100 MY LIFE [Chap. 



of the islands of the globe had been carefully studied botani- 

 cally by such eminent botanists as Sir Joseph Hooker, for 

 the Galapagos, New Zealand, Tasmania, 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, Ber- 

 muda, and St. Helena. With such excellent materials, and 

 with the further assistance of Sir Joseph Hooker's invaluable 

 essays on the relations of the southern and northern floras, I 

 felt that my work would be mainly of a statistical nature, as 

 interpreted by those general principles of organic evolution 

 which were my especial study. 



But I also found it necessary to deal with a totally dis- 

 tinct branch of science — recent changes of climate as depen- 

 dent 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 astro- 

 nomical causes of glacial and interglacial periods, and I had 

 much correspondence with him on difficult points of his 

 theory. While differing on certain details, I adopted the 

 main features of his theory, combining with it the effects of 

 changes in height and extent of land which form an important 

 adjunct to the meteorological agents. To this subject I 

 devoted two of my longest and most argumentative chapters, 

 introducing many considerations not before taken account of, 

 and leading, I still think, to a more satisfactory explanation 

 of the causes that actually brought about the glacial epoch 

 than any which have since been put forth. 



Besides this partially new theory of the causes of glacial 

 epochs, the work contained a fuller statement of the various 

 kinds of evidence proving that the great oceanic basins are 

 permanent features of the earth's surface, than had before 

 been given ; also a discussion of the mode of estimating the 

 duration of geological periods, and some considerations lead- 

 ing to the conclusion that organic change is now less rapid 



