THE VIEWPOINT OF A PHYSIOLOGIST 



midity ( Pittendrigh, 1950 ) . This difference is secondarily derived; 

 that is, the species were originally separated by some other fac- 

 tor. 



Temperature. More different physiological criteria have been 

 used with temperature than with other environmental stresses. 

 Compensation for cold climates by poikilotherms has been re- 

 viewed and population differences have been described recently 

 (Bullock, 1955). The low lethal temperature of some temperate 

 zone marine animals may be above the high lethal temperature 

 of closely related arctic animals. However, lethal temperatures 

 can be altered markedly by acclimation, and familiar seasonal 

 variations in lethal temperatures reflect acclimation. Statements 

 about lethal temperatures of local populations need to include 

 information regarding previous temperature experience. 



Many rate functions have been compared for animals from 

 different latitudes measured over a range of temperatures (Bul- 

 lock, 1955). The rate of oxygen consumption by cold-acclimated 

 poikilothermic animals is usually higher than for warm-accli- 

 mated ones, whether the acclimation is seasonal (Fundulus, 

 Wells, 1935; crustaceans, Edwards and Irving, 1943, as discussed 

 by Rao and Bullock, 1954), or latitudinal (Pandalus, Fox and 

 Wingfield, 1937), or altitudinal (planaria, Biasing, 1953). Ab- 

 sence of metabolic compensation for temperature change lias 

 been reported for other animals (hot springs fish, Sumner and 

 Lanham, 1942; certain terrestrial insects, Scholander et at., 1953; 

 an Alaskan pond gammarid, Krug 1954; and Rana pipiens, Fromm 

 and Johnson, 1955). Other rate functions such as heart rate, 

 breathing rate, and cruising speed indicate some compensation 

 by animals from cold environments. For example, My til us caJi- 

 fornianus from latitude 48°21' pumped water at the same rate 

 at 6.5°C. as those from 38°31' at 10°C. and those from 34°0' at 

 12°C. (Fig. 3) (Rao, 1953). Also the animals from colder waters 

 are less affected by temperature, i.e., their Qio is lower than for 

 animals from warmer regions. Adaptation by alteration in the 

 Qio, i.e., shift in slope as well as position of the rate-temperature 

 curve, occurs in some animals but not in others (Rao and Bullock, 

 1954; Scholander et ai, 1953; Bullock, 1955). In southern Cali- 



