96 RESULTS OF THE ACTION OF [Chap. IV. 



I see no reason to limit the process of modification, as now ex- 

 plained, to the formation of genera alone. If, in the diagram, we 

 suppose the amount of change represented by each successive group 

 of diverging dotted lines to be great, the forms marked a 1 * to p 1 *, 

 those marked b u and /", and those marked o u to m u , will form 

 three very distinct genera. We shall also have two very distinct 

 genera descended from (I), differing widely from the descendants 

 of (A). These two groups of genera will thus form two distinct 

 families, or orders, according to the amount of divergent modifica- 

 tion supposed to be represented in the diagram. And the two new 

 families, or crders, are descended from two species of the original 

 genus, and these are supposed to be descended from some still more 

 ancient and unknown form. 



We have seen that in each country it is the species belonging 

 to the larger genera which oftenest present varieties or incipient 

 species. This, indeed, might have been expected ; for, as natural 

 selection acts through one form having some advantage over other 

 forms in the struggle for existence, it will chiefly act on those which 

 already have some advantage ; and the largeness of any group 

 shows that its species have inherited from a common ancestor some 

 advantage in common. Hence, the struggle for the production of 

 new and modified descendants will mainly he between the larger 

 groups which are all trying to increase in number. One large group 

 will slowly conquer another large group, reduce its numbers, and 

 thus lessen its chance of further variation and improvement. 

 Within the same large group, 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 have 

 as yet suffered least extinction, will, for a long period, continue to 

 increase. But which groups will ultimately prevail, no man can 

 predict ; for we know tbat 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 as, according to this view, extremely few of the 



