CHAPTER XIII. 



PHYLOGENESIS IN CLASSIFICATION. 



Principles of Classification Illustrated by the Mollusca and 

 Molluscoidea. In order to reach a closer view of the meaning of 

 the relationship of organic form to the place in the time scale 

 at which it appears, we must examine more particularly the 

 principles underlying the classification of organisms. 



The groups of organisms from which examples will be 

 chosen are the Mollusca and the Molluscoidea; chiefly for the 

 reason that they present hard parts which are abundantly 

 preserved in the rocks, and therefore afford more satisfactory 

 records of their geological history than any furnished by any 

 other class of organisms. A second reason for selecting them 

 is the fact that the statistics, regarding the relation of their 

 forms to conditions of external environment, are so satisfac- 

 tory as to be at least equal to those regarding any other 

 group of animals. 



The Author's Philosophy Reflected in his Classification. From 

 what has already been said it will have been perceived that 

 form and function are both regarded in the classification of 

 organisms ; but hitherto the fact has not been emphasized that 

 the classification of organisms, i.e., the description and or- 

 derly arrangement of the characters which one is supposed to 

 see in particular examples of organisms, is affected by the phi- 

 losophy of the classifier. At best the classification expresses 

 only the author's interpretation of the laws of association of 

 different things ; hence if we know the theory by which the 

 association is reached we are better prepared to learn truth 

 from the resulting classification. 



Effect of Theories of Phylogenesis upon Classification. Much 

 is found in the modern literature about phylogenesis as a 

 basis of classification, and it is supposed to supersede quite 



237 



