Isidore of Seville was born in the latter half of the sixth century (the exact date is unknown) and died around 636 CE. Little is known of his early life. A biography of Isidore was supposedly written in the thirteenth century by Lucas Tudensis (in the Acta Sanctorum), but it is mostly fable and cannot be trusted. His family originated in Cartagena; they were orthodox Catholic and probably Roman, and were likely a family with some power and influence. Isidore's parents died while he was young, leaving him in the care of his older brother, Leander. Isidore probably received a classical education, intended to prepare him for a life in service to the Church. His brother Leander became a monk, and later was raised to the position of bishop of Seville; Isidore succeeded him as bishop around 600 CE, a post he held to the end of his life. He was a respected figure in the Church, as can be seen from the introduction to his works written by Braulio, bishop of Saragossa:

"Isidore, a man of greate distinction, bishop of the church of Seville, successor and brother of bishop Leander, flourished from the time of Emperor Maurice and King Reccared. In him antiquity reasserted itself--or rather, our time laid in him a picture of the wisdom of antiquity: a man practiced in every form of speech, he adapted himself in the quality of his words to the ignorant and the learned, and was distinguished for unequalled eloquence when there was fit opportunity. Furthermore, the intelligent reader will be able to understand easily from his diversified studies and the works he has completed, how great was his wisdom."  (Brehaut, p. 23)

Isidore was known mostly for his writing. He was the author of several books: the Differentiae, an exposition on the differences of words and things; the Proemia, a description of the books of the Bible; the De Ortu et Obitu Patrum, on the works and deaths of the Church fathers; the Officia, an explanation of church ritual; the De Natura Rerum, "on the nature of things," a description of the universe as it was known in the seventh century; the Liber Numerorum, nominally on the "science of arithemetic" but actually on the mystical meaning of numbers used in the Bible; the Chronica, a history of the world from creation to his own time; The Allegoriae, on the allegories attributed to Bible stories; the Sententiae, a treatise on Christian morals and doctrine, based chiefly on the Moralia of Gregory the Great; the De Ordine Creaturarum, on the spiritual and material universe; and the Etymologiae, an encyclopedia of all knowledge.

It is the Etymologies that proved most useful to the later compilers of the bestiaries. In it Isidore attempted to set down, in twenty volumes, the basics of all that was known on a vast range of topics, including grammar, rhetoric and logic; arithematic, geometry, and astronomy; law, military science and theology; cosmology; and agriculture, mineralogy, physiology and zoology, among others. There is very little that is original in the Etymologies Isidore "borrowed" from the works of such earlier writers as Pliny the Elder, Solinus, Aristotle, Justinius, Lucretius, Cassiodorus, Servius, Suetonius, Hyginus, Ambrose, Augustine, Orosius, Tertullian, Sallust, Hegesippus, and many others. While Isidore was certainly well-read, his reading was not critical: he accepted most of what he read without question, only rarely expressing doubt about the information he appropriated. His main goal throughout the Etymologies is not only to record facts, but to assign meaning, usually, as the title suggests, through etymology.

Etymology is the study of the histories of words, an attempt to trace their development back to their origins. Isidore believed that the names of things gave some insight into the properties of those things; he further believed that the original names were assigned in the "first language," Hebrew. In the introduction the book on animals, Isidore says:

"Adam first named all living creatures, assigning a name to each in accordance with its purpose at that time, in view of the nature it was to be subject to. But the nations have named all animals in their own languages. But Adam did not give those names in the language of the Greeks or Romans or any barbaric people, but in that one of all languages which existed before the flood, and is called Hebrew."  (Brehaut translation)

For the most part, Isidore's etymological analysis is fanciful at best, generally linking the names of things to unrelated words that merely have a similar sound or form, in order to get the meaning he wants. The derivations are based on Latin or Greek words; for example: "Bees [in Latin apes] are so called either because they bind themselves together with their feet [in Latin pes] or because they are born without feet [a-pes]..."  (Grant translation); or "The eagle [aquila] is so called from its sharpness [acumine] of sight."  (Brehaut translation).

Book 12 of the Etymologies is about animals. Isidore took much of his information from Aristotle and Pliny, who also wrote about real and imaginary animals. Isidore, as usual, accepted whatever his sources told him; observation of the real world has little part in his "zoology." However, unlike the earlier Physiologus, Isidore did not include any moralizations or allegory in his beast stories. The compilers of the later bestiaries quoted Isidore extensively and added the allegory Isidore left out. For example, in the Aberdeen Bestiary (f. 37v) we find: "In his book of Etymologies, Isidore says that the raven picks out the eyes in corpses first, as the Devil destroys the capacity for judgement in carnal men, and proceeds to extract the brain through the eye. The raven extracts the brain through the eye, as the Devil, when it has destroyed our capacity for judgement, destroys our mental faculties." In fact, Isidore supplied only the basic "facts" about the raven; the allegory was from other sources. Philippe de Thaon in his Bestiaire quotes Isidore on the ant: "...Isidore speaks of the ant in his writing, and shows the reason well why it is named formica It is fortis (strong), and carries mica (a particle), that is the meaning of the name; there is no creature of so small a shape, which carries by its own force so great a burden..."  (Wright translation), in this case repeating Isidore's etymology, as was often done.

While it is easy to be critical of Isidore's credulity, we must remember that he was not a "scientist" in the modern sense, and that his purpose in gathering the information found in the Etymologies was not the same as that of the compilers of modern encyclopedias. As  Brehaut (p. 33) says:

"Throughout the Etymologies there is a leading principle which guides Isidore in his handling of the different subjects, namely, his attitude toward words. His idea was that the road to knowledge was by way of words, and further, that they were to be elucidated by reference to their origin rather than to the things they stood for. ... His confidence in words really amounted to a belief, strong though perhaps somewhat inarticulate, that words were transcendental entities. All he had to do, he believed, was to clear away the misconceptions about their meaning, and set it forth in its true original sense; then, of their own accord, they would attach themselves to the general scheme of truth. The task of first importance, therefore, in treating any subject, was to seize upon the leading terms and trace them back to the meanings which they had in the beginning, before they had been contaminated by the false usage of the poets and other heathen writers; thus the truth would be found."

Other medieval encyclopedias:

Bartholomeus Anglicus : De proprietatibus rerum

Hrabanus Maurus : De rerum naturis

Lambert of Saint-Omer : Liber floridus

Brunetto Latini : Li Livres dou Tresor

Jacob van Maerlant : Der Naturen Bloeme

Konrad von Megenberg : Das Buch der Natur

Thomas de Cantimpré : Liber de natura rerum