time, would have gone far towards suggesting
the answer, which was in fact delayed for more than half a century. I
refer to Cuvier's investigation of the mammalian fossils yielded by
the quarries in the older tertiary rocks of Montmartre, among the chief
results of which was the bringing to light of two genera of extinct
hoofed quadrupeds, the _Anoplotherium_ and the _Palaeotherium._ The rich
materials at Cuvier's disposition enabled him to obtain a full
knowledge of the osteology and of the dentition of these two forms, and
consequently to compare their structure critically with that of existing
hoofed animals. The effect of this comparison was to prove that the
_Anoplotherium,_ though it presented many points of resemblance with the
pigs on the one hand and with the ruminants on the other, differed from
both to such an extent that it could find a place in neither group. In
fact, it held, in some respects, an intermediate position, tending to
bridge over the interval between these two groups, which in the existing
fauna are so distinct. In the same way, the _Palaeotherium_ tended to
connect forms so different as the tapir, the rhinoceros, and the horse.
Subsequent investigations have brought to light a variety of facts of
the same order, the most curious and striking of which are those
which prove the existence, in the mesozoic epoch, of a series of forms
intermediate between birds and reptiles--two classes of vertebrate
animals which at present appear to be more widely separated than any
others. Yet the interval between them is completely filled, in the
mesozoic fauna, by birds which have 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
hi