 193
Extracted dominant, 274
Extracted recessive, 274
Eye, of Arthropod, 199; of Cuttle-
fish, 199; development of, 200;
human, diagram of, 201 ; Inver-
tebrate, 199; optic stalk, 199;
origin of, 198-200; rods and
cones, 201; Vertebrates vs. In-
vertebrates, 200
Fabricius, 401
Factors, multiple, 286
Faeces, 156, 175
Fallopian tubes, 205 (see Oviduct)
Fats, 13, 14, 42, 157, 158
Fat body, 149
Feather, 138
Fermentation, 47; alcoholic, 310
Ferns, classification of, 414; fer-
tilization in, 240; life history,
103, 104, 107
Fertilization, 34, 113, 114, 214,
231, 235, 237-242, 245, 249-251;
Protista, 243-248; significance
of, 242-251
Fibrous root, 65
Fibula, 144
Fig, 67
•Filial regression, law of , 270, 301,
303
Fins, 138, 141
Fish, 117, 136; brain, 189; circu-
lation, 164, 165, 171; classifi-
cation, 117, 135, 416; dissec-
tion of, 148; embryo, 866; res-
piratory current, 170; skeleton,
141
Fission, binary, 212; multiple, 212
Fixity of species, 346
Flagellum, 33
Flatworm, 414; fission, 217; re-
generation, 223
Flax, 110
Flexures, cranial, 188
Floral parts, 75, 107, 108
Flower, 107, 112; staminate, 355;
vertical section, 110
Flowering plants, 61, 105; classi-
fication, 414; life history,
107-114; physiology, 84-90;
structure, 65-84
Fluctuations, 300, 302
'Flying Lemur,' 318
Foetal membranes, 205
Food, 157, 158, 308-311; of ani-
mal and green plant contrasted,
42; stuffs, 14, 42; utilization,
Flowering Plants, 89, 90
Fore-brain, 186, 187, 188, 189
Four-o'clock, 283, 284
Fovea centralis, 201
Fragmentation, 113
Frequency curve, 299-305
Frog, brain, 189; circulation, 165;
dissection of, 149; section of in-
testine, 59 (see Amphibian)
Frond, 39, 103, 104
Fructose, 13, 36 ^
Fruit, 66, 110, 112
Fucus, 62
Fungi, classification of, 413 (see
Bacteria)
G
Galen, 382, 394, 396
Galeopithecus, 318
464
INDEX
Galileo, 387
Gallapagos Islands, 369
Gallbladder, 148, 149, 155
Galton, 301, 303, 404
Gallon's Laws, 269-271, 303
Gamete, 54, 112, 228-230, 236;
formation, 94-96; evolution of,
237 (see Egg and Sperm)
Gametophyte, 64, 101, 105; fe-
male, 106, 110} male, 106, 110
Ganglion, 133, 185, 191
Ganong, W. F., 61, 111
Gastric juice, 157, 158
Gastric vacuole, 40
Gastroliths, 131
Gastrula, 57, 58, 156, 252
Geddes, P., 307
Gel, 8
Gene, 236, 280, 286, 377; altera-
tion of, 298; modifying, 304;
multiple, 285, 286; segregation
of, 290
Genetics, 4, 261-306; history of,
403-406
Genital duct, 137
Genotype, 275, 277, 279, 281
Genus, in classification, 349
Geological time table, 356
Geomelrid Moth, larva, 322
Germ cells, 215, 229; origin, 223-
242; primordial, 224
Germ layer, 58, 128; theory, 403
Germ plasm, 222, 265, 377, 404
Germinal continuity, 216, 222,
264, 265, 377, 404
Gesner, 384
Gill, 175; pouches, 155, 169; slits,
137, 147, 164, 366
Giraffe, 266
Gizzard, of Earthworm, 123] of
Bird, 151
Gland, Cowper's, 152; diagram
of, 159} ductless, 159] endo-
crine, 159; oil, 151} prostate,
152} salivary, 159; sebaceous,
139] sweat, 139, 176; thymus,
150, 152, 155, 159; thyroid,
152, 155, 159, 182; unicellular,
59, 158
Glossary, 429-455
Glottis, 149, 150, 151
Glucose, 13, 36
Glycerine, 14
Goethe, 410
Goitre, 182
