20 TABLE OF CONTENTS 



CHAPTER XVIII 



PAGE 



Isolation 282 



Geographical isolation and physiological isola- 

 tion. — Moritz Wagner's theory. — Wallace on the 

 distribution of American butterflies and island 

 fauna. — Gulick and Romanes. — Two modes of 

 isolation. — D. S. Jordan. — Physiological selection. 

 — Vernon's theory of reproductive divergence. — 

 Discussion. 



CHAPTER XIX 

 Orthogenesis 296 



Variations occurring in determined sequence. — 

 Exaggerated development of certain structures; 

 giant reptiles; the tusks of the mammoth and of 

 the Babirussa, the antlers of the Irish stag, the 

 protective coloring of the Kallima. — Eimer's or- 

 thogenesis and his laws of organic growth. — 

 Cope's archsesthetism ; the role of consciousness, — 

 Naegeli's views. 



CHAPTER XX 

 Mutation 306 



Continuous variation and discontinuous variation. 

 Examples of discontinuous variation. — Theories 

 based upon it. — Korshinsky's heterogenesis. — De 

 Vries' experiments; the theory of mutation. Its 

 theoretical importance according to De Vries. — 

 Formation of new varieties and species. — The mu- 

 tations of (Enothera. — The laws of mutation. — 



V 



