90 KESULTS OF THE ACTION OF [CHAP. IV. 



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 modification supposed to be represented in the diagram. 

 And the two new families, or orders, 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 lie 

 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 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 mil 

 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 

 wore 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 exists 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. 



