"Differences in reaction to toxic substances vary considerably between species so that the value of these tests remains doubtful. Although thalidomide was extensively tested on animals in several countries, its terrible properties were not discovered. Conversely, penicillin, the greatest medical discovery of the century, was not extensively tested on animals before its miraculous therapeutic qualities were demonstrated in human patients. If it had been fully tested on animals its high toxicity for guinea pigs would have almost certainly prevented its clinical use."