FUNDULUS AND THE SURROUNDING MEDIUM. 129 



until it is dead at the end of the forty-hour period having shown 

 a gain of 16 per cent. It is well known that fishes absorb water 

 and increase in weight after death. Some of the final weight 

 increases in the dead specimens may have been due to post- 

 mortem processes. But there is ample evidence that the increase 

 in weight began before death. This is shown by specimens 10, 

 II, 12 and 18. On the whole, selection takes place when Fundu- 

 lus is placed in fresh water. This experiment shows at least one 

 phenomenon accompanying the acclimatization of Fundidus to 

 fresh water. As was said above, it is now well known that when 

 Fundidus is put in fresh water from sea water, that some die and 

 some survive. The present experiments show that the survivors 

 after the first gain weight incidental to being placed in fresh 

 water, recover from this gain, lose weight and survive. Others 

 either at the beginning or later begin to steadily increase in 

 weight. This may be sudden and at any time during the course 

 of the experiment. In another experiment, where Fundidus 

 was placed in a solution of one half sea water plus one half 

 fresh water, some specimens gained somewhat as the survivors 

 in the present experiments. They later lost. Others showed a 

 slight loss from the beginning. Those that died resembled those 

 that died in sea water, that is, the changes in weight gave no clue 

 to the cause of death. The effect of this diluted sea water is not 

 one half the effect of the sea water. The osmotic pressure of the 

 diluted sea water is about half that of full strength sea water. 

 In other words the effect of the two solutions is not proportional 

 to the differences in their osmotic pressures. The result is 

 similar to that found by Sumner, '05. Some of the specimens 

 in the diluted sea water died but showed no peculiar weight 

 differences. 



Finally individual records were kept of changes in weight when 

 fishes of this same species were placed in sea water to which 

 know r n quantities of sea salt had been added. These experiments 

 also revealed great variations in weight of one individual as 

 compared with another. As w r ould be expected the survival 

 time decreased as the strength of the solution increased but not 

 in proportion to the increase in the density of the surrounding 

 medium. The results of some of these experiments are show r n 



