CLASSIFICATION 221 



done, not because further research has detected important 

 structural difi'erences, at first overlooked, but because 

 numerous allied species with slightly different grades of 

 diflerence have been subsequently discovered. 



All the foregoing rules and aids and difficulties in 

 classification may be explained, if I do not greatly 

 deceive myself, on the view that the Natural System 

 is founded on descent with modification; — that the char- 

 acters which naturalists consider as showing true affinity 

 between any two or more species are those which have 

 been inherited from a common parent, all true classifi- 

 cation being genealogical; — that community of descent 

 is the hidden bond which naturalists have been uncon- 

 sciously seeking, and not some unknown plan of crea- 

 tion, or the enunciation of general propositions, and the 

 mere putting together and separating objects more or less 

 alike. 



But I must explain my meaning more fully. I be- 

 lieve that the arrangement of the groups within each 

 class, in due subordination and relation to each other, 

 must be strictly genealogical in order to be natural; but 

 that the amount of difference in the several branches 

 or groups, though allied in the same degree in blood to 

 their common progenitor, may differ greatly, being due 

 to the different degrees of modification which they have 

 undergone; and this is expressed by the forms being 

 ranked under different genera, families, sections, or 

 orders. The reader will best understand what is meant, 

 if he will take the trouble to refer to the diagram in the 

 fourth chapter. We will suppose the letters A to L to 

 represent allied genera existing during the Silurian epoch, 

 and descended from some still earlier form. In three of 



