Chap. VII. Theory of Natural Selection. 197 



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 mnst 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 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 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 as- 

 cend 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 flow T ers, 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 sensitiveness, it ought to be found in a 

 nascent condition in many plants which have not become climbers. 

 This is the case : I observed that the young flower-peduncles of 

 the above Maurandia curved themselves a little towards the side 

 which was touched. Morren found in several species of Oxalis that 

 the leaves and their foot-stalks moved, especially after exposure 

 to a hot sun, when they were gently and repeatedly touched, or 

 when the plant was shaken. I repeated these observations on some 

 other species of Oxalis with the same result ; in some of them the 

 movement was distinct, but was best seen in the young leaves ; in 

 others it was extremely slight. It is a more important fact that 

 according to the high authority of Hofmeister, the young shoots and 

 leaves of all plants move after being shaken ; and with climbing 

 plants it is, as we know, only during the early stages of growth that 

 the foot-stalks and tendrils are sensitive. 



