20 Richard H. Boerker 
either synthetize an artificial environment and proceed to study 
the plant under definitely measured differences of light and water, 
or we may measure the physical factors influencing the same plant 
under various natural conditions. The observational method is 
il suited for most work on habitat relations because the habitat 
involves an extremely variable array of uncontrolled physical 
factors, and it is practically impossible to determine without actual 
measurements which factor has the controlling influence and what 
the relative importance of the others are. The most desirable 
method for problems which will allow its application is the one 
in which we synthetize an artificial environment. In this case we 
keep certain factors constant and measure the variable one; in 
this way, it is quite obvious, the environment 1s comparatively easy 
to analyze. This method, of course, presupposes a greenhouse 
and on this account is only of limited application. 
There is no question that all these methods have their value in 
their proper places ; the choice of one must vary with the problem 
and the circumstances. The method of measuring the factors 
influencing the same plant under various natural complexes is the 
one probably of widest application in the field. The purely ob- 
servational method, for work on the determination of habitat 
factors, while of some value when other methods are impossible 
of application has unsurmountable objections. Observers in vari- 
ous parts have no common basis or standard; their mentai equip- 
ment and fund of ecological knowledge vary greatly and they may 
even have very different points of view. Some of these ob- 
jections might be summed up in the term “ personal equation.” 
Another danger in this method is that of applying local observa- 
tions to large areas, in other words, in generalizing on the basis 
of too meager observations. The conclusions drawn in the ob- 
servational method are largely in the nature of opinions modified 
as indicated above by the personal equation, while the experi- 
mental method produces conclusions based upon actual figures 
which are indisputable and carry the weight of scientifically 
proven facts. 
Another objection to the observational method in determining 
the effect of habitat factors is that this method studies the effect 
