252 ORIGIN OF SPECIES 



to a highly complex pollinium, admirably adapted for trans- 

 portal 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 differ- 

 ent insects. In this, and in almost every other case, the en- 

 quiry 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 be- 

 ings, 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 ar- 

 ranged 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 twin- 

 ing, 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 modi- 

 fied 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 grada- 

 tions 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 se- 

 lection. The power of twining depends, firstly, on the stems 

 whilst young being extremely flexible (but this is a character 

 common to many plants which are not climbers) ; and, sec- 



