282 ON THE COMMON PLAN OF ANIMAL FORMS 



The Lecturer proceeded to show how the celebrated Camper 

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

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

 constituent 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 phrase- 

 ology are HomologoiLs : — 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 Kingdom — the 

 Aiiiiulosa. 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) \h.& 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 

 Laindlibrandis, Brachiopoda, Tunicata, and Polysoa. 



We have then a common plan of the Vertebrata, of the Artieulata, 

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

 the controversy in the Academic des Sciences — are all these common 

 plans identical, or are they not ? 



Now if v.'e confine ourselves to the sole method which Cuvier 

 admitted — the method of the insensible gradation of forms — there 

 can be no 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 h3'pothetical 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 t 