Gonad, 137, 164, 179, 203 (see
Ovary and Testis)
Gorilla, skeleton of, 354
Grafting, 221
Grape, 70
Grape sugar, 87
Grass, 65, 70
Greek natural philosophers, 345-
406
Greek science, 2, 379-382
Green gland, 131
Green plants, 30-38
Gregarious animals, 331
Grew, 388
Growing point, 76, 79, 81, 82, 87
Growth, by accretion, 16; by
intussusception, 11, 16, 19
Growth zone, 78
Guard cells, 22, 83
Gulfweed, 63
Gullet, 40
Gymnura, 315
H
Hair, 139] character, inheritance
of, 278
Hales, 397
Haller, 395
Haploid number, 229, 234, $35
Harvey, 161, 243, 385, 386, 389,
394, 401
INDEX
465
Haustoria, 68
Hay infusion microcosm, 50-51
Head, 130, 190; of Honey Bee, 326
Heart, 131, 137, 148-151, 153, 163,
165, 172; evolution of, 365;
work of, 172
Heat, animal, 15 (see Temperature)
Hedgehog, 313, 416
Hematochrome, 35
Henderson, L. J., 308
Hen's egg, 238, 253
Hepatic artery and vein, 164, 165,
167
Hepatic portal system, 150, 165,
167, 168
Herbalists, 385
Herbals, 383
Heredity, 251; and evolution,
376; 'social,' 297 (see Inherit-
ance)
Heritage of the individual, 261-
306
Hermaphrodite, 204
Hertwig, O., 21
Heterospory, 106
Heterozygote, 277, 279, 281
Heterozygous, 276
Hickory, 72
Hind-brain, 186 (see Brain)
Hippocrates, 381
Histology, 4; history of, 398-400;
plant, 75-84 (see Tissue)
History, of biology, 379-411; com-
parative anatomy, 392-394;
embryology, 401-403; genetics,
403-406; histology, 398-400;
organic evolution, 406-411;
physiology, 394-398; taxonomy,
390-392
Holdfast, 63
Homologous chromosome (see
Chromosome)
Homologous organs, 130, 852
Homology, serial, 353
Homothermal, 174, 176, 312
Homozygote, 277, 279, 281
Homozygous, 276, 302
Honey Bee (see Bee)
Hoofs, 138
Hooke, 387
Hormone, 181, 206 (see Endo-
crine)
Horns, 138
Horse Chestnut bud, 72
Horse, evolution, 361, 362; skele-
ton of leg, 352
Host, 334
Human, body, chemical composi-
tion, 11; median section, 153;
ear, 198; egg, 239; eye, 201;
kidney, 180; skeleton, 354;
sperm, 239 (see Man)
Humerus, 144
Hutton, 410
Huxley, 30, 54,211, 261, 345, 356,
393, 394
Hybrid, 272, 306 (see Heterozy-
gote)
Hydra, 118-121, 157, 169, 194,
219, 332, 414; asexual reproduc-
tion, 217; discovery of, 388;
feeding instinct, 342; longitudi-
nal section, 119; nerve cell, 183;
receptor-effector system, 184;
regeneration and grafting, 221;
reproductive organs, 204; trans-
verse section, 22, 120
Hydranth, 218; comparison with
medusa, 219
Hydrochloric acid, 157
Hydroid, 115, 117, 145; colony,
218; life history, 218
I
Iliac artery and vein, 164
Ilium, 144, 145, 354
466
INDEX
Immunity, 338, 339
Incus, 198
Indian Corn, 67, 263
Individual, adaptability, 339-344;
origin of, 209-260; heritage of,
261-306
Infundibulum, 187-189
Ingenhousz, 398
Inheritance, 261, 403; alterna-
tive, 268, 405; blending, 268,
283, 286; of human hair char-
acters, 278; mosaic, 268; sex-
linked, 268; of size in Peas, 278
Insects, 117, 415; traps, 73
Instincts, 342
Integumentary system, 116
Intercellular digestion, 156
Internal receptor, 193
Intestine, 123, 131, 137, 148, 156;
nerve supply, 186; section of,
59
Intracellular digestion, 156
Invertebrates, 117, 414, 415
Invertebrate eye, 199, 200
Iris, 201
