144 RESULTS OP 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 modifica- 

 tion 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^ to a^" 

 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 any fresh branches or races. 



After ten thousand generations, species (A) is sup- 

 posed to have produced three forms, a^°, f^", and m^°, 

 which, from having diverged in character during the suc- 

 cessive generations, will have come to differ largely, but 

 perhaps unequally, from each other and from their com- 

 mon parent. If we suppose the amount of change be- 

 tween each horizontal line in our diagram to be exces- 

 sively 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 last into well-defined species. Thus the 

 diagram illustrates the steps by which the small differ- 

 ences 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^* and m^*, all descended from (A). Thus as 

 I believe, species are multiplied and genera are formed. 



