CHAPTER V ORGANIC EVOLUTION 



THE morphology of plants, as presented in the preceding chapters, is in 

 reality a somewhat detailed illustration of the evolution of the plant 

 kingdom. The theory of descent is the working theory of modern biol- 

 ogy, and no student of morphology should omit some consideration of it. 

 The subject has developed so extensively that it can be presented here 

 only in very brief outline, an outline that may serve as an extended defi- 

 nition, and also as an introduction to a real study of organic evolution. 

 Many names connected with the doctrine of evolution and many impor- 

 tant views in reference to it must be passed over, and only the most con- 

 spicuous features presented. 



Definition. The doctrine of organic evolution claims that the exist- 

 ing plants and animals are the modified descendants of earlier forms; 

 that in some way new forms have arisen from the old ones, and have 

 given rise in turn to still other forms. According to this view, the whole 

 plant kingdom, for example, may be likened to a profusely branching 

 tree, the tips of whose myriad branchlets represent our present flora. 

 The morphologist attempts to trace these branchlets from their tips, 

 which he sees, to their connections, which he can only infer. His proofs 

 are obtained from the structure and development and behavior of living 

 plants, and also from the form and structure of ancient plants, so far as 

 they are available in suitable fossil form. His conclusions, it must be 

 remembered, are reasonable inferences, and cannot be based upon actual 

 demonstrations. It is evident that opinions may differ widely as to the 

 actual historical connections of plant groups; but it is practically unani- 

 mous that there are such connections. 



The idea of organic evolution is not modern, being in the thought of 

 man as far back as records of thought have been found; but it is only 

 in modern times that it has been based upon direct observation of the 

 facts, that is, has become scientific. It is not necessary to recite the 

 facts that underlie the widespread belief in organic evolution, for many 

 of these facts, so far as they concern plants, have been given in the 

 preceding chapters. To believe in organic evolution, however, is one 



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