One of these submarine worlds was exceptionally interesting. Early in the life of our galaxy, when few of the stars had yet condensed from the "giant" to the solar type, when very few planetary births had yet occurred, a double star and a single star in a congested cluster did actually approach one another, reach fiery filaments toward one another, and spawn a planet brood. Of these worlds, one, an immense and very aqueous sphere, produced in time a dominant race which was not a single species but an intimate symbiotic partnership of two very alien creatures. The one came of a fish-like stock. The other was in appearance something like a crustacean. In form it was a sort of paddle-footed crab or marine spider. Unlike our crustaceans, it was covered not with a brittle carapace but with a tough pachydermatous hide. In maturity this serviceable jerkin was more or less rigid, save at the joints; but in youth it was very pliant to the still-expanding brain. This creature lived on the coasts and in the coastal waters of the many islands of the planet. Both species were mentally of human rank, though each had specific temperament and ability. In primitive times each had attained by its own route and in its own hemisphere of the great aqueous planet to what might be called the last stage of the subhuman mentality. The two species had then come into contact, and had grappled desperately. Their battle-ground was the shallow coastal water. The "crustaceans," though crudely amphibian, could not spend long under the sea; the "fish" could not emerge from it. The two races did not seriously compete with one another in economic life, for the "fish" were mainly vegetarian, the "crustaceans" mainly carnivorous; yet neither could tolerate the presence of the other. Both were sufficiently human to be aware of one another as rival aristocrats in a subhuman world, but neither was human enough to realize that for each race the way of life lay in cooperation with the other. The fish-like creatures, which I shall call "ichthyoids," had speed and range of travel. They had also the security of bulk. The crab-like or spider-like "crustaceans," which I shall call "arachnoids," had greater manual dexterity, and had also access to the dry land. Cooperation would have been very beneficial to both species, for one of the staple foods of the arachnoids was parasitic to the ichthyoids.
In spite of the possibility of mutual aid, the two races strove to exterminate one another, and almost succeeded. After an age of blind mutual slaughter, certain of the less pugnacious and more flexible varieties of the two species gradually discovered profit in fraternization with the enemy.
This was the beginning of a very remarkable partnership. Soon the arachnoids took to riding on the backs of the swift ichthyoids, and thus gained access to more remote hunting grounds.
As the epochs passed, the two species molded one another to form a well-integrated union. The little arachnoid, no bigger than a chimpanzee, rode in a snug hollow behind the great "fish's" skull, his back being stream-lined with the con-tours of the larger creature. The tentacles of the ichthyoid were specialized for large-scale manipulation, those of the arachnoid for minute work. A biochemical interdependence also evolved. Through a membrane in the ichthyoid's pouch an exchange of endocrine products took place. The mechanism enabled the arachnoid to become fully aquatic. So long as it had frequent contact with its host, it could stay under water for any length of time and descend to any depth. A striking mental adaptation also occurred in the two species. The ichthyoids became on the whole more introvert, the arachnoids more extrovert.
Up to puberty the young of both species were free-living individuals; but, as their symbiotic organization developed, each sought out a partner of the opposite species. The union which followed was life-long, and was interrupted only by brief sexual matings. The symbiosis itself constituted a kind of contrapuntal sexuality; but a sexuality that was purely mental, since, of course, for copulation and reproduction each individual had to seek out a partner belonging to his or her own species. We found, however, that even the symbiotic partnership consisted invariably of a male of one species and a female of the other; and the male, whichever his species, behaved with parental devotion to the young of his symbiotic partner.
I have not space to describe the extraordinary mental reciprocity of these strange couples. I can only say that, though in sensory equipment and in temperament the two species were very different, and though in abnormal cases tragic conflicts did occur, the ordinary partnership was at once more intimate than human marriage and far more enlarging to the individual than any friendship between members of distinct human races. At certain stages of the growth of civilization malicious minds had attempted to arouse widespread interspecific conflict, and had met with temporary success; but the trouble seldom went as deep even as our "sex war," so necessary was each species to the other. Both had contributed equally to the culture of their world, though not equally at all times. In creative work of every kind one of the partners provided most of the originality, the other most of the criticism and restraint. Work in which one partner was entirely passive was rare. Books, or rather scrolls, which were made from pulped seaweed, were nearly always signed by couples. On the whole the arachnoid partners dominated in manual skill, experimental science, the plastic arts, and practical social organization. The ichthyoid partners excelled in theoretical work, in literary arts, in the surprisingly developed music of that submarine world, and in the more mystical kind of religion. This generalization, however, should not be interpreted very strictly.
The symbiotic relationship seems to have given the dual race a far greater mental flexibility than ours, and a quicker aptitude for community. It passed rapidly through the phase of inter-tribal strife, during which the nomadic shoals of symbiotic couples harried one another like hosts of submarine-cavalry; for the arachnoids, riding their ichthyoid mates, attacked the enemy with bone spears and swords, while their mounts wrestled with powerful tentacles. But the phase of tribal warfare was remarkably brief. When a settled mode of life was attained, along with submarine agriculture and coral-built cities, strife between leagues of cities was the exception, not the rule. Aided no doubt by its great mobility and ease of communication, the dual race soon built up a world-wide and unarmed federation of cities. We learned also with wonder that at the height of the pre-mechanical civilization of this planet, when in our worlds the cleavage into masters and economic slaves would already have become serious, the communal spirit of the city triumphed over all individualistic enterprise. Very soon this world became a tissue of interdependent but independent municipal communes.