=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 determine 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 indispensable 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.