92 RESULTS OF THE ACTION OF [Chap. IT, 



that the most divergent varietiss 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 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 num- 

 bered 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 inter- 

 vals 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 tond 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 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 

 modification Avill 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 he represented in the diagram, if all the lines pro- 

 ceeding 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 w , 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 be- 

 tween 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 



