Ryder used the term again in his contribution to the Godlovitches/Harris book, in an essay called "Experiments on Animals" (1971). He wrote in the essay that animal researchers seek to have it both ways: they defend the scientific validity of animal experiments on the grounds of the similarity between humans and nonhumans, while defending the morality of it on the grounds of the differences. He argued that speciesism is as illogical as racism, writing that "species" and "race" are both vague terms, and asked: "If, under special conditions, it were one day found possible to cross a professor of biology with an ape, would the offspring be kept in a cage or in a cradle?"