Against organisms of other species (e.g. bacteria) we have barriers, which are both external (e.g. personal hygiene), and internal (e.g. the immune response). The preceding chapter outlined the origin of the clonal selection theory which is fundamental for understanding how our bodies (“self”) detect and destroy foreign organisms (“not-self”) in immune responses. Another, no less subtle form of self/not-self discrimination, involves our detection of a mate (“near-self”) who will be our “physiological complement” such that the union will produce healthy offspring (“hybrids”). An incestuous relationship with a close relative (“too near-self”) will probably result in less healthy offspring. On the other hand, extreme out-breeding, such as with an ape (not-self), is prohibited by species barriers.