of the radiated zoophytes is of a nature quite different from all the rest, and approaching, as we have suggested, to the kind of Symmetry found in plants. Some naturalists have doubted whether these zoophytes are not referrible to two types (acrita or polypes, and true radiata,) rather than to one. This fourfold division was introduced by Cuvier, 10 Before him, naturalists followed Linnæus, and divided non-vertebrate animals into two classes, insects and worms. 'I began,' says Cuvier, 'to attack this view of the subject, and offered another division, in a Memoir read at the Society of Natural History of Paris, the 21st of Floreal, in the year III. of the Republic, (May 10, 1795) printed in the Decade Philosophique: in this, I mark the characters and the limits of molluscs, insects, worms, echinoderms, and zoophytes. I distinguished the red-blooded worms or annelides, in a Memoir read to the Institute, the 11th Nivose, year X. (December 31, 1801). I afterwards distributed these different classes into three branches, each co-ordinate to the branch formed by the vertebrate animals, in a Memoir read to the Institute in July, 1812, printed in the Annales du Muséum d'Histoire Naturelle, tom. xix.' His great systematic work, the Règne Animal, founded on this distribution, was published in 1817; and since that time the division has been commonly accepted among naturalists. [2nd Ed.] [The question of the Classification of Animals is discussed in the first of Prof. Owen's Lectures on the Invertebrate 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 10 Règne An. 61. 9 Brit. Assoc. Rep. iv. 227. divisions; the Nematoneura, in which the nervous system can be traced in a filamentary form (including Echinoderma, Ciliobrachiata, Calelmintha, Rotifera,) and the Acrita or lowest division of the animal kingdom, including Acalepha, Nudibrachiata, Sterelmintka, Polygastria.] 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,11 who had previously endeavoured 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 vertebrae 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. Another application of the principle, according to which creatures the most different are developements of the same original type, may be discerned12 in the doctrine, that the embryo of the higher forms of animal life passes by gradations through those forms 11 Mr. Jenyns, Brit. Assoc. Rep. iv. 150. which are permanent in inferior animals. Thus, according to this view, the human foetus 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;'13 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. Cuvier 14 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 branchia 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.' Whether it may be possible to establish, among the four great divisions of the 'Animal Kingdom,' some analogies of a higher order than those which prevail within each division, I do not pretend to conjecture. If this can be done, it is clear that it must be by comparing the types of these divisions under their most general forms: and thus Cuvier's arrangement, so far as it is itself rightly founded on the unity of composition of each branch, is the surest step to the discovery of a unity pervading and uniting these branches. But though those who generalize surely, and those who generalize rapidly, may travel in the same direction, they soon separate so widely, that they 14 Laurillard, Elog. de Cuvier, p. 66. 13 Dr. Clark, p. 114. appear to move from each other. The partisans of a universal unity of composition' of animals, accused Cuvier of being too inert in following the progress of physiological and zoological science. Borrowing their illustration from the political parties of the times, they asserted that he belonged to the science of the resistance, not to the science of the movement. Such a charge was highly honourable to him; for no one acquainted with the history of zoology can doubt that he had a great share in the impulse by which the 'movement' was occasioned; or that he himself made a large advance with it; and it was because he was so poised by the vast mass of his knowledge, so temperate in his love of doubtful generalizations, that he was not swept on in the wilder part of the stream. To such a charge, moderate reformers, who appreciate the value of the good which exists, though they try to make it better, and who know the knowledge, thoughtfulness, and caution, which are needful in such a task, are naturally exposed. For us, who can only decide on such a subject by the general analogies of the history of science, it may suffice to say, that it appears doubtful whether the fundamental conceptions of affinity, analogy, transition, and developement, have yet been fixed in the minds of physiologists with sufficient firmness and clearness, or unfolded with sufficient consistency and generality, to make it likely that any great additional step of this kind can for some time be made. We have here considered the doctrine of the identity of the seemingly various types of animal structure, as an attempt to extend the correspondencies which were the basis of Cuvier's division of the animal kingdom. But this doctrine has been put forward in another point of view, as the antithesis to the doctrine of final causes. This question is so important a one, that we cannot help attempting to give some view of its state and bearings. CHAPTER VIII. THE DOCTRINE OF FINAL CAUSES IN PHYSIOLOGY. Sect. 1.-Assertion of the Principle of Unity of Plan. W E have repeatedly seen, in the course of our historical view of Physiology, that those who have studied the structure of animals and plants, have had a conviction forced upon them, that the organs are constructed and combined in subservience to the life and functions of the whole. The parts have a purpose, as well as a law;- -we can trace Final Causes, as well as Laws of Causation. This principle is peculiar to physiology; and it might naturally be expected that, in the progress of the science, it would come under special consideration. This accordingly has happened; and the principle has been drawn into a prominent position by the struggle of two antagonist schools of physiologists. On the one hand, it has been maintained that this doctrine of final causes is altogether unphilosophical, and requires to be replaced by a more comprehensive and profound principle: on the other hand, it is asserted that the doctrine is not only true, but that, in our own time, it has been fixed and developed so as to become the instrument of some of the most important discoveries which have been made. Of the views of these two schools we must endeavour to give some account. The disciples of the former of the two schools express their tenets by the phrases unity of plan, unity of composition; and the more detailed developement of these doctrines has beem termed the Theory of Analogies, by Geoffroy Saint-Hilaire, who claims this theory as his own creation. According to this theory, the structure and functions of animals are to be studied by the guidance of their analogy only; our attention is to be turned, not to the fitness of the organization for any |