It seems, therefore, to me extremely probable that they will have taken
the places of, and thus exterminated, not only their parents (A) and
(I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will
have transmitted offspring to the fourteen-thousandth generation. We
may suppose that only one (F), of the two species which were least
closely related to the other nine original species, has transmitted
descendants to this late stage of descent.
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species _a_14 and _z_14 will be much greater than that between
the most different of the original eleven species. The new species,
moreover, will be allied to each other in a widely different manner. Of
the eight descendants from (A) the three marked _a_14, _q_14, _p_14,
will be nearly related from having recently branched off from _a_10;
_b_14 and _f_14, from having diverged at an earlier period from _a_5,
will be in some degree distinct from the three first-named species; and
lastly, _o_14, _e_14, and _m_14, will be nearly related one to the
other, but from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six descendants
from (I) will, owing to inheritance, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct,
and have left no descendants. Hence the six new species descended from
(I), and the eight descended from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same genus.
And the two or more
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