438 CLASSIFICATION. 



common progenitor, may differ greatly, being due to 

 the different degrees of modification whicli 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 represent 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 trans- 

 mitted modified descendants to the present day, repre- 

 sented by the fifteen genera {a^^ to z'^^) on the uppermost 

 horizontal line. N"ow, 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 dif- 

 ferent degrees from each other. The forms descended 

 from A, now broken up into two or three families, consti- 

 tute a distinct order from those desceuded 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 parent I. But the existing 

 genus F^* may be supposed to have been but slightly mod- 

 ified, 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 suc- 

 cessive 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; 

 SQ will it be with each subordinate branch of descendants 

 at each successive stage. If, however, we suppose any de- 

 scendant 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 svstem 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 supposed to have been but little modi- 

 fied, and they form a single genus. But this genus. 



