The story Weinbaum did not write
...I have an odd hunch that if we knew the secret of the little cape-clothed imp, we’d know the mystery of the vast abandoned city and of the decay of Martian culture.
Valley of Dreams

“Ten-H fail,” said Parker, the engineer on duty. “Repair crew to section H.”
There was no strong emotion in his voice; such things happened at least twice a day, and  moreover, the engineer was not expected to get out of the observation dome and fix the protective fence under slinker fire.
“Gas-throwers ready,” he added.
No sane man will venture outside the dome without protection from the gas-throwers. Fortunately, the slinkers were bad chemists and therefore had no gas masks - yet. But then they had no firearms five years ago.
Grant Calthorpe sighed. It all looked so promising just ten years ago, when the agronomists of Neilan Drug managed to cultivate ferva in the polar zones of Io, so promising and so quiet. No more slinkers with their endless store of impish tricks and poisoned darts. No more trading with the utterly irresponsible and infuriatingly silly loonies. Officially, the loonies were a decadent remnant of a great race, but then the Martians were decadent as well and nobody called them silly. Crazy, nutty, queer they were all right; the loonies were not even stupid but plain silly. Calthorpe was not a sociologist, but he thought decadence was too mild - or too noble a word for this.
Ten years ago he was a young manager of one of the ferva plantations, and married to the beautiful Lee Neilan to boot. This, at least, did not change: they were still happily married, though they long ago decided to send their children to school someplace safe on Earth. The children were now in the Alice in Wonderland stage, and a small fortune was spent daily on interplanetary radio discussions of the subject. The Calthorpes could afford it; the profits of Neilan Drug were still fantastic, in spite of the decrease of the last few years.
And this decrease was due not to poor management but rather to something as inevitable and as unpredictable as a major natural disaster, to the unexpected aggressive onslaught of the slinkers. These small creatures, rather resembling cloaked rats and rather ratty in temper, were the only intelligent species on Io. Or maybe the only surviving one since intelligence is not something you usually associate with a present-day loony, in spite of their past glory. Ten years ago numerous slinker tribes and slinker nations lived in the equatorial jungles in a state that passed for peace - slinker peace, it is. No one could expect them to turn into slinker hordes and slinker armies relentlessly fighting humans wherever they could find them. The polar towns themselves were still safe, unlike the ferva plantations. Electric fences would hold for several days; afterwards you had to go to the damaged section and gas or burn all the surroundings, including as many slinkers as possible. For men, it was a permanent source of frustration; for the slinkers, it was total warfare, the like of which humans have not seen for centuries.
The slinkers were numerous, inventive, cunning and deadly - at close quarters. Instead of spectacular miniature medieval towns, complete to towers and battlements, they now dwelt in underground bunkers, well-hidden and not that easy to destroy by aerial bombardment. There was not much metal for the slinkers  to mine on Io, so they used tiny rifles with poisoned bullets but no serious artillery. Planes and caterpillar cars could resist slinker attack. In fact, as far as Calthorpe could remember, planes were never attacked at all.
“What the devil!” the engineer suddenly exclaimed. “It’s going to crush the fence!”
Calthorpe looked outside, in the direction of the ferva plantation. A small plane was moving jerkily over the jungles, much lower than the safe altitude, meaning safe under normal conditions, not over the battlefield.
“Can you see who it is?” he asked.
“No, but it carries the Committee emblem. Must be Stern, though I thought him to be a better pilot.”
Stern was also practically the only man on Io mad enough for long sightseeing excursions, or scientific expeditions, if you like it better. With this tiny world almost owned by several huge drug companies intent on producing shiploads of potent alkaloids, it was inevitable that the League of Nations’ Drug Committee kept its representatives here. Since the companies took care of all the possible illegal drug dealing which was scarce to begin with, these representatives had very little to do and virtually no chance for a good career. Consequently the ones to survive had some other interests in life. Sergei Stern was known to be fond of Ionian plants. Sometimes, however, he sounded as if he were fond even of loonies, though Calthorpe firmly believed that only a loony can feel to another loony anything better than utter disgust. But Parker was right: Stern would have known better than to make a lawn mower of his plane.  Or maybe he simply could not manage it? Ridiculous, anyone can fly a plane, even a drunk loony, if he manages to fit his neck inside.
