Chap. VIE.] THEORY OF NATURAL SELECTION. 301 



appropriateness this resemblance as a special difficulty ; 

 or even their resemblance to the head and beak of a bird. 

 The avicularia are believed by Mr. Busk, Dr. Smitt, and 

 Dr. Nitsche — naturalists who have carefully studied this 

 group — to be homologous with the zooids and their cells 

 which compose the zoophyte; the moveable lip or lid 

 of the cell corresponding with the lower and moveable 

 mandible of the avicularium. Mr. Busk, however, does 

 not know of any gradations now existing between a zooid 

 and an avicularium. It is therefore impossible to con- 

 jecture by what serviceable gradations the one could 

 have been converted into the other : but it by no means 

 follows from this that such gradations have not existed. 



As the chelce of Crustaceans resemble in some decree 

 the avicularia of Polyzoa, both serving as pincers, it 

 may be worth while to show that with the former a 

 long series of serviceable gradations still exists. In 

 the first and simplest stage, the terminal segment of a 

 limb shuts down either on the square summit of the broad 

 penultimate segment, or against one whole side; and 

 is thus enabled to catch hold of an object ; but the limb 

 still serves as an organ of locomotion. We next find 

 one corner of the broad penultimate segment slightly 

 prominent, sometimes furnished with irregular teeth; and 

 against these the terminal segment shuts down. By an 

 increase in the size of this projection, with its shape, 

 as well as that of the terminal segment, slightly modified 

 and improved, the pincers are rendered more and more 

 perfect, until we have at last an instrument as efficient 

 as the chelse of a lobster ; and all these gradations can 

 be actually traced. 



Besides the avicularia, the Polyzoa possess curious 

 organs called vibracula. These generally consist of long 



