HEAT 



119 



almost complete absence in laboratory 

 dishes. In different locations and despite in- 

 dividual variations, the general form of the 

 helmet is characteristic for the several pop- 

 ulations. 



Two main theories have been advanced 

 to explain the phenomenon of cyclomor- 

 phosis in cladocerans (Coker, 1939). The 

 first, the buoyancy theory, is based on the 



related species from warmer waters. The 

 low temperatures retard the rate of growth 

 and delay the appearance of sexual activity; 

 this delay tends to produce larger forms. In 

 marine copepods, for example, there is an 

 inverse correlation between body size and 

 temperature. The relation to temperature 

 may be more indirect, since the viscosity of 

 warm water is so much lower than that of 



Fig. 22. The decreasing size of ears of Lepus from south to north. A, Arizona, jack rabbit ( L. 

 alleni); B, jack rabbit from Oregon (L. californicus); C, varying hare from northern Minnesota 

 (L. americanus); D, Arctic hare from the Barren Grounds (L. arcticus). (Redrawn from 

 Hamilton. ) 



fact that the floating power of warm water 

 is much less than that of cold water, and 

 there is the suggestion that protuberances, 

 whether spines or helmets, will aid the flo- 

 tation process in summer. The other theory 

 holds that the protuberances are directive 

 and stabilizing surfaces that function as do 

 rudders or keels. 



Jordan's Rule 



Jordan's rule that fishes in low tempera- 

 tures tend to have more vertebrae than do 

 those in warmer waters holds true in gen- 

 eral; however, this is not the only factor 

 that affects the number of vertebrae of 

 closely related fish. One of the exceptions 

 is illustrated by the observation that the 

 average number of vertebrae of young coal- 

 fish, Gadus viens, is lower for small fish 

 than it is for large ones of the same year 

 class. A possible, though unproved, explana- 

 tion for the relations found in coalfish may 

 be that small eggs produce smaller fish lar- 

 vae than do larger eggs and that such ef- 

 fects persist in later life. In such an in- 

 stance, temperature is involved only indi- 

 rectly (Dannevig, 1933). 



Cold-water forms of many sorts are fre- 

 quently larger than are individuals or 



cold water that the larger forms would be 

 handicapped in their efforts to maintain po- 

 sition in warmer seas (Hesse, Allee, and 

 Schmidt, 1937; Coker, 1934). 



Bergmann's Rule and Allen's Rule 



Homoiothermal animals from colder cli- 

 mates tend to be larger in size and hence 

 to have less surface in proportion to body 

 weight than do their relatives from warmer 

 regions. This phenomenon occurs widely 

 even though not universally among birds 

 and mammals and is usually interpreted in 

 relation to heat conservation in the north 

 and to heat radiation in the south. This is 

 Bergmann's rule. Allen's rule is correlated 

 with it and is concerned with the marked 

 tendency toward the lessening of extremi- 

 ties in colder climates (see Fig. 22.) Allen 

 based his conclusions on measurements of 

 animals killed in nature. His observations 

 have many confirmations both from field 

 and laboratory studies, especially when 

 rather large differences in temperatvire are 

 considered. For example, mice reared at 

 31 to 33.5° C. have longer tails than those 

 of the same strain reared at 15.5 to 20° 

 (Allen's rule), and the latter have larger 

 and stockier bodies and hence are decidedly 



