But the untruth of this method was early perceived by some systematists who insisted that structure was the true foundation on which to erect what they styled a "natural" classification. Hence arose a classifying of classifications into two categories — the artifi cial, for convenience, as a scaffolding for study; and the natural as an expression of real truth. To some extent these lines of thought and work still exist, but the former has been nearly abandoned.
Seekers after truth of relationship by study of structure — forms of organs, morphology increased in number from the early 17th century onward, and their accumulated in formation, published in various partial schemes, together with his own exten sive investigations, gave material for the first grand generalization in zoology — that by Georges Cuvier (1769-1832), whose fame was popularized in this country by the most talented of all his pupils, Louis Agassiz, himself a great investigator and the author of a remarkable "Essay on Classification." Cuvier's " Plans of Structure."-- Cuvier thought himself able to separate animals into four groups, distinguished by four "plans of structure" which had been assigned by the divine Creator, and which were of equal rank. These were: (1) Vertebrata, char acterized by an internal skeleton, an essen tial part of which was a backbone ; it com prised mammals, birds, reptiles (including batrachians) and fishes. (2) Mollusca, char acterized by a massive type of body, without bones; the mollusks, brachiopods, tunicates, etc. (3) Articulata, with bodies composed of ring-like segments; the insects, crustaceans, annelids and spiders. (4) Radiata, character ized by a radial arrangement of all the parts around a vital focus; star-fishes, polyps, worms, and animalcules.
Those four "plans of structure," ordained from the beginning of things, Cuvier regarded as great facts; and Agassiz objected to all those like Leuckart, Vogt and others, who criticized or modified this arrangement, that they were considering too much complexity of structure and losing sight of "plan" or "type." Agassiz pointed out that Cuvier's divisions combined their various subdivisions; and that this was a great step forward, even if he had not the correct measures for all his groups.
"For we must remember," said Agassiz, "that at the time he wrote naturalists were bent upon establishing one continuous, uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals.'" The watchword of their school was "Nature makes no leaps"; they called their system the "Chain of Being." Nevertheless these views were not accepted by all investigators. One after another the leaders in biological science proposed modifica tions, especially as to the Radiata. Ehrenberg in 1836 departed altogether from the Cuvierian notion, and laid down the principle that the type of development is one and the same in all animals from nomad to man; that is, he set aside the idea of ((plan," and erected a classi fication on purely anatomical grounds. That of Owen, a few years later, had a similar basis. Both assembled in their schemes groups that were heterogeneous, and gave varying rank to similar aggregations. The result was that neither was much accepted. Von Siebold, in 1845, added to Cuvier's four plans or types of structure two more —Zoophyta and Vermes — asserting that they had equal rank with the others, and Leuckart accepted them. Later Von Baer, the great embryologist, offered a classi fication based on mode of development from an egg to maturity, and other embryological systems were made by Van Beneden, Rolliker and Vogt.
All these men multiplied facts, cleared up subordinate relationships, and, by the different angles from which the subject was viewed, broadened knowledge immensely. The effort of all was to find a standard that could be applied uniformly, and would necessarily reveal the true place in nature of every living thing. If a perfect order in organic nature existed, then approaches from the point of view of the embryologist and from the point of view of the anatomist ought to arrive at the same result the real condition — the truth. Thus far it had not done so except in favorable cases. One method would logically assign an animal, a plant or a well-defined group to a certain place in the system which would be quite negatived by other considerations.