tested by its application to facts
already known, and it was found that animals whose affinities had been
questionable before were now at once referred to their true relations with
other animals by ascertaining whether they were built on one or another of
these plans. Of such plans or structural conceptions Cuvier found in the
whole animal kingdom only four, which he called _Vertebrates_, _Mollusks_,
_Articulates_, and _Radiates_.

With this new principle as the basis of investigation, it was no longer
enough for the naturalist to know a certain amount of features
characteristic of a certain number of animals,--he must penetrate deep
enough into their organization to find the secret of their internal
structure. Till he can do this, he is like the traveller in a strange
city, who looks on the exterior of edifices entirely new to him, but knows
nothing of the plan of their internal architecture. To be able to read in
the finished structure the plan on which the whole is built is now
essential to every naturalist.

There have been many criticisms on this division of Cuvier's, and many
attempts to change it; but though some improvements have been made in the
details of his classification, all departures from its great fundamental
principle are errors, and do but lead us away from the recognition of the
true affinities among animals.

Each of these plans may be stated in the most general terms. In the
_Vertebrates_ there is a vertebral column terminating in a prominent head;
this column has an arch above and an arch below, forming a double internal
cavity. The parts are symmetrically arranged on either side of the
longitudinal axis of the body. In the _Mollusks_, also, the parts are
arranged according to a bilateral symmetry on either side of the body, but
the body has but one cavity, and is a soft, concentrated mass, without a
distinct individualization of parts. In the _Articulates_ there is but one
cavity, and the parts are here again arranged on either side of the
longitudinal axis, but in these animals the whole body is divided from end
to end into transverse rings or joints movable upon each other. In the
_Radiates_ we lose sight of the bilateral symmetry so prevalent in the
other three, except as a very subordinate element of structure; the plan
of this lowest type is an organic sphere, in which all parts bear definite
relations to a vertical axis.

It is not upon any special features, then, that these largest divi