ADAPTATION- TO ENVn^ONMENT WAEDLAW 407 



allowed to vary unrestricted. Still less, under these circumstances, 

 can the organisni attain that freedom from external variation of 

 the rate of its activities which seems to be one of the principal objects 

 of adaptive mechanisms. The rates of chemical reactions vary 

 rapidly with the temperature at which they take \Aace. The effect of 

 these variations upon the activities of organisms without a tempera- 

 ture-regulating mechanism is so striking and so familiar as to require 

 no further reference. 



The effect of variation of temperature on the composition of living 

 tissues and of their immediate environment is not so obvious, but is 

 none the less important. Variations of temperature alter the equi- 

 librium constants of chemical reactions. In this way they alter the 

 proportions between the reacting materials which will exist under 

 given conditions. For example, the proportions between the ions in 

 the circulating fluids and cells will not be the same at different tem- 

 peratures. We have seen the permeability of the living cell is largely 

 controlled by the proportions of the ions present in its immediate 

 vicinity. As it is this permeability which determines many of 

 the fundamental properties of living matter, these properties must be 

 modified by changes of temperature, quite apart from any changes 

 in the rate of the vital activities which they may bring about. 



The temperature-regulating mechanisms of all the warm-blooded 

 animals are by no means equally effective. In the higher mammals 

 this mechanism continues to function as long as the external condi- 

 tions of temperature remain within limits compatible with the life 

 of the animal. In other species, however, the mechanism goes out 

 of action if the temperature of the surroundings falls below certain 

 levels which the animal can still survive. At these lower tempera- 

 tures the animals behave like cold-blooded animals and have body 

 temperatures close to those of their external environment. This is 

 the phenomenon of hibernation. The ease with which this mecha- 

 nism is thrown out of action by a fall of temperature differs among 

 different species. It is interesting to observe that in Echidna^ which, 

 on morphological grounds, is regarded as the most primitive of 

 mammals, the action of the heat-regulating mechanism is peculiarly 

 susceptible to disturbance. It ceases to function at external temper- 

 atures several degrees higher than those at which an effect is to be 

 seen in other hibernating animals. (Wardlaw, 1915, 1921.) 



It is well known also that the effectiveness of the mechanism for 

 the regulation of body temperature of warm-blooded animals is much 

 less eflicient in the immature individuals than in the adults. Even 

 in normal infants, for example, the fluctuations of body temperatures 

 are much greater than those of adults, while in premature infants 

 this mechanism is so ineffective that survival is often impossible 



