160 Temperature 



a. The temperature must never be so high or so low at any time as 

 to kill the organism. 



b. The temperature must be high enough, or in some instances low 

 enough, for a sufficient period to permit the reproduction and 

 growth of the species. 



Limiting minimal temperatures for survival ordinarily occur in 

 winter, and minimal heat required for reproduction and growth will 

 be found in summer. Either or both of these influences determine 

 boundaries of distribution toward the poles or higher altitudes. 

 Limiting maximal temperatures for survival commonly occur in sum- 

 mer and minimal chilling necessary for reproduction and growth will 

 be found in winter. These influences determine boundaries of dis- 

 tribution toward the equator or low altitudes. On the basis of the 

 foregoing generalities Hutchins ( 1947 ) has delineated four basic types 

 of zonation in geographical distribution and has discussed them par- 

 ticularly in relation to the ranges of marine animals. As we shall see, 

 in these common types of temperature zonation as well as in more 

 irregular situations, failure to meet the thermal requirements listed 

 above may occur at any stage of the life cycle and at various seasons 

 of the year. 



Control by Extremes. The simplest type of control of geographical 

 distribution by temperature extremes is that in which the polar limit 

 of the range of a species is determined by the minimum temperature 

 in winter and the equatorial limit is fixed by the maximum summer 

 value. Many illustrations of this type of control will occur to the 

 reader and can be found in treatments of plant and animal geography. 

 Sometimes the lowest temperatures tolerated by tropical species are 

 encountered far to the south (in the northern hemisphere), and the 

 highest temperatures tolerated by arctic species are found well to the 

 north. There is evidence, for example, that the southward (and east- 

 ward ) distribution of three flowering alpine perennials in Scandinavia 

 is correlated with the maximum temperatures in summer as indicated 

 in Fig. 5.12. Although the exact causal relationship in such instances 

 is not known, it is possible that the protoplasm of the northern species 

 is adversely affected, that respiration is increased faster than assimila- 

 tion, or that temperatures are too high for the proper transport of as- 

 sirnilated products (cf. Went, 1950). 



Intensive study of certain species has revealed the fact that control 

 by extremes may involve unexpected complications and that other 

 aspects of temperature influence may be in operation simultaneously. 

 A multiphased action of temperature will probably be found in many 



