The great art of the singer consists in acquiring the ability to render imperceptible to the ear, the passing from the one register to the other. In other words, to unite the two, so as to have perfect quality of voice throughout the whole range, each tone being on a level with your best and purest tone. This is art and it is not easy to reach the goal. It takes study, work and industry to correct the defects originated from the more or less strong constitution of the vocal organs, and it requires ability and such a careful use of the voice to render it equally sonorous and agreeable, that few students succeed. Many teachers do not know the practical rules, or else thy do not know how to apply them. It is of no use or reason for one to reproach Nature, saying she has not given one a voice of even register. In such a case, the singer should accuse himself, or make a complaint against those teachers who did not know how to guide him the right way.
Giambattista Mancini, “Practical Reflections on the Figurative Art of Singing” published in 1774.