Allan Kardec. The book of spirits (№ 397212)

Question: is the progress of mankind takes place steadily and slowly?
Answer: There is a continuous and slow progress, carried out by the force of things; but when people are not progressing fast enough, God produces it from time to time some physical or mental, is a concussion, it is transformational.
Man cannot forever remain in ignorance, because it is intended to achieve the objective set by Providence, and he's gaining knowledge by the power of the things themselves. Moral revolution as the revolution of social, penetrate little by little in ideas; they grow for centuries, then suddenly explode and crush the old building of the past, are not more in harmony with new needs and new aspirations.