222 THE ORIGIN OF SPECIES 



these genrni (A, F, and I), u species has transmitted 

 modiiicd descendiuits to the present day, represented by 

 the fifteen genera (a" to z") on the uppermost horizontal 

 line. Now all these moditied descendants from a single 

 species are related in blood or descent in the same de- 

 gree; 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, con- 

 stitute a distinct order irom 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 belong 

 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 genea- 

 logical nrranriement remains strictly true, not only at the 

 present time, but at each successive period of descent. 

 All the modified descendants from A will have inherited I 

 something in common from their common parent, as willJ 

 all the descendants from I; so will it be with each sub-l 

 ordinate branch of descendants at each successive stage.] 

 If, however, we suppose any descendant of A, or of 1,1 

 to have become so much modified as to have lost all] 

 traces of its parentage, in this case its place in the nat- 

 ural system will be lost, as seems to have occurred witl 

 some few existing organisms. All the descendants of th( 

 genus F, along its whole line of descent, are supposed! 



