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 to 
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 
