Ill 



" The Mollusca pass from the condition of the ciliated Monad to 

 that of the shell-less Acephalan, and in like manner either remain 

 to work out the perfections of that stage, or diverge to achieve the 

 development of shells, of a head, of a ventral foot, or of cephalic 

 arms. 



" The vertebrated ovum having manifested its monadiform rela- 

 tions by the spontaneous fission, growth, and multiplication of the 

 primordial nucleated cells, next assumes, by their metamorphosis 

 and primary arrangement, the form and condition of the finless 

 cartilaginous fish, from which fundamental form development ra- 

 diates in as many and diversified directions and extents, and attains 

 more extraordinary heights of complication and perfection than any 

 of the lower secondary types appear to be susceptible of/' 



To the second application of the principle, I must more particularly 

 refer, as the subject on which perhaps Professor Owen's investiga- 

 tions have been more fully and elaborately and systematically carried 

 out, and have exercised a more important and universal influence on 

 these sciences than any other, — I mean the doctrine of Homologies, 

 or the correspondency of parts and of plan in the construction of 

 animals. This had been the subject of close and sharp discussions 

 in the Academy of Sciences between Cuvier and Geoff"roy St. 

 Hilaire, which are summed up by the latter in the ' Principes de 

 Philosophic Zoologique,' published in 1830 ; and it can be no matter 

 of surprise, that with an antagonist so strong in his v/ell-founded 

 reputation, as a great master in science, and so skilful in applying 

 the w^eapons of a severe and sarcastic logic, GeofFroy St. Hilaire 

 should have failed to impress the physiological world with those 

 views which Cuvier objected to, as being based upon a piiori specu- 

 lation. 



The effect of these discussions may be traced in most of the ablest 

 works on Anatomy and Physiology which subsequently appeared, 

 as, e. g. those by Prof. John MuUer, Prof. Wagner, Milne-Edwards, 

 Siebold and Stannius ; and in the ' Outline of the Animal Kingdom ' 

 and ' Manual of Comparative Anatomy ' by Professor Jones of 

 King's College, London. By all these authors, the principle of 

 Unity of Organization, as it has been attempted to be illustrated 

 and applied by Geofiroy St. Hilaire, and by the German Anatomists 

 of the Trancendental School of Schelling, is tacitly or avowedly 

 abandoned. By M. Agassiz it was directly opposed. 



Nevertheless, the question whether the principle of a common 

 pattern, or the principle of final causes, or, as Cuvier called it, ' con- 

 ditions of existence,' — I say, which of these two principles, or in 

 what degree both have governed the development of organization of 

 animals — the greatest question which can occupy the philosophical 

 anatomist — was still far from having been satisfactorily decided. 



It enforced itself upon the most serious consideration of Professor 

 Owen, when he was called upon to prepare the Catalogue of the 

 Osteological Collections in the Museum of the Royal College of 

 Surgeons : and the results of this consideration were promulgated 



