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to suggest that the general level of the sea is constant； while
that of the solid land fluctuates up and down through thousands
of feet in a secular ground swell； it may well have appeared far
less hazardous to conceive that fossils are sports of nature
than to accept the necessary alternative； that all the inland
regions and highlands； in the rocks of which marine shells had
been found； had once been covered by the ocean。 It is not so
surprising； therefore； as it may at first seem； that although
such men as Leonardo da Vinci and Bernard Palissy took just
views of the nature of fossils； the opinion of the majority of
their contemporaries set strongly the other way； nor even that
error maintained itself long after the scientific grounds of the
true interpretation of fossils had been stated； in a manner that
left nothing to be desired； in the latter half of the
seventeenth century。 The person who rendered this good service
to palaeontology was Nicolas Steno； professor of anatomy in
Florence； though a Dane by birth。 Collectors of fossils at that
day were familiar with certain bodies termed 〃glossopetrae；〃 and
speculation was rife as to their nature。 In the first half of
the seventeenth century； Fabio Colonna had tried to convince his
colleagues of the famous Accademia dei Lincei that the
glossopetrae were merely fossil sharks' teeth； but his arguments
made no impression。 Fifty years later； Steno re…opened the
question； and； by dissecting the head of a shark and pointing
out the very exact correspondence of its teeth with the
glossopetrae； left no rational doubt as to the origin of the
latter。 Thus far； the work of Steno went little further than
that of Colonna； but it fortunately occurred to him to think out
the whole subject of the interpretation of fossils； and the
result of his meditations was the publication； in 1669； of a
little treatise with the very quaint title of 〃De Solido intra
Solidum naturaliter contento。〃 The general course of Steno's
argument may be stated in a few words。 Fossils are solid bodies
which； by some natural process； have come to be contained within
other solid bodies； namely； the rocks in which they are
embedded； and the fundamental problem of palaeontology； stated
generally； is this： 〃Given a body endowed with a certain shape
and produced in accordance with natural laws； to find in that
body itself the evidence of the place and manner of its
production。〃 The only way of solving this problem is by the
application of the axiom that 〃like effects imply like causes；〃
or as Steno puts it； in reference to this particular case； that
〃bodies which are altogether similar have been produced in the
same way。〃 Hence； since the glossopetrae are altogether
similar to sharks' teeth； they must have been produced by
sharklike fishes； and since many fossil shells correspond； down
to the minutest details of structure； with the shells of
existing marine or freshwater animals； they must have been
produced by similar animals； and the like reasoning is applied
by Steno to the fossil bones of vertebrated animals； whether
aquatic or terrestrial。 To the obvious objection that many
fossils are not altogether similar to their living analogues；
differing in substance while agreeing in form； or being mere
hollows or impressions； the surfaces of which are figured in the
same way as those of animal or vegetable organisms； Steno
replies by pointing out the changes which take place in organic
remains embedded in the earth； and how their solid substance may
be dissolved away entirely； or replaced by mineral matter； until
nothing is left of the original but a cast； an impression； or a
mere trace of its contours。 The principles of investigation thus
excellently stated and illustrated by Steno in 1669； are those
which have； consciously or unconsciously； guided the researches
of palaeontologists ever since。 Even that feat of palaeontology
which has so powerfully impressed the popular imagination； the
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
horse's grinder may be as sure that it had one complete toe on
each foot and did not ruminate； but if ruminants and horses were
extinct animals of which nothing but the grinders had ever been
discovered； no amount of physiological reasoning could have
enabled us to reconstruct either animal； still less to have
divined the wide differences between the two。 Cuvier； in the
〃Discours sur les Revolutions de la Surface du Globe；〃 strangely
credits himself； and has ever since been credited by others；
with the invention of a new method of palaeontological research。
But if you will turn to the 〃Recherches sur les Ossemens
Fossiles〃 and watch Cuvier； not speculating； but working； you
will find that his method is neither more nor less than that of
Steno。 If he was able to make