THEORY OF NATURAL SELECTION. 231 



ceous 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 candicle, even when of considerable 

 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 cau- 

 dicle, 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 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 an ordinary flower, to a highly com- 

 plex pollinium, admirably adapted for transportal by 

 insects; nor will he deny that all the gradations in the sev- 

 eral species are admirably adapted in relation to the 

 general structure of each flower for its fertilization by 

 different insects. In this, and in almost every other case, 

 the inquiry may be pushed further backward; and it may 

 be asked how did the stigma of an ordinary flower become 

 viscid, but as we do not know the full history of any one 

 group of beings, it is as useless to ask, as it is hopeless to 

 attempt answering, such questions. 



