Chap. VII.] THEORY OF NATURAL SELECTION. 303 



cell. Hence we can understand how it is that these 

 organs graduate in some cases, as I am informed by 

 Mr. Busk, into each other. Thus with the avicularia 

 of several species of Lepralia, the moveable mandible 

 is so much produced and is so like a bristle, that the 

 presence of the upper or fixed beak alone serves to de- 

 termine its avicularian nature. The vibracula may 

 have been directly developed from the lips of the cells, 

 without having passed through the avicularian stage; 

 but it seems more probable that they have passed 

 through this stage, as during the early stages of the 

 transformation, the other parts of the cell with the in- 

 cluded zooid could hardly have disappeared at once. 

 In many cases the vibracula have a grooved support at 

 the base, which seems to represent the fixed beak; 

 though this support in some species is quite absent. 

 This view of the development of the vibracula, if trust- 

 worthy, is interesting; for supposing that all the species 

 provided with avicularia had become extinct, no one 

 with the most vivid imagination would ever have thought 

 that the vibracula had originally existed as part of an 

 organ, resembling a bird's head or an irregular box or 

 hood. It is interesting to see two such widely different 

 organs developed from a common origin; and as the 

 moveable lip of the cell serves as a protection to the 

 zooid, there is no difficulty in believing that all the 

 gradations, by which the lip became converted first into 

 the lower mandible of an avicularium and then into an 

 elongated bristle, likewise served as a protection in dif- 

 ferent ways and under different circumstances. 



• 



In the vegetable kingdom Mr. Mivart only alludes 

 to two cases, namely the structure of the flowers of 



