126 NATURAL SELECTION. Chap. IV. 
groupj the later and more highly perfected sub-groups, 
from branching out and seizing on many new places 
in the polity of Nature, will constantly tend to supplant 
and destroy the earlier and less improved sub-groups. 
Small and broken groups and sub-groups will finally 
disappear. Looking to the future, we can predict that 
the groups of organic beings which are now large and 
triumphant, and which are least broken up, that is, 
which as yet have suffered least extinction, will for a 
long period continue to increase. But which groups 
will ultimately prevail, no man can predict; for we 
well know that many groups, formerly most extensively 
developed, have now become extinct. Looking still 
more remotely to the future, we may predict that, 
owing to the continued and steady increase of the larger 
groups, a multitude of smaller groups will become 
utterly extinct, and leave no modified descendants; and 
consequently that of the species living at any one period, 
extremely few will transmit descendants to a remote 
futurity. I shall have to return to this subject in the 
chapter on Classification, but I may add that on this 
view of extremely few of the more ancient species 
having transmitted descendants, and on the view of all 
the descendants of the same species making a class, 
we can understand how it is that there exist but very 
few classes in each main division of the animal and 
vegetable kingdoms. Although extremely few of the 
most ancient species may now have living and modified 
descendants, yet at the most remote geological period, 
the earth may have been as well peopled with many 
species of many genera, families, orders, and classes, as 
at the present day. 
Summary of Chapter ,—If during the long course of 
ages and under varying conditions of life, organic beings 
