250 ORIGIN OF SPECIES 



in appearance than a bristle or vibraculum, and an avicu- 

 larium like the head of a bird ; yet they are almost certainly 

 homologous and have been developed from the same common 

 source, namely a zooid with its cell. Hence we can under- 

 stand 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 determine 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 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 ab- 

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

 worthy, is interesting; for supposing that all the species pro- 

 vided 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, resem- 

 bling a bird's head or an irregular box or hood. It is inter- 

 esting 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 differ- 

 ent 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 thoroughly 

 unsatisfactory— utterly insufficient to explain the incipient, 

 infinitesimal beginnings 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 



