All of the creatures in Species are bilaterally symmetrical, so their left side is a mirrored duplicate of their right. Interestingly, this didn’t actually need to be the case. I could have allowed the limbs and eyes, for example, to break symmetry without too much modification from the code I’m using currently: but I decided not to.
I chose to do this for several reasons:
1. Simplicity. Unlike real life, creatures in Species don’t consist of potentially infinite variables with a phase space of “everything that’s possible with organic chemestry”. A computer simply can’t run a real-time simulation of organic life. So the creatures in Species have to be simplified representations: I need to be able to condense them to as few variables as possible. Bilateralism allows me to condense two variables into one. In addition, I don’t have to program contingencies for lopsidedness: a better leg means a faster creature, not a lopsided one.
2a. It’s actually logical (No, really). In real life, bilateralism evolved fairly early: after plants and fungi split off, but before vertebrates evolved. As a result, all surviving animals on earth are bilateral. So it’s a fair enough assumption that the creatures in Species; are decendants of a bilaterally symmetrical species, so bilateral symmetry is built into their genetic code.
2b. It’s not just earth. Yes, other potential bodyplans exist: many Trees and plants, and even some early animals, use/used a fractal bodyplan, and animals like jellyfish and sea-anenomes exhibit radial symettry. But other body plans don’t easily allow for mobility or for versatility: jellyfish can only move in one axis, tree’s can’t move at all except by growth. It’s very plausible that life evolving elsewhere in our universe will also stumble upon bilateral symmetry.
3. Evolution rate. Non-symmetrical mutations are almost invariably harmful: to improve a leg for example you would have to mutate both of them, not just one. In real life, bilateralism gets around this on a genetic level: when a body is developing, it often uses the same genes to make both the left and right sides of the body, so a mutation to these genes will affect both sides in the same fashion. This allows for beneficial mutations to be taken advantage of immediately, rather than waiting for the other side to develop a coinciding mutation to fix the lopsidedness. The end result of all this is that symmetry speeds up the process of evolution. By replicating this in Species, natural selection will be faster and more apparent than if non-symmetrical creatures were allowed for.
4. Intuition. We expect our animals to be bilaterally symmetrical. Our mind instinctively objects to exceptions to this rule, like sponges and flounders and Yog Sothoth. We only accept trees and flowers because we’re used to them, and because they don’t move: this is at least partly why the idea of walking tree’s is creepy on a fundamental level. Also, because we all subconsiously realise that sooner or later tree’s will develop sentience and the ability to move, and when that happens we’re all going to die the moment they realise what paper’s made out of. Seriously, have you seen some of the weapons on vegetation? We’re totally screwed.
Now, whether all of these count as valid and sensible reasons to use bilateral symmetry, or whether they’re just the weak-willed excuses of someone too lazy to properly implement other types of body-plans, is up to you to decide. I’m not telling. I am laughing maniacally, but I spend a good quarter of my time doing that anyway. It’s kinda a thing with me.