THE VIEWPOINT OF A PHYSIOLOGIST 



the opposite range limits of one species. Most physiologists seek 

 homogeneous material, whereas in population studies, controlled 

 heterogeneity is desired. When differences are observed in similar 

 animals, the differences may be among individuals within a ge- 

 netically similar population, they may be in mutants carried at 

 some level in the population, they may be racial between eco- 

 types of the same species, or they may be between the total ge- 

 netic systems of true species. Our operative assumption has been 

 that speciation progresses from individual variation through 

 strains and races to species. Since the complete sequence cannot 

 be observed in any single species but is inferred, it is necessary 

 to piece together as much indirect evidence as possible to test 

 the basic assumption. 



Physiological Criteria of Variation 



Before examining hypotheses and experimental data it is im- 

 portant to consider what physiological criteria can best be used 

 in laboratory comparisons of animals drawn from different popu- 

 lations. We may list seven general categories of criteria of physio- 

 logical variation. 



Survival. Tolerance of sudden environmental extremes is the 

 easiest and most widely used criterion of physiological variation. 

 Percentage survival at different times after application of a stress 

 is plotted for different intensities of stress, and LD.-,<> (median 

 lethal dose) values for the stress are obtained. More important 

 is the establishment of the maximum stress tolerated indefinitely. 

 The causes of death within short times after exposure to extreme 

 stresses are usually different from those after longer times, and 

 it is important that prior history ol the animals be known because 

 acclimation may strongly influence survival. For example, Fry, 

 Brett, and Clawson (1942) and Brett (1944) constructed useful 

 polygons that gave the upper and lower limits of temperature 

 tolerated by fish which had been differently acclimated, and they 

 found in goldfish a 1 C change in lethal temperatures for each 

 3°C. acclimation difference. When a stress is applied gradually, 

 the acclimation process increases the degree of stress which 

 can be tolerated, i.e., the rate of application of the' stress affects 



