310 ELLINOR HELENE BEHRE. 



head-frequency in regulation all agree in indicating that when 

 individuals of Planaria dorotocephala are subjected to changes 

 in temperature which are not too extreme, two sorts of change in 

 metabolic condition are distinguishable. The first of these is 

 the change in oxidation rate which is the direct effect of the 

 change in temperature, and is either an increase or a decrease in 

 rate according as the change consists in a rise or a fall in tem- 

 perature. The second is a gradual process of acclimation extend- 

 ing over days or weeks and apparently gradually approaching a 

 limit. This process involves an alteration opposite in direction 

 to the first, consisting in a gradual decrease in oxidation rate if 

 the change is from a lower to a higher temperature, an increase 

 if the change is from a higher to a lower temperature. 



That this alteration in rate is a real acclimation and not merely 

 a recovery from the shock of a change in temperature is indicated 

 first by the fact that extreme changes are not necessary to bring 

 it about; second, by the fact that it occurs when the temperature 

 is raised the same number of degrees gradually as well as when 

 it is raised abruptly, and third, that it only begins to be appreci- 

 able after twelve hours or more and may extend over a period of 

 weeks. Recovery from any shock that might conceivably result 

 from a change of 10 in temperature in an animal like Planaria 

 might be expected to occur within a much shorter period of time 

 than this acclimation process. 



The acclimatory change in rate of oxidation, a decrease follow- 

 ing rise, an increase following fall in temperature, represents 

 the working of a regulatory mechanism which shows some re- 

 semblances in its results to the mechanism of temperature regula- 

 tion in the warm-blooded animals. There, too, a rise in tempera- 

 ture brings about a decrease in rate of oxidation, and a fall, an 

 increase in rate. It seems probable that a regulatory mechanism 

 working in much the same way as that of Planaria will be found 

 in other cold-blooded animals, and that such a general mechanism 

 of acclimation to temperature in cold-blooded animals is the 

 basis from which the temperature-regulating mechanism of warm- 

 blooded animals is developed and elaborated. 



The main differences between a general mechanism of acclima- 

 tion to temperature such as that of Planaria and a temperature- 



