The Lecturer commenced by referring to a short essay by Göthethe last which proceeded from his pencontaining a critical account of a discussion bearing upon the doctrine of the Unity of Organization of Animals, which had then (1830) just taken place in the French Academy. Göthe said that, for him, this controversy was of more importance than the Revolution of July which immediately followed ita declaration which might almost be regarded as a prophecy; for while the Charte and those who established it have vanished as though they had never been, the Doctrine of Unity of Organization retains a profound interest and importance for those who study the science of life.
It would be the object of the Lecturer to explain, how the controversy, in question arose, and to show what ground of truth was common to the combatants.
The variety of Forms of Animals is best realised, perhaps, by reflecting, that there are certainly 200,000 species, and that each species is, in its zoological dignity, not the equivalent of a family or a nation of men, merely, but of the whole Human Race. It would be hopeless to attempt to gain a knowledge of these forms, therefore, if it were not possible to discover points of similarity among large numbers of them, and to classify them into groups,one member of which might be taken to represent the whole. A rough practical classification, based on obvious resemblances, is as old as language itself; and the whole purpose of Zoology and Comparative Anatomy has consisted chiefly in giving greater exactness to the definition and expression of these intuitive perceptions of resemblance.
 The Lecturer proceeded to show how the celebrated Camper illustrated these resemblances of the organs of animals, by drawing the arm of a man, and then by merely altering the proportions of its constituent parts, converting it into a bird's wing, a horse's foreleg, &c. &c. Organs which can in this way be shown to grade into one another, are said to be the same organs, or in anatomical phraseology are Homologous:and by thus working out the homologies of all the organs of the Vertebrate class, Geoffroy, Oken, and Owen,to the last of whom, we are indebted for, by far, the most elaborate and logical development of the doctrine,have demonstrated the homology of all the parts of the Vertebrata, or in other words, that there is a common plan on which all those animals which possess back-bones are constructed.
Precisely the same result has been arrived at, by the same methods, in another great division of the Animal Kingdomthe Annulosa. As an illustration, the Lecturer showed how the parts of the mouth of all insects were modifications of the same elements, and briefly sketched the common plan of the Annulosa, as it may be deduced from the investigations of Savigny, Audouin, Milne-Edwards, and Newport.
Leaving out of consideration (for want of time merely), the Radiate animals, and passing to the remaining great division, the Mollusca,it appears that the same great principle holds good even for these apparently unsymmetrical and irregular creatures: and the Lecturer, after referring to the demonstration of the common plan upon which those Mollusks possessing heads are constructed,which he had already given in the Philosophical Transactions,stated that he was now able to extend that plan to the remaining orders, and briefly explained in what way the 'Archetypal Mollusk' is modified in the Lamellibranchs, Brachiopoda, Tunicata, and Polyzoa.
We have then a common plan of the Vertebrata, of the Articulata, of the Mollusca, and of the Radiata,and to come to the essence of the controversy in the Académie des Sciencesare all these common plans identical, or are they not?
Now if we confine ourselves to the sole method which Cuvier admittedthe method of the insensible gradation of formsthere can be doubt that the Vertebrate, Annulose, and Molluscan plans are sharply and distinctly marked off from one another, by very definite characters; and the existence of any common plan, of which they are modifications, is a purely hypothetical assumption, and may or may not be true. But is there any other method of ascertaining a community of plan beside the method of Gradation?
 The Lecturer here drew an illustration from Philologya science which in determining the affinities of words also employs the method of gradation. Thus unus, uno, un, one, ein, are said to be modifications of the same word, because they pass gradually into one another. So Hemp, Hennep, Hanf, and Cannabis, Canapa, Chanvreare respectively modifications of the same word; but suppose we wish to make out what, if any, affinity exits between Hemp and Cannabis the method of gradations fails us. It is only by all sorts of arbitrary suppositions that one can be made to pass into the other.
Nevertheless modern Philology demonstrates that the words are the same, by a reference to the independently ascertained laws of change and substitution for the letters of corresponding words, in the Indo-Germanic tongues: by showing in fact, that though these words are not the same, yet they are modifications by known developmental laws of the same root.
Now Von Bär has shown that the study of development has a precisely similar bearing upon the question of the unity of organization of animals. He indicated, in his masterly essays published five and twenty years ago, that though the common plans of the adult forms of the great classes are not identical, yet they start in the course of their development from the same point. And the whole tendency of modern research is to confirm his conclusion.
If then, with the advantage of the great lapse of time and progress of knowledge, we may presume to pronounce judgment where Cuvier and Geoffroy St. Hilaire were the litigantsit may be said that Geoffroy's inspiration was true, but his mode of working it out false. An insect is not a vertebrate animal, nor are its legs free ribs. A cuttlefish is not a vertebrate animal doubled up. But there was a period in the development of each, when insect, cuttlefish, and vertebrate were undistinguishable and had a Common Plan.
The Lecturer concluded by remarking, that the existence of hotly controverted questions between men of knowledge, ability, and especially of honesty and earnestness of purpose, such as Cuvier and his rival were, is an opprobrium to the science which they profess. He would feel deeply rewarded if he had produced in the minds of his hearers the conviction that these two great menfriends as they were to one anotherneed not be set in scientific opposition; that they were both true knights doing battle for science; but that as the old story runs, each came by his own road to a different side of the shield.