It is not easy to tell, for the animal kingdom at large, what sense-organs came first. Many coelenterates have eyes, while certain annelids, standing far higher in the line of morphologic development, have no eyes. But it is fortunate for our present purpose that, though the exact line of human ancestry is not known, biological opinion now speaks conclusively for an annelid type at one stage, having a simple nervous system. The earthworm, though probably not our identical ancestor, may show us what that creature would be like.
This worm has a well-defined and relatively complex nervous system. This system comprises a lot of nerves running to and from segmental ganglia which are connected by bilateral cords. The forward ganglion is the largest, and may be called the brain. The peripheral ends of its sensory nerves terminate, so far as can be discovered, in simple cells of like nature throughout. There is no evidence of other specific sense energies than those which may be mediated by this apparently homogeneous sense system. By means of this, the creature digs, eats, procreates, is a respectable traveller, and leads a complicated life. It does not see, hear, or smell. To say it does not experience touch, taste, heat, cold, pain, or pleasure would raise criticism respectively from the champions of each of these as the primary sense. We will only assume, then, what all seem inclined to agree to, that the worm does all that it does in terms of some one sense. And for our ancestral worm, we will leave the kind, or psychic quality, of its single sense for the present unnamed.
Our worm being provided with this kind of a nervous system, we may note that from the outset its neuro-muscular functions would divide into two contrasting classes: those which would tend to continue or to promote beneficial experiences, and those which would tend to discontinue or to inhibit baneful ones. At
sense than would pleasure. Pleasure, of all the senses we know anything about, best satisfies the demands of a primary sense, is open to more influences of the proper range. Any creature, having the pleasure sense as its primary sense, would therefore have advantages over others from the start. Man's line of ancestry certainly became possessed of the pleasure sense at some time, and appears to have had the lead since some very important epoch. It does not seem unfair, then, to use the above as a slight argument in favor of pleasure as the primary sense of our line of ancestry.
