Chap. IV.] NATURAL SELECTION. 93 



into doubtful or at last into well-defined species. Thus the diagram 

 illustrates the steps by which the small differences distinguishing 

 varieties are increased into the larger differences distinguishing spe- 

 cies. By continuing the same process for a greater number of gene- 

 rations (as shown in the diagram in a condensed and simplified 

 manner), we get eight species, marked by the letters between a H 

 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 hori- 

 zontal lines. After fourteen thousand generations, six new species, 

 marked by the letters n u to z u , are supposed to have been produced. 

 In any genus, the species which are already very different in cha- 

 racter 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 de- 

 scendants ; and this is shown in the diagram by the dotted lines 

 unequally prolonged upwards. 



But during the process of modification, represented in the dia- 

 gram, 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 advan- 

 tage in the struggle for life over other forms, there will be a constant 

 tendency in the improved descendants of any one species to sup- 

 plant 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, constitution, 

 and structure. Hence all the intermediate forms between the earlier 

 and later states, that is between the less and more improved states 

 of the same species, as well as the original parent-species itself, will 

 generally tend to become extinct. So it probably will be with many 

 whole collateral lines of descent, which will be conquered by later 

 and improved lines. If, however, the modified offspring of a species 

 get into some distinct country, or become quickly adapted to some 



