NATURAL SELECTION 169 



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 multi- 

 plying 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 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 re- 

 moved, excepting that from a' to a'". In the same way 

 the English racehorse and English pointer have ap- 

 parently 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 sup- 

 posed to have produced three forms, a'", /'", and m'", 

 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 ex- 

 cessively small, these three forms may still be only well- 

 marked varieties; but we have only to suppose the steps 



—Science — 8 





