To be honest with you, I have a hard time believing or comprehending that this could happen by chance mutation.
Well, let's look at this.
If poison / venom developed as a chance mutation what would we expect to find.
I would expect to find a variety of venoms/poisons and a variety of delivery systems. I would expect that species which are closely related would have similiar delivery systems, while species very distantly related might have totally unrelated delivery systems. I would expect that those types of animals which have been around the longest my have developed poison over animals which haven't been around as long.
Now, let's see what we find.
Types of Poison:
Snakes venom, jellyfish poison, toad poison, wasp poison, etc. There are literally hundreds if not thousands of different kinds of poisons. Their mechanics are so varied in fact that we have to develop dozens of different kinds of anti-venoms just to combat those we come into contact that most. Snake poisons kill their prey, spider poisons dissolve the insides of their prey, jellyfish poison paralyzes and kills extremely fast, toad poison makes predators vomit.
Delivery systems:
Ants/Bees/Wasps have detachable stingers on their abdomins. Snakes have hollow syringe-like fangs. Spiders also deliver poison through "fangs" though there fangs aren't true teeth. Toads excrete their poison through glands on their skin. Scorpions have stingers mounted on their long tails.
Where poisons are found:
The vast majority of poisonous/venomous creatures are found in the sea. They make up the most kinds of creatures, most kinds of poison, and by far the deadliest of poisons. Those animals on the ground with poison are mostly in the groups of insects, arachnids and snakes.
Seems like everything we'd predict to see, we're seeing.
What would we predict to see if poison was put in place by an intelligent designer?
As for the question - "why do snakes still crawl along the ground?"
You're going under the assumption that crawling on their bellies is not advantageous for snakes. That's just wrong. Snakes can get into places I could never hope to go. They can climb straight up limbless trees. Swim with great effeciency. Swallow meals so big, that even the fattest man wouldn't be able to come close to finishing it. Snake can kill by constriction, a particularly nifty skill.
Snakes are incredibly successful, largely due to the gains from losing their legs.
Your next question should be - "well, if being legless is so good, why aren't more things legless?"
Well, there are legless lizards (phoney snakes). But the biggest problem a species would have becoming like snakes is that snakes are already filling that niche very effectively. It's hard to dislodge them from their place in the chain of life.
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