212 CLASSIFICATION. [Chap. XIV. 



been done, not because further research has detected 

 important structural differences, at first overlooked, but 

 because numerous allied species with slightly differ- 

 ent grades of difference, have been subsequently dis- 

 covered. 



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 affin- 

 ity between any two or more species, are those which 

 have been inherited from a common parent, all true 

 classification being genealogical; — that community of 

 descent is the hidden bond which naturalists have been 

 unconsciously seeking, and not some unknown plan of 

 creation, 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 



