Callas's voice has been difficult to place in the modern vocal classification or Fach system, especially since in her prime, her repertoire contained the heaviest dramatic soprano roles as well as roles usually undertaken by the highest, lightest and most agile coloratura sopranos. Regarding this versatility, Serafin said, "This woman can sing anything written for the female voice". Michael Scott argues that Callas's voice was a natural high soprano, and going by evidence of Callas's early recordings, Rosa Ponselle likewise felt that "At that stage of its development, her voice was a pure but sizable dramatic coloratura—that is to say, a sizable coloratura voice with dramatic capabilities, not the other way around." On the other hand, music critic John Ardoin has argued that Callas was the reincarnation of the 19th-century "soprano sfogato" or "unlimited soprano", a throwback to Maria Malibran and Giuditta Pasta, for whom many of the famous bel canto operas were written. He avers that like Pasta and Malibran, Callas was a natural mezzo-soprano whose range was extended through training and willpower, resulting in a voice which "lacked the homogeneous color and evenness of scale once so prized in singing. There were unruly sections of their voices never fully under control. Many who heard Pasta, for example, remarked that her uppermost notes seemed produced by ventriloquism, a charge which would later be made against Callas". Ardoin points to the writings of Henry Chorley about Pasta which bear an uncanny resemblance to descriptions of Callas: