From “Hints on Singing”:
Q: While the faults of emission are mending, is there any other study to be pursued?
A: The acquirement of agility.
Q: How is this to be obtained?
A: By the study of diatonic scales, passages of combined intervals, arpeggios, chromatic scales, turns, shakes, light and shade.
Q: How long will this study take?
A: Not less than two years.
Q: Is agility the only result of this study?
A: When properly directed, it renders the organ flexible, even, mellow, besides strengthening and preparing it for the florid style as well as for the plain and declamatory (canto fiorito, canto spianato, canto declamato).
Q: Cannot singers avoid all that trouble?
A: They cannot, but they do. Anyone who wishes to obtain proficiency in the art can no more avoid this amount of study than a violinist, a pianist, or any other instrumentalist. A less ambitious singer may be content with ballads or nota e parola pieces. But even if the singer be gifted with a fine voice and talent, the organ will show the absence of culture, by the uncertain and irregular manner of uniting and colouring the vowels.
Image below from the Garcia School of Singing Method, published by Ditson.