The animal kingdom constantly seeks to improve itself through "survival of the fittest". The feeble and diseased are preyed upon by predators, thus cleansing the stock, leaving behind the strongest to compete for the attention of mates in order to reproduce. One could argue that over many generations, this constant "improvement" would lead to super animals within a given species, but in fact it hasn't.
Ecosystems eventually reach an equilibrium with their environment, where the weak and diseased are still "trimmed" from the pack, but the overall strength of the entire system remains constant.
This all brings three questions to mind:
1) Is there some sort of external constant downward entropy that gets stronger as groups of animals improve? Such a thing would cause a natural limit to be reached.
2) Are their natural limits of the physical biology that the animals are hitting?
3) Are families of animals that exceed natural limits the ones who go on to speciate?