THOMAS HUNT MORGAN
Reading edition
PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY
LECTURES DELIVERED AT PRINCETON UNIVERSITY FEBRUARY 24, MARCH 1, 8, 15, 1916
PRINCETON UNIVERSITY PRESS PRINCETON LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1916
Copyright, 1916, by Princeton University Press Published October, 1916
PREFACE
Occasionally one hears today the statement that we have come to realize that we know nothing about evolution. This point of view is a healthy reaction to the over-confident belief that we knew everything about evolution. There are even those rash enough to think that in the last few years we have learned more about evolution than we might have hoped to know a few years ago. A critique therefore not only becomes a criticism of the older evidence but an appreciation of the new evidence.
In the first lecture an attempt is made to put a new valuation on the traditional evidence for evolution. In the second lecture the most recent work on heredity is dealt with, for only characters that are inherited can become a part of the evolutionary process. In the third lecture the physical basis of heredity and the composition of the germ plasm stream are examined in the light of new observations; while in the fourth lecture the thesis is developed that chance variation combined with a property of living things to manifold themselves is the key note of modern evolutionary thought.
T. H. Morgan
July, 1916
TABLE OF CONTENTS
CHAPTER I
A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
PAGE
Preface v
1. Three Kinds of Evolution 1-7
2. The Evidence for Organic Evolution 7-27
a. The Evidence from Comparative Anatomy 7-14
b. The Evidence from Embryology 14-23
c. The Evidence from Paleontology 24-27
3. The Four Great Historical Speculations 27-39
a. The Environment 27-31
Geoffroy St. Hilaire
b. Use and Disuse 31-34
From Lamarck to Weismann
c. The Unfolding Principle 34-36
Nägeli and Bateson
d. Natural Selection 36-39
Darwin
CHAPTER II
THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITY CHARACTERS
1. Mendel's First Discovery—Segregation 41-52
2. Mendel's Second Discovery—Independent Assortment 52-59
3. The Characters of Wild Animals and Plants Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals and Plants 59-84
a. Sexual Dimorphism 61-64
Eosin eye color of Drosophila 61-62
Color of the Clover Butterfly, Colias philodice 62-63
Color of Papilio turnus 63
Color pattern of Papilio polytes 63-64
b. Duplication of parts 65-66
Thorax of Drosophila 65
Legs of Drosophila 65-66
c. Loss of characters 66-68
"Eyeless" of Drosophila 66-67
Vestigial wings of Drosophila 67
Bar eye of Drosophila 67-68
d. Small changes of characters 68-70
"Speck" 68
Bristles of "club" 70
e. Manifold effects of same factor 71
f. Constant but trivial effects may be the product of factors having other vital aspects 73
g. Sex-linked inheritance 75-80
in Drosophila ampelophila 75-76
in the wild species D. repleta 76
in man 77
in domesticated Fowls 77-78
in the wild moth, Abraxas 78-80
h. Multiple allelomorphs 81-84
in the wild Grouse Locust 81-83
in domesticated mice and rabbits 83
in Drosophila ampelophila 84
4. Mutation and Evolution 84-88
CHAPTER III
THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
1. The Cellular Basis of Organic Evolution and Heredity 89-98
2. The Mechanism of Mendelian Heredity Discovered in the Behavior of the Chromosomes 98-102
3. The Four Great Linkage Groups of Drosophila Ampelophila 103-118
a. Group I. 104-109
b. Group II. 109-112
c. Group III. 112-115
d. Group IV. 115-118
4. Localization of Factors in the Chromosomes 118-142
a. The Evidence from Sex Linked Inheritance 118-137
b. The Evidence from Interference 137-138
c. The Evidence from Non-Disjunction 139-142
5. How Many Genetic Factors are there in the Germ-plasm of a Single Individual? 142-143
6. Conclusions 144
CHAPTER IV
SELECTION AND EVOLUTION
1. The Theory of Natural Selection 145-161
2. How has Selection in Domesticated Animals and Plants brought about its Results? 161-165
3. Are Factors Changed through Selection? 165-187
4. How does Natural Selection Influence the course of Evolution? 187-193
5. Conclusions 193-194
Index 195-197