120 RESULTS OF NATURAL SELECTION. Chap. IV. 



naturalist will be able to bring some such case before his 

 mind. 



In the diagram, each liorizonal line has hitherto been sup- 

 posed to represent a tliousand generations, but each may rep- 

 resent a million or several million generations ; it may also 

 represent a section of the successive strata of the earth's crust 

 including extinct remains. ' We shall, when we come to our 

 chapter on Geology, have to refer again to this subject, and I 

 think we shall then sec that the diagram throws light on the 

 affinities of extinct beings, Avhich, though generally belonging 

 to the same orders, or families, or genera, ■with those now liv- 

 ing, yet are often, in some degree, intermediate in character 

 between existing groups ; and we can understand this fact, for 

 the extinct species lived at very ancient epochs when the 

 branching lines of descent had diverged less. 



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

 explained, to the formation of genera alone. If, in the diagram, 

 we suppose the amount of change represented by each succes- 

 sive group of diverging dotted lines to be great, the forms 

 marked a'* to^J>", those marked i'* and/"'\ and those marked 

 o'* to ni^*, will form three very distinct genera. We shall also 

 have two very distinct genera descended from (1), differing 

 widely from the descendcnts of (A). These two groups of 

 genera will thus form two distinct families, or orders, accord- 

 ing 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 suj^posed to be descended from some still more 

 ancient and unknown form. 



We have seen that in each countrj^ it is the species belong- 

 ing to the larger genera which oftcnest present varieties or in- 

 cipient species. This, indeed, might have been expected ; for 

 as natural selection acts throvigh one form having some advan- 

 tage over other forms in the struggle for existence, it will 

 chiefly act on those which already have some advantage ; and 

 the largeness of any grouji shows that its species have inherited 

 from a connnon ancestor some advantage in common. Hence, 

 the struggle for the production of new and modified descend- 

 ants will mainly lie between the larger groups which are all 

 trying to increase in number. One large grouj) will slowl}' 

 ct)nfjuer another large group, reduce its numbers, and thus les- 

 sen its chance of further variation and improveuKMit. Within 

 the same large group, the later and more highly -perfected su]> 



