GEOLOGICAL SUCCESSION 297 



We may suppose that the numbered letters represent 

 genera, and the dotted lines diverging from them 

 the species in each genus. The diagram is much too 

 simple, too few genera and too few species being 

 given, but this is unimportant for us. The horizontal 

 lines may represent successive geological formations, 

 and all the forms beneath the uppermost line may 

 be considered as extinct. The three existing genera, 

 a 14 , <j 14 , p 14 , will form a small family ; 6 14 and / 14 a 

 closely allied family or sub-family ; and o 14 , e u , m 14 , a 

 third family. These three families, together with the 

 many extinct genera on the several lines of descent 

 diverging from the parent-form (A), will form an 

 order ; for all will have inherited something in 

 common from their ancient and common progenitor. 

 On the principle of the continued tendency to diver- 

 gence of character, which was formerly illustrated by 

 this diagram, the more recent any form is, the more 

 it will generally differ from its ancient progenitor. 

 Hence we can understand the rule that the most 

 ancient fossils differ most from existing forms. We 

 must not, however, assume that divergence of char- 

 acter is a necessary contingency ; it depends solely on 

 the descendants from a species being thus enabled to 

 seize on many and different places in the economy of 

 nature. Therefore it is quite possible, as we have seen 

 in the case of some Silurian forms, that a species might 

 go on being slightly modified in relation to its slightly 

 altered conditions of life, and yet retain throughout a 

 vast period the same general characteristics. This is 

 represented in the diagram by the letter f 14 . 



All the many forms, extinct and recent, descended 

 from (A), make, as before remarked, one order ; and 

 this order, from the continued effects of extinction 

 and divergence of character, has become divided into 

 several sub -families and families, some of which are 

 supposed to have perished at different periods, and 

 gome to have endured to the present day. 



By looking at the diagram we can see that if many 

 of the extinct forms, supposed to be embedded in the 