“Oh, dear”, Parker said almost in a whisper. “He’s past.”
Past the fence, he meant. The dome could withstand not only collision with a plane but a falling spaceship or maybe some decent-sized meteor. Stern, if Stern it was, had to reach the airfield and land - preferably safely. And then he’d have to do his best to explain what happened. Fortunately, there was nothing high between this plantation and the airfield.
“O-o-oh”, Parker gasped. “He must have some problems.”
Now that Calthorpe had several seconds to think, he saw that damage to a plane was the only reasonable explanation for such an eccentric flying performance. But what on earth - or on Io - could have damaged a plane? Better to check on the spot and to question Stern or whoever was piloting.

The pilot turned out to be Stern, tired but undamaged at that, and his plane was somewhat different from what the manufacturers intended. There was no tail to speak of; what was present, or rather what was left of it gave virtually no possibility for either vertical or horizontal maneuvering. There were also some pox-marks on the hull, none of them going deeper than the paint.
“Imagine yourself”, Stern said in his most Russian-sounding English, “Zat rat chose to chew ze tail of mine when he saw the bullets didn’t damage me.”
“You mean the slinkers?” Calthorpe asked.
“I mean slinker plane. It tried to shoot at me, and when he saw that good Terran metal was too much for him, zat son of a bitch chose the collision course and got his propeller right into my tail. I’d say it would be death for an average pilot.”
“Slinkers have planes?”
“And rockets, maybe real strong ones, but the bastards missed me. I’ve always said ze slinkers were beyond our control, and now I have proof.”
“Proof? For what?”
“OK, let me have half an hour for myself. I think I deserve it as the first surviving participant of a dogfight on Io.”

Calthorpe did his best to bring the news to everyone of importance in Neilan Drugs, so when Stern was ready he had a wide audience including Gustavus Neilan himself. By then Stern really looked - and sounded - much better; at least he seemed to recover his ths though as yet not all the prepositions. 
“You know,” Stern said, “the origin of slinkers have always been of utmost interest for me..”
“To me,” Neilan corrected.
“Yes, to me. These analytical languages they always say it’s easy to manage. But what is important, is that there is no serious difference, on some deeper level of biological analysis, between slinkers and Terran rodents.”
“Meaning that slinkers are rats? It’s common knowledge.”
“Not as common as should have been. There is absolutely no way for one and the same species to develop independently on two worlds, no way at all; so if there are slinkers on Mars and slinkers on Io, there was someone who transported them to both planets.”
“Both? It is certainly more economic to assume that they originated on Mars - or on  Io for that matter,” one of the engineers ventured.
“It would have been economic were the slinkers related, let it be even distantly, to any of the local animals. By the way, just being animals makes them alien to Mars where every organism is part animal, part plant.”
“So you may mistrust scientists, but if they dubbed the slinkers Mus sapiens, it meant simply that slinkers are of Terran origin, no questions asked. Before the commence of human expansion there had been but one race capable of interplanetary travel, the Martians. Unfortunately it is not at all easy to question a Martian. Several decades after the first Martian expedition we are no much closer to the understanding how the Martians think than Jarvis himself was.”
“But one can make guesses. There are some technological constants which override any kind of psychological differences, you see, as productive forces determining relations of production. First of all, with no radioactive elements around, the Martians could rely only on chemical propellants - and on solar energy.”
“Solar light? But it can never be sufficient for a space ship!” Calthorpe had his share of space travel and he really didn’t believe this.
“No, it most certainly can. In interplanetary space high acceleration is the only benefit you gain from a powerful engine, and the Martians probably cared little about increased weight anyway. If they had big concave mirrors to heat and evaporate water, they could get enough thrust for space travel though the travel was possibly very long. Well, we know that being to some extent plants they can live for centuries. But they certainly had to use something like stratosphere planes to explore the planet of their destination.”
“I still don’t  buy it,” Neilan countered. “How could they assemble such a queer contraption? No chance for a normal take-off even with some good old chemical fuel.”
