I take this opportunity of expressing my thanks to the Councils of the Royal Society and the Zoological Society for permission to reproduce the figures in the Plates. I also desire to thank Professor Dendy, F.R.S., of King's College for his sympathetic interest in the publication of the book, and Messrs. Constable and Co. for the care they have taken in its production.

J. T. CUNNINGHAM.
London, June 1921.

CONTENTS

INTRODUCTION - Historical Survey Of Theories Or Suggestions Of Chemical
Influence In Heredity

CHAPTER I - Classification And Adaptation
CHAPTER II - Mendelism And The Heredity Of Sex
CHAPTER III - Influence Of Hormones On Development Of Somatic Sex-Characters
CHAPTER IV - Origin Of Somatic Sex-Characters In Evolution
CHAPTER V - Mammalian Sexual Characters, Evidence Opposed To The Hormone Theory
CHAPTER VI - Origin Of Non-Sexual Characters: The Phenomena Of Mutation