WATER RELATIONS IN HIBERNATION. 255 



conditions can exist ; to hold the water or to let go of it, and there 

 are no other possibilities. In other words, the alteration is 

 adaptive and directive in its response, because from the physical 

 nature of the materials no other relations are physically possible. 



A crucial question is, why has the reaction not been as 

 easily reversible on return to Chicago conditions? In this lies 

 interesting possibilities for investigations now in progress. Up 

 to the present the reversibility of this action has not certainly 

 been obtained, although I have no doubt but that if I can dis- 

 cover the proper relations between the internal mechansim of 

 these beetles and their medium that the reverse alteration of the 

 Tucson condition to that at Chicago will be found. 



Whether the reversal ancestralward, or what may be assumed 

 to be an ancestralward direction, is easier than progressively, is 

 for the present is a matter of opinion, and I am unable to discover 

 any physico-chemical reason why it should be so, but we know 

 many reversible actions that go easily through one series of 

 events in one direction, and with more difficulty in the reverse 

 and through another series of operations. In these purely phys- 

 ical and chemical reactions, ancestral influence is not at all in- 

 volved, and I see no reason to assume the influence of ancestry 

 and phylogenetic relationship in living reactions of the same 

 general physical type. 



EVOLUTIONARY SIGNIFICANCE. 



In the evolution of populations, in their habitat adjustments 

 as in their distributional phenomena, reactions, accommodations 

 of this sort may well have played a by no means minor role, 

 and the value of adjustments of the kind found in these intro- 

 duced populations at Tucson in these operations in nature are 

 at once too obvious to need mention. Physiological adaptations 

 of many sorts may well have their origin and owe their diversifica- 

 tion to operations of precisely this general type, the fact of their 

 heritability or behavior Mendelianwise having no significance 

 or indication of the method of origin. In fact this and other 

 experiences with these animals through many years strongly 

 inclines me to the conclusion, that Mendelian alternativeness is 



