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.
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