Stine opened the door of the pharmacy. "This talk of thirst makes me dry." With economically efficient motions he poured grain alcohol into a beaker, thinned it with distilled water and flavored it with some flavor crystals from a bottle. He filled two glasses and handed Brion one. It didn't taste bad at all. "How do you mean parasitic, doctor? Aren't we all parasites of the lower life forms? Meat animals, vegetables and such?" "No, no--you miss the point! I speak of parasitic in the exact meaning of the word. You must realize that to a biologist there is no real difference between a parasitism, symbiosis, mutualism, biontergasy, commensalism--" "Stop, stop!" Brion said. "Those are just meaningless sounds to me. If that is what makes this planet tick, I'm beginning to see why the rest of the staff has that lost feeling." "It is just a matter of degree of the same thing. Look. You have a kind of crustacean living in the lakes here, very much like an ordinary crab. It has large claws in which it holds anemones, tentacled sea animals with no power of motion. The crustacean waves these around to gather food, and eats the pieces they capture that are too big for them. This is biontergasy, two creatures living and working together, yet each capable of existing alone. Now, this same crustacean has a parasite living under its shell, a degenerated form of a snail that has lost all powers of movement. A true parasite that takes food from its host's body and gives nothing in return. Inside this snail's gut there is a protozoan that lives off the snail's ingested food. Yet this little organism is not a parasite as you might think at first, but a symbiote. It takes food from the snail, but at the same time it secretes a chemical that aids the snail's digestion of the food. Do you get the picture? All these life forms exist in a complicated interdependence." * * * * * Brion frowned in concentration, sipping at the drink. "It's making some kind of sense now. Symbiosis, parasitism and all the rest are just ways of describing variations of the same basic process of living together. And there is probably a grading and shading between some of these that make the exact relationship hard to define." [Illustration] "Precisely. Existence is so difficult on this world that the competing forms have almost died out. There are still a few left, preying off the others. It was the co-operating and interdependent life forms that really won out in the race for survival. I say life forms with intent; the creatures here are mostly a mixture of plant and animal, like the lichens you have elsewhere. The Disans have a creature they call a vaede that they use for water when traveling. It has rudimentary powers of motion from its animal parts, yet uses photosynthesis and stores water like a plant. When the Disans drink from it the thing taps their blood stream for food elements."