328 Adaptation to Environment 



ponds with a concentration of salt so high that most 

 marine animals would succumb if suddenly transferred 

 to such a solution we have a limited fauna and flora. 

 The common idea is that marine animals become 

 adapted to fresh water or vice versa; or to the condi- 

 tions in salt lakes; especially if the changes take place 

 gradually. Yet it can be shown that the existence of 

 these different faunas can be explained without the 

 assumption of an adaptive effect of the environment. 

 The writer has worked with a marine fish Fundulus 

 whose eggs develop naturally in sea water which, how- 

 ever, will develop just as well in distilled water; and 

 the young fish hatching in distilled water live and grow 

 in this medium. Most of the adult fish die after several 

 days, when put suddenly into distilled water, but they 

 can live in fresh water which contains only a trace of 

 salt. They can also live in very concentrated sea 

 water, e. g., twice the normal concentration. Suppose 

 that a bay of the ocean containing such fish should 

 suddenly become landlocked and the concentration 

 of the sea water be thus raised to twice its natural 

 amount; the majority of forms would die and only 

 Fundulus and possibly a few other species with the 

 same degree of resistance would survive. An investi- 

 gator examining the salinity of the water and not know- 

 ing the natural resistance of Fundulus to changes in 

 concentration would be inclined to assume that he had 

 before him an instance of a gradual adaptation of the 



