Discover subclasses of a given class in Obj-C

Question

Is there any way to discover at runtime which subclasses exist of a given class?

Edit: From the answers so far I think I need to clarify a bit more

Answer 1

Marc and bbum hit it on the money. This is usually not a good idea.

However, we have code on our CocoaHeads wiki that does this: http://cocoaheads.byu.edu/wiki/getting-all-subclasses