Scientist: In our study, chemical R did not cause cancer in laboratory rats. But we cannot conclude from this that chemical R is safe for humans. After all, many substances known to be carcinogenic to humans cause no cancer in rats; this is probably because some carcinogens cause cancer only via long-term exposure and rats are short lived.
It illustrates the claim that rats are too short lived to be suitable as test subjects for the carcinogenic properties of substances to which humans are chronically exposed most precisely describes the role played in the scientist's argument by the statement that chemical R did not cause cancer in laboratory rats.