The value of this principle was soon 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 divisions of the animal kingdom are based, but simply upon the general structural idea.  Striking as this statement was, it was coldly received at first by contemporary naturalists:  they could hardly grasp Cuvier’s wide generalizations, and perhaps there was also some jealousy of the grandeur of his views.  Whatever the cause, his principle of classification was not fully appreciated; but it opened a new road for study, and gave us the keynote to the natural affinities among animals.  Lamarck, his contemporary, not recognizing the truth of this principle, distributed the animal kingdom into two great divisions, which he calls Vertebrates and Invertebrates.  Ehrenberg also, at a later period, announced another division under two heads,—­those with a continuous solid nervous centre, and those with merely scattered nervous swellings.[3]