144 RESULTS OF THE ACTION OF [Chap. IV. 



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 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 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 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 anv fresh branches or races. 



After ten thousand generations, species (A) is supposed 

 to have produced three forms, a 10 , f 10 , and ??i 10 , winch, 

 from having diverged in character during the successive 



o o o 



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 varieties are increased into the larger 

 differences distinguishing species. By continuing the 

 same process for a greater number of generations (as 

 shown in the diagram in a condensed and simplified 

 manner), we get eight species, marked by the letters 

 between a u and m l4: , all descended from (A). Thus, as 

 I believe, species are multiplied and genera are formed. 