Irritability, 18, 181, 183
Ischium, 144, 145, 354
Islands, continental, 369; coral,
372
Island faunas and floras, 369
Ivy, 67
Jaws, 143
Jennings, H. S., 342
Johannsen, 303
Jugular vein, 164, 165
Jurassic period, 358
K
Kallima, 321
Karyolymph, 27
Karyosome, 26, 28
Kata holism, 16
Katydid, 319
Kelley, H. A., 148-152
Kelp, 63
Kidney, 137, 153, 175, 180
Kinetic energy, 6, 35, 42
Labyrinth, 196
Lacteals, 168
Lagena, 197
Lamarck, 3, 39, 267, 374, 393, 409
Lamina, 71
Lamprey, 416; egg and sperm, 236
Laplace, 15
Larynx, 153, 356
Latent character, 268
Lateral line organs, 195
Lavoisier, 15, 396
Law of probability, 299-30-5
Leaf, 65, 71-75, 82-84, 112; air
spaces, 83] base, 71; develop-
ment, 82; epidermis, 83; pali-
sade layer, 83; section of, 83;
vein, 83; vertical section, 22
Leeuwenhoek, 387, 388
Legs of Bee, 327
Lens, 201
Lichen, 332
Liebig, 398
Life, 6, 19; definition, 15; origin,
28, 209; physical basis, 6, 400;
'triangle' of, 298; web of, 330
Limb, pentadactyl, 144 (see Skele-
ton)
Linin, 26, 27
Linkage, 293-296
Linnaeus, 346, 390-392
Linville, H. R., 148-152
Liver, 137, 148, 149, 156, 168, 175
Living matter, adaptation, 17-18;
characteristics of, 10-20; chemi-
INDEX
467
cal composition, 11-14; organ-
ization, 18, 19
Lizard, dissection of, 150
Lockjaw, 311
Loiseleuria, 75
Lumbricus (see Earthworm)
Lungs, 150, 153, 161, 175
Lyell, 410
Lymph, 156, 163, 168, 173
M
Macronucleus, 40, 41, 245, 248
Malaria, 336
Malarial Parasite, life history, 335
Malleus, 198
Malpighi, 388, 389, 401
Mammal, 117, 136; adaptive radi-
ation of, 313-319; brain, 189;
circulation, 165; copulatory
organs, 152', classification of,
416; dissection of, 152] egg of,
238, 239; Eutherian, 313, 416
Mammary glands, 206
Man, body temperature, 312;
digestion, 158; education, 297,
344; embryo, 366; inheritance
in, 297; skeleton, 354; skeleton
of arm, 352 (see Human)
Mandible, 132, 145
Marsilia, 106
Mastodon, 370
Mathews, A. P., 181, 344
Matter, non-living and living as-
sociated, 7
Maturation, 288 (see Oogenesis
and Spermatogenesis)
Maxilla, 132, 145
Maxilliped, 131, 132
Mechanism, 257
Medicine and biology, 381
Medieval science, 382, 383
Medulla, 153, 187, 189 (see Brain)
Medusa, 218, compared with hy-
dranth, 219
Megasporangium, 108, 112
Megaspore, 106, 107, 110, 112
Megasporophyll, 106, 108, 112
Mendel, 271-274, 404, 405
Mendelism, 271-306; general prin-
ciples, 280-282; in Man, 278;
mechanism of, 287-296, 405;
laws rediscovered, 405; ratio,
272 (see Monohybrid, Dihybrid,
and Trihybrid)
Meristem, 77, 78, 79
Mesenteric artery, 164
Mesentery, 137, 149
Mesoderm, 57, 126, 215; bands,
126; somatic, 126; splanchnic,
186
Mesogloea, 118, 120, 219
Mesohippus, 361, 368
Mesonephric duct, 148, 179
Mesoriephros, 137, 148, 179
Mesozoic era, 358
MeiahoJism, 15^1^.175, 181, 307;
animals, 39-43; Bacteria, 44-
53; colorless plants, 44-53;
green plants, 34-38
Metagenesis, 220
Metamere, 121-183, 190
Metamerism, 121, 124, 127, 130,
191
Metanephric duct, 179
Metanephros, 150, 151, 152, 153,
179
Metaphase, 225
Metaphyta, 54
Metaplasm, 26
Metazoa, 54, 116
Metencephalon (see Medulla)
Microcentrum, 319
Micronucleus, 40, 41, 245, 248
