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 @étre 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 wamber, 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 Linnzus, 
namely, that the characters do not give the genus, 
D2 
