Evidence of recreational swimming date back to prehistoric times with Stone Age paintings, dated to around 10000 years ago, showing the activity. The first book about swimming was written by Nikolaus Wynmann in 1538. Swimming at a competitive level started to develop in England in the 1830s. The National Swimming Society began to hold regular swimming competitions around London in 1837. A swimming stroke which would later develop into front crawl was introduced to a European audience by two Native American participants at a swimming competition in 1844. Men's swimming was included at the first modern Olympics in 1896. Butterfly was developed in the 1930s having begun as a variant of breaststroke before it was accepted as a separate style in 1952. Water polo developed from demonstrations of swimming strength and skill. The first rules of water polo were codified by William Wilson in the late 19th century. Water polo was introduced at the Olympics in 1900.