I think I recall reading in Peter Singer's "Animal Liberation" that humans, by nature, have a Continuous Moral Improvement. This was in reference to how sexism and racism are become less common, and how this could one day be the case for speciesism.
However, there was another, related term used by Singer to express the same concept. I thought it might be Kaizen, but this seems more business orientated.
So, what is the other term used by Singer?