Philosophical Institute of Canterbury. 71 



Any great and continuous change in a vital part of the environment, if not fatal to 

 the organism, if the balance is preserved, must be accompanied by a corresponding 

 •change in the organism. 



The part of jilants mainly concerned in the progressive diminution of CO 2 is the 

 surface, and balance might be retamed in two ways — ( 1 ) by increased effectiveness of a 

 given area as a COo collector : (2) by increase of surface-area. The second — increase of 

 plant-surface— is claimed to have occurred. 



The earliest-known dominant plants were relatively massive, branchless, and leafless, 

 with a small ratio of surface to mass. Branching and leaf-development came in 

 gradually. Leafy plants were not dominant till late Carboniferous times, while the 

 present dominant plants are excessively leafy. 



Ferns developed considerable surface very earlj^ owmg possibly to the large surface 

 necessary to shade-jjlants in order to collect sufficient light. This factor — deficiency of 

 light — was probably at work long before CO 2 became scarce. Hence the early develop- 

 ment of surface in ferns. 



The subject-matter of the paper was subsequently discussed by Drs. Cockajnie and 

 Chilton and ilj-. Speight, and the author replied to the criticisms which had been directed 

 against his general conclusions. 



