344 
GEOLOGICAL SUCCESSION. 
Chap. X. 
descendants, and thus new sub-groups and groups are 
formed. 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 
may often be a very 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 the spreading of the domi¬ 
nant forms of life, which are those that oftenest vary, 
will in the long run tend to people the world with 
allied, but modified, descendants; and these will gene¬ 
rally succeed in taking the places of those groups of 
species which are their inferiors in the struggle for 
existence. Hence, after long intervals of time, the 
productions of the world will appear to have changed 
simultaneously. 
We can understand how it is that all the forms of 
life, ancient and recent, make together one grand 
system; for all are connected by generation. 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 previ¬ 
ously classed as distinct into one; but more commonly 
only bringing them a little closer together. The more 
ancient a form is, the more often, apparently, it dis¬ 
plays characters 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 be- 
