Platypus vanish from river where Charles Darwin pondered evolution

Almost 190 years on, in the place where Charles Darwin first spotted a platypus, the egg-laying mammal seems to have disappeared.

According to documents in the State Library of NSW, the fateful platypus encounter led the famous naturalist to question the prevailing idea of a singular creator: a key step on the way to his famous theory of evolution.

But anyone strolling along the banks of the Cox's River today would be unlikely to spy the notoriously shy monotremes.

Scientists and environmental groups have blamed unnaturally high salinity levels in a stretch of the river near Lithgow, on the western side of the Blue Mountains.

"There are no platypus in that section of the Cox's now because it is heavily polluted with salt and other metal pollutants," Associate Professor Ian Wright said.