and this is shown in the
diagram by the dotted lines not prolonged far upwards 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 constant 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, constitution, 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 {122} modified offspring of a species get into some
distinct country, or become quickly adapted to some quite new station, in
which child and parent do not come into competition, both may continue to
exist.

If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a^{14} to m^{14}); and
(I) will have been replaced by six (n^{14} to z^{14}) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D, than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A) and (I), were also supposed to be very common
and widely diffused species, so that they must originally have had some
advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation, will
probably have inherited some of the same advantages: they have also been
modified and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural economy of
their country.