My question concerns poincare invariance (I have left out the accent) in bosonic string theory. As far as I know, action of a 1-d String is described by poincare invariance. So my question is: why poincare invariance? And here comes the more ambarassing question: What is poincare invariance(deriviations are more than welcome)? I am refering to Polchinskis first volume of two ,named "Introduction to bosonic string theory". The page is given below...Thanks for any help!