Pleasure does not play any such quantitative part in our mental experiences as the aforesaid multiplications would at first indicate. Another genetic neural process accounting for this matter must now be noted.
Our worm had a sensory nervous system of a particular kind, ramifying throughout its body. Pleasure nerves do not ramify through our body to a like extent. "If the worm's nerves were pleasure nerves," you ask, "what has become of these in us?" As well you might ask what has become of the particular muscles that controlled the worm's various segments, or the segments themselves. They have been lost in morphologic modifications. Each sensory nerve of the worm may, so far as I know, now have its representation in us. But, if so, it could by no means play the same part in mediating our sensory life as it did in that of the worm. In the worm most if not all of these nerves terminated peripherally. If they were pleasure nerves, we must conceive that they there in some vague way mediated pleasure in response to the kinds of influence to which pleasure now responds in us; that is, to most of those kinds of influence which in us cause sight, sound, smell, taste, touch, heat, and cold. But as new sense systems came in, and new sense-organs were developed specially to mediate these separate kinds of influences, the old systems became pushed aside or down, or were left behind in growth, and buried from where their former influences would commonly reach them. In view of consequent disuse, therefore, we might say they would atrophy; only, 'inherited atrophy' is not a happy biological postulate at present. It seems better to conceive that as new organs developed, — organs of all kinds, sensory and muscular, legs, arms, eyes, ears, tongue, — as the organism as a whole grew, and developed to special forms and functions, each new sense system grew along with and into its appropriate organs, while the old sense did not grow, at least did not grow to anything like the extent that the other special sense systems did.
In the last Review we pointed out that 'in us' pleasure sensations seem to be developed in those sensory functions which are most fundamental, and in due proportion to their being
