But let me for the moment confine myself to the simplest and most common situation, involving only two sexes. Recent investigations indicate that there is an apparently inevitable psychological effect of combining two essentially distinct sub-species in one genetic unit. (Sparks like mad.) I perceive that many of you have just experienced the same delight-dismay the first researchers felt at recognizing this so-obvious and so-overlooked parallel with the familiar cases of symbiosis. The Terrans, mentioned earlier, are in many ways prototypical of sexuality in an intelligent species, and the usual and rather dramatic events on Aldebaran VI have added greatly to our insights into the psychology of sexuality in general. In this culture, dualism is very deeprooted, affecting every aspect of the i-e-s complex: not just philosophy and engineering, but mathematics, for instance, and mystique. This cultural attitude starts with a duality, or two-sided symmetry, of body structure. (Throughout this discussion he uses visual material--photos, diagrams, etc., of human bodies, anatomy, physiology, habitat, eating and mating habits, etc. Also goes off into some intriguing speculation of the chicken-or-egg type: is physical structure influenced by mental attitudes, or is it some inherent tendency of a chromosome pattern with _pairs_ of genes from _pairs_ of parents?) In this respect, the Terrans are almost perfect prototypes, with two pairs of limbs, for locomotion and manipulation, extending from a central--single--abdominal cavity, which, although containing some single organs as well as some in pairs, is so symmetrically proportioned that the first assumption from an exterior view would be that everything inside was equally mirror-imaged. Actually, the main circulatory organ is single--though consisting of two valves; the main breathing apparatus is paired; the digestive system is single--although food intake is through an orifice with paired lips and two rows of teeth. In both "male" and "female" types, the organ of sexual contact is single, whereas the gamete-producers are pairs. There is a single, roundish head set on top of the abdomen, containing the primary sensory organs, all of which occur in pairs. Even the brain is paired!