CHAPTER XVII. 

 CLASSIFICATION OF PLANTS. 



230. Basis of Classification. The number of diverse 

 plants is so enormous that a systematic classification is a 

 necessity for their identification and for a concise indica- 

 tion of their relationships. It is plain that since plants 

 are descended from generation to generation it is best, if 

 possible, to classify them according to their lineal descent ; 

 those plants which give evidence of a recent common 

 ancestor being placed together in a group, and assem- 

 blages of this sort which appear still further back to have 

 descended from a common form, being classed under a 

 still more comprehensive family, and so on in widening 

 tribes of remoter relationships until the ultimate groups 

 are finally embraced under one all-comprehensive designa- 

 tion, the Vegetable Kingdom. Properly to determine the 

 degrees of relationship among plants is, in many cases, 

 impossible, and our classification on grounds of consan- 

 guinity can be only approximately right. 



As we have already learned, plants have been subjected 

 to great vicissitudes in their long residence on the earth ; 

 many forms have become extinct, while many others have 

 wandered far from their ancestral homes, and have become 

 profoundly modified under changed environments ; and 

 other modifications have come about by intercrossing. The 

 chain of evidence may, therefore, be incomplete or obscure. 

 Where, however, forms are related by lineal descent, there 



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