FACTORS 2I 
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 ohserved 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 cbtained 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 certain 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 
