C. L. PROSSER 343 



deviation from its "normal" internal state, and under natural 

 conditions every animal operates close to some "optimum." For 

 example, a homeotherm can be given either a fever or hypo- 

 thermia and then, when the stress is removed, it returns to its 

 "normal" temperature. An animal can have its normal water 

 load (water content) altered by dehydration or by excessive 

 hydration after which in its usual environment it will gain or 

 lose water until it arrives at its "normal" water load. One of the 

 most obvious and yet least understood generalizations in physiol- 

 ogy is that life processes go best within limited ranges of osmo- 

 concentration, at certain oxygen pressures, within narrow limits 

 of temperature and of concentration of specific ions; the condi- 

 tions which are most favorable differ for different animals, but 

 in a given animal there is always a tendency to return to the 

 "optimum" state after deviation. Numerous patterns of return 

 exist, some with an overshoot (Adolph, 1943), and comparison 

 of recovery patterns is a useful way to differentiate different kinds 

 of animals. Most of the comparisons thus far made have been 

 between distantlv related forms. 



Rate Functions. Probably the most subtle and quantitative 

 method of differentiating physiological types is to compare reac- 

 tion rates within narrow and natural environmental ranges. Rates 

 of oxygen consumption, rates of heartbeat and breathing, rates of 

 growth, are affected by many factors, some more critical than 

 others. The most familiar use of rate functions is in respect to 

 temperature effects, and numerous natural populations have been 

 compared with respect to temperature characteristics (Qw, the 

 rate increase calculated per 10 °C.) for various functions. There 

 is need to extend rate measurements from organ systems to en- 

 zyme systems in order to learn to what extent physiological 

 variation depends on integration in the intact animal and to what 

 extent it may occur at the cellular or subcellular level. Rate func- 

 tions have meaning for speciation only if related to a critical en- 

 vironmental stress. 



Behavior. Specific differences in reproductive behavior, par- 

 ticularly as concerns mating and care of young, have been useful 

 for differentiating certain insects and birds. In addition, taxic 



