Simplify the following text for a Grade 12 native English-speaking person.
Original: Several decades later , Wade Davis , a Harvard ethnobotanist , presented a pharmacological case for zombies in two books , The Serpent and the Rainbow ( 1985 ) and Passage of Darkness : The Ethnobiology of the Haitian Zombie ( 1988 ) .
Simplified: Wade Davis , a Canadian ethnobotanist ( someone who studies what effect plants have on people ) , wrote about zombies in two books , The Serpent and the Rainbow ( 1985 ) and Passage of Darkness : The Ethnobiology of the Haitian Zombie ( 1988 ) .