336 THE ORIGIN OF SPECIES 



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

 difficalty 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 selec- 

 tion. It is, however, probable, from reasons which I have 

 assigned 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 endeavored 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 efiects 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 nat- 

 uralist, 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 difficulty 

 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 subject, 

 which was not treated at sufficient length in the former 

 editions of this work. I will now briefly recapitulate the 

 foregoing cases. 



With the giraffe, the continued preservation of the 



I 



