Lord May also pointed out that it was very difficult to make
accurate estimates as nobody knew how many species of
animals lived on the planet. So far 1.5 million species had
been named, but the true figure might be as high as 100
million. Our ignorance of this made it almost impossible
to work out the actual rate of extinction. However, the use
of intelligent guesses suggests that losses over the past
century were comparable with the extinctions of earlier
periods, evidence of which is found in the fossil record.