As is gravitation.

The chicken and egg thing: if it's any egg, then the answer is egg, as reptiles etc. came before chickens. If it's chicken eggs... the only ambiguity is how you define the question.

If a chicken egg is an egg made by a chicken (i.e. the species change from pre-chicken to chicken involved genes in egg-creation), then although the first chicken foetus obviously preceded the first chicken, it wasn't in an egg from another chicken (bearing in mind that the egg shell etc. is made by the mother, not the child), hence the chicken was first, which made the first true chicken egg.

On the other hand, if the genetic change which caused the transition from pre-chicken to chicken did not involve the egg-making genes, then the answer is egg, because the eggs are all the same and the only thing which differentiates them is if they have a chicken foetus inside or not, so the egg precedes the fully grown chicken.

In reality though there is not really a single moment at which a species can be said to change into another species. It's transitory.