CHAP. VII.] THEORY OF NATURAL SELECTION. 185 



It is scarcely possible that the above slight movements, due to 

 a touch or shake, in the young and growing organs of plants, can 

 be of any functional importance to them. But plants possess, in 

 obedience to various stimuli, powers of movement, which are of 

 manifest importance to them ; for instance, towards and more 

 rarely from the light. in opposition to, and more rarely in the 

 direction of, the attraction of gravity. When the nerves and 

 muscles of an animal are excited by galvanism or by the absorp- 

 tion of strychnine, the consequent 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 

 certain stimuli, they are excited in an incidental manner by a 

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

 of some extinct high-reaching ruminant, which had the longest 

 necks, legs, &c., and could browse a little above the average 

 height, and the continued destruction of those which could not 

 browse so high, would have sufficed for the production of this 

 remarkable quadruped; but the prolonged use of all the parts 



