1 10 BBS UL T OF THE A CTION 



more modified condition, some producing two or three 

 varieties, and some failing to produce any. Thus the 

 Tttrieties or modified descendants of the common parent 

 (A), will generally go on increasing in number and diverg- 

 ing in character. In the diagram the process is repre- 

 sented up to the ten-thousandth generation, and under a 

 condensed and sim23lified form up to the fourteen-thous- 

 andth generation. 



But I must here remark that I do not suppose that the 

 process aver goes on so regularly as is represented in the 

 diagj'am, 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 infinitely complex rela- 

 tions. But as a general rule, the more diversified in struc- 

 ture 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 succession 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 

 anywhere, after intervals long enough to allow the accumu- 

 lation of a considerable amount of divergent variation. 



As all the modified descendents from a common and 

 widely-dilfused 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 multi- 

 plying 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 improved branches: 

 this is represented in the diagram by some of the lower 

 branches not reaching to the upper horizontal lines, lu 



