212 CLASSIFICATION. [Chap. XIV. 



this has been done, not because further research has 

 detected important structural differences, at first over- 

 looked, but because numerous allied species with slightly 

 different grades of difference, 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 ^vith modification ; — that the cha- 

 racters 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 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 proposi- 

 tions, and the mere putting together and separating 

 objects more or less alike. 



But I must explain my meaning more fully. I 

 believe 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 



