"I'm only guessing now, but there is plenty of evidence that gives us an idea of its function. I'm willing to bet that the symbiote itself is not a simple organism, it's probably an amalgam of plant and animal like most of the other creatures on Dis. The thing is just too complex to have developed since mankind has been on this planet. The magter must have caught the symbiotic infection by eating some Disan animal. The symbiote lived and flourished in its new environment. Well protected by a bony skull in a long-lived host. In exchange for food, oxygen and comfort, the brain-symbiote must generate hormones and enzymes that enable the magter to survive. Some of these might aid digestion, enabling the magter to eat any plant or animal life they can lay their hands on. The symbiote might produce sugars, scavenge the blood of toxins—there are so many things it could do. Things it must have done, since the magter are obviously the dominant life form on this planet. They paid a high price for their symbiote, but it didn't really matter to race-survival until now. Did you notice that the magter's brain is no smaller than normal?"
"It must be—or how else could that brain-symbiote fit in inside the skull with it?" Brion said.
"If the magter's total brain were smaller in volume than normal, it could fit into the remaining space in the cranial hollow. But the brain is full-sized—it is just that part of it is missing, absorbed by the symbiote."
"The frontal lobes," Brion said with sudden realization. "This hellish growth has performed a prefrontal lobotomy!"
"It's done even more than that," Lea said, separating the convolutions of the gray matter with her scalpel to uncover a green filament beneath. "These tendrils penetrate farther back into the brain, but always remain in the cerebrum. The cerebellum appears to be untouched. Apparently just the higher functions of mankind have been interfered with, selectively. Destruction of the frontal lobes made the magter creatures without emotions or ability for really abstract thought. Apparently they survived better without these. There must have been some horrible failures before the right balance was struck. The final product is a man-plant-animal symbiote that is admirably adapted for survival on this disaster world. No emotions to cause complications or desires that might interfere with pure survival. Complete ruthlessness—mankind has always been strong on this anyway, so it didn't take much of a push."
"The other Disans, like Ulv here, managed to survive without turning into such a creature. So why was it necessary for the magter to go so far?"
"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 come 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."