120 NATUKAL SELECTION. Chap. IV. 



of divergent modification may have been increased in the 

 successive generations. 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, for instance, the English race-horse and 

 English pointer have apparently both gone on slowly 

 diverging in character from their original stocks, with- 

 out 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 ; or they may have arrived at the doubtful 

 category of sub-species ; 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 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 14 and m 14 , all descended from (A). Thus, as I 

 believe, species are multiplied and genera are formed. 



In a large genus it is probable that more than one 

 species would vary. In the diagram I have assumed 

 that a second species (I) has produced, by analogous 

 steps, after ten thousand generations, either two well- 

 marked varieties (tv 10 and z 10 ) or two species, according 

 to the amount of change supposed to be represented be- 



