Features differ in their mutability. For example, a robin could still be a robin even if it lacked a red breast; but it would probably not count as one if it lacked bones. One hypothesis to explain this differential transformability is that having bones is more critical to a biological theory than having a red breast is. W e reject this hypothesis in favor of a theory of mutability based solely on local dependency links and expressed in the form of an iterative equation. W e hypothesize that features are immutable to the extent other features depend on them and offer supporting data.
