8 



earliest stages of its existence until its maturity and death. Eggs re- 

 spire as surely as larvae and adults, and the chemical, physical, and 

 physiological changes within them vary with their growth and develop- 

 ment. Some of these changes are primarily dependent on the orderly 

 course of development during the life cycle, and are therefore irrever- 

 sihle processes, because no higher animal which is mature may reverse 

 its development and become young again. At different stages of de- 

 velopment different enzymes and harmones appear which modify the 

 phvsiological conditions of growth, development, and behavior. Envi- 

 ronmental changes, persistent and uniform, or periodic in character, 

 tend to modify and alter these internal processes, and are an additional 

 source of change, which is particularly shown in behavior. 



It is interesting to observe in this connection that certain factors 

 are important as thev hasten or retard other processes. Thus enzymes 

 hasten chemical changes which without them would take place at a 

 very slow rate, and they set free much energy in a relatively short 

 time. Temperature is another hastener of chemical reaction. Not 

 only is it a condition which sets limitations on the chemical reaction in 

 animals, but it also influences their optimum, and with increasing tem- 

 perature chemical changes take place within the animal irrespective of 

 the control of the animal, except in the warm-blooded animals, where 

 a mechanism exists which regulates, within certain limits, temperature 

 conditions. 



3. OPTIMA AND I^IMITING FACTORS 



We have seen that the animal is dependent upon its environment 

 for both substance and energy. If, therefore, the environment does 

 not contain, in available form, both substance and energy, animals will 

 not be able to live in it permanently, although with energy stored in 

 their bodies they may be able to make more or less prolonged and suc- 

 cessful invasions into such an environment. The optimum is the most 

 favorable condition for any function. We may consider optima cor- 

 responding to units of different rank: a single cell or tissue in action, 

 an organ or system of organs, the animal as a whole, a taxonomic 

 unit — and so on, to an animal community or association. There are, 

 then, many kinds of oj)tima. and the study of the conditions which pro- 

 duce them is a complex subject. The optima for different functions 

 may differ much; for example, that for growth is often different from 

 that for reproduction, and the optima may also change greatly with the 

 development of the animal. Optima, therefore, are not fixed condi- 

 tions, even though they do represent a condition of physiological rela- 

 tivc equUibrinm. The amount or intensity of substance and energy 

 which produces an optimum is limited above by the maximum and be- 

 low by the iiiiniinuni. Thus departures from the optimum, toward an 



