reptilian characters, on the one side, and reptiles which have
ornithic characters, on the other. So again, while the group of
fishes, termed ganoids, is, at the present time, so distinct
from that of the dipnoi, or mudfishes, that they have been
reckoned as distinct orders, the Devonian strata present us with
forms of which it is impossible to say with certainty whether
they are dipnoi or whether they are ganoids.

Agassiz's long and elaborate researches upon fossil fishes,
published between 1833 and 1842, led him to suggest the
existence of another kind of relation between ancient and modern
forms of life. He observed that the oldest fishes present
many characters which recall the embryonic conditions of
existing fishes; and that, not only among fishes, but in several
groups of the invertebrata which have a long palaeontological
history, the latest forms are more modified, more specialised,
than the earlier. The fact that the dentition of the older
tertiary ungulate and carnivorous mammals is always complete,
noticed by Professor Owen, illustrated the same generalisation.

Another no less suggestive observation was made by Mr. Darwin,
whose personal investigations during the voyage of the
Beagle led him to remark upon the singular fact, that the
fauna, which immediately precedes that at present existing in
any geographical province of distribution, presents the same
peculiarities as its successor. Thus, in South America and in
Australia, the later tertiary or quaternary fossils show that
the fauna which immediately preceded that of the present day
was, in the one case, as much characterised by edentates and, in
the other, by marsupials as it is now, although the species of