NATURAL SELECTION 109 



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 10 ,/ 10 , 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 

 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 (w 10 and z 10 ) or two species, according 

 to the amount of change supposed to be represented 

 between the horizontal lines. After fourteen thousand 

 generations, six new species, marked by the letters ra 14 

 to z u , are supposed to have been produced. In each 

 genus, the species, which are already extremely dif- 

 ferent in character, will generally tend to produce the 

 greatest number of modified descendants ; for these 

 will have the best chance of filling new and widely 

 different places in the polity of nature : hence in the 

 diagram I have chosen the extreme species (A), and the 

 nearly extreme species (I), as those which have largely 



