I 



CLASSIFICATION. 409 



All the foregoing rules and aids and difficulties in classifi- 

 cation 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 characters 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 com- 

 munity 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 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 modifica- 

 tion which they have undergone ; and this is expressed by 

 the forms being ranked under different genera, families, sec- 

 tions, 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 and L 

 to represent allied genera existing during the Silurian epoch, 

 and descended from some still earlier form. In three of these 

 genera (A, F, and I) a species has transmitted modified 

 descendants to the present day, represented by the fifteen 

 genera (a u to z u ) on the uppermost horizontal line. Now, 

 all these modified descendants from a single species are 

 related in blood or descent in the same degree. They may 

 metaphorically be called cousins to the same millionth degree, 

 yet they differ widely and in different degrees from each 

 other. The forms descended from A, now broken up into two 

 or three families, constitute a distinct order from those de- 

 scended from I, also broken up into two families. Nor can 

 the existing species descended from A be ranked in the same, 

 genus with the parent A, or those from I with the parent I. 

 But the existing genus f 14 may be supposed to have been but 

 slightly modified, and it will then rank with the parent genus 

 F, just as some few still living organisms belong to Silurian 

 genera. So that the comparative value of ^he differences be- 

 tween these organic beings, which are all related to each other 

 in the same degree in. blood, has come to be widely different 



