Classification. 35 



fication the apparent paradox is at once explained. 

 For it is evident that organs of functional importance 

 are, other things equal, the organs which are most 

 likely to undergo different modifications in different 

 lines of family descent, and therefore in time to have 

 their genetic relationships in these different lines 

 obscured. On the other hand, organs or structures 

 which are of no functional importance are never called 

 upon to change in response to any change of habit, or 

 to any change in the conditions of life. They may, 

 therefore, continue to be inherited through many 

 different lines of family descent, and thus afford 

 evidence of genetic relationship where such evidence 

 fails to be given by any of the structures of vital 

 importance, which in the course of many generations 

 have been required to change in many ways according 

 to the varied experiences of different branches of the 

 same family. Here, then, we have an empirically 

 discovered rule in the science of classification, the 

 raison (tetre of which we are at once able to appre- 

 ciate upon the theory of evolution, whereas no 

 possible explanation of why it should ever have 

 become a rule could be furnished upon the theory of 

 special creation. 



Here, again, is another empirically determined rule. 

 The larger the number, as distinguished from the 

 importance, of structures which are found common 

 to different groups, the greater becomes their value 

 as guides to the determination of natural affinity. 

 Or, as Darwin puts it, " the value of an aggre- 

 gate of characters, even when none are important, 

 alone explains the aphorism enunciated by Linnaeus, 

 namely, that the characters do not give the genus, 



D 2 