Microscope, 7; invention of, 386,
387
468
INDEX
Microscopists, 386-389
Microsporangium, 108, 110, 112
Microspore, 106, 107, 110, 112
Microsporophyll, 106, 112
Mid-brain, 186 (see Brain)
Millipede, 129, 415
Mimicry, protective, 323
Mirabilis, 284
Mistletoe, 68
Mitosis, 29, 224-228; typical
stages in, 225
Mitral valve, 165, 172
Modifications, 265-267, 302, 304,
444
von Mohl, 7
Mole, 317
Mollusc, 117, 415; development
of, 255; nerve cells, 194
Monads, 52
Monographers, 385
Monohybrid, 272-276
Morphogenesis, 403
Morphology, 3
Mosaic inheritance, 268
Mosquito, 336
Moss, 105; classification, 413,
414; fertilization, 240; life
history, 100-103
Motor nerve, 191
Mouth, 41, 119, 121
Movement, amoeboid, 19; ciliary,
19; power of, 19
Mulatto, 283-285; recombination
square, 285
Muller, 396
Muscles, 18, 138, 139; cells, 59,
183; involuntary, 139; smooth,
25; striated, 25; voluntary,
139; of eye, 201
Muscular system, 116
Mutations, 269, 298, 306, 377
Mycelium, 332
Myotome, 139
Myrsiphyllum, 71
Myxedema, 182
N
Nails, 138
Nares (see Nostrils)
Natural history, 2
Natural philosophy, 3
Natural selection, 306, 374-376,
378, 411
Nature versus nurture, 296-299
Neo-Mendelism, 282-306
Nephridium, 122, 123, 124, 181,
177, 207
Nephrostome, 124, 177, 208
Nerve, 191; afferent and efferent,
192; auditory, 197; cranial,
152, 188; motor, 191; optic,
152; sensory, 191 ; sensory end-
ing, 139; spinal, 188, 190; tem-
perature, 312; trophic, 312;
vagus, 190; vasomotor, 174, 312
Nerve cells, differentiation, 185
Nerve cord, 123, 134, 190
Nerve fibers, 185
Nerve net, 184, 186
Nerve plexus, 186, 190, 191
Nervous impulse, 191, 193, 194
Nervous system, 116, 133; coordi-
nation by, 183-193; Crayfish,
134; Earthworm, 134; Frog, 190
Neural arch, 137, 143
Neural canal, 137, 143, 153
Neural groove, 186
Neural tube, 186, 187, 190
Neuro-muscular mechanism, 183
Neuron, 184-186, 193, 194
Nictitating membrane, 355
Nitrates, 36, 43
Nitrogen cycle, 49
Nitrogen-fixing Bacteria, 49, 333
Nitrogenous wastes, 43
Nomenclature, Binomial, 391
INDEX
469
Nostrils, 149, 152, 153
Notochord, 137, 142, 146
Nucleolus, 26
Nucleus, 9, 26, 27, 28, 77; during
conjugation, 245; during endo-
mixis, 248
Nurture versus nature, 296-299
Nutrition of animals, 154-160
Nutritional chain, 330
Obelia, 414; life history, 218
Oedogonium, 97, 98
Oesophagus, 123, 131, 148-155
Oil, 14; gland, 151
Olfactory, bud, 196; lobe, IJfr,
187, 189, 190; pouches, 196;
sense, 196
Oligocene period, 358
Onion leaf, 72
Onoclea, 74
Ontogeny, 364 (see Embryology)
Oocytes, primary and secondary,
231
Oogenesis, 231-233
Oogonium, 224, 232
Operculum, 141
Opsonin, 338
Optic capsule, 141, 145; cup, 199;
lobes, 188, 189', nerve, 152,
190] stalk, 199
Order, in classification, 349
Organ, 59; organ-forming sub-
stances, 256, 259
Organic evolution, 262 (see Evolu-
tion)
Organisms, adaptation of, 307-
344; colonial, 55, 56; micro-
cosm, 20; structure of multi-
cellular, 54-60
Organization, 11, 18
Organ systems, 60, 116
Origin of the germ cells, 223-242
Origin of the individual, 209-260
Origin of species, 345-378, 410, 411
Osmosis, 86, 88
Osteology, 140 (see Skeleton)
Ovary, 119, 123, 148, 203
