Chap. VII.] THEORY OF NATURAL SELECTION. 307 



part of a stem strikes against any object and is stopped, 

 the upper part still goes on bending and rcYolving, and 

 thus necessarily twines round and up the support. The 

 revolving movement ceases after the early growth of 

 each shoot. As in many widely separated families of 

 plants, single species and single genera possess the power 

 of revolving, and have thus become twiners, they must 

 have independently acquired it, and cannot have in- 

 herited it from a common progenitor. Hence I was led 

 to predict that some slight tendency to a movement of 

 this kind would be found to be far from uncommon with 

 plants which did not climb; and that this had afforded 

 the basis for natural selection to work on and improve. 

 When I made this prediction, I knew of only one imper- 

 fect case, namely, of the young flower-peduncles of a 

 Maurandia which revolved slightly and irregularly, like 

 the stems of twining plants, but without making any use 

 of this habit. Soon afterwards Fritz Miiller discovered 

 that the young stems of an Alisma and of a Linum, — 

 plants which do not climb and are widely separated in 

 the natural system, — revolved plainly, though irregular- 

 ly; and he states that he has reason to suspect that this 

 occurs with some other plants. These slight movements 

 appear to be of no service to the plants in question; any- 

 how, they are not of the least use in the way of climb- 

 ing, which is the point that concerns us. Nevertheless 

 we can see that if the stems of these plants had been 

 flexible, and if under the conditions to which they are ex- 

 posed it had profited them to ascend to a height, then the 

 habit of slightly and irregularly revolving might have 

 been increased and utilised through natural selection, 

 until they had become converted into well-developed 

 twining species. 



