Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

298 CREAAT~OV BY LA W.
Intermediate or generalized Forms of extinct Animals,
an indication of Transmutation or Development.
The misconception of this writer illustrates another
point very frequently overlooked. It is an essential
part of Mr. Darwin's theory, that one existing animal
has not been derived from any other existing animal,
but that both are the descendants of a common ancestor, which was at once different from either, but,
in essential characters, intermediate between them both.
The illustration of the duck and the gull is therefore
misleading; one of these birds has not been derived
from the other, but both from a common ancestor.
This is not a mere supposition invented to support the
theory of natural selection, but is founded on a variety
of indisputable facts. As we go back into past time,
and meet with the fossil remains of more and more
ancient races of extinct animals, we find that many
of them actually are intermediate between distinct
groups of existing animals. Professor Owen continually dwells on this fact: he says in his 1" Palaeontology," p. 284': "A more generalized vertebrate
structure is illustrated, in the extinct reptiles, by
the affinities to ganoid fishes, shown by Ganocephala,
Labyrinthodontia, and Icthyopterygia; by the affinities
of the Pterosauria to Birds, and by the approximation
of the Dinosauria to Mammals. (These have been recently shown by Professor Huxley to have more affinity
to Birds.) It is manifested by the combination of
modern crocodilian, chelonian, and lacertian characters