120 
NATUEAL SELECTION. 
Chap. IV. 
of divergent modification may have been increased in the 
successive generations. This case would be represented 
in the diagram, if all the lines proceeding from (A) 
were removed, excepting that from to In the 
same way, for instance, the English race-horse and 
English pointer have apparently both gone on slowly 
diverging in character from their original stocks, with¬ 
out either having given off any fresh branches or races. 
After ten thousand generations, species (A) is supposed 
to have produced three forms, a and m which, 
from having diverged 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 excessively 
small, these three forms may still be only well-marked 
varieties; or they may have arrived at the doubtful 
category of sub-species; 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 well-defined species: thus the diagram 
illustrates the steps by which the small differences 
distinguishing 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 simplified 
manner), we get eight species, marked by the letters 
between a and m 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 and or two species, according 
to the amount of change supposed to be represented be- 
