This author is a veteran of sci-fi. He's written scores of novels, and done many novelizations of movies (such as the rebooted Star Trek, the Alien movies, the Transformers movies, and so on). This makes it intriguing that I found poor writing and errors in this novel, such as his use of the term 'googolplex' which he renders as 'googaplex'!
Midworld is a 1975 novel set in a Foster-created universe and is a part of a series comprised of almost a dozen standalone novels. Why Foster never launched a lawsuit against James Cameron and others associated with the 2009 movie Avatar is a bit of a mystery, because the similarities between this novel and that movie are quite startling.
The borrowing (to put it politely) from Foster's book is extensive, including six-legged native species, an intensely harsh jungle environment with luminescent plants, arboreal living quarters which are actually named Hometree, interloping humans intent upon exploiting the planet, the planet's living things all connected in a web of life, and so on. The differences are also notable. In this case, the natives that the interlopers encounter are actually humans from a colony ship who were stranded on this planet centuries before. They have quickly evolved somehow to be smaller, although they still speak English. There is also a second species on the planet which is both native and sentient (and six-legged), and which seems to have partnered-up with the humans who have now become native.
That said, I adored the Avatar movie. I discovered recently there is now a planned four sequels to it, running through 2023 for release dates, and I'm really looking forward to them. The first Avatar earned almost three billion dollars. My guess is that they're going to re-release it when the sequel comes out, so it could top three billion when it's done. I'd certainly like to see it in 3D again in the movie theater. It's the best exponent of 3D in a movie that I've ever seen.
But I digress! This story is of a tribe of diminutive humans (not hobbits!) living in a hellish hostile jungle, where the ground is deemed too dangerous to inhabit, so the humans live in the trees, hence the name 'Midworld": there are several levels in the canopy from ground to sky, and this one has proved the safest, despite it still being a nightmare. Here's where problems may arise for some readers because although Foster evidently understands evolution, which is a refreshing change from a disturbing number of other authors, particularly YA ones, he still had inexplicable organisms which make little sense even in context.
Just as it is in Avatar, although less extreme there, this earlier work has nature so hostile that it exists at war zone levels. You can argue that it's dangerous on Earth, for example in a jungle where plants, insects, and predators make life highly risky, but here in Midworld, it's like every single step risks an encounter with a virulently deadly organism of one sort or another, each of which seems to have highly-developed poison or predatory traits.
I found it hard to believe that anything could survive on a planet like this except for the apex predators, who would quickly be forced into cannibalism as their hapless prey became extinct. Normally organisms only evolve to a level at which they can survive (or they become extinct because they fail to adapt). There is no impetus to evolve beyond that because evolution involves no intelligence whatsoever, regardless of the clueless claims of the brain-dead creationist community, and no planning for the future.
You can argue that snakes have no need for their venom to be so potent, and this is a good argument if your 'science' background consists of the book of Genesis, but in the real world, this view actually ignores evolution. For example, snakes did not evolve with mammals, which are a big component of their prey today. Snakes evolved with other reptiles whose metabolism was much slower than that of mammals, and so the toxins needed to be overwhelming and fast-acting. Snakes which had such toxins survived better than those without them.
When mammals came along later, these poisons worked even better on the hyped metabolism of this new prey. This is why you cannot ignore evolution when world-building in a story like this. For me it was more of an annoyance than it was a fail initially, because some of it was interesting and inventive. It was the extension of this into sheer idiocy which turned me off the story eventually. The real problem though, was that the author seemed to have become quite carried away with his own creation and like a parent obsessed with their young child, expended far too much time telling us stories about it, writing pages on the locals' battle with flora and fauna, at the neglect of getting on with the larger story.
Another issue I had was with the names given to the local life. Historically, when humans have expanded into new areas, they have carried with them the baggage of their previous life, and this would have been the case with the colonists who landed on the planet all those centuries ago, so it made no sense that the local life was not named after life on Earth. I can see some new names coming along for things which had no good counterpart on Earth, but when we're introduced to a creature described as reminiscent of a pig, which lives in the trees, why was it called a Brya instead of a Tree-pig? From a writing perspective, it bears thinking about, and evidently this author didn't think enough.
The way Foster would have it is that pretty much everything in this world is an apex predator and that's impossible. You can't have organisms this deadly without having a completely different ecosystem than the one that's presented here. Predators must necessarily be fewer than their prey otherwise they would die out from lack of same, yet here we see only predators, they're always hungry, and there's virtually no prey save the small group of indigent humans! It makes no sense. It was done only for "drama" but it was way the hell too dramatic to be either realistic or entertaining.
Additionally, Foster seems to forget that you not only have to give a serious nod (and no winking!) to evolution, but you also have to stay within the bounds of physics, unless you're positing an entirely different universe than the one Foster created here. One example of this is the ridiculous height of the 'trees'. The tallest known tree on Earth is close to the maximum limit. It's around 115 meters, and the limit is about 122, so it's pretty much there already. Taller than this, the trees cannot suck up water to the top, but Foster is claiming the trees on this world are half a kilometer, or over four times as tall as is practical and realistic. That's not gonna happen!
Here's a poor writing example from about sixty percent through the novel: "The Silverslith was moving slowly, deliberately, playing with its intended prey." The intended prey were the humans who were sleeping and unaware of the predator, so how, in any sense, was this playing with them?! And what's with the Silverslith name? Was this a snake of some sort? The description is too vague to determine properly what it was, but whatever it was, why was it called a slith instead of a snake or whatever?! Worse than this is that this is yet another example of the dangerous wildlife hijacking the story, and some of the wildlife, such as this and the ant-like (in behavior but not in size) Akadi hoard are far too improbable to exist in any reality.
Of course, the Silverslith is only a poor excuse to make the humans travel to a lower level of the forest so Foster can exhibit even more insane predators than the ones which exist in the upper canopy. It was so transparent and amateurish that I began to dislike the story at this point. Even when the danger of the Silverslith was over, these people stayed down there! I'd had it repeatedly drilled into me, during the entire first half of the story that it was far, far, far too dangerous to travel down to the lower levels, yet this group of travelers stayed down there for several hours for no reason! I'm sorry, but this was not only unnecessary, this amateurish approach rendered all the previous talk into pure bullshit! If a first time writer had submitted this story, it would have been rejected, but because Foster was established by then, he could get away with it.
One amusing part was when one of the visiting humans felt death was near. The panicked statement came out, "Not like this...not this way" which was very reminiscent of what Belinda McClory's character Switch's last words from The Matrix said, right before she died! But that kind of humor was unintentional and very rare. Unlike in Avatar there was no humor here, and the story suffered for it.
Part-way through chapter ten, or around 65% in, I'd had enough of this endless onslaught of absurd and improbably predatory creatures and lack of a direction to the story, so I quit reading this as a waste of my time. I can't recommend it. It's #4 in the so-called 'humanx' commonwealth series, but I will not be reading any more. I recommend watching Avatar instead. It's more realistic (for its framework) and inventive, and it tells an amusing and much more engaging story.