THEORY OF NATURAL SELECTION 249 



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 chela: of Crustaceans resemble in some degree 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 im- 

 proved, the pincers are rendered more and more perfect, un- 

 til we have at last an instrument as efificient 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 bristles, 

 capable of movement and easily excited. In one species ex- 

 amined by me the vibracula were slightly curved and ser- 

 rated along the outer margin ; and all of them on the same 

 polyzoary often moved simultaneously ; so that, acting like 

 long oars, they swept a branch rapidly across the object- 

 glass of my microscope. When a branch was placed on its 

 face, the vibracula became entangled, and they made violent 

 efforts to free themselves. They are supposed to serve as a 

 defence, and may be seen, as Mr. Busk remarks, "to sweep 

 slowly and carefully over the surface of the polyzoary, re- 

 moving what might be noxious to the delicate inhabitants of 

 the cells when their tentacula are protruded." The avicu- 

 laria, like the vibracula, probably serve for defence, but they 

 ?lso catch and kill small living animals, which it is believed 

 are afterwards swept by the currents within reach of the 

 tentacula of the zooids. Some species are provided with 

 avicularia and vibracula ; some with avicularia alone, and a 

 few with vibracula alone. 



It is not easy to imagine two objects more widely different 



