86 RESULTS OF THE ACTION OF [CHAP. IV. 



suppose that the most divergent varieties are invariably preserved : 

 a medium form may often long endure, and may or may not pro- 

 duce 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 

 10 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 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 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 90 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 appa- 

 rently 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 diverged 

 in character during the successive generations, 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 in our diagram to be excessively small, these 

 three forms may still be only well-marked varieties ; but we have 

 only to suppose the steps in the process of modification to be 

 more numerous or greater in amount, to convert these three forms 

 into doubtful or at least into well-defined species. Thus the diagram 

 illustrates the steps by which the small differences distinguishing 



