reason the discussion of adaptation in the pages that follow is practically confined to modification of structure. This is particularly desirable, since growth has long been the theme of physiological study, while modification has too often been considered from the structural standpoint alone. The comparatively few studies that have taken function into account have been largely empirical; in them neither stimulus nor adaptation has received anything approaching adequate treatment. =148. The measurement of response.= The amount of response to a stimulus is proportional to the intensity of the factor concerned. This does not mean that the same stimulus produces the same response in two distinct species, or necessarily in two plants of one species. In these cases the rule holds only when the plants or species are equally plastic. For each individual, however, this quantitative correspondence of stimulus and response is fundamental. It is uncertain whether an exact or constant ratio can be established between factor and function; the answer to this must await the general use of quantitative methods. There can be no doubt, however, that within certain limits the adjustment is proportional to the amount of stimulus, whereas reaction is well known to be abnormal or inhibited beyond certain extremes. It is quite erroneous to think that reaction is independent of quantity of stimulus, or to liken the stimulating factor to “the smallest spark (which) by igniting a mass of powder, produces an enormous mechanical effect.”[8] Such a statement is only apparently true of the action of mechanical stimuli upon the few plants that may properly be said to possess irritability, such as sensitive plants and certain insectivorous ones. Of the normal relation of response to direct factors, water, light, etc., it is entirely untrue. Axiomatically, there is ordinarily an essential correspondence, also, between the amount of adjustment and of adaptation. This correspondence is profoundly affected, however, by the structural stability of the plant. From the preceding it follows that the measurement of response and the relating it to definite amounts of direct factors as stimuli are two of the most fundamental tasks of ecology. The exact determination of physical factors has no value apart from its use for this purpose. It is perfectly clear that precise methods of measuring stimuli call for similar methods in determining the amount of adjustment and of adaptation. The problem is a difficult one, and it is possible at present only to indicate the direction which its development should take, and to describe a few methods which will at least serve as a beginning. To cover the ground adequately it is necessary to measure response by adjustment and by adaptation separately, and in the latter to find a measure for the individual and one for the species. The one is furnished by the methods of morphology and the other by biometry.