Chap. XIY.] CLASSIFICATION. 213 



three of these genera (A, F, and I), a species has transmit- 

 ted modified descendants to the present day, represented 

 hy the fifteen genera (a^* to z^*) on the uppermost hori- 

 zontal Une. ISTow 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 famihes. 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 differ- 

 ences between these organic beings, which are all re- 

 lated 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 pres- 

 ent time, but at each successive period of descent. All 

 the modified descendants from A will have inherited 

 something in common from their common parent, as 

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

 subordinate branch of descendants, at each successive 

 stage. If, however, we suppose any descendant of A, 

 or of I, to have become so much modified as to have 

 lost all traces of its parentage, in this case, its place in 

 the natural system will be lost, as seems to have occurred 

 with some few existing organisms. All the descendants 

 of the genus F, along its whole line of descent, are 



