Chap. VII.] THEORY OF NATURAL SELECTION. 303 



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 transforma- 

 tion, the other parts of the cell with the included 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 develop- 

 ment of the vibracula, if trustworthy, is interesting ; for 

 supposing that all the species provided with avicularia . 

 had become extinct, no one with the most vivid iniagina- 

 tion 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 different ways and under 

 different circumstances. 



In the vegetable kingdom Mr. Mivart only alludes 

 to two cases, namely the structure of the flowers of 

 orchids, and the movements of climbing plants. With 

 respect to the former, he says, " the explanation of their 

 origin is deemed throughly unsatisfactory — utterly in- 

 sufficient to explain the incipient, infinitesimal begin- 

 nings of structures which are of utility only when they 

 are considerably developed." As I have fully treated 

 this subject in another work, I will here give only a few 

 details on one alone of the most striking peculiarities of 



