[100] Règne A. 61.
[2nd Ed.] [The question of the Classification of Animals is discussed in the first of Prof. Owen’s Lectures on the Invertebrate [480] Animals (1843). Mr. Owen observes that the arrangement of animals into Vertebrate and Invertebrate which prevailed before Cuvier, was necessarily bad, inasmuch as no negative character in Zoology gives true natural groups. Hence the establishment of the sub-kingdoms, Mollusca, Articulata, Radiata, as co-ordinate with Vertebrata, according to the arrangement of the nervous system, was a most important advance. But Mr. Owen has seen reason to separate the Radiata of Cuvier into two divisions; the Nematoneura, in which the nervous system can be traced in a filamentary form (including Echinoderma, Ciliobrachiata, Cœlelmintha, Rotifera,) and the Acrita or lowest division of the animal kingdom, including Acalepha, Nudibrachiata, Sterelmintha, Polygastria.] ~Additional material in the [3rd edition].~
Sect. 3.—Attempts to establish the Identity of the Types of Animal Forms.
Supposing this great step in Zoology, of which we have given an account,—the reduction of all animals to four types or plans,—to be quite secure, we are then led to ask whether any further advance is possible;—whether several of these types can be referred to one common form by any wider effort of generalization. On this question there has been a considerable difference of opinion. Geoffroy Saint-Hilaire,[101] who had previously endeavored to show that all vertebrate animals were constructed so exactly upon the same plan as to preserve the strictest analogy of parts in respect to their osteology, thought to extend this unity of plan by demonstrating, that the hard parts of crustaceans and insects are still only modifications of the skeleton of higher animals, and that therefore the type of vertebrata must be made to include them also:—the segments of the articulata are held to be strictly analogous to the vertebras of the higher animals, and thus the former live within their vertebral column in the same manner as the latter live without it. Attempts have even been made to reduce molluscous and vertebrate animals to a community of type, as we shall see shortly.
[101] Mr. Jenyns, Brit. Assoc. Rep. iv. 150.
Another application of the principle, according to which creatures the most different are developments of the same original type, may be discerned[102] in the doctrine, that the embryo of the higher forms of animal life passes by gradations through those forms which are [481] permanent in inferior animals. Thus, according to this view, the human fœtus assumes successively the plan of the zoophyte, the worm, the fish, the turtle, the bird, the beast. But it has been well observed, that “in these analogies we look in vain for the precision which can alone support the inference that has been deduced;”[103] and that at each step, the higher embryo and the lower animal which it is supposed to resemble, differ in having each different organs suited to their respective destinations.
[102] Dr. Clark, Report, Ib. iv. 113.
[103] Dr. Clark, p. 114.
Cuvier[104] never assented to this view, nor to the attempts to refer the different divisions of his system to a common type. “He could not admit,” says his biographer, “that the lungs or gills of the vertebrates are in the same connexion as the branchiæ of molluscs and crustaceans, which in the one are situated at the base of the feet, or fixed on the feet themselves, and in the other often on the back or about the arms. He did not admit the analogy between the skeleton of the vertebrates and the skin of the articulates; he could not believe that the tænia and the sepia were constructed on the same plan; that there was a similarity of composition between the bird and the echinus, the whale and the snail; in spite of the skill with which some persons sought gradually to efface their discrepancies.”
[104] Laurillard, Elog. de Cuvier, p. 66.