CLASSIFICATION AND DISTRIBUTION 375 



on through the whole series, until we get back to the common 

 starting point, or the common center from which all animals and 

 plants diverge. Classification is thus an expression of history. 

 The following is an outline of the classification of animals and 

 plants. The classification accepted by science is ever under- 

 going changes, as a more complete knowledge of relations is 

 obtained, and the classification accepted to-day is different in 

 many respects from that adopted a generation ago. In turn, 

 the classification used to-day will doubtless be modified by 

 future study, until it becomes practically perfect. But even 

 though we recognize that it is not yet perfect, it is quite necessary 

 to have such a classification in order to understand the living 

 world. It must not be inferred that our present classification 

 represents an accurate history of organisms. The classification 

 that biologists are aiming at is a genetic one, i. e., one that repre- 

 sents actual relationships, and to a considerable extent the classi- 

 fication outlined below does represent such relationships. But 

 the difficulties of determining the actual history of organisms 

 have been so great as to seem in some respects almost insur- 

 mountable. The classification of organisms given to-day rep- 

 resents, therefore, only an attempt to express genetic relation- 

 ships, and is recognized as being only in part successful. 



AN OUTLINE OF THE CLASSIFICATION OF THE LIVING WORLD 

 THE PLANT KINGDOM: 



Phylum I. THALLOPHYTA: plants without distinction of root, 



stem, or branch. 

 Sub-phylum 1. Algae: thallophytes possessing chlorophyll: 



including unicellular forms, pond weeds, seaweeds, etc. 

 Class I. Diatomacece: the diatoms (Fig. 68 A). 

 Class II. Cyanophycece: the blue-green algae (Fig. 68 C). 

 Class III. Chlorophycece: the green algae (Fig. 30). 

 Class IV. Phceophycece: the brown algae. 

 Class V. Rhodophycece: the red algae. 



