THEORY OF NATURAL SELECTION. '^31 



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 caudicle, 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. 



Vv'ith 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 cau- 

 dicle — to others in which the caudicle becomes firmly at- 

 tached to the viscid matter, with the sterile stigma itself 

 much modified. In this latter case we have a polliuium 

 in its most highly developed and perfect condition. He 

 who will carefully examine the flowers of orchids for 

 himself will not clenv 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 the ordinary flowers, to a highly comjilex 

 pollinium, admirably adapted for transportal by insects ; 

 nor will he deny that all the gradations in the several 

 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 in- 

 quiry 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 biow the full history of any one 

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

 attempt answering, such questions. 



