they are also used in active flight. As another example, Alan Hayward stated in Creation and Evolution (1985) that "Darwinists rarely mention the whale because it presents them with one of their most insoluble problems. They believe that somehow a whale must have evolved from an ordinary land-dwelling animal, which took to the sea and lost its legs ... A land mammal that was in the process of becoming a whale would fall between two stools—it would not be fitted for life on land or at sea, and would have no hope for survival." The evolution of whales has been documented in considerable detail, with Ambulocetus, described as looking like a three-metre long mammalian crocodile, as one of the transitional fossils. The hippopotamus, the whale's closest living ancestor, exemplifies how an animal might be well-suited for both land and water. Although transitional fossils elucidate the evolutionary transition of one life-form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of living beings, only a very small percentage of all life-forms that ever have existed can be expected to be discovered. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never be known in detail. Progressing research and discovery managed to fill in several gaps and continues to do so. Critics of evolution often cite this argument as being a convenient way to explain off the lack of 'snapshot' fossils that show crucial steps between species. The theory of punctuated equilibrium developed by Stephen Jay Gould and Niles Eldredge is often mistakenly drawn into the discussion of transitional fossils. This theory pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. For example, the change from a creature the size of a mouse, to one the size of an elephant, could be accomplished over 60,000 years, with a rate of change too small to be noticed over any human lifetime. 60,000 years is too small a gap to be identified or identifiable in the fossil record. Experts in evolutionary theory have pointed out that even if it were possible