392 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 19 31 



environment shall be restricted within certain limits. The more 

 effective the mechanism is, the closer together will these limits be. 



Broadly speaking, we may say that the living organism first pro- 

 tects itself against the variations of its surroundings by placing 

 barriers between itself and its environment. The process of encyst- 

 ment and the formation of spores are what appear to be simple 

 examples of this type of reaction which are shown by very simple 

 organisms. Even more highly developed organisms make use of 

 devices of this kind at some stage of their life histories, as in the 

 formation of seeds by plants. 



There is no doubt about the effectiveness of such a mechanism for 

 protecting an organism against unfavorable changes of its environ- 

 ment. The extreme difficulty with which the spores of certain 

 microorganisms are destroyed, even by the most drastic treatment, 

 is well known. The tenacity with which the seeds of certain plants 

 retain their viability has been demonstrated by Cambage (1928), 

 who showed that seeds of Acacia melanoxylon were capable of ger- 

 mination after 10 years' soaking in sea water. In its simplest form, 

 however, a protective mechanism of this kind imposes severe restric- 

 tions upon the organism using it. At times such an organism must 

 purchase its survival by an almost complete suspension of its vital 

 activities. The mechanism can do no more than protect the organ- 

 ism from destruction b}^ extremes of variation in its environment, 

 and appears to display the phenomenon of adaptation in its crudest 

 form. It is a regulatory mechanism which permits of wide varia- 

 tion in the rate at which the organism is able to carry on its activities. 

 Except for this power of passive resistance, an organism limited to 

 this kind of adaptive mechanism is still very largely at the mercy 

 of its environment. 



Another way in which an organism may place a barrier between 

 itself and certain parts of its surroundings is by removing itself from 

 those parts. It is able to do this wdien possessed of the property of 

 motility which is shown even by some of the most primitive forms of 

 life. As an adaptive mechanism, motility in many ways is a distinct 

 advance beyond processes similar to encystment. The motile organ- 

 ism is able to make use of one part of its environment to protect itself 

 against another. Instead of erecting about itself, when conditions 

 become unfavorable, barriers composed of its own substance, it is 

 able to place parts of its environment between itself and those 

 conditions. 



It is evident that the freedom of an organism possessing motility 

 must be much greater than that of similar organisms without this 

 mechanism. Its effect is to render unnecessary many of the varia- 

 tions of activity to which the nonmotile organism must be subject. 



