134 THE ORIGIN OF SPECIEi^ 



run that all undergo modification to some extent. The 

 extinction of old forms is the almost inevitable conse- 

 quence of the production of new forms. We can under- 

 stand 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 belonging 

 to large and dominant groups tend to leave many modi- 

 fied descendants, which form new sub-groups and groups. 

 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 has some- 

 times been a slow process, from the survival of a few 

 descendants, lingering in protected and isolated situations. 

 When a group has once wholly disappeared, it does not 

 reappear; for the link of generation has been broken. 



We can understand how it is that dominant forms 

 which spread widely and yield the greatest number of 

 varieties tend to people the world with allied, but modi- 

 fied, descendants; and these will generally succeed in 

 displacing the groups which are their inferiors in the 

 struggle for existence. Hence, after long intervals of 

 time, the productions of the world appear to have 

 changed simultaneously. 



We can understand how it is that all the forms of 

 life, ancient and recent, make together a few grand 

 classes. We can understand, from the continued ten- 

 dency to divergence of character, why, the more ancient 

 a form is, the more it generally diflEers from those now 



