Chap. IV. 
DIVERGENCE OF CHARACTER. 
121 
tween the horizontal lines. After fourteen thousand 
generations, six new species, marked by the letters n 
to z are supposed to have been produced. In each 
genus, the species, which are already extremely dif¬ 
ferent in character, will generally tend to produce the 
greatest number of modified descendants; for these 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 a long period continue to 
transmit unaltered descendants; and this is shown in 
the diagram by the dotted lines not prolonged far up¬ 
wards from want of space. 
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 con¬ 
stant tendency in the improved descendants of any one 
species to supplant and exterminate in each stage of 
descent their predecessors and their original parent. 
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, constitu¬ 
tion, and structure. Hence all the intermediate forms 
between the earlier and later states, that is between the 
less and more improved state of a 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 of descent. If, however, the 
