420 CLASSIFICATION. Chap. XIII. 



All the foregoing rules and aids and difficulties in 

 classification are explained, if I do not greatly deceive 

 myself, on the view that the natural system is founded 

 on descent with modification ; that the characters which 

 naturalists consider as showing true affinity between 

 any two or more species, are those which have been 

 inherited from a common parent, and, in so far, all true 

 classification is genealogical ; that community of descent 

 is the hidden bond which naturalists have been un- 

 consciously 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 

 believe that the arrangement of the groups within each 

 class, in due subordination and relation to the other 

 groups, 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 

 of referring to the diagram in the fourth chapter. We 

 will suppose the letters A to L to represent allied 

 genera, which lived during the Silurian epoch, and these 

 have descended from a species which existed at an un- 

 known anterior period. Species of three of these genera 

 (A, F, and I) have transmitted modified descendants to 

 the present day, represented by the fifteen genera (a 14 to 

 2») on the uppermost horizontal line. Now all these 

 modified descendants from a single species, are repre- 

 sented as related in blood or descent to the same 



