20 Richard H. Bocrkcr 



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 

 ill 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 is 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 ^insurmountable objections. Observers in vari- 

 ous parts have no common basis or standard ; their mental 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 



