One of the observations in Micrographia was of fossil wood, the microscopic structure of which he compared to ordinary wood. This led him to conclude that fossilised objects like petrified wood and fossil shells, such as Ammonites, were the remains of living things that had been soaked in petrifying water laden with minerals. Hooke believed that such fossils provided reliable clues to the past history of life on Earth, and, despite the objections of contemporary naturalists like John Ray who found the concept of extinction theologically unacceptable, that in some cases they might represent species that had become extinct through some geological disaster.
