CHAP. IV.] NATURAL SELECTION. 87 



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 sim- 

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

 a 14 and i 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 2 10 ) or two species, 

 according to the amount of change supposed to be represented 

 between the horizontal lines. After fourteen thousand genera- 

 tions, six new species, marked by the letters n w to 2 14 , 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 dif- 

 ferent 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 upwards. 



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 pre- 

 decessors and their original progenitor. For it should be remem- 

 bered 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 quite new station, in which off- 

 spring and progenitor do not come into competition, both may 

 continue to exist. 



If, then, our diagram be assumed to represent a considerable 

 amount of modification, species (A) and all the earlier varieties 



