Through long ages of exposure to the stimuli of cold and pain came response in the form of cold and pain spots, after minute struggles between the static conservative tissues of the skin and the dynamic force of repeated assaults upon them. In due time then receptors appeared and each became connected with the central organs, by which means better adapted motor reactions against “nocuous” cold and pain became possible. In 1900 Professor Sherrington summed up the evidence in Schafer’s work on Physiology against the existence “of separate afferent fibres with their specific end-organs entrusted specifically with carrying painful impressions to a pain centre,” but Professor Starling in his later work on Human Physiology speaks of “a distinct sense of pain,” probably subserved by a distinct set of nerve fibres, but for the present purpose it is not necessary that agreement on such a problem should be reached, for it is alone with pain _spots_ that we are concerned. He also points out that on the one hand the cornea is sensitive to only one of the four stimuli in question, that is, pain, and on the other that the surface of the glans penis is sensitive to cold and pain, but tactile sensation and warmth sensations are almost entirely absent. _Touch._--This form of stimulus and its response can only be reckoned as useful to the organism, except that it may be, and often must be indifferent. The great number of the touch spots can be understood when it is declared by Professor Sherrington that almost invariably there are one or more touch spots close to the emergence of each hair,[87] and that they are very numerous also on the palmar and plantar surfaces of the hand and foot. Of the four forms of cutaneous stimuli those of touch are the only kind that have so far been proved to have specialised corpuscles, the other three having developed the physiological equivalent of cold, pain and warmth _spots_. [87] _Schafer_, p. 922. _Warmth spots_ are decidedly the least numerous of the four, those of pain being, as stated by Professor Sherrington, the most numerous. It is obvious that unless thermal stimuli become somewhat excessive they hardly can be described as “stimuli,” being more or less neutral in their action on a warm-blooded animal. This cannot be entirely so, because it has been shown quite conclusively that warmth spots _do_ exist, though much less numerous than others. There is a significant fact as to thermal reaction and that is that there are no pure _heat_ spots like those of cold, for the stimuli of about 49° C are so associated with those of pain that warmth spots alone are distinguished, and among primitive man no stimuli of heat could impinge on his skin, until he had learned the use of fire, more powerful than those of solar heat.