"Nothing is necessary in evolution, you know that," Lea said. "Many variations are possible, and all the better ones continue. You might say that Ulv's people survive, but the magter survive better. If offworld contact hadn't been re-established, I imagine that the magter would slowly have become the dominant race. Only they won't have the chance now. It looks as though they have succeeded in destroying both races with their suicidal urge." "That's the part that doesn't make sense," Brion said. "The magter have survived and climbed right to the top of the evolutionary heap here. Yet they are suicidal. How does it happen they haven't been wiped out before this?" "Individually, they have been aggressive to the point of suicide. They will attack anything and everything with the same savage lack of emotion. Luckily there are no bigger animals on this planet. So where they have died as individuals, their utter ruthlessness has guaranteed their survival as a group. Now they are faced with a problem that is too big for their half-destroyed minds to handle. Their personal policy has become their planetary policy--and that's never a very smart thing. They are like men with knives who have killed all the men who were only armed with stones. Now they are facing men with guns, and they are going to keep charging and fighting until they are all dead. "It's a perfect case of the utter impartiality of the forces of evolution. Men infected by this Disan life form were the dominant creatures on this planet. The creature in the magters' brains was a true symbiote then, giving something and receiving something, making a union of symbiotes where all were stronger together than any could be separately. Now this is changed. The magter brain cannot understand the concept of racial death, in a situation where it must understand to be able to survive. Therefore the brain-creature is no longer a symbiote but a parasite." "And as a parasite it must be destroyed!" Brion broke in. "We're not fighting shadows any more," he exulted. "We've found the enemy--and it's not the magter at all. Just a sort of glorified tapeworm that is too stupid to know when it is killing itself off. Does it have a brain--can it think?" "I doubt it very much," Lea said. "A brain would be of absolutely no use to it. So even if it originally possessed reasoning powers they would be gone by now. Symbiotes or parasites that live internally like this always degenerate to an absolute minimum of functions." "Tell me about it. What is this thing?" Ulv broke in, prodding the soft form of the brain-symbiote. He had heard all their excited talk but had not understood a word. "Explain it to him, will you, Lea, as best you can," Brion said, looking at her, and he realized how exhausted she was. "And sit down while you do it; you're long overdue for a rest. I'm going to try--" He broke off when he looked at his watch.