104 RESULT OF THE ACTION 



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 relations. 

 But as a general rule, the more diversified in structure the 

 descendants from any one species can be rendered, the *. re 

 places they will be enabled to seize on, and the more heir 

 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 any- 

 where, 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 

 nunjber 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 dia- 

 gram by some of the lower branches not reaching to the 

 upper horizontal lines. In some cases no doubt the process 

 of modification will be confined to a single line of descent, 

 and the number of modified descendants will not be in- 

 creased ; 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 apparently 

 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 , f 10 , and m 10 , which, from 

 having diverged in character during the successive genera- 

 tions, will have come to differ largely, but perhaps unequally, 

 from each other and from their common parent. If we 

 suppose the amount of change between each horizontal line 



