168 THE ORIGIN Of SPECIES 



tions, producing only a single variety, but in a more and 

 more modified condition, some producing two or three 

 varieties, and some failing to produce any. Thus the 

 varieties or modified descendants of the common parent 

 (A) will generally go on increasing in number and 

 diverging in character. In the diagram the process is 

 represented up to the ten-thousandth generation, and 

 under a condensed and simplified form up to the 

 fourteen -thousandth generation. 



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 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 

 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. 



