94 THE ORIGIN OF SPECIES 



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 continu- 

 ously; 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 pre- 

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

 not produce more than one modified descendant; for natural selec- 

 tion 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 relations. But as a gen- 

 eral 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 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 accumulation 

 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 multiplying in number as well as diverg- 

 ing 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. In some cases no doubt the process of modifica- 

 tion will be confined to a single line of descent, and the number of 

 modified descendants will not be increased ; although the amount 

 of divergent modification may have been augmented. This case 

 would be represented in the diagram, if all the lines proceeding 

 from (A) were removed, excepting that from a 1 to a 10 . In the 

 same way the English race-horse and English pointer have appar- 

 ently both gone on slowly diverging in character from their original 

 stocks, without either having given off any fresh branches or races. 



After ten thousand generations, species (A) is supposed to have 

 produced three forms, a 10 , / 10 , and m 10 , which, from having di- 



