Royal Institution. 73 



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 con- 

 troversy in question arose, and to show what ground of truth was 

 common to the combatants. 



The variety of Forms of Animals is best realized, perhaps, by re- 

 flecting, 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 illus- 

 trated these resemblances of the organs of animals, by drawing the 

 arm of a man, and then by merely altering the proportions of its con- 

 stituent parts, converting it into a bird's wing, a horse's fore-leg, 

 &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, Geoffrey, Oken, and Uwen,— to 

 the last of whom we are indebted for by far the most elaborate and 

 logical development of the doctrine, — have demonstrated the homo- 

 logy 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 Kingdom— the 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 Ra- 

 diate 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 bad 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 Articutafa, 

 of the Mollusca, and of the Iiadiata, — and to come to the essence of 



