Chap. IY.] NATURAL SELECTION 143 



But I must here remark that I do not suppose that 

 the process ever goes on so regularly as is represented 

 in the diagram, though in itself made somewhat irregular, 

 nor that it goes on continuously ; it is far more probable 

 that each form remains for long periods unaltered, and 

 then again undergoes modification. Nor do I suppose that 

 the most divergent varieties are invariably preserved : 

 a medium form may often long endure, and may or may 

 not produce more than one modified descendant; for 

 natural selection will always act according to the nature 

 of the places which are either unoccupied or not perfectly 

 occupied by other beings ; and this will depend on infi- 

 nitely complex relations. But as a general rule, the 

 more diversified in structure the descendants from any 

 one species can be rendered, the more places they will 

 be enabled to seize on, and the more their modified 

 progeny will increase. In our diagram the line of suc- 

 cession is broken at regular intervals by small numbered 

 letters marking the successive forms which have become 

 sufficiently distinct to be recorded as varieties. But these 

 breaks are imaginary, and might have been inserted any- 

 where, after intervals long enough to allow the accumula- 

 tion of a considerable amount of divergent variation. 



As all the modified descendants from a common and 

 widely-diffused species, belonging to a large genus, will 

 tend to partake of the same advantages which made their 

 parent successful in life, they will generally go on mul- 

 tiplying in number as well as diverging in character : 

 this is represented in the diagram by the several 

 divergent branches proceeding from (A). The modified 

 offspring from the later and more highly improved 

 branches in the lines of descent, will, it is probable, often 



take the place of, and so destroy, the earlier and less 

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