THE WAYS OF LIFE 239 



crossed the boundary line ! The same applies to an 

 aquarium half white, half black. The order of their prefer- 

 ences is thus : 



Black -> red -> yellow -* blue -> violet -> green -> white + 



The next result reached by Minkiewiez was very remark- 

 able, that a change in the nature of the medium brings 

 about a reversion of the attractions to the various rays. 

 When distilled water was added to the sea water (25-80 

 cubic centimetres to 100 cubic centimetres), the Nemertean 

 worm, Linens ruber, turned towards the most refrangible 

 rays of the spectrum as decidedly as it had previously 

 avoided them ! The change in the medium disturbed the 

 physiological condition of the animal, but on the fourth 

 day the original attractions manifested themselves again. 

 When a worm which had got accustomed to the diluted 

 water and showed its normal preference for red was put back 

 again, after two or three weeks, into ordinary sea water, 

 it Avas again disturbed, and made for the violet. 



Hermit-crabs left in a basin without change of water 

 become gradually intoxicated with their own waste pro- 

 ducts, and all their preferences are inverted. The scale 

 of values remains in the same sequence, but the direction 

 of movement has changed to the opposite, as in the follow- 

 ing line : 

 + Black <- red <- yellow <- blue <- violet <- green <- white 



Professors Keeble and Gamble have shown that young 

 prawns (Hippolyte varians), almost colourless to start 

 with, rapidly assume the coloration of the seaweed on 

 which they are placed. What is more, the adults that have 

 put on some definite colour, are able to change this and 

 assume a new one in harmony with a new environment. 

 Minkiewiez got similar results : any colour can be changed 



