soning from a shell, or
a tooth, or a bone, to the nature of the animal to which it belonged,
rests absolutely on the assumption that the likeness of this shell,
or tooth, or bone, to that of some animal with which we are already
acquainted, is such that we are justified in inferring a corresponding
degree of likeness in the rest of the two organisms. It is on this
very simple principle, and not upon imaginary laws of physiological
correlation, about which, in most cases, we know nothing whatever, that
the so-called restorations of the palaeontologist are based.

Abundant illustrations of this truth will occur to every one who is
familiar with palaeontology; none is more suitable than the case of the
so-called _Belemnites._ In the early days of the study of fossils,
this name was given to certain elongated stony bodies, ending at one
extremity in a conical point, and truncated at the other, which were
commonly reputed to be thunderbolts, and as such to have descended from
the sky. They are common enough in some parts of England; and, in the
condition in which they are ordinarily found, it might be difficult to
give satisfactory reasons for denying them to be merely mineral bodies.

They appear, in fact, to consist of nothing but concentric layers
of carbonate of lime, disposed in subcrystalline fibres, or prisms,
perpendicular to the layers. Among a great number of specimens of these
Belemnites, however, it was soon observed that some showed a conical
cavity at the blunt end; and, in still better preserved specimens, this
cavity appeared to be divided into chambers by delicate saucer-shaped
partitions, situated at regular intervals one above the other. Now there
is no mineral body which presents any structure comparable to this, and
the conclusion suggested itself that the Belemnites must be the effects
of causes other than those which are at work in inorganic nature. On
close examination, the saucer-shaped partitions were proved to be all
perforated at one point, and the perforations being situated exactly
in the same line, the chambers were seen to be traversed by a canal, or
_siphuncle,_ which thus connected the smallest or aphical chamber with
the largest. There is nothing like this in the vegetable world; but an
exactly corresponding structure is met with in the shells of two kinds
of existing animals, the pearly _Nautilus_ and the _Spirula,_ and
only in them. These animals belong to the same division--the
_Cephal