164 NATURAL SELECTION vu 
mediate 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 Paleontology, p. 284: “A more generalised vertebrate 
structure is illustrated, in the extinct reptiles, by the affinities 
to ganoid fishes, shown by Ganocephala, Labyrinthodontia, and 
Ichthyopterygia ; 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 combina- 
tion of modern crocodilian, chelonian, and lacertian characters 
in the Cryptodontia and the Dicynodontia, and by the com- 
bined lacertian and crocodilian characters in the Thecodontia 
and Sauropterygia.” In the same work he tells us that “the 
Anoplotherium, in several important characters, resembled 
the embryo Ruminant, but retained throughout life those 
marks of adhesion to a generalised mammalian type;” and 
assures us that he has “never omitted a proper opportunity 
for impressing the results of observations showing the more 
generalised structures of extinct as compared with the more 
specialised forms of recent animals.” Modern paleontologists 
have discovered hundreds of examples of these more generalised 
or ancestral types. In the time of Cuvier, the Ruminants 
and the Pachyderms were looked upon as two of the most 
distinct orders of animals; but it is now demonstrated that 
there once existed a variety of genera and species, connecting 
by almost imperceptible grades such widely different animals 
as the pig and the camel. Among living quadrupeds we can 
scarcely find a more isolated group than the genus Equus, 
comprising the horses, asses, and zebras; but through many 
species of Paloplotherium, Hippotherium, and Hipparion, and 
numbers of extinct forms of Equus found in Europe, India, 
and America, an almost complete transition is established with 
