As early as 1902 Rollins wrote, almost despairingly, that his warnings about the dangers involved in careless use of X-rays were not being heeded by industry or by his colleagues. By this time Rollins had proved that X-rays could kill animals, could cause a pregnant guinea pig to abort, and that they could kill a fetus. He also stressed that "animals vary in susceptibility to the external action of X-light" and warned that these differences should be considered when patients were treated by means of x-rays.