THEORY OF NATURAL SELECTION 251 



only a few details on one alone of the most striking pecu- 

 liarities 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 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 secretes a little viscid matter. Now in cer- 

 tain 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 mul- 

 titude 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 condition. 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 an ordinary flower. 



