Chap. IX. SUMMAKY OF OHAPTEE. 491 



is described nearer and nearer to its source ; tlien the 

 loops are dra\^Ti out into a strongly pronounce! zigzag 

 line, with here and there a small loop still formed. 

 At the same time that the movement towards the light 

 is increased in extent and accelerated, that in the 

 opposite direction is lessened and retarded, and at last 

 stopped. The zigzag movement to either side is 

 likewise gradually lessened, so that finally the course 

 becomes rectilinear. Thus under the stimulus of a 

 fairly bright light there is no useless expenditure of 

 force. 



As with plants every character is more or less 

 variable, there seems to be no great difficulty in be- 

 lieving that their circumnutating movements may 

 have been increased or modified in any beneficial 

 manner by the preservation of varying individuals. 

 The inheritance of habitual movements is a necessary 

 contingent for this process of selection, or the survival 

 of the fittest ; and we have seen good reason to believe 

 that habitual movements are inherited by plants. In 

 the case of twining species the circumnutating move- 

 ments have been increased in amplitude and rendered 

 more circular ; the stimulus being here an internal 

 or innate one. ^A'ith sleeping plants the movements 

 have been increased in amplitude and often changed 

 in direction ; and here the stimulus is the alternation 

 of light and darkness, aided, however, by inheritance. 

 In the case of heliotropism, the stimulus is the unequal 

 illumination of the two sides of the plant, and this 

 determines, as in the foregoing cases, the modifica- 

 ticjn of the circumnutating movement in such a manner 

 that the organ bends to the light. A plant which 

 has been rendered heliotropic by the above means, 

 might readily lose this tendency, judging from the 

 cases already given, as soon as it became useless or 



