O INTRODUCTION. 



We have attempted to carry out these principles in the 

 different groups of invertebrates. With the increase of 

 knowledge certain animals which are here described as 

 primitive will doubtless be found to be reduced forms, 

 while certain reduced forms may in reality be primitive. 

 Notwithstanding these changes in the position of species, 

 the principles of this genealogical classification will remain 

 essentially the same. 



In order to bring out these principles clearly and for- 

 cibly in the descriptive text of the Guide, it has been neces- 

 sary to abandon altogether the use of certain terms, while 

 the meaning of other terms has been restricted. Among 

 those given up are the words "high," "low," "highest," 

 "lowest." Textbooks and manuals have usually consid- 

 ered animals as either "high" or "low." Generally 

 speaking, vertebrates have been studied first, as the 

 "highest" representatives of animal life, and all other 

 forms have been "low" in comparison. In other cases 

 the study of a class has begun with the so called " lowest" 

 forms ; for instance, the Crustacea with the barnacles. 



These classifications were the best that were possible at 

 the time they were made, and they met the necessities of 

 the period. But the time is now ripe, as we have already 

 said, for a more natural system of classification which shall 

 embody and set forth the history of animal life on our 

 globe. In a genealogical classification that illustrates the 

 broad natural relationships which bind animals together 

 genetically there can be no " high " nor " low. rt There 

 can be only simple, primary forms from which have been 

 evolved in the process of the ages complex and secondary 

 forms. This, in reality, is the fundamental principle of 

 our classification. Granted that this is true, then the 

 most profound knowledge is needed concerning the prob- 

 lems of heredity and of variation. Immense strides have 

 been made the past quarter of a century, and epoch-mak- 

 ing monographs on the development of certain animals 

 have thrown strong light on these difficult problems. 



