Chap. IV.] NATURAL SELECTION. I43 



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 irregu- 

 lar, nor that it goes on continuously; it is far more prob- 

 able that each form remains for long periods unaltered, 

 and then again undergoes modification. Nor do I sup- 

 pose that the most divergent varieties are invariably pre- 

 served: 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 na- 

 ture of the places which are either unoccupied or not per- 

 fectly occupied by other beings; and this will depend on 

 infinitely 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 pro- 

 geny will increase. In our diagram the line of succession 

 is broken at regular intervals by small numbered letters 

 marking the successive forms which have become suffi- 

 ciently 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 accumu- 

 lation 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 diver- 

 gent branches proceeding from (A). The modified off- 

 spring 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 



