William Ernest Castle was born on a farm in Ohio and took an early interest in natural history.
Formulated in the terms "as soon as selection is arrested the race remains stable at the degree of purity then attained", it appeared in his paper of November that year.
In 1908 Castle moved from the Harvard Museum of Comparative Zoology to the Bussey Institution for Applied Biology.
His work with hooded rats provided important evidence that evolution could occur by the action of selection on small variations in traits.
Other biologists (including T. H. Huxley and William Bates) had doubted Darwin's belief in the sufficiency of small variations (acted upon by natural selection over long periods of time) to explain evolution.
(2) Castle retired from Harvard in 1936 when the Bussey Institution closed, and took up a position at the University of California, Berkeley, as Research Associate in mammalian genetics.