w 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 parent-species are supposed to have descended from
some one species of an earlier genus. In our diagram, this is indicated
by the broken lines, beneath the capital letters, converging in
sub-branches downwards towards a single point; this point representing a
single species, the supposed single parent of our several new sub-genera
and genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case, its affinities to the other fourteen
new species will be of a curious and circuitous nature. Having descended
from a form which stood between the two parent-species (A) and (I),
now supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended from these
species. But as these two groups have gone on diverging in character
from the type of their parents, the new species (F14) will not be
directly intermediate between them, but rather between types of the two
groups; and every naturalist will be able to bring some such case before
his mind.

In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when we
come to our chapter on Geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the
same orders, or families, or genera, with those now living, yet are
often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at very ancient epochs when the branching lines of descent had diverged
less.

I see no reason to limit the process of modification, as now explained,
to the formation of genera alone. If, in our diagram, we suppose the
amount of change represented by each success