THE WAYS OF LIFE 195 



lasting change or registration in the protoplasmic organiza- 

 tion. 



Changes of Reaction dependent on External 

 Changes. — A small crustacean called Gammarus, very 

 common in fresh water, where it plays the part of a cleaner- 

 up, has the habit of avoiding the light. It frequents 

 shaded corners and gets under things. To avoid using 

 question-begging terms, we say that it is negatively 

 heliotropic. Its habitual reaction or tropism is to move 

 away from the light. But add the least trace of acid to 

 the water, so that the solution is no stronger than tUtj of 

 one per cent., and Gammarus moves towards the light. 

 It seems almost hke magic, changing the creature's 

 ingrained habit by a tiny drop — not of some potent philtre 

 — but of commonplace acid ! 



This case would be extremely puzzling if it stood alone, 

 but there are related facts which throw some hght on it. 

 Loeb has experimented with some smaller Crustaceans, 

 Copepods, which do not seem in ordinary circumstances 

 to be much aSected by the hght. When they are put 

 into an aquarium hghted from one side only, they do not 

 behave in any special way. But if some water rich in 

 carbonic acid be poured slowly into the aquarium, the 

 scene is changed. The Copepods become positively and 

 strongly heUotropic ; they form a group in the brightest 

 part of the aquarium and dispose themselves, as best they 

 can, in the direction of the hght. Loeb suggests that the 

 acid, acting as a catalyzer, increases the amount of the 

 material affected by the light in the animal's eye from a 

 previously minimal and negUgible quantity to a quantity 

 that cannot be disregarded. The difference between the 

 more illumined and the less illumined side of the animal 



