OF NATURAL SELECTION. 109 



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 

 have as yet suffered least extinction, will, for a long period, 

 continue to increase. But which groups will ultimately pre- 

 vail, no man can predict ; for we know that many groups, 

 formerly most extensively developed, have now become ex- 

 tinct. 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 clas- 

 sification, but I may add that as, according to this view, 

 extremely few of the more ancient species have transmitted 

 descendants to the present day, and as all the descendants 

 of the same species form a class, we can understand how it 

 is that there exist so few classes in each main division of the 

 animal and vegetable kingdoms. Although few of the most 

 ancient species have left modified descendants, yet, at remote 

 geological periods, the earth may have been almost as well 

 peopled with species of many genera, families, orders, and 

 classes, as at the present time. 



ON THE DEGREE TO WHICH ORGANIZATION TENDS TO 



ADVANCE. 



Natural selection acts exclusively by the preservation and 

 Hccumulation of variations, which are beneficial under the 

 organic and inorganic conditions to which each creature is 

 exposed at all periods of life. The ultimate result is that 

 each creature tends to become more and more improved in 

 relation to its conditions. This improvement inevitably leads 

 to the gradual advancement of the organization of the greater 

 number of living beings throughout the world. But here we 

 enter on a very intricate subject, for naturalists have not 

 defined to each other's satisfaction what is meant by an 

 advance in organization. Among the vertebrata the degree 

 of intellect and an approach in structure to mm eJe&rly come 



