"Why don't more animals regenerate? Although it's useful, regeneration isn't the best survival strategy because healing wounds is a trade-off between speed and accuracy. Perfect repairs can take time, whereas immediately closing an open wound minimizes the risk of infection or bleeding to death, although it does leave scar tissue. Animals use a fibrous connective protein called collagen to holds cells together (one-third of the human body consists of collagen). It's arranged in a basketweave (criss-cross) pattern in regular tissue, while scar tissue fibres are aligned side-by-side. Regular tissue is like fabric material, whereas scars are stitches holding two pieces of fabric together. Stitching is faster than weaving, so it's better for rapid repair."