128 ORIGL^ OF SPECIES 



If, then, these two varieties be variable, the most divergent 

 of their variations will generally be preserved during the 

 next thousand generations. And after this interval, variety 

 a^ is supposed in the diagram to have produced variety a", 

 which will, owing to the principle of divergence, differ more 

 from (A) than did variety a.\ Variety m^ is supposed to 

 have produced two varieties, namely m^ and /^ differing from 

 each other, and more considerably from their common parent 

 (A). We may continue the process by similar steps for any 

 length of time; some of the varieties, after each thousand 

 generations, 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 varie- 

 ties or modified descendants of the common parent (A), will 

 generally go on increasing in number and diverging in char- 

 acter. In the diagram the process is represented up to the 

 ten-thousandth generation, and under a condensed and sim- 

 plified 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 di- 

 vergent 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 al- 

 ways act according to the nature of the places which are 

 either unoccupied or not perfectly occupied by other beings; 

 and this will depend on infinitely complex relations. But as 

 a general rule, the more diversified in structure the descend- 

 ants 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 succession 

 is broken at regular intervals by small numbered letters mark- 

 ing the successive forms which have become sufficiently dis- 

 tinct to be recorded as varieties. But these breaks are 

 imaginary, and might have been inserted anywhere, after 

 intervals long enough to allow the accumulation of a con- 

 siderable amount of divergent variation. 



