140 THE ACTION OF THE LIVING CELL 



species when the external environment of the organ- 

 ism is not permitted to undergo marked changes. Con- 

 versely, external changes v^ill result in internal varia- 

 tions which tend to compensate for the former and 

 thus to a degree prevent serious impairment of the 

 body cells. This type of compensation is well il- 

 lustrated in the warm blooded animals, which have 

 developed during the course of evolution a tempera- 

 ture-regulating mechanism which permits them to 

 maintain a body temperature very nearly constant 

 throughout a wide range of external temperature vari- 

 ations. 



Similarly all living organisms have developed to a 

 certain degree various mechanisms which enable them 

 to sufifer environmental changes which do not vary 

 too far from the norm to which they are best adapted, 

 although in some few instances this inherent adaptive 

 ability is exceedingly elastic. In general, individual 

 cells such as protozoans and the cells of higher ani- 

 mals in tissue culture demand for their normal ac- 

 tivities a rigorously constant environment, while 

 higher animals, due to the compensating and inter- 

 related activities of their component tissue and organs, 

 are much better able to withstand marked environ- 

 mental changes such as those of temperature, radia- 

 tion, and oxygen pressure. 



In many instances such ability is sufficiently pro- 

 nounced to permit an actual acclimatization to a sud- 

 den shift in environment. To this end, living beings 

 are provided with certain defense mechanisms which 

 as a rule do not become apparent until called upon 

 to defend the organism against an environmental 

 change or a sudden new environmental factor which 



