22 THE HABITAT 



efficient in producing a recognizable structural change in plant or formation. 

 Investigations made by the writer have shown that standard instruments 

 will measure differences of quantity quite too small to produce a visible re- 

 action. Efficient differences are not the same for different factors, and 

 perhaps also for the same factor when found in various combinations. They 

 vary widely for different species, being in direct relation to the plasticity of 

 the latter. The point necessary to bear in mind in formulating methods for 

 habitat investigation and in making use of instruments is that standard in- 

 struments should be used for the very reason that we do not yet know the 

 relation between determinable and efficient differences. On the other hand, 

 it is unnecessary to insist upon absolute exactness as soon as it is found that 

 the determinable difi'erence lies well within the efficient one. This by no 

 means indicates that instruments are not to be carefully standardized and 

 frequently checked, or that accurate readings should not be made. It 

 means that a slight margin of error may be permitted in a machine which 

 registers well within the efficient difference for that factor, and that instru- 

 ments that read to the last degree of nicety are not absolutely necessary. In 

 the fundamental work of determining efficient differences, however, instru- 

 ments can not have too great precision. Moreover, these differences must be 

 based upon the most plastic species of a formation, and the readings must 

 be made under normal conditions. 



Instrumentation 



35. Methods. In the field use of instruments two methods have been de- 

 veloped. The lirst in point of time was the method of simple instruments, 

 devised especially for class work, and capable of being used only where a 

 number of trained students are available. The method of automatic instru- 

 ments was an immediate outgrowth of this, due to the necessity which con- 

 fronts the solitary investigator of being in different habitats at the same 

 time. In the gradual evolution of this subject, it has become possible to 

 combine the two methods in such a way as to retain all the advantages of 

 the avitomatic method, and most of those of the method of simple 

 instruments. 



36. Method of simple instruments. By simple instruments are denoted 

 those that do not record, but must be read by the observer at the time. 

 They are standard instruments of precision, but possess the disadvantage 

 of requiring an observer for each one. They are well illustrated by the 

 thermometers and psychrometers used by the Weather Bureau. In the 

 hands of trained 'observers the results obtained are unimpeachable ; in fact, 

 standard simple instruments must be constantly employed to check automatic 