“So what? They probably used something like steam catapults to launch the components and assembled the ship in space. Not an easy task for humans, I agree. Yet we know that the Martians in their prime were equal to such marvels of cooperation and organization.”
“In any case, the Martian ships had to be really big and therefore had to employ the lightest materials possible. Plastics, but most certainly light alloys as well, and quite probably as much beryllium as they could find. I decided to put my  bet on beryllium simply because I knew that it  was toxic to Terran life. Fortunately, it is even more toxic to all life forms here on Io, and so I found several spots where the jungles did not completely recover even after fifteen millennia. Beryllium was certainly long gone, but I found some minor parts made of very resistant materials - platinum wire and also plastics I think we humans still don’t have. The Martians were great material scientists if nothing else.”
“On my way from this site I was attacked by the slinker fighter plane. Their first fighter plane as far as I know, but it means air travel is not safe over Io anymore. But, well, it’s not the most important thing.”
“OK, I may be just a mundane business man, but I’ve been to the university and I know the look of a man with a newly-hatched theory. Spill it!” Neilan said.
Stern smiled.
 “Well, it’s not much of a  theory, rather a piece of historical speculation. I am possibly nowhere near historical truth. I just don’t believe in too many random coincidences. But I’m almost sure I know how slinkers evolved from common rodents to an intelligent species capable of aerial combat, if it is any measure of intelligence.”
“First, the Martians were certainly fascinated with Terran civilization even at its stone age level. I don’t think our ancestors were an imposing lot but then I’ve never pretended to understand the intricacies of Martian mind. Or of an anthropologist’s for that matter. Yet in spite of this fascination there was possibly only one Martian expedition to Earth. What if that it was their last interplanetary expedition and that the Martian space programme collapsed soon after its return?”
“For no reason? Do you mean that fifteen thousand years ago they were already decadent, ready for decline and fall just for the hack of it?”
“No,” Stern said, “ for a very impressive reason. And the reason was that they brought rats from Earth, or mice, probably. Not only rats, of course, maybe quite a respectable sample of Terran flora and fauna. Do you know that there is still no explanation for these immense Martian buildings which look like so many empty boxes big enough for a space liner but containing nothing at all?”
“Some are still used as factory buildings, and some were maybe used in this Martian space programme of yours.”
“And some are just libraries, pure and simple. But these functional ones are a minor fraction, while all the rest simply enclose some considerable volume of thin Martian air. So what if they were intended precisely for this purpose, to contain the atmosphere and the living beings of other worlds?”
“Do you mean that all these gargantuan structures were a Martian zoo? Impossible!”
“Impossible for humans, yes, but not for the Martians. I don’t believe they ever had more than half a dozen spaceships, and they had to spend in space not months, as we do, not even years, as was the case of our early atomic rockets, but decades. Mind you, the Martians are adapted to low gravity and they most probably never used acceleration higher than their force of gravity. Their expeditions could have been even centuries apart. To take as many samples as possible for further study on Mars was for them the only practical option.”
“You hypothesis doesn’t hold air,” one of the witty officials said. “If these buildings were really meant to reproduce extra-Martian conditions, they had to be gas-tight at the very least.”
“And they most probably were. Say, our acclimatization chambers contain a lot of delicate and vulnerable things apart from thick walls and solid doors, and I believe the same applied to the giant Martian incubators. But imagine a population of rats contained in such a chamber. They would have most certainly attacked rubber. So what if several rats not only left the Terran incubator but penetrated some other one? The one where ferva grew?”
Stern paused, maybe just because he was tired of soliloquizing, but Calthorpe could not resist temptation.
“So what, indeed? Those hypothetical rats of yours had a cure for blancha provided they had enough brains to catch it.”
It was a mistake. Stern was as enthusiastic about local plants as any Neilan biochemist.
“Well, it is known that ferva acts on Terran organisms like gin-seng; it increases their resistance to local toxic agents including the ubiquitous fern spores causing white fever. By the way, do you know that the Drug Committee is monitoring their source, Iodryoptreis gigas? It can never become a smugglers’ drug, fortunately it has too many side effects for this; but it can endanger the psychical health of this colony.”
