130 ORIGIN OF SPECIES 



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 sup- 

 posed to be represented between the horizontal lines. After 

 fourteen thousand generations, six new species, marked by 

 the letters n" 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 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 choeen 

 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 im- 

 proved descendants of any one species to supplant and ex- 

 terminate 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 quite new station, in which oflfspring and 



