I have a question. I wonder if anyone has a good answer. Since the Cretaceous era, it seems reasonable to believe that there will freqently have been ecological niches in which greatest size offered greatest advantage -- as one assumes from the evidence was the case in the era of the great Sauropods. At the present time, one assumes that the Blue Whale has evolved within a niche favourable to greatest size. Yet, as I understand it, the largest land animals to evolve during the entire Cenozoic era have not been greatly more massive than the present day African Bush Elephant. So my problem is this: Why has nature never pulled out all the stops (so to speak) to create a land animal of, say, 30 or 40 tons or more since the dinosaurs? Surely the size incentive must sometimes (across several continents and 65m years) have been present! Is there something about the basic structure of land mammals that puts an upper limit on size? And what was it about the Sauropods that let them defy this limit?