Chap. VII.] THEORY OF NATURAL SELECTION. 195 



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, 

 1 will here give only a few details on one alone of the most striking 

 pecu]iarities 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 trans- 

 ported 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 Cypri- 

 pedium, we can see how the threads were probably first developed. 

 In other orchids the threads cohere at one end of the pollen-masses ; 

 and this forms the first or nascent trace of a caudicle. That this 

 is the origin of the caudicle, even when of considerable length and 

 highly developed, we have good evidence in the aborted pollen- 

 grains which' can sometimes be detected embedded 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 se- 

 cretes 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- 

 condition, which differs but little from that of a multitude of 

 common 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, 

 with the sterile stigma itself much modified. In this latter case 

 we have a pollinium in its most highly developed and perfect con- 

 dition. He who will carefully examine the flowers of orchids for 

 himself will not deny the existence of the above series of gradations 

 — from a mass of pollen-grains merely tied together by threads, 

 with the stigma differing but little from that of $u ordinary flower, 



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