THE AXIAL GRADIENTS IN HYDROZOA. 383 



may be purely chemical; but it is very doubtful that they ever 

 are solely chemical in nature, and, of course, equally doubtful 

 that they are ever the consequence of purely physical changes- 

 In all probability biological processes are neither complexes of 

 purely chemical reactions nor purely the resultants of physical 

 changes but they involve both types of changes occurring simul- 

 taneously and mutually interacting. On a priori grounds, how- 

 ever, it may be accepted that the chemical processes are of 

 paramount importance in living things, since, while substances 

 having physical properties similar to or identical with those of 

 protoplasm exist which are not alive, in no case do non-living 

 materials carry on the chemical reactions characteristic of proto- 

 plasm; further, the "signs of life" are chemical or of chemical 

 origin, and protoplasm in which the chemical reactions have 

 fallen to a low level is to all intents dead. Granting, therefore, 

 that chemical reactions play the most important roles in life 

 processes and that in many cases physical changes are insignifi- 

 cant, it may be valid to draw conclusions from the value of the 

 temperature coefficient. But it must always be borne in mind 

 that the chemical reactions which occur in living things are 

 subject to processes of regulation in the organism. The relation 

 of chemical changes to temperature is therefore in the organism 

 a variable quantity. Thus Behre ('18) found that the rate of 

 respiratory metabolism of Planaria is lowered when the animals 

 are maintained at a high temperature and raised when they are 

 maintained at low temperatures. The temperature coefficient 

 for the rate of respiration of Planaria is therefore not a fixed 

 value for a certain range of temperature but depends to some 

 extent upon the temperature at which the animals had been 

 living previous to the experimental test. Since such modifica- 

 tions or regulations are known for emulsoid colloids, their be- 

 havior at any given time depending upon the conditions to 

 which they had previously been exposed (phenomenon of hys- 

 teresis), it is possible that this ability of organisms to modify 

 the rate of processes presumably chiefly chemical with reference 

 to temperature is due to the colloidal substratum in which the 

 chemical reactions take place. 



In Tubularia similar regulations to temperature are observable. 



