344 GEOLOGICAL SUCCESSION. Chap. X. 



descendants, and thus new sub-groups and groups are 

 formed. As these are formed, the species of the less 

 vigorous groups, from their inferiority inherited from a 

 common progenitor, tend to become extinct together, and 

 to leave no modified offspring on the face of the earth. 

 But the utter extinction of a whole group of species 

 may often be a very slow process, from the survival of a 

 few descendants, lingering in protected and isolated 

 f situations. When a group has once wholly disappeared, 

 | it does not reappear ; for the link of generation has 

 been broken. 



We can understand how the spreading of the domi- 

 nant forms of life, which are those that oftenest vary, 

 will in the long run tend to people the world with 

 allied, but modified, descendants ; and these will gene- 

 rally succeed in taking the places of those groups of 

 species which are their inferiors in the struggle for 

 existence. Hence, after long intervals of time, the 

 productions of the world will appear to have changed 

 simultaneously. 



We can understand how it is that all the forms of 

 life, ancient and recent, make together one grand 

 system ; for all are connected by generation. We can 

 understand, from the continued tendency to divergence 

 of character, why the more ancient a form is, the more 

 it generally differs from those now living. Why ancient 

 and extinct forms often tend to fill up gaps between 

 existing forms, sometimes blending two groups previ- 

 ously classed as distinct into one ; but more commonly 

 only bringing them a little closer together. The more 

 ancient a form is, the more often, apparently, it dis- 

 plays characters in some degree intermediate between 

 groups now distinct ; for the more ancient a form is, 

 the more nearly it will be related to, and consequently 

 resemble, the common progenitor of groups, since be- 



