large body size and short appendages give less sur- 

 face area per volume of body and thus minimize heat 

 loss from the body, in cold climates. It is doubtful, 

 however, that the smaller surface area thus attained 

 gives enough reduction of heat loss to be significant 

 in warm-blooded animals and would not apply to 

 cold-blooded ones. Rather the ability of warm- 

 blooded animals to live in cold climates depends more 

 importantly on the insulation of the body surface, its 

 exposure, its vascularization, and its ability to tolerate 

 a cold tissue temperature (Scholander 1955, Irving 

 1957). The older explanations of selective value of 

 many of these rules are therefore doubtful. 



PHYSIOLOGICAL ADJUSTMENTS 



Nature of adjustments 



Probably the first response of any organism to 

 a change in the environment is physiological. A 

 physiological response must certainly precede any 

 change in form or structure which requires growth. 

 Even a change in behavior must follow a change in 

 some receptor or sense organ followed by nervous 

 function ; a fall in air temperature, for instance, 

 brings a drop in the metabolic rate of cold-blooded 

 organisms but a rise in the rate of warm-blooded or- 

 ganisms. Cold may stimulate nerve endings in the 

 skin of birds or mammals and produce shivering and 

 a search for protective cover. Transference from the 

 dark to light may immediately initiate photosynthesis 

 in resting chloroplastids within a plant cell, or a 

 change in turgescence on opposite sides of a sessile 

 zooid may result in a turning movement, an orienta- 

 tion to or away from the light source. Physiological 

 responses are thus internal responses to factors of the 

 environment. Often they are difficult to detect. 



Types of response 



Environmental factors influence organisms 

 physiologically in various ways (Fry 1947). These 

 effects may be classified as follows : 



Lethal : causing death ; for instance, extreme heat 

 or cold, lack of moisture, and so forth. 



Masking : modifying the effect of some other fac- 

 tor. Low relative humidity increases the rate 

 of evaporation of moisture from body sur- 

 faces so that warm-blooded animals are able 

 to survive at otherwise intolerably high air 

 temperatures. 



Directive: producing an orienting response in 

 relation to some environmental response so 



that the organism gets itself into favorable 

 conditions. 



Controlling: influencing the rate at which some 

 process functions, but not entering the reac- 

 tion. Temperature, pressure, and viscosity, 

 for instance, affect secretion, locomotion, and 

 metabolism. 



Deficient : curtailing an activity because some es- 

 sential ingredient, such as a salt, oxygen, or 

 the like is absent or at unfavorably low con- 

 centration. 



The same environmental factor may produce dif- 

 ferent effects at different times and under different 

 conditions. Temperature may be lethal, if extreme ; 

 masking, as when cold reduces the demand of cold- 

 blooded organisms for food ; directive, by inducing a 

 search for more favorable locations ; or controlling, 

 as a modifier of the rate of metabolism. Often the 

 distinction between controlling and deficient factors 

 is not made, or they are considered as together con- 

 stituting limiting factors. 



Threshold and rate 



Every environmental factor varies through a 

 wider range of intensivity than any single organism 

 could tolerate. Characteristically, there is for each in- 

 dividual organism a lower and an upper limit in the 

 range of an environmental factor between which it 

 functions efficiently. For any one factor, different or- 

 ganisms find optimal conditions for existence at dif- 

 ferent points along the range : hence their segregation 

 into different habitats. 



The threshold is the minimum quantity of any 

 factor that produces a perceptible effect on the or- 

 ganism. It may be the lowest temperature at which 

 an animal remains active, the least amount of mois- 

 ture in the soil that permits growth of a plant, the 

 minimum intensity of light at which a photoreceptor 

 is stimulated, and so forth. Above the threshold, the 

 rate of a function increases more or less rapidly as 

 the quantity of heat, moisture, light, or other environ- 

 mental factor is augmented, until a maximum rate is 

 attained. Above the maximum, there is usually a de- 

 cline in the rate of a process either because of some 

 deleterious effect produced, the interference of some 

 other factor, or exhaustion. The curve of decline at 

 high temperatures is usually steeper than the curve 

 of acceleration at low temperatures. 



Law of toleration 



For each species there is a range in an environ- 

 mental factor within which the species functions at 

 or near an optimum. There are extremes, both maxi- 

 mum and minimum, towards which the functions of 



10 Background 