Oviduct, 128, 179, 205, 208
Ovule, 110, 112
Ovule case, 108
Owen, 394
Ox, skeleton of leg, 352
Oyster, 135, 415
Pain, sense of, 195
Paleontology, 4, 356-364, 393
Palm, 67
Pancreas, 149, 152, 155, 158
Pancreatic duct, 151, 155
Paramecium, 39, 52, 116, 157, 169;
aurelia, 40, 244; behavior, 340,
341; calkinsi, 40; caudatum,
40; conjugation, 41, 245; con-
tractile vacuole, 40; digestion,
42; division, 41 i ectoplasm, 40;
endomixis, 41, 248; endoplasm,
40; excretion, 43; focd taking,
42; gastric vacuole, 40; gullet,
40; heredity in, 264; irrita-
bility, 194; macronucleus, 40,
41, 245, 248; metabolism, 41-43;
micro-nuclei, 40, 41, 245, 248;
mouth, 40, 41; neuromotor ap-
paratus, 40; peristome, 40;
power of reproduction, 375; re-
production, 212, 246; respira-
tion, 43; species, 40, 414; struc-
ture and life history, 39-41;
trichocysts, 40
Parasitism, 68, 220, 334-338
Parencephala, 188 (see Cerebral
hemispheres)
Parthenogenesis, 243, 249
Pasteur, 210
470
INDEX
Patella, 145
Peas, inheritance in, 272-282
Pectoral girdle, 141, 144, 14$,
354
Peduncle, 107
Pelvic bones, 317, 144, 153
Perca, 141, 148
Perch, dissection, 148
Pericardial cavity, 140
Peripheral nervous system, 186
Peristalsis, 157
Peristome, 40
Peritoneum, 59, 137
Permian period, 358
Perspiration, 176
Petal, 75, 107, 108, 112
Petiole, 71
Phagocyte, 338
Pharynx, 123, 152, 153, 155
Phenotype, 275, 277, 279, 281
Phloem, 60, 76, 81, 87
Phosphates, 36
Photosynthesis, 35, 87, 332; chem-
ical equation, 36
Phylogeny, 364
Physcia, 332
Physical basis of life, 7-10, 400
Physical sciences, 1
Physics, origin of, 395
Physiological division of labor
(see Division of labor)
Physiologus, 383
Physiology, 3, 4, 367; compara-
tive, 396; Flowering Plant, 84-
90; history, 394-398
Pigeon, compared with Archaeop-
teryx, 360; brain, 189; dissec-
tion of, 151; domestic varieties,
373
Pineal body, 187, 188, 189
Pinna, 152, 198
Pistil, 75, 107; compound, 108
Pitcher-plant, 73
Pith, 60, 76, 81, 87
Pituitary body, 183, 447
Placenta, 205
Planaria, 223
Plant, body, 61-90; classification,
413, 414; chromosome reduc-
tion, 228; colorless, 38; evolu-
tion, 105, 112-114; food, 397;
green, 30; gross structure, 65-
75; histology, 75-84; ideal
vertical section, 76; physiologi-
cal activities, 87; reproduction,
91-114; stem, 81; unicellular,
30, 39
Plantigrade, 316
Plasma, 163
Plastid, 26, 27
Plexus, nerve, 190, 191
Pliny the Elder, 382
Pliocene period, 358
Polar body, 231, 233
Polar lobe, 255
Pole cells, 126
Pollen, 110, 112; basket, 329;
brush, 326; combs, 329; grain,
108; tube, 109, 110
Pollination, 111
Polygon, frequency, 299-305
Polyhybrid, 276
Polymorphism, 218
Polytrichum, 101
Pond Scum, 92, 413
Population and pure lines, 300
Porpoise, 317, 416
Portal vein, 150, 167, 168
Porto Santo Rabbits, 369
Potato, 66, 70
Potential energy, 6, 15, 35, 42
Precaval vein, 164
Preformation, 253, 257, 259, 401,
406
Pressure, 313
Prickly Pear, 70
INDEX
471
Priestley, 398
Primary cylinder, 76
Primates, 358, 416
Pronephric duct, 179
Pronephros, 179
Pronuclei, 241
Prophase, 225
Prosencephalon, 187, 188
Prostomium, 123
Proteins, 12, 14, 36, 42, 87, 157
Proterozoic era, 358
