308 ON THE ORIGIN OF SPECIES 



leading facts in palaeontology seem to me simply to 

 follow on the theory of descent with modification 

 through natural selection. We can thus understand 

 how it is that new species come in slowly and succes- 

 sively ; how species of different classes do not neces- 

 sarily change together, or at the same rate, or in the 

 same degree ; yet in the long run that all undergo 

 modification to some extent. The extinction of old 

 forms is the almost inevitable consequence of the pro- 

 duction of new forms. We can understand why when 

 a species has once disappeared it never reappears. 

 Groups of species increase in numbers slowly, and 

 endure for unequal periods of time ; for the process of 

 modification is necessarily slow, and depends on many 

 complex contingencies. The dominant species of the 

 larger dominant groups tend to leave many modified 

 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 sur- 

 vival of a few descendants, lingering in protected and 

 isolated situations. When a group has once wholly dis- 

 appeared, it does not reappear ; for the link of genera- 

 tion 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 gener- 

 ally 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 



