its an easily provable fact. If you just check the written record, with say google's ngram detector (or any other method), you can see when and where the phrase comes into use. Previous to the 1700's, people described themselves as Anglo or English or Irish, or Protestant, pale, rosy, Romanesque, fair-skinned, swarthy, African, negroid, mongoloid, Asiatic, dark-skinned, etc. but no one thought they were "white", self identified as white. Its kind of an odd adjective to choose for most continental Europeans whose skin is not as pale as their teeth.