EXPERIMENTAL EVOLUTION LECT. 



cess is carefully and slowly accomplished, the age of 

 the tadpoles experimented upon must be taken into 

 consideration, for tadpoles of three weeks' age with- 

 stand conditions which younger larvae cannot resist. 

 For instance, young tadpoles, aged two or three days 

 at the beginning of the experiment, all died when the 

 water contained eleven grammes of salt per litre, 

 while those which were three weeks old died only when 

 the percentage was fifteen grammes per litre. Rather 

 strangely, tadpoles of four or five weeks which had 

 been accustomed to live in water containing fourteen 

 grammes of salt per litre, died rapidly when I trans- 

 ferred them to their normal element, fresh water : they 

 had become so well adapted to the new environment 

 that their normal medium had become deadly for them. 

 Young eels may also be accustomed to live in salt 

 water, although they are very sensitive to the influence 

 of salt, even if there is less than two grammes of salt 

 per litre of water : at the beginning they feel the 

 change, and during some ten or twelve hours remain 

 sluggish, seemingly half paralysed ; but they become 

 quite well after twelve or fifteen hours, and recover 

 their usual activity, and may be by gradual addi- 

 tions of salt accustomed to live in water containing at 

 least five grammes of salt per litre. P. Bert, Plateau, 

 Hugo Eisig, and many others have performed similar 

 experiments with similar results. They all show that 



