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Not long ago, all that was known in zoology, botany, geology and related subjects was studied under the heading of natural history. With the rapid advance in science, zoology alone has grown to such magnitude that it must be divided into numerous special fields. The number of described species of animals is said to range from 840,000 to more than a million, and one writer has said that there are probably two million species of living animals, to say nothing of the large numbers of extinct forms. The subdivision of zoology which deals with the classification of organisms according to their natural relationships is taxonomy. This textbook classifies the animal kingdom after first giving a historical review of zoology, with brief biographies and photographs of some famous zoologists. Then follows a discussion of protoplasm and many other chapters, each devoted to some one phylum or class of animals. Chapter four
Textbook of Zoology. JAMA. 1938;111(26):2417. doi:10.1001/jama.1938.02790520073027