Classification. 37 



that these characters have been inherited from a 

 common ancestor ; and we know that such aggregated 

 characters have especial value in classification V 



It is true that even a single character, if found 

 common to a large number of forms, while uniformly 

 absent from others, is also regarded by naturalists as 

 of importance for purposes of classification, although 

 they recognise it as of a value subordinate to that of 

 aggregates of characters. But this also is what we 

 should expect on the theory of descent. If even any 

 one structure be found to run through a number of 

 animals presenting different habits of life, the readiest 

 explanation of the fact is to be found in the theory of 

 descent ; but this does not hinder that if several such 

 characters always occur together, the inference of 

 genetic relationship is correspondingly confirmed. 

 And the fact that before this inference was ever drawn, 

 naturalists recognised the value of single characters in 

 proportion to their constancy, and the yet higher 

 value of aggregates of characters in proportion to 

 their number this fact shows that in their work of 

 classification naturalists empirically observed the 

 effects of a cause which we have now discovered, to 

 wit, hereditary transmission of characters through 

 ever- widening groups of changing species. 



There is another argument which appears to tell 

 strongly in favour of the theory of descent. We have 

 just seen that non-adaptive structures, not being 

 required to change in response to change of habits or 

 conditions of life, are allowed to persist unchanged 

 through many generations, and thus furnish excep- 

 tionally good guides in the science of classification 

 1 Origin of Species, p. 372. 



