ADAPTATION TO ENVrROlSrMENT WARDLAW 393 



by avoiding many of the occasions on which those variations would 

 occur. It is an adaptive mechanism which avoids many adaptive 

 modifications on the part of the organism. In addition it is capable 

 of being much more selective in action than a mechanism which 

 withdraws the organism from a condition of interchange with its 

 environment. The working of this mechanism is seen most clearly 

 perhaps in the tropisms which many of the more primitive living 

 forms display. The movements of the more complex organisms do 

 not always bear such an evident relation to the effects of environment 

 as do those of the simpler. They are complicated as a rule by the 

 simultaneous action of many other adaptive mechanisms. The large- 

 scale movements, however, even of the higher animals, such as migra- 

 tions, are sufficiently analogous to tropisms to suggest that they may 

 be the results of some common underlying mechanism. 



An organism possessed of the property of motility is considerably 

 more independent of its surroundings than an organism limited to the 

 type of adaptive mechanism first discussed, but its chances of survival 

 are not necessarily greater. While it does survive, however, its vital 

 activities are likely to be much less subject to variation than those of 

 an organism whose only protection is quiescence. 



Although this mechanism shows a great advance over that previ- 

 ously discussed, its effectiveness is still decidedly limited. It is, no 

 doubt, adequate for those simple organisms whose normal environ- 

 ment is not subject to much simultaneous variation in several com- 

 ponents. Such a mechanism is likely to break down, if not assisted by 

 other regulatory devices, when the organism is faced with concurrent 

 changes of different factors of its environment. Removal of the 

 organism from a portion of its environment unfavorable in one re- 

 spect may deprive it of conditions which may be favorable in other 

 respects. An organism restricted to this type of adaptive mechanism, 

 or even possessing it in addition to the power of becoming encysted, 

 would often find itself in a dilemma. 



PRIMITIVE RELATIONS BETWEEN ORGANISM AND ENVIRONMENT 



The action of all of the mechanisms which regulate the chemical 

 relations of the organism is essentially to control the exchange of 

 material which takes place between the organism and its surround- 

 ings. In its crudest form this mechanism acts simply by abolishing 

 interchange between organism and environment when the characters 

 of the latter become unsuitable. As these mechanisms develop in 

 effectiveness, and, incidentally, in complexity, so do they increase in 

 selectivity. They become able to control independently the exchange 

 of a wide variety of substances with the outside world, and so to regu- 

 late the concentrations of these substances in contact with the living 



