CHAPTER XXIII 

 PRINCIPLES OF CLASSIFICATION 



MANY classifications have been suggested ; each was used for 

 a time and then gave way to another. These early classifications 

 were " artificial," but in the course of time we have developed 

 what we now call a " natural system." It is the product of the 

 Darwinian theory of " natural selection " and is based primarily 

 on phy.logenetic relationships. Phylogeny is the term used to 

 indicate the origin of a plant or animal from some previous 

 form of plant or animal life. It necessitates a comparative study 

 of the form, arrangement and structure of the organs of the 

 plant or animal. Since the flowers are much more constant in 

 character than the vegetative organs of the plant, they are of 

 much greater importance in classification than the other organs. 

 For example, the bean and the locust tree are very different in 

 size and form, yet the reproductive organs show striking resem- 

 blances which lead to their being placed in the same family. 

 The fruit of the apple and peach are quite different, but the 

 study of the entire flower indicates a very close relationship. 



Plants of the same kind are said to belong to the same 

 species. Similar species are grouped into a genus. The generic 

 and specific names combined constitute the scientific name of 

 the plant. For example, all oak trees belong to the genus 

 Quercus. The white oak is Quercus alba; the scarlet oak is 

 Q. coccinea; the bur oak is Q. macrocarpa; the black oak is 

 Q. nigra; the pin. oak is Q. palustns, and the red oak is Q. rubra. 

 This is known as the binominal system and was brought into gen- 

 eral use by the work of the celebrated naturalist, Linnseus 

 (Fig. 117). 



In this same manner the genera which show resemblances are 

 collected into families, the families into orders and the orders 

 17 257 



