If a curiously selective plague came along and killed all people of intermediate height, 'tall' and 'short' would come to have just as precise a meaning as 'bird' or 'mammal'. The same is true of human ethics and law. Our legal and moral systems are deeply species-bound. The director of a zoo is legally entitled to 'put down' a chimpanzee that is surplus to requirements, while any suggestion that he might 'put down' a redundant keeper or ticket-seller would be greeted with howls of incredulous outrage. The chimpanzee is the property of the zoo. Humans are nowadays not supposed to be anybody's property, yet the rationale for discriminating against chimpanzees in this way is seldom spelled out, and I doubt if there is a defensible rationale at all. Such is the breathtaking speciesism of our attitudes, the abortion of a single human zygote can arouse more moral solicitude and righteous indignation than the vivisection of any number of intelligent adult chimpanzees! [T]he only reason we can be comfortable with such a double standard is that the intermediates between humans and chimps are all dead.