But we must understand this matter yet more clearly. In our ancestral worm was developed a pretty complex nervous organism, for the mediation of a pretty complex life of physical activities and of mental experiences. We must now examine how new sense systems would join in upon the old, and, in consequence, what relationships the mental experiences mediated by the new would bear to those mediated by the old. Suppose a pair of eyes should develop in the worm's head. They would be of no use biologically unless joined in some way with the motor apparatus of the creature. Either a new set of motor nerves must be developed correspondent to the new optic fibres, or the new fibres must be grafted upon the old nervous centres. We do not fear contradiction when we say that, in general, the latter was the method of rise of our different senses.
This being so, it is to be noted that, excepting those of the new organs of sight, no new acts are made possible to the body by reason of its new sensations. Its old acts, however, are stimulated to far more numerous and complicated occurrence. In so far, therefore, as formerly these acts were in turn incitive to mental states, so the kind of states formerly incited and their peculiar characteristics would now be multiplied to far more numerous occurrence. The like would occur with the birth of each new sense system. Whatever the peculiar characteristics of our worm's primary sense, therefore, we should expect to discover them preserved to it and multiplied over and over again into the increased mental life of the developing creature.
It is probable, also, that the nerves of the primary system would, to some degree, extend themselves into the new organs of the new senses. And, to the degree that this happened and its nerves were stimulated by the same occurrences as worked upon the new sense, would the developed characteristics of the primary sense be revived and multiplied, both directly and by association, in the new mental experiences, and would thus become woven still more widely into the increasing mental life of the developing creature.
Having identified our primary sense with that of pleasure, it may now be suspected that we have accounted for far too much.
