458 ORIGIN OF SPECIES 



But I must explain my meaning more fully. I believe that 

 the arrangement of the groups 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 differ- 

 ence 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 repre- 

 sent 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 de- 

 scendants to the present day, represented by the fifteen 

 genera (a" to /^) on the uppermost horizontal line. Now 

 all these modified descendants from a single species, are re- 

 lated in blood or descent in the same degree ; they may meta- 

 phorically 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 

 descended 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" 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 be- 

 long to Silurian genera. So that the comparative value of 

 the differences between these organic beings, which are all 

 related to each other in the same degree in blood, has come 

 to be widely different. Nevertheless their genealogical 

 arrangement remains strictly true, not only at the present 

 time, but at each successive period of descent. All the modi- 

 fied descendants from A will have inherited something in 

 common from their common parent, as will all the descend- 

 ants from I ; so will it be with each subordinate branch of 

 descendants, at each successive stage. If, however, we sup- 

 pose any descendant of A, or of I, to have become so much 



