iv Contents 



CHAPTER PAGE 



VIII. THE PROBLEMS OF PLANT DISTRIBUTION . . 236 

 Antiquity of the Principal Plant Types Uni- 

 formity of Early Floras Origin of Angio- 

 sperms Factors in Plant Distribution Alpine 

 Floras Island Floras Agents in Plant Dis- 

 tribution Plant Distribution in the United 

 States. 



IX. THE HUMAN FACTOR IN PLANT EVOLUTION . . 277 

 Antiquity of Cultivated Plants Man's Spread 

 over the Earth due to Agriculture Plant 

 Breeding. 



X. THE ORIGIN OF SPECIES 301 



Mutability of Plants Parallel Development 

 of Reproduction in Plants and Animals Vari- 

 ation Orthogenesis Theories of Evolution 

 Changes Due to Environment The Mutation 

 Theory of De Vries Heredity Mendel's Law 

 of Heredity Evolution of Sex Significance 

 of Sex Hybrids Experimental Morphology 

 Conclusion. 



INDEX 349 



