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. 



The 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 



