tted
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 improved descendants of any one species to supplant and exterminate
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 a 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
offspring and progenitor 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, being replaced by eight new species (a14 to m14); and species
(I) will be replaced by six (n14 to z14) 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. It seems, therefore,
extremely probable that they will have taken the pl