582 - Heredity and Evolution 



matter. If protoplasm freezes, 2 the formation 

 of ice crystals tends to isolate the water as a 

 separate phase — and without water, the 

 protoplasmic structure disintegrates beyond 

 repair. Conversely, if protoplasm continues 

 to absorb heat, even though the heat capac- 

 ity is high, a point is finally reached where 

 the enzymes and other protein components 

 of the cell become denatured, and the proto- 

 plasmic structure is destroyed. 



Although living organisms can remain ac- 

 tive only when the body temperature keeps 

 within certain limits, many species possess 

 adaptations that permit them to inhabit 

 regions of unusual heat or cold. This is 

 especially true of homeothermic animals — 

 namely the birds and mammals — which pos- 

 sess mechanisms for controlling the body 

 temperature at a relatively steady level even 

 when there are great changes in the environ- 

 mental temperature. But even the birds and 

 mammals vary widely as to their capacity to 

 endure the extremes of cold and heat. Cold- 

 adapted species (Figs. 30-8 and 30-9) tend to 

 possess specialized integumentary coverings 

 (feathers or fur) and thickened layers of sub- 

 cutaneous fatty tissue — which are effective in 

 insulating the body against the loss of heat; 

 and polar species are apt to maintain a very 

 high basal metabolism — which produces heat 

 as a by-product (p. 436). Conversely, tropical 

 species tend to limit their basal metabolism 

 to a minimum, and such organisms usually 

 display a well-developed capacity for dissipat- 

 ing heat from the body surfaces. 



Poikilothermic organisms, in which the 

 body temperature changes constantly with 

 the surrounding temperature, likewise are 

 variously adapted in relation to the tempera- 



2 By the "quick-freezing" technique, which involves 

 a very rapid reduction to subfreezing temperatures, 

 protoplasm may solidify without the separation of ice 

 crystals, and without a disruption of structure. How- 

 ever, the conditions for "quick freezing" seldom pre- 

 vail in nature, and probably this phenomenon has 

 little evolutionary significance. Also many cells can 

 tolerate subfreezing temperatures temporarily, if the 

 formation of ice crystals is delayed (as in superocxiled 

 solutions) by an absence of crystallizing foci, or if 

 much of the intracellular water is in "bound" form. 



Fig. 30-8. Polar bear on an iceberg off Greenland. 

 (Courtesy of the American Museum of Natural History, 

 New York.) 



ture range of their native habitat. In most 

 cases the metabolic enzymes are keyed to 

 operate efficiently only within some rather 

 restricted range of temperature, which may 

 be high or low according to the evolutionary 

 experience of the species. In fact, there are 



" " • <m 



Fig. 30-9. Female fur seals. (Courtesy of the Amer- 

 ican Museum of Natural History, New York.) 