“As for ferva, it has many effects, some of them very intricate and not at all beneficial. I believe you’ve heard of the Harvard experiment when some of the ferva alkaloids were demonstrated to induce telepathy in humans, not to mention all sorts of hallucinations. So the ferva-eating rats who hardly had brains for hallucinations got the best of their diet. They became telepathic.”
 “Can’t be,” snapped Calthorpe. “I’ve  been taking ferverin almost daily while in the jungles, and it never made a telepath of  me. You may say that ferverin is to other ferva alkaloids what papaverin is to morphine, and you will even be right, but I’ve eaten raw ferva leaves, and nothing strange happened.”
“First of all,” Stern said, “you are not a rat.”
“Thank you.”
“Nor are you a cat, and therefore you will never enjoy valerian to the extent your cat does. By the way, do you know that some couple of centuries ago its effect was explained away as pure rationalization? If psychoanalysis were left to cats, psychopharmacology would have been different. So, well, telepathic effects in humans are erratic and capricious, but laboratory mice behaved quite differently. Or rather, when only one mouse was treated, nothing special happened, but in a group, every single mouse became several times more intelligent. There was even some... rumor, never confirmed officially, that they could predict the researchers’ intentions. ”
“Meaning that telepathy can overcome the interspecies barrier?” Neilan asked. “Nuts! We’ve been here for decades and nothing happened.”
“Absolutely nothing, only the slinkers have always been happy with their tiny bows. Then, all of a sudden, they develop firearms, then rockets and airplanes. They’ve been stealing ideas from us! Or do you need a slinker atom bomb to convince you?”
“But we are digressing. As for these experiments with mice, they were fortunately aborted, but then we humans know how dangerous rodents can be. The Martians had no such knowledge, and I believe that wild rats were much more intelligent than modern laboratory mice are. So in some historically short time a new intelligent species flourished on Mars, endowed with all the morality and charm of a cornered rat. They probably spent centuries hiding in places only a rat could find attractive - the Martians never had sewers, but there certainly were some underground constructions. The Martian atmosphere is thin, so there is much more space radiation to cause mutations. They most certainly mutated, gaining somewhat in stature and even more in intelligence, and finally the Martians could ignore them no more. These slinkers of old were, I suppose, basically the same beings they are now - dishonest, troublesome, stealing. Knowing what to steal. By the way, do you seriously think they have a purely aesthetic passion for our textiles?”
“Well, why not? And who cares?”
“We should have. No one seems to remember that our twenty-second century textiles contain all sorts of artificial fibers. I’ve not investigated it personally, it takes a suicide to dig into a slinker dwelling, but I’m absolutely sure threads of artificial silk, heating wire and cooling tubes are hoarded there. And I’m sure they’ve being stealing from the Martians as well, albeit with greater consequences since Mars have always been a poor planet.”
“This cohabitation could not have lasted indefinitely. We will probably never know what means the Martians used against the slinkers, but they were enough to infuriate this vermin. You know it’s not difficult to turn them into your enemies; but that time they were battling in earnest. God save us from their hate!”
Calthorpe thought that it was a somewhat peculiar phrasing for a Russian, but then Stern have always been eccentric.
“After I don’t know how much serious damage, they captured all the space ships available and set sail for Io. This would have maybe been tolerable if not for the rear guard that destroyed all the ground-based space facilities. By the way, that’s why the Martians hate their local slinkers bitterly. I don’t have data on the subject, but I suspect that without ferva they became duller than their brethern here. Or maybe they were dull to begin with and therefore were left behind? It’s hard for me to imagine a self-sacrificing rat. Anyway, the Martians never had a chance in that war, and it took them millennia to recover at least partially.”
“And you mean we are doomed to lose too?” Calthorpe said. “Ridiculous!”
“It would have been poor publicity indeed,” said Neilan. “A drug company defeated by rodents! As if men can not conquer rats!”
