520 AI^NUAL EEPORT SMITHSONIAN INSTITUTION, 1917. 



It is interesting to observe in this connection that certain factors 

 are important as they hmten 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 

 temperature chemical changes take place within the animal irrespec- 

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

 where a mechanism exists w^hich regulates, within certain limits, 

 temperature conditions. 



3. OPTIMA AND LIMITING 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 ma}^ be able to make more or less prolonged and 

 successful invasions into such an environment. The optimum is the 

 most favorable condition for any function. We may consider optima 

 corresponding to units of different rank; a single cell or tissue in 

 action, an organ or system of organs, the animal as a whole, a taxo- 

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

 There are, then, many kinds of optima, and the study of the condi- 

 tions which produce 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, there- 

 fore, are not fixed conditions, even though they do represent a condi- 

 tion of physiological relative equilibrmm. The amount or intensity 

 of substance and energy which produces an optimum is limited above 

 by the maximum and below by the minimum. Thus departures from 

 the optimum, tow^ard an increase or a decrease, are departures from 

 the most favorable conditions toward less favorable conditions, and 

 hence toward limiting conditions. This form of expression is mainly 

 that of the laboratory ; it is desirable therefore, in addition, to express 

 it in terms of the normal habitat. In nature we look upon the opti- 

 mum as that complex of habitat factors which is the most favorable, 

 and departure in any direction from this optimum intensity is in the 

 direction of a less favorable degree of intensity or into unfavorable 

 conditions. From this standpoint any unfavorable condition is a 

 limiting factor and may retard, hasten, or prevent vital and eco- 

 logical activities. Optima are thus almost ideal conditions, and are 

 probably realized in nature only to a limited degree ; in other words 



