20 THE HABITAT 
affecting water-content and the amount of water absorbed. Earthworms 
and plant parts change the texture of the soil, and thereby the water-content. 
Indirect factors often exert a remote influence also, as may be seen in the 
effect which temperature and wind have in increasing evaporation from the 
soil, and thus reducing the water-content. This distinction between. factors 
may.seem insufficiently grounded. In this event, it should be noted that it 
centers the effects of all factors upon the three direct ones, water-content, 
humidity, and light. If it further be recalled that these are the only factors 
which produce qualitative structural changes, and that the classification of 
ecads and formations is based upon them, the validity of the distinction is 
clear. 
Tue DETERMINATION OF FACTORS. 
31, The need of exact measurement. Any serious endeavor to find in the 
habitat those-causes which are producing modification in the plant and in 
vegetation can not stop with the factors merely. The next step is to de- 
termine the quantity of each, It is not sufficient to hazard a guess at this, 
or to make a rough estimate of it. Habitats differ in all degrees, and it is 
impossible to institute comparisons between them without an exact measure 
of each factor. Similarly, one can not trace the adaptations of species to 
their proper causes unless the quantity of each factor is known. It is of 
little value to know the general effect of a factor, unless it is known to 
what degree this effect is exerted. For this purpose it becomes necessary 
to appeal to instruments, in order to determine the exact amount of each 
factor that is present in a particular habitat, and hence to determine the 
ratio between the stimulus and the amount of structural adjustment which 
results. The employment ‘of instruments of precision is clearly indispen- 
sable for the task which we have set for ecology, and every student that 
intends to strike at the root of the subject, and to make lasting contributions 
to it, must familiarize himself with instrumental methods. One great benefit 
will accrue to ecology as soon as this fact is generally recognized. The use 
of instruments and the application of results obtained from them demand 
much patience and seriousness of purpose upon the part of the student. As 
a consequence, there will be a general exodus from ecology of those that 
have been attracted to it as the latest botanical fad, and have done so much 
to bring it into disrepute. 
32. The value of meteorological methods. At the outset there must be 
a very clear understanding that weather records and readings have only a 
very general value. This is in spite of the fact that the instruments em- 
ployed are of standard precision. An important reason for this lack of 
