Chap. VII.] THEORY OP NATURAL SELECTION. 305 



is secreted, but in much larger quantities by one alone 

 of the three stigmas; and this stigma, perhaps in con- 

 sequence of the copious secretion, is rendered sterile. 

 Wben an insect visits a flower of this kind, it rubs o£E 

 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 multi- 

 tude 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, 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 ordinary flower become viscid, but as we do not know 

 the full history of any one group of beings, it is as use- 

 less 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 



