OF NA TUBAL SELECTION, 1 1 1 



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 tbe 

 amount of divergent modification may have been augmented. 

 This case would be represented in the diagram, if all the 

 lines proceeding from (A) were removed, excepting that 

 from «! to «!". In the same way the English racehorse 

 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 supposed 

 to have produced three forms, a^^,f^^, 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 hori- 

 zontal 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 modi- 

 fication 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 con- 

 densed 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. 



In a large genus it is probable that more than one sj^e- 

 cies 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 

 {w'^^ and z'^^) or two species, according to the amount of 

 change supposed to be represented between the horizontal 

 lines. After fourteen thousand generations, six new spe- 

 cies, marked by the letters n'^'^ to 2;i*, are supnosed to 

 have been produced. In any genus, the species which are 

 already very ditt'erent in character from each other, will 

 generally tend to produce the greatest number of modified 

 descendants; for these will have the best chance of seizing 



