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 successive group of diverging
dotted lines to be very great, the forms marked _a_14 to _p_14, those
marked _b_14 and _f_14, and those marked _o_14 to _m_14, will form
three very distinct genera. We shall also have two very distinct genera
99