TY PRESS
Published October, 1916

[Illustration]

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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_

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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
Naegeli 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