CuAP. XIV.] CLASSIFICATION. 213 



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

 mitted modified descendants to the present day, repre- 

 sented by the fifteen genera (a^* to z^*) on the uppermost 

 horizontal line. Now 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 

 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 genealogical 

 arrangement remains strictly true, not only at the 

 present time, but at each successive period of descent. 

 Al l the modified descendants from A will have inherited 

 sometliing 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 



