Chap. VII.] THEORY OF NATURAL SBLrCTION. 305, 



quent movements may be called an incidental result, for 

 the nerves and muscles have not been rendered specially 

 sensitive to these stimuli. So with plants it appears that, 

 from having the power of movement in obedience to cer- 

 tain stimuli, they are excited in an incidental manner by 

 a touch, or by being shaken. Hence there is no great dif- 

 ficulty in admitting that in the case of leaf-climbers and 

 tendril-bearers, it is this tendency which has been taken 

 advantage of and increased through natural selection. 

 It is, however, probable, from reasons which I have as- 

 signed in my memoir, that this will have occurred only 

 with plants which had already acquired the power of 

 revolving, and had thus become twiners. 



I have already endeavoured to explain how plants 

 became twiners, namely, by the increase of a tendency 

 to slight and irregular revolving movements, which were 

 at first of no use to them; this movement, as well as that 

 due to a touch or shake, being the incidental result of 

 the power of moving, gained for other and beneficial 

 purposes. Whether, during the gradual development of 

 climbing plants, natural selection has been aided by the 

 inherited effects of use, I will not pretend to decide; but 

 we know that certain periodical movements, for instance 

 the so-called sleep of plants, are governed by habit. 



I have now considered enough, perhaps more than 

 enough, of the cases, selected with care by a skilful 

 naturalist, to prove that natural selection is incompetent 

 to account for the incipient stages of useful structures; 

 and I have shown, as I hope, that there is no great diffi- 

 culty on this head. A good opportunity has thus been 

 afforded for enlarging a little on gradations of structure, 

 often associated with changed functions, — an important 



