X PREFACE. 



largely confined to the distribution of species. Each species was of 

 importance, because its distribution threw light upon the fioristic 

 affinities of the region in which it grew. Only a historical interest 

 now attaches to the discussions of "centers of creation" which occupied 

 de Candolle and his contemporaries. The inevitable questions as to 

 the origin and significance of the floral regions of the world were 

 immediately given a new trend by the publication of The Origin of 

 Species. Two features of the Darwinian conceptions and method 

 were destined to be of the most fundamental influence upon the further 

 development of plant geography and its later outgrowths. The first 

 was the demonstration — which it is now difficult for us to realize as 

 so recent — that the present features of plant distribution have grown 

 out of the distributional features of the past, and the second was the 

 emphasis which it laid upon the importance to the plant of the entire 

 complex or constellation of its environmental conditions. 



In the hands of Darwin the great store of distributional facts served 

 as a source of material for aiding in the demonstration of his new 

 principle. The multifarious structures of plants, as yet incompletely 

 investigated by physiologists and anatomists, assumed a new signifi- 

 cance as having an important role in the existence of the individual 

 and the race. 



For 20 years after the appearance of The Origin of Species there 

 was keen activity in the reinterpretation of distributional facts and 

 in the fresh interpretation of plant structures as related to environ- 

 mental conditions. The study of plant distribution and of the differ- 

 ences between the great fioristic areas was now carried on as a dynamic 

 subject, correlated with our knowledge of the geological past, and 

 interpreted in terms of the importance of areas and periods of evolu- 

 tionary activity, of paths of migration, of barriers, of the importance 

 of isolation, and of relict forms. This path of investigation was tra- 

 versed up to the point at which it became obscure and difficult. Its 

 culminating achievements are recorded in Engler's Entwickelungs- 

 geschichte der Pflanzenwelt, a work which would even now admit of 

 only minor revisions, 39 years after its first appearance. The fresh 

 interpretation of plant structures, to which stimulus was given by the 

 work of Darwin, was due to a new appreciation of the importance of 

 these structures in relation to environmental conditions; but it was 

 unfortunately the course of events for many years that the structures 

 themselves received attention, to the great neglect of the environment. 

 The principal attempt of the outdoor workers who had become imbued 

 with the Darwinian conceptions was to attach a significance to every 

 structure and habit in plants, no matter whether such significance 

 had been experimentally demonstrated or merely seemed to be highly 

 plausible. One of the most energetic and ingenious of these workers 

 was Kerner von Marilaun, to whom we owe many acute observations, 



