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 F^{14}, 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 (F^{14}) 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 {125} 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 descended from (I); and
as these latter two genera, both from continued divergence of ch