Natural selection played a major role in the development of the breed, as large numbers of horses died from lack of food and exposure to the elements. Between 874 and 1300 AD, during the more favorable climatic conditions of the medieval warm period, Icelandic breeders selectively bred horses according to special rules of color and conformation. From 1300 to 1900, selective breeding became less of a priority; the climate was often severe and many horses and people died. Between 1783 and 1784, around 70% of the horses in Iceland were killed by volcanic ash poisoning and starvation after the 1783 eruption of Lakagígar. The eruption lasted eight months, covered hundreds of square miles of land with lava, and rerouted or dried up several rivers. The population slowly recovered during the next hundred years, and from the beginning of the 20th century selective breeding again became important. The first Icelandic breed societies were established in 1904, and the first breed registry in Iceland was established in 1923.