FACTORS 21 



value is that readings are not made in a particular habitat ; as a rule, indeed, 

 they are made in towns and cities, and hence are far removed from masses 

 of vegetation. They are usually taken at considerable heights, and give but 

 a general indication of the conditions at the level of vegetation. The chief 

 difficulty, however, is that the factors observed at weather stations tem- 

 perature, pressure, wind, and precipitation are those which have the least 

 value for the ecologist. It is true that a knowledge of the temperature and 

 rainfall of a great region will afford some idea of the general character of 

 its vegetation. A proper understanding of such a vegetation is, however, 

 to be gained only through the exact study of its component formations. 

 Ecology has already incurred sufficient censure as a subject composed of 

 very general ideas, and the use of meteorological data, which can never be 

 connected definitely with anything in the plant or the formation, should be 

 discontinued. This must not be understood to mean that meteorological in- 

 struments can not be used in the proper place and manner, i. e., in the habitat. 



33. Habitat determination. It is self-evident that determinations of 

 factors by instruments can only be of value, in the habitat where they are 

 made. In other words, a habitat is a unit for purposes of measuring its 

 factors, and measures of one habitat have no exact value in another. This 

 fact can not be overstated. Thus, -while it is perfectly legitimate, and indeed 

 highly desirable, to locate thermographs in different mountain zones for 

 ascertaining the rate at which temperature decreases with altitude, the 

 data obtained in this way are not directly applicable in explanation of plant 

 or formation changes, except when the same species occurs at different al- 

 titudes. Special methods are valuable and often absolutely necessary, but 

 in view of the fact that the plant as well as the formation is the definite 

 product of a definite habitat, the fundamental rule in instrumentation is 

 that complete readings must be made within a habitat for that habitat alone. 

 This necessarily presupposes a certairt preliminary acquaintance with the 

 habitat to be investigated, as it is imperative that the station for making 

 readings be located well within the formation, in order to avoid transition 

 conditions. In vegetation, there are as many habitats as formations, and 

 in addition a large number of new and denuded habitats upon which suc- 

 cessions have not yet started ; a knowledge of each formation or succession 

 must rest ultimately upon the factors of its particular habitat. 



34. Determinable and efficient differences. The instruments employed in 

 studying habitats can not be too exact, as there is no adequate knowledge as 

 yet concerning the real differences which exist between related or con- 

 tiguous formations. This is particularly true of differences which are 



