THEORY OF NATURAL SELECTION 253 



ondly, on their continually bending to all points of the com- 

 pass, one after the other in succession, in the same order. By 

 this movement the stems are inclined to all sides, and are 

 made to move round and round. As soon as the lower part 

 of a stem strikes against any object and is stopped, the upper 

 part still goes on bending and revolving, 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 inherited 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 imperfect case, namely of 

 the young flower-peduncles of a Maurandia v/hich 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 

 irregularly ; 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 ; anyhow, 

 they are not of the least use in the way of climbing, 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 exposed 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. 



With respect to the sensitiveness of the foot-stalks of the 

 leaves and flowers, and of tendrils, nearly the same remarks 

 are applicable as in the case of the revolving movements of 

 twining plants. As a vast number of species, belonging to 

 widely distinct groups, are endowed with this kind of sensi- 

 tiveness, it ought to be found in a nascent condition in many 



