THEORY OF NATURAL SELECTION. 217 



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 grada- 

 tions — 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 to- 

 gether by threads, with the stigma differing but little from 

 that of an ordinary flower, to a highly complex pollinium, 

 admirably adapted for transportal by insects ; nor will he 

 deny that all the gradations in the several species are admir- 

 ably 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 fur- 

 ther 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. 



We will now turn to climbing plants. These can be 

 arranged in a long series, from those which simply twine 

 round a support, to those which I have called leaf-climbers, 

 and to those provided with tendrils. In these two latter 

 classes the stems have generally, but not always, lost the 

 power of twining, though they retain the power of revolv- 

 ing, which the tendrils likewise possess. The gradations 

 from leaf-climbers to tendril bearers are wonderfully close, 

 and certain plants may be indifferently placed in either 

 class. But in ascending the series from simple twiners to 

 leaf -climbers, an important quality is added, namely sensi' 

 tiveness to a touch, by which means the foot-stalks of the 



