3>art 2] anniversary address of the president. lxix 



assumption. There is something attractive in the idea of deriving 

 plants of the land from highly organized inhabitants of the ocean- 

 floor; and, in my view at least, there is no insuperable objection to 

 the conception that terrestrial vegetation received additions from 

 a^raised portions of the crust at more than one epoch in the 

 •history of the earth. 



One may hazard the opinion that palajobotanical research has 

 'not made it easy to picture the course of evolution through the 

 •ages as a single and continuous process. Dynasties rise with 

 apparent suddenness into prominence ; after a longer or shorter 

 period of existence, during which comparatively slight changes 

 • occur, they are superseded, again with apparent suddenness, by 

 newer and more vigorous races. It would almost seem that 

 ■* missing links ' never existed. The vegetation of the land that 

 has passed across the world's stage cannot be correctly represented 

 as a tree with many branches. Its development, seen in perspec- 

 tive, appears as a series of separate lines, some stretching into a 

 remote past, others of more recent origin ; the impression received 

 ds that the starting-points of new lines are not scattered irregularly 

 over the path of geological history, but rather that there is a more 

 or less definite grouping at certain clearly marked periods, nodal 

 points in the history of evolution. I have elsewhere expressed the 

 opinion that the late Pakeozoic floras differ considerably from those 

 preserved in Triassic strata : the older lines of evolution came to 

 -an end ; a new series began. In a word, the history of vegetation, 

 like the history of the rocks, is characterized by cycles ; there 

 were revolutions in the organic as in the inorganic world. As the 

 late Dr. J. Barrell said, 'Nature vibrates with rhythms'. There is 

 no absolute break in continuity, but apparently sudden changes in 

 the dominant types. The conception of 'Nature's unchanging 

 harmony ' is not inconsistent with variations in the manner of its 

 manifestation. We cannot hope to follow the unfolding of plant- 

 life, unless we keep before us the historical background. 



The Pie-Cambrian Era. 



For the sake of simplicity, the term Archaean is used in this 

 sketch as equivalent to Pre-Cambrian, without always attempting 

 to draw a distinction between Archaean in the narrower sense, the 

 Lewisian of British geologists, and an upper, Algonkian or 

 Proterozoic series. In order to realize the duration of the 

 Archaean Era as a whole, it is important to bear in mind that the 



