170 THE ORIGIN OF SPECIES 



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 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. 



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 (ly'" 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 species, marked by the letters w" to 

 g'*, are supposed to have been produced. In any genus, 

 the species which are already very different in character 

 from each other will generally tend to produce the great- 

 est number of modified descendants; for these will have 

 the best chance of seizing on new and widely different 

 places in the polity of nature: hence in the diagram I 

 have chosen the extreme species (A), and the nearly ex- 

 treme species (I), as those which have largely varied, 

 and have given rise to new varieties and species. The 

 other nine species (marked by capital letters) of our orig- 

 inal genus may for long but unequal periods continue to 

 transmit unaltered descendants; and this is shown in the 

 diagram by the dotted lines unequally prolonged upward. 