“We can use atomic blast and thus sterilize Io completely. We can gas this tiny world to the same degree. In both cases you will lose your plantations. Possibly all the slinkers too, though it is very difficult to kill a rat even with an atom bomb. And remember: we’ve never even aspired to crush an alien civilization completely. The slinkers succeeded twice.”
“Twice? You believe they’ve beaten somebody else after they’ve been done with Mars?”
“There is certainly something wrong with this explorer and exploiter type flourishing in our space colonies,” Stern said almost dejectively. “Some intellectual vice. Do you really think any intelligent species can ever become decadent to the point of pure imbecility? Primitive people on Earth were primitive only in a material sense; they retained perfectly normal human brains and were absolutely capable of using them. The loonies here are just plain idiots. It makes no sense.”
“Furthermore, they are stupid, but the slinkers are certainly not, and the human settlers are reputedly intelligent too. So how could it happen that the slinkers’ darts and arrows were poisoned with something toxic to humans but not to loonies and no one ever cared why? Or do you suppose that was a specific anti-human poison?”
“Well, why not? Humans and loonies have biochemical differences.”
“And to find it out they needed a human for experimentation, preferably a live human at that, and preferably not a single one. Mind you, they were barely medieval when we arrived. Or do you suspect that they remembered their planet of origin, knew we came from that same world, and deduced that anything affecting them had to be harmful to us too? This is ridiculous, if you know what I mean!”
“No, they are still using the same anti-loony poison, and it is just our bad luck that our bodies don’t like it. Or, if you prefer, it’s our good luck it doesn’t affect our minds. The loonies’ mind likes it less. It certainly took a long time to turn the whole race to the dim-wits they are now, but the slinkers succeeded. Maybe the city in Idiots’ Hills was the last loony citadel, and maybe its culture really has that melancholy tint the archaeologists claim. If I’m right, it had to.”
“Anyway, I don’t see what can be done against these creatures. We can hold for some  time, maybe for several years, but they are getting smarter and more technically advanced. With our help, too; we can not seal our thoughts. They don’t need to invent anything, just to adopt to their size, and they are quite adept in this. So my advice is to try and cultivate ferva someplace else, no matter how bad the publicity. I know it’s common knowledge that it does not grow anywhere but on Io, but remember that the Martians managed to cultivate it, for better or for worse.”

“Darling,” said Calthorpe that evening, “it is very likely we’d better start packing. Looks like we can’t beat the slinkers after all.” 
“Do you mean this Stern’s slinker history? I remember they used to be horrible years ago, and I realize that they are even worse now. Is there maybe some way to convince them we are not enemies?” Lee asked.
“First of all, we are enemies, and have been for years. For maybe twenty Earth years and for I don’t know haw many slinker generations. And please don’t forget: they are rats. They’ve been living side by side with the Martians who are the most moral race we know, and they remained rats. We have pretty little chance to reform them.”
“But... cats maybe?”
“We’ve discussed it today, but it won’t work. Cats are individualists, and they will never gain such a boost from ferva as to be on par with the slinkers. Stern still hopes some colonial carnivores could help, South African suricats maybe. But they are small, too small. Moreover, this can lead to two belligerent species here instead of one.”
Lee was leafing through a book absentmindedly.
“But... A! “It’s worth a hundred pounds! He says it kills all the rats and -” she said, somewhat unexpectedly. “Don’t you see? A dog, a terrier! They are small, they are pack animals, they’ve been hunting rats for centuries, and they are on friendly terms with man, if any animal ever was.”
“Carroll again?” asked Calthorpe. “But this can really help!”

Next morning Neilan was somewhat surprised when he saw a request for two hundred terriers to be shipped from Earth.
“An interesting project,” he agreed. “Maybe it will even work. The only problem is that I’ll have to fire every man on Io who is not fond of dogs. And I mean genuinely fond, for these telepaths will sniff the wrong emotion.”
“A good idea,” Stern said. “Shame I’ve forgotten about dogs. I hope you realize what will be their first and most important duty?”
“Chewing ferva leaves to develop collective intelligence.”
“No,” Stern sneered. “Standing on guard in the spaceport to make sure none of the slinkers sneaks on board an Earth-going ship.”