It cannot be too strongly impressed upon, or too frequently pointed out to, the singer (no matter what may be the stage of his or her artistic development) how desirable and advantageous it is to be constantly singing exercises and solfeggi in preference to songs. It is a popular fallacy, especially among amateurs, that the practice of scales and intervals should be left behind as soon as possible. Pray do not be mistaken. The never-failing daily practice of singing open chords in solfeggi, scales, and exercises, is fraught with advantages which cannot be gained by the study of yards, or even miles, of song tunes. As an instance of how much may be done in the study of scale practice, the writer would point out that this particular exercise should not be left until the student can sing the diatonic scales throughout the whole extent the voice in one unbroken breath, and with one quality, character, and volume of voice. When this point has been reached, its peculiar efficacy will be so apparent as  not to require any recommendation or advice for its daily continuance.

Anonymous. Advice to singers, by a singer. 1882.