Prothallus, 103, 104
Protista, 214, 216; fertilization
in, 243-248
Protohippus, 361, 862
Protonema, 101, 102
Protophyta, 39, 214
Protoplasm, 4, 7-10, 19; alveolar
structure, 10; appearance, 8;
chemical composition, 11; con-
cept, 400; and environment,
9, 17
Protoplast, 31
Protopodite, 132
Protozoa, 39, 116, 214; classifica-
tion, 414; discovery, 388; fer-
tilization in, 335 ; malarial para-
site, 335; maturation in, 233
(see Paramecium)
Pseudopodium, 9
Psychology, 4
Psychozoic era, 358
Pteridophyta, 103, 112, 414
Pubis, 144, 14$, 354
Pulmonary artery and vein, 150-
153, 165, 171, 172
Pulvillus, 328
Pupil, 201
Pure lines, 300, 302-306; versus
population, 300, 305
Purkinje, 7
Pyloric, caecum, 148; valve, 156
Python, 355
R
Rabbits, Porto Santo, 369
Radial symmetry, 118
Radius, 144
Rana, 149 (see Frog)
Ray, 390
Reaumur, 395
Recapitulation theory, 364, 403
Receptor-effector system, 183, 184
Receptor, external, 193; internal,
193 (see Sense organs)
Recessive character (see Charac-
ter)
Recombinations, 268
Rectum, 155
Redi, 210, 388
Reducing division, 231 (see Oogen-
esis and Spermatogenesis)
Reduction, 228, 229
Reflex action, 193
Reflex arc, 184
Reflexes, 342
Regeneration, 217; in Crayfish,
Earthworm, Flat worm, Para-
mecium, Salamander, Snail,
221; and grafting in Hydra, 221
Rejuvenation, 244
Renaissance science, 346, 384-386
Renal artery, 180
Renal portal system, 164, 165, 167
Reproduction, 11, 17, 212-222;
in animals, 203-208; asexual,
113; biparental, 54; versus
fertilization, 243, 251; in plants,
91-114; uniparental, 54
Reproductive organs, 98-100;
Crayfish, 131; Earthworm, 204;
Hydra, 204; system, 116; evo-
lution of, 179, 208, 365; Verte-
brate, 137, 148-152
Reptile, 117, 136; versus Bird,
359; brain, 189; dissection of,
150
472
INDEX
Respiration, 37, 87, 89, 396; in
animals, 43, 161-174; chemical
equation, 37; in Invertebrates,
169; in Vertebrates, 169
Respiratory, currents, paths of,
170; membranes, 175; system,
116
Response, 18; organic, 307 (see
Adaptation)
Retina, 200, 201
Reversion, 269
Rhinencephala, 188 (see Olfactory
lobes)
Rhizoids, 104
Rhizome, 70, 74, 103
Rib, 137, 141
Ricinus, 81
Rockweed, 62, 63
Rodentia, 349
Rods and cones, 201
Roman science, 382
Root, 65-69, 76, 78-80, 112; cap,
79; hair, 80; primary, 65; tip,
79, 87
Rostrum, 131
Rotifer, 135; parthenogenesis in,
249
Round Worms, parthenogenesis
in, 249, 415
Runners, 70; of Strawberry, 69
Sacculus, 196, 197
Salivary glands, 155, 157, 159
Sap cavity, 77
Saprophytic, 51
Sarcode, 7
Sargassum, 64
de Saussure, 393
Scales, 72
Scapula, 141, 144
Sceloporus, 150
Schizogony, 335
Schleiden, 399
Schuchert, C., 358
Schultze, 7, 400
Schwann, 242, 399, 400
Scientific method, 2
Sciurus, 152, 349
Sclerotic coat, 201
Sea Lettuce, 63
Sea Urchin, 135, 259, 415; devel-
opment, 58; egg, 256
Seaweeds, 62, 63
Sebaceous gland, 139
Secondary root, 66
Secretion, 158
Seed, 66, 110, 111, 112; coat, 111;
plants, 61
Segregation, 274, 280-283, 286,
290, 306
Selaginella, 107
Selection, 299-306, 377; artificial,
373; natural, 306, 324, 374, 376,
378, 410, 411; in population,
300; in pure lines, 300
Semicircular canals, 190-198
Seminal fluid, 152, 207
Seminal vesicle, 123, 151
Senile degeneration, 244
Sense, auditory, 196; cutaneous,
195; cells, differentiation of,
194; organs, 116, 193-202;
pain, 195; sight, 198; smell,
196; taste, 195; temperature,
195
Sensitive Fern, 74
Sensory nerve, 191
Sepal, 107, 112
Septa, 121
Setae, 124
Sex, 34, 113; chromosome, 292-
295, 377; determination, 291-
293; differentiation, 96-98;
linked characters, 268, 291, 293;
origin, 94, 95
INDEX
473
Sexual characters, secondary, 206
Shark, 164, 416
Sheep, 263
Shoot, 65, 76, 81
Simplex group, 234
Sinus venosus, 165, 166
Skeleton, appendicular, 143; axial,
143; Bat's wing, 352; Bird's
wing, 352; Cat, 145; Fish, 141;
Gorilla, 354; Horse's leg, 352;
Man, 354; Man's arm, 352;
Ox's leg, 352; Vertebrate, 140-
146; Vertebrate limbs, 352;
Whale's flipper, 352
Skin, 138, 175; human, 139
Skull, 141, 143; bones, ^/'evo-
lution, in Camel, 363; evolution,
in Horse, 362
Sloth, 317
Smell, sense of, 196
'Smilax,' 70
Snake, 416; hind limbs, 355
'Social heredity,' 297
Sociology, 4
Sol, 8
Somatic cells, 229, 246 (see
Germ plasm and Germinal
continuity)
Somatoplasm, 265
Somite (see Metamere)
Sorus, 104
Spallanzani, 395
Special creation, 210, 346
Species, 262, 345; classification,
391, 392; concept, 390; muta-
bility, 410
Specific form, 11
Spencer, 15, 114, 115, 375
Sperm, 101, 104, 106, 112, 236;
discovery, 388; human, 239;
Snake, 25
Spermatic fluid, 152, 207
Spermatid, 231
Spermatocytes, primary, second-
ary, 231
Spermatogenesis, 230-232; dia-
gram of, 231
Spermatogonia, 224
Spermatophytes, 61, 112, 414
Sphaerella, 30-38, 52, 169, 413
Spider, 129, 415
Spinal cord, 137, 142, 152, 153;
paths of nervous impulses, 193
Spinal nerves, 188, 190, 193
Spines, 72
Spireme, 225
Spirogyra, 61
Spleen, 137, 148, 149, 150, 152
Splint bones, 355, 361, 362
Spondylomorum, 55
Sponges, 115, 117, 414
Spontaneous generation, 209, 210,
388
Sporangium, 65, 74, 112
Spore, 31, 101-113; formation, 92,
93, 312, 332
Sporogony, 335
Sporophyll, 65, 74, 106, 112
Sporophyte, 64, 101, 104, 105
Sporulation, 213, 335
Squeteague, food of, 330
Squid, 330, 415
Squirrel, 349; classification, 350;
dissection, 152
Stamen, 107, 108, 112
Staminate flower, 355
Stapes, 198
Starch, 13, 36
Starfish, 221, 415
Statolith, 131
Stele, 76
Stem, 60, 65, 69-71, 81, 103, 112
Sternum, 145, 151, 153
Stigma, 108
Stimulus, 18
Stipule, 71
474
INDEX
Stoma, 28, 88
Stomach, 137, 148-158, 155
Storage root, 67
Struggle for existence, 330, 375,
411
Style, 108
Subclavian artery, 171; vein, 164
Subgenus, 351
Suboesophageal ganglion, 122, 123,
130, 131
Suborder, 351
Subspecies, 351
Sugar, 13, 36, 158
Sulfates, 36
Sulfur Bacteria, 309
Sundew, 73
Survival of the fittest, 375, 406
Swammerdam, 388
Sweat gland, 139, 176
Swimming foot, 131, 132
Sylvius, 395
Symbiosis, 331-334; Alga and
Fungus, 332
Symmetry, bilateral, 124; radial,
118
Sympathetic nervous system, 186;
~192
Synapse, 185
Synapsis, 281, 234, 235, 287, 289,
290, 296
Synaptic mates, 232
Syngamy, 244 (see Fertilization)
Synkaryon, 241, 244, 250
Systems, of organs, 116
Tactile corpuscle, 195
Talpa, 317
Tamias, 349
Tap root, 65
Tapeworm, 135, 414
Tapirs, distribution of, 368
