PHYSIOLOGICAL 247 



of years a balancing or equilibrating organ, before it became a 

 hearing ear, so the eye was for ages a light-and-shade organ, or a 

 movement-detecting organ, or something else, before it could be 

 said that its possessors were able to form a picture of their sur- 

 soundings. That power came very late. 



The electro-magnetic radiations that we call light have many 

 direct effects on living creatures. Thus they bring about photo- 

 synthesis in green plants; they evoke pigment-formation in many 

 animals; and they seem to have a tonic influence on growth and 

 health. But by light-sense is meant a special photochemic suscepti- 

 bility of certain receptor nerve-cells to light-rays in general or to 

 certain light-rays in particular, with the result that the explosive 

 thrill, evoked by the absorbed rays, is passed on to other parts of 

 the organism which react in some definite way, notably by move- 

 ment. The living matter of a simple Unicellular animal or Protozoon 

 may be sensitive to light, without there being any appreciable 

 differentiation of structure that could be dignified even with the 

 name of "eye-spot". The function comes before the organ! But one 

 of the first progressive steps was the accumulation of a little splash 

 of pigment, which may have various uses — for instance, in absorbing 

 rays to which the receptor cell or protoplasm is not attuned, or in 

 surrounding the sensitive spot so that the light enters only from 

 straight in front. A second step was the formation of something in 

 the way of a lens which focuses the rays of light. In the simplest 

 cases the lens is not even cellular, so primitively do things begin! 

 A third step was the fashioning of the "light-organ" into something 

 like a little cup or camera, with the receptor cells on the posterior 

 concave surface and the lens in front. In some of the sea-worms, for 

 instance, we start with diagrammatically simple "cup-eyes", Hke 

 prentice-work, and gradually pass to very elaborate "cup-eyes", the 

 grading of the series being in itself a quite convincing "evidence of 

 evolution". 



With the development of a minute optic skin-cup, so readily 

 brought about by inequalities of growth, there must be associated 

 {a) the beginning of the perception of the direction from which the 

 light comes; (6) the first dim awareness of moving objects whose 

 shadows flit across the sensitive wall of the tiny camera; and 

 (c) the first experiences in the visual detection of obstacles. But 

 long before animals showed directed movements, definitely oriented 

 in relation to the light-stimuli, there were vaguer reactions. Thus 

 many animals, such as tube-inhabiting worms, sea-urchins, rock- 

 barnacles, gnat larvae, burrowing bivalves with their siphons pro- 

 jecting out of the sand, and the long-horned snails, answer back by 

 reflex retraction to the shadow of one's hand held ever so gently 

 above them. Many do this, although they have no eyes in the 

 ordinary sense of the term. It should be noted that the shadow- 



