|
nearly at the extreme
of the original genus, the six descendants from (I) will, owing to
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
with modification, from two or more species of the same genus. And the two or more parent-species are supposed to
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
a
species, the supposed
of our several
sub-genera and genera. |
|
| It is worth while to reflect for a moment on the character of the new species
→
F
14
,
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.
descended from a form which stood between the
(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
But as these two groups have gone on diverging in character from the type of their parents, the new species
→
(F
14
)
not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to
such
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
→more generations; it may also represent
a section of the successive strata of the
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,
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
epochs when the branching lines of descent had diverged
|
