Caw. IV. ADVANCE OF ORGANIZATION. 121 



j^roups, from hraneliiiig" out and seizinc^ on many new plar-cs in 

 the polity of Nature, will constantly tend to sujiplant and de- 

 stroy the earlier and Icss-iinp.roved svib-<^oups. Small and 

 broken ""roups and sul>irroups will hnally disappear, Lookinnf 

 to the future, Ave can ])redict that the groups of organic beings 

 which are now large and triumphant, and Avbich.are least 

 broken u]i, that is, which as yet have sulTered 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 develojied, 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 (gw will transmit descendants to a remote futurity. 

 I shall have to return to this subject in the chapter on Classifi- 

 cation, but I may add that as according to this view extremely 

 few of the more ancient species have transmitted descendants, 

 and as :ill the descendants of the same species form a class, we 

 can imdcrstand 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 motlified descendants, 

 yet, at remote geological periods, the earth may have been 

 almost as well jjcopled with species of many genera, families, 

 oiders, and classes, as at the; present day. 



O71 the Degree to which Organization tends to advance. 



Natural Selection exclusively acts by the preservation and 

 accunmlation of variations, which are beneficial under the or- 

 ganic and inorganic conditions of life to which each creature is 

 exposed at each successive period 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 bi'ings 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 org-anization. Among the vertcbrata the degree of 

 intellect and approac-h in structure to man clearly come into 

 play. It might be thought that the amount of change Avhich 

 the various ])arts and organs pass through in their develop- 

 G 



