contravariance

The reversal of the order of data types acted upon by an operator.

I get why with covariance I can return a Dog (a Dog is a Mammal), but why does contravariance work the other way? Shouldn't it accept a derived type when it expects a base type? Contravariance is consistent with Postel's Law: […]

A functor which reverses composition.

Of vectors, the property of scaling inversely with a change of basis, as opposed to covariance.