THE EVOLUTION OF PLANTS 



dude that they have come into existence by descent fror. 

 preexisting organisms. 



2. Gradual change. Either conditions have always been 

 as they now are, or else there has been a change throughout 

 the vast aeons of geological time. The evidence on this 

 subject consists of fossils found in the rocks of the earth's 

 crust. The more recently formed rocks (such as those of 

 the Tertiary period) contain fossil remains of plants very 

 similar to those now living — in fact species of the same 

 genera. But as we go down the geological scale to older 

 and older rocks all evidence of species now living gradually 

 disappears. Moreover, we find abundant evidence in the 

 older rocks of the existence of forms not represented in the 

 fossils of the more recent rocks and not found at all in the 

 vegetation of to-day. Obviously, there has been a profound 

 change in the vegetation of the earth. Forms appear, persist 

 for awhile, and then die out, giving place to new forms. 

 Moreover, the geological and biological evidence forces us 

 to the conclusion that this change has come about gradually. 



3. Evolution. By studying the comparative anatomy of 

 forms in successive geological periods we learn that they 

 resemble one another just as they would if they were related 

 to one another like parents and offspring. When we con- 

 template this fact in the light of the principle of biogenesis 

 the only logical conclusion we can reach is that the plants 

 of one geological period have been derived from those of a 

 preceding period by a process of descent with gradual modi- 

 fication. This is what is meant by organic evolution. 



4. Hypothesis, theory, fact. When men study the 

 phenomena of nature they get ideas suggesting explanations 

 of what they observe. These ideas are of the nature of 

 guesses, but they are rational guesses, which are fully war- 

 ranted by the contemplation of the facts observed. Such a 



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