OP NATURAL SELECTION. 105 



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 modification to be more 

 numerous or greater in amount, to conveit 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 differ 

 ences distinguishing species. By continuing the same pro- 

 cess 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 u , all 

 descended from (A). Thus, as I believe, species are multi- 

 plied, 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 thou- 

 sand generations, either two well-marked varieties (w 1Q and 

 £ 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 n u to z li , 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 

 greatest 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 extreme 

 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 original 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. 



But during the process of modification, represented in the 

 diagram, another of our principles, namely that of extinction, 

 will have played an important part. As in each fully 

 stocked country natural selection necessarily acts by the 

 selected form having some advantage in the struggle for life 

 over other forms, there will be a constant tendency in the 

 improved descendants of any one species to supplant and 

 exterminate in each stage of descent their predecessors and 

 their original progenitor. For it should be remembered that 

 the competition will generally be most severe between those 

 forms which are most nearly related to each other in habits. 



