Bates, Henry Walter (1825-1892), a British naturalist and explorer, participated in a scientifically important expedition up the Amazon River in South America. In 1861, Bates presented a famous paper based on some of his discoveries, which firmly supported Charles Robert Darwin's theory of evolution. He is best known for his theory of mimicry.

Although he was a gifted pupil and musician, Bates, the eldest son of a Leicester hosiery manufacturer, was apprenticed at 13 to one of his father's colleagues. He continued to take night classes at the local Mechanic's Institute, however, where he studied classical languages, French, drawing, and composition. He would later learn German and Portuguese while traveling in South America.

Bates also showed an early interest in entomology and, at 18, published a short paper on beetles in the first issue of The Zoologist. In about 1844, he became acquainted with Alfred Russel Wallace, then a master at Leicester's Collegiate School, whom he introduced to entomology. After Wallace moved the following year, the friends continued to exchange specimens and to discuss the origin of species. In 1847, Wallace made a bold suggestion: together they should travel to tropical jungles to gather specimens, send them back to England for sale, and collect data that might illuminate the problem of the origin of species. Wallace and Bates were inspired by William H. Edwards, author of the recently published Voyage Up the River Amazon, Including a Residence at Pará, and they were fortunate enough to meet him and get help on their plan. Edward Doubleday of the British Museum also encouraged them by showing them some specimens of new butterfly species from the environs of Pará (Belém). Brazil, near the mouth of the Amazon.

The two amateur naturalists set out for Pará in late April 1848 and arrived there in late May. Wallace returned to England four years later, while Bates remained for a total of 11 years, traveling almost 2,000 miles (3,219 kilometers) into the wilderness. Of the nearly 15,000 animal species Bates collected over the years, more than 8,000 had never been known to science before.

Bates was headquartered at Santarém, a small town at the mouth of the Tapajós River, from 1849 through 1851, and at Ega (Tefé), at the foot of the Andes, from autumn 1851 to the end of his South American sojourn. On Feb. 11, 1859, he set out for the voyage home. That November, a few months after Bates's arrival in England, Darwin's pioneering masterpiece, On the Origin of Species by Means of Natural Selection, was published. Bates read Darwin's book and believed he had considerable additional evidence to support Darwin's thesis.

Two years after the publication of Origin of Species, Bates read a paper, “Contributions to an Insect Fauna of the Amazon Valley,” to the Linnean Society (before whom Darwin had first expounded his views in 1858). The paper set forth Bates's discovery that several different butterfly species have nearly identical color patterns on their wings, only some of which deter bird predators. Bates argued that natural selection and survival of the fittest caused butterfly wings to mimic the patterns that increase survival chances. Before Bates published his paper, other naturalists had noticed the similarity in color patterns among certain butterfly groups inhabiting the same geographical area. Bates was the first to offer a full scientific explanation for it. This phenomenon, now called Batesian mimicry, fully confirmed Darwin's theory of natural selection. Using Bates's data, Darwin was able to rebut his critics in a short, unsigned review in the Natural History Review of 1863.

In 1863, Bates published The Naturalist on the River Amazon, in which he described both his expedition and his scientific discoveries.

In 1864, Bates became assistant secretary of the Royal Geographical Society of London, a post he held for 28 years. He edited its journal and proceedings and served as consultant or assistant to other scientific journals, including The Entomologist. Bates contributed more than 100 scientific papers on entomology to scholarly journals, which earned him worldwide fame as a preeminent authority on Coleoptera, the order comprising the beetles.