32 THE EYE IN EVOLUTION 



(a) " Dermatoptic sensitivity'', a "primary" reaction of proto- 

 plasm to light evident throughout the cell in Protozoa and particularly 

 in the surface layer in Metazoa ; and (6) "visual sensitivity", a 

 characteristic of specific photoreceptor organs. 



The first tyjDe of reaction is concerned with simple attraction 

 towards (or repulsion from) light ; the second is concerned with orienta- 

 tion. The first has two distinct and reciprocal phases : the essential 

 reaction is attraction towards the light (phototropism), purposeful in 

 nature, elicited most readily by short-waved light, the response varying 

 as the logarithm of the intensity of the stimulus (the Weber-Fechner 

 law). Repulsion from light (photoi^hobism), on the other hand, is a 

 negative reaction, a phase of adaptation and recuperation in which 

 the animal flees from light at its own particular speed. Visual sen- 

 sitivity, on the other hand, is confined to the eyes and, concerning 

 itself solely with visual orientation, responds most readily to stimula- 

 tion by the mid-region of the spectrum. The first type of reaction is 

 prepotent in lowly forms (such as Hydra) but becomes masked in 

 higher forms by the second, although it again determines the animal's 

 conduct when it is blinded (Crustaceans such as Dajylmia, Rotifers 

 such as Asplanchna) ; it cannot be elicited in forms higher than 

 Amphibians. The second type of reaction does not appear in the 

 lowest forms and in the higher adds visual apperception to its original 

 function of spatial orientation. This is an interesting although some- 

 what speculative philosophy, and although all the complex story of 

 orientation to light cannot be fitted into it as it stands, it may perhaps 

 contain much truth. 



In the scheme to be adopted here, the motorial responses of 

 organisms to light will be divided into two main classes : 



(a) PHOTOKINESES, non-cUrectional changes in random movements. 

 This implies merely a change of activity depending on the intensity of 

 the stimulation, not on its direction ; for its initiation a mechanism 

 is required sensitive only to changes in intensity ; there is no true 

 orientation and the direction of the response is merely a matter of 

 weighted chance. 



(6) DIRECTIONAL ORIENTATIONS towards (positive) or away from 

 {negative) the stimulating light. The term phototropism will be 

 retained to indicate the directional orientation of parts of sessile plants 

 and animals ; while the translatory movements of motile organisms 

 will be described as phototaxes. It is obvious that these directional 

 responses are more efficient and purposive than the more primitive 

 changes ir-, --ndom activity, since they allow the organism to adapt 

 itself mo: rapidly to the most favourable location in its 

 environmcii . 



