CHAPTER III 

 THE EFFECT OF LIGHT ON MOVEMENT 



The control of the movements of living organisms, both plants 

 and animals, by light is a fundamental function of great phylogenetic 

 age, preceding the acquirement of vision and, indeed, leading directly 

 to its development ^ ; it will be remembered that the association of 

 the functions of equilibration and orientation with the visual system 

 of the higher animals is in every sense basic. This primitive control of 

 movement by light is undoubtedly an adaptive process, directing the 

 organism to regions in the environment which are favourable to it, 

 and has originated and evolved in the same way as other biologically 

 useful reactions. 



Historical development 



It was originally held that the orientation of primitive organisms 

 in space depended on the exercise of those "vital forces " the presence 

 of which were considered to differentiate living creatures from the 

 inanimate world ; and it was not until the time of the Cambridge 

 clergyman-journalist, John Ray (1693), that a mechanistic explanation 

 was offered to account for this aspect of the behaviour of plants. This 

 English botanist suggested that plants placed before a window turned 

 towards the light because the side towards the window was cooler than 

 that towards the room and consequently grew more slowly so that the 

 plant became bent by the relatively greater growth on the warmer side. 

 The Huguenot botanist, August de Candolle (1832), on the other hand, 

 introduced the conception that light rather than heat was the respon- 

 sible agent, a concept elaborated and rationalized by Sachs (1882), 

 the botanist of Wiirzburg ; he maintained that orientation was 

 determined by the directional incidence of the light and so formulated 

 the interpretation of these phenomena generally current today. 



Meantime, similar reactions in the animal world were considered 

 to be dominated by a vital force usually conceived as acting automatic- 

 ally and thoughtlessly, a view ej^itomized by the great French 

 philosopher, Rene Descartes (1650). The publication of Darwin's 

 Origin of Sjiecies in 1859, however, caused a revolution in biological 

 thinking so that contemporaneous writers spent much ingenuity in 

 interpreting the behaviour of the lower animals in an anthropomorphic ^ 

 way, attributing their reactions to primitive psychic activities which 



» p. 105. 



* avOpwTTos, man ; ^lop^-q, form. 

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