Works on Biology, etc. 



means of dispersal and colonisation of animals is so con- 

 nected with, and often dependent on, that of plants, that 

 a consideration of the latter is essential to any broad views 

 as to the distribution of life upon the earth, while they 

 throw unexpected light upon those exceptional means of 

 dispersal which, because they are exceptional, are often 

 of paramount importance in leading to the production of 

 new species and in thus determining the nature of insular 

 floras and faunas. 



Having no knowledge of scientific botany, it needed some 

 courage, or, as some may think, presumption, to deal with 

 this aspect of the problem ; but ... I had long been exces- 

 sively fond of plants, and . . . interested in their distribu- 

 tion. The subject, too, was easier to deal with, on account 

 of the much more complete knowledge of the detailed dis- 

 tribution of plants than of animals, and also because their 

 classification was in a more advanced and stable condition. 

 Again, some of the most interesting islands of the globe had 

 been carefully studied botanically by such eminent botanists 

 as Sir Joseph Hooker for the Galapagos, New Zealand, Tas- 

 mania, and the Antarctic islands; Mr. H. C. Watson for the 

 Azores; Mr. J. G. Baker for Mauritius and other Mascarene 

 islands ; while there were floras by competent botanists of the 

 Sandwich Islands, Bermuda and St. Helena. . . . 



But I also found it necessary to deal with a totally 

 distinct branch of science — recent changes of climate as 

 dependent on changes of the earth's surface, including the 

 causes and effects of the glacial epoch, since these were 

 among the most powerful agents in causing the dispersal 

 of all kinds of organisms, and thus bringing about the 

 actual distribution that now prevails. This led me to a 

 careful study of Mr. James Croll's remarkable works on 

 the subject of the astronomical causes of the glacial and 

 interglacial periods. . . . While differing on certain 

 details, I adopted the main features of his theory, com- 

 bining with it the effects of changes in height and extent 

 of land which form an important adjunct to the meteoro- 

 logical agents. . . . 



Besides this partially new theory of the causes of glacial 



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