Modern genetics poses some of the most significant ethical problems that science has ever faced. As a result of recent advances, scientists are now able to engineer living organisms with genes taken from unrelated species. Proponents of genetic engineering claim that this technology is a natural continuity of older breeding practices. This is, however, clearly not true. The new genetic technology is a human intervention that must not be confused with any previous intrusions upon nature; like animal and plant breeding for agricultural purposes, or the induction of mutations with x rays in laboratories. All earlier procedures worked within single or closely related species while the genetic engineering of today is unprecedented in that it allows human beings to direct the crossing of species barriers in entirely new ways. Though scientific morality up to now has been to proceed without...