CHANPtES TN ENVIUONMENT of DAPHNIA MAGNA. 223 



their state of health at the time being. From this we may 

 infer that if a natural piece of fresh water containing Daphnia 

 were suddenly inundated with a certain quantity of salt 

 water, whether all the animals would be killed or not would 

 depend on the physiological condition of the animals at that 

 particular period. Supposing, for example, the water were 

 choked with excessive vegetation, then the population of 

 Daphnia would become extinct. 



The physiological condition arrived at by acclimatising 

 Daphnia to '25 per cent, solution might certainly be expected 

 a priori to increase the resisting power to a stronger solu- 

 tion. In the experiments described this was found not to be 

 the case. Probably, although the acclimatised animals 

 appeared perfectly healthy, yet there was a certain weakness 

 in the constitution which caused them to succumb quicker 

 to the stronger solution than ordinary unacclimatised 

 Daphnia. 



The effect of living in a confined volume of water on the 

 length of the spine is somewhat surprising, but I think the 

 evidence for it is quite overwhelming. The apparent direct 

 action of the environment on a character which has no 

 obvious connection with any change in the environment has 

 been observed by Darwin and others. Doubtless the length 

 of the spine is correlated with something which is acted upon 

 directly by the state of the water, and the difference observed 

 in the spinal length is merely the expression of this correla- 

 tion. 



Living in a confined volume of water has a very marked 

 effect in decreasing the power of reproduction, both in the 

 number of generations and in the number of offspring in a 

 brood. 



Sudden change from New River water to Canterbury 

 water, and vice versa, produced a pause in growth, and 

 some of the younger Daphnia died. Doubtless the mineral 

 character of a water is one of the numerous causes which 

 decide whether or not Daphnia are to be found living in any 

 particular pond or ditch. 



