314 SUMMARY OF THE [Chap. XI, 



together, or at the same rate, or in the same degree ; yet in the 

 long run that all undergo modification to some extent. The ex- 

 tinction of old forms is the almost inevitable consequence of the 

 production 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 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 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 sometimes 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 modified, descendants ; and these will gene- 

 rally 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 under- 

 stand, 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, pre- 

 viously 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 pro- 

 genitor 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 are intermediate in 

 character. 



