Wll, if Evolution is as blind and random as Darwin said it was, then even if Crocodiles don't need to evolve, they will still evolve. They can't just turn off the "Evolve"button once they get to perfection. They will still evolve, but not into a whole new kind of creature, just into new species of crocs that are variations of the basic croc kind...
You must remember. A creature 'evolving' is an abstract term.
A species does not 'EVOLVE' as an entity.
That is called Intelligent design, where something controls how a whole species develops. As a theory, it isn't very popular, mainly because the idea that a species could colaboratively change themselves is just stupid.
Evolution, on it's most basic basic form, is everywhere all around us.
Look around your class room at school. Look at how different and unique everyone of us is. Different hair colours, skin tones, tall people, smallish people, heavily built people, skinny people etc etc.
Everything constantly changes, but it's not called evolution.
It only becomes evolution, when selective design comes into play. Be it intelligent design or Natural Selection.
For example: you have an artic fox, white, fereal, furry fox.
This fox has a litter of pups.
Each one of these pups is unique. Some are smaller, some are bigger etc.
now we introduce Intelligent Design.
Lets say, some breeders take about 50 foxes and begin breeding them, and then choosing only the cutest and fluffiest pups, which are then bred together and so forth. Within a few generations, as few as 5, the resulting pups are ridiculously cuddely and tame.
The fereal artic fox, having been domesticated into a cuddely artic fox. Essentially, evolved.
This is the same way Dogs were domesticated.
Natural Selection works in a similar, but slower and less exclusive way. In the natural world, survival by any means possible, is what allows an animal to breed, and is therefore, a success.
Essentially, the animals which aren't capable of surviving dissappear, and those who do better, evolve.
However, if nobody does significantly better, then nothing happens.
Crocs didn't turn off the 'Evolve' button, nature did.
The Crocs stopped evolving because the weak crocs stopped dying.
If you were to take lets say, 1000 crocs, and released them into an artificial environment, then establish a food chain, only to replace their primary food source with a scarcer and tougher food source, you'd soon find crocs dying. Their numbers would drop, as it would be harder for them to find food.
Eventually the number of crocs would even out and stop dropping.
I garantee you, if you were to examine these crocs then, you'd find they had evolved. Not a new leg or anything stupid like that, but you'd probably find them to be stronger, or equipped with more suitible camoflauge.