But if the determination of the taste organs and their localization offers difficulties in the adult human being, these are multiplied many times when the study is carried to the lower animals. The method of stimulation is greatly limited, to what extent depending upon the kind of animal studied, because the results of the application of stimuli must be interpreted from the forms of behavior exceedingly crude, compared with the language behavior of man. This handicap in the study of taste is great, as compared with sight and hearing, at least, on account of the extremely close relation between the taste and smell organs in position and in the nature of their appropriate stimuli. In case of certain of the lower animals the assumption of distinct states of consciousness corresponding to our experiences of taste and smell is unwarranted, as is even the assumption of any consciousness at all. The study of taste structures reduces itself largely to the search for sense organs resembling those of man, and in the same neighborhood as they are found in man. Here again the difficulty is especially great in taste, because the taste organs even in man are not very highly differentiated from other structures, and the really essential part of the organ is not definitely known. (See Chapter VI.) In the search for taste in lower animals one must rely much upon the expectation of finding the taste mechanism in the mouth or its immediate neighborhood. Structures found here and not known to function otherwise are likely to be looked upon as taste organs. These assumptions from location are then tested by stimulation of the parts with sapid substances and looking for characteristic responses, and further by extirpating these organs and noting the effect upon behavior. Other criteria of sensitivity, which can be used especially in the study of sight and hearing, such as rate of fatigue, reaction time to stimulation, and the like, are of little use on account of the above-mentioned close relation between taste and smell. When looked at from the evolutionary point of view, all of the senses are seen to have developed through modification of the _sensitivity_ of a single structure, the cell, with its additional properties of conductivity and motility. In the simplest living organisms, for instance, one finds sensitivity to consist in the irritability common to all living cells, and the sense organ to be represented by the whole cell. Still, the amœba, one of these unicellular organisms, reacts differently to the contact of food substances and to purely mechanical stimulation. And the white corpuscles of the blood in the human body are said to adjust their behavior according to the chemical composition of their surroundings. So, even in this earliest stage of evolution, before any differentiation of structure appears, one sees a reaction analogous to the taste reactions of the higher animals. “_The Chemical Sense_”