THE EVOLUTION OF PLANTS 



and elaborated the pioneer work done on this problem by 

 MendeL 



In summary it may be said that the plant kingdom presents 

 itself to us as a multitude of organisms of various degrees 

 of complexity, ranging from one-celled algae to multi-celled 

 organisms such as orchids and chrysanthemums. The present 

 vegetation of the earth differs profoundly from that of pre- 

 ceding geological ages; and amid all the present and past 

 diversity of form there is evidence that leads to only one 

 conclusion, namely, that the various forms of plant life are 

 genetically related — that the newer and more complex types 

 have been derived by descent (with modification) from older 

 and simpler types. A mechanism has been worked out along 

 lines suggested particularly by Darwin, de Vries, and Mendel, 

 which offers a partial but rational explanation as to how 

 these evolutionary changes may have been and probably 

 have been accomplished. 



In particular the fact to stress in such problems as those 

 here discussed is that they cannot be solved by philosophical 

 speculation; they can be solved only by first-hand study of 

 plants themselves. In our quest of the elusive thing we call 

 truth, whether in science, religion, politics, or any other 

 department of human thought, the most conspicuous his- 

 torical feature we note is change, revision, and continued 

 research for new and more reliable information and interpre- 

 tation. What one generation ties to, the next rejects, but not 

 in toto. A residuum remains, which we believe represents 

 the truth. Some progress is made by each generation. The 

 discovery of new facts may necessitate the radical revision or 

 even the abandonment of old ideas, but the only things that 

 should cause us grave concern would be the cessation of the 

 discovery of such facts (for each revision takes us one step 

 nearer to ultimate truth) and the closing of our minds, by 



[153] 



