356 THE VIEWPOINT OF A PHYSIOLOGIST 



northern and southern Rana pipiens are genetic races or sub- 

 species; in fact, hybrid abnormalities are so great that species 

 status is approached (Volpe, 1954). 



In several species of DrosopJiila, e.g., pseudoobscura and per- 

 similis, natural populations contain individuals with several dis- 

 tinct arrangements of chromosome bandings. The proportion of 

 each pattern may vary with season and altitude, and clines are 

 observed with respect to certain inversions. Controlled breeding 

 in cages at constant temperature and humidity shows that the 

 percentage of certain types is in part temperature-dependent. 

 For example, when several types of D. persimilis were reared to- 

 gether, the high-altitude type became predominant at 16° C. but 

 not at 25° C. ( Spiess, 1950). When two gene arrangements of 

 D. pseudoobscura from the same locality were bred through 

 many generations, the heterozygote became more common than 

 the homozygotes. However, if the same two forms from different 

 localities were bred there was sometimes stabilization with one 

 homozygote predominating. Hence geographic differences in 

 gene contents must exist separate from the differences in chromo- 

 some pattern (Dobzhansky, 1954). The selective effects of tem- 

 perature on the chromosome polymorphism have been demon- 

 strated to occur in both pupae and adults (Levine, 1952), and 

 physiological studies are needed to learn the bases for the tem- 

 perature adaptedness. 



Both genetic and nongenetic differences are shown in the fol- 

 lowing experiments on fish in which the zones of temperature 

 tolerance were tested after acclimation to different temperatures. 

 Fourteen species of freshwater fish from different latitudes 

 (Ontario, Tennessee, and Florida) were compared (Hart, 1952). 

 The tolerance polygons showed genetic differences for three of 

 these, the common shiner (Notropis cornutus), the large- 

 mouthed bass (Micwptcrus salmoides), and Gambusia affinis. 

 Morphological differences between northern and southern forms 

 of the first two are such that they are considered subspecies. Sev- 

 eral of the others such as golden shiner and blacknose dace are 

 morphological subspecies but they fail to show differences in 

 their temperature tolerance polygons. 



