CONTENTS 



Chapter I. [ntroduction. 



Facts Calling for Explanation 1-3 



Small Areas of Distribution for Species of Hawaiian Snails 1 



The Intergrading of Species 2 



Selection not Always the Cause of Divergence 3-6 



In Many Cases Sexual Selection not the Cause 3 



In Many of the Same Cases Natural Selection not t he Cause 4 



May not the Prevention of Free Crossing be an Explanation 6 



Investigation of Causes and Effects of Segregation 7 



Segregation the Unifying Principle in Evolution 7 



Chapter II. — Bionomic Laws. 



Method of Investigation 9 _I 3 



vScope of Bionomics 9 



Why we Commence with the Method of Evolution 9 



Need of Investigation of all Factors 10 



Natural and Sexual Selection not the Only Factors 11 



Comparison of Conditions in Natural Species and in Domestic Varie- 

 ties 12 



Production of Domestic Races 1 3 - -- 



Continuance of Races 13 



Transformation of Races 14 



Divergence of Races 17 



Stability of Races 19 



Amalgamation of Races 20 



Influence of Acquired Characters on Racial Characters 20 



Chapter III. — The Evolution of Natural Species. 



Unity and Diversity 23-29 



Darwin's Explanation of Unity 23 



Divergence Through Variation under Isolation and Unity Through 



Community of Descent 24 



Facts in Distribution of Hawaiian Snails 26 



Diversity of Natural Selection not a Sufficient Explanation 27 



Divergence through Independent Transformation 28 



Natural Selection as Explanation of Evolution 29-34 



What Natural Selection does not Explain 29 



Selection, how Far Determined by External Nature 3 1_ 34 



Discontinuity of Species 34 - 3" 



Chapter IV. — Divergence Under the Same Environment. 



Explanation of Plate I 37 



Explanation of Plate II 39 



vii 



