830 THE ORIGIN OF SPECIES 



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 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 development 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 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 movable 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 con- 

 verted 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 cir- 

 cumstances. 



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 thoroughly unsatisfactory — utterly insufficient 

 to explain the incipient, infinitesimal beginnings of struc- 

 tures which are of utility only when they are consider- 

 ably developed." As I have fully treated this subject 

 in another work, I will here give only a few details on 



