Worse, thanks to what is called “shifting baseline syndrome,” changes that initially seem extreme come to be accepted as normal over time. The next generation of children, who will never remember how many birds (or butterflies or hedgehogs or manatees) once existed in the wild, will not hear anything eerie in the silence. Once a mass murder is complete, its victims tell no tales. When humans killed the last of the giant “Steller sea cows” several hundred years ago, the memory of these mega-manatees faded away quickly, and now they seem like something fantastical. We know that “in just over 100 years the world lost as much forest as it had in the previous 9,000 years,” but when nobody is left alive who remembers the old forests, the loss is not felt.