182 



THE AMEBIC AN NATURALIST 



[Vol. LIII 



for a long time in rain water containing but a trace of salts, and 

 those from the brackish ponds will live equally well when sud- 

 denly transferred to sea water of full salinity (36 per mille). 



Now, the fundulus living in the brackish ponds have been 

 there for an indefinitely long period. They reproduce there, 

 and must be regarded as "adapted" to the low salinity of the 

 ponds. There is consequently no reason to expect, on the 

 adaptation hypothesis, that they should be as resistant to con- 

 centrated sea water as the individuals living in Fairyland Creek, 

 for example, where the water is of normal salinity. Yet this 

 appears to be the case. It is true that this species inhabits other 

 brackish swamp pools at Bermuda, where the salinity under- 

 goes considerable changes. But if the high resistance of the 

 isolated-pond fundulus were to be explained as the result of a 

 persisting mechanism inherited from ancestors adapted to with- 

 stand changes in salinity, then it will be noted that the appeal 

 to adaptation in the first place becomes not merely superfluous, 

 but inconsistent. 



IV. There is another explanation available, which probably 

 accounts for the high resistance of the sea-water and brackish- 

 pond fundulus to concentrated solutions. This explanation con- 

 siders that the conditions of temperature and the composition 

 of the water (especially in the brackish ponds) have shifted the 

 protoplasmic equilibria which determine the composition (and 

 hence the permeability and the resistance) of the limiting mem- 

 branes of the fish 's body. 



Loeb and Wasteneys (12, '15) found that fundulus taken 

 from a temperature of 10° died in the course of several hours 

 wlien kept at 29°, in a few minutes at 35° ; whereas those main- 

 tained at 27° would live indefinitely if transferred to 35°. The 

 fundulus at Bermuda living in the mangrove creeks are at a 

 temperature of 26°-27° (during the summer months). They 

 withstood for some hours a temperature of at least 37°, and died 

 when heated to 40.9°. In the shallow landlocked ponds the sur- 

 face temperature was 30°-33°. Fundulus from these ponds 

 withstood for several hours a temperature of 39°-40°, and died 

 when heated to 42.6°. The upper temperature limit was also 

 determined for fundulus from the brackish ponds which (at 

 27°) had for two weeks been living in M sea water; they 

 withstood 40°, and died at 42.5°. Other individuals living for 

 the same period in % M sea water withstood 40°, and died quickly 



