198 THE ORIGIN OF SPECIES 



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 ex- 

 planation 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 only a few details on one alone of the most strik- 

 ing peculiarities of the flowers of orchids, namely, their pollinia. 

 A pollinium, when highly developed, consists of a mass of pollen- 

 grains, affixed to an elastic foot-stalk or caudicle, and this to a 

 little mass of extremely viscid matter. The pollinia are by this 

 means transported by insects from one flower to the stigma of 

 another. In some orchids there is no caudicle to the pollen-masses, 

 and the grains are merely tied together by fine threads; but as 

 these are not confined to orchids, they need not here be considered; 

 yet I may mention that at the base of the orchidaceous series, in 

 Cypripedium, we can see how the threads were probably first de- 

 veloped. In other orchids the threads cohere at one end of the 

 pollen-masses; and this forms the first or nascent trace of a cau- 

 dicle. That this is the origin of the caudicle, even when of con- 

 siderable length and highly developed, we have good evidence in 

 the aborted pollen-grains which can sometimes be detected em- 

 bedded within the central and solid parts. 



With respect to the second chief peculiarity, namely, the little 

 mass of viscid matter attached to the end of the caudicle, a long 

 series of gradations can be specified, each of plain service to the 

 plant. In most flowers belonging to other orders the stigma secretes 

 a little viscid matter. Now, in certain orchids similar viscid matter 

 is secreted, but in much larger quantities, by one alone of the three 

 stigmas; and this stigma, perhaps in consequence of the copious 

 secretion, is rendered sterile. When an insect visits a flower of this 

 kind, it rubs off some of the viscid matter, and thus at the same 

 time drags away some of the pollen-grains. From this simple con- 

 dition, which differs but little from that of a multitude of com- 

 mon flowers, there are endless gradations — to species in which the 

 pollen-mass terminates in a very short, free caudicle — to others in 

 which the caudicle becomes firmly attached to the viscid matter, 



