108 ON THE ORIGIN OF SPECIES 



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 

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

 more their modified progeny will be increased. In our 

 diagram the line of succession is broken at regular 

 intervals by small numbered letters marking the suc- 

 cessive forms which have become sufficiently distinct to 

 be recorded as varieties. But these breaks are imagi- 

 nary, and might have been inserted anywhere, after 

 intervals long enough to have allowed the accumulation 

 of a considerable amount of divergent variaticu. 



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 diverging in char- 

 acter : this is represented in the diagram by the several 

 divergent branches proceeding from (A). The modi- 

 fied 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 I do not doubt 

 that the process of modification will be confined to a 

 single line of descent, and the number of the de- 

 scendants will not be increased ; although the amount 

 of divergent modification may have been increased in 

 the successive generations. This case would be repre- 

 sented in the diagram, if all the lines proceeding 

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

 In the same way, for instance, the English race-horse 

 and English pointer have apparently both gone on 

 slowly diverging in character from their original 



