NATURAL SELECTION 95 



verged in character during the successive generations, will have 

 come to differ largely, but perhaps unequally, from each other 

 and from their common parent. If we suppose the amount of 

 change between each horizontal line in our diagram to be exces- 

 sively 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 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 dif- 

 ferences 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 14 and w 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, ac- 

 cording to the amount of change supposed to be represented be- 

 tween the horizontal lines. After fourteen thousand generations, 

 six new species, marked by the letters » 14 to z 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 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 vari- 

 eties and species. The other nine species (marked by capital let- 

 ters) of our original genus, may for long but unequal periods con- 

 tinue 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 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 

 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 prede- 

 cessors 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, con- 



