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 difference 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 first 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 automatic 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 
