The Historia animalium was Gessner's magnum opus, and was the most widely read of all the Renaissance natural histories. The generously illustrated work was so popular that Gessner's abridgement, Thierbuch, was published in Zurich in 1563, and in England Edward Topsell translated and condensed it as a Historie of foure-footed beastes. Gessner’s monumental work attempts to build a connection between the ancient knowledge of the animal world, its title the same as Aristotle's work on animals, and what was known at his time. He then adds his own observations, and those of his correspondents, in an attempt to formulate a comprehensive description of the natural history of animals. Gessner’s Historia animalium is based on classical sources. It is compiled from ancient and medieval texts, including the inherited knowledge of ancient naturalists like Aristotle, Pliny the Elder, and Aelian. Gessner was known as "the Swiss Pliny." For information he relied heavily on the Physiologus. In his larger works Gessner sought to distinguish fact from myth and popular misconceptions, and so his encyclopedic work included both extinct creatures and newly discovered animals of the East Indies, those of the far north and animals brought back from the New World. The work included extensive information on mammals, birds, fish, and reptiles. It described in detail their daily habits and movements. It also included their uses in medicine and nutrition. Historia animalium showed the animals' places in history, literature and art. Sections of each chapter detailed the animal and its attributes, in the tradition of the emblem book. Gessner's work included facts in different languages such as the names of the animals.
Fantastical creatures
There have been various academic studies relating to Gessner's inclusion of some fantastical-looking creatures in the volumes, such as sea monk, sea bishop, or ichthyocentaur. , United Kingdom. Gessner was aware of fakery in the curio shops market, where dried rays were manipulated to look like dragons. There may have also been fake mermaid-like creatures being imported from China by the Dutch. Also, commercial interests may also have motivated publishers or authors such as Gessner to include such creatures to boost sales. But Gessner was known for meticulously checking facts, and it has been suggested that publishers may have interpolated material when Gessner was in no condition to gainsay them, since the author was already morbidly ill by the time of these publications. In fact there is the example of the Su of Patagonia, posthumously inserted in the 1603 Frankfurt edition.
The colored woodcut illustrations were the first real attempts to represent animals in their natural environment. It is the first book to illustrate fossils. Gessner acknowledges one of his main illustrators was Lucas Schan, an artist from Strasbourg. He likely used other illustrators as well as himself; the book is however famous for copying illustrations from other sources, including Durer's Rhinoceros from a well-known woodcut. Gessner's natural history was unusual for sixteenth centuryreaders in providing illustrations.
