"He told me that a German doctor named Wolff figured it out in the 1800s by studying X-rays of infants' hips as they transitioned from crawling to walking. "A whole new evolution of bone structure takes place to support the mechanical loads associated with walking," said Lang. "Wolff had the great insight that form follows function." Alas, Wolff did not have the great insight that cancer follows gratuitous X-raying with primitive nineteenth-century X-ray machines."