By late 1941, Adolf Hitler decided that the Jews of Europe were to be exterminated, so Birkenau, originally intended to house slave laborers, was re-purposed as a combination labor camp / extermination camp. Prisoners were transported there by rail from all over German-occupied Europe, arriving in daily convoys. By July 1942, the SS were conducting "selections". Incoming Jews were segregated; those deemed able to work were admitted into the camp, and those deemed unfit for labor were immediately killed in the gas chambers. The group selected to die, about three-quarters of the total, included almost all children, women with small children, pregnant women, all the elderly, and all those who appeared on brief and superficial inspection by an SS Doctor not to be completely fit. Mengele, a member of the team of doctors assigned to do selections, undertook this work even when he was not assigned to do so in the hope of finding subjects for his experiments. He was particularly interested in locating sets of twins. In contrast to most of the doctors, who viewed undertaking selections as one of their most stressful and horrible duties, Mengele undertook the task with a flamboyant air, often smiling or whistling a tune.