The Rise and Progress of Palaeontology
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reconstruction of an extinct animal from a tooth or a bone, is
based upon the simplest imaginable application of the logic of
Steno. A moment's consideration will show, in fact, that Steno's
conclusion that the glossopetrae are sharks' teeth implies the
reconstruction of an animal from its tooth. It is equivalent to
the assertion that the animal of which the glossopetrae are
relics had the form and organisation of a shark; that it had a
skull, a vertebral column, and limbs similar to those which are
characteristic of this group of fishes; that its heart, gills,
and intestines presented the peculiarities which those of all
sharks exhibit; nay, even that any hard parts which its
integument contained were of a totally different character from
the scales of ordinary fishes. These conclusions are as certain
as any based upon probable reasonings can be. And they are so,
simply because a very large experience justifies us in believing
that teeth of this particular form and structure are invariably
associated with the peculiar organisation of sharks, and are
never found in connection with other organisms. Why this should
be we are not at present in a position even to imagine; we must
take the fact as an empirical law of animal morphology, the
reason of which may possibly be one day found in the history of
the evolution of the shark tribe, but for which it is hopeless
to seek for an explanation in ordinary physiological reasonings.
Every one practically acquainted with palaeontology is aware
that it is not every tooth, nor every bone, which enables us to
form a judgment of the character of the animal to which it
belonged; and that it is possible to possess many teeth, and
even a large portion of the skeleton of an extinct animal, and
yet be unable to reconstruct its skull or its limbs. It is only
when the tooth or bone presents peculiarities, which we know by
previous experience to be characteristic of a certain group,
that we can safely predict that the fossil belonged to an animal
of the same group. Any one who finds a cow's grinder may be
perfectly sure that it belonged to an animal which had two
complete toes on each foot and ruminated; any one who finds a