COMPOUND EYES OF AUTURol'ODS. 



IV. — PlIYI.or.KNY OK ■rilK OMMATIDiaM. 



I do not pretend to enter into a detiiiled discussion of this 

 dillicult prolilein. In order, however, to give a more intelligent 

 idea of what I have thus far been considering, I will devote the 

 following pages to the reconsideration of facts given in tlio 

 preceding pages from a more comprcliensive standi)oint. 



Balfour' has given a sketch of the possible evolution of a visual 

 organ. He starts with a simple organism in which a spot on 

 the surface of the body ma}' become spontaneously pigmented 

 and therefore become specially sensitive to light. The cuticular 

 covering of the body may become thickened at this sjjot and act 

 as an apparatus for condensing the light upon tlic pigmented 

 spot lying beneatli it. He further expresses his view elsewliere^ 

 that the lens-like dioptric apparatus of the eye, formed either 

 as a thickening of the cuticle or as a mass of cells, was at first 

 formed simply to concentrate the light on the sensitive spot; the 

 power to throw an image of external objects on the perceptive 

 part of the eye was acquired gradually afterward. 



The part which is played by pigment in the ])hysiology 

 of vision is considered a most obscure problem. 1 quote the 

 following, clearly put forward by Foster,' as a physiological 

 aspect of the question bearing upon the discussion at issue: 

 " But in order that light may produce chemical effects (upon 

 protoplasm), it must be absorbed ; it must be spent in doing the 

 chemical work. Accordingly, the first step towards the forma- 

 tion of an organ of vision is the differentiation of a portion of 

 protoplasm into a pigment at once capable of absorbing light 

 and sensitive to light — i. e. undergoing decomposition upon 

 exposure to light. An organism, a portion of whose protojilasm 

 had thus become differentiated into such a pigment, would be 

 able to react towards light. The light falling on the organism 

 would be in part absorbed by the pigment, and the rays thus 

 absorbed would produce a chemical action and set free chemical 



'P. M. Balfour: Address to the Depftrtment of Anatomy and Physiology, 

 British Association, 1880; Nalure, Vol. XXII, p. 417, 18S0. 



'■Comparative, Embryology, Vol. II, Chapter XVI, Organs of Vision, p. 470. 



"JI. Poster: ,4 Text Book of Phy.nology, 4th Edition, Book III, Chap. 11: 

 Sight ; Tlie Photochemistry of Ketina, pp. 51.')-516. 



