Classification. Bu 
that these characters have been inherited from a 
common ancestor ; and we know that such aggregated 
characters have especial value in classification }.” 
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. 
