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
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become extinct, having been replaced by eight new species (_a_14 to
_m_14); and (I) will have been replaced by six (_n_14 to _z_14) new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.
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