DAWSON, W. R. 



regulation has been mainly significant in reducing the energetic cost 

 of homeothermy and in extending the range of environmental tem- 

 peratures over which this condition can be maintained. Modification 

 of this aspect of temperature regulation has, of course, subsequently 

 comprised a major theme in the climatic adaptation of homeotherms. 



It is appropriate in connection with the evolution of physical 

 thermoregulation to mention Bergmann's and Allen's Rules, which 

 state that forms from higher latitudes tend to be larger and to have 

 relatively smaller appendages than their counterparts from lower 

 latitudes. The validity and significance of these rules in climatic ad- 

 aptation have recently been the subjects of some controversy (Scho- 

 lander, 1955, 19 56; Mayr, 1956; Newman, 19 56; and Irving, 1957). 

 King and Earner's (1960:267) comments on these rules appear use- 

 ful. 



Neither of the rules appears to hold generally for most species 

 with extensive latitudinal (and hence temperature) distributions. 

 Furthermore, the relatively slight differences in the size of the body 

 and length of the appendages are quite trivial with respect to adjust- 

 ment of heat dissipation (Hutchinson, 1954; and Scholander, 1955, 

 1956). This is not meant to argue for the invalidity of the "rules" in 

 species in which such clines do clearly occur, for it is quite plausi- 

 ble that these clines may have developed because of the slight en- 

 ergy-conserving advantages conferred by these differences. It must 

 be emphasized, however, that the magnitude of the changes in bodily 

 dimension necessary to provide adequate ad justmentofheatdissipa- 

 tion, or evenany appreciable fraction thereof, far exceeds the genet- 

 ic potential of any species. 



PATTERNS IN THE ONTOGENY OF TEMPERATURE REGULATION 



Considerable variation in the state of development of birds at 

 hatching is evident, and this is reflected indifference in thermore- 

 gulatory capacities. At one extreme are the young of altricial spe- 

 cies, e. g., passerines, which are hatched in a very immature state 

 and do not develop effective temperature regulation until a week or 

 more after emergence from the egg. At the other extreme are the 



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