Chap. XIII. CLASSIFICATION. 3-79 



cation arc explained, if I do not greatly deceive myself, on the 

 view that the Natural System is founded on descent Avitli 

 modification; that the characters which naturalists consider as 

 showing true allinity between any two or more species, are 

 those which have been inherited from a connnon parent, all true 

 classification being genealogical ; that community of descent 

 is the hidden bond Avhich naturalists have been unconsciously 

 seeking, and not some unknown plan of creation, or the enun- 

 ciation 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 arret)} rjcment of the groujis within each class, in due sub- 

 ordination and relation to each other, must be strictly genea- 

 logical in order to be natural ; but that the amount of diflerence 

 in the several branches or groups, though allied in the same 

 (h'grce in blood to their common progenitor, may diller greatly, 

 In'ing duo to the different degrees of modification Avhich they 

 have luidergone ; 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 sup]iose the letters A to L to rcpi'csent during the Silurian 

 epoch allied genera, descended from some still earlier forms. 

 In three of these genera (A, F, and I) the species have trans- 

 mitted modified descendants to the present day, represented l)y 

 the fifteen genera («'* to s'*) on the uppermost horizontal line. 

 Now all these modified descendants from a single species, are 

 related in blood or descent to the same degree ; they may 

 metaphorically be called cousins to the same millionth degTce ; 

 y(}t tiiey differ widely and in diU'ercnt degrees from each other. 

 The forms descended from A, now broken up into two or three 

 families, constitute a distinct order from those descended from 

 I, also broken up into two families. Nor can the existing sjie- 

 cies, desecnded from A, be ranked in the same genus with the 

 parent A; or those from I, with the parent I. But the exist- 

 ing geiuis f" may be supposed to have been but slightly modi- 

 tied ; and it will then rank with tlie parent-genus F ; just as 

 some few still living organisms belong to Silurian genera. So 

 Ihat the amount or value of the differences between these or- 

 ganic beings which arc all related to each other in the same 

 degree in blood, has come to be widely different. Nevertheless, 

 their genealogi(Ml arranffcment remains strictly true, not only 

 at tlie present time, but at each successive period of descent. 



