Chap. XL] PRECEDING AND PRESENT CHAPTERS. 127 



ants, lingering iff 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 domi nant form s 

 which spread widely and yield the, greatest n umber of 

 varieties tend to people the world with a^llied^but modi- 

 fied, descendants; and these will generally succeed in 

 displacing the groups which are their inferiors m the 

 struggle for existence. Hence, after long intervals of 

 time, the productions of the world appear_ to .have 

 changeSTsimultaneousIy. 



We TfanTunderstandTiow 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 

 d 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 to- 

 gether. 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 ex- 

 tinct 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. 



