Chap. VII.] THEORY OF NATURAL SELECTION. 305 



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, to a 

 highly complex pollinium, admirably adapted for trans- 

 portal by insects ; nor will he deny that all the grada- 

 tions in the several species are admirably adapted in 

 relation to the general structure of each flower for its 

 fertilisation by different insects. In this, and in almost 

 every other case, the enquiry may be pushed further 

 backwards ; and it may be asked how did the stigma of 

 an ordinarv 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 revolving, 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 sensitiveness to a touch, by which means 

 the foot-stalks of the leaves or flowers, or these modified 



