2. Endocrines and Populations 325 



III. Conclusion 



The first part of this chapter reviewed briefly the endocrine adaptive 

 mechanisms which are of unquestionable importance in enabling mammals 

 to meet and adapt to their constantly changing environment and the visic- 

 situdes of daily existence. The mechanisms discussed are by no means the 

 only adaptive responses which are evoked by environmental exigencies, 

 but they are the better known ones and at the present time those which are 

 most Hkely to be of interest to mammalogists, ecologists, and students of 

 mammalian population dynamics. The first part of the chapter was in itself 

 a summary review of these responses; so no useful purpose would be served 

 by further condensing and .summarizing at this point. However, it should 

 be emphasized that one is not justified in regarding any single response, for 

 example increased adrenocortical activity, as an isolated phenomenon 

 complete unto itself and independent of any other changes. Any adaptive 

 response initiates and is a part of an extremely complex series of physio- 

 logic changes which probably involve every aspect of the host's physiology 

 and metabolism. Furthermore, the adaptive responses are not static affairs, 

 but a system of dynamic changes in the nature of feedback mechanisms 

 which operate to maintain a constant internal environment and life. As 

 stated earlier, these mechanisms are undoubtedly constantly active and 

 responding to even minor changes encountered in daily life. Emphasis is 

 lent to this statement by the hypoactive physiologic state observed in 

 closely confined, inactive, and overfed mammals. Therefore one should 

 regard the adaptive responses that are customarily studied and reported 

 merely as quantitative deviations from "normal" daily experience. Finally, 

 we should regard physiologic adaptations as flexible in that the available 

 evidence indicates that qualitatively different stimuli probably do not 

 elicit qualitatively similar responses even though cursory examination may 

 seem to contradict this statement. The components of the responses prob- 

 ably differ proportionally from each other with differing stimuli. It is true 

 that certain organ systems are primarily responsive to the demands of 

 external change, but their responses appear not to be identical, either 

 quantitatively or qualitatively, to all stimuli. For example, we have seen 

 that heat and cold elicit quite different responses, while emotional stimuli 

 evoke a third and different set of reactions, although all three may have 

 certain features in common. Therefore a plea is again made to examine 

 adaptive responses in the light of the specific stimuli by which they were 

 elicited. Generalization serves an extremely useful purpose conceptually, 

 but it may be misleading in the interpretation of results from critical 

 studies designed to explore physiologic adjustments to specific stimuli. 



