LONDON — It might sound like tomorrow's medicine, but legend has it that the first documented case of xenotransplantation dates back to 1682, when bone from a dog was successfully used to repair the skull of an injured Russian aristocrat. The potential for use of animal organs and tissue to benefit humankind remains as controversial today as it was then. But after decades of failed attempts to successfully marry the two, new signs of progress are beginning to emerge in the ability to achieve successful transplantation between different species.