CHAF. VII.] THEORY OF NATURAL SELECTION. 183 



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 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 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 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 whick 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 and converted into tendrils, are excited to bend 

 round and clasp the touching object. He who will read my 

 memoir on these plants will, I think, admit that all the many 

 gradations in function and structure between simple twiners and 

 tendril-bearers are in each case beneficial in a high degree to the 

 species. For instance, it is clearly a great advantage to a twining 

 plant to become a leaf -climber ; and it is probable that every 

 twiner which possessed leaves with long foot-stalks would have 

 been developed into a leaf-climber, if the foot-stalks had 

 possessed in any slight degree the requisite sensitiveness to 

 a touch. 



As twining is the simplest means of ascending a support, and 

 forms the basis of our series, it may naturally be asked how did 

 plants acquire this power in an incipient degree, afterwards to be 

 improved and increased through natural selection. The power of 

 twining depends, firstly, on the stems whilst young bdng ex- 

 tremely flexible (but this is a character common to many plants 

 which are not climbers) ; and, secondly, on their continually 

 bending to all points of the compass, one after the other in 

 succession, in the same order. By this movement the stems are 

 inclined to all sides, and are made to move round and round. As 

 soon as the lower part of a stem strikes against any object and is 

 stopped, the upper part still goes on bending and revolving, and 



