THE KEIATION UKTWEKN JJCIHT AND PKIMENT-EOKMATION. 571 



that in the production of a green Hippolyte on Ulva the 

 yellow and blue pigments are encouraged, the red discouraged. 

 We do know that this effect occurs in pure red light, but 

 in this case few red rays are reflected. We are driven to the 

 conclusion that in daylight the yellow of Ulva directs the 

 expansion and development of yellow pigment, and the green 

 the expansion and development of blue pigment. In other 

 words, we have here Wiener's eli'ect or conclusion confirmed. 

 But when the water deepens, the red (vermilion) pigment, no 

 longer inhibited by light rays, develops more strongly, and 

 yellow and brown, and even blackish, Hippolyte occur in 

 response to the diffused background of brown weeds, the light 

 from which contains chiefly red and yellow-green rays. At this 

 depth the incident light has lost some of its red and yellow 

 rays, and is of a more bluish-green colour. From this depth 

 onwards the action of diffused light becomes more and more 

 apparent, that of the background less so. In the dominantly 

 green water the crimson and diffuse blue pigments of 

 Hippolyte develop to the exclusion and repression of the 

 red and yellow oues, thus giving the various shades of 

 carmine, purple and violet, that characterise Hippolyte 

 taken in deeper water and in deep, shady crevices near the 

 shore. In a greater depth than that to which light extends, 

 Hippolyte varians is not found. Indeed, it does not 

 appear to extend beyond the range of some ten fathoms. In 

 deeper Avater the genus is represented by Spirontocaris, the 

 colour problems of which have not yet been investigated. 



If we accept this conclusion, that carmine, purple, violet, 

 are colour effects, related directly to the diffuse green light 

 in which many animals of deeper water live, an explana- 

 tion may be found for the prevalence of these colours in many 

 other groups. For example, carmine is a tint acquired by 

 some tish, Crustacea, many echinids, starfish, and corals. 

 Violet or purple is an even more characteristic pigment of 

 the deep-sea fauna. This purplish tint is complementary to 

 green, and the relation has given rise to much speculation, 

 but, so far as I am aware, the above experiments with 



