C.IAP. XL] PRECEDING AND PRESENT CHAPTERS. 295 



successively ; how species of different classes do not necessarily 

 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 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 

 generally succeed in displacing the groups which are their in- 

 feriors 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, previously classed as distinct, into one ; but more com- 

 monly bringing them only a little closer together. The more 

 ancient a form is, the more often it stands in some degree inter- 

 mediate 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 be- 

 tween 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 inter- 

 mediate formation are intermediate in character. 



The inhabitants of the world at each successive period in its 



