Chap XI.] PRECEDING AND PRESENT CHAPTEES. 127 



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 ten- 

 dency 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 inter- 

 mediate 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. 



The inhabitants of the world at each successive 

 29 



