The real reason polar bears are white:
It may be perpended whether [pigmentation] might not fall out the same way that Jacobs cattel became speckled, spotted and ring-straked, that is, by the Power and Efficacy of Imagination; which produceth effects in the conception correspondent unto the phancy of the Agents in generation; and sometimes assimilates the Idea of the Generator into a reality in the thing ingendred. For, hereof there pass for current many indisputed examples, so in Hippocrates we read of one, that from the view and intention of a Picture conceived a Negro; and in the history of Heliodore of a Moorish Queen, who upon aspection of the Picture of Andromeda, conceived and brought forth a fair one. And thus perhaps might some say it was the beginning of this complexion: induced first by Imagination, which having once impregnated the seed, found afterward concurrent co-operations, which were continued by Climes, whose constitution advantaged the first impression. Thus Plotinus conceiveth white Peacocks first came in. Thus many opinion that from aspection of the Snow, which lieth long in Northern Regions, and high mountains, Hawks, Kites, Bears and other creatures become white
--Thomas Browne, Pseudodoxia Epidemica, 1646, Bk. VI, ch. 10, 275-76.