The original series, which ran from 1922 until 1933. You can view a complete list of these films here, and many of them are available to stream online.
The sequel to Secrets of Nature, made by many of the same cinematographers, directors and producers, released between 1934 and 1950.
In the 1930s, many of the same individuals produced instructional films for Gaumont-British Instructional Films, with a focus mainly on biology.
Here you can view a list of related titles available to watch online, including other films made by the Secrets producers, additional natural history productions from the period, and amateur films.