39 books • 12 series
Book #1: How to Begin the Study of Amphibians
Book #2: How to Begin the Study of Birds
Book #3: How to Begin the Study of Spiders
Book #3: The Size of Animals
Book #10: Form and Function in Animals
Book #15: Why the Dinosaurs Became Extinct
Book #17: Biological Periodicities
Book #16: Terrestrial Environments
Book #6: Microecology