The quasiconformal equivalence of Riemann surfaces and the universal Schottky space

In the theory of Teichm"uller space of Riemann surfaces, we consider the set of Riemann surfaces which are quasiconformally equivalent. For topologically finite Riemann surfaces, it is quite easy to examine if they are quasiconformally equivalent or not. On the other hand, for Riemann surfaces of topologically infinite type, the situation is rather complicated. In this paper, after constructing an example which shows the complexity of the problem, we give some geometric conditions for Riemann surfaces to be quasiconformally equivalent. Our argument enables us to obtain the universal Schottky space which contains all Schottky spaces, the deformation spaces of Schottky groups as the universal Teichm"uller space contains all Teichm"uller spaces.