Sir David Attenborough’s hushed but excited tones have been heard in living rooms around the world. In honour of Sir David’s 100th birthday on 8 May , here are some of his most memorable quotes: – “It seems to me that the natural world is the greatest source of excitement; the greatest source of visual beauty; the greatest source of intellectual interest. It is the greatest source of so much in life that makes life worth living.” – “I just wish the world was twice as big and half of it was still unexplored.” – “The question is, are we happy to suppose that our grandchildren may never be able to see an elephant except in a picture book?” – “It is vital that there is a narrator figure whom people believe. That’s why I never do commercials.…