SUMMARY 393 



and depends on many complex contingencies. The dominant 

 species belonging to large and dominant groups tend to leave 

 many modified descendants, which form new sub-groups and 

 groups. As these are formed, the species of the less vig- 

 orous groups, from their inferiority inherited from a com- 

 mon 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 sometimes been a 

 slow process, from the survival of a few descendants, lin- 

 gering 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 modified, 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 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, previously classed 

 as distinct, into one; but more commonly bringing them only 

 a little closer together. The more ancient a form is, the 

 more often it stands 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 become widely 

 divergent. Extinct forms are seldom directly intermediate 

 between existing forms ; but are intermediate only by a long 

 and circuitous course through other extinct and different 

 forms. We can clearly see why the organic remains of 

 closely consecutive formations are closely allied; for they 

 are closely linked together by generation. We can clearly 

 see why the remains of an intermediate formation arc inter- 

 mediate in character. 