Tarsus, 144, 145
Taste, sense of, 195
Taxonomy, 4, 348-351, 390-392
Teeth, 138; evolution, in Camel,
363; evolution, in Horse, 362
Telophase, 225
Temperature, body, 174; limits
for life, 311, 312; regulation,
174, 312; sense, 195
Tendril, 70, 72
Tennyson, 203
Tentacles of Hydra, 119
Testis, 119, 123, 131, 149-152, 203
Tetanus, 311
Thallophytes, 112, 413
Thallus, 62, 63, 112, 113
Theophrastus, 2, 381
Thistle, 72
Thomson, J. A., 6, 154, 297, 307,
364
Thoracic duct, 168
Thorax, 130, 140, 153
Thrush, 369
Thymus gland, 150, 152, 155, 159
Thyroid gland, 152, 155, 159, 182
Tibia, 144, 145, 354
Time, geologic, 358; cosmic, 358
Tissue, 59; connective, 59, 142;
systems, 60, 116
Tonsils, 155
Totipotent, 258
Toxin, 338
Trachea, 149, 153, 155
Tracheid, 86
Transpiration, 87, 88
Transverse process, 137, 143
Treviranus, 3, 410
Trichocyst, 40
Tricuspid valve, 165, 172
Trihybrid, 280, 281
Trillium, 70
Trypanosome, 337, 338, 414
Tunicate, 146, 415
Turgor, 84
INDEX
475
Turnip, 66
Tympanic membrane, 197, 193
Typhlosole, 124
U
Ulna, 144, 145, 352, 354
Ulothrix, 61, 94, 95, 96, 413
Ulva, 63, 413
Umbilical cord, 205
Underwing Moth, 320
Ungulata, 349
Unguligrade, 316
Uniformitarian doctrine, 376, 406
Unit character, 262, 280, 282, 286,
306
Urea, 43, 176
Ureter, 148-153, 179, 180
Urinary bladder, 137, 148-153,
179
Urinary and reproductive systems,
interrelationship, 179, 207
Urogenital canal, 207; pore, 148;
system, 207; system of Verte-
brates, 179
Uropod, 132
Urostyle, 149
Uterus, human, 205
Utriculus, 196
Vaccination, 339
Vacuole, 26, 77; contractile, 9,
40; food, 9; gastric, 40
Valves, 163, 165, 172
Variation, 237, 261; adaptive,
378; fluctuating, 301; herita-
bility of, 264-269; universality
of, 410
Varieties, 351
Vascular bundle, 77, 78, 83
Vascular plants, 64, 103
Vascular system (see Circulation)
Vaso-motor nerves, 174
Vein, 163; of leaf, 77 (see Circula-
tion)
Veinlets, 163
Vena cava, 152
Venous system, 166 (see Circula-
tion)
Ventral aorta, 164, 171
Ventral root, 192
Ventricle, 163
Vermiform appendix, 153, 155, 355
Vertebra, 137, 141, 145, 153;
human, 143
Vertebral column, 141
Vertebrates, 117, 135-153; body
plan, 136-138; brains of, 189;
characters, 146, 147; circula-
tion and respiration, 161-174;
classification, 350, 416; coelom,
140; labyrinth, diagram of,
197; limb, plan of, 144> longi-
tudinal section, 137; skeleton,
140-146; skin, 138; transverse
section, 137; urogenital system,
179
Vesalius, 384, 385, 394
Vespertilio, 318
Vestigial organs, 317, 355, 361, 362
Violet seed, 111
Vitamines, 14
Vitreous humor, 201
Vocal organ of Bird, 151
Volant, 316
Volvox, 55, 56, 217
de Vries, 269
W
Walking-stick, 322
Wallace, 410
Warm-blooded animals, 176 (see
Homothermal)
Waste, nitrogenous, 43; and re-
pair, 10
476
INDEX
Water Lily, 108
Weismann, 267, 404
Whale, 116; skeleton of flipper,
352
Wheat, 263
Wood, 88
Working hypothesis, 5, 396
X chromosome, 292, 293, 377
Xylem, 60, 76, 81, 87
Yeast, SIS, 310
Yolk, 238
Zoogeography, 368
Zoology, 3, 4
Zygote, 32, 95, 98; chromosomes
in, 289, 290, 293; in genetics,
275-284; organization of, 251-
260
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