INTRODUCTION 3 



Primitive vs. Degenerate. — Simplicity of organization is not 

 always a safe guide in the interpretation of relationships, for sim- 

 plicity of form and structure may be either primitive or secon- 

 darily derived as a consequence of degeneracy. A tapeworm 

 (Fig. 67) is simpler in many points of organization than a plana- 

 rian (Fig. 60 c), because the tapeworm lacks a digestive system 

 and special sense organs, both of which are present in the planarian. 

 Primitively, every organism must be able to digest and assimilate 

 its own food materials. Dependence upon some other individual 

 for performing a process essential to life cannot be a primitive 

 condition but is secondarily acquired and has been accompanied 

 by degeneration of the digestive organs in perfectly adjusted 

 parasites. The tapeworm is consequently degenerately and not 

 primitively more simple than the planarian. 



Mutability of Group Concepts. — Throughout the entire system 

 of classification, different premises and definitions lead to wholly 

 divergent conclusions. A group which has been considered as 

 a class by one zoologist may be set apart as an independent 

 phylum by another, just because the same observed facts receive 

 different interpretation and the same terms are defined differ- 

 ently. In elementary zoological courses, a rigid system of classi- 

 fication is frequently taught, but the advanced student must 

 sooner or later appreciate the fact that group concepts are man- 

 made devices adopted for man's convenience in his discussions 

 of organisms which seem to be related. Shift of emphasis or 

 new facts and new interpretations play an important part in 

 formulating any scheme of classification, for, after all, group con- 

 cepts are constructed to include organisms and are not necessarily 

 expressions of natural laws with which the organisms must of 

 necessity agree. 



The Law of Priority. — Scientific names of the larger subdivi- 

 sions of the animal kingdom are subject to considerable differ- 

 ences in usage, for there are no fixed rules governing the acceptance 

 or rejection of names pertaining to phyla, classes, and orders. 

 In contrast with this, the use of names for species, genera, and 

 families is definitely controlled by rules or laws. An Inter- 

 national Commission on Zoological Nomenclature was established 

 by the International Zoological Congress (1895) to formulate a 

 code of rules or laws governing the problems of naming animals. 

 One of the basic principles of this code has been the law of pri- 

 ority. According to this law the valid name of a genus or of a 



