Chap. IV. NATURAL SELECTION. 117 



ill tiie process of modification to be more numerous or j^rcatcr 

 in amount, to convert these three forms into doubtful or at 

 last wcU-denncd species : thus the diao^ram iUustratcs the 

 steps by wliich the small diflerences distinguishing varieties 

 are increased into the larger differences distinguishing species. 

 By continuing the same process for a greater number of gener- 

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

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

 «" and m^\ all descended from (A). Thus, as I believe, spe- 

 cies 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 thou- 

 sand generations, either two well-marked varieties (w'° and z'") 

 or two species, according to the amount of change supposed to 

 be'rcjircsented between the horizontal lines. After fourteen 

 thousand generations, six new species, marked by the letters 

 w'* to z'*, 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 num- 

 ber of modified descendants ; for they 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 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 con- 

 tinue to transmit unaltered descendants ; -and this is shown in 

 tlu; 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 selecti(jn necessarily acts by the selected form 

 having some advantage in the struggle for life over other forms, 

 there will be a constant tondcMicy in the improved descendants 

 of any one species to supplant and exterminate in each stage 

 of descent their pred(X"essors and their original progenitor. 

 For it should be remembered that the competition will gen- 

 erally be most severe between those forms which are most 

 nearly related to each other in habits, constitution, and struc- 

 ture. Hence all the intermediate forms between the earlier 

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

 states of a species, as well as the original parent-species itself. 



