12 



INTRODUCTION 



and varied qualifications that should pre- 

 vent us from making dogmatic generaliza- 

 tions. 



The inadequacy of the framework of 

 ecological principles presented in the fol- 

 lowing chapters is evident; supplementa- 

 tion and correction are urgent needs for 

 the advancement of ecology. But it is also 

 important to point out that it is often im- 

 possible to find exact and well-chosen data 

 concerning a given point. The minimum 



temperature at which death occurs imme- 

 diately for any population of a species of 

 animals is a good illustration. For that mat- 

 ter, the limits of toleration for all elements 

 in the physical environment except in gen- 

 eral terms are unknown for any one species 

 of animal, even for man. With all our em- 

 phasis on the need of ecological principles, 

 it must be emphasized again that in the 

 formulation of principles, as in testing and 

 extending them, evidence is basic. 